Abstract

The magnetic anisotropy and rotational hysteresis loss in Ni–Fe/Mn–Ir bilayers were investigated for films prepared by an ultraclean sputtering deposition process. An in-plane field of 30 Oe during deposition served to define the antiferromagnetic (AF) alignment axis for the Mn–Ir layer. The Ni–Fe layer thickness was maintained at 50 Å and the Mn–Ir layer thickness ranged from 20 to 200 Å. Room temperature magnetization and torque measurements were made as a function of the Mn–Ir layer thickness dAF and the applied field. The magnetization data were obtained for fields applied in the same direction as during deposition. The magnetization data indicate a critical dAF value of 37 Å, taken as dAFcr. For dAF>dAFcr, the data show hysteresis loops which are displaced along the field axis. The torque response and rotational hysteresis characteristics are sensitive to both dAF and the measuring field. (1) When dAF is much less than dAFcr, the torque curves have a sin θ characteristic at fields below 30–40 Oe or so which suddenly changes to a sin 2θ characteristic at higher fields. With the onset of the sin 2θ torque response, rotational hysteresis loss also appears but then vanishes for fields above 100 Oe or so. (2) As dAF approaches dAFcr from below, the torque response is the same as above. Here, however, the rotational hysteresis appears for fields well below the field at which the torque response assumes a sin 2θ character and persists to the maximum available measuring field of 15 kOe or so. (3) When dAF exceeds dAFcr, the torque has a predominant sin θ character at all fields and a small sin 2θ component and rotational hysteresis which only around a field of 400 Oe or so. These results, while somewhat complicated, are in accord with responses evaluated from the simple exchange anisotropy model of W. H. Meiklejohn and C. P. Bean [Phys. Rev. 102, 1413 (1956); 105, 904 (1957)]. Among other things, one may conclude that a rotational hysteresis which persists to high field is not intrinsic to exchange anisotropy.

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