A granular level model of the magnetic properties of coupled ferromagnetic/antiferromagnetic layers is used to calculate the temperature dependence of the exchange bias. The predicted results are in good qualitative agreement with experiment. Agreement with experiment requires the introduction of the temperature dependence of the auisotropy constant of the antiferromagnetic layer.
The magnetization reversal in square lattice cobalt antidot arrays with the applied field at 45° to the anti dot rows was investigated using Lorentz electron microscopy in the Fresnel mode. While the hysteresis loops from magneto-optical Kerr effect measurements only reflect the easy axis character of the reversal, several different reversal processes were identified in the Fresnel images depending on the field history. Details of this complex magnetization reversal were elucidated with micromagnetic simulations.When a regular array of holes is introduced into a continuous ferromagnetic thin film, the magnetic properties are significantly changed. I -5 In such ferromagnetic antidot arrays, domain wall behavior and interactions can be investigated, topics which are currently of interest for future magnetic devices based on the domain walls. 6 ,7 The basic domain configuration for square lattice antidot arrays with antidot size"'" antidot separation is shown in the schematic diagrams I to 3 in Fig. 3. This configuration results from the fact that the spins adjacent to the antidots align parallel to the antidot borders in order to reduce the stray field energy and is observed with photoemission electron microscopy (PEEM) as a periodic checked domain contrast commensurate with the antidot lattice. 8 The hysteresis loops with the applied field along the hard and easy axes, i.e., parallel and at 45° to the antidot rows (see schematic diagram in Fig. 1), have been measured and the basic domain states have been observed. 1,5 However, very little is known about the details of magnetization reversal processes. Recently, we showed that on application of an in-plane magnetic field parallel to the antidot columns, magnetization reversal occurs by nucleation and propagation of chains of magnetic domains which have discrete lengths corresponding to multiples of the antidot period. 9 In addition, when the ends of two orthogonal do- main chains coincide, a stable domain wall configuration is formed giving a pinning center for propagating domain walls. Depending on the film thickness, this domain wall configuration comprises either a 180° wall or four 90° walls forming an antivortex. 9 ,10 Here, we report on the magnetization reversal in square lattice cobalt antidot arrays on application of a magnetic field at an angle of 45° to the antidot rows.Cobalt antidot arrays were fabricated by electron beam lithography. For observations with Lorentz electron microscopy, it was necessary to place the antidot arrays on substrates which allow the transmission of electrons. We therefore used silicon nitride (Si 3 N 4 ) membrane chips with 500 X 500 f.J.,m 2 membrane windows which are 50-nm-thick (from Silson Ltd., UK). It was not possible to use lift-off for pattern transfer since this method is assisted with an ultrasound bath which will break the fragile membranes. Instead, arrays of holes were created in the Si3N4 membranes using electron beam lithography in combination with reactive ion etching (RIE) processes and the cobalt thin...
Transmission electron microscopy ͑TEM͒ has been used to study magnetization processes in four high moment CoFe films. While all films were of similar total thickness, 50 nm, the differences between them were the inclusion or otherwise of a seed layer and the introduction of nonmagnetic spacers to form laminated films. The detailed reversal mechanism for easy and hard axis reversals of each film was investigated. As expected cross-tie walls were observed in the films with thicker CoFe layers and wall displacements between layers were seen with the introduction of one or more spacer layers. Magnetization dispersion was reduced as multilayering was introduced. In the laminated film with three spacer layers, defect areas where the local magnetization distribution differed markedly from the surrounding film were observed. Cross-sectional TEM showed that layer roughness increased through the stack and this was the probable cause of the localized magnetic anomalies.
Electron microscopy has been used to determine directly the effect of artificially introduced roughness on the micromagnetic processes that occur in soft high moment multilayer films. The nanodefects, introduced to roughen the substrate, and the local magnetic domain structure were identified in the same images, leading to unambiguous information on the role played by the former. Characteristic of the micromagnetic state of the samples was a high density of 360° wall segments that showed quite remarkable resistance to annihilation. Following a description of the domain walls generated in both easy and hard axis magnetization cycles, a model is proposed for the way the observed domain walls respond when their ends are strongly pinned and, using this, we account for their continued existence over an extended number of magnetization cycles. Finally, the implications for device performance are discussed briefly.
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