Magnetoresistance In Spin Valves Research Articles

Electrically controlled exchange bias for spintronic applications

Exchange coupling between a magnetoelectric (111)-oriented Cr2O3 single crystal and a CoPt multilayer with perpendicular anisotropy exhibits an exchange bias field proportional to the applied axial electric field. Extrapolation from bulk to thin film magnetoelectric pinning system suggests promising spintronic applications due to coupling between the electric field-controlled magnetization and the magnetization of a neighbor ferromagnetic layer. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is an attractive alternative to current-induced magnetization switching.

Influence of the annealing field strength on exchange bias and magnetoresistance of spin valves with IrMn

We report on field annealing effects in spin valves with an IrMn pinning layer and spin valves with a synthetic antiferromagnet. The exchange bias field and magnetoresistance of spin valves with an IrMn∕CoFe bilayer at the bottom improve drastically upon annealing in large magnetic fields. The evolution of the exchange bias field with annealing field strength shows a rapid increase up to an applied field of 0.5 T, which is followed by a more gradual improvement up to an annealing field of 5.5 T. The increase of the exchange bias field in large magnetic fields indicates that the interfacial spin structure of the IrMn layer is directly influenced by the annealing field strength.

Low-current sensing with specular spin valve structures

While magnetoresistive spin valve (SV) layered structures have been widely used in both analogue and digital applications, more recent specular nano-oxide-layer spin valves (NOL-SV) have been used in read head and related digital applications. In the paper, the authors report on the design, fabrication and fundamental characterisation of NOL-SV-based magnetoresistive sensing elements for low-current monitoring purposes. The sensitivity of NOL-SV devices is improved in two ways. On the one hand, NOL-SV structures display higher magnetoresistance levels when compared with standard SVs. On the other hand, the current paths are patterned into the chip during the microelectronics fabrication process, therefore decreasing the gap space between the current paths and the sensing magnetoresistors. The authors describe in detail the fabrication process and characterise the first fabricated prototypes. Currents of the order of 1 mA have proved to be measured. Moreover, the temperature drift and the frequency response of the elements have been measured.

Magnetic biosensors for genetic screening of cystic fibrosis

The paper reports on the development of a magnetic biosensing platform for genetic analysis. The sensing technique is based on the on-chip manipulation of magnetically labelled biomolecules by the magnetic field gradient force created by current-carrying conductors; on biomolecular reactions such as the binding of complementary DNA material; and on the detection of the fringe field of the magnetic label by a magnetoresistive spin valve sensor. Different experiments will demonstrate the advantages, difficulties, prospects and potential of the combination of magnetic bead manipulation and detection on a single microchip. Early detection of DNA coding for the cystic fibrosis (CF) gene is used as a model system for proof-of-concept of the magnetic DNA biosensor platform.

Rotating-field magnetoresistance of exchange-biased spin valves

We investigate the magnetoresistance (MR) of spin valves by (i) varying the strength of the field applied in a fixed direction and (ii) rotating the field with fixed strength. The latter data reflect in general a mixture of giant and anisotropic magnetoresistance (GMR and AMR). We present an experimental procedure to suppress the AMR contributions of all ferromagnetic layers in the spin valve without disturbing the GMR response. The resulting angular MR curves are fitted with a single-domain model to determine with high precision the exchange bias field, the uniaxial anisotropies, the GMR ratio, and the interlayer coupling field. The application of the method to differently prepared Ta(5.0nm)∕NiFe(3.0nm)∕FeMn(15.5nm)∕NiFe(3.0nm)∕Co(2.0nm)∕Cu(3.5nm)∕Co(2.0nm)∕NiFe(7.0nm) spin valves with GMR ratios of 1.8% and 4% demonstrates the sensitivity and reveals differences of the order of a few percents of the exchange bias field for the uniaxial anisotropy fields of the free and pinned layer as well as for the interlayer coupling field.

CPP-GMR enhancement in spin valves using a thin Ru layer

Current perpendicular to plane giant magnetoresistance (CPP-GMR) of single spin valves with two kinds free layers has been investigated. One is a single ferromagnetic layer (Conventional structure) and the other is a ferromagnetic layer with thin Ru cap layer (Ru cap structure). Both structure had increasing resistance and resistance changes with decreasing element size. The CPP-GMR ratio for a Ru cap structure was enhanced from 0.70 to 4.3% at room temperature. We think the MR enhancement is caused by the spin dependent scattering due to strong reflection of majority spins at the Co 90Fe 10/Ru interface.

Spin transfer effects in exchange-biased spin-valves for current-perpendicular-to-plane magnetoresistive heads

Magnetoresistive heads used in computer disk drives are nowadays based on the giant magnetoresistance of spin-valves. In the present technology, the current is flowing parallel to the interfaces of the spin-valve stack (the so-called “current-in-plane geometry”). However, due to the steady increase in the storage areal density of disk drives, this technology is close to its physical limit. Consequently, more and more efforts are made aiming at using the current-perpendicular-to-plane geometry which offers larger signal amplitude and better spatial resolution. However, new physical effects appear when the current flows perpendicular to the plane in a magnetic multilayer. Of particular interest are the spin transfer effects which were predicted by Slonczewski and Berger in 1996. When they traverse the pinned layer of the spin-valve, the conduction electrons acquire a spin polarization. This spin current exerts a torque on the magnetization of the free layer which induces very peculiar dynamical phenomena. This spin torque can induce the switching of the magnetization of the free layer or the generation of magnetic excitations which contribute to the noise of the head. Therefore in the context of magnetoresistive heads, it is important to understand and master these effects which perturb the behavior of the heads. On the contrary, these effects can be used in a positive way as a new write scheme in non-volatile magnetic random access memories or to generate a steady precession of the magnetization in RF oscillators.

Exchange bias in NiFe∕FeMn∕NiFe trilayers

Ni Fe ∕ Fe Mn ∕ Ni Fe trilayer structure forms an integral part of many conventional and tunneling magnetoresistance spin valve structures with FeMn antiferromagnetic layer. A systematic investigation of the exchange bias variations of the seed and top pinned NiFe layers in the NiFe∕FeMn∕NiFe trilayer structure is reported as a function of thickness of all the three constituting layers, in multilayers prepared by rf magnetron sputtering. X-ray diffraction patterns show the (111) texture for the NiFe and FeMn layers, necessary for the development of antiferromagnetic γ-fcc phase. In thickness variation studies of all the three magnetic layers, seed NiFe layer shows greater bias (150Oe) than the top pinned NiFe layer (80Oe only). The exchange bias shows the expected 1∕t behavior for increasing NiFe layer thickness after initial maxima at low thickness. In the FeMn antiferromagnet layer thickness variation on the other hand, the large bias values attained around 5nm thickness is nearly retained up to a thickness of 25nm and the bias for the top NiFe layer is again substantially lower. The greater bias observed for the seed NiFe layer in all the three thickness variation studies is attributed to its growth over a saturated (111) oriented NiFe seed layer, which induces formation of interfacial FeMn layers with a net parallel spin ordering, in presence of the constant applied field. On the other hand, at the top FeMn∕NiFe surface, the rigid FeMn surface with compensated bulk spin ordering formed already, is not easily biased and reoriented along the top NiFe layer, to develop as much parallel net spins in the antiferromagnetic material, and hence lower bias.

Nano-oxide layer and magnetic properties in Mn–Ir–Pt-based spin valve multilayers

In giant magnetoresistance (MR) spin valve (SV), the thin nano-oxide layer (NOL) has been used for the confinement of conduction electrons, which contributes to spin scattering in film and its interfaces. In this article, we have investigated the effects of NOLs, which were fabricated by a plasma oxidation of Co–Fe layer on the magnetic properties and MR in a Mn–Ir–Pt-based SV. The NOLs were located at the middle of pinned layers and between a free and a metal-capping layer. The adjusted NOL could result in high MR and strong exchange coupling field (Hex). From a high-resolution electron microscopy analysis, the oxide was formed at about 1 nm. For SVs employing an oxide-capping layer, the strong reflectivity at the interface of a free and an oxide-capping layer should lead to the large decrease of an interlayer coupling field, which could possibly improve Hex.

Annealing temperature dependence on magnetoresistance of single and dual specular spin valve

We studied the changes of magnetoresistance ratio (MR ratio), exchange bias field ( H ex), and microstructure with annealing in conventional, specular, and dual specular spin valves. MR ratio of conventional spin valve was 7.88% and that of specular spin valve was 11.8% at their optimal annealing conditions. Conventional spin valve had less thermal stability than specular spin valve due to the diffusion mainly of Mn at between 200 and 305 °C. Exchange bias field of both samples was improved rapidly with increasing temperature. Dual specular spin valve showed higher MR ratio than single specular spin valve due to two nano-oxide layers (NOLs) in the top- and bottom-pinned layers.

Optimization of Nano-Oxide Layer in CIP Spin Valves

The influence of nano-oxide layer (NOL) on interlayer coupling (Hin) and magnetoresistance (MR) of bottom spin-valves (SV) were studied by inserting NOL at four positions: 1) before the Cu spacer; 2) in the middle of the Cu spacer; 3) after the Cu spacer; and 4) after the Cu cap layer. The NOL smoothens the surface of the interface at position 1), 3), and 4), except for 2). Surface flatten by NOL is associated with CoFeOx. Ten percent of MR enhancement can be achieved with NOL at the position 3) and 4). Weak antiferromagnetic interlayer coupling was observed with NOL at position 3). A 27.2% MR ratio has been reached in dual spin valve with two NOLs in two pinned layers.

MR Ratio Enhancement by NOL Current-Confined-Path Structures in CPP Spin Valves

We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 m/spl Omega//spl mu/m/sup 2/, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 m/spl Omega//spl mu/m/sup 2/. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 m/spl Omega//spl mu/m/sup 2/. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording.

Magnetoresistance and thermal stability enhancement in FeCr-based spin valves

In a synthetic spin valve (SV), a thin FeCr layer inserted in the pinned layer closer to the antiferromagnetic layer was found to improve the magnetoresistance, the pinning field strength, and thermal stability. Single SVs with 1-nm-thick FeCr inserted in the pinned layer shows a magnetoresistance ratio (MR) of 18% and a pinned field of about 1700Oe. This letter reports on the high MR in an all-metal single SV without specular scattering using a nano-oxide layer or spin filtering. This result, suggests that FeCr with high resistance and low spin polarization is effective in increasing the spin scattering and maximize the contribution of the pinned layer closer to spacer on the magnetotransport while keeping its antiferromagnetic exchange coupling high. The FeCr thin layer may also have an effect on minimizing Mn diffusion into the pinned layer and spacer as revealed by the increase of the pinning field.

A Novel Spin-Valve Bridge Sensor for Current Sensing

Magnetic field measurement constitutes a good method to know some magnitudes related to it. In this work, we present a novel spin-valve bridge magnetoresistance sensor and the design of an instrumentation system with the aim to characterize several properties of it. Experimental results derived from the characterization process are shown, and a novel measurement and control application of these sensors is presented.

Current-induced magnetization dynamics in current perpendicular to the plane spin valves

We observe magnetization dynamics induced by spin momentum transfer in the noise spectra of current perpendicular to the plane giant magnetoresistance spin valves. The dynamics are observable only for those combinations of current direction and magnetic configuration in which spin transfer acts to reorient the free layer magnetization away from the direction set by the net magnetic field. Detailed measurements as a function of magnetic configuration reveal an evolution of the noise spectra, going from a spectrum with a well-defined noise peak when the free layer is roughly collinear with the pinned layer to a spectrum dominated by 1/f noise when the free layer is in an orthogonal configuration. Finally, the amplitude of the corresponding resistance noise increases rapidly with increasing current until it saturates at a value that is a substantial fraction of the magnetoresistance between parallel and antiparallel states.

Giant magnetoresistance in organic spin-valves.

A spin valve is a layered structure of magnetic and non-magnetic (spacer) materials whose electrical resistance depends on the spin state of electrons passing through the device and so can be controlled by an external magnetic field. The discoveries of giant magnetoresistance and tunnelling magnetoresistance in metallic spin valves have revolutionized applications such as magnetic recording and memory, and launched the new field of spin electronics--'spintronics'. Intense research efforts are now devoted to extending these spin-dependent effects to semiconductor materials. But while there have been noteworthy advances in spin injection and detection using inorganic semiconductors, spin-valve devices with semiconducting spacers have not yet been demonstrated. pi-conjugated organic semiconductors may offer a promising alternative approach to semiconductor spintronics, by virtue of their relatively strong electron-phonon coupling and large spin coherence. Here we report the injection, transport and detection of spin-polarized carriers using an organic semiconductor as the spacer layer in a spin-valve structure, yielding low-temperature giant magnetoresistance effects as large as 40 per cent.

Microstructures of magnetic nanoscale metallic multilayer and granular alloys with GMR effects

AbstractAmong the magnetic metallic multilayer and granular alloys displaying a giant magnetoresistance effect are a wide variety of inhomogeneous magnetic nanostructures. The two characteristic length scales in these systems—spin‐dependent electron mean free path and exchange coupling—vary from a few tens of nanometres to interatomic distances. Advanced methods for the characterization of surface and interface structures have been applied: x‐ray diffraction with a synchrotron radiation source; high‐resolution electron microscopy; field ion microscopy with an atom probe, etc. Some major achievements of research performed mainly at the Institute of Physics, Beijing, are reviewed with an emphasis on the roles of the interface structure and local morphology of nanoscale structures. For example, the structure of the nano‐oxide layers and its physical mechanism in an advanced specular giant magnetoresistance spin valve are characterized. The morphology of the nanoscaled granules has been determined directly in a CuCo granular alloy with giant magnetoresistance effects. The columnar crystallites in the multilayers, which is a very general phenomenon, were confirmed. Copyright © 2004 John Wiley & Sons, Ltd.

Enhancement of magnetoresistance in spin valves with ion sputtered Si substrates

The pattern of dots with height about 1.2 nm is formed on Si substrates by ion sputtering. Top, bottom spin valves, and bottom spin valves with a nano-oxide layer are fabricated on both non-ion sputtered and ion sputtered Si substrates. Magnetoresistance ratio increase around 10% has been achieved in the spin valves on the ion sputtered substrates.

Magnetoresistance of Ir 22Mn 78-based top, bottom, and dual spin valves

The magnetoresistive properties of Co 75Fe 25/Ir 22Mn 78-based top (TSV), bottom (BSV), and dual type spin valves (DSV) by DC magnetron sputtering were investigated. The magnetoresistance (MR) ratio and exchange coupling field ( H ex) of TSV were 5.6% and 430 Oe, respectively. For BSV the H ex values (1180 Oe) is much higher while MR ratio (3.6%) is reduced with respect to TSV. The blocking temperatures ( T B) of the TSV and BSV were 250°C and 270°C, respectively. DSV, with MR ratio of 7.8% and H ex of 850/510 Oe, showed two different exchange loops.

極薄Ｒｕキャップ層を用いたスピンパルブ素子のＣＰＰ‐ＧＭＲ特性

We have studied the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) of single spin-valve (SV) films with two different free layers : one is a single ferromagnet layer (conventional), and the other is a ferromagnetic layer with a ruthenium cap layer (Ru cap). When the spacer Cu layer thickness is 2.5 nm and the Ru layer is 0.45 nm, the Ru cap-type greatly enhances the CPP-GMR, with increasing its value up to 4.3%. We argue that the MR enhancement is probably caused by the spin-dependent scattering due to the strong reflection of majority spins at the Co90Fe10/Ru interface.