CLEAR EVIDENCE FOR FIELD-INDUCED UNIDIRECTIONAL EXCHANGE SURFACE ANISOTROPY IN NIMN ALLOYS

Weak secondary modes which were observed [1] beside the main ferro-magnetic resonance (FMR) bulk mode of water quenched Ni76Mn24 alloy from 850-degrees-C, are highly pronounced by annealing the same sample at room temperature for about one year in the atmosphere. After cooling the sample in the presence of external magnetic field in the plane of the sample, FMR absorption lines were recorded by applying the measurement field either along the cooling field direction (n-FC case) or opposite to it (r-FC case). The highly pronounced secondary absorption peaks at the higher field side of main resonance peak for r-FC case are different than those for n-FC case in terms of intensities and separation from main FMR bulk modes. These secondary modes were identified as surface modes by using simulated spectra calculated from well-known Rado and Weertman exchange boundary conditions together with unidirectional field induced exchange surface anisotropy energy, as well as unidirectional field induced bulk anisotropy field in classical equation of motion for magnetization. This simulation qualitatively accounts for the directional behavior of both surface and bulk modes. However, because of field training effects on the spectra for such a metastable system during the recording of the spectra as a function of magnetic field, no attempt has been made to analyze the data as a function of temperature.