R. W. Cochrane

Magnetoimpedance measurements of ferromagnetic resonance and antiresonance

We report the observation of both ferromagnetic resonance and antiresonance in a magnetic metal using a magnetoimpedance technique. In this experiment, the magnetoimpedance was measured as the frequency was swept from 30 MHz to 11 GHz at constant magnetic fields ranging up to 1.1 kOe (88 kA/m). The sample was an amorphous NiCo-rich soft-magnetic wire with a saturation magnetization sufficiently small to meet both the resonance and antiresonance conditions at frequencies below 10 GHz. A saturation magnetization, very close to that obtained through magnetometry, was deduced using a simultaneous fit to the field dependence of the resonance and antiresonance frequencies. This experiment clearly demonstrates that magnetoimpedance provides a powerful tool for characterizing the intrinsic properties of magnetic metals, with several advantages compared to standard ferromagnetic resonance techniques.

Ion-beam modification of Co/Ag multilayers I: Structural evolution and magnetic response

We describe the effects of 1 MeV Si+ ion-beam irradiation on a sputtered Co/Ag multilayer with layer thicknesses of 5 A for Co and 25 A for Ag, thicknesses for which the magnetoresistance is maximum in the as-deposited sample. X-ray diffraction, magnetization, and magnetoresistance measurements all point to the conclusion that the Co is initially dispersed through the Ag and segregates completely upon ion-beam bombardment. Throughout the process both Ag and Co grains maintain a high degree of texture, essentially face centered cubic (111). The magnetization behavior evolves from superparamagnetic to mixed superparamagnetic–ferromagnetic with ion dose whereas the room-temperature magnetoresistance decreases from 12% to 1.5% upon irradiation up to 5×1016 Si+/cm2. Simple models taking into account the size distribution of the Co particles have been used to analyze these phenomena in order to quantify the particle size distribution.

HIGH FREQUENCY BEHAVIOR OF SOFT MAGNETIC WIRES USING THE GIANT MAGNETOIMPEDANCE EFFECT

We have investigated the high frequency properties of several amorphous and polycrystalline wires mounted as inner conductors in coaxial lines. A static magnetic field was applied along the wire axis. The impedance spectra of the wires, measured using a network analyzer, show peaks in the real part of the impedance, which shift to higher frequency with the strength of the static field, a behavior typical of ferromagnetic resonance. The theoretical resonance condition predicts a straight line on an f02−H0 plot, where f0 is the resonance frequency and H0 is the resonant field, whose slope depends only on the saturation magnetization, Ms, of the material. All our wires obey this relation, and the values of Ms calculated from the slopes are in good agreement with those measured directly using a vibrating sample magnetometer.

MeV ion irradiation of Co/Cu multilayers

We examine the effect of MeV ion-beam irradiation on the giant magnetoresistance and related interlayer magnetic coupling in sputtered Co/Cu multilayers. At ion doses higher than 1013/cm2, the resistivity of the multilayers increases noticeably, well beyond that measured for pure copper or cobalt films. This increase in resistivity of the multilayers is tentatively ascribed to ion-beam-induced interface disorder. With increasing dose, the magnetic interlayer coupling passes systematically from a mainly antiferromagnetic (AF) coupling to a ferromagnetic one and, in parallel, the giant magnetoresistance (GMR) is progressively destroyed. A linear relationship between the GMR and the volume fraction of AF coupled regions is observed up to an ion dose of 2×1014/cm2.

Modeling of domain structure and anisotropy in glass-covered amorphous wires

Giant magnetoimpedance (GMI) at 10 MHz and longitudinal magnetization curves are used to investigate the anisotropy of Co68.15Fe4.35Si12.5B15 wires, glass-covered and after glass removal. The high resolution GMI response to the field shows hysteresis and large Barkhausen jumps, in good agreement with those observed in the magnetization curves. These are modeled through superposition of the response of the inner core and outer shell of the wires. The GMI response is calculated using the differential susceptibility deduced from the model, thus relating the domain structure to the observed magnetoimpedance.

First-Order Reversal Curves Diagrams of Ferromagnetic Soft Nanowire Arrays

First-order reversal curves (FORC) diagrams map the statistical distributions of magnetic hysterons, based on their critical fields (H c) and local interaction fields (Hu), throughout the magnetization reversal. This technique has been adapted to soft nanowire arrays deposited into alumina templates, with axes perpendicular to the template plane (diameter=50--175 nm, length=5--60 mum). From the topology obtained, the intrinsic physical properties of the array can be determined, notably the coercivities of individual nanowires and the interaction field at saturation. However, the FORC diagram may also yield information on other properties, such as the existence of a distribution of wire lengths

Ion-beam modification of Co/Ag multilayers II: Variation of structural and magnetic properties with Co layer thickness

The structural, magnetic and transport properties of rf sputtered Co/Ag multilayers with Co-layer thicknesses ranging from 1 to 14 A have been studied by a combination of x-ray diffraction, magnetic and transport measurements. The magnetoresistance at room temperature has a maximum value of more than 12% for a Co-layer thickness around 5 A. Magnetic measurements demonstrate that samples near this Co-layer thickness are in the transition region from superparamagnetic to ferromagnetic behavior. X-ray analysis indicates that, during deposition, a significant quantity of Co is dispersed throughout a highly textured Ag matrix. Upon irradiation with 1 MeV Si+ ions up to a dose of 5×1016 Si+/cm2, an initial demixing of the Co is followed by segregation into grains with the same texture as the Ag. The resulting changes in the magnetization and magnetoresistance are characterized on the basis of a log-normal distribution of the volume of the magnetic particles. As the particle sizes increase, a systematic evolutio...

Ion-beam irradiation of Co/Cu nanostructures: Effects on giant magnetoresistance and magnetic properties

We have studied the effects of ion irradiation at low doses (<5×1014 ions/cm2) on the structural properties, giant magnetoresistance (GMR), and interlayer magnetic coupling in Co/Cu multilayers. X-ray analysis combined with magnetic and resistivity measurements reveal that intermixing is promoted by ion irradiation while the periodic structure and crystallographic properties of the multilayers are not significantly altered. The GMR ratio of a multilayer decreases monotonically with ion dose. However, thermal annealing on an irradiated multilayer results in sharp recovery of the reduced GMR, and can be associated with a backdiffusion process in metastably intermixed regions. Hence, using ion irradiation and subsequent annealing, the GMR of a single multilayer can be altered reversibly over a wide range. The variation of GMR upon irradiation (or annealing) is accompanied by significant suppression (or improvement) of the antiferromagnetic interlayer coupling. The correlation between GMR and AF coupling, as ...

Modeling the magnetoimpedance in anisotropic wires

We have developed a theory of giant magnetoimpedance (GMI) in ideal anisotropic magnetic wires, which is valid over a broad field and frequency range. The emphasis is put on the moderate frequency GMI response in the low field region, where the wire is not saturated. The model agrees with experimental data on amorphous CoFeSiB wires, over broad frequency and field ranges, but does not correspond to an experiment at low field.

Giant magnetoresistance with low saturation field in (NixCo100−x/Cu) multilayers

We have investigated giant magnetoresistance (GMR) in NixCo100−x/Cu multilayers for x in the range 20–100. The GMR at room temperature is greater than 12% for magnetic alloys with x near 80, where the magnetocrystalline anisotropy is small. The smallest saturation fields are found near x=60 in the region where the magnetostriction vanishes. This combination of large GMR and small saturation field results in large MR field sensitivities up to 0.16%/Oe at room temperature. In order to maximize these parameters, we have also studied the dependence of the MR on the magnetic layer thickness and the number of bilayers.