Parallel Pumping of Europium and Yttrium Iron Garnets at Low Temperatures

Abstract

The spin-wave dispersion and relaxation properties as well as magnetoelastic and elastic properties of europium iron garnet (EuIG) and yttrium iron garnet (YIG) are measured at low temperatures and at about 17 Gc/sec using the parallel-pump technique. ${\mathrm{Eu}}^{3+}$ ions have unusual magnetic and relaxation characteristics in large exchange fields, and the effect of these ions on the spin-wave spectrum of the iron lattice is of interest. Also, the large effective magnetoelastic coupling constant of EuIG along [111] should have a marked effect on the character of the threshold curve as well as the magnon-phonon interaction notch. We find that the spin-wave linewidth of YIG is less than that of EuIG and that the three-magnon confluence process does not account quantitatively for the $k$-dependent linewidth at low temperatures. The exchange constant of EuIG is predicted quite well from the known magnetic properties of the ${\mathrm{Eu}}^{3+}$ ion and its expected effect on the spin-wave spectrum of the iron lattice. The large magnetoelastic coupling constant of EuIG along [111] makes the threshold curve quite anomalous. It shifts the longitudinal notch to higher $k$ values, yielding an erroneously high exchange constant. Also, the shear-wave notch is absent, possibly because the spin-wave instability merges smoothly into the elastic-wave instability. Along [100], the coupling constant of EuIG is much smaller and a normal threshold curve is obtained. For YIG, the magnetoelastic coupling constants derived from parallel-pump measurements are significantly higher than those obtained from strain-gauge measurements. This might be due to the lower temperature at which the parallel-pump measurements were done. For EuIG, the agreement between the two kinds of measurements was reasonably good. The elastic $Q$ increases with decreasing temperature for both garnets and is larger for YIG than for EuIG, as expected.