A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

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Multiferroic and magnetoelectric materials

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

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Exchange bias in nanostructures

Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

A voltage-induced symmetry change in a ferromagnetic material can change its magnetization or magnetic anisotropy, but these effects are too weak to be used in memory devices. Researchers have now shown that a relatively small electric field can cause a large change in the magnetic anisotropy of a few atomic layers of iron. The results could lead to low-power logic devices and non-volatile memory cells.

Large voltage-induced magnetic anisotropy change in a few atomic layers of iron

We have synthesized Co fine particles with the average diameter $(D)$ of less than 500 \AA{} by sputtering Co in a somewhat high inert-gas pressure. It has been found that there is a close relationship between the particle size and the crystal phase; that is, pure fcc (\ensuremath{\beta}) phase for $Dl~200\AA{},$ a mixture of hcp (\ensuremath{\alpha}) and \ensuremath{\beta} phases for $D\ensuremath{\sim}300\AA{},$ and \ensuremath{\alpha} phase with inclusion of a very small amount of \ensuremath{\beta} phase for $Dg~400\AA{}.$ Precise structural characterizations have revealed that the \ensuremath{\beta} particles are multiply twinned icosahedrons and the \ensuremath{\alpha} particles are perfect single crystals with external shape of a Wulff polyhedron. In order to explain the size effect on the crystal phase of Co fine particles, we have performed theoretical calculations for total free energies of an \ensuremath{\alpha} single crystal, a \ensuremath{\beta} single crystal, and a multiply twinned \ensuremath{\beta} icosahedron. The present calculations well explain the size dependence of the crystal phase of the Co fine particles, and have revealed that the stabilization of \ensuremath{\beta} phase, confirmed by previous studies, is the intrinsic effect caused by the small dimensionality of fine particles. Moreover, the phase transformations that occurred in annealing experiments can also be explained by the theory.

Size effect on the crystal phase of cobalt fine particles

Anomalous Hall voltage was measured for FePt ${L1}_{0}$ films having very high magnetic anisotropy. The magnetic anisotropy ${K}_{1}$ and ${K}_{2}$ were determined with high accuracy by analyzing the magnetization curves obtained from the Hall voltage measurement. The saturation magnetization ${M}_{s}$ of the samples with different chemical-order parameter (S) exhibits a different temperature dependence, implying that the Curie temperature weakly depends on S. The first-order anisotropy ${K}_{1}$ gradually increases with S, while the second-order anisotropy ${K}_{2}$ remains almost constant of about $5\ifmmode\times\else\texttimes\fi{}{10}^{6}\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{c}.$ The temperature dependence of ${K}_{1}$ is correlated with S, that is, ${K}_{1}$ with a small S is more temperature dependent than that with a large S. These behaviors are quite similar to the temperature dependence of ${M}_{s}$ with different S, and can be explained by the conventional model based on thermal spin fluctuations. The domain wall energy ${\ensuremath{\sigma}}_{w}$ evaluated by the theoretical analysis of the stripe-domain structure tends to increase linearly with S, in a similar manner as that of ${K}_{1},$ whereas the exchange stiffness constant A of about $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}$ deduced from ${\ensuremath{\sigma}}_{w}$ and ${K}_{u}{(=K}_{1}{+K}_{2})$ hardly depends on S.

Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films

We have explored magnetic properties and structural characteristics of Fe/Pt multilayers after annealing at various temperatures in order to clarify the growth process of the ordered Fe–Pt phase in the Fe/Pt multilayer structures. It is found that rapid diffusion at Fe/Pt interface occurs at temperatures 275–325 °C and the multilayer structure directly transforms to the ordered (fct) phase when Fe and Pt layer thickness is almost equal. The ordering parameter S is evaluated to be 0.50–0.65 after annealing at 300–325 °C, and then is significantly enhanced to ∼1.00 with increasing annealing temperatures. By comparing the thermal processes of these different multilayer structures, it is found that the rapid formation of the fct phase in the multilayers with Fe/Pt≅1 is due to relatively rapid diffusion at the interface.

Lowering of ordering temperature for fct Fe–Pt in Fe/Pt multilayers

Thin-film inductors for 1 GHz-drive mobile communication handset application has been demonstrated. This is the possible practical application of soft magnetic films in 1 GHz range. Fe61Al13O26 with Ms=1.2 T, ρ=500 μΩ cm, fr=2 GHz were used for the inductors. L=7.6 nH, R=6.5 Ω, and Q=7.4 were obtained at 1 GHz in a 370 μm×370 μm square four turn spiral of line/space=11 μm/11 μm covered with 0.1-μm-thick slitted Fe61Al13O26 film with Cr underlayer. The L was increased over the flux saturated inductor by 8.6% without any degrade of quality factor.

Microfabrication and characteristics of magnetic thin-film inductors in the ultrahigh frequency region

We have studied the effect of additional elements of Sn, Pb, Sb, and Bi on the ordering of L10–CoPt films. All of these additives are demonstrated to be very effective to promote the ordering and developing of a very large coercivity of the samples annealed at 400 °C. It is worth noting that this annealing temperature for ordering is 200 °C lower than that of pure CoPt. The crystallographic and chemical analyses have revealed that these additives easily diffuse and segregate onto the film surfaces by postannealing because of their very low surface free energy and extremely low solubility in Co. Therefore, it seems reasonable to conclude that the ordering in the CoPt film is significantly promoted at much lower temperature by the aid of a lot of defects produced by the additives excreted by postannealing.

Yutaka Shimada

Papers

Size effect on the crystal phase of cobalt fine particles

Chemical-order-dependent magnetic anisotropy and exchange stiffness constant of FePt (001) epitaxial films

Lowering of ordering temperature for fct Fe–Pt in Fe/Pt multilayers

Microfabrication and characteristics of magnetic thin-film inductors in the ultrahigh frequency region

Low-temperature ordering of L10?CoPt thin films promoted by Sn, Pb, Sb, and Bi additives