The spin-lattice relaxation time T, of Eu'" in powder EuO was measured at temperatures from 1.7 to 20 K and external fields above magnetic saturation from 24 to 60 kOe. The results were analyzed using the multimagnon scattering theory previously proposed by Beeman and Pincus. The observed relaxation rates are well explained by the dipolar-induced two-magnon process and exchange-scattering-enhanced three-magnon process. It is found that the 6rst is dominant at low temperatures (T & 14 K) and the second at higher temperatures.

The existing theoretical model proposed by Hone, Jaccarino, Ngwe, and Pincus (HJNP) to treat spin-spin relaxations in inhomogeneous lines is extended in order to~»e it applicable to experimental situations often encountered in real ferromagnetic systems. The transverse rehLxation of Eu""" in ferromagnetic EuO and of Co~~i n ferromagnetic Co metal was studied by the spin~ho technique as function of external magnetic 6eld Ho & 4aM at low temperatures. Nonexponential echo decays were observed in all cases and the relaxation rates were found to be field dependent. The extended HJNP model was successfully applied to account for the observations in EuO, verifying in a quantitative @~oner the important role played by the indirect spin-spin (Suhl-N»natura) interaction in the relaxation processes of this system. The~&ysis also yields detailed information concerning the character of the local distribution of the effective fields acting on Eu nuclei in EuO.

We report the phase locking of an oscillating GaAs/AlGaAs resonant tunneling diode (RTD) to a train of optical pulses. Locking was achieved by direct illumination of a specially designed RTD with a diode laser pulse train. Locking occurs at all rational frequency ratios and with excitation energies as low as 0.1 pJ/pulse for the principle lock.

The kinematical effect can be estimated as follows: One takes a quadratic operator like S { + Sj~ and expresses this operator in terms of boson operators via the Dyson-Maleev transformation or the Hoi stein-Primakoff expansion, and then expresses these operators in terms of spin-wave operators obtained by a Bogoliubov transformation. Such a series of steps have been carried out for the simple Heisenberg Hamiltonian by Harris, 11 and four-magnon creation terms appear. The final expressions are quite complicated, and it is difficult to determine the size of the 4M terms. A much simpler method is to again start with the Ising states listed in Fig. 3 and calculate via perturbation theory the admixture of two spindeviation states with these states and with the ground states. Assuming that the light introduces a perturbation of the form S/S/, where i and.; are nearest neighbors, the 4M states all have the spin deviations grouped in pairs as in states h, i, Z, m, or «, and the matrix element is zero for the states of type h. For a state of type I the matrix element is -&-J~ l/2z 2 times the matrix element of 2M creation, where z is the number of nearest neighbors. Thus the total intensity is ~z~A~ 10" s of the 2M peak. If the kinematical effect made the dominant contribution to the matrix element, this ratio of the 4M intensity should be the same for both NiO and KNiF 3 . Experimentally, the ratio is 7 times larger in NiO than in KNiF 3 and this is only consistent with some form of excited-state exchange mechanism as discussed in the previous paragraph. Since in KNiF 3 the ratio of 4M to 2M intensities --jfe, it is possible that some kinematical effect is important for this case.

No abstract