Dual Spin Valves Research Articles

All-Organic Dual Spin Valves with Well-Resolved Four Resistive-States.

The formation of all-organic dual spin valves (DSVs) with three organic spin-selective layers, that is, spin-injection, spin-detection, and an additional spin-filtering layer at the intermediate, is reported. As spin-selective layers, manganese- and cobalt phthalocyanines, which are well-known single-molecule magnets, are used in their immobilized forms, so that all-organic DSVs can be prefabricated for characterization. The three spin-selective layers have provided four configurations with at most two spin-flip interfaces enforcing spin-flipping at the two nonmagnetic organic spacer layers, for which copper phthalocyanine is used. Since a couple of the four configurations have exhibited similar resistivities, the degeneracy in the resistive-states is broken through asymmetric spin-injection and spin-detection layers and also through asymmetric thickness of the nonmagnetic spacer layers. When both the spin-flip interfaces are made operative independently, a 2-bit logic with four distinct resistive states can be achieved.

Stretchable Spin Valve with Stable Magnetic Field Sensitivity by Ribbon-Patterned Periodic Wrinkles.

A strain-relief structure by combining the strain-engineered periodic wrinkles and the parallel ribbons was employed to fabricate flexible dual spin valves onto PDMS substrates in a direct sputtering method. The strain-relief structure can accommodate the biaxial strain accompanying with stretching operation (the uniaxial applied tensile strain and the induced transverse compressive strain due to the Poisson effect), thus significantly reducing the influence of the residual strain on the giant magnetoresistance (GMR) performance. The fabricated GMR dual spin-valve sensor exhibits the nearly unchanged MR ratio of 9.9%, magnetic field sensitivity up to 0.69%/Oe, and zero-field resistance in a wide range of stretching strain, making it promising for applications on a conformal shape or a movement part.

Enhanced stability against spin torque noise in current perpendicular to the plane self-biased differential dual spin valves

We present a detailed study of spin-transfer torque induced noise in self-biased differential dual spin valves (DDSV) which could be potentially used as magnetic read-heads for hard-disk drives. Micromagnetics studies of DDSV were performed in all the major magnetic configurations experienced by read-heads and we show that in every case, self-biased DDSV provide a much stronger stability against spin-transfer torque noise than conventional spin valves. Provided are also insights on the influence of the dipolar interlayer coupling, shape anisotropy, exchange bias and relative orientation between the 2 free layers. Our results demonstrate the viability of DDSV read-heads for future hard disk drives generations.

Gap Layer Effect on Performances of Differential Dual Spin Valve

Several materials (Ta, Cu, Al, Ru) are investigated for the gap layer (GL) in a differential dual spin valve (DDSV) as a function of GL thickness in terms of the differential effect and interlayer coupling. High field measurements in current in the plane geometry show that similar GMR effect can be obtained for two spin valves (SVs) with Ta, Ru, and Cu GLs, while for Al GL, GMR ratio of the upper SV decreases as Al thickness increases. For Ta GL, the two free layers (FLs) may switch either simultaneously or separately, depending on which FL switches first due to the weak ferromagnetic (FM) Neel coupling between the FLs. For Ru GL, the switching behavior depends on the thickness of Ru, due to the oscillation of interlayer exchange coupling. Cu GL causes additional GMR resulted from the FL/GL/FL structure. Detailed investigation on thickness dependence of Ru GL shows that FM interlayer coupling is achieved with Ru thickness of 1.6, 2.4, and 4 nm. The good differential effect can be obtained till Ru = 8 nm, above which MR of the upper SV decreases. Patterned DDSV sensors with Ru GL = 2.4 nm shows an overall FM coupling between the FLs, implying FM exchange coupling is larger than the antiferromagnetic magnetostatic coupling at edges.

Transverse spin penetration length in metallic spin valves

A semiclassical description of diffusive spin transport in spin valves, which takes into account the transverse components of spin accumulation, is used to calculate the second harmonic voltage response to a low-frequency current. The description is applied to single as well as dual spin valves, with the magnetic moment of the sensing layer slightly tilted out of the equilibrium position by an in-plane external magnetic field. In the case of double spin valves, only the antiparallel configuration is considered since the spin torque in this configuration is enhanced, while in the parallel configuration it is significantly reduced. In both cases considered, the second harmonic voltage response and the relevant magnetoresistance are shown to be significantly dependent on the transverse spin penetration length.

The Respective Effects of the Free Layer Thickness and Width on the Downtrack and Crosstrack Responses of Read Heads

Miniaturization is very important in magnetic recording. Here, we have micromagnetically evaluated the respective effects of the free layer (FL) thickness and width on the downtrack and crosstrack responses for single spin valves (SSVs) and differential dual spin valves (DDSVs). Our micromagnetic results show that the linear resolution scales down with the free layer thickness for DDSVs, as is the case for SSVs found by the reciprocity theory. We have also found that the crosstrack resolution scales down with free layer width in both SSVs and DDSVs (correspondingly, the side reading increases, which needs to be mitigated). The corresponding scalings of the signal amplitudes with FL thickness and width in SSVs and DDSVs are also discussed in terms of the shape anisotropy of the free layers. Finally, we have found that side shields can provide bias fields for the free layers and, meanwhile, the improvement in track density and the reduction in side reading are not compromised; this bias point of view of side shields is an additional appreciation of the usual roles played by side shields.

Band-structure-dependent nonlinear giant magnetoresistance in Ni1−xFexdual spin valves

Conventional giant magnetoresistance (GMR) in spin valves is current-independent, so the resistance of a device depends only on the relative orientation of the magnetic layers. In dual spin valves consisting of three ferromagnetic (FM) layers separated by nonmagnetic (NM) spacers (i.e., a FM1/NM/FM2/NM/FM1), GMR can be current-dependent if spin can accumulate in FM2 when outer FM1 layers are aligned antiparallel. Currently the underlying physics is poorly understood, although spin accumulation in FM2 is likely to depend on the gradient in the density of states at the Fermi energy of the ferromagnet. To investigate this hypothesis, we have measured a series of dual spin valves with Ni${}_{1\ensuremath{-}x}$Fe${}_{x}$ as FM2 layers of varying composition. We show that both the magnitude and sign of the nonlinear GMR depend strongly on the Fe content and thus on the band structure of the ferromagnet FM2.

Domain wall configuration and magneto-transport properties in dual spin-valve with nanoconstriction

We investigated the effect of external field on magneto-transport properties of synthetic antiferromagnet dual spin-valve with nanoconstriction with focus on domain wall (DW) configuration and magnetization reversal process. As magnetic field rotated from in-plane to out-of-plane along hard axis configuration, the magnetization reversal mode changed from a vortex to a transverse type, and a multistep switching process appeared due to the development of a transverse magnetization reversal mode with DW pushing towards the higher anisotropy region. The difference in the shape of nanoconstriction made an asymmetric energy barrier to the DW propagation which resulted in an asymmetry depinning field.

Self-Biased Differential Dual Spin Valve Readers for Future Magnetic Recording

For future magnetic recording, one challenge is that the shield to shield spacing (SSS) of readers cannot be scaled down to achieve the linear density requirement. A differential dual spin valve (DDSV) has been proposed to improve the linear resolution as it does not rely on the magnetic shields to diminish the interference from adjacent bits. The side reading becomes more and more challenging as the track width shrinks to below SSS to accommodate high track density, in particular for a DDSV reader in which no magnetic shield or larger SSS is used. A self-biased DDSV structure is proposed to replace the conventional abutted junction stabilization scheme. In the self-biased DDSV readers, two side shields are put at the both sides of the sensor across the track direction in the replacement of the hard bias (HB). The stray fields from the two free layers in the DDSV sensor bias each other to stabilize the domain structure of the free layers. Preliminary analysis shows that the self-bias DDSV is also robust against the spin torque-induced magnetic instability. Simulation results indicate that the self-biased DDSV reader has much better reading sensitivity for the opposing fields generated by magnetic transitions than HB-stabilized sensor. It is found that for DDSV readers, the mag-noise is less significant due to a larger signal field and thicker free layer. Current perpendicular to the plane (CPP)-DDSV sensors have been fabricated and show very good differential effect in the real operating mode. The magnetoresistance performance of individual spin valve in CPP-DDSV sensors has been obtained through measurements with applied field and pinning field parallel to the easy axis of the free layer.

Effect of interlayer coupling on the reversal process of differential dual spin valves

Differential dual spin valve (DDSV) is a potential read sensor for ultrahigh density magnetic recording. Recently, it has been found that self-biased DDSVs in which the two free layers (FLs) align in flux closure configuration could potentially remove permanent hard bias. In the present work, we have carried out a systematic study of the effect of the interlayer coupling through gap layer on the reversal process of a self-biased DDSV. From the uniform field response of the DDSV, it was found that the competition among magnetostatic coupling, interlayer coupling and the shape anisotropy of the FLs gives rise to distinctive reversal behaviors, and the magnetostatic coupling could not be perfectly compensated by the interlayer exchange coupling. The down track response of the DDSV reveals that interlayer coupling could play a dominant role on the performance of the DDSV.

Controllable spin-transfer torque on an antiferromagnet in a dual spin-valve

We consider current-induced spin-transfer torque on an antiferromagnet in a dual spin-valve setup. It is demonstrated that a net magnetization may be induced in the AFM by partially or completely aligning the sublattice magnetizations via a current-induced spin-transfer torque. This effect occurs for current densities ranging below 10$^6$ A/cm$^2$. The direction of the induced magnetization in the AFM is shown to be efficiently controlled by means of the magnetic configuration of the spin-valve setup, with the anti-parallell configuration yielding the largest spin-transfer torque. Interestingly, the magnetization switching time-scale $\tau_\text{switch}$ itself has a strong, non-monotonic dependence on the spin-valve configuration. These results may point toward new ways to incorporate AFMs in spintronic devices in order to obtain novel types of functionality.

Spin transfer torque switching for multi-bit per cell magnetic memory with perpendicular anisotropy

A novel multi-bit dual pseudo spin valve with perpendicular magnetic anisotropy is investigated for spin transfer torque (STT) switching. The structure consists of two free layers and one reference layer, and all are based on Co/Pd multilayer. STT switching of the multi-bit device shows distinct four resistance levels. The selection of intrinsic properties of each ferromagnetic layer can be controlled for distinct separation of the resistance levels as well as the respective STT switching current. Reversible transitions between different states can be achieved by a pulsed current, in which its critical value is found to be linearly dependent on pulse duration.

Relaxed Head Media Spacing by High Resolution DDSV Reader at Multi- $\hbox{Tb/in}^2$ Areal Density

At 10 Tb/in2 areal density and bit aspect ratio (BAR) of 4, the 1 T bit length is only 4 nm. By proportional scaling, it requires the head media spacing (HMS) to be around 2 nm. In such a small spacing, it is extremely hard to squeeze in the medium overcoat and lubricant for having a reliable head disk interface. This small HMS demand is mainly determined by the readback process for the reader to have large signal intensity and high signal resolution. The differential dual spin valve (DDSV) reader incorporates a metallic gap layer between the free layers of two spin valves. The opposite pinned reference layers generate the differential operation and reproduce the pulse signal from the transition of perpendicular magnetizations. With the readback resolution defined by the thicknesses of gap layer and two free layers, the DDSV reader gains more resolution than the conventional reader. Unlike conventional reader also using two shields to define signal resolution, the shields of DDSV reader is only for the elimination of inter-symbol interference. This allows the shield to shield spacing of DDSV reader to be much larger, which has smaller shunting effect and promotes the signal strength. With stronger signal intensity and higher readback resolution, the DDSV reader is able to reproduce good quality signal at much higher HMS than the conventional reader.

Spin transfer torque enhancement in dual spin valves in the ballistic regime

The spin transfer torque in an all-metal dual spin valve, in which two antiparallel-aligned pinned ferromagnetic layers are on the two sides of a free ferromagnetic layer with two thin normal metal spacers in between, is studied in the ballistic regime. It is argued that, similar to the results in the diffusive regime, the spin transfer torque is dramatically enhanced in comparison to that in a conventional spin valve in the ballistic regime. Within the Slonczewski approach, an analytical expression of the torque on the free magnetic layer is obtained, which may serve as a theoretical model for the micromagnetic simulation of the spin dynamics in a dual spin valve. Depending on the orientation of the free layer and the degree of electron polarization, the spin transfer torque enhancement could be tenfold. The general cases when transmission and reflection probabilities of the free layer are different from zero or one are also numerically calculated.

Interlayer couplings in a differential dual spin valve

As a differential dual spin valve (DDSV) consists of two SVs separated by a gap layer, in addition to the interlayer couplings between the free layer (FL) and its reference layer in each SV, it is important to understand the interaction between the two FLs and its effect on reading performances. Systematical studies on FL’s switching behavior have been performed in both thin films and small devices. For thin-film samples, FL’s switching is governed by interlayer couplings while for a patterned DDSV device it is dominated by the strong magnetostatic edge coupling of the two FLs.

Development of current perpendicular to plane differential dual spin valve for ultrahigh resolution

Recently, a differential dual spin valve (DDSV) has been proposed to enhance the downtrack resolution for the application in the hard disk drive recording at 10 Tb/in2 and beyond. The following key issues for the implementation of a high-quality DDSV have been addressed. I. Formation of the antiparallel arrangement of the reference magnetization of two spin valves (SVs) with high pinning stability. II. Differential effect of a DDSV. To achieve perfect differential effect in a DDSV, not only should two SVs have the same giant magnetoresistance effect, but their free-layer (FL) magnetization should also respond to a field identically. Using 2.4 nm ruthenium as the gap layer to form a ferromagnetic interlayer coupling between FLs, a good differential effect is achieved. III. Quasistatic characterization of a DDSV. As there is no output from a DDSV under a uniform field, it would be difficult to evaluate performances of a DDSV. By varying the annealing field direction, field response of each SV as well as differential effect can be identified.

Magnetostatic coupling effect on the reversal process of differential dual spin valves

Differential dual spin valves (DDSVs) are a potential candidate for read sensors in ultrahigh density data storage. In this work, we show that the magnetostatic coupling (MC) plays a dominant role in the reversal process of DDSVs at a recording density of 10 Tb/in.2 It has been demonstrated that the conventional bias scheme using a pair of permanent magnet abutted junctions is not suitable in DDSVs due to MC preferring a flux closure configuration between the two free layers (FLs). The unidirectional longitudinal bias field causes the magnetization of the FLs to vary in an asymmetrical way, resulting in undesirable fluctuation when the sensor is subject to a uniform external field. Further studies on the bias field strength revealed that the overall output signal differs in shape and fluctuation magnitude with field. In the differential field test, the MC-induced reversal process was clearly observed, which leads to synchronized magnetization reversal between the two FLs. The single-magnetization transition evaluation shows that DDSV sensors do not respond to the differential fields with a linear dependence, but have an output influenced by the absolute magnitude of the fields. The results presented here are useful for optimization of DDSV sensors for future hard disk recording.

Thickness dependence and the role of spin transfer torque in nonlinear giant magnetoresistance of permalloy dual spin valves

The recent discovery of nonlinear current-dependent magnetoresistance in dual spin valve devices [A. Aziz, O. P. Wessely, M. Ali, D. M. Edwards, C. H. Marrows, B. J. Hickey, and M. G. Blamire, Phys. Rev. Lett. 103, 237203 (2009)] opens up the possibility for distinct physics which extends the standard model of giant magnetoresistance. When the outer ferromagnetic layers of a dual spin valve are antiparallel, the resulting accumulation of spin in the middle ferromagnetic layer strongly modifies its bulk and interfacial spin asymmetry and resistance. Here, we report experimental evidence of the role of bulk spin accumulation in this nonlinear effect and show that interfacial spin accumulation alone cannot account for the observed dependence of the effect on the thickness of the middle ferromagnetic layer. It is also shown that spin torque acting on the middle ferromagnetic layer combined with the nonlinear effect might be useful in understanding the dynamical features associated with the nonlinear behavior.

Nonlinear magnetotransport in dual spin valves

Recent experimental measurements of magnetoresistance in dual spin valves [A. Aziz et al., Phys. Rev. Lett. 103, 237203 (2009)] reveal some nonlinear features of transport, which have not been observed in other systems. We propose a phenomenological model describing current-dependent resistance (and giant magnetoresistance) in double spin valves. The model is based on a modified Valet-Fert approach, and takes into account the dependence of bulk/interface resistance and bulk/interface spin asymmetry parameters for the central magnetic layer on spin accumulation, and consequently on charge current. Such a nonlinear model accounts for recent experimental observations.

Readback Resolution of Differential Dual CPP Spin Valve Reader

The readback resolution of conventional CPP spin valve reader is mainly determined by the shield to shield spacing and partially affected by the head media spacing change. As the linear density grows continuously, the bit length will be reduced to sub-10 nm or even sub-5 nm in the future. In such, the shield to shield spacing has to be below 20 nm or even 10 nm. It gradually becomes not feasible to squeeze all the functional layers used by current reader, such as: AFM layer, SAF, tunnel barrier, free layer, and capping layer, in such a small spacing. The differential dual spin valve (DDSV) read head is one of the new reader structures that can provide better readback resolution. This work studied the sensitivity function of DDSV reader without and with shields, then, identified the waveform characteristics of readback signal at different head media spacing (HMS) and gap layer (GL) thickness. The readback resolution of DDSV reader is mainly determined by the thickness of the gap layer between the two free layers. The two shields are still necessary for the DDSV reader in order to have the free layers picking up more localized media flux. The shield to shield spacing at 35 nm helps to constrain the free layers to pick up media flux mainly from ±5 nm region.