IIT Jodhpur
Webinar Series "Deep Diving into Physics" by Prof Subodh Kumar De titled "Exchange Anisotropy in Heterogeneous Magnetic System" on 26 January 2021

The Department started a series of webinars with the theme Deep Diving into Physics. The first speaker Prof Subodh Kumar De from Department of Material Sciences, Indian Association for the Cultivation Science, Jadavpur, Kolkata introduces the students to the world of Magnetism.


Exchange anisotropy arises due to competing magnetic interactions across the interface of heterogeneous magnetic phases. Such an anisotropy leads to a shift of hysteresis loop with respect to origin which is commonly known as exchange bias effect. A number of magnetic heterostructure is designed by dispersion of nanosized ferromagnetic BiFeO3 (BFO) nanoparticles into antiferromagnetic (AFM) NiO and CuO matrix. [1-4]
All the zero field cooled (ZFC) and field cooled (FC) magnetization curves for BFO-NiO and BFO-CuO show an irreversible behavior at a temperature which is much less than Neel temperature (TN) of bulk NiO and CuO and Curie temperature (TC) of BFO nanocrystals. Quite different behavior in ZFC and FC magnetizations in comparison with pure NiO, CuO and BFO suggests that the magnetization originates from interfacial exchange interactions. The temperature dependence of ZFC and FC has been modeled as a ferromagnetic core with disordered spins constituting the shell. The radius for the effective ferromagnetic ordered region is less than the actual particle size determined using TEM analysis. This certainly establishes that a large fraction of ferromagnetic spins are involved in the interfacial magnetism. The BFO-CuO interface produces a maximum exchange bias field (HEB) of 1840 Oe, while for BFO-NiO it is 1550 Oe at the same temperature and applied cooling magnetic field. The larger HEB for BFO-CuO indicates that the interfacial magnetic coupling is stronger than that for BFO-NiO. The higher spin freezing temperature of the BFO-CuO interface as compared with BFO-NiO suggests that the spins are highly disordered across the BFO-CuO interface. Comprehensive studies indicate that the exchange coupling at the BFO/AFM interface is mainly controlled by the crystalline and spin structures of the AFM component.
Strong exchange coupling between the uncompensated surface spins of AFM and the disordered spins of BFO nanocrystals, as well as the discontinuity in crystal structure at the interface, result in interesting magnetic properties and a significant exchange bias field in the BFO-AFM nanocomposite due to exchange anisotropy. The large exchange bias effect and multiferroicity in BFO make the nanocomposite more attractive for development of multifunctional devices.
1. Journal of Physics D: Applied Physics 47, (2014) 325002
2. Journal of Applied Physics 115, (2014) 013906
3. Nanotechnology 24 (2013) 505711
4. Applied Physics Letters 101 (2012) 042401