Low-field Behavior Research Articles

Crossover from relativistic to non-relativistic net magnetization for MnTe altermagnet candidate

Abstract We experimentally study magnetization reversal curves for MnTe single crystals, which is the altermagnetic candidate. Above 85 K temperature, we confirm the antiferromagnetic behavior of magnetization M, which is known for α–MnTe. Below 85 K, we observe anomalous low-field magnetization behavior, which is accompanied by the sophisticated M(α) angle dependence with beating pattern as the interplay between M(α) maxima and minima: in low fields, M(α) shows ferromagnetic-like 180° periodicity, while at high magnetic fields, the periodicity is changed to the 90° one. This angle dependence is the most striking result of our experiment, while it can not be expected for standard magnetic systems. In contrast, in altermagnets, symmetry allows ferromagnetic behavior only due to the spin-orbit coupling. Thus, we claim that our experiment shows the effect of weak spin-orbit coupling in MnTe, with crossover from relativistic to non-relativistic net magnetization, and, therefore, we experimentally confirm altermagnetism in MnTe.

Magnetic field induced ab -plane rotation of the Eu magnetic moments in trigonal EuMg2Bi2 and EuMg2Sb2 single crystals below their Néel temperatures

The thermodynamic and electronic-transport properties of trigonal ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$ in $ab$-plane magnetic fields ${\mathbf{H}}_{x}$ and the A-type antiferromagnetic structure have recently been reported. The Eu magnetic moments with spin $S=7/2$ remain locked in the $ab$ plane up to and above the $ab$-plane critical field ${\mathbf{H}}_{x}^{\mathrm{c}}=27.5$ kOe at which the Eu moments become parallel to ${\mathbf{H}}_{x}$. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field ${H}_{c1}\ensuremath{\approx}465$ Oe where the Eu moments remain in the $ab$ plane but become perpendicular to ${H}_{x}$. At higher fields, the moments cant toward the field resulting in $M\ensuremath{\propto}{H}_{x}$ up to ${\mathbf{H}}_{x}^{\mathrm{c}}$. Similar results are reported from measurements of the magnetic properties of ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ single crystals, where ${H}_{\mathrm{c}1}\ensuremath{\approx}220$ Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. The $ab$-plane trigonal anisotropy in ${\mathrm{EuMg}}_{2}{\mathrm{Sb}}_{2}$ is found to be significantly smaller than in ${\mathrm{EuMg}}_{2}{\mathrm{Bi}}_{2}$.

Thermodynamics of the Nonrelativistic Free-Electron Fermi Gas in One, Two, and Three Dimensions from the Degenerate to the Nondegenerate Temperature Regime

The thermodynamic and electronic-transport properties of trigonal EuMg2Bi2 in ab-plane magnetic fields Hx and the A-type antiferromagnetic structure have recently been reported. At a temperature of 1.8 K < TN, the Eu magnetic moments with spin S = 7/2 remain locked in the ab plane up to and above the ab-plane critical field Hxc = 27.5 kOe at which the Eu moments become parallel to Hx. Here additional measurements at low fields are reported that reveal a new spin-reorientation transition at a field Hc1 = 465 Oe where the Eu moments remain in the ab plane but become perpendicular to Hx. At higher fields, the moments cant towards the field resulting in M proportional to Hx up to Hxc. Similar results are reported from measurements of the magnetic properties of EuMg2Sb2 single crystals, where Hc1 = 220 Oe is found. Theory is formulated that models the low-field magnetic behavior of both materials, and the associated anisotropies are calculated. The ab-plane trigonal anisotropy in EuMg2Sb2 is found to be significantly smaller than in EuMg2Bi2.

Fe-Rich Ferromagnetic Cleavable Van der Waals Telluride Fe5AsTe2

Transition metal-based layered compounds with van der Waals gaps between the structural layers are a rich source of magnetic materials for spintronic applications. Bulk crystals can be cleaved, providing high-quality two-dimensional nanomaterials, which are promising for the manipulation of spins in spintronic devices and low power quantum logic interfaces. The layered van der Waals telluride Fe5AsTe2 can be synthesized by the high-temperature reaction of elements. In the crystal structure, Fe-rich structural layers with the composition of Fe4.58(4)AsTe2 are separated by the van der Waals gaps with no atoms in the interstitial region. Crystal growth employing chemical vapor transport reactions yields bulk cleavable crystals, which exhibit weak inherent ferromagnetism below the Curie temperature of TC = 48 K. In the ordered state, the magnetization shows a dual-slope behavior in low magnetic fields, indicating the compensated or canted nature of magnetism. Magnetic susceptibility and magnetization measurements reveal perpendicular magnetic anisotropy. The large Rhodes-Wohlfarth ratio of 4.6 indicates the itinerant nature of ferromagnetism in Fe5AsTe2.

Investigation of the high-field transport, Joule-heating-driven conductivity improvement and low-field resistivity behaviour in lightly-reduced free-standing graphene oxide papers

Free-standing reduced graphene oxide (rGO) has been gaining popularity for its use in supercapacitors and battery applications due its facile synthesis, multi-layered structure, and high-current carrying capacity. Pertinent to the successful implementation of such applications, however, is the need to develop a thorough understanding of the electrical properties of such materials when subject to high applied electric fields. In this work, we undertake a detailed study of high-field electrical properties of mm-scale, lightly-reduced, rGO papers. Our results reveal that the I–V curves exhibit substantial nonlinearity with associated hysteresis that depends strongly on the applied electric field. The nonlinear behaviour which was interpreted using conventional transport models of Fowler–Nordheim tunnelling and space charge limited conduction revealed that while these models provided good qualitative fits to our data, they were quantitatively lacking, thus leaving the issue of high-field transport mechanisms in rGO open for debate. Careful I–V cycling experiments with measurement time-delay introduced between cycles revealed that the observed hysteresis contained recoverable and non-recoverable parts that we identified as arising from charge trapping and Joule heating effects, respectively. Time-dependent measurements showed that these effects were characterized by two distinct time scales. Importantly, the Joule heating was found to cause a permanent conductivity improvement in the rGO via the ‘current annealing’ effect by effectively eliminating oxygenated groups from the rGO. The analysis of the electrical breakdown in our samples resembled a thermal runaway-like event that resulted in premature damage to the rGO. Finally, we investigated the low-field resistivity in the 80 K–300 K temperature range. The reduced activation energy analysis revealed a robust power law behaviour below 230 K, while deviating from this trend at higher temperatures. For samples that received current annealing treatment, a reduced value for the power law exponent was obtained, confirming the effective lowering of disordered regions.

Effect of active layer thickness variation on scaling response in a-IGZO thin film transistors under Schottky limited operation

Active layer thickness variation in highly-doped amorphous indium-gallium-zinc oxide thin film transistors with molybdenum-chromium contacts is studied to reveal parametric dependencies under both channel length and gate-contact overlap length scaling. Devices with thickness variations from 5 nm to 30 nm, channel length variation from 3 µm to 100 µm and gate-contact overlap length variation from 1 µm to 10 µm were fabricated, characterized and analyzed. Analysis from a field effect transistor perspective show typical thickness variation trends where threshold voltage shifts positively, subthreshold slope improves and I on/I off ratio increases as the active layer thickness is scaled down. Peculiar observations like extremely low saturation voltage, insensitivity towards channel length variations, two different slopes in the subthreshold region and peaks in the transconductance characteristics can be explained only by invoking the principle of Schottky transistor operation. Thinner devices exhibit high field Schottky operation with barrier lowering which causes monotonously increasing transconductance, while thicker devices show low field behavior with no barrier lowering signified by peaks with saturated behavior of transconductance. Channel length modulates this dependence of transconductance on thickness. Sensitivity towards gate-contact overlap length scaling increases with thickness. Devices with active layer thickness of 5 nm can withstand overlap length scaling up to 5 µm without transconductance degradation, while devices of 10 nm thickness can withstand overlap length scaling only upto 10 µm. Devices of 30 nm thickness show contact limited operation even at 10 µm overlap length. Conventional FET operating principles cannot explain these observations and the phenomenon of Schottky contact transistor operation has been invoked. The results point towards a thin borderline in low dimensional transistors, differentiating a FET from Schottky contact transistors, through a field dependent barrier lowering mechanism, which is modulated by active layer thickness, channel length and gate contact overlap length.

The physicochemical properties and stability of flaxseed oil emulsions: effects of emulsification methods and the ratio of soybean protein isolate to soy lecithin.

The properties and stability of emulsion rely greatly on the emulsification method and emulsifier. In this study, different emulsification methods (high-speed homogenization, ultrasonic treatment and their combination) were employed for the preparation of emulsions stabilized by soybean protein isolate (SPI) and soy lecithin (SLT) at three ratios. The microstructure, hydrodynamic average diameter, ζ-potential, creaming stability and low-field nuclear magnetic resonance relaxation behaviors of emulsions were investigated. The results indicated that the influence of emulsification method was closely related to the ratio of SPI/SLT. Overall, the SPI-SLT-stabilized emulsion treated by ultrasound showed better stability and uniformity, while the combined treatment of high-speed homogenization and ultrasound was helpful in improving the uniformity and stability of SPI-stabilized Pickering emulsion. However, the SLT-stabilized emulsions all exhibited worse uniformity in terms of particle size distribution and polydispersity index. These results will be helpful for selecting an appropriate emulsification method and emulsifier to improve the stability of emulsions. © 2021 Society of Chemical Industry.

Low-temperature thermal transport measurements of oxygen-annealed Yb2Ti2O7

Low-temperature thermal conductivity measurements have been conducted on an oxygen-annealed single crystal of ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ from 60 mK to 50 K and in magnetic fields up to 8 T applied in the [111] crystallographic direction. The temperature dependence of the conductivity in zero field shows a significant peak in thermal conductivity at $T\ensuremath{\sim}13$ K and a sharp anomaly at ${T}_{c}\ensuremath{\sim}0.2$ K suggesting that the sample's behavior is representative of the high-purity limit, with low levels of disorder. The magnetic field dependence of the thermal conductivity close to ${T}_{c}$ reveals a reentrant magnetic phase for a field in the [111] direction. With this information, analysis of the very low magnetic field behavior of the thermal conductivity suggests the presence of significant fluctuations close to the phase line.

Second-Harmonic Voltage Response for the Magnetic Weyl Semimetal Co3Sn2S2

We experimentally investigate longitudinal and transverse second-harmonic voltage response to ac electrical current for a magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$. In contrast to the previously observed Berry-curvature induced non-linear Hall effect for non-magnetic Weyl and Dirac semimetals, the second-harmonic transverse voltage demonstrates sophisticated interplay of different effects for Co$_3$Sn$_2$S$_2$. In high magnetic fields, it is of Seebeck-like square-B law, while the low-field behavior is found to be linear and sensitive to the direction of sample magnetization. The latter can be expected both for the non-linear Hall effect and for the surface state contribution to the Seebeck effect in Weyl semimetals. Thus, thermoelectric effects are significant in Co$_3$Sn$_2$S$_2$, unlike non-magnetic Weyl and Dirac materials.

Investigation of the electrical charge transport mechanism and magnetoresistance response in chloride-doped polyaniline–Fe composite nanofibers

One-dimensional composite nanostructures of chloride-doped polyaniline (PANI-Cl) and Fe nanoparticles (NPs) were fabricated via deposition of Fe NPs onto the PANI nanofiber matrix at room temperature. Morphological and structural characterization performed using microscopic techniques, e.g. scanning electron microscopy and transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy confirmed effective deposition of crystalline Fe NPs onto the amorphous PANI-Cl nanofiber matrix. The temperature-dependent magnetic property measurement results revealed the ferromagnetic nature of the prepared PANI-Cl–Fe composite nanofibers (CNFs) structure. Depending on the Fe NP loading, the PANI-Cl–Fe CNFs showed both metallic and semiconducting behaviour. The temperature-dependent resistivity of semiconducting PANI-Cl–Fe CNFs was best described by Efros–Shklovskii variable range hopping (ES-VRH) and Mott three-dimensional variable range hopping (Mott-3D-VRH) mechanisms at low and high temperature regimes. Magnetoresistance (MR) investigation was executed at different temperatures and magnetic fields for the PANI-Cl–Fe CNF pellets in semiconducting form. Room temperature (300 K) negative MR values were detected in both the low and high magnetic field regions. A significant increase in MR values was noted with a decrease in temperature from 300 K to 5 K. Meanwhile, the observed low-field negative MR behaviour of PANI-Cl–Fe CNFs was explained by the forward interference model. Therefore, the implications of these findings might be significant for the application of PANI-Cl–Fe CNFs as sensing materials for magnetic field sensor devices.

Field-induced quantum magnetism in the verdazyl-based charge-transfer salt[o−MePy−V−(p−Br)2]FeCl4

We successfully synthesized a verdazyl-based charge-transfer salt $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$, which has an $S_{\rm{V}}$=1/2 on the radical $o$-MePy-V-($p$-Br)$_2$ and an $S_{\rm{Fe}}$=5/2 on the FeCl$_4$ anion. $Ab$ $initio$ molecular orbital calculations indicate the formation of an $S_{\rm{V}}$=1/2 honeycomb lattice composed of three types of exchange interaction with two types of inequivalent site. Further, the $S_{\rm{V}}$=1/2 at one site is sandwiched by $S_{\rm{Fe}}$=5/2 spins through antiferromagnetic (AF) interactions. The magnetic properties indicate that the dominant AF interactions between the $S_{\rm{V}}$ = 1/2 spins form a gapped singlet state, and the remaining $S_{\rm{Fe}}$ = 5/2 spins cause an AF order. The magnetization curve exhibits a linear increase up to approximately 7 T, and an unconventional 5/6 magnetization plateau appears between 7 T and 40 T. We discuss the differences between the effective interactions associated with the magnetic properties of the present compound and ($o$-MePy-V)FeCl$_4$. We explain the low-field linear magnetization curve through a mean-field approximation of an $S_{\rm{Fe}}$ = 5/2 spin model. At higher field regions, the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced by the $S_{\rm{V}}$ = 1/2 AF dimer, in which a particular internal field is applied to one of the spin sites. The ESR resonance signals in the low-temperature and low-field regime are explained by conventional two-sublattice AF resonance modes with easy-axis anisotropy. These results demonstrate that exchange interactions between $S_{\rm{V}}$ = 1/2 and $S_{\rm{Fe}}$ = 5/2 spins in $[$$o$-MePy-V-($p$-Br)$_2]$FeCl$_4$ realize unconventional magnetic properties with low-field classical behavior and field-induced quantum behavior.

Effect of heat treatment condition on magnetic, electrical transport and magnetoresistance properties of La0.67Sr0.33MnO3 manganite coatings

Effects of three different heat treatment conditions (air, vacuum and nitrogen) on the structure, magnetic, electrical transport and low-field magnetoresistance behaviors of La0.67Sr0.33MnO3 manganite coatings synthesized by sol-gel and screen printing methods have been systematically investigated. The detailed crystallographic analyses revealed that the diffraction peak positions of the samples subjected to annealing in vacuum and nitrogen ambients shifted to lower-angle regions, and lattice parameters and structure distortion increased as compared with those of the sample annealed in air, which were due to the introduction of oxygen deficiency under the annealing conditions of vacuum and nitrogen. The content of oxygen deficiency of the samples could be determined by the change of lattice parameters. Furthermore, the vacuum and nitrogen annealed samples exhibited a dramatic increase in the resistivity, accompanied by the reduction in ferromagnetism, the resistivity peak temperature and the ferromagnetism transition temperature. These phenomena were attributed to the decrease of average manganese oxidation state, as a result the mobile charge carriers were reduced, the ferromagnetic coupling and the double exchange effect between manganese ions decreased while the magnetic inhomogeneity increased. In the meanwhile, magnetoresistance ratios of the oxygen-deficient samples were enhanced around the percolation threshold.

Electron transport in some transition metal di-chalcogenides: MoS2 and WS2

The transition metal di-chalcogenides are promising single monolayer materials that hold promise for applications in several fields, including nanoelectronics. Here, I study the transport of electrons in two of these materials, MoS2 and WS2. While the low-field behavior shows very low mobility, due mostly to impurity scattering, the high-field behavior shows a relatively high saturated velocity and a high breakdown field. Complications arise due to the relative narrowness of the conduction band, and the effect of this on the transport is discussed.

Magnetotransport properties of Fe0.8Ga0.2 films with stripe domains

Magnetotransport properties of ${\mathrm{Fe}}_{0.8}{\mathrm{Ga}}_{0.2}$ films with stripe domains are studied. The anisotropic magnetoresistance dominates the low field behavior, which is extremely dependent on the magnetic domains configuration. The magnetoresistance measured at different temperatures displays qualitatively different behaviors depending on the measurement configuration. When the stripes are oriented along the electric current, the low-field magnetoresistance changes sign with temperature, while when the stripes are perpendicular to the electric current the magnetoresistance curves are nearly temperature independent. A simple model considering parallel (series) conduction along (across) the stripes, plus the temperature dependence of anisotropic magnetoresistance and domains configuration, accounts for these experimental results.

Study of magnetic transitions in Dy by means of reversible Villari effect

Application of a new method—high-sensitivity measurements of periodic stress-induced induction (reversible Villari effect, RVE)—allows us to uncover new effects in the low-field behaviour of different magnetic phases of polycrystalline Dy. A loss of magnetoelastic coupling (Villari critical point) is found around 166 K, close to the temperature which was supposed to separate the helical structure from the possible vortex one. We show that the low-field magnetic hysteresis emerges immediately below the Néel temperature and, below the Villari critical point, demonstrates a qualitative difference for cooling and heating from the ferromagnetic state. It has been found that, below the Villari critical point, in the helical phase, polycrystalline Dy is in essentially non-equilibrium state, revealed as a time-dependent relaxation of RVE. We relate the effects observed in the helical phase with thermal internal stresses and existence of lattice defects which inherit ferromagnetic order upon heating from the ferromagnetic state.

Engineering of ion-doping on the ground states and Bose–Einstein condensation of Sr3Cr2O8

Single crystals of Sr3Cr2O8, Sr3Cr1.9V0.1O8 and Sr3Cr1.9Fe0.1O8 were grown using the floating zone technique, and their magnetization behaviors were systematically investigated under pulsed field up to 54 T at various temperatures ranging from 2 K to 20 K. Temperature-induced transition from singlet ground state to triplet state and magnetic-field-induced Bose–Einstein condensation were observed, of which the critical temperature and the critical field are correlative to ion-dopants. The analysis of the ground state and low-field magnetization behaviors indicates that the ion-dopants lead to an enhancement of the intradimer exchange J0, which changes the transition temperatures, and increases the critical fields Hc1. The magnetic Fe ion dopant makes the transition from singlet to triplet states more sensitive to the applied magnetic field, in clearly contrast to the situation of the nonmagnetic V5+ dopant.

Heat capacity measurements on UBe13 in rotated magnetic fields: Anisotropic response in the normal state and absence of nodal quasiparticles

In order to gain insight into the superconducting (SC) gap of UBe13, we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C(H)∝H behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe13 is fully open over the Fermi surfaces. Furthermore, we observed a characteristic oscillation of heat capacity both in the SC and non-Fermi-liquid normal states above ∼2T, and the angular variation of heat capacity possibly originates from anisotropic magnetic response of the heavy-electron state. Our result regarding the low-energy quasiparticle excitations in the SC and normal states will be a clue to understand the unusual nature of UBe13.

Robust surface state transport in thin bismuth nanoribbons.

While a two-dimensional (2D) metallic surface state in bismuth has been proposed, experimental 2D evidence of quantum transport, e.g., angular dependent Shubnikov-de Haas (SdH) oscillations is still lacking. Here, we report the angular-dependent magnetoresistance measurements in single-crystal Bi nanoribbons, and found that both the low-field weak antilocalization behavior and the high-field angle-dependent SdH oscillations follow exactly the 2D character, indicative of the 2D metallic surface states which dominate the transport properties of thin Bi nanoribbons. Moreover, by controllable exposing the ribbons to ambient environment (1 atm and room temperature), the metallic surface states were found to be robust to the oxidation although the carrier density in the surface states are modified after the exposures. These results suggest that the metallic surface states in Bi nanoribbons should be topologically protected which can provide key information in understanding the surface properties of Bi in nanometer scale.

Low-field microwave absorption behaviors on single layer magnetic film and exchange coupled multilayer magnetic film

This study examined the magnetization reversal effects on low-field microwave absorption in a Fe91.6B2.5N5.9 single layered film with in-plane uniaxial magnetic anisotropy and a multi-layered film with giant magnetoresistivity using ferromagnetic resonance measurements at 9.84 GHz. Two different kinds of absorption modes were observed at near zero dc field and high dc field. The signals at high-field showed all the features of ferromagnetic resonance due to spin precession. However, the absorption signals at low-field should be associated with the switching field at unsaturated magnetic field region.

Low temperature and magnetic field behaviour of the (Cr84Re16)89.6V10.4 alloy

Measurements of the temperature (T) dependence of the magnetic susceptibility (χ) and electrical resistance (R) on an antiferromagnetic (AFM) (Cr84Re16)89.6V10.4 alloy are reported in order to probe the existence of quantum critical behaviour (QCB) utilizing static magnetic fields (H) as a tuning parameter. The results indicate that an increase in H suppresses TN in such a way that it varies exponentially with increasing H. R(T) measurements show evidence of possible superconducting (SC) behaviour below 1 K at H = 0 T. These results therefore indicate the coexistence of the AFM and SC phases in the (Cr84Re16)89.6V10.4 alloy.