NMR Study of Iron Sublattice Magnetization in YIG and GdIG

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Abstract
The 57Fe NMR frequencies in the two iron sublattices of YIG and GdIG have been measured accurately at 5° to 15°K intervals from 4.2° to 500°K. We have also measured the pressure dependence of the 57Fe NMR frequencies in YIG to 10 kbar at 196°, 273°, 299°, and 346°K. In YIG, we find the pressure dependence of the critical temperature to be ∂Tc/∂P = 0.69°±0.02°K/kbar. The results of the pressure measurements have been used to correct the NMR frequencies for thermal expansion. From these data the temperature dependence at constant volume of each sublattice magnetization has been determined to about 0.01% precision. A spin‐wave analysis of the low‐temperature data gives the dispersion constant for the acoustic spin‐wave mode as D=31.8±1 cm−1. Our NMR measurements of the iron sublattice magnetizations in GdIG have been combined with total magnetization data to obtain the Gd sublattice magnetization up to 0.9 Tc. The Gd magnetization can be fit quite well by a molecular field calculation and yields estimates of the exchange constants to be Jcd=−3.46 cm−1, Jca=−0.22 cm−1, and Jcc=−0.13 cm−1.