Spin-valve Films Research Articles

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.

Sub-100-nm Current-Perpendicular-to-Plane Sensor Fabrication

Two sets of processes for sub-100-nm current-perpendicular-to-plane (CPP) spin-valves (SVs) were developed. In the first method, a CPP SV was fabricated by using via-hole instead of small cell patterning. This method makes it easy to characterize the CPP spin valve films. The shunting effect has been analyzed in this method. In the second method, a resist layer for planarization, ion-milling, and wet etching for self-opening via-hole in a self-aligned fashion by making use of the height difference between the cell and surrounding regions was introduced. This method is suitable for the actual recording heads with small dimension even without an ion-beam-deposition system. CPP sensors with size from 0.05 /spl mu/m/spl middot/0.05 /spl mu/m to 0.4 /spl mu/m/spl middot/ 0.5 /spl mu/m with different free- layer structures have been investigated by means of the first method.

Effects of underlayer on one-directional anisotropy in spin-valve films without any antiferromagnetic layers

The magnetoresistive (MR) characteristics of spin-valve films with no antiferromagnetic layer were investigated. The films were deposited with various underlayer materials and the one-directional anisotropy in MR curves of the film with Ta underlayer turned out very strong anisotropy compared with that of other materials. It seemed that the strong one-directional anisotropy was caused by increase of coercivity and magnetic anisotropy induced in the pinned layer by external dc magnetic field during deposition. The calculated MR curves by using a torque equation clarified a relation between the magnetic characteristics of the pinned layer and the one-directional anisotropy in measured MR curves.

Magnetoresistance of antiferromagnetic Ir22Mn78-pinned spin filter specular spin valves

Specular spin valves (SSVs) having the spin filter layer (SFL) in contact with the ultrathin free layer of composition Ta3/NiFe2/IrMn7/CoFe1/(NOL1)/CoFe2/Cu1.8/CoFe(tF)/Cu(tSF)/(NOL2)/Ta3.5 (in nm) deposited by magnetron sputtering were studied. For these antiferromagnetic Ir22Mn78-pinned spin filter specular spin valve (SFSSV) films, an optimal magnetoresistance (MR) ratio of 11.9% was obtained when both the free layer thickness (tF) and the SFL thickness (tSF) were 1.5 nm, and a MR ratio higher than 11% was maintained even when tF was reduced to 1.0 nm. This was due to an increase of specular electrons by the nano-oxide layer (NOL) and of current shunting through the SFL. Moreover, the interlayer coupling field (Hint) between the free layer and pinned layer could be explained by considering the RKKY and magnetostatic coupling. The coercivity of the free layer (Hcf) was significantly reduced as compared to traditional spin valves (TSV), and remained as low as 4 Oe when tF varied from 1 to 4 nm. It was found that the SFL made it possible to reduce the free layer thickness and enhance the MR ratio without degrading the soft magnetic property of the free layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal Stability and Domain Structure in Spin Valve Films with IrMn Exchange Biased Layers

We have investigated the magnetic domain structure and the thermal stability of magnetotransport properties of IrMn biased spin-valves containing Co, CoFe and NiFe. The magnetic domain structures were imaged using a magneto-optical indicator film(MOIF) technique. To investigate the thermal stability, magnetoresistance(MR) was measured at annealing temperature(TANN) and room temperature(<TEX>$T_{RT}$</TEX>) followed by the annealing. Domain imaging reveal that the increase of annealing temperature led to changes in the exchange coupling between the two ferromagnet(FM) layers through nonmagnetic layer rather than between FM and antiferromagnet. unlike the NiFe biased IrMn spin valve with large domains, MOIF pictures of Co and CoFe biased IrMn spin valve structures show the formation of many small microdomains. The magnetic structure, as revealed by the domain images, appeared unchanged while the MR dropped dramatically. From the combined giant magnetoresistance(GMR) and MOIF results, it was apparent that the decrease of MR ratio was not related to the spin valve magnetic structure up to about <TEX>$350^{\circ}C$</TEX>(<TEX>$T_{RT}$</TEX> ).

Readout Performance of Confined-Current-Path Current-Perpendicular-to-Plane Heads

Current-perpendicular-to-plane (CPP) spin valve (SV) films with nano-oxide layers have been developed. The enhancement of giant magnetoresistance by inserting the current confining layer between free and pinned layers was confirmed by the calculation of the current-flow distribution using a finite-element model. For the bottom-type spin valve films with the structure of PdPtMn/CoFeB/Ru/CoFeB/Cu/oxidized CoFeB(t nm)/Cu/CoFeB, MR ratio of 5.0% and RA of 0.6 /spl Omega//spl middot//spl mu/m/sup 2/ were obtained at t=1.2 nm. The merged head using the CPP-SV film with a nano-oxide layer whose MR ratio was 3.2% was fabricated. The magnetic read core width was 130 nm when the physical free-layer width was 100 nm. 0.91 mV/sub p-p/ output voltage was obtained at the sense current of 4.8 mA which corresponds to the average current density of 19 MA/cm/sup 2/.

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.

Perpendicular giant magnetoresistance and magnetic switching properties of a single spin valve with a synthetic antiferromagnet as a free layer

We study the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) and magnetic switching behavior of single spin-valve (SV) films with two different free layers---one is a single ferromagnet (FM) layer, while the other is a synthetic antiferromagnet (SyAF) consisting of ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}/{\mathrm{R}\mathrm{u}/\mathrm{C}\mathrm{o}}_{90}{\mathrm{Fe}}_{10}.$ When the interlayer Cu thickness is 2.5 nm, the SyAF as a free layer greatly enhances the CPP-GMR of SVs from 0.8% to 3.6%. The GMR enhancement effect decreases with increasing interlayer Cu thickness. We argue that the MR enhancement by the SyAF is probably because of strong reflection of the majority spins by the interface between ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$ and ruthenium. Experimental and theoretical studies of the magnetic switching behavior show that the SVs with SyAF have a much better tendency to form a single magnetic domain than the conventional ones. The single domain structure results in a size-independent magnetic switching field of the SVs with SyAF at the low aspect ratio 1.

High-sensitivity GMR with low coercivity in top-IrMn spin-valves

Top-IrMn spin-valves with a structure of Ta/NiFe/CoFe/Cu/CoFe/IrMn/Ta have been investigated. The spin-valves were deposited by high vacuum DC magnetron sputtering at room temperature. The magnetoresistance ratio reaches 9.12% at room temperature. The coercivity of the free layer and the exchange bias field is 1.04 and 180 Oe, respectively. The maximum sensitivity of the spin-valves is 8.36%/Oe. A reduction of 33.2% of the coercivity was obtained after a 2-min RIE process. Utilizing standard integrated circuit (IC) process, mass production of robust giant magnetoresistance sensors can be achieved with these spin-valve thin films.

Enhancement of current-perpendicular-to-plane giant magnetoresistance by synthetic antiferromagnet free layers in single spin-valve films

Current-perpendicular-to-plane (CPP) geometrical giant magnetoresistance (GMR) single spin-valve (SPV) structures with synthetic antiferromagnets (SyAFs) as free layers were fabricated. The sample sizes range from micron to submicron. We demonstrate that SyAFs as free layers dramatically enhances the CPP-GMR ratio from 0.83% to 3.56% at room temperature and also double the resistance-area product in single SPV films. A study of the magnetic switching behavior by both experiment and single-domain thermal activation modeling shows that the CPP-GMR SPV with SyAF structure approaches that of a single-domain magnet even with low aspect ratio of 1 as long as its size diminishes to ∼0.18 μm2. The single-domain structure brings size-independent magnetic switching field when the aspect ratio is 1. The large GMR ratio at room temperature, single-domain structure and size-independent magnetic switching field demonstrate the great potential of CPP-GMR SPVs with SyAF free layers for use in future ultrahigh density magnetic storage devices.

Comparison between semiconducting and oxide layers as a reflection layer in spin-valve films

It is well established that appropriate oxide capping is effective in forming nano-oxide layers (NOL) in spin-valve films for specular enhancement of giant magnetoresistance (GMR) effect. However, the beneficial effect of a NOL is strongly dependent on its process of formation. Therefore, we are interested to use a nano-semiconducting layer (NSL) for specular reflection instead of oxide layers because its achievement is easier since no specific growth conditions are needed. Moreover, we intend to compare the efficiency of the electronic confinement inside the spin valve induced either by NSL or NOLs for structures with the same stack. We have prepared hard-soft spin valve structures by sputtering on glass substrates with the following stacking sequence: Fe6 nm/Cu3 nm/CoFe1.8 nmRu0.8 nmCoFe3 nmCu2 nmRu2 nm. The reflecting layers have been inserted in the middle of the Fe soft layer and on the top of the spin valve. The GMR effect is enhanced by 60% and 75% respectively for the NSL and the NOL. This shows that the NOL is more efficient in term of electronic confinement. To understand the origin of the difference between the NOL and NSL magnetization measurements as well as transmission electron microscopy are presented.

High-frequency measurements of spin-valve films and devices (invited)

High-frequency measurements of spin-valve films and devices, made using several different measurement techniques, are presented and compared. Pulsed inductive measurements were made on sheet films and provide insight into the intrinsic dynamical properties of the component films and multilayer stacks. The damping parameter, in the completed spin-valve stack, is larger than in the constituent films. Direct time and frequency domain measurements of the dynamical response of micrometer-size spin-valve devices, made using high-bandwidth magnetoresistance techniques, showed damping parameters comparable to these measured on spin-valve sheet films. The small-angle magnetization response was also determined by high-frequency magnetic noise measurements. The damping parameters were smaller than those obtained by direct susceptibility measurements. The device-level measurements show a different dependence of the damping parameter on the easy-axis field as compared to sheet-level measurements. In addition to the uniform rotation mode, other peaks can be observed in the noise spectra that correspond to fluctuation modes arising from the micromagnetic structure. Electrical device measurements have much greater sensitivity than other high-frequency magnetic measurement techniques, which allow the direct observation of magnetization motion in submicrometer elements without averaging. This technique is used to directly examine thermally activated events and nonrepetitive dynamical motions.

Effect of N2 addition in sputter gas on giant magnetoresistance response of PtMn bottom spin-valve films

Interlayer coupling in spin-valve films is usually dominated by the ferromagnetic Néel coupling of a magnetostatic nature due to interfacial roughness. The addition of N2 as a second sputter gas species during deposition of thin layers in PtMn bottom spin-valve films has shown effectiveness in the reduction of interfacial roughness and, therefore, control of ferromagnetic Néel coupling. The interlayer coupling field has been more effectively reduced from an original 38.8 Oe down to 6.1 Oe at a Cu spacer layer thickness of 24 Å with the addition of 4 sccm N2 gas during the Cu spacer layer deposition. In addition to higher giant magnetoresistance values over 10% at thinner Cu spacer layer thicknesses, a pronounced oscillatory dependence of the interlayer coupling field on Cu spacer layer thickness has been observed. Apparently, nitrogen serves as surfactant and helps layer-by-layer growth of Cu on a CoFe pinned layer, resulting in smoother CoFe/Cu and Cu/CoFe interfaces.

Interface and microstructure evolutions in synthetic ferrimagnet-based spin valves upon exposure to postdeposition annealing

Synthetic ferrimagnet-based spin-valve (SV) films comprising Ta 5/NiFe 2/IrMn 9/CoFe (P1) 1.5/Ru 0.7/CoFe (P2) 3/Cu 2.8/CoFe 1.6/NiFe 3.2/Ta 5 (in nanometers) were examined with an emphasis given on understanding the interface evolution of IrMn/CoFe (P1) upon postdeposition annealing. A 300 °C, 5 h annealing resulted in 12% and 42% decreases in effective pinning field and 29% and 53% decreases in MR ratio for the bottom (B) and top (T) SV, respectively. Because both structures exhibit different thermal degradation behaviors, we were curious to uncover the underlying causes by employing various analytical methods. The column sizes of the B–SV and T–SV were about 30 and 15 nm, respectively. No major difference in the (111) crystal texture was observed. We found that the main cause was due to the interlayer diffusion of atomic constituents such as Mn, most likely through grain boundaries and IrMn/CoFe interface, where the degree of the diffusion was considerably higher for the T–SV. Due to the surface free energy differences depending on the deposition sequence, two types of SV structures behaved differently upon exposure to annealing. The B–SV featuring less grain boundaries and nonabrupt IrMn/CoFe (P1) interface could effectively impede interlayer diffusion and, as a result, exhibited less thermal degradation in MR properties.

Ion-assisted oxidationによる極薄FeCo系酸化物を有するスペキュラースピンバルブ膜の高性能化

Ion-assisted oxidationによる極薄FeCo系酸化物を有するスペキュラースピンバルブ膜の高性能化

Batch-fabricated spin-injection magnetic switches

A method is developed for the fabrication of sub-100 nm current-perpendicular spin-valve junctions with low contact resistance. The approach is to use a batch-fabricated trilayer template with the junction features defined by a metal stencil layer and an undercut in the insulator. The spin-valve thin film stack is deposited afterwards into the stencil, with the insulator undercut providing the necessary magnetic isolation. Using this approach, reproducible spin-current-induced magnetic switching is demonstrated for junctions down to 50 nm×100 nm in size.

High exchange-coupling field and thermal stability of antiferromagnetic Ir-Mn pinned spin-valve films

IrMn pinned spin-valve (SV) films with stacks of Ta-NiFe-IrMn-CoFe-Cu-CoFe-NiFe-Ta were prepared by dc sputtering onto thermally oxidized Si[111] substrates at room temperature under a magnetic field of about 100 Oe. The annealing cycle number and temperature dependence of the exchange-coupling field (H/sub ex/), magnetoresistance (MR) ratio, and coercivity (H/sub c/) were investigated. By optimizing the process of deposition and the post-thermal annealing condition, we obtained the IrMn-based SV films with MR ratio of 3.6%, H/sub ex/ of 1180 Oe for the pinned layer. The H/sub ex/ is stabilized after the second annealing cycle, and it is thought that this SV reveals high thermal stability. The H/sub ex/ maintained its strength of 600 Oe in operation up to 240/spl deg/C and decreased monotonically to zero at 270/spl deg/C.

Multilayer Structures with Giant Magnetoresistance

The phenomenological description of the giant magnetoresistance effect as well as the discussion of the requirements which must be fulfilled in giant magnetoresistance thin film structures are given in the first part of our review. In the second part the magnetization reversal and giant magnetoresistance effect of antiferromagnetically coupled multilayers, spin valve and pseudo-spin valve thin film structures are explained. For these structures we also discuss the influence of the structure defects such as surface roughness and pinholes on the giant magnetoresistance effect.

Nano-oxide-layer specular spin valve heads with synthetic pinned layer: Head performance and reliability

To implement the specular nano-oxide-layer (NOL) spin valve (SV) heads for use in practical applications, it is key to simultaneously achieve a good specular effect of the NOL inserted in the synthetic ferrimagnet pinned layer (i.e., high magnetoresistance MR performance) and a strong pinning field through the NOL. By using CoFe+X as a substance to be subjected to oxidation, we obtained the NOL specular SV films simultaneously achieving a high MR ratio of 17%–18% and a high pinning field of 1100–1500 Oe. Narrow track (0.12 μm) heads were fabricated and they showed a high sensitivity of 10 mV/μm. Several reliability tests were done both at the sheet film level and the actual head level. The oxygen inside NOL was found to be stable up to 350 °C, and pinned layer magnetization canting after orthogonal field annealing was found to be almost the same as today’s non-NOL SV films. An electrostatic discharge test and accelerated lifetime test were also performed and NOL specular heads were demonstrated to have almost the same robustness as today’s non-NOL heads.

Physical properties of spin-valve films grown on naturally oxidized metal nano-oxide surfaces

The physical properties of spin-valve films NiFe 25 Å/CoFe 10 Å/Cu(tCu)/CoFe 30 Å/IrMn 70 Å/Ta 20 Å with graded Cu layer thickness (tCu=18–45 Å) grown on the surface of metal nano-oxide layers (NOLs) were studied. The NOLs were formed from ultrathin Al, Cr, Cu, Nb, Ta, CoFe, NiFe, and NiFeCr layers by natural oxidation. The growth of the spin-valve films on NOLs has led to an enhancement in giant magnetoresistance value by up to 48%. A corresponding reduction in minimum film resistance by over 10% confirms that this enhancement originates from an increase in the mean free path of spin-polarized electrons due to the resultant specular reflection at the nano-oxide surfaces. A wide spectrum of oscillatory interlayer exchange coupling dependence on tCu for these NOL-bearing films suggests that a specular nano-oxide surface does not necessarily result in a smoother multilayer structure. The observation of an enhanced exchange biasing among these spin-valve films appears in contradiction to the observed deterioration of their crystallographic quality. As an important application, TaOx, CrOx, and NbOx could be employed as an alternative to AlOx as the barrier layer for magnetic tunnel junctions.