Anisotropic magnetization and dynamic susceptibility sign change in single-crystalNa0.85CoO2

Abstract

The dc and ac magnetic susceptibilities of ${\text{Na}}_{0.85}{\text{CoO}}_{2}$ single crystals were measured for the different crystal orientations of $H\ensuremath{\parallel}(ab)$ and $H\ensuremath{\parallel}(c)$ axis. The dc-magnetic susceptibility for $H\ensuremath{\parallel}(c)$ direction exhibited the antiferromagnetic transition at ${T}_{N}=22\text{ }\text{K}$. The thermal hysteresis between the zero-field-cooled and the field-cooled magnetization below ${T}_{N}$ and the large frustration parameter indicated the spin frustration along the $c$ axis. For an applied magnetic field in $H\ensuremath{\parallel}(ab)$ plane, the dc magnetic susceptibility exhibited the logarithmic divergent behavior at low temperatures $(T\ensuremath{\le}6.8\text{ }\text{K})$. This could be understood by the impurity spin effect dressed by the spin fluctuation. From the ac magnetic susceptibility measurements, the real part of the ac susceptibility for $H\ensuremath{\parallel}(ab)$ exhibited the spin glasslike behavior at low temperatures $(T\ensuremath{\le}4\text{ }\text{K})$. Remarkably, for an applied ac magnetic field with $H\ensuremath{\parallel}(c)$ axis, the sign of the ac magnetic susceptibility changed from a positive to a negative value with increasing ac magnetic field frequency $(f\ensuremath{\ge}3\text{ }\text{kHz})$ at low temperatures $(T\ensuremath{\le}7\text{ }\text{K})$. We interpret the sign change of ac magnetic susceptibility along the $c$ axis in terms of a sudden sign reversal of the phase difference $\ensuremath{\phi}$ from in-phase to out-of-phase response with an applied ac magnetic field in the ac-susceptibility phase space.