Research Projects

In recent times, there is a quest for materials with large magnetocaloric effect that can be for magnetic refrigeration. The present proposal aims at design of a suitable magnetocaloric material in which a change in temperature, ΔT can be produced by an external magnetic field, H through the magnetic interactions. It includes to (i) observe a huge change in magnetization, ΔM that produces a ΔT for a finite but small adiabatic change in H, (ii) observe the magnetic transitions like spin-reorientation (SR) phenomena that involves high ΔM, (iii) bring the spin-reorientation/spin switching temperature, T_SR and the specific heat anomaly near to ambient conditions, and (iv) compare the observations in nano regime with the corresponding bulk counterpart in antiferromagnetic perovskite oxide through specific heat, C_P measurements. Rare-earth orthochromites, RCrO3 (R= rare-earth, Y, and La) (and isostructural orthoferrites, RFeO3) belong to a class of perovskite oxides of the form ABO3 in which SR phenomenon has been observed with a high ΔM. They crystallize into orthorhombic structure with space group Pbnm, D_4h^19 symmetry. The R3+-ion (4f1-14) is in a nine-coordinated A-site and the Cr3+-ion (4s03d3) occupy the octahedral B-site. The Cr3+ spins in paramagnetic RCrO3 compounds order antiferromagnetically with a canting of spins at T_N^Cr, below the room temperature. This is due to antisymmetric Dzyaloshinskii-Moriya (DM) interaction that gives rise to a weakferromagnetic (WFM) moment. The R3+ ions order antiferromagnetically at T_N^R, at low temperatures. The WFM moment increases with decrease in temperature that makes RCrO3 to exhibit intriguing phenomena like magnetization reversal, negative magnetization, and spin-reorientation (SR) in the presence of magnetic anisotropy. The novel aspect is that the anisotropy may be brought by a magnetic R and/or suitable doping/substitution at A and/or B-sites, the later helps in tuning the T_SR near to RT, without altering the structure of RCrO3. And this involve a high ΔM that reflects as an anomaly specific heat measurement. Though many groups have been focused on the magnetic properties and ferroelectric polarization of RCrO3, little attention has been paid to the magneto-caloric effect and heat capacity measurements around the spin-reorientation temperature, T_SR in RCrO3 and isostructural RFeO3. Moreover, the magnetization and changes in specific heat of RCrO3 nanoparticles is expected to vary with size and that has a direct bearing on the magneto-caloric effect. The present proposal aims at a thorough investigation of specific heat anomalies with respect to the 3d-4f interaction in RCrO3.

Antiferromagnetism, Heat Capacity, Spin-reorientation, Perovskite, MagnetoCaloric effect