arXiv subjects

Peter Bencok

Publications and source records attributed to Peter Bencok.

13 recordsLinked to original sources

arXiv2026

Semi-Dirac States and Quantum Linear Magnetoresistance in Helimagnetic Pnictide MnP

Large linear positive magnetoresistance (LPMR) in topological and magnetic materials remains a subject of intense debate, particularly in noncollinear spin systems where spin-dependent scattering complicates charge transport. Manganese phosphide (MnP), a helimagnetic binary pnictide with multiple field-induced magnetic transitions, provides a useful platform to investigate the interplay between complex magnetism and electronic topology. Here, we present a comprehensive experimental and theoretical investigation of phase-dependent magnetotransport in high-quality MnP single crystals. Hall measurements reveal an anomalous Hall effect dominated by skew scattering at high temperatures and a finite topological Hall effect in the noncollinear fan (FAN) and low-temperature screw (SCR) phases. At low temperatures, we observe a large, non-saturating LPMR reaching nearly 800 percent at 4 K and 15 T, with a pronounced linear field dependence in the field-polarized ferromagnetic (FM2) state. First-principles calculations reveal a strongly anisotropic semi-Dirac-like band at the Y point that progressively approaches the Fermi level from the SCR to FAN and FM2 states. Our analysis indicates that the resulting small Fermi pocket can access the extreme quantum-limit regime at experimentally accessible fields, providing a microscopic framework for the observed LPMR within Abrikosov's quantum magnetoresistance theory.

cond-mat.str-el↗

arXiv2026

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways.

cond-mat.mtrl-sci↗

arXiv2026

Revealing the origin of XMCD in an altermagnet via three-dimensional control of spins

Altermagnets are an emerging class of collinear antiferromagnets that exhibit unconventional spin-polarised electronic bands, potentially unlocking new functionalities that do not rely on spin-orbit coupling (SOC). Experimental signatures traditionally associated with spin polarisation, like X-ray magnetic circular dichroism (XMCD), are thus being used as a validation of altermagnetism. However, unlike altermagnetic spin-splitting, these responses require SOC and are not invariant under spin-space rotations. This brings into question the extent to which they can be considered direct signatures of altermagnetism. Here, we exploit the g-wave altermagnet $\alpha$-Fe$_{2}$O$_{3}$ to demonstrate that XMCD is governed precisely by the spin-direction-induced symmetry breaking that altermagnetic spin groups are designed to ignore. Strikingly, the XMCD is highly anisotropic and is decoupled from the weak magnetic canting. We show that this anomalous XMCD can be described by on-site Faraday tensors capturing the locally uncompensated spin-orbital anisotropies - a scenario that can be applied to other altermagnets. Leveraging this, we reconstruct complete vectorial maps of nanoscale textures in $\alpha$-Fe$_{2}$O$_{3}$ thin films, including domain walls and topological solitons, which are promising for building future spintronics and magnonics devices.

cond-mat.mtrl-sci↗

arXiv2026

The Evolution of Magnetism in a Thin Film Pyrochlore Ferromagnetic Insulator

The pyrochlore vanadates are compelling candidates for next-generation dissipationless devices. Lu2V2O7 and Y2V2O7 are ferromagnetic insulators (Tc ~ 70 K) that are believed to exhibit the magnon Hall effect and are expected to host topological magnons. Their completely dissipationless magnon edge states could be harnessed to realize low-power information transport in spintronic or magnonic devices. As a crucial step in the realization of devices, we synthesize the first thin films of pyrochlore Y2V2O7 on isostructural Y2Ti2O7 substrates and explore the evolution of their magnetic properties down to the ultrathin limit. All films are insulating ferromagnets with transition temperatures of up to the bulk value (Tc ~ 68 K) that decrease with thickness according to finite-size effects. Our films also exhibit a change in anisotropy from in-plane to out-of-plane easy axis coincident with the development of partial strain relaxation and nonzero magnetic hysteresis in an applied field. This evolution demonstrates the impact of strain on magnetic anisotropy and paves the way to tunable magnon topology.

cond-mat.mtrl-sci↗

arXiv2024

From ferromagnetic semiconductor to anti-ferromagnetic metal in epitaxial Cr$_x$Te$_y$ monolayers

Chromium ditelluride, CrTe$_2$, is an attractive candidate van der Waals material for hosting 2D magnetism. However, how the room-temperature ferromagnetism of the bulk evolves as the sample is thinned to the single-layer limit has proved controversial. This, in part, reflects its metastable nature, vs. a series of more stable self-intercalation compounds with higher relative Cr:Te stoichiometry. Here, exploiting a recently-developed method for enhancing nucleation in molecular beam epitaxy growth of transition-metal chalcogenides, we demonstrate the selective stabilisation of high-coverage CrTe$_2$ and Cr$_{2+\varepsilon}$Te$_3$ epitaxial monolayers. Combining X-ray magnetic circular dichroism, scanning tunnelling microscopy, and temperature-dependent angle-resolved photoemission, we demonstrate that both compounds order magnetically with a similar Tc. We find, however, that monolayer CrTe$_2$ forms as an anti-ferromagnetic metal, while monolayer Cr$_{2+\varepsilon}$Te$_3$ hosts an intrinsic ferromagnetic semiconducting state. This work thus demonstrates that control over the self-intercalation of metastable Cr-based chalcogenides provides a powerful route for tuning both their metallicity and magnetic structure, establishing the Cr-Te system as a flexible materials class for future 2D spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Covalency, correlations, and inter-layer interactions governing the magnetic and electronic structure of Mn$_3$Si$_2$Te$_6$

Mn$_3$Si$_2$Te$_6$ is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations, to probe the elemental-selective electronic structure and magnetic order in Mn$_3$Si$_2$Te$_6$. Through these, we identify a marked Mn-Te hybridisation, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in Mn$_3$Si$_2$Te$_6$, which is key to stabilising its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbours and anti-symmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilising the ferrimagnetic order in Mn$_3$Si$_2$Te$_6$.

cond-mat.mtrl-sci↗

arXiv2022

Temperature-Dependent Dynamic Disproportionation in LiNiO$_2$

Nickelate materials offer diverse functionalities for energy and computing applications. Lithium nickel oxide (LiNiO$_2$) is an archetypal layered nickelate, but the electronic structure of this correlated material is not yet fully understood. Here we investigate the temperature-dependent speciation and spin dynamics of Ni ions in LiNiO$_2$. Our ab initio simulations predict that Ni ions disproportionate into three states, which dynamically interconvert and whose populations vary with temperature. These predictions are verified using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and resonant inelastic x-ray scattering at the Ni L$_{3,2}$-edge. Charge-transfer multiplet calculations consistent with disproportionation reproduce all experimental features. Together, our experimental and computational results support a model of dynamic disproportionation that explains diverse physical observations of LiNiO$_2$, including magnetometry, thermally activated electronic conduction, diffractometry, core-level spectroscopies, and the stability of ubiquitous antisite defects. This unified understanding of the fundamental material properties of LiNiO$_2$ is important for applications of nickelate materials as battery cathodes, catalysts, and superconductors.

cond-mat.mtrl-sci↗

arXiv2022

A Detailed Investigation of the Onion Structure of Exchanged Coupled Magnetic Fe(3-delta)O4@CoFe2O4@Fe(3-delta)O4 Nanoparticles

Nanoparticles (NPs) which combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide NPs which hampers potential applications. In this manuscript, we report on the design of NPs with an onion-like structure which have been scarcely reported yet. These NPs consist in a Fe(3_delta)O4 core covered by a first shell of CoFe2O4 and a second shell of Fe(3-delta)O4. They were synthesized by a multi-step seed mediated growth approach. Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increase. We investigated very precisely the structure of NPs in performing high resolution (scanning) TEM imaging and GPA. The chemical composition and spatial distribution of atoms were studied by EELS. The chemical environment and oxidation state of cations were investigated by Mössbauer spectrometry, soft XAS and XMCD. These techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, while the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe(3-delta)O4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell NPs.

cond-mat.mtrl-sci↗

arXiv2020

Incipient antiferromagnetism in the Eu-doped topological insulator Bi$_2$Te$_3$

Rare earth ions typically exhibit larger magnetic moments than transition metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet, in a recent photoemission study of Eu-doped Bi$_2$Te$_3$ films, the spectra remained gapless down to $T = 20\;\text{K}$. Here, we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the $M_{4,5}$ x-ray absorption and magnetic circular dichroism spectra reveals a Eu$^{2+}$ valence and confirms a large magnetic moment, consistent with a $4f^7 \; {^8}S_{7/2}$ ground state. At temperatures below $10\;\text{K}$, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth doped Bi$_2$Te$_3$ and call for spectroscopic studies in the kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.

cond-mat.str-el↗

arXiv2019

Full control of Co valence in isopolar LaCoO3 / LaTiO3 perovskite heterostructures via interfacial engineering

We report charge-transfer up to a single electron per interfacial unit cell across non-polar heterointerfaces from the Mott insulator LaTiO3 to the charge transfer insulator LaCoO3. In high-quality bi- and tri-layer systems grown using pulsed laser deposition, soft X-ray absorption, dichroism and STEM-EELS are used to probe the cobalt 3d-electron count and provide an element-specific investigation of the magnetic properties. The experiments prove a deterministically-tunable charge transfer process acting in the LaCoO3 within three unit cells of the heterointerface, able to generate full conversion to 3d7 divalent Co, which displays a paramagnetic ground state. The number of LaTiO3 / LaCoO3 interfaces, the thickness of an additional "break" layer between the LaTiO3 and LaCoO3, and the LaCoO3 film thickness itself in tri-layers provide a trio of sensitive control knobs for the charge transfer process, illustrating the efficacy of O2p-band alignment as a guiding principle for property design in complex oxide heterointerfaces.

cond-mat.mtrl-sci↗

arXiv2018

Tuning the hybridization and magnetic ground state of electron and hole doped CeOs$_2$Al$_{10}$: an x-ray spectroscopy study

Here we present linear and circular polarized soft x-ray absorption spectroscopy (XAS) data at the Ce $M_{4,5}$ edges of the electron (Ir) and hole-doped (Re) Kondo semiconductor CeOs$_2$Al$_{10}$. Both substitutions have a strong impact on the unusual high N$\acute{e}$el temperature, $T_N$=28.5\,K, and also the direction of the ordered moment in case of Ir. The substitution dependence of the linear dichroism is weak thus validating the crystal-field description of CeOs$_2$Al$_{10}$ being representative for the Re and Ir substituted compounds. The impact of electron- and hole-doping on the hybridization between conduction and 4$f$ electrons is related to the amount of $f^0$ in the ground state and reduction of x-ray magnetic circular dichroism. A relationship of $cf$-hybridization strength and enhanced $T_N$ is discussed. The direction and doping dependence of the circular dichroism is in agreement with strong Kondo screening along the crystallographic $a$ direction.

cond-mat.str-el↗

arXiv2015

Enhancing Magnetic Ordering in Cr-doped Bi2Se3 using High-TC Ferrimagnetic Insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 K to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

cond-mat.mtrl-sci↗

arXiv2006

Unravelling the origin of the controversial magnetic properties of BiFeO3 thin films

Single phase (001)-oriented BiFeO3 (BFO) thin films grown by pulsed laser deposition can only be obtained in a narrow window of deposition pressure and temperature and have a low magnetic moment. Out of the stability window Fe- or Bi-rich impurity phases form, which has a strong impact on the physical and structural properties of the films, even for impurity concentrations hardly detectable by standard X-ray diffraction measurements. By using more sensitive tools such as X-Ray absorption spectroscopy and X-ray magnetic circular dichroism and performing advanced X-ray diffraction characterization, we show that in non-optimal conditions Fe forms ferrimagnetic gamma-Fe2O3 precipitates that are responsible for virtually all the ferromagnetic signal measured on such BFO films by standard magnetometry. This confirms that the BFO phase has a very low intrinsic moment that does not depend on strain. We also study the influence of film thickness on the nucleation of parasitic phases and find that epitaxial strain can stabilize the pure BFO phase in slightly over-oxidizing growth conditions.

cond-mat.mtrl-sci↗