Showing papers on "Magnetoresistance published in 1994"

Thousandfold Change in Resistivity in Magnetoresistive La-Ca-Mn-O Films

Abstract: A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

Giant magnetoresistance in magnetic multilayered nanowires

Abstract: Giant magnetoresistance (GMR) is observed in a new type of nanostructured material consisting of magnetic multilayered nanowires formed by electrodeposition into nanometer-sized pores of a template polymer membrane. The composition of these nanowires is modulated over nanometer length scales with distinct magnetic and nonmagnetic metallic layers. Magnetoresistance measurements with the current perpendicular to the layers were performed on the array of parallel nanowires. GMR of about 15% was observed at room temperature on Co/Cu multilayered nanowires. (C) 1994 American Institute of Physics.

Giant magnetic field dependent impedance of amorphous fecosib wire

Abstract: The impedance of an amorphous Fe4.3Co68.2Si12.5B15 wire (100 μm diameter) exhibits an extraordinarily large frequency dependent resistance in addition to the previously reported frequency dependent wire reactance. The frequency response of both the resistance and reactance is almost entirely suppressed by an axial magnetic field HA<150 Oe, resulting in a typical magnetoresistance for frequencies f<1 MHz of the order of the dc wire resistance. The magnetoresistance at f=1 MHz is ΔR/Rsat=370%. As the bulk of the magnetic response occurs for HA<5 Oe, this system shows great technological promise. We give a quantitative analysis of the phenomenon, which is rooted in classical electrodynamics.

Giant Magnetotransport Phenomena in Filling-Controlled Kondo Lattice System: La1-xSrxMnO3

Abstract: Giant magnetotransport phenomena including the field-induced nonmetal-metal transition have been found in single crystals of La 1- x Sr x MnO 3 near the critical composition ( x ≈0.17) for the nonmetal-metal transition and in the temperature region around the magnetic phase transition. Change of the resistivity shows a universal curve as a function of the magnitude of temperature- or field-induced magnetization, the most of which agrees with the prediction by the D =∞ and S =∞ Kondo lattice model with strong ferromagnetic (Hund) coupling.

Theory of Magnetic Recording

Abstract: Preface 1. Overview 2. Review of magnetostatic fields 3. Inductive head fields 4. Medium magnetic fields 5. Playback process: general concepts, single transitions 6. Playback process: multiple transitions 7. Magnetoresistive heads 8. Record process: transition models 9. Record process: non-linearities and overwrite 10. Medium noise mechanisms: general concepts, modulation noise 11. Medium noise mechanisms: particulate noise 12. Medium noise mechanisms: transition noise References Index.

Giant magnetoresistance in La1-xSrxMnOz films near room temperature

Abstract: We report the observation of giant magnetoresistance near room temperature in ferromagnetic films of La1−xSrxMnOz for 0.16≤x≤0.33. For B=5 T, the maximum magnetoresistance ratio [R(0)−R(B)]/R(0) of an annealed film is 60% at 260 K for x=0.2, and 35% at 330 K for x=0.33. Annealed films have higher Curie temperature (Tc), a larger saturation moment and a larger magnetoresistance effect near Tc than do as‐grown films. The temperature dependence of resistivity for all the samples investigated is unusual, activated above Tc and metallic below Tc. This and the giant magnetoresistance are possibly explained by scattering from magnetic polarons which dominate the transport near Tc.

Giant magnetoresistance of nanowires of multilayers

Abstract: A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.

Very large magnetoresistance in perovskite‐like La‐Ca‐Mn‐O thin films

Abstract: Colossal magnetoresistance with more than a thousandfold change in resistivity (ΔR/RH=127 000% at 77 K, H=6 T) has been obtained in epitaxially grown La‐Ca‐Mn‐O thin films. This magnetoresistance value is about three orders of magnitude higher than is typically seen in the giant‐magnetoresistance‐type metallic, superlattice films. The temperature of peak magnetoresistance is located in the region of metallic resistivity behavior. As the magnetoresistance peak occurs not at the temperature of magnetic transition but at a temperature where the magnetization is still substantial, the spin‐disorder scattering is not likely to be the main mechanism in these highly magnetoresistive films. The peak can be shifted to near room temperature by adjusting processing parameters. Near‐room‐temperature ΔR/RH values of ∼1300% at 260 K and ∼400% at 280 K have been observed. The presence of grain boundaries appears to be very detrimental to achieving large magnetoresistance in the lanthanum manganite compounds. The fact th...

Colossal magnetoresistance in La‐Ca‐Mn‐O ferromagnetic thin films (invited)

Abstract: A colossal magnetoresistance effect with more than a thousandfold change in resistivity (ΔR/RH=127 000% at 77 K, H=6 T) has been obtained in epitaxially grown La‐Ca‐Mn‐O thin films. The effect is negative and isotropic with respect to the field orientations. The magnetoresistance is strongly temperature dependent, and exhibits a sharp peak that can be shifted to near room temperature by adjusting processing parameters. Near‐room‐temperature ΔR/RH values of ∼1300% at 260 K and ∼400% at 280 K have been observed. The presence of grain boundaries appears to be detrimental to achieving very large magnetoresistance in the lanthanum manganite films. The orders of magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.

Transport Properties of the Kondo Lattice Model in the Limit S=∞ and D=∞

Abstract: The Kondo lattice model with Hund's ferromagnetic spin coupling is investigated as a microscopic model of the perovskite-type 3 d transitionmetal oxide La 1- x Sr x MnO 3 . In the classical spin limit S =∞ and the infinite-dimensional limit D =∞, the one-body Green's function is exactly calculated. Transport properties of the system in the presence of magnetic fields are obtained. The giant magnetoresistance of this model, which is in good agreement with the experimental data of La 1- x Sr x MnO 3 , is explained by the spin-disorder scattering process.

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

Abstract: Several series of thin films (∼3000 A) 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 A with a minimum average separation of ∼10 A. The large MR values attained in both the PM and FM components a...

Magnetic field sensors using GMR multilayer

Abstract: Wheatstone bridge magnetic field sensors using giant magnetoresistive ratio (GMR) multilayers were designed, fabricated, and evaluated. The GMR ranged from 10% to 20% with saturation fields of 60 Oe to 300 Oe. The multilater resistances decreased linearly with magnetic field and showed little hysteresis. In one sensor configuration, a permanent magnet bias was placed between two pairs of magnetoresistors, each pair representing opposite legs of the bridge. This sensor gave a bipolar bridge output whose output range was approximately GMR times the bridge source voltage. The second sensor configuration used shielding on one resistor pair, and it gave a bridge output dependent on the magnetic field magnitude, but not polarity, and the output range was approximately one half GMR tines the bridge source voltage. Field amplifications of 3 to 6 were accomplished by creating a gap in a low reluctance magnetic path, thus providing the full range of outputs with 1/3 to 1/6 of the intrinsic saturation fields of the GMR multilayers. >

Giant Magnetoresistance in Magnetic Layered and Granular Materials

Abstract: Publisher Summary Magnetoresistance (MR) is the change in electrical resistance of a material in response to a magnetic field. All metals have an inherent, albeit small, MR owing to the Lorentz force that a magnetic field exerts on moving electrons. Although earlier studies reported unusual magnetoresistive effects in layered structures, it was discovered that the application of magnetic fields to atomically engineered materials known as magnetic superlattices greatly reduced their electrical resistance, that is, superlattices had a giant magnetoresistance. Superlattices are a special form of multilayered structures, artificially grown under ultrahigh-vacuum conditions by alternately depositing on a substrate several atomic layers of one element, say, iron, followed by layers of another, such as chromium. The original observation of giant magnetoresistance was made on iron–chromium superlattices with nearly perfect crystallinity, which was grown by molecular beam epitaxy (MBE). Giant magnetoresistance observed in layered and granular structures arises from the dependence of the resistivity on their internal (local) magnetic configuration.

Magnetoresistance of symmetric spin valve structures

Abstract: A spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned ferromagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure. The symmetric spin valve is shown to increase the magnetoresistance by 50% over the values of individual spin valves. The increase is attributed to a reduction of spin-independent outer boundary scattering and doubling the number of spin-dependent scattering interfaces. The magnetoresistance and coupling fields of the spin valves that comprise the symmetric structure have been measured as a function of Cu spacer thickness. Oscillatory antiferromagnetic to ferromagnetic coupling was observed in standard spin valves, which had /spl Delta/R/R as high as 13%. >

Ultra high density, non-volatile ferromagnetic random access memory

Abstract: A random access memory element utilizes giant magnetoresistance. The element includes at least one pair of ferromagnetic layers sandwiching a nonmagnetic conductive layer. At least one of the two ferromagnetic layers has a magnetic moment oriented within its own plane. The magnetic moment of at least the first ferromagnetic layer of the pair has its magnetic moment oriented within its own plane and is typically fixed in direction during use. The second ferromagnetic layer of the pair has a magnetic moment which has at least two preferred directions of orientation. These preferred directions of orientation may or may not reside within the plane of the second ferromagnetic layer. The bit of the memory element may be set by applying to the element a magnetic field which orients the magnetic moment of the second ferromagnetic layer in one or the other of these preferred orientations. Once the bit is set, the value of the determined by the relative alignment of the magnetic moments of the first and second ferromagnetic layers. This value may be read by applying an interrogating current across the memory element, perpendicular to the plane within which the magnetic moment of the first ferromagnetic layer is oriented, and observing the variation in resistance. These ferromagnetic elements may be fabricated using conventional photolithography. Groups of these ferromagnetic element may be organized into word trees and other arrays.

Spin flip diffusion length and giant magnetoresistance at low temperatures.

Abstract: Measurements at 42 K of the magnetoresistance of Co/AgMn, Co/CuMn, Co/AgPt, and Co/CuPt multilayers with the current perpendicular to the layer planes (CPP-MR) show effects of reduced spin diffusion lengths due to alloying of the nonmagnetic metal with impurities that produce spin-spin (Mn) or spin-orbit (Pt) scattering Combining the data with a theory by Valet and Fert gives the spin diffusion lengths in the alloys

Bridge circuit magnetic field sensor having spin valve magnetoresistive elements formed on common substrate

Abstract: A magnetic field sensor uses four individual magnetoresistive spin valve elements electrically connected in a bridge circuit. The spin valve elements are lithographically formed on the same substrate with their free layers having their magnetization axes parallel to one another. An electrically conductive fixing layer is formed on the substrate but is insulated from the spin valve elements. The application of current through the fixing conductor during fabrication of the field sensor fixes the direction of magnetization of two of the pinned layers to be antiparallel to the direction of magnetization of the other two pinned layers. The bridge circuit output voltage is responsive to an external magnetic field in the plane of the sensor. By appropriate fixing of the direction of magnetization of the pinned layers during sensor fabrication, and appropriate connection to the input and output leads, the bridge circuit output voltage is a measure of either the magnetic field or field gradient. The fixing conductor, or a separate current strap formed on the substrate, can be used to pass an unknown current over the sensor, in which case the bridge circuit output voltage is a measure of the unknown current.

Multilayer magnetoresistance effect-type magnetic head

Abstract: The magnetoresistance effect element is of a multilayered structure having at least magnetic layers and an intermediate layer of an insulating material, a semiconductor or an antiferromagnetic material against the magnetic layers, and the magnetoresistance effect element has terminals formed at least on the opposite magnetic layers, respectively, so that a current flows in the intermediate layer. The film surfaces of all the magnetic layers constituting the magnetoresistance effect element are opposed substantially at right angles to the recording surface of a magnetic recording medium. Therefore, the area of the magnetic layers facing the recording surface of the magnetic recording medium can be extremely reduced, and thus the magnetic field from a very narrow region of the high-density recorded magnetic recording medium can be detected by the current which has a tunneling characteristic and passes through the intermediate layer.

Giant magnetoresistance in Fe/Cr superlattices with very thin Fe layers

Abstract: Carefully tailored Fe/Cr epitaxial superlattices with extremely thin Fe layers have been grown on MgO(100) by molecular beam epitaxy. The low‐angle x‐ray spectra reveal the presence of sharp interfaces down to an Fe layer thickness of a few monolayers. An [Fe(4.5 A)/Cr(12 A)]50 superlattice shows a 220% magnetoresistance at 1.5 K, and a saturation field of 110 kOe. A further decrease of the Fe layer thickness produces a drastic decrease in the magnetoresistance.

Nanolithography with an atomic force microscope for integrated fabrication of quantum electronic devices

Abstract: We describe a novel technique using an atomic force microscope (AFM) for integrated nanometer‐scale lithography on various mask materials such as photoresist or gold covering a mesa‐etched GaAs‐AlGaAs heterostructure at ambient conditions. The generated patterns can be transferred to the two‐dimensional electron gas by wet chemical etching or by ion beam irradiation. We succeed in fabricating hole arrays with a periodicity down to 35 nm and a hole diameter of only a few nanometers. In magnetoresistance studies on so‐called antidot devices with 95 nm period at T=4.2 K we can clearly observe commensurability oscillations, demonstrating the successful pattern transfer to the electron system. With the AFM we can also pattern lines of varying width and depth into prefabricated devices.

Inverse spin-valve-type magnetoresistance in spin engineered multilayered structures.

Abstract: The resistivity of magnetic multilayers is generally smaller when the magnetizations of successive layers are parallel, which is the so-called giant magnetoresistance or spin-valve effect. %'e have been able to reverse this eA'ect and to obtain a smaller resistivity for an antiparallel arrangement by intercalating thin Cr layers within half of the Fe layers in Fe/Cu multilayers. This inverse spin-valve elfect is due to the inverse spin asymmetries of the electron scattering in successive Fe layers with and without Cr. This is a confirmation of the fundamental mechanism of the giant magnetoresistance.

Intrinsic giant magnetoresistance of mixed valence La‐A‐Mn oxide (A=Ca,Sr,Ba) (invited)

Abstract: A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.

Two oscillatory behaviors as functions of ferromagnetic layer thickness in Fe/Cr(100) multilayers.

Abstract: Oscillatory magnetoresistance as a function of the Fe layer thickness has been found in Fe/Cr(100) multilayers deposited on MgO(100). It is mainly attributed to an oscillatory interlayer exchange coupling between adjacent Fe layers as a function of the Fe layer thickness. Further, oscillatory variation of the saturation resistivity with Fe layer thickness was also observed. Both the oscillation periods are approximately 8 \AA{}. These two oscillations can be understood in terms of the partial confinement of the perpendicular motion to the layer plane of Fe majority spin electrons.

Magnetic field sensor constructed from a remagnetization line and one magnetoresistive resistor or a plurality of magnetoresistive resistors

Abstract: Described is a sensor based on the magnetoresistive effect and integrated into the thin-film arrangement of a remagnetization line in the form of a meander. In an adaptation to this meandering structure, the magnetoresistive film strips are provided in regions with alternating positive- and negative-inclined Barber pole structures. When periodic remagnetization of the regions takes place, a drift-free AC voltage is obtained as a sensor output signal. This lack of drift is the presupposition for the use of the magnetic field sensor for precise measurement of weak magnetic fields.

Multilayer magnetoresistive sensor

Abstract: A magnetoresistive read sensor incorporates a multilayer sensing element formed of one or more magnetoresistive elements in a planar array, each magnetoresistive element having a multilayer structure of at least two ferromagnetic layers separated by a nonmagnetic layer. The ferromagnetic layers are coupled antiferromagnetically by magnetostatic coupling at opposing edges of the ferromagnetic layers. A bias layer separated from the magnetoresistive sensing element by a spacer layer provides a magnetic field to bias the magnetoresistive sensing element at a desired non-signal point for linear response. The magnetoresistive sensing element is formed by alternatively depositing layers of ferromagnetic material and layers of nonmagnetic material on a substrate and then patterning the resulting structure using photolithographic techniques to provide a planar array of magnetoresistive elements. A conductive layer is deposited over the array filling in the spaces separating the magnetoresistive elements to provide electrical conductivity between the elements in the plane of the structure.

Ferromagnetic La0.6Pb0.4MnO3 thin films with giant magnetoresistance at 300 K

Abstract: Highly textured, as-deposited $La_{0.6}Pb_{0.4}MnO_3$ thin films have been grown on $LaAlO_3$ by pulsed laser deposition. The films are ferromagnetic metals below 300 K. Giant negative magnetoresistance of over 40% is observed at 300 K at 6 T.

Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe4.6Si15B10

Abstract: ac magnetoresistance (MR) measurements performed in thin ribbons of the amorphous soft ferromagnet Co70.4Fe4.6Si15B10 are reported with a longitudinal dc applied magnetic field H varying from 0 to ±15 Oe. The dependence of the MR with the frequency f of the measuring ac current was investigated for 0.4≤f≤100 kHz. No significant frequency dependence in the resistivity ρ for H=0 and H=±15 Oe was found. For intermediate values of H, ρ(H) presents a peak in H which increases linearly in frequency. The MR peak in H at room temperature varies from typical values of 0.13% in the low‐frequency range to a giant value of 27% at 100 kHz. Two possible sources for the frequency dependence of the magnetoresistance in amorphous alloys are discussed.

Thin-film magnetic sensor using high frequency magneto-impedance (HFMI) effect

Abstract: This study reports on the performance of a thin-film magnetic sensor which uses the high frequency magneto-impedance (HFMI) effect. In order to obtain a high sensitivity and a large voltage change ratio (/spl Delta/Vpp/Vpp(0): corresponds to the MR ratio), a strip pattern, a closed magnetic circuit, and a NiFe/SIO/sub 2/ multilayer film structure are adopted for the magnetic films of the sensor. A /spl Delta/Vpp/Vpp(0) of 60-70% is achieved by applying an external magnetic field of several Oe. Moreover there is no hysteresis or no Barkhausen noise in this sensor, which has a magnetic film width of 10 /spl mu/m. In terms of linearity, the sensor exhibits modulation degree (m) of 12% and a total harmonic distortion (THD) of 0.8%. >

Magnetic wire and box arrays (invited)

Abstract: Ferromagnetic wire and box arrays with widths and spacings in the range of 2–06 μm were successfully fabricated utilizing high‐resolution electron‐beam lithography and lift‐off techniques The box arrays possessed a unique magnetic switching mechanism As the magnetic field decreased, the magnetic coherency between the boxes which are in a row first disappeared Domain wall motion in each box occurred in the next step The demagnetizing fields observed in the wire arrays with short spacings were different from those calculated by a model, in which the wires are isolated from each other Ferromagnetic resonance measurement implied the appearance of interwire dipole‐dipole interaction Multilayered wire arrays were also prepared to study the magnetoresistive characteristics

Magnetic field induced confinement in strongly correlated anisotropic materials.

Abstract: We have previously argued that strong correlation effects in highly anisotropic materials can lead to "confinement" in the sense that single particle motion in some directions is intrinsically incoherent. The presence of a magnetic field transverse to the direction of the hopping would enhance this effect and could potentially change the nature of hopping itself if the material were close to the coherence-incoherence transition. We believe that such an effect has already been observed in the unusual commensurability effects observed in the magnetoresistance of the highly anisotropic organic conductor ${(\mathrm{TMTSF})}_{2}$P${\mathrm{F}}_{6}$.