Layer Of Spin Valve Research Articles

A Spin Valve-Based Rhombus-Shaped Micro-Object Implementing a Full Wheatstone Bridge.

Spin valves with a synthetic antiferromagnet were fabricated via magnetron sputtering. It was shown that the fabricated spin valve layers had a perfect microstructure and smooth interfaces, and therefore, an RKKY interaction dominated in the coupling of the ferromagnetic layers separated by a copper spacer. Rhombus-shaped micro-objects were fabricated from a single spin valve film. The thermomagnetic treatment procedure was found to form unidirectional anisotropy in the micro-object such that the values of the exchange bias fields in the rhombus' nonparallel sides were opposite in sign. For the CoFeNi/Ru/CoFeNi synthetic antiferromagnet, we determined the differences between the ferromagnetic layer thicknesses at which the thermomagnetic treatment formed the same exchange bias all over each rhombus' side. We also fabricated a sensor element in which each side of the rhombus was the shoulder of a Wheatstone bridge. After the thermomagnetic treatment procedure, each shoulder worked as an active magnetosensitive element, enabling the device to operate as a full Wheatstone bridge. The sensor output exhibited a step shape, high sensitivity to field changes, and significant magnetic hysteresis. Such characteristics are suitable for switching devices.

Large stress-induced anisotropy in soft magnetic films for synthetic spin valves

We obtain a large in-plane magnetic uniaxial anisotropy in the soft magnetic Fe60Co20B20 (FeCoB) thin films prepared by rotational sputtering. The anisotropy field (Ha) of 75 to 175 Oe was found in the films with wide ranged thickness from 2.5 to 100 nm, which was attributed to the magnetoelastic energy in association with anisotropic tensile stress. This stress-induced anisotropy has outstanding thermal stability that survives up to 350 °C in the annealing process. The similar large uniaxial anisotropy can be realized in other soft magnetic thin films, such as Fe, Co, Ni, FeCo, and NiFe, with the same synthesis technique. The anisotropic FeCoB film was used as a free layer in a synthetic spin valve. A linear resistance change against external field with a range wider than ±100 Oe together with a significantly reduced coercivity of ∼1.1 Oe (∼8.5 Oe in the case with isotropic free layer) was observed in the transfer curve. The results of this work not only confirm the feasibility of films with large stress-induced magnetic anisotropy as a functional layer in spin-valve devices but also demonstrate a simple synthesis route to induce the magnetic anisotropy, which provides an additional control parameter for the spintronic device design.

Electronic structure, optical and magnetic properties of double Perovskites La2MTiO6 (M = Co, Ni, Cu and Zn)

Structural, electronic, optical and magnetic properties of titanium based perovskites La2MTiO6 (M = Co, Ni, Cu and Zn) are investigated in monoclinic phase with space group (P21/n) in the domain of density functional theory (DFT). The generalized gradient approximation plus Hubbard potential (GGA + U) is utilized to study these high correlated electrons systems. Structural parameters are found in closed agreement with experiments. Cohesive energy reveals that La2CoTiO6 is more stable among these compounds. The electronic properties show the semiconductor behavior of these compounds with band gap varies from 1.02 to 2.3 eV. Optical properties demonstrate that all these compounds are potential candidates for optoelectronic devices. The electrical resistivity which increases in a sequence from Co to Zn also confirms the semiconductor nature of these compounds. The magnetic susceptibility curves show that La2CoTiO6, La2NiTiO6 and La2CuTiO6 are antiferromagnetic whereas La2ZnTiO6 is a paramagnetic material. Based on the semiconductor antiferromagnetic nature of La2CoTiO6, La2NiTiO6 and La2CuTiO6 it is expected that these compounds could be used as pinning layers in spin valves and storage devices.

Effect of α-Fe2O3 microbeads on CoFeB/Ta/CoFeB magnetic switching and magnetic instabilities

Effect of cell microbeads labelled with commercial α-Fe2O3 nanoparticles (NPs) on magnetic properties of MgO/CoFeB/Ta/CoFeB/MgO/GaAs spin valves and monolayer system MgO/CoFeB/MgO/GaAs was observed. Magnetic scattering field of α-Fe2O3 microbeads shifts critical magnetic field (CMF) of magnetization switching of free layer in spin valves. The CMF is controlled by energy balance of magnetic anisotropy, interlayer exchange coupling, Zeeman energy and scattering magnetic field of microbeads. Local magnetic field of microbeads and their effect on CMF were estimated accordingly with the magnetic profile of the surface scanned by magnetic force microscope. Magnetic instability of film magnetization caused by magnetic microbeads deposited on the spin valve surface was revealed.

Spin valves with Conetic based synthetic ferrimagnet free layer

The detection of low magnetic field requires hysteresis-free magnetoresistive sensor with high sensitivity. Giant magnetoresistance spin valve (SV) sensors with CoFeB based synthetic antiferromagnetic (SAF) pinned layer and Conetic based synthetic ferrimagnet (SF) free layer are developed in this paper. Low free layer coercivity (HC) of 0.27 Oe is achieved through adapting synthetic antiferromagnetic pinned layer in SV with Conetic single free layer. In SV with SAF pinned layer and SF free layer, the MR ratio, HC, switching field (HSW) and offset field (HO) increase with the thickness of the Conetic alloy layer right above the Cu spacer. The HC, HO, and HSW show oscillatory behavior with the Ru spacer thickness in the SF free layer. Through introducing CoFeB/SF free layer, an increased sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained, which holds promise for low field sensing applications.

Magnetic transitions and butterfly-shaped hysteresis of Sm-Fe-Al-based perovskite-type orthoferrite

Al doped SmFeO3 (SmFe1–xAlxO3; 0.0≤x≤0.15; step 0.05) were prepared by double sintering ceramic technique. The obtained samples were crystallized in single phase structure except the sample with x=0.15. The unit cell volume was found to decrease with increasing Al substitution in orthoferrite. The effective magnetic moment (<ι>μ</ι>eff) and the Curie constant (C) were calculated from the reciprocal of the molar magnetic susceptibility (χ−1) versus absolute temperature plot and found to attain maximum value for the parent sample. The magnetic behavior showed two different magnetic transitions, viz, Néel temperature (TN) and spin reorientation (TSR) transitions. The M-H hysteresis loop of the parent sample took butterfly-shape as a result of different contributions anisotropies. From the magnetic properties measurements, it was obviously found that B-site cation dilution resulted in a drastic decrease in the magnetization. Surprisingly large value of the coercive field was obtained for the undoped sample; Hc=6198.8 Oe. Based on the mentioned results, one can recommend the use of such orthoferrite in magnetic recording media and as pining layer in spin valve for spintronic applications.

Unidirectional Anisotropy in Nanostructures with Antiferromagnetic NiFeMn Layer

Magnetic properties of nanostructures including an antiferromagnetic (NiFe)1-хMnx alloy have been studied for various modes of this AFM layer preparation. The possibility for application of the AFM (NiFe)1-хMnx alloy as a material of the pinning layer in spin valves is discussed.

Optimum Laser Exposure for Setting Exchange Bias in Spin Valve Sensors

The specific local alignment of the magnetization of a reference layer in spin valves has a rapidly growing significance in the implementation of monolithically integrated sensor devices. The spatial sensitivity of such sensors is defined by the exchange bias effect arising in strongly coupled ferromagnetic (FM) and antiferromagnetic (AFM) heterostructures. The (re-)setting of the orientation of the exchange bias direction can be achieved through local annealing by means of a focused laser beam and subsequent cooling in the presence of a magnetic field. The parameters of such a laser exposure will have a significant influence to the FM/AFM system as well as the remaining layers, playing therefore an important role on the sensing characteristics. In order to analyse this effect, bottom-pinned IrMn / CoFe / Cu / CoFe/ NiFe spin valves are patterned into a meander shape and further annealed using a focused pulsed laser beam. Different processing peak intensities are tested. The analysis of the magnetotransport properties after this exposure shows how strongly the electrical conductivity is affected for higher intensities, as well as the effect of the laser peak intensity on the magnetic coupling between the ferromagnetic electrodes. These changes are discussed as a result of the structural modifications induced by the laser exposure on the spin valve structures.

Investigation of selective realignment of the preferred magnetic direction in spin-valve layer stacks using laser radiation

Selective realignment of the preferred magnetization direction in a laser micro structured GMR spin-valve layer system (Ni81Fe19/Co90Fe10/Cu/Co90Fe10/IrMn/Ni81Fe19) with a total film thickness of 23nm was studied. For this, patterns of isolated microstructures (500μm×200μm) were fabricated by laser ablation. These micropatterns were annealed using laser irradiation at a temperature above the IrMn Néel temperature. During laser annealing, the sample was subjected to an external magnetic field in order to selectively realign the magnetic direction of the reference layer. Two different laser assisted annealing techniques were investigated applying either continuous or pulsed laser systems. After laser annealing, the magnetic properties of the micropatterns were investigated using a magnetic microsensor and magneto optical Kerr effect set up.

Wheatstone bridge giant-magnetoresistance based cell counter

A Wheatstone bridge giant magnetoresistance (GMR) biosensor was proposed here for the detection and counting of magnetic cells. The biosensor was made of a top-pinned spin-valve layer structure, and it was integrated with a microchannel possessing the function of hydrodynamic focusing that allowed the cells to flow in series one by one and ensured the accuracy of detection. Through measuring the magnetoresistance variation caused by the stray field of the magnetic cells that flowed through the microchannel above the GMR biosensor, we can not only detect and count the cells but we can also recognize cells with different magnetic moments. In addition, a magnetic field gradient was applied for the separation of different cells into different channels.

Asymmetry of Magnetization Reversal of Pinned Layer in NiFe/Cu/NiFe/IrMn Spin-Valve Structure

Two types of asymmetry in giant magnetoresistance (GMR) are observed which are not related to a training effect, but indicate different mechanisms of magnetization reversal of the pinned layer in spin-valve (SV) structures for ascending and descending field scans. GMR, exchange bias and coercivity in Si/Ta/NiFe/Cu/NiFe/IrMn/Ta SV-structures were investigated as functions of the thickness of the nonmagnetic spacer. The spacer thickness effects are discussed in correlation with layers microstructure and interfaces morphology variations.

First-principles study of spin-transfer torque in Co2MnSi/Al/Co2MnSi spin-valve

The spin-transfer torque (STT) in Co2MnSi(CMS)/Al/Co2MnSi spin-valve system with and without interfacial disorder is studied by a first-principles noncollinear wave-function-matching method. It is shown that in the case of clean interface the angular dependence of STT for CoCo/Al (the asymmetry parameter Λ≈4.5) is more skewed than that for MnSi/Al (Λ≈2.9), which suggests the clean CoCo/Al architecture is much more efficient for the application on radio frequency oscillation. We also find that even with interfacial disorder the spin-valve of half-metallic CMS still has a relatively large parameter Λ compared to that of conventional ferromagnet. In addition, for clean interface the in-plane torkance of MnSi/Al is about twice as large as that of CoCo/Al. However, as long as the degree of interfacial disorder is sufficiently large, the CoCo/Al and MnSi/Al will show approximately the same magnitude of in-plane torkance. Furthermore, our results demonstrate that CMS/Al/CMS system has very high efficiency of STT to switch the magnetic layer of spin-valve.

Monte Carlo simulations of the fcc kagome lattice: Competition between triangular frustration and cubic anisotropy

The impact of local cubic anisotropy on the magnetic states of the Heisenberg model on the fcc kagome lattice are examined through classical Monte Carlo simulations. Previous simulations revealed that the macroscopic degeneracy of the two-dimensional (2D) kagome exchange-coupled co-planar spin system partially persists in the 3D case of ABC stacked layers giving rise to a discontinuous phase transition. Local cubic anisotropy is shown to remove this degeneracy by re-orienting the spins out of the co-planar configuration. In addition, the re-oriented states are shown to carry a uniform magnetic moment. Simulation results indicate that the effect of anisotropy is to transform the first-order phase transition to a continuous one. These results are relevant to Ir-Mn alloys which have been widely used by the magnetic storage industry in thin-film form as the antiferromagnetic pinning layer in spin valves.

Study of the possibility of using Ni-Fe-Mn alloys as material for pinning layers in spin valves

Magnetic properties of nanostructures based on Ni80Fe20/(Ni80Fe20)1 − x Mn x bilayers are studied at various compositions and thicknesses of the antiferromagnetic (AFM) layer. The possibility of using triple compounds based on Ni-Fe-Mn in spin valves is considered. Regularities of the formation of unidirectional anisotropy in permalloy/manganese bilayers during thermomagnetic treatment are revealed.

Investigation of nanostructures based on Ni80Fe20/(Ni80Fe20)20Mn80 bilayers with a unidirectional exchange anisotropy

The magnetic properties of nanostructures based on Ni80Fe20/(Ni80Fe20)20Mn80 bilayers have been investigated depending on the thickness of the antiferromagnetic layer and the material of the substrate. The possibility of using the antiferromagnetic alloy (Ni80Fe20)20Mn80 as the material for the pinning layer in spin valves has been considered.

Large-scale Solution Processable Graphene-based Thin Film Devices

ABSTRACTThe purpose of this work is to fabricate large-scale solution processable graphene-based films from graphene oxide (GO) solution and to characterize the transport properties of these films. The graphene like film is produced by annealing of the GO film to form reduced graphene oxide (rGO) thin films. The conductive rGO thin films are useable as spacer layers in spin valves and as organic electrodes. Atomic Force Microscope (AFM) characterizations on the film thickness and morphology have been carried out and simple electrical transport studies performed on spin coated rGO thin films. We have fabricated rGO thin films ranging from few to tens of nanometers in thickness with conductivities in the order of 1-100 S/m. We also show that the morphology of the films play an important role in facilitating higher conductivities for rGO thin films.

Enhanced difference in switching fields for perpendicular magnetic spin valves with a composite [Co/Ni]N/TbCo reference layer

A perpendicularly exchange-coupled composite structure, which consists of a [Co/Ni]N multilayer and a ferrimagnetic TbCo layer, displays specific features when it is utilized as a reference layer in spin valves. By slightly increasing the TbCo thickness, the coercivity of the coupled structure greatly increases, thereby giving an effective way to widen the difference in switching fields for the free and reference layers, and making it possible to manipulate magnetizations parallel or antiparallel for a wide range of fields. Such composite reference layer cannot only maintain a high perpendicular giant magnetoresistive (GMR) ratio, but it also minimizes the field offset in the minor GMR curves due to the negligible stray field generated by the antiferromagnetically aligned moments in the reference layer.

Effect of grain size on the properties of NiFe/PtMn bilayers

The structural and magnetic properties of NiFe/PtMn bilayers with a (Ni81Fe19)100−xCrx seed layer were investigated. It is found that the pinning field Hex of NiFe/PtMn bilayers is very sensitive to the Cr content of the (Ni81Fe19)100−xCrx seed layer, which can effectively influence the grain size, L10 ordering phase transformation and the critical thickness of PtMn as well as the blocking temperature (Tb). When the Cr content is 40 at%, PtMn shows a large grain size, a small critical thickness and a high blocking temperature, but the degree of L10 ordering phase transformation is weak after annealing, which results in a low pinning field Hex. When the Cr content is below 34 at% or above 50 at%, PtMn has a small grain size, a large critical thickness and a low blocking temperature, but the degree of L10 ordering phase transformation is very good after annealing, resulting in a high pinning field Hex. These results indicate that (Ni81Fe19)100−xCrx can be a promising seed layer in spin valves based on Mn-alloyed antiferromagnets.

Spin-torque driven ferromagnetic resonance of Co∕Ni synthetic layers in spin valves

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co∕Ni synthetic layers with perpendicular anisotropy confined in spin valve based nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude rf excitation, from 1to20GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co∕Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth, and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements.

Magnetic properties of spin valves having extremely thin underlayers

AbstractMagnetic properties of spin valve elements having Mo(N) and Ta(N) underlayers were studied by varying their thickness. Spin valve structure was Si/SiO2/Underlayer(tÅ)/NiFe(21 or 42Å)/CoFe(28Å)/Cu(22Å)/CoFe(18Å)/IrMn(65Å)/Ta(25Å). Spin valve elements having exteremely thin Mo(N) and Ta(N) underlayers showed high MR ratio of about 7 ‐ 8%. Annealing of such spin valve elements having underlayer thickness of 7 to 8 Å showed comparable behavior with the spin valve elements with thicker (35Å) underlayer, which can be utilized to reduce overall device thickness. Also, it was found that Mo(N) underlayers for spin valve elements may be used as diffusion barriers between Si substrate and ensuing active spin valve layers, simultaneously. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)