Magnetic Nature of a Ni Dopant in $La_{1.85}Sr_{0.15}CuO_4$: Spin-Glass Behavior

The magnetic properties of $La_{1.85}Sr_{0.15}CuO_4$ doped with Ni was investigated in the field up to 5 T and in the temperature range from 2 K to 400 K using both dc and ac techniques. For Ni content larger than 0.05 the system exhibits irreversibility of low-field susceptibility χ(T) below a certain temperature depending on y and a cusp at $T_{g}$ in χ(T) measured after zero-field cooling. The decay of remnant magnetization below $T_{g}$ with time is described by a stretched-exponential function. In accordance with scaling theory, all the χ(T) data for y = 0.50 sample taken in the vicinity of $T_{g}$ at different fields collapse onto two separate curves when plotted as $q|t|^{-β}$ vs. B$^2 |t|^{-β - γ}$, where q is the spin-glass order parameter, t = (T - $T_{g}$)/$T_{g}$, and β and γ are the critical exponents. All these features taken together reveal existence of spin-glass phase below $T_{g}$. Variation of $T_{g}$ with y is linear below y = 0.25 and $T_{g}$ extrapolates to 0 K for y → 0 what strongly suggests that spin-glass phase extends into superconducting region of the phase diagram.