Research Papers
Magnetic order and magnetic properties of the oxygen deficient SmBaMn2O5 layered perovskite

https://doi.org/10.1016/j.materresbull.2022.111780Get rights and content

Highlights

  • SmBaMn2O5 exhibits a checker board pattern of compressed and expanded MnO5 pyramids due to the condensation of a breathing mode.

  • The magnetic structure of SmBaMn2O5 has been determined thanks to the use of hot neutrons.

  • The magnetic structure is ferrimagnetic with the Mn moments align antiparallel along the c axis.

  • The polarization of Sm moments leads to a change in the direction of the easy magnetization axis below 10 K.

Abstract

Magnetism in SmBaMn2O5 was investigated on a single crystal by magnetic and neutron diffraction measurements. This is an oxygen deficient perovskite with a layered ordering of Sm and Ba cations. Mn atoms are coordinated with five oxygens forming a square pyramid and they are ordered in a checkerboard pattern of expanded-compressed pyramids in the ab-plane. The neutron diffraction study revealed a ferrimagnetic ordering of Mn moments below TN=134 K. Macroscopic measurements reveal a very anisotropic behavior. Measurements with the external magnetic field parallel (M||c) and perpendicular (M⊥c) to the c-axis confirm that this is the easy axis above 10 K. Below this temperature, the Sm sublattice begins to polarize and the magnetization M||c decreases while M⊥c experiences a huge increase. This indicates that Sm moments begin to order around 10 K in the ab-plane with a minor component on the c-axis that opposes the overall magnetization from Mn sublattices.

Introduction

Manganese oxides with a perovskite structure have been attracting considerable interest because of their magnetic and electrical properties [1,2]. The close interplay among charge, orbital and spin degrees of freedom give rise to rich phase diagrams including different types of charge, orbital and spin ordered phases [3,4]. In this way, by varying the chemical composition of the sample, materials with either giant magnetoresistance or metal-insulating transitions can be obtained [3], [4], [5], [6]. Later on, the discovery of layered LnBaMn2O6-δ perovskites with Ln-Ba order along the c axis added a new ingredient to their complex set of interactions [7,8]. Compared to disordered or simple manganites, the layered Ln-Ba ordering strengthens ferromagnetic interactions in LnBaMn2O6 compounds with light Ln [9,10] while reinforces the charge and orbital orderings in samples with heavy Ln or Y [11], [12], [13], [14], [15]. In the latter compounds, the combination of layered order and the cooperative tilts of MnO6 octahedra to relieve the enhanced structural strain (small size of Ln3+ versus Ba2+) gives rise to multiferroic materials that present improper ferroelectricity [14], [15], [16].

LnBaMn2O6 compounds are synthesized by topotactic oxidation of LnBaMn2O6-δ (0.5≤δ≤1) ones that are prepared by solid state reaction in reducing conditions [9,17]. The undistorted layered perovskite phase LnBaMn2O6 adopts a tetragonal P4/mmm structure [18] with lattice parameters ap × ap × 2ap (ap being the cell parameter of the simple pseudocubic perovskite). It is composed of an alternating sequence of BaO12 and LnO12 cube-octahedra layers stacked along the c-axis. Mn atoms have octahedral coordination occupying a single non-equivalent site in the unit cell. In the reduced phase, LnBaMn2O5, oxygen vacancies are concentrated in the apical position of the MnO6 octahedra but the symmetry of the aristotype phase is preserved. Accordingly, the Mn atoms have now a square-pyramidal coordination MnO5. The Ba atom preserves the same coordination whereas the Ln atom, located on the same plane as the oxygen vacancies, is now 8-fold coordinated. Both phases contain Mn in a mixed valence state: Mn (III) / Mn (IV) in the oxidized phase and Mn (II) / Mn (III) in the reduced one. Therefore, electron localization can lead to charge ordering (CO) transitions giving rise to multiple non-equivalent sites for Mn atoms. CO transitions in LnBaMn2O6 phases have been the subject of many studies [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and recently, the crystal structures of the different successive CO phases found in SmBaMn2O6 have been determined [19]. In the case of LnBaMn2O5 manganites, a CO phase has been reported in La- and Y-based compounds [9,20,21]. It is characterized by the appearance of (h/2, k/2, l)T superstructure peaks (T stands for undistorted tetragonal cell) leading to a new tetragonal cell with lattice parameters √2ap × √2ap × 2ap and P4/nmm symmetry [9,21].

The topotactic oxidation/reduction processes between LnBaMn2O6 and LnBaMn2O5 phases confer a remarkable capability to intake and release oxygen to this system [22], [23], [24], [25]. This property has attracted attention because of their potential ability for a precise control of redox reactions [26] and recent studies report that SmBaMn2O5+δ is a promising electrode material for symmetrical solid oxide fuel cells [27]. Therefore, a thorough study on the structural and physical properties of these materials is of great importance. These studies have been extensively carried out on the oxidized compounds LnBaMn2O6 but are very scarce in the reduced samples LnBaMn2O5. A study on LaBaMn2O6-δ samples reported that LaBaMn2O5 adopts a P4/nmm structure and undergoes a magnetic transition at 130 K. It develops a ferrimagnetic order of Mn(II) and Mn(III) moments although the experimental saturation moment was somewhat less than the theoretical one [9,21]. This type of anisotropic ferrimagnetic materials might be suitable in the construction of microwave devices like resonant isolators or circulators [28] although its low TN (well below room temperature) suggests that fuel cell applications are more promising. The magnetic structure of G-type agreed with theoretical calculations [29] and similar results compatible with that ordering were found for YBaMn2O5 [21]. No information is yet reported about the magnetic properties of SmBaMn2O5 in spite of large amount of studies devoted to the oxidized SmBaMn2O6 compound [12,13,19]. One reason may be the difficulty of studying Sm compounds using neutron techniques due to the high neutron absorption cross section of natural Sm [30]. There are two ways to avoid this problem. The first possibility is to prepare compounds enriched in 154Sm isotope. However, the cost and availability of the oxides with this isotope prevent single-crystal growth. Nevertheless, the neutron absorption strongly depends on the neutron energy [31] and high-energy neutrons (so-called hot neutrons) offer a second alternative that has successfully been used to solve magnetic structures in different Sm-based compounds [32], [33], [34], [35], [36].

We here report on the structural and magnetic properties of SmBaMn2O5 using a single crystal grown by floating zone and aligned along the c-axis. This has allowed us to measure the magnetic properties in the directions parallel and perpendicular to this axis, determining its great magnetic anisotropy. Moreover, the use of hot neutrons with a significant smaller absorption cross section for natural Sm permitted us to study the magnetic structure of this crystal at 12 K. Our study reveals that there are two non-equivalent Mn sites in SmBaMn2O5. This is produced by condensation of a breathing mode of the oxygen sublattice that gives rise to a checkerboard pattern of compressed and expanded MnO5 pyramids in the ab-plane. The charge segregation between the two sites is close to the expected for a couple of Mn2+ and Mn3+ cations. The magnetic structure of Mn moments is ferrimagnetic with an antiparallel coupling of Mn(II) and Mn(III) moments along the c-axis which is the easy magnetization axis. The magnetic space group has been identified as P4/nm'm'. At very low temperature, the polarization onset of Sm3+ moments lead to a change in the direction of the easy magnetization axis that is now perpendicular to c-axis.

Section snippets

Experimental

Single crystals of SmBaMn2O5 were grown using the floating zone method from polycrystalline precursors. Stoichiometric amounts of dried Sm2O3, BaCO3 and Mn2O3 were mixed, ground and heated at 1000 °C overnight. The resulting powder was reground, pressed into pellets and sintered at 1250 °C in a gas flow of H2/Ar mixture (2 % of H2) saturated in water vapor to achieve a reductive atmosphere (PO2 ≈10−11). This is required to prevent the formation of BaMnO3 impurity [9]. Thus, the pellets are

Results and discussion

Thermogravimetric analysis (TGA) reveals that the growth conditions of our crystal give rise to a sample, SmBaMn2O5+δ, with a small oxygen excess compared to the ideal formula SmBaMn2O5. Fig 1(a) displays the TGA curve unveiling that oxidation of SmBaMn2O5+δ begins at ≈500 K and finish at ≈600 K. The entire process occurs in a single step and the final product was identified as SmBaMn2O6 [19]. Assuming this phase to be stoichiometric, the calculated value of δ is 0.11(1). This small excess of

Conclusions

The structural and magnetic properties of SmBaMn2O5+δ have been studied on a single crystal specimen with a small oxygen excess (δ≈0.1). This compound adopts a tetragonal structure with P4/nmm symmetry and a checkerboard CO of Mn(II)O5 and Mn(III)O5 pyramids at room temperature. A symmetry analysis reveals that the main distortion associated with the stabilization of the CO is a breathing mode of the basal oxygens. The sample undergoes a ferrimagnetic transition on cooling developed in two

Author statement

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare no competing financial interest.

Aknowledgments

Authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación from Universidad de Zaragoza. Granted beam time at ILL, is appreciated (Experiment No. 5–41–1121). For financial support, we thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Projects No. RTI2018–098537-B-C22 cofunded by ERDF from EU), Severo Ochoa” Programme for Centres of Excellence in R&D [SEV- 2015-0496 and FUNFUTURE (CEX2019-000917-S)] and Diputación General de Aragón (Project

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