Tunnel-type Magnetoresistance Research Articles

Magnetic, magnetoresistive and structural properties of Cox(CoO)100-x thin film composites

The structural, magnetic, and magnetoresistive properties of Cox(CoO)100-x thin film composites synthesized by ion-beam sputtering of a composite target in an argon and mixed atmosphere (argon + oxygen) has been studied. The experimentally determined percolation threshold was significantly lower than that for most of the studied previously nanocomposites: ∼ 12.2 at % Co and ∼8.3 at % Co for films fabricated in inert and mixed atmosphere, respectively. This is explained by the special morphology of the films when small metallic Co nanoparticles are located on the surface of larger CoO particles. A negative tunnel-type magnetoresistance reaching 2.4% in 9 kOe field at room temperature was found for composites with close to the percolation threshold Co content. The magnetoresistance is proportional to the square of magnetization and is characterized by hysteresis, which correlates well with magnetic hysteresis. Magnetic hysteresis with a coercivity of the order of 700 Oe is observed for dielectric compositions up to 4 at. % Co. A possible reason for this behavior, along with the special morphology, is the appearance of thin layers of antiferromagnetic oxide at the interface between Co nanoparticles and CoO particles, which causes a significant increase in magnetic anisotropy.

Increased low field magnetoresistance in electron doped system Sr0.4Ba1.6−xLaxFeMoO6

Magnetotransport properties of electron doped polycrystalline system Sr0.4Ba1.6−xLaxFeMoO6 are presented in this paper. We have observed increased low field magnetoresistance values with significant Curie temperature increase in the electron doped system Sr0.4Ba1.6−xLaxFeMoO6. The low field magnetoresistance value (at 1000 Oe) in 20% La3+ doped sample is observed to be 2.17% at 300 K. At 0.64 T and 80 K, the magnetoresistance change measured in this sample is 21.4%. This sample also showed 50 K increase in Curie temperature over the pristine sample. The increased low field magnetoresistance values are associated with modified grain boundary barriers of the system. The results confirm the fact that the modification of grain boundary barriers has enough potential to possess high low field magnetoresistance values even in the systems with lower spin polarization values. The role of grain to grain connectivity is observed to be dominantly determining the low field magnetoresistance values in grain to grain tunnel type magnetoresistance.

Mössbauer Studies for Exploring CMR and TMR Perovskites

Doped transition metal compounds with complex electronic and magnetic structures show a wide range of new physical phenomena like high-temperature superconductivity, room temperature magnetic semiconductivity, or colossal magnetoresistance (CMR). Following the discovery of the effect of colossal negative magnetoresistance in manganese based perovskites, several different classes of transition metal compounds such as Sr2FeMoO6 double perovskites or La1-xSrxCoO3 perovskites were found to exhibit unusually high magnetoresistance (MR) partly as a peak around their Curie-temperature (CMR effect), partly as an increasing feature with decreasing temperature (tunneling-type magnetoresistance, TMR)(Table1). The most important difference between these two types of MR is their temperature dependence: while CMR effect manifests only around the magnetic temperature as a peak in the MR vs. T plot, TMR increases monotonically with decreasing temperature. Due to the complexity of the underlying physical and chemical processes in these materials, the understanding of their electronic and magnetic structure is one of the most vital topics in condensed matter physics nowadays. The first models aiming to shed light on the CMR effect found in manganite perovskites, and to explain the unusually strong correlation between the magnetic state of the material and its electric transport properties were based on the theories of double exchange model and strong electron-lattice interactions. The former was based on the fact that in La1-xCaxMnO3 perovskites the doping divalent ions (usually Ca) introduce a number of x extra electrons to the system, either oxidizing Mn ions into Mn or creating oxygen vacancies, although for low doping rates the latter effect was found to be negligible. As a Mossbauer Studies for Exploring CMR and TMR Perovskites

Tunneling-type magnetoresistance characteristics of nanosized Fe–Co granules in α-Al2O3 synthesized by ion implantation

Nanometer-sized clusters of Fe–Co alloy in α-Al2O3 matrices were synthesized by sequential implantation of Fe and Co ions. The implantations were performed at room temperature with a total dose of around (1.2–1.5)×1017ions∕cm2 with ion energy of 100keV. It is demonstrated for the Fe–Co granular layers that the alloying improves the tunneling-type magnetoresistance (TMR) effect and results in an eminent increase of the MR ratio to the value of 13.2% at an external field of 1.1T at room temperature. The magnetic property of the Fe–Co granules was investigated by means of conversion electron Mössbauer spectroscopy at room temperature and 4.2K. It is shown that the enhancement of the TMR effect in the Fe–Co granules is correlated with the granule size, its dispersion in the insulating matrix, and the magnetic characteristics in the granular layers.

Spintronics in metal–insulator nanogranular magnetic thin films

Magnetic and electrical properties of metal–insulator nanogranular thin films are overviewed, from the spintronics point of view, by presenting our recent results. (1) The metal-rich ferromagnetic ones possess ultrahigh-frequency (MHz–GHz) permeabilities μ′, μ″, mainly owing to large induced magnetic anisotropy field H k . They are useful for various magnetic devices, such as thin-film inductors and noise suppressors. (2) The insulator-rich superpramagnetic ones exhibit tunnel-type magnetoresistance, being large in (Mg or Al)-fluoride base films. The Granular-In-Gap (GIG) devices with improved field sensitivity are useful for very low magnetic-field sensors.

High-frequency spin-dependent tunnelling in magnetic nanocomposites: Magnetorefractive effect and magnetoimpedance

Since the dielectric permittivity is linear with frequency-dependent conductivity, high-frequency properties for any kind of magnetic materials with the high magnetoresistance depend on magnetization. It manifests as magnetorefractive effect (MRE) in the infrared region of spectrum and as magnetoimpedance (MI) in the frequency range between radio and microwaves. The main mechanism of both MRE and MI in nanocomposites with tunnel-type magnetoresistance is high-frequency spin-dependent tunnelling. We report on recent results of theoretical and experimental investigations of MRE and MI in nanocomposites Co 51.5Al 19.5O 29, Co 50.2Ti 9.1O 40.7, Co 52.3Si 12.2O 35.5 and (Co 0,4Fe 0,6) 48(MgF) 52. Most of the obtained experimental data for MRE and MI are consistent with the theory based on considering the tunnel junction between adjacent granules in percolation cluster as a capacitor.

Spin-dependent tunnelling at infrared frequencies: magnetorefractive effect in magnetic nanocomposites

Abstract We present a brief review of recent experimental and theoretical results on magnetorefractive effect in magnetic metal–insulator nanogranular alloys with tunnel-type magnetoresistance focusing on its relation with high-frequency spin-dependent tunnelling.

Control of Fe valence state and magnetoresistance by means ofT=Taand W substitution inSr2Fe(Mo1−xTx)O6

High-purity ${\mathrm{Sr}}_{2}\mathrm{Fe}({\mathrm{Mo}}_{1\ensuremath{-}x}{T}_{x}){\mathrm{O}}_{6}$ samples with $T=\mathrm{W},$ Ta and $0l~xl~1$ were obtained by means of encapsulation synthesis. For the nonsubstituted samples earlier ${}^{57}\mathrm{Fe}$ M\ossbauer spectroscopy measurements indicate that the Fe ions occupy a fluctuating mixed-valence state of $+2.5.$ [J. Lind\'en et al. Appl. Phys. Lett. 76 (2000) 2925.] ${\mathrm{W}}^{\mathrm{VI}}$ substitution causes increasing amounts of Fe to enter the II state, whereas ${\mathrm{Ta}}^{\mathrm{V}}$ substitution yields increasing amounts of ${\mathrm{Fe}}^{\mathrm{III}}.$ Both substitution schemes lead to a decrease in the intensity of the component assigned to ${\mathrm{Fe}}^{2.5+}.$ Nonsubstituted samples exhibit a characteristic tunneling-type magnetoresistance below ${T}_{\mathrm{C}}.$ Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the N\'eel temperature of the pure ${\mathrm{Sr}}_{2}{\mathrm{FeWO}}_{6}$ and ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{TaO}}_{6}$ phases, respectively. The enhancement appears to be related to the colossal magnetoresistance (CMR) effect at the paramagnetic to antiferromagnetic transitions in the areas rich in W or Ta. The transition and consequently the region of non-zero CMR effect are rather broad due to the glass-like behavior of the highly-substituted samples within the low-temperature region. Ta substitution had a stronger influence on the transport properties, magnetization and mixed valency than W substitution had. It is suggested that ${\mathrm{Ta}}^{\mathrm{V}}$ disrupts the double-exchange interaction responsible for the magnetism in the ${\mathrm{Sr}}_{2}\mathrm{Fe}{\mathrm{MoO}}_{6}$ more efficiently than ${\mathrm{W}}^{\mathrm{VI}}.$

기판 효과에 따른 저 자장 영역에서의 자기저항 효과에 관한 연구

The low-field tunnel-type magnetoresistance (MR) properties of sol-gel derived thin film deposited on different substrate have been investigated. Polycrystalline thin films were fabricated by spin-coating on substrate and that with yttria-stabilized zirconia (YSZ) buffer layer, while c-axis-oriented thim film was grown on (LAO) single crystal substrate. The full width half maximum (FWHM) of the rocking curve scan of LPMO/LAO film is . Tunnel-type MR ratio is 0.52 % in (100) film and that of (100) film is as high as 0.68 %, whereas that of LPMO/LAO(001) film is less than 0.4 % under the applied field of 500 Oe at 300 K. Well-pronounced MR hysteresis was registered with an MR peak in the vicinity of the coercive field. The low-field tunnel-type MR characteristics of thin films deposited on different substrates originates from the behavior of grain boundary properties.

Influence of W/Ta substitution at the Mo site on the Fe valence and magnetoresistive properties of Sr2FeMoO6

High-purity Sr2Fe(Mo1−xTx)O6 samples with T=W, Ta and 0⩽x⩽1 were synthesized using a solid-state reaction scheme. During synthesis a strongly reducing atmosphere was maintained by confining the precursors and an Fe/FeO oxygen getter to fused-silica tubes. Sample quality and homogeneity were confirmed by X-ray diffraction and energy-dispersive X-ray analysis. For the non-substituted samples, 57Fe Mössbauer spectra indicate that the Fe ions occupy a mixed-valence state of +2.5. W6+ forces increasing amounts of Fe to enter the +2 state, whereas Ta5+ substitution yields increasing amounts of Fe3+. Non-substituted samples exhibit a characteristic tunneling-type magnetoresistance below TC. Both W and Ta substitution were found to enhance the low-temperature magnetoresistance around the Néel temperature of the pure Sr2FeWO6 and Sr2FeTaO6 phases, respectively. The enhancement is related to the colossal magnetoresistance effect at the paramagnetic to antiferromagnetic transitions of areas rich in W or Ta.

Giant room-temperature magnetoresistance in polycrystalline Zn(0.41)Fe(2.59)O4 with alpha-Fe2O3 grain boundaries.

A tunneling-type magnetoresistance (MR) as large as 158% is observed at T = 300 K in a polycrystalline Zn0.41Fe2.59O4 sample, in which the Zn0.41Fe2.59O4 grains are separated by insulating alpha-Fe2O3 boundaries. The huge room-temperature MR is attributed to the high spin polarization of Zn(0.41)Fe(2.59)O4 grains and antiferromagnetic correlations between magnetic domains on both sides of the insulating alpha-Fe2O3 boundary. The MR exhibits strong temperature dependence below 100 K and its magnitude is enhanced to reach 1280% at 4.2 K, which may arise from the Coulomb blockade effect.

Complex impedance analysis of granular La 2/3Sr 1/3MnO 3 manganite thin films

Low-field tunnel-type magnetoresistance (MR) was enhanced through annealing in the air, in contrast to the intrinsic CMR where the optimized MR is associated with the annealing in O 2, and 10 times larger MR was obtained. Impedance analysis showed that the grain boundary resistance was 2 times larger in the film annealed in the air than that in the O 2, while the grain resistance was invariant in the films. From these results, it can be concluded that the characteristics of grain boundaries play a crucial role in low-field tunnel-type MR of manganite LSMO/SiO 2/Si thin films. Therefore, we suggest that the complex impedance analysis can provide us with useful information about not only transport at grain boundaries, but also the separated contribution of grains and grain boundaries to MR.

Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr2Fe(Mo1−xWx)O6−w system

In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6−w system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1−xWx)O6−w samples with 0⩽x⩽1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN≈50 K for x⩾0.6. In samples with 0.7⩽x⩽0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistance effect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMoO6−w. The coexistence of the two types of magnetoresistance effect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para- to ferrimagnetic transition at Tc.

斜めスパッタ薄膜下地層を用いたグラニュラー薄膜の軟磁性化

Magnetic softening of the tunnel-type magnetoresistance (TMR) effect of Co-Sm-O granular magnetic films was attempted by employing FeNi under layer films that were sputtered under an oblique cathode arrangement. The obliquely sputtered FeNi films had a nano-scale corrugated structure whose height and spacing were approximately 10 nm. Because of the stray field acting between neighboring corrugations of the FeNi film, the resistivity change of the Co-Sm-O granular film formed on the obliquely sputtered FeNi film showed good low-field response, reaching about 240 times higher than that of the single-layer Co-Sm-O film.

Layered Magnetic Manganites

▪ Abstract We review the recent progress in the study of layered magnetic manganites La2−2xSr1+2xMn2O7, which have many unique features of charge transport and magnetism inherent to the quasi-two-dimensional electronic structure. The system shows a wide variety of magnetic-field-induced phenomena due to the layered crystal and magnetic structure, such as the highly anisotropic ferromagnetic metallic ground state, the colossal magnetoresistance (CMR) effect, and the tunneling-type magnetoresistance (TMR). The charge transport properties, as well as the magnetic ones, strongly depend on the carrier-doping level and the applied pressure, which reflects the variation of the orbital-dependent occupancy of itinerant eg-like electrons. Although the layered manganite is one of a new class of CMR materials, the study of this system may also reveal some of the key issues for understanding the CMR effect in mixed-valent manganese oxide.

MORE EVIDENCES FOR DOUBLE EXCHANGE-RELATED INTERFACIAL TUNNELING IN NANOSTRUCTURED PEROVSKITE La0.82Sr0.18MnO3

The intergranular tunneling in nanostructured perovskites La 0.82 Sr 0.18 MnO 3 has been investigated. A series of experimental evidences including magnetization, heat capacity, structure distortions and magnetoresistance, have been given to demonstrate that the intergranular tunneling originates from an interfacial effect associated with the double exchange interaction, and the related tunnel-type magnetoresistance results from a field-induced change of interfacial magnetic configuration.

Discussion again of the interfacial tunneling in granular perovskite La 0.82Sr 0.18MnO 3

The spin-dependent intergranular tunneling in granular perovskites La 0.82Sr 0.18MnO 3 has been investigated. A series of experimental evidences, including magnetization, heat capacity, structure distortions and magnetoresistance, have been given to demonstrate that the intergranular tunneling stems from a spin-dependent surface effect, which is related to the magnetic difference between the core and the surface of the perovskite grain, and the related tunnel-type magnetoresistance results from a field-induced change of interfacial magnetic configuration.

Tunnel-type Magnetoresistance in Matel-nonmetal Granular Filmes Prepared by Tandem Deposition Method

The relationship between the tunnel-type magnetoresistance(TMR) and the structure of metal-nonmetal granular system (Fe-Co)-Al-O films has been examined. Films were prepared by a tandem deposition method with rotating a substrate holder. The average diameter of metallic granules is found to decrease with an increase in the rotating speed of substrate holder without any change of film composition. And the MR ratio increases with a decrease of the diameter of metallic granules. The influence of the granule diameter on the MR ratio is discussed by so-called coulomb blockade effect. Furthermore, we studied the effect of a substrate temperature(Tsub.) on the MR and the structure of films. The MR ratio and electrical resistivity of the films increase with an increase of Tsub.. The magnetization measurements suggests that these increases are related with the decrease of the amount of magnetic impurities in the Al-oxide intergranule. The phase separation between the metallic granule and the oxide intergranule may take place in the film prepared on the higher Tsub..

Double exchange-related interfacial tunneling and the corresponding giant magnetoresistance in granular La[sub 1−x]Sr[sub x]MnO[sub 3] perovskites

Fine grain manganese perovskites La1−xSrxMnO3 have been synthesized by using the inorganic sol-gel method. The grain size-dependent and the doping level-dependent magnetic transport properties in the fine grain system have been studied. The interfacial tunneling associated with the double exchange interaction and the corresponding tunnel-type magnetoresistance have been observed in the samples with an average grain size no greater than 0.12 μm in the doping range 0.05⩽x⩽0.45 for temperatures below the Curie point. The magnetization, the heat capacity, and the oxygen stoichiometry for the sample system have been measured to demonstrate that the interfacial tunneling originates from the difference in magnetic configuration between the surfaces and the cores of the grains, and the corresponding giant magnetoresistance results from the field-induced change of the surface magnetic ordering.

Spin-dependent interfacial tunnelling and tunnel-type GMR in granular perovskite family

The granular perovskites , with the concentration x from 0.05 to 0.5, have been fabricated by the sol-gel method. The concentration-dependent transport properties in the granular perovskites have been investigated. Spin-dependent interfacial tunnelling and the corresponding tunnel-type magnetoresistance have been observed in the samples sintered at lower temperatures in the doping range from x = 0.05 to 0.45 for the temperatures below the Curie point. The magnetization, the heat capacity and the oxygen stoichiometry for the sample family have been measured to show that the interfacial tunnelling stems from the magnetic difference between the surfaces and the cores of the grains, and that the tunnel-type GMR originates from the field-induced change of the surface magnetic configuration.