Spin-Glass Transition in the $RCoGaO_{4}$ (R=Lu, Yb) Layered Cobaltites

The frequency, temperature, and the dc and ac dependences of the magnetic susceptibility of the $YbCoGaO_{4}$ and $LuCoGaO_{4}$ single crystals are investigated. The $YbCoGaO_{4}$ behaves like an Ising spin glass with a strong uniaxial anisotropy. In contrast $LuCoGaO_{4}$ is Heisenberg-like spin glass. Dynamical scaling reveals a three dimensional phase transition near Tg and yields critical exponent values between those of Heisenberg- and Ising-like systems.