Structure Magnetoresistance 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.

Application of TEM to the development of information storage materials

One of the most spectacular examples of nanomagnetic systems is that of devices based on the giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) phenomena. The device response depends critically on parameters such as layer thickness and chemical abruptness of the interfaces between layers, which are nanometre-scale in thickness. We have used high resolution electron microscopy (HREM) and TEM chemical mapping to understand the microstructural origins of the magnetic and transport properties of magnetoresistive structures. We have also used Lorentz TEM and in situ magnetising experiments to analyse their magnetic structure and magnetisation reversal processes.

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.

Thermal diffusion studies of MBE-grown ZnSe/Fe/ZnSe and ZnS/Fe/ZnS structures

This study focused on the investigation of the thermal stability of Fe on ZnSe and ZnS matrix using secondary ion mass spectroscopy (SIMS). Sandwiched three layer source structures of ZnSe/Fe/ZnSe and ZnS/Fe/ZnS were grown on GaAs and GaP substrates, respectively, by the molecular beam epitaxy technique. The bottom II–VI layers and the Fe- sandwiched layers in these structures are single crystalline while the top II–VI layers are polycrystalline. Thermal annealing was conducted in a range covering from 320 to 550 °C. The SIMS depth profiles of the as-grown and annealed structures reveal that (1 0 0) oriented single crystalline Fe/ZnSe interface is thermally stable at temperature as high as 450 °C while its polycrystalline counterpart suffers from fast diffusion even at the growth temperature. In contrast, (1 0 0) oriented polycrystalline Fe/ZnS interface is quite stable at least up to 200 °C. For both ZnSe/Fe/ZnSe and ZnS/Fe/ZnS systems, (1 1 1) oriented structures were found to have lower thermal stability than (1 0 0) oriented ones. These results provide important findings towards the optimization of Fe-based tunneling magneto-resistance structures using a II–VI semiconductor barrier.

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.

Current-perpendicular-to-the-plane giant magnetoresistance in structures with half-metal materials laminated between CoFe layers

A current-perpendicular-to-the-plane spin-valve (CPP-SV) structure having half-metal materials laminated between CoFe layers was investigated. A CPP-SV film having a CoMnGe free layer had a resistance-change-area product ΔRA of 0.9 mΩ μm2, almost the same as that of a CPP-SV film having a simple CoFe free layer. But the ΔRA of a CPP-SV film with a CoFe/CoMnGe/CoFe sandwiched free layer was 1.6 mΩ μm2. CPP-SV films having CoFe/iron-added-magnetite/CoFe sandwiched layers were also investigated and found to have a ΔRA of ≈2 mΩ μm2. We found that a CoFe/half-metal/CoFe sandwiched layer was a stronger source of spin-dependent scattering than a CoFe layer and simple half-metal layer.

Skewed ion beam etching: a simple method to obtain a reduction in critical dimensions for magnetoelectronic sensors

The skewed ion beam etching process (SIBE) was contrived as well as implemented to generate a lead-overlaid giant magnetoresistive read sensor structure in submicrometer scale. The key concept of this process is based on the shadowing effect arising from the pattern height and incident ion beam angle. Due to shadowing, the desired critical dimension such as lead-to-lead distance can be smaller compared to the distance between a set of photoresist patterns. The SIBE was performed under Ar and CHF 3 reactive environment where the etch selectivity between the photoresist and sacrificial alumina layer was high. A lead-to-lead distance of 0.35 μm was accomplished starting with a 0.9 μm photoresist pattern.

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.

Room-temperature preparation and magnetic behavior of Co2MnSi thin films

Our study presents experimental results on Co2MnSi thin-film preparation and resulting magnetic properties of the Co2MnSi Heusler alloy. The focus of our work is on the important role of the microstructure and the magnetic properties relationships of Co2MnSi thin films prepared using dc magnetron sputtering. We examined the microstructure evolution determined with x-ray diffraction for various substrates, e.g., MgO, SrTiO3, Si and SiO2, at different substrate temperatures. Polycrystalline growth observed at high substrate temperatures is independent of the nature and orientation of the substrate. These films show soft magnetic behavior at a net magnetization of 4.12μB. In contrast, textured growth is obtained at room temperature by introducing a vanadium seed layer. These samples are magnetically harder but possess a magnetization of 0.25μB only. This behavior indicates a two phase film consisting of an amorphous and textured volume. Consequently, sputtering at low argon pressure at high temperature result in very smooth Co2MnSi Heusler films, enabling the Co2MnSi Heusler alloys to serve as electrodes in tunnel magnetoresistance structures.

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.

Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB.

Spin-valve magnetoresistive structures based on Co/Tb multilayer films

The effect of the layer thickness on the magnetic properties of {Co/Tb}n, {Co/Tb}n/Co, and {Co/Tb}n/Co/Cu/Co multilayer films is studied. The dependence of the hysteresis and magnetoresistive properties of {Co(1 nm)/Tb(1 nm)}n/Co(5 nm)/Cu(LCu)/Co(5 nm) structures on the thickness of the {Co/Tb}n layer and copper spacing are obtained. The feasibility of spin-valve structures based on {Co/Tb}n multilayer films with in-plane anisotropy is demonstrated.

Magnetotransport of a quasi-three-dimensional electron gas in the lowest Landau level

We have observed features in the magnetoresistance of a wide parabolic quantum well in the presence of the in-plane magnetic field at field three times larger than fundamental field corresponding to the depopulation of the last Landau level. The magnetoresistance structures shift with a specific sample parameter, such as potential width. We suggested the formation of correlated states of a three-dimensional electron gas at Landau filling factor 1/3, in analogy with a two-dimensional fractional quantum Hall effect.

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.

Magnetoresistive Structures for Artificial Neurons

Principles of operation, structure, and theoretical characteristics of an artificial neuron without activating function, which consists of series-connected memory elements based on multilayer thin-film anisotropic, spin-valve, and spin-tunneling magnetoresistive structures, are investigated. Examples of such neurons based on memory elements with optimal parameters are given. Prospects for three types of neurons are outlined.

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%.

Integrated magnetic field sensor based on magnetoresistive spin valve structures

Abstract A new integrated magnetic sensor, which is based on a micro-patterned spin valve bridge design, is described in this paper. The main advantage of the design is that an adjacent soft magnetic layer guides the magnetic flux, allowing a balanced bridge sensor to be sensitive to the direction of the external magnetic field. This new design avoids some of the additional circuitry and processing steps required in previous spin valve-based field sensors. We report a significant increase in the device sensitivity (∼15 mV/(VxOe)), even in non-optimised prototype devices compared to other commercially available lake NVE (15 mV/(V·Oe)).

Co/Cu spin valves electrodeposited on Si

Co/Cu spin-valve structures have been electrodeposited from a single bath directly onto n-type (1 0 0) Si substrates. The structures were based on the fact that Co layers on Si show a dependence of coercive field on layer thickness. By sandwiching a stack of 3, 5 or 8 hard antiferromagnetic-coupled Co/Cu multilayers between two soft Co layers, it was possible to obtain low-field-sensitive magnetoresistive structures, showing MR ratios ranging from 5.9% to 8.6%, as well as field sensitivities in the range of 0.10% Oe at 15–40 Oe. Samples with 8.6% MR ratio were obtained by stacking up to 10 magnetic layers.