The complex (2,3-dmpyH)2CuBr4 has been synthesized and its crystal packing determined by single-crystal X-ray diffraction (2,3-dmpyH = 2,3-dimethylpyridinium). The compound crystallizes in the triclinic space group P1. The crystal packing is characterized by the formation of a ladder structure for the CuBr4 anions showing short Br...Br contacts. The rungs of the ladder are formed via a crystallographic inversion center, while the rails are formed via unit cell translations. Variable temperature magnetic susceptibility measurements agree very well with the ladder model [Jrung = -3.10 cm-1 (-4.34 K) and Jrail = -6.02 cm-1 (-8.42 K)]. The assignment as a magnetic ladder is confirmed by first principles bottom-up theoretical calculations which conclude that Jrung = -3.49 cm-1 (-4.89 K) and Jrail = -7.79 cm-1 (-10.9 K), in very good agreement with the experimental values. They also support the absence of additional significant magnetic exchange within the crystals. Thus, (2,3-dmpyH)2CuBr4 represents the second reported example of a weak-exchange limit magnetic ladder (that is, one in which the exchange along the rail is stronger than that across the rung).

(100) and (111) oriented single-crystal CoPt 3 films were deposited over a range of growth temperatures from 250 to 800 ± C. The Curie temperature is increased by 200 ± C over the value expected for the homogeneous alloy in the as-deposited films (of both orientations) grown near 400 ± C. We interpret this as evidence for a previously unobserved, surface-mediated, magnetically driven miscibility gap in vapor-deposited CoPt 3 films. Large perpendicular magnetic anisotropy is also observed in the as-deposited films (of both orientations) grown near 400 ± C.

͑111͒-oriented CoPt 3 and Co 0.35 Pt 0.65 films were deposited at various growth rates and over a range of growth temperatures from Ϫ50 to 800°C. Films grown at moderate temperatures ͑200-400°C͒ exhibit remarkable growth-induced properties: perpendicular magnetic anisotropy and large coercivity, as well as enhanced Curie temperature and low-temperature saturation magnetization. Magnetic measurements indicate significant Co clustering in these epitaxial fcc films. These properties are independent of crystallographic orientation, increase with increasing growth temperature, and vanish with annealing. We propose that the correlation between magnetic inhomogeneity, magnetic anisotropy, and enhanced moment is explained by clustering of Co into thin platelets in a Pt-rich lattice. This clustering occurs at the growth surface and is trapped into the growing film by low bulk atomic mobility. ͓S0163-1829͑99͒09041-4͔

Small-angle neutron scattering ͑SANS͒ and magnetic-force microscopy ͑MFM͒ have been used to characterize the temperature dependence of the ferromagnetic correlation length and the domain structure in amorphous TbFe 2 below its magnetic ordering temperature. Amorphous TbFe 2 is classified as a random anisotropy magnet, in the exchange-dominated limit, and previous SANS observations had shown a correlation length limited to 50 Å at low temperatures. In the present study, samples were prepared by both sputtering and electron beam coevaporation and were either grown or preannealed at 200°C in order to permit measurements above T c without structural relaxation. Samples grown by vapor deposition processes possess a large macroscopic perpendicular anisotropy constant K u , which can be reduced or eliminated by annealing. A strong SANS signal is seen in all samples, with a magnitude strongly correlated with the temperature-dependent sample magnetization and with the inverse length scale of the domain structure seen in MFM. For all samples, the magnetic correlation length determined from SANS is 300-500 Å in the thermally demagnetized state, and increases beyond measurement range after magnetizing. This long correlation length is consistent with theoretical predictions of a ferromagnetic ground state in exchange-dominated random anisotropy magnets in the presence of coherent anisotropy. The SANS signal is dominated by a Lorentzian squared term, which is best understood as resulting from ferromagnetic domains with meandering domain walls, similar to the Debye-Bueche model developed for materials consisting of two strongly segregated, interpenetrating phases.

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