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Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure
Chemical Physics 528 (2020) 110541
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure
T
S.E. Kichanova, D.P. Kozlenkoa, L.H. Khiemb, N.X. Nghiab, N.T.T. Lieuc, M.T. Vud, E.V. Lukina, ⁎ D.T. Khane, N.Q. Tuane, B.N. Savenkoa, N.T. Dangf, a
Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Moscow Reg., Russia Institute of Physics, Vietnam Academy of Science and Technology, 100000 Ha Noi, Viet Nam c Posts and Telecommunications Institute of Technology, 100000 Ha Noi, Viet Nam d Department of Physics, University of Ulsan, 44610 Ulsan, South Korea e The University of Danang – University of Science and Education, 550000 Da Nang, Viet Nam f Institute of Research and Development, Duy Tan University, 550000 Da Nang, Viet Nam b
ARTICLE INFO
ABSTRACT
Keywords: Manganites Neutron diffraction High pressure Magnetic phase separation
The high-pressure evolution of structural and magnetic properties of the single valence Mn3+ manganite La0.8Sr0.2Mn0.9Sb0.1O3 was studied by means of neutron powder diffraction up to 4.5 GPa in the temperature range of 10–300 K. At ambient pressure, a ferromagnetic ground state with the magnetic ordering temperature TC ≈ 175 K is stabilized. Application of high pressure suppresses the initial FM phase and leads to the appearance of a new A-type antiferromagnetic AFM phase with TN ~ 130 K at P = 2.2 GPa. Under compression, both TC and TN almost linearly increases with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. It is assumed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to the eg d x 2 z 2 orbital polarization at high pressures.
1. Introduction Mixed valence manganites R1−xAxMnO3 (R – rare earth, A – alkali earth elements) with perovskite-like structure are widely known for their unusual rich variety of challenging physical phenomena like colossal magnetoresistance effect, orbital and charge ordering, insulatormetal transition, rich magnetic phase diagrams [1–3]. Unusual physical properties of manganites are a result of complex coupling of spin, charge, and orbital degrees of freedom [4]. Thus, the realization of different magnetic structures in the manganites corresponds to complicated balance between the ferromagnetic (FM) double exchange interaction in a chain Mn3+-O2−-Mn4+ mediated by charge carriers of the eg nature and the antiferromagnetic (AFM) superexchange one between localized magnetic moments of the t2g nature, coupled to lattice distortion effects and orbital degrees of freedom [5–8]. Recently, it has been found that the ferromagnetic FM state can be formed in manganites containing only Mn3+ ions, for examples, LaMnO3 doped by Ga3+ or Sc3+ ions [9–11]. It is well-known that the undoped LaMnO3 exhibits ferromagnetic interactions between Mn3+ ions above the orbital order-disorder transition around To-d ~ 750 K [9,10]. At lower temperatures, a static cooperative Jahn-Teller (JT) ⁎
distortion removes the degeneracy of half-filled eg orbitals of Mn3+ ions and leads to the long-range eg antiferrodistorsive d3x 2 r 2 /d3z 2 r 2 orbital order, which is the origin of A-type antiferromagnetic (AFM) insulating ground state below TN = 140 K [6,12,13]. As mentioned above, the long-range ferromagnetic order was observed in the Mn3+-site substituted LaMnO3 by diamagnetic Ga3+ and Sc3+ ions, preserving the trivalent state of manganese ions [9–11]. The origin of the FM state in these single valence Mn3+ manganites was assumed to be associated with dynamic JT correlations between neighboring manganese ions [9,11]. A formation of dielectric FM state was also found in the orbitally disordered manganites R0.7Sr0.3Mn0.85Nb0.155+O3 (R = La, Nd, Sm, and Pr), where the content of Mn4+ ions is fully negligible [10,14,15]. With decreasing the ionic radius of the rare-earth ion R, an increase of lattice distortions provokes the magnetic phase transition from the FM state to a spin-glass one [10]. Besides chemical composition, the magnetic properties of materials are also strongly dependent on other factors such as their dimensions or external perturbations like temperature and pressure, and electric or magnetic fields [16–23]. Among these factors, the high-pressure application can directly vary relevant inter-atomic distances and angles in a large range, tuning the balance between the magnetic interactions
Corresponding author. E-mail address: [email protected] (N.T. Dang).
https://doi.org/10.1016/j.chemphys.2019.110541 Received 18 May 2019; Received in revised form 22 August 2019; Accepted 26 September 2019 Available online 26 September 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 528 (2020) 110541
S.E. Kichanov, et al.
and the magnetic structure of the materials. It has been found that in the mixed valence manganites, the pressure-induced variation of the above-mentioned structural parameters leads to enhancement of the double exchange interactions and an increase of the Curie temperature TC, which is approximately equal to the insulator-metal transition temperature TIM [24–26]. Moreover, a pressure-induced appearance of the A-type AFM state coexisting with the initial FM one was observed in several orthorhombic manganites [15,25,27,28]. Recently, a similar effect has been observed in La0.7Sr0.3Mn0.83Nb0.17O3 containing only Mn3+ ions under high pressures above 2 GPa [15]. For further and comparative insight into structural and magnetic properties of another single valence Mn3+ manganite La0.7Sr0.3Mn0.85Sb0.15O3 [29] as well as the underlying relationship of its structure-magnetic properties, we have performed detailed studies using neutron diffraction at high pressures up to 4.5 GPa in a temperature range of 10–300 K.
at ambient conditions the studied compound adopts a rhombohedral crystal structure with R3¯c symmetry (Fig. 2a), which then upon cooling transforms into an orthorhombic one with Pnma symmetry (Fig. 2b) [29]. The obtained structural parameters for the room-temperature rhombohedral and low-temperature orthorhombic phases of La0.8Sr0.2Mn0.9Sb0.1O3 are in good agreement with those previously reported [10,29]. Moreover, at the structural transition point, it has been also evidenced a magnetic contribution to intensity of the nuclear peaks (0 2 0)/(1 0 1) and (1 2 1)/(2 0 0)/(0 0 2) with dhkl ~ 2.76 Å and 3.91 Å, indicating the onset of the long-range FM order [9,10,15]. The average value of the magnetic moment of the Mn3+ ions is about 3.21(15) μB per one manganese ion. As can be seen in Fig. 1, under compression at P = 2.2 GPa below the Neel temperature TN ≈ 130 K, in addition to the FM peaks, it was observed an appearance of a new magnetic reflection located at dhkl ≈ 3.6 Å. The data analysis has shown that the change corresponds to a formation of the A-type AFM state coexisting with the initial FM state. Contrast to the three-dimensional parallel spin alignment of the FM structure, this A-type AFM structure consists of FM (ac) planes antiferromagnetically coupled along the b-axis (see insets of Fig. 3). As can be seen in Fig. 1, a combination of the orthorhombic structural and two A-type AFM and FM magnetic phases fits the low-temperature and highpressure neutron diffraction patterns well with good values of the Rfactors, e.g., Rp = 9.69% and Rwp = 11.05% for the pattern at T = 10 K and P = 2.2 GPa. Moreover, the ordered magnetic moment has been refined to be 2.65(17) and 1.78(15) µB for the FM and AFM phases at 10 K, respectively. Upon further compression up to 4.5 GPa, the ordered Mn magnetic moment of the FM state decreases to 2.27(16) μB, whereas the magnetic moment of the AFM phase increases gradually to 2.21(14) µB. It indicates the development of the pressure-induced AFM phase and the suppression of the initial FM one. Based on the obtained ordered magnetic moments, the volume fraction of the AFM phase has been estimated to 31(5)% at 2.2 GPa and 49(7)% at 4.5 GPa. The temperature dependence of the ordered magnetic moments of the FM and AFM phases at different pressures is shown in Fig. 3a. In order to obtain the pressure dependence of the Curie temperature TC of the FM phase, the MFM(T) data were fitted by the function [34,35]:
2. Experimental details The powder sample La0.8Sr0.2Mn0.9Sb0.1O3 was prepared by solidstate reaction technique using high purity oxides La2O3, Mn2O3, Sb2O5 and carbonate SrCO3 taken in a stoichiometric ratio and thoroughly mixed in a planetary mill. La2O3 was preliminarily annealed at a temperature of 1100 °C to remove moisture. The synthesis was performed at 1550 °C for 7 h in air, using a two-step procedure with interim annealing at 1400 °C for 5 h followed by a thorough grinding. More details of the synthesis procedure were described in several previous works [10,15]. The neutron powder diffraction measurements at pressures up to 4.5 GPa were performed at several temperatures in the range 15–300 K using the DN-12 diffractometer [30] at the IBR-2 pulsed reactor, JINR, Russia. The sample with a volume of about 2 mm3 was loaded into a high-pressure cell with sapphire anvils [31,32]. The pressure was determined by a standard ruby fluorescence technique. In this procedure, several tiny ruby chips were placed at different points of the sample surface. Measurements of pressure distribution on the sample yield typical pressure inhomogeneities of ± 15%. The diffraction patterns were collected at the scattering angle of 90° with the resolution of Δd/ d = 0.022. The neutron diffraction patterns were analysed by the Rietveld method using the Fullprof software [33].
MFM 3S MFM Tc = Bs , MFM 0 S + 1 MFM 0 T
3. Results and discussion
(1)
where BS is the Brillouin function, S is the spin of the system (S = 2 for La0.8Sr0.2Mn0.9Sb0.1O3 manganite) and MFM0 is the magnetic moment of corresponded magnetic phase at T = 0. The pressure dependence of the Néel temperature TN was evaluated by fitting the MAFM(T) data by the function [36]:
The neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3 manganite measured at different temperatures and pressures are shown in Fig. 1. Consistent with previous studies [10,29], we have observed that
Fig. 1. (a) Neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3 at selected pressures at low temperature and room temperature. The experimental data processed by the Rietveld method are shown. Lower and upper vertical ticks represent the calculated positions of the structural peaks of the rhombohedral at ambient conditions and orthorhombic at T = 10 K and P = 4.5 GPa phases, respectively. The most intense peaks of the FM and AFM phases are denoted by symbol “FM” and “AFM”, respectively. (b) An enlarged region of neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3.
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Chemical Physics 528 (2020) 110541
S.E. Kichanov, et al.
Fig. 2. Representation of crystal structure of (a) R-3c rhombohedral and (b) Pnma orthorhombic phases of La0.8Sr0.2Mn0.9Sb0.1O3.
Fig. 3. (a) Temperature dependence of ordered magnetic moments of manganese ions for ferromagnetic FM and AFM phases of La0.8Sr0.2Mn0.9Sb0.1O3 obtained at different pressures. The solid lines are fitting of experimental data of the FM and AFM phases using the function (1) and (2), respectively. The inset pictures show the spin configuration of the FM and AFM orderings. (b) The Curie and Neel temperature of La0.8Sr0.2Mn0.9Sb0.1O3 as a function of pressure.
MAFM (T ) = MAFM0 (1
(T / TN ) )2 ,
(2)
10 K is present in Fig. 4a and b, respectively. As can be seen in Fig. 4a, the lattice compression is anisotropic with the most compressibility along the axis b. The obtained linear compressibility coefficients of the lattice parameters are ka = 0.0033(4) GPa−1, k ai = (1/ ai0 )(dai / dP )T (ai = a, b, c ) −1 ka = 0.0042(1) GPa and kc = 0.0030(2) GPa−1. The unit-cell volume compressibility data (Fig. 4b) were fitted by the third-order BirchMurnaghan equation of state [37,38]:
where MAFM0 is the magnetic moment of the AFM phase at T = 0. The obtained values of TC and TN as the function of pressure are shown in the insets of Fig. 3b. Under compression, both TC and TN almost linearly increase with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. These values are close to those previously obtained for La0.7Sr0.3Mn0.82Nb0.18O3 [15]. The coexistence of the FM and AFM phases reflects the competition between anti-sign magnetic interactions, which consequently are dependent on structural parameters such as bond distances and bond angles. Thus, in order to understand the origin of the observed magnetic phenomena it is necessary to establish the pressure evolution of such structural parameters of the low-temperature orthorhombic phase. The pressure dependence of the lattice parameters and unit-cell volume at
P=
(
3 B0 x 2
7 3
x
5 3
)
1+
3 (B 4
(
4) x
2 3
1
)
,
(3)
where x = V / V0 is the relative volume change, V0 is the unit cell volume at P = 0 GPa , B0 = V (dP / dV )T is the bulk modulus, and B = (dB0/ dP )T is the derivative of the bulk modulus with respect to pressure. The fitted values are B0 = 101(3) GPa and B′ = 4.0(7) for the
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Chemical Physics 528 (2020) 110541
S.E. Kichanov, et al.
slightly increase under pressure (Fig. 5b). The uniaxial octahedra compression can be characterized by a distortion parameter Sa = (Mn–Ob)/〈Mn–Oa〉 [25]. The Sa parameter reduces from close to perfect value 0.998 to 0.979, as pressure increases from 0 to 4.5 GPa. Such an effect reflects an enhancement of the incoherent JT lattice distortions at Mn3+ sites [39–41]. Moreover, for the uniaxial octahedra compression along the b axis a preferential polarization of the eg d x 2 z 2 orbitals is expected [9,11,25,34]. At low temperatures, onset of the d x 2 z 2 orbital order would result in a formation of the A-type AFM state embedded in the orbital disordered FM matrix [9,11,25,34]. In addition, the magnetic and transport properties of manganites in the FM phase are determined by the charge carrier bandwidth W and simple approach is TC ~ W [24,25]. In this case, bandwidth W depends on the average 〈Mn–O〉 bond length l and the average Mn–O–Mn angle 〈φ〉 as W =
cos 2
l 3.5
[34,42]. We can estimate those pressure depen-
(1/ TC )(dTC / dP ) = 3.5kl 2 ·tan( )·k , dence as where kl = (1/ l 0)(dl/ dP )|T andk = (1/ 0)(d /dP )|T . The calculated value of dTC/dP is 2.1 K GPa−1, which is in good agreement with the experimental one of 1.9(2) K GPa−1. Taking into account the present and previous results [15], the qualitatively similar high-pressure behavior is expected in general orthorhombically distorted manganites with single valence Mn3+ and ferromagnetic insulating ground state [10].
Fig. 4. Pressure dependence of lattice parameters (a) and unit cell volume (b) of La0.8Sr0.2Mn0.9Sb0.1O3 at 10 K. The solid lines are linear fitting of lattice parameters and calculated curve of unit cell volume by the Birch-Murnaghan equation (3). The experimental error does not exceed the symbol sizes.
low-temperature Pnma phase of La0.8Sr0.2Mn0.9Sb0.1O3. Notably, the obtained bulk moduli value is close to that B0 = 118(3) GPa reported for the orthorhombic La0.7Sr0.3Mn0.83Nb0.17O3, containing only Mn3+ ions [15]. Furthermore, the pressure dependence of Mn–O bond lengths and Mn–O–Mn bond angles for the orthorhombic phase of La0.8Sr0.2Mn0.9Sb0.1O3 is also established and shown in Fig. 5a and b, respectively. Notably, at ambient pressure the orthorhombic crystal structure of La0.8Sr0.2Mn0.9Sb0.1O3 contains nearly regular MnO6 octahedra with approximately equal Mn–O bonds: one axial Mn–Ob directed along b axis and two apical Mn–O1a and Mn–O2a lying in the crystallographic plane (ac). The rapid compression of the bond lengths Mn-Ob was observed, where linear compressibility coefficients kMn(i = b, 1a, 2a) are kMnOi = (−(1/lMn-Oi)dlMn-Oi/dP)T −1 , kMn-O2a = 0.0033(1) GPa−1 and kMnOb = 0.0051(2) GPa −1 . Both Mn–O1–Mn and Mn–O2–Mn bond angles O1a = 0.0028(2) GPa
4. Conclusions The results of this study show that the FM ground state in La0.8Sr0.2Mn0.9Sb0.1O3 becomes unstable under high pressure and the A-type AFM ground state is gradually developed instead. It has been revealed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to orbital ordering effects, provoking mesoscopic structural separation due to local incoherent JT lattice distortions at Mn3+ sites. The comparable positive values of the pressure coefficient of the Curie and Néel temperatures reflect the strongly competing character of the FM and AFM Mn3+-O-Mn3+ superexchange interactions on the distorted Mn3+ lattice.
Fig. 5. Pressure dependence of Mn–O bond lengths (a) and Mn–O–Mn bond angles (b) measured at T = 10 K and linear fit of experimental points. The scheme of the oxygen octahedron around the manganese with labeled Mn–O bonds is presented.
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Chemical Physics 528 (2020) 110541
S.E. Kichanov, et al.
Acknowledgements
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We dedicate this manuscript to the memory of our respected colleague Prof. I.O. Troyanchuk, who has stimulated this research and recently passed away. This work was supported in part by Vietnam Academy of Science and Technology under the project “Utilization of neutron diffraction on IBR-2M pulse nuclear reactor for studying the crystal and magnetic structures of the composite materials under variable pressure and temperature” numbered VAST.CTVL.01/17-20. References [1] Y. Tokura, Colossal Magnetoresistive Oxides, Gordon and Breach Science Publishers, Amsterdam, The Netherlands, 2018. [2] J.M.D. Coey, M. Viret, S. Von Molnár, Adv. Phys. 58 (2009) 571. [3] J.M.D. Coey, M. Viret, L. Ranno, K. Ounadjela, Phys. Rev. Lett. 75 (1995) 3910. [4] J. Bonc̆a, Open Problems in Strongly Correlated Electron Systems, Kluwer Academic Publishers, 2011. [5] C. Zener, Phys. Rev. 82 (1951) 403. [6] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675. [7] P.G. De Gennes, Phys. Rev. 118 (1960) 141. [8] R. Maezono, S. Ishihara, N. Nagaosa, Phys. Rev. B 58 (1998) 11583. [9] I.O. Troyanchuk, M.V. Bushinsky, N.V. Volkov, V. Sikolenko, E.A. Efimova, C. Ritter, J. Exp. Theor. Phys. 120 (2015) 97. [10] I.O. Troyanchuk, M.V. Bushinsky, H. Szymczak, K. Bärner, A. Maignan, Eur. Phys. J. B 28 (2002) 75. [11] I.O. Troyanchuk, D. Karpinsky, V. Efimov, V. Sikolenko, O. Prokhnenko, M. Bartkowiak, J. Phys.: Condens. Matter 26 (2014) 396002. [12] H.Y. Hwang, S.W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [13] I. Loa, P. Adler, A. Grzechnik, K. Syassen, U. Schwarz, M. Hanfland, G. Rozenberg, P. Gorodetsky, M.P. Pasternak, Phys. Rev. Lett. 87 (2001) 125501/1. [14] I.O. Troyanchuk, M.V. Bushinsky, V. Efimov, S. Schorr, C. Ritter, V. Sikolenko, Powder Diffr. 30 (2015) S97. [15] M.T. Vu, D.P. Kozlenko, S.E. Kichanov, I.O. Troyanchuk, E.V. Lukin, L.H. Khiem, B.N. Savenko, J. Alloys Compd. 681 (2016) 527. [16] M. Yarmohammadi, B.D. Hoi, J. Magn. Magn. Mater. 464 (2018) 103. [17] B. Dinh Hoi, M. Yarmohammadi, M. Davoudiniya, Solid State Commun. 271 (2018) 21.
5
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Magnetic phase transition in La0.8Sr0.2Mn0.9Sb0.1O3 manganite under pressure
T
S.E. Kichanova, D.P. Kozlenkoa, L.H. Khiemb, N.X. Nghiab, N.T.T. Lieuc, M.T. Vud, E.V. Lukina, ⁎ D.T. Khane, N.Q. Tuane, B.N. Savenkoa, N.T. Dangf, a
Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Moscow Reg., Russia Institute of Physics, Vietnam Academy of Science and Technology, 100000 Ha Noi, Viet Nam c Posts and Telecommunications Institute of Technology, 100000 Ha Noi, Viet Nam d Department of Physics, University of Ulsan, 44610 Ulsan, South Korea e The University of Danang – University of Science and Education, 550000 Da Nang, Viet Nam f Institute of Research and Development, Duy Tan University, 550000 Da Nang, Viet Nam b
ARTICLE INFO
ABSTRACT
Keywords: Manganites Neutron diffraction High pressure Magnetic phase separation
The high-pressure evolution of structural and magnetic properties of the single valence Mn3+ manganite La0.8Sr0.2Mn0.9Sb0.1O3 was studied by means of neutron powder diffraction up to 4.5 GPa in the temperature range of 10–300 K. At ambient pressure, a ferromagnetic ground state with the magnetic ordering temperature TC ≈ 175 K is stabilized. Application of high pressure suppresses the initial FM phase and leads to the appearance of a new A-type antiferromagnetic AFM phase with TN ~ 130 K at P = 2.2 GPa. Under compression, both TC and TN almost linearly increases with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. It is assumed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to the eg d x 2 z 2 orbital polarization at high pressures.
1. Introduction Mixed valence manganites R1−xAxMnO3 (R – rare earth, A – alkali earth elements) with perovskite-like structure are widely known for their unusual rich variety of challenging physical phenomena like colossal magnetoresistance effect, orbital and charge ordering, insulatormetal transition, rich magnetic phase diagrams [1–3]. Unusual physical properties of manganites are a result of complex coupling of spin, charge, and orbital degrees of freedom [4]. Thus, the realization of different magnetic structures in the manganites corresponds to complicated balance between the ferromagnetic (FM) double exchange interaction in a chain Mn3+-O2−-Mn4+ mediated by charge carriers of the eg nature and the antiferromagnetic (AFM) superexchange one between localized magnetic moments of the t2g nature, coupled to lattice distortion effects and orbital degrees of freedom [5–8]. Recently, it has been found that the ferromagnetic FM state can be formed in manganites containing only Mn3+ ions, for examples, LaMnO3 doped by Ga3+ or Sc3+ ions [9–11]. It is well-known that the undoped LaMnO3 exhibits ferromagnetic interactions between Mn3+ ions above the orbital order-disorder transition around To-d ~ 750 K [9,10]. At lower temperatures, a static cooperative Jahn-Teller (JT) ⁎
distortion removes the degeneracy of half-filled eg orbitals of Mn3+ ions and leads to the long-range eg antiferrodistorsive d3x 2 r 2 /d3z 2 r 2 orbital order, which is the origin of A-type antiferromagnetic (AFM) insulating ground state below TN = 140 K [6,12,13]. As mentioned above, the long-range ferromagnetic order was observed in the Mn3+-site substituted LaMnO3 by diamagnetic Ga3+ and Sc3+ ions, preserving the trivalent state of manganese ions [9–11]. The origin of the FM state in these single valence Mn3+ manganites was assumed to be associated with dynamic JT correlations between neighboring manganese ions [9,11]. A formation of dielectric FM state was also found in the orbitally disordered manganites R0.7Sr0.3Mn0.85Nb0.155+O3 (R = La, Nd, Sm, and Pr), where the content of Mn4+ ions is fully negligible [10,14,15]. With decreasing the ionic radius of the rare-earth ion R, an increase of lattice distortions provokes the magnetic phase transition from the FM state to a spin-glass one [10]. Besides chemical composition, the magnetic properties of materials are also strongly dependent on other factors such as their dimensions or external perturbations like temperature and pressure, and electric or magnetic fields [16–23]. Among these factors, the high-pressure application can directly vary relevant inter-atomic distances and angles in a large range, tuning the balance between the magnetic interactions
Corresponding author. E-mail address: [email protected] (N.T. Dang).
https://doi.org/10.1016/j.chemphys.2019.110541 Received 18 May 2019; Received in revised form 22 August 2019; Accepted 26 September 2019 Available online 26 September 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 528 (2020) 110541
S.E. Kichanov, et al.
and the magnetic structure of the materials. It has been found that in the mixed valence manganites, the pressure-induced variation of the above-mentioned structural parameters leads to enhancement of the double exchange interactions and an increase of the Curie temperature TC, which is approximately equal to the insulator-metal transition temperature TIM [24–26]. Moreover, a pressure-induced appearance of the A-type AFM state coexisting with the initial FM one was observed in several orthorhombic manganites [15,25,27,28]. Recently, a similar effect has been observed in La0.7Sr0.3Mn0.83Nb0.17O3 containing only Mn3+ ions under high pressures above 2 GPa [15]. For further and comparative insight into structural and magnetic properties of another single valence Mn3+ manganite La0.7Sr0.3Mn0.85Sb0.15O3 [29] as well as the underlying relationship of its structure-magnetic properties, we have performed detailed studies using neutron diffraction at high pressures up to 4.5 GPa in a temperature range of 10–300 K.
at ambient conditions the studied compound adopts a rhombohedral crystal structure with R3¯c symmetry (Fig. 2a), which then upon cooling transforms into an orthorhombic one with Pnma symmetry (Fig. 2b) [29]. The obtained structural parameters for the room-temperature rhombohedral and low-temperature orthorhombic phases of La0.8Sr0.2Mn0.9Sb0.1O3 are in good agreement with those previously reported [10,29]. Moreover, at the structural transition point, it has been also evidenced a magnetic contribution to intensity of the nuclear peaks (0 2 0)/(1 0 1) and (1 2 1)/(2 0 0)/(0 0 2) with dhkl ~ 2.76 Å and 3.91 Å, indicating the onset of the long-range FM order [9,10,15]. The average value of the magnetic moment of the Mn3+ ions is about 3.21(15) μB per one manganese ion. As can be seen in Fig. 1, under compression at P = 2.2 GPa below the Neel temperature TN ≈ 130 K, in addition to the FM peaks, it was observed an appearance of a new magnetic reflection located at dhkl ≈ 3.6 Å. The data analysis has shown that the change corresponds to a formation of the A-type AFM state coexisting with the initial FM state. Contrast to the three-dimensional parallel spin alignment of the FM structure, this A-type AFM structure consists of FM (ac) planes antiferromagnetically coupled along the b-axis (see insets of Fig. 3). As can be seen in Fig. 1, a combination of the orthorhombic structural and two A-type AFM and FM magnetic phases fits the low-temperature and highpressure neutron diffraction patterns well with good values of the Rfactors, e.g., Rp = 9.69% and Rwp = 11.05% for the pattern at T = 10 K and P = 2.2 GPa. Moreover, the ordered magnetic moment has been refined to be 2.65(17) and 1.78(15) µB for the FM and AFM phases at 10 K, respectively. Upon further compression up to 4.5 GPa, the ordered Mn magnetic moment of the FM state decreases to 2.27(16) μB, whereas the magnetic moment of the AFM phase increases gradually to 2.21(14) µB. It indicates the development of the pressure-induced AFM phase and the suppression of the initial FM one. Based on the obtained ordered magnetic moments, the volume fraction of the AFM phase has been estimated to 31(5)% at 2.2 GPa and 49(7)% at 4.5 GPa. The temperature dependence of the ordered magnetic moments of the FM and AFM phases at different pressures is shown in Fig. 3a. In order to obtain the pressure dependence of the Curie temperature TC of the FM phase, the MFM(T) data were fitted by the function [34,35]:
2. Experimental details The powder sample La0.8Sr0.2Mn0.9Sb0.1O3 was prepared by solidstate reaction technique using high purity oxides La2O3, Mn2O3, Sb2O5 and carbonate SrCO3 taken in a stoichiometric ratio and thoroughly mixed in a planetary mill. La2O3 was preliminarily annealed at a temperature of 1100 °C to remove moisture. The synthesis was performed at 1550 °C for 7 h in air, using a two-step procedure with interim annealing at 1400 °C for 5 h followed by a thorough grinding. More details of the synthesis procedure were described in several previous works [10,15]. The neutron powder diffraction measurements at pressures up to 4.5 GPa were performed at several temperatures in the range 15–300 K using the DN-12 diffractometer [30] at the IBR-2 pulsed reactor, JINR, Russia. The sample with a volume of about 2 mm3 was loaded into a high-pressure cell with sapphire anvils [31,32]. The pressure was determined by a standard ruby fluorescence technique. In this procedure, several tiny ruby chips were placed at different points of the sample surface. Measurements of pressure distribution on the sample yield typical pressure inhomogeneities of ± 15%. The diffraction patterns were collected at the scattering angle of 90° with the resolution of Δd/ d = 0.022. The neutron diffraction patterns were analysed by the Rietveld method using the Fullprof software [33].
MFM 3S MFM Tc = Bs , MFM 0 S + 1 MFM 0 T
3. Results and discussion
(1)
where BS is the Brillouin function, S is the spin of the system (S = 2 for La0.8Sr0.2Mn0.9Sb0.1O3 manganite) and MFM0 is the magnetic moment of corresponded magnetic phase at T = 0. The pressure dependence of the Néel temperature TN was evaluated by fitting the MAFM(T) data by the function [36]:
The neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3 manganite measured at different temperatures and pressures are shown in Fig. 1. Consistent with previous studies [10,29], we have observed that
Fig. 1. (a) Neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3 at selected pressures at low temperature and room temperature. The experimental data processed by the Rietveld method are shown. Lower and upper vertical ticks represent the calculated positions of the structural peaks of the rhombohedral at ambient conditions and orthorhombic at T = 10 K and P = 4.5 GPa phases, respectively. The most intense peaks of the FM and AFM phases are denoted by symbol “FM” and “AFM”, respectively. (b) An enlarged region of neutron diffraction patterns of La0.8Sr0.2Mn0.9Sb0.1O3.
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Fig. 2. Representation of crystal structure of (a) R-3c rhombohedral and (b) Pnma orthorhombic phases of La0.8Sr0.2Mn0.9Sb0.1O3.
Fig. 3. (a) Temperature dependence of ordered magnetic moments of manganese ions for ferromagnetic FM and AFM phases of La0.8Sr0.2Mn0.9Sb0.1O3 obtained at different pressures. The solid lines are fitting of experimental data of the FM and AFM phases using the function (1) and (2), respectively. The inset pictures show the spin configuration of the FM and AFM orderings. (b) The Curie and Neel temperature of La0.8Sr0.2Mn0.9Sb0.1O3 as a function of pressure.
MAFM (T ) = MAFM0 (1
(T / TN ) )2 ,
(2)
10 K is present in Fig. 4a and b, respectively. As can be seen in Fig. 4a, the lattice compression is anisotropic with the most compressibility along the axis b. The obtained linear compressibility coefficients of the lattice parameters are ka = 0.0033(4) GPa−1, k ai = (1/ ai0 )(dai / dP )T (ai = a, b, c ) −1 ka = 0.0042(1) GPa and kc = 0.0030(2) GPa−1. The unit-cell volume compressibility data (Fig. 4b) were fitted by the third-order BirchMurnaghan equation of state [37,38]:
where MAFM0 is the magnetic moment of the AFM phase at T = 0. The obtained values of TC and TN as the function of pressure are shown in the insets of Fig. 3b. Under compression, both TC and TN almost linearly increase with the pressure coefficient of 1.9(2) and 0.9(3) K/GPa, respectively. These values are close to those previously obtained for La0.7Sr0.3Mn0.82Nb0.18O3 [15]. The coexistence of the FM and AFM phases reflects the competition between anti-sign magnetic interactions, which consequently are dependent on structural parameters such as bond distances and bond angles. Thus, in order to understand the origin of the observed magnetic phenomena it is necessary to establish the pressure evolution of such structural parameters of the low-temperature orthorhombic phase. The pressure dependence of the lattice parameters and unit-cell volume at
P=
(
3 B0 x 2
7 3
x
5 3
)
1+
3 (B 4
(
4) x
2 3
1
)
,
(3)
where x = V / V0 is the relative volume change, V0 is the unit cell volume at P = 0 GPa , B0 = V (dP / dV )T is the bulk modulus, and B = (dB0/ dP )T is the derivative of the bulk modulus with respect to pressure. The fitted values are B0 = 101(3) GPa and B′ = 4.0(7) for the
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slightly increase under pressure (Fig. 5b). The uniaxial octahedra compression can be characterized by a distortion parameter Sa = (Mn–Ob)/〈Mn–Oa〉 [25]. The Sa parameter reduces from close to perfect value 0.998 to 0.979, as pressure increases from 0 to 4.5 GPa. Such an effect reflects an enhancement of the incoherent JT lattice distortions at Mn3+ sites [39–41]. Moreover, for the uniaxial octahedra compression along the b axis a preferential polarization of the eg d x 2 z 2 orbitals is expected [9,11,25,34]. At low temperatures, onset of the d x 2 z 2 orbital order would result in a formation of the A-type AFM state embedded in the orbital disordered FM matrix [9,11,25,34]. In addition, the magnetic and transport properties of manganites in the FM phase are determined by the charge carrier bandwidth W and simple approach is TC ~ W [24,25]. In this case, bandwidth W depends on the average 〈Mn–O〉 bond length l and the average Mn–O–Mn angle 〈φ〉 as W =
cos 2
l 3.5
[34,42]. We can estimate those pressure depen-
(1/ TC )(dTC / dP ) = 3.5kl 2 ·tan( )·k , dence as where kl = (1/ l 0)(dl/ dP )|T andk = (1/ 0)(d /dP )|T . The calculated value of dTC/dP is 2.1 K GPa−1, which is in good agreement with the experimental one of 1.9(2) K GPa−1. Taking into account the present and previous results [15], the qualitatively similar high-pressure behavior is expected in general orthorhombically distorted manganites with single valence Mn3+ and ferromagnetic insulating ground state [10].
Fig. 4. Pressure dependence of lattice parameters (a) and unit cell volume (b) of La0.8Sr0.2Mn0.9Sb0.1O3 at 10 K. The solid lines are linear fitting of lattice parameters and calculated curve of unit cell volume by the Birch-Murnaghan equation (3). The experimental error does not exceed the symbol sizes.
low-temperature Pnma phase of La0.8Sr0.2Mn0.9Sb0.1O3. Notably, the obtained bulk moduli value is close to that B0 = 118(3) GPa reported for the orthorhombic La0.7Sr0.3Mn0.83Nb0.17O3, containing only Mn3+ ions [15]. Furthermore, the pressure dependence of Mn–O bond lengths and Mn–O–Mn bond angles for the orthorhombic phase of La0.8Sr0.2Mn0.9Sb0.1O3 is also established and shown in Fig. 5a and b, respectively. Notably, at ambient pressure the orthorhombic crystal structure of La0.8Sr0.2Mn0.9Sb0.1O3 contains nearly regular MnO6 octahedra with approximately equal Mn–O bonds: one axial Mn–Ob directed along b axis and two apical Mn–O1a and Mn–O2a lying in the crystallographic plane (ac). The rapid compression of the bond lengths Mn-Ob was observed, where linear compressibility coefficients kMn(i = b, 1a, 2a) are kMnOi = (−(1/lMn-Oi)dlMn-Oi/dP)T −1 , kMn-O2a = 0.0033(1) GPa−1 and kMnOb = 0.0051(2) GPa −1 . Both Mn–O1–Mn and Mn–O2–Mn bond angles O1a = 0.0028(2) GPa
4. Conclusions The results of this study show that the FM ground state in La0.8Sr0.2Mn0.9Sb0.1O3 becomes unstable under high pressure and the A-type AFM ground state is gradually developed instead. It has been revealed that the appearance of the A-type AFM state is mediated by the strong anisotropic distortions of MnO6 octahedra, coupled to orbital ordering effects, provoking mesoscopic structural separation due to local incoherent JT lattice distortions at Mn3+ sites. The comparable positive values of the pressure coefficient of the Curie and Néel temperatures reflect the strongly competing character of the FM and AFM Mn3+-O-Mn3+ superexchange interactions on the distorted Mn3+ lattice.
Fig. 5. Pressure dependence of Mn–O bond lengths (a) and Mn–O–Mn bond angles (b) measured at T = 10 K and linear fit of experimental points. The scheme of the oxygen octahedron around the manganese with labeled Mn–O bonds is presented.
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Acknowledgements
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