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Optical detection of spin-bath relaxation in some paramagnetic crystals Glattli, Hans

Abstract

The magneto-optical Faraday effect has been used to observe the spin-bath relaxation at low temperatures in CeES and in Eu-doped CaF₂. The paramagnetic Faraday rotation ϩ is an instantaneous measure of the magnetization M and it is shown that in CeES and Eu²+ : CaF₂, ϩ∾ M at the light frequencies employed. The apparatus is the same as that previously described by Rieckoff and by Griffiths. Pulsed MW-power at X-band has been used to disturb the equilibrium between spin system and bath. In CeES, the observed relaxation time Շ is of the order of a few msec, which is several orders of magnitude longer than the theoretical estimate of T₁. This suggests a severe bottleneck in the energy transfer spin-bath. Շis found to be environment-dependent. In HeII, the relaxation is exponential. Շ is in good agreement with nonresonant relaxation measurements by Van den Broek and Van der Marel. It is explained as arising from the Kapitza boundary resistance at the CeES-HeII interface. In HeI, the relaxation is non-exponential and is slower than in He gas at the same temperature. This suggests that in this case the thermal diffusion in the helium around the crystal is the bottleneck. The same relaxation behaviour is found when the crystal is heated dielectrically with MW power far off resonance. This supports the assumption that the energy transfer spin-bath is limited by spatial diffusion. If the crystal is surrounded by a He film at a temperature below the λ-point, Շ is found to be the same as in HeII up to a well defined average MW power level. For higher powers the relaxation behaviour is similar to that of CeES immersed in HeI. In Eu²+ ; CaF₂, T₁ is expected to have the form T₁= AT + BT⁵. The observed relaxation time Շ , however, is found to be concentration dependent. All measurements have been done on the +½⇢ -½ transition with H II [100]. For the three lowest concentrations, the temperature dependence of Շ from 1.5 to 4.2°K can be fitted with the expression Շ ¯¹=CT with C = 2.75 (sec°K) ¯¹ for 0.02% Eu, C = 3.5for 0.8% and C = 5 for 0.2%, Շ is shorter and Շ ¯¹ ∾ T ² from 1.5°K to 7°K. The concentrations given correspond to the total Eu content. The Eu²+ concentration has been inferred from the magnitude of the saturation rotation. Շ(T) seems to depend on both Eu²+ and Eu³+ concentrations. It is suggested that exchange coupled pairs of Eu²+ and clusters involving Eu³+ may account for the concentration dependence of Շ. Upper limits of A = 2.5 and B = 5 x 10¯⁵ are found for T₁ by extrapolating the lowest concentrations investigated. These values are somewhat lower than both measured and calculated values found by Huang.

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