Research Summary of Arup Ghosh

Research Highlights

My research works are mainly focused on the transition metal based alloys and oxides to study their multifunctional properties like Anomalous Nernst effect (ANE), Spin Seebeck effect (SSE), magnetocaloric effect (MCE), exchange bias (EB), magnetoresistance (MR), ac magnetic properties, etc. Most of my reported works are related to the compositional study in Ni-Mn based Heusler alloys to maximize their MCE which may advance the magnetic refrigeration technology to achieve an environmentally clean and energy saving cooling as compared to gas refrigeration. I have developed experimental setups for room temperature ac hysteresis loop measurement and temperature dependent ac susceptibility measurement. Presently, I have developed temperature dependent ANE/SSE measurement setup both in Indian Institute of Science Education and Research (IISER) Pune and National university of Singapore (NUS). I have been able to obtain large SSE and ANE signals in some intermetallics and oxide materials

Anomalous Nernst Effect (ANE)

During my Ph.D., I have studied the magnetic, magnetocaloric and magneto-transport properties of The Ni-Mn based Heusler alloys. These materials are very effective magnetocaloric material amongst the other magnetic refrigerants. They are cheap in cost and their structural transition can be tuned easily by varying the composition. We have doped Co by replacing Ni in Ni-rich Ni2MnSn off-stoichiometric alloys and successfully enhanced the magnetic entropy change (∆SM) across their first order magneto-structural transition (FOMST) under a small change in magnetic field (~ 15 kOe), but the net refrigerant capacity (RC) starts to fall as the doping concentration exceeds 1 at%. The reasons behind such results are small transition width and large field induced hysteresis loss (Ghosh et al, J. Alloys Compd. 579, 295 (2013); Ghosh et al, Eur. Phys. J. B 86, 378 (2013)).

ANE in NiCoMnSn Heusler alloy

Magnetocaloric Effect (MCE)

In order to enhance the RC, we have prepared a Ni-rich Ni2MnSn alloy and annealed it for different times. The ∆SM is found to increase as the structural ordering increases with increasing the duration of annealing, but net RC is the maximum for the material which is partially disordered. A favourable value of ∆SM and relatively larger transition width due to disorder lead to such result. So, certain degree of disorder is appreciated in these alloys for their better applicability (Ghosh et al, Appl. Phys. Lett. 104, 031905 (2014)).

Ni50Mn36.5Sn13.5 alloy Mn50.5-xNi41Sn8.5+x alloys Mn-Ni-Fe-Sn alloys

annealed for different times

Next, we have prepared some Mn-rich Mn2NiSn off-stoichiometric Heusler alloys and obtained expected increase in magnetic correlation, better magneto-structural coupling, smaller thermal hysteresis and a significantly large magnetocaloric effect (MCE) with almost 50% higher net RC as compared to the similar Ni-rich alloys. Moreover, we have doped Fe independently in the place of Ni and Mn in Mn-rich such alloys. Results conclude that smaller concentration of doping (1-2 at %) can be effective to enhance the MCE with almost double net RC comparative to undoped similar materials (Ghosh et al, J. Phys. D: Appl. Phys. 46, 435001 (2013); Ghosh et al, J. Appl. Phys. 117, 093909 (2015)).

MCE as calculated from high field M-T curves. M-H curves across the forrward and reverse FOMST.

Despite of all the above mentioned improvements, these materials still suffer from hysteresis losses. For further investigation, We have carried out a detail MCE study on a Mn-rich Mn-Ni-Sn-Si off-stoichiometric Heusler alloy by measuring the MCE across its magneto-structural transition in both the heating and cooling mode. Finally, we have concluded that the cooling protocol can provide a better and more accurate MCE values as the field induced and temperature induced effects oppose each other, which corresponds to a negligible hysteresis loss. Large magnetoresistance has been obtained in these alloys across their FOMST and increased with increasing the ferromagnetic correlations (Ghosh et al, J. Appl. Phys., 119, 183902 (2016)).

Temperature dependent MCE (isothermal measurements) and its fittings

I have developed a closed cycle refrigerator (CCR) based temperature dependent ac susceptibility measurement setup and an ambient temperature ac hysteresis setup during my Ph.D. to study the dynamic magnetic properties of magnetic materials in their bulk and nano-structures. Thus, Fe3O4 nano-hollow spheres and CoFe2O4 nanoparticles have been studied for their ac magnetic properties and potential in hyperthermia treatment (Sarkar et al, J. Magn. Magn. Mater., 393, 192-198 (2015); Dey et al, J. Magn. Magn. Mater., 427, 168-174 (2017)).

Setup for temperature dependent ac susceptibility measurement

Setup for room temperature ac hysteresis measurement

Zn vacancy induced ferromagnetism and photoluminescence have been also studied in alkali-metal substituted p-type ZnO thin films (Ghosh et al, Cryst. Eng. Comm., 15, 7748 (2013)).

Presently, I have developed a CCR based (10 K to 300 K) experimental setup for spin caloritronic measurements. Some of the literature reports have been reproduced to standardize the setup.