arXiv subjects

Johanna Fischer

Publications and source records attributed to Johanna Fischer.

8 recordsLinked to original sources

arXiv2026

Spin Hall Magnetoresistance in Antiferromagnetic Insulators

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. We show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators alpha-Fe2O3 (hematite) and NiO in bilayer heterostructures with a Pt heavy-metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR effect allows to readout the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. We demonstrate that the SMR amplitude scales with the sum of the absolute sublattice magnetizations in ferrimagnetic and antiferromagnetic materials. In alpha-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of 2.5E-3, twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Design optimization of flux concentrators for magnetic tunnel junctions-based sensors

Miniaturized, ultra-sensitive and easily integrable magnetometers are needed for many applications, like space exploration or health monitoring. Achieving this goal requires a magnetic sensor with high sensitivity and low noise. High sensitivity (>1000 %/mT) can be obtained by integrating high gain permalloy flux concentrators (FC). And reducing the magnetic 1/f noise can be realized by increasing the number of magnetic tunnel junctions (MTJs) in the air-gap of the FC. However, this is obtained at the expense of a wider air-gap and consequently a decrease of the magnetic gain and thus of the sensitivity. In this paper, we explore a design optimization scheme in order to find the best trade-off between high FC gain and low magnetic noise. To model the gain of the flux concentrator, we propose two complementary approaches; one is based on finite elements simulations of the FC gain where the influence of geometrical parameters of the air-gap is investigated. Then, in a second step, we propose an analytical formula consistent with all our simulations results and based on magnetic reluctance. Finally, we derive an analytical model of the sensor detectivity from which we can extract the optimal sensor design which allows an improvement by three orders of magnitude of the performances compared to a single junction.

cond-mat.mes-hall↗

arXiv2025

Dzyaloshinskii-Moriya interaction chirality reversal with ferromagnetic thickness

In ultrathin ferromagnetic films sandwiched between two distinct heavy metal layers or between a heavy metal and an oxide layer, the Dzyaloshinskii-Moriya interaction (DMI) is of interfacial origin. Its chirality and strength are determined by the properties of the adjacent heavy metals and the degree of oxidation at the interfaces. Here, we demonstrate that the DMI chirality can change solely with variations in the thickness of the ferromagnetic layer - an effect that has not been experimentally studied in details or explained until now. Our experimental observation in the trilayer system Ta/FeCoB/TaOx is supported by ab initio calculations: they reveal that variations in orbital filling and inter-atomic distances at the interface, driven by the structural relaxations in the ultrathin regime, lead to an inversion of DMI chirality. We hence propose a new degree of freedom to tune DMI chirality and the associated chiral spin textures by tailoring crystal structure e.g. using strain or surface acoustic waves.

cond-mat.mtrl-sci↗

arXiv2025

Spin Hall magnetoresistance at the altermagnetic insulator/Pt interface

The resistance of a heavy metal can be modulated by an adjacent magnetic material through the combined effects of the spin Hall effect, inverse spin Hall effect, and dissipation of the spin accumulation at the interface. This phenomenon is known as the spin Hall magnetoresistance. The dissipation of the spin accumulation can occur via various mechanisms, with spin-transfer torque being the most extensively studied. In this work, we report the observation of spin Hall magnetoresistance at the interface between platinum and an insulating altermagnetic candidate, Ba$_2$CoGe$_2$O$_7$. Our findings reveal that this heterostructure exhibits a relatively large spin Hall magnetoresistance signal, which is anisotropic with respect to the crystal orientation of the current channel. We explore and rule out several potential explanations for this anisotropy and propose that our results may be understood in the context of the anisotropies of the spin current channels across the Pt/altermagnetic Ba$_2$CoGe$_2$O$_7$ interface.

cond-mat.mes-hall↗

arXiv2022

Gate-controlled skyrmion and domain wall chirality

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2 - 3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities.

cond-mat.mes-hall↗

arXiv2020

Spin Hall magnetoresistance in antiferromagnetic insulators

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. Here, we show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators $α$-Fe2O3 (hematite) and NiO in bilayer heterostructures with a Pt heavy metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR allows us to understand the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. Furthermore, in $α$-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of $2.5 \times 10^{-3}$, twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Large spin Hall magnetoresistance in antiferromagnetic α-Fe2O3/Pt heterostructures

We investigate the spin Hall magnetoresistance (SMR) at room temperature in thin film heterostructures of antiferromagnetic, insulating, (0001)-oriented alpha-Fe2O3 (hematite) and Pt. We measure their longitudinal and transverse resistivities while rotating an applied magnetic field of up to 17T in three orthogonal planes. For out-of-plane magnetotransport measurements, we find indications for a multidomain antiferromagnetic configuration whenever the field is aligned along the film normal. For in-plane field rotations, we clearly observe a sinusoidal resistivity oscillation characteristic for the SMR due to a coherent rotation of the Neel vector. The maximum SMR amplitude of 0.25% is, surprisingly, twice as high as for prototypical ferrimagnetic Y3Fe5O12/Pt heterostructures. The SMR effect saturates at much smaller magnetic fields than in comparable antiferromagnets, making the alpha-Fe2O3/Pt system particularly interesting for room-temperature antiferromagnetic spintronic applications.

cond-mat.mtrl-sci↗

arXiv2018

Spin Hall magnetoresistance in antiferromagnet/heavy-metal heterostructures

We investigate the spin Hall magnetoresistance in thin film bilayer heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO. While rotating an external magnetic field in the easy plane of NiO, we record the longitudinal and the transverse resistivity of the Pt layer and observe an amplitude modulation consistent with the spin Hall magnetoresistance. In comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by 90° and its amplitude strongly increases with the magnitude of the magnetic field. We explain the observed magnetic field-dependence of the spin Hall magnetoresistance in a comprehensive model taking into account magnetic field induced modifications of the domain structure in antiferromagnets. With this generic model we are further able to estimate the strength of the magnetoelastic coupling in antiferromagnets. Our detailed study shows that the spin Hall magnetoresistance is a versatile tool to investigate the magnetic spin structure as well as magnetoelastic effects, even in antiferromagnetic multidomain materials.

cond-mat.mes-hall↗