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a nearly constant exchange contribution, and so this term, and thus the spin-wave energy, will not go to zero with the anisotropy contribution on approaching the transition temperature.In summary, we have proposed a model in which the cone to spiral transition in holmium is caused by an instability of the spin-wave mode of wave vector E 0 arising from the strong temperature dependence of the anisotropy parameters. Calculations of these parameters lend support to our arguments, and we have carried out neutron inelastic scattering measurements which demonstrate the influence of this instability on a spinwave mode of wave vector near £ 0 .The authors wish to thank their colleagues at Chalk River for advice and encouragement. E. A. Glaser and R. Campbell gave valuable technical assistance. of the effect of short-range spin ordering on the electrical resistivity near a magnetic transition. To date, there have not been any experimental data which can be reasonably described in terms of these theories. These studies have used a molecular-field or Ornstein-Zernicke 3 approximation for the long-range behavior of the spinspin correlation function and the Born approximation to relate this correlation function to the electrical resistivity. The basic model used is that of an "s-d" exchange interaction between the conduction (s) electrons and the assumed lo-Phys. Rev. 127, 57 (1962). . Phys. 40, 1452Phys. 40, (1969.calized d or f electrons. Recently, the measurements of Craig et al. 4 on the electrical resistivity of Ni near its Curie point showed a behavior very different from that predicted by these theories. Fisher and Langer 5 have argued that these theories are inadequate because the anomalous behavior of the electrical resistivity near a magnetic critical point is determined largely by the short-range rather than the longrange spin fluctuations, at least for temperatures above the transition temperature T c . In this paper we report the observation of the effect of the long-range nature of the critical fluctuations of the short-range spin ordering on the tempera-We have measured the temperature dependence of the electrical resistivity of the cubic, Laves-phase, ferromagnetic, intermetallic compounds GdNi 2 , GdPt 2 , and GdRh 2 , and for GdNi 2 , we have analyzed the temperature derivative of the electrical resistivity in the neighborhood of the magnetic transition. Above the Curie temperature our data are very well described by the molecular-field treatment of the long-range spin fluctuations of the short-range order, giving for the first time an experimental result in agreement with this treatment. 83

The Gd hyperfine field, in the magnetically ordered state of several GdX 2 cubic Lavesphase compounds, shows a large variation in magnitude and sign. A comparison of our results with those on other rare-earth systems supports the supposition of a constant 4/ core-polarization field in the rare earths.We have measured the gadolinium hyperfine field in the magnetically ordered state at 4.2°K for several GdX> (X = A1, Rh, Pt, Fe, Mn) intermetallic Laves-phase compounds by nuclear-magnetic-resonance spin-echo techniques. Our results show a large variation in the magnitude of the Gd hyperfine field throughout this series. This is in sharp contrast to the essentially constant value of the Gd hyperfine field previously reported for Gd nuclei in a variety of environments, namely, Gd metal, 1 ' 2 GdN, 3 and Gd 3+ ions. 4 ' 5 ' 6 As a result of the earlier investigation, which yielded a field of approximately -340 kOe (the negative sign implying that H e ff is directed opposite to the local magnetization), it was believed that the major contribution to the Gd hyperfine field comes from core polarization by the unfilled 4/ shell. Our results point out the importance of other contributions to the hyperfine field which we will call "valence" polarization.Our experimental results on the Gd hyperfine fields in the cubic Laves-phase intermetallic compounds and in GdN are summarized in Table

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