Spin-valve Multilayers Research Articles

Microstructure and magnetism of Ga+-irradiated IrMn-based spin-valve multilayered film

An IrMn top-nailed spin-valve multilayer film was synthesized using direct-current magnetron sputtering on a naturally oxidized silicon substrate. The multilayered film microstructure and magnetism were analyzed using x-ray diffraction, transmission, and atomic force microscopies as well as vibrating sample magnetometry as functions of ion irradiation. The reverse saturation field residence time of the spin-valve multilayered sample was used to assess its magnetic stability. Ga+ distribution in the layers after irradiation was also simulated using the SRIM2003 package. Ga+ irradiation weakened the intensity of the (111) peak of the antiferromagnetic IrMn (111) layer, decreased surface roughness as well as exchange bias field and pinned layer coercive force, while increasing the free layer coercive force. However, the multilayer exchange bias field of the spin-valve film under the reverse saturation field after Ga+ irradiation did not change with time.

Bias current dependence of superconducting transition temperature in superconducting spin-valve nanowires

Competition between superconducting and ferromagnetic ordering at interfaces between ferromagnets (F) and superconductors (S) gives rise to several proximity effects such as odd-triplet superconductivity and spin-polarized supercurrents. A prominent example of an S/F proximity effect is the spin switch effect (SSE) observed in S/F/N/F superconducting spin-valve multilayers, in which the superconducting transition temperature T$_c$ is controlled by the angle $\phi$ between the magnetic moments of the F layers separated by a nonmagnetic metallic spacer N. Here we present an experimental study of SSE in Nb/Co/Cu/Co/CoO$_x$ nanowires measured as a function of bias current flowing in the plane of the layers. These measurements reveal an unexpected dependence of T$_c(\phi)$ on the bias current: T$_c(\pi)$--T$_c(0)$ changes sign with increasing current bias. We attribute the origin of this bias dependence of the SSE to a spin Hall current flowing perpendicular to the plane of the multilayer, which suppresses T$_c$ of the multilayer. The bias dependence of SSE can be important for hybrid F/S devices such as those used in cryogenic memory for superconducting computers as device dimensions are scaled down to the nanometer length scale.

Influence of temperature on the texture and magnetic properties of IrMn-based spin-valve multilayers

Top-pinning, spin-valve multilayer films with the structure Ta (5 nm)/Co75Fe25 (5 nm)/Cu (2.5 nm)/Co75Fe25 (5 nm)/Ir20Mn80 (12 nm)/Ta (8 nm) were prepared by high-vacuum DC magnetron sputtering. The effects of temperature on the film microstructure and magnetic properties were investigated by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and vibrating sample magnetometry. The effects of temperature on the thermomagnetic stability of the spin-valve multilayers were studied based on the residence time of the sample in a reverse saturation field. The following effects were observed with increasing temperature: the IrMn(111) texture weakened; surface/interface roughness increased; interfacial diffusion was enhanced; the coercivities of the exchange bias field and pinned layer decreased; the coercivity of the free layer increased; relaxation became more obvious; and thermomagnetic stability decreased.

Inverse Magnetoresistance Characteristic of Hybrid-Type Multilayer Structure of IrMn-Based Giant-Magnetoresistance Spin Valve and High-Tc Superconductor YBa2Cu3O7−x Film

An antiferromagnetic-IrMn-based giant-magnetoresistance spin-valve (GMR-SV) multilayer on a high-temperature superconductor YBa2Cu3O7−x (YBCO) film was prepared using an ion-beam deposition system. The exchange bias coupling field (Hex), coercivity (HC), and magnetoresistance ratio (MR (%)) of the hybrid YBCO/GMR-SV multilayer structure were measured and compared at room temperature and 77 K. The MR ratio (%) of the hybrid YBCO/GMR-SV structure measured by the four-probe method at 77 K was − 9.3%, different from that of the conventional GMR-SV structure. The phenomenon of a negative MR (%) below the critical temperature (Tc) can be explained by the current-in-plane tunneling effect, when the resistance of the middle gap (G) layer between the high-temperature superconductor YBCO and GMR-SV multilayer reaches a value comparable to the plane resistance of the upper metal layer. In order to analyze the inversed MR (%) of the hybrid YBCO/GMR-SV multilayer structure, the equivalent circuit for each layer was constructed and an equation of the output voltage was derived.

Reduction of shunt current in buffer-free IrMn based spin-valve structures

The presence of thick buffer layers in magnetic sensor devices decreases sensor sensitivity due to shunt currents. With this motivation, we produced IrMn-based spin-valve multilayers without using buffer layer. We also studied the effects of post-annealing and IrMn thickness on exchange bias field (HEB) and blocking temperature (TB) of the system. Magnetization measurements indicate that both HEB and TB values are significantly enhanced with post-annealing of IrMn layer. In addition, we report that IrMn thickness of the system strongly influences the magnetization and transport characteristics of the spin-valve structures. We found that the minimum thickness of IrMn layer is 6 nm in order to achieve the lowest shunt current and high blocking temperature (>300 K). We also investigated the training of exchange bias to check the long-term durability of IrMn-based spin-valve structures for device applications.

Spin-Valve based magnetoresistive nanoparticle detector for applications in biosensing

A magnetoresistive sensor platform for the detection of individual magnetic reporter particles is presented. The sensing scheme is based on the detection of the shift in the sensor’s magnetoresistance-switching field in the presence of magnetic particle(s). A bottom-pinned spin-valve multilayer (Co/Ru/Co)/Cu/(Co/Ru/Co) device structure (with Ta/Ru seed and Ta capping layers) used for the sensor was deposited using ultra-high vacuum magnetron sputtering and patterned using conventional microfabrication techniques. Inexpensive electronics to measure device magnetoresistance (using DC currents) was used for testing. The detection of individual 500nm Fe3O4 nanoparticles as well as the detection of up to ten (10) nanoparticles was demonstrated and verified using scanning electron microscopy (SEM). The developed sensors are readily integratable into a biosensing platform for the detection of biomolecules at ultra-low concentration using magnetic nanoparticles as reporters.

Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

Hall effect-induced acceleration of electromigration failures in spin valve multilayers under magnetic field

It was observed that electromigration (EM)-induced failures in spin valve multilayers were severely accelerated by an externally applied magnetic field. The theoretical and experimental analysis results confirmed that Hall effect-induced Lorentz force applied to the perpendicular-to-the-film-plane direction is primarily responsible for the severe acceleration of the EM failures due to its dominant contribution to abruptly increasing local temperature and current density. The proposed failure model and the theoretical calculations were demonstrated to agree well with the experimental observations.

Stochastic and complex depinning dynamics of magnetic domain walls

We study the thermally induced depinning process of a single magnetic domain wall (DW) under applied field when the DW is trapped by a notch made in a nanowire shaped from a spin-valve multilayer with in-plane magnetization. In such devices depinning is typically a stochastic process and the depinning field distribution exhibits complex features. By analyzing simultaneously depinning field distributions and relaxation data, we observe two distinct sources of ``complexity'' in DW depinning processes: multiplicity in the DW structure and fundamental complexity in the depinning process.

IrMn 스핀밸브 박막소자의 폭 크기에 의존하는 자장감응도

NiFe/Cu/NiFe/IrMn 스핀밸브 박막에 대해 Cu의 두께에 의존하는 자장감응도를 조사하였다. Ta(5 nm)/NiFe(8 nm)/Cu(3.5 nm)/NiFe(4 nm)/IrMn(8 nm)/Ta(2.5 nm) 다층박막 구조에 대해 측정한 Minor 자기저항 곡선에서 자기저항비, 자장감응도, 보자력, 층간상호교환결합력은 각각 1.46 %, 2.0 %/Oe, 2.6 Oe, 0.1 Oe 이었다. 광 리소그래피 공정으로 제작한 10가지 다른 폭 크기와 <TEX>$4.45\;{\mu}m$</TEX>의 길이를 갖는 GMR-SV 소자의 자장감응도는 폭 크기가 <TEX>$10\;{\mu}m$</TEX>에서 <TEX>$1\;{\mu}m$</TEX>까지 작아짐에 따라 0.3 %/Oe에서 0.06 %/Oe로 감소하였다. The Cu thickness dependence of magnetic sensitivity for the NiFe/Cu/NiFe/IrMn spin valve multilayer was investigated. The magnetic properties measured by minor MR curves for the Ta(5 nm)/NiFe(8 nm)/Cu(3.5 nm)/NiFe(4 nm)/IrMn(8 nm)/Ta(2.5 nm) multilayer is MR = 1.46 %, MS = 2.0 %/Oe, <TEX>$H_c\;=\;2.6\;Oe$</TEX>, and <TEX>$H_{int}\;=\;0.1\;Oe$</TEX>. The magnetic sensitivities of GMR-SV devices with ten different widths and a same length of <TEX>$4.45\;{\mu}m$</TEX> by fabricated by photo lithography decreased from 0.3 %/Oe to 0.06%/Oe as from a width of <TEX>$10\;{\mu}m$</TEX> to <TEX>$1\;{\mu}m$</TEX>.

Focused ion beam fabrication of spintronic nanostructures: an optimization of the millingprocess

Focused ion beam (FIB) milling has been used to fabricate magnetic nanostructures(wires, squares, discs) from single magnetic layers (Co, permalloy) and spin-valve(permalloy/Cu/Co) multilayers (thicknesses 5–50 nm) prepared by ion beam sputteringdeposition. Milled surfaces of metallic thin films typically exhibit residual roughness, whichis also transferred onto the edges of the milled patterns. This can lead to domain wallpinning and influence the magnetization behaviour of the nanostructures. We haveinvestigated the milling process and the influence of the FIB parameters (incidence angle,dwell time, overlap and ion beam current) on the roughness of the milled surface. It hasbeen found that the main reasons for increased roughness are different sputter yields forvarious crystallographic orientations of the grains in polycrystalline magnetic thin films.We have found that the oblique ion beam angle, long dwell time and overlap < 1 are favourable parameters for suppression of this intrinsic roughness. Finally, we haveshown how to determine the ion dose necessary to mill through the whole thin film upto the silicon substrate from scanning electron microscopy (SEM) images only.

Current-driven vortex oscillations in metallic nanocontacts: zero-field oscillations and training effects

We present an experimental and theoretical study of the low-field dynamics of current-driven vortex oscillations in nanocontacts based on spin-valve multilayers. These oscillations appear as low-frequency (250–500 MHz) excitations in the electrical power spectrum which arise from variations in the giant magnetoresistance. We show that the vortex oscillations, once nucleated at large fields applied perpendicular to the film plane, persist at zero applied magnetic fields. Some training effects on the oscillation frequency and linewidth are also observed for small in-plane magnetic fields.

Magnetic instability of giant magnetoresistance spin-valves due to electromigration-induced inter-diffusion

Electromigration-induced magnetic failures in NiFe/(Co)/Cu/(Co)/NiFe spin-valve (SV) multilayers have been investigated. Electrically stressed NiFe/Cu/NiFe SV multilayers showed a dramatic reduction of magnetic moment up to 41%, and a shift of interlayer coupling characteristics, while no obvious magnetic degradation was observed in the NiFe/Co/Cu/Co/NiFe SV multilayers. It was experimentally confirmed that the magnetic degradation of the NiFe/Cu/NiFe SV multilayers is primarily due to the formation of Ni–Cu intermixing caused by the electromigration-induced Cu spacer inter-diffusion. Furthermore, it was demonstrated that an ultra thin Co diffusion barrier at the NiFe/Cu interface is promisingly effective to improve the magnetic stability of NiFe/(Co)/Cu/(Co)/NiFe SV multilayers against electromigration.

Current-sweep-rate dependence of spin-torque driven dynamics in magnetic nanopillars

We report on the measurements of current-driven dynamics in magnetic nanopillars as a function of the current-sweep rate. These measurements were intended to verify the recently predicted thermal-activation nature of current-induced precessional states in spin-valve multilayers. For a fixed temperature and field (applied perpendicular to the layers), the critical current for dynamic excitations was found to increase logarithmically with the sweep rate as expected for a thermally activated process. However, we observed little if any dependence of this behavior on temperature. We discuss the implications of this in terms of a simple model of spin-torque driven magnetization dynamics.

Electromigration-Induced Failure Characteristics of Spin-Valve Multilayers for Metallic-Based Spintronic Devices

Electromigration (EM) failure characteristics of patterned NiFe/Cu/NiFe spin-valve (SV) multilayers (MLs) with or without thin Co diffusion barrier inserted at the interface between NiFe and Cu layers have been investigated by applying a constant dc current with different current densities to predict the electrical and magnetic stabilities of metallic SV-based spintronic devices. It was experimentally verified that the Ni-Cu intermixing caused by the EM-induced Cu interdiffusion to the top or bottom NiFe layer plays a dominant role in determining the EM-induced failure characteristics of the NiFe/(Co)/Cu/(Co)/NiFe SV multilayered devices. In addition, it was clearly demonstrated that the interlayer (indirect) exchange coupling field and the magnetic moment of the NiFe/(Co)/Cu/(Co)/NiFe SV-MLs are strongly dependent on the EM-failure induced Ni-Cu intermixing directly relevant to the Cu spacer interdiffusion. The failure mechanism of NiFe/(Co)/Cu/(Co)/NiFe SV multilayered devices showed ldquobimodal failure characteristics.rdquo The critical current density (Jc) for such a bimodal failure mechanism was found to be determined at Jc = 7 times107A/cm2 When J les Jc, the failure was mainly caused by the electrostatic force (or electron wind force) accelerating an interdiffusion through grain boundaries that leads to forming typical EM failures such as voids and hillocks. Whereas, when J > Jc, a melting or a vaporization dominantly accelerated by the Joule heating played more significant role and caused the catastrophic failures. The NiFe/Co/Cu/Co/NiFe SV-MLs showed a much longer mean time-to-failure (t50) than that of NiFe/Cu/NiFe SV-MLs. This experimental result implies that an ultrathin Co insertion layer is effective to improve the EM-induced failure lifetime due to its restraining effects as a diffusion barrier to prevent Cu interdiffusion to both the top and bottom NiFe layers. Furthermore, this result directly verified that Ni-Cu intermixing is the dominant factor in determining the EM characteristics of the NiFe/Cu/NiFe SV multilayered spintronic devices.

Ni 80Fe 20/Cu/Co/Cu] spin-valve multilayers electrodeposited on NiFe buffer layers

[Ni 80Fe 20/Cu/Co/Cu] n spin-valve multilayers were fabricated on n-Si(111) substrates by double bath electrodeposition. A well-defined periodical structure has been demonstrated by X-ray diffraction (XRD). Scanning electron microscopy morphology shows that the electrodeposited NiFe buffer layer was continuous with a nominal thickness over 25 nm. XRD and atomic force microscopy results reveal that NiFe buffer layer has a significant effect on the texture and microstructure of spin valves. Giant magnetoresistance (GMR) of spin-valve multilayers was tested by four probe technique. The maximum room temperature GMR ratio of 5.4% was achieved at the range of 1.8–2.8 kA/m with a sensitivity of up to 2.5%/(kA/m) for NiFe(25 nm)/[Cu(3.6 nm)/Co(1.2 nm)/Cu(3.6 nm)/Py(2.8 nm)] 30. GMR performance is proved to be highly related to the thickness of each component layer. It is considered that the variation in sublayer thickness will change the coercivity difference between adjacent magnetic layers, which drives the antiparallel alignment process of magnetizations.

Low energy ion beam assisted deposition of a spin valve

The spin dependent electron transport in giant magnetoresistive (GMR) multilayers is significantly affected by the atomic scale structure of their interfaces. Devices with atomically flat and chemically sharp interfaces are preferred for magnetic sensor and memory applications. Recent atomic simulations of the atom-by-atom assembly of these devices indicate that near optimal interfacial structures can be created using low energy, ion assisted vapor deposition techniques with ion energies in the 5–10eV range. A recently developed biased target ion beam deposition system has been used to experimentally test this hypothesis. Prototypical Ta∕NiFe∕Co∕Cu∕Co∕FeMn∕Cu spin valve structures were first grown using (simultaneous) argon ion assistance during deposition of the Co∕Cu∕Co trilayer part of the spin valve multilayer. Assisting ion energies of around 10eV resulted in structures with a 30% higher magnetoresistance ratio and significantly reduced coupling field compared to samples grown with no ion assistance or with ion energies above 15eV. These results are consistent with the atomistic simulation predictions. Other promising ion assistance schemes identified by the simulations were then used to deposit the Ta, NiFe, FeMn, and the top copper layer. A near optimal strategy was identified that resulted in the further improvement of the GMR ratio.

Electrochemical Preparation and Giant Magnetoresistance Effect of Spin-valve Multilayers

[Ni_(80)Fe_(20) /Cu/Co/Cu]_n spin-valve multilayers were fabricated onto n-Si(111) substrates covered with NiFe buffer layers by means of double bath potentiostatic electrodeposition. The electrochemical conditions were determined. A well-defined superlattice structure was characterized by high-angle X-ray diffraction, and the influence of NiFe buffer layer upon the orientation of crystalline growth was studied. Magneto-transport properties of the spin valves were investigated by four-probe technique. Hysteresis loops were tested by vibrating sample magnetometer (VSM) at room temperature. With the increase of the Cu layer thickness (t_(Cu)), giant magnetoresistance(GMR) ratio showed a periodical oscillation at first and achieved the maximum at tCu=3.6 nm, then declined gradually. With the increase of the Co and NiFe layer thickness, GMR ratio rose at first, and dropped after the peak point. The maximum room temperature GMR ratio of 5.4% was obtained with a sensitivity up to 0.2%·Oe~(-1) and a saturation field of 350 Oe for NiFe(25 nm)/[Cu(3.6 nm)/Co(1.2 nm)/Cu(3.6 nm)/NiFe(2.8 nm)]_30 structures.

Huge exchange-coupling field observed in FeMn-pinned spin valve with ultra-thin pinned NiFe layer

A conventional Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve multilayer was prepared to investigate the exchange bias variations of the pinned NiFe layer. An exchange bias field of 560 Oe has been found in a valve multilayer with ultra-thin pinned NiFe layers (1 nm), in which a large constant magnetic field of 700 Oe was applied during film deposition procession. The observed results are attributed to the large applied magnetic field, which produced more net spins of the antiferromagnet at the interface. These interfacial uncompensated spins provide the net spin moments required for exchange coupling and bias.

Layer thickness effect on the magnetoresistance of a current-perpendicular-to-plane spin valve

We performed a theoretical study and analysis of the effect of modifying the layer thicknesses of a current-perpendicular-to-plane (CPP) spin valve multilayer on its magnetoresistance (MR) ratio. An increase in the ferromagnetic (FM) layer thickness results in (i) an increase in the spin-dependent component of its total resistance, thereby resulting in higher MR, but also leads to (ii) greater spin relaxation in that layer and (iii) an anomalous MR effect in the high resistance regime, both of which suppress the MR ratio. The interplay of these effects results in a complex MR dependence on FM thickness, instead of the simple monotonic MR increase predicted by the two-current model. It also explains the existence of an optimum FM thickness for maximum MR ratio, as evidenced by experimental data. Finally, we consider the MR dependence on the strength and spin selectivity of interfacial resistances, which can either arise naturally or be engineered in the spin valve structure. The study of the combined effects of the FM layer thickness and resistivity, the MR suppression in the high resistive limit, and the competitive spin-dependent scattering in the bulk and at the interfaces is essential for optimizing the structure and material of a practical CPP spin valve to achieve the maximum MR ratio.