arXiv subjects

A. V. Timonina

Publications and source records attributed to A. V. Timonina.

At least 19 recordsLinked to original sources

arXiv2026

Spin-polarized electron transport for the altermagnet CrSb

We experimentally investigate spin-polarized electron transport for the centrosymmetric altermagnet CrSb, which is known to reveal both altermagnetic and topological features. We demonstrate pronounced first-harmonic anomalous and second-harmonic non-linear Hall effects for a single-crystal CrSb flake with ferromagnetic nickel contacts, while both effects can not be seen for the reference samples with non-magnetic gold ones. For the anomalous Hall effect, we demonstrate bow-tie hysteresis loop in Hall voltage, which is usually ascribed to surface spin textures in magnetic materials. The slope of the Hall curve changes a sign for two orientations of the Hall-bar contact configuration for the same sample, i.e. for the same sign of the charge carriers. We interpret the observed sign inversion and bow-tie hysteresis as the joint effect of the alternating bulk spin splitting and spin-polarized topological surface states in CrSb. The pronounced non-linear Hall effect with hysteresis in magnetic field confirms finite Berry curvature dipole under injection of spin-polarized electrons, i.e. the topological features for the altermagnetic candidate CrSb.

cond-mat.mes-hall↗

arXiv2026

Josephson diode and spin-valve effects on the surface of altermagnet CrSb

We experimentally investigate charge transport in In-CrSb and In-CrSb-In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of altermagnet CrSb. For double In-CrSb-In junctions, $dV/dI(B)$ curves are mirrored in respect to zero field for two magnetic field sweep directions, which is characteristic behavior of a Josephson spin valve. Also, we demonstrate Josephson diode effect by direct measurement of the critical current for two opposite directions in external magnetic field. We interpret these observations as a joint effect of the spin-polarized topological surface states and the altermagnetic spin splitting of the bulk bands in CrSb. For a single In-CrSb interface, the superconducting gap oscillates in magnetic field for both field orientations, which strongly resembles the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) behavior. FFLO is based on finite-momentum Cooper pairing, therefore, it is fully compatible with the requirements for the Josephson diode effect.

cond-mat.mes-hall↗

arXiv2025

Altermagnetic bulk and topological surface magnetizations for CrSb single crystals

We experimentally investigate the angle dependence of magnetization $M(α)$ for single crystals of CrSb. CrSb belongs to a new class of altermagnetic materials, the small net magnetization is accompanied by alternating spin splitting in the k-space. In addition, CrSb reveals also topological features with Weyl surface states originating from bulk band topology. We observe, that $M(α)$ oscillates around zero value, so magnetization is positive for $M(α)$ maxima and it is negative for $M(α)$ minima. The magnetization reversal curves $M(H)$ are non-linear with low-field hysteresis, but with almost linear high-field branches. The slope of the linear branches well correlates with $M(α)$ oscillations, so it is positive for $M(α)$ maxima and negative for $M(α)$ minima. We demonstrate, that the interplay between the positive and the negative $M(H)$ slopes originates from several magnetic phases in CrSb. In particular, current-carrying topological surface states are responsible for the diamagnetic-like $M(H)$ negative slope, which dominates for the directions of full spin compensation in the bulk CrSb altermagnetic spectrum. Due to the spin-momentum locking, topological surface states are spin-polarized, which is responsible for the low-field hysteresis. Thus, we experimentally demonstrate both the altermagnetic bulk and the topological surface magnetizations for the altermagnetic candidate CrSb.

cond-mat.mes-hall↗

arXiv2025

Magnetocaloric effect for the altermagnetic candidate MnTe

We experimentally investigate magnetocaloric effect for single crystals of MnTe altermagnet at the transition to the state with spontaneous spin polarization, i.e. well below the Néel temperature of MnTe. The isothermal magnetic entropy change $ΔS$ is calculated from the experimental magnetization curves by using Maxwell relation. We observe well-defined magnetocaloric effect as a narrow $ΔS$ peak around the cricital temperature $T_c\approx 81$~K, which is accompanied by sharp magnetization jump. This behavior is unusual for standard ferromagnetic transitions, so it confirms the predicted spin-orbit-induced spin polarization in the MnTe altermagnetic state.

cond-mat.mes-hall↗

arXiv2025

Magnetization symmetry for the MnTe altermagnetic candidate

We experimentally investigate the magnetization angle dependence $M(α)$ for single crystals of MnTe altermagnetic candidate. In high magnetic fields, experimental $M(α)$ curves mostly reflect standard antiferromagnetic spin-flop processes, which are allowed below the Néel vector reorientation field. In low magnetic fields and at low temperatures, spontaneous magnetization appears as a sharp $M(T)$ magnetization jump around 81~K. In this regime, $M(α)$ dependence is quite unusual: the easy magnetization axis is $π/2$ rotated either by increasing the field above 1~kOe or the temperature above 81~K. The observed behavior cannot be expected for antiferromagnetics, e.g. it differs strongly from the well known weak ferromagnetism. Thus, it requires to take into account the formation of the altermagnetic ground state for MnTe altermagnetic candidate. Despite MnTe is expected to have g-wave order parameter, $M(α)$ magnetization symmetry confirms the prevailing population of one from three easy axes, as it has been shown previously by temperature-dependent angle-resolved photo-emission spectroscopy.

cond-mat.mes-hall↗

arXiv2025

Spin-valve effect for spin-polarized surface states in topological semimetals

We experimentally investigate magnetoresistance of a single GeTe-Ni junction between the $α$-GeTe topological semimetal and thick nickel film at room and liquid helium temperatures. For the magnetic field parallel to the junction plane, we demonstrate characteristic spin-valve hysteresis with mirrored differential resistance $dV/dI$ peaks even at room temperature. In contrast, for normal magnetic fields spin-valve effect appears only at low temperatures. From the magnetic field anisotropy, observation of the similar effect for another topological semimetal Cd$_3$As$_2$, and strictly flat $dV/dI(H)$ magnetoresistance curves for the reference GeTe-Au junction, we connect the observed spin-valve effect with the spin-dependent scattering between the spin textures in the topological surface states and the ferromagnetic nickel electrode. For the topological semimetal $α$-GeTe, room-temperature spin-valve effect allows efficient spin-to-charge conversion even at ambient conditions.

cond-mat.mes-hall↗

arXiv2025

Demonstration of the third-order nonlinear Hall effect in topological Dirac semimetal NiTe$_2$

We experimentally investigate third-order nonlinear Hall effect for three-dimensional NiTe$_2$ single crystal samples. NiTe$_2$ is the recently discovered type-II Dirac semimetal, so both the inversion and the time-reversal symmetries are conserved in the bulk. As a result, the well known second-order nonlinear Hall effect does not expected for this material, which we confirm as negligibly small second-harmonic transverse Hall voltage response to the longitudinal ac electric current. As the main experimental result, we demonstrate the unsaturated third-harmonic Hall response in NiTe$_2$, which well corresponds to the theoretically predicted third-order nonlinear Hall effect in Dirac semimetals. We also demonstrate, that the third harmonic signal does not depend on the external magnetic field, in contrast to the field-depended first-order and second-order Hall effects.

cond-mat.mes-hall↗

arXiv2024

Magnetocaloric effect for the topological semimetal Co$_3$Sn$_2$S$_2$ due to the antiferromagnetic coupling of the bulk and surface spin-polarized phases

We experimentally investigate magnetocaloric effect for the topological magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ in a wide temperature range. The isothermal magnetic entropy change $ΔS$ is calculated from the experimental magnetization curves by using Maxwell relation. In addition to the expected $ΔS$ peak at the Curie temperature $T_C$, we obtain another one at the temperature $T_{inv}$ of the hysteresis inversion, which is the main experimental result. The inverted hysteresis usually originates from the antiferromagnetic coupling between two magnetic phases. For Co$_3$Sn$_2$S$_2$ topological magnetic Weyl semimetal these phases are the ferromagnetic bulk and the spin-polarized topological surface states. Thus, the pronounced magnetocaloric effect at $T_{inv}$ is determined by the bulk magnetization switching by the exchange bias field of the surface spin-polarizad phase, in contrast to the ferromagnetic-paramagnetic transition at the Curie temperature $T_C$. For possible applications of magnetocaloric effect, Weyl semimetals open a new way to shift from ferromagnetic to the antiferromagnetic systems without loss of efficiency, but with higher reversibility and with smaller energy costs.

cond-mat.mes-hall↗

arXiv2024

Andreev reflection for MnTe altermagnet candidate

We experimentally study electron transport across a single planar junction between the indium electrode and MnTe altermagnet candidate. We confirm standard Ohmic behavior with strictly linear current-voltage curves above the indium critical field or temperature, although with high, about 100~kOhm, junction resistance. At low temperatures and in zero magnetic field, we observe a well-developed Andreev curve with the pronounced coherence peaks, which cannot be normally expected for these high values of normal junction resistance. The conclusion on the Andreev reflection is also supported by suppression in magnetic field, as well as by universality of the observed behavior for all of the investigated samples. The experimental results can be explained by specifics of Andreev transport through the disordered region at the superconductor-altermagnet interface. Due to a different set of restrictions on the possibility of Andreev reflection, an altermagnet suffers from the presence of disorder less than a normal spin-degenerate metal, so the conductance enhancement is retained throughout the superconducting gap.

cond-mat.mes-hall↗

arXiv2024

Fractional ac Josephson effect as evidence of topological hinge states in a Dirac semimetal NiTe2

We experimentally investigate Josephson current between two 5~$μ$m spaced superconducting indium leads, coupled to a NiTe$_2$ single crystal flake, which is a type-II Dirac semimetal. Under microwave irradiation, we demonstrate a.c. Josephson effect at millikelvin temperatures as a number of Shapiro steps. In addition to the integer ($n=1,2,3,4...$) steps, we observe fractional ones at half-integer values $n=1/2,3/2,5/2$ and 7/2, which corresponds to $π$ periodicity of current-phase relationship. In contrast to previous investigations, we do not observe $4π$ periodicity (disappearance of the odd $n=1,3,5...$ Shapiro steps), while the latter is usually considered as a fingerprint of helical surface states in Dirac semimetals and topological insulators. We argue, that our experiment confirms Josephson current through the topological hinge states in NiTe$_2$: since one can exclude bulk supercurrent in 5~$μ$m long Josephson junctions, interference of the hinge modes is responsible for the $π$ periodicity, while stable odd Shapiro steps reflect chiral character of the topological hinge states.

cond-mat.mes-hall↗

arXiv2024

Crossover from relativistic to non-relativistic net magnetization for MnTe altermagnet candidate

We experimentally study magnetization reversal curves for MnTe single crystals, which is the altermagnetic candidate. Above 85~K temperature, we confirm the antiferromagnetic behavior of magnetization $M$, which is known for $α$--MnTe. Below 85~K, we observe anomalous low-field magnetization behavior, which is accompanied by the sophisticated $M(α)$ angle dependence with beating pattern as the interplay between $M(α)$ maxima and minima: in low fields, $M(α)$ shows ferromagnetic-like 180$^\circ$ periodicity, while at high magnetic fields, the periodicity is changed to the 90$^\circ$ one. This angle dependence is the most striking result of our experiment, while it can not be expected for standard magnetic systems. In contrast, in altermagnets, symmetry allows ferromagnetic behavior only due to the spin-orbit coupling. Thus, we claim that our experiment shows the effect of weak spin-orbit coupling in MnTe, with crossover from relativistic to non-relativistic net magnetization, and, therefore, we experimentally confirm altermagnetism in MnTe.

cond-mat.mes-hall↗

arXiv2023

Surface spin polarization in the magnetic response of GeTe Rashba ferroelectric

We experimentally investigate magnetization reversal curves for a GeTe topological semimetal. In addition to the known lattice diamagnetic response, we observe narrow magnetization loop in low fields, which should not be expected for non-magnetic GeTe. The hysteresis is unusual, so the saturation level is negative in positive fields, and the loop is passed clockwise, in contrast to standard ferromagnetic behavior. The experimental hysteresis curves can not be obtained from usual ferromagnetic ones by adding/subtracting of any linear dependence, or even by considering several interacting magnetic phases. The possibility of several phases is also eliminated by the remanence plots technique (Henkel or δM plots). We explain our results as a direct consequence of the correlation between ferroelectricity and spin-polarized surface states in GeTe, similarly to magnetoelectric structures.

cond-mat.mes-hall↗

arXiv2023

Ferromagnetic response of thin NiI$_2$ flakes up to room temperatures

We investigate the magnetic response of thin NiI$_2$ flakes for temperatures above 80~K. Since no magnetic ordering is expected for bulk NiI$_2$, we observe clear paramagnetic response for massive NiI$_2$ single crystals. In contrast, thin NiI$_2$ flakes show well-defined ferromagnetic hysteresis loop within $\pm2$~kOe field range. The value of the response does not scale with the sample mass, ferromagnetic hysteresis can be seen for any flake orientation in the external field, so it originates from the sample surface, possibly, due to the anisotropic exchange (Kitaev interaction). The observed ferromagnetism is weakly sensitive to temperature up to 300~K. If a flake is multiply exposed to air, ferromagnetic hysteresis is accompanied by the periodic modulation of the magnetization curves, which is usually a fingerprint of the multiferroic state. While NiI$_2$ flakes can not be considered as multiferroics above 80~K, surface degradation due to the crystallohydrate formation decreases the symmetry of NiI$_2$ surface, which produces the surface ferroelectric polarization in addition to the described above ferromagnetic one.

cond-mat.mes-hall↗

arXiv2023

Reentrant proximity-induced superconductivity for GeTe semimetal

We experimentally investigate charge transport in In-GeTe and In-GeTe-In proximity devices, which are formed as junctions between superconducting indium leads and thick single crystal flakes of $α$-GeTe topological semimetal. We observe nonmonotonic effects of the applied external magnetic field, including reentrant superconductivity in In-GeTe-In Josephson junctions: supercurrent reappears at some finite magnetic field. For a single In-GeTe Andreev junction, the superconducting gap is partially suppressed in zero magnetic field, while the gap is increased nearly to the bulk value for some finite field before its full suppression. We discuss possible reasons for the results obtained, taking into account spin polarization of Fermi arc surface states in topological semimetal $α$-GeTe with a strong spin-orbit coupling. In particular, the zero-field surface state spin polarization partially suppresses the superconductivity, while it is recovered due to the modified spin-split surface state configuration in finite fields. As an alternative possible scenario, the transition into the Fulde-Ferrell-Larkin-Ovchinnikov state is discussed. However, the role of strong spin-orbit coupling in forming the nonmonotonic behavior has not been analyzed for heterostructures in the Fulde-Ferrell-Larkin-Ovchinnikov state, which is crucial for junctions involving GeTe topological semimetal.

cond-mat.mes-hall↗

arXiv2023

Gate-dependent non-linear Hall effect at room temperature in topological semimetal GeTe

We experimentally investigate non-linear Hall effect as zero-frequency and second-harmonic transverse voltage responses to ac electric current for topological semimetal GeTe. A thick single-crystal GeTe flake is placed on the Si/SiO$_2$ substrate, where the p-doped Si layer serves as a gate electrode. We confirm, that electron concentration is not gate-sensitive in thick GeTe flakes due to the gate field screening by bulk carriers. In contrast, by transverse voltage measurements, we demonstrate that the non-linear Hall effect shows pronounced dependence on the gate electric field at room temperature. Since the non-linear Hall effect is a direct consequence of a Berry curvature dipole in topological media, our observations indicate that Berry curvature can be controlled by the gate electric field. This experimental observation can be understood as a result of the known dependence of giant Rashba splitting on the external electric field in GeTe. For possible applications, the zero-frequency gate-controlled non-linear Hall effect can be used for the efficient broad-band rectification.

cond-mat.mes-hall↗

arXiv2022

Surface ferromagnetism in a chiral topological semimetal CoSi

Despite the chiral topological semimetal CoSi is known as bulk diamagnetic, it shows unusual surface ferromagnetism of debatable origin. The ferromagnetic ordering has been attributed to the distorted bonds, the superlattice of ordered vacancies, or even to topological surface textures due to the spin polarization in the neighboring Fermi arcs. We experimentally compare magnetization reversal curves for initially oxidized CoSi single crystals and cleaved samples with a fresh, oxide-free surface. While the oxidized CoSi samples do not show sizable ferromagnetism, the fresh CoSi surface gives a strong ferromagnetic response, which is accompanied by the pronounced modulation of the angle dependence of magnetization, as it can be expected for easy and hard axes in a ferromagnet. In addition to the first order reversal curves analysis, this observation allows us to distinguish between different mechanisms of the ferromagnetic ordering in CoSi single crystals. We conclude that the surface states-induced RKKY interaction between distorted bonds near the sample surface is responsible for the strong ferromagnetic multi-domain behavior for freshly cleaved samples.

cond-mat.mes-hall↗

arXiv2022

Evidence for surface spin structures from first order reversal curves in Co3Sn2S2 and Fe3GeTe2 magnetic topological semimetals

We study magnetization reversal and first order reversal curves for two different magnetic topological semimetals, Co3Sn2S2 and Fe3GeTe2, in a wide temperature range. For the magnetization reversal, we observe strong temperature dependence of the initial (low-temperature) step-like magnetization switchings, so the inverted hysteresis appears at high temperatures. Usually, the inverted hysteresis is a fingerprint of material with two independent magnetic phases, the inversion reflects the phase interaction. First order reversal curve analysis confirms the two-phase behavior even at the lowest temperatures of the experiment. While the bulk ferromagnetic magnetization shows strong temperature dependence, one of the observed phases demonstrates perfect stability below the Curie temperature. The obtained hysteresis loops are of the bow-tie type, which is usually ascribed to appearance of the skyrmionic phase. The described two-phase behavior is mostly identical for Co3Sn2S2 and Fe3GeTe2 magnetic topological semimetals, only the characteristic temperatures differ for these materials. The specifics of our experiment is the excellent temperature stability of the second phase, while the skyrmions are usually observed near the Curie temperature. On the other hand, temperature stability can be expected for surface-state induced spin textures due to the topological protection of surface states in topological semimetals. This also explains the universal behavior of the second phase for two different topological semimetals Co3Sn2S2 and FGT. Both these materials have strongly different bulk properties, the only similarity is the presence of the topological surface states. Thus, we can ascribe the second, temperature-stable magnetic phase to the surface states in topological semimetals.

cond-mat.mes-hall↗

arXiv2022

Interface superconductivity in a type-II Dirac semimetal NiTe$_2$

We experimentally investigate charge transport through a single planar junction between a NiTe$_2$ Dirac semimetal and a normal gold lead. At millikelvin temperatures we observe non-Ohmic $dV/dI(V)$ behavior resembling Andreev reflection at a superconductor -- normal metal interface, while NiTe$_2$ bulk remains non-superconducting. The conclusion on superconductivity is also supported by suppression of the effect by temperature and magnetic field. In analogy with the known results for Cd$_3$As$_2$ Dirac semimetal, we connect this behavior with interfacial superconductivity due to the flat-band formation at the Au-NiTe$_2$ interface. Since the flat-band and topological surface states are closely connected, the claim on the flat-band-induced superconductivity is also supported by the Josephson current through the topological surface states on the pristine NiTe$_2$ surface. We demonstrate the pronounced Josephson diode effect, which results from the momentum shift of topological surface states of NiTe$_2$ under an in-plane magnetic field.

cond-mat.mes-hall↗