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  Pramanik, P.; Joshi, D.C.; Reehuis, M.; Hoser, A.; Hoffmann, J.U.; Manna, R.S.; Sarkar, T.; Thota, S.: Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti_1-xMn_xCo₂O₄ spinel. Journal of Physics: Condensed Matter 32 (2020), p. 245803/1-17

10.1088/1361-648x/ab71a6
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Abstract:
A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1−xMnxCo2O4 in the temperature interval 1.6 K < T < 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1−xMnxCo2O4 system for all the Jahn–Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d1, 3d4 and 3d6, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet–Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length dc(TB-O) is longer than the equatorial bond length dab(TB-O) due to the splitting of the eg level of Mn3+ ions into dx2−y2 and dz2. For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at TCOMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below TCOMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.