Hall Resistivity Ρxy Research Articles

Identifying and clearing individual oxygen impurities on graphene through the use of NO2 as a radical scavenger

We present a novel method to detect and eliminate oxygen adatoms on a graphene basal plane, which can be observed as individual single-electron transfer (SET) events through graphene Hall measurements. The central idea, supported by first-principles calculations, is to use NO2 as a radical scavenger, effectively targeting and removing oxygen impurities on graphene. The reaction between NO2 and the oxygen adatom produces a negatively charged NO3− species and creates a hole carrier in graphene. Subsequently, NO3− reacts with NO2 to form N2O5, accompanied by an electron back-donation to graphene. The thermal decomposition of N2O5 produces neutral NO3, which can accept an electron from graphene and return to the NO3− state. Under conditions of low oxygen impurity levels and NO2 partial pressures, these SET reactions induce intermittent, step-like changes in the graphene's Hall resistivity ρxy at room temperature. Notably, the simulated ρxy patterns during NO2 adsorption closely resemble previously observed patterns [Schedin et al., Nat. Mater. 6 (2007) 652–655], suggesting a connection to the proposed SET reactions for unintentionally introduced oxygen impurities. Furthermore, we find that NO2 also reacts with hydroxyl and peroxide groups on the graphene basal plane, removing them by forming HNO3 and N2O5, respectively. Our findings offer a promising approach to remove oxygen-containing functional groups from graphene, paving the way for obtaining high-quality graphene from reduced graphene oxide.

Observation of fractionally quantized anomalous Hall effect.

The integer quantum anomalous Hall (QAH) effect is a lattice analogue of the quantum Hall effect at zero magnetic field1-3. This phenomenon occurs in systems with topologically non-trivial bands and spontaneous time-reversal symmetry breaking. Discovery of its fractional counterpart in the presence of strong electron correlations, that is, the fractional QAH effect4-7, would open a new chapter in condensed matter physics. Here we report the direct observation of both integer and fractional QAH effects in electrical measurements on twisted bilayer MoTe2. At zero magnetic field, nearfilling factor ν = -1 (one hole per moiré unit cell), we see an integer QAH plateau in the Hall resistance Rxy quantized to h/e2 ± 0.1%, whereas the longitudinal resistance Rxx vanishes. Remarkably, at ν = -2/3 and -3/5, we see plateau features in Rxy at [Formula: see text] and [Formula: see text], respectively, whereas Rxx remains small. All features shift linearly versus applied magnetic field with slopes matching the corresponding Chern numbers -1, -2/3 and -3/5, precisely as expected for integer and fractional QAH states. Additionally, at zero magnetic field, Rxy is approximately 2h/e2 near half-filling (ν = -1/2) and varies linearly as ν is tuned. This behaviour resembles that of the composite Fermi liquid in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field8-14. Direct observation of the fractional QAH and associated effects enables research in charge fractionalization and anyonic statistics at zero magnetic field.

Hall effect of RCo2 (R= Er and Lu) compounds

The Hall effect of RCo2 with R = Er and Lu has been studied. LuCo2 is an exchange-enhanced Pauli paramagnet. ErCo2 is ferrimagnetic below TC = 32 K and undergoes a first-order magnetic transition at TC. The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. We estimated the normal (ordinary) Hall coefficient RN and the anomalous Hall coefficient RA from the analyses of the ρxy – B curves and the magnetization curves. For LuCo2, ρxy increases linearly with increasing field and its slope decreases rapidly with increasing temperature below 70 K. This is attributable to strong temperature dependence of RN. The ρxy – B curve of ErCo2 depends on a magnetic state. In the ferrimagnetic state, both RN and RA have positive values, while RN is positive and RA is negative in the paramagnetic state. We calculated RN's of nonmagnetic and ferromagnetic LuCo2 from the energy band structures and compared them with the experimental values. The origin of a sign change in RA of ErCo2 is briefly discussed.

An anomalous Hall effect in edge-bonded monolayer graphene.

An anomalous Hall effect (AHE) is usually presumed to be absent in pristine graphene due to its diamagnetism. In this work, we report that a gate-tunable Hall resistance Rxy can be obtained in edge-bonded monolayer graphene without an external magnetic field. In a perpendicular magnetic field, Rxy consists of a sum of two terms: one from the ordinary Hall effect and the other from the AHE (RAHE). Plateaus of Rxy ∼ 0.94h/3e2 and RAHE ∼ 0.88h/3e2 have been observed while the longitudinal resistance Rxx decreases at a temperature of 2 K, which are indications of the quantum version of the AHE. At a temperature of 300 K, Rxx shows a positive, giant magnetoresistance of ∼177% and RAHE still has a value of ∼400 Ω. These observations indicate the existence of a long-range ferromagnetic order in pristine graphene, which may lead to new applications in pure carbon-based spintronics.

Transport properties and phase diagrams of FeSe1−S (0 ≤ x ≤ 1) single crystals

Based on the high quality FeSe1−xSx (0 ≤ x ≤ 1) single crystals synthesized via a hydrothermal method, we carried out systematic measurement of the normal state magnetoresistance and Hall effect. Through the maximum entropy mobility spectrum analysis (MEMSA), we investigate the evolution of mobility spectrum, which reflects the multi-band structures for different doping content x and temperature T. The relationships between Tc and carrier properties calculated by MEMSA, as well as bond angles and anion height calculated by first-principle method are investigated. Nonlinear to linear transition of Hall resistivity(ρxy(B)) is mainly attributed to the decreasing mobilities and merging of mobility peaks with the reductive x or increasing T. The relationship between Tc and carrier density n show weak interaction for the orthogonal phases when x ≤ 0.2, but nearly positive correlation for the tetragonal phases when x > 0.2. Bond angle as well as the Se/S anion height change almost linearly with x within the tetragonal phases, and the V-shaped relationships with Tc indicate that structural factors play a key role in the regulation of superconductivity. Our researches are helpful for understanding the multi-band and complex phase diagram of iron-based superconductors.

Hall effect of itinerant electron metamagnet Co(S1-xSex)2

We have studied the Hall effect of Co(S1-xSex)2 with 0.08 ≤ x ≤ 0.14, which shows itinerant electron metamagnetism (IEM). The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. It is found that ρxy decreases nearly linearly with increasing B in the paramagnetic state, while it has a positive slope in the ferromagnetic state. Abrupt drops were observed at around the metamagnetic transition field in the ρxy – B curves, which are attributable to the anomalous Hall effect. From the analyses of the ρxy – B curves and the magnetization curves, we estimated the anomalous Hall coefficient, the normal (ordinary) Hall coefficient in the ferromagnetic state, and that in the paramagnetic state. Appearance of both ferromagnetic and paramagnetic states depending on a magnetic field in IEM makes it possible to estimate these values, simultaneously. The estimated normal Hall coefficients are compared with the theoretical values calculated from the energy band structures. The present study has demonstrated that IEM provides a suitable stage to understand the electronic structures of both ferromagnetic and paramagnetic states of the itinerant electron system at the same temperature.

Hall Effect Study of the Metamagnetic Transition in the Sr4(Ru0.99Fe0.01)3O10 Nanosheet

Sr4(Ru0.99Fe0.01)3O10 shows a ferromagnetic (FM) transition at TC ∼ 105 K with the magnetic easy axis in the ab plane, followed by a metamagnetic transition (MMT) at low temperatures when the magnetic field H is applied along the c axis, which is in sharp contrast to that of the pure Sr4Ru3O10, where the easy axis is along the c axis and the MMT is in the ab plane. Here, we studied the MMT in the Sr4(Ru0.99Fe0.01)3O10 nanosheet by the Hall effect. It was found that the ordinary Hall coefficient of Sr4(Ru0.99Fe0.01)3O10 is almost the same as that of the pure Sr4Ru3O10, while a sudden increase in the Hall resistance Rxy is observed below ∼50 K, above which the Rxy presents the conventional anomalous Hall effect up to TC. Analysis of the results indicates that the MMT has no direct correlation to the electronic structure but closely relates to the magnetic moment locking, where the magnetic-field-induced breakdown of the locked moments is responsible for the MMT.

Thickness-dependent magnetic properties in Pt/[Co/Ni] n multilayers with perpendicular magnetic anisotropy

We systematically investigated the Ni and Co thickness-dependent perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain structures, and magnetization dynamics of Pt(5 nm)/[Co(t Co)/Ni(t Ni)]5/Pt(1 nm) multilayers by combining the four standard magnetic characterization techniques. The magnetic-related hysteresis loops obtained from the field-dependent magnetization M and anomalous Hall resistivity (AHR) ρxy showed that the two serial multilayers with t Co = 0.2 nm and 0.3 nm have the optimum PMA coefficient K U as well as the highest coercivity H C at the Ni thickness t Ni = 0.6 nm. Additionally, the magnetic domain structures obtained by magneto-optic Kerr effect (MOKE) microscopy also significantly depend on the thickness and K U of the films. Furthermore, the thickness-dependent linewidth of ferromagnetic resonance is inversely proportional to K U and H C, indicating that inhomogeneous magnetic properties dominate the linewidth. However, the intrinsic Gilbert damping constant determined by a linear fitting of the frequency-dependent linewidth does not depend on the Ni thickness and K U. Our results could help promote the PMA [Co/Ni] multilayer applications in various spintronic and spin-orbitronic devices.

Impact of Dysprosium doping on the structural and electrical properties of BiSbTe3 single crystals

Breaking of time reversal symmetry in 3D topological insulators and tuning of Fermi level in the vicinity of the Dirac point is a prerequisite for unravelling various exotic physical phenomena which might be leveraged for potential technological device applications. Here we account effective single crystal growth of (BiSb)2-xDyxTe3 where x = 0, 0.005, 0.05 using modified Bridgman method. The structural, morphological, electrical and Hall measurements were performed on the synthesized single crystals. X-ray Diffraction (XRD) measurements confirmed high quality crystal growth with unidirectional orientation of the synthesized crystals. The lattice parameters were found decreasing with Dysprosium (Dy) doping thereby decreasing the lattice volume,indicating the incorporation of Dy atoms in the lattice of BiSbTe3. Scanning Electron Microscopy (SEM) confirmed the layered morphology of synthesized crystals while Energy Dispersive X-ray Spectroscopy (EDX) measurements revealed the uniform distribution of all the elements as well as stoichiometry of prepared crystals. The resistivity versus temperature (ρ-T) plot from 4 K to 300 K showed metallic behaviour of crystals with an increase in resistivity with Dy doping. The Hall resistivity (ρxy) demonstrates that holes are the predominant charge carriers. The Hall data revealed displacement of Fermi level with Dy doping. These results are indicative to the shift in the Fermi level in the vicinity of the Dirac point.

Magnetism and transport behavior of Ni42Co8Mn38Sb12: Magnetization, electrical resistivity and Hall effect measurements

Magnetic and transport behavior of Ni42Co8Mn38Sb12 alloy is discussed using the combined results of magnetization, electrical resistivity and Hall effect measurements. The alloy undergoes a ferromagnetic transition at 364 K, followed by a martensitic transition at 110 K. Large positive Curie-Weiss temperature, soft-ferromagnetic behavior with negative magnetic entropy change below 50 K and a well-scaled normalized magnetic-entropy onto a single universal curve confirm the predominant ferromagnetic exchange interactions in the alloy. Normal Hall coefficients were found to be nearly constant in ferromagnetic region in the temperature range of 120–300 K. The electrical transport properties are largely governed by electrons with a carrier concentration of about 1023/cc. Quadratic dependence of anomalous Hall resistivity (ρxy) on longitudinal resistivity (ρxx) indicates the dominant intrinsic contribution. In a magnetic field of 50 kOe, magnetoresistance is found to be about 34% near the martensitic transition temperature while it is about 2.8% near the second-order transition.

Hydrothermal Synthesis and Transport Properties of FeS1-xTex (0 \u2264 x \u2264 0.15) Single Crystals

In this work, a series of FeS1-xTex (0 ≤ x ≤ 0.15) single crystals were successfully synthesized by a hydrothermal method for the first time. According to the measurement of in-plane resistivity, Hall effect, and magnetoresistance (MR), we find that the superconducting transition temperature Tc is rapidly suppressed with the increasing Te substitution, and finally the superconductivity disappears when x > 0.05. With the substitution of Te for S, the residual resistivity ρ0 increases while the residual resistivity ratio (RRR) decreases monotonously. Meanwhile, the MR of FeS1-xTex is also reduced by Te doping. All these results reveal that the Te substitution introduces more impurity scattering. In consequence, the non-linear field-dependent of Hall resistivity ρxy at low temperature region is suppressed and a linear behavior is restored upon Te doping. The negative Hall coefficients RH for all the FeS1-xTex samples suggest that the electron-type carrier dominates the electrical conduction. Moreover, the MR of FeS1-xTex obviously follows Kohler’s law, indicating the isotropic scattering rates in the Fermi surface.

Magnetoresistance and Kondo Effect in Nodal-Line Semimetal VAs2 * *Supported by the National Key R&D Program of China (Grant No. 2016YFA0300402), the National Natural Science Foundation of China (Grant Nos. 12074335 and 11974095), the Zhejiang Natural Science Foundation (Grant No. LY16A040012), and the Fundamental Research Funds for the Central Universities.

We performed calculations of the electronic band structure and the Fermi surface, measured the longitudinal resistivity ρxx (T,H), Hall resistivity ρxy (T,H), and magnetic susceptibility as a function of temperature at various magnetic fields for VAs2 with a monoclinic crystal structure. The band structure calculations show that VAs2 is a nodal-line semimetal when spin-orbit coupling is ignored. The emergence of a minimum at around 11 K in ρxx (T) measured at H = 0 demonstrates that some additional magnetic impurities (V4+, S = 1/2) exist in VAs2 single crystals, inducing Kondo scattering, evidenced by both the fitting of ρxx (T) data and the susceptibility measurements. It is found that a large positive magnetoresistance (MR) reaching 649% at 10 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of ρxx (T) also emerge in VAs2, although MR is not so large due to the existence of additional scattering compared with other topological nontrivial/trivial semimetals. The observed properties are attributed to a perfect charge-carrier compensation, which is evidenced by both the calculations relying on the Fermi surface and the Hall resistivity measurements. These results indicate that the compounds containing V (3d 3 4s 2) element can be as a platform for studying the influence of magnetic impurities to the topological properties.

Current Distributions in the Vicinity of Electrical Contacts with a Two Dimensional Electron Gas under High Magnetic Fields

Current distributions near electrodes with a two-dimensional electron gas were investigated by low temperature measurement of the longitudinal resistivity ρxx, and the Hall resistivity ρxy, of a Ha...

Physical and magnetic properties of (Ce1−xNdx)3Al (x = 0.3): Coexisting ferromagnetism and Kondo effect

By means of detailed electrical, magnetic and thermal studies, we illustrate the unusual coexistence of ferromagnetism and Kondo effect for the critical Nd concentration of x = 0.3 in (Ce1−xNdx)3Al. Magnetic susceptibility χ(T) confirms the ferromagnetic character with TC ~ 8 K along with Kondo effect which appears below 35 K. Non-saturating nature of magnetization (M (H)) and Hall resistivity (ρxy (H)) isotherms indicates the Kondo screening. Resistivity ρ (T) curves exhibit nonlinear temperature dependence; ~Tn (0.48 < n < 1) below 40 K. Higher values of Sommerfeld coefficient γ (=321 mJ/Ln - mol-K2) and Seebeck coefficient S (T) as compared to the parent compound Ce3Al and other Nd doped compounds in the series (Ce1−xNdx)3Al (x = 0–1) indicate the role of critical Nd concentration namely, 0.3 in the anomalous behavior. The high value of γ observed in this critical concentration is unusual for a ferromagnetic compound and appears to be partially contributed by the exchange interaction due to Nd doping along with the Kondo hybridization. Suppression of γ value with field corroborates with the resistivity data i.e., weakening of Kondo screening. Like in the case of Ce3Al, Nd doped compound also shows structural transition attributed to the valence fluctuation of Ce. χ (T), ρ (T), S (T) and 4f part of specific heat, C4f (T) data show peculiar temperature dependence and confirm the non-Fermi liquid behavior due to Ce (4f) – Nd (4f) interaction. Change in the relative strengths of Coulomb repulsion (U) and Kondo hybridization (Vfd) is found to determine the temperature dependence of thermal and thermoelectric behavior in this compound.

Scaling analysis of anomalous Hall resistivity in the Co2TiAl Heusler alloy

A comprehensive magnetotransport study including resistivity (ρxx), isothermal magnetoresistance, Hall resistivity (ρxy) and magnetization have been carried out at different temperatures on the Co2TiAl Heusler alloy. Co2TiAl alloy shows a paramagnetic to ferromagnetic (FM) transition below the Curie temperature (TC) ∼ 125 K. In the FM region, resistivity and magnetoresistance reveal a spin flip electron–magnon scattering and the Hall resistivity unveils the anomalous Hall resistivity. Scaling of anomalous Hall resistivity with resistivity establishes the extrinsic scattering process responsible for the anomalous Hall resistivity; however skew scattering is the dominant mechanism compared to the side-jump contribution. A one to one correspondence between magnetoresistance and side-jump contribution to anomalous Hall resistivity verifies the electron–magnon scattering being the source of side-jump contribution to the anomalous Hall resistivity.

Bending Strain-Tailored Magnetic and Electronic Transport Properties of Reactively Sputtered γ'-Fe4N/Muscovite Epitaxial Heterostructures toward Flexible Spintronics.

The strain modulation on the magnetic and electronic transport properties of the ferromagnetic films is one of the hot topics due to the practical applications in flexible and wearable spintronic devices. However, the large strain-induced saturation magnetization and resistance change is not easy to achieve because most of the ferromagnetic films deposited on flexible substrates are polycrystalline or amorphous. Here, the flexible epitaxial γ'-Fe4N/mica films are fabricated by facing-target reactive sputtering. At a tensile strain with a radius of curvature (ROC) of 3 mm, the saturation magnetization (Ms) of the γ'-Fe4N/mica film is tailored significantly with a maximal variation of 210%. Meanwhile, the magnetic anisotropy was broadly tunable at different strains, where the out-of-plane Mr/Ms at a tensile strain of ROC = 2 mm is six times larger than that at the unbent state. Besides, the strain-tailored longitudinal resistance Rxx and anomalous Hall resistivity ρxy appear where the drop of Rxx (ρxy) reaches 5% (22%) at a tensile strain of ROC = 3 mm. The shift of the nitrogen position in the γ'-Fe4N unit cell at different bending strains plays a key role in the strain-tailored magnetic and electronic transport properties. The flexible epitaxial γ'-Fe4N films have the potential applications in magneto- and electromechanical wearable spintronic devices.

Hump-like structure in Hall signal from ultra-thin SrRuO3 films without inhomogeneous anomalous Hall effect

A controversy arose over the interpretation of the recently observed hump features in Hall resistivity ρxy from ultra-thin SrRuO3 (SRO) film; it was initially interpreted to be due to topological Hall effect but was later proposed to be from existence of regions with different anomalous Hall effect (AHE). In order to settle down the issue, we performed Hall effect as well as magneto-optic Kerr-effect measurements on 4 unit cell SRO films. Clear hump features are observed in ρxy, whereas neither hump feature nor double hysteresis loop is seen in the Kerr rotation which should be proportional to the magnetization. In addition, magnetization measurement by superconducting quantum interference device shows no sign of multiple coercive fields. These results show that inhomogeneous AHE alone cannot explain the observed hump behavior in ρxy data. We suggest that emergence of the hump structure in ρxy is closely related to the growth condition.

Ultrahigh Hall mobility and suppressed backward scattering in layered semiconductor Bi2O2Se

We report on an ultrahigh Hall mobility exceeding 40 000 cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.

Transport Studies on GaAs/AlGaAs Two-Dimensional Electron Systems Modulated by Triangular Array of Antidots**Supported by the National Basic Research Program of China under Grant Nos 2015CB921101 and 2014CB920904, and the Strategic Priority Research Program B of Chinese Academy of Sciences under Grant No XDB07010200.

Triangular antidot lattices of various periods and aspect ratios are fabricated on high mobility GaAs/AlGaAs two-dimensional electron systems (2DESs), and are characterized by magneto-transport measurements at low temperatures down to 300 mK. Commensurability peaks are generally observed in the magneto-resistivity ρxx, and remarkable similarity between dρxy/dB and ρxx is found. In samples of relatively large aspect ratio d/a, the Aharonov–Bohm-type oscillations are clearly observed in both ρxx and ρxy, as well as the quenching of the Hall resistivity ρxy in the vicinity of B = 0. These observations evince the good quality of our samples, and attest to the adequate preparation for fabricating antidot lattices of a reduced period to realize artificial graphene from GaAs/AlGaAs 2DESs.

Anomalous Hall effect in two-dimensional non-collinear antiferromagnetic semiconductor Cr0.68Se

Cr0.68Se single crystals with two-dimensional (2D) character have been grown, and the detailed magnetization M(T), electrical transport properties (including longitudinal resistivity ρxx and Hall resistivity ρxy), and thermal transport properties [including heat capacity Cp(T) and thermoelectric power S(T)] have been measured. There are some interesting phenomena: (i) Cr0.68Se presents a non-collinear antiferromagnetic (AFM) semiconducting behavior at the Néel temperature of TN = 42 K and with the activated energy of Eg = 3.9 meV; (ii) it exhibits the anomalous Hall effect (AHE) below TN and large negative magnetoresistance about 83.7% (2 K, 8.5 T). The AHE coefficient RS is 0.385 cm−3/C at T = 2 K, and the AHE conductivity σH is about 1 Ω−1 cm−1 at T = 40 K; (iii) the scaling behavior between the anomalous Hall resistivity ρxyA and the longitudinal resistivity ρxx is linear, and further analysis implies that the origin of the AHE in Cr0.68Se is dominated by the skew-scattering mechanism. Our results may be helpful for exploring the potential application of these kinds of 2D AFM semiconductors.