C. Bansal | Searcharxiv

arXiv2013

Magnetic Phase Transition and Relaxation Effects in LiFePO4

We report the observation of para - antiferromagnetic transition at ~ 50 K in lithium iron phosphate, LiFePO4 through DC magnetization and Mössbauer spectroscopy. The Ferrous ion Fe2+ (3d6, 5D) in LiFePO4 exhibits relaxation effects with a relaxation frequency ~1.076 \times 10(rise to 7) s-1 at 300 K. The temperature dependence of the frequency suggests the origin of the relaxation is spin-lattice type. The quadrupole splitting at low temperatures indicates the excited orbital states mix strongly to the orbital doublet ground state via spin-orbit coupling. Modified molecular field model analysis yields a saturation value for hyperfine field ~125 kOe. The anomaly in magnetization and Mössbauer parameters below 27 K may be ascribed to contribution of orbital angular momentum. The high value of the asymmetry parameter (η ~ 0.8) of the electric field gradient obtained in the antiferromagnetic regime indicates a strongly distorted octahedral oxygen neighbourhood for the ferrous sites.

cond-mat.mtrl-sci↗

arXiv2002

Magnetic enhancement of Co$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$ spinel oxide by mechanical milling

We report the magnetic properties of mechanically milled Co$_{0.2}$Zn$_{0.8}$Fe$_2$O$_4$ spinel oxide. After 24 hours milling of the bulk sample, the XRD spectra show nanostructure with average particle size $\approx$ 20 nm. The as milled sample shows an enhancement in magnetization and ordering temperature compared to the bulk sample. If the as milled sample is annealed at different temperatures for the same duration, recrystallization process occurs and approaches to the bulk structure on increasing the annealing temperatures. The magnetization of the annealed samples first increases and then decreases. At higher annealing temperature ($\sim$ 1000$^{0}$C) the system shows two coexisting magnetic phases {\it i.e.}, spin glass state and ferrimagnetic state, similar to the as prepared bulk sample. The room temperature Mössbauer spectra of the as milled sample, annealed at 300$^{0}$C for different durations (upto 575 hours), suggest that the observed change in magnetic behaviour is strongly related with cations redistribution between tetrahedral (A) and octahedral (O) sites in the spinel structure. Apart from the cation redistribution, we suggest that the enhancement of magnetization and ordering temperature is related with the reduction of B site spin canting and increase of strain induced anisotropic energy during mechanical milling.

cond-mat.soft↗