J. Fassbender

Modified Gilbert damping due to exchange bias in NiFe∕FeMn bilayers

A picosecond all-optical pump-probe technique is used to investigate magnetization dynamics and magnetic damping behavior in the Ni81Fe19∕Fe50Mn50 exchange bias system. Within the laser excitation the spin temperature is elevated resulting in a collapse of the exchange bias field. Thus, a fast anisotropy field pulse is launched which triggers the magnetization precession. The extracted Gilbert damping increases linearly with the exchange bias field magnitude which can be understood by taking the local fluctuations of the interfacial exchange coupling as an additional dissipation mechanism into account.

Memory effect of Mn5Ge3 nanomagnets embedded inside a Mn-diluted Ge matrix

Crystalline Mn5Ge3 nanomagnets are formed inside a Mn-diluted Ge matrix using Mn ion implantation. A temperature-dependent memory effect and slow magnetic relaxation are observed below the superparamagnetic blocking temperature of Mn5Ge3. Our findings corroborate that the observed spin-glass-like features are caused by the size distribution of Mn5Ge3 nanomagnets, rather than by the interparticle interaction through the Mn-diluted Ge matrix.

Domain structure during magnetization reversal of PtMn/CoFe exchange bias micropatterned lines

The magnetic domain configuration and the magnetization reversal behavior of micropatterned exchange bias elements were investigated by means of magnetic force microscopy. In addition to the unidirectional anisotropy the shape anisotropy determines the overall magnetization reversal behavior. In order to modify the ratio between both anisotropy contributions, the exchange bias field strength was reduced by means of 5keV He+ ion irradiation. For the as-prepared samples, a monodomain magnetization state with the magnetization direction aligned along the exchange bias field direction was found regardless of the element shape. After irradiation the unidirectional anisotropy contribution is reduced and hence the previously homogeneous magnetization state breaks up into small domains with 360° domain walls in between. The appearance of these domain walls, which was mainly observed for the descending branch of the magnetization reversal, is found to depend strongly on the structure width and orientation.

Exchange bias on rippled substrates ¢?? step induced uniaxial versus unidirectional anisotropy

The angular dependence of the magnetization reversal curves reflect a complicated behavior due to the competition between uniaxial and unidirectional anisotropy contributions. Here we present an alternative route to study these phenomena by employing rippled substrates prepared by ion erosion. The magnetization reversal behavior of the F layer and the exchange bias system was then investigated by means of magneto-optic Kerr effect. A complete angular dependency of the magnetization reversal behavior has been investigated in both cases and compared to simulations based on a Stoner-Wohlfarth coherent rotation model.

Magnetic domain structure of micro-patterned PtMn/NiFe exchange bias bilayers

The magnetic domain structure of the micropatterned PtMn/NiFe exchange bias bilayers during magnetization reversal prepared by ion beam and physical vapor deposition were investigated. Using magneto-optic Kerr effect magnetometry (MOKE) and Kerr microscopy, the magnetic properties of the stripe array created using a 25 keV Ga/sup +/ focused ion beam (FIB) were also analyzed.

Small and large angle precession in exchange biased bilayers

Both small and large angle precession of a ferromagnetic layer upon photoexcitation can be modeled with reasonable values of the Gilbert parameter within the Landau-Lifshitz and Gilbert framework. Employing the known antiferromagnetic thickness dependence of the exchange bias field, the exchange bias field dependence of the Gilbert parameter is investigated. For this purpose, a wedge shaped exchange bias bilayer with a fixed thickness of the ferromagnetic layer and a varying thickness of the antiferromagnetic layer along the sample is prepared and measured. The extracted Gilbert parameter from the time-resolved Kerr traces, hence the dissipation rate, increases linearly with the exchange bias field magnitude. Local fluctuations of the interfacial coupling due to interface roughness can increase the two-magnon relaxation probability, which in terms of an additional dissipation channel finally leads to an increased Gilbert damping parameter.