J O. Oti

High-Speed Characterization of Submicrometer Giant Magnetoresistive Devices

A microwave test structure has been designed to measure the high-speed response of giant magnetoresistive (GMR) devices. The test structure uses microwave transmission lines for both writing and sensing the devices. Pseudo-spin-valve devices, with line widths between 0.4 and 0.8 μm, were successfully switched with pulses whose full width at half-maximum was 0.5 ns. For small pulse widths τpw the switching fields are observed to increase linearly with 1/τpw. The increase in switching fields at short pulse widths is characterized by a slope which, for the current devices, varies between 4 and 16 μA s/m (50–200 Oe ns). The magnetoresistive response during rotation and switching was observed. For small rotations (∼45° between layer magnetizations) the GMR response pulses had widths of 0.46 ns, which is at the bandwidth limit of our electronics. For larger rotations (∼90°) the response pulses broadened considerably as the magnetic layers were rotated near the unstable equilibrium point perpendicular to the dev...

Magnetostatic effects in giant magnetoresistive spin‐valve devices

We report on magnetotransport measurements of spin valve films that have been fabricated into rectangular stripes with Au current leads. The spin valve films consisted of two magnetic NiFe layers separated by a nonmagnetic Cu layer. The top NiFe layer was magnetically pinned by a FeMn layer with an effective pinning field of 12 kA/m (150 Oe). After device fabrication, the transport properties changed dramatically as the stripe‐height of the device was decreased below 1 μm. Internal demagnetizing fields and magnetostatic interactions between the magnetic layers dominated the magnetic response. These interactions change the biasing point and the linearity, and cause a decrease in sensitivity to field changes. We have developed a simple single‐domain rotation model that includes magnetostatic, anisotropy, and exchange interactions to describe the magnetic behavior, from which we calculate the transport response.

Size effects in submicron NiFe/Ag GMR devices

We have measured the magnetoresistive response of submicron NiFe/Ag giant magnetostrictive (GMR) devices as a function of current density and field angle. In addition to magnetostatic broadening, we observe large lumps in the magnetoresistive response (Barkhausen jumps) due to domain switching. These effects lead to irregular device-specific magnetoresistive response curves, The large Barkhausen jumps are more pronounced at low current density while at high current densities the response is smoother due to self field stabilization. The detailed structure of the Barkhausen jumps is very sensitive to the angle of the applied magnetic field. These effects are general properties of a wide class of GMR materials that rely on incoherent reversal of many small magnetic domains. We compare the experimental data with a micromagnetic simulation which incorporates a phenomenological GMR transport model. The model qualitatively describes the experimental data and provides insight into the detailed micromagnetic behavior of these films.

Size effects and giant magnetoresistance in unannealed NiFe/Ag multilayer stripes

We have observed giant magnetoresistance (GMR) in unannealed NiFe/Ag multilayer thin‐film stripes. Rectangular stripes having constant thickness and a constant 11:1 length‐to‐width aspect ratio, but varying widths down to 0.5 μm, were measured. Two types of multilayer configurations were tested, a system of five NiFe/Ag bilayers with 5.5‐nm‐thick Ag spacer layers, and a system of nine bilayers with 4.4‐nm‐thick Ag layers. In contrast to the characteristic of annealed NiFe/Ag multilayer stripes, the unnannealed stripes produced increasing GMR ratios for decreasing stripe sizes, with the 0.5‐μm‐wide stripe of the five‐bilayer system exhibiting a ΔR/R of 2.5%. Barkhausen noise and response broadening also increased with decreasing stripe size, however. The results are discussed in terms of magnetostatic coupling of the NiFe layers within the stripes.

Simulating device size effects on magnetization pinning mechanisms in spin valves

The effects of magnetostatic interactions on the giant magnetoresistive (GMR) response of NiFe/Cu/NiFe spin valves are studied using an analytical model. The model is applicable to devices small enough for the magnetic layers to exhibit single‐domain behavior. Devices having lengths in the track‐width direction of 10 μm and interlayer separations of 4.5 nm are studied. Stripe heights are varied from 0.5 to 2 μm. The magnetization of one magnetic layer is pinned by a transverse pinning field that is varied from 0 to 24 kA/m (300 Oe). GMR curves for transverse fields are calculated. At zero external field the magnetization of the layers shows a tendency to align themselves antiparallel in the transverse direction. This results in an offset from the ideal biasing of the device. Broadening of the curves due to shape anisotropy occurs with decreasing stripe height and increasing magnetic layer thickness, and the magnetization in the pinned layer becomes less stable.

Magnetoresistance of thin-film NiFe devices exhibiting single-domain behavior

Rectangular NiFe stripes as small as 1/spl times/5 /spl mu/m were fabricated and characterized as a function of film thickness. Gold current leads were sputtered and patterned onto the stripes so that magnetoresistance measurements could be performed. A uniform in-plane magnetic field was applied transverse to the stripe length and at various angles from the perpendicular direction. For film thicknesses greater than 10 nm, the magnetoresistance for all of the devices had large jumps and hysteresis due to domain formation. As the thickness of the film decreased below 10 nm, the domain structure disappeared for stripe heights 2 /spl mu/m or less. Theoretical calculations of the magnetization reversals were obtained using a numerical implementation of the Stoner-Wohlfarth model for the switching of a single-domain ellipsoidal particle. The calculations were used to predict the switching field where the magnetization reaches an unstable threshold, causing a jump in the magnetization and magnetoresistance. The model was in close agreement with experimental results for various field orientations.

Proposed antiferromagnetically coupled dual‐layer magnetic force microscope tips

A magnetic force microscope tip designed from dual‐layer magnetic films of antiferromagnetically coupled magnetic layers is proposed. A theoretical analysis of the possible advantages of such a tip over conventional single‐layer tips is given, using an extension to dual layers of a previously described micromagnetic model of single‐layer tips. In contrast to single‐layer tips, the magnetic domains of dual‐layer tips are less sensitive to the fringing fields of the specimen, and the tips’ stray fields are greatly reduced, thus minimizing the likelihood of erasure of the sample magnetization. These properties of dual‐layer tips should lead to improved resolution of magnetic force microscopy images.

Field angle and current density effects in submicrometer spin valves for digital applications

We have characterized the magnetoresistive response of giant magnetoresistive spin valve devices, designed for digital applications, as a function of current density and magnetic field angle. The devices are designed to have only two stable states and are characterized by their positive and negative switching fields. The variations in the switching fields of submicrometer devices are compared with a multilayer single-domain model to determine how accurately the switching fields can be predicted. Significant deviation from single domain behavior is observed. Structure in the magnetoresistive response curve, indicating stable micro-domains, is seen in devices with small line widths and small aspect ratios. At large field angles, the micro-domains are stable to high field values and can dramatically affect the switching process. The variation of the switching fields with bias current and field angle depart considerably from the single domain model predictions.

Micromagnetic simulations of tunneling stabilized magnetic force microscopy

A micromagnetic model simulating the images produced by tunneling stabilized magnetic force microscopy is described. The images are related to the force interactions between the fringing fields from the imaged surface and stray fields from the sensing probe. The model, which allows for variations in the magnetization states of the probe, is used to examine the dependence of the interaction forces on varying probe‐film separations for a magnetic garnet film. The results show that the images are sensitive to separation and that the changing probe magnetization plays an important role in determining the final image.

Models of granular giant magnetoresistance multilayer thin films

Phenomenological micromagnetic and large-scale magnetization-dependent models of resistivity that produce giant magnetoresistance in granular multilayer magnetic thin films are described. Included in the models are intralayer and interlayer scattering components formulated explicitly in terms of the microstructural properties and characteristic transport lengths of the medium. The micromagnetic model provides insight into the influence of the magnetization distribution on the giant magnetoresistance response of the medium. The large-scale model which is derived from the micromagnetic model, is useful for obtaining media transport parameters from experimental data. Both models are used to study a set of annealed NiFe/Ag multilayer films.