Size effects on the magnetization reversal behavior of exchange bias modulated thin films

Abstract

Thin films of NiFe/IrMn/Ta with spatially modulated unidirectional anisotropy have been prepared by local oxidation of the antiferromagnetic IrMn layer. Varying the lateral dimensions and orientation of the stripe structures with respect to the direction of exchange bias, the films exhibited different magnetization reversal behavior. While stripes aligned parallel to the unidirectional anisotropy direction display a two-step hysteresis loop, those with perpendicular orientation show a single-step, shifted hysteresis loop. Magnetic domain observation reveals separate switching of the magnetically different stripes for the parallel structures. For decreased lateral dimensions as well as perpendicularly aligned stripes quasidomain states are observed. The different magnetization reversal phenomena are discussed in terms of direct exchange coupling of the bimodal magnetic properties at the stripe interfaces.

Competing magnetic interactions in exchange-bias-modulated films

“…3͑e͔͒. During the remagnetization process, stripes of the same magnetic properties reverse in a correlated manner, forming so-called quasidomains 10,20 or hyperdomains. 21 However, the true vectorial magnetization orientation on the hysteresis step, i.e., the plateau region with oppositely stripe magnetization, is not easily determined from the longitudinal magnetization and domain images.…”

Magnetization Reversal in Ferromagnetic Films Patterned with Antiferromagnetic Gratings of Various Sizes

“…In comparison, 30-nm NiFe patterned with ð2-20Þ-μm IrMn stripes [18] shows only single-step reversal for exchange bias and field perpendicular to the stripes. The lack of two-step switching is attributed to the interstripe extension of domain walls and overlapping tails of the Néel walls.…”

“…For example, continuous NiFe films with overlaid FeMn stripes of micrometer-scale period showed asymmetrical hysteresis loops [16,17] suggesting the pinned and unpinned regions reverse at different fields. Exchange bias parallel to the stripes produced switching in two steps, whereas exchange bias perpendicular to the stripes produced switching in one step [18]. Understanding and manipulating the reversal process of a magnetic film with local exchange bias is, therefore, a key part of magnetic device design, providing, for example, the ability to trap a domain wall or to initiate reversal in a magnetic nanostructure.…”

“…This can be accomplished by ion-beam etching [16], but this is a nonselective etch which can also damage the FM [19]. Ion bombardment of an AF or FM bilayer through a resist mask can locally alter magnetic properties [14,15,17], for example, by oxidizing an AFM in select portions of the film [18]. However, the lateral resolution of this technique is limited by ion straggle, and it has been limited to introducing exchange bias into features larger than 1 μm.…”

Magnetization reversal in ferromagnetic wires patterned with antiferromagnetic gratings

“…[11][12][13][14] In general, exchange bias and the blocking temperature decrease with decreasing dimensions, which has been attributed to formation of spin glass-like regions with weaker exchange bias at the edges of the patterned features. 17,18 Structures in which the FM and AFM layer are patterned into different dimensions [20][21][22][23][24] are much less well studied, even though FM layers with local regions of exchange bias may be useful in controlling domain walls in racetrack memory and logic devices 23 and in magnetic bit encoders 25 and mangnonic crystals. 26,27 For example, the magnetic properties of a continuous FM film partly covered with patterned AFM regions depend on the dimensions of the AFM and the strength of the exchange bias within the pinned regions, as shown in NiFe/FeMn 24 and NiFe/IrMn, 28 and Co films with O implantation showed local EB due to the formation of CoO in the implanted regions.…”