Spatial inhomogeneity of magnetic moments in the cobalt oxide spinelCo3O4

Abstract

The antiferromagnetic ͑AF͒ nature of the normal spinel Co 3 O 4 with Néel temperature ͑T N ͒ =30 K was investigated by means of positive muon spin rotation and relaxation ͑ + SR͒ techniques using a polycrystalline sample. Clear muon spin precession signals due to a quasistatic long-range AF order were found in the zero-field + SR spectra below T N. The spectra consist of two oscillating signals with frequencies at T → 0 K of 80 and 60 MHz, respectively, indicating an incommensurate ͑IC͒ AF order in Co 3 O 4. A possible reason for the appearance of the IC-AF order in Co 3 O 4 would be local structural transitions due to a charge and/or a spin state change of Co ions.

Evolution of magnetic states in frustrated diamond lattice antiferromagneticCo(Al1−xCox)<

“…The best fits presented in Fig. 4 Our findings give a natural explanation of the SR results 8 on Co 3 O 4 as due to spin-liquid physics rather than incommensurate magnetic order. Also, based on the position of CoAl 2 O 4 in the J 2 / J 1 phase diagram, we suggest that even in an ideal sample, with no inversion or other perturbation, the ground state would be a collinear AFM, though one might need very low temperatures to reach it.…”

“…Existing experiments on Co-Al oxide spinels do not provide a clear picture. An early neutron-diffraction study 7 on Co 3 O 4 showed that the magnetic moments of the Co 2+ ions located at the tetrahedral sites form a simple collinear AFM below the Neel temperature T N = 40 K. This picture has been questioned by a recent muon spin rotation and relaxation ͑SR͒ study 8 13 attempted to answer these questions based on bulk macroscopic measurements of Co͑Al 1−x Co x ͒ 2 O 4 system. They proposed that the spin-liquid state is realized in CoAl 2 O 4 ͑x =0͒, while with increasing Co substitution x the second-neighbor coupling J 2 decreases and collinear AFM long-range order develops.…”

The effects of size and orientation on magnetic properties and exchange bias in Co3O4 mesoporous nanowires

“…19,20 Therefore, for our samples, especially W350, it is most likely that Co 3þ cations on the exposed surface planes in the O position of Co 3 O 4 nanostructures are reduced to Co 2þ cations by surface oxygen deficiency. The subsequent x-ray photoelectron spectroscopy (XPS) results (not shown here) further confirmed this hypothesis, and in addition, several groups of researchers 21,22 have found room temperature ferromagnetism in Co 3 O 4 and identified the original microscopic mechanism as due to Co 2þ ions replacing Co 3þ in the O positions by different methods. The XPS indicates that (1) the W350 sample is high purity, which can rule out impurity and metallic Co magnetic particle effects; and (2) W350 presents a higher amount of Co 2þ , which is caused by the Co 3þ -oxygen vacancy complex, since the satellite peak of Co 2p3/2 in W350 is higher than for the other samples.…”

Spin frustration and magnetic exchange in cobalt aluminum oxide spinels

“…The next-nearest neighbor interactions are particularly important in Co-Al-O spinels; therefore they also might be responsible for the complex arrangement of the A-site spins. After original submission of our paper, Ikedo et al 42 reported on the µSR studies of Co 3 O 4 which were interpreted in terms of an incommensurate spin configuration. By analogy with the strongly frustrated MnSc 2 S 4 spinel 20 they attributed the incommensurate spin order in Co 3 O 4 to a structural transition due to a charge-and/or spin-state change of the Co ions.…”