Investigation of the magnetic and magnetodielectric properties of few complex spinels

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The present Ph.D. thesis focus on the nature of ferromagnetic behavior of few complex inverse-spinels such as Co2SnO4 (CSO), Co2TiO4 (CTO) and the magnetodielectric response of tetragonal spinel Mn3O4. The electronic structure evaluated form the X-ray photo electron spectroscopy revels the following configuration [Co2+][Co2+Sn4+]O4 and [Co2+][Co3+Ti3+]O4 for CSO and CTO, respectively. The temperature dependence of dc- and ac-magnetic susceptibilities χ'(T) and χʺ(T) reveals that both Co2SnO4 and Co2TiO4 ferrimagnetic ordering due to the unequal magnetic moments of Co2+ on the 'A' sites and 'B' sites of spinel lattice with ferrimagnetic Néel temperature TFN ~ 41 K and 47.8 K for CSO and CTO, respectively. Below the TFN, CSO exhibits spin-glass (SG) phase for T < TSG =39.1 K in which only the transverse spin components are frozen below TSG. This co-existence of longitudinal ferrimagnetic order below TFN = 41 K and transverse SG state below TSG =39.1 K is suggested to result from the presence of non-magnetic Sn4+ ions on the 'B' sites. It is interesting to note that CTO exhibits a dramatic magnetic compensation effect across 32K due to the complete balance of the magnetization of both the sublattices, which is an unusual scenario for the CSO system. Due to a very high magnetocrystalline-anisotropy both the systems CSO and CTO exhibits giant sign reversible exchange bias effect, cervicitis, and pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5K≤T≤32 K. In the case of tetragonally distorted Mn3O4 a sequence of transitions at TN ~ 42.75 K, T1 ~ 39 K, and T2 ~ 34 K are observed together with a new anomaly at 38 K (T*) which was successfully probed by χac(T) and εr(T) measurements. A clear hysteresis of about 5.15 K was observed in the temperature dependence of dielectric εr(T) for bulk Mn3O4 which is consistent with the previously reported first-order transition across this temperature. Whereas, the magneto-dielectric Δεr/εr(T,H) studies in zero-field εr(T) across the incommensurate-to-commensurate transition T2 ~ 34 K which provides dominant role of magneto-crystalline anisotropy in Hausmannite Mn3O4.
Supervisor: Subhash Thota