Anisotropy driven spin–reorientation, and two–step magnetic ordering in cubic semiconducting spinel Cr0.1Mn0.9Fe0.2Co1.8O₄

The nature of magnetism in the cubic spinel Cr0.1%Mn0.9Fe0.2Co1.8O₄ is reported based onsystematic investigations by means of magnetization (M), ac susceptibility (χ) and heat capacity (CP) measurements, as well as by neutron diffraction. Structural characterization of the samplewas done using x-ray absorption spectroscopy, neutron and synchrotron diffraction. The M vs. Tvariation in different magnetic fields indicate ferrimagnetic ordering below TC = 230 K,followed by a magnetic field and anisotropy induced spin reorientation at TSR ∼150 K. Withincreasing T starting from 2 K, the coercivity HC and anisotropy field HK decrease and becomenegligible for T > TSR. A model to explain the HC vs. T data shows that TSR is due toreorientation of M along H when H > HK. The CP vs. T data shows a weak λ -type anomaly atTC with changes in magnetic entropy smaller than those observed below TSR suggesting thatlong-range magnetic ordering is completed below TSR. For TSR < T < TC, the presence ofweakly interacting magnetic clusters having weak short-range interactions is evident based onanalysis of magnetization and ac susceptibilities. Exchange constants (JAA/kB = 7.9 K, JAB/kB =22.6 K and JBB/kB = –5.3 K) are determined from the temperature dependence of paramagneticsusceptibility for T > TC. This analysis also shows the low-spin S = 0 state of Co³⁺ ions on the B sites which along with negligible HK for TSR < T < TC produces weakly interacting magneticclusters in this magnetic semiconductor with bandgap ∼0.57 eV.