Maximum Magnetoresistance Ratio Research Articles

Iron Clustering and the Magnetic Properties in α-Al 2 O 3 Crystals after Iron Implantation to High Doses

Magnetization and magnetoresistance were studied in iron-ion-implanted aluminum oxide crystals. The implantation was done up to a total dose of 3.5 x 1017 ions/cm2 into Al2O3 substrates with projectile energy of 100 keV. Using conversion electron Mossbauer spectroscopy, it is shown that most of the implanted ions precipitate as α-Fe clusters and that the nano-size clusters exhibit a transition from superparamagnetic to ferromagnetic state at the dose range of (1.5–2.0) x 1017 ions/cm2. A maximum magnetoresistance ratio of 7.5% was observed around a dose of 1.5 x 1017 ions/cm2 at a field of H=1.2 T.

Characteristics of La 0.7 Ca 0.3 MnO 3 Powders Prepared by the Solution Combustion and Solid State Reaction Methods for Colossal Magnetoresistance Applications

La0.7Ca0.3MnO3 powders were prepared by both the solution combustion method and the solid state reaction method and were calcinated at various calcination temperatures and time intervals in air atmosphere. In the solid state reaction method, single-phase La0.7Ca0.3MnO3 was obtained after heat treatment of the powder at 1000°C for 24 hr. In the solution combustion method, however, single-phase La0.7Ca0.3MnO3 powder could be obtained easily when the powder was heat-treated at 650°C for only 30 min. Polycrystalline La0.7Ca0.3MnO3 powder, using the solution combustion method, showed good powder characteristics, such as an average grain size of 50 nm and a specific surface area of 92 m2/g. The resistance as a function of temperature and the magnetoresistance ratio in La0.7Ca0.3MnO3 thin films were attempted to examine the colossal magnetoresistance characteristics. These thin films also showed excellent colossal magnetoresistance properties in that 96% of the maximum magnetoresistance ratio was obtained at 97K.

The Effect of the Interface Impurity in Fabrication of Spin Dependent Tunnel Junction

Ferromagnetic-insulator-ferromagnetic tunneling was measured in two types of Co/Al2O3/Co/NiFe junctions. For type 1 junction (two times of interface exposure to air), the maximum tunnel junction magnetoresistance (TMR) ratio was 16.5% at room temperature. Junctions with a relatively thicker Al layer showed a lower resistance and a higher TMR ratio. Whereas, junctions with a relatively thinner Al layer showed a fast increase of barrier width. The passivation effect of Al2O3 in oxidation process enhanced the TMR ratio. As the oxidation time is increased, the coercivity of Co bottom electrode is also increased. So the effect of CoO formation in the interface of the Co bottom electrode and the Al2O3 is discussed in three possible assumptions. For type 2 junction (just one time of interface exposure to air), the maximum TMR ratio was 21% at room temperature. And the TMR ratio was decreased to half at 550 mV. These junctions have potential use as low-power field sensors and memory elements.

Topography and magnetoresistance in Nd 0.6Sr 0.4MnO films

The highly textured Nd 0.6Sr 0.4MnO 3 thin films with different thickness t were obtained. The maximum magnetoresistance (MR) ratio, defined as [R(0)−R(1.5 T)]/R(0) at the insulating- to metallic-phase transition temperature, was found to increase with decreasing t, reaching a value of 75% for t=500 Å. However, when t was further decreased down to 100 Å, the MR ratio was not significantly affected. In conjunction with our topographic results, we have attributed the saturation of MR ratio to the achieving of an optimized interface strain.

Magnetoresistive behaviour of La 0.67(Ca 0.33− xPb x)MnO 3 nanopowders prepared at lower temperature

We have been able to prepare ultrafine powders of pure and doped La 0.67Ca 0.33MnO 3 (LCMO) at a relatively lower temperature of 550°C using a simple, cost effective and versatile PVA-based chemical route. A maximum magnetoresistance (MR) ratio of −25% is obtained at 20 K in a magnetic field of 3 kOe. Magnetoresistive behaviour being retained in the nanopowders may have implications for enhancing data storage capacity. The semicondutor-to-metal transition temperature, T p, however, shifts towards the lower temperature as the particle size is reduced. To improve T p and near-room-temperature MR additives like NH 4NO 3 and citric acid have been used. In addition, Pb is substituted for Ca.

Electronic transport and magnetic properties in (La 1− xGd x) 0.67 Ca 0.33MnO δ perovskites

Polycrystalline samples of La-Ca-Mn-O doped with Gd have been prepared. We have systematically examined their electronic transport, low-field magnetoresistance, and magnetization behaviors in a series of (La 1− x Gd x ) 0.67Ca 0.33MnO δ with 0.10≤ x≤1.00. The maximum magnetoresistance ratio at B=0.85 T with ( R 0− R B )/ R 0=92.5% occurs approximately at x=0.3. It is also found that the Gd-doped samples show a novel memory effect. The effects of gadolinium ion composition on the electric and magnetoresistance behaviors of manganese perovskites (La 1− x Gd x ) 0.67Ca 0.33MnO δ could arise as from changes in the average ionic radius of the La site 〈 r A〉.

Magnetic properties and electron-transport properties in Fe 0.92Cr 2S 4

Sample of Fe 0.92Cr 2S 4 has been studied with Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), SQUID magnetometer, and magnetoresistance (MR). The crystal structure was cubic spinel with its lattice constant a 0=9.9925(2) Å. The maximum MR ratio was observed at 183 K about 12% under 1.6 T. The Mössbauer spectra were recorded from 18 K to room temperature. Below the Curie temperature the asymmetric line broadening is observed and considered to be dynamic Jahn–Teller distortion. Isomer shift value of the sample at room temperature was 0.53 mm/s, which means that charge state of Fe ions is ferrous in character. The conduction mechanism in this sample is different from the double exchange mechanism in a point that there were no mixed iron charge valences.

A study on tunneling magnetoresistance in magnetic tunnel junctions oxidized by ozone

Magnetic tunnel junctions of Ta/NiFe/FeMn/NiFe/CoFe/Al 2O 3/CoFe/NiFe structure were studied. Two different types of oxidation atmosphere were used to form the barrier layer; plasma oxidation and ozone oxidation. The maximum tunneling magnetoresistance ratio of 32% was observed at room temperature for 50 min ozone oxidation. The junction resistance was several hundred ohms at 0.1 mm×0.2 mm junction. This is about one order lower than that of the plasma oxidized junctions. The plasma free oxidation process would eliminate the possible structural damage by an oxygen ion bombardment and give improved control over the tunnel barrier growth.

Microstructure and Giant Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers

Direct current plating, pulse plating, two-pulse plating, and reverse pulse plating were used to produce electrodeposited Co-Cu alloys and Co-Cu/Cu multilayers under galvanostatic control from an electrolyte containing and Atomic force microscopy, X-ray diffraction, and transmission electron microscopy were used to study the sample structure and morphology. Direct current plating resulted in a alloy with nearly equal amounts of face-centered cubic (fcc) and hexagonal close packed phases, while all pulsed current methods yielded multilayers with fcc structure. Giant magnetoresistance (GMR) behavior was observed in the multilayers with a maximum magnetoresistance (MR) ratio of about 9% as measured at 8 kOe. The shape of the MR curves and the magnitude of the GMR were very similar, regardless of the sign of the current between the Co deposition pulses. The results of structural studies also confirmed the formation of a multilayer structure for each pulsed electrodeposition mode. The conclusion was that the spontaneous exchange reaction between Co and is responsible for the formation of a pure Cu layer even under reverse pulse plating conditions. The GMR of the multilayer deposits decreased with increasing bilayer number, due to the deterioration of the microstructure as the deposit grew. © 2001 The Electrochemical Society. All rights reserved.

Bias voltage and annealing-temperature dependences of magnetoresistance ratio in Ir–Mn exchange-biased double tunnel junctions

Dual-spin-valve-type double tunnel junctions (DTJs) of sputtered Ir–Mn/Co–Fe/AlO x /Co 90Fe 10/AlO x /Co–Fe/Ir–Mn were fabricated using photolithography and ion-beam milling. The DTJs were annealed at various temperatures (150–400°C) to introduce interdiffusion. The magnetoresistance (MR) ratio and DC bias voltage value at which the MR ratio decreases in half value ( V 1/2) were measured before and after annealing. A maximum MR ratio and V 1/2 obtained after annealing at ∼320°C was 42.4% and 952 mV, respectively, at room temperature. There is a correlation between the loss of the MR ratio and that of V 1/2 above 320°C. The loss of the MR ratio and that of V 1/2 are well explained by considering two phenomena, i.e., interdiffusion of O and Mn at the AlO x /Co–Fe/Ir–Mn interfaces. The mechanism for the loss of MR ratio is not only related to the loss of interface polarization, but is also related to the barrier properties, taking into account the spin-independent two-steps tunneling via defect states in the barrier. These results are consistent with the X-ray photoelectron spectroscopy and cross-sectional transmission electron spectroscopy measurements, which indicate the existence of an Al–Mn–O barrier above 320°C.

Magnetoresistive and magnetic properties of RF-magnetron sputter deposited La-Ca,Sr-Mn-O films

Epitaxial and polycrystalline La-Ca,Sr-Mn-O thin films about 1000 /spl Aring/ thick were grown on LaAlO/sub 3/[100], MgO[100] and SiO/sub 2//Si[100] substrates by RF magnetron sputtering. Changes in the resistivity and magnetoresistance of the films on LaAlO/sub 3/ [100] substrate were investigated as the Ca and Sr ratio was varied. The crystal structure of the La-Ca,Sr-Mn-O films was determined to be orthorhombic perovskite. Lattice constants of the crystalline La/sub 0.82/Ca/sub 0.1/Sr/sub 0.08/MnO/sub 3/ film were a/sub 0/=5.466 /spl Aring/, b/sub 0/=5.392 /spl Aring/, c/sub 0/=7.725 /spl Aring/. When the amount of Sr was increased, the results of x-ray diffraction patterns showed no appreciable change of the lattice parameter. However, the semiconductor-metal transition temperature (T/sub SC-M/) of the epitaxial films on LaAlO/sub 3/ [100] increased and the resistivity decreased as the amount of Sr was increased. For the epitaxial La/sub 0.82/Ca/sub 0.1/Sr/sub 0.08/MnO/sub 3/ film on LaAlO/sub 3/, the temperature dependence of the resistivity under zero and 1.5 T applied fields showed that semiconductor-metal transition occurs at 246 K and the relative maximum magnetoresistance ratio was about 172% at 230 K.

Colossal magnetoresistance effect in perovskite-type La–Sn–Mn–O epitaxial films

La–Sn–Mn–O (LSnMO) thin films epitaxially grown on single-crystal substrates by pulsed-laser deposition are reported. The films have a perovskite structure and perform the colossal magnetoresistance effect with the maximum magnetoresistance (MR) ratio of 103% (at 233 K and 6 T). The dependence of electrical transport and magnetic properties on the film thickness has been studied. The analyses reveal that the electrical transport, in contrast with the magnetic phase transition, is more sensitive to the thickness of the films.

Aperiodical oscillation of interlayer coupling in epitaxial Co/Ir(001) superlattices

High quality epitaxial cobalt-iridium superlattices were successfully grown on MgO(001) substrates via molecular-beam epitaxy and were found to exhibit anomalous oscillatory interlayer exchange coupling with a strong coupling constant. The aperiodical antiferromagnetic (AF) peaks appear at Ir layer thicknesses of 5 \AA{}, 15 \AA{}, and 33 \AA{}. Maximum magnetoresistance (MR) ratios of $\ensuremath{\approx}1.1%$ are obtained and are almost the same for the first and the second AF peaks. The interlayer coupling corresponding to the first AF peak is so strong that the magnetization did not saturate even in an applied field up to 90 kOe. The estimated value of the interlayer exchange constant by MR curves at room temperature is more than 2.2 ${\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}}^{2}$ close to the first AF peak. A distinction of the in-plane magnetization process between [100] and [110] directions is observed and the easy axis of magnetization is found to be along the [110] axis in plane. The aperiodic oscillations are thought to indicate multiperiodicity of the interlayer exchange constants.

Magnetoresistance Effect of Co–Cu Nanostructure Prepared by Electrodeposition Method

It was possible to produce Co–Cu multilayer and alloy films controlled in the atomic scale by electrodeposition. We have examined the magnetoresistance (MR) in the Co–Cu multilayers and the alloy films produced by electrodeposition and have investigated the relationships between the magnetoresistance and film thickness, composition, and magnetization. For the Co–Cu multilayer films the maximum MR ratio at 300 K is about 16% (21 kOe) and at 5 K it increases to about 24%. The MR ratio is more strongly dependent on the Co alloy ferromagnetic layer thickness than the change of the composition near the interface between Co alloy and Cu layers. The giant magnetoresistance has also been observed for Co–Cu alloy films. The maximum MR ratio of the Co–Cu alloy films increases to 6.3% after annealing the film at 723 K for 1 hour.

Heteroepitaxial growth of La 0.7Ca 0.3MnO 3/SrTiO 3/TiN/Si by pulsed laser deposition

Epitaxial perovskite magnetoresistive La 0.7Ca 0.3MnO 3 (LCMO) films have been successfully grown on SrTiO 3(STO)/TiN buffered (100)Si substrates by pulsed laser deposition method. The structural quality of the LCMO/STO/TiN/Si heterostructure was characterized by a four-circle X-ray diffractometer. The crystallinity of the epitaxial layers was found to improve with increased substrate temperature. An epitaxial relationship of 〈100〉LCMO//〈100〉STO//〈100〉TiN//〈100〉Si was obtained at a deposition temperature of 700°C. The electrical properties and the magnetoresistance ratio (MR) of the LCMO films were measured by a standard four-probe technique. A maximum MR ( MR= ΔR B=1.2 T /R B=0 ) of −50% was observed at 116 K in the as-grown LCMO films.

Colossal magnetoresistance in Ce doped manganese oxides

In this study we investigate the effects of Ce doping in R1−xAxMnO3 (R=La, Ce, and A=Sr, Ce) on the magnetic and transport properties of this system. For La1−xCexMnO3 (LCMO), an increase in Ce concentration is accompanied by an increase in TC from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system Ce1−xSrxMnO3 (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.

Microstructure and Electrical Characteristics of La1−xSrxMnO3 (0.19≤x≤0.31) Thin Films Prepared by Sputter Techniques

AbstractLa1−x SrxMnO3(0.19≤x≤0.31) thin films were prepared on silicon wafers by sputter techniques. The effect of substrate temperature, chemical composition and post-deposition heat-treatment on the crystalline structure and electrical characteristics of the films was investigated. The films grown at a substrate temperature of 500°C were found to be of the pseudo-tetragonal system (0.97≤a/c≤1) and exhibited a strong tendency of {001} planes to lie parallel to substrate surface. With the increase of x, the electrical resistivity of the films decreased and the transition temperature between the metallic and semiconducting electrical transport behaviors shifted to high temperature. With a magnetic field of 0.18 Tesla, the maximum magneto-resistance ratio (MR%) of La0.69Sr0.31MnO3 polycrystalline thin films was about 390%.

Enhanced tunnel magnetoresistance at high bias voltage in double-barrier planar junctions

Single Co/Al2O3/NiFe and double Co/Al2O3/Co/Al2O3/NiFe planar tunnel junctions were grown by sputtering and subsequently patterned in a four-step process using optical lithography. The Al2O3 barriers are formed by radio frequency plasma oxidation of 1.5 nm aluminum layers. The double junctions exhibit three clear resistance levels depending on the relative configuration of the magnetizations. Both single and double junctions exhibit maximum magnetoresistance (MR) ratios above 10% at room temperature and 20% at 30 K and a decrease of MR with increasing bias voltage. With regard to its low bias value, the MR is reduced by a factor of 2 at 0.26 V for the single junctions and at values above 0.8 V for the double junctions. The decay of the MR of double junctions with bias voltage is significantly slower than expected from two independent junctions in series.

Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films

We have studied the tunneling resistivity and magnetoresistance of reactive sputter deposited FeHfO and FeHfSiO thin granular films. Maximum magnetoresistance ratios at room temperature of 2% and 3.2% were observed for films with compositions of Fe47Hf10O43 and Fe40Hf6Si6O48, respectively. The magnetoresistance shows a decrease with temperature, which cannot be explained by spin-dependent tunneling only. We propose that spin-flip scattering in the amorphous FeHf(Si)O matrix causes this decrease as function of temperature. A two current model for the tunnel magnetoresistance, taking into account spin-flip scattering, is presented which can describe the observed temperature dependence of the magnetoresistance.

Low temperature processing of epitaxial La 1− xCa xMnO 3 thin films by pulsed laser deposition

Magnetoresistive perovskite-like La 0.7Ca 0.3MnO 3 (LCMO) thin films have been successfully grown on (100)LaAlO 3 (LAO) at a relatively low substrate temperature of 600°C–750°C by pulsed laser deposition (PLD) method. Epitaxial LCMO films are obtained at deposition temperature of 650°C or above without post-annealing. The as-deposited films grown at higher substrate temperatures showed lower resistance and higher semiconductor-to-metal transition temperature. For films post-annealed for 1 h at their respective deposition temperatures, a large reduction in their electrical resistivity and an up-shifting of their transition temperature were observed. The maximum magnetoresistance ratio (MR), − Δ R(1 T)/ R(0), for the films deposited and annealed at 650°C and 750°C were 37% and 50%, respectively.