Hanle Effect Research Articles

Observation of the Hanle effect in giant magnetoresistance measurements

We report the observation of the Hanle effect in the giant magnetoresistance measurement of metallic lateral nanostructures. By applying magnetic fields perpendicularly to the magnetization of the electrodes, we obtain resistance values lower than that of the parallel magnetization state, due to the extra decoherence provided by the random propagation time of the carriers. An analytical model is developed, enabling us to extract the characteristic transport parameters of CoFe, Al, and Cu from the fitting of the experimental results. We compare the obtained parameters to that of standard non-local Hanle measurements in lateral spin-valves, with the interest that this measurement configuration gives much larger signal variation.

Spatial variations of the Sr I 4607 Å scattering polarization peak

Context. The scattering polarization signal observed in the photospheric Sr I 4607 Å line is expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect), but also to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular scales, but observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required. Aims. We aim to provide observational evidence of the polarization peak spatial variations, and to analyze the correlation they might have with granulation. Methods. Observations conjugating high spatial resolution and high spectropolarimetric precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the adaptive optics system and the newly installed image derotator. Results. Spatial variations of the scattering polarization in the Sr I 4607 Å line are clearly observed. The spatial scale of these variations is comparable with the granular size. Small correlations between the polarization signal amplitude and the continuum intensity indicate that the polarization is higher at the center of granules than in the intergranular lanes.

Tuning of the Hanle effect from EIT to EIA using spatially separated probe and control beams

We demonstrate a technique for continuous tuning of the Hanle effect from electromagnetically induced transparency (EIT) to electromagnetically induced absorption (EIA) by changing the polarization ellipticity of a control beam. In contrast to previous work in this field, we use spatially separated probe and control beams. The experiments are done using magnetic sublevels of the Fg = 4 → Fe = 5 closed hyperfine transition in the 852 nm D2 line of 133Cs. The atoms are contained in a room temperature vapor cell with anti-relaxation (paraffin) coating on the walls. The paraffin coating is necessary for the atomic coherence to be transported between the beams. The experimental results are supported by a density-matrix analysis of the system, which also explains the observed amplitude and zero-crossing of the resonances. Such continuous tuning of the sign of a resonance has important applications in quantum memory and other precision measurements.

Hanle model of a spin-orbit coupled Bose-Einstein condensate of excitons in semiconductor quantum wells

We present a theoretical model of a driven-dissipative spin-orbit coupled Bose-Einstein condensate of indirect excitons in semiconductor quantum wells (QW's). Our steady-state solution of the problem shares analogies with the Hanle effect in an optical orientation experiment. The role of the spin pump in our case is played by Bose-stimulated scattering into a linearly-polarized ground state and the depolarization occurs as a result of exchange interaction between electrons and holes. Our theory agrees with the recent experiment [A. A. High et al., Phys. Rev. Lett. 110, 246403 (2013)], where spontaneous emergence of spatial coherence and polarization textures have been observed. As a complementary test, we discuss a configuration where an external magnetic field is applied in the structure plane.

Pure spin current transport in a SiGe alloy

Using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve devices with Si0.1Ge0.9, we study pure spin current transport in a degenerate SiGe alloy (n ∼ 5.0 × 1018 cm−3). Clear nonlocal spin-valve signals and Hanle effect curves, indicating generation, transport, and detection of pure spin currents, are observed. The spin diffusion length and spin lifetime of the Si0.1Ge0.9 layer at low temperatures are reliably estimated to be ∼0.5 µm and ∼0.2 ns, respectively. This study demonstrates the possibility of exploring physics and developing spintronic applications using SiGe alloys.

Influence of the Atmospheric Model on Hanle Diagnostics

We clarify the uncertainty in the inferred magnetic field vector via the Hanle diagnostics of the hydrogen Lyman-$\upalpha$ line when the stratification of the underlying atmosphere is unknown. We calculate the anisotropy of the radiation field with plane-parallel semi-empirical models under the nonlocal thermal equilibrium condition and derive linear polarization signals for all possible parameters of magnetic field vectors based on an analytical solution of the atomic polarization and Hanle effect. We find that the semi-empirical models of the inter-network region (FAL-A) and network region (FAL-F) show similar degrees of anisotropy in the radiation field, and this similarity results in an acceptable inversion error (e.g., ${\sim}\, 40~\mbox{G}$ instead of 50 G in field strength and ${\sim}\,100^{\circ}$ instead of $90^{\circ}$ in inclination) when FAL-A and FAL-F are swapped. However, the semi-empirical models of FAL-C (averaged quiet-Sun model including both inter-network and network regions) and FAL-P (plage regions) yield an atomic polarization that deviates from all other models, which makes it difficult to precisely determine the magnetic field vector if the correct atmospheric model is not known (e.g., the inversion error is much larger than 40% of the field strength; ${>}\,70~\mbox{G}$ instead of $50~\mbox{G}$). These results clearly demonstrate that the choice of model atmosphere is important for Hanle diagnostics. As is well known, one way to constrain the average atmospheric stratification is to measure the center-to-limb variation of the linear polarization signals. The dependence of the center-to-limb variations on the atmospheric model is also presented in this paper.

Spatial variation of dynamic nuclear spin polarization probed by the non-local Hanle effect

The spatial distribution of dynamic nuclear spin polarization (DNP) has been investigated in a lateral all-semiconductor spin-injection device based on a (Ga,Mn)As/n+-GaAs spin-Esaki diode. The DNP induced by the hyperfine interaction has been probed via satellite peaks in non-local Hanle-type spin precession signals, indicating the recovery of electron spin polarization. A quantitative analysis using the self-consistent calculation reproduces the magnetic field position of the satellite peaks as a function of spin injection bias and injector-detector separation. The distance dependence of the Hanle curves reveals that the spin-lattice relaxation rather than the hyperfine interaction is the dominant mechanism of nuclear spin relaxation in the non-local region.

Magneto-optical Effects in the Scattering Polarization Wings of the Ca i 4227 Å Resonance Line

Abstract The linear polarization pattern produced by scattering processes in the Ca i 4227 Å resonance line is a valuable observable for probing the solar atmosphere. Via the Hanle effect, the very significant Q/I and U/I line-center signals are sensitive to the presence of magnetic fields in the lower chromosphere with strengths between 5 and 125 G, approximately. On the other hand, partial frequency redistribution (PRD) produces sizable signals in the wings of the Q/I profile, which have always been thought to be insensitive to the presence of magnetic fields. Interestingly, novel observations of this line revealed a surprising behavior: fully unexpected signals in the wings of the U/I profile and spatial variability in the wings of both Q/I and U/I. We show that the magneto-optical (MO) terms of the Stokes-vector transfer equation produce sizable signals in the wings of U/I and a clear sensitivity of the Q/I and U/I wings to the presence of photospheric magnetic fields with strengths similar to those that produce the Hanle effect in the line core. This radiative transfer investigation on the joint action of scattering processes and the Hanle and Zeeman effects in the Ca i 4227 Å line should facilitate the development of more reliable techniques for exploring the magnetism of stellar atmospheres. To this end, we can now exploit the circular polarization produced by the Zeeman effect, the magnetic sensitivity caused by the above-mentioned MO effects in the Q/I and U/I wings, and the Hanle effect in the line core.

Spin relaxation and proximity effect in WS2/graphene/fluorographene non-local spin valves

The mechanisms leading to spin relaxation in graphene and its heterostructures continue to be debated. Control of the spin relaxation in graphene-based structures is necessary to achieve the envisioned utility of graphene in future spintronic devices beyond Moore's law. Proximity induced spin relaxation caused by contact to a high spin-orbit material, such as WS2, offers a promising avenue to manipulate the spin lifetime. We demonstrate the operation of WS2/graphene/fluorographene non-local spin valves and extract the spin lifetimes for a range of carrier concentrations by Hanle effect measurements. Four-terminal charge transport measurements allow us to calculate the momentum relaxation time as a function of carrier concentration and compare it to the spin lifetime. These data show that the D'yakonov-Perel’ mechanism is the dominant spin relaxation mechanism for WS2/graphene/fluorographene devices, while, for reference graphene/fluorographene devices, linear scaling between the spin and momentum lifetimes points to spin-flip scattering during strong elastic scattering events where the scattering event is strongly coupled to the electron spin. We attribute the change in spin relaxation type in part with the inclusion of WS2 as a substrate to proximity induced spin-orbit coupling due to the adjacent WS2 layer, and we compare our data to the literature.

Circularly polarized photoluminescence and Hanle effect measurements of spin relaxation in organic–inorganic hybrid perovskite films

Partial circularly polarized photoluminescence with polarization degree of 2% from CH3NH3PbBr3 film at 77 K is observed, excited using a circularly polarized 532 nm laser. Moreover, the Hanle effect is measured to study spin relaxation in CH3NH3PbBr3 film, yielding a spin lifetime of ∼240 ps at 77 K.

ВИМІРЮВАННЯ ПРОСТОРОВО НЕРОЗДІЛЬНИХ МАГНІТНИХ ПОЛІВ ЗМІШАНОЇ ПОЛЯРНОСТІ ЗА СПОСТЕРЕЖЕННЯМИ ЕФЕКТУ ЗЕЄМАНА В СОНЯЧНОМУ ФАКЕЛІ

It is shown that it is quite possible to measure spatially unresolved magnetic fields of mixed polarity by the Zeeman effect if their intensity exceeds 150-200 G. This means that one can interconnect the ranges of the registration of these fields based on the Hanle and Zeeman effects. This thesis is illustrated by the results of calculations of the half-width of the Stokes profile I for the FeI 5247.1 and 5250.2 lines. Parameter I is sensitive to the presence of magnetic fields, regardless of whether they have the same magnetic polarity within the input aperture of the instrument, or the opposite one. According to the calculations, if the accuracy of the measurement of the half-width ratio of these two lines is increased to 0.5 %, then it is possible to measure the magnetic fields from 70-100 G. On the basis of the proposed method, an estimate of the intensity of the magnetic fields of mixed polarity in a solar faculae in the tail part of the active region NOAA 1809 was made, which on the day of observations, August 6, 2013, was located not far from the Sun’s disc center. The Echelle Zeeman-spectrogram of this region was obtained on the Horizontal Solar Telescope of Astronomical Observatory of Taras Shevchenko National University of Kyiv. The magnetic field in the faculae was measured in two ways: by shift of of the “center of gravity”of the profiles of the FeI 5247.1 and FeI 5250.2 Ǻ lines in the I + V i I – V spectra and also by the half-width ratio of the profiles I of these lines. The first method allows to measure the effective magnetic field Beff, which turned out to be 280 G by FeI 5250.2 and 360 G by FeI 5247.1. The corresponding ratio Beff (5247.1) / Beff (5250.2) ≈ 1.3 indicates the existence of the sub-telescopic flux tubes with kilogauss fields. However, the ratio of the half-width of the Stokes I profiles of these two lines is 1.08, which corresponds to the magnetic field ± 650 G, if magnetic field is purely longitudinal. Based on the analysis of these data as well as the results of other studies, one can conclude that in the investigated faculae there were probably three magnetic field field components: the fluxtubes with kG field Bfluxtube, the areas of background field Bbackgr of regular magnetic polarity, and the areas of subtelescopic fields Bmixpol of mixed magnetic polarity. In absolute value the magnetic flux of tangled mixed-polarity field exceeds the flux of entire regular field with the intensities of Bfluxtube and Bbackgr at least 2-fold.

Spin accumulation in permalloy-ZnO heterostructures from both electrical injection and spin pumping

We report the results of room temperature spin injection and detection studies in ZnO using both electrical injection and spin pumping. At ferromagnetic resonance, an interfacial voltage with a constant polarity upon magnetization reversal is observed in permalloy-ZnO heterostructures, which is attributed to spin accumulation after ruling out other origins. Simultaneous electrical injection during spin pumping is achieved in samples with large interface resistance or insertion of a thin MgO layer at the interface of permalloy and ZnO. From the pumping frequency dependence of detected voltage, a spin lifetime of 32 ps is extracted for ZnO at room temperature, despite the fact that there was no Hanle effect observed in the same device using the conventional three-terminal DC measurement.

Anomalous Nonlocal Resistance and Spin-Charge Conversion Mechanisms in Two-Dimensional Metals.

We uncover two anomalous features in the nonlocal transport behavior of two-dimensional metallic materials with spin-orbit coupling. First, the nonlocal resistance can have negative values and oscillate with distance, even in the absence of a magnetic field. Second, the oscillations of the nonlocal resistance under an applied in-plane magnetic field (the Hanle effect) can be asymmetric under field reversal. Both features are produced by direct magnetoelectric coupling, which is possible in materials with broken inversion symmetry but was not included in previous spin-diffusion theories of nonlocal transport. These effects can be used to identify the relative contributions of different spin-charge conversion mechanisms. They should be observable in adatom-functionalized graphene, and they may provide the reason for discrepancies in recent nonlocal transport experiments on graphene.

Electrical spin transport in cylindrical silicon nanowires with CoFeB/MgO contacts

We examined electrical spin transport in cylindrical silicon nanowires (Si NWs) using the lateral nonlocal spin-valve (NLSV) geometry with CoFeB/MgO contacts. The use of a thin MgO layer as the tunnel barrier in the NLSV devices provided an optimum resistance-area product for spin transport measurements in the Si NWs. A robust NLSV spin signal of over 3.95 kΩ and clear minor loops were observed at 1.8 K in the Si NWs heavily doped with phosphorous. Furthermore, the NLSV magnetoresistance was strongly influenced by the local magnetizations resulting from the ferromagnetic (FM) electrodes being attached to the cylindrically shaped Si NW, with these magnetizations differing from those of bulk ferromagnets. These local micro-magnetic configurations of the FM electrodes led to intriguing NLSV spin signals associated with the Hanle effect. Our study of spin transport in the heavily doped Si NWs provides a sound basis for developing applications of nanoscale semiconductor spintronic devices.

Weak temperature and bias dependence of spin accumulation in epitaxial Co70Fe30/MgO tunnel contacts to p-type Ge

Electrical spin accumulation that was robust against changes in temperature and bias voltage was achieved in epitaxial Co70Fe30/MgO tunnel contacts to p-type Ge, with negligible Schottky barrier formation. Hanle and inverted Hanle effects, which are characteristic features of non-equilibrium spin accumulation in the tunnel contacts, were clearly observed up to room temperature for both hole spin injection and extraction. Notably, the obtained spin signal showed weak temperature dependence even at a low bias voltage, and symmetric behavior with respect to bias polarity, which are inconsistent with spin-polarized tunneling via localized states in the contact.

Polarized Line Formation in Arbitrary Strength Magnetic Fields Angle-averaged and Angle-dependent Partial Frequency Redistribution

Abstract Magnetic fields in the solar atmosphere leave their fingerprints in the polarized spectrum of the Sun via the Hanle and Zeeman effects. While the Hanle and Zeeman effects dominate, respectively, in the weak and strong field regimes, both these effects jointly operate in the intermediate field strength regime. Therefore, it is necessary to solve the polarized line transfer equation, including the combined influence of Hanle and Zeeman effects. Furthermore, it is required to take into account the effects of partial frequency redistribution (PRD) in scattering when dealing with strong chromospheric lines with broad damping wings. In this paper, we present a numerical method to solve the problem of polarized PRD line formation in magnetic fields of arbitrary strength and orientation. This numerical method is based on the concept of operator perturbation. For our studies, we consider a two-level atom model without hyperfine structure and lower-level polarization. We compare the PRD idealization of angle-averaged Hanle–Zeeman redistribution matrices with the full treatment of angle-dependent PRD, to indicate when the idealized treatment is inadequate and what kind of polarization effects are specific to angle-dependent PRD. Because the angle-dependent treatment is presently computationally prohibitive when applied to realistic model atmospheres, we present the computed emergent Stokes profiles for a range of magnetic fields, with the assumption of an isothermal one-dimensional medium.

High-speed optical three-axis vector magnetometry based on nonlinear Hanle effect in rubidium vapor

The magnetic-field-compensation optical vector magnetometer based on the nonlinear Hanle effect in alkali metal vapor allowing two-axis measurement operation has been further elaborated for three-axis performance, along with significant reduction of measurement time. The upgrade was achieved by implementing a two-beam resonant excitation configuration and a fast maximum searching algorithm. Results of the proof-of-concept experiments, demonstrating 1 μTB-field resolution, are presented. The applied interest and capability of the proposed technique is analyzed.

Spatiotemporal Evolution of Hanle and Zeeman Synthetic Polarization in a Chromospheric Spectral Line

Abstract Due to the quick evolution of the solar chromosphere, its magnetic field cannot be inferred reliably without accounting for the temporal variations of its polarized light. This has been broadly overlooked in the modeling and interpretation of the polarization, due to technical problems (e.g., lack of temporal resolution or of time-dependent MHD solar models) and/or because many polarization measurements can apparently be explained without dynamics. Here, we show that the temporal evolution is critical for explaining the spectral-line scattering polarization because of its sensitivity to rapidly varying physical quantities and the possibility of signal cancellations and attenuation during extended time integration. For studying the combined effect of time-varying magnetic fields and kinematics, we solved the 1.5D non-LTE problem of the second kind in time-dependent 3D R-MHD solar models and synthesized the Hanle and Zeeman polarization in forward scattering for the chromospheric λ4227 line. We find that the quiet-Sun polarization amplitudes depend on the periodicity and spectral coherence of the signal enhancements produced by kinematics, but that substantially larger linear polarization signals should exist all over the solar disk for short integration times. The spectral morphology of the polarization is discussed as a combination of Hanle, Zeeman, partial redistribution and dynamic effects. We give physical references for observations by degrading and characterizing our slit time series in different spatiotemporal resolutions. The implications of our results for the interpretation of the second solar spectrum and for the investigation of the solar atmospheric heatings are discussed.

Pattern recognition in spectra

We present a new automated procedure that simultaneously derives the effective temperature Teff, surface gravity log g, metallicity [Fe/H], and equatorial projected rotational velocity ve sin i for stars. The procedure is inspired by the well-known PCA-based inversion of spectropolarimetric full-Stokes solar data, which was used both for Zeeman and Hanle effects. The efficiency and accuracy of this procedure have been proven for FGK, A, and late type dwarf stars of K and M spectral types. Learning databases are generated from the Elodie stellar spectra library using observed spectra for which fundamental parameters were already evaluated or with synthetic data. The synthetic spectra are calculated using ATLAS9 model atmospheres. This technique helped us to detect many peculiar stars such as Am, Ap, HgMn, SiEuCr and binaries. This fast and efficient technique could be used every time a pattern recognition is needed. One important application is the understanding of the physical properties of planetary surfaces by comparing aboard instrument data to synthetic ones.

Large spin accumulation and crystallographic dependence of spin transport in single crystal gallium nitride nanowires

Semiconductor spintronics is an alternative to conventional electronics that offers devices with high performance, low power and multiple functionality. Although a large number of devices with mesoscopic dimensions have been successfully demonstrated at low temperatures for decades, room-temperature operation still needs to go further. Here we study spin injection in single-crystal gallium nitride nanowires and report robust spin accumulation at room temperature with enhanced spin injection polarization of 9%. A large Overhauser coupling between the electron spin accumulation and the lattice nuclei is observed. Finally, our single-crystal gallium nitride samples have a trigonal cross-section defined by the (001), () and () planes. Using the Hanle effect, we show that the spin accumulation is significantly different for injection across the (001) and () (or ()) planes. This provides a technique for increasing room temperature spin injection in mesoscopic systems.