Dual Spin Valves Research Articles

Magnetoresistance effect in asymmetric dual spin-valves—a device for three-state logic

We show that for asymmetric dual spin-valves MnIr/CoFe1/Cu/Py1/Cu/Py2/CoFe2/MnIr (Py denotes Ni 80Fe 20 Permalloy alloy), the application of two different ferromagnetic materials as pinned layers can result in appearance of three well-separated stages in giant magnetoresistance (GMR) effect reflecting three different and almost independent magnetization reversal processes. For the correct choice of the constituent layer thicknesses, the GMR amplitude of the dual spin-valve can depend on the field sweep direction.

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.

Narrow Track Current-Perpendicular-To-Plane Spin Valve GMR Heads

We have investigated the performance of dual spin valve heads in the current-perpendicular-to-plane (CPP) geometry on dynamic electrical tests (DET) and quasi-static tests (QST). Shielded CPP reader and stitched pole writer composite heads were fabricated. A various physical read track width and area of the dual spin valve element were deliberately obtained in the range of 41-175 nm and 0.0034-0.022 /spl mu/m/sup 2/, respectively. Materials used for the CPP dual spin valve are Co/sub 90/Fe/sub 10/, Ni/sub 80/Fe/sub 20/, Cu, Ru, Ta, and Pt/sub 50/Mn/sub 50/, which are common in industrial current-in-plane (CIP) spin valves. The shape and smoothness of QST transfer curve, readback waveform, micro-track, and full-track profiles of the narrow read track CPP heads are essentially just the same as those of CIP spin valve. A track-average-amplitude of 0.6 mV was obtained at the bias current density of element around 100 MA/cm/sup 2/ in the CPP head with the physical read track width of 47 nm. Overall reader resistance of the CPP heads at head gimbals assembly (HGA) level was in the range of 10-35 /spl Omega/ and proved to be well appropriate.

Extension of the semiclassical theory of current-perpendicular-to-plane giant magnetoresistance including spin flip to any multilayered magnetic structures

In the framework of the Valet and Fert theory of current perpendicular to plane (CPP) giant magnetoresistance (GMR) in metallic multilayers [Valet and Fert, Phys. Rev. B 48, 7099 (1993)], the calculation of the CPP resistance and magnetoresistance has been generalized to any multilayered stacks including spin valves with synthetic free layers, laminated free and pinned layers, and dual spin valves. The theory takes into account bulk and interfacial spin-dependent scattering as well as spin flip in all layers. It also considers the effect of scattering at lateral edges of submicron multilayered pillars which can be viewed as a current in plane effect intruding on CPP transport. This latter effect plays a role when the diameter of the pillar becomes of the order of the elastic mean-free paths (i.e., below ∼30 nm). Based on the theory, a code has been developed to compute the CPP resistance and CPP magnetoresistance from the transport parameters of each material involved in the stack (spin-dependent resistivities, spin-dependent interfacial resistances, and spin-diffusion length in each layers). As examples, we compare the results of the calculations with various already published CPP experiments. In particular, we interpret experiments previously carried out on (NiFe/Cu/Co/Cu)N and (Co 6 nm/Cu/Co 1 nm/Cu)N multilayers in which the order of the layers in the stack had been shown to affect the CPP resistance and magnetoresistance, a property which could not be explained in a simple two-channel serial resistance model. We also investigate the influence of the thickness of the various layers and underline the key role of the spin diffusion length in these thickness variations. Unexpected predictions are made with this theory such as the existence of a maximum in CPP–MR as a function of the thickness of the antiferromagnetic pinned layer. This type of calculations should allow a faster optimization of CPP–GMR in metallic multilayers.

Bottom IrMn-based spin valves by using oxygen surfactant

Bottom IrMn-based spin valves through exposure of part of the CoFe pinned layer to O2 with the structure Ta30/NiFe20/IrMn70/CoFet/oxidation/CoFe(30−t)/Cu20/CoFe30/Ta35 (all thickness in angstroms) (oxidation indicates exposure to different O2 flows) were deposited by dc magnetron sputtering on thermally oxidized Si (111) substrates. Within the range of O2 flows (0.4–2.8 sccm) studied, the value of the magnetoresistance (MR) ratio is seen to be quite low at a relatively large t of >16 Å and a exchange coupling field (Hex) decreases when the O2 exposed CoFe surface is too close to IrMn. Under optimal conditions, a MR ratio of 8.6% with Hex∼275 Oe for the bottom single spin valve and a MR ratio 12.2% with Hex∼268/220 Oe for the dual spin valve are obtained. X-ray reflectivity data show smoother interfaces for the spin valves subjected to O2 exposure. The enhanced MR ratio coupled with the lower interlayer coupling field (Hint), sheet resistance, and magnetic moment of the specular spin valve can be attributed to an enhanced specularity as a result of smoother interfaces after O2 exposure.

Dual spin valves with nano-oxide layers

A novel method is proposed for increasing the giant magnetoresistance (GMR) of dual spin valves of the CoFe/Cu/CoFe/Cu/CoFe type by inserting nano-oxide layers (NOLs) into the pinned layers. Using this method, MR ratio of 23.5% was obtained, a value equal to those of specular spin valves with antiferromagnetic oxide, e.g., NiO. This method allows the selection of metallic materials for the antiferromagnetic layer. In addition, we obtained the specularity factor of upper NOLs, 0.8, and that of lower NOLs, 0.7, by calculating Boltzmann equations taking into account the roughness of each interlayer. This model shows that the MR ratio would be 27.5% for dual spin valves with ideal NOLs.

Perpendicular giant magnetoresistance of CoFeB/Cu single and dual spin-valve films

We present giant magnetoresistance measurements in the current-perpendicular-to-plane mode on single and dual spin valves whose areas are from 0.1 to 1.0 μm2. Dependence on the magnetic layer thickness and the structure was investigated. It was revealed that magnetoresistance change increases 20% by insertion of a nonmagnetic spacer layer in the free layer. The results were analyzed within the framework of the two-current series-resistor model, and proved to be consistent with the theory.

Study on requirements for shielded current perpendicular to the plane spin valve heads based on dynamic read tests

Shielded current perpendicular to the plane (CPP) spin valve heads with about 20 Gbit/in.2 dimensions were fabricated and tested both quasistatically and dynamically. Dual synthetic spin valves with 2.3 mΩ μm2 of resistivity change and area product (dRA) was used. Quasistatic tests were conducted to determine the current efficiency loss caused by the specific CPP head structure. Dynamic tests were performed to examine the flux efficiency of the readback process in the spin valve element, in order to confirm the calculation results which take into account the increase of effective anisotropy field caused by the circular sense current field. Output voltage of about 260 μV (peak to peak) was successfully obtained from the dynamic tests. Finite element modeling analysis of current distribution and temperature rise, together with degradation experiments with excessive sense current, was conducted to predict the maximum allowable current density in the 200 Gbit/in.2 dimension, where read track width and stripe height are assumed to be 100 nm. With these results, requirements of spin valve material properties (dRA, RA) and the CPP structure was predicted. Essential issues to satisfy the requirements and possible solutions are also discussed.

Barrier dependence of magnetoresistance enhancement in nano-oxide added spin valves

The effects of different types of nano-oxides on the properties of spin valves have been studied. The nano-oxides that have been investigated in this study include NiFe–O, Al–O, CoFe–O, Ta–O, and Cu–O. From the point of view of overall properties (magnetoresistance ratio, change of sheet resistance, exchange coupling field, and interlayer coupling field), CoFe–O turned out to be the most desirable oxide when being used as the bottom oxide for top spin valves. As a capping layer for bottom spin valves, however, it was found that Al–O is the best choice among those investigated. Under optimum conditions, IrMn-based dual spin valves with a magnetoresistance ratio up to 21.8% has been successfully fabricated by incorporating oxide layers in both pinned ferromagnetic layers.

The dual spin valve head for high density recording

A dual spin valve head with synthetic antiferromagnetic pinned layers, DSSV, consists of a reader and a writer. The key fundamental concept of the magnetic materials for the reader and the writer of the DSSV will be discussed in this paper. The reader consists of a sensor stripe, a pair of hard magnetic bias films for the purpose of stabilization, and a pair of current leads. The reader has a high sensor sensitivity because it allows a longer coherent spin dependent electron mean free path that enhances the giant magnetoresistance change. The sensor is a symmetrical multilayer film structure that was optimized to have a high GMR ratio. The writer is a conventional inductive head with high magnetic moment poles consisting a thin layer of laminated high moment FeRhN film material The multilayer film sturcture of the DSSV sensor, its magnetic properties, and the head performance will be discussed in detail. The possible future development will also be discussed.

The effect of annealing on the MR and exchange bias characteristics in dual spin valve with nano-oxide layer

We investigated magnetoresistance (MR) and exchange bias properties by annealing in the dual spin valve (SV) with nano-oxide layer (NOL). By analyzing effects of NOL in top and bottom pinned simple SVs, MR enhancement effect of NOL inserted in the bottom pinned layer was higher than that of NOL in the top pinned layer with annealing. By the enhanced specular scattering of electrons by NOL, the MR ratio of dual SV with NOL was increased to 15.5–15.9% with an annealing of 200–250°C. Exchange coupling constant J ex was improved rapidly as 0.13–0.16 erg/cm 2 by annealing in the bottom pinned layer, whereas the effect of annealing was not large in the top pinned layer with J ex of about 0.09–0.116 erg/cm 2.

Oxygen-enhanced IrMn spin valves deposited by ion-beam and magnetron sputtering

Enhancement of giant magnetoresistance properties of single (bottom) and dual IrMn-based spin valves through exposure of part of the CoFe pinned layer to O2 is reported. Under optimal conditions, a ΔR/R of 10.4% [Hua=460 Oe, Hf=5.1 Oe, and Hc=4.7 Oe for a free and pinned layer thickness (permalloy equivalent) of 50 Å each] for an ion beam sputtered single spin valve, and a ΔR/R of as high as 20.5% for a magnetron sputtered dual spin valve having a 30 Å thick CoFe free layer are observed, compared to a value of 6.5% and 10.6% for the corresponding spin valve without O2 exposure, respectively. Transmission electron microscopy results reveal the presence of a thin (10 Å) crystalline oxygen-containing layer near the IrMn–CoFe pinned layer interface as a result of O2 exposure. X-ray reflectivity data show smoother interfaces for the spin valves subjected to O2 exposure, consistent with the lower Hf and smaller sheet resistance observed for these samples. The enhanced ΔR/R thus can be attributed to improved growth after O2 exposure.

Band-structure calculations of specular reflection in spin valves

Band structure calculations are carried out on three types of spin-valve structures: (1) simple spin valves, (2) dual spin valves, and (3) spin valves with a synthetic antiferromagnet. The effect of specular reflection is studied by comparing the transport properties of spin valves with resistive metallic and insulating layers at the outer boundaries. In the spin valve with a synthetic antiferromagnet, an insulating layer needs to be inserted inside the reference layer to achieve similar enhancement to the giant magnetoresistance as the other two types of spin valves with insulating outer boundaries. Results are analyzed in terms of s–d scattering in the different spin channels with different boundary conditions.

Giant magnetoresistance in ion beam deposited spin-valve films with specular enhancement

Three different techniques, natural oxidation, remote plasma oxidation and low energy ion beam oxidation, have been proved to be equally effective in forming nano-oxide layers (NOLs) in spin-valve films for specular enhancement of giant magnetoresistance (GMR) effect. GMR values over 12% have been routinely obtained in spin-valve films with NOL, corresponding to a 30% specular enhancement over those without NOL. The consistency and robustness of the oxidation processes has been demonstrated by a very large GMR value ∼19% in a dual spin-valve film with the NOLs formed in both pinned layers, the oscillatory dependence of the interlayer coupling field on Cu layer thickness in specular enhanced spin-valve films and the uniform and repeatable film performance over 5 in. substrates.

スピンバルブ膜を用いたＣＰＰ（Ｃｕｒｒｅｎｔ‐ｐｅｒｐｅｎｄｉｃｕｌａｒ‐ｔｏ‐ｐｌａｎｅ）素子のＧＭＲ特性

The CPP (current-perpendicular-to-plane) element using spin-valve (SV) film is adaptable to many applications because it does not exhibit the problems of multilayer GMR, such as generation of hysteresis and difficulty in controlling the magnetic domain. We investigated the ΔRS and MR ratio of CPP elements, which are determined by the structure of the SV film and thickness of the magnetic layers. By introducing synthetic ferrimagnetic single SV to the CPP element, we kept ΔRS small (0.49 mΩμm2) while achieving an MR ratio of 15% (1.6 times the value for the CIP). With dual SV structure for the CPP element, the ΔRS and MR ratio peaked at 3.85 mΩμm2 and 40%, when the total thickness of the free side pinned layer and free layer was around 18 nm. This result was caused by increasing the number of magnetic/non-magnetic interfaces and enhancing of bulk scattering. When the total thickness of free side pinned layer and free layer exceeds 18 nm, ΔRS and MR ratio drop because of increased spin diffusion. Applying a synthetic ferrimagnetic dual SV structure would provide a ΔV of 3.2 mV with an element size of 0.01 μm2.

CoFe/IrMn exchange biased top, bottom, and dual spin valves

The present work is a comparative study of CoFe/IrMn exchange biased top, bottom, and dual spin valves. For as-deposited top spin valves the exchange bias energy Jk was determined to be 0.14 erg/cm2 for IrMn 50 Å. The exchange field (Hex) decreases rapidly upon heating, resulting in a blocking temperature (Tb) of 250 °C. For bottom IrMn based spin valves the exchange bias energy is much higher (Jk=0.28 erg/cm2) while ΔR/R (6.9%) is reduced with respect to top spin valves (>8.0%). This is interpreted as showing that Hex is dominantly dependent on fcc(111) crystalline texture while ΔR/R is sensitive to both crystalline texture and interface roughness. Dual spin valves based on the above results showed two exchange loops due to the difference in the exchange bias energy between the top and bottom pinning layers.

Sensor properties of a robust giant magnetoresistance material system at elevated temperatures

The temperature dependence of the giant magnetoresistance (GMR) ratio, resistance and exchange-biasing field for a spin valve comprising an Ir19Mn81-biased artificial antiferromagnet (AAF) has been studied up to 325 °C. Up to 200–250 °C the temperature effects are reversible, at higher temperatures gradual irreversible changes are observed, probably due to atomic diffusion. The magnetoresistance effect is even at 200 °C still higher than for anisotropic magnetoresistance sensors at room temperature. The resistance of the multilayer shows a maximum around 250 °C. We found that this is due to the peculiar behavior of Ir–Mn, which has a negative temperature coefficient of the resistance. This provides a possibility to tune the temperature coefficient for the complete multilayer by varying the thickness of the Ir–Mn layer. The relative decrease of the exchange-biasing field as a function of temperature is much smaller for spin valves with AAF than for conventional spin valves (without AAF). Furthermore, it was demonstrated that the GMR ratio can be increased to 12% at room temperature by using a dual spin valve with two AAFs.

Effects of pumping time on GMR and coercivity of RF-sputtered MRAM dual spin-valves

The effects of residual oxygen in a deposition vacuum chamber on the GMR performance of RF-sputtered MRAM dual spin-valves have been studied by varying the pumping time while maintaining a constant base pressure of 5/spl times/10/sup -7/ Torr. By increasing only the pumping time from 1.5 to 8 hours, the GMR ratio of Si/NiFe/Cu/Co/Cu/NiFe dual spin-valves increased from 1.2 to 2.9%, and coercivity of NiFe decreased from 11 to 5 Oe and coercivity of Co decreased from 80 to 15 Oe. According to depth profiles of the chemical composition using AES and XPS, the oxygen content decreased substantially with increasing pumping time. This result implies that the partial oxidation inside or at the interfaces of multilayers is a "GMR killer" which causes incomplete anti-parallel spin alignment of the dual spin valves.

Reliability characteristics of IrMn dual synthetic spin valves

Dual synthetic spin valves using IrMn as the antiferromagnetic layer have been fabricated with areal densities from 10 to 36 Gbit/in/sup 2/. Tests for the environmental corrosion resistance, thermal stability, and ESD damage thresholds show these devices are reliable and viable as high density spin valve designs.

Demonstration of 1 Gbit/sec magnetic recording using an integrated inductive write/GMR read head

1 Gbit/sec magnetic recording was demonstrated with integrated inductive write/GMR read heads on a thin film disk with a coercivity of 3200 Oe and an Mrt of 0.34 memu/cm/sup 2/ with a spinstand tester. The write heads were made with high moment materials on both P1 and P2 and a densely packed coil using conventional manufacturing processes, and the read heads were made of dual synthetic spin valve sensor used for the 36 Gbit/in/sup 2/ areal density demonstration. The interconnect between the head and pre-amplifier is a pair of twisted wires of about 2 inch length. The demonstration was done at the outer diameter of the disk rotating at 15000 RPM. With a modest 35 mA (0-P) write current, we achieved an overwrite of about 30 dB and an on track error rate of better than 10/sup -7/. BER test on a disk with a coercivity of 4000 Oe also showed very good results.