Magnetic anisotropy and rotational hysteresis loss in exchange coupled Ni-Fe/Mn-Ir films

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 Angstrom and the Mn-Ir layer thickness ranged from 20 to 200 Angstrom. Room temperature magnetization and torque measurements were made as a function of the Mn-Ir layer thickness d(AF) 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 d(AF) value of 37 Angstrom, taken as d(AF)(cr). For d(AF)> d(AF)(cr), the data show hysteresis loops which are displaced along the field axis. The torque response and rotational hysteresis characteristics are sensitive to both d(AF) and the measuring field. (1) When d(AF) is much less than d(AF)(cr), the torque curves have a sin theta characteristic at fields below 30-40 Oe or so which suddenly changes to a sin 2 theta characteristic at higher fields. With the onset of the sin 2 theta torque response, rotational hysteresis loss also appears but then vanishes for fields above 100 Oe or so. (2) As d(AF) approaches d(AF)(cr) 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 2 theta character and persists to the maximum available measuring field of 15 kOe or so. (3) When d(AF) exceeds d(AF)(cr), the torque has a predominant sin theta character at all fields and a small sin 2 theta 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. (C) 2000 American Institute of Physics. [S0021-8979(00)04609-0].