- Email: [email protected]
Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6
Materials Chemistry and Physics xxx (2015) 1e13
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6 Yan Zhang a, *, Vincent Ji b, Ke-Wei Xu c a
School of Materials Science and Engineering, Chang'an University, Xi'an 710061, Shaanxi, PR China ICMMO/SP2M, UMR CNRS 8182, Universit e Paris-Sud, 91405 Orsay C edex, France c State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, PR China b
h i g h l i g h t s Half-metallic and magnetic properties of the disordered Pb2FeMoO6 are studied by GGAþU. No structural changes are observed for FeMo, MoFe , FeeMo, VPb cases. The 6 (8) nearest oxygen neighbors move away from (close to) VFe or VMo (VO ) vacancy. Half-metal holds for FeMo, VFe , VO , VPb , but vanishes for MoFe, FeeMo, VMo , even C ¼ 6.25%. Total moments decrease in sequence of VPb , VMo , FeeMo, VO , VFe , FeMo , MoFe .
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 25 May 2015 Accepted 24 June 2015 Available online xxx
In point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the disordered Pb2FeMoO6 compound containing seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU). No obvious structural changes are observed for the cases of the FeMo and MoFe antisites, FeeMo interchange, and VPb vacancy, however, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO ) vacancy. The half-metallic character is maintained for the FeMo antisite, VFe , VO or VPb vacancy cases, while it vanishes in the MoFe antisite, FeeMo interchange or VMo vacancy cases even the defect concentration reduces down to C ¼ 6.25%. So the MoFe antisite, FeeMo interchange or VMo vacancy defects have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound and thus usable in magnetoresistive and spintronics devices. In FeMo or MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. On the contrary, in FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe , VO or VPb vacancy cases, a ferromagnetic coupling is observed within each cation sublattice, while the two cation sublattices are coupled antiferromagneticly. But in VMo vacancy case, a ferromagnetic coupling is obtained not only within each cation sublattice but also between Fe and Mo sublattices. The saturation magnetization of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb , VMo , FeeMo, VO , VFe , FeMo and MoFe cases. © 2015 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Magnetic materials Ab initio calculations Electronic structures Magnetic properties
1. Introduction Double perovskites, which were firstly discovered by Ward et al.
* Corresponding author. E-mail address: [email protected] (Y. Zhang).
in 1961 [1,2], are a broad class of compounds with a chemical formula of A2BB0 O6 [3]. The A-site is usually an alkaline-earth metal atom (Ca, Sr, Ba) or rare-earth metal atom (La, Ce, Nd), the B-site is a 3d (Cr, Mn, Fe, Co, Ni, Zn) and B0 -site is a 4d (Mo, Te, Ru) or 5d (W, Re, Os) transition-metal (TM) atom. Most of the double perovskites have been found to take a rock-salt crystal structure with alternate
http://dx.doi.org/10.1016/j.matchemphys.2015.06.046 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
2
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
perovskite units ABO3 and AB0 O3 along three crystallographical axes. The corners of each perovskite unit are in turn occupied by the TM atoms B and B0 with oxygen atoms located in between, forming alternate BO6 and B0 O6 octahedra. The large alkaline-earth metal or rare-earth metal atom A occupies the body-centered site with a 12fold oxygen coordination in each unit. A recent finding of the intrinsic tunneling magnetoresistance (TMR) effect at room temperature in ordered Sr2FeMoO6 [4] and Sr2FeReO6 [5] revives the extensive studies on the double perovskites. The calculated energy band structures reveal that they are ferrimagnetic half-metal with highly spin-polarized transport properties at the Fermi level [3e5]. The Curie temperatures TC of the Sr2FeMoO6 and Sr2FeReO6 are found to be as fairly high as 415 and 401 K, respectively, making them potential candidates for industrial applications in magnetoresistive [6] and spintronics [7] devices at room temperature. Besides experimental works [4,5,8], various first-principles calculation methods [4,5,9e14] including the local spin density approximation (LSDA), the generalized gradient approximation (GGA), and both schemes with on-site Coulomb correlation correction (LSDAþU and GGAþU) have been used to investigate the electronic and magnetic properties of the ordered Sr2FeMoO6 and Sr2FeReO6. It has been found that at the ground state, the Fe3þ (3d5 ) are in the high spin state of S ¼ 5/2 according to Hund's rule, Mo5þ(4d1 ) and Re5þ(5d2 ) are highly ionized with valence spin states of S ¼ 1/2 and S ¼ 1, respectively. Each of the two TM sites, namely, the Fe3þ (3d5 , S ¼ 5/2) and Mo5þ (4d1 , S ¼ 1/2) or Re5þ (5d2 , S ¼ 1) sites, are believed to be ferromagnetically (FM) arranged within each sublattice, while the two sublattices are coupled antiferromagnetically (AFM), giving rise to the total spin magnetic moments for the ferrimagnetic (FiM) states are thus 4 and 3 mB per formula unit (f.u.) for Sr2FeMoO6 and Sr2FeReO6, respectively. The smaller saturation magnetizations of 3 and 2.7 mB /f.u. at 4.2 K for Sr2FeMoO6 [4] and Sr2FeReO6 [5], respectively, are attributed to the mis-site-type disorder on the TM sites [15e17]. As is well known that the choice of a suitable combination of the atoms A, B and B0 in double perovskites A2BB0 O6 is a key issue to obtain the desired electronic and magnetic properties and ultimately industrial applications. Researches are still being performed to seek for A2BB0 O6 materials with optimized properties. Since the physical properties of double perovskites A2BB0 O6 are not only depended on the TM elements B and B0 but also associated with the characteristics of A-site atom [18,19] and the ionic radius of Pb2þ (1.63 Å) is comparable with that of Sr2þ (1.58 Å) [20], it is likely suitable for the A-site atom of the double perovskites A2BB0 O6. In our previous paper [21], we proposed to substitute Sr2þ ion with Pb2þ ion in Sr2FeMoO6 and the structural, electronic and magnetic properties of the double perovskite Pb2FeMoO6 have been studied in detail. The half-metallic (HM) ferromagnetic nature implies a potential application of this new compound in magnetoresistive and spintronics devices. One year later, the magnetic properties and magnetoresistive effects of the Pb2FeMoO6 have been systematically studied in experiment [22]. The sample shows ferromagnetic behavior with Curie temperature TC about 243 K. Recently, the HM character has also been observed in numerous double perovskites, such as Ba2CrMoO6 and Ba2FeMoO6 [23], which presented completely spin polarization of the conduction electrons crossing the Fermi level. Usually, the HM materials are characterized by the coexistence of metallic behavior in one electron spin channel and insulating behavior in the other spin channel. Their electronic density of states (DOS) is completely spin polarized at the Fermi level, and conductivity is dominated by these metallic single-spin charge carriers. Therefore, the HM materials offer potential technological applications in the realm of single-spin electron source and high-efficiency magnetic sensors. Similar to the Sr2FeMoO6 case [4,13,15e17,24e33], the
fabrication of the double perovskite Pb2FeMoO6 would necessarily lead to a certain degree of the defects and thus reduce the saturation magnetization and even destroy the half-metallic character. To the best of our knowledge, there are no works done on the effects of the defects on the structural, electronic and magnetic properties of the Pb2FeMoO6. In this paper, we make a comparative study on the disordered Pb2FeMoO6 compound with seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies. With this analysis, we try to understand the effect of defects on half metallicity and magnetization. 2. Calculation methods and models The calculations are performed using the Vienna ab-initio simulation package (VASP) based on the density function theory (DFT) [34e37]. The electron-ionic core interaction is represented by the projector augmented wave (PAW) potentials [38] which are more accurate than the ultra-soft pseudopotentials. To treat electron exchange and correlation, we chose the Perdew-BurkeErnzerhof (PBE) [39] formulation of the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU) (U ¼ 2.0 eV for Fe and 1.0 eV for Mo [10,40]), since the TM Fe and Mo have localized electrons in their d-orbitals and these localized d-electrons have energies near the Fermi level. A conjugate-gradient algorithm is used to relax the ions into their ground states, and the energies and the forces on each ion are converged within 1.0 104 eV/atom and 0.02 eV/Å, respectively. The cutoff energy for the plane-waves is chosen to be 450 eV. The Pb 6s2 6p2 , Fe 3d6 4s2 , Mo 4s2 4p6 4d5 5s1 and O 2s2 2p4 electrons are treated as valence electrons. The k-point is sampled according to the Monkhorst-Pack automatic generation scheme with their origin at G point [41], together with a Gaussian smearing broadening of 0.1 eV. In order to compare the electronic structures of the ordered and the disordered Pb2FeMoO6 on the same footing, calculations are performed by constructing the supercells of four f.u.. As compared with Fig. 1(a) for the perfect Pb8Fe4Mo4O24, seven different defects (indicated by circles) are considered: (b) Fe antisite (FeMo ), one Fe atom substitutes for one Mo atom at (0.5, 0.5, 0.5) site (Pb8Fe5Mo3O24), (c) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0, 0.5, 0.5) site (Pb8Fe3Mo5O24), (d) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0, 0) and (0, 0, 0.5) sites (Pb8Fe4Mo4O24), (e) Fe vacancy (VFe ), removing one Fe atom from (0, 0.5, 0.5) site (Pb8Fe3Mo4O24), (f) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb8Fe4Mo3O24), (g) O vacancy (VO ), removing one O atom from (0.5, 0.5, 0.25) site (Pb8Fe4Mo4O23) and (h) Pb vacancy (VPb ), removing one Pb atom from (0.75, 0.75, 0.75) site (Pb7Fe4Mo4O24). 3. Results and discussions 3.1. Optimized structures The optimized structures of the four f.u. Pb2FeMoO6 with seven different defects (indicated by circles) are shown in Fig. 1(b)e(h) together with perfect one (a) for comparison. From Fig. 1(b)e(d) we can see that, for FeMo antisite, MoFe antisite and FeeMo interchange, no obvious structural changes are observed due to Fe3þ and Mo5þ ions having similar radii of 0.63 and 0.60 Å, respectively. However, from Fig. 1(e) and (f) we can see that, for VFe (VMo ) vacancy, the vanished attractions of the removed Fe (Mo) atom make its six nearest neighbor oxygen atoms move close to their nearest Mo (Fe) neighbors, so that the corresponding MoeO bond lengths reduce to 1.901 Å (ab plane) and 1.903 Å (c axis) (the corresponding
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Fig. 1. Optimized structures of ordered and disordered (indicated by circles) Pb2FeMoO6 with four f.u., (a) Perfect (Pb8Fe4Mo4O24), (b) Fe antisite (FeMo ), one Fe atom substitutes for one Mo atom at (0.5, 0.5, 0.5) site (Pb8Fe5Mo3O24), (c) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0, 0.5, 0.5) site (Pb8Fe3Mo5O24), (d) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0, 0) and (0, 0, 0.5) sites (Pb8Fe4Mo4O24), (e) Fe vacancy (VFe ), removing one Fe atom from (0, 0.5, 0.5) site (Pb8Fe3Mo4O24), (f) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb8Fe4Mo3O24), (g) O vacancy (VO ), removing one O atom from (0.5, 0.5, 0.25) site (Pb8Fe4Mo4O23) and (h) Pb vacancy (VPb ), removing one Pb atom from (0.75, 0.75, 0.75) site (Pb7Fe4Mo4O24).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
4
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
FeeO bond lengths reduce to 1.835 Å (ab plane) and 1.837 Å (c axis)). In case of ordered Pb2FeMoO6 as shown in Fig. 1(a), the optimized bond lengths are 1.950 Å (ab plane) and 1.954 Å (c axis) for MoeO bonds and 2.011 Å (ab plane) and 2.016 Å (c axis) for FeeO bonds and thus the lattice constants of a ¼ b ¼ 5.60 Å and c ¼ 7.94 Å for the corresponding body-centered tetragonal (BCT) structure. On the contrary, Fig. 1(g) shows the vanished repulsive forces of the removed oxygen atom make its eight nearest neighbor oxygen atoms move close to O vacancy site. From Fig. 1(h), we find that there are no obvious structural changes for Pb vacancy case. This is because the interactions between Pb atom and the other nearest neighbor atoms are negligible. 3.2. Total density of states (TDOS)ehalf-metallic characters In order to investigate the effects of the various defects on the spin polarization of the conduction electrons and half-metallic character of the Pb2FeMoO6 compound, we show in Fig. 2 the plots of total density of states (TDOS) of four f.u. Pb2FeMoO6 containing seven different defects, (a) Fe antisite (FeMo ), (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo), (d) Fe vacancy (VFe ), (e) Mo vacancy (VMo ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. We can see that the half-metallic character is maintained for the disordered Pb2FeMoO6 compound containing (a) Fe antisite (FeMo ), (d) Fe vacancy (VFe ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ), because around the Fermi level EF the TDOS of the down-spin channel (red lines) cross the Fermi level EF and those of the up-spin upspin channel (black lines) open the gaps Eg of 1.190, 1.177, 1.098 and 1.367 eV, respectively, although they are all slightly smaller than that of 1.370 eV for the perfect Pb2FeMoO6 [21]. On the contrary, the half-metallic character is lost for the disordered Pb2FeMoO6 compound containing (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo) and (e) Mo vacancy (VMo ), since the TDOS of both downspin channel (red lines) and up-spin channel (black lines) cross the Fermi level EF . So we conclude that the Mo antisite (MoFe ), FeeMo interchange (FeeMo) and Mo vacancy (VMo ) defects have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound. We present here the reasons for disappearing of the halfmetallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite defect (reference Figs. 1(c) and 2(b) for the structure and TDOS, respectively). The detailed orbitaldecomposed density of states (ODDOS) projected onto the concerned inequivalent Fe atoms at (0, 0, 0) and (0.5, 0, 0.5) regular Fe sites and Mo atoms at (0, 0.5, 0.5) MoFe antisite and (0.5, 0.5, 0.5) regular Mo site are shown in Fig. 3. The black and red lines represent up-spin and down-spin, respectively. It is clearly that, in Fig. 2(b) for the TDOS of the disordered Pb2FeMoO6 compound containing MoFe antisite defect, the TDOS of down-spin channel (red line) crossing the Fermi level EF are resulted from the hybridizations of t2g (dxy , dyz and dzx ) states of the Fe and Mo atoms at regular sites and antisite, which is similar to the perfect Pb2FeMoO6 case [21]. Since there are no states around the Fermi level EF for both channels of the s, p (px , py and pz ) and eg (dz2 and dx2 y2 ) orbitals of the Fe and Mo atoms at regular sites and antisite, the emerging of a new up-spin state (indicated by red arrow in Fig. 2(b)) in the band gap region of the ordered Pb2FeMoO6, i.e. the disappearing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite defect, is attributed to the small bonding-antibonding splitting of the up-spin t2g (dxy , dyz and dzx ) states of both antisite MoFe at (0, 0.5, 0.5) (especially) and near regular site Mo at (0.5, 0.5, 0.5) with positive and negative magnetic moments (see Table 1), respectively.
Since a FeeMo interchange defect consists of one FeMo antisite and one MoFe antisite, so we can predict the appearance of a new up-spin state (indicated by red arrow in Fig. 2(c)) in the band gap region of the ordered Pb2FeMoO6, i.e. the disappearing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing FeeMo interchange defect, is also attributed to the small bonding-antibonding splitting of the up-spin t2g (dxy , dyz and dzx ) states of both antisite MoFe at (0, 0, 0) (especially) and near antisite FeMo at (0, 0, 0.5) (reference Figs. 1(d) and 2(c) for the structure and TDOS, respectively). Then we examine the vanishing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect (reference Figs. 1(f) and 2(e) for the structure and TDOS, respectively). The detailed ODDOS projected onto the near neighbors Fe atom at regular site (0.5, 0, 0.5) and O atom at regular site (0.5, 0.23, 0.5) (after relaxation) are shown in Fig. 4. The black and red lines represent up-spin and down-spin, respectively. Since there are no up-spin states (black lines) around the Fermi level EF for p (px , py and pz ) and t2g (dxy , dyz and dzx ) orbitals of the near neighbor Fe atom at (0.5, 0, 0.5) regular site as well as the s, px and pz orbitals of the near neighbor O atom at (0.5, 0.23, 0.5) regular site, the occurrence of the up-spin state (indicated by red arrow in Fig. 2(e)) in the band gap region of the ordered Pb2FeMoO6, i.e. the vanishing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect, is attributed to the hybridization of the up-spin s and eg (dz2 and dx2 y2 ) states of the near neighbor Fe atom at (0.5, 0, 0.5) regular site and the up-spin py (in fact along the connecting line of the near neighbor O atom and the VMo site) state of the near neighbor O atom at (0.5, 0.23, 0.5) regular site. 3.3. Difference/spin charge densitiesespin coupling and saturation magnetizations In order to investigate the effects of the defects on the spin coupling and magnetic moments, the difference or spin charge densities are shown in Fig. 5 for four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ) at (0.5, 0.5, 0.5) site, (b) Mo antisite (MoFe ) at (0, 0.5, 0.5) site, (c) FeeMo interchange (FeeMo) at (0, 0, 0) and (0, 0, 0.5) site, (d) Fe vacancy (VFe ) at (0, 0.5, 0.5) site, (e) Mo vacancy (VMo ) at (0.5, 0.5, 0.5) site, (f) O vacancy (VO ) at (0.5, 0.5, 0.25) site and (g) Pb vacancy (VPb ) at (0.75, 0.75, 0.75) site. The yellow (turquoise) isosurfaces represent positive (negative) charge density of 0.004/Å3 and thus up-spin (downspin) moment. In the (100) cross sections, the colors blue, turquoise and green represent the value of charge density in increasing order. We can see that, firstly in (a) FeMo antisite and (b) MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiparallel/antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. We also make a parallel/ferromagnetic configuration calculation for each case, i.e. all Fe (Mo) moments are positive (negative), but the total energies are 0.85 and 0.15 eV higher than the antiparallel/antiferromagnetic ground states for (a) FeMo antisite and (b) MoFe antisite, respectively. On the contrary, for (c) FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. The total energy of the antiparallel/antiferromagnetic configuration is 0.81 eV higher than the parallel/ferromagnetic ground state. Second, in (d) VFe vacancy, (f) VO vacancy and (g) VPb vacancy cases, a parallel/ferromagnetic coupling is observed within each sublattice, while the two sublattices are coupled antiparallel/antiferromagnetic. But in (e) VMo vacancy case, a parallel/ferromagnetic coupling is obtained not only within each sublattice but also between Fe and Mo sublattices, although an
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
5
Fig. 2. Total density of states (TDOS) of four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ), (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo), (d) Fe vacancy (VFe ), (e) Mo vacancy (VMo ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
antiparallel/antiferromagnetic coupling is initially designed between Fe and Mo sublattices. So we conclude that a completely parallel/ferromagnetic coupling exists in Pb2FeMoO6 containing VMo vacancy. Thirdly, as expected each cation defect affects mainly its six nearest neighbor oxygen atoms. For examples, in rich Fe
cases including (a) FeMo antisite and (e) VMo vacancy, a partial negative spin moment (turquoise) presents for the six nearest neighbor oxygen atoms along connecting lines of the oxygen and defect. While in rich Mo cases including (b) MoFe antisite and (d) VFe vacancy both at (0, 0.5, 0.5), no spin moments are distributed on
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
6
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 3. Orbital-decomposed density of states (ODDOS) projected onto the concerned inequivalent Fe atoms at (0, 0, 0) and (0.5, 0, 0.5) regular sites and Mo atoms at (0, 0.5, 0.5) MoFe antisite and (0.5, 0.5, 0.5) regular Mo site for the disordered Pb2FeMoO6 compound containing MoFe antisite defect (reference Figs. 1(c) and 2(b) for the structure and TDOS, respectively). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the six nearest neighbor oxygen atoms of the defect. Finally, similar to the perfect Pb2FeMoO6 case [21], the absence of the spin density distributions at Pb sites also indicates the contributions to the magnetic moment from Pb atoms are negligible. The local magnetic moments (mB /atom) on the Fe and Mo atoms at different initial fractional coordinates x, y, z, the total magnetic moment mtot (mB /f.u.), half-metallic character and up-spin band gap Eupspin (eV) of the disordered Pb2FeMoO6 compound with seven g different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies are summarized in Table 1. The values corresponding to the perfect Pb2FeMoO6 [21] are also listed in the last column for comparison. It can be seen that for Fe antisite
(FeMo ), Mo antisite (MoFe ), FeeMo interchange (FeeMo), Fe vacancy (VFe ) and O vacancy (VO ) cases, the total magnetic moments mtot of the disordered Pb2FeMoO6 systems, which correspond to the saturation magnetizations of the samples, are smaller than that of the perfect Pb2FeMoO6. It is clearly that for FeMo antisite case, the reduced saturation magnetization is resulted from the occurrence of antiparallel aligned magnetic moment (3.930 mB ) on FeMo antisite. For MoFe antisite case, the reduced saturation magnetization is attributed to the supplied parallel aligned magnetic moment (0.948 mB ) by MoFe antisite is smaller than the disappeared moment (3.870 mB ) of the substituted Fe atom. In stoichiometric FeeMo interchange (in fact one FeMo antisite plus one MoFe antisite) case,
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
7
Fig. 3. (continued).
the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites, so result in a slightly smaller saturation magnetization of 3.91 mB /f.u than the theoretical magnetization value of 4 mB /f.u. (3.95 mB /f.u. here for perfect Pb2FeMoO6). The vanishing of a regular site Fe atom and thus vanishing of its parallel aligned magnetic moment (3.870 mB ) is clearly responsible for the reduced saturation magnetization of the Pb2FeMoO6 containing Fe vacancy (VFe ). Although removing one O atom from (0.5, 0.5, 0.25) site causes a smaller parallel aligned (positive) magnetic moment of 3.634 mB for its nearest Fe atom at (0.5, 0.5, 0) site and a smaller antiparallel aligned (negative) magnetic moment of 0.331 mB for its nearest Mo atom at (0.5, 0.5, 0.5) site simultaneously, the
induced nearly as two times large antiparallel aligned (negative) magnetic moments as perfect case on the other three Mo sites should be responsible for the reduced saturation magnetization of the Pb2FeMoO6 containing O vacancy (VO ). The local magnetic moment of the Mo5þ ions is easily influenced by the defects than the Fe3þ ions which can be readily understood from their states (4d1 , S ¼ 1=2) and (3d5 , S ¼ 5=2), respectively, i.e. there is only one loosely bounded electron on Mo(4d) orbital with larger radius while there are five (half-filled) tightly bounded electrons on Fe(3d) orbital with small radius. On the contrary, in either Mo vacancy (VMo ) or Pb vacancy (VPb ) cases, the total magnetic moments mtot of the disordered Pb2FeMoO6 systems are slightly larger than that of the perfect one. This is
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
8
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Table 1 The local magnetic moments (mB /atom) on the Fe and Mo atoms at different initial fractional coordinates x, y, z, the total magnetic moment mtot (mB /f.u.), half-metallic character and up-spin band gap Eupspin (eV) of the disordered Pb2FeMoO6 compound with seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and g VPb vacancies. The values corresponding to the perfect Pb2FeMoO6 [21] are also listed in the last column for comparison. Atoms
x, y, z
Fe
0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.0, 0.0,
Mo
0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5,
mtot (mB /f.u.) Half-metallic character upspin Eg (eV)
0.0 0.0 0.5 0.5 0.5 0.5 0.0 0.0 0.5 0.5 0.0, 0.5
FeMo
MoFe
FeeMo
VFe
VMo
VO
VPb
Perfect [21]
4.026 4.005 4.004 3.999
3.922 3.930 3.928 MoFe
MoFe 3.978 3.977 4.018 4.024 FeMo
4.057 4.027 4.039 VFe
4.092 3.527 3.522 3.533
3.873 3.634 3.891 3.851
4.002 4.001 4.002 4.001
3.870 3.870 3.870 3.870
3.930 FeMo 0.039 0.077 0.063 FeMo
1.029 0.111 0.085 0.121
0.501 0.505 FeMo 0.270 1.001 MoFe
0.084 0.043 0.042 0.060
0.058 0.032 0.044 VMo
0.737 0.708 0.783 0.331
0.139 0.139 0.139 0.140
0.380 0.380 0.380 0.380
3.41 yes 1.190
0.948 MoFe 3.21 no e
3.91 no e
3.41 yes 1.177
4.19 no e
3.52 yes 1.098
4.39 yes 1.367
3.95 yes 1.370
because in Mo vacancy (VMo ) case, a completely parallel/ferromagnetic coupling is existed not only within each sublattice but also between Fe and Mo sublattices as mentioned above. In Pb vacancy (VPb ) case, the reason is attributed to not only the parallel
aligned (positive) magnetic moment of about 4.001 mB for four Fe atoms is larger than that of 3.870 mB for four Fe atoms in perfect Pb2FeMoO6 [21], but also the antiparallel aligned (negative) magnetic moment of about 0.140 mB for four Mo atoms is smaller than
Fig. 4. Orbital-decomposed density of states (ODDOS) projected onto the near neighbors Fe atom at regular site (0.5, 0, 0.5) and O atom at regular site (0.5, 0.23, 0.5) for the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect (reference Figs. 1(f) and 2(e) for the structure and TDOS, respectively).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Fig. 5. The difference or spin charge density of four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ) at (0.5, 0.5, 0.5) site, (b) Mo antisite (MoFe ) at (0, 0.5, 0.5) site, (c) FeeMo interchange (FeeMo) at (0, 0, 0) and (0, 0, 0.5) sites, (d) Fe vacancy (VFe ) at (0, 0.5, 0.5) site, (e) Mo vacancy (VMo ) at (0.5, 0.5, 0.5) site, (f) O vacancy (VO ) at (0.5, 0.5, 0.25) site and (g) Pb vacancy (VPb ) at (0.75, 0.75, 0.75) site. The yellow (turquoise) isosurfaces represent positive (negative) charge density of 0.004/Å3 and thus up-spin (downspin) moment. In the (100) cross sections, the colors blue, turquoise and green represent the value of charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
10
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 6. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 2 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0, 0.5) site (Pb4FeMo3O12), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.5, 0) and (0, 0, 0) sites (Pb4Fe2Mo2O12), (c) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb4Fe2MoO12).
that of 0.380 mB for four Mo atoms in perfect Pb2FeMoO6 [21], so results in the largest saturation magnetization of 4.39 mB /f.u. among all configurations. The saturation magnetization (mB /f.u.) of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb vacancy (4.39), VMo vacancy (4.19), FeeMo interchange (3.91), VO vacancy (3.52), VFe vacancy (3.41), FeMo antisite (3.41) and MoFe antisite (3.21).
3.4. Effects of the defect concentrations The TDOS shown in Fig. 2 show that the half-metallic character is lost in 4 f.u. Pb2FeMoO6 compound containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo), or one Mo vacancy (VMo ). In these configurations, the corresponding defect concentration C ¼ 25%, since one Mo vacancy is formed among four Mo atoms for example. Following we further check whether the halfmetallic character is also lost in 2 f.u. (C ¼ 50%), 8 f.u. (C ¼ 12.5%) and 16 f.u. (C ¼ 6.25%) Pb2FeMoO6 compound each containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo), and one Mo vacancy (VMo ). The optimized structures of the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing (a) one Mo antisite (MoFe ), (b) one pair of FeeMo interchange (FeeMo) and (c) one Mo vacancy (VMo ) defects (indicated by circles) are shown in Figs. 6e8 respectively. Similar to the 4 f.u. (C ¼ 25%) cases above, for MoFe antisite and FeeMo interchange cases, no obvious structural changes are observed due to Fe3þ and Mo5þ ions having similar radii of 0.63 and 0.60 Å, respectively. While for VMo vacancy case, the vanished attractions of the removed Mo atom make its six nearest neighbor oxygen atoms move close to their nearest Fe neighbors. The TDOS of the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing (a) one Mo antisite (MoFe ), (b) one pair of FeeMo interchange (FeeMo) and (c) one Mo vacancy (VMo ) are shown in Figs. 9e11 respectively. The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy. We can see that the half-metallic character is also lost for the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo) or one Mo vacancy (VMo ). So we conclude that, the half-metallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite, FeeMo interchange or VMo vacancy is also lost even the defect concentration reduces down to C ¼ 6.25%.
4. Conclusions In point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the disordered Pb2FeMoO6 compound containing seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies have been studied by using the firstprinciples projector augmented wave (PAW) potential within the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU). Following conclusions have been obtained. (1) No obvious structural changes are observed for the disordered Pb2FeMoO6 compound containing FeMo and MoFe antisites, FeeMo interchange and VPb vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO ) vacancy. (2) The half-metallic character is maintained for the disordered Pb2FeMoO6 compound containing FeMo antisite, VFe , VO or VPb vacancy, while it vanishes when MoFe antisite, FeeMo interchange or VMo vacancy are presented even the defect concentration reduces down to C ¼ 6.25%. So we conclude that the MoFe antisite, FeeMo interchange or VMo vacancy have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound and thus usable in magnetoresistive and spintronics devices. (3) In FeMo or MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiparallel/ antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. On the contrary, in FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe , VO or VPb vacancy cases, a parallel/ferromagnetic coupling is observed within each cation sublattice, while the two cation sublattices are coupled antiparallelly/antiferromagneticly. While in VMo vacancy case, a parallel/ferromagnetic coupling is obtained not only within each cation sublattice but also between Fe and Mo sublattices. (4) The mechanisms and degrees of the saturation magnetization reduction have also been analyzed for seven different defects. The saturation magnetization (mB /f.u.) of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb vacancy (4.39), VMo vacancy (4.19), FeeMo interchange (3.91), VO vacancy (3.52), VFe vacancy (3.41), FeMo antisite (3.41), MoFe antisite (3.21).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
11
Fig. 7. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 8 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0.5, 0.5) site (Pb16Fe7Mo9O48), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.5, 0) and (0, 0, 0) sites (Pb16Fe8Mo8O48), (c) Mo vacancy (VMo ), removing one Mo atom from (0.25, 0.5, 0.25) site (Pb16Fe8Mo7O48).
Fig. 8. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 16 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0.25, 0.5) site (Pb32Fe15Mo17O96), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.25, 0) and (0, 0, 0) sites (Pb32Fe16Mo16O96), (c) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb32Fe16Mo15O96).
Fig. 9. Total density of states (TDOS) of 2 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
12
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 10. Total density of states (TDOS) of 8 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. Total density of states (TDOS) of 16 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
References [1] A.W. Sleight, R. Ward, J. Am. Chem. Soc. 83 (1961) 1088e1090. [2] J. Longo, R. Ward, J. Am. Chem. Soc. 83 (1961) 2816e2818.
[3] M.T. Anderson, K.B. Greenwood, G.A. Taylor, K.R. Poeppelmeier, Prog. Solid State Chem. 22 (1993) 197e233. [4] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677e680. [5] K.I. Kobayashi, T. Kimura, Y. Tomioka, H. Sawada, K. Terakura, Y. Tokura, Phys.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13 Rev. B 59 (1999) 11159e11162. [6] C.N.R. Rao, B. Raveau, Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Publishers, Singapore, 1998. r, [7] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molna M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488e1495. [8] M.S. Moreno, J.E. Gayone, M. Abbate, A. Caneiro, D. Niebieskikwiat, R.D. S anchez, A. de Siervo, R. Landers, G. Zampieri, Phys. B 320 (2002) 43e46. [9] D.D. Sarma, P. Mahadevan, T. Saha-Dasgupta, S. Ray, A. Kumar, Phys. Rev. Lett. 85 (2000) 2549e2552. [10] Y. Moritomo, S. Xu, T. Akimoto, A. Machida, N. Hamada, K. Ohoyama, E. Nishibori, M. Takata, M. Sakata, Phys. Rev. B 62 (2000) 14224e14228. [11] Z. Fang, K. Terakura, J. Kanamori, Phys. Rev. B 63 (2001) 180407. [12] H. Wu, Phys. Rev. B 64 (2001) 125126. [13] I.V. Solovyev, Phys. Rev. B 65 (2002) 144446. [14] H.T. Jeng, G.Y. Guo, Phys. Rev. B 67 (2003) 094438. [15] C. Meneghini, S. Ray, F. Liscio, F. Bardelli, S. Mobilio, D.D. Sarma, Phys. Rev. Lett. 103 (2009) 046403. [16] S. Ray, A. Kumar, D.D. Sarma, R. Cimino, S. Turchini, S. Zennaro, N. Zema, Phys. Rev. Lett. 87 (2001) 097204. [17] L. Balcells, J. Navarro, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Appl. Phys. Lett. 78 (2001) 781e783. [18] W.H. Song, J.M. Dai, S.L. Ye, K.Y. Wang, J.J. Du, Y.P. Sun, J. Appl. Phys. 89 (2001) 7678e7680. [19] J.M. De Teresa, D. Serrate, J. Blasco, M.R. Ibarra, L. Morellon, J. Magn. Magn. Mater. 290e291 (2005) 1043e1049. [20] R.D. Shannon, Acta Cryst. A32 (1976) 751e767.
[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
13
Y. Zhang, V. Ji, J. Phys. Chem. Solids 73 (2012) 1116e1121. X.P. Yuan, M.X. Xu, Y.B. Chen, Appl. Phys. Lett. 103 (2013) 052411. H.-E.M. Musa Saad, M. El-Hagary, J. Magn. Magn. Mater. 360 (2014) 229e233. A.S. Ogale, S.B. Ogale, R. Ramesh, T. Venkatesan, Appl. Phys. Lett. 75 (1999) 537e539. T. Saha-Dasgupta, D.D. Sarma, Phys. Rev. B 64 (2001) 064408. X.F. Zhu, Q.F. Li, L.F. Chen, Solid State Commun. 144 (2007) 230e235. D. Stoeffler, S. Colis, J. Magn. Magn. Mater. 290e291 (2005) 400e404. D. Stoeffler, S. Colis, Mater. Sci. Eng. B 126 (2006) 133e138. A.B. Munoz-Garcia, M. Pavone, E.A. Carter, Chem. Mater. 23 (2011) 4525e4536. E. Burzo, I. Balasz, M. Valeanu, I.G. Pop, J. Alloys Compd. 509 (2011) 105e113. A. Calleja, X.G. Capdevila, M. Segarra, C. Frontera, F. Espiell, J. Eur. Ceram. Soc. 31 (2011) 121e127. €pfer, J. Solid State Chem. 185 (2012) 76e81. R. Kircheisen, J. To W.J. Ji, J.F. Wang, J. Xu, L. Jiao, J. Zhou, Y.B. Chen, Z.B. Gu, S.H. Yao, S.T. Zhang, Y.F. Chen, J. Phys. D. Appl. Phys. 46 (2013) 015001. G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558e561. G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251e14269. G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15e50. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169e11186. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758e1775. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865e3868. T. Saitoh, M. Nakatake, A. Kakizaki, H. Nakajima, O. Morimoto, Sh Xu, Y. Moritomo, N. Hamada, Y. Aiura, Phys. Rev. B 66 (2002) 035112. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188e5192.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6 Yan Zhang a, *, Vincent Ji b, Ke-Wei Xu c a
School of Materials Science and Engineering, Chang'an University, Xi'an 710061, Shaanxi, PR China ICMMO/SP2M, UMR CNRS 8182, Universit e Paris-Sud, 91405 Orsay C edex, France c State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, PR China b
h i g h l i g h t s Half-metallic and magnetic properties of the disordered Pb2FeMoO6 are studied by GGAþU. No structural changes are observed for FeMo, MoFe , FeeMo, VPb cases. The 6 (8) nearest oxygen neighbors move away from (close to) VFe or VMo (VO ) vacancy. Half-metal holds for FeMo, VFe , VO , VPb , but vanishes for MoFe, FeeMo, VMo , even C ¼ 6.25%. Total moments decrease in sequence of VPb , VMo , FeeMo, VO , VFe , FeMo , MoFe .
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 25 May 2015 Accepted 24 June 2015 Available online xxx
In point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the disordered Pb2FeMoO6 compound containing seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies have been studied by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU). No obvious structural changes are observed for the cases of the FeMo and MoFe antisites, FeeMo interchange, and VPb vacancy, however, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO ) vacancy. The half-metallic character is maintained for the FeMo antisite, VFe , VO or VPb vacancy cases, while it vanishes in the MoFe antisite, FeeMo interchange or VMo vacancy cases even the defect concentration reduces down to C ¼ 6.25%. So the MoFe antisite, FeeMo interchange or VMo vacancy defects have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound and thus usable in magnetoresistive and spintronics devices. In FeMo or MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. On the contrary, in FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe , VO or VPb vacancy cases, a ferromagnetic coupling is observed within each cation sublattice, while the two cation sublattices are coupled antiferromagneticly. But in VMo vacancy case, a ferromagnetic coupling is obtained not only within each cation sublattice but also between Fe and Mo sublattices. The saturation magnetization of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb , VMo , FeeMo, VO , VFe , FeMo and MoFe cases. © 2015 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Magnetic materials Ab initio calculations Electronic structures Magnetic properties
1. Introduction Double perovskites, which were firstly discovered by Ward et al.
* Corresponding author. E-mail address: [email protected] (Y. Zhang).
in 1961 [1,2], are a broad class of compounds with a chemical formula of A2BB0 O6 [3]. The A-site is usually an alkaline-earth metal atom (Ca, Sr, Ba) or rare-earth metal atom (La, Ce, Nd), the B-site is a 3d (Cr, Mn, Fe, Co, Ni, Zn) and B0 -site is a 4d (Mo, Te, Ru) or 5d (W, Re, Os) transition-metal (TM) atom. Most of the double perovskites have been found to take a rock-salt crystal structure with alternate
http://dx.doi.org/10.1016/j.matchemphys.2015.06.046 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
2
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
perovskite units ABO3 and AB0 O3 along three crystallographical axes. The corners of each perovskite unit are in turn occupied by the TM atoms B and B0 with oxygen atoms located in between, forming alternate BO6 and B0 O6 octahedra. The large alkaline-earth metal or rare-earth metal atom A occupies the body-centered site with a 12fold oxygen coordination in each unit. A recent finding of the intrinsic tunneling magnetoresistance (TMR) effect at room temperature in ordered Sr2FeMoO6 [4] and Sr2FeReO6 [5] revives the extensive studies on the double perovskites. The calculated energy band structures reveal that they are ferrimagnetic half-metal with highly spin-polarized transport properties at the Fermi level [3e5]. The Curie temperatures TC of the Sr2FeMoO6 and Sr2FeReO6 are found to be as fairly high as 415 and 401 K, respectively, making them potential candidates for industrial applications in magnetoresistive [6] and spintronics [7] devices at room temperature. Besides experimental works [4,5,8], various first-principles calculation methods [4,5,9e14] including the local spin density approximation (LSDA), the generalized gradient approximation (GGA), and both schemes with on-site Coulomb correlation correction (LSDAþU and GGAþU) have been used to investigate the electronic and magnetic properties of the ordered Sr2FeMoO6 and Sr2FeReO6. It has been found that at the ground state, the Fe3þ (3d5 ) are in the high spin state of S ¼ 5/2 according to Hund's rule, Mo5þ(4d1 ) and Re5þ(5d2 ) are highly ionized with valence spin states of S ¼ 1/2 and S ¼ 1, respectively. Each of the two TM sites, namely, the Fe3þ (3d5 , S ¼ 5/2) and Mo5þ (4d1 , S ¼ 1/2) or Re5þ (5d2 , S ¼ 1) sites, are believed to be ferromagnetically (FM) arranged within each sublattice, while the two sublattices are coupled antiferromagnetically (AFM), giving rise to the total spin magnetic moments for the ferrimagnetic (FiM) states are thus 4 and 3 mB per formula unit (f.u.) for Sr2FeMoO6 and Sr2FeReO6, respectively. The smaller saturation magnetizations of 3 and 2.7 mB /f.u. at 4.2 K for Sr2FeMoO6 [4] and Sr2FeReO6 [5], respectively, are attributed to the mis-site-type disorder on the TM sites [15e17]. As is well known that the choice of a suitable combination of the atoms A, B and B0 in double perovskites A2BB0 O6 is a key issue to obtain the desired electronic and magnetic properties and ultimately industrial applications. Researches are still being performed to seek for A2BB0 O6 materials with optimized properties. Since the physical properties of double perovskites A2BB0 O6 are not only depended on the TM elements B and B0 but also associated with the characteristics of A-site atom [18,19] and the ionic radius of Pb2þ (1.63 Å) is comparable with that of Sr2þ (1.58 Å) [20], it is likely suitable for the A-site atom of the double perovskites A2BB0 O6. In our previous paper [21], we proposed to substitute Sr2þ ion with Pb2þ ion in Sr2FeMoO6 and the structural, electronic and magnetic properties of the double perovskite Pb2FeMoO6 have been studied in detail. The half-metallic (HM) ferromagnetic nature implies a potential application of this new compound in magnetoresistive and spintronics devices. One year later, the magnetic properties and magnetoresistive effects of the Pb2FeMoO6 have been systematically studied in experiment [22]. The sample shows ferromagnetic behavior with Curie temperature TC about 243 K. Recently, the HM character has also been observed in numerous double perovskites, such as Ba2CrMoO6 and Ba2FeMoO6 [23], which presented completely spin polarization of the conduction electrons crossing the Fermi level. Usually, the HM materials are characterized by the coexistence of metallic behavior in one electron spin channel and insulating behavior in the other spin channel. Their electronic density of states (DOS) is completely spin polarized at the Fermi level, and conductivity is dominated by these metallic single-spin charge carriers. Therefore, the HM materials offer potential technological applications in the realm of single-spin electron source and high-efficiency magnetic sensors. Similar to the Sr2FeMoO6 case [4,13,15e17,24e33], the
fabrication of the double perovskite Pb2FeMoO6 would necessarily lead to a certain degree of the defects and thus reduce the saturation magnetization and even destroy the half-metallic character. To the best of our knowledge, there are no works done on the effects of the defects on the structural, electronic and magnetic properties of the Pb2FeMoO6. In this paper, we make a comparative study on the disordered Pb2FeMoO6 compound with seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies. With this analysis, we try to understand the effect of defects on half metallicity and magnetization. 2. Calculation methods and models The calculations are performed using the Vienna ab-initio simulation package (VASP) based on the density function theory (DFT) [34e37]. The electron-ionic core interaction is represented by the projector augmented wave (PAW) potentials [38] which are more accurate than the ultra-soft pseudopotentials. To treat electron exchange and correlation, we chose the Perdew-BurkeErnzerhof (PBE) [39] formulation of the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU) (U ¼ 2.0 eV for Fe and 1.0 eV for Mo [10,40]), since the TM Fe and Mo have localized electrons in their d-orbitals and these localized d-electrons have energies near the Fermi level. A conjugate-gradient algorithm is used to relax the ions into their ground states, and the energies and the forces on each ion are converged within 1.0 104 eV/atom and 0.02 eV/Å, respectively. The cutoff energy for the plane-waves is chosen to be 450 eV. The Pb 6s2 6p2 , Fe 3d6 4s2 , Mo 4s2 4p6 4d5 5s1 and O 2s2 2p4 electrons are treated as valence electrons. The k-point is sampled according to the Monkhorst-Pack automatic generation scheme with their origin at G point [41], together with a Gaussian smearing broadening of 0.1 eV. In order to compare the electronic structures of the ordered and the disordered Pb2FeMoO6 on the same footing, calculations are performed by constructing the supercells of four f.u.. As compared with Fig. 1(a) for the perfect Pb8Fe4Mo4O24, seven different defects (indicated by circles) are considered: (b) Fe antisite (FeMo ), one Fe atom substitutes for one Mo atom at (0.5, 0.5, 0.5) site (Pb8Fe5Mo3O24), (c) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0, 0.5, 0.5) site (Pb8Fe3Mo5O24), (d) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0, 0) and (0, 0, 0.5) sites (Pb8Fe4Mo4O24), (e) Fe vacancy (VFe ), removing one Fe atom from (0, 0.5, 0.5) site (Pb8Fe3Mo4O24), (f) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb8Fe4Mo3O24), (g) O vacancy (VO ), removing one O atom from (0.5, 0.5, 0.25) site (Pb8Fe4Mo4O23) and (h) Pb vacancy (VPb ), removing one Pb atom from (0.75, 0.75, 0.75) site (Pb7Fe4Mo4O24). 3. Results and discussions 3.1. Optimized structures The optimized structures of the four f.u. Pb2FeMoO6 with seven different defects (indicated by circles) are shown in Fig. 1(b)e(h) together with perfect one (a) for comparison. From Fig. 1(b)e(d) we can see that, for FeMo antisite, MoFe antisite and FeeMo interchange, no obvious structural changes are observed due to Fe3þ and Mo5þ ions having similar radii of 0.63 and 0.60 Å, respectively. However, from Fig. 1(e) and (f) we can see that, for VFe (VMo ) vacancy, the vanished attractions of the removed Fe (Mo) atom make its six nearest neighbor oxygen atoms move close to their nearest Mo (Fe) neighbors, so that the corresponding MoeO bond lengths reduce to 1.901 Å (ab plane) and 1.903 Å (c axis) (the corresponding
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Fig. 1. Optimized structures of ordered and disordered (indicated by circles) Pb2FeMoO6 with four f.u., (a) Perfect (Pb8Fe4Mo4O24), (b) Fe antisite (FeMo ), one Fe atom substitutes for one Mo atom at (0.5, 0.5, 0.5) site (Pb8Fe5Mo3O24), (c) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0, 0.5, 0.5) site (Pb8Fe3Mo5O24), (d) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0, 0) and (0, 0, 0.5) sites (Pb8Fe4Mo4O24), (e) Fe vacancy (VFe ), removing one Fe atom from (0, 0.5, 0.5) site (Pb8Fe3Mo4O24), (f) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb8Fe4Mo3O24), (g) O vacancy (VO ), removing one O atom from (0.5, 0.5, 0.25) site (Pb8Fe4Mo4O23) and (h) Pb vacancy (VPb ), removing one Pb atom from (0.75, 0.75, 0.75) site (Pb7Fe4Mo4O24).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
4
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
FeeO bond lengths reduce to 1.835 Å (ab plane) and 1.837 Å (c axis)). In case of ordered Pb2FeMoO6 as shown in Fig. 1(a), the optimized bond lengths are 1.950 Å (ab plane) and 1.954 Å (c axis) for MoeO bonds and 2.011 Å (ab plane) and 2.016 Å (c axis) for FeeO bonds and thus the lattice constants of a ¼ b ¼ 5.60 Å and c ¼ 7.94 Å for the corresponding body-centered tetragonal (BCT) structure. On the contrary, Fig. 1(g) shows the vanished repulsive forces of the removed oxygen atom make its eight nearest neighbor oxygen atoms move close to O vacancy site. From Fig. 1(h), we find that there are no obvious structural changes for Pb vacancy case. This is because the interactions between Pb atom and the other nearest neighbor atoms are negligible. 3.2. Total density of states (TDOS)ehalf-metallic characters In order to investigate the effects of the various defects on the spin polarization of the conduction electrons and half-metallic character of the Pb2FeMoO6 compound, we show in Fig. 2 the plots of total density of states (TDOS) of four f.u. Pb2FeMoO6 containing seven different defects, (a) Fe antisite (FeMo ), (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo), (d) Fe vacancy (VFe ), (e) Mo vacancy (VMo ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. We can see that the half-metallic character is maintained for the disordered Pb2FeMoO6 compound containing (a) Fe antisite (FeMo ), (d) Fe vacancy (VFe ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ), because around the Fermi level EF the TDOS of the down-spin channel (red lines) cross the Fermi level EF and those of the up-spin upspin channel (black lines) open the gaps Eg of 1.190, 1.177, 1.098 and 1.367 eV, respectively, although they are all slightly smaller than that of 1.370 eV for the perfect Pb2FeMoO6 [21]. On the contrary, the half-metallic character is lost for the disordered Pb2FeMoO6 compound containing (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo) and (e) Mo vacancy (VMo ), since the TDOS of both downspin channel (red lines) and up-spin channel (black lines) cross the Fermi level EF . So we conclude that the Mo antisite (MoFe ), FeeMo interchange (FeeMo) and Mo vacancy (VMo ) defects have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound. We present here the reasons for disappearing of the halfmetallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite defect (reference Figs. 1(c) and 2(b) for the structure and TDOS, respectively). The detailed orbitaldecomposed density of states (ODDOS) projected onto the concerned inequivalent Fe atoms at (0, 0, 0) and (0.5, 0, 0.5) regular Fe sites and Mo atoms at (0, 0.5, 0.5) MoFe antisite and (0.5, 0.5, 0.5) regular Mo site are shown in Fig. 3. The black and red lines represent up-spin and down-spin, respectively. It is clearly that, in Fig. 2(b) for the TDOS of the disordered Pb2FeMoO6 compound containing MoFe antisite defect, the TDOS of down-spin channel (red line) crossing the Fermi level EF are resulted from the hybridizations of t2g (dxy , dyz and dzx ) states of the Fe and Mo atoms at regular sites and antisite, which is similar to the perfect Pb2FeMoO6 case [21]. Since there are no states around the Fermi level EF for both channels of the s, p (px , py and pz ) and eg (dz2 and dx2 y2 ) orbitals of the Fe and Mo atoms at regular sites and antisite, the emerging of a new up-spin state (indicated by red arrow in Fig. 2(b)) in the band gap region of the ordered Pb2FeMoO6, i.e. the disappearing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite defect, is attributed to the small bonding-antibonding splitting of the up-spin t2g (dxy , dyz and dzx ) states of both antisite MoFe at (0, 0.5, 0.5) (especially) and near regular site Mo at (0.5, 0.5, 0.5) with positive and negative magnetic moments (see Table 1), respectively.
Since a FeeMo interchange defect consists of one FeMo antisite and one MoFe antisite, so we can predict the appearance of a new up-spin state (indicated by red arrow in Fig. 2(c)) in the band gap region of the ordered Pb2FeMoO6, i.e. the disappearing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing FeeMo interchange defect, is also attributed to the small bonding-antibonding splitting of the up-spin t2g (dxy , dyz and dzx ) states of both antisite MoFe at (0, 0, 0) (especially) and near antisite FeMo at (0, 0, 0.5) (reference Figs. 1(d) and 2(c) for the structure and TDOS, respectively). Then we examine the vanishing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect (reference Figs. 1(f) and 2(e) for the structure and TDOS, respectively). The detailed ODDOS projected onto the near neighbors Fe atom at regular site (0.5, 0, 0.5) and O atom at regular site (0.5, 0.23, 0.5) (after relaxation) are shown in Fig. 4. The black and red lines represent up-spin and down-spin, respectively. Since there are no up-spin states (black lines) around the Fermi level EF for p (px , py and pz ) and t2g (dxy , dyz and dzx ) orbitals of the near neighbor Fe atom at (0.5, 0, 0.5) regular site as well as the s, px and pz orbitals of the near neighbor O atom at (0.5, 0.23, 0.5) regular site, the occurrence of the up-spin state (indicated by red arrow in Fig. 2(e)) in the band gap region of the ordered Pb2FeMoO6, i.e. the vanishing reason of the half-metallic character of the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect, is attributed to the hybridization of the up-spin s and eg (dz2 and dx2 y2 ) states of the near neighbor Fe atom at (0.5, 0, 0.5) regular site and the up-spin py (in fact along the connecting line of the near neighbor O atom and the VMo site) state of the near neighbor O atom at (0.5, 0.23, 0.5) regular site. 3.3. Difference/spin charge densitiesespin coupling and saturation magnetizations In order to investigate the effects of the defects on the spin coupling and magnetic moments, the difference or spin charge densities are shown in Fig. 5 for four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ) at (0.5, 0.5, 0.5) site, (b) Mo antisite (MoFe ) at (0, 0.5, 0.5) site, (c) FeeMo interchange (FeeMo) at (0, 0, 0) and (0, 0, 0.5) site, (d) Fe vacancy (VFe ) at (0, 0.5, 0.5) site, (e) Mo vacancy (VMo ) at (0.5, 0.5, 0.5) site, (f) O vacancy (VO ) at (0.5, 0.5, 0.25) site and (g) Pb vacancy (VPb ) at (0.75, 0.75, 0.75) site. The yellow (turquoise) isosurfaces represent positive (negative) charge density of 0.004/Å3 and thus up-spin (downspin) moment. In the (100) cross sections, the colors blue, turquoise and green represent the value of charge density in increasing order. We can see that, firstly in (a) FeMo antisite and (b) MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiparallel/antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. We also make a parallel/ferromagnetic configuration calculation for each case, i.e. all Fe (Mo) moments are positive (negative), but the total energies are 0.85 and 0.15 eV higher than the antiparallel/antiferromagnetic ground states for (a) FeMo antisite and (b) MoFe antisite, respectively. On the contrary, for (c) FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. The total energy of the antiparallel/antiferromagnetic configuration is 0.81 eV higher than the parallel/ferromagnetic ground state. Second, in (d) VFe vacancy, (f) VO vacancy and (g) VPb vacancy cases, a parallel/ferromagnetic coupling is observed within each sublattice, while the two sublattices are coupled antiparallel/antiferromagnetic. But in (e) VMo vacancy case, a parallel/ferromagnetic coupling is obtained not only within each sublattice but also between Fe and Mo sublattices, although an
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
5
Fig. 2. Total density of states (TDOS) of four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ), (b) Mo antisite (MoFe ), (c) FeeMo interchange (FeeMo), (d) Fe vacancy (VFe ), (e) Mo vacancy (VMo ), (f) O vacancy (VO ) and (g) Pb vacancy (VPb ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
antiparallel/antiferromagnetic coupling is initially designed between Fe and Mo sublattices. So we conclude that a completely parallel/ferromagnetic coupling exists in Pb2FeMoO6 containing VMo vacancy. Thirdly, as expected each cation defect affects mainly its six nearest neighbor oxygen atoms. For examples, in rich Fe
cases including (a) FeMo antisite and (e) VMo vacancy, a partial negative spin moment (turquoise) presents for the six nearest neighbor oxygen atoms along connecting lines of the oxygen and defect. While in rich Mo cases including (b) MoFe antisite and (d) VFe vacancy both at (0, 0.5, 0.5), no spin moments are distributed on
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
6
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 3. Orbital-decomposed density of states (ODDOS) projected onto the concerned inequivalent Fe atoms at (0, 0, 0) and (0.5, 0, 0.5) regular sites and Mo atoms at (0, 0.5, 0.5) MoFe antisite and (0.5, 0.5, 0.5) regular Mo site for the disordered Pb2FeMoO6 compound containing MoFe antisite defect (reference Figs. 1(c) and 2(b) for the structure and TDOS, respectively). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the six nearest neighbor oxygen atoms of the defect. Finally, similar to the perfect Pb2FeMoO6 case [21], the absence of the spin density distributions at Pb sites also indicates the contributions to the magnetic moment from Pb atoms are negligible. The local magnetic moments (mB /atom) on the Fe and Mo atoms at different initial fractional coordinates x, y, z, the total magnetic moment mtot (mB /f.u.), half-metallic character and up-spin band gap Eupspin (eV) of the disordered Pb2FeMoO6 compound with seven g different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies are summarized in Table 1. The values corresponding to the perfect Pb2FeMoO6 [21] are also listed in the last column for comparison. It can be seen that for Fe antisite
(FeMo ), Mo antisite (MoFe ), FeeMo interchange (FeeMo), Fe vacancy (VFe ) and O vacancy (VO ) cases, the total magnetic moments mtot of the disordered Pb2FeMoO6 systems, which correspond to the saturation magnetizations of the samples, are smaller than that of the perfect Pb2FeMoO6. It is clearly that for FeMo antisite case, the reduced saturation magnetization is resulted from the occurrence of antiparallel aligned magnetic moment (3.930 mB ) on FeMo antisite. For MoFe antisite case, the reduced saturation magnetization is attributed to the supplied parallel aligned magnetic moment (0.948 mB ) by MoFe antisite is smaller than the disappeared moment (3.870 mB ) of the substituted Fe atom. In stoichiometric FeeMo interchange (in fact one FeMo antisite plus one MoFe antisite) case,
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
7
Fig. 3. (continued).
the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites, so result in a slightly smaller saturation magnetization of 3.91 mB /f.u than the theoretical magnetization value of 4 mB /f.u. (3.95 mB /f.u. here for perfect Pb2FeMoO6). The vanishing of a regular site Fe atom and thus vanishing of its parallel aligned magnetic moment (3.870 mB ) is clearly responsible for the reduced saturation magnetization of the Pb2FeMoO6 containing Fe vacancy (VFe ). Although removing one O atom from (0.5, 0.5, 0.25) site causes a smaller parallel aligned (positive) magnetic moment of 3.634 mB for its nearest Fe atom at (0.5, 0.5, 0) site and a smaller antiparallel aligned (negative) magnetic moment of 0.331 mB for its nearest Mo atom at (0.5, 0.5, 0.5) site simultaneously, the
induced nearly as two times large antiparallel aligned (negative) magnetic moments as perfect case on the other three Mo sites should be responsible for the reduced saturation magnetization of the Pb2FeMoO6 containing O vacancy (VO ). The local magnetic moment of the Mo5þ ions is easily influenced by the defects than the Fe3þ ions which can be readily understood from their states (4d1 , S ¼ 1=2) and (3d5 , S ¼ 5=2), respectively, i.e. there is only one loosely bounded electron on Mo(4d) orbital with larger radius while there are five (half-filled) tightly bounded electrons on Fe(3d) orbital with small radius. On the contrary, in either Mo vacancy (VMo ) or Pb vacancy (VPb ) cases, the total magnetic moments mtot of the disordered Pb2FeMoO6 systems are slightly larger than that of the perfect one. This is
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
8
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Table 1 The local magnetic moments (mB /atom) on the Fe and Mo atoms at different initial fractional coordinates x, y, z, the total magnetic moment mtot (mB /f.u.), half-metallic character and up-spin band gap Eupspin (eV) of the disordered Pb2FeMoO6 compound with seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and g VPb vacancies. The values corresponding to the perfect Pb2FeMoO6 [21] are also listed in the last column for comparison. Atoms
x, y, z
Fe
0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.5, 0.0, 0.0, 0.5, 0.0, 0.0,
Mo
0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5,
mtot (mB /f.u.) Half-metallic character upspin Eg (eV)
0.0 0.0 0.5 0.5 0.5 0.5 0.0 0.0 0.5 0.5 0.0, 0.5
FeMo
MoFe
FeeMo
VFe
VMo
VO
VPb
Perfect [21]
4.026 4.005 4.004 3.999
3.922 3.930 3.928 MoFe
MoFe 3.978 3.977 4.018 4.024 FeMo
4.057 4.027 4.039 VFe
4.092 3.527 3.522 3.533
3.873 3.634 3.891 3.851
4.002 4.001 4.002 4.001
3.870 3.870 3.870 3.870
3.930 FeMo 0.039 0.077 0.063 FeMo
1.029 0.111 0.085 0.121
0.501 0.505 FeMo 0.270 1.001 MoFe
0.084 0.043 0.042 0.060
0.058 0.032 0.044 VMo
0.737 0.708 0.783 0.331
0.139 0.139 0.139 0.140
0.380 0.380 0.380 0.380
3.41 yes 1.190
0.948 MoFe 3.21 no e
3.91 no e
3.41 yes 1.177
4.19 no e
3.52 yes 1.098
4.39 yes 1.367
3.95 yes 1.370
because in Mo vacancy (VMo ) case, a completely parallel/ferromagnetic coupling is existed not only within each sublattice but also between Fe and Mo sublattices as mentioned above. In Pb vacancy (VPb ) case, the reason is attributed to not only the parallel
aligned (positive) magnetic moment of about 4.001 mB for four Fe atoms is larger than that of 3.870 mB for four Fe atoms in perfect Pb2FeMoO6 [21], but also the antiparallel aligned (negative) magnetic moment of about 0.140 mB for four Mo atoms is smaller than
Fig. 4. Orbital-decomposed density of states (ODDOS) projected onto the near neighbors Fe atom at regular site (0.5, 0, 0.5) and O atom at regular site (0.5, 0.23, 0.5) for the disordered Pb2FeMoO6 compound containing Mo vacancy (VMo ) defect (reference Figs. 1(f) and 2(e) for the structure and TDOS, respectively).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Fig. 5. The difference or spin charge density of four f.u. Pb2FeMoO6 containing seven different defects of (a) Fe antisite (FeMo ) at (0.5, 0.5, 0.5) site, (b) Mo antisite (MoFe ) at (0, 0.5, 0.5) site, (c) FeeMo interchange (FeeMo) at (0, 0, 0) and (0, 0, 0.5) sites, (d) Fe vacancy (VFe ) at (0, 0.5, 0.5) site, (e) Mo vacancy (VMo ) at (0.5, 0.5, 0.5) site, (f) O vacancy (VO ) at (0.5, 0.5, 0.25) site and (g) Pb vacancy (VPb ) at (0.75, 0.75, 0.75) site. The yellow (turquoise) isosurfaces represent positive (negative) charge density of 0.004/Å3 and thus up-spin (downspin) moment. In the (100) cross sections, the colors blue, turquoise and green represent the value of charge density in increasing order. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
10
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 6. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 2 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0, 0.5) site (Pb4FeMo3O12), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.5, 0) and (0, 0, 0) sites (Pb4Fe2Mo2O12), (c) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb4Fe2MoO12).
that of 0.380 mB for four Mo atoms in perfect Pb2FeMoO6 [21], so results in the largest saturation magnetization of 4.39 mB /f.u. among all configurations. The saturation magnetization (mB /f.u.) of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb vacancy (4.39), VMo vacancy (4.19), FeeMo interchange (3.91), VO vacancy (3.52), VFe vacancy (3.41), FeMo antisite (3.41) and MoFe antisite (3.21).
3.4. Effects of the defect concentrations The TDOS shown in Fig. 2 show that the half-metallic character is lost in 4 f.u. Pb2FeMoO6 compound containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo), or one Mo vacancy (VMo ). In these configurations, the corresponding defect concentration C ¼ 25%, since one Mo vacancy is formed among four Mo atoms for example. Following we further check whether the halfmetallic character is also lost in 2 f.u. (C ¼ 50%), 8 f.u. (C ¼ 12.5%) and 16 f.u. (C ¼ 6.25%) Pb2FeMoO6 compound each containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo), and one Mo vacancy (VMo ). The optimized structures of the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing (a) one Mo antisite (MoFe ), (b) one pair of FeeMo interchange (FeeMo) and (c) one Mo vacancy (VMo ) defects (indicated by circles) are shown in Figs. 6e8 respectively. Similar to the 4 f.u. (C ¼ 25%) cases above, for MoFe antisite and FeeMo interchange cases, no obvious structural changes are observed due to Fe3þ and Mo5þ ions having similar radii of 0.63 and 0.60 Å, respectively. While for VMo vacancy case, the vanished attractions of the removed Mo atom make its six nearest neighbor oxygen atoms move close to their nearest Fe neighbors. The TDOS of the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing (a) one Mo antisite (MoFe ), (b) one pair of FeeMo interchange (FeeMo) and (c) one Mo vacancy (VMo ) are shown in Figs. 9e11 respectively. The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy. We can see that the half-metallic character is also lost for the 2 f.u., 8 f.u. and 16 f.u. Pb2FeMoO6 each containing one Mo antisite (MoFe ), one pair of FeeMo interchange (FeeMo) or one Mo vacancy (VMo ). So we conclude that, the half-metallic character of the disordered Pb2FeMoO6 compound containing MoFe antisite, FeeMo interchange or VMo vacancy is also lost even the defect concentration reduces down to C ¼ 6.25%.
4. Conclusions In point of view of the half-metallic character and magnetization reduction, the structural, electronic and magnetic properties of the disordered Pb2FeMoO6 compound containing seven different defects of FeMo and MoFe antisites, FeeMo interchange, and VFe , VMo , VO and VPb vacancies have been studied by using the firstprinciples projector augmented wave (PAW) potential within the generalized gradient approximation taking into account on-site Coulomb repulsive energy (GGAþU). Following conclusions have been obtained. (1) No obvious structural changes are observed for the disordered Pb2FeMoO6 compound containing FeMo and MoFe antisites, FeeMo interchange and VPb vacancy defects. However, the six (eight) nearest oxygen neighbors of the vacancy move away from (close to) VFe or VMo (VO ) vacancy. (2) The half-metallic character is maintained for the disordered Pb2FeMoO6 compound containing FeMo antisite, VFe , VO or VPb vacancy, while it vanishes when MoFe antisite, FeeMo interchange or VMo vacancy are presented even the defect concentration reduces down to C ¼ 6.25%. So we conclude that the MoFe antisite, FeeMo interchange or VMo vacancy have to be avoided in order to preserve the half-metallic character of the Pb2FeMoO6 compound and thus usable in magnetoresistive and spintronics devices. (3) In FeMo or MoFe antisite cases, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in an antiparallel/ antiferromagnetic coupling with those of Fe (Mo) cations on the regular sites. On the contrary, in FeeMo interchange case, the spin moments of Fe (Mo) cations situated on Mo (Fe) antisites are in a parallel/ferromagnetic coupling with those of Fe (Mo) cations on the regular sites. In VFe , VO or VPb vacancy cases, a parallel/ferromagnetic coupling is observed within each cation sublattice, while the two cation sublattices are coupled antiparallelly/antiferromagneticly. While in VMo vacancy case, a parallel/ferromagnetic coupling is obtained not only within each cation sublattice but also between Fe and Mo sublattices. (4) The mechanisms and degrees of the saturation magnetization reduction have also been analyzed for seven different defects. The saturation magnetization (mB /f.u.) of the disordered Pb2FeMoO6 compound decreases in the sequence of VPb vacancy (4.39), VMo vacancy (4.19), FeeMo interchange (3.91), VO vacancy (3.52), VFe vacancy (3.41), FeMo antisite (3.41), MoFe antisite (3.21).
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
11
Fig. 7. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 8 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0.5, 0.5) site (Pb16Fe7Mo9O48), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.5, 0) and (0, 0, 0) sites (Pb16Fe8Mo8O48), (c) Mo vacancy (VMo ), removing one Mo atom from (0.25, 0.5, 0.25) site (Pb16Fe8Mo7O48).
Fig. 8. The optimized structures of disordered (indicated by circles) Pb2FeMoO6 with 16 f.u., (a) Mo antisite (MoFe ), one Mo atom substitutes for one Fe atom at (0.5, 0.25, 0.5) site (Pb32Fe15Mo17O96), (b) FeeMo interchange (FeeMo), exchange Fe and Mo positions respectively at (0, 0.25, 0) and (0, 0, 0) sites (Pb32Fe16Mo16O96), (c) Mo vacancy (VMo ), removing one Mo atom from (0.5, 0.5, 0.5) site (Pb32Fe16Mo15O96).
Fig. 9. Total density of states (TDOS) of 2 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
12
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13
Fig. 10. Total density of states (TDOS) of 8 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. Total density of states (TDOS) of 16 f.u. Pb2FeMoO6 containing three different defects of (a) Mo antisite (MoFe ), (b) FeeMo interchange (FeeMo), and (c) Mo vacancy (VMo ). The black and red lines represent up-spin and down-spin, respectively, and the Fermi level EF is set at zero energy and indicated by vertical blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
References [1] A.W. Sleight, R. Ward, J. Am. Chem. Soc. 83 (1961) 1088e1090. [2] J. Longo, R. Ward, J. Am. Chem. Soc. 83 (1961) 2816e2818.
[3] M.T. Anderson, K.B. Greenwood, G.A. Taylor, K.R. Poeppelmeier, Prog. Solid State Chem. 22 (1993) 197e233. [4] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677e680. [5] K.I. Kobayashi, T. Kimura, Y. Tomioka, H. Sawada, K. Terakura, Y. Tokura, Phys.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046
Y. Zhang et al. / Materials Chemistry and Physics xxx (2015) 1e13 Rev. B 59 (1999) 11159e11162. [6] C.N.R. Rao, B. Raveau, Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Publishers, Singapore, 1998. r, [7] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molna M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488e1495. [8] M.S. Moreno, J.E. Gayone, M. Abbate, A. Caneiro, D. Niebieskikwiat, R.D. S anchez, A. de Siervo, R. Landers, G. Zampieri, Phys. B 320 (2002) 43e46. [9] D.D. Sarma, P. Mahadevan, T. Saha-Dasgupta, S. Ray, A. Kumar, Phys. Rev. Lett. 85 (2000) 2549e2552. [10] Y. Moritomo, S. Xu, T. Akimoto, A. Machida, N. Hamada, K. Ohoyama, E. Nishibori, M. Takata, M. Sakata, Phys. Rev. B 62 (2000) 14224e14228. [11] Z. Fang, K. Terakura, J. Kanamori, Phys. Rev. B 63 (2001) 180407. [12] H. Wu, Phys. Rev. B 64 (2001) 125126. [13] I.V. Solovyev, Phys. Rev. B 65 (2002) 144446. [14] H.T. Jeng, G.Y. Guo, Phys. Rev. B 67 (2003) 094438. [15] C. Meneghini, S. Ray, F. Liscio, F. Bardelli, S. Mobilio, D.D. Sarma, Phys. Rev. Lett. 103 (2009) 046403. [16] S. Ray, A. Kumar, D.D. Sarma, R. Cimino, S. Turchini, S. Zennaro, N. Zema, Phys. Rev. Lett. 87 (2001) 097204. [17] L. Balcells, J. Navarro, M. Bibes, A. Roig, B. Martinez, J. Fontcuberta, Appl. Phys. Lett. 78 (2001) 781e783. [18] W.H. Song, J.M. Dai, S.L. Ye, K.Y. Wang, J.J. Du, Y.P. Sun, J. Appl. Phys. 89 (2001) 7678e7680. [19] J.M. De Teresa, D. Serrate, J. Blasco, M.R. Ibarra, L. Morellon, J. Magn. Magn. Mater. 290e291 (2005) 1043e1049. [20] R.D. Shannon, Acta Cryst. A32 (1976) 751e767.
[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]
13
Y. Zhang, V. Ji, J. Phys. Chem. Solids 73 (2012) 1116e1121. X.P. Yuan, M.X. Xu, Y.B. Chen, Appl. Phys. Lett. 103 (2013) 052411. H.-E.M. Musa Saad, M. El-Hagary, J. Magn. Magn. Mater. 360 (2014) 229e233. A.S. Ogale, S.B. Ogale, R. Ramesh, T. Venkatesan, Appl. Phys. Lett. 75 (1999) 537e539. T. Saha-Dasgupta, D.D. Sarma, Phys. Rev. B 64 (2001) 064408. X.F. Zhu, Q.F. Li, L.F. Chen, Solid State Commun. 144 (2007) 230e235. D. Stoeffler, S. Colis, J. Magn. Magn. Mater. 290e291 (2005) 400e404. D. Stoeffler, S. Colis, Mater. Sci. Eng. B 126 (2006) 133e138. A.B. Munoz-Garcia, M. Pavone, E.A. Carter, Chem. Mater. 23 (2011) 4525e4536. E. Burzo, I. Balasz, M. Valeanu, I.G. Pop, J. Alloys Compd. 509 (2011) 105e113. A. Calleja, X.G. Capdevila, M. Segarra, C. Frontera, F. Espiell, J. Eur. Ceram. Soc. 31 (2011) 121e127. €pfer, J. Solid State Chem. 185 (2012) 76e81. R. Kircheisen, J. To W.J. Ji, J.F. Wang, J. Xu, L. Jiao, J. Zhou, Y.B. Chen, Z.B. Gu, S.H. Yao, S.T. Zhang, Y.F. Chen, J. Phys. D. Appl. Phys. 46 (2013) 015001. G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558e561. G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251e14269. G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15e50. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169e11186. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758e1775. J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865e3868. T. Saitoh, M. Nakatake, A. Kakizaki, H. Nakajima, O. Morimoto, Sh Xu, Y. Moritomo, N. Hamada, Y. Aiura, Phys. Rev. B 66 (2002) 035112. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188e5192.
Please cite this article in press as: Y. Zhang, et al., Effects of the defects on the half-metallic characters and magnetic properties in double perovskite Pb2FeMoO6, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.06.046