The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.

We report on interfacial contributions to the anisotropic magnetoresistance (AMR) in Co layers sandwiched between Pt. Utilizing the Fuchs-Sondheimer formalism interface contributions can be separated from bulklike AMR. We demonstrate that for all-metal systems interfacial AMR is also present when varying the magnetization within the film plane. This interfacial in-plane AMR is two times smaller than the contribution that arises when the magnetization is varied within the plane perpendicular to the current direction. This finding is in contrast to the spin Hall MR found for ferromagnetic insulator/Pt bilayers revealing the existence of different MR effects at the interfaces of Pt with conducting and insulating ferromagnets.

The structural properties and magnetic anisotropy of Pt/Co/Pt trilayers grown on thermally oxidized (Si/SiO2) and naturally oxidized silicon (Si/Siox) are presented. Although similar substrates and identical preparation conditions are used distinct differences in the structural composition are found which stem from the Pt seed layer created via ion assisted sputtering. While for thermal oxidized Si a Pt/Co/Pt trilayer is formed, for systems grown on naturally oxidized Si a complex PtSi alloy formation within the seed layer is observed as a consequence of the high ion energies of ion assisted sputtering. The composition of the PtSi alloy varies along the growth direction with a low Si content at the interface to Co and the lattice constant is similar to bulk Pt. The latter provides a much higher magnetic interface anisotropy constant compared to Pt/Co/Pt on thermal oxidized Si of about 0.9 mJ/m2 which is comparable to the highest values found for MBE grown Co on single crystalline Pt(111).

The dependence of the longitudinal thermoelectric power on the orientation of magnetization in Pt/Co/Pt sandwiches is investigated. In the Co thickness range <= 6 nm, where interface scattering is relevant, the thermoelectric power depends on the orientation of magnetization in the plane perpendicular to the temperature gradient. This behavior reveals the thermoelectric analog to the anisotropic interface magnetoresistance. It is shown that this interfacial magnetothermoelectric power fulfills Mott's formula, however, significant deviations from the bulk anisotropic magnetothermoelectric behavior are reported. The dependence of the Seebeck effect on magnetization orientation therefore provides experimental evidence of differences in the electronic states of the bulk and interface. We demonstrate that for the very same system the Seebeck coefficient does not show a one-to-one correspondence to the conductivity.

Wave transmission media created by a periodically modulated magnetic material are referred to as magnonic crystals [1]. The dynamics of magnonic crystals are described by common concepts of solid state physics, i.e., group velocity, density of states, and band structure. We investigate so-called magnonic vortex crystals created via rectangular arrangements of magnetic vortices. Here we aim at the control of vortex-core polarizations by perpendicularly aligned bias fields and use magnetic force microscopy and broadband-ferromagnetic transmission spectroscopy. In the first step, arrays of CoPt-multilayer disks arranged in a checkerboard pattern are prepared by electron-beam lithography, sputter deposition, and lift-off processing, compare figure 1(a). Two layer types A (Pt/0.7 nm Co/Pt) and B (Pt/2x(0.8 Co/1.1 nm Pt)/Pt) of the pattern differ in the magnetic anisotropy and thus yield different switching fields of the perpendicularly magnetized disks [2]. The switching fields are investigated by Kerr microscopy. After saturation in negative field direction, disks of type B start to switch at a field strength of μ0 H = +29 mT visible in the hysteresis loop shown in figure 1(b). The switching is completed at +37 mT where the greyscale intensity stays constant. A stable state of antiparallel magnetization of both types of disks persists up to +56 mT, where disks of type A start to switch. At +68 mT the magnetizations of disks of type A and B are aligned parallel with the positive field direction, compare figure 1(c). The switching of the multilayer disks follows a normal distribution and shows no dependence on the interdisk distance. In the next step, permalloy disks are prepared on top of the CoPt disks by electron-beam lithography, thermal evaporation, and lift-off processing. A thin Si interlayer is used to avoid direct contact of the perpendicularly and in-plane magnetized ferromagnets. The magnetization of t- e CoPt disks is adjusted using a perpendicularly aligned magnetic field. Subsequently an in-plane field is used to nucleate vortices in the permalloy disks. It is expected that due to stray field coupling the polarization of the vortex core in each permalloy disk is determined by the subjacent CoPt disk and coincides with the direction of the magnetization of the CoPt disk. The vortex-core polarizations are investigated using magnetic force microscopy (not shown).

We characterize the magnetic domain structure of Co/Pt multilayer films on length scales below one hundred nanometers using resonant magnetic scattering and magnetic force microscopy. The extreme ultraviolet light for the scattering experiment is created by a laser-based high-order harmonic generation source. After illumination with intense ultrashort infrared laser pulses, we observe pronounced changes in the magnetic structure and morphology. This study points out the importance of a detailed analysis of the different laser-induced modifications of a magnetic thin film that influence the scattering patterns.

In order to switch its direction, a macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier which is due to magnetic anisotropies in the material. The switching rate is usually described by a Neel-Brown-Arrhenius law, in which the energy barrier is a constant for a given temperature. However, magnetic anisotropies are in general depending on temperature. We take this temperature dependence into account and derive a generalized Neel-Brown theory for the switching rate. Importantly enough, this generalization has a tremendous effect on the physical interpretation of switching processes obtained in experiments on the basis of the conventional Arrhenius law. In particular, the assumption of a temperature-independent anisotropy may result in an overestimation of the attempt frequency by several orders of magnitude.

We study the magnetostatic interaction of submicron Ni-81 Fe-19 rectangles arranged in a linear chain by measuring the anisotropic magnetoresistance (AMR) of a single rectangle. The rectangles have a lateral aspect ratio of 2 : 1 and are lined up with the long axis oriented side by side varying the interelement distance down to 60 nm. The energy density of the Landau state is determined from the hard-axis magnetization reversal for a field applied along the chain direction. As a second approach identical energy densities are deduced from the switching field (Landau to quasisingle domain state) like in the case of a Stoner-Wohlfarth particle. The results show that the impact of the magnetostatic interaction on the energy density of the Landau state in remanence is negligibly small (<1 kJ/m(3)). The magnetostatic interaction between field-distorted Landau states, however, is the same as for rectangles in a single domain state and is therefore governed by the compensation of surface charges at the rim. By studying rectangles with only one neighbor, the important role of symmetry on the magnetostatic interaction is shown.

Received 15 April 2014Revised 24 June 2014DOI:https://doi.org/10.1103/PhysRevB.90.016402©2014 American Physical Society

We present results of single-shot resonant magnetic scattering experiments of Co/Pt multilayer systems using 100 fs long ultraintense pulses from an extreme ultraviolet (XUV) free-electron laser. An x-ray-induced breakdown of the resonant magnetic scattering channel during the pulse duration is observed at fluences of 5 J/cm(2). Simultaneously, the speckle contrast of the high-fluence scattering pattern is significantly reduced. We performed simulations of the nonequilibrium evolution of the Co/Pt multilayer system during the XUV pulse duration. We find that the electronic state of the sample is strongly perturbed during the first few femtoseconds of exposure leading to an ultrafast quenching of the resonant magnetic scattering mechanism.

We report on domain nucleation in nanowires consisting of Co/Pt multilayers with perpendicular magnetic anisotropy that are patterned by electron-beam lithography, sputter deposition, and lift-off processing. It is found that the nucleation field can be tuned by changing the geometry of the wire ends. A reduction of the nucleation field by up to 60% is achieved when the wire ends are designed as tips. This contrasts with the behavior of wires with in-plane anisotropy where the nucleation field increases when triangular-pointed ends are used. In order to clarify the origin of the reduction of the nucleation field, micromagnetic simulations are employed. The effect cannot be explained by the lateral geometrical variation but is attributable to a local reduction of the perpendicular anisotropy caused by shadowing effects due to the resist mask during sputter deposition of the multilayer.

We present a method to accurately determine the canting angle of magnetization in Co/Pt multilayers by utilizing magnetoresistance effects. In a current-in-plane geometry, the longitudinal voltage drop is determined as a function of the direction of an externally applied magnetic field. The field strength is sufficient to prevent domain decay. Measuring the change of resistance for two slightly differing field strengths allows the determination of the canting angle with high accuracy.

An endstation for pump-probe small-angle X-ray scattering (SAXS) experiments at the free-electron laser in Hamburg (FLASH) is presented. The endstation houses a solid-state absorber, optical incoupling for pump-probe experiments, time zero measurement, sample chamber, and detection unit. It can be used at all FLASH beamlines in the whole photon energy range offered by FLASH. The capabilities of the setup are demonstrated by showing the results of resonant magnetic SAXS measurements on cobalt-platinum multilayer samples grown on freestanding Si(3)N(4) membranes and pump-laser-induced grid structures in multilayer samples.

During ultrafast demagnetization of a magnetically ordered solid, angular momentum has to be transferred between the spins, electrons, and phonons in the system on femto-and picosecond timescales. Although the intrinsic spin-transfer mechanisms are intensely debated, additional extrinsic mechanisms arising due to nanoscale heterogeneity have only recently entered the discussion. Here we use femtosecond X-ray pulses from a free-electron laser to study thin film samples with magnetic domain patterns. We observe an infrared-pump-induced change of the spin structure within the domain walls on the sub-picosecond timescale. This domain-topography-dependent contribution connects the intrinsic demagnetization process in each domain with spin-transport processes across the domain walls, demonstrating the importance of spin-dependent electron transport between differently magnetized regions as an ultrafast demagnetization channel. This pathway exists independent from structural inhomogeneities such as chemical interfaces, and gives rise to an ultrafast spatially varying response to optical pump pulses.

It is shown in terms of a fully relativistic spin-polarized ab initio-type approach that in Pt/Co/Pt trilayers two types of anisotropic magnetoresistance (AMR) have to be distinguished: an in-plane and an out-of-plane AMR. The obtained results, namely the magnetic field dependence as well as the thickness dependence of both AMR types are in very good agreement with a very recent experimental study, in which the in-plane as well as the out-of-plane AMR was reported for this system. The difference between the two types of AMR is visualized in terms of layer-resolved resistivities. In particular, it is confirmed that the anisotropic interface magnetoresistance (AIMR) introduced in the recent publication mainly originates in the vicinity of the Co/Pt interfaces.

We report on an effect of reduced dimensionality on the magnetotransport in cobalt layers sandwiched by platinum. In a current in-plane geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current direction. The resistivity shows a symmetry adapted cos(2) dependence on the angle to the surface normal, with the maximum along the surface normal. The Co thickness dependence of the effect in Pt/Co/Pt sandwiches clearly points out that the mechanism behind this effect originates at the Co/Pt interfaces and is disparate to the texture induced geometrical size effect.