Exchange-biased Spin Valves Research Articles

Bringing flexibility to giant magnetoresistive sensors directly grown onto commercial polymeric foils

The emergence of augmented reality, robotics and point-of-care biosensors has pushed forward the frontiers of compliant sensors with mechanical resilience and capability of being arbitrarily shaped upon demand. Here, we report exchange-biased spin valve structures directly fabricated on 25 μm thick commercial polymeric substrates. Linear electrical response with low coercivity was demonstrated for sensors grown on polymers. Despite the higher roughness of polymers ≃0.75 nm, a Néel coupling field comparable to that of samples grown on conventional SiO2 substrates was shown. Nevertheless, significant changes in the linear range of polymeric samples were observed, together with changes in the shift of the transfer curve. The measurements also indicate deviations from fully orthogonal magnetization orientation achieved in patterned linear sensors. Such results were ascribed to the presence of a non-negligible magnetostrictive component, most likely due to residual mechanical stress in the sensor's free- and pinned-layers. To support the study, a macrospin model was developed, considering the magnetoelastic anisotropy, to address in particular the impact of mechanical stress on final sensor output.

Temperature-Driven Pinned Layer Magnetization Reversal in Exchange Biased Fe₃O₄/Alq₃/Co/CoO Hybrid Spin Valve

In this article, we provide an initial qualitative description of the basic magnetization reversal processes behind temperature and cooling field-dependent exchange bias effect at Co/CoO interface of Fe3O4/Alq3/Co/CoO/Au hybrid spin valve (SV) device structure. The exchange field at Co/CoO interface is observed to be inversely proportional to temperature, indicating enhanced magnetic field sensitivity of Co reverse domain nucleation at the high-temperature region. The exchange field, at constant temperature, is found to be insensitive to change in the cooling field, which may be due to efficient ferromagnetic coupling at the Co/CoO interface of the device. Sudden decrease of exchange field with training cycle number ( $n$ ) has been observed at lower values of $n$ , which may be attributed to rearrangement of Co pinned layer spin configuration toward equilibrium. Interestingly, incorporation of antiferromagnetic CoO layer has not been observed to bring any change in magnetoresistance (MR) behavior of the exchange biased SV device, which shows a two-step switching pattern of MR at 100 K.

Magnetoresistive properties of exchange biased spin valve caused by helical magnetic ordering in dysprosium layer

Spin valves containing CoFe/Dy/CoFe nanostructure as a pinned layer were prepared by magnetron sputtering. Investigations of microstructure and magnetoresistive properties were performed. The magnetoresistive properties of the spin valve were used as the instrument to study the changes in magnetic state of the dysprosium layer. The existence of noncollinear magnetic ordering in dysprosium polycrystalline nanolayer was observed. The angle between the magnetic moments in a top and bottom part of the dysprosium layer was estimated.

A flexible exchange-biased spin valve for sensing strain direction

We have demonstrated that the strain direction can be detected sensitively using an exchange-biased spin valve (SV) formed on a flexible substrate. The flexible giant magnetoresistive SV used here contains a strain-sensitive Co free layer and a strain-insensitive exchange-biased NiFe pinned layer. The exchange bias is imparted from an antiferromagnetic FeMn layer stacked on the NiFe layer. When the strain is applied in a direction deviating from the exchange bias field, the free layer magnetization rotates along this directly only, resulting in a resistance change. Compared to the previously reported flexible pseudo SV, the present device allows strain direction sensing at a much smaller strain magnitude because no external magnetic field assistance is needed to fix the pinned layer magnetization, which hampers the rotation of the free layer magnetization. The robustness of the exchange bias to the strain in the simple NiFe/FeMn bilayer structure is also discussed.

Magnetoresistive performances in exchange-biased spin valves and their roles in low-field magnetic sensing applications

Abstract The magnetoresistive properties of pinned spin valves (SV) and their roles in low-field sensing applications were characterized. The magnetoresistive parameters were extracted, including the exchange bias (Heb) field as a function of the iron content in the CoFe layer and the antiferromagnetic (AFM) thickness, the magnetoresistance (MR) ratio versus the spacer thickness, the coercivity (Hc) as a function of the seed layer, and the composite layer [NiFe/Co] used. These parameters are crucial in determining the features of the magnetic sensors. Eventually, the selected SV film structure of (Si/SiO2)/Ta(50 A)/[NiFe(30 A)/Co(15 A)]/Cu(24 A)/Co80Fe20(25 A)/IrMn(100 A)/Ta(50 A) was found significant, and the SV elements were patterned using the lithographic lift-off method with the active cell dimensions of 2 μm × 150 μm. To define a pinning axis, a cool-field anneal was applied at 250 °C for 30 min in a magnetic field of 2 kOe. A Wheatstone half bridge was engineered using two SV elements and two external resistors. The operation point of the sensor was well tuned using a tiny permanent magnet. A sensitivity of 5 V/T was observed with a linear range of ±2 mT. To demonstrate the performance of the designed sensor, a measurement of the Earth magnetic field was carried out. The engineered SV sensor finds its usefulness in low-field magnetometer and electronic compass applications.

Synthesis, Structural, Electronic, and Magnetic Properties of Cubic Perovskite La1−xBaxMnO3 (0.125 ≤ x ≤ 0.875) for Spintronic Devices

In this study, La0.5Ba0.5MnO3 has been synthesized by hydrothermal method. Crystal structure and morphology are determined by x-ray diffraction and scanning electron microscopy. It is observed that Ba-doped LaMnO3 (LMO) has the same cubic crystal structure as the host material with space group $\text {Pm}\bar {3}\mathrm {m}$ . Half of the La atoms are replaced by Ba in the composition. Further, the electronic and magnetic structures are investigated by solving the Kohn-Sham equation with the full-potential linearized augmented plane wave method (FP-LAPW), with GGA+U approximation for exchange and correlation, for Ba concentration from 12.5 to 87.5%. The system is half-metallic ferromagnetic (FM) up to 55% and then converts into anti-ferromagnetic (AFM). This happens due to the onset of super exchange on increasing Ba concentrations. It is suggested that Ba-doped LMO is the best material for the fabrication of an exchange-biased spin valve. The spacer material can be any non-magnetic cubic perovskite such as BaTiO3 while the FM and AFM layers can be made of Ba-doped LMO with different concentrations of Ba.

59Co NMR analysis of CoFeB-MgO based magnetic tunnel junctions

59Co Nuclear Magnetic Resonance (NMR) and X-Ray Diffraction (XRD) has been used to investigate annealing effects on crystallographic structure and magnetic properties of ferromagnetic CoFeB layer in pseudo spin valves (P-SV): Ta/CoFeB/MgO/CoFeB and in an exchange bias spin valves (EB-SV): PtMn/CoFe/Ru/CoFeB/MgO/CoFeB magnetic tunnel junctions. It was found, that the as-deposited CoFeB layers in the P-SV and EB-SV are amorphous. The annealing treatment at 340 °C and 360 °C induces in all studied systems progressive phase segregation in CoFeB layer to areas of bcc CoFe regions displaying different degree of B2 order and to areas of an amorphous magnetic reminder with a higher B content. In the CoFeB layers grown on Ru (EB-SV) and Ta (P-SV) and covered by MgO in the upper interface, after annealing at 500 °C a state of substantially full phase segregation to B2 ordered bcc CoFe phase has been observed. The annealing at 500 °C of the CoFeB layers grown on MgO and covered by Ru in P-SV and EB-SV triggers extensive thermal diffusion and induces mixing at the CoFeB/Ru interface which transforms the CoFeB into a non-magnetic layer. Moreover, it has been found that CoFeB crystallizes better into the bcc CoFe (100) texture when deposited on Ta than on a Ru layer. XRD profiles show that while CoFeB layer is growing on Ru it crystallizes into bcc CoFe (110) oriented texture, and the one grown on Ta crystallizes into bcc CoFe (200) when annealed at 500 °C. 59Co NMR restoring field analysis (Hrest - magnetic stiffness) showed that the insertion of the antiferromagnetic PtMn layer into the EB-SV system increases the magnetic hardness of the CoFeB layer by an order of the magnitude. However, adding CoFeB/MgO layers between PtMn/CoFe/Ru and CoFeB layers in the EB-SV decouples the CoFeB from the hard-magnetic system making it magnetically soft.

Magnetization reversal and inverted magnetoresistance of exchange-biased spin valves with a gadolinium layer

FeMn-based spin valves with a gadolinium layer have been fabricated by magnetron sputtering. The magnetoresistive properties of the spin valves have been investigated at temperatures of 80–293 K. Temperature-induced switching between low- and high-resistance magnetic states has been revealed. Realization of the low- or high-resistance states depends on which magnetic moment dominates in the exchange-coupled Gd/CoFe, of Gd or CoFe. It has been shown that the switching temperature depends on the thickness of the gadolinium layer.

Current-perpendicular-to-the-plane giant magnetoresistance in spin-valves with AgSn alloy spacers

We investigate the use of AgSn alloys as the spacer layer in current-perpendicular-to-the-plane magnetoresistance devices. Alloying with Sn increases resistivity but results in a reasonably long (>10 nm) spin-diffusion length, so large magnetoresistance can be achieved with thin AgSn spacers. Compared to Ag thin films, AgSn forms smaller grain sizes, reduced roughness, and exhibits less interdiffusion upon annealing, resulting in decreased interlayer magnetic coupling in exchange biased spin-valves. AgSn also shows improved corrosion resistance compared to Ag, which is advantageous for nanofabrication, including magnetic recording head sensors. Combining a AgSn spacer with Co-based Heusler alloy ferromagnet in an exchange biased, polycrystalline trilayer thinner than 12 nm results in magnetoresistance values up to 15% at room temperature.

Current-driven non-linear magnetodynamics in exchange-biased spin valves

This work investigates the excitation of parametric resonance in exchange-biased spin valves (EBSVs). Using a mechanical point contact, high density dc and microwave currents were injected into the EBSV sample. Observing the reflected microwave power and the small rectification voltage that develops across the contact allows detecting the current-driven magnetodynamics not only in the bulk sample but originating exclusively from the small contact region. In addition to ferromagnetic resonance (FMR), parametric resonance at twice the natural FMR frequency was observed. In contrast to FMR, this non-linear resonance was excited only in the vicinity of the point contact where current densities are high. Power-dependent measurements displayed a typical threshold-like behavior of parametric resonance and a broadening of the instability region with increasing power. Parametric resonance showed a linear shift as a function of applied dc bias which is consistent with the field-like spin-transfer torque induced by current on magnetic moments in EBSV.

Spin-flipping in Pt and at Co/Pt interfaces

There has been recent controversy about the magnitude of spin-flipping in the heavy metal Pt, characterized by the spin-diffusion length lsfPt. We propose a resolution of this controversy, and also present evidence for the importance of a phenomenon neglected in prior studies of transport across sputtered Ferromagnet/Pt (F/Pt) interfaces, spin-flipping at the interface. The latter is characterized by an interface spin-flipping parameter, δCo/Pt, that specifies the probability P=[1−exp(−δ)] of a conduction electron flipping its spin direction as it traverses a Co/Pt interface. From studies of the Current-Perpendicular-to-Plane (CPP) Resistances and Magnetoresistances of sputtered ferromagnetically coupled Co/Pt multilayers by themselves, and embedded within Py-based Double Exchange-biased Spin-Valves, we derive values at 4.2K of δCo/Pt=0.9−0.2+0.5, interface specific resistance, ARCo/Pt⁎=0.74±0.15fΩm2, and interface spin-scattering asymmetry, γCo/Pt=0.53±0.12. This value of δCo/Pt is much larger than ones previously found for five other interfaces involving Co but not Pt. To derive δ requires knowledge of lsfPt for our sputtered Pt, which we obtain from separate measurements. Combining our results with those from others, we find that lsfPt for Pt is approximately proportional to the inverse resistivity, 1/ρPt.

Mathematical Modelling of Magnetic Hysteresis in Exchange-Bias Spin Valves

In this work we present a physical-related model of vector hysteresis for magnetic simulation of nanometric structures. The mathematical formulation, based on a generalization of the Preisach and Stoner-Wohlfarth models, is very simple and elegant. The numerical implementation is very easy and the computation time required for the simulations is extremely fast. However the accuracy is not comparable with the codes based on the micromagnetic theory, but it is sufficient for the use as preprocessing of more accurate and more complex methods, in order to speed up the convergence. In this paper the model identification is performed for a nanoscale spin valve using the data generated by a 3D micromagnetic code.

Spin flipping at sputtered Co/Ag interfaces

We measured at 4.2 K the current-perpendicular-to-plane magnetoresistances of sputtered ferromagnetically coupled [Co(3 nm)/Ag(1.8 or 2.0 nm)]${}_{n}$Co(3 nm) multilayers with $n$ = 0--8 that were imbedded in the middle of symmetric, Py-based, double exchange-bias spin valves. The measurements yielded the parameter for spin flipping at the Co/Ag interface, ${\ensuremath{\delta}}_{\mathrm{Co}/\mathrm{Ag}}$ = 0.33\ifmmode\pm\else\textpm\fi{}0.1. Despite the expected larger lattice-mismatch-induced disorder at the Co/Ag interface, this value is similar to that reported earlier for the Co/Cu interface, suggesting that such disorder does not play a major role in \ensuremath{\delta} for these metal pairs.

Co-Based Heusler Alloys for CPP-GMR Spin-Valves With Large Magnetoresistive Outputs

We studied various Heusler alloy compositions for the ferromagnetic layers of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valves (SVs) to obtain large MR outputs for the read sensor applications. The exchange biased SV with Co2 Fe(Al0.5 Si0.5) alloy showed a resistance change-area product (ΔRA) of 6 m Ωμm2 and an MR ratio of 21% at room temperature when the Co2 Fe(Al0.5 Si0.5) layer was well-B2-ordered by a high temperature annealing at 500°C. The critical bias current density for the spin torque effect on the reduction of ΔRA was ~ 3 × 107 A/cm2. Based on the high spin polarization directly measured by the point contact Andreev reflection method, Co2 (Cr0.1 Fe0.9 )Si, Co2 Mn(Ga0.5 Sn0.5 ), and Co2 Fe(Ge0.5 Ga0.5) alloy thin films were investigated for CPP-GMR SVs. Among them, the Co2Fe(Ge0.5Ga0.5) alloy showed a high CPP-GMR ratio in the pseudo SVs exceeding 40% at RT and 120% at 10 K.

Enhancement of interfacial specific resistance by the insertion of Fe(001) layers between alternate monatomic [Fe/Co]n superlattices and Ag spacer

The bulk and interface spin scattering asymmetric coefficients, βF and γF/N, respectively, as well as the interfacial specific resistance, AR*F/N, contribute to the total resistance change-area product, ΔRA, in current-perpendicular-to-plane (CPP) geometry. In this paper, we report the effect of the insertion of a 1 nm Fe layer into ferromagnetic (F)/non-ferromagnetic (N) interfaces on the spin dependent transport properties. Using the Valet-Fert theory based on the two-current model in which ΔRA is obtained as a function of thicknesses of the ferromagnetic layer (3, 4, and 5 nm), we experimentally deduced the values of βF, γF/N, and AR*F/N of exchange-biased spin valves (EBSVs) for an artificially ordered B2 state Fe50Co50 alloy as a magnetic layer and an Ag spacer layer.

A Study on the Sensitivity of a Spin Valve with Conetic-Based Free Layers

An exchange-biased spin valve with Conetic-based free layers of Co90Fe10, Co90Fe10/Conetic and Conetic was investigated. The spin valve with the Co90Fe10 free layer showed the highest giant magnetoresistance (GMR) ratio of 4% but showed the lowest normalized sensitivity of 0.02 Oe-1. The GMR ratio of 3% and the normalized sensitivity of 0.07 Oe-1 were obtained for the spin valve with the Co90Fe10/Conetic free layer after annealing. The spin valve having the Conetic free layer showed softer magnetic properties and well-defined smaller anisotropy than the other spin valves. Though the spin valve showed the lowest GMR of 0.4% after annealing, it showed the highest normalized sensitivity of 0.14 Oe-1. Our study shows that further improvement in MR response of spin valves with Conetic-based free layers can make a spin valve sensor promising for detecting extremely low fields.

Dependence of exchange coupling direction on cooling-field strength

We studied the dependence of exchange coupling on cooling-field strength in an exchange-biased spin valve with a synthetic antiferromagnetic layer by experiment and theory. Our theory calculates magnetic anisotropy energies in each magnetic layer composing the spin valve during the field-cooling process, finds the minimum state of total energy, and explains how the magnetizations in the layers interact with one another during field-cooling under various cooling-field strengths. Calculations based on the theory well match results of the experimental measurements. Our observation shows that one has to carefully choose the cooling-field strength optimal for designing exchange-biased spin devices having a synthetic antiferromagnetic layer; otherwise the exchange coupling direction can significantly deviate from the cooling-field direction, which impairs performance.

Magnetoresistance in CuPc Based Organic Magnetic Tunnel Junctions

We investigate the magnetoresistance of exchange biased spin valves with hybrid barrier layers, MgO(2 nm)/CuPc (t = 0, 1, 2, nm) where CuPc is the organic semiconductor copper phthalocyanine, which are patterned into micron size tunnel junctions by UV lithography. The stacks are produced by sputtering in a Shamrock tool and the organic layer is subsequently deposited by thermal evaporation. Devices in the first set showed 22 %, 10 % and 5% MR at 300 K for t = 0, 1 and 2 nm, respectively. These values increased up to 33 %, 28 % and 23% for the same junctions at 50 K. From these data we infer that the spin polarization is reduced by CuPc in the barrier and the spin scattering by Cu2+ is strongly temperature-dependent. By comparison, if an Alq3 layer is used, where there are no paramagnetic scattering centres, the magnetoresistance is independent of the thickness of the organic layer, at least up to 8 nm.

Uniaxial spin-transfer torque in an exchange-biased spin valve

We study the effects of uniaxial spin-transfer torque (USTT) on the ferromagnetic (F) aswell as antiferromagnetic (AF) layers in an exchange-biased (EB) spin valve. Byanalytically treating the free-energy functional of the F/AF bilayer and numerically solvingthe Landau–Lifshitz–Gilbert equation for magnetic moments, we can reproduceand explain two existing experimental facts relevant to USTT: one is that theEB field can be reversed by both positive and negative pulsed currents, and theother is that the critical current to excite the F moments is greatly increased inthe presence of an AF layer and independent of external fields. We also derivethe angular dependence of the critical currents to excite AF and F moments,which suggests a possible way to quantitatively determine USTT in experiments.

Physical nature of anomalous peaks observed in extraordinary Hall effect measurement of exchange biased spin-valves with perpendicular anisotropy

We experimentally and theoretically demonstrate that the abrupt change in magnetostatic energy during the free and pinned [Co/Pd] layer reversal by an external magnetic field and the magnetic field dependent extraordinary Hall effect (EHE) coefficient are the main physical reasons for the anomalous peaks observed in EHE measurement of exchange biased [Pd/Co]/Cu/Co/[Pd/Co]/FeMn giant magnetoresistance spin-valves with perpendicular anisotropy (EBPA-SVs). The correlation of the anomalous EHE peaks to the extrinsic magnetic properties of EBPA-SVs paves a way to indirectly evaluate these properties.