P. Anil Kumar

Ferrimagnetism, antiferromagnetism, and magnetic frustration in La2-xSrxCuRuO6 (0 <= x <= 1)

We studied structural and magnetic properties of a series of insulating double perovskite compounds, La2-xSrxCuRuO6 (0 0). Heat capacity measurements indicate the absence of any long-range magnetic order in any of these compounds. The magnetic relaxation and memory effects observed in all compounds suggest glassy dynamical properties associated with magnetic disorder and frustration. We show that the observed magnetic properties are dominated by the competition between the nearest-neighbor Ru-O-Cu 180 degrees superexchange interaction and the next-nearest-neighbor Ru-O-O-Ru 90 degrees superexchange interaction as well as by the formation of antisite defects with interchanged Cu and Ru positions. Our calculated exchange interaction parameters from first principles calculations for x = 0 and x = 1 support this interpretation.

Superspin glass state and exchange bias in amorphous Fe/Fe-O core/shell nanoparticles

Nanoparticles of iron and iron oxide are widely explored in several biomedical and technological applications. We report on the magnetic properties of amorphous Fe/Fe–O core/shell nanoparticles compared to those of a reference system with crystalline Fe–O nanoparticles. These nanoparticles are prepared by thermal decomposition of iron precursor, where the amorphous and crystalline nature of core and shell is determined by the choice and concentration of the ligand. The crystalline system exhibits a blocking temperature higher than 300 K and negligible exchange bias effect. In contrast, the amorphous systems display large exchange bias, and collective magnetic behavior at low temperatures, with features of magnetic frustration and disorder reminiscent of those observed in spin glass and superspin glass systems. We discuss the origin of the dynamical magnetic behavior of the amorphous particles and study the dependence of the exchange bias field on the cooling field.

Magnetoresistance and electroresistance effects in Fe3O4 nanoparticle system

Nearly monodisperse spherical magnetite (Fe3O4) nanoparticles are prepared by colloidal chemistry route. Magnetic and electronic transport properties of the annealed pellets of these nanoparticles are reported. Effect of external magnetic and electric fields on the magnetic and transport properties of the material are studied as a function of temperature. We find that the highest resistance state of the ferromagnetic system occurs at a magnetic field which is approximately equal to its magnetic coercivity; this establishes the magnetoresistance (MR) in this system to be of the conventional tunnelling type MR as against the spin-valve type MR found more recently in some ferromagnetic oxide systems. The material also shows electroresistance (ER) property with its low-temperature resistance being strongly dependent on the excitation current that is used for the measurement. This ER effect is concluded to be intrinsic to the material and is attributed to the electric field-induced melting of the charge-order state in magnetite.