FIELD-DEPENDENCE OF SPIN-LATTICE RELAXATION OF ND3+ IONS IN Y3AL5O12 CRYSTALS

Our studies involve measuring spin-lattice relaxation times for Nd3+ i ons in yttrium-aluminum garnets over the temperature range 4-50 K at 9 .25 and 36.4 GHz for different orientations of the external magnetic f ield in relation to the crystallographic axes. The temperature depende nce of the relaxation rate is described by T-1(-1)=AT(n) + b exp(-Delt a/kT), where n varies from sample to sample, with n=2 for ''perfect'' samples (i.e., with the longest relaxation times). Here Delta is appro ximately 130 cm(-1), which is the energy of the excited Kramers double t of the neodymium ion closest to the ground state, and this makes it possible to interpret the second term in T-1(-1) as the contribution o f two-stage relaxation proceeding through the intermediate level Delta . A strong field dependence of these processes has been discovered: wh en the frequency was increased fourfold, the relaxation rate increased by a factor of 10. The effect is a specific manifestation of the dege neracy of the excited level, breaking of the symmetry of the crystalli ne field due to lattice defects, and the prevalence of deformations of a certain type in the spin-lattice interaction. (C) 1997 American Ins titute of Physics.