Negative Magnetoresistance Values Research Articles

Magnetic Properties of Silicon with Paramagnetic Impurity Atoms

One of the possible ways to obtain silicon with magnetic properties is the introduction of paramagnetic impurities into silicon: Cr, Mn, Fe, Ni, and Co. In our opinion, silicon materials containing magnetic nanosized clusters are most suitable for spintronic devices. The possibility of obtaining silicon with magnetic properties by diffusion doping was studied in this work. To obtain silicon doped with Cr, Mn, Fe and Ni impurity atoms, p-type single-crystal silicon with a specific resistance of ρ = 5 Ohm·cm and ρ = 0.5 Ohm·cm was used, and for doping with Co atoms, n-type silicon with resistivity ρ=10 Ohm·cm was used. The diffusion temperature and time were chosen such that, after diffusion annealing, the samples with impurity Cr, Fe, and Mn atoms remained highly compensated p-type, and when doped with impurity Co atoms, they remained high-resistance n-type. The results of the study showed that with decreasing temperature, the value of the negative magnetoresistance Δρ/ρ in the Si<Mn> samples increases and reaches its maximum value (about 800%) at T = 240 K, a further decrease in temperature leads to a decrease in the magnetoresistance, and at a temperature T = 170 K, the sign of the magnetoresistance is inverted. In Si <Cr> samples, with decreasing temperature, the positive magnetoresistance turns into a negative one, the value of which increases with decreasing temperature, and is achieved at T=100 K Δρ/ρ = 45–50%. In Si<Fe> samples, with decreasing temperature, the value of negative magnetoresistance increases monotonically and at T=100 K its value is Δρ/ρ = (100÷120) %. The study in Si<Сo> samples showed that with decreasing temperature the value of positive magnetoresistance increases and at Т=100 K it reaches Δρ/ρ = (17÷20) %. The study of magnetoresistance in samples - Si<Ni> showed that with decreasing temperature the value of positive magnetoresistance increases and at T=100 K it reaches Δρ/ρ = (10÷15) %. When studying the magnetic properties of p-Si <B, Mn> samples at low temperatures (below T=30 K), a ferromagnetic state was found, i.e. succeeded in obtaining a magnetic semiconductor material by the method of diffusion of a paramagnetic impurity. In the overcompensated Si <B, Mn> (n‑type) samples, no magnetic hysteresis was found. This shows a significant effect on the magnetic properties of the manganese impurity in silicon of its charge and, accordingly, spin state. Based on the results obtained, it can be argued that diffusion doping of silicon with manganese can be used to obtain silicon with magnetic properties.

Magnetotransport property of oxygen-annealed Fe1+y Te thin films

Fe-based superconductors are one of the current research focuses. FeTe is unique in the series of FeSe1−x Te x , since it is nonsuperconducting near the FeTe side in the phase diagram in contrast to the presence of superconductivity in other region. However, FeTe thin films become superconducting after oxygen annealing and the mechanism remains elusive. Here, we report the temperature dependences of resistivity, Hall effect and magnetoresistance (MR) of a series of FeTe thin films with different amounts of excess Fe and oxygen. These properties show dramatic changes with excess Fe and oxygen incorporation. We found the Hall coefficients are positive for the oxygen-annealed samples, in contrast to the transition from positive to negative below 50 K for the vacuum-annealed samples. For all samples, both the resistivity and Hall coefficient show a dramatic drop, respectively, at around 50 K–75 K, implying coexistence of superconductivity and antiferromagnetic order for the oxygen-annealed samples. The vacuum-annealed samples show both positive and negative values of MR depending on temperature, while negative MR dominates for the oxygen-annealed samples. We also found that oxygen annealing reduces the excess Fe in FeTe, which has been neglected before. The results are discussed in terms of several contributions, and a comparison is made between the oxygen-annealed FeTe thin films and FeSe1−x Te x . This work is helpful for shedding light on the understanding of oxygen-annealed FeTe thin films.

Interlayer-enhanced room temperature in-plane magnetoresistance in graphene nano-crystalline carbon (GNC) film/SiO2/p-Si heterostructures

Graphene nano-crystalline carbon film/SiO2/p-Si heterostructures were fabricated with SiO2 interlayer thicknesses varying from 0 to 1000 nm, and their in-plane magnetoresistance (MR) was investigated. The results showed that, compared to the 0 nm SiO2 case, the presence of a 5 nm SiO2 interlayer significantly improved the room temperature MR by more than eight times (from 2.5% to 20.9% at 280 K). For SiO2 layers with thicknesses of 50 nm and above, only small negative MR values were observed. For all SiO2 thicknesses, the variations in the mobility of heterostructures were highly consistent with their MR performance. The results of this study provide a practical route to building graphene-based high-performance MR devices suitable for mass production as magnetic sensors and magnetic logic devices.

Effect of nonuniform microstructure on magnetoresistance and field sensitivity in Co–MgO nanocomposite films

Co–MgO metal-insulator nanocomposite films have attracted increasing interest for their high resistivity and negative magnetoresistance at room temperature. In this study, nonuniform-microstructure Co–MgO nanocomposite films were produced, in which superparamagnetic particles and ferromagnetic clusters coexist. At the Co content of 36 at.%, the film exhibits a high room-temperature negative magnetoresistance value of over 2.2%, and a high resistivity of 7.0 × 104 μΩ m. When Co content increases, the size and quantity of Co clusters gradually increase, which causes the peaks shifting of magnetoresistance field sensitivity to higher field. Moreover, we have established a mechanism mode to explain this phenomenon, by inducing the localized magnetization flux effect and inhomogeneous current paths effect. The high-resistivity nonuniform-microstructure Co–MgO nanocomposites are realized, which can be used in different application scenarios by modulating magnetoresistance field sensitivity.

Giant magnetoresistance effect in Fe-doped SrCoO2.9-δF0.1 perovskites

Excellent magnetoresistive materials have attracted people's attention due to their wide applications. Unfortunately, few cobalt-based perovskite oxides can demonstrate good magnetoresistance (MR) properties. Herein, a new series of SrCo 1-x Fe x O 2.9-δ F 0.1 (x = 0, 0.1, 0.2, and 0.3) perovskites are prepared and studied as potential magnetoresistive materials. Room temperature X-ray diffraction (XRD) and Rietveld refinement confirm the simple cubic structure of all samples. The magnetization vs temperature measurement reveals that the magnetic transition temperature goes down with the increase of Fe content, suggesting the weakened ferromagnetism caused by Fe 4+ -O-Co 3+ , Fe 3+ -O-Fe 3+ and Fe 4+ -O-Fe 4+ interactions. And the resistivity data of all Fe doped samples can be better fitted using Mott's variable range hopping (VRH) model, suggesting its disordering semi-conduction behavior. Astonishingly, the SrCo 0.8 Fe 0.2 O 2.9-δ F 0.1 ceramic sample owns the giant negative magnetoresistance value of approximately 90% at 5 K in 90 kOe, which is almost the largest negative magnetoresistance in cobalt-based perovskite oxides, indicating that cobalt-based perovskites can be an underlying material system for the research of the potential material applying to magnetic sensors or other spintronic devices.

Magnetoresistance, magnetothermopower, magnetothermal conductivity and magnetostriction in La1-xNaxMnO3[formula omitted

We report the influence of the magnetic field on electrical resistivity (ρ), thermal conductivity (κ), thermopower (S), and linear thermal expansion (ΔL/L0) in La1-xNaxMnO3 series (x = 0, 0.01, 0.02, and 0.05) over a wide range of temperature. While LaMnO3 is an antiferromagnetic insulator, the doping with Na weakens the inter-planner antiferromagnetic coupling and transforms it into a ferromagnetic metal by promoting the double exchange coupling between Mn3+ and Mn4+ ions. ρ and S show peaks, whereas, κ shows a sudden increase at the onset of ferromagnetic spin ordering in the samples. Dominant charge carriers are holes in these doped compounds as indicated by the positive sign of S. High-temperature transport is explained in the context of the small polaron hopping model. External magnetic field influences thermal properties (S and κ), and ΔL/L0 mostly around the Curie temperature (TC), while it affects ρ to as low as 10 K. Enhancement of κ at the insulator-metal transition, on the application of external magnetic field, suggests a decrease in spin-wave scattering. The highest value of negative magnetoresistance (~69.7%) and negative magnetothermopower (~80.4%) is obtained for x = 0.02 and the value decreases for higher dopant concentration.

Colossal Negative Magnetoresistance Effect in a La1.37Sr1.63Mn2O7 Single Crystal Grown by Laser-Diode-Heated Floating-Zone Technique

We have grown La 1.37 Sr 1.63 Mn 2 O 7 single crystals with a laser-diode-heated floating-zone furnace and studied the crystallinity, structure, and magnetoresistance (MR) effect by in-house X-ray Laue diffraction, X-ray powder diffraction, and resistance measurements. The La 1.37 Sr 1.63 Mn 2 O 7 single crystal crystallizes into a tetragonal structure with space group I4/mmm at room temperature. At 0 T, the maximum resistance centers around ∼166.9 K. Below ∼35.8 K, it displays an insulating character with an increase in resistance upon cooling. An applied magnetic field of B = 7 T strongly suppresses the resistance indicative of a negative MR effect. The minimum MR value equals −91.23% at 7 T and 128.7 K. The magnetic-field-dependent resistance shows distinct features at 1.67, 140, and 322 K, from which we calculated the corresponding MR values. At 14 T and 140 K, the colossal negative MR value is down to −94.04(5)%. We schematically fit the MR values with different models for an ideal describing of the interesting features of the MR value versus B curves.

Magnetotransport mechanisms and magnetoresistive properties in La0.75Dy0.05Sr0.2MnO3 polycrystalline manganite

We have studied in this work the electrical and magnetoresistive properties of polycrystalline La0.75Dy0.05Sr0.2MnO3. Magnetic study has shown the presence of a paramagnetic–ferromagnetic transition at Curie temperature TC = 225 K. Temperature-dependent resistivity shows two resistivity peaks. The magnetotransport in the metallic phase can be described by Zener’s polynomial law, while small polaron hopping mechanism is more suitable to describe resistivity evolution in the insulating phase. The resistivity upturn observed around 75 K was ascribed to electron–electron scattering process at low temperature. We have recorded important values of negative magnetoresistance (MR) around TC (MR (T) ~ 22% and MR (H) ~ 19%) for only 1 T applied magnetic field. This suggests the possibility of using our sample for magnetic field sensing and spintronics.

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.

Quantization in magnetoresistance of strained InSb whiskers

Strain influence on the longitudinal magnetoresistance for the n-type conductivity InSb whiskers doped by Sn to concentration 6·1016–6·1017 сm–3 was studied in the temperature range 4.2–40 K and magnetic field up to 10 T. The Shubnikov–de Haas oscillations at low temperatures were observed in the strained and unstrained samples in all range of doping concentrations and magnetic fields. The character of longitudinal magnetoresistance dependences was analyzed and compared with theoretical one. The whisker magnetoresistance alters its sign with increasing magnetic field. It is positive at weak magnetic fields and becomes negative at higher magnetic fields. Possible mechanism of the large value of negative magnetoresistance (NMR) was discussed in the InSb whiskers with doping concentration in the vicinity to metal–insulator transition. The origin of large NMR was explained by the existence of classical size effect and boundary scattering during conductance in subsurface whisker layers.

Systems of Manganites with Enhanced Electromagnetic Parameters

The effects of paired substitution of 3d10 cations ( $$\rm{Zn{_{0.5}^{2+}}Ge{_{0.5}^{4+}}}$$ ) or 2p6 and 3p6 cations ( $$\rm{Mg{_{0.5}^{2+}}Ti{_{0.5}^{4+}}}$$ ) for manganese in polycrystalline manganites of specifically designed systems $$\text{L}{\text{a}_{0.8 - x}}\text{S}{\text{r}_{0.2 + x}}\text{Mn}_{0.8 - 2x -2\gamma}^{3 +}\text{Mn}_{0.2 + x + 2\gamma}^{4 +} \times {\left( {\text{Me}_{0.5}^{2 +}\text{Me}_{0.5}^{4 +}} \right)_x}{\text{O}_{3 + \gamma}}$$ on their electromagnetic parameters are studied and compared. Temperature dependences of the resistance, magnetoresistance and magnetostriction constant are measured. In the proposed systems, the linear rise of strontium concentration simultaneously with increasing number of substituents allows high characteristics of manganites to be maintained owing to the increase in the concentration of free charge carriers, through which ferromagnetic double exchange interaction is carried out. The data obtained are also compared with the properties of Ga3+(3d10)-substituted manganites as peculiar standards. All sintered (Zn,Ge)-containing manganites in the temperature range from 100 to 293 K have metallic type of conductivity, while (Mg,Ti)-substituted samples exhibit semiconducting features, and Ga-containing compositions reveal a “metal-semiconductor” transition point (Tms). The annealing under conditions ensuring stoichiometric oxygen content leads to the shift of this transition toward lower temperatures in manganites with 3d10 substituents and to the increase in Tms in compositions with substituting p6 cations. In the investigated systems, the absolute values of negative magnetoresistance up to ~90–200% at 120–150 K in the field of 9.2 kOe are obtained, and the linear magnetostriction constant reaches record values of about 10–3 in the field of 4.6 kOe. Manganite La0.65Sr0.35Mn0.85Ga0.15O3 with a weak temperature dependence of magnetoresistance of ~20% at room temperatures is of particular service for magnetic field sensors. Possible approaches to the interpretation of established regularities demonstrating the role of electronic configurations of substituents for manganese in the formation of the properties of manganites and interesting for the development of new materials for electronics are discussed.

Effect of charge ordering and phase separation on the electrical and magnetoresistive properties of polycrystalline La0.4Eu0.1Ca0.5MnO3

We have investigated the effect of charge ordering and phase separation on the electrical and magnetotransport properties of La0.4Eu0.1Ca0.5MnO3 polycrystalline sample. Temperature dependence of resistivity shows a metal–insulator transition at transition temperature Tρ. A hysteretic behavior is observed for zero field resistivity curves with Tρ = 128 K on cooling process and Tρ = 136 K on warming process. Zero field resistivity curves follow Zener polynomial law in the metallic phase with unusual n exponent value ∼9. Presence of resistivity minimum at low temperatures has been ascribed to the coulombic electron–electron scattering process. Resistivity modification due to the magnetic field cycling testifies the presence of the training effect. Magnetization and resistivity appear to be highly correlated. Magnetoresistive study reveals colossal values of negative magnetoresistance reaching about 75% at 132 K under only 2T applied field. Colossal values of magnetoresistance suggest the possibility of using this sample for magnetic field sensing and spintronic applications.

Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials.

Electronic state transitions, especially metal-insulator transitions (MIT), offer physical properties that are useful in intriguing energy applications and smart devices. But to-date, very few simple metal oxides have been shown to undergo electronic state transitions near room temperature. Herein, we demonstrate experimentally that chemical induction of double-exchange in two-dimensional (2D) nanomaterials brings about a MIT near room temperature. In this case, valence-state regulation of a 2D MnO2 nanosheet induces a Mn(III)-O-Mn(IV) structure with the double-exchange effect, successfully triggering a near-room-temperature electronic transition with an ultrahigh negative magneto-resistance (MR). Double-exchange in 2D MnO2 nanomaterials exhibits an ultrahigh MR value of up to -11.3% (0.1 T) at 287 K, representing the highest reported negative MR values in 2D nanomaterials approaching room temperature. Also, the MnO2 nanosheet displays an infrared response of 7.1% transmittance change on going from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure promises novel magneto-transport properties and sensitive responses for smart devices.

Magnetoresistance of nanocomposite copper/carbon thin films

Nanocomposite thin films made of partially oxidized Cu nanoparticles embedded into hydrogenated amorphous carbon, with different thicknesses and Cu/C ratio, were prepared by means of radio frequency plasma enhanced chemical vapor deposition and radio frequency sputtering using acetylene gas and copper target. The surface roughness was investigated using atomic force microscopy, revealing the fractal geometry of the Cu/carbon thin films at the nanoscale with fractal dimensions around 2.7. In addition, the electrical properties of these films and their dependence on the application of low magnetic fields were explored at room temperature. It was found that when the Cu nanoparticles are separated by gaps, the electrical conduction is governed by tunneling effects. In these conditions, the samples exhibit negative magnetoresistance values, displaying steps in the dependences on the magnetic field. These properties suggest the potential use of these films as magnetic sensors in spintronics.

Interlayer dependent polarity of magnetoresistance in graphene spin valves

We have studied the polarity of magnetoresistance (MR) in three types of magnetic junctions. While the NiFe/single layer graphene (SLG)/Co and NiFe/Al2O3/Co junctions showed spin valve signals with positive MR, the NiFe/Al2O3/SLG/Co junction revealed negative MR values.

Ferromagnetism and impurity band in a magnetic semiconductor: InMnP

We have synthesized ferromagnetic InMnP, a member of the III-Mn-V ferromagnetic semiconductor family, by Mn ion implantation and pulsed laser annealing. Clear ferromagnetic hysteresis loops and a perpendicular magnetic anisotropy are observed up to a Curie temperature of 42 K. Large values of negative magnetoresistance and magnetic circular dichroism as well as an anomalous Hall effect are further evidence of a ferromagnetic order in InMnP. An effort is made to understand the transport mechanism in InMnP using the theoretical models. We find that the valence band of InP does not merge with the impurity band of the heavily doped ferromagnetic InMnP. Our results suggest that impurity band conduction is a characteristic of Mn-doped III-V semiconductors which have deep Mn-acceptor levels.

Transport and magnetoresistance effect in an oxygen-deficient SrTiO3/La0.67Sr0.33MnO3 heterojunction

An oxygen-deficient SrTiO3/La0.67Sr0.33MnO3 heterojunction is fabricated on an SrTiO3 (001) substrate by a pulsed laser deposition method. The electrical characteristics of the heterojunction are studied systematically in a temperature range from 80 K to 300 K. The transport mechanism follows I ∞ exp(eV/nkT) under small forward bias, while it becomes space charge limited and follows I ∞ Vm(T) with 1.49 < m < 1.99 under high bias. Such a heterojunction also exhibits magnetoresistance (MR) effect. The absolute value of negative MR monotonically increases with temperature decreasing and reaches 26.7% at 80 K under H = 0.7 T. Various factors, such as strain and oxygen deficiency play dominant roles in the characteristics.

Enhanced Room Temperature Magnetoresistance and Spin Injection from Metallic Cobalt in Co/ZnO and Co/ZnAlO Films

Co/ZnO and Co/ZnAlO films were prepared by depositing ultrathin cobalt layers and semiconductor layers on glass substrates at room temperature. The films consist of metallic Co particles, semiconductor matrix, and an interfacial magnetic semiconductor with the substitution of Co(2+) for Zn(2+) in the ZnO lattice at the interface between Co particles and the semiconductor matrix. Large room temperature negative tunneling magnetoresistance was observed in the films. In addition, the magnetism and magnetoresistance were obviously enhanced by adding aluminum to the ZnO, and in one Co/ZnAlO sample, the room temperature negative magnetoresistance value reaches -12.3% at 18 kOe (compared with -8.4% of the corresponding Co/ZnO film) and the spin polarization of the tunneling electrons is about 37.5% which is characteristic of metallic Co. This enhancement of the tunneling spin polarization has been ascribed to the tunneling through an interfacial magnetic semiconductor, which causes the robust spin injection from cobalt metal into the semiconductors at room temperature resulting from the spin filter effect of the interfacial magnetic semiconductors.

Electric field-induced magnetic changes in La0.7Sr0.3MnO3 thin film using electric field-induced resistance phenomenon

We achieved the electrical control of a magnetic state by using an external magnetic field via electric field-induced resistance (EIR) change phenomena. We prepared a La0.7Sr0.3MnO3 (LSMO) film which showed EIR changes. Before the EIR treatment, the sample barely showed negative magnetoresistance (MR). After the EIR treatment, the value of negative MR became six times larger than that before the EIR treatment. The magnetization curves also changed from paramagnetism to superparamagnetism. These behaviors revealed that the EIR treatment changed not only the resistance state but also the magnetic state in LMSO film.

Charge transfer processes and magnetoresistance in strontium ferromolybdate with dielectric barriers

The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie temperature, large values of negative magnetoresistance at room temperature and a practically 100% spin polarization of conduction electrons. We have studied the role of grain boundaries on the electrical transport in Sr2FeMoO6−δ in applied magnetic fields up to 8 T. The compound was synthesized out of partially reduced SrFeO3−x, SrMoO4−y precursors (sample I). At the first oxidation stage upon annealing at 700 K in argon with a partial oxygen pressure p(O2) = 10 Pa for 15 h (sample II), the internal structure of the Sr2FeMoO5.82 grains did not change. Sample II exhibits a mixed type of electrical conduction. In a magnetic field B its resistivity decreases without changing the mixed regime of charge transport, only shifting the temperature of minimum resistivity (TmB) to lower values. At temperatures above TmB the conductivity is predominantly metallic, whereas below TmB down to 4.2 K it is of semiconductor type. The increase of the annealing time up to 30 h (sample III) brings about an increase of the resistivity and the appearance of semiconductor-type conductivity at T = 300–4.2 K, which indicates the formation of a continuous insulating interlayer between the grains. In a magnetic field, the resistivity decreases, with the conductivity changing from the semiconducting to a mixed one. In the latter case the charge transport can occur both through point metallic contacts and by means of spin-dependent tunneling across dielectric interlayers between Sr2FeMoO5.82 grains.