Exchange-biased Spin Valves Research Articles

Experimental study of the local magnetization reversal in exchange-biased spin-valve sensors

During the write action in magnetoresistive random access memories, the bit status is set by applying current pulses through on-chip word lines. The magnetic fields generated in this way are sufficient for switching the bit status. In this paper, an experimental study of the switching process in small spin-valves is initiated by assessing the effect of magnetic fields, generated by the on-chip word lines, on the nucleation of magnetic domains. The transport properties of micron-scale exchange-biased spin-valve sensors were used to detect the influence of these local magnetic fields.

Perpendicular giant magnetoresistance of Co 91Fe 9/Cu exchange-biased spin-valves: further evidence for a unified picture

Measurements are reported of the increase in specific resistance, AΔ R, with increasing Co 91Fe 9 layer thickness in current-perpendicular to the plane (CPP) exchange-biased spin-valves (EBSVs) of Co 91Fe 9 and Cu. Analysis of these measurements yields a spin anisotropy parameter for Co 91Fe 9 of β=0.65±0.05, and a spin diffusion length of l CoFe sf=12±1 nm. The value of β agrees reasonably well with those obtained experimentally and theoretically for dilute CoFe alloys, thus providing additional support for a unified picture of spin-polarized transport in CPP-MR and bulk alloys. This value of l CoFe sf, and the previously determined l Py sf for Permalloy, scale approximately inversely with the residual resistivities of the two alloys.

Metastable permalloy monoxide based spin valves and pseudo-spin valves

We report the reactive magnetron sputter deposition of a metastable antiferromagnetic oxide, (Ni81Fe19)O or permalloy monoxide (PyO), suitable for exchange biasing permalloy (Ni81Fe19 or Py) film layers at room temperature or for manipulating metallic Py film coercivities. Examples are presented demonstrating how PyO can be incorporated in Py/Cu/Py-based exchange biased spin valves (SV) or nonexchange biased pseudo-spin valves (PSV) using the same metallic Py target for fabricating both the PyO pinning layer and the Py metallic layers. The presence of a Ti seed layer beneath the PSV or SV film structures and the oxidized silicon substrate is shown to have an insignificant effect on the film magnetic properties, demonstrating the feasible use of Ti as an adhesion or diffusion barrier layer in hybridized microelectronic devices based on silicon. Glancing incidence parallel beam x-ray diffraction data are presented supporting the existence of the antiferromagnetic monoxide rocksalt phase for the PyO. In addition, vibrating sample magnetometry and four point probe magnetoresistance analysis data are presented for the SV and PSV films.

Test of unified picture of spin-dependent transport in perpendicular (CPP) giant magnetoresistance and bulk alloys

Measurements of current perpendicular (CPP) magnetoresistance (MR) on hybrid and exchange-biased spin valves allow determination of the layer anisotropy parameter β for ferromagnetic alloys, thus allowing testing of whether this CPP β is similar to values obtained from direct measurements on, and theoretical estimates for, dilute bulk ferromagnetic alloys. Of special interest are alloys where β is expected to be negative. In this article, we derive a value of β for a Ni97Cr3 alloy using such spin valves based upon permalloy. We confirm prior CPP-MR results that this β is negative, but our inferred value, β=−0.35±0.1, is closer to bulk and theoretical estimates. Our data thus provide new support for a unified picture of β in the CPP-MR and bulk alloys. We also derive a short spin-diffusion length in NiCr, lsf=3±1 nm.

Resistance and spin-direction memory loss at Nb/Cu interfaces

A new method is described for measuring the loss of spin-direction memory (spin relaxation) in nonmagnetic materials and at nonmagnetic interfaces. The method involves inserting the material of interest into an exchange-biased spin valve and monitoring the associated reduction in the spin valve’s magnetoresistance measured with the current perpendicular to the planes. This technique is tested by investigating spin relaxation near Nb/Cu interfaces. A surprisingly large loss per interface is found, between 15% and 25% with larger values being associated with thicker Nb layers.

Angular dependence of giant magnetoresistance properties of exchange biased spin valves

Giant magnetoresistance (GMR) responses were studied as a function of applied measurement field angle in FeMn and NiMn pinned spin valves. As the measurement field varies away from the pinning field direction, the peak GMR ratio and the free layer coercivity decrease. The GMR curve changes from unsymmetrical to symmetrical with respect to the applied field polarity. The GMR responses from the component parallel to the applied field are proportional to the cosine of the angle between the pinning and the applied field. The results can be described well by a simple vector model, and an empirical method to measure the pinning field rotation is established.

Giant magnetoresistance properties of patterned IrMn exchange biased spin valves

Giant magnetoresistance (GMR) responses were studied in micron-sized IrMn pinned spin valve stripes. As the stripe height is decreased both the pinning field and free layer bias field increase while the GMR ratio stays almost constant. The free layer coercivity becomes zero for the shortest stripe while the pinned layer coercivity drops slightly at high aspect ratio. The pinned layer switching field is enhanced in the patterned films, indicating a strong magnetostatic coupling between the free and pinned layer that was further simulated micromagnetically.

Fabrication of exchange-biased spin valves with CoFeB amorphous layers

Amorphous Co72Fe8B20 is a soft ferromagnetic material with a high electrical resistivity and is, therefore, unique for use as the active layer in giant magnetoresistance (GMR) spin-valve structures. CoFeB/Cu/CoFeB/FeMn spin-valve structures were prepared by magnetron sputtering with varying FeMn thickness, deposition sequence, CoFeB deposition rate, Cu deposition rate, applied magnetic field, and annealing treatment, and their magnetic and magnetotransport properties were investigated by superconducting quantum interference device magnetometry and four-terminal magnetoresistance measurements. FeMn, above a critical thickness of 100 Å, is found to be a suitable biasing layer only if deposited on top of CoFeB. Optimum CoFeB and Cu thicknesses and deposition rates were also determined. A modest GMR ratio of 1.2% in a field range 10 Oe<H<15 Oe is achieved at T=10 K in a CoFeB (40 Å)/Cu(30 Å)/CoFeB(20 Å)/FeMn(100 Å) structure. However, we expect that the size of the GMR effect can be tailored independently by suitable engineering of the CoFeB/Cu interface.

Improvement of Magnetoresistive Character of NiO/NiFeCo/Cu/NiFeCo Exchange-Biased Spin-Valves Through Annealing in Magnetic Field

NiO(70 nm)/NiFeCo(5 nm)/Cu(2 nm)/NiFeCo(5 nm) spin-valve multilayers were prepared by electron beam evaporation. It is found that the magnetoresistive (MR) ratio of the as-deposited films is relatively small (about 1.6%) and there is no apparent biasing in MR curves of as-deposited multilayers due to weak exchange coupling. Through heat treatment at a proper temperature in a magnetic field, the exchange field of the materials increases and the multilayers show good exchange-biased spin-valve MR character with MR value reaching about 2.5%. The proper temperature range for annealing is 200-250° C. With further increasing temperature, MR ratio drops rapidly.

Enhanced exchange biasing in ion-beam sputtered bottom spin-valve films

IrMn exchange biased bottom spin-valve films of structure Ta/underlayer/IrMn/CoFe/Cu/CoFe/NiFe/Ta were prepared using ion beam deposition techniques. The exchange bias field exhibits strong underlayer thickness dependence. For the first time, a large exchange energy of 0.29 erg/cm/sup 2/ was measured in spin-valve films exchange biased by a disordered antiferromagnet, comparable to the values usually obtained in spin valve films exchange biased by an ordered antiferromagnet. We have conducted a comparative study on both bottom and top exchange biased spin-valve and ferromagnetic/antiferromagnetic bilayer films. The results indicate that the exchange field obeys very well the inverse pinned layer thickness law over a thickness range from 200 /spl Aring/ down to 10 /spl Aring/. The exchange energy for bottom spin-valve films is, however, a factor of two larger than that for top spin-valve films. When normalized, the exchange field exhibits the same temperature dependence for both bottom and top spin valve films. The enhancement in exchange biasing is mainly attributed to an enhanced texture for fcc [111] crystallographic orientation of the IrMn layer in bottom spin-valve films.

Giant magnetoresistance of current-perpendicular exchange-biased spin-valves of Co/Cu

We present the first measurements of the specific resistance change, A/spl Delta/R, with increasing Co layer thickness, t/sub Co/, of current-perpendicular (CPP) exchange biased spin-valves (EBSVs) of Co and Cu. A/spl Delta/R grows about as predicted for a long spin diffusion length in Co until about t/sub Co/=20 nm, after which it grows more slowly than predicted. Possible reasons for this deviation from prediction are considered, including a short spin diffusion length in Co.

Stress effects on the magnetic properties of FeMn and NiMn spin valves

The effects of stress on exchange coupling field, H/sub ex/, coercivity, H/sub c/, and pinning field, H/sub p/, in NiFe/FeMn and NiFe/NiMn exchange-biased spin valves deposited using ion beam sputtering have been examined. The samples were uniaxially strained (compressive and tensile) along the easy and hard axis directions. The hysteresis loops of the stressed spin valves were measured in situ along the easy and hard axes using a vibrating sample magnetometer. It is found that the NiFe/FeMn spin valves are more susceptible to the effects of stress than NiFe/NiMn, with the most significant effects occurring along the hard axis. In addition to being less susceptible to the effects of stress, NiFe/NiMn spin valves displayed more isotropic effects to the stress along the easy and hard axis directions.

Effect of roughness slope on exchange biasing in NiO spin valves

Exchange biasing field Hex and coercive field Hc of the exchange-biased NiO spin valves deposited over differently etched glass substrates and glass Si3N4 buffer layers were measured in order to investigate the effect of roughness at the NiO/NiFe interfaces. The magnetoresistive (MR) ratio, Hex, and Hc were not influenced by etching time, even though the rms roughness Rrms increased from 4.7 to 33 Å. However, the MR ratio, Hex, and Hc increased with Si3N4 buffer thickness, even when the Rrms had almost the same values. To explain this ambiguous dependence of Rrms, we consider an effect of the average slope of roughness instead of Rrms in an atomic force microscope image. The steep slope of roughness played an important role in Hex and Hc in NiO spin valves due to an increase in magnetostatic energy and the decrease in antiferromagnetic domain size.

Exchange-biased spin valves combining a high magnetoresistance ratio with soft-magnetic behavior

We report on the preparation of bottom spin valves combining a high giant magnetoresistance effect (above 10%) with a soft-magnetic behavior (coercivities of a few hundreds of A/m). By optimization of film composition and preparation conditions, it is possible to obtain materials having magnetoresistance values around 16%, which display minor loop coercivities of 0.2 kA/m, and sensitivities of 11%/kA/m.

Switching field interval of the sensitive magnetic layer in exchange-biased spin valves

The switching field interval, ΔHs, of Ni–Fe–Co-based thin films and spin-valve layered structures, sputter-deposited on a Ta-buffer layer, was studied. The switching field interval is the field range in which the magnetization reversal of a ferromagnetic layer takes place. In thin films, ΔHs is determined by the uniaxial anisotropy, induced by growth in a magnetic field. This anisotropy increases with the ferromagnetic layer thickness and saturates at a thickness of 10–25 nm. It also depends on the alloy composition as well as on the choice of the adjacent layers. In exchange-biased spin valves, an additional contribution to ΔHs was observed, which increases monotonically with increasing interlayer coupling. We explain this in terms of the effect on the magnetization reversal of the sensitive layer due to a simultaneous small, but temporary, magnetization rotation in the exchange-biased layer and lateral variations of the interlayer coupling. In addition, the effect of biquadratic coupling on ΔHs is discussed. Finally, the thermal stability of ΔHs is investigated.

Perpendicular-current exchange-biased spin-valve evidence for a short spin-diffusion lenght in permalloy

We present the first measurements of giant magnetoresistance with current perpendicular to the layer planes (CPP-MR) of an exchange-biased spin-valve (EBSV) - a controllable spin-switch. Interpreted with the theory of Valet and Fert, data on EBSVs of permalloy (Py) with Cu yield an unexpectedly short spin-diffusion length in Py −lsfPy=5.5± nm. Such a short length has implications for the interpretation of Johnson's data on a Py/Au/Py spin-switch.

Ion beam deposition and structural characterization of GMR spin valves

NiO exchange-biased "bottom" spin valves of the type NiO/NiFe/Co/Cu/Co/NiFe and FeMn exchange-biased "top" spin valves of the type NiFe/Co/Cu/Co/NiFe/FeMn were deposited by ion-beam deposition (except the NiO layer). Their magnetic properties, magneto-transport, and microstructures are characterized and compared with corresponding GMR spin valves deposited by dc magnetron sputtering. High-resolution cross-sectional transmission electron microscopy and X-ray diffraction reveal microstructural differences between ion-beam-deposited and dc magnetron sputtered spin valves. In particular, film texture, surface morphology, GMR ratio, exchange bias, interlayer coupling strength, and coercivity vary widely, but property-structure-processing correlation can be identified. A GMR ratio of /spl sim/9.7% was obtained on random textured NiO exchange-biased bottom spin valves by ion-beam deposition.

Comparison of hysteresis loops from giant magnetoresistance and magnetometry for perpendicular-current exchange-biased spin valves

We compare for permalloy (Py)/Cu/Py or Py/Ag/Py exchange-biased spin valves, hysteresis loops of: (1) giant magnetoresistance (GMR) with current perpendicular to the layer planes (CPP-MR), and (2) magnetization M measured for the same portion of the sample used to measure the CPP-MR. As expected, the two kinds of loops are mostly very similar. There are, however, some slight differences, the source of which is not yet clear.

Improvement of thermal stability and magnetoresistance recovery of Tb25Co75 biased spin-valve heads

The thermal stability of TbCo exchange biased spin valve sensors has been extended to 230 °C, by using a Co pinned layer rather than NiFe. Magnetoresistance (MR) remains essentially stable up to annealing temperatures TA of 230 °C and degrades to 1% at TA=240 °C. Providing TA does not exceed 230 °C, any loss in the MR signal can be fully recovered by applying a field perpendicular to the film plane. This improved thermal stability allows processing at higher temperatures. The unidirectional exchange in the TbCo/Co interface becomes uniaxial for TA>100 °C. Even though the unidirectional character was not recovered by any field/anneal treatment, uniaxial anisotropy above 120 Oe is maintained for TA=260 °C. Devices were fabricated using a process with a maximum temperature of 180 °C. No MR loss is found during process in test structures at wafer level. A prototype 10-μm-track width spin valve tape head for QIC applications gave a maximum output of 2.3 mVpp at 20 kfci.

Perpendicular-current transport in exchange-biased spin-valves

We have measured giant magnetoresistance (GMR) with current perpendicular to the layer planes (CPP-MR) in exchange-biased (EB) spin-valves-a controllable spin-switch. We will present CPP data on EB-spin valves of Permalloy (Py) and Cu, and explain how the data are analyzed. Interpreted with the theory of Valet and Fert, the data yield an unexpectedly short value for the length over which electrons lose spin-memory in Py, the spin-diffusion length, l/sub sf//sup Py/=5.5/spl plusmn/1 nm. We will show that the CPP parameters derived with the aid of single EB-spin-valves predict rather well the CPP-MRs of double EB spin-valves.