We reinvestigate the ferrimagnetic spiral ordering of the normal spinel ferrimagnets CoCr2O4 (TC≃93 K) and MnCr2O4 (TC≃51 K), in which magnetic Co2+ and Mn2+ ions occupy the A sites and magnetic Cr 3+ 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 7C, while the satellite reflections are diffusive even in the lowest temperature phase below TF (≃13 K for CoCr2O4 and ≃14 K for MnCr2O 4). 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 MnCr 2O4. 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 (5μB spin-only value) is larger than that of Co2+ ions at the A sites (3μB 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.
|Number of pages||12|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 2004 Dec 1|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics