Giant Magnetoresistance Structures Research Articles

Dual current-perpendicular-to-plane giant magnetoresistive sensors for magnetic recording heads with reduced sensitivity to spin-torque-induced noise

Dual IrMn-pinned current-perpendicular-to-plane giant magnetoresistive sensor structures have been investigated, and their signal and noise characteristics are compared to similar standard single sensors, using test structures with sizes down to 50nm. The intrinsic signal ΔRA product is found to be increased by a factor of nearly 2 between single and dual spin valves, although the parasitic resistance due to the second 80-nm-thick IrMn-pinned layer stack limits the net increase in ΔR∕R to a factor of 1.5. For the single spin valve with antiparallel free and reference layers, the noise power spectral density shows a large asymmetry between positive and negative current flows due to strong spin-torque effects for positive (electrons flow from reference layer to free layer) current densities above 1×107A∕cm2. For the dual sensor, however, the critical current density is symmetric in polarity and is increased to above 5×107A∕cm2, resulting in a 5-fold increase in practical sensor output voltage.

GMR and magnetodynamics of MnIr spin valves depending on growth order of FM and AFM Layers

We have investigated the influence of deposition order of spin-valve (SV) layers on the giant magnetoresistance and magnetic domain structures of top-deposited Si-Ta-Co-Cu-Co-Ir/sub 20/Mn/sub 80/-Ta (TSV) and bottom-deposited Si-Ta-Ir/sub 20/Mn/sub 80/-Co-Cu-Co-Ta (BSV). The TSV has a maximum MR ratio of about 9.6%, whereas that of the BSV is about 3%. Magnetooptical imaging revealed that the magnetization reversal of both samples occurs by nucleation, extension, and annihilation of microsize domains. It was found that, unlike the TSV, the BSV changes its unidirectional anisotropy in an applied magnetic field. The difference in the domain behavior and in the GMR of the spin valves with top and bottom antiferromagnetic layer deposition could be attributed to differences in defect structures.

Giant magnetoresistive structures based on CrO2 with epitaxial RuO2 as the spacer layer

Epitaxial ruthenium dioxide (RuO2)∕chromiumdioxide(CrO2) thin film heterostructures have been grown on (100)-TiO2 substrates by chemical vapor deposition. Both current-in-plane (CIP) and current-perpendicular-to-plane (CPP) giant magnetoresistive stacks were fabricated with either Co or another epitaxial CrO2 layer as the top electrode. The Cr2O3 barrier, which forms naturally on CrO2 surfaces, is no longer present after the RuO2 deposition, resulting in a highly conductive interface that has a resistance at least four orders of magnitude lower. However, only very limited magnetoresistance (MR) was observed. Such low MR is due to the appearance of a chemically and magnetically disordered layer at the CrO2 and RuO2 interfaces when Cr2O3 is transformed into rutile structures during its intermixing with RuO2.

Current-perpendicular-to-the-plane spin-valve films with iron-added magnetite layers

A current-perpendicular-to-the-plane (CPP)-spin-valve sensor with iron-added magnetite layers (IMLs) was investigated. We prepared bottom synthetic-pin spin-valve films with CoFe∕IML∕CoFe sandwiched layers as both free layers and pinned layers. The slope of resistance-change-area product (ΔRA) versus IML thickness is around 0.5–0.6Ωμm2∕nm, and this indicates that using IMLs results in large bulk scattering. Magnetite and iron grains were observed in transmission electron microscopy images even in a 2-nm-thick IML in a CPP giant magnetoresistance structure. As a result, ΔRA up to 2–2.5mΩμm2 and MR ratio of 2.0%–2.6% at RA of 0.08–0.1Ωμm2 were obtained by using IML in CPP-spin-valve films.

Positive/negative magnetostrictive GMR trilayer systems as strain gauges

Recently, highly sensitive strain gauges were developed, which are based on tunnel magnetoresistance (TMR) or giant magnetoresistance (GMR) effects combined with the inverse magnetostriction. GMR and TMR structures generally possess a symmetrical characteristic, which reflects the switching fields of the soft and hard layers, respectively. This characteristic can be changed by a stress field if the soft layer is replaced by a suitable magnetostrictive layer leading to a stress-induced rotation of the magnetostrictive layer with respect to the reference layer. Alternatively, both magnetic layers can be soft magnetic, one being positive and the other negative magnetostrictive. In this case, a stress applied on the stack leads to a reverse rotation of both layers due to the different sign in magnetostriction. This new approach is especially attractive since no reference layer is required which allows multilayering for GMR effect enhancement. This paper presents the stress biased characteristics of (FeCo/Cu/Ni) GMR trilayers in which the positive magnetostrictive FeCo and the negative magnetostrictive Ni replace the sensing and reference layer of a conventional GMR stack. The results can be interpreted by a simple model taking into account the magnetization direction of the individual layers and their response to mechanical strain in the range of 0.1–1‰.

Submicron giant magnetoresistive sensors for biological applications

We have fabricated submicron giant magnetoresistive (GMR) structures and evaluated their sensitivity for biomagnetic applications. GMR devices were fabricated using electron beam lithography, with minimum dimensions below 100 nm. We developed a new characterization technique for these sensors, using a scanned nanoscale magnetic probe and monitoring the resulting response of the sensors. The magnetic field from the scanned probe is similar to that generated by the magnetic particles used to tag bioanalytes. The devices demonstrated extremely high magnetic field resolution. Noise measurements, combined with a local field sensitivity from the scanned probe measurements, predict a sensitivity of 2 × 1 0 − 16 emu/Hz 1/2 for a magnetic particle 100 nm above the sensor surface. This corresponds to detection of single 100 nm commercially available magnetic labels, which are the lowest size scale of labels now used in biological studies, with a signal-to-noise of unity. Additionally, we predict detection of single 200 nm magnetic labels with a position sensitivity of 93 nm/Hz 1/2, allowing proximity detection for particles not directly bound to the sensor surface, with a corresponding signal-to-noise of 10.

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.

Switching behavior of cross contacting current perpendicular to plane giant magnetoresistance structure fabricated by two-step ion milling technique

A current perpendicular to plane giant magnetoresistance structure with a cross contact geometry was fabricated by a two-step ion-milling technique. Multilayer pillars with various layer structures and a lateral dimension down to 0.4 μm were embedded between orthogonal top and bottom electrodes. The microfabricated cross point structure minimizes the influence of electrode resistance in measurements of magnetoresistance in current perpendicular to plane geometry. Individual switching of Co layer was observed in a pillar with layer structure of [Co(10 nm)/Cu(4 nm)/Co(2 nm)/Cu(10 nm)]3 and lateral dimension of 0.4×2.0 μm2. Distinct three-step resistance change was observed for the rising part of the magnetoresistance (MR) curve, corresponding to the switching of Co(2 nm) layers. Somewhat complicated MR behavior in the falling part suggests the existence of a local minimum configuration during the switching of Co(10 nm) layers. The switching behavior of magnetic layers with interlayer dipole coupling was studied by micromagnetic simulations. Markedly different switching field of Co layers can be attributable to the variation of the dipole fields at each layer.

Spin-dependent sensors

The discovery and development of structures using the spin dependence of conduction electrons has led to products and product developments in recording heads and nonvolatile memory. It has also led to the introduction of several new products which sense magnetic fields using these new structures. This article describes giant magnetoresistance and magnetic tunnel junction structures and how they are used in several applications.

Effect of annealing and ion implantation on interlayer exchange coupling in Fe/Cr multilayers

Antiferromagnetic interlayer coupling in layered structures exhibiting giant magnetoresistance (GMR) is frequently destroyed by pinholes in the nonmagnetic layers. Therefore pinholes play a key role in limiting performance of GMR structures or magnetic tunnel junctions. We report temperature measurements of magnetization reversal in annealed and ion beam mixed [Fe-3.0 nm/Cr-1.1 nm] multilayers. Analysis of the experimental data shows that both annealing and ion mixing lead to similar subtle changes in the layered structure related to creating pinholes, changes in their density and average volume.

Giant magnetoresistance effect in Ni buffered Co/Cu/Co sandwich

The effects of Ni buffer layer on the giant magnetoresistance structure of Co/Cu/Co sandwich are investigated systematically in this paper. It is found that Ni buffer layer can induce the crystallization of the lower Ni/Co layer and produce small coercivity, thus enlarging the difference in the magnetic behavior between the two magnetic layers in the sandwich. Moreover, the use of the Ni buffer layer can also improve the interface flatness in the sandwich. All these factors enhance the sensitivity of the Ni buffered sandwich.

Micro-sensor coupling magnetostriction and magnetoresistive phenomena

Giant magnetoresistance or tunnel magnetoresistance (TMR) structures with magnetostrictive free layers are attractive approaches for the realization of novel strain sensors. It has been found that the strain response can be enhanced by using magnetostrictive materials such as Fe 50Co 50. First experiments have further revealed that the tunnel barrier of TMR structures withstands a strain level of at least 0.1%.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

Funding the Nanotech Frontier

T he burgeoning field of nanotechnology is certain to play a critically important role in science and the global economy. Neal Lane, currently Assistant to the President for Science and Technology, has stated that nanoscale science and engineering are the areas most likely to produce the breakthroughs of tomorrow. Thus, it is important that a new national nanotechnology initiative, proposed by President Clinton with a budget of $500 million for fiscal year 2001, be implemented. Nanotechnological devices have features with dimensions that range from about 10−9 to 10−7 meters [1 to 100 nanometers (nm)]. Because atoms or molecules at surfaces are often reactive, the behavior of nanostructures, which typically have high ratios of surface area to volume, is expected to differ from that of ordinary materials. For example, recent Japanese research involving nanoscale gold particles has demonstrated that they have potential practical applications. Gold particles that are 3 to 5 nm in size catalyze the oxidation of carbon monoxide at temperatures as low as -70\_C. Gold nanoparticles are highly selective in partial oxidation reactions and show 100% selectivity in converting propylene to propylene oxide at 50\_C. An existing commercial example of nanostructure catalysis is the synthesis and use of a series of nanoporous zeolites by the Mobil Corporation. Their zeolites play a substantial role in the global $210 billion catalyst market. Evidence is mounting that materials science and technology will use multicomponent nanostructures often in the future. The giant magnetoresistance (GMR) effect is a commercialized example. GMR structures consist of alternating nanolayers of magnetic and nonmagnetic metal films. The transport of spin-polarized electrons that occurs between the magnetic layers is responsible for the ability of the structures to sense magnetic fields such as those created by magnetic bits stored on computer disks. GMR structures are currently revolutionizing the hard disk drive magnetic storage industry, which is worth $30 billion to $40 billion per year. A future application of GMR is nonvolatile random access memory that will compete in a $100 billion worldwide market. The mechanical behavior of deliberately created nanostructures is now being investigated. Some of the materials are prepared by a gas condensation method. Among these products have been pure metals with high hardness and strength and a grain size of about 10 nm. Their hardness and strength are two to seven times greater than those of larger-grained metals. An ongoing goal is to create nanoscale building blocks with precisely controlled size and composition and assemble them into larger useful structures. It should be obvious from this necessarily limited picture of nanotechnology that additional R&D is needed to create more ideas and possibilities for applications. Some of it will take place elsewhere. The Japanese are leaders in creating nanodevices and consolidated nanostructures. Europe is strong in new instrumentation, dispersions, and coatings. The United States is active in the synthesis and assembly of nanostructures to form materials with large surface areas, and in the creation of dispersions and coatings. President Clinton's proposed $500 million budget for a national nanotechnology initiative has been accompanied by the release of federal reports and recommendations. [*][1] Some of the latter are directed toward universities. In order to speed the development of U.S. nanotechnology, an interagency working group has recommended that universities involve multiple departments in interdisciplinary work, foster on-campus nanotechnology centers for greater interactions, introduce nanoscience and engineering in existing and new courses, and establish graduate and postdoctoral fellowships for interdisciplinary work. However, implementation of these recommendations will be expedited only when they are accompanied by money. The past history of U.S. federal support for nanotechnology has not been impressive. In fiscal 1997, U.S. government funds totaled $116 million; the comparable number for Japan was $120 million, and for Western Europe, $128 million. In these countries as well as in the United States, industry is also supporting R&D at levels comparable to or greater than that of government. In light of the opportunities offered by nanoscience and engineering and the realities of global competition, the proposed appropriation of $500 million for fiscal 2001 should be accorded a high national priority. [1]: #fn-1

Ferromagnetic resonance as a probe of anisotropy in giant magnetoresistance structures

The ferromagnetic resonance spectra of thin alloy films, which are often used as components of giant magnetoresistance structures, were observed. Face-centered-cubic films with Cu underlayers and single magnetic layers of permalloy, permalloy containing Co; or Co rich CoFe alloys were deposited by magnetron sputtering. The magnetic field position of the resonance provides information about the magnetic moment and the growth induced anisotropies of the films, and was affected by magnetostriction when stresses were induced. Linewidths and coercivity increased rapidly as a function of the total (Cu+magnet) layer thickness above about 20 nm. The linewidth is sensitive to the damping parameter for the thinner films and to magnetocrystalline anisotropy and crystallographic texture for the thicker Co containing films in which the grains become large. Based on observations of the resonances in stressed films, several nonmagnetostrictive compositions were determined.

Mössbauer study of the influence of thermal treatment on giant magnetoresistance and interface structure in Fe/Cr multilayers

The dependence of the giant magnetoresistance effect (GMR) on the interface structure in Fe/Cr multilayers was studied by magnetoresistivity and Mössbauer spectroscopy. The Fe/Cr multilayers consisting of Fe(6 nm)+[Cr(1.1 nm)/Fe(3 nm)]60+Cr(1.1 nm) were deposited by dc magnetron sputtering. Samples were annealed for 1 h at temperatures ranging from 200 to 500 °C. The interface structure was characterized by conversion electron Mössbauer spectroscopy (CEMS). Various different Fe sites: the bulk, step and perfect interface positions were identified. The evolution of the fraction of Fe atoms in different environments vs annealing temperature revealed that annealing at 300 °C induces bulk and in-plane diffusion of atoms. Higher annealing temperature causes substantial roughening of the interface related to a strong bulk diffusion of atoms. Microstructural changes observed in the CEMS spectra correspond well to the GMR measurements which reveal an enhancement of magnetoresistivity in Fe/Cr multilayers annealed at about 300 °C and its decrease due to deterioration of the interface at higher annealing temperatures.

Temperature effects on the switching of small ultrathin magnetic structures

Mechanisms for the switching of the magnetization in ultrathin small structure are summarized. The temperature dependence of the switching field for different sizes and anisotropies is studied and found to be significant. Much of the current interpretations of finite temperature experimental results on giant magnetoresistance and tunnel structures in terms of zero temperature micromagnetics calculations have not considered this problem.

In-situ Lorentz microscopy studies of spin-valve structures

Lorentz electron microscopy, including a quantitative magnetisation-mapping technique, has been used to study the local magnetisation reversal mechanism in active spin-valve devices for correlation with the giant magnetoresistance and domain structure. Varying various parameters such as device size, applied current value and direction of easy-axis have been found to have an effect on the magnetisation reversal mechanism and on the GMR curve.

Magnetic field sensing utilizing GMR materials

The Giant MagnetoResistance (GMR) effect, discovered in France in 1988, has already been applied in magnetic sensors and has promise in other applications. The rapid acceptance of this technology is due to GMR’s unique characteristics such as high sensitivity, good temperature stability, and excellent linearity over a wide sensing range. In this article GMR materials are described as are their application in magnetic field sensors. New GMR structures utilizing spin valves and spin dependent tunneling (SDT) will offer even more potential for expanding the horizon of solid state magnetic sensing. Comparisons are made to sensors using conventional technology. Integrated GMR sensors that have signal conditioning and output electronics monolithically integrated with the sensor offer further uses of this new technology. Beyond the sensor itself, other control system functions have the potential for using the same GMR materials to make magnetic isolators and nonvolatile memories.

Metal Organic CVD of Cobalt Thin Films Using Cobalt Tricarbonyl Nitrosyl

The combination of Cu and Co in giant magnetoresistance (GMR) structures is something of great interest. Here the same precursor is used as in the previous paper, but for deposition of Co rather than an oxide. Shiny, adherent films can be formed on a range of substrates. SEM analysis shows that the layers are polycrystalline with equiaxial grains (see Figure). Resistivity is reported to be comparable with that of bulk Co.