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Ferromagnetic ordering in La0.7Sr0.3Mn3+ 0.85Nb5+ 0.15O3 manganite

Published online by Cambridge University Press:  05 May 2015

I. O. Troyanchuk ,
M. V. Bushinsky ,
V. Efimov ,
S. Schorr ,
C. Ritter  and
V. Sikolenko
I. O. Troyanchuk
Affiliation:
Scientific-Practical Material Research Center of NAS of Belarus, Minsk, Belarus
M. V. Bushinsky
Affiliation:
Scientific-Practical Material Research Center of NAS of Belarus, Minsk, Belarus
V. Efimov
Affiliation:
Joint Institute for Nuclear Research, Dubna, Russia
S. Schorr
Affiliation:
Helmholtz Center Berlin, Berlin, Germany
C. Ritter
Affiliation:
Institute Laue Langevin, Grenoble, France
V. Sikolenko*
Affiliation:
Joint Institute for Nuclear Research, Dubna, Russia Helmholtz Center Berlin, Berlin, Germany
*
a) Author to whom correspondence should be addressed. Electronic mail: vadim.sikolenko@helmholtz-berlin.de

Abstract

Structural measurements have been performed on the La0.7Sr0.3Mn3+ 0.85Nb5+ 0.15O3 compound with oxidation state of manganese close to +3. The composition undergoes a structural transition from rhombohedral to orthorhombic symmetry below room temperature. The calculated structural parameters show that the orthorhombic phase is not long-range orbitally ordered and that the structural transition is associated with a steric effect. The compound is ferromagnetic with a Curie point of around 150 K and a magnetic moment of 3.1 μ B/Mn. It is suggested that ferromagnetism is originated from superexchange interactions via oxygen. Covalence enhances the positive part of the superexchange interactions whereas structural disorder leads to suppression of ferromagnetism.

Keywords

neutron diffractionRietveld analysesmanganitessuperexchange

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Type
Technical Articles
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Powder Diffraction , Volume 30 , Supplement S1 , June 2015 , pp. S97 - S100
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Copyright
Copyright © International Centre for Diffraction Data 2015 

I. INTRODUCTION

La1− x A x MnO3 (A = Ca, Sr, Ba) mixed valence manganites have been of interest for many years since they exhibit very intriguing magnetic and magnetotransport properties (Zener, Reference Zener1951; Pirogov et al., Reference Pirogov, Teplykh, Voronin, Balagurov and Petrov1999; Dagotto et al., Reference Dagotto, Hotta and Moreo2001; Şen et al., Reference Şen, Alvarez and Dagotto2007). To explain the interplay between magnetic and transport properties Zener introduced a special form of exchange interactions through charge carriers (Mn4+) – double exchange (Zener, Reference Zener1951). However, it was found that the ferromagnetic state can be realized even in compounds containing only manganese of valence 3 (Bents, Reference Bents1957; Goodenough, Reference Goodenough1963; Troyanchuk et al., Reference Troyanchuk, Khalyavin and Szymczak1997; Zhou et al., Reference Zhou, Yin and Goodenough2001; Blasco et al., Reference Blasco, Garcıa, Campo, Sanchez and Subıas2002; Deisenhofer et al., Reference Deisenhofer, Paraskevopoulos, Krug von Nidda and Loidl2002; Troyanchuk et al., Reference Troyanchuk, Bushinsky, Szymczak, Barner and Maignan2002; Zhou et al., Reference Zhou, Uwatoko, Matsubayashi and Goodenough2008). For example the parent LaMnO3 exhibits ferromagnetic interactions in the orbitally disordered phase (T > 750 K) with an approximate Curie point around 160 K (Zhou and Goodenough, Reference Zhou and Goodenough1999). Long-range ferromagnetism is observed in the LaMn1− x Ga x O3 (0.2 < x < 0.6) and LaMn1− x Cr x O3 (0.2 < x < 0.6) series containing only Mn3+. Both these series show a gradual transition into an orbitally disordered state upon Ga3+(x > 0.5) or Cr3+(x > 0.35) substitution. The origin of the ferromagnetic state in single valent manganites is a matter of discussion. It has been suggested that ferromagnetism can occur in the d z 2-orbitally ordered state by mixing of e g-orbitals with different symmetry whereas orbital disorder leads to the frustration of magnetic interactions (Zhou et al., Reference Zhou, Uwatoko, Matsubayashi and Goodenough2008). However, it is an apparent contradiction to the fact that ferromagnetism is very strong in the orbitally disordered phases in LaMn1− x Ga x O3 (x = 0.6) and LaMn1− x Cr x O3 (0.35 < x < 0.6) (Blasco et al., Reference Blasco, Garcıa, Campo, Sanchez and Subıas2002; Deisenhofer et al., Reference Deisenhofer, Paraskevopoulos, Krug von Nidda and Loidl2002).

Optimally doped La0.7Sr0.3(Mn3+ 0.7Mn4+ 0.3)O3 has the highest critical temperature (T C = 380 K) of the transition into the ferromagnetic state among mixed-valence manganites. The substitution of manganese ions with five-valence ions such as Nb5+ or Sb5+ leads to the reduction of the average manganese valence and hence, Mn3+–O–Mn3+superexchange interactions via oxygen should be dominant. In this work, we report the structure and properties of La0.7Sr0.3Mn0.85Nb0.15O3 stoichiometric compound containing only Mn3+ species.

II. EXPERIMENTAL

Ceramic sample of La0.7Sr0.3Mn0.85Nb0.15O3 series were prepared by a solid-state reaction technique using high purity oxides La2O3, Mn2O3, Nb2O5 and carbonate SrCO3 taken in a stoichiometric ratio and thoroughly mixed in a planetary mill (Retsch, 300 rpm, 30 min). La2O3 was preliminary annealed at 1100 °C in air in order to remove moisture. The synthesis was performed at 1550 °C for 7 h in air, using a two-step procedure with an interim annealing at 1400 °C for 5 h followed by a thorough grinding. The sample was cooled from the synthesis temperature with a rate of 300 °C/h down to 300 °C. Neutron powder diffraction (NPD) measurements were performed on the high intensity D1B (λ = 2.520 Å) and high resolution D2B (λ = 1.594 Å) diffractometers (Institute Laue-Langevin, Grenoble).

III. RESULTS AND DISCUSSION

Neutron powder diffraction measurements show that the crystal structure at room temperature can be successfully described in the frame of the rhombohedral space group R-3c (Figure 1, Table I). However, the compound shows a structural transition with temperature decrease. This transition occurs above 180 K as evidenced by NPD data recorded at different temperatures. Rietveld refinement of the neutron diffraction patterns at low temperature has been performed using the Pnma space group resulting in a satisfied agreement between experimental data and calculated patterns (Figure 1). The connection between the lattice and orbital degrees of freedom, as investigated in several orthorhombic manganites (Deisenhofer et al., Reference Deisenhofer, Paraskevopoulos, Krug von Nidda and Loidl2002; Zhou and Goodenough, Reference Zhou and Goodenough2008) suggests that the development of orbital ordering results in a contraction of the b parameter, and if b/√2 < c ≤ a the occurrence of orbital ordering can be conjectured. On the other hand, if c > a ≈ b/√2, orbital disorder is expected. The observed relationship between the determined structural parameters (c > a ≈ b/√2) is in agreement with the absence of orbital order in the orthorhombic phase of the sample. Rietveld refining of the neutron diffraction patterns using high-resolution data indicates that the refined oxygen contents correspond to a stoichiometric composition.

Figure 1.

(Color online) NPD patterns were recorded at 300 and 2 K for La0.7Sr0.3Mn0.85Nb0.15O3. The line and points refer to calculated and observed profiles and the bottom line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the structural phase whereas the bottom row denotes ferromagnetic reflections. The inset of the top panel shows the temperature dependence of reflection (110).

Table I.

The results of crystal and magnetic structures refinement of La0.7Sr0.85Nb0.15− x Mg x O3 samples

The additional intensity contribution to some structural peaks observed below 150 K for the x = 0 sample is associated with the ferromagnetic ordering [inset of Figure 1(a)]. The refined magnetic moment is 3.1 μ B/Mn at 10 K.

The determined structural parameters prove that the orthorhombic distortion of the crystal lattice found at low temperatures is not caused by a long-range orbital ordering. Unit cell parameters correspond to the O-type orthorhombic phase. Apparently the orthorhombic distortion is caused by steric effects similar to the case in optimally doped La1− x Ca x MnO3 (Huang et al., Reference Huang, Santoro, Lynn, Erwin, Borchers, Peng, Ghosh and Greene1998).

Therefore, the ferromagnetism of the studied sample cannot be caused by orbital ordering or double exchange and is not associated with charge carriers. According to the Goodenough-Kanamori rules the sign of the 180°-superexchange interaction between Mn(e g)–O–Mn(e g) cannot be determined for the Mn3+ ion if the orbital ordering is removed (Goodenough, Reference Goodenough1963; Zhou and Goodenough, Reference Zhou and Goodenough1999). So, the antiferromagnetic and ferromagnetic components of the interactions can be equal. However, this statement is correct only in the case of a purely ionic bond (Troyanchuk, Reference Troyanchuk, Bushinsky and Lobanovsky2013a). However the chemical bond includes a covalent component and hybridization occurs between the e g orbitals of manganese and the 2p orbitals of oxygen. This leads to a decrease of formal population of filled e g orbitals of Mn as e g electrons are partially located at the oxygen site and to an increase of the ferromagnetic component of the superexchange interactions. In the ionic model a similar effect results by partially replacing La3+ ions with two-valent alkaline earth ions, in this case Mn4+ ions appear. This substitution leads to the decrease of the antiferromagnetic contribution in the superexchange interactions as the e g orbitals of Mn4+ are empty.

On the other hand structural disorder can decrease the covalence because of local variations of the bond angle Mn–O–Mn. This angle controls the hybridization of 2p(O) and 3d(Mn) orbitals (Goodenough, Reference Goodenough1963). There is a critical value of the Mn–O–Mn angle associated with a change in sign of the superexchange interaction from positive to negative (Goodenough, Reference Goodenough1963; Akahoshi et al., Reference Akahoshi, Uchida, Tomioka, Arima, Matsui and Tokura2003). The decrease of the Mn–O–Mn angle leads to gradual collapse of the long-range ferromagnetic order in both mixed and single valent manganites (Troyanchuk, Reference Troyanchuk, Lobanovsky, Kasper, Hervieu, Maignan, Michel, Szymczak and Szymczak1998).

We will now try to discuss the interplay between magnetism and orbital ordering. The orbital order–disorder transition in the parent compounds LnMnO3 (Ln-lanthanide) has a martensitic character (Kasper and Troyanchuk, Reference Kasper and Troyanchuk1996; Colin et al., Reference Colin, Buurma, Zimmermann and Palstra2008). This means that there is a two phase regime under both doping and temperature variation. The orbitally ordered LaMnO3 is A-type antiferromagnetic whereas the orbitally disordered LaMnO3 has isotropic ferromagnetic interactions (Trokiner et al., Reference Trokiner, Verkhovskii, Gerashenko, Volkova, Anikeenok, Mikhalev, Eremin and Pinsard-Gaudart2013). The experimental data for lightly doped La1− x A x MnO3 (A = Ca, Sr; x < 0.15) provide no evidence for any homogeneous ferromagnetic state within a formally d z 2-orbital ordered phase (Allodi et al., Reference Allodi, De Renzi, Guidi, Licci and Pieper1997; Kumagai et al., Reference Kumagai, Iwai, Tomioka, Kuwahara, Tokura and Yakubovskii1999; Korolyov et al., Reference Korolyov, Arkhipov, Gaviko, Mukovskii, Arsenov, Lapina, Bader, Jiang and Nizhankovskii2000; Biotteau et al., Reference Biotteau, Hennion, Moussa, Rodrıguez-Carvajal, Pinsard and Revcolevschi2002; Choi et al., Reference Choi, Zhou, Kuhns, Reyes and Dalal2010) Moreover, the ferromagnetic ordering in the lightly doped manganites leads to a magnetostructural first-order transition into a less distorted low temperature phase and concomitant localization of the charge carriers (Korolyov et al., Reference Korolyov, Arkhipov, Gaviko, Mukovskii, Arsenov, Lapina, Bader, Jiang and Nizhankovskii2000; Biotteau et al., Reference Biotteau, Hennion, Moussa, Rodrıguez-Carvajal, Pinsard and Revcolevschi2002; Hennion and Moussa, Reference Hennion and Moussa2005). The magnetostructural transition is accompanied by a significant increase of the spontaneous magnetic moment. However, the low temperature phase is not homogeneous. It contains inclusions of pure A-type antiferromagnetic phases as it was proved by NMR (Choi et al., Reference Choi, Zhou, Kuhns, Reyes and Dalal2010) and NPD methods (Lia et al., Reference Lia, Su, Xiao, Persson, Meuffels and Brückel2009; Troyanchuk, Reference Troyanchuk, Bushinsky, Sikolenko, Emov, Ritter, Hansen and Többens2013b). Naturally one can suggest that these A-type antiferromagnetic inclusions arise from the local d z 2 orbital ordering inherent to parent LaMnO3. So the static d z 2 orbital ordering seems to be incompatible with a pure ferromagnetic ordering in spite of mixing of d z 2 and d x−y 2 2 orbitals.

The ferromagnetic state in single-valent manganites likely arises from the removal of the static Jahn-Teller distortion. It was suggested that the formally Jahn-Teller distorted LaMn0.5Ga0.5O3 (b/√2 < c < a) is a homogeneous ferromagnet. According to magnetic study, this compound is magnetically inhomogeneous. In comparison to LaMn0.5Ga0.5O3 the completely orbitally disordered LaMn0.4Ga0.6O3 is characterized by a larger Weiss constant thus indicating an enhancement of the ferromagnetic correlations (Blasco et al., Reference Blasco, Garcıa, Campo, Sanchez and Subıas2002). One can suggest that Ga3+doping leads to nanoscale structural separation into nanodomains with fast and slow orbital dynamic. The fast orbital dynamic corresponds to ferromagnetic ordering whereas the slow orbital dynamic favors antiferromanetism.

ACKNOWLEDGEMENT

This work was supported by Russian Foundation for Basic Research (grant 12-02-01252).

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Figure 0

Figure 1. (Color online) NPD patterns were recorded at 300 and 2 K for La0.7Sr0.3Mn0.85Nb0.15O3. The line and points refer to calculated and observed profiles and the bottom line represents their difference. The upper row of vertical ticks marks the Bragg reflections of the structural phase whereas the bottom row denotes ferromagnetic reflections. The inset of the top panel shows the temperature dependence of reflection (110).

Figure 1

Table I. The results of crystal and magnetic structures refinement of La0.7Sr0.85Nb0.15−xMgxO3 samples