In order to investigate grain size effects on ferro–antiferromagnetic coupling in NiFe/FeMn systems, samples of glass/Cu zÅ/NiFe 75 Å/FeMn 150 Å/Ta 30 Å were made and the grain size of FeMn was changed by changing the Cu thickness z. Increasing z from 0 to 315 Å, the mean grain size increases from 18 to 50 Å. The blocking temperature, at which the coupling field NiFe/FeMn disappears, increases from 330 to 430 K with the increase of the mean grain size. The curve shape of the temperature dependence of the coupling field changes from a concave type to a convex type with the increase of the mean grain size. Temperature dependence of the coupling field were calculated using a thermal fluctuation model of antiferromagnet. The calculated curve of the temperature dependence of the coupling field agrees with the experiment qualitatively. However, the calculated curves of the temperature dependence of coercivity does not agree with the experiment.
The complementary nature of the exchange bias field and the coercivity enhancement in ferromagnetic (F) -antiferromagnetic (AF) exchange-coupled layer systems is explained based on an extended Fulcomer & Charap's model, in which the directional distribution of the easy axes plays a decisive role in governing the magnetization behaviors. The model was further extended to deal with the AF-layer thickness ranging from zero to infinity taking into account of NCel's spin fanning model. A comprehensive explanation of the influence of exchange coupling on the magnetization behavior of the F-layer is given. It is shown that a rotationalAlthough there have been controversies about the microscopic mechanism causing the unidirectional anisotropy, the interfacial uncompensated AF-spins seem to be responsible for the manifestation of exchange biasing in most cases [4,5,6]. The enhancement of the coercivity, AHc, can often be understood as the effect of irreversible changes in the AFspin directions accompanying the switch of the F-magnetization, M. Other mechanisms, in which some incoherent magnetization process is taken into consideration, have also been proposed [7]. This paper will give a comprehensive magnetization procesi is inherent for the samples in which the explanation of the effect of the exchange coupling on the exchange coupling effect is dominant. The same model explains magnetization behavior of soft F-layers stressing the the temperature dependence Of the exchange bias field and complementary nature of Heb and H,. The role of AF-spincoercivity and the memory effect observed after annealing. The fanning along the thickness direction, the rotational nature of the magnetization reversal process and the temperature varieties of phenomena are mostly ascribed to the existence of distribution in the magnetic properties, net spin moment at the dependence of Heb and Hc will also be explored. AF-grain surface and AF-grain size.
To investigate thermal decay of ferro/antiferromagnetic coupling, spin valve films with Co/(Cr0.4Mn0.6)90Pt10 exchange coupling layers were heat treated at various temperatures under a magnetic field whose direction was opposite to the pinning direction, and the pinning fields were evaluated at 35 °C. The pinning field decayed as the treatment time increased. During the first 10 min of the heat treatment, the pinning field decreased rapidly and then decreased gradually. The initial decrease in the pinning field became larger and the rate of the change of the pinning field after 10 min became larger at the higher treatment temperatures. A thermal fluctuation model, which assumes coherent rotations and grain size distributions in the antiferromagnets, was used to obtain the activation energy and relaxation times for reversal of the antiferromagnetic moment. Smaller antiferromagnetic grains had lower activation energies and shorter relaxation times. As a result, the smaller grains reversed their moments at an earlier time stage, which gave rise to the rapid decay of the pinning field during the first 10 min of the heat treatment. The model explains the thermal decay of the pinning field when the temperature is lower than 130 °C. However, a narrower distribution of the activation energy than can be expected from the model is required to explain the thermal decay at 150 °C, which suggests that a incoherent magnetic rotation occurs in antiferromagnets above 150 °C.
To investigate ferro/antiferromagnetic coupling in Co/CrMnPt layers of spin valve films, spin valve films of glass/Ta/NiFe/CoFe/Cu/Co 30 Å/CrMnPt D Å/Ta 30 Å were made with various CrMnPt thicknesses indicated D. Shift fields of the hysteresis loop indicated Hp and coercivities indicated Hc were evaluated. With the increase of D from 150 to 1000 Å, the blocking temperature increases from RT to 340 °C. By increasing the temperature, the coercivity Hc increases, peaks at around the blocking temperature and then decreases. The temperature at which Hc peaks increases with an increase of D. The maximum value of Hc decreases with the increase of D. The grain size distribution of CrMnPt was measured and temperature dependencies of Hp and Hc were calculated based on a thermal fluctuation model using the grain size distribution. Quantitatively, the calculated results agree with the experimental results. The change of temperature dependence of Hp and Hc are explained by the change of the local blocking temperature distribution. With the increase of D, the mean blocking temperature becomes higher and the local blocking temperature shows narrower distributions. The anisotropy constant of the antiferromagnet Ka0, which is a fitting parameter, increases with the increase of D and saturates at above 400 Å.
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