M.T. Johnson

Magnetic anisotropy in metallic multilayers

Ferromagnetic materials exhibit intrinsic `easy and `hard directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

Exchange biasing in MBE-grown Ni80Fe20/Fe50Mn50 bilayers

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy (MBE) grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001] and [110] orientations were investigated by Kerr effect measurements. A strong dependence of Heb and Hc on the growth orientation is observed. A strong uniaxial in-plane anisotropy introduced by the Fe50Mn50 layer was found for the [110]-oriented sample. Conversion electron Mossbauer studies revealed a roughness of 2–3 A and no significant moment reduction at the Ni80Fe20/Fe50Mn50 interface.

A novel interpretation of the complex permeability in polycrystalline ferrites

Abstract The complex permeability in polycrystalline ferrites is known to depend strongly on the size of the grains. A model has recently been developed to account for this, which considers a microstructure of crystalline ferrite grains with an intrinsic complex permeability μ i , each surrounded by thin, non-magnetic grain boundaries. It is demonstrated in the present paper that our model successfully accounts for the observation that in polycrystals the temperature variation of the initial permeability around the secondary maximum is observed to decrease with decreasing grain size. Further consequences of our model relating to such technologically important parameters as the temperature factor and the loss factor are also discussed.

Relation between grain size and domain size in MnZn ferrite studied by neutron depolarisation

Abstract The domain size has been determined by means of neutron depolarisation in a series of polycrystalline MnZn ferrites of varying grain size but unaltered composition. For ferrite samples with grain sizes between 0.3 and 3 μm, it is found that the domain size is identical to the grain size i.e. no magnetic domain walls are present within the grains.

Magnetic layer thickness dependence of the interlayer exchange coupling in (001) Co/Cu/Co

A dependence of the strength of the antiferromagnetic coupling across Cu on the Co layer thickness has been observed. The Co thickness dependence displays two clear peaks consistent with the recently predicted oscillation period of 6.2 A Co. Apart from the two peaks also several small peaks are visible on a scale of about 1 monolayer Co. Free‐electron calculations indicate that these rapid variations in strength may result from slight differences between the slopes and starting points of the two Co wedges that were involved in the experiment.

Magnetic layer thickness dependence of the interlayer exchange coupling

Recent progress in the research on interlayer exchange coupling, focusing on the effect of the magnetic layer, is reviewed. Several theoretical works have shown that the exchange coupling does not just involve an interaction localized at the interfaces, but the electronic properties of the sandwich as a whole. The important prediction that the strength of the exchange coupling should oscillate as a function of the magnetic layer thickness has been confirmed recently for the Co/Cu(100) and the Fe/Cr(100) system. The oscillation periods are strongly believed to be related to wave vectors that span extremal parts of the spin-resolved Fermi surfaces of the ferromagnets.

Loose spins in Co/Cu(100)

Using a double wedge sample, the effect of Co loose spins, introduced by J. Slonczewski, on the interlayer coupling in Co/Cu(100)/Co was studied. The predicted biquadratic coupling was not observed. However, a gradual increase of the maximum bilinear antiferromagnetic coupling strength with increasing loose spins layer thickness was found. This can be understood within the loose spins model, but also by assuming that the loose spins layer behaves like a ferromagnetic layer.

Lifshitz-type domain patterns observed in CoNbZr structures

Abstract A Lifshitz-type domain pattern containing wedge-shaped easy-axis oriented domains is shown which was observed using the Kerr effect in a 250 x 160 μm 2 pattern of amorphous soft-magnetic CoNbZr with a thickness of 0.75 μm. This pattern is similar to the one recently observed for a 140 μm wide NiFe element, but displays a small additional hard-axis oriented domain on the tip of the wedge-shaped domains. This domain pattern changes into an in-plane closure domain pattern when the width of the CoNbZr structure is decreased, and into an easy-axis domain pattern without closure domains when the CoNbZr is laminated by a nonmagnetic spacer having a thickness of 30 nm. For spacer thicknesses of 50 and 100 nm, an intermediate domain pattern containing smaller wedge-shaped domains and possessing some flux closure of the large easy-axis oriented domains around the nonmagnetic spacer along the edges of the structures was observed.

Geometrically induced domain pattern variations in laminated CoNbZr structures

Abstract Domain patterns of the upper magnetic layers of CoNbZr structures laminated with SiO2 or Al2O3 spacers were observed with a Kerr-effect microscope. In these relatively long laminated structures the width was varied in the range 20 μm ≤ w ≤ 150 μm, the spacer thickness in the range 8 nm ≤ b ≤ 100 nm and the magntic layer thickness in the range 0.35 mum ≤ D ≤ 2 μm. For the larger spacer thicknesses and/or the smaller pattern widths, an in-plane closure domain pattern similar to that of an unlaminated layer was observed, which suggests that the CoNbZr layers are magnetically uncoupled. For the other geometries, domain patterns featuring large easy-axis oriented domains were found, indicating that the CoNbZr layers are magnetically coupled through flux closure around the spacer. The transition from uncoupled to coupled magnetic layers takes place through an intermediate domain pattern incorporating enlarged easy-axis orientated domains which are only partially closed in the film plane by closure domains. The region in which this intermediate domain pattern occurs was found to coincide with the boundary between an in-plane closure domain pattern and a single easy-axis domain pattern as calculated according to Slonczewski et al. (IEEE Trans. Magn. MAG-24 (1988) 2045). Magnetic coupling between upper and lower CoNbZr layers is also shown to modify the behaviour of thin-film transformers having laminated CoNbZr cores.

Enhanced magnetic behaviour of narrow track thin-film heads determined from thin-film transformers

Abstract Measurements are presented in which the mutual magnetic inductance of a thin-film transformer has been measured as a function of the track width in the frequency range from 1 to 8 MHz. The thin-film transformer was made using a 2 μm structured amorphous CoNbZr layers as flux guide. It was found that the relative permeability at 1 MHz, determined from the mutual inductance, increased by a factor of 2.5 when the track width decreased from 250 to 10 μm. At 7 MHz, however, this effect was found to have almost disappeared. Domain observations using a Kerr-effect microscope were made of CoNbZr patterns which were similar in lay-out to those applied in the transformers. For all track widths, a closure domain pattern was observed, the periodicity of which changed with the track width. No relationship between both the relative area of the rotationally sensitive domains or of the closure domains and the measured mutual inductance was found for these layers. Instead, a relationship was found between the length of the 90° walls of the closure domains per unit area and the increased mutual inductance observed at 1 MHz.