W. Jung

Mesoscopic thin-film magnetic rings (invited)

The magnetic properties and magnetoresistance of thin-film circular and elliptical magnetic rings made from Co, NiFe, NiFe∕FeMn, and Co∕Cu∕NiFe have been explored. Single-layer rings show stable onion and vortex states and metastable twisted states containing a 360° wall. For NiFe rings, four-point magnetotransport results can be explained quantitatively by anisotropic magnetoresistance. NiFe∕FeMn exchange-biased rings show offset hysteresis loops, and the easy axis is determined by a combination of the ring ellipticity and the exchange coupling. In Co∕Cu∕NiFe multilayer rings the behavior is dominated by the magnetostatic coupling between the domain walls in the Co and NiFe. In the major loop the giant magnetoresistance varies between three distinct levels corresponding to combinations of onion and vortex states in the NiFe and Co layers.

Multilayer magnetic antidot arrays from block copolymer templates

Antidot arrays (films with periodic arrays of holes) with periodicity of 26 or 40nm have been prepared from Co and Co∕Cu∕NiFe films using a block copolymer templating method. The magnetic properties of the antidot arrays differ from those of continuous films. The holes raise the coercivity of single-layer Co films and in the multilayers lead to an antiparallel alignment of the moments in the Co and NiFe layers at remanence, as a result of the strong magnetostatic interactions between the layers. These results are confirmed by micromagnetic modeling and the trend in coercivity is explained in terms of the interactions between the nanoscale holes and the domain walls in the films.

Magnetization reversal in single-layer and exchange-biased elliptical-ring arrays

Arrays of elliptical rings with long axis of 3μm, short axis of 1.8μm, and widths of 400nm and above were fabricated by zone-plate-array lithography and lift-off processing. Hysteresis loops of NiFe elliptical rings with different widths indicate the “vortex” state is more stable for narrower rings. Micromagnetic simulations reveal that the magnetization configurations of wider rings are more complicated than that of narrower ones. Elliptical-ring arrays fabricated from exchange-biased thin film structures display shifted hysteresis loops and the same width dependence of the stability of the vortex state.

Elliptical-ring magnetic arrays fabricated using zone-plate-array lithography

Zone-plate-array lithography and lift-off processing were employed to fabricate large arrays of elliptical-ring thin film magnets with widths of 600nm and above. An undercut profile was created using WiDE™ antireflection coating spun underneath a PFI-88 resist layer. The process allowed for up to 60-nm-thick sputtered magnetic multilayered structures. The magnetic properties of the elliptical ring arrays clearly show that the shape anisotropy induced by the ellipticity of the ring creates different magnetization reversal depending on the applied field direction. Magnetic force microscopy shows that the rings display magnetic states characteristic of ring structures, as well as sharp transitions between them. A fabrication process to produce magnetic memory prototypes based on these elliptical rings is presented.

Inhomogeneities in spin states and magnetization reversal of geometrically identical elongated Co rings

The magnetic configurations and magnetic reversal processes in arrays of geometrically identical rounded rectangular Co rings have been investigated. Magnetic imaging reveals a range of configurations, including diagonal onion, horseshoe onion, and vortex states. Reversal from the onion to the vortex state can occur via different routes involving domain wall motion within the rings, and the mechanism depends on the applied field orientation.

Influence of contact geometry on the magnetoresistance of elliptical rings

Room temperature magnetotransport measurements have been carried out on NiFe single layer and NiFe∕Cu∕Co∕Au multilayer elliptical rings. The shape of the magnetoresistance response is strongly dependent on the contact configuration and the direction of the applied field with respect to the easy axis of the ellipse. The magnetization states and magnetoresistance can be quantitatively modeled.

Current-in-plane magnetoresistance of spin valve elliptical rings

The giant magnetoresistance of NiFe∕Cu∕Co∕IrMn spin valve elliptical rings with 3.2∕1.9μm major/minor diameter and widths of 340–370nm has been characterized in a current-in-plane geometry. Spin valve rings show asymmetric magnetoresistance curves with three different resistance levels. Minor loop magnetoresistance measurements, which correspond to the switching of only the NiFe free layer, demonstrate that individual control of the chirality of the magnetization vortex in each ferromagnetic layer is possible, enabling at least 16 distinct magnetic configurations to be formed in a spin valve ring.

Angular dependence of the giant magnetoresistance in multilayer rings with different contact configurations

We report on the effects of electrical contact configuration on the giant magnetoresistance (GMR) response of NiFe∕Cu∕Co pseudo-spin-valve elliptical rings. Room temperature GMR measurements show different approaches to remanence, depending on the position of the voltage leads with respect to the curvature of the rings. Computational analysis of micromagnetic simulations reproduces these experimental trends, which result from changes in the relative alignment between the NiFe and Co layers in different regions of the rings.

Spin waves in exchange-biased NiFe∕IrMn circular nanorings

We present the experimental investigation of the static and dynamical properties of NiFe(20nm)∕IrMn(5nm) rings. Magneto-optic Kerr effect measurements show that the IrMn introduces a sizable exchange-bias field HEB≈80Oe, which shifts the hysteresis loop along the excange-bias direction. Measured loops were satisfactorily reproduced using a micromagnetic calculation which includes the presence of HEB. Measurement of high frequency normal modes, by Brillouin light scattering, shows that the presence of the IrMn layer causes an overall up-shift of the spin wave frequency, in qualitative agreement with pervious studies of other exchange-biased systems. Moreover, the frequency asymmetry of the spin wave frequency upon reversing the external field is quantitatively consistent with the presence of the exchange-bias field measured by the Kerr effect.

Soft x-ray resonant magnetic scattering investigation of stable magnetic configurations in patterned rings

The first experimental results of soft x-ray resonant magnetic scattering from a pattern of microscopic Permalloy rings are reported. Experimental measurements and simulations of the scattering from the stable magnetic configurations such as “vortex” and “onion” commonly observed for the ring structures are presented. Variations in the diffraction profile as a function of field are discussed in the context of the simulation results, which show a good agreement with the behavior of the experimental data.