Magnetic short-range order and reentrant-spin-glass-like behavior in CoCr2O4 and MnCr2O4 by means of neutron scattering and magnetization measurements -: art. no. 214434

We reinvestigate the ferrimagnetic spiral ordering of the normal spinel ferrimagnets CoCr2O4 (T(C)similar or equal to93 K) and MnCr2O4 (T(C)similar or equal to51 K), in which magnetic Co2+ and Mn2+ ions occupy the A sites and magnetic Cr3+ ions occupy the B sites, by neutron scattering experiments and magnetization measurements on single crystal specimens. Neutron scattering experiments revealed that the fundamental reflections show coherent Bragg peaks at all temperatures below T-C, while the satellite reflections are diffusive even in the lowest temperature phase below T-F (similar or equal to13 K for CoCr2O4 and similar or equal to14 K for MnCr2O4). These facts indicate the simultaneous formation of a long-range order of the ferrimagnetic component and of a short-range order of the spiral component in the lowest temperature phase. The correlation length of the ferrimagnetic long-range order is estimated to be larger than 50 nm below T-C, while that of the spiral short-range order is estimated to be 3.1 nm at 8 K for CoCr2O4 and 9.9 nm at 4 K for MnCr2O4. In magnetization measurements, a reentrant-spin-glass-like behavior of the ferrimagnetic domains was found in the two chromites. In order to explain these magnetic properties comprehensively, we propose the concept of weak magnetic geometrical frustration; magnetic geometrical frustration among the B sites forming the pyrochlore lattice survives even if magnetic ions occupy the other sublattices (A sites). The weak magnetic geometrical frustration leads to the spiral short-range order. Since the magnitude of magnetic moments of Mn2+ ions at the A sites (5mu(B) spin-only value) is larger than that of Co2+ ions at the A sites (3mu(B) spin-only value), the degree of magnetic geometrical frustration among the B sites in MnCr2O4 is weaker than that in CoCr2O4; therefore, the correlation length of the spiral component in MnCr2O4 (9.9 nm) is larger than that in CoCr2O4 (3.1 nm). The reentrant-spin-glass-like behavior of the ferrimagnetic domains is caused by freezing and fluctuation of the spiral component.