Zero-field-cooled exchange bias effect in phase-segregated La2−xAxCoMnO6−δ (A=Ba,Ca,Sr

Abstract

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La1.5Sr0.5CoMnO6 stands out as presenting the largest ZEB reported so far, while for La1.5Ca0.5CoMnO6 the exchange bias field (HEB) is one order of magnitude smaller. Here we show that La1.5Ba0.5CoMnO6 also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate HEB value in respect to Ca-and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La2CoMnO6 was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca-and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co 2+ /Co 3+ and Mn 4+ /Mn 3+ are already present at the La2CoMnO6 parent compound, and that Ba 2+ /Ca 2+ /Sr 2+ partial substitution at La 3+ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the L-edge XAS indicate the presence of oxygen vacancies in all samples (0.05≤ δ ≤0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn. arXiv:1909.05287v1 [cond-mat.mtrl-sci]

Zero-field cooling exchange bias induced by spin metastable state in MnNiGa system

“…随 后, 关于ZEB的研究, 涌现出很多出色的工作. 研究者 相继在Ni 2 Mn 1.4 Ga 0.6 [14] , Ni 50 Mn 36 Co 4 Sn 10 [15] , Ni 50 Mn 50−x -Sn x [16] , Ni 50 Mn 38 Ga 12−x Sb x [17] , Mn 50 Ni 40 Sn 10−x Si x [18] , Mn 50 -Ni 50−x Al x [19] , Ni 50 Mn 34 In 13 Fe 3 [20] 等Heusler合金体系, PdNCr 3 [21] , Mn 3 Co 1−x Mn x N [22,23] 等反钙钛矿氮化物体系, 以及La 1.5 Sr 0.5 CoMnO 6 [24] , Pr 1-x Ca x MnO 3 [25] , YMnO 3 [26] , Y 0.9 Pr 0.1 CrO 3 [27] , Co 0.8 Cu 0.2 Cr 2 O 4 [28] , La 2−x A x CoMnO 6−δ (A = Ba, Ca, Sr; x = 0, 0.5) [29] 等钙钛矿、尖晶石氧化 物体系中观测到ZEB. 最近, Zhang等人 [30] 在La 0.7 Sr 0.3 - [31] .…”

Absence of zero-field-cooled exchange bias effect in single crystalline La2−xAx CoMnO6 Macchiutti1, Jesus2, Carneiro3 et al. 2021Phys. Rev. MaterialsSelf Cite

“…There is great interest in this family of compounds due to the possible applications in spintronic devices. Particularly interesting is the zero-field-cooled (ZFC) exchange bias (ZEB) effect recently observed for hole-doped members of this family of compounds [2][3][4][5][6][7][8][9]. The conventional exchange bias (CEB) effect is characterized as a shift in hysteresis loop measurements, observed after cooling the system in the presence of an external magnetic field [10].…”

“…The conventional exchange bias (CEB) effect is characterized as a shift in hysteresis loop measurements, observed after cooling the system in the presence of an external magnetic field [10]. In materials in which the ZEB effect is seen, the shift in magnetization as a function of the applied field is detected spontaneously in zero-field cooling [2][3][4][5][6][7][8][9]. In these double perovskites, the ZEB effect may result from a reentrant spin-glass-like phase in which magnetic relaxation of unusual glassy moments affects the magnetization loop shifts [11,12].…”

“…The Curie temperature (T C ) for ordered samples can reach up to 226 K [18] with a saturation spin moment M sat = 6 μB/f.u., whereas the disordered ones, e.g., with oxygen deficiency and/or antisite defects, present lower T C and M sat [2,18,19]. If changes in the B-cation valence also occur, a Co 3+ -O-Mn 3+ ferromagnetic interaction may happen, and a distinct ferromagnetic transition is present [5,14,17,18]. Also, the disorder can generate antiferromagnetic clusters, giving rise to an antiferromagnetic transition temperature T N because of exchange Co 3+ -O-Mn 4+ coupling and/or antiphase boundaries of Co 2+ -Co 2+ or Mn 4+ -Mn 4+ pairs [14,20].…”

“…Also, the disorder can generate antiferromagnetic clusters, giving rise to an antiferromagnetic transition temperature T N because of exchange Co 3+ -O-Mn 4+ coupling and/or antiphase boundaries of Co 2+ -Co 2+ or Mn 4+ -Mn 4+ pairs [14,20]. Regarding its crystal structure, LCMO is mostly described as a monoclinic P2 1 /n space group for an ordered population of the B sites and orthorhombic Pnma when the disorder is present [5,18].…”