arXiv subjects

A. A. Avakyants

Publications and source records attributed to A. A. Avakyants.

8 recordsLinked to original sources

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

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↗

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↗

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

Second-harmonic response as a tool to detect spin textures in magnetic nodal-line semimetal Fe$_3$GeTe$_2$

We experimentally investigate second-harmonic transverse voltage response to ac electrical current for a magnetic nodal-line semimetal Fe$_3$GeTe$_2$. For zero magnetic field, the observed response depends as a square of the longitudinal current, as it should be expected for non-linear Hall effect. The magnetic field behavior is found to be sophisticated: while the first-harmonic response shows the known anomalous Hall hysteresis in FGT, the second-harmonic Hall voltage is characterized by the pronounced high-field hysteresis and flat ($B$-independent) region with curves touching at low fields. The high-field hysteresis strongly depends on the magnetic field sweep rate, so it reflects some slow relaxation process. For the lowest rates, it is also accomplished by multiple crossing points. Similar shape of the second-harmonic hysteresis is known for skyrmion spin textures in non-linear optics. Since skyrmions have been demonstrated for FGT by direct visualization techniques, we can connect the observed high-field relaxation with deformation of the skyrmion lattice. Thus, the second-harmonic Hall voltage response can be regarded as a tool to detect spin textures in transport experiments.

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