Localization correction to the anomalous Hall effect in amorphous CoFeB thin films

An obvious weak localization correction to anomalous Hall conductance(AHC) in very thin CoFeB film is reported.We find that both the weak localization to AHC and the mechanism of the anomalous Hall effect are related to the CoFeB thickness.When the film is thicker than 3 nm,the side jump mechanism dominates and the weak locaUzation to AHC vanishes.For very thin CoFeB films,both the side jump and skew scattering mechanisms contribute to the anomalous Hall effect,and the weak localization correction to AHC is observed.     

Hall effect of tetragonal and orthorhombic SrRuO3 films

High quality orthorhombic and tetragonal SrRuO₃ thin films were grown by pulsed laser deposition on SrTiO₃(001) and Ba_0.75Sr_0.25TiO₃ buffered LaAlO₃(001) substrates. Resistivity vs. temperature curves showed a slope change at a Curie temperature of 147.5 ± 2 K for 40 nm thick films irrespective of crystalline symmetry. The Hall resistivity of both films contained an anomalous Hall contribution. The anomalous Hall coefficient was positive throughout the whole temperature range for the tetragonal film, whereas it showed a sign change at 143 K for the orthorhombic film. This is a strong indication that the Berry‐phase mechanism is the dominant anomalous Hall effect mechanism in SrRuO₃. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strong nonlinearity and hysteresis of Hall resistance versus magnetization in nickel thin films

The anomalous Hall effect (AHE) in ferromagnetic materials is perhaps one of the oldest unresolved mysteries in physics. First observed in 1881, its mechanism is still a controversial topic today. The question remains whether AHE is caused by intrinsic (Berry phase and band structure) or extrinsic (defect scattering) effects or a combination of both. Here we present experimental observation in nickel thin films that seems to add to the mystery, but may in fact provide crucial clues for ultimately resolving the controversy. The key observation is that the Hall resistivity of nickel films is a strongly nonlinear function of the magnetization and displays clear hysteresis with respect to M. Specifically, at low temperatures, the anomalous Hall coefficient switches between two saturated values under the magnetic field with a narrow transition region, but with a strong hysteresis, in contrast to the slow saturation of the magnetization. The nonlinearity and the hysteresis become more apparent with decreasing temperature or film thickness. Despite the simplicity of the lattice and magnetic structure of nickel films, these results are outside our current understanding of AHE, whether using intrinsic or extrinsic mechanisms of AHE. It presents a challenge for these models, and may be used as a test of validity for both types of theories.     

The spin Hall effect in single-crystalline gold thin films

The spin Hall effect has been investigated in 10-nm-thick epitaxial Au(001) single crystal films via H-pattern devices,whose minimum characteristic dimension is about 40 nm. By improving the film quality and optimizing the in-plane geometry parameters of the devices, we explicitly extract the spin Hall effect contribution from the ballistic and bypass contribution which were previously reported to be dominating the non-local voltage. Furthermore, we calculate a lower limit of the spin Hall angle of 0.08 at room temperature. Our results indicate that the giant spin Hall effect in Au thin films is dominated not by the interior defects scattering, but by the surface scattering. Besides, our results also provide an additional experimental method to determine the magnitude of spin Hall angle unambiguously.     

Giant Hall effect in (GaAs)Fe granular films

A series of (GaAs)_1 − xFe_x (x: volume fraction) films with Fe granules embedded in GaAs matrix were prepared by magnetron sputtering. Hall Effect of the films was characterized. The largest saturated Hall resistivity of was observed in (GaAs)₃₀Fe₇₀ film at room temperature, which is over 2 orders larger than that of pure Fe, about 1 order larger than that of (NiFe)–(Al₂O₃) and (NiCo)–(SiO₂) granular films prepared under the same preparation conditions, and 150% larger than that of Ge₃₀Fe₇₀.     

Thickness dependence of the anomalous Hall effect in disordered face-centered cubic FePt alloy films

The anomalous Hall effect in disordered face-centered cubic(fcc) FePt alloy films is experimentally studied. The longitudinal resistivity independent term of the anomalous Hall conductivity(AHC) increases and approaches saturation with increasing film thickness. The contribution of side jump scattering is suggested to decrease monotonically with increasing film thickness, which can be ascribed to the variation of the surface scattering with the film thickness. The sign of the skew scattering contribution to the AHC is opposite to that of the intrinsic contribution in the system.     

Tb doping induced enhancement of anomalous Hall effect in NiFe films

Tbx(Ni0.8Fe0.2)1-x films with x≤0.14 are fabricated and the anomalous Hall effect is studied.The intrinsic anomalous Hall conductivity and the extrinsic one from the impurity and phonon induced scattering both increase with increasing x.The enhancement of the intrinsic anomalous Hall conductivity is ascribed to both the weak spin–orbit coupling enhancement and the Fermi level shift.The enhancement of the extrinsic term comes from the changes of both Fermi level and impurity distribution.In contrast,the in-plane and the out-of-plane uniaxial anisotropies in the Tb Ni Fe films change little with x.The enhancement of the Hall angle by Tb doping is helpful for practical applications of the Hall devices.     

Co:BaTiO3纳米复合薄膜的制备及其结构

用脉冲激光沉积技术(PLD)在MgO(100)基底上生长了嵌埋Co纳米晶的BaTiO3复合薄膜. 分别利用x射线衍射(XRD)、原子力显微镜(AFM)以及拉曼光谱(Raman)对薄膜的微观结构、表面 形貌进行了表征. 结果表明该薄膜为c轴取向的四方晶体结构，薄膜表面均匀、致密、 具有原子尺度的光滑性，其均方根表面粗糙度(RMS)达到015nmCo以纳米晶形式嵌埋BaTi O3基体中，呈单分散性均匀分布，其粒径随激光脉冲数的增加而增大. Co：BaTiO3纳米 复合薄膜拉曼峰的强度随钴纳米晶粒径的增加明显减弱，但是峰的宽度逐渐增加. 关键词： Co：BaTiO3 纳米复合薄膜 脉冲激光沉积     

Prediction of quantum anomalous Hall effect in CrI3/ScCl2 bilayer heterostructure

Based on first-principles calculations, a two-dimensional (2D) van der Waals (vdW) bilayer heterostructure consisting of two topologically trivial ferromagnetic (FM) monolayers CrI₃ and ScCl₂ is proposed to realize the quantum anomalous Hall effect (QAHE) with a sizable topologically nontrivial band gap of 4.5 meV. Its topological nature is attributed to an interlayer band inversion between the monolayers and critically depends on the symmetry of the stacking configuration. We further demonstrate that the topologically nontrivial band gap can be increased nearly linearly by the application of a perpendicular external pressure and reaches 8.1 meV at 2.7 GPa, and the application of an external out-of-plane electric field can also modulate the band gap and convert the system back to topologically trivial via eliminating the band inversion. An effective model is developed to describe the topological phase evolution in this bilayer heterostructure. This work provides a new candidate system based on 2D vdW materials for realization of potential high-temperature QAHE with considerable controllability.     

On the origin of the anomalous sign reversal in the Hall effect in Nb thin films

We re-visit the anomalous sign reversal problem in the Hall effect of the sputtered Nb thin films. We find that the anomalous sign reversal in the Hall effect is extremely sensitive to a small tilting of the magnetic field and to the magnitude of the applied current. Large anomalous variations are also observed in the symmetric part of the transverse resistance R_xy. We suggest that the surface current loops on superconducting grains at the edges of the superconducting thin films may be responsible for the Hall sign reversal and the accompanying anomalous effects in the symmetric part of R_xy.     

Scaling behavior of mixed-state hall effect in MgB2 thin films

The Hall resistivity (ρ_xy) and the longitudinal resistivity (ρ_xx) in c-axis-oriented superconducting MgB₂ thin films have been investigated in extended fields up to 18 T. We have observed a scaling behavior between the Hall resistivity and the longitudinal resistivity, , where the exponent (β) is observed to be independent of the temperatures and the magnetic fields. For a wide magnetic field region from 1 to 18 T and a wide temperature region from 10 to 28 K, a universal power law with β = 2.0 ± 0.1 was observed in c-axis-oriented MgB₂ thin films. These results can be well interpreted by using recent models.     

Anomalous Hall effect and perpendicular magnetic anisotropy in Sm28Fe72 and Sm32Fe68 thin films

Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films of 100 nm thickness were grown using DC magnetron sputter deposition and their structure, magnetization, electrical and Hall resistance characteristics were investigated. An increase in electrical resistivity from 4.75×10⁻⁶ to 5.62×10⁻⁶ Ω m and from 2.26×10⁻⁶ to 2.84×10⁻⁶ Ω m for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, with decrease in temperature from 300 to 40 K is attributed to the strain induced anisotropy that dominates at lower temperatures. The positive extraordinary Hall coefficients (R_S) are observed for both films at 300 and 80 K. The existence of hysteresis indicates that Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films possess perpendicular anisotropy at 300 K. Hysteresis loop becomes narrow at 80 K for both Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films. Magnetization measurements at 300 K exhibiting small coercive field values of 31 and 49 Oe for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, confirm the existence of perpendicular anisotropy at 300 K.     

Anomalous strain effect in heteroepitaxial SrRuO3 films on (111) SrTiO3 substrates

We report comprehensive investigations into the structure of high-quality (111)-oriented SrRuO₃ films on SrTiO₃ substrates to elucidate the effect of (111) heteroepitaxial strain. We found that SrRuO₃ film with a thickness of ～ 40 nm is compressively strained in plane on the substrate with full coherency. Nevertheless, the out-of-plane spacing is almost the same as in the bulk, which is at odds with the conventional paradigm. By probing a series of half-order Bragg reflections using synchrotron-based x-ray diffraction combined with analyses of the scanning transmission electron microscopy images, we discovered that the heteroepitaxial strain is accommodated via significant suppression of the degree of c⁺ octahedral tilting and the formation of three equivalent domain structures on the (111) SrTiO₃ substrate. This anomalous effect sheds light on the understanding of an unconventional paradigm of film-substrate coupling for the (111) heteroepitaxial strain.     

Magnetization reversal process in Fe/Si (001) single-crystalline film investigated by planar Hall effect

A planar Hall effect(PHE) is introduced to investigate the magnetization reversal process in single-crystalline iron film grown on a Si(001) substrate.Owing to the domain structure of iron film and the characteristics of PHE,the magnetization switches sharply in an angular range of the external field for two steps of 90° domain wall displacement and one step of 180°domain wall displacement near the easy axis,respectively.However,the magnetization reversal process near the hard axis is completed by only one step of 90° domain wall displacement and then rotates coherently.The magnetization reversal process mechanism near the hard axis seems to be a combination of coherent rotation and domain wall displacement.Furthermore,the domain wall pinning energy and uniaxial magnetic anisotropy energy can also be derived from the PHE measurement.     

分子束外延Mn4N单晶薄膜的反常霍尔效应研究

Mn₄N是立方相的反钙钛矿型晶体,具有显著的亚铁磁性和反常霍尔效应.该文利用等离子辅助分子束外延在MgO(100)衬底上生长厚度为40 nm的Mn₄N(100)单晶薄膜,通过X射线衍射θ扫描和φ扫描,证实外延层的结构符合Mn₄N单晶的空间结构特征;化学态测试结果表明Mn₄N(100)薄膜内部存在Mn⁰、Mn²⁺和Mn⁴⁺等几种价态,其实际化学式为Mn_3.6N,薄膜中存在富余的N元素;电学测试数据表明Mn₄N(100)薄膜具有以电子为载流子的反常霍尔效应(在正磁场中得到负的霍尔电阻率),正常霍尔效应的贡献约占千分之六,其反常霍尔电阻率随着测试温度升高5 K～350 K单调增大,说明温度升高导致电子散射现象加剧.通过对测试数据分析可以推断,在5 K～50 K和50 K～75 K温度范围,反常霍尔效应的来源可分别归结为电子斜散射机制和电子边跳机制.在75 K～350 K这一温度范围内,反常霍尔效应...     

Effect of oxygen pressure on structure and properties of Bi1.5Zn1.0Nb1.5O7 pyrochlore thin films prepared by pulsed laser deposition

The Bi₂O₃-ZnO-Nb₂O₅ (BZN) cubic pyrochlore thin films were prepared on Pt/TiO₂/SiO₂/Si(1 0 0) substrates by using pulsed laser deposition process. The oxygen pressure was varied in the range of 5-50 Pa to investigate its effect on the structure and dielectric properties of BZN thin films. It is found that oxygen pressure during deposition plays an important role on structure and other properties of BZN films. The BZN films deposited at temperature of 650 °C and at O₂ pressure of 5 Pa have an amorphous BZN and Nb₂O₅ phases but exhibits a cubic pyrochlore structure with a preferential (2 2 2) orientation when the oxygen pressure increases to 10 Pa. Dielectric constant and loss tangent of the films deposited at 10 Pa are 185 and 0.0008 at 10 kHz, respectively. The dielectric tunability is about 10% at a dc bias field of 0.9 MV/cm.     

Large enhancement of domain wall-induced anomalous magnetoresistance in ferrimagnetic Tb/Co wires: The effect of injecting spin Hall current

We demonstrated domain wall (DW)-induced anomalous magnetoresistance (MR) generated in asymmetric and symmetric ferrimagnetic Tb/Co multilayered, and Tb–Co alloyed wires. The extraordinary Hall effect (EHE)-induced circulating currents in the vicinity of DWs between longitudinal voltage probes are assigned to the anomalous MR. A large anomalous MR ~1.5% was obtained in the asymmetric Tb/Co multilayered wire. The large MR can be attributed to an addition of spin Hall current with a long coherence length from an adjacent Pt layer. These results open new possibilities for the use of ferrimagnetic multilayered wires beyond multi-function devices.     

Pressure tuning of the anomalous Hall effect in the kagome superconductor CsV3Sb5

Controlling the anomalous Hall effect(AHE)inspires potential applications of quantum materials in the next generation of electronics.The recently discovered quasi-2D kagome superconductor CsV₃Sb₅ exhibits large AHE accompanying with the charge-density-wave(CDW)order which provides us an ideal platform to study the interplay among nontrivial band topology,CDW,and unconventional superconductivity.Here,we systematically investigated the pressure effect of the AHE in CsV₃Sb₅.Our high-pressure transport measurements confirm the concurrence of AHE and CDW in the compressed CsV₃Sb₅.Remarkably,distinct from the negative AHE at ambient pressure,a positive anomalous Hall resistivity sets in below 35 K with pressure around 0.75 GPa,which can be attributed to the Fermi surface reconstruction and/or Fermi energy shift in the new CDW phase under pressure.Our work indicates that the anomalous Hall effect in CsV₃Sb₅ is tunable and highly related to the band structure.     

Quenching of surface traps in Mn doped ZnO thin films for enhanced optical transparency

The structural and photoluminescence analyses were performed on un-doped and Mn doped ZnO thin films grown on Si (1 0 0) substrate by pulsed laser deposition (PLD) and annealed at different post-deposition temperatures (500-800 °C). X-ray diffraction (XRD), employed to study the structural properties, showed an improved crystallinity at elevated temperatures with a consistent decrease in the lattice parameter ‘c’. The peak broadening in XRD spectra and the presence of Mn 2p3/2 peak at ∼640 eV in X-ray Photoelectron Spectroscopic (XPS) spectra of the doped thin films confirmed the successful incorporation of Mn in ZnO host matrix. Extended near band edge emission (NBE) spectra indicated the reduction in the concentration of the intrinsic surface traps in comparison to the doped ones resulting in improved optical transparency. Reduced deep level emission (DLE) spectra in doped thin films with declined PL ratio validated the quenching of the intrinsic surface traps thereby improving the optical transparency and the band gap, essential for optoelectronic and spintronic applications. Furthermore, the formation and uniform distribution of nano-sized grains with improved surface features of Mn-doped ZnO thin films were observed in Field Emission Scanning Electron Microscopy (FESEM) images.     

Room-temperature anomalous Hall effect and magnetroresistance in (Ga, Co)-codoped ZnO diluted magnetic semiconductor films

This paper reports that the(Ga,Co)-codoped ZnO thin films have been grown by inductively coupled plasma enhanced physical vapour deposition.Room-temperature ferromagnetism is observed for the as-grown thin films.The x-ray absorption fine structure characterization reveals that Co 2+ and Ga 3+ ions substitute for Zn 2+ ions in the ZnO lattice and exclude the possibility of extrinsic ferromagnetism origin.The ferromagnetic(Ga,Co)-codoped ZnO thin films exhibit carrier concentration dependent anomalous Hall effect and positive magnetoresistance at room temperature.The mechanism of anomalous Hall effect and magneto-transport in ferromagnetic ZnO-based diluted magnetic semiconductors is discussed.