Long ferromagnetic correlation length in amorphous TbFe2

Small-angle neutron scattering (SANS) and magnetic-force microscopy (MFM) have been used to characterize the temperature dependence of the ferromagnetic correlation length and the domain structure in amorphous TbFe2 below its magnetic ordering temperature. Amorphous TbFe2 is classified as a random anisotropy magnet, in the exchange-dominated limit, and previous SANS observations had shown a correlation length limited to 50 Angstrom 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 degrees 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 Angstrom 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. [S0163-1829(99)01317-X].