Reduction Of Magnetoresistance Research Articles

Tunable anomalous resistance and large magnetoresistance in HfTe5 by atom doping

The Dirac layered material HfTe5 renews significant interest due to its exotic band structure, leading to abundant transport properties, such as the anomaly resistance peak and its large magnetoresistance. Here, we prepared single crystals HfTe5 and Cr-doped CrxHf1−xTe5 and carried out their electrical transport measurements to explore the underlying physical origin of the anomaly resistance behavior and the large magnetoresistance. An anomalous resistance peak was observed in both intrinsic HfTe5 and the Cr-doped ones. Specifically, the peak temperature in the doped ones experiences an obvious shift from 52 to 34 K as the doping concentration x increases from 0 to 0.15, as well as the magnitude of the peak resistance is significantly enhanced. Furthermore, the magnetoresistance of CrxHf1−xTe5 is reduced by more than one order of magnitude compared with the intrinsic one. The significant reduction in magnetoresistance after Cr doping is attributed to the breaking of the balance between electron and hole carriers, which is confirmed by Kohler's plots. Meanwhile, in the sample where the magnetoresistance was minimized, we observed Shubnikov–de Haas oscillations. These observations illustrate that the large magnetoresistance is primarily contributed by the compensation of electrons and holes rather than the high mobility. Our findings provide valuable insight into how to engineer HfTe5 to achieve large magnetoresistance and its further applications in magnetic sensors and spintronics.

Role of Fe (x = 0–0.15) Substitution on Structural, Magnetic, and Transport Properties of the La0.7Sr0.3FexMn1−xO3 System

This paper investigates the effect of Fe substitution on the structural, magnetic and transport properties of the La0.7Sr0.3MnxFe1−xO3 (LSMO) system where x ranged between 0 and 0.15. Samples were prepared using a non-aqueous sol-gel synthesis method. Structural and chemical analysis confirmed the Fe3+ substitution at Mn3+ sites without any impurity phase resulted in a small change in structural parameters of the LSMO. The change in magnetic behavior and Curie temperature of the Fe-doped LSMO is explained through competitive exchange interactions developed in the system. The temperature-dependent resistivity demonstrated that the resistivity of the samples increased with Fe concentration due to different conduction mechanisms related to the ferromagnetic-metallic and paramagnetic-insulating regions. The magneto-transport measurement showed significant improvement in the magnetoresistance due to Fe doping at Mn site, which was attributed to enhancement of the spin-glass phase in Fe doped LSMO system. The reduction in magnetoresistance for higher Fe concentration is explained with the help of percolation threshold mechanism.

Pressure effect on the magnetoresistivity of topological semimetal RhSn

RhSn is a topological semimetal with chiral fermions. At ambient pressure, it exhibits large positive magnetoresistance (MR) and field-induced resistivity upturn at low temperatures. Here we report on the electrical transport properties of RhSn single crystal under various pressures. We find that with increasing pressure the temperature-dependent resistivity ρ(T) of RhSn varies minutely, whereas the value of MR at low temperatures decreases significantly. The ρ(T) data was fitted with the Bloch–Grüneisen model and the Debye temperature was extracted. Analyses of the nonlinear Hall conductivity with two-band model indicate that the carrier concentrations do not change significantly with pressure, but the mobilities for both electron and hole carriers are reduced monotonically, which can account for the significant reduction of MR under high pressures.

Magnetotransport Behaviour of Nanocrystalline Pr1-xSrxMnO3 (0.40≤x≤0.60)

The nanocrystalline samples of Pr1-xSrxMnO3 (PSMO) (x=0.40, 0.50, 0.55, and 0.60) were synthesized by wet-chemical sol-gel route. Structural, magnetic, and magnetotransport properties have been studied systematically. It is found that Pr1-xSrxMnO3 samples with Sr content x=0.40 and 0.50 show paramagnetic to ferromagnetic (PM-FM) transition at TC~308 K with no trace of FM-AFM transition within the temperature range of 77–350 K. However, interestingly a second transition is observed at T~ 273 and 255 K, respectively, for samples x=0.55 and 0.60 correspond to an A-AFM magnetic structure. This indicates that samples x=0.40 and 0.50 are ferromagnetic below TC, while other samples (x=0.55 and 0.60) have a mixed phase consisting of FM and A-type AFM phases. Resistivity versus temperature (ρ-T) curve shows that the resistivity of all the samples is much larger than the single crystals of corresponding compositions due to large contribution of grain boundaries in the present nanocrystalline samples. Moreover, the decrease in metallic component at higher Sr concentration is also evidenced by the successive reduction in magnetoresistance (MR) with increasing Sr content from x=0.40 to 0.60.

Influence of Fe segregation at grain boundaries on the magnetoresistance of Sr2Fe1+δMoO6 polycrystals

In this paper, we report the influence of Fe impurities on the magnetoresistance (MR) of double perovskite Sr2Fe1+δMoO6 (0.00 ≤ δ ≤ 0.20) system. The significant Fe impurities have been created by two different approaches: one by adding the extra amount of Fe2O3 at starting of the synthesis and other by sintering the pristine Sr2FeMoO6 sample at high temperature (1300 °C) in high reducing environment (∼8% H2 + 92% Ar). A remarkable 11% magnetoresistance at room temperature under the presence of low magnetic field (0.72 T) has been observed in the pristine sample. The achieved high low field magnetoresistance value in the sample may be due to the optimized synthesis conditions to get better inter-granular tunneling through grain boundaries. However, the presence of Fe impurity results into the sharp reduction in magnetoresistance because of reduced spin polarized tunneling. The loss of insulating nature of the grain boundaries and the inelastic scattering of the charge carriers through the metallic impurities of iron at grain boundaries may be the reason of reduced spin polarized tunneling.

Dependence of magnetoresistance on dopant density in phosphorous doped silicon

The transverse magnetoresistance at 280 K is reported as a function of phosphorous dopant density in silicon ranging from 1011 to 4×1018 cm−3. A reduction in magnetoresistance from 55% to 1.8% in an 8 T magnetic field occurred for dopant densities greater than 1016 cm−3. This reduction is related to the decrease in mobility due to increased scattering from ionized donors. A quadratic relationship of magnetoresistance with mobility is proposed in agreement with classical theory.

Ga Implantation in a MgO-based Magnetic Tunnel Junction With $\hbox{Co}_{60}\hbox{Fe}_{20}\hbox{B}_{20}$ Layers

A Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> -based tunneling magnetoresistance multilayer stack with an MgO barrier has been exposed to 30 keV Ga ions at doses corresponding to ion etching and metal deposition in a focused ion beam (FIB) instrument, to study the applicability of these processes to magnetic tunnel junction (MTJ) fabrication. MTJs were fabricated and irradiated to investigate how the exposures affected their coercivity and magnetoresistance. Elemental depth profiles, acquired using electron spectroscopy for chemical analysis, showed that Ga gathered in and around the two Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> layers. Correlated with the results of the magnetic measurements, this Ga presence was found to cause a reduction of magnetoresistance and an increase in coercivity. Quantitatively, a dose of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> reduced the magnetoresistance by 60%, whereas a dose of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> reduced the magnetoresistance by 67% and also increased the coercivity by 2 mT and changed the dipole coupling between the sensing and the pinning layers by 1.6 mT. The latter was attributed to an imbalance in the synthetic antiferromagnetic structure, where the stack's Ru spacer served as an implantation barrier. The magnetoresistance was lost at a dose of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . Annealing reduced the content of Ga around the magnetic layers but also caused diffusion of Cu from one of the layers in the stack. Apart from the observation and explanation of implantation damages in the multilayer, this work concludes on the applicability of FIB processes for prototyping of MTJs.

Enhancement in Curie temperature and reduction in magnetoresistance of Sr 2(Fe 1− xNi x)MoO 6 (0≤ x ≤0.15)

The polycrystalline Sr 2(Fe 1− x Ni x )MoO 6 (0≤ x ≤0.15) samples with ordered double perovskite structure were prepared by solid-state reaction process. Structural, magnetic and magneto-electrical transport properties have been studied systematically. The X-ray diffraction results show gradual decrease in degree of cationic order with increasing Ni content and increase in the anti-site defects. The Curie temperature ( T C) in the Sr 2(Fe 1− x Ni x )MoO 6 is enhanced systematically from 408 K for x = 0 to 424 K for x = 0.15. However, magnetization is observed to decrease with increasing Ni content. The resistivity of all the samples exhibits semiconducting behavior in measured temperature range (80–300 K). The low field magnetoresistance is observed to decrease with increasing the Ni concentration. The observed decrease in low field magnetoresistance has been attributed to the increased disorder and anti-site defects.

Magnetic and transport properties of Co-doped Fe3O4 films

We present a systematic study of the structural, magnetic, and magnetotransport properties of Co-doped Fe3O4 films deposited on MgO (100) substrates by cosputtering technique. Transmission electron microscopy images suggest that the undoped and Co-doped Fe3O4 films are polycrystalline in nature and consist of a well defined grain boundary network. The temperature dependence of resistance also shows that the transport mechanism in our films is dominated by electron tunneling across antiferromagnetically coupled grain boundaries. We observed that the magnetic properties of the doped films are markedly sensitive to the Co doping concentration, with the magnetization curves showing drastic changes in coercivity with increasing doping concentration. In-plane magnetoresistance curves show linear magnetic field dependence for the undoped Fe3O4 films while a reduction in magnetoresistance and a departure from linear field dependence are observed for the Co-doped films.

Competing Exchange Interactions in Magnetic Multilayers

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

Study of magnetoresistance behavior near percolation threshold in CrO2/Fe3O4 binary half-metal composites

The electrical properties and magnetoresistance (MR) of CrO2/Fe3O4 composites with opposite spin polarization have been investigated as a function of CrO2 content. Experimental results indicate that the conductivity exhibits an abrupt increase when CrO2 volume fraction ϕ is close to 0.15, where a percolation behavior happens. Magnetoresistance measurement shows a minimum value in the vicinity of percolation threshold at a temperature of 77 K, while this phenomenon cannot be observed at 150 K. Due to spin-dependent tunneling through grain boundary, the reduction in MR at 77 K mainly results from the opposite sign of spin polarizations in the binary half-metal granular CrO2/Fe3O4 composite.

Effect of cobalt doping concentration on the structural and magnetic properties of Fe 3O 4

In this work, we investigate the effects of cobalt doping on the structural, magnetic and transport properties of half metallic Fe 3O 4 films fabricated on MgO (100) substrates by co-sputtering. The doped films exhibit a highly broadened metal–insulator Verwey transition as compared to that of undoped Fe 3O 4 films. The magnetic and transport properties of the doped films are markedly sensitive to the Co doping concentration; films with high Co doping exhibit almost negligible coercivity at room temperature. Magnetoresistance (MR) curves show linear magnetic field dependence for the undoped films, while a reduction in MR and departure from linear field dependence is observed for Co-doped films.

Influence of nonmagnetic Al ions on magnetoresistance of double-perovskite Sr2Fe1−xAlxMoO6 (0≤x≤0.30)

The structural, magnetic, and magnetoresistance properties of the double-perovskite series Sr2Fe1−xAlxMoO6 (0≤x≤0.30) were systematically investigated in order to clarify the influence of nonmagnetic Al ions on the magnetoresistance. The structural refinements of these samples show that the degree of cationic order increases gradually from 88.5% for x=0 to 92% for x=0.30 without any change in the crystal structure. The magnetization measurements reveal that the substitution of nonmagnetic Al ion for Fe ion enhances the magnetic moment per Fe ion significantly. In addition, the magnetic-field dependence of magnetization and magnetoresistance of these Sr2Fe1−xAlxMoO6 samples were all fitted excellently by taking into account the contributions from ferromagnetic-coupled Fe–O–Mo region and nonferromagnetic-coupled regions. The fitting results indicate that the low-field magnetoresistance can be greatly enhanced due to the separation of the cationic-ordered Fe–O–Mo regions by the paramagnetic Mo–O–Al–O–Mo chains introduced through Al doping. Furthermore, doping nonmagnetic Al ions also suppress the formation of antiferromagnetic Fe–O–Fe antiphase boundaries, and then lead to the improvement of cation ordering and the reduction of magnetoresistance under high field.

Giant tunneling magnetoresistance effect in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions for read-head applications

The giant tunneling magnetoresistance effect has been achieved in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions (MTJs) at room temperature. A magnetoresistance (MR) ratio as high as 138%, seven times that of state-of-the-art MTJs for magnetic sensor application, was obtained at room temperature in MTJs with a resistance-area product (RA) as low as 2.4Ωμm2. Such a high MR ratio at such a low resistance was made possible by introducing an ultrathin Mg metal layer with a thickness of 4 Å between the CoFeB bottom electrode layer and the MgO(001) tunnel barrier layer. The Mg layer was slightly but not fully oxidized, which resulted in a reduction in MR for a thicker MgO barrier (high RA) region and in an increase in MR for a thinner barrier (low RA) region. The Mg layer improves the crystalline orientation of the MgO(001) layer when the MgO(001) layer is thin. These MTJs will accelerate the realization of highly sensitive read heads for ultrahigh-density hard-disk drives.

Low-field magnetoresistance of Fe/Cr multilayers

Sputtered {Fe/Cr} multilayers with a fixed Cr individual layer thickness t Cr=2 nm and variable Fe individual layer thickness (1 nm⩽ t Fe⩽6 nm) are investigated by means of X-ray diffraction, magnetoresistance and magnetisation measurements. At room temperature, the initial magnetoresistive susceptibility of the as-deposited samples is almost constant. However, the saturation field increases with decreasing Fe-layer thickness, therefore, a maximal magnetoresistance ratio Δ R/ R of 0.7% is reached in the sample with t Fe=1 nm. After annealing at 350°C, a Δ R/ R value as large as 2.3% was obtained. Further annealing causes a reduction of magnetoresistance. As the temperature is decreased, the Δ R/ R ratio measured in μ 0 H=0.3 T increases linearly. At 77 K, the magnetoresistance ratio is about four times larger than that at 300 K. Results are discussed in terms of the scattering located at interfaces and the formation of a ferromagnetic state at high-temperature heat treatments.

Electron tunneling and noise studies in ferromagnetic junctions

Transport properties of exchange-biased magnetic tunnel junction structures are reported as a function of tunnel barrier thickness. The temperature dependence of the tunneling conductance and magnetoresistance (MR) is consistent with recent theories relating it to the magnon excitations at the electrode–barrier interface or the temperature-dependent surface magnetization. We have also measured the bias voltage dependence of the MR. For CoFe magnetic electrodes, the reduction in MR is approximately 50% at biases of ∼250 mV. At low temperatures, we observe a cusp-like dip in the tunneling conductance at zero-bias. The conductance increases as the square-root of the bias voltage, indicating that electron–electron interactions as in disordered media may be important. Coating a magnetic electrode or the oxide barrier with an alternate, thin magnetic layer of higher spin polarization is shown to generally increase the MR by several percent but at the expense of higher 1/ f noise.

Pr-doping of the high-magnetoresistance perovskite [formula omitted

We have investigated the effect of Pr-doping on the Nd-sites in the high magnetoresistance (MR) perovskite Nd 2 3 Sr 1 3 MnO 3 . For polycrystalline materials, negative MR as high as ∼ 235% is observed near 237K in pure Nd 2 3 Sr 1 3 MnO 3 . The substitution of Pr for Nd leads to a reduction in the overall MR, and an increase of the MR-peak temperature. We investigate the possibility that the large negative MR originates from the formation of localized magnetic polarons.