Giant Magnetoresistance Multilayers Research Articles

Interface Engineering of GMR Multilayers and Exchange-Biased Spin-Valves Using Ag as a Surface Modifier

Giant magnetoresistance (GMR) type Co-Cu multilayers and exchange-biased spin-valve (EBSV) type Co-Cu-Co tri-layers are fabricated by sputtering with a thin layer of Ag inserted between Co and Cu as a surface modifier (SM). Their magnetoresistances (MR) are measured in both current-perpendicular-to-plane (CPP) and current-in-plane (CIP) configurations. In all situations, the size of the MR change decreases as the Ag SM is introduced, and the effect is interface related

Role of heterostructures and multiple magnetic phases in the low-field magnetization of Fe–Cr giant magnetoresistive multilayers

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations along with ac magnetization versus temperature and M-H loop measurements are reported for two series of ion beam sputtered Fe–Cr giant magnetoresistive (GMR) (maximum of 33%) multilayers where the interface roughness is different. The ZFC and FC magnetization data follow different curves below an irreversibility temperature (Tirr). The FC data show a T3∕2 thermal demagnetization behavior at lower temperatures with a very small spin-wave stiffness constant but they go as 1∕T at higher temperatures (above Tirr). The ZFC data show maxima at Tm(H), from which we find the glass transition temperature Tg using the de Almeida and Thouless plot. ac magnetization versus T also shows broad peaks roughly near the same Tg. This behavior is interpreted in terms of the coexistence of spin-glass∕superparamagnetic, FM and AF phases. Most of the above exotic low-field features are absent in molecular beam epitaxy grown Fe–Cr multilayers (GMR >50%) with very smooth interfaces. The relatively rough interfaces in the present sputtered samples give rise to frustrations of Fe and Cr moments, leading to spin-glass-like behavior.

Relaxation of thermo-remanent magnetization in Fe–Cr GMR multilayers

The time decay of the thermo-remanent magnetization (TRM) in Fe–Cr giant magnetoresistive (GMR) multilayers has been investigated. The magnetization in these multilayers relaxes as a function of time after being cooled in a small magnetic field of 100 Oe to a low temperature and then the magnetic field is switched off. Low-field ( < 500 Oe ) magnetization of these samples has shown history dependence. This spin-glass-like behavior may originate from structural imperfections at the interfaces and in the bulk. We find that the magnetization relaxation is logarithmic. Here the magnetic viscosity is found to increase first with increasing temperature, then it reaches a maximum around T g , and further it decreases with increasing temperature. This behavior is different from that of conventional spin glasses where the logarithmic creep rate is observed to increase with temperature. The dynamical effects of these multilayers are related to the relaxation of thermally blocked superparamagnetic grains and magnetic domains in the film layers.

Mechanisms of Ag as a surfactant in giant magnetoresistance multilayer growth and thermal stability

The mechanisms played by Ag as a surfactant in giant magnetoresistance multilayers were investigated using interface sensitive x-ray anomalous scattering techniques. Analysis on [Cu∕Ni70Co30]20 and [Cu∕Ag∕Ni70Co30]20 multilayers revealed that 6Å thick NiCu and 6Å thick CuNi3Co intermixing regions are formed at the Ni70Co30-on-Cu interfaces of undoped and Ag-doped multilayers, respectively. The Cu-on-Ni70Co30 interfaces in both multilayers are sharp. Annealing causes severe diffusion across both types of interfaces in the undoped multilayer. But the interfaces in the Ag-doped multilayer do not change significantly upon annealing, except that Ag atoms diffuse into the whole Ni70Co30 layer and some parts of the Cu layer. The results suggest that addition of Ag during the deposition suppresses interfacial intermixing. X-ray diffuse scattering profiles show that the interfacial lateral correlation length of the Ag-doped multilayer is longer than that of the undoped multilayer and does not change significantly after annealing, suggesting that the addition of Ag gives rise to smoother interfaces and results in a good thermal stability.

Modeling and measurement of spin torques in current-perpendicular-to-plane giant magnetoresistive (invited)

This paper discusses some aspects of practical modeling of spin-torque effects in current-perpendicular-to-plane (CPP) giant magnetoresistive magnetic multilayers, with emphasis on considerations of primary importance for sensor and/or read head applications of CPP spin valves (SVs). These include an extension of the well-known Valet-Fert transport model [Phys. Rev. B 48, 7099 (1993)] to the general case of noncollinear magnetization vectors in CPP multilayer of arbitrary layer structure as well as the application of the model to realistic CPP-SV structures. Simple analytical models for the angular dependence of spin-torque critical currents in CPP-SV devices are reviewed and compared with numerical simulations which include thermal fluctuations. Finally, the paper also contains some recent high frequency spectral measurements of spin-torque-induced noise in dual spin-valve sensors.

Optimization of magnetoresistive sensitivity in electrodeposited FeCoNi/Cu multilayers

Giant magnetoresistive (GMR) multilayers with high GMR sensitivity might find application in low-end magnetic sensors. Electrodeposition of FeCoNi/Cu GMR multilayers directly onto n-type silicon was carried out from a single electrolyte and their magnetotransport properties were investigated. Inhomogeneous dissolution of the ferromagnetic (FM) layer during Cu sublayer deposition has an important role in determining the shape of the GMR characteristics; lower dissolution promotes sharper interfaces and larger slopes of the GMR versus field curve at low fields. An increase of the Fe content in the FM sublayers reduces coercivity but promotes selective dissolution and degrades interface sharpness. Electrodeposition conditions are optimized to achieve a maximum GMR of 9% saturated at <0.5 kOe, and a maximum GMR sensitivity of over 0.11%/Oe in the field range 5-15 Oe with a saturation GMR of 6.1%.

Giant-Magnetoresistive/Superconducting Contacts and Josephson Junction Devices

We have been investigating superconductor-normal metal-superconductor (SNS) (Proximity-Effect) Josephson Junctions in which the normal metal is a magnetic heterostructure in which the magnetic character can be actively controlled. As part of this program we have investigated the use of giant magnetoresistive (GMR) Cu-Py multilayers as the barrier in variable thickness planar bridge SNS junctions in addition to in-plane S-GMR contacts. In this paper we report on the latest results from our devices which have been fabricated using our established focussed ion beam (FIB) processes.

Evidence of spin-wave demagnetization in Fe–Cr giant magnetoresistance multilayers

The temperature dependence of the saturation magnetization of Xe ion-beam sputtered Fe–Cr multilayers has been measured between 5 and 300K in the presence of a field greater than the saturation field (Hsat). With the application of Hsat, the zero-field antiferromagnetically coupled ferromagnetic Fe layers are forced to align ferromagnetically. We find that the thermal demagnetization of such magnetically composite structure follows the Bloch formula for spin waves with anharmonic term in the magnon dispersion relation, viz., M(T)=M(0)[1+AT3∕2+BT5∕2]. Comparisons are presented between the multilayers and bulk crystalline Fe.

Magnetoresistive multilayers deposited on the AAO membranes

Silicon and GaAs wafers are the most commonly used substrates for deposition of giant magnetoresistive (GMR) multilayers. We explored a new type of a substrate, prepared electrochemically by anodization of aluminum sheets, for deposition of GMR multilayers. The surface of this AAO substrate consists of nanosized hemispheres organized in a regular hexagonal array. The current applied along the substrate surface intersects many magnetic layers in the multilayered structure, which results in enhancement of giant magnetoresistance effect. The GMR effect in uncoupled Co/Cu multilayers was significantly larger than the magnetoresistance of similar structures deposited on Si.

A new experimental design for noncontact giant magnetoresistance measurements using the magnetorefractive effect

Electrical magnetoresistance measurements of spin dependent materials or devices are very difficult without surface damaging or sample contamination, especially for thin-film giant magnetoresistance (GMR) multilayers. Moreover, the in situ determination of the GMR profile is almost impossible using electrical measurements. We propose a novel experimental design that allows the convenient measurement of magnetoresistance profiles using a noncontact method based on the magnetorefractive effect. This technique is applicable to metallic samples or devices showing magnetotransport properties and is also suitable for in situ measurements. The experiment involves infrared (IR) reflectivity measurements as a function of the applied magnetic field. By introducing IR optical fibers, the experimental setup has been substantially simplified while the need for costly optical components and time consuming alignments has been eliminated. Theoretical simulations of this experiment are also presented prior to the introduction of the proposed design.

Decomposition of the magnetoresistance of multilayers into ferromagnetic and superparamagnetic contributions

An analysis of both magnetic and magnetotransport properties is presented for electrodeposited multilayers prepared intentionally under conditions to make the superparamagnetic (SPM) magnetization contribution comparable to or larger than the ferromagnetic term. Based on a model elaborated for the giant magnetoresistance (GMR) of granular metals [N. Wiser, J. Magn. Magn. Mater. 159, 119 (1996); B. Hickey et al., Phys. Rev. B. 51, 667 (1995)], it is shown that in such multilayers both the magnetization and the GMR can be decomposed into ferromagnetric and superparamagnetic contributions where the latter term is described by a Langevin function. The size of the SPM regions estimated from the experimental data is in the nanoscale regime. It is believed that the method applied here gives a quantitative answer to the problem of the often observed nonsaturating behavior of GMR in multilayers. Electrodeposited multilayers are particularly prone to this feature although the occurrence of SPM regions is quite common in multilayers prepared by any technique. Therefore, this type of analysis should help better understanding of the factors influencing the GMR of multilayer films.

Preparation and Characterisation of Electrodeposited Ni–Cu/Cu Multilayers

Ni–Cu/Cu multilayers have been, grown from a single electrolyte under potentiostatic conditions at different electrolyte pH values. The current-time transients recorded during deposition indicated different growth modes of the Ni–Cu layers. Structural characterisation by X-ray diffraction revealed that the multilayers have the same crystal structure and texture as their (1 0 0) textured polycrystalline Cu substrate. Scanning electron microscopy showed that the films grown at low pH (2.2) have smoother surfaces than those grown at high pH (3.0). Energy dispersive X-ray analysis revealed that the magnetic layers of the multilayers electrodeposited at high pH contain much more Cu compared to those deposited at low pH. Anisotropic magnetoresistance was found for nominal Cu layer thicknesses below 0.6 nm, and giant magnetoresistance (GMR) above 0.6 nm. The shape of the magnetoresistance curves for GMR multilayers indicated the predominance of a superparamagnetic contribution, possibly due to the discontinuous nature of the magnetic layer. For multilayers with the same bilayer and total thicknesses, the GMR magnitude decreased as the electrolyte pH increased. Besides possible structural differences, this may have come from a strong increase in the Cu content of the magnetic layers since this causes a nearly complete loss of ferromagnetism at room temperature.

GMR Multilayers on a New Embossed Surface

It has been shown that the deposition of magnetoresistive multilayers on stepped, corrugated or V-grooved surfaces can increase the magnitude of giant magnetoresistance (GMR). The primary reason for this enhancement of GMR is that the in-the-substrate-plane current crosses multiple magnetic layers which results in the mixed current-in-plane and current perpendicular to plane modes called current at an angle to the plane mode. In our studies, we use a novel substrate consisting of nano-hemispheres organized in a regular hexagonal array. The substrate was produced by anodization of Al and subsequent etching of alumina membrane. Scanning electron microscopy was used to investigate larger areas and cross-sectional images of the embossed surface, whereas detailed analysis of the surface structure was made by high resolution atomic force microscopy. We deposited uncoupled Co/Cu multilayers on the alumina substrate with an 8-nm-thick Fe buffer using magnetron sputtering. Our preliminary studies of the magnetotransport using a physical property measurement system (quantum design) demonstrated that the samples on the new substrate have an enhanced GMR effect compared to the samples with similar composition deposited on smooth (100) Si wafers. Because of the inexpensive method of fabrication of the embossed substrate, the GMR structures deposited on this substrate have a potential for use in magnetic sensors.

Studies of Deep-Submicrometer Ultranarrow-Track Recording by Composite Simulation Models

This paper presents an analysis of the signal and noise in deep-submicrometer ultranarrow-track disk recording by composite simulation models, including the micromagnetics in longitudinal media and giant magnetoresistive (GMR) multilayers and the surface finite-element methods for inductive thin film heads and GMR head shields. The analysis assumes that the linear density of the recording process is in a range of 57 to 1200 kilo-flux changes per inch (kfci) and the width/gap ratio of the write head is between 4 and 2, corresponding to current industry practice. The first to fifth harmonics of all-one signals are found by the simulated spectrum. The signal and noise at different track densities and linear densities are analyzed by the harmonics in the spectrum. The composite heads in the disk recording system with widths 160, 240, and 320 nm are evaluated by the signal-to-noise ratio at a linear density around 1000 kfci.

Wavelength dispersive grazing incidence X-ray fluorescence of multilayer thin films

We developed the grazing incidence X-ray fluorescence (GIXRF) technique based on the wavelength dispersive (WD) fluorescence equipment with a high brilliance synchrotron radiation at SPring-8. With a good energy resolution and a high count rate of the WD detector, a high quality data of an incident angle dependence of fluorescence yields for the composition elements were obtained. Data analysis based on the multilayer model has been developed. On the applications of this technique, two examples are discussed. The interfacial completeness has been evaluated for the 10-layer structure of the giant magnetoresistance (GMR) multilayer. On the other hand, the diffusion of atoms at interface has been evaluated for the Ta 2O 5 metal gate structures.

Transport properties of sharp antiferromagnetic boundaries in Gd/Fe multilayers

The transport properties of sharp antiferromagnetic boundaries between Gd and Fe have been studied. The measurements were carried out with the current perpendicular to the plane at 20 K and up to 8 T in multilayer Gd/Fe of different thicknesses. We have extended the Valet and Fert model for giant magnetoresistive multilayers to explain the results in terms of spin-dependent scattering at the interface that changes its magnitude when the external field decreases the angle of the interface.

Extraordinary Hall effect in Fe-Cr giant magnetoresistive multilayers

We show that for giant magnetoresistive Fe-Cr multilayer samples, the scaling law for the extraordinary Hall constant, ${R}_{S}\ensuremath{\sim}{\ensuremath{\rho}}^{n}$ gives unrealistically large values of $n(n=2.8--3.3)$ if we use the resistivity $\ensuremath{\rho}$ at an external field $B=0$ as done by previous investigators. With B perpendicular to the film plane, the transverse magnetoresistance saturates at $B\ensuremath{\approx}3$ T in the 4.2--300 K temperature range. This indicates that the Fe layers, although antiferromagnetically coupled at $B=0,$ are ferromagnetically aligned only above $B\ensuremath{\approx}3$ T. Therefore, we use $\ensuremath{\rho}=\ensuremath{\rho}(B=3\mathrm{T})$ in ${R}_{S}\ensuremath{\sim}{\ensuremath{\rho}}^{n}$ and obtain $n=1.96\ifmmode\pm\else\textpm\fi{}0.02,$ which implies that the side-jump mechanism $n=2$ is the dominant scattering process. We emphasize that our interpretation of ${R}_{S}(\ensuremath{\rho})$ not only evolves from a proper understanding of the magnetic state of the multilayer system but also yields physically meaningful values of the exponent n.

Microfabrication and test of a magnetic field sensor using electrodeposited thin film of giant magnetoresistive (Cu/Co) x multilayers

Giant magnetoresistive (GMR) films of (Cu/Co) x multilayers have been successfully electrodeposited on a silicon substrate. An effort has also been made to microfabricate a sensor based on the electroplated GMR multilayers. Fabrication of the microsensor was implemented through computer-controlled electrodeposition of GMR thin films and lithographic patterning technology. The prototype microsensor shows a magnetoresistance ratio of ΔR/R=4% in a 3000 Gauss magnetic field. It was also found that the sensitivity of the prototype sensor is significantly affected at low fields by the patterning process.

Magnetoresistance of electrodeposited iron–cobalt–nickel–copper multilayers

Giant magnetoresistance (GMR) is observed in electrodeposited FeCoNiCu/Cu multilayers with Cu layer thickness varying from 1.0 to 2.5 nm at a fixed alloy layer thickness of 2.0 nm. A maximum GMR occurred at a copper layer thickness of 1.8 nm, which corresponds to the second maximum peak reported for similar multilayers fabricated by vapor deposition. The saturation magnetic field was considerably larger than that for the vapor deposited counterparts. The GMR of multilayers with a thicker alloy layer was sensitive to the magnetic field history, indicating the onset of anisotropic magnetoresistance.

Current-perpendicular-to-plane multilayer sensors for magnetic recording

We investigated current-perpendicular-to-plane giant magnetoresistance multilayer (CPP-ML) sensors with an active region of (1.0-nm CoFe/1.8-nm Cu) /spl times/ 15 nm. These sensors would allow a shield-to-shield spacing of less than 50 nm. Square CPP-ML devices ranging in size from 120 to 365 nm on a side have been fabricated and tested. In this paper, we focus on the magnetotransport properties of the 140 nm devices, which were measured at room temperature. The average device characteristics were found to be R/sub max/=1.0 /spl Omega/, R/sub min/=0.81 /spl Omega/, DR=191.1 /spl Omega/, and DR/R/sub min/=23.7. These values were measured by using a four-point probe geometry; the data were not corrected for lead or contact resistance and no current crowding was observed. After correction for buffer and seed layer resistances, the magnetoresistance had an intrinsic DR/R/sub min/ value of 55.6%. Our measured results are in good agreement with values obtained with a simple two-current series resistance model. We demonstrate that our CPP-ML structures are viable candidates to replace current-in-plane spin valves as the next generation magnetic recording readback sensor.