Granular Giant Magnetoresistance Research Articles

Magnetoresistive behaviour of ternary Cu-based materials processed by high-pressure torsion

Severe plastic deformation using high-pressure torsion of ternary Cu-based materials (CuFeCo and CuFeNi) was used to fabricate bulk samples with a nanocrystalline microstructure. The goal was to produce materials featuring the granular giant magnetoresistance effect, requiring interfaces between ferro- and nonmagnetic materials. This magnetic effect was found for both ternary systems; adequate subsequent annealing had a positive influence. The as-deformed states, as well as microstructural changes upon thermal treatments, were studied using scanning electron microscopy and X-ray diffraction measurements. Deducing from electron microscopy, a single-phase structure was observed for all as-deformed samples, indicating the formation of a supersaturated solid solution. However, judging from the presence of the granular giant-magnetoresistive effect, small ferromagnetic particles have to be present. The highest drop in room temperature resistivity (2.45% at 1790 kA/m) was found in Cu62Fe19Ni19 after annealing for 1 h at 400 °C. Combining the results of classical microstructural studies and magnetic measurements, insights into the evolution of ferromagnetic particles are accessible.

Tuneable Magneto-Resistance by Severe Plastic Deformation

Bulk metallic samples were synthesized from different binary powder mixtures consisting of elemental Cu, Co, and Fe using severe plastic deformation. Small particles of the ferromagnetic phase originate in the conductive Cu phase, either by incomplete dissolution or by segregation phenomena during the deformation process. These small particles are known to give rise to granular giant magneto-resistance. Taking advantage of the simple production process, it is possible to perform a systematic study on the influence of processing parameters and material compositions on the magneto-resistance. Furthermore, it is feasible to tune the magneto-resistive behavior as a function of the specimens’ chemical composition. It was found that specimens of low ferromagnetic content show an almost isotropic drop in resistance in a magnetic field. With increasing ferromagnetic content, percolating ferromagnetic phases cause an anisotropy of the magneto-resistance. By changing the parameters of the high pressure torsion process, i.e., sample size, deformation temperature, and strain rate, it is possible to tailor the magnitude of giant magneto-resistance. A decrease in room temperature resistivity of ~3.5% was found for a bulk specimen containing an approximately equiatomic fraction of Co and Cu.

Lab-on-a-Chip Magneto-Immunoassays: How to Ensure Contact between Superparamagnetic Beads and the Sensor Surface.

Lab-on-a-chip immuno assays utilizing superparamagnetic beads as labels suffer from the fact that the majority of beads pass the sensing area without contacting the sensor surface. Different solutions, employing magnetic forces, ultrasonic standing waves, or hydrodynamic effects have been found over the past decades. The first category uses magnetic forces, created by on-chip conducting lines to attract beads towards the sensor surface. Modifications of the magnetic landscape allow for additional transport and separation of different bead species. The hydrodynamic approach uses changes in the channel geometry to enhance the capture volume. In acoustofluidics, ultrasonic standing waves force µm-sized particles onto a surface through radiation forces. As these approaches have their disadvantages, a new sensor concept that circumvents these problems is suggested. This concept is based on the granular giant magnetoresistance (GMR) effect that can be found in gels containing magnetic nanoparticles. The proposed design could be realized in the shape of paper-based test strips printed with gel-based GMR sensors.

Giant magnetoresistance effects in gel-like matrices

We present transport measurements with magnetoresistance effect amplitudes of up to 260% at room temperature obtained in granular systems consisting of Co nanoparticles embedded in conductive gels as a non-magnetic matrix. In order to gain a better understanding of the transport mechanism in gel during measurement, the granular system was simultaneously monitored by optical microscopy. Gel-like matrices with different conductivities and viscosities were tested and will allow us to realize a highly sensitive granular giant magnetoresistance sensor without the need for lithographic techniques.

Low Field Magnetoresistance at Room Temperature in CrO2 Doped Polyvinyl Alcohol Films

ABSTRACTFerromagnetic CrO2 nanoparticles embedded in a diamagnetic matrix of polyvinyl alcohol (PVA) presents an example of granular giant magnetoresistance (GMR) in a diluted magnetic composite structure. Usefully significant MR occurs at room temperature, viz. -6.8% in 3.0 wt% CrO2-PVA, at field as small as 1.1 kOe. We suggest that the system is a natural analogue to dilute ferro-fluid structures composed of nanoscale magnetic particles in a viscous organic fluid (diamagnetic as well as an electrical insulator), i.e., this material displays a GMR effect without an introduction of chemical interfaces. It has great potential in various applications and can also facilitate studies of magnetotransport properties in GMR materials.

TEM and electron holography analyses of granular and thin layered Cu–Co magnetic materials

The paper shows the examples of application of transmission electron microscopy (TEM) techniques for characterization of two types of copper–cobalt magnetic nanomaterials: Cu-10 wt% Co granular giant magnetoresistance (GMR) thin ribbons and thin nanocrystalline Co films deposited on Cu substrate. Quantitative TEM microstructural analyses were used for determination of Co particle size distributions in GMR ribbons. It was demonstrated that the relative resistivity depends on the mean diameter of the cobalt nanoparticles. For nanocrystalline thin Co layers, off-axis electron holography was used to investigate their magnetic structure. The mean in-plane component of the magnetic field in cobalt was calculated from the phase gradient.

Giant magnetoresistance, structural and magnetic properties of glass-coated Fe–Ni–Cu microwires

Glass-coated Fe–Ni–Cu microwires prepared by Taylor's technique exhibit negative magnetoresistance (MR) of 8.15% at 77 K and 6.35% at 300 K in a magnetic field of 9.0 T. The MR is of the same origin found in the granular giant magnetoresistance (GMR) materials and is distinguishable from the giant magnetoimpedance (MI) commonly seen in soft magnetic microwires. MI displays a peak at zero field for RF currents with frequencies less than 20 MHz and it crosses over to a sharp dip at higher frequencies. This crossover is ascribed to the skin-depth-limited response primarily governed by the field dependence of the permeability. Micro-X-ray absorption near edge structure (micro-XANES) spectroscopy data were collected at the K edges of Cu, Ni and Fe and revealed that the Fe atoms in the as-cast sample are in FCC configuration and they remain in the FCC phase throughout the annealing processes. The MR decreases to ∼2.5% as the annealing temperature increases to 500°C. The loss of GMR upon annealing is attributed to the growth of FCC Fe–Ni-rich magnetic particles. The increase in the Fe–Ni particle size also results in higher room temperature coercivity. When the annealing temperature is increased to 500°C, a wasp-waisted hysteresis loop is observed which arises from the locking-in of the domain walls by the directional order of atoms due to diffusion under the influence of the local magnetic field. A magnetically hard glass-coated microwire with coercivity of 600 Oe is obtained after annealing at 700°C for 1 h.

Direct measurement of the dependence of granular giant magnetoresistance on the relative orientation of magnetic granules

Experiments have been designed to vary the relative angle between the magnetic moments of different Co granules in Cu80Co20 granular system. The moments of granules are mostly aligned in the same direction by field cooling to low temperature in a high magnetic field. A small field applied at an angle relative to the cooling field rotates the moments of a portion of the granules that have small particle size and coercivity. It is found that the giant magnetoresistance (GMR) varies linearly with cos φ, where φ is the relative angle between the magnetic axes of granules. This behavior disappears if the sample is cooled in zero fields, or if the rotating field is too large or small, or if the measuring temperature is higher than the blocking temperature. Our results show that the GMR in granular structures has the same angular dependence as the layered films and confirm the existing theories and recent microscopic models of granular GMR suggesting a crucial role of the relative orientations of the magnetic granules in determining the spin dependent scattering.

Magnetoresistance in glass-coated Fe–Ni–Cu microwires

Glass-coated Fe–Ni–Cu microwires were prepared by Taylor’s technique. X-ray diffraction data show patterns corresponding to fcc Cu. Negative magnetoresistance has been observed, which reaches 8.15% at 77 K and 6.35% at 300 K in magnetic field of 9.0 T. There is no significant difference between I⊥H and I∥H configurations. Both dc and low frequency ac (f<1 kHz) measurements exhibit essentially the same values of magnetoresistance. The observed magnetoresistance in the Fe–Ni–Cu microwires is of the same origin found in the granular giant magnetoresistance materials and is distinguishable from the giant magnetoimpedance commonly seen in soft magnetic microwires.

Giant Magnetoresistance in Granular Perovskite La0.8Sr0.2MnO3

The electric and magnetic properties in granular perovskite La0.8Sr0.2MnO3 have been investigated. A giant magnetoresistance (GMR) has been observed at temperatures well below TC, the Curie temperature, associated with crystal intrinsic magnetic properties. The granular GMR effect is also isotropic but shows some characteristics different from those in the films and crystals of similar perovskites. This suggests an effect of interfacial spin-dependent tunneling in the granular perovskite at temperatures much lower than TC. In addition, with the reduction of grain size, an evident superparamagnetic behavior has been observed.

The transition between granular and discontinuous GMR in sputtered Co/sub 90/Fe/sub 10//Ag multilayers

The giant magnetoresistance (GMR), structure, and magnetic properties of DC magnetron sputtered (Co/sub 90/Fe/sub 10/ 9.2 /spl Aring//Ag X /spl Aring/) multilayer films have been investigated. This CoFe thickness is unique in that a Ag thickness and annealing condition dependent transition is observed between layered granular GMR and discontinuous GMR behavior.

Thin film structures for low field granular giant magnetoresistance

Giant magnetoresistance (GMR) is a relatively new field of study pursued by scientists and technologists alike. A potential application of GMR thin films is the replacement of NiFe alloy thin films (having anisotropic magnetoresistance) to obtain greater signal. Applications of this type require that the GMR be obtained at low fields, on the order of 10 Oe. In this paper three types of granular GMR thin film structures are described: co-deposited heterogeneous alloys, granular bi-layers, and discontinuous multilayers. Experimental examples of each type are presented, and the field dependence of GMR for each structure is discussed. Only discontinuous multilayer thin film structures have shown suitable GMR at low fields, and the properties and microstructure of these thin films will be described in greater detail.

Absolute Magnetometry Using Electron Holography: Magnetic Superlattices and Small Particles

Synthesized magnetic structures are of interest due to their unique and unusual properties, which are governed by their micromagnetic structure. For example, giant-magnetoresistance (GMR) multilayer structures composed of magnetic layers separated by nonmagnetic spacers, and granular GMR films composed of magnetic and nonmagnetic metals exhibit phenomena whose interpretation requires knowledge of both the physical and micromagnetic structure at nanometer-length scales. Techniques for magnetic-microstructure imaging are based on the interaction between a probe and either the magnetic microstructure itself (magnetization) or a physical quantity related to the magnetization distribution (e.g., magnetostriction, magnetic induction). Transmission methods are sensitive to bulk magnetic microstructure averaged along the direction of the incident probe; surface structure is lost. Reflection techniques interact with the near-surface region and no information is obtained about the bulk structure aside from those properties that can be inferred from appropriate boundary conditions.Electron-optical methods represent the widest class of high-spatial-resolution, magnetic-domain imaging techniques. The most advanced techniques provide the highest contrast, sensitivity, and point resolution (1 nm). Electron holography offers quantitative micromagnetic information at high spatial resolution, a feature missing in most magnetic-imaging techniques. Quantitative information can be extracted from the absolutely calibrated electron wavelength and a knowledge of electron phase shifts in electromagnetic fields. High sensitivity, nanometer spatial resolution, and absolute calibration make electron holography a powerful tool for examining magnetic microstructure. In electron holography, both the amplitude and phase of the transmitted electron waves can be recovered in contrast to conventional electron microscopy where only the intensity is available. The phase, containing information about the local distribution of electromagnetic fields, can be retrieved from an electron hologram.

Hybrid NiFeCo-Ag/Cu multilayers: Giant magnetoresistance, structure, and magnetic studies

Giant magnetoresistance (GMR), crystal structure, and magnetic properties have been investigated in sputtered Ni66Fe16Co18-Ag/Cu hybrid granular multilayer thin films. High angle x-ray diffraction (HXRD) was used to reveal the overall film structure and growth texture and low angle XRD was used to investigate the periodicity and flatness of the multilayer structures. Hysteresis loops for the as-deposited Ag-rich films show superparamagnetic behavior (and conventional granular GMR) which does not saturate in 14 kOe. Very NiFeCo-rich films are magnetically soft and exhibit induced in-plane uniaxial anisotropy.

Granular giant magnetoresistance at low fields in bilayer thin films

Granular giant magnetoresistance is observed at saturation fields of 350–1100 Oe in thin-film structures consisting of a discontinuous layer of Co or NiFe deposited on an insulating substrate and overcoated with Cu. The phenomena are similar to that recently observed in codeposited immiscible alloys such as Co-Cu, although at lower fields. The bilayer structure does not require immiscibility and thus allows a greater materials choice. Magnetoresistance values up to 3% are observed and the reduced saturation fields are attributed to the controlled size and shape of the ferromagnetic islands.

Modeling Giant Magnetoresistance and Relative Permeability in Granular Films

ABSTRACTA Model for the field dependence of giant Magnetoresistance (GMR) in ‘granular’ co-sputtered alloy thin films (based on a relatively simple spin-dependent scattering concept appropriate to superparamagnetic and weakly ferromagnetic films) is applied to new experimental data from the Co90Fe10-Ag system. The Model and the experimental data can be shown to compare very well with the help of a single adjustable parameter related to spin correlation of adjacent Co-Fe clusters. A careful fit of field-dependent MR data and theory leads to a fairly reliable determination of spin-cluster radius. An analysis of the relative permeability of granular GMR films derived from the generalized form of the Clausius-Mossoti relationship is also presented. For a non-Magnetic Matrix the effective relative permeability is shown to be materials independent. The permeability model is applied to Co-Au granular films.