arXiv subjects

Benjamin Zingsem

Publications and source records attributed to Benjamin Zingsem.

7 recordsLinked to original sources

arXiv2023

Influence of amorphous phase on coercivity in SmCo5-Cu nanocomposites

Severe plastic deformation of powder blends consisting of SmCo5-Cu results in magnetically hardened nanocomposite bulk materials. The microstructure is continuously refined with increasing torsional deformation, yet, coercivity saturates at a certain level of strain. Transmission electron microscopy (TEM) investigation of the microstructure reveals a partial amorphization of the SmCo5 phase due to high-pressure torsion by 20 applied rotations. In this amorphous matrix nanocrystals are embedded. The effect of these experimentally observed microstructural features on the magnetic properties are investigated by micromagnetic simulations, which show that an increasing volume fraction of nanocrystals is beneficial for higher coercivities. For a fixed volume fraction of nanocrystals the simulations reveal an increasing coercivity with decreasing the size of the nanocrystals due to increasing number of interfaces acting as pinning sites. Furthermore, our micromagnetic simulations disclose the mechanisms of the saturation and decline of magnetic hardening due to the strain induced by high-pressure torsion. The calculated coercivity fits very well to the experimentally observed coercivity of Hc=1.34 T. The knowledge can also be used to develop and provide optimization strategies from the microstructure perspective.

cond-mat.mtrl-sci↗

arXiv2023

Emergence of net magnetization by magnetic-field-biased diffusion in antiferromagnetic L1$_0$ NiMn

NiMn is a collinear antiferromagnet with high magneto crystalline anisotropy ($K_2=-9.7\times10^5\;\text{J m}^{-3}$). Through magnetic annealing of NiMn with excess Ni, strongly pinned magnetic moments emerge due to an imbalance in the distribution of Ni in the antiferromagnetic Mn-sublattices. The results are explained with a model of magnetic-field-biased diffusion, supported by ab initio calculations. Another observation is the oxidation of Mn at the surface, causing an enrichment of Ni in the sub-surface region. This leads to an additional ferromagnetic response appearing in the magnetization measurements, which can be removed by surface polishing.

cond-mat.mtrl-sci↗

arXiv2019

Imaging non-standing spin-waves in confined micron-sized ferromagnetic structures under uniform excitation

A non-standing character of directly imaged spin-waves in confined micron-sized ultrathin permalloy (Ni\textsubscript{80}Fe\textsubscript{20}) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations the presence of the spin-wave modes excited in the permalloy stripes along with the quasi-uniform modes were calculated. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing character of the spin-waves. Also it was shown by micromagnetic simulations and confirmed with STXM-FMR results that the observed character of the spin-waves can be influenced by the local magnetic fields in different sample geometries.

cond-mat.mes-hall↗

arXiv2019

Element-specific visualization of dynamic magnetic coupling in a Co/Py bilayer microstructure

We present the element-specific and time resolved visualization of uniform ferromagnetic resonance excitations of a Permalloy (Py) disk - Cobalt (Co) stripe bilayer microstructure. The transverse high frequency component of the resonantly excited magnetization is sampled in the ps regime by a combination of ferromagnetic resonance (FMR) and scanning transmission X-ray microscopy (STXM- FMR) recording snapshots of the local magnetization precession of Py and Co with nanometer spatial resolution. The approach allows us to individually image the resonant dynamic response of each element, and we find that angular momentum is transferred from the Py disk to the Co stripe and vice versa at their respective resonances. The integral (cavity) FMR spectrum of our sample shows an unexpected additional third resonance. This resonance is observed in the STXM-FMR experiments as well and our microscopic findings suggest that it is governed by magnetic exchange between Py and Co, showing for the Co stripe a difference in relative phase of the magnetization due to stray field influence.

cond-mat.mes-hall↗

arXiv2019

Unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling

We report strong unidirectional anisotropy in bulk polycrystalline B20 FeGe measured by ferromagnetic resonance spectroscopy. Bulk and micron-sized samples were produced and analytically characterized. FeGe is a B20 compound with inherent Dzyaloshinskii-Moriya interaction. Lorenz microscopy confirms a skyrmion lattice at $190 \; \text{K}$ in a magnetic field of 150 mT. Ferromagnetic resonance was measured at $276 \; \text{K} \pm 1 \; \text{K}$, near the Curie temperature. Two resonance modes were observed, both exhibit a unidirectional anisotropy of $K=1153 \; \text{J/m}^3 \pm 10 \; \text{J/m}^3$ in the primary, and $K=28 \; \text{J/m}^3 \pm 2 \; \text{J/m}^3$ in the secondary mode, previously unknown in bulk ferromagnets. Additionally, about 25 standing spin wave modes are observed inside a micron-sized FeGe wedge, measured at room temperature ($\sim \; 293$ K). These modes also exhibit unidirectional anisotropy.

cond-mat.mtrl-sci↗

arXiv2019

Evaluation protocol for revealing magnonic contrast in STXM-FMR measurements

We present a statistically motivated method to extract magnonic contrast from STXM-FMR measurement with microwave frequencies of the order of \unit[10]{GHz}. With this method it is possible to generate phase and amplitude profiles with a spatial resolution of about \unit[30]{nm} given by the STXM resolution, furthermore this method allows fo a rigoros transformation to reciprocal \vec{k}-space, revealing \vec{k}-dependent magnon properties.

cond-mat.str-el↗

arXiv2015

Enhanced and Tunable Spin-Orbit Coupling in Tetragonally Strained Fe-Co-B Films

We have synthesized 20 nm thick films of tetragonally strained interstitial Fe-Co-B alloys epitaxially grown on Au55Cu45 buffer layer. The strained axis is perpendicular to the film plane and the corresponding lattice constant c is enlarged with respect to the in-plane lattice parameter a. By adding the interstitial boron with different concentrations 0, 4, and 10 at.% we were able to stabilize the tetragonal strain in 20 nm Fe-Co films with different c/a ratios of 1.013, 1.034 and 1.02, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the orbital magnetic moment increases with increasing the c/a ratio, pointing towards the enhancement of spin-orbit coupling (SOC) at larger strain. Our results show that careful doping of ferromagnetic films allows to control the SOC by stabilizing anisotropic strain states. These findings are applicable in material design for spintronics applications. We also discuss the influence of B doping on the Fe-Co film microstructure, its magnetic properties and magnetic relaxation.

cond-mat.mtrl-sci↗