Spin-valve Elements Research Articles

Detection of Surface and Subsurface Flaws with Miniature GMR-Based Gradiometer.

The eddy-current (EC) testing method is frequently utilized in the nondestructive inspection of conductive materials. To detect the minor and complex-shaped defects on the surface and in the underlying layers of a metallic sample, a miniature eddy-current probe with high sensitivity is preferred for enhancing the signal quality and spatial resolution of the obtained eddy-current images. In this work, we propose a novel design of a miniature eddy-current probe using a giant magnetoresistance (GMR) sensor fabricated on a silicon chip. The in-house-made GMR sensor comprises two cascaded spin-valve elements in parallel with an external variable resistor to form a Wheatstone bridge. The two elements on the chip are excited by the alternating magnetic field generated by a tiny coil aligned to the position that balances the background output of the bridge sensor. In this way, the two GMR elements behave effectively as an axial gradiometer with the bottom element sensitive to the surface and near-surface defects on a conductive specimen. The performance of the EC probe is verified by the numerical simulation and the corresponding experiments with machined defects on metallic samples. With this design, the geometric characteristics of the defects are clearly visualized with a spatial resolution of about 1 mm. The results demonstrate the feasibility and superiority of the proposed miniature GMR EC probe for characterizing the small and complex-shaped defects in multilayer conductive samples.

Granular Hall Sensors for Scanning Probe Microscopy.

Scanning Hall probe microscopy is attractive for minimally invasive characterization of magnetic thin films and nanostructures by measurement of the emanating magnetic stray field. Established sensor probes operating at room temperature employ highly miniaturized spin-valve elements or semimetals, such as Bi. As the sensor layer structures are fabricated by patterning of planar thin films, their adaption to custom-made sensor probe geometries is highly challenging or impossible. Here we show how nanogranular ferromagnetic Hall devices fabricated by the direct-write method of focused electron beam induced deposition (FEBID) can be tailor-made for any given probe geometry. Furthermore, we demonstrate how the magnetic stray field sensitivity can be optimized in situ directly after direct-write nanofabrication of the sensor element. First proof-of-principle results on the use of this novel scanning Hall sensor are shown.

Use of a Spin-Flop State for the Creation of Spin-Valve Elements for a Full Wheatstone Bridge

A method of universal thermomagnetic treatment forming a pairwise opposite exchange bias in spin valves, which are sensor elements of a Wheatstone bridge, has been developed. The method is based on the formation of two magnetic phases in a spin valve upon the transition of a synthetic antiferromagnet into a spin-flop state. Thermomagnetic treatment in a two-phase state leads to the formation of a pairwise mutually opposite exchange bias in different elements of a Wheatstone bridge. The direction of a formed exchange bias is governed by the uniaxial anisotropy of every element.

Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

Interrelated emission and spin–spin relaxation feature mediated by VO+ defects in Gd2O3 nanorods subjected to swift ion impact

We report on the manifestation and interconnected photoluminescence and electron paramagnetic resonance responses in gadolinium oxide (Gd2O3) nanorods subjected to 80 MeV carbon ion irradiation. On increasing the irradiation fluence between 1 × 1011 and 3 × 1012 ions/cm2, the emission associated with neutral oxygen vacancies (VOx), positioned at ~350 nm, undergoes a steady increase compared to that associated with singly charged vacancies (VO+), located at ~414 nm. The enhancement of spin–spin relaxation time (τss) is ascribed to a substantial changeover from VO+ to VOx defects with irradiation, the former being recognized as the major contributor to paramagnetic centres. Interconnected luminescence and spin–spin relaxation could provide insight for making advanced nanophosphors and spin valve elements.

Multi-bits memory cell using degenerated magnetic states in a synthetic antiferromagnetic reference layer

We newly developed a magnetic memory cell having multi-bit function. The memory cell composed of a perpendicularly magnetized magnetic tunnel junction (MB-pMTJ) and a synthetic antiferromagnetic reference layer. The multi-bit function is realized by combining the freedom of states of the magnetic free layer and that in the antiferromagnetically coupled reference layer. The structure of the reference layer is (FeB/Ta/[Co/Pt]3)/Ru/([Co/Pt]6); the top and the bottom layers are coupled through Ru layer where the reference layer has two degrees of freedom of a head-to-head and a bottom-to-bottom magnetic configuration. A four-state memory cell is realized by combination of both degrees of freedom.The states in the reference layer however is hardly detected by the total resistance of MB-pMTJ, because the magnetoresistance effect in the reference layer is negligibly small. That implies that the resistance values for the different states in the reference layer are degenerated. On the other hand, the two different states in the reference layer bring different stray fields to the free layer, which generate two different minor loop with different switching fields.Therefore, the magnetic states in the reference layer can be differentiated by the two-step reading, before and after applying the appropriately pulsed magnetic field which can identify the initial state in the reference layer. This method is similar to distinguishing different magnetic states in an in-plane magnetized spin-valve element. We demonstrated that four different states in the MB-pMTJ can be distinguished by the two-step read-out. The important feature of the two-step reading is a practically large operation margins (large resistance change in reading) which is equal to that of a single MTJ. Even though the two-step reading is a destructive method by which 50% of the magnetic state is changed, this MB-pMTJ is promising for high density non-volatile memory cell with a minor cost of operation speed.

Dynamics of the reference layer driven by spin-transfer torque: Analytical versus simulation model

The magnetization orientation of the polarizing reference layer would fluctuate between the parallel and antiparallel states when the reference layer loses stability. In this paper, a combined analytical and simulation study is presented to predict the magnetic dynamics of the spin valve element with single-domain magnets as the free and reference layers. We acquire a complete phase diagram that includes the normal spin-torque switching and random magnetization fluctuations by tuning the spin torque strength between the free and reference layers. The phase dynamics strongly depends on the magnetization state of the reference layer, showing that the instability of the reference layer could be responsible for the random fluctuation events.

Spin scattering asymmetric coefficients and enhanced specific interfacial resistance of fully epitaxial current-perpendicular-to-plane giant magnetoresistance spin valves using alternate monatomic layered [Fe/Co]n and a Ag spacer layer

Using current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) measurement, we have evaluated the bulk and interface spin scattering asymmetric coefficients, βF and γF/N and the specific interfacial resistance, AR*F/N, for exchange-biased spin-valves consisting of artificially ordered B2 structure Fe50Co50 and Ag spacer layer. Artificially epitaxial ordered Fe50Co50 superlattices have been successfully fabricated on MgO (001) substrate by alternate monatomic layer (AML) deposition at a substrate temperature of 75 °C. The structural properties of the full epitaxial trilayer, AML[Fe/Co]n/Ag/AML[Fe/Co]n, on the Ag electrode have been confirmed by in situ reflection high-energy electron diffraction and transmission electron diffraction microscopy. A considerably large resistance-area product change and MR ratio (ΔRA > 3 mΩμm2 and MR ratio ∼5%) were confirmed even at thin AML[Fe/Co]n layer at room temperature (RT) in our spin-valve elements. The estimated values of βF and γF/N were 0.80 and 0.84 ± 0.02, respectively, from the Valet–Fert theory analysis of ΔRA as a function of thickness of the ferromagnetic layer (3, 4, and 5 nm) on the basis of the two-current model.

Current-driven chaotic magnetization dynamics in microwave assisted switching of spin-valve elements

The switching process of a uniformly magnetized spin-valve is considered. The system is subject to external dc applied fields and injected radio-frequency (RF) spin-polarized currents. The possibility of using the RF power to obtain a reduced coercivity of the particle is related to the onset of chaotic magnetization dynamics for moderately low values of the RF current amplitude. Perturbation technique for the estimation of the reduced coercive field is developed and applied to the microwave assisted switching of the particle. Numerical simulations confirm the predictions of the theory.

Optimization of spin-valve parameters for magnetic bead trapping and manipulation

Magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements were used to measure the switching characteristics of spin-valve (SV) arrays currently being developed to trap and release superparamagnetic beads within a fluid medium. The effect of SV size on switching observed by MOKE showed that a 1 μm×8 μm SV element was found to have optimal switching characteristics. MR measurements on a single 1 μm×8 μm SV switched with either an external applied magnetic field or a local magnetic field generated by an integrated write wire (current density ranging from 10 6 to 10 7 A/cm 2) confirmed the MOKE findings. The 1 μm×8 μm SV low field switching was observed to be +8 and −2 mT with two stable states at zero field; the high field switching was observed to be −18 mT. The low switching fields and the large magnetic moment of the SV trap along with our observation of minimal magnetostatic effects for dense arrays are necessary design characteristics for high-force, “switchable-magnet,” microfluidic bead trap applications.

Design of Element Configuration on Magnetic Rotation Angle Sensor with Spin-Valve Film

To reduce the error of a magnetic rotation angle sensor, we investigated the effect of sensor element configuration on the output signal of the sensor by using numerical calculations. The sensor elements were made of spin-valve films with a large shape-induced anisotropy. The magnetization of elements was calculated by minimizing the magnetic energy in a framework of the Stoner–Wohlfarth model. The sensor response was calculated as a function of the relative angle between the spin-valve elements. The error of signals was minimized when the relative angle was 99.5°. It is clear that this configuration is robust against the variation of shape-induced anisotropy.

High Bulk Spin Scattering Asymmetry in CPP Spin Valves With Alternated Monatomic $[\hbox{Fe/Co}]_{\rm n}$ Superlattice

We confirmed the structural properties of trilayered epitaxial films with AML [Fe/Co] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> and Au spacer on Au electrode by RHEED and TEM. A considerably large DeltaRA (2.61 mOmegamum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) resistance area product was observed in all metallic current perpendicular-to-plane giant magneto-resistance (CPP-GMR) spin-valve elements by using alternate monatomic epitaxial [Fe/Co] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> ferromagnetic layer with IrMn layer on the top. The estimated spin scattering asymmetry coefficient beta and interface asymmetry coefficient gamma was 0.81 and 0.43 plusmn 0.05, respectively. This value of beta is higher than that of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu laminated with Cu spacer or Heusler alloy which is largest value ever reported at room temperature.

Direct visualization of three-step magnetization reversal of nanopatterned spin-valve elements using off-axis electron holography

The remanent states and magnetization reversal mechanisms for nanopatterned Co/Cu/Py (Py=Permalloy) spin-valve elements with slotted-ring shapes have been studied using off-axis electron holography. The hysteresis loops of individual elements were quantitatively determined for slots parallel to the applied field, indicating that three identifiably separate steps occurred during reversal. Magnetization configurations at each stage were directly visualized, suggesting that Co and Py layers undergo coherent switching from onion state to flux-closure state and finally to reverse onion state. The effective loop rotation direction for the Py layer was clockwise.

Remanent states and magnetization reversal of nanopatterned spin-valve elements using off-axis electron holography

Remanent states and reversal behavior of nanopatterned spin-valve elements have been investigated using off-axis electron holography and Lorentz microscopy. Arrays of 30-nm-thick Co∕Cu∕Py (12, 6, and 12nm, respectively) slotted rings with typical outer diameters ranging from 300to600nm and slot angles of 30°, 60°, and 90° were fabricated by electron-beam lithography. Flux-closure configurations, with zero or one vortex, were the most common remanent states when magnetic fields were applied perpendicular to the slot direction. More complex configurations were introduced for fields parallel to the slot direction. Slotted nanorings with 300∕100nm of outer or inner diameter had the most uniform remanent configuration of fully closed flux. Holographic phase images confirmed that the Co and Py layers were coupled in opposite directions at remanence to minimize demagnetization energy. Magnetization reversal processes were investigated: the in-plane switching fields needed to fully reverse both ferromagnetic layers were typically on the order of 600Oe.

High-frequency giant magnetoresistance measurement to study the dynamics of a micron-scale spin-valve sensor

We report on the study of the magnetization dynamic of a single micrometric spin-valve element by high-frequency giant magnetoresistance measurement. We have combined transport measurements in the microwave range and large band ferromagnetic resonance with microantennas. The spin valve is used as an in situ demodulator to probe the individual dynamics of each layer and the relative phase of their oscillations. We deduce the conditions for coupling the hard and free layer dynamics and show the influence of a static magnetic configuration on the sensitivity of spin valves in the microwave range. This work offers a different approach for studying the dynamics of single isolated micron-sized systems.

TaN underlayers for spin valves deposited directly on top of Si substrates

TaN underlayers for spin valves were studied, which were deposited directly on top of Si substrates. The experimental results obtained with the TaN underlayer were compared with those obtained with other (Ta, Mo, and MoN) underlayers. The spin valve structure was Si/Underlayer(tÅ)/NiFe(21 Å)/CoFe(28 Å)/Cu(22 Å)/CoFe(18 Å)/IrMn(65 Å)/Ta(25 Å). The TaN underlayer for a spin valve element exhibited good adhesion to the Si substrate. The XRD patterns of the annealed TaN on bare Si substrate at 900 °C showed no Ta silicide phases, which suggests that the TaN layer may also be used as a diffusion barrier between Si substrate and the ensuing spin valve active layers, as well as an underlayer. A spin valve element having TaN underlayer deposited directly on top of a Si substrate showed a high MR ratio of about 8.3% after annealing at 200 °C. It is concluded that it is advantageous to use a TaN underlayer if one wants to fabricate spin valve elements directly on top of Si substrates.

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)

Magnetization dynamics in microscopic spin-valve elements: Shortcomings of the macrospin picture

We have studied ultrafast magnetodynamics in micropatterned spin-valve structures using time-resolved x-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism. Exciting the system with ultrafast field pulses of $250\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ width, we find the dynamic response of the free layer to fall into two distinctly different contributions. On the one hand, it exhibits localized spin wave modes that strongly depend on the shape of the micropattern. A field pulse applied perpendicular to the exchange bias field along the diagonal of a square pattern leads to the excitation of a standing spin wave mode with two nodes along the field direction. This mode is strongly suppressed for a pattern of elliptical shape. On the other hand, the integrated response of the free layer roughly follows a single-spin model with a damping constant of $\ensuremath{\alpha}=0.025$ independent of the shape and resembles the response of a critically damped forced oscillator.

Spin transfer switching in current-perpendicular-to-plane spin valve observed by magneto-optical Kerr effect using visible light

The authors have succeeded in optically observing the spin transfer switching (STS) in a current-perpendicular-to-plane spin-valve device. The device consists of three spin-valve elements, each of which comprises of a transparent top electrode, free and pinned magnetic layers separated by a copper spacer, and a bottom copper electrode. Despite a relatively large device resistance, spin transfer switching of the free layer was carried out using the spin injection from the transparent top electrode. Magneto-optical Kerr effect measurements performed through the transparent top electrode show clear changes in the signal synchronized with the resistance change as a result of the STS.

Multichannel Write and GMR Heads for Over 1 TB Tape System

This paper reports on the structure and performance of multichannel write and giant magnetoresistive (GMR) heads for a helical-scan tape system. The four-channel write head was fabricated with four layers stacked and shifted by a track pitch. The eight-channel read head was also stacked four layers, each of which has two spin-valve elements. The write head and read head were assembled into a commercial drive. Four channels are written in one revolution without a guard band and each of the eight channels is read, in half of the track pitch. The feasibility of a recording density over 4.3 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was established, which would make possible a tape system of over 1 TB using an 8 mm cassette