arXiv subjects

Andrea Ehrmann

Publications and source records attributed to Andrea Ehrmann.

4 recordsLinked to original sources

arXiv2026

Stable Asymmetric Magnetization Reversal in Epitaxial Co(001)/CoO(001) Bilayer

The exchange bias (EB) in ferromagnetic/antiferromagnetic (FM/AFM) bilayer systems causes a shift of the magnetic hysteresis curve after field cooling through the N\'eel temperature of the AFM. In some cases, this shift is accompanied by an asymmetry between ascending and descending branches. In the past, this asymmetric magnetization reversal has been studied in different bilayer systems, including polycrystalline Co/CoO thin films. Here we investigate the asymmetric magnetization reversal in an epitaxial fcc-Co(001)/CoO(001) thin film grown on MgO(001) by molecular beam epitaxy at varying temperatures up to room temperature after field cooling along the easy and the hard axes. Room temperature measurements of the longitudinal and transverse magneto-optic Kerr effect show different magnetization reversal processes via stable intermediate states for different angles between external magnetic field and magnetic easy axes. Once the sample is cooled below the blocking temperature, a pronounced asymmetric magnetization reversal can be observed. We show that in contrast to polycrystalline bilayers, the loop asymmetry stays constant after multiple training cycles and that the magnitude of the asymmetry is directly correlated with the magnitude of the EB.

cond-mat.mtrl-sci↗

arXiv2026

Cubic magneto-optic Kerr effect in Co(111) thin films

The magneto-optic Kerr effect (MOKE) is often applied as a tool for the magnetic characterization of thin films. Here, the change in polarization upon reflection from the magnetized sample is mainly regarded as being linearly proportional to the magnetization $\mathbf{M}$ (LinMOKE). MOKE contributions of second order in $\mathbf{M}$, also known as quadratic MOKE (QMOKE), which are proportional to $\mathbf{M}^2$, have also been studied in the past and used in thin film characterization. Recently, we reported on a systematic investigation of third-order MOKE contributions, named cubic MOKE (CMOKE) in Ni(111) thin films. This CMOKE manifests itself as an anisotropic contribution to the MOKE signal (with regard to the crystallographic orientation) measured in longitudinal or transversal configuration in full magnetic saturation. While LinMOKE (odd in $\mathbf{M}$) and QMOKE (even in $\mathbf{M}$) can easily be separated by methods based on magnetization parity, this no longer holds true for LinMOKE and CMOKE (odd in $\mathbf{M}$). It is therefore crucial to be aware of CMOKE contributions in order to correctly interpret MOKE data. Here, we report on the observation of CMOKE in thin film heterostructures with structurally twinned and untwinned Co(111) layers, demonstrating that a large CMOKE is not only present in Ni thin films. Additionally, we show that the observed anisotropic contributions cannot stem from LinMOKE by analyzing their dependence on the angle of incidence (AoI) of light. While the QMOKE is almost vanishing in Co(111) using light with wavelengths of 635\,nm and 406\,nm, the CMOKE contributions reach up to about 30\% of the LinMOKE contribution at an AoI of 45 degrees and become even more dominant towards normal AoI, which emphasizes the importance of higher-order MOKE effects in magneto-optic experiments.

physics.optics↗

arXiv2017

Magnetization reversal in bent nanofibers of different cross-sections

Artificial ferromagnetic nanofiber networks with new electronic, magnetic, mechanical and other physical properties can be prepared by electrospinning and may be regarded as the base of bio-inspired cognitive computing units. For this purpose, it is necessary to examine all relevant physical parameters of such nanofiber networks. Due to the more or less random arrangement of the nanofibers and the possibility of gaining bent nanofibers in this production process, elementary single nanofibers with varying bending radii, from straight fibers to those bent along half-circles, were investigated by micromagnetic simulations, using different angles with respect to the external magnetic field. As expected from the high aspect ratios and the resulting strong shape anisotropy, all magnetization reversal processes took place via domain wall processes. Changing the cross-section from circular to a circle-segment or a rectangle significantly altered the coercive fields and its dependence on the bending radius, especially for the magnetic field oriented perpendicular (90 deg) to the fiber axes. In all three cross-sections, an angle of 45° between the fiber orientation and the external magnetic field resulted in the smallest influence of the bending radius. The shapes of the longitudinal and transverse hysteresis curves showed strong differences, depending on cross-section, bending radius and orientation to the magnetic field, often depicting distinct transverse magnetization peaks perpendicular to the fibers for fibers which were not completely oriented parallel to the magnetic field. Varying these parameters thus provides a broad spectrum of magnetization reversal processes in magnetic nanofibers and correspondingly scenarios for a variety of fiber-based information processing.

cond-mat.mtrl-sci↗

arXiv2012

Anatomy of Demagnetizing and Exchange Fields in Magnetic Nanodots Influenced by 3D Shape Modifications

Hysteresis loops of 3D ferromagnetic permalloy nano-half-balls (dots) with 100 nm base diameter have been examined by means of LLG micromagnetic simulations and finite element methods. Tests were carried out with two orthogonal directions of the externally applied field at 10 kA/(m.ns) field sweeping speed. The comparison of samples with different 3D modifications at the sub-10nm scale, accessible by nowadays lithographic techniques, enables conclusions about different mechanisms of competition between demagnetizing and exchange fields. Design paradigms provided here can find possible applications in magneto-electronic devices.

cond-mat.mes-hall↗