Giant Magnetoresistance Behavior Research Articles

Zigzag boron nitride nanoribbon doped with carbon atom for giant magnetoresistance and rectification behavior based nanodevices

Using the principles of density functional theory (DFT) and nonequilibrium Green’s function (NEGF), We thoroughly researched carbon-doped zigzag boron nitride nanoribbons (ZBNNRs) to understand their electronic behavior and transport properties. Intriguingly, we discovered that careful doping can transform carbon-doped ZBNNRs into a spintronic nanodevice with distinct transport features. Our model showed a giant magnetoresistance (GMR) up to a whopping 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} under mild bias conditions. Plus, we spotted a spin rectifier having a significant rectification ratio (RR) of 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document}. Our calculated transmission spectra have nicely explained why there’s a GMR up to 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} for spin-up current at biases of -1.2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.2$$\\end{document} V, -1.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.1$$\\end{document} V, and -1.0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1.0$$\\end{document} V, and also accounted for a GMR up to 103\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}–105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^5$$\\end{document} for spin-down current at biases of 1.0 V, 1.1 V, and 1.2 V. Similarly, the transmission spectra elucidate that at biases of 1.0 V, 1.1 V, and 1.2 V for spin-up, and at biases of 1.1 V and 1.2 V for spin-down in APMO, the RRs reach 104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^4$$\\end{document}. Our research shines a light on a promising route to push forward the high-performance spintronics technology of ZBNNRs using carbon atom doping. These insights hint that our models could be game-changers in the sphere of nanoscale spintronic devices.

Perfect Spin Filtering, Giant Magnetoresistance, and Rectification Behavior Induced by V‐Doped Zigzag Graphene Nanoribbons

AbstractEmploying the constructs of density functional theory (DFT) and the Nonequilibrium Green's Function (NEGF), the investigation extensively explores the electronic and transport properties of zigzag graphene nanoribbons (ZGNRs) doped with vanadium (V). Notably, this inquiry unveils that strategic doping can transform V‐doped ZGNRs into spintronic nanodevices with distinctive transport attributes. Initially, the simulations showcase remarkably high spin‐filtering efficiencies (SFEs) at certain bias voltages. Furthermore, a giant magnetoresistance (GMR) peaking at 6.87 10 is detected. In conclusion, the examination discerns a spin rectifier that exhibits a significant rectification ratio (RR) of 9.62 10. This research delineates a viable trajectory for the refinement of high‐performance spintronics in ZGNRs via vanadium doping. The implications of this study indicate that the model harbors considerable promise for application in miniature spintronic devices.

DFT Analysis of Hydrogenated Zigzag Aluminum Nitride Nanoribbons for Spintronic Devices

Density functional theory (DFT) and nonequilibrium Green’s function (NEGF) framework are used to explore the structural, spin-polarized electronic, and spin-based transport properties of edge-hydrogenated zigzag aluminum nitride nanoribbons (ZAlNNRs). The proposed ZAlNNR is observed to be structurally stable and exhibits half-metallic nature in the magnetic state. The quantum transport property of the proposed two-terminal device model of 1H-AlN-1H demonstrates the bipolar spin-filter characteristics along with giant magnetoresistance (GMR), spin-based peak to valley current ratio (spin-PVCR), and spin-based rectification ratio (spin-RR) of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> , 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> , and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> , respectively. The calculated GMR and spin-RR of the 1H-AlN-1H device are 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> times higher than zigzag silicene nanoribbon (ZSiNR) and doped-zigzag graphene nanoribbon (doped ZGNR), respectively. The observed GMR, spin-PVCR, and spin-rectifying behavior of the reported ZAlNNR device could be deployed for multifunctional spintronic device applications.

Giant Magnetoresistance and Rectification Behavior in Fluorinated Zigzag Boron Nitride Nanoribbon for Spintronic Nanodevices

In the present work, the spin-polarized structural and electronic properties of fluorine (F) passivated zigzag boron nitride nanoribbons (ZBNNRs) at the selective boron (B) and nitrogen (N) edge atoms are investigated. This study is based on the density functional theory (DFT) along with non-equilibrium Green function (NEGF) formalism. Our study predicts that half-metallic property can be obtained in ZBNNRs via F passivation at selective edges. The F-passivated ZBNNRs are found to be structurally stable in both non-magnetic as well as magnetic ground states irrespective of their width. Hence, the transport properties of F-passivated ZBNNRs are also studied as fluorinated structures are reported to be more stable. The current-voltage characteristic of F-passivated ZBNNRs based devices exhibit the perfect spin-filter characteristics with magnificently high spin-filtering efficiency (SFE) even under a low bias. It is worth mentioning here that giant magnetoresistance resistance (GMR), and rectification ratio of the order of 10⁸and 10⁵ have also been observed for F-BN-F devices. This is because dangling bonds break the edge states' symmetry and induce some localized states, which suppress the electron transmission and reduce the current. The observed perfect spin-filtering characteristics, GMR, and rectifying characteristics suggest that Fpassivated ZBNNRs have immense potentials to be deployed for nanoscale spintronic devices.

Selective Edge-Hydrogenated Zigzag Boron Nitride Nanoribbons for Giant Magnetoresistance and Rectifying Behavior

The structural stability, spin-polarized electronic and transport properties of edge-hydrogenated zigzag boron nitride nanoribbons (ZBNNRs) are investigated using density functional theory (DFT) combined with a non-equilibrium Green’s function (NEGF) approach. The considered structures are observed to be energetically stable in the magnetic ground state. The calculated spin-dependent transport properties indicate that a selective-edge hydrogenated ZBNNR device shows bipolar spin filter characteristics. Interestingly, high spin giant magnetoresistance (GMR) and a rectification ratio (RR) are observed of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> , respectively. The observed GMR and rectification behavior in edge-hydrogenated ZBNNRs have potential applications for designing future spintronic nanodevices.

Magnetic and electronic properties of metallic multilayers: monolayer thickness effects

Giant magnetoresistance (GMR) is discussed at RT for the Co(111)/Cu(111) metallic bilayer system. The theoretical approach based on the original model developed by Hood Falicov and Penn has been used and extended in this analysis. We study the effect of the Cu and Co monolayer thickness on the behavior and amplitude of GMR. An identical decreasing behavior in the evolution of GMR is observed by varying the thickness of each monolayer. The resistivitiy of a ferromagnetic configuration (up-up, $$^{{\uparrow \uparrow }})$$ behaves differently. The influence of phenomenological scattering on the variation of GMR as a function of the interlayer thickness is also taken into account. We analyze experimental data giving the bilayer number dependence on the GMR ratio of Co/Cu multilayers at RT.

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

Bulk nanocrystalline materials of small and medium ferromagnetic content were produced using severe plastic deformation by high-pressure torsion at room temperature. Giant magnetoresistive behavior was found for as-deformed materials, which was further improved by adjusting the microstructure with thermal treatments. The adequate range of annealing temperatures was assessed with in-situ synchrotron diffraction measurements. Thermally treated Cu–Co materials show larger giant magnetoresistance after annealing for 1 h at 300 °C, while for Cu-Fe this annealing temperature is too high and decreases the magnetoresistive properties. The improvement of magnetoresistivity by thermal treatments is discussed with respect to the microstructural evolution as observed by electron microscopy and ex-situ synchrotron diffraction measurements.

Quasi one-dimensional van der Waals gold selenide with strong interchain interaction and giant magnetoresistance

The atomic structure of quasi one-dimensional (1D) van der Waals materials can be regarded as the stacking of atomic chains to form thin flakes or nanoribbons, which substantially differentiates them from typical two-dimensional (2D) layered materials and 1D nanotube/nanowire array. Here we present our studies on quasi 1D gold selenide (AuSe) that possesses highly anisotropic crystal structure, excellent electrical conductivity, giant magnetoresistance, and unusual reentrant metallic behavior. The low in-plane symmetry of AuSe gives rise to its high anisotropy of vibrational behavior. In contrast, quasi 1D AuSe exhibits high in-plane electrical conductivity along the directions of both atomic chains and perpendicular one, which can be understood as a result of strong interchain interaction. We found that AuSe exhibits a near quadratic nonsaturating giant magnetoresistance of 1841% with the magnetic field perpendicular to its in-plane. We also observe unusual reentrant metallic behavior, which is caused by the carrier mismatch in the multiband transport. Our works help to establish fundamental understandings on quasi 1D van der Waals semimetallic AuSe and identify it as a new candidate for exploring giant magnetoresistance and compensated semimetals.

Magnetoelectric P(VDF-HFP)-CoFe2O4 films and their giant magnetoresistance properties

Abstract Magnetoelectric (ME) nanocomposite flexible films composed of electroactive poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) and ferromagnetic cobalt ferrite (CoFe2O4) are fabricated. Single-phase nano CoFe2O4 with cubic spinel structure, synthesized by sol-gel auto combustion technique, is reinforced in different weight percentages to fabricate P(VDF-HFP)-CoFe2O4 films through spin coating technique. Effect of CoFe2O4 on structural, morphological, magnetic, ferroelectric, magnetoelectric properties, and giant magnetoresistance (GMR) behavior has been investigated. It is observed that the reinforcement of CoFe2O4 in P(VDF-HFP) matrix has strongly influenced the formation of β-phase conformation and the degree of structural order. The co-existence of ferroelectric and ferromagnetic orderings in the films has been demonstrated by the P-E and M-H plots. The variations in the polarization effects in the presence of a magnetic field provide the evidence for magnetoelectric cross-coupling in P(VDF-HFP)-CoFe2O4. The films exhibit a GMR effect from 7% to 100%. In order to demonstrate the use of ME induced GMR in magnetic field sensing, P(VDF-HFP)-CoFe2O4 film (10 wt%) is used in a prototype sensor circuit. A good sensing ability and linear response obtained prove that the prepared film can be used as field sensor.

Spin-dependent scattering and magnetic proximity effect in Ni-doped Co/Cu multilayers as a probe of atomic magnetism

We investigate the spin transport and ferromagnetic resonance properties of giant magnetoresistance (GMR) Co/Cu-Ni multilayers with variable levels of Ni doping in the Cu spacer. We present an experimental evidence for a magnetic-to-diamagnetic transition in the atomic magnetic moment of Ni in the Cu matrix for concentrations below 15 at. % Ni. As its concentration is increased, Ni atoms turn into spin scattering centers, which is manifested experimentally as a step-like change in the GMR of the multilayers. This behavior is observed in multilayers with gradient-doped Cu spacers, where only the inner region was doped with Ni. In the uniformly doped spacers, the GMR decreases monotonously with increasing Ni content, indicating that Ni atoms are magnetic and act as spin relaxation centers in the entire dopant-concentration range studied. We explain the difference in the observed GMR behavior due to a strong magnetic proximity effect in the uniform spacers, which is efficiently suppressed in the gradient spacers. The observed magnetic phase transition is fully supported by our detailed ab initio calculations, taking into consideration structural relaxation in the system as well as potential Ni clustering. Controlling the loss or gain of the atomic magnetism for a specific dopant can be a tool in probing and controlling spin relaxation in materials and devices for spin-valve and spin-torque based applications.

Structure and Giant Magnetoresistance of Co-Fe/Cu Multilayer Films Electrodeposited from Various Bath Formulations

Detailed new results are reported on the preparation and giant magnetoresistance (GMR) of electrodeposited Co-Fe/Cu multilayer films by using four different baths (sulfamate, sulfate, ammonium chloride and sodium citrate type solutions). Two-pulse plating was applied for Co-Fe(5nm)/Cu(5nm) multilayer preparation by using galvanostatic pulses for the deposition of the magnetic layer. The Cu layer deposition potential was electrochemically optimized for each bath formulation by analyzing the current transients during the deposition of the non-magnetic layers. The optimal Cu deposition potential was found to be dependent both on the bath formulation and the Fe2+/Co2+ ion concentration ratio. The results of X-ray diffraction (XRD) measurements were in agreement with the composition of the samples. At low Fe content (about 10 at.% Fe) in the magnetic layer, an fcc structure was formed (in some cases, even multilayer satellites were observed). In samples with high Fe content (about 33 at.% Fe) in the magnetic layer, both fcc and bcc phases were present. A GMR behavior was observed for all multilayers, with a maximum GMR of about 4% in some cases. For multilayers from the sulfamate, sulfate and chloride baths, the GMR exhibited a multilayer-type behavior whereas the GMR of samples from the citrate bath was rather similar to the behavior of granular magnetic alloys containing also superparamagnetic regions.

Oscillatory Giant Magnetoresistance of Electrodeposited Fe/Cu/Ni Multilayered Structures—Effect of Non-Magnetic and Magnetic Layer Thicknesses

Giant magnetoresistance (GMR) behavior in electrodeposited N[Fe/Cu/Ni] multilayered (ML) structures is investigated as a function of non-magnetic (Cu) and ferromagnetic (Fe and Ni) layers’ thicknesses. Detailed structural and magnetic analyses of the ML structures are studied before looking into the GMR behavior. Structural analyses reveal that all peaks are well matched with Ni (face-centered cubic) being the top most layer during ML deposition. Shifting of preferred orientation, between (220) and (111) planes, is observed with the thickest layer of copper spacer. Refined structural parameters are calculated and details of fittings are discussed with reference to layer thicknesses. Crystallite size, dislocation density, stacking fault probability, and strain all show oscillatory behavior with variation in thicknesses. Magnetic behavior shows strong dependence of magnetization on thickness of each layer. Saturation magnetization ( $M_{s})$ increases up to a value of 10.12 emu/cm3 with increasing Fe and Ni layer thicknesses. Whereas, oscillatory behavior of magnetization is observed with variation in Cu layer thickness. Magnetoresistance (MR) measurements show oscillatory GMR behavior as a function of intervening layer thickness. Highest value of GMR ~12% is observed at a Cu layer thickness of 9.6 nm. Structural, magnetic, and MR properties of N[Fe/Cu/Fe] are observed on the basis of variation of thicknesses of non-magnetic (Cu) and ferromagnetic (Fe and Ni) layers. It is observed that layer thicknesses play a dominant effect on the nature (i.e., oscillatory) and on the value of GMR.

Azulene-like molecular devices with high spin filtering, strong spin rectifying, and giant magnetoresistance effects

Abstract By using the nonequilibrium Green's function method combined with density functional theory, we investigate the spin transport properties of an azulene-like molecule sandwiched between two ferromagnetic zigzag graphene nanoribbon (ZGNR) electrodes. The results show that the width of electrodes and the magnetization configurations of electrodes are critical to spin transport. By tuning the spin configurations of ZGNR electrodes, excellent spin filtering, spin rectifying, and giant magnetoresistance behaviors can be obtained simultaneously in devices with 4-ZGNR electrodes. Analysis of symmetric of Bloch wave functions, spin resolved band structures of the electrodes with applied bias, transmission spectra, as well as the projected density of states reveals the mechanism of these intriguing properties. These results might be useful in the design of high performance multifunctional spintronic devices.

Valley Polarization of Trions and Magnetoresistance in Heterostructures of MoS2 and Yttrium Iron Garnet.

Manipulation of spin degree of freedom (DOF) of electrons is the fundamental aspect of spintronic and valleytronic devices. Two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit an emerging valley pseudospin, in which spin-up (-down) electrons are distributed in a +K (-K) valley. This valley polarization gives a DOF for spintronic and valleytronic devices. Recently, magnetic exchange interactions between graphene and magnetic insulator yttrium iron garnet (YIG) have been exploited. However, the physics of 2D TMDCs with YIG have not been shown before. Here we demonstrate strong many-body effects in a heterostructure geometry comprising a MoS2 monolayer and YIG. High-order trions are directly identified by mapping absorption and photoluminescence at 12 K. The electron doping density is up to ∼1013 cm-2, resulting in a large splitting of ∼40 meV between trions and excitons. The trions exhibit a high circular polarization of ∼80% under optical pumping by circularly polarized light at ∼1.96 eV; it is confirmed experimentally that both phonon scattering and electron-hole exchange interaction contribute to the valley depolarization with temperature; importantly, a magnetoresistance (MR) behavior in the MoS2 monolayer was observed, and a giant MR ratio of ∼30% is achieved, which is 1 order of magnitude larger than the reported ratio in MoS2/CoFe2O4 heterostructures. Our experimental results confirm that the giant MR behaviors are attributed to the interfacial spin accumulation due to YIG substrates. Our work provides an insight into spin manipulation in a heterostructure of monolayer materials and magnetic substrates.

Field-induced metamagnetic transition and nonsaturating magnetoresistance in the antiferromagnetic semimetal NdSb

We have performed a joint study of magnetization and magnetoresistance in the antiferromagnetic (AFM) semimetal NdSb under pulsed high magnetic field up to 60 T. We show that NdSb displays extremely large magnetoresistance (XMR), reaching values of $1.77\ifmmode\times\else\texttimes\fi{}{10}^{6}%$ at 1.3 K and 60 T without any sign of saturation. The field-dependent magnetization at 4.2 K reveals a field-induced metamagnetic transition from the AFM to the ferromagnetic state around 9 T. During the metamagnetic transition, a steplike reduction of resistance occurs, resembling the giant magnetoresistance (GMR) behavior in metallic multilayers. The investigation of the Shubnikov--de Haas effect in NdSb suggests that a particular orbital texture might contribute to the XMR effect in addition to the perfect electron-hole compensation.

Giant magnetoresistance (GMR) behavior of electrodeposited NiFe/Cu multilayers: Dependence of non-magnetic and magnetic layer thicknesses

Giant magnetoresistance (GMR) behavior in electrodeposited NiFe/Cu multilayers was investigated as a function of non-magnetic (Cu) and ferromagnetic (NiFe) layer thicknesses, respectively. Prior to the GMR analysis, structural and magnetic analyses of the multilayers were also studied. The elemental analysis of the multilayers indicated that the Cu and Ni content in the multilayers increase with increasing Cu and NiFe layer thickness, respectively. The structural studies by X-ray diffraction revealed that all multilayers have face centred cubic structure with preferred (110) crystal orientation as their substrates. The magnetic properties studied with the vibrating sample magnetometer showed that the magnetizations of the samples are significantly affected by the layer thicknesses. Saturation magnetisation, Ms increases from 45 to 225emu/cm3 with increasing NiFe layer thickness. The increase in the Ni content of the multilayers with a small Fe content causes an increase in the Ms. And, the coercivities ranging from 2 to 24 Oe are between the soft and hard magnetic properties. Also, the magnetic easy axis of the multilayers was found to be in the film plane. Magnetoresistance measurements showed that all multilayers exhibited the GMR behavior. The GMR magnitude increases with increasing Cu layer thickness and reaches its maximum value of 10% at the Cu layer thickness of 1nm, then it decreases. And similarly, the GMR magnitude increases and reaches highest value of pure GMR (10%) for the NiFe layer thickness of 3nm, and beyond this point GMR decreases with increasing NiFe layer thickness. Some small component of the anisotropic magnetoresistance was also observed at thin Cu and thick NiFe layer thicknesses. It is seen that the highest GMR values up to 10% were obtained in electrodeposited NiFe/Cu multilayers up to now. The structural, magnetic and magnetoresistance properties of the NiFe/Cu were reported via the variations of the thicknesses of Cu and NiFe layers with stressing the role of layer thicknesses on the high GMR behavior.

Lateral Magnetically Modulated Multilayers by Combining Ion Implantation and Lithography.

The combination of lithography and ion implantation is demonstrated to be a suitable method to prepare lateral multilayers. A laterally, compositionally, and magnetically modulated microscale pattern consisting of alternating Co (1.6 µm wide) and Co-CoO (2.4 µm wide) lines has been obtained by oxygen ion implantation into a lithographically masked Au-sandwiched Co thin film. Magnetoresistance along the lines (i.e., current and applied magnetic field are parallel to the lines) reveals an effective positive giant magnetoresistance (GMR) behavior at room temperature. Conversely, anisotropic magnetoresistance and GMR contributions are distinguished at low temperature (i.e., 10 K) since the O-implanted areas become exchange coupled. This planar GMR is principally ascribed to the spatial modulation of coercivity in a spring-magnet-type configuration, which results in 180° Néel extrinsic domain walls at the Co/Co-CoO interfaces. The versatility, in terms of pattern size, morphology, and composition adjustment, of this method offers a unique route to fabricate planar systems for, among others, spintronic research and applications.

Tunable Giant Magnetoresistance Behavior and Electron Conductivity of Conjugated Naopolyaniline

Due to the widely applications of giant magnetoresistance (GMR) phenomenon in magnetic recording, biosensing, and biochip systems, and the unique physical and chemical properties of polyaniline (PANI), the MR behavior of conjugated conductive PANI nanoparticles is widely studied in recent years to widen its applications in new area. The nanostructure, doping level, temperature, magnetic field direction, and types of dopants effects on the MR behavior of PANI nanoparticles are systematically studied in this work. The temperature (from 50 to 290K) dependent electrical conductivity of PANI were controlled by adjusting the doping level (0.5, 1.0, and 2.0 M), dopants type (HCL, H2SO4, H3PO4 and PTSA), and nanostructure (nanofiber and nanosphere of PANI). The Mott variable range hopping (VRH) model was used to study the electron transport mechanism of PANI. And the quasi 2 and 3-dimensionals variable range hopping (VRH) electrical conduction mechanism are observed in PANI nanofibers and nanospheres, respectively. The positive and negative MR are observed in the synthesized PANI at room temperature, and analyzed by the wave-function shrinkage model and orbital magnetoconductivity theory (forward interference model), respectively. The scanning electron microscope (SEM) is used to characterize the surface morphology of PANI nanoparticles. The chemical structure of PANI nanoparticles is characterized by Fourier transform infrared (FT-IR) spectroscopy. The thermal stability of PANI is assessed by thermogravimetric analysis (TGA). X-ray diffraction (XRD) is used to study the crystal structure of PANI. The dielectric permittivity and optical property are also reported in this work. Experimental PANI nanofibers were prepared by an interfacial polymerization method with an aniline: ammonium persulfate (APS):PTSA(p-toluene sulfonic acid) ratio of 8:2:25. Briefly, aniline (3.2 mmol) was dissolved in chloroform (10 mL) as Solution 1, and Solution 2 was prepared by dissolving APS (0.8 mmol) in 1.0 M PTSA aqueous solution (10 mL). Then, Solution 2 was added into Solution 1 quickly and maintained still for 2 h of polymerization at room temperature.1 PANI nanospheres were synthesized by oxidation polymerization method. Briefly, the molar ratio was aniline: APS: PTSA = 6:3:5. For solution 1, PTSA (30 mmol) and APS (18 mmol) were dissolved in deionized water (200 mL) in a beaker, which was treated by sonication in the ice water bath for 1 hour mechanical stirring (300 rpm). Solution 2 was aniline (36 mmol) dissolved in deionized water (50 mL). Solution 2 was then added into solution 1, and the mixture was sonicated for an additional 1.5 hour mechanical stirring (300 rpm) in the ice water bath for polymerization of aniline. After the polymerization, the suspensions contains PANI nanoparticles were vacuum filtered and washed acetone to remove the oligomer, then washed with ammonia to remove the doped acid, then wash with different concentrations (0, 0.5, 1.0, and 2.0 M) of dopant acid (PTSA, HCL, H2SO4, and H3PO4). The final product was dry in the oven at 50 oC. Results and Discussion The FT-IR, XRD and SEM images indicates that the PANI (emeraldine salt form) nanofibers and nanospheres are successfully synthesized by interfacial and oxidation polymerization methods, respectively. The temperature dependent resistivity results indicate that morphology, dopant acid, doping level have effect on the electrical conductivity of PANI nanoparticles. The resistivity decreases with increasing the temperature, indicating the typical behavior of a semiconductor. The Mott variable range hopping (VRH) model was used to study the electron transport mechanism of PANI nanoparticles, and the quasi 2 and 3-dimensionals variable range hopping (VRH) electrical conduction mechanism are observed in PANI nanofibers and nanospheres, respectively. The PANI nanoparticles shows different MR behavior due to the difference of morphology, doping level, and dopant acid. The positive and negative MR are observed in the synthesized PANI at room temperature and analyzed by the wave-function shrinkage model and orbital magnetoconductivity theory (forward interference model), respectively. The localization length(a0 ), average hopping length (R hop), and density of states at the Fermi level N(EF ) are calculated. In a word, the MR and electrical conductivity of the PANI nanoparticles is tunable by controlling the morphology of PANI, doping level, and dopant acid, which has significant effect on the application of MR based recording system. Acknowledgments The financial supports from University of Tennessee Knoxville are kindly acknowledged. Reference 1. X. Zhang, Q. He, H. Gu, H. A. Colorado, S. Wei and Z. Guo; ACS Applied Materials & Interfaces , 5(3) 898 (2013) Figure 1

Effect of thermal deformation on giant magnetoresistance of flexible spin valves grown on polyvinylidene fluoride membranes**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374312, 51401230, 51522105, and 51471101) and the Ningbo Science and Technology Innovation Team, China (Grant No. 2015B11001).

We fabricated flexible spin valves on polyvinylidene fluoride (PVDF) membranes and investigated the influence of thermal deformation of substrates on the giant magnetoresistance (GMR) behaviors. The large magnetostrictive Fe81Ga19 (FeGa) alloy and the low magnetostrictive Fe19Ni81 (FeNi) alloy were selected as the free and pinned ferromagnetic layers. In addition, the exchange bias (EB) of the pinned layer was set along the different thermal deformation axes α31 or α32 of PVDF. The GMR ratio of the reference spin valves grown on Si intrinsically increases with lowering temperature due to an enhancement of spontaneous magnetization. For flexible spin valves, when decreasing temperature, the anisotropic thermal deformation of PVDF produces a uniaxial anisotropy along the α32 direction, which changes the distribution of magnetic domains. As a result, the GMR ratio at low temperature for spin valves with EB∥ α32 becomes close to that on Si, but for spin valves with EB∥ α31 is far away from that on Si. This thermal effect on GMR behaviors is more significant when using magnetostrictive FeGa as the free layer.

Magnetostrictive GMR spin valves with composite FeGa/FeCo free layers

We have fabricated strain-sensitive spin valves on flexible substrates by utilizing the large magnetostrictive FeGa alloy to promote the strain sensitivity and the composite free layer of FeGa/FeCo to avoid the drastic reduction of giant magnetoresistance (GMR) ratio. This kind of spin valve (SV-FeGa/FeCo) displays a MR ratio about 5.9%, which is comparable to that of the conventional spin valve (SV-FeCo) with a single FeCo free layer. Different from the previously reported works on magnetostrictive spin valves, the SV-FeGa/FeCo displays an asymmetric strain dependent GMR behavior. Upon increasing the lateral strain, the MR ratio for the ascending branch decreases more quickly than that for the descending branch, which is ascribed to the formation of a spiraling spin structure around the FeGa/FeCo interface under the combined influences of both magnetic field and mechanical strain. A strain sensitivity of GF = 7.2 was achieved at a magnetic bias field of -30 Oe in flexible SV-FeGa/FeCo, which is significantly larger than that of SV-FeCo.