Significant Enhancement In Magnetoresistance Research Articles

Significant enhancement of magnetoresistance with the reduction of particle size in nanometer scale.

The Physics of materials with large magnetoresistance (MR), defined as the percentage change of electrical resistance with the application of external magnetic field, has been an active field of research for quite some times. In addition to the fundamental interest, large MR has widespread application that includes the field of magnetic field sensor technology. New materials with large MR is interesting. However it is more appealing to vast scientific community if a method describe to achieve many fold enhancement of MR of already known materials. Our study on several manganite samples [La1−xCaxMnO3 (x = 0.52, 0.54, 0.55)] illustrates the method of significant enhancement of MR with the reduction of the particle size in nanometer scale. Our experimentally observed results are explained by considering model consisted of a charge ordered antiferromagnetic core and a shell having short range ferromagnetic correlation between the uncompensated surface spins in nanoscale regime. The ferromagnetic fractions obtained theoretically in the nanoparticles has been shown to be in the good agreement with the experimental results. The method of several orders of magnitude improvement of the magnetoresistive property will have enormous potential for magnetic field sensor technology.

Revealing the origin of magnetoresistance in unipolar amorphous organic field-effect transistors

We report on the magnetoresistance (MR) effect in a unipolar p-channel field-effect transistor based on amorphous thin film of low molecular weight 2,2′,7,7′-tetrakis(diphenylamino)-9,9′-spirobifluorene (Spiro-TAD). To scrutinize the origin of this effect two themes have been studied: (i) the influence of gate dielectric SiO2 surface treatment and (ii) the importance of organic molecular p-dopant 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP) thin films sandwiched between SiO2 and Spiro-TAD. A device fabricated on bare SiO2 shows larger MR than one fabricated on hexamethyldisilazane-treated SiO2, suggesting the bipolaron species as the origin of this effect. This could be understood through two aspects. (i) The stabilization of bipolaron, i.e. trapped charge-stabilized bipolaron. Due to large energy cost for bipolaron formation, the stabilization of bipolaron by a trap with opposite sign is favored. Additionally, interface doping of Spiro-TAD with F6-TNAP reveals a significant enhancement in MR caused by an increase in trap density and charge carriers, boosting the effectiveness of bipolaron stabilization. (ii) Larger energetic disorder that might compensate the energy cost for bipolaron formation is expected to be obtained for a device fabricated on bare SiO2 caused by dipolar disorder at the corresponding interface. Adding a thin layer of F6-TNAP is expected to increase the energetic disorder as well.

Ferro-Antiferromagnetic Coupling and Enhancement of Magnetoresistance in La<sub>0.67</sub>Ca<sub>0.33</sub>MnO<sub>3</sub>/LaMnO<sub>3</sub> Composites

The samples with the nominal composition of (1-x)La0.67Ca0.33MnO3/xLaMnO3 with x=0.00, 0.05, 0.15 and 0.25 were fabricated using a special experimental method. The electrical transport behaviour and magnetoresistance (MR) were studied for the composites in magnetic fields H=0.3T, 3T. Experimental results show that with the increasing LaMnO3 doping level, the metal–insulator（M-I） transition temperature TP shifts to lower temperature and the resistivity increases sharply in zero magnetic field. Meanwhile, a significant enhancement in MR is observed for the composites especially in the low temperature range（below TP）. Specially, the maximum MR at 3 T increased from 35% for the pure La0.67Ca0.33MnO3 to 92% for the sample with x=0.25. We suggest that such enhancement in MR is attributed to the strong ferro-antiferromagnetic coupling effects in the composite system, which increase the magnetic disorder at the grain surface and boundary, will improve the spin-polarized tunneling process of the conducting electron between adjacent grains, and thus enhance the MR effects.

Low-field magnetoresistance in La 0.7Sr 0.3MnO 3/CuCrO 2 composites

Two-phase composites of (1− x)La 0.7Sr 0.3MnO 3/ xCuCrO 2 ( x is mass%) are synthesized by the sol–gel method. The microstructure, and magnetic and transport properties of the composites were investigated. Magnetic measurements reveal that the ferromagnetic order of La 0.7Sr 0.3MnO 3 (LSMO) is weakened by addition of CuCrO 2. Magnetization drops from 80.1 emu/g in pure La 0.7Sr 0.3MnO 3( x=0) to 40.8 emu/g for x=0.5. Compared with pure LSMO, enhancement of low-field magnetoresistance (LFMR) is observed in the composites. A significant enhancement in magnetoresistance (MR) under an applied field of 5 KOe is observed from low temperature up to room temperature. The MR value at 300 K for the sample with x=0.3 is about 8 times larger than that of pure LSMO. We suggest that the improved MR should be arising from enhanced spin-polarized tunneling caused by magnetic disorder.

Ferro–antiferromagnetic coupling and enhancement of magnetoresistance in La0.7Ca0.3MnO3/LaMnO3 composites

AbstractThe samples with the nominal composition of (La0.7Ca0.3MnO3)1–x /(LaMnO3)x with x = 0.00, 0.05, 0.15 and 0.25 were fabricated using traditional solid‐state reaction method. The electrical‐magnetic transport behavior and magnetoresistance (MR) were studied for the composites in magnetic fields H = 0.3 T, 3 T. Experimental results show that with the increasing of LaMnO3 (LMO) content x, the metal–insulator (M–I) transition temperature TP shifts to lower temperature and the resistivity increases sharply in zero magnetic field. Meanwhile, a significant enhancement in MR is observed for the composites, especially in the low temperature range (below TP). Specially, the maximum MR at 3 T increased from 35% for the pure La0.7Ca0.3MnO3 (LCMO) to 92% for the sample with x = 0.25. We suggest that such enhancement in MR is attributed to the strong ferro–antiferromagnetic coupling effects in the composite system, which increase the magnetic disorder at the grain surface and boundary, will improve the spin‐polarized tunneling process of the conducting electron between adjacent grains, and thus enhance the MR effect. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of grain boundary on electrical, magnetic and magnetoresistance properties in La 2/3Ca 1/3MnO 3/CuMn 2O 4 composites

Composites of La 2/3Ca 1/3MnO 3 (LCMO)/ CuMn 2O 4 were synthesized through a sol-gel method. The effect of ferrimagnetic semiconducting CuMn 2O 4 on electrical and magnetic properties has been investigated in detail. The structure analysis shows CuMn 2O 4 mainly segregates at the grain boundaries of LCMO. With the increasing content of CuMn 2O 4, the electrical transport behavior turns from T 2 dependent to T 3 dependent at low temperatures, which suggests electron–phonon and electron–magnon scattering are becoming more important. In addition, the suppression of magnetization and magnetic transition demonstrates the magnetic coupling has a sizeable influence on the composites. A significant enhancement in magnetoresistance (MR) under an applied field 0.3 T is observed over a wide temperature range, which is considered to arise from the enhanced spin-polarized tunneling caused by the addition of CuMn 2O 4 at the grain boundaries.

Improved magnetotransport in LCMO-Polymer (PPS) composite

Polymer embedded, La 0.7Ca 0.3MnO 3/polyphenylene sulfide (LCMO) 1− x /(PPS) x (with x ∼ 0 , 0.10, 0.20 and 0.30, x is the weight fraction of PPS), composites were prepared and their magnetotransport properties were investigated. X-ray diffraction and scanning electron microscopy observations indicate that there is no reaction between the LCMO and PPS. It has been observed that the incorporation of PPS phase into the LCMO matrix sharply increases the resistivity and lowers the metal–insulator transition temperature ( T IM ). Magnetic measurement reveals that the ferromagnetic order of LCMO is suppressed by the addition of nonmagnetic PPS. The significant enhancement in magnetoresistance (MR) is observed at low temperature below 175 K for the composites with x = 0.10 and 0.20 with respect to pure LCMO at magnetic field H ∼ 3 kOe . We suggest that such enhancement in MR is because of spin disorder caused through enhanced spin-polarized tunneling at the grain boundaries in the composite samples.

Giant magnetoresistance in La 0.67Ca 0.33MnO 3 granular system with CuO addition

We report on a heterogeneous precipitation method to modify the surfaces of La 0.67Ca 0.33MnO 3 (LCMO) grains with CuO. It is shown that such a modification causes transport and magnetoresistance (MR) properties of the composites largely different from that observed in pure LCMO granular system. Especially, a significant enhancement in MR is observed near the insulator–metal transition temperature ( T IM). The maximum MR reaches as high as ∼88 and ∼90% at a low magnetic field of 0.3 T for the modified samples of LCMO/ xCuO with x = 4 and 15 mol%, respectively. Compared to pure LCMO, the CuO-modified samples have a substantial decrease in resistivity ( ρ) at the temperature regions apart T IM. Furthermore, for the x = 4% sample, a considerable thermal hysteresis is observed at the same temperature region where abnormal MR effect appears. On the basis of magnetization measurement and structural analysis, a possible interpretation for the experimental observations is presented.

Fluctuation-induced tunneling conductance and enhanced magnetoresistance in polycrystallineCrO2and its composites

Intergranular conduction in half metallic ${\mathrm{CrO}}_{2}$ is known to occur through a combination of spin dependent tunneling [driven by Coulomb blockade (CB) effects] together with certain spin independent (SI) hopping processes. We present evidence that in polycrystalline ${\mathrm{CrO}}_{2}$ with enhanced grain size, both these process (CB effect and SI hopping) are suppressed and the functional form of conductance is best described by fluctuation-induced tunneling (FIT) below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, FIT is seen to be retained when pure ${\mathrm{CrO}}_{2}$ is diluted by two different insulators ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{5}$. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a step in conductance is observed in pure ${\mathrm{CrO}}_{2}$ which is seen to be modulated by the magnetic state of the grain boundary as well as applied magnetic field. In the vicinity of this step, a significant enhancement of magnetoresistance is observed. Overall, magnetotransport studies in these compounds reveal that a systematic variation in crystallographic microstructure in granular ${\mathrm{CrO}}_{2}$ leads to a different functional form of conductance which is intimately coupled to the enhanced magnetoresistive properties as observed here.

Ultralow resistance-area product of 0.4Ω(μm)2 and high magnetoresistance above 50% in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

An ultralow resistance-area (RA) product of 0.4Ω(μm)2 was achieved in CoFeB∕MgO∕CoFeB magnetic tunnel junctions with a high magnetoresistance ratio of 57% at room temperature. Various growth conditions for polycrystalline MgO(001) tunneling barrier were optimized to improve the crystalline orientation of the MgO(001) layer, which resulted in a significant enhancement of magnetoresistance in an ultralow RA region below 1Ω(μm)2. Removal of residual H2O molecules from a growth chamber was especially effective in improving the crystalline orientation. The present achievements will enable the development of highly sensitive read heads for ultrahigh-density hard disk drives.

Antiferromagnetic-coupling-induced magnetoresistance enhancement in Fex(TiO2)1−x films

Fe-incorporated amorphous TiO2 films with different Fe volume fractions of 0.46⩽x⩽0.76 were deposited by cosputtering iron and Ti targets in an Ar+O2 mixture. X-ray diffraction and x-ray photoelectron spectroscopy analyses give a structure of nanosized Fe particles embedded in amorphous TiO2 matrix for the Fex(TiO2)1−x films. Magnetic measurements show antiferromagnetic coupling between nanoscaled Fe granules when x<0.60. The magnetoresistance of Fe0.46(TiO2)0.54 is about −7.6% at room temperature, which increases dramatically with decreasing temperature below ∼100K and reaches −29.3% at 3K. This significant enhancement of magnetoresistance can be qualitatively explained by antiferromagnetic coupling between Fe granules.

Influence of sputtering pressure on the giant magnetoresistance and structure in Fe–Cu–Ni granular thin films

Composition, structure and giant magnetoresistance in Fe x Cu y Ni z films prepared at different sputtering pressures were investigated. X-ray diffraction studies showed only Cu (1 1 1) peak from the Cu grain. The shifts in the d-spacing d 111 of Cu indicates a progressive substitution of Ni in the Cu lattice. Similar trends in d 111 of Cu observed for the samples prepared at different sputtering pressures indicate that the structural behaviour of the samples is nearly independent of the sputtering pressure. A significant enhancement of magnetoresistance (MR) for the samples sputtered at 0.001 mbar as compared to that sputtered at 0.02 mbar is attributed to the reduced role of residual gas impurities in the films upon lowering the sputtering pressure. An interesting observation is that the MR did not significantly decrease even with a large substitution of Ni in the Cu grains.

Enhanced magnetoresistance in La 0.7Sr 0.3MnO 3/Nd 0.7Sr 0.3MnO 3 nanocomposites

La 0.7Sr 0.3MnO 3/Nd 0.7Sr 0.3MnO 3 (LSMO/NSMO) nanocomposites were prepared by a method based on the sol–gel coating of powder. Microstructural, magnetic and transport properties of the composites were investigated. A significant enhancement in magnetoresistance is observed in the composites compared with parent LSMO and NSMO. It is suggested that enhanced magnetic disorder at interfaces and grain boundaries is responsible for the MR enhancement.

Enhanced magnetoresistance at room temperature in La0.7Sr0.3Mn1−xScxO3perovskites

The uncommon Sc ions are employed to partially replace Mn in La0.7Sr0.3MnO3perovskite. X-ray diffraction patterns indicate a pure single phase withrhombohedral lattice symmetry for La0.7Sr0.3Mn1−xScxO3 (0 ≤ x ≤ 0.1). The Curietemperature (TC)is tuned to near room temperature by Sc substitution withx = 0.03–0.05,and consequently a significant enhancement in magnetoresistance (MR) at roomtemperature is obtained in the Sc-doped compounds. Magnetic measurementshows no spin coupling between Sc and Mn ions. We suggest that the reduction inTCand the enhancement in MR are ascribable to the suppression of ferromagneticorder due to the structural distortion caused by the large Sc3+ ions.

Enhanced room temperature magnetoresistance in La0.7Sr0.3MnO3/Sm0.7Sr0.3MnO3 nanocomposites

La 0.7 Sr 0.3 MnO 3 / Sm 0.7 Sr 0.3 MnO 3 (LSMO/SSMO) nanocomposites were prepared by a sol–gel coating method. The microstructural, magnetic, and transport properties of the composites were investigated. A significant enhancement in magnetoresistance (MR) under an applied field of 5 T is observed from low temperature up to room temperature. In the vicinity of room temperature, the MR values of the composites are larger than those of both LSMO and SSMO. Magnetic measurement reveals that the ferromagnetic order of LSMO is weakened by addition of SSMO. We suggest that the improved MR should be arisen from the enhanced spin-polarized tunneling caused by the magnetic disorder.

Enhanced magnetoresistance in granular La2/3Ca1/3MnO3/ polymer composites

Polymer embedded granular composites, La2/3Ca1/3MnO3/polyparaphenylene (LCMO)1−x(PPP)x (x is the weight fraction of PPP), were prepared and their magnetoresistance (MR) properties were investigated. A significant enhancement in MR is observed for the composites especially in the low temperature range. The MR reaches the maximum at x=0.1 when T=5 K and at x=0.2 when T=260 K. We argue that such enhancement in MR is attributed to the enhanced spin-polarized tunneling, which is manipulated by the spin disorder at the LCMO surfaces caused by PPP.