arXiv subjects

Y. Myasoedov

Publications and source records attributed to Y. Myasoedov.

At least 19 recordsLinked to original sources

arXiv2023

Imaging the Ettingshausen effect and cryogenic thermoelectric cooling in a van der Waals semimetal

Attaining viable thermoelectric cooling at cryogenic temperatures is of major fundamental and technological interest for novel electronics and quantum materials applications. In-device temperature control can provide a more efficient and precise thermal environment management as compared to the conventional global cooling. Here we develop nanoscale cryogenic imaging of a magneto-thermoelectric effect and demonstrate absolute cooling and an ultrahigh Ettingshausen effect in exfoliated WTe2 Weyl semimetal flakes at liquid He temperatures. Application of a current and perpendicular magnetic field gives rise to cooling via generation of electron-hole pairs on one side of the sample and heating by their recombination at the opposite side. In contrast to bulk materials, the cooling process is found to be nonmonotonic in magnetic field and device size. The derived model of magneto-thermoelectricity in mesoscopic semimetal devices shows that the cooling efficiency and the induced temperature profiles are governed by the interplay between sample geometry, electron-hole recombination length, magnetic field, and flake and substrate heat conductivities. The findings open the way for direct integration of microscopic thermoelectric cooling and for temperature landscape engineering in novel van der Waals devices.

cond-mat.mes-hall↗

arXiv2020

SQUID-on-tip with single-electron spin sensitivity for high-field and ultra-low temperature nanomagnetic imaging

Scanning nanoscale superconducting quantum interference devices (nanoSQUIDs) are of growing interest for highly sensitive quantitative imaging of magnetic, spintronic, and transport properties of low-dimensional systems. Utilizing specifically designed grooved quartz capillaries pulled into a sharp pipette, we have fabricated the smallest SQUID-on-tip (SOT) devices with effective diameters down to 39 nm. Integration of a resistive shunt in close proximity to the pipette apex combined with self-aligned deposition of In and Sn, have resulted in SOT with a flux noise of 42 n$Φ_0$Hz$^{-1/2}$, yielding a record low spin noise of 0.29 $μ_B$Hz$^{-1/2}$. In addition, the new SOTs function at sub-Kelvin temperatures and in high magnetic fields of over 2.5 T. Integrating the SOTs into a scanning probe microscope allowed us to image the stray field of a single Fe$_3$O$_4$ nanocube at 300 mK. Our results show that the easy magnetization axis direction undergoes a transition from the (111) direction at room temperature to an in-plane orientation, which could be attributed to the Verwey phase transition in Fe$_3$O$_4$.

cond-mat.mes-hall↗

arXiv2015

Visualization of superparamagnetic dynamics in magnetic topological insulators

Quantized Hall conductance is a generic feature of two dimensional electronic systems with broken time reversal symmetry. In the quantum anomalous Hall state recently discovered in magnetic topological insulators, time reversal symmetry is believed to be broken by long-range ferromagnetic order, with quantized resistance observed even at zero external magnetic field. Here, we use scanning nanoSQUID magnetic imaging to provide a direct visualization of the dynamics of the quantum phase transition between the two anomalous Hall plateaus in a Cr-doped (Bi,Sb)$_2$Te$_3$ thin film. Contrary to naive expectations based upon macroscopic magnetometry, our measurements reveal a superparamagnetic state formed by weakly interacting magnetic domains with a characteristic size of few tens of nanometers. The magnetic phase transition occurs through random reversals of these local moments, which drive the electronic Hall plateau transition. Surprisingly, we find that the electronic system can in turn drive the dynamics of the magnetic system, revealing a subtle interplay between the two coupled quantum phase transitions.

cond-mat.mes-hall↗

arXiv2013

The effect of uniaxial pressure on the magnetic anisotropy of the Mn_{12}-Ac single-molecule magnet

We study the effect of uniaxial pressure on the magnetic hysteresis loops of the single-molecule magnet Mn_{12}-Ac. We find that the application of pressure along the easy axis increases the fields at which quantum tunneling of magnetization occurs. The observations are attributed to an increase in the molecule's magnetic anisotropy constant D of 0.142(1)%/kbar. The increase in D produces a small, but measurable increase in the effective energy barrier for magnetization reversal. Density-functional theory calculations also predict an increase in the barrier with applied pressure.

cond-mat.mes-hall↗

arXiv2012

Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena

We describe a new type of scanning probe microscope based on a superconducting quantum interference device (SQUID) that resides on the apex of a sharp tip. The SQUID-on-tip is glued to a quartz tuning fork which allows scanning at a tip-sample separation of a few nm. The magnetic flux sensitivity of the SQUID is 1.8 μ_0/Hz^{1/2} and the spatial resolution is about 200 nm, which can be further improved. This combination of high sensitivity, spatial resolution, bandwidth, and the very close proximity to the sample provides a powerful tool for study of dynamic magnetic phenomena on the nanoscale. The potential of the SQUID-on-tip microscope is demonstrated by imaging of the vortex lattice and of the local AC magnetic response in superconductors.

cond-mat.supr-con↗

arXiv2009

Controlled suppression of local magnetization hysteresis by dc in-plane field in Bi2Sr2CaCu2O8

Local magnetization hysteresis is among the most extensively studied features of high-temperature superconductors (HTSC). The usual source of hysteresis in superconductors is bulk vortex pinning due to material defects. Two additional known mechanisms of magnetic irreversibility are the Bean-Livingston surface barrier and the geometrical barrier (GB). GB arises due to a competition between the line energy of a vortex penetrating into the sample and the Lorentz force of Meissner currents which focuses vortices in the sample's center to form a dome-shaped vortex distribution. This work demonstrates that dc in-plane field overcomes hysteresis mainly through the GB suppression in a region of phase diagram under consideration (high temperatures and low fields).

cond-mat.supr-con↗

arXiv2009

Transport Properties of Vortex Matter Governed by the Edge Inductance of Superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ Single Crystals

We study the distribution of transport current across superconducting Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals and the vortex flow through the sample edges. We show that the $T_x$ transition is of electrodynamic rather than thermodynamic nature, below which vortex dynamics is governed by the edge inductance instead of the resistance. This allows measurement of the resistance down to two orders of magnitude below the transport noise. By irradiating the current contacts the resistive step at vortex melting is shown to be due to loss of c-axis correlations rather than breakdown of quasi-long-range order within the a-b planes.

cond-mat.supr-con↗

arXiv2009

Magnetic avalanches of minor fast-relaxing species of Mn12-acetate

Using micron-sized thermometers and Hall bars, we report time-resolved studies of the local temperature and local magnetization for two types of magnetic avalanches (abrupt spin reversals) in the molecular magneti Mn12-acetate, corresponding to avalanches of the main slow-relaxing crystalline form and avalanches of the fast-relaxing minor species that exists in all as-grown crystals of this material. An experimental protocol is used that allows the study of each type of avalanche without triggering avalanches in the other, and of both types of avalanches simultaneously. In samples prepared magnetically to enable both types of avalanches, minor species avalanches are found to act as a catalyst for the major species avalanches. magnetically to enable both types of avalanches, minor species avalanches are found to act as a catalyst for the major species avalanches.

cond-mat.mtrl-sci↗

arXiv2009

Tuning Magnetic Avalanches in Mn12-ac

Using micron-sized Hall sensor arrays to obtain time-resolved measurements of the local magnetization, we report a systematic study in the molecular magnet Mn$_{12}$-acetate of magnetic avalanches controllably triggered in different fixed external magnetic fields and for different values of the initial magnetization. The speeds of propagation of the spin-reversal fronts are in good overall agreement with the theory of magnetic deflagration of Garanin and Chudnovsky \cite{Garanin}.

cond-mat.mtrl-sci↗

arXiv2009

Mott insulator phases and first-order melting in BSCCO crystals with periodic surface holes

We measured the effects of periodic surface holes, created using a focused ion beam, on the phase diagram of the vortex matter in high-T_c Bi_2Sr_2CaCu_2O_{8+δ} crystals. Differential magneto-optical measurements show that the irreversibility line is shifted to higher fields and temperatures, with respect to the pristine melting line. The irreversibility line displays weak field dependence between integer matching fields indicating multiple-flux-quanta pinning at holes. We find reduced equilibrium compressibility of the vortex matter at integer matching fields, which is strong evidence for the existence of thermodynamic Mott insulator phases. Shaking with a transverse ac field surprisingly reveals first-order melting that is not shifted with respect to the pristine melting line and that seems to occur within the Mott insulator regions. This melting is understood to be the first-order transition in the bulk of the crystal beneath the surface holes. The transition is visible at the surface, despite the reduced vortex compressibility in the top layer.

cond-mat.supr-con↗

arXiv2008

Experimental determination of the dipolar field in Mn12-acetate

Crystals of the molecular magnet Mn12-acetate are known to contain a small fraction of low- symmetry (minor) species with a small anisotropy barrier against spin reversal. The lower barrier leads to faster magnetic relaxation and lower coercive field. We exploit the low coercive fields of the minor species to make a direct determination of the dipole field in Mn12-ac. We find that the dipolar field of a fully magnetized crystal is 51.5 \pm 8.5 mT, consistent with theoretical expectations.

cond-mat.other↗

arXiv2007

Ignition and Propagation of Magnetic Avalanches in Mn$_{12}$-Acetate: the effect of quantum tunneling

Using a wire heater to ignite magnetic avalanches in fixed magnetic field applied along the easy axis of single crystals of the molecular magnet Mn$_{12}$-acetate, we report fast local measurements of the temperature and time-resolved measurements of the local magnetization as a function of magnetic field. In addition to confirming maxima in the velocity of propagation, we find that avalanches trigger at a threshold temperature which exhibits pronounced minima at resonant magnetic fields, demonstrating that thermally assisted quantum tunneling plays an important role in the ignition as well as the propagation of magnetic avalanches in molecular magnets.

cond-mat.mes-hall↗

arXiv2007

Angular dependence of the melting of the porous vortex matter in $\mathrm{Bi_{2}Sr_{2}CaCu_{2}O_{8}}$

The behavior of vortex matter in $\mathrm{Bi_{2}Sr_{2}CaCu_{2}O_{8}}$ in the presence of a low concentration of tilted columnar defects (CDs) was studied using magneto-optical measurements and molecular dynamics simulations. We find that while the dynamic properties are significantly affected by tilting the field away from CDs, the thermodynamic lines are angle independent. Our simulations reveal that vortex pancakes remain localized along CDs even at large tilting angles thus preserving the thermodynamic features, while suppression of irreversible properties is caused by vortex kink sliding.

cond-mat.supr-con↗

arXiv2007

Interplay of Anisotropy and Disorder in the Doping-Dependent Melting and Glass Transitions of Vortices in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant. Yet, in the region where disorder breaks that scaling, the glass lines are still collapsed. A quantitative fit to melting and replica symmetry breaking lines of a 2D Ginzburg-Landau model further reveals that disorder amplitude weakens with doping, but to a lesser degree than thermal fluctuations, enhancing the relative role of disorder.

cond-mat.supr-con↗

arXiv2007

Spatial Determination of Magnetic Avalanche Ignition Points

Using time-resolved measurements of local magnetization in the molecular magnet Mn12-ac, we report studies of the propagation of magnetic avalanches (fast magnetization reversals) that originate from points inside the crystals rather than at the edges. The curved nature of the fronts produced by avalanches is reflected in the time-of-arrival at micro-Hall sensors placed at the surface of the sample. Assuming that the avalanche interface is a spherical bubble that grows with a radius proportional to time, we are able to locate the approximate ignition point of each avalanche in a two-dimensional cross-section of the crystal. For the samples used in these studies, avalanches in a given crystal are found to originate in a small region with a radius of roughly 150 microns.

cond-mat.mtrl-sci↗

arXiv2006

Photon-Induced Magnetization Changes in Single-Molecule Magnets

Microwave radiation applied to single-molecule magnets can induce large magnetization changes when the radiation is resonant with transitions between spin levels. These changes are interpreted as due to resonant heating of the sample by the microwaves. Pulsed-radiation studies show that the magnetization continues to decrease after the radiation has been turned off with a rate that is consistent with the spin's characteristic relaxation rate. The measured rate increases with pulse duration and microwave power, indicating that greater absorbed radiation energy results in a higher sample temperature. We also performed numerical simulations that qualitatively reproduce many of the experimental results. Our results indicate that experiments aimed at measuring the magnetization dynamics between two levels resonant with the radiation must be done much faster than the >20-microsecond time scales probed in these experiments.

cond-mat.mes-hall↗

arXiv2005

Equilibrium First-Order Melting and Second-Order Glass Transitions of the Vortex Matter in Bi$_2$Sr$_2$CaCu$_2$O$_8$

The thermodynamic $H-T$ phase diagram of Bi$_2$Sr$_2$CaCu$_2$O$_8$ was mapped by measuring local \emph{equilibrium} magnetization $M(H,T)$ in presence of vortex `shaking'. Two equally sharp first-order magnetization steps are revealed in a single temperature sweep, manifesting a liquid-solid-liquid sequence. In addition, a second-order glass transition line is revealed by a sharp break in the equilibrium $M(T)$ slope. The first- and second-order lines intersect at intermediate temperatures, suggesting the existence of four phases: Bragg glass and vortex crystal at low fields, glass and liquid at higher fields.

cond-mat.supr-con↗

arXiv2005

Propagation of Avalanches in Mn$_{12}$-acetate: Magnetic Deflagration

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.

cond-mat.other↗