In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

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.

https://iopscience.iop.org/article/10.1088/0034-4885/59/11/002/pdf

Magnetic anisotropy in metallic multilayers

The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.

Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features

(1993). Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions. Advances in Physics: Vol. 42, No. 5, pp. 523-639.

Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions

An overview is given of the material properties of exchange-biased spin-valves. More specifically, we discuss the microstructure and magnetic properties of these materials as relevant for (future) industrial application in magnetoresistive read heads for rigid disk and tape recording and Magnetic Random Access Memory (MRAM) devices.

Exchange-biased spin-valves for magnetic storage

The ferromagnetic interlayer coupling in sputter-deposited permalloy/copper/permalloy exchange-biased spin valve multilayers has been measured as a function of the copper thickness. The variation with thickness may, for t/sub cu/>1.7 nm, be analyzed in terms of the Neel model for magnetostatic coupling due to correlated interface roughness, using parameters which are consistent with the observed microstructure.

/pdf/on-the-ferromagnetic-interlayer-coupling-in-exchange-biased-2mgtlcto9y.pdf

On the ferromagnetic interlayer coupling in exchange-biased spin-valve multilayers

Columnar microstructures in magnetron-sputtered refractory metal thin films of tungsten, molybdenum and W-Ti-(N)

PbZrxTi1−xO3 films have been grown heteroepitaxially onto (001)SrTiO3 and SrRuO3/(001)SrTiO3 by organometallic chemical vapor deposition. As a start, the microstructure of PbZrxTi1−xO3 films on (001)SrTiO3 was studied as a function of the zirconium fraction, x. Rutherford backscattering spectrometry, including channeling experiments, and transmission electron microscopy have shown that the microstructure is dominated by the crystal structure of the PbZrxTi1−xO3. In the case of tetragonal PbZrxTi1−xO3 the films may contain a‐axis oriented regions. These regions have not been observed for films with a composition giving a rhombohedral unit cell. Despite the rather large mismatch of rhombohedral PbZrxTi1−xO3 with the (001)SrTiO3, values as low as 4% for the minimum channeling yield have been obtained. For a rhombohedral film the ferroelectric properties have been measured. To this end a single crystalline PbZr0.8Ti0.2O3 film was grown onto (001)SrTiO3 provided with a heteroepitaxial SrRuO3 electrode grown by...

Structural and electrical characterization of heteroepitaxial lead zirconate titanate thin films

In situ X-ray diffraction: a useful tool to investigate hydride formation reactions

Inverted spin valves (Si substrate/Ta/X/Fe50Mn50/Ni80Fe20/Cu/Ni80Fe20/Ta with X=Cu or Ni80Fe20) have been investigated by transmission electron microscopy and magnetoresistance measurements. The newly proposed Ni80Fe20 buffer layer provides a significantly increased magnetoresistance ratio compared to the usual Cu buffer. This makes it possible to use inverted spin valves instead of conventional spin valves in magnetic heads and sensors, offering reduced corrosion sensitivity and a higher GMR ratio. First test structures for tape heads have been fabricated and evaluated.

A.E.M. De Veirman

Papers

On the ferromagnetic interlayer coupling in exchange-biased spin-valve multilayers

Columnar microstructures in magnetron-sputtered refractory metal thin films of tungsten, molybdenum and W-Ti-(N)

Structural and electrical characterization of heteroepitaxial lead zirconate titanate thin films

In situ X-ray diffraction: a useful tool to investigate hydride formation reactions

Inverted spin valves for magnetic heads and sensors