Magneto-optical study of magnetic field dependence of spin-lattice relaxation times in CaF2:Eu2+

Abstract

A magneto-optical technique was developed to measure the spin-lattice relaxation time of paramagnetic ions in a crystal lattice as a function of applied static magnetic field strength. This method utilized the Faraday effect at low temperatures. An ac magnetic field modulated the Faraday rotation of plane polarized light traversing the sample. The phase angle between the diamagnetic rotation and the total rotation was measured as a function of frequency of the modulating magnetic field. When the frequency of modulating field exceeded the inverse spin-lattice relaxation time, the modulation of the light due to paramagnetic contributions was diminished leaving only the modulation due to diamagnetic contributions. From these measurements the spin-lattice relaxation time was determined. Second order effects were also considered for the case where tau was assumed to have a time-varying component. The spin-lattice relaxation times for divalent europium ions in a CaF2 lattice at 9ºK were measured as a function of applied dc magnetic field strength from 0 - 5760 Oe, The results indicated tau = 9.6 +- 0.1 msec at 5760 Oe and tau = 0 msec at 0 Oe. A linear relationship was found between tau and the strength of the applied static magnetic field. This is contradictory to the commonly accepted theories of spin-lattice relaxation of paramagnetic ions. The second order effects produced by assuming tau contained an additional time-varying term were negligible in this experiment. The results obtained by the above method agreed quite satisfactorily with other reported spin-lattice relaxation times obtained by electron paramagnetic resonance techniques at the same magnetic field.