Long ferromagnetic correlation length in amorphous TbFe2

Small-angle neutron scattering (SANS) and magnetic-force microscopy (MFM) h ave been used to characterize the temperature dependence of the ferromagnet ic correlation length and the domain structure in amorphous TbFe2 below its magnetic ordering temperature. Amorphous TbFe2 is classified as a random a nisotropy magnet, in the exchange-dominated limit, and previous SANS observ ations had shown a correlation length limited to 50 Angstrom at low tempera tures. In the present study, samples were prepared by both sputtering and e lectron beam coevaporation and were either grown or preannealed at 200 degr ees C in order to permit measurements above T-c without structural relaxati on. 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 an d with the inverse length scale of the domain structure seen in MFM. For al l samples, the magnetic correlation length determined from SANS is 300-500 Angstrom in the thermally demagnetized state, and increases beyond measurem ent range after magnetizing. This long correlation length is consistent wit h theoretical predictions of a ferromagnetic ground state in exchange-domin ated random anisotropy magnets in the presence of coherent anisotropy. The SANS signal is dominated by a Lorentzian squared term, which is best unders tood 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. [S0163-1829(99)01317-X].