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 Cr0.1Mn0.9Fe0.2Co1.8O4 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 theB sites which along with negligible HK for TSR < T < TC produces weakly interacting magneticclusters in this magnetic semiconductor with bandgap ∼0.57 eV.