Nuclear Quadrupole Resonance of Cu in the Paramagnetic State of CuO

The 63 Cu nuclear quadrupole resonance (NQR) technique has been utilized to investigate the dynamical properties of CuO in paramagnetic states. The NQR frequency increases linearly with increasing temperature. The spin-lattice relaxation rate 1/ T 1 asymptotically approaches a constant value 1/ T 1∞ of 1.16±0.02×10 3 sec -1 at high temperatures. Below about 350 K, the rate begins to increase, and near the Néel temperature of 229∼230 K, it diverges due to the critical slowing down. The critical region is found to be not very wide (( T - T N )/T_N\lesssim0.5). A n e s t i m a t e d v a l u e f o r 1/T_1∞ i n t e r m s o f e x c h a n g e c o u p l e d l o c a l m o m e n t s i s i n g o o d a g r e e m e n t w i t h t h e e x p e r i m e n t a l v a l u e . F r o m t h e v i e w p o i n t o f t h e s p i n - l a t t i c e r e l a x a t i o n e x p e r i m e n t s , C u O a p p e a r s t o b e a w e l l - l o c a l i z e d m a g n e t i c i n s u l a t o r .