Thermomagnetisation studies in the temperature range of 800K to 3000K on three representative samples of dry milled zinc ferrites by vibration sample magnetometer in zero field cooled conditions at 50 Oe field. The samples selected for this study are unmilled sample, 30 minutes milled sample and 10 hours milled sample. The results are shown in figure. Figure shows the 10 hours milled sample shows a broad transition within our temperature range of experiments while a transition in other two samples are expected at lower temperatures.
Since we have observed a spin glass like transition in 10 hours milled sample, we have carried out FC-ZFC thermomagnetisation experiments in this sample. The results are shown in figure. The curves show a spin glass like behaviour suggesting the existence of magnetic spin clusters at the surface and establishes a short range ferrimagnetic ordering. The deviation from ideal spin glass property may be due to the size distribution of spin clusters. The broad transition peak of ZFC curve lies at around 1700 K. Below this spin glass transition temperature electron spins in a random direction and the clusters occupy different metastable states. Also below the transition temperature the intra sublattice exchange interactions that are antiferromagnetic plays a major role to decrease the magnetisation. These B-B and A-A antiferromagnetic interactions compete with the ferrimagnetic interactions and freeze the clusters below the transition temperature. The magnetic clusters at the surface are as a result of broken O2- superexchange bonds, which increase with milling. Thus the oxygen deficiency will increase the transition temperature.
In the field cooled magnetisation curve, a continuous increase of magnetization is observed on decreasing the temperature and finally almost stabilises at around 800K. This is due to the strong A-A and B-B antiferromagnetic interactions, which is reduced by the orientation of the clusters along the field direction.
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