D. Engel

Suppression of exchange bias by ion irradiation

The exchange bias effect in ferromagnetic/antiferromagnetic sandwich structures is generally believed to be sensitive on the interface exchange interaction, the magnetization, and the thickness of the ferromagnetic layer. Also the interface structure plays a crucial role. We show that, by irradiating samples with He ions, we can manipulate the exchange bias field in a controlled manner. Depending on the dose (1014–1017 ions/cm2) and the acceleration voltage (10–35 kV) of the ions, the shift of the hysteresis can be reduced or even fully suppressed. Potential applications of this effect for magnetic patterning on the nanoscale will be discussed.

Magnetic micropatterning of FeNi/FeMn exchange bias bilayers by ion irradiation

Ion irradiation is an excellent tool to modify magnetic properties on the submicrometer scale, without modification of the sample topography. We utilize this effect to magnetically pattern exchange bias double layers using resist masks patterned by electron-beam lithography. Ion irradiation through the masks leads to a lateral modification of the magnetization reversal behavior and allows one to study the magnetization reversal as a function of the exchange bias field strength on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and magnetic force microscopy, respectively.

Asymmetric Magnetization Reversal of Stripe‐Patterned Exchange Bias Layer Systems for Controlled Magnetic Particle Transport

Domain wall movement assisted transport of particles: exchange-biased samples with designed stripe-domains show strong stray fields and an asymmetric magnetization reversal. Using these characteristics superparamagnetic particles can be trapped and transported directly on the sample over large-scale areas. High particle velocities, small external fields, and automatically reduced particle clustering allow broad applicability of this transport method.

On the origin of ion bombardment induced exchange bias modifications in polycrystalline layers

A model for the modifications of the exchange bias (EB) field of antiferromagnet (AF)/ferromagnet (F) bilayers with a polycrystalline or multidomain AF layer induced by ion bombardment (IB) in an external magnetic field is proposed. The model is based on a known two-energy level model for an antiferromagnetic grain or domain in contact with a ferromagnet where two free energy minima are separated by an energy barrier. The model explains the as yet unexplained increase of upon IB on the basis of the grain/domain size and magnetic anisotropy constants distributions in the antiferromagnetic layer after its deposition and on the basis of a twofold effect of the IB on the antiferromagnetic grains/domains: (1) IB acts like local hyperthermal heating leading to an almost immediate increase of the samples EB. (2) Defects induced by IB in the antiferromagnetic grains/domains lead to a decrease of the energy barrier between the two minima, resulting in a slow additional increase of with time (with temperature T as a parameter) after the bombardment. The model is tested by experiments on the time dependence of the EB and coercive fields after the IB of NiO/NiFe bilayers.

Controlled movement of superparamagnetic bead rows for microfluid mixing

The controlled movement of magnetic beads trapped on a surface in the moving inhomogeneous stray fields of moving domain walls between artificial domains of exchange bias layer systems has been applied for mixing of two aqueous fluids in a microfluidic device of small volume. The mixing of the two fluids can be considerably accelerated by transporting full rows of beads and use them as micro stirrers. The mixing speed in the current experiment is tripled in the first 6 min of mixing as compared to normal diffusion even for ratios of 250 between container height and bead diameter.

Magnetization Reversal of Exchange Bias Double Layers Magnetically Patterned by Ion Irradiation

He + ion irradiation is an excellent tool to modify the magnitude and direction of the exchange bias field on the sub-micrometer scale without affecting the sample topography. This effect has been utilized to magnetically pattern NiFe/FeMn exchange bias double layers using resist masks patterned by electron beam lithography. Ion irradiation through the masks leads to a local modification of the magnetization reversal behavior and allows to study the magnetization reversal as a function of the exchange bias field strength and the pattern dimensions on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and Lorentz microscopy.

Tuning exchange bias and coercive fields in ferromagnet/antiferromagnet bilayers with ion irradiation

The effect of He ion irradiation on the magnetic properties of NiFe exchange coupled to different antiferromagnetic alloys (FeMn, CrMn, and PtMn) with the same layer thickness is investigated. All systems exhibit an enhanced coercivity prior to irradiation. An exchange bias field is only observed for FeMn and PtMn. Upon ion irradiation the FeMn-based system shows with increasing ion dose an enhancement followed by a decrease and finally a full suppression of the exchange bias field. For systems exchange coupled to PtMn only a decrease and suppression of the bias field is found. This can be attributed to the ion induced chemical disordering of the antiferromagnetic phase in the latter case. In the case of CrMn the antiferromagnetic layer thickness is too small to induce an exchange bias field, but an enhanced coercivity is observed which is caused by the exchange coupling between the antiferromagnetic and ferromagnetic layers. For all systems, this enhanced coercivity of the exchange coupled bilayer system is modified by ion irradiation.

Domain propagation in He-ion-bombarded magnetic wires with opposite exchange bias

Exchange-biased IrMn/CoFe full films are magnetically structured with He-ion bombardment into stripes with antiparallel-aligned loop shift. The patterning results in a two-step magnetization loop corresponding to two regions of oppositely aligned exchange bias. The longitudinal magnetization reversal through head-on domain-wall motion and partial penetration of magnetization from neighboring strips is highly asymmetric involving ripplelike domain structures and incoherent rotation of magnetization. In addition, Neel-wall-like structures with a preferred sense of rotation are formed at the edges of the strips. Along the transverse direction the reversal is dominated by the switching of the magnetic border structures between the strips. Complicated domain patterns are generated under other external field angles.

Magnetic field induced transition from weak to strong ferromagnetic coupling in NiFe∕Au∕Co∕Au multilayers

We report on a specific magnetostatic coupling in sputter deposited (Ni80Fe20∕Au∕Co∕Au)10 multilayers of alternating in-plane and out-of-plane magnetic anisotropies. We demonstrate on the basis of complementary studies (magnetoresistance, conventional magnetometry, and element specific soft x-ray resonant magnetic scattering hysteresis measurements) that the magnetization reversal of the Ni–Fe layers is strongly influenced by a magnetostatic coupling originating from the out-of-plane stripe domain stray fields of the Co layers.

Exchange anisotropy modification in NiO/NiFe bilayers by ion bombardment

Abstract The dose dependence of the exchange bias field modification by He-ion bombardment in an applied magnetic field was investigated for an oxidic antiferromagnet/ferromagnet bilayer system consisting of NiO/NiFe in continuation of previous investigations for metallic antiferromagnet/ferromagnet systems (FeMn/NiFe, PtMn/NiFe, CrMn/NiFe). For low ion doses ( 15 ions / cm 2 ), in a magnetic field applied parallel to the as-grown exchange bias field direction, the exchange bias field H eb is enhanced compared to the as-grown field H eb,0 , i.e. H eb / H eb,0 >1. For higher doses, the relative exchange bias field H eb / H eb,0 is found to decrease gradually with increasing dose to roughly zero. Bombardment with low doses in a magnetic field applied antiparallel to the direction of H eb,0 results in magnitudes of | H eb / H eb,0 |>1 and reverses the sign of H eb . For higher ion doses H eb again approaches zero. These findings are similar to the findings for the bilayer system FeMn/NiFe, but differ to those for the systems PtMn/NiFe and CrMn/NiFe. The experimental results are compared with a simulation using the SRIM code.