Spin-valve Elements Research Articles

Scanning tunneling microscopy for laterally resolved measurements of magnetoresistance through a point contact

Using a scanning tunneling microscope for point contact measurements, we obtained laterally resolved information of the magnetoresistive properties of nanostructured spin-valve elements. A good correlation is found between magnetization and magnetoresistance curves of single-domain elliptical elements (450nm by 150nm), for magnetic fields applied along their long and short axes. In ring-shaped elements (inner and outer diameters 1.8 and 2.2μm), different magnetoresistance curves are acquired as different points around the ring are probed. The observed switching can be related to the onion state of the rings, and it clearly demonstrates a lateral resolution ⩽100nm.

Two-dimensional patterns of spin-wave radiation by rectangular spin-valve elements

The radiation of spin waves by rectangular spin-valve elements into a surrounding magnetic film has been studied experimentally using a novel micro-focus Brillouin light scattering setup allowing for spatially resolved measurements of the intensity of dynamic magnetization with a resolution better than 300nm. The investigated elements have lateral dimensions of 1.3×2.3μm2 and consist of a 10nm thick Co80Fe20 and a 5nm thick Ni81Fe19 layer separated by a 4nm thick Cu spacer layer. It was found, that the elements radiate spin waves at frequencies corresponding to their laterally quantized spin-wave eigenmodes. Two-dimensional distributions of intensities of spin waves radiated by different eigenmodes were recorded outside the element. It was shown, that the radiation patterns consist of several rays intersecting each other and forming spots where the amplitude of variable magnetization locally increases. A theoretical model qualitatively explaining the observed radiation patterns has been suggested.

Local precessional reversal of a spin-valve element

Magnetisation reversal by precession has been observed for a micron-sized (4 μm×8 μm) spin-valve element in response to 250 ps field pulses. Due to the dispersion of the magnetisation orientation at remanence, the magnetisation of only a small fraction of the sample is aligned perpendicular to the excitation field and can undergo precessional reversal. Although this leads to an overall reduced magneto-resistance ratio, two well-defined signal levels can be established.

Narrow Track Current-Perpendicular-To-Plane Spin Valve GMR Heads

We have investigated the performance of dual spin valve heads in the current-perpendicular-to-plane (CPP) geometry on dynamic electrical tests (DET) and quasi-static tests (QST). Shielded CPP reader and stitched pole writer composite heads were fabricated. A various physical read track width and area of the dual spin valve element were deliberately obtained in the range of 41-175 nm and 0.0034-0.022 /spl mu/m/sup 2/, respectively. Materials used for the CPP dual spin valve are Co/sub 90/Fe/sub 10/, Ni/sub 80/Fe/sub 20/, Cu, Ru, Ta, and Pt/sub 50/Mn/sub 50/, which are common in industrial current-in-plane (CIP) spin valves. The shape and smoothness of QST transfer curve, readback waveform, micro-track, and full-track profiles of the narrow read track CPP heads are essentially just the same as those of CIP spin valve. A track-average-amplitude of 0.6 mV was obtained at the bias current density of element around 100 MA/cm/sup 2/ in the CPP head with the physical read track width of 47 nm. Overall reader resistance of the CPP heads at head gimbals assembly (HGA) level was in the range of 10-35 /spl Omega/ and proved to be well appropriate.

Precessional magnetization reversal in microscopic spin valve cells

We demonstrate stable and reversible precessional switching of the magnetization in microscopic spin valve cells. Magnetization reversal is achieved by applying sub-ns-hard axis magnetic field pulses under compensation of the easy axis offset field. Multiple application of a given hard axis field pulse leads to a stable and reversible switching back and forth of the cell's magnetization. Applications of this precessional switching process in ultra fast low power magnetic random access memories can be foreseen.

Study on requirements for shielded current perpendicular to the plane spin valve heads based on dynamic read tests

Shielded current perpendicular to the plane (CPP) spin valve heads with about 20 Gbit/in.2 dimensions were fabricated and tested both quasistatically and dynamically. Dual synthetic spin valves with 2.3 mΩ μm2 of resistivity change and area product (dRA) was used. Quasistatic tests were conducted to determine the current efficiency loss caused by the specific CPP head structure. Dynamic tests were performed to examine the flux efficiency of the readback process in the spin valve element, in order to confirm the calculation results which take into account the increase of effective anisotropy field caused by the circular sense current field. Output voltage of about 260 μV (peak to peak) was successfully obtained from the dynamic tests. Finite element modeling analysis of current distribution and temperature rise, together with degradation experiments with excessive sense current, was conducted to predict the maximum allowable current density in the 200 Gbit/in.2 dimension, where read track width and stripe height are assumed to be 100 nm. With these results, requirements of spin valve material properties (dRA, RA) and the CPP structure was predicted. Essential issues to satisfy the requirements and possible solutions are also discussed.

Coherently suppressed ringing of the magnetization in microscopic giant magnetoresistive devices

We study the magnetotransport response of 1 μm×4 μm size spin valve elements to ultrashort (pulse length 0.2–4 ns) easy axis magnetic field pulses. When applying an additional static hard axis field, pronounced magnetic precession is observed in the magnetoresistance. Damped precession occurs after the setting of the pulse and after pulse termination (“ringing”). The strength of the ringing can be controlled by adjusting the ratio of the precession period to the effective pulse length. When the pulse length matches the precession period, the ringing after pulse termination is effectively suppressed.

Coherent suppression of magnetic ringing in microscopic spin valve elements

We demonstrate the coherent suppression of magnetic precession in microscopic spin valve elements after the decay of ultrashort magnetic field pulses. The magnetization dynamics in 1 μm×4 μm wide giant magnetoresistance devices are studied by measuring the magnetotransport response to ultrashort magnetic field pulses (pulse length 0.2–4 ns). Under the influence of a static field perpendicular to the pulsed field, pronounced magnetic precession is observed after the onset of the pulse as well as upon pulse termination. The precession after the pulse decay (“ringing”) can be effectively suppressed by adapting the effective pulse length to the precession period.

Recording processes in perpendicular patterned media using longitudinal magnetic recording heads

An experimental study of the recording processes in patterned magnetic media is presented. The reading of patterned media using spin-valve elements is compared to the signal levels from magneto-resistive sensors. Writing and reading of patterned columnar media at high areal densities is demonstrated. A new experimental technique has been developed that allows precise determination of the location of the write gap poles with respect to the patterned media column during the write process. Implications for patterned media write synchronization and the write head field requirements are discussed.

Magnetic switching in 100 nm patterned pseudo spin valves

Progress in developing operative high-density magnetoresistive random access memory (MRAM) devices relies critically on tailoring the magnetic switching occurring in arrays of small patterned pseudo spin valve (PSV) elements. Co/Cu/NiFe PSV films, produced by sputtering in the presence of a magnetic field, have an in-plane anisotropy and switching fields of typically 10 Oe for the soft NiFe and 40 Oe for the hard Co. These films were patterned into arrays of elliptical and circular elements with dimensions of 80 nm to 10 /spl mu/m. The layered structure in these large-area arrays of compositionally modulated PSV elements is preserved through the patterning processes. Hysteresis measurements of the dot arrays show that the switching field of the hard layer increases significantly with decreasing element size, reaching 600 Oe for the smallest elements. Additionally, in patterned elements the soft layer switches prior to field reversal due to magnetostatic coupling between the layers, leading to antiparallel alignment at remanence.

Write induced magnetic field in spin-valve heads

The magnetic field at a spin valve (SV) element is measured by exciting the inductive write head with a pulse current. The measured write-induced magnetic field can be used as an indicator of write-induced stability. A piggyback type SV head and a merged type SV head are compared in terms of the magnetic field at the SV. While the write-induced magnetic field of the merged head saturates at 40 mA as current increases, that of piggyback head does not saturate even at 120 mA. The method reported here can be used to evaluate the SV magnetic performance and to obtain the yoke saturation curve of the inductive write head in a merged SV head.

Magnetic characterization and modeling of FeMn/Co/Ru/Co artificial antiferromagnets

FeMn/Co/Ru/Co artificial antiferromagnets for use in giant magnetoresistive spin-valve elements were fabricated and magnetically characterized. The magnetization behavior of the films can be understood by coherent rotation of the magnetizations of the layers. A simple model is presented which reproduces all the basic characteristics of the magnetization loops, e.g., shape, switching fields, and magnitude of the pinning field as a function of thickness. The model is also extended to a complete spin-valve system, including the effects of coupling between the biased layer and the free layer.

Extremely clean sputtering process for spin valves:: exchange anisotropy and spin-dependent transport

The extremely clean sputtering process (XC process) was newly introduced in the fabrication of ultra-thin spin valves to control the microstructure and to establish the excellent magnetic properties. The relation between cleanness during film deposition process and the magnetic properties of spin valve elements has been discussed in connection with their microstructure. As results we found that (1) the purification of the sputtering atmosphere facilitates the growth of grains; (2) the enlargement of the antiferromagnetic grains in the XC processed films enhances the exchange anisotropy at room temperature; and (3) the reduction of the grain boundaries, impurities, and defects in the XC processed spin valves results in the reduction of saturated resistivity and the enlargement of the magnetoresistance (MR) ratio. The spin valve with 9.7% of the MR ratio even in the total thickness of 148 Å except for the capping Ta layer was realized under the XC process.

Highly sensitive giant magnetoresistive sensor consisting of NiO spin-valve element and AC bias current line

We proposed a new sensitive giant magnetoresistive sensor consisting of a multilayered structure including a spin-valve element and an AC bias current line. Micro patterned spin-valve film exhibits a step-shape MR loop, in which free layer magnetization switches like a single domain. High sensitivity is attained utilizing only the edge of this MR loop. The magnetic field induced by the bias current was set to be slightly smaller than the rising field of the MR loop. The sensor detected an external field change of 0.5 Oe, and operated using a bias current having a pulse width of 30 ns.

Effect of stray-field coupling on active spin-valve elements analyzed by Lorentz transmission electron microscopy

In situ experiments using Lorentz transmission electron microscopy have been performed on 10 μm×30 μm active spin-valve elements through which a current is applied during observation of the magnetization reversal of the sense layer. It is shown that the reversal mechanism and the change in resistance are very different depending on the relative orientations of the easy axis versus the applied field. The results show clearly that the contributions of the magnetostatic fields, due to either ferromagnetic coupling or stray-field coupling, are higher than that of the field induced by the sensing current. A model is proposed to predict the offset of the giant magnetoresistance curves with respect to zero field. This model has been found to be in good agreement with the experimental results.

Spin-Valve Film for Sensitive Giant Magnetoresistive Sensor Using AC Bias Magnetic Field

A giant magnetoresistive material for a new sensitive magnetic sensor element was developed by a NiO spin-valve film. The magnetic sensor is composed of a multilayered structure that includes the spin-valve element and a bias current line. High sensitivity is obtained by using only the (rising or falling) edge of the magnetoresistive (MR) loop. An AC bias magnetic field induced by a square-wave bias current (plus or minus) is applied to the spin-valve element. The bias magnetic field is set to be slightly smaller than the rising field. The resistance of the spin-valve element is constant, if an external magnetic field is not applied. The output signal synchronizes with the input bias current, if the bias field is made to cross the rising edge of the MR loop by applying an external magnetic field. The spin-valve film: [NiO/NiFeCo/Cu/NiFeCo] exhibits the appropriate properties for the sensor element, such as an MR ratio of 5.1%, a sensing field of 0.2 kA/m, a driving field of 1.4 kA/m, an element destroying field of 32 kA/m and a high corrosion resistance. Moreover, the spin-valve element, patterned to a microscopic size, exhibits a step-shaped MR loop in which a free-layer magnetization switches like a single domain. The magnetic sensor using this spin-valve film detects an external field change of 0.04 kA/m, and operates at 30 ns.

ＥＳＤによるスピンバルブヘッドのピン層磁化反転過程

We measured the micro-track profiles of spin-valve heads after damage by ESD. The output was reduced and the profiles showed several peaks depending on the ESD voltage. We proposed two models for the magnetization reversal process of the pinned layer in the spin-valve heads, and calculated the micro-track profiles by using 3D micro-magnetic simulation. In the first model, the pinned layer magnetization is gradually rotated as the ESD voltage is increased. In the second model, the direction of the pinned layer magnetization is reversed in the center of the spin-valve element and the reversal region width is increased as the ESD voltage is increased. The calculation results for the second model agreed fairly well with the experimental results. We think that at first the pinned layer magnetization is locally reversed in the center of the element after ESD in the spin-valve head.

Contribution of the magnetic field induced by the current passing through a spin-valve element

In situ experiments in a Lorentz microscope have been performed on active spin-valve elements, and a correlation between magnetoresistance and the magnetic domain structure is shown. The presence of a magnetic field induced by the applied current is clearly demonstrated. A simple model is presented, which is in good agreement with the experimental data. In addition, the effect of stray-field coupling between the ferromagnetic layers, arising because of the free magnetic poles at the edges of the element, has been observed.

Correlation between domain structure and magnetoresistance in an active spin-valve element

In situ experiments in a Lorentz transmission-electron microscope have been performed on spin-valve elements through which a current is being applied. The ability to simultaneously measure the resistance and observe the domain structure of the element has allowed us to correlate magnetoresistance with the angle between the magnetization directions in the ferromagnetic layers. The angles have been obtained from summed image differential-phase-contrast images of the domains. A model is proposed to calculate the resulting magnetoresistance. The calculated data are in good agreement with the experimental magnetoresistance measured during observation of the magnetization reversal mechanism.

Anisotropy control in fabrication process for NiMn spin-valve dual element heads

NiMn antiferromagnets with a high blocking temperature are attractive for application to the pinning layer in spin-valve heads, because a highly stable exchange coupling field is expected through their use. We have fabricated NiMn spin-valve heads having a CoZrTa bottom shield and a CoZrTa/NiFe write element, using an anisotropy control process which provides a desirable magnetization configuration in each magnetic layer. These heads showed highly stable, symmetrical, and Barkhausen noise free readback waveforms with output voltage of over 800 /spl mu/V/sub pp///spl mu/m.