Pronounced Magnetoresistance Research Articles

Electric current noise in mesoscopic organic semiconductors induced by nuclear spin fluctuations

We demonstrate that nuclear spin fluctuations lead to the electric current noise in the mesoscopic samples of organic semiconductors showing the pronounced magnetoresistance in weak fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radiofrequency fields, which allows one to prove the role of nuclei in magnetoresistance experimentally.

Spontaneous and field-induced magnetic phase transitions inDy2Co3Al9: Effects of exchange frustration

Due to the long-range oscillatory character of RKKY exchange interactions, for ${\mathrm{Dy}}_{2}{\mathrm{Co}}_{3}{\mathrm{Al}}_{9}$ there exist positive and negative couplings between the Dy magnetic moments that lead to magnetic frustration. As a result, the ground state can be easily perturbed, and the system displays a number of spontaneous and field-induced phase transitions. We performed magnetization, magnetic-susceptibility, specific-heat, and electrical-resistivity measurements as well as neutron-diffraction experiments on a single crystal. We find two transitions to distinct incommensurate antiferromagnetic phases at 6.2 and 5.2 K that evolve to a commensurate phase at 3.7 K. In applied magnetic field, new phases emerge. Field-dependent magnetization exhibits a multistep metamagnetic process with sharp transitions accompanied by pronounced magnetoresistance changes. The large number of phases and their complex magnetic structures suggest that the physical properties of ${\mathrm{Dy}}_{2}{\mathrm{Co}}_{3}{\mathrm{Al}}_{9}$ are ruled by exchange frustration in the presence of a large magnetocrystalline anisotropy.

Ferromagnetic–antiferromagnetic transition and magnetotransport properties of La0.7Sr0.3Mn1−xNixO3 perovskites

The crystal structure and magnetic properties of polycrystalline compounds La0.7Sr0.3Mn1−xNixO3 (x ⩽ 0.37) were studied by neutron powder diffraction and magnetometry measurements, and the magnetotransport properties were also investigated, focusing on the magnetic state of the compounds. The crystal structure of the compounds remains rhombohedral upon chemical doping with Ni ions in the mentioned concentration range within the temperature interval 5–300 K. Doping with nickel ions causes a decrease in magnetization, and the Curie point drops from ~290 K for x = 0.1 compound down to ~70 K for x = 0.3 wherein the electrical resistivity of the compounds increases. The compounds with the dopant concentration x < 0.2 are characterized by pronounced magnetoresistance near the Curie points; the compounds with x ⩾ 0.2 show a gradual decrease in the magnetoresistance upon temperature increase. Neutron diffraction measurements of the compounds with x ⩾ 0.25 have showed a gradual reduction in ferromagnetic component, while an antiferromagnetic component of the G-type appears and progressively strengthens with an increase in the dopant concentration. The inhomogeneous magnetic state of the compounds with 0.25 < x ⩽ 0.3, attested by magnetization and neutron diffraction measurements, is governed by a competition between positive and negative exchange interactions formed between Ni and Mn ions depending on their oxidation state and electron configuration.

Vortex crossing and trapping in doubly connected mesoscopic loops of a single-crystal type-II superconductor

Numerical calculations on a mesoscopic ring of a type II superconductor in the London limit suggest that an Abrikosov vortex can be trapped in such a structure above a critical magnetic field and generate a phase shift in the magnetoresistance oscillations. We prepared submicron-sized superconducting loops of single-crystal, type II superconductor NbSe$_2$ and measured magnetoresistance oscillations resulting from vortices crossing the loops. The free energy barrier for vortex crossing determines the crossing rate and is periodically modulated by the external magnetic flux threading the loop. We demonstrated experimentally that the crossing of vortices can be directed at a pair of constrictions in the loop, leading to more pronounced magnetoresistance oscillations than those in a uniform ring. The vortex trapping in both a simple ring and a ring featuring two constrictions was found to result in a phase shift in the magnetoresistance oscillations as predicted in the numerical calculations. The controlled crossing and trapping of vortices demonstrated in our NbSe$_2$ devices provide a starting point for the manipulation of individual Abrikosov vortices, which is useful for future technologies.

Large positive room temperature magnetoresistance in nanogranular FeCo–Si–N thin films

A large positive room temperature magnetoresistance (MR) up to ~30% at 50kOe was obtained in nanogranular FeCo–Si–N thin film prepared by magnetron sputtering. The quasi-linear dependence of MR on the magnetic field is observed for the fields above 20kOe. The properties of the films depend on both the nonmagnetic component volume and film thickness. To achieve high MR, a proper Si–N content and a critical film thickness are required. The large positive MR may result from the combination of geometric effect as well as spin-dependent scattering. The FeCo–Si–N films with pronounced MR and linear MR-H behavior can be good candidates for future magnetoelectric devices.

Gate-controlled linear magnetoresistance in thin Bi2Se3 sheets

We explore the emergence of linear magnetoresistance in thin Bi2Se3 sheets upon tuning the carrier density using a back gate. With increasingly negative gate voltage, a pronounced magnetoresistance of ∼100% is observed, while the associated B-field dependence changes from quadratic to linear. Concomitantly, the resistance-versus-temperature curves evolve from metallic to semiconductor-like, and increasingly strong weak anti-localization behavior is manifested. Analysis of the magnetoresistance data reveals two contributions, namely from the bulk conduction band and from a state inside the bulk gap. The latter is responsible for the linear magnetoresistance and likely represents the topologically protected surface state.

Phase diagram of the strongly disordereds-wave superconductor NbN close to the metal-insulator transition

We present a phase diagram as a function of disorder in three-dimensional NbN thin films, as the system enters the critical disorder for the destruction of the superconducting state. The superconducting state is investigated using a combination of magnetotransport and tunneling spectroscopy measurements. Our studies reveal 3 different disorder regimes. At low disorder the (k_{F}l~10-4), the system follows the mean field Bardeen-Cooper-Schrieffer behavior where the superconducting energy gap vanishes at the temperature where electrical resistance appears. For stronger disorder (k_{F}l<4) a "pseudogap" state emerges where a gap in the electronic spectrum persists up to temperatures much higher than Tc, suggesting that Cooper pairs continue to exist in the system even after the zero resistance state is destroyed. Finally, very strongly disordered samples (k_{F}l<1) exhibit a pronounced magnetoresistance peak at low temperatures, suggesting that localized Cooper pairs continue to survive in the system even after the global superconducting ground state is completely destroyed.

Magnetic-field-inducedlog-Tinsulating behavior in the resistivity of fluorine-dopedSmFeAsO1−xFx

We report the resistivity of a series of nominally fluorine-doped ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ polycrystalline superconductors in magnetic fields up to 60 T. For samples where $x&lt;0.15$, the low-temperature resistive state is characterized by pronounced magnetoresistance including an upturn at low temperatures. The ``insulating behavior'' is characterized by a $\text{log-}T$ divergence observed over a decade in temperature, a behavior strikingly similar to the underdoped cuprates. The normal state for samples with doping $x&gt;0.15$ behaves very differently: metallic behavior with little magnetoresistance, where intense magnetic fields broaden the superconducting transition rather than significantly suppressing ${T}_{c}$. The doping at which the insulating behavior disappears coincides with the reported collapse of the structural phase transition in the phase diagram for ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ series.

Soft magnetic lithography and giant magnetoresistance in superconducting/ferromagnetic hybrids

We demonstrate an approach to create a tunable pinning potential in a superconducting/ferromagnetic (SC/FM) hybrid, allowing the switching of their electronic properties through the application of a small magnetic field. Using direct magneto-optical imaging, macroscopic transport, and magnetic measurements, we show that the alignment of stripe domains in the ferromagnet provides a remarkable directionality for the superconducting vortex motion. An analysis of the anisotropic flux motion demonstrates a substantial critical current anisotropy in the superconductor. The possibility of aligning stable lattices of stripe domains in select directions using in-plane magnetic fields allows the realization of soft magnetic lithography for efficient manipulation of supercurrent flow in SC/FM bilayers. Furthermore, in our samples we observed a pronounced magnetoresistance effect yielding 4 orders of magnitude resistivity change in a few millitesla in-plane field.

Collective Josephson vortex dynamics in a finite number of stacked intrinsic Josephson junctions

We report the experimental confirmation of the collective transverse plasma modes excited by the Josephson vortex lattice in stacks of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ single crystals. The excitation was confirmed by analyzing the temperature ($T$) and magnetic field ($H$) dependencies of the multiple sub-branches in the Josephson-vortex-flow region of the current-voltage characteristics of the system. In the near-static Josephson vortex state for a low tunneling bias current, pronounced magnetoresistance oscillations were observed, which represented a triangular-lattice vortex configuration along the c axis. In the dynamic vortex state in a sufficiently high magnetic field and for a high bias current, splitting of a single Josephson vortex-flow branch into multiple sub-branches was observed. Detailed examination of the sub-branches for varying $H$ field reveals that sub-branches represent the different modes of the Josephson-vortex lattice along the c axis, with varied configuration from a triangular to a rectangular lattices. These multiple sub-branches merge to a single curve at a characteristic temperature, above which no dynamical structural transitions of the Josephson vortex lattice is expected.

Stray-fields-based magnetoresistance mechanism inNi80Fe20−Nb−Ni80Fe20trilayered hybrids

We report on the transport and magnetic properties of hybrid trilayers and bilayers that consist of low spin-polarized Ni80Fe20 exhibiting in-plane but no uniaxial anisotropy and low-Tc Nb. We reveal a magnetoresistance effect that is pronounced. In our trilayers the magnetoresistance exhibits an increase of two orders of magnitude when the superconducting state is reached: from the conventional normal-state values 0.6 % it goes up to 1000 % for temperatures below Tc. In contrast, in the bilayers the effect is only minor since from 3% in the normal state increases only to 70 % for temperatures below Tc. Magnetization data of both the longitudinal and transverse magnetic components are presented. Most importantly, we present data not only for the normal state of Nb but also in its superconducting state. Strikingly, these data show that below its Tc SC the Nb interlayer under the influence of the outer Ni80Fe20 layers attains a magnetization component transverse to the external field. By comparing the transport and magnetization data we propose a candidate mechanism that could motivate the pronounced magnetoresistance effect observed in the trilayers. Adequate magnetostatic coupling of the outer Ni80Fe20 layers is motivated by stray fields that emerge naturally in their whole surface due to the multidomain magnetic structure that they attain near coercivity. Atomic force microscopy is employed in order to examine the possibility that such magnetostatic coupling could be promoted by interface roughness. Referring to the bilayers, although out-of-plane rotation of the magnetization of the single Ni80Fe20 layer is still observed, in these structures magnetostatic coupling does not occur due to the absence of a second Ni80Fe20 one so that the observed magnetoresistance peaks are only modest.

Magnetoresistance of semiconductor-metal hybrid structures: The effects of material parameters and contact resistance

We have used the finite element method to study the extraordinary magnetoresistance (EMR) effect of semiconductor-metal hybrid structures in rectangular device geometries. These have recently been found to exhibit intriguing properties interesting for magnetic-field sensors. Current and potential distributions in the devices are calculated in an applied magnetic field. By these means, we investigate the EMR effect, in particular, as a function of material parameters and of the contact resistance ρc between the semiconductor and the metal. In our calculations we find that ρc should be within a specific operation regime in order to obtain a pronounced magnetoresistance effect. We show that by means of the electron mobility in the semiconductor the voltage and the current sensitivity of a hybrid device can be optimized with respect to an operation field in the 10-mT range.

Magnetisation and magnetoresistance of YMn 6Sn 6− xIn x compounds ( x=0.10, 0.18, 0.21, 0.38)

Polycrystalline samples of HfFe 6Ge 6-type YMn 6Sn 6− x In x compounds (0.10≤ x≤0.38) have been studied by X-ray diffraction, high field magnetisation and magnetoresistance measurements. The low In content compounds order antiferromagnetically ( T N=328, 321 and 309 K for x=0.10, 0.18 and 0.21, respectively) while YMn 6Sn 5.62In 0.38 orders ferromagnetically ( T c=329 K) and undergoes a ferro- to antiferromagnetic transition on cooling ( T t=240 K). All compounds display metamagnetic behaviour at 10 K associated with giant magnetoresistance (Δ R/ R∼−30% for x=0.10). The different magnetoresistance effects measured in the x=0.18 and 0.21 compound seems to be related to different textures suggesting a more pronounced magnetoresistance along the [001] direction. Results are discussed and compared with properties of isotypic series.

Internal magnetic focusing in an array of ballistic cavities

We study the ballistic motion of electrons in an array of submicron circular cavities, fabricated by electron beam lithography and dry etching techniques. Pronounced magnetoresistance oscillations are observed that can be understood in terms of internal magnetic focusing or commensurability between the electron trajectories and the geometry of the cavities. The depletion distance in the two-dimensional electron gas caused by the dry etching process can be deduced by analysis of the resonance peak positions within a simple magnetic focusing picture.

Magnetotransport in two-dimensional lateral superlattices.

Two-dimensional electron systems on GaAs heterostructures subjected to a two-dimensional superlattice potential reveal novel features in the magnetoresistance. Strong density modulation causing the formation of antidots is accompanied by a pronounced magnetoresistance maximum. On weakly modulated systems, oscillations are observed that reflect the commensurability of the cyclotron diameter and the superlattice period. A classical model of ballistic motion resulting in magnetic-field-dependent diffusion is found to explain our and other recent magnetotransport observations.

Oscillators by Means of Magnetoresistance Effect

This paper describes the application of the magnetoresistance effect to oscillators using the high mobility semiconducting compound InSb as the active element. The first stable oscillator consisted of an InSb Corbino disk in a ferrite pot core reduced to a temperature of -40°C. The frequency was 210 cps and the open-circuited ac voltage was 540 mv, peak to peak. In order to construct a stable oscillator at room temperature, the shape of the semiconductor specimen was modified to a rectangular form composed of narrow strips of InSb joined together with metallic indium. This specimen has a comparatively higher internal resistance and a more pronounced magnetoresistance effect at low magnetic flux densities. A second oscillator using this specimen as an active element in an E-form ferrite core was tested and found to operate stably at a temperature of 25°C, in a frequency range from a few cps up to 300 cps. The maximum output power available was 2.5 mw, at 70 cps. There is no heating-up time required by the oscillator and the oscillation can start immediately under certain initial conditions. The elimination of the equipment to provide the low temperature, i.e., below 0° C for the oscillator has shown that the magnetoresistance effect can be made good use of in electronic devices without requiring a complicated circuit arrangement.