arXiv subjects

Julie Karel

Publications and source records attributed to Julie Karel.

13 recordsLinked to original sources

arXiv2026

Omnidirectional magnetic imaging of magnetic anisotropy and phase transitions

Micron scale imaging of magnetic fields is an important tool for understanding the evolution of magnetism through phase transitions and as a result of interactions inside of heterostructures. However, most imaging platforms, like the nitrogen-vacancy (NV) centre in diamond, are restricted to applying magnetic fields along the quantisation axis of the quantum sensor. This greatly restricts the utility of these systems for exploring materials that emit strong fields or exhibit variable response with respect to the applied field direction. Here we explore an alternative approach using weakly coupled spin-pairs in hBN that exhibit a spin-1/2-like behaviour and an isotropic response to magnetic field. We demonstrate that the spin-pair system can operate in the presence of strong fields from a thin film magnet which were incompatible with NV diamond imaging even with applied fields along the quantisation axis. Further, we demonstrate that using this platform allows for imaging with an arbitrary applied magnetic field direction, allowing us to probe the anisotropy and spin-reorientation transition in the ferrimagnet TbMn$_6$Sn$_6$. Finally, we propose an improved geometry for imaging small anisotropy contributions such as crystalline anisotropy. These results demonstrate how this or similar spin-1/2 systems might be used for imaging magnetic materials that are incompatible with other techniques despite the reduction in sensitivity compared with NV in diamond imaging.

cond-mat.mes-hall↗

arXiv2025

Room temperature quantum metric effect in TbMn6Sn6

Quantum geometry, including Berry curvature and the quantum metric, of the electronic Bloch bands has been studied via nonlinear responses in topological materials. Naturally, these material systems with intrinsic strong nonlinear responses also form the key component in nonlinear electronic devices. However, the previous reported quantum geometry effects are mainly observed at cryogenic temperatures, hindering their application in practical devices. Here we report the tuneable strong room-temperature second-harmonic transport response in a quantum magnet, TbMn6Sn6, which is governed by the quantum metric and can be tuned with applied magnetic fields. We show that around room temperature, which is close to the spontaneous spin-reorientation transition, the magnetic configurations, and therefore the related symmetry breaking phases, are easily controlled via magnetic fields. Our results also show that manipulation of the symmetries of the magnetic structure presents an effective route to tuneable quantum-geometry-based devices.

cond-mat.mtrl-sci↗

arXiv2023

Magnetotransport and Berry phase Tuning in Gd-doped Bi2Se3 Topological Insulator Single Crystals

The Berry phase is an important concept in solids, correlated to the band topology, axion electrodynamics and potential applications of topological materials. Here, we investigate the magnetotransport and Berry phase of rare earth element Gd doped Bi2Se3 (Gd_Bi2Se3) topological insulator at low temperatures and high magnetic fields. Gd_Bi2Se3 single crystals show Shubnikov-de Haas (SdH) oscillations with nontrivial Berry phase while Bi2Se3 single crystals show zero Berry phase in SdH oscillations. The temperature dependent magnetization curves can be well fitted with the Curie-Weiss law in 3-300 K region, indicating no magnetic ordering in Gd_Bi2Se3 crystals. Moreover, Gd doping has limited influence on the quantum oscillation parameters (e.g., frequency of oscillations, the area of the Fermi surface, effective electron mass, Fermi wave vectors etc.), but has an impact on the Hall mobility, carrier density, and band topology. Our results demonstrate that Gd doping can tune the Berry phase of topological insulators effectively, which may pave a way for the future realization of many predicted exotic transport phenomena of topological origin.

cond-mat.mtrl-sci↗

arXiv2023

Coexistence of Logarithmic and SdH Quantum Oscillations in Ferromagnetic Cr-doped Tellurium Single Crystals

We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr_Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (< 3.8 K) and low field (< 0.15 T) region, and high Hall mobility, e.g., 1320 cm2 V-1 s-1 at 30 K and 350 cm2 V-1 s-1 at 300 K, implying that Cr_Te crystals are ferromagnetic elemental semiconductors. When B // c // I, the maximum negative MR is -27% at T = 20 K and B = 8 T. In the low temperature semiconducting region, Cr_Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic field B is parallel to the [100] crystallographic direction and show Landau quantization dominated Shubnikov-de Haas (SdH) oscillations for B // [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr_Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena.

cond-mat.mtrl-sci↗

arXiv2023

Antiferromagnetic topological insulating state in Tb$_{0.02}$Bi$_{1.08}$Sb$_{0.9}$Te$_2$S single crystals

Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantized anomalous Hall insulator, which has been observed inmagnetic-topological-insulator-based devices. In this work, we report a successful doping of rare earth element Tb into Bi$_{1.08}$Sb$_{0.9}$Te$_2$S topological insulator single crystals, in which the Tb moments are antiferromagnetically ordered below ~10 K. Benefiting from the in-bulk-gap Fermi level, transport behavior dominant by the topological surface states is observed below ~ 150 K. At low temperatures, strong Shubnikov-de Haas oscillations are observed, which exhibit 2D-like behavior. The topological insulator with long range magnetic ordering in rare earth doped Bi$_{1.08}$Sb$_{0.9}$Te$_2$S single crystal provides an ideal platform for quantum transport studies and potential applications.

cond-mat.mtrl-sci↗

arXiv2022

Increased Phase Coherence Length in a Porous Topological Insulator

The surface area of Bi2Te3 thin films was increased by introducing nanoscale porosity. Temperature dependent resistivity and magnetotransport measurements were conducted both on as-grown and porous samples (23 and 70 nm). The longitudinal resistivity of the porous samples became more metallic, indicating the increased surface area resulted in transport that was more surface-like. Weak antilocalization (WAL) was present in all samples, and remarkably the phase coherence length doubled in the porous samples. This increase is likely due to the large Fermi velocity of the Dirac surface states. Our results show that the introduction of nanoporosity does not destroy the topological surface states but rather enhances them, making these nanostructured materials promising for low energy electronics, spintronics and thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2022

Hot 2DHG states in tellurium

Element semiconductor Te is very popular in both fundamental electronic structure study, and device fabrication research area due to its unique band structure. Specifically, in low temperatures, Te possesses strong quantum oscillations with magnetic field applied in basal plane, either following Shubnikov-de Haas (SdH) oscillation rule or following log-periodic oscillation rule. With magnetic field applied along the [001] direction, the SdH oscillations are attributed to the two-dimensional hole gas (2DHG) surface states. Here we reported an interesting SdH oscillation in Te-based single crystals, with the magnetic field applied along the [001] direction of the crystals, showing the maximum oscillation intensity at ~ 75 K, and still traceable at 200 K, which indicates a rather hot 2DHG state. The nontrivial Berry phase can be also obtained from the oscillations, implying the contribution from topological states. More importantly, the high temperature SdH oscillation phenomena are observed in different Te single crystals samples, and Te single crystals with nonmagnetic/magnetic dopants, showing robustness to bulk defects. Therefore, the oscillation may be contributed by the bulk symmetry protected hot 2DHG states, which will offer a new platform for high-temperature quantum transport studies.

cond-mat.mtrl-sci↗

arXiv2021

Formation of a stable surface oxide in MnBi$_2$Te$_4$ thin films

Understanding the air-stability of MnBi$_2$Te$_4$ thin films is crucial for the development and long-term operation of electronic devices based around magnetic topological insulators. In the present work, we study MnBi$_2$Te$_4$ thin films upon exposure to atmosphere using a combination of synchrotron-based photoelectron spectroscopy, room temperature electrical transport and atomic force microscopy to determine the oxidation process. After 2 days air exposure a 2 nm thick oxide passivates the surface, corresponding to oxidation of only the top two surface layers, with the underlying layers preserved. This protective oxide layer results in samples that still exhibit metallic conduction even after several days air exposure. Furthermore, the work function decreases from 4.4 eV for pristine MnBi$_2$Te$_4$ to 4.0 eV after the formation of the oxide, along with only a small shift in the core levels indicating minimal doping as a result of air exposure. With the oxide confined to the top surface layers, and the underlying layers preserved, it may be possible to explore new avenues in how to handle, prepare and passivate future MnBi$_2$Te$_4$ devices.

cond-mat.mtrl-sci↗

arXiv2020

Recent Progress in Proximity Coupling of Magnetism to Topological Insulators

Inducing long-range magnetic order in three-dimensional topological insulators can gap the Diraclike metallic surface states, leading to exotic new phases such as the quantum anomalous Hall effect or the axion insulator state. These magnetic topological phases can host robust, dissipationless charge and spin currents or unique magnetoelectric behavior, which can be exploited in low-energy electronics and spintronics applications. Although several different strategies have been successfully implemented to realize these states, to date these phenomena have been confined to temperatures below a few Kelvin. In this review, we focus on one strategy, inducing magnetic order in topological insulators by proximity of magnetic materials, which has the capability for room temperature operation, unlocking the potential of magnetic topological phases for applications. We discuss the unique advantages of this strategy, the important physical mechanisms facilitating magnetic proximity effect, and the recent progress to achieve, understand, and harness proximity-coupled magnetic order in topological insulators. We also highlight some emerging new phenomena and applications enabled by proximity coupling of magnetism and topological materials, such as skyrmions and the topological Hall effect, and we conclude with an outlook on remaining challenges and opportunities in the field.

cond-mat.mes-hall↗

arXiv2020

Spin-orbit torque generated by amorphous Fe$_{x}$Si$_{1-x}$

While tremendous work has gone into spin-orbit torque and spin current generation, charge-to-spin conversion efficiency remains weak in silicon to date, generally stemming from the low spin-orbit coupling (low atomic number, Z) and lack of bulk lattice inversion symmetry breaking. Here we report the observation of spin-orbit torque in an amorphous, non-ferromagnetic Fe$_{x}$Si$_{1-x}$ / cobalt bilayer at room temperature, using spin torque ferromagnetic resonance and harmonic Hall measurements. Both techniques provide a minimum spin torque efficiency of about 3 %, comparable to prototypical heavy metals such as Pt or Ta. According to the conventional theory of the spin Hall effect, a spin current in an amorphous material is not expected to have any substantial contribution from the electronic bandstructure. This, combined with the fact that Fe$_{x}$Si$_{1-x}$ does not contain any high-Z element, paves a new avenue for understanding the underlying physics of spin-orbit interaction and opens up a new class of material systems - silicides - that is directly compatible with complementary metal-oxide-semiconductor (CMOS) processes for integrated spintronics applications.

cond-mat.mes-hall↗

arXiv2016

Magnetic properties and Curie temperatures of disordered Heusler compounds: Co(1+x)Fe(2-x)Si

The local atomic environments and magnetic properties were investigated for a series of Co(1+x)Fe(2-x)Si (0<x<1) Heusler compounds. While the total magnetic moment in these compounds increases with the number of valance electrons, the highest Curie temperature (Tc) in this series was found for Co1.5Fe1.5Si, with a Tc of 1069 K (24 K higher than the well known Co2FeSi). 57Fe Mössbauer spectroscopy was used to characterize the local atomic order and to estimate the Co and Fe magnetic moments. Consideration of the local magnetic moments and the exchange integrals is necessary to understand the trend in Tc.

cond-mat.mtrl-sci↗

arXiv2016

Large anomalous Hall effect driven by non-vanishing Berry curvature in non-collinear antiferromagnetic Mn3Ge

It is well established that the anomalous Hall effect that a ferromagnet displays scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. Here we show that the non-collinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is 500 per ohm per cm at 2 K and 50 per ohm per cm at room temperature. The angular dependence of the anomalous Hall effect measurements confirm that the small residual in-plane magnetic moment has no role in the observed effect. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a non-vanishing Berry curvature that arises from the chiral spin structure, and which also results in a large spin Hall effect, comparable to that of platinum. The present results pave the way to realize room temperature antiferromagnetic spintronics and spin Hall effect based data storage devices.

cond-mat.str-el↗

arXiv2015

Development of hard x-ray photoelectron SPLEED-based spectrometer applicable for probing of buried magnetic layer valence states

A novel design of high-voltage compatible polarimeter for spin-resolved hard x-ray photoelectron spectroscopy (Spin-HAXPES) went into operation at beamline BL09XU of SPring-8 in Hyogo, Japan. The detector is based on the well-established principle of electron diffraction from a W(001) single-crystal at a scattering energy of 103.5 eV. Its special feature is that it can be operated at a high negative bias potential up to 10 kV, necessary to access the HAXPES range. The polarimeter is operated behind a large hemispherical analyzer (Scienta R-4000). It was optimized for high transmission of the transfer optics. The exit plane of the analyzer contains a delay-line detector (20 mm dia.) for conventional multichannel intensity spectroscopy simultaneously with single-channel spin analysis. The performance of the combined setup is demonstrated by the first spin-resolved data for the valence-region of a FeCo functional layer of a tunneling device, buried beneath 3 nm of oxidic material. The well-structured spin polarization spectrum validates Spin-HAXPES in the valence energy range as powerful method for bulk electronic structure analysis. The spin polarization spectrum exhibits a rich structure, originating from clearly discernible transitions in the majority and minority partial spin spectra.

physics.ins-det↗