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Phase stabilization of Fe substituted NdMn2O5 ceramics and their properties
Solid State Sciences 46 (2015) 1e6
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Phase stabilization of Fe substituted NdMn2O5 ceramics and their properties K. Saravana Kumar a, b, *, C. Venkateswaran b a b
Department of Physics, SRM University, Ramapuram Campus, Chennai, 600 089, India Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai, 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2014 Received in revised form 8 March 2015 Accepted 16 May 2015 Available online 19 May 2015
An oxide of stoichiometry, NdFeMnO5, has been synthesized using a two-step process. First the precursor oxides are high-energy ball milled and the as-milled powders are then sintered to obtain the NdFeMnO5 phase. Rietveld refinement of the XRD pattern, by replacing the Mn3þ sites with Fe3þ, shows the formation of stoichiometric NdFeMnO5. Nd & Mn are found to be in þ3 and þ4 states, respectively from the XPS study. Agglomeration of fine grains is evident from the electron micrographs. Fe3þ has a magnetic moment higher than Mn3þ, and hence affects the magnetic property. Thermo-magnetization measurements show the existence of magnetic ordering below 290 K. An increase in magnetization at low temperature is also observed due to ordering of R3þ moments in addition to Fe and Mn spin moments. A linear increase in conductivity and stable dielectric response is observed from impedance study at high temperature. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: NdMn2O5 High-energy ball milling Magnetic study X-Ray Photoelectron Spectroscopy
1. Introduction Materials of the RMn2O5 (R ¼ Rare earths, Bi,Y) family have Mn at two different crystallographic sites. Mn4þ ions occupy the octahedral pyramidal (4f) sites where it is coordinated with six oxygen atoms and Mn3þ ions are present at tetragonal pyramidal (4h) sites having coordination with five oxygen atoms [1]. They have complex magnetic structures which give rise to magnetodielectric effect around the magnetic transition temperatures. Generally, RMn2O5 systems have orthorhombic structure with Pbam spacegroup and exhibit paramagnetic behavior at room temperature [1]. They have a Neel transition temperature around ~40 K below which antiferromagnetic ordering exists. A magnetoelectric coupling behavior is exhibited at the magnetic transition at low temperatures which is reportedly very weak in nature. The Fe substitution has its effect on the structural and magnetic properties of RMn2O5 family due to its high spin-only effective magnetic moment [2e4]. Such a study describes the Fe3þ substitution at the Mn3þ (4h) site and the corresponding change in properties are compared with parent system [2e8]. Most of the reports indicate a minor structural modification along with change
* Corresponding author. Department of Physics, SRM University, Ramapuram Campus, Chennai, 600 089, India. E-mail address: [email protected] (K.S. Kumar). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.003 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
in bond lengths and bond angles in Mn layers of RFeMnO5 depending on the type of R ion (R ¼ Y, Tb, Dy, Ho, Er, Bi). But only few of the R(Y, Er, Dy) ions exhibit a ferrimagnetic behavior as a result of Fe substitution into the RMn2O5 structure [5e7]. Remaining other systems in the family report a change in transition temperature or modification in the long range magnetic ordering established below the transition temperature. It is understood from literature that NdMn2O5 is one of the less studied system and the effect of Fe substitution is not yet reported [9,10]. Hence, this paper deals with the structural and magnetic properties of phase stabilized NdFeMnO5 obtained from a two-step synthesis procedure. 2. Experimental Neodymium oxide, manganese mono-oxide and iron oxide were taken in stoichiometric proportions and milled for a total of 5 h at 450 RPM with a ball to powder ratio of 28:2. The obtained reddish brown powders were then pressed into dense pellets of 8 mm diameter and sintered at 750 C for 24 h. Pellets were furnace cooled and then ground in mortar to obtain the black powders of NdFeMnO5. X-Ray diffraction (XRD) data was collected with CueKa radiation at room temperature. A DAR400-XM1000 (OMICRON Nanotechnologies, Germany) instrument equipped with dual Al/Mg anodes as the x-ray source was used to perform X-Ray
2
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
Fig. 1. Rietveld refinement of the XRD pattern of NdFeMnO5. * indicates un-indexed peaks.
Photoelectron Spectroscopy (XPS) measurement. Al anode was used to obtain the survey and elemental spectrums. The peak of C 1s at 284.5 eV was used as reference to calibrate the binding energy of elements to exclude charging effect on the sample. Pass energy of 50 eV was used to obtain survey spectrum and 20 eV pass energy was used for elemental spectra. High resolution scanning electron microscopy (HR-SEM) was done in FEI Quanta FEG 200 HR Microscope. Isothermal magnetization and Zero field cooled (ZFC)-Field Cooled (FC) measurements were carried out in the range 20e300 K in a Lakeshore VSM 7410 instrument. A field of 1000 Oe was applied for the ZFC-FC experiment and for the iso-thermal magnetization curves a maximum field of 15 KOe was applied. Impedance measurements were done in Solatron Impedance Analyzer (S-1260) in 1 Hze1 MHz frequency range and 300 Ke523 K temperature range. A NdFeMnO5 pellet of 8 mm diameter and 2.8 mm thickness obtained by pressing with 2.5 ton and then coated with silver on both faces for better electrical contact. The coated pellet was heat treated at 350 C for 4 h to obtain dense pellet, and used for the measurements. 3. Results and discussion
Table 1 Structural parameters obtained from the Rietveld refinement of NdFeMnO5. Label
Atom
Position
x
y
z
Occupation
Nd Mn1 Fe2 O1 O2 O3 O4
Nd3þ Mn4þ Fe3þ O2 O2 O2 O2
4g 4f 4h 4e 4g 4h 8i
0.1411 0.50000 0.4064 0.00000 0.15855 0.18839 0.4023
0.1700 0.00000 0.3482 0.00000 0.4425 0.43967 0.2109
0.00000 0.2687 0.50000 0.2687 0.00000 0.50000 0.2330
1.0 1.0 1.0 1.0 1.0 1.0 2.0
a ¼ 7.5037(2) Å, b ¼ 8.6002(2) Å, c ¼ 5.6973(1) Å c2 ¼ 1.10, Rp ¼ 7.76, Rwp ¼ 10.3. Weight percentage of phases: NdFeMnO5 (93.98%) and NdMnO3 (6.02%).
Rietveld refinement plot of X-ray diffraction (XRD) pattern is given in Fig 1, indicating the formation of stoichiometric NdFeMnO5 which is iso-structural with NdMn2O5. The structural parameters obtained from the refinement are listed in Table 1. The structural model of NdMn2O5 was taken as the starting model for the Rietveld refinement, done using the using the FULLPROF suite, by substituting Fe3þ in Mn3þ (4h) sites. The calculated pattern of the NdFeMnO5 phase matches with the experimental pattern. NdMnO3 is present as a minor secondary phase and does not contribute to the magnetic property due to the paramagnetic nature at room temperature. Secondary phases related to Fe are not evident from
Fig. 2. (a) Structural model of NdFeMnO5 showing chains of Mn4þO6 and Fe3þO5 extending along c-axis. (b) Model showing arrangement of Nd, Mn and Fe along c-axis (extending into plane of paper) without O ions. (c) Arrangement of layers of Nd, Mn and Fe layers with a-axis extending into plane of paper.
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
the XRD pattern. From the structural point of view, illustrated in Fig 2a, NdFeMnO5 has orthorhombic (Pbam) structure as that of its parent NdMn2O5. Parent system has the Mn4þ octahedra network connected through oxygen atoms, extending along c-axis and, has Mn3þ & Nd3þ layers alternating between these Mn4þ layers. In the case of NdFeMnO5, it is considered that the Mn3þ layers in the parent system are completely replaced by Fe3þ layers connecting Mn4þ ions through oxygen ions. The arrangement of Nd, Mn and Fe atoms in a unit cell of NdFeMnO5 is illustrated in Fig 2, along the caxis (b) and a-axis (c). Even though the NdFeMnO5 is iso-structural with NdMn2O5, still there is a change in the bond angles and bond lengths of the Mn4þ, Fe3þ and Nd3þ layers with the surrounding oxygen atoms. This change is also brought about by the removal of Mn3þ, a Jahn Teller ion whose effect decreases in NdFeMnO5. Table 2 shows the refined cell parameters of NdFeMnO5 along with those of NdMn2O5 from selected references. As discussed elsewhere [9,10], it is inferred that a small variation in cell parameters of NdMn2O5 depend on the synthesis procedure also. There is insignificant variation in cell parameters of NdFeMnO5 from the parent system as expected, since the radii of Fe3þ and Mn3þ are comparable in a 5-fold coordination environment [11]. Hence from Table 2, it is evident that the Fe substitution has minimum possible effect on the crystal structure. In spite of the assumption that Fe3þ replaces Mn3þ ions, there is a possibility of anti-site disorder which enables few Fe3þ ions to be found at Mn4þ sites and Mn4þ/Mn3þ to be found at Fe3þ sites also. XPS study was carried out to determine the possible oxidation state of Mn ions and substituted Fe ions. Survey spectrum, Fig 3a, shows the presence of elements in the NdFeMnO5. The binding energy value of C 1s peak at 284.5 eV was taken as the reference for charge correction and all the binding energy values determined by fitting the spectra were charge corrected. CASA XPS software was used to analyze the elemental spectra with the fitting function GL(30) [GL(p) - product of Gaussian(0)-Lorentzian(100)]. ‘Linear’ and ‘Shirley’ functions were used appropriately for background fitting. The Nd 3d5/2 peak at 980.5 eV, close to the value of Nd2O3, indicates that Nd is in þ3 oxidation state [12]. A very weak spectrum for Fe was obtained which was not suitable for fitting. A reasonable fit for the single oxidation state of þ4 was obtained from the Mn spectrum as shown in Fig 3b. The binding energy value is 641.7 eV for 2p3/2 peak of Mn, which corresponds to the energy value for Mn in MnO2 [12]. Hence the inference that Mn at þ4 states is consistent with XRD results serving as an indirect evidence for the Fe3þ states along with the insignificant variation in cell parameters due to Fe3þ substitution. High resolution scanning electron microscopic (HR-SEM) image of NdFeMnO5 is shown in Fig 4. Image shows the agglomeration of irregular shaped fine grains with a distribution of grains sizes. It is also observed that the grain sizes are not below the critical size in nanometers and hence do not affect the magnetic property due to finite size effect. The magnetic behavior of the RMn2O5 family is dominated by the Mn4þeOeMn4þ(J1) chain with antiferromagnetic nature and Mn4þeOeMn3þeOeMn4þ(J2) chain with ferromagnetic nature [1]. The low temperature zero field cooled-field cooled (ZFC-FC) curves are shown in Fig 5, which exhibits a bifurcation below 290 K and an increase in FC magnetization values till 20 K. This behavior is Table 2 Comparison of the NdFeMnO5 and NdMn2O5 cell parameters.
NdFeMnO5 NdMn2O5 (Ref. [10]) NdMn2O5 (Ref. [9])
a (Å)
b (Å)
c (Å)
Volume (Å3)
7.5037(2) 7.5051(2) 7.501(1)
8.6002(2) 8.6209(2) 8.620(2)
5.6973(1) 5.7022(1) 5.701(1)
367.6 368.9 368.6
3
Fig. 3. XPS spectrum of NdFeMnO5, (a) survey spectrum [Inset: Elemental spectrum of Fe], and (b) Mn elemental spectrum.
indicative of the magnetic ordering below ~290 K and is supported by the linear dependence of magnetization on the applied magnetic field obtained at 20 K (Fig 6). The parent NdMn2O5 has a Neel transition (TN) temperature of ~75 K, below which it is reported to have antiferromagnetic ordering [9,10]. NdMnO3 has reported magnetic ordering temperature of ~82 K in which the Nd ions order at around ~20 K [13]. The inset of Fig. 5 shows the (dM/dT) plot for the ZFC curve, in which there is evidence for the three different transitions (~95 K, ~70 K and ~40 K). The first transition (~95 K) corresponds to NdMnO3 phase and the second (~70 K) relates to the NdMn2O5 phase, which may be present in minor percentages, not evident from the XRD data. Transition observed at ~40 K corresponds to the ordering of Nd3þ spins. Evidently, NdMnO3 phase does not have major influence on magnetic property observed at low temperatures. Hence, it is inferred that the magnetic ordering temperature of NdFeMnO5 is at ~290 K when compared to the transition temperature of NdMn2O5 (~70 K). Isothermal magnetization measurements at 20 K, 100 K and 300 K are given in Fig 6 indicating an increase in magnetization value at 15 kOe. Initially a weak ferromagnetic behavior is observed at low fields and later a linear dependence on magnetic field is observed for curves at 20 K and 100 K. Weak ferromagnetic behavior is evident from the inset of Fig. 6 indicating the presence of coercivity and retentivity at low fields. The magnetization curve
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K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
at 300 K hints on the onset of magnetic ordering with a weak ferromagnetic response at low fields and the bifurcation observed in ZFC-FC curves at ~290 K may be an indicator of the completion of moment ordering. The magnetic transition is due to the moment ordering in Mn4þ and Fe3þ layers, where the moment of Nd3þ layer also contributes to the observed magnetic ordering below the transition temperature of ~75 K. Global magnetic behavior in NdFeMnO5 is based on the relative alignment of spins in Mn4þeOeMn4þ(J3) and Mn4þeOeFe3þeOeMn4þ(J4) chains determined through superexchange interaction, where type of ordering in J3 is influenced by the alignment of Nd3þ moments. The spin only effective magnetic moments of Mn4þ, Mn3þ and Fe3þ are 3.87 mB, 4.89 mB and 5.92 mB [14], respectively. The magnetic ordering arises due to the alignment of the magnetic Nd3þ, Fe3þ and Mn4þ moments in the unit cell. Due to Fe3þ substitution at Mn3þ sites, the magnetic ordering of the chain Mn4þeOeFe3þeOeMn4þ through the superexchange interaction is affected as the magnetic moment of Fe3þ is higher than Mn3þ. Also, Fe3þeOeFe3þ having an angle close to 90 shows a short range ferromagnetic ordering in accordance with GoodenougheKanamorieAnderson [15e17] rules which requires higher fields for spin reversal. Generally, Mn4þeOeMn4þ has an AFM ordering in RMn2O5 structures. In the case of NdFeMnO5, the Mn4þeOeMn4þ interaction is definitely affected by the moment ordering of Nd3þ ions below the transition temperatures. Inferences from magnetic studies indicate that the Fe
Fig. 5. Low temperature ZFC-FC curves at an applied field of H ¼ 1000 Oe showing bifurcation at ~290 K [Inset: dM/dT curve for ZFC data indicating three different magnetic transitions].
substitution has major influence on the magnetic ordering in NdFeMnO5 with no major structural variation. Impedance spectroscopy measurements were carried out to determine the electrical properties. In a parallel RC combination, shown in Fig. 7, the real and imaginary parts are, Zr ¼ R/(1 þ (uRC)2) and Zi ¼ uR2C/(1 þ (uRC)2) Elimination of u from the above equations leads to,
2 R 2 R Zr þ Zi2 ¼ 2 2 which is the equation of a circle of radius R/2 with center at (R/2,0). From the derived resistance values, the conductivity values are calculated using the formula,
s¼
d R$A
Where, s is the conductivity, d is the thickness of sample, R is the resistance value obtained from fit of ColeeCole plot and A is the
Fig. 4. HR-SEM micrographs of NdFeMnO5 at two different magnifications.
Fig. 6. Isothermal magnetization curve at 20 K, 100 K and 300 K [Inset: Magnetization curve at low fields indicating a weak ferromagnetic behaviour.].
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
Fig. 7. (a) A common RC circuit and (b) its impedance plane plot.
area of the sample. The values of Arrhenius plot is then derived using the formula,
Ea s ¼ s0 exp kB T from which the fit for linear region gives the value of activation energy, Eg. A resistivity value of the order of ~107 ohm-cm is observed from Fig. 8a at a temperature of 333 K. The Arrhenius Plot of NdFeMnO5 to determine the activation energy is given in Fig 8b. A linear increase in conductivity is observed in the entire measurement temperature range. Activation energy (Ea) value of 0.57 eV is determined from the linear fit of the data. The conduction increases with temperature due to the activation of defects and charge
Fig. 8. (a) Resistivity plot of NdFeMnO5 obtained from impedance measurement. (b) Arrhenius plot of NdFeMnO5 from which activation energy is derived.
5
carriers. Contribution towards conduction from the secondary phases present in the samples have to be considered at higher temperatures, which is also evident from the lower activation energy values. The dielectric constant values have no major variation with temperature, as evident from the almost flat response from temperature dependent dielectric constant curves given in Fig 9a. This trend indicates the dielectric stability of the sample with increasing temperatures for various frequencies. The dielectric constant values decreases with increasing frequency as observed from the frequency dependent dielectric constant graph given in Fig 9b. This is a normal behavior as the defects and charge carriers follow the field at low frequencies, which will not happen at high frequencies. Consequently, it is observed that the sample shows a stable dielectric response throughout the measurement temperature range with linear increase in conductivity. 4. Conclusion The compound of stoichiometry, NdFeMnO5, has been synthesized by initial high-energy ball milling of oxides and then sintering the milled powders. The oxidation states of Nd, Mn and Fe are found to be þ3, þ4 and þ3, respectively from the combined XPS and XRD studies, determining the existence of Fe ions at Mn3þ sites in the NdMn2O5 structure. The magnetic property is affected significantly as observed from the dramatic rise in the magnetic
Fig. 9. (a) Temperature-dependent dielectric constant plot of NdFeMnO5 at various frequencies. (b) Frequency-dependent dielectric constant plot of NdFeMnO5 at various temperatures.
6
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
ordering temperature, around 290 K. This is due to the definite change in interaction dynamics in Mn4þeOeMn4þ and Mn4þeOeFe3þeOeMn4þ chains due to Fe substitution and Nd3þ moment ordering at low temperatures. A stable dielectric response with linear increase in conductivity is observed in the measurement temperature range. Acknowledgment KSK thanks DRDO, India for support through fellowship. Authors acknowledge SAIF, IIT Madras for low temperature VSM measurements; NFMTC, IIT Madras for HR-SEM measurement and NCNSNT, University of Madras for XPS measurements. References ~ oz, J.A. Alonso, M.T. Casais, M.J. Martínez-Lope, J.L. Martínez, [1] A. Mun ndez-Díaz, Magnetic structure and properties of BiMn2O5 oxide: a M.T. Ferna neutron diffraction study, Phys. Rev. B 65 (14) (2002) 144423. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis and study of [2] A. Mun the crystallographic and magnetic structure of HoFeMnO5, Eur. J. Inorg. Chem. 2007 (14) (2007) 1972e1979. [3] K.S. Kumar, C. Venkateswaran, Effect of Fe substitution on the magnetic properties of BiMn2O5, J. Phys. D Appl. Phys. 44 (32) (2011) 325001. ~ oz, T. Ruskov, I. Spirov, K. Krezhov, [4] M. Retuerto, M.J. Martínez-Lope, A. Mun M.T. Fern andez-Díaz, M. García-Hern andez, J.A. Alonso, Synthesis, structural study and magnetic properties of TbFeMnO5, Solid State Commun. 150 (37e38) (2010) 1831e1836. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis, structural, [5] A. Mun and magnetic characterization of a new ferrimagnetic oxide: YFeMnO5, Chem.
Mater. 16 (21) (2004) 4087e4094. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis and study of [6] A. Mun the crystallographic and magnetic structure of the ferrimagnetic oxide ErFeMnO5, Phys. Rev. B 72 (18) (2005) 184402. [7] M.J. Martínez-Lope, M. Retuerto, J.A. Alonso, V. Pomjakushin, Synthesis and study of the crystallographic and magnetic structure of DyFeMnO5: a new ferrimagnetic oxide, J. Solid State Chem. 181 (9) (2008) 2155e2160. ndez-Díaz, J.A. Alonso, [8] M. Retuerto, M.J. Martínez-Lope, K. Krezhov, M.T. Ferna Structural and magnetic characterization of BiFexMn2exO5 oxides (x¼0.5, 1.0), J. Solid State Chem. 184 (9) (2011) 2428e2433. [9] Y. Chen, H. Yuan, G. Tian, G. Zhang, S. Feng, Hydrothermal synthesis and magnetic properties of RMn2O5 (R¼La, Pr, Nd, Tb, Bi) and LaMn2O5þd, J. Solid State Chem. 180 (4) (2007) 1340e1346. [10] J.A. Alonso, M.T. Casais, M.J. Martınez-Lope, I. Rasines, High oxygen pressure preparation, structural refinement, and thermal behavior of RMn2O5 (R¼La, Pr, Nd, Sm, Eu), J. Solid State Chem. 129 (1) (1997) 105e112. [11] R. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (5) (1976) 751e767. [12] J.F. Moulder, J. Chastain, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of Xray Photoelectron Spectroscopy, Physical Electronics, 1995. [13] C. Tapan, O. Bachir, B. Dipten, Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO3, J. Phys. Condens. Matter 21 (30) (2009) 306001. [14] M.J. Martínez-Lope, M. Retuerto, J.A. Alonso, M. García-Hern andez, K. Krezhov, I. Spirov, T. Ruskov, M.T. Fern andez-Díaz, Crystal structure and magnetism of €ssbauer spectroscopy study, Solid YbFeMnO5: a neutron diffraction and Mo State Commun. 149 (13e14) (2009) 540e545. [15] J.B. Goodenough, Theory of the role of covalence in the perovskite-type manganites [La,M(II)]MnO3, Phys. Rev. 100 (2) (1955) 564. [16] J. Kanamori, Superexchange interaction and symmetry properties of electron orbitals, J. Phys. Chem. Solids 10 (2e3) (1959) 87e98. [17] P.W. Anderson, Antiferromagnetism. Theory of superexchange interaction, Phys. Rev. 79 (2) (1950) 350.
́
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Phase stabilization of Fe substituted NdMn2O5 ceramics and their properties K. Saravana Kumar a, b, *, C. Venkateswaran b a b
Department of Physics, SRM University, Ramapuram Campus, Chennai, 600 089, India Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai, 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2014 Received in revised form 8 March 2015 Accepted 16 May 2015 Available online 19 May 2015
An oxide of stoichiometry, NdFeMnO5, has been synthesized using a two-step process. First the precursor oxides are high-energy ball milled and the as-milled powders are then sintered to obtain the NdFeMnO5 phase. Rietveld refinement of the XRD pattern, by replacing the Mn3þ sites with Fe3þ, shows the formation of stoichiometric NdFeMnO5. Nd & Mn are found to be in þ3 and þ4 states, respectively from the XPS study. Agglomeration of fine grains is evident from the electron micrographs. Fe3þ has a magnetic moment higher than Mn3þ, and hence affects the magnetic property. Thermo-magnetization measurements show the existence of magnetic ordering below 290 K. An increase in magnetization at low temperature is also observed due to ordering of R3þ moments in addition to Fe and Mn spin moments. A linear increase in conductivity and stable dielectric response is observed from impedance study at high temperature. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: NdMn2O5 High-energy ball milling Magnetic study X-Ray Photoelectron Spectroscopy
1. Introduction Materials of the RMn2O5 (R ¼ Rare earths, Bi,Y) family have Mn at two different crystallographic sites. Mn4þ ions occupy the octahedral pyramidal (4f) sites where it is coordinated with six oxygen atoms and Mn3þ ions are present at tetragonal pyramidal (4h) sites having coordination with five oxygen atoms [1]. They have complex magnetic structures which give rise to magnetodielectric effect around the magnetic transition temperatures. Generally, RMn2O5 systems have orthorhombic structure with Pbam spacegroup and exhibit paramagnetic behavior at room temperature [1]. They have a Neel transition temperature around ~40 K below which antiferromagnetic ordering exists. A magnetoelectric coupling behavior is exhibited at the magnetic transition at low temperatures which is reportedly very weak in nature. The Fe substitution has its effect on the structural and magnetic properties of RMn2O5 family due to its high spin-only effective magnetic moment [2e4]. Such a study describes the Fe3þ substitution at the Mn3þ (4h) site and the corresponding change in properties are compared with parent system [2e8]. Most of the reports indicate a minor structural modification along with change
* Corresponding author. Department of Physics, SRM University, Ramapuram Campus, Chennai, 600 089, India. E-mail address: [email protected] (K.S. Kumar). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.003 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
in bond lengths and bond angles in Mn layers of RFeMnO5 depending on the type of R ion (R ¼ Y, Tb, Dy, Ho, Er, Bi). But only few of the R(Y, Er, Dy) ions exhibit a ferrimagnetic behavior as a result of Fe substitution into the RMn2O5 structure [5e7]. Remaining other systems in the family report a change in transition temperature or modification in the long range magnetic ordering established below the transition temperature. It is understood from literature that NdMn2O5 is one of the less studied system and the effect of Fe substitution is not yet reported [9,10]. Hence, this paper deals with the structural and magnetic properties of phase stabilized NdFeMnO5 obtained from a two-step synthesis procedure. 2. Experimental Neodymium oxide, manganese mono-oxide and iron oxide were taken in stoichiometric proportions and milled for a total of 5 h at 450 RPM with a ball to powder ratio of 28:2. The obtained reddish brown powders were then pressed into dense pellets of 8 mm diameter and sintered at 750 C for 24 h. Pellets were furnace cooled and then ground in mortar to obtain the black powders of NdFeMnO5. X-Ray diffraction (XRD) data was collected with CueKa radiation at room temperature. A DAR400-XM1000 (OMICRON Nanotechnologies, Germany) instrument equipped with dual Al/Mg anodes as the x-ray source was used to perform X-Ray
2
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
Fig. 1. Rietveld refinement of the XRD pattern of NdFeMnO5. * indicates un-indexed peaks.
Photoelectron Spectroscopy (XPS) measurement. Al anode was used to obtain the survey and elemental spectrums. The peak of C 1s at 284.5 eV was used as reference to calibrate the binding energy of elements to exclude charging effect on the sample. Pass energy of 50 eV was used to obtain survey spectrum and 20 eV pass energy was used for elemental spectra. High resolution scanning electron microscopy (HR-SEM) was done in FEI Quanta FEG 200 HR Microscope. Isothermal magnetization and Zero field cooled (ZFC)-Field Cooled (FC) measurements were carried out in the range 20e300 K in a Lakeshore VSM 7410 instrument. A field of 1000 Oe was applied for the ZFC-FC experiment and for the iso-thermal magnetization curves a maximum field of 15 KOe was applied. Impedance measurements were done in Solatron Impedance Analyzer (S-1260) in 1 Hze1 MHz frequency range and 300 Ke523 K temperature range. A NdFeMnO5 pellet of 8 mm diameter and 2.8 mm thickness obtained by pressing with 2.5 ton and then coated with silver on both faces for better electrical contact. The coated pellet was heat treated at 350 C for 4 h to obtain dense pellet, and used for the measurements. 3. Results and discussion
Table 1 Structural parameters obtained from the Rietveld refinement of NdFeMnO5. Label
Atom
Position
x
y
z
Occupation
Nd Mn1 Fe2 O1 O2 O3 O4
Nd3þ Mn4þ Fe3þ O2 O2 O2 O2
4g 4f 4h 4e 4g 4h 8i
0.1411 0.50000 0.4064 0.00000 0.15855 0.18839 0.4023
0.1700 0.00000 0.3482 0.00000 0.4425 0.43967 0.2109
0.00000 0.2687 0.50000 0.2687 0.00000 0.50000 0.2330
1.0 1.0 1.0 1.0 1.0 1.0 2.0
a ¼ 7.5037(2) Å, b ¼ 8.6002(2) Å, c ¼ 5.6973(1) Å c2 ¼ 1.10, Rp ¼ 7.76, Rwp ¼ 10.3. Weight percentage of phases: NdFeMnO5 (93.98%) and NdMnO3 (6.02%).
Rietveld refinement plot of X-ray diffraction (XRD) pattern is given in Fig 1, indicating the formation of stoichiometric NdFeMnO5 which is iso-structural with NdMn2O5. The structural parameters obtained from the refinement are listed in Table 1. The structural model of NdMn2O5 was taken as the starting model for the Rietveld refinement, done using the using the FULLPROF suite, by substituting Fe3þ in Mn3þ (4h) sites. The calculated pattern of the NdFeMnO5 phase matches with the experimental pattern. NdMnO3 is present as a minor secondary phase and does not contribute to the magnetic property due to the paramagnetic nature at room temperature. Secondary phases related to Fe are not evident from
Fig. 2. (a) Structural model of NdFeMnO5 showing chains of Mn4þO6 and Fe3þO5 extending along c-axis. (b) Model showing arrangement of Nd, Mn and Fe along c-axis (extending into plane of paper) without O ions. (c) Arrangement of layers of Nd, Mn and Fe layers with a-axis extending into plane of paper.
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
the XRD pattern. From the structural point of view, illustrated in Fig 2a, NdFeMnO5 has orthorhombic (Pbam) structure as that of its parent NdMn2O5. Parent system has the Mn4þ octahedra network connected through oxygen atoms, extending along c-axis and, has Mn3þ & Nd3þ layers alternating between these Mn4þ layers. In the case of NdFeMnO5, it is considered that the Mn3þ layers in the parent system are completely replaced by Fe3þ layers connecting Mn4þ ions through oxygen ions. The arrangement of Nd, Mn and Fe atoms in a unit cell of NdFeMnO5 is illustrated in Fig 2, along the caxis (b) and a-axis (c). Even though the NdFeMnO5 is iso-structural with NdMn2O5, still there is a change in the bond angles and bond lengths of the Mn4þ, Fe3þ and Nd3þ layers with the surrounding oxygen atoms. This change is also brought about by the removal of Mn3þ, a Jahn Teller ion whose effect decreases in NdFeMnO5. Table 2 shows the refined cell parameters of NdFeMnO5 along with those of NdMn2O5 from selected references. As discussed elsewhere [9,10], it is inferred that a small variation in cell parameters of NdMn2O5 depend on the synthesis procedure also. There is insignificant variation in cell parameters of NdFeMnO5 from the parent system as expected, since the radii of Fe3þ and Mn3þ are comparable in a 5-fold coordination environment [11]. Hence from Table 2, it is evident that the Fe substitution has minimum possible effect on the crystal structure. In spite of the assumption that Fe3þ replaces Mn3þ ions, there is a possibility of anti-site disorder which enables few Fe3þ ions to be found at Mn4þ sites and Mn4þ/Mn3þ to be found at Fe3þ sites also. XPS study was carried out to determine the possible oxidation state of Mn ions and substituted Fe ions. Survey spectrum, Fig 3a, shows the presence of elements in the NdFeMnO5. The binding energy value of C 1s peak at 284.5 eV was taken as the reference for charge correction and all the binding energy values determined by fitting the spectra were charge corrected. CASA XPS software was used to analyze the elemental spectra with the fitting function GL(30) [GL(p) - product of Gaussian(0)-Lorentzian(100)]. ‘Linear’ and ‘Shirley’ functions were used appropriately for background fitting. The Nd 3d5/2 peak at 980.5 eV, close to the value of Nd2O3, indicates that Nd is in þ3 oxidation state [12]. A very weak spectrum for Fe was obtained which was not suitable for fitting. A reasonable fit for the single oxidation state of þ4 was obtained from the Mn spectrum as shown in Fig 3b. The binding energy value is 641.7 eV for 2p3/2 peak of Mn, which corresponds to the energy value for Mn in MnO2 [12]. Hence the inference that Mn at þ4 states is consistent with XRD results serving as an indirect evidence for the Fe3þ states along with the insignificant variation in cell parameters due to Fe3þ substitution. High resolution scanning electron microscopic (HR-SEM) image of NdFeMnO5 is shown in Fig 4. Image shows the agglomeration of irregular shaped fine grains with a distribution of grains sizes. It is also observed that the grain sizes are not below the critical size in nanometers and hence do not affect the magnetic property due to finite size effect. The magnetic behavior of the RMn2O5 family is dominated by the Mn4þeOeMn4þ(J1) chain with antiferromagnetic nature and Mn4þeOeMn3þeOeMn4þ(J2) chain with ferromagnetic nature [1]. The low temperature zero field cooled-field cooled (ZFC-FC) curves are shown in Fig 5, which exhibits a bifurcation below 290 K and an increase in FC magnetization values till 20 K. This behavior is Table 2 Comparison of the NdFeMnO5 and NdMn2O5 cell parameters.
NdFeMnO5 NdMn2O5 (Ref. [10]) NdMn2O5 (Ref. [9])
a (Å)
b (Å)
c (Å)
Volume (Å3)
7.5037(2) 7.5051(2) 7.501(1)
8.6002(2) 8.6209(2) 8.620(2)
5.6973(1) 5.7022(1) 5.701(1)
367.6 368.9 368.6
3
Fig. 3. XPS spectrum of NdFeMnO5, (a) survey spectrum [Inset: Elemental spectrum of Fe], and (b) Mn elemental spectrum.
indicative of the magnetic ordering below ~290 K and is supported by the linear dependence of magnetization on the applied magnetic field obtained at 20 K (Fig 6). The parent NdMn2O5 has a Neel transition (TN) temperature of ~75 K, below which it is reported to have antiferromagnetic ordering [9,10]. NdMnO3 has reported magnetic ordering temperature of ~82 K in which the Nd ions order at around ~20 K [13]. The inset of Fig. 5 shows the (dM/dT) plot for the ZFC curve, in which there is evidence for the three different transitions (~95 K, ~70 K and ~40 K). The first transition (~95 K) corresponds to NdMnO3 phase and the second (~70 K) relates to the NdMn2O5 phase, which may be present in minor percentages, not evident from the XRD data. Transition observed at ~40 K corresponds to the ordering of Nd3þ spins. Evidently, NdMnO3 phase does not have major influence on magnetic property observed at low temperatures. Hence, it is inferred that the magnetic ordering temperature of NdFeMnO5 is at ~290 K when compared to the transition temperature of NdMn2O5 (~70 K). Isothermal magnetization measurements at 20 K, 100 K and 300 K are given in Fig 6 indicating an increase in magnetization value at 15 kOe. Initially a weak ferromagnetic behavior is observed at low fields and later a linear dependence on magnetic field is observed for curves at 20 K and 100 K. Weak ferromagnetic behavior is evident from the inset of Fig. 6 indicating the presence of coercivity and retentivity at low fields. The magnetization curve
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at 300 K hints on the onset of magnetic ordering with a weak ferromagnetic response at low fields and the bifurcation observed in ZFC-FC curves at ~290 K may be an indicator of the completion of moment ordering. The magnetic transition is due to the moment ordering in Mn4þ and Fe3þ layers, where the moment of Nd3þ layer also contributes to the observed magnetic ordering below the transition temperature of ~75 K. Global magnetic behavior in NdFeMnO5 is based on the relative alignment of spins in Mn4þeOeMn4þ(J3) and Mn4þeOeFe3þeOeMn4þ(J4) chains determined through superexchange interaction, where type of ordering in J3 is influenced by the alignment of Nd3þ moments. The spin only effective magnetic moments of Mn4þ, Mn3þ and Fe3þ are 3.87 mB, 4.89 mB and 5.92 mB [14], respectively. The magnetic ordering arises due to the alignment of the magnetic Nd3þ, Fe3þ and Mn4þ moments in the unit cell. Due to Fe3þ substitution at Mn3þ sites, the magnetic ordering of the chain Mn4þeOeFe3þeOeMn4þ through the superexchange interaction is affected as the magnetic moment of Fe3þ is higher than Mn3þ. Also, Fe3þeOeFe3þ having an angle close to 90 shows a short range ferromagnetic ordering in accordance with GoodenougheKanamorieAnderson [15e17] rules which requires higher fields for spin reversal. Generally, Mn4þeOeMn4þ has an AFM ordering in RMn2O5 structures. In the case of NdFeMnO5, the Mn4þeOeMn4þ interaction is definitely affected by the moment ordering of Nd3þ ions below the transition temperatures. Inferences from magnetic studies indicate that the Fe
Fig. 5. Low temperature ZFC-FC curves at an applied field of H ¼ 1000 Oe showing bifurcation at ~290 K [Inset: dM/dT curve for ZFC data indicating three different magnetic transitions].
substitution has major influence on the magnetic ordering in NdFeMnO5 with no major structural variation. Impedance spectroscopy measurements were carried out to determine the electrical properties. In a parallel RC combination, shown in Fig. 7, the real and imaginary parts are, Zr ¼ R/(1 þ (uRC)2) and Zi ¼ uR2C/(1 þ (uRC)2) Elimination of u from the above equations leads to,
2 R 2 R Zr þ Zi2 ¼ 2 2 which is the equation of a circle of radius R/2 with center at (R/2,0). From the derived resistance values, the conductivity values are calculated using the formula,
s¼
d R$A
Where, s is the conductivity, d is the thickness of sample, R is the resistance value obtained from fit of ColeeCole plot and A is the
Fig. 4. HR-SEM micrographs of NdFeMnO5 at two different magnifications.
Fig. 6. Isothermal magnetization curve at 20 K, 100 K and 300 K [Inset: Magnetization curve at low fields indicating a weak ferromagnetic behaviour.].
K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
Fig. 7. (a) A common RC circuit and (b) its impedance plane plot.
area of the sample. The values of Arrhenius plot is then derived using the formula,
Ea s ¼ s0 exp kB T from which the fit for linear region gives the value of activation energy, Eg. A resistivity value of the order of ~107 ohm-cm is observed from Fig. 8a at a temperature of 333 K. The Arrhenius Plot of NdFeMnO5 to determine the activation energy is given in Fig 8b. A linear increase in conductivity is observed in the entire measurement temperature range. Activation energy (Ea) value of 0.57 eV is determined from the linear fit of the data. The conduction increases with temperature due to the activation of defects and charge
Fig. 8. (a) Resistivity plot of NdFeMnO5 obtained from impedance measurement. (b) Arrhenius plot of NdFeMnO5 from which activation energy is derived.
5
carriers. Contribution towards conduction from the secondary phases present in the samples have to be considered at higher temperatures, which is also evident from the lower activation energy values. The dielectric constant values have no major variation with temperature, as evident from the almost flat response from temperature dependent dielectric constant curves given in Fig 9a. This trend indicates the dielectric stability of the sample with increasing temperatures for various frequencies. The dielectric constant values decreases with increasing frequency as observed from the frequency dependent dielectric constant graph given in Fig 9b. This is a normal behavior as the defects and charge carriers follow the field at low frequencies, which will not happen at high frequencies. Consequently, it is observed that the sample shows a stable dielectric response throughout the measurement temperature range with linear increase in conductivity. 4. Conclusion The compound of stoichiometry, NdFeMnO5, has been synthesized by initial high-energy ball milling of oxides and then sintering the milled powders. The oxidation states of Nd, Mn and Fe are found to be þ3, þ4 and þ3, respectively from the combined XPS and XRD studies, determining the existence of Fe ions at Mn3þ sites in the NdMn2O5 structure. The magnetic property is affected significantly as observed from the dramatic rise in the magnetic
Fig. 9. (a) Temperature-dependent dielectric constant plot of NdFeMnO5 at various frequencies. (b) Frequency-dependent dielectric constant plot of NdFeMnO5 at various temperatures.
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K.S. Kumar, C. Venkateswaran / Solid State Sciences 46 (2015) 1e6
ordering temperature, around 290 K. This is due to the definite change in interaction dynamics in Mn4þeOeMn4þ and Mn4þeOeFe3þeOeMn4þ chains due to Fe substitution and Nd3þ moment ordering at low temperatures. A stable dielectric response with linear increase in conductivity is observed in the measurement temperature range. Acknowledgment KSK thanks DRDO, India for support through fellowship. Authors acknowledge SAIF, IIT Madras for low temperature VSM measurements; NFMTC, IIT Madras for HR-SEM measurement and NCNSNT, University of Madras for XPS measurements. References ~ oz, J.A. Alonso, M.T. Casais, M.J. Martínez-Lope, J.L. Martínez, [1] A. Mun ndez-Díaz, Magnetic structure and properties of BiMn2O5 oxide: a M.T. Ferna neutron diffraction study, Phys. Rev. B 65 (14) (2002) 144423. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis and study of [2] A. Mun the crystallographic and magnetic structure of HoFeMnO5, Eur. J. Inorg. Chem. 2007 (14) (2007) 1972e1979. [3] K.S. Kumar, C. Venkateswaran, Effect of Fe substitution on the magnetic properties of BiMn2O5, J. Phys. D Appl. Phys. 44 (32) (2011) 325001. ~ oz, T. Ruskov, I. Spirov, K. Krezhov, [4] M. Retuerto, M.J. Martínez-Lope, A. Mun M.T. Fern andez-Díaz, M. García-Hern andez, J.A. Alonso, Synthesis, structural study and magnetic properties of TbFeMnO5, Solid State Commun. 150 (37e38) (2010) 1831e1836. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis, structural, [5] A. Mun and magnetic characterization of a new ferrimagnetic oxide: YFeMnO5, Chem.
Mater. 16 (21) (2004) 4087e4094. ~ oz, J.A. Alonso, M.J. Martínez-Lope, J.L. Martínez, Synthesis and study of [6] A. Mun the crystallographic and magnetic structure of the ferrimagnetic oxide ErFeMnO5, Phys. Rev. B 72 (18) (2005) 184402. [7] M.J. Martínez-Lope, M. Retuerto, J.A. Alonso, V. Pomjakushin, Synthesis and study of the crystallographic and magnetic structure of DyFeMnO5: a new ferrimagnetic oxide, J. Solid State Chem. 181 (9) (2008) 2155e2160. ndez-Díaz, J.A. Alonso, [8] M. Retuerto, M.J. Martínez-Lope, K. Krezhov, M.T. Ferna Structural and magnetic characterization of BiFexMn2exO5 oxides (x¼0.5, 1.0), J. Solid State Chem. 184 (9) (2011) 2428e2433. [9] Y. Chen, H. Yuan, G. Tian, G. Zhang, S. Feng, Hydrothermal synthesis and magnetic properties of RMn2O5 (R¼La, Pr, Nd, Tb, Bi) and LaMn2O5þd, J. Solid State Chem. 180 (4) (2007) 1340e1346. [10] J.A. Alonso, M.T. Casais, M.J. Martınez-Lope, I. Rasines, High oxygen pressure preparation, structural refinement, and thermal behavior of RMn2O5 (R¼La, Pr, Nd, Sm, Eu), J. Solid State Chem. 129 (1) (1997) 105e112. [11] R. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A 32 (5) (1976) 751e767. [12] J.F. Moulder, J. Chastain, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of Xray Photoelectron Spectroscopy, Physical Electronics, 1995. [13] C. Tapan, O. Bachir, B. Dipten, Neutron diffraction investigation of the magnetic structure and magnetoelastic effects in NdMnO3, J. Phys. Condens. Matter 21 (30) (2009) 306001. [14] M.J. Martínez-Lope, M. Retuerto, J.A. Alonso, M. García-Hern andez, K. Krezhov, I. Spirov, T. Ruskov, M.T. Fern andez-Díaz, Crystal structure and magnetism of €ssbauer spectroscopy study, Solid YbFeMnO5: a neutron diffraction and Mo State Commun. 149 (13e14) (2009) 540e545. [15] J.B. Goodenough, Theory of the role of covalence in the perovskite-type manganites [La,M(II)]MnO3, Phys. Rev. 100 (2) (1955) 564. [16] J. Kanamori, Superexchange interaction and symmetry properties of electron orbitals, J. Phys. Chem. Solids 10 (2e3) (1959) 87e98. [17] P.W. Anderson, Antiferromagnetism. Theory of superexchange interaction, Phys. Rev. 79 (2) (1950) 350.
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