Large Magnetoresistance Values Research Articles

Enhanced spin-valve giant magneto-resistance in non-exchange biased sandwich films

A large giant magnetoresistance (GMR) value of 7.5% has been measured in simple NiFeCo(1)/Cu/NiFeCo(2) sandwich films grown on a 30 /spl Aring/ Cr seed layer. This spin-valve GMR effect is consistent with the differential switching of the two NiFeCo layers due to an enhanced coercivity of the NiFeCo(1) layer grown on the Cr seed layer. A change in growth texture of the NiFeCo(l) layer from fcc [111] to bcc [110] crystallographic orientation leads to an increase in magnetic anisotropy and an enhancement in coercivity. The GMR value increases to 8.7% when a thin CoFe interfacial enhancing layer is incorporated. Further enhancement in GMR values up to 14% is seen in the sandwich films by nano-oxide layer formation. The specular reflection at the oxide/magnetic layer interface further extends the mean free path of spin-polarized electrons.

EFFECTS OF DIMENSIONALITY IN THE RARE-EARTH MANGANATES

We report measurements of magnetization, magnetoresistance and ESR on systems with the 3-D and layered structures, to study the effect of reduced dimensionality. Compositions of the systems Pr 0.7 Sr 0.3- x Ca x MnO 3 and La 2-2 y Sr 1+2 y Mn 2- z Cr z O 7 were synthesized, which have the 3D and quasi-2D structures, respectively. The magnetic and transport properties are markedly affected by the reduction in dimensionality. Large values of magnetoresistance have been observed in the layered materials even at low temperature, in contrast with the behavior of the 3D compositions. ESR measurements on the Pr 0.7 Sr 0.3- x Ca x MnO 3 system show a single resonance line in the temperature range of our study. However, compositions of the La 2-2 y Sr 1+2 y Mn 2 O 7 system with y =0.3, 0.4 and 0.5 show a complex behavior. As the samples are cooled, a single resonance line is first observed, which can be described by a Lorentzian. Below ~2 T c, for compositions with y =0.3 and 0.4, a complex lineshape evolves, which can be resolved into two Gaussian lines. This crossover in the lineshape indicates a transition from a homogeneous to an inhomogeneous spin system, which can be attributed to the nature of the ferromagnetic ordering between the bilayers. The composition with y =0.5 shows a different behavior, which might be due to the antiferroma.gnetic ordering exhibited by this composition. A detailed analysis of the experiments is presented.

Ion beam sputtered spin-valve films with improved giant magnetoresistance response

A large value of giant magnetoresistance ΔR/R=9% with an exchange field Hex=350 Oe has been measured from simple NiFe/CoFe/Cu/CoFe/IrMn top spin-valve films prepared by ion beam deposition (IBD) techniques. The exchange biasing was greatly enhanced when a synthetic pinned layer, CoFe/Ru/CoFe, is used in the spin-valve structures. Apparent exchange field values in excess of 2000 Oe and ΔR/R values above 8% have been obtained in synthetic spin-valve films. These IBD spin-valve films show excellent thermal stability and they are suitable for the applications in high density magnetic recording heads.

Preparation dependence of the magnetic behaviour in La–Ca–Mn–O films

The influence of deposition temperature ( T s) on the structure, transport property and magnetoresistance (MR) behaviour of pulsed-laser-deposited (PLD) La–Ca–Mn–O films on Si substrates has been studied. An increase in resistivity and a decrease in the peak resistivity temperature are found to occur simultaneously with increasing T s. The largest MR value, with a 40% (Δ ρ/ ρ o) change in resistivity at 4.2 K, appeared in films deposited at 900°C, whilst the largest MR of 20% (Δ ρ/ ρ o) near room temperature was exhibited in films deposited at 500°C. The stoichiometry of the films was also examined. Both calcium and oxygen deficiencies are found in the films deposited at higher temperature, which can be used to explain the observed magnetoresistance results.

Magnetic studies on electrodeposited Cu 1− xCo x alloy films

Six Cu 1− x Co x alloy films were prepared by an electrodeposition technique. The compositions of the films were determined to be x=0.06, 0.13, 0.17, 0.19, 0.21, 0.26 by an atomic absorption spectrophotometer. The crystal structure is FCC-Cu for all films but a shift in the diffraction lines is observed with increasing Co content. The giant magnetoresistance effect was determined in some of the films below 200 K. Magnetisation curves showed no saturation at 10 kOe and the curves of the samples which have a large magnetoresistance value are inclined more than the curves of the other low magnetoresistance samples. This may be due to the degree of magnetic moment distribution in these samples.

Large tunneling magnetoresistance enhancement by thermal anneal

Spin tunnel junctions with tunneling magnetoresistance of 36.5%±0.5%, resistance-area product of 35±6 kΩ×μm2, and junction area between 6 and 75 μm2 were fabricated. The barrier height is 2.5±0.3 eV and the barrier thickness is 7.7±0.3 Å. Large tunneling magnetoresistance (TMR) values are obtained by vacuum anneal (at temperatures from 100 to 240 °C for over 5 h) of junctions prepared with as-deposited TMR of 21%±1.7%, and an as-deposited resistance-area product of 25±6 kΩ×μm2. Two regimes occur during anneal. The first one occurs for anneals up to 200 °C where TMR and junction resistance increase, but the barrier parameters are unaltered. The second occurs above 200 °C, where TMR increases faster, together with an increase in barrier height. At 240 °C, TMR starts to decrease. Rutherford backscattering analysis indicates an asymmetry in the oxygen distribution in the as-deposited barrier. The oxygen distribution becomes homogeneous for anneals above 150 °C.

Interlayer coupling across an alloy spacer: Co/Cu 75Au 25 multilayers

The magnetic coupling in DC magnetron sputtered [Co 10 Å/Cu 75Au 25( t)] 20 multilayers grown on Si(1 0 0) substrates, with the Cu 75Au 25 spacer thickness ( t) ranging from 8 to 33 Å, has been investigated by magneto resistance and magnetic remanence measurements. Large magneto resistance values were observed in this system, with a peak of the order of 25% at 10 K for the Cu 75Au 25 spacer thickness t = 12 A ̊ . An oscillatory behaviour with a period of 8 ± 1 A ̊ is exhibited by the magnetic remanence and saturation magnetic field as a function of the Cu 75Au 25 thickness, attributed to the corresponding oscillations in the interlayer magnetic coupling.

Phase segregation and giant magnetoresistance behavior in as-deposited Co-Ag film grown by pulsed laser deposition

The growth of Co-Ag granular films on Si (100) substrates by pulsed laser deposition (PLD) is reported. It is found that phase segregation can be directly produced in as-deposited Co-Ag granular films. This is attributed to the high quench rates induced by PLD, unlike the film deposited by sputtering, where post-annealing is required to achieve the required phase segregation. A large magnetoresistance (MR) value of around 27% was obtained at 4.2 K under a magnetic field of 4.7 T in as-deposited Ag0.70Co0.30 films. Our results suggest that the large magnetoresistance values obtained are strongly determined by phase segregation and particle size in association with the laser fluences used.

Determination of Spin-Dependent Scattering Parameters of NiFe/Cu and Co/Cu Multilayers

ABSTRACTWe present magnetoresistance (MR) data of high-vacuum magnetron sputtered NiFe/Cu multilayers (NiFe=Ni80Fe20) grown on Si(100) substrates with a Cu buffer layer and compare these with earlier results on Co/Cu(100) multilayers [1]. Measured MR values are directly proportional to the antiferromagnetically coupled fraction in the multilayers. Extrapolating to full antiparallel alignment, we can make a reliable comparison of the MR with the magnetoresistance model of Levy, Zhang, and Fert [2,3]. For the NiFe/Cu multilayers the deduced spin-asymmetry parameters are for interface and bulk scattering, respectively. Although much smaller than in our Co/Cu multilayers, , it is still the spin dependence of the interface scattering that is the main cause for the large MR values.

Large magnetoresistance in La-Ca-Mn-O films

A very large magnetoresistance value in excess of 10/sup 6/% has been obtained at 110 K, H=6T in La-Ca-Mn-O thin films epitaxially grown on LaAlO/sub 3/ substrates by pulsed laser deposition. The as-deposited film exhibits a substantial magnetoresistance value of 39,000%, which is further improved by heat treatment. A strong dependence of the magnetoresistance on film thickness was observed, with the value reduced by orders of magnitude when the film is made thicker than /spl sim/2000 /spl Aring/. This behavior is interpreted in terms of lattice strain in the La-Ca-Mn-O films.

Determination of para- and ferromagnetic components of magnetization and magnetoresistance of granular Co/Ag films (invited)

Several series of thin films (∼3000 Å) were fabricated by coevaporation of Co and Ag in a dual e-beam UHV system at substrate temperatures, TS, between 300 and 500 K. The composition was varied between 23 and 54 at. % Co. The maximum measured magnetoresistance was 31% at room and 67% at nitrogen temperature. Magnetization and Rutherford backscattering measurements showed that the Co and Ag atoms are completely segregated for films deposited at TS≥400 K. The magnetoresistance (MR) and magnetization curves have been analyzed taking into account both the paramagnetic (PM) and ferromagnetic (FM) contributions that are observed to be present. The films having ≤32 at. % Co are entirely paramagnetic. The fraction of Co atoms in the ferromagnetic component increases monotonically with increasing TS and/or at. % Co. The average radii of the PM granules in the films having an MR effect of 25%–31% are ∼20–22 Å with a minimum average separation of ∼10 Å. The large MR values attained in both the PM and FM components are attributed to the small effective domain sizes, ∼40–50 Å. The MR values are large because the MR effect varies inversely as the mean-free-path for magnetic boundary scattering which is the average distance between domain boundaries. The variation of the resistive and magnetic behavior of the films was also studied as a function of the annealing time for an annealing temperature of 300 °C. A striking behavior seen in these films is the long relaxation times when a FM component is present.

Giant magnetoresistance in sputter deposited Co/Cu multilayer systems

This paper presents some results of magnetoresistance measurements on sputter deposited Co/Cu thin film multilayers. We find that large values of magnetoresistance approaching 40% can be obtained and that these values oscillate in magnitude with the thickness of the non-magnetic spacing layer with a period of about 1.1 nm. It appears that the multilayer must be grown on a particular form of underlayer to realize these large values of magnetoresistance ratio.

Giant Magnetoresistance and Oscillatory Interlayer Exchange Coupling in Copper Based Multilayers

AbstractGiant magnetoresistance values exceeding 65% at room temperature are reported in antiferromagnetically coupled Co/Cu multilayers grown on Fe buffer layers using magnetron sputter deposition. For such samples the saturation fields exceed 10 kOe. However by taking advantage of the rapid decrease of the Co interlayer magnetic exchange coupling with increasing Cu spacer layer thickness, large magnetoresistance values exceeding 35% can be obtained at room temperature for saturation fields of just a few hundred oersted.

Giant magnetoresistance in sputtered magnetic superlattices (invited) (abstract)

Recently, large values of saturation magnetoresistance (SMR) were reported in single-crystal Fe/Cr superlattice structures prepared by molecular-beam-epitaxy (MBE) techniques.1 We have prepared similar polycrystalline superlattice structures by dc magnetron sputtering in a high-vacuum deposition system and find comparably high SMR values of up to 50% for Cr-layer thicknesses of ≂8 Å. Magnetization-versus-field data suggest that successive Fe layers are coupled antiferromagnetically with one another, as for the MBE-grown films. Magnetic fields of up to 20 kOe are required to align Fe layers 20 Å thick, implying substantial exchange-coupling energies between the Fe layers. While the magnetization curves have a weak temperature dependence even for temperatures above the Néel temperature of bulk Cr, the SMR decreases as the temperature is increased. Work on related structures which do and do not show giant SMR effects will be discussed.

Electrical and thermal properties of fluorine-intercalated graphite fibers.

We report measurements on the electrical- and thermal-transport properties of fluorine-intercalated vapor-grown carbon fibers ${\mathrm{C}}_{\mathit{x}}$F with the carbon-to-fluorine-atom ratio x between 4.1 and 5.8. The electrical resistivity is found to be very sensitive to the fluorine concentration. Indeed, while the resistivities of the ${\mathrm{C}}_{5.2}$F and the ${\mathrm{C}}_{5.8}$F compounds are well below that of the pristine graphite fibers, the tendency is reversed when x\ensuremath{\le}4.5 and large resistivity values are obtained. All ${\mathrm{C}}_{\mathit{x}}$F compounds exhibit a logarithmic resistance increase at low temperature, which can consistently be explained by weak-localization and carrier-carrier interaction effects for two-dimensional electron systems. Large negative magnetoresistance values have been measured on the ${\mathrm{C}}_{\mathit{x}}$F compound with x=4.1. At higher temperature, an anomalous resistance-versus-temperature behavior appears for each fluorine-intercalated vapor-grown carbon fiber, possibly resulting from a phase transition. Heat transport in ${\mathrm{C}}_{\mathit{x}}$F (with x=4.1) is almost entirely governed by lattice vibrations between 2.5 and 300 K while the thermoelectric power is positive, and its temperature dependence is similar to that previously observed in other acceptor graphite intercalation compounds. Our results indicate that the high resistivity of the ${\mathrm{C}}_{\mathit{x}}$F compounds (with x\ensuremath{\le}4.5) is due to strong defect scattering rather than a low free-charge-carrier density.

Magnetoresistance of Very Pure Polycrystalline Aluminum

The behavior of the resistance of polycrystalline aluminum wires as a function of magnetic field and purity at temperatures of 4, 15, and 19.6 K is reported. Both longitudinal and transverse configurations were measured. The residual resistance ratios of the specimens varied from 1600 to 31 000. The measured magnetoresistance ($\frac{\ensuremath{\Delta}R}{{R}_{0}}$) is separated into a saturating and a linear part. The value of the saturating component is high at 19.6 K but is shown to be less than 6, even in the limit of infinite specimen purity. The linear component varies with both temperature and purity. Possible sources for the large saturating magnetoresistance values and for the variations observed in the linear portion are discussed. An analysis scheme is presented which allows prediction of the saturating component from zero-field resistance values. A deviation from Matthiessen's rule observed here, and by several other experimenters, is presented and discussed.