R.J. Pollard

Giant magnetoresistance in sputter deposited Co/Cu multilayer systems

This paper presents some results of magnetoresistance measurements on sputter deposited Co/Cu thin film multilayers. We find that large values of magnetoresistance approaching 40% can be obtained and that these values oscillate in magnitude with the thickness of the non-magnetic spacing layer with a period of about 1.1 nm. It appears that the multilayer must be grown on a particular form of underlayer to realize these large values of magnetoresistance ratio.

Magnetic properties of cobalt layers and Co/Pt multilayers

Abstract This paper presents results from an investigation of the development of the structure-dependent magnetic hysteresis properties of sputter-deposited Co/Pt multilayers. It shows that perpendicular magnetisation can be obtained in layered structures containing single and double Co layers, and that hysteresis properties are improved with the number of Co/Pt bilayers until, in our structures, near-optimum properties are obtained in a ten-period Co/Pt multilayer.

A magnetic and high resolution structural investigation of Pt/Co multilayers

Abstract The paper discusses the results of experiments on sputter-deposited polycrystalline Pt/Co multilayer films. A particular aim was to measure the magnetic properties of multilayers with differing periods and layer thicknesses and to examine their atomic scale structure using high resolution electron microscopy. We show that the magnetic properties of the multilayers are strongly dependent of their microstructure. Perpendicular magnetisation at small cobalt layer thickness corresponding to one or two monolayers is associated with a strong surface anisotropy and a columnar and coherent crystal growth texture. At greater bilayer thicknesses this effective anisotropy and mode of growth is lost.

Giant magnetoresistance in Co/Cu multilayer thin films

Abstract This paper presents and discusses magnetic, magnetoresistive and structural properties of small grained, polycrystalline sputter-deposited Co/Cu multilayers. Of particular interest is the effect of different buffer-layers. GMR values are obtained in multilayers with little texture but with a well-defined layered structure.

Magnetic hyperfine fields in Fe/Co and Fe/Cu multilayers

Abstract Hyperfine fields in magnetic multilayer samples of Fe/Co and Fe/Cu having equal layer thickness of Fe and Co and Fe and Cu were measured in samples having layer thickness in the range 10–150 A by 57Fe Mossbauer spectroscopy. Measurements were made at 290, 77 and 4.2 K. The mean hyperfine field B hf at Fe sites in Fe/Co multilayers was found to increase with decreasing layer thickness. The mean hyperfine field at Fe sites in Fe/Cu multilayers was observed to decrease with decreasing layer thickness. The decrease in B hf between 4.2 and 290 K was found to be much greater in Fe/Cu samples than in Fe/Co. The Mossbauer spectra are well fitted with two sextet components, one corresponding to interface Fe sites and the other to midlayer Fe sites. Analysis of the relative intensity of these components measures the interface layer thickness. The variation of hyperfine field with sample layer thickness can be well accounted for using the measured thickness of the interface layers.

Giant magnetoresistance in highly oriented sputter deposited (111) Co/Cu multilayers

Abstract This Letter reports the results of giant magnetoresistance (GMR) and magnetic hysteresis measurements on sputter-deposited Co/Cu multilayers with random or highly oriented (111) textures. The multilayers were designed to be at the second maximum of the oscillatory exchange coupling and were deposited on to ion beam etched (111) Si substrates. We find that randomly oriented multilayers exhibit clear antiferromagnetic coupling and ∼ 20% GMR, whereas (111) oriented multilayers are ferromagnetically coupled and show near-zero GMR.

The effect of quench rate on the microstructure and coercivity of some Nd-Fe-B based ribbons

Abstract The microstructure of some melt spun NdFeB and NdFeBNb ribbons has been examined using transmission electron microscopy. The correlation between microstructure and coercivity has been investigated as a function of quench rate. In general the coercivity of these ribbons increased as the grain size became smaller, reaching an upper limit just before the ribbon became amorphous at the higher quench rates. However, in a narrow range of wheel speeds (≈8–9 m/s using a Cu wheel) we observed a microstructure containing fairly large grains (≈0.5 μm) of the hard magnetic Nd 2 Fe 14 B phase surrounded by an amorphous matrix. These ribbons had coercivities of less than 16 kA/m and we attribute the fall in coercivity to the existence of a ferromagnetic amorphous grain boundary. In the ribbons containing Nb additions we observed fine coherent precipitates in the hard magnetic phase. Similar precipitates have been observed in sintered NdFeB magnets containing Nb additions. The precipitate size decreased with increasing quench rate and attained dimensions comparable with the domain wall width in the hard phase (≈5 nm) at wheel speeds in the range 5–7 m/s. We attribute the enhanced coercivities of the under-quenched Nb-containing ribbons to the presence of coherent precipitation in the hard magnetic grains. The coercivity of the ternary ribbons fell in this range of wheel speeds.

Magnetoresistive remanence curves in Co/Cu multilayers

Abstract We report measurements of magnetoresistive remanence curves and initial magnetoresistance curves on Co/Cu multilayers. The initial magnetoresistance curve shows an enhance magnetoresistance change over that of the hysteretic magnetoresistance curve. Using remanence curves we are able to separate out the irreversible and reversible components contributing to the magnetoresistance. We show that it is irreversible changes of resistance within the multilayers from an initial low energy demagnetised state that is responsible for this ‘extra’ magnetoresistance.

Magnetic coupling and magnetic anisotropy in Fe/Co and Fe/Cu multilayers

Abstract Magnetisation measurements of magnetic multilayer samples of Fe/Co and Fe/Cu having equal layer thickness of Fe and Co and Fe and Cu, respectively, were carried out at room temperature using an alternating gradient force magnetometer (AGFM). Comparison of the saturation magnetisation measurements with calculated values of magnetisation indicated that the magnetic moments of the Fe layer and the Co layer are ferromagnetically coupled. Mossbauer spectroscopy studies at room temperature and at 4.2 K show that in the Fe/Cu multilayers and some of the Fe/Co multilayers the magnetic easy axis lies in the plane of sample. In several other Fe/Co multilayers a normal component of magnetisation is observed and it is necessary to introduce a magnetic anisotropy term favouring a magnetic easy axis normal to the sample plane. Measurement of hysteresis cycles and applied field Mossbauer studies enable the magnetic anisotropy of the samples to be evaluated.

Giant magnetoresistance and microstructure in highly oriented Co/Cu multilayers sputter-deposited on (111) silicon

This paper reports the results of giant magnetoresistance (GMR) and magnetic hysteresis measurements on sputter-deposited Co/Cu multilayers with random or highly oriented (111) textures. These results are correlated with microstructural information obtained by high-resolution transmission electron microscopy and by X-ray diffraction. The multilayers were designed to be at the second maximum of the oscillatory exchange coupling and were deposited onto ion beam etched (111) Si substrates. We find that randomly oriented multilayers exhibit clear antiferromagnetic coupling and ∼20% GMR, whereas (111)-oriented multilayers are ferromagnetically coupled and show near-zero GMR.