Magnetization data are presented on sputter-deposited amorphous RFe, materials, where R represents the rare-earth elements Tb, Gd, and Y. Both the Tb and Gd compounds exhibit ferrimagnetic order with Curie temperatures approximately 40% lower than the equivalent crystalline Laves phase compounds. The 0 K saturation magnetizations of the Gd amorphous and crystalline materials are both equal to 3.9p,~, while in the TbFe, the magnetization is significantly lower in the amorphous phase (4.2pz vs 5. 4pz for crystalline Tbpe, ). The YFe, exhibits no long-range magnetic order above 3.5 K. At low temperatures large "coercive" fields appear for the TbFe, and increase greatly in magnitude at temperatures below 40 K, reaching 30 kG at 4 K. A possible model to explain this and other eAects associated with a large random-directional local anisotropy field is presented.
A systematic investigation was made of magnetostriction and magnetization in amorphous thin films (~l micron) of TbxFel_x with x = 0 to 0.5 prepared by electron-beam co-evaporation. The amorphous or crystalline character of the films was determined using x-ray and Mossbauer techniques. Net magnetization, coercivity, and the initial in-plane and outof-plane distribution of magnetization was determined using a vibrating sample magnetometer. Both the sign and magnitude of the magnetostriction was measured as a function of magnetic field using a capacitance method. Over most of the concentration range the amorphous samples have a positive magnetostriction with a maximum of A f,/ f, = 285 x 10-6 at x = 0.4 and going to zero at the compensation point (x = .22) and near x = .5 where the amorphous alloys are no longer magnetically ordered at room temperature.In the region x = 0.1 to 0.2 the magnetostriction is more complex, being negative for fields up to 5 kOe and positive at higher fields. All the films studied with x :?: 0.1 were magnetically very hard and requi red fields much greater than 15 kOe to saturate. The relatively large magnetostrictions compared with crystalline materials indicate that the large Tb single -ion magnetostriction mechanism continues to playa strong role in the amorphous lattice structure.
We have synthesized, for the first time, amorphous alloy films of (Fe,Co)1–xBix. The amorphous alloys are most stable in a narrow composition range near x = 0.20, while above or below this composition (for 0.10⩽x⩽0.30), a predominant amorphous alloy along with Fe or Co and Bi metal is observed. The alloys are ferromagnetic and magnetization data indicate that the Fe or Co moments are little affected by the Bi content near x = 0.20 and are at least 95% of that for the pure magnetic metals. For the Fe-Bi alloys, a detailed characterization was made using x-ray, magnetization, Mössbauer, FMR, and differential scanning calorimetry (DSC) techniques. The DSC measurements show irreversible exothermic or combined exothermic-endothermic transitions over narrow temperature ranges up to 830 K. Their magnetization versus temperature plots exhibit discontinuous changes associated with the DSC transitions and Curie temperatures near 1020 K. The films display large uniaxial anisotropies perpendicular to the film planes. They are highly resistant to oxidation or tarnishing at room temperature.
The heat capacity of powdered MnBr2·4H2O has been measured between 1.4° and 20°K in applied magnetic fields up to 24 kOe. The shift of the Néel point to lower temperatures with increasing field has been observed. A series of adiabatic magnetization measurements from initial temperatures above and below the zero field Néel point (TN(0)=2.13°K) has been carried out. For Ti<TN(0) the anticipated cooling effect is observed for applied fields smaller than that required to make the salt paramagnetic. Adiabatic data have been used to locate the curves of entropy vs temperature computed from the heat capacity relative to the curve for H=0.
The heat capacity C p of a single-crystal specimen of|MnBr 2 -4H 2 0 has been measured between 1.4 T and 8°K with magnetic fields from 0 to 15 kOe applied along the c r axis. This axis, which is orthogonal to the'a and b axes of this monoclinic substance, is thought to be close to the direction of preferred spin orientation in the antiferromagnetic state [7V(#=0) = 2.13°K], Temperature changes associated with adiabatic magnetization (magnetocaloric effect) have also been observed. For H^O, C p exhibits at TN(H) SL X anomaly whose maximum shifts to lower temperature with increasing H, tracing out in the H-T plane a portion of the boundary between antiferromagnetic and paramagnetic phases. Isentropic lines originating within the antiferromagnetic region of this diagram show cooling with initial increase of H, appear to intersect the phase boundary at their inflection points with common tangent, and exhibit temperature minima in the paramagnetic region. The phase boundary is nearly parabolic, with the same curvature as is deduced thermodynamically from zero-field susceptibility and heat-capacity data.
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