Giant Magnetoresistance Multilayers Research Articles

Tools and strategies for Developing New Materials

AbstractThe synergistic dependence of materials properties and performance upon structure and processing provides effective strategies for developing new materials for advanced technologies. This intimate interplay provides the central theme of this article. Advanced synthesis techniques, controlled microstructure, compositional tuning and theoretical predictions are powerful tools for searching for new materials or improved properties. These are discussed by selected examples including fullerenes, giant magnetoresistance multilayers, diamond and nitride films. The article also provides a brief overview of these materials and discusses their technological significance.

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.

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.

Change of magnetoresistance characteristic and crystal structure by ion bombardment to interfaces in [Ni81Fe19/Cu] multilayers (abstract)

Many practical applications using giant magnetoresistance (GMR) have been developed, such as read-out heads and spin valve devices; however, the origin of GMR is not still fully understood. The GMR effect seems to be originated from the spin scattering mechanism whether at the interfaces or at the layer bodies. Therefore, it is necessary to investigate the spin scattering behaviors at the interfaces in GMR multilayers. It is regarded that the mixing and the diffusion of atoms at the interfaces in multilayers can be well promoted by ion bombardment to the interfaces at proper energy in dual ion beam sputtering (DIBS) method. In this study, change of magnetoresistance (MR) and crystallographic characteristics induced by interfacial mixing of atoms were investigated. Specimen films were deposited by DIBS. Additional ion bombardment to the growing surfaces arranges the mixing effect at the interfaces between Ni-Fe and Cu layers. Acceleration voltage of sputtering ion source Vmg were set at 500 V and that of bombarding ion source Vsg was varied in the range of 0–300 V. Si wafers were used as substrates. Ni–Fe/Cu multilayers with GMR were deposited on 50 Å thick Fe buffer layers. Only two monolayers at the interfaces in Ni–Fe/Cu multilayers were exposed to ion bombardment to cause the local interfacial mixing. X-ray diffraction diagrams, showed that (111) orientations of Ni–Fe and Cu crystallites are obtained at Vsg below 200 V and that, on the contrary, (100) orientation became dominant in the films deposited at Vsg above 200 V. There is no apparent differences in MR ratio with increase of Vsg. However, the field sensitivity and saturation field properties were drastically degraded at Vsg above 200 V, while the crystal structure seemed to be changed. These results indicate that spin scattering were mainly occurred at the layer bodies in Ni–Fe/Cu multilayers. These results implied that the interfacial mixing is not so effective for changing MR ratio of Ni–Fe/Cu multilayers. However, the behavior of magnetization vectors are much influenced by change of local structure at the interfaces. These results seem to confirm the proposed theory; i.e., the spin scattering in layer bodies is the dominant mechanism on GMR in Ni–Fe/Cu 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.

Design and fabrication of GMR multilayers with enhanced thermal stability

Enhanced thermal stability of NiFeCo/Cu multilayers has been observed for multilayer stacks prepared with ‘overdesigned’ magnetic layers. Multilayers such as these show almost no giant magnetoresistance (GMR) in the as-deposited state, but exhibit huge improvements upon annealing. This methodology represents a considerable improvement over conventional methods.

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%.

Recent results on the giant magnetoresistance in magnetic multilayers (anisotropy, thermal variation and CCP-GMR)

We present some recent results obtained on the electrical transport properties in magnetic multilayers. Three points are addressed. The first one is an experimental demonstration of the existence of an intrinsic anisotropy of the giant magnetoresistance (GMR). The experiments have been carried out on spin-valve samples for which there is no contribution of the usual anisotropic magnetoresistance to the observed magnetoresistance. The GMR amplitude is found to be larger (lower) in the direction perpendicular (parallel) to the sensing current. The second point concerns a quantitative analysis of the thermal variation of the CIP (current-in-plane) GMR in magnetic multilayers. This analysis is based on a semi-classical theory including the spin-intermixing due to spin-flip scattering by magnons. This approach allows quantitatively evaluation of the respective weights of the various contributions to the thermal decrease in GMR: (i) scattering by magnons in the bulk of the ferromagnetic layers; (ii) phonon scattering in the non-magnetic spacer layer; and (iii) interfacial scattering by paramagnetic interfacial layers which may form as the temperature is increased. The third point is a theoretical investigation of the CPP (current perpendicular to the plane) electrical transport through an interface between two semi-infinite metallic materials. It is shown that when a potential step U exists at such an interface, this step gives rise to an interfacial resistance proportional to U 2. It also leads to the existence of large oscillations in the electric fields on both sides of the interface.

Ab initio calculations of the giant magnetoresistance.

Ab initio calculations are presented for the giant magnetoresistance (GMR) of Fe/Cr multilayers. The electronic structure of Fe/Cr superlattices is calculated within the spin density functional theory. The spin-dependent bulk impurity scattering is treated in a fully quantum mechanical way using a Green function formalism. The transport is described quasiclassically. The calculations suggest that the GMR in thin multilayers is mainly determined by the electronic structure of the layered system. Spin-dependent impurity scattering can reduce, increase, or invert the effect.

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.

Effect of ion bombardment and bias fielding for [Ni/sub 81/Fe/sub 19//Cu] multilayers with giant magnetoresistance deposited by dual ion beam sputtering

Ni-Fe/Cu multilayers with giant magnetoresistance (GMR) mere deposited by dual ion beam sputtering method. Simultaneous ion bombardment during the film deposition changed the crystallite orientation in GMR multilayers. The specimen films with ion-bombarded GMR multilayer and unbombarded Fe buffer layer exhibited larger MR ratio and higher field sensitivity /spl Delta/MR//spl Delta/H. The films deposited under ion bombardment and bias fielding revealed dominant (100) orientation, and these (100)-oriented films seems to possess relatively better GMR characteristics.

Stm Studies of GMR Spin Valves

ABSTRACTWe have investigated the surface roughness and the grain size in giant magnetoresistance (GMR) spin valve multilayers of the general type: FeMn/Ni80Fe20Co/Cu/Co/Ni80Fe20 on glass and aluminum oxide substrates by scanning tunneling microscopy (STM). The two substrates give very similar results. These polycrystalline GMR multilayers have a tendency to exhibit larger grain size and increased roughness with increasing thickness of the metal layers. Samples deposited at a low substrate temperature (150 K) exhibit smaller grains and less roughness. Valleys between the dome-shaped individual grainsare the dominant form of roughness. This roughness contributes to the ferromagnetic, magnetostatic coupling in these films, an effect termed “orange peel” coupling by Néel. We have calculated the strength of this coupling, based on our STM images, and obtain values generally within about 20% of the experimental values. It appears likely that the ferromagnetic coupling generally attributed to so-called “pinholes” in the Cu when the Cu film thickness is too small is actually “orange peel” coupling caused by these valleys.

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.