%e investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to = 20 A. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.

Lai.85Bao.i5Cu04 exhibits high-7" c superconductivity with an onset temperature near 33 K as measured by resistivity and magnetic susceptibility. The nonsuperconducting end-member compound, La2CuC>4, which has an orthorhombic, Cmca, structure, exhibits semiconducting behavior. The orthorhombic distortion is proposed to result from a Peierls 2&F instability or a soft zone-boundary phonon mode. The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm, structure which is metallic and superconducting.

We report measurements of the specific heat of paramagnetic UA1 2 between 0.8 and 25 K in magnetic fields up to 43 kOe. A nearly field-independent T 3 lnT term dominates the temperature region below 10 K. The susceptibility was measured between 3 and 10 K and varies as 1-AT 2 . These results, and earlier resistivity measurements, are consistent with the predictions of the spin-fluctuation model. This is the first unambiguous observation of the specific heat predicted by the spin-fluctuation model for an atomic ally ordered paramagnet.Experimental verification of the existence of persistent spin fluctuations in exchange-enhanced paramagnets has been unsuccessful for the most part. In particular, the theoretically predicted contributions to the specific heat and magnetic susceptibility had never been unambiguously observed in any uniform metal prior to this work. 1 In this Letter we report low-temperature measurements of the specific heat and susceptibility of the narrow-band intermetallie compound UA1 2 which strongly support the theoretical calculations and indicate the presence of spin fluctuations associated with a narrow 5/ band. The existence of a low-temperature resistivity proportional to T 2 previously led to the proposal of spinfluctuation scattering in UA1 2 . 2 The spin-fluctuation modification of the electronic specific heat has been considered by a number of authors G Doniach and Engelsberg 3 and Berk and Schrieffer 4 have shown that the absorption and re-emission of spin fluctuations renormalizes the electronic self-energy, leading to an enhanced effective mass at low temperatures. This effect manifests itself as a low-temperature enhancement of the electronic specific-heat coefficient, y, which falls off with increasing temperature as T 3 ln(T/T SF ) for T«T SF . Here T S ?=T ¥ /S is the characteristic spin-fluctuation temperature, and T F and S are the degeneracy temperature of the band and the Stoner exchange-enhancement factor. Brinkman and Engelsberg 5 and BealMonod, Ma, and Fredkin 6 showed that this effect will be nearly insensitive to the presence of an external field.Exchange-enhanced pure metals, notably Pd, exhibit enhanced low-temperature values of y, but a spin-fluctuation contribution to the enhancement is difficult to separate from that due to the electron-phonon interaction, and quantitative comparisons with spin-fluctuation theory have been largely unsuccessful. The known spin-fluctuation system He 3 exhibits a T 3 lnT term in the specific heat at very low temperatures, 7 but such terms have not been observed in any metal prior to this worko 8 Very large exchange enhancements are found in some alloys near the critical concentration for ferromagnetism. Up turns in C/T for CttNi and RhNi have been fitted by T 3 InT terms and attributed to spin fluctuations. 9 ' 10 However, these effects are now known to be due to magnetic clustering (superparamagnetism)o 1,11 In the case of RhNi this was demonstrated by the observation that the upturn in C/T is suppressed by applied fields much smaller...

%e investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to = 20 A. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.

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