Exchange paths were investigated for unidirectional exchange coupled 40 mn Nis,Fe,dSO mn NiO films by performing several field cooling experiments. Our experimental data were very consistent with the assumed existence of a variety of exchange paths. Each exchange path seemed to produce its own local unidirectional anisotropy and different local blocking temperature. The measureable exchange coupling could be described as consisting of the sum of the respective exchange paths, each with its own local blocking temperature. On the other hand, an observed blocking temperature of about 230 "C was determined from the exchange paths having the highest local blocking temperature. The local blocking temperatures were thought to be widely distributed, ranging from room temperature to about 230 "C, and the maximum existence probability was most likely at about 215 "C. This indicated that the exchange paths having the local blocking temperature of 215 "C made the largest contribution to the exchange coupling field at room temperature. According to cross sectional transmission electron microscopy observations, this variety of exchange paths was caused by inhomogeneous N&Fe,,-NiO interfaces associated with inter-facial disorder and fluctuating atomic arrangement.

Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19 film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19 film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature for tNiO<50 nm (tNiO:NiO film thickness) seems to originate from a decrease of antiferromagnetic NiO anisotropy with decreasing tNiO.

No abstract

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.

Exchange coupling between a ferromagnetic film and an antiferromagnetic NiO film was investigated. Bilayered ferromagnetic Ni81Fe19/antiferromagnetic NiO films had a large exchange coupling field and blocking temperature of about 200 °C. In trilayered Ni81Fe19/ferromagnetic (Ni81Fe19)100−xNbx/NiO films, a way could be developed to control the exchange coupling field at a small value by increasing the Nb of the intermediate film. The most important factor in the control seemed to be that the numbers of magnetic Fe and Ni atoms of (Ni81Fe19)100−xNbx, which contributed to the exchange coupling between (Ni81Fe19)100−xNbx and NiO, varied with the existence of nonmagnetic Nb at their interface. From experimental results with other trilayered Ni81Fe19/ferromagnetic (Ni100−xFex)93Nb7/NiO films, it was ascertained that the exchange coupling field seemed to be independent of the magnetic moment of the ferromagnetic film although unidirectional anisotropy constant was proportional to it. As for blocking temperature, it did not seem to be determined by the magnetism of the ferromagnetic material but the magnetism of the antiferromagnetic material.