Magnetic behavior of Ni substituted LiCoO2, magnetization and electron paramagnetic resonance studies

Single phase Ni substituted LiCo1-xNixO2 solid solutions with x < 0.15 have been synthesized to study the effect of substitution on the magnetic behavior. Two different techniques, magnetic susceptibility and electron paramagnetic resonance, EPR which provide information in two different time windows have been used. The solid solutions have been found to be single phase with large grains conforming to R-3m rhombohedral structure. The lattice parameters c and a increase with increasing Ni substitution but with a nearly constant c/a ratio of about 4.99 in all the cases indicating that the CoO6 and LiO6 octahedra do not undergo any Jahn-Teller distortions. The room temperature EPR absorption spectra clearly shows a peak in all the compounds at a field of 314 mT corresponding to a g-factor of 2.14. The peak width however is found to be a strong function of Ni substitution; increasing with increasing Ni from 3.6 mT to 6.5 mT for x = 0.08. The magnetization increases with decreasing temperature in all the compounds, a paramagnetic behavior, unlike Li-deficient compounds which show a Pauli paramagnetic susceptibility. Also, the magnetization exhibits thermal irreversibility which vanishes at large magnetic fields in all the compounds. The unsubstituted compound has discontinuities at 200 K and 50 K corresponding to magnetic transitions which disappear on substitution with 2 % Ni for Co. The effective magnetic moment is found to increase from 0.32 Bohr magnetons to 0.66 Bohr magnetons on increasing the substitution to 0.15. The unique feature however is that all the compounds exhibit a clear magnetic hysteresis at room temperature with a finite coercivity. The coercivity increases from 31 Oe to 168 Oe for x = 0.04 and then decreases on further increasing of x. A deconvolution of the hysteresis loops clearly shows an increasing paramagnetic component with substitution.