High In-plane Fields Research Articles

High Magnetic Field Stability in a Planar Graphene-NbSe2 SQUID.

Thin NbSe2 retains superconductivity at a high in-plane magnetic field up to 30 T. In this work we construct a novel atomically thin, all van der Waals SQUID, in which current flows between NbSe2 contacts through two parallel graphene weak links. The 2D planar SQUID remains uniquely stable at high in-plane field, which enables tracing critical current interference patterns as a function of the field up to 4.5 T. From these we extract the evolution of the current distribution up to high fields, demonstrating sub-nanometer sensitivity to deviation of current flow from a perfect atomic plane and observing a field-driven transition in which supercurrent redistributes to a narrow channel. We further suggest a new application of the asymmetric SQUID geometry to directly probe the current density in the absence of phase information.

Evolution of switching fields caused by reorientation of GdFeCo/Ir/GdFeCo synthetic ferrimagnet in magnetic field

Anomalous Hall effect (AHE) in GdFeCo/Ir/GdFeCo multilayered structures attracts great interest because all optical switching, spin-torque, and other effects promise effective application for ultrafast memory element creation. Since AHE is controlled by GdFeCo magnetization, domain dynamics has importance for practical applications. In our work, magnetization reversal in perpendicular GdFeCo/Ir/GdFeCo synthetic ferrimagnets is characterized by AHE measurements. The AHE hysteresis loop obtained with the field applied perpendicular to the sample plane is composed of three sub-loops, and two of them are symmetrically biased with respect to the third one. Switching magnetic fields for two of the three transitions are found to be dependent on magnetic history. In particular, exposure of the sample in the in-plane field leads to reduction of the out-of-plane switching fields in side sub-loops. A multiple series of perpendicular hysteresis loops recorded after exposure under high in-plane field reveals gradual (within 30 min) relaxation of the out-of-plane switching fields to their initial values observed in a non-magnetized sample. Domain wall mobility, limiting switching of the bilayer devices, is complicated due to the coupling between partial domains in each single layer. Unusual dynamics of double domain walls results in unexpected new phenomena affecting electrical processes in bilayer structures.

Formation of relocatable umbilical defects in a liquid crystal with positive dielectric anisotropy induced via photovoltaic fields

Abstract All optically generated, relocatable umbilic defects in a liquid crystal (LC) with positive dielectric anisotropy are reported. The studied samples are made with a Fe-doped LiNbO3 substrate showing the bulk photovoltaic effect and an ITO coated glass plate as covering substrate. Umbilics are created upon exposure with a focused laser beam: They are induced via photo generated fields at the relocatable exposure beam position. The field distribution of these fields is normal to the sample plane at the exposure spot center and has increasingly high in-plane field components in radial direction: Ideal conditions to induce director reorientations with radial symmetry in a LC with positive dielectric anisotropy. The irradiated regions are studied with polarized probe light revealing the characteristic for umbilic defects and the typical doughnut-shaped intensity distributions expected for optical vortex beams. The experimental findings are compared to calculated transmission profiles. Moreover, a LC with negative dielectric anisotropy was studied in the same type of sample for reference.

Domain-wall motion and interfacial Dzyaloshinskii-Moriya interactions in Pt/Co/Ir(t Ir)/Ta multilayers.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a Pt/Co/Ta multilayer between the Co and Ta layers. There is a marked increase in magnetic moment, due to the suppression of the dead layer at the interface with Ta, but the perpendicular anisotropy is hardly affected. All samples show a universal scaling of the field-driven DW velocity across the creep and depinning regimes. Asymmetric bubble expansion shows that DWs in all of the samples have the left-handed Néel form. The value of in-plane magnetic field at which the creep velocity shows a minimum drops markedly on the introduction of Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in Brillouin light scattering (BLS) measurements. Despite this qualitative similarity, there are quantitative differences in the DMI strength given by the two measurements, with BLS often returning higher values. Many features in bubble expansion velocity curves do not fit simple models commonly used, namely a lack of symmetry about the velocity minimum and no difference in velocities at high in-plane fields. These features are explained by the use of a new model in which the depinning field is allowed to vary with in-plane field in a way determined from micromagnetic simulations. This theory shows that the velocity minimum underestimates the DMI field, consistent with BLS giving higher values. Our results suggest that the DMI at an Ir/Co interface has the same sign as the DMI at a Pt/Co interface.

Novel vortex phases and vortex manipulation in highly anisotropic superconductors by Scanning Hall Probe Microscopy (SHPM)

Scanning Hall probe microscopy (SHPM) has been used to demonstrate the interaction of pancake vortices with the Josephson vortex lattice in Bi2Sr2CaCu2O8+δ (2212) single crystals at large in-plane fields in the ‘‘crossing lattices” regime of highly anisotropic cuprate superconductors. SHPM has been used to study vortex structures in the interacting crossing lattices regime under applied in and out-of- plane magnetic fields. The interactions between the pancake and Josephson vortices (JV) depend on the values of crossing fields and temperature and the space between chains varies inversely with the in-plane field. These chains are clearly visible at very high in-plane fields HII~315 G over a range of out-of-plane fields 1.7- 6.8 G at 85 K. The study of such chains allows a better understand the regime of crossing vortex lattices in highly anisotropic cuprate superconductors.

Probing electron interactions in a two-dimensional system by quantum magneto-oscillations

We have experimentally studied the renormalized effective mass ${m}^{*}$ and Dingle temperature ${T}_{D}$ in two spin subbands with essentially different electron populations. Firstly, we found that the product ${m}^{*}{T}_{D}$ that determines the damping of quantum oscillations, to the first approximation, is the same in the majority and minority subbands even at a spin polarization degree as high as 66%. This result confirms the theoretical predictions that the interaction takes place at high energies $\ensuremath{\sim}{E}_{F}$ rather than within a narrow strip of energies ${E}_{F}\ifmmode\pm\else\textpm\fi{}{k}_{B}T$. Secondly, to the next approximation, we revealed a difference in the damping factor of the two spin subbands, which causes skewness of the oscillation line shape. In the absence of the in-plane magnetic field ${B}_{\ensuremath{\parallel}}$, the damping factor ${m}^{*}{T}_{D}$ is systematically smaller in the spin-majority subband. The difference, quantified with the skew factor $\ensuremath{\gamma}=({T}_{D\ensuremath{\downarrow}}\ensuremath{-}{T}_{D\ensuremath{\uparrow}})/2{T}_{D0}$ can be as large as 20%. The skew factor tends to decrease as ${B}_{\ensuremath{\parallel}}$ or temperature grow, or ${B}_{\ensuremath{\perp}}$ decreases; for low electron densities and high in-plane fields, the skew factor even changes sign. Finally, we compared the temperature and magnetic field dependencies of the magneto-oscillation amplitude with predictions of the interaction correction theory, and found, besides some qualitative similarities, several quantitative and qualitative differences. To explain qualitatively our results, we suggested an empirical model that assumes the existence of easily magnetized triplet scatterers on the Si/${\mathrm{SiO}}_{2}$ interface.

Nonmonotonic magnetic field and density dependence of in-plane magnetoresistance in dilute two-dimensional holes inGaAs/AlxGa1−xAs

We studied low temperature $(T=50$ mK) in-plane magnetoresistance of a dilute two-dimensional hole system in GaAs/AlGaAs heterostructure that exhibits an apparent metal-insulator transition. We found an anisotropic magnetoresistance, which changes dramatically at high in-plane fields ${(B}_{\ensuremath{\Vert}}\ensuremath{\gtrsim}5$ T) as the hole density is varied. At high densities where the system behaves metallic at ${B}_{\ensuremath{\Vert}}=0,$ the transverse magnetoresistance is larger than the longitudinal magnetoresistance. With decreasing the hole density the difference becomes progressively smaller, and at densities near the ``critical'' density and lower, the longitudinal magnetoresistance becomes larger than the transverse magnetoresistance.

Strongly Anisotropic Electronic Transport at Landau Level Filling Factorν=9/2andν=5/2under a Tilted Magnetic Field

We have investigated the influence of an increasing in-plane magnetic field on the states of half filling of Landau levels ( $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11/2$, 9/2, 7/2, and 5/2) of a two-dimensional electron system. In the electrically anisotropic phase at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9/2$ and 11/2 an in-plane magnetic field of $\ensuremath{\sim}1--2\mathrm{T}$ overcomes its initial pinning to the crystal lattice and reorients this phase. In the initially isotropic phases at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ and 7/2 an in-plane magnetic field induces a strong electrical anisotropy. In all cases, for high in-plane fields the high-resistance axis is parallel to the direction of the in-plane field.

The dynamical behavior of magnetic domain walls and magnetic bubbles in single-, double-, and triple-layer garnet films

The dynamical behavior of straight magnetic domain walls and propagating magnetic bubbles in single-, double-, and triple-layer lanthanum gallium garnet films is reported. In the study of straight walls in the single-layer films, a good agreement is obtained between the observed and the calculated values of the domain-wall width parameter Δ, the derivative of the peak velocity to in-plane field at high in-plane fields, and the derivative of the saturation velocity to in-plane field at a moderate drive field and high in-plane fields H1. The saturation velocity (at H1=0) decreases with decreasing film thickness, which is in disagreement with theoretical predictions. A propagating bubble in a single-layer film exhibits an overshoot. In double- and triple-layer films, the observed and calculated Δ values disagree in most cases, and the straight wall and propagating bubble saturation velocities are significantly higher than in single-layer films. Overshoots and low-frequency oscillations are observed in the saturation regime of straight walls. The resulting high wall masses are attributed to stacking of horizontal Bloch lines on the film surface(s). From the measurements in in-plane fields, we conclude that the saturation velocity and the peak velocity in a double- and a triple-layer film depend on the polarity of the in-plane field, which is always perpendicular to the straight wall. It is found that the saturation region in double- and triple-layer films can be subdivided in three parts. This is explained qualitatively, and a calculation is made of the saturation velocity in double- and triple-layer films. This calculation shows good agreement with experimental results.