Hall Bar Devices Research Articles

Strong perpendicular anisotropic ferromagnet Fe3GeTe2/graphene van der Waals heterostructure

Two-dimensional magnets offer a new platform for exploring fundamental properties in van der Waals (vdW) heterostructures and their device applications. Here, we investigated heterostructure devices of itinerant metallic vdW ferromagnet Fe3GeTe2 (FGT) with monolayer chemical vapor deposited graphene. The anomalous Hall effect measurements of FGT Hall-bar devices exhibit robust ferromagnetism with strong perpendicular anisotropy at low temperatures. The electrical transport properties measured in FGT/graphene heterostructure devices exhibit a tunneling transport with weak temperature dependence. We assessed the suitability of such FGT/graphene heterostructures for spin injection and detection and investigated the presence of FGT on possible spin absorption and spin relaxation in the graphene channel. These findings will be useful for engineering spintronic devices based on vdW heterostructures.

Topological Hall Effect in Thin Films of an Antiferromagnetic Weyl Semimetal Integrated on Si.

Since the large room-temperature anomalous Hall effect was discovered in noncollinear antiferromagnets, Mn3Sn has received immense research interest as it exhibits abundant exotic physical properties including Weyl points and enormous potential for antiferromagnetic spintronic device applications. In this work, we report the emergence of the topological Hall effect in Mn3Sn films grown on Si that is the workhorse for the modern highly integrated information technology. Importantly, through a series of systematic comparative experiments, the intriguing topological Hall effect phenomenon related to the appearance of the noncoplanar chiral spin structure is found to be induced by the Mn3Sn/SiO2 interface. Furthermore, it was found that the current injection to a Pt/Mn3Sn bilayer Hall bar device can effectively manipulate the chiral spin structure of Mn3Sn, which demonstrates the feasibility of Si-based noncollinear antiferromagnetic spintronics.

Study of slow magnetization relaxation in Hf/GdFeCo/SiN Hall bar by anomalous Hall resistance measurements

The race-track memory concept relies on domain wall propagation through a magnetic track, which has the potential to change the future course of memory-related applications. Therefore, the materials where one can control the propagation of domain walls are crucial. Further, the stochastic nature of magnetization switching has potential applications in neuromorphic computing. Here we study the propagation of magnetic domains and the stochastic magnetization relaxation in the Gd-Fe-Co Hall bar. The magnetic relaxation of Hall bar devices is studied using anomalous Hall resistance measurements. This study highlights the behavior of magnetization relaxation under various parameters viz., at different magnitudes and directions of sensing currents, Hall probe aspect ratios, and the presence of a notch in the Hall bar. This study shows that the stochastic nature and delay in magnetization relaxation can be controlled by the sensing currents and the Hall bar geometry, which is crucial for magnetization switching-based applications.

Charge-mediated voltage modulation of magnetism in Hf0.5Zr0.5O2/Co multiferroic heterojunction

We construct the Hall-bar device with the size of several hundred nanometers based on the HZO/Co multiferroic heterojunction. A remarkable voltage-controlled magnetism is observed in the device that possesses both ferroelectric property and perpendicular magnetic anisotropy (PMA). The nucleation field and coercivity can be modulated by voltage pulse while saturation field keeps stable. The non-volatile and reversible voltage-controlled magnetism is ascribable to interfacial charges caused by ferroelectric polarization. Meanwhile, the effective anisotropy energy density (K u) can also be controlled by voltage pulse, a decrease of 83% and increase of 28% in K u are realized under –3-V and 3-V pulses, respectively. Because the energy barrier is directly proportional to K u under a given volume, a decreased or enhanced energy barrier can be controlled by voltage pulse. Thus, it is an effective method to realize low-power and high-stability magneto-resistive random-access memory (MRAM).

Extrinsic spin-valley Hall effect and spin-relaxation anisotropy in magnetized and strained graphene

The implementation of the quantum spin-valley Hall effect is a critical challenge for the spintronics and valleytronic experimentalists because it requires breaking both time-reversal symmetry ($\mathcal{T}$) and spatial inversion symmetry ($\mathcal{P}$) while preserving the joint symmetry $\mathcal{O}=\mathcal{T}\mathcal{P}$. Here, we demonstrate an extrinsic spin-valley Hall effect by the magnetic field and temperature modulation of the nonlocal resistance in a Hall bar device consisting of magnetized and strained graphene. Besides, we achieve a striking crossover from positive to negative nonlocal magnetoresistance owing to the magnetic field dependence of spin-valley relaxation instead of the usual Hanle spin precession. Moreover, we microscopically derive a large and tunable spin-relaxation anisotropy of the magnetized graphene within the Born-Markov and the Weiss-field approximations. Our findings offer fascinating opportunities to manipulate the spin and valley degrees of freedom and design novel electronic devices.

Joule heating and the thermal conductivity of a two-dimensional electron gas at cryogenic temperatures studied by modified 3ω method

During the standard ac lock-in measurement of the resistance of a two-dimensional electron gas (2DEG) applying an ac current I=2I0sin(ωt), the electron temperature Te oscillates with the angular frequency 2ω due to the Joule heating ∝I2. We have shown that the highest (TH) and the lowest (TL) temperatures during a cycle of the oscillations can be deduced, at cryogenic temperatures, exploiting the third-harmonic (3ω) component of the voltage drop generated by the ac current I and employing the amplitude of the Shubnikov–de Haas oscillations as the measure of Te. The temperatures TH and TL thus obtained allow us to roughly evaluate the thermal conductivity κxx of the 2DEG via the modified 3ω method, in which the method originally devised for bulk materials is modified to be applicable to a 2DEG embedded in a semiconductor wafer. κxx thus deduced is found to be consistent with the Wiedemann–Franz law. The method provides a convenient way to access κxx using only a standard Hall-bar device and the simple experimental setup for the resistance measurement.

High-field magnetotransport studies of surface-conducting diamonds

The observation of a strong and tunable spin-orbit interaction (SOI) in surface-conducting diamond opens up a new avenue for building diamond-based spintronics. Herein we provide a comprehensive method to analyze the magnetotransport behavior of surface-conducting hydrogen-terminated diamond (H-diamond) Hall bar devices and $\mathrm{Al}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/{\mathrm{V}}_{2}{\mathrm{O}}_{5}/\mathrm{H}$-diamond metal-oxide semiconductor field-effect transistors, respectively. By adopting a significantly improved theoretical magnetotransport model, the reduced magnetoconductance can be accurately explained both within and outside the quantum diffusive regime. The model is valid for all doping strategies of surface-conducting diamond tested. From this analysis, we find that the orbital magnetoresistance, a classical effect distinct from the SOI, dominates the magnetotransport in surface-conducting diamond at high magnetic fields. Furthermore, local hole mobilities as high as $1000\text{--}3000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ have been observed in this work, indicating the possibility of diamond-based electronics with ultrahigh hole mobilities at cryogenic temperatures.

Nonlocal thermoelectric resistance in vortical viscous transport

The pursuit for clearly identifiable signatures of viscous electron flow in the solid state systems has been a paramount task in the search of the hydrodynamic electron transport behavior. In this work, we investigate theoretically the nonlocal electric and thermal resistances for the generic non-Galilean-invariant electron liquids in the multiterminal Hall-bar devices in the hydrodynamic regime. The role of the device inhomogeneity is carefully addressed in the model of the disorder potential with the long-range correlation radius. We obtain analytic expressions for the thermoelectric resistances that are applicable in the full crossover regime from charge neutrality to high electron density. We show that the vortical component of the electron flow manifests in the thermal transport mode close to the charge neutrality where vorticity is already suppressed by the intrinsic conductivity in the electric current. This behavior can be tested by the high-resolution thermal imaging probes.

Electron transport properties of a narrow-bandgap semiconductor Bi2O2Te nanosheet

A thin, narrow-bandgap semiconductor Bi2O2Te nanosheet is obtained via mechanical exfoliation, and a Hall-bar device is fabricated from it on a heavily doped Si/SiO2 substrate and studied at low temperatures. Gate transfer characteristic measurements show that the transport carriers in the nanosheet are of n-type. The carrier density, mobility, and mean free path in the nanosheet are determined by measurements of the Hall resistance and the longitudinal resistance of the Hall-bar device, and it is found that the electron transport in the nanosheet is in a quasi-two-dimensional (2D), strongly disordered regime. Magnetotransport measurements for the device at magnetic fields applied perpendicular to the nanosheet plane show dominantly weak antilocalization (WAL) characteristics at low fields and a linear magnetoresistance (LMR) behavior at high fields. We attribute the WAL characteristics to strong spin–orbit interaction (SOI) and the LMR to the classical origin of strong disorder in the nanosheet. Low-field magnetoconductivity measurements are also performed and are analyzed based on the multi-channel Hikami–Larkin–Nagaoka theory with the LMR correction being taken into account. The phase coherence length, spin relaxation length, effective 2D conduction channel number, and coefficient in the linear term due to the LMR in the nanosheet are extracted. It is found that the spin relaxation length in the Bi2O2Te nanosheet is several times smaller than that in its counterpart Bi2O2Se nanosheet, and thus, an ultra-strong SOI is present in the Bi2O2Te nanosheet. Our results reported in this study would greatly encourage further studies and applications of this emerging narrow-bandgap semiconductor 2D material.

Quantum oscillations in a hexagonal boron nitride-supported single crystalline InSb nanosheet

A gated Hall-bar device is made from an epitaxially grown, free-standing InSb nanosheet on a hexagonal boron nitride (hBN) dielectric/graphite gate structure and the electron transport properties in the InSb nanosheet are studied by gate-transfer characteristic and magnetotransport measurements at low temperatures. The measurements show that the carriers in the InSb nanosheet are of electrons and the carrier density in the nanosheet can be highly efficiently tuned by the graphite gate. The mobility of the electrons in the InSb nanosheet is extracted from low-field magneotransport measurements and a value of the mobility exceeding ∼ 1.8 × 104 cm2⋅V−1⋅s−1 is found. High-field magentotransport measurements show well-defined Shubnikov–de Haas (SdH) oscillations in the longitudinal resistance of the InSb nanosheet. Temperature-dependent measurements of the SdH oscillations are carried out and key transport parameters, including the electron effective mass m* ∼ 0.028m 0 and the quantum lifetime τ ∼ 0.046 ps, in the InSb nanosheet are extracted. It is for the first time that such experimental measurements have been reported for a free-standing InSb nanosheet and the results obtained indicate that InSb nanosheet/hBN/graphite gate structures can be used to develop advanced quantum devices for novel physics studies and for quantum technology applications.

Fabrication and characterization of InSb nanosheet/hBN/graphite heterostructure devices

Semiconductor InSb nanosheet/hexagonal boron nitride (hBN)/graphite trilayers are fabricated, and single- and double-gate devices made from the trilayers are realized and characterized. The InSb nanosheets employed in the trilayer devices are epitaxially grown, free-standing, zincblende crystals and are in micrometer lateral sizes. The hBN and graphite flakes are obtained by exfoliation. Each trilayer is made by successively stacking an InSb nanosheet on an hBN flake and on a graphite flake using a home-made alignment stacking/transfer setup. The fabricated single- and double-gate devices are characterized by electrical and/or magnetotransport measurements. In all these devices, the graphite and hBN flakes are employed as the bottom gates and the gate dielectrics. The measurements of a fabricated single bottom-gate field-effect device show that the InSb nanosheet in the device has an electron field-effect mobility of ∼7300 cm2 V−1 s−1 and a low gate hysteresis of ∼0.05 V at 1.9 K. The measurements of a double-gate Hall-bar device show that both the top and the bottom gate exhibit strong capacitive couplings to the InSb nanosheet channel and can thus tune the nanosheet channel conduction effectively. The electron Hall mobility in the InSb nanosheet of the Hall-bar device is extracted to be larger than 1.1 × 104 cm2 V−1 s−1 at a sheet electron density of ∼6.1 × 1011 cm −2 and 1.9 K and, thus, the device exhibits well-defined Shubnikov–de Haas oscillations.

Hydrodynamic electron transport in graphene Hall-bar devices

We consider hydrodynamic electron transport in the Hall-bar geometry. The theory is developed for systems with non-Galilean-invariant electron liquids. We show that inhomogeneity of the electron density induced by long-range disorder and gating leads to mixing between the hydrodynamic transport mode and transport relative to the electron liquid. For graphene systems near charge neutrality, these effects lead to strong coupling of the hydrodynamic flow to charge transport. As a result, the effective electrical conductivity of the system may significantly exceed the intrinsic conductivity of the electron liquid. We obtain analytic expressions for the thermoelectric transport coefficients of the system as a function of density in the full crossover region between clean and disorder-dominated regimes.

Nanosecond ultralow power spin orbit torque magnetization switching driven by BiSb topological insulator

Topological insulators (TIs) are promising for spin–orbit torque (SOT) switching thanks to their giant spin Hall angle. SOT switching using TIs has been studied so far in the thermal activation regime by direct currents or relatively long pulse currents (≥10 ns). In this work, we studied SOT magnetization switching of (Pt/Co) multilayers with strong perpendicular magnetic anisotropy by the BiSb topological insulator in both thermal activation and fast switching regime with pulse width down to 1 ns. We reveal that the zero-Kelvin threshold switching current density Jth0BiSb is 2.5 × 106 and 4.1 × 106 A/cm2 for the thermal activation regime and fast switching regime in a 800 nm-wide Hall bar device via domain wall depining. From time-resolved measurements using 1 ns pulses, we find that the domain wall velocity is 430–470 m/s at JBiSb = 1.6 × 107–1.7 × 107 A/cm2. Our work demonstrates the potential of the BiSb thin film for ultralow power and fast operation of SOT-based spintronic devices.

Novel behaviors of coercivity in GdFeCo/Hf/MgO heterostructure

It is commonly known that the coercivity (Hc) of the rare-earth/transition-metal (RE–TM) compound is tuned based on the varying RE content as compared to the TM component. The drawback of this approach is that the Hc changes are permanent. In this work, we investigate the coercivity behaviors of the GdFeCo/Hf/MgO heterostructure where the heavy metal Hf layer is inserted in the middle not only to convert charge current into spin current but also to prevent the oxygen diffusion effect. A strong geometry dependence of coercivity detected on Hall bar devices is attributed to the intrinsic properties of GdFeCo and elucidates that the oxidation issue from MgO on GdFeCo is prevented. By selecting a proper Gd content, we demonstrate that the coercivity can be altered flexibly in a wide range Hcmax/Hcmin≈200% via electric current. We develop a simple model to simulate the roles of Joule heating and spin–orbit torques in the novel behaviors of coercivity. It is verified that the excellent magnetic characteristics of GdFeCo are still preserved while manipulating coercivity by the electric current. Our findings broaden the new magnetic behaviors of RE–TM alloys, making them attractive for tunable magnetic anisotropy.

Hall bar measurements of topological surface states of (001) cadmium arsenide thin films interfaced with superconductors

Hybrid structures consisting of a topological insulator brought in proximity to an $s$-wave superconductor are one of the most promising routes to topological superconductivity and Majorana quasiparticles. A key challenge is the superconductor/topological insulator interface, which must be transparent to Cooper pairs while also preserving high-mobility topological states. Here, we investigate Hall bar devices fabricated on the topological insulator-like surfaces of (001) cadmium arsenide films, which we partially cover with different superconductors. We show that, depending on the magnitude of a perpendicular magnetic field, devices with sputter-deposited Nb exhibit superconductivity and the quantum Hall effect from the topological surface states, respectively. By contrast, devices with thermally evaporated Sn do not show superconductivity, indicating poor interface transparency. We discuss the coupling between the superconductor and the surface states.

Gate tuning of fractional quantum Hall states in an InAs two-dimensional electron gas

We report the observation of fractional quantum Hall (FQH) effects in a two-dimensional electron gas (2DEG) confined to an InAs/AlGaSb quantum well, using a dual-gated Hall-bar device allowing for the independent control of the vertical electric field and electron density. At a magnetic field of 24 T, we observe FQH states at several filling factors, namely $\ensuremath{\nu}=5/3, 2/3$, and $1/3$, in addition to the $\ensuremath{\nu}=4/3$ previously reported for an InAs 2DEG. The $\ensuremath{\nu}=4/3$ and $5/3$ states, which are absent at zero back-gate voltage, emerge as the quantum well is made more symmetric by applying a positive back-gate voltage. The dependence of zero-field electron mobility on the quantum-well asymmetry reveals a significant contribution of interface-roughness scattering, with much stronger scattering at the lower InAs/AlGaSb interface. However, the dependence of the visibility of the FQH effects on the quantum-well asymmetry is not entirely consistent with that of mobility, suggesting that a different source of disorder is also at work.

Spin‐Torque Switching in Rare‐Earth‐Free Compensated Ferrimagnet Mn4N Films

AbstractIn this paper, the growth of rare‐earth‐free compensated ferrimagnet (C‐FIM) Mn4N single crystalline films is reported, with perpendicular magnetic anisotropy (PMA) on the MgO(001) substrates by reactive magnetron sputtering. Subsequently, the current‐induced spin‐orbit torque (SOT) switching is experimentally demonstrated through passing currents in the Hall bar device made of Mn4N/Pt bilayer, with the thickness of Mn4N approaching 60 nm. Furthermore, the accompanied SOT efficiencies are characterized by employing the harmonic Hall voltage measurements. In particular, an enhanced effective SOT efficiency with the increased thickness of Mn4N films is observed, suggesting the existence of a bulk‐like spin torque in the present material system. The results demonstrate that the perpendicularly magnetized C‐FIM Mn4N films can provide a promising material platform for developing the rare‐earth‐free FIM spintronics devices.

Negative nonlocal vicinity resistance of viscous flow in a two-dimensional electron system

We perform a quantum transport study of Hall viscous liquid in multiterminal narrow Hallbar devices of a high-mobility two-dimensional electron system (2DES) in GaAs/AlGaAs heterostructure. In the nonlocal transport measurements of vicinity geometry under magnetic fields $(B)$, we observe that the absolute negative magnetoresistance $({R}_{xx})$ value $({R}_{\text{min}}<0)$ and the corresponding magnetic field $({B}_{\text{min}})$ are both inversely proportional to the adjacent Hallbar arm distance $(L)$. The occurrence of negative resistance is dependent on the characteristic lengths of the devices and the electron flow direction under $B$-fields. The minimal resistance ${R}_{\text{min}}$ occurs when the cyclotron radius ${R}_{C}$ approximates $L$. Multiples of high-order ($n\mathrm{th}$-order) ${R}_{\text{min},n}$'s persist from low to high magnetic fields in a large sample size of $L=5\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. Our experimental study reveals the transport behaviors in the vicinity regime, where the negative resistances depict viscous electronic flows in the high-mobility 2DES. In addition, the negative and high-order minimal resistances expand to a magnetic field of several kGs.

Bilayer Quantum Hall States in an n-Type Wide Tellurium Quantum Well.

Tellurium (Te) is a narrow bandgap semiconductor with a unique chiral crystal structure. The topological nature of electrons in the Te conduction band can be studied by realizing n-type doping using atomic layer deposition (ALD) technique on two-dimensional (2D) Te film. In this work, we fabricated and measured the double-gated n-type Te Hall-bar devices, which can operate as two separate or coupled electron layers controlled by the top gate and back gate. Profound Shubnikov-de Haas (SdH) oscillations are observed in both top and bottom electron layers. Landau level hybridization between two layers, compound and charge-transferable bilayer quantum Hall states at filling factor ν = 4, 6, and 8, are analyzed. Our work opens the door for the study of Weyl physics in coupled bilayer systems of 2D materials.

Probing the magnetization switching with in-plane magnetic anisotropy through field-modified magnetoresistance measurement

Effective probing current-induced magnetization switching is highly required in the study of emerging spin–orbit torque (SOT) effect. However, the measurement of in-plane magnetization switching typically relies on the giant/tunneling magnetoresistance measurement in a spin valve structure calling for complicated fabrication process, or the non-electric approach of Kerr imaging technique. Here, we present a reliable and convenient method to electrically probe the SOT-induced in-plane magnetization switching in a simple Hall bar device through analyzing the MR signal modified by a magnetic field. In this case, the symmetry of MR is broken, resulting in a resistance difference for opposite magnetization orientations. Moreover, the feasibility of our method is widely evidenced in heavy metal/ferromagnet (Pt/Ni20Fe80 and W/Co20Fe60B20) and the topological insulator/ferromagnet (Bi2Se3/Ni20Fe80). Our work simplifies the characterization process of the in-plane magnetization switching, which can promote the development of SOT-based devices.