Figure 1. x (REDUCED WAVE VECTOR) Inelastic magnetic scattering in HoCo, at 4 K for the [q,q,0] and [q,q,q] propagation directions. The lower mode spin wave groups were measured about the [111] and [220] reflection and the flat modes at the [002] and [222] respectively, as determined from dynamic structure factor considerations. The solid line is the result of a RPA model calculation using the exchange and crystal field parameters shown in the figure. 1. 2. D. Gignoux, F. Givord and R. LeMaire, Phys. Rev. B. 12, 3878 (1975). R. M. Moon, W. C. Koehler and J. Farrell, J. Appl. Phys. 36, 978 (1965). MAGNETIC EXCITATIONS IN ErCo2 N. C. Koon (Naval Research Laboratory, Washington, DC) and B. N. Das (Naval Research Laboratory, Washington, DC) and J. J. Rhyne We have measured the magnetic excitation spectrum of ErCo2 at 4.2 K using neutron inelastic scattering and compared the results to a Green's function RPA theory of magnetic excitations in a ferrimagnet with crystalline electric fields. At the zone center [111] two modes are clearly resolved, one at 8.2 meV and another at 11 meV. The width of the 8.2 meV is almost equal to the instrumental resolution, while the 11 meV mode is considerably broader, and about 5 times more intense than the 8.2 meV mode. Both modes exhibit a weak q dependence in the [111] zone. Based on dynamic structure factor considerations we conclude that each of the observed scattering peaks consists of two nearly degenerate excitations, one of which corresponds to out-of-phase precession of the two rare earths in the primitive cell, while the other corresponds to in-phase precession. The in-phase excitations are observed at [111] but not at [002] and have a weak q dependence, while the out-of-phase modes are essentially q independent. By comparing these results with a Green's function RPA theory we are able to determine the Er crystal field parameters as well as the Er-Co mean exchange field. From the lack of dispersion in the out-of-phase modes we conclude that the Er-Er exchange is very weak, just as was found for RFe, compounds. The mean field exchange and crystal field constants for the rare earth spins are gugHexch 2.0 meV, A = 4.3 meV, and A = -0.15 meV. 0 Аб 0 A spin wave mode due mainly to excitation of the Co spins is predicted but not observed, probably due to much weaker scattering intensity than the predominantly rare earth modes. NEUTRON SCATTERING MEASUREMENTS ON AMORPHOUS NdFe2 H. A. Alperin (Naval Surface Weapons Center, White Oak, MD) and (National Bureau of Standards, Washington, DC) (University of Rhode Island, Kingston, RI) and J. J. Rhyne Earlier measurement [1,2,3,4,5] on bulk amorphous rare-earthiron alloys incorporating heavy rare earths showed the following general features: i) A dense random close-packed amorphous structure, ii) Lorentzian neutron scattering above T characterized by a correlation C length which remained finite at To, iii) Below Te, intense small angle scattering due to the presence of small magnetic inhomogenieties, -3 characterized by a q dependence of the intensity on wave vector, q, iv) Large coercive fields, H increasing with decreasing temperature. In the present study, the effect of a light rare earth element on these characteristics was investigated. C The sample was a disc 1 mm in thickness and 5 cm in diameter, prepared by rapid sputtering. An incident wavelength of 0.91 A was used to determine the amorphous structure. Small angle scattering was observed between 5 and 400 K with two techniques: a double axis spectrometer with a 10' slit system with 2.38 A neutrons covering the wavevector range .03<q<.12 A and a small angle scattering spectrometer with a linear position-sensitve detector with neutrons of 7.2 A for q down to .01 A. Inelastic neutron scattering measurements were made at room temperature with a triple axis spectrometer at 2.36 A. A diffraction pattern taken at room temperature shows a split first peak and pronounced structure at higher angles; features not present in the patterns for the heavy rare earth alloys which have been explained by a dense random packing (DRP) of iron atoms (with radii of 1.27 A) and rare earth atoms with their 12-fold coordination radii. The pair correlation function for NdFe2 obtained by Fourier inversion of the diffraction pattern is given in figure 1 and shows first neighbor peaks at 2.54, 3.15, and 4.25 A. The peak at 2.54 A is at the position expected for the nearest neighbor Fe atoms in the DRP model; however, the other peaks cannot be correlated with combinations of the radii of Nd (1.83 A) and Fe atoms as predicted by the DRP model. This lack of agreement is certainly unexpected particularly in light of the other amorphous rare earth alloy results and is not understood at present. The small angle scattering as a function of temperature taken on the double axis instrument is plotted in figure 2. The critical scattering peak (as determined from a measurement at q = .03 with finer temperature control) is well-defined at 305 K and is more pronounced than that seen in any of the heavy rare earth alloys. The intense scattering at low temperatures is comparable in magnitude to that observed 1 2 in TbFe2, and bacause it is not present above T and scales with o it must be magnetic in origin. Above T the data can be fitted with a C C -1 Lorentzian in q, which yields correlation length K ~60 A at 313 K. The intensities at low temperature can be analyzed as an inverse power law dependence on wavevector q (= 4πsine/λ). We find approximately that the -3 intensity I a q-3. α Inelastic neutron scattering was measured at room temperature where σ has attained ~0.4 of its full value. Nevertheless, no energy broadening of the elastic peak and no well-defined excitations (at wave vectors 0.1 < q < 0.28 A ̄1) could be observed. The latter result is not completely unexpected since the wings of the particularly large elastic small angle scattering component can effectively mask the low q spin wave scattering. The usual negative exchange between the rare earth and iron aligns their spins oppositely. In the heavy rare earth alloys where the total this leads to a ferrimagnetic alignment of the quantum number J = L+S iron and rare earth moments (p=8JJ), but for light rare earth atoms |