Magnetoresistance In Multilayers Research Articles

Magnetoresistive characteristics of multilayered wire arrays

Wire arrays comprising three types of giant magnetoresistive (GMR) multilayers were fabricated by high-resolution electron-beam lithography and lift-off techniques. For antiferromagnetic coupling-type GMR multilayer, a magnetic field applied parallel to the stripes produced larger magnetoresistance (MR) change and a higher response than in corresponding multilayered film with no patterns. In contrast, the wire structures worsened the MR characteristics of different coercive fields-type and spin-valve-type GMR materials.

Magnetic structure and magnetoresistance of metallic multilayers

Giant magnetoresistance (GMR) phenomena are briefly introduced and the relationship between the magnetic structure and the magnetoresistance of multilayers is argued. The merits of non-coupled-type multilayers, for technical applications and also for fundamental studies, are described. The results achieved on samples with doped interface layers indicate that the spin-dependent scattering occurs primarily at the interface sites. MR measurements in a new geometry, with the current at an angle to the plane, were carried out by depositing non-coupled-type multilayers on substrates with V-shaped microgrooves, and enhanced MR ratios were observed.

Angular dependence of the perpendicular giant magnetoresistance of multilayers.

We present measurements of the angular dependence of the giant magnetoresistance in multilayers with the current applied perpendicular to the layer plane. The samples used are (${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$/Ag/Co/Ag) multilayers sandwiched between superconducting underlayers and overlayers. The angle between the magnetizations of the ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ and Co layers is continuously varied. We find that the current-perpendicular-to-the-plane magnetoresistance of these multilayers varies as 1-a ${\mathrm{cos}}^{2}$(\ensuremath{\theta}/2)+b ${\mathrm{cos}}^{4}$(\ensuremath{\theta}/2), where \ensuremath{\theta} is the angle between the magnetization of successive magnetic layers and b/a is between 0.1 and 0.3. We discuss the theoretical interpretation of this variation. \textcopyright{} 1996 The American Physical Society.

Resistance noise in uncoupled giant magnetoresistive multilayers

Various uncoupled, magnetoresistive multilayer systems show quasiequilibrium resistance noise associated with ‖dR(H)/dH‖. The size of this 1/f noise, particularly in comparison with Johnson noise in various frequency ranges, is discussed for all the different systems. All the observed samples exhibit resistive Barkhausen noise, allowing the measurement of domain sizes very similar to those observed in coupled samples. In addition, the asymmetric hysteresis of the Barkhausen noise in uncoupled multilayers suggests the same type of hysteretic domain structure observed in antiferromagnetically coupled multilayers.

Giant magnetoresistance effect in multilayered wire arrays

Three wire arrays, each comprised of a different type of giant magnetoresistive (GMR) multilayer, were fabricated by high-resolution electron-beam lithography and lift-off techniques. With a GMR multilayer of the biquadratic-coupling type, a magnetic field applied parallel to the wires produced a larger MR change and higher response than in the corresponding unpatterned multilayered film; however, the wire structures worsened the MR characteristics of two other types of GMR multilayers.

Quarternary giant magnetoresistance random access memory

We designed a quarternary memory using weakly coupled giant magnetoresistance (GMR) multilayers based on the fact that there are four stable states when the applied field is zero. Compared with conventional binary memory, the major advantage of the quarternary GMR memory is that we can simply double its capacity. This design’s feasibility has been proved by experiments.

Giant magnetoresistance multilayers of high thermal stability with thicker magnetic layers

The problem of poor thermal stability in NiFeCo/Cu multilayers can be resolved by utilizing thicker magnetic layers than those giving optimum giant magnetoresistance (GMR), in the as-deposited state. This type of structure exhibits very little GMR in the as-deposited state, but upon appropriate post-deposition anneal, produces huge improvements in GMR magnitude and sensitivity. The described technique provides an improvement necessary for retaining the magnetic properties of the multilayers at temperatures that would be encountered in the fabrication and operation of GMR field sensors.

Magnetoresistance of multilayers on microstructured substrates

A new class of multilayers is fabricated on microstructured substrates. The GMR effect in a new geometry (CAP, with a current at an angle to the plane) was studied using multilayers prepared on micro-scale V-shaped groove structures. The measured CAP-MR is greater than the CIP-MR. The results are discussed in terms of the theory proposed by Levy et al.

Magnetoresistance studies of multilayers including hard magnetic CoSm layers

Cobalt-samarium alloy films were prepared by alternate deposition of Co and Sm in the presence of an external magnetic field to obtain thin magnetic layers with large in-plane coercive fields. These anisotropic CoSm layers were used as hard magnetic layers of noncoupled-type magnetoresistance multilayers or as pinning layers of spin-valve structures. Large magnetoresistance changes up to 4.6% were attained in the CoSm/Co/Cu/NiFe system.

Theory of the bipolar spin switch.

We extend the Valet-Fert model of the perpendicular magnetoresistance in magnetic multilayers to describe the spin accumulation and relaxation effects in the spin switch structure introduced by Johnson and calculate the output voltage of the device. In contrast to the usual treatment by Johnson, we take into account the spin relaxation in the ferromagnetic layers, and also the influence of interface resistances and interface spin flips. We show that, for thin nonmagnetic layers, the output voltage is limited by the spin relaxation in the ferromagnetic layers. We find that the interpretation of the experimental data requires surprisingly long spin diffusion lengths in both the magnetic and nonmagnetic layers, much longer than those derived from perpendicular magnetoresistance in multilayers of similar materials. \textcopyright{} 1996 The American Physical Society.

Perpendicular current giant magnetoresistance in 0.4 μm diameter Co-Cu multilayer sensors

We have fabricated a novel giant magnetoresistive (GMR) multilayer (ML) flux sensor that is designed to operate in the current perpendicular to the plane (CPP) mode. The CPP-GMR sensor is a 0.4 /spl mu/m diameter, 0.09 /spl mu/m tall Co-Cu ML pedestal. The sensors are patterned using electron beam lithography. The Al/sub 2/O/sub 3/-TiC substrate is coated with a sputter deposited Al/sub 2/O/sub 3/ film that is polished to <0.2 nm root mean square (RMS) roughness. Contact to the bottom of the CPP-GMR sensor is made by depositing and Co-Cu multilayers onto a smooth 0.45 /spl mu/m thick Mo-Si ML stack. The top contact is self-aligned to the CPP-GMR sensor. This is accomplished, in part, by chemical mechanical polishing (CMP). The top and bottom contacts are electrically isolated by a plasma enhanced chemical vapor deposited (PECVD) Si/sub 3/N/sub 4/ film. The configuration of the contacts allows four point probe resistance measurements. The CPP-GMR coefficient for these 0.4 /spl mu/m diameter sensors is 13%.

Ion beam sputtering of magnetoresistive multilayers: Co/Cu and Co-modified Ni/sub 81/Fe/sub 19//Cu systems

The ion beam sputtering method was applied to the preparation of Co/Cu and related multilayers. Improvements were obtained by changing the sputtering gas from Ar to Xe and by increasing the growth rate. By these means we achieved a magnetoresistance effect of 51% at room temperature in Co/Cu multilayers of only 16 periods. With these optimized working conditions Ni/sub 81/Fe/sub 19//Cu multilayers with cobalt interfaces of various thicknesses were prepared. A linear correlation between the magnetoresistance ratio and the antiferromagnetically coupled volume fraction is found indicating a predominant interface contribution to the magnetoresistance effect. On the other hand, cobalt was found to degrade the coupling quality and to enhance the coercivity.

Magnetic properties of ion-beam sputter deposited NiFe ultrathin films

Studies have been made of the magnetic properties of NiFe ultrathin films 15–100 Å thick. The NiFe films were deposited on Cu, Ti, and Ta underlayers by an ion-beam sputtering method. In the NiFe films deposited on the Cu layers, the saturation magnetization shows little change with thickness down to 60 Å, but below this it shows a gradual decrease with decreasing thickness. The coercivity in the hard axis is almost unaltered with the film thickness, whereas the coercivity in the easy axis and the anisotropy field decrease with decreasing film thickness. Both values greatly decrease below 30 Å. Thickness dependence of the induced uniaxial anisotropy constant and the direct-current permeability were obtained from these parameters. Remarkable decreases of these parameters were observed in the films on the Ti and Ta layers likely owing to the formation of fine islands. Magnetic properties and giant magnetoresistance in multilayers are discussed on the basis of the present experimental results.

Low magnetic field spin flops in giant magnetoresistance multilayers

Sputtered FeCoNi(Cu)/Cu giant magnetoresistance multilayers on glass show growth-induced uniaxial in-plane magnetic anisotropy. We controlled the anisotropy and made multilayers with various anisotropy and antiferromagnetic coupling fields. A multilayer with a low spin flop field of 9 Oe can be obtained with a resistance change of 5% for a 10 Oe external field.

Observation of giant magnetoresistance in multilayers

Abstract The giant magnetoresistance (GMR) in magnetron sputtered Fe Mo multilayers was reported. At 4.2 K, the maximum value of MR reaches 12%. Our present results confirm that the giant magnetoresistance is associated with the existence of antiferromagnetic interlayer coupling in Fe Mo multilayers.

The role of interface impurities in Co/Cu giant magnetoresistance multilayers

Abstract We have deposited impurities (Au, Ge and Cu) at the interface of Co/Cu multilayers grown by MBE. It was found that small fractions of a monolayer of impurities dramatically affect the GMR.

Photolithographic structuring of giant magnetoresistive CoCu multilayers

Abstract A manufacturing process for structuring giant magnetoresistive multilayers based on dry etching techniques is described. The samples were deposited on oxidized silicon with the sensitive areas varying in width between 5 and 100 μm. The structured multilayers exhibited magnetic properties in accordance with unstructured macroscopic control samples of 100 mm 2 total area.

Micromagnetic study of narrow track orthogonal giant magnetoresistive heads

Orthogonal narrow track magnetoresistive heads using giant magnetoresistive multilayers have been studied by micromagnetic modeling. The modeling shows that the hysteresis arises from irreversible switching of the edge-curling wall (ECW) on the side edges of the giant magnetoresistive (GMR) sensor layers. Effects of thickness and interlayer exchange coupling on the switching of the ECW are analyzed. It is also found that if the interlayer exchange coupling is ferromagnetic, the GMR orthogonal head has significant high signal amplitude.

Electron microscope observations of the magnetic structures in magnetoresistive multilayer films

A detailed investigation of the magnetic domain structures present in Cu-based magnetoresistive multilayers was carried out using transmission electron microscopy. The Cu thickness was varied to give multilayer films exhibiting a wide range of magnetoresistance values. The domain structures and magnetization reversal processes in samples with low magnetoresistance values were found to be typical of predominantly uniaxial, ferromagnetically coupled multilayer films. The giant magnetoresistance samples did not show a pronounced anisotropy. Rather than a complete antiparallel alignment of the magnetic layers at zero magnetization, the giant magnetoresistance samples displayed a significant parallel alignment. Samples with moderate magnetoresistance values exhibited a mixture of the structures observed in the high and low magnetoresistance films depending on the applied field orientation.

Structural, magnetic and transport properties of NiFe χAg( 1−χ) heterogeneous alloys

We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni 80Fe 20) χAg( 1−χ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni 80Fe 20) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni 80Fe 20 and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni 80Fe 20 concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.