http://files.janapv.webnode.cz/200000097-8a03f8afdf/ex_bias_nano.pdf

Exchange bias in nanostructures

bases were introduced in [Smy77] where they are called “R-structures”. Examples of abstract bases are concrete bases of continuous domains, of course, where the relation≺ is the restriction of the order of approximation. Axiom (INT) is satisfied because of Lemma 2.2.15 and because we have required bases in domains to have directed sets of approximants for each element. Other examples are partially ordered sets, where (INT) is satisfied because of reflexivity. We will shortly identify posets as being exactly the bases of compact elements of algebraic domains. In what follows we will use the terminology developed at the beginning of this chapter, even though the relation ≺ on an abstract basis need neither be reflexive nor antisymmetric. This is convenient but in some instances looks more innocent than it is. An idealA in a basis, for example, has the property (following from directedness) that for everyx ∈ A there is another element y ∈ A with x ≺ y. In posets this doesn’t mean anything but here it becomes an important feature. Sometimes this is stressed by using the expression ‘ A is a round ideal’. Note that a set of the form↓x is always an ideal because of (INT) but that it need not contain x itself. We will refrain from calling ↓x ‘principal’ in these circumstances. Definition 2.2.21. For a basis〈B,≺〉 let Idl(B) be the set of all ideals ordered by inclusion. It is called theideal completionof B. Furthermore, leti : B → Idl(B) denote the function which maps x ∈ B to ↓x. If we want to stress the relation with whichB is equipped then we write Idl(B,≺) for the ideal completion. Proposition 2.2.22.Let 〈B,≺〉 be an abstract basis.

Domain theory

Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum and silicon have attracted significant interest in recent years. While scientific thrust is often directed towards the elucidation of the principles of the self-organization phenomena, technological efforts target applications based on the direct use of the high surface area, for example, for sensing 6] or controlled catalysis, exploit the optical properties in photonic crystals, waveguides, or in 3D arranged Bragg-stack type of reflectors. The highly organized structures may be used indirectly as templates for the deposition of other materials such as metals, semiconductors, or polymers. Over the past few years, nanoporous TiO2 structures have also been formed by electrochemical anodization of titanium. Although several applications have been proposed, a wider impact of these structures has been hampered by the fact that the layers could only been grown to a limiting thickness of a few hundreds of nanometers. Herein we demonstrate for the first time how high-aspectratio, self-organized, TiO2 films can be grown by tailoring the electrochemical conditions during titanium anodization. Figure 1 shows scanning electron microscope (SEM) images of self-organized porous titanium oxide formed to a thickness of approximately 2.5 mm in 1m (NH4)2SO4 electrolyte containing 0.5 wt.% NH4F. From the SEM images it is evident that the self-organized regular porous structure consists of pore arrays with a uniform pore diameter of approximately 100 nm and an average spacing of 150 nm. It is also clear that pore mouths are open on the top of the layer while on the bottom of the structure the tubes are closed by presence of an about 50-nm thick barrier layer of TiO2. The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO2 by localized acidification at the pore bottom while a protective environment is maintained along the pore walls and at the pore mouth. In our previous work in HF and NaF solutions it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO2 at the pore bottom and the chemical dissolution of this TiO2 in an F ion containing solution (Figure 2). The solubility of TiO2 in HF, forming [TiF6] 2 , is essential for pore formation, however, it is also the reason that previous attempts to form porous layers in HF electrolytes always resulted in layer thicknesses in the range of some 100 nm. We tackled the problem by controlling the self-induced acidification of the pore bottom that is caused by the electrochemical dissolution of the metal (Figure 2 a–c). Main reason for the localized acidification is the oxidation and hydrolysis of elemental titanium [Eq. (1), in Figure 2]. The chemical dissolution rate of TiO2 is highly dependent on the pH value (see Figure 2 d). Using a numerical simulation of the relevant ion fluxes we can construct the pH profile in the pore (such as in Figure 2b), in other words, the ideal ion flux for the desired pH profile can then be determined. Furthermore we can tune the dissolution rate by the dissolution current. In other words, using a buffered neutral solution as electrolyte and adjusting the anodic current flow to an ideal value, acid can be created where it is needed, that is, at the pore bottom, while higher pH values are established at the pore mouth as a result of migration and diffusion effects of the pH buffer species (NH4F, (NH4)2SO4). Assuming equilibrium, the flux of dissolving species (leading to acidification at the pore bottom) and the flux of buffering species are equal. The calculations show that for the experimental conditions given in Figure 1 the pH value at the pore bottom is around 2 and increase to about 5 at the pore mouth, this corresponds to a drop in the local chemical etch rate of about 20 times. We used a voltage-sweep technique to achieve a steadystate current and to establish the desired pH profile. The reason to use a voltage-sweep technique rather than a Figure 1. SEM images of porous titanium oxide nanotubes. The crosssectional (a), top (b), and bottom (c) views of a 2.5-mm thick selforganized porous layer. The titanium sample was anodized up to 20 V in 1m (NH4)2SO4 + 0.5 wt. % NH4F using a potential sweep from open-circuit potential to 20 V with sweep rate 0.1 Vs . The average pore diameter is approximately 100 nm and the average pore spacing is approximately 150 nm.

High‐Aspect‐Ratio TiO2 Nanotubes by Anodization of Titanium

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Materials with a coexistence of magnetic and ferroelectric order — multiferroics — provide an efficient route for the control of magnetism by electric fields. The study of multiferroics dates back to the 1950s, but in recent years, key discoveries in theory, synthesis and characterization techniques have led to a new surge of interest in these materials. Different mechanisms, such as lone-pair, geometric, charge-ordering and spin-driven effects, can support multiferroicity. The general focus of the field is now shifting into neighbouring research areas, as we discuss in this Review. Multiferroic thin-film heterostructures, device architectures, and domain and interface effects are explored. The violation of spatial and inversion symmetry in multiferroic materials is a key feature because it determines their properties. Other aspects, such as the non-equilibrium dynamics of multiferroics, are underrated and should be included in the topics that will define the future of the field. Multiferroic materials exhibit magnetic and ferroelectric order at the same time and provide a way to control magnetism with electric fields. We discuss the mechanisms supporting multiferroicity, multiferroic thin films and heterostructures, the non-equilibrium dynamics of multiferroics, fundamental symmetry issues and the impact of multiferroics on other research areas.

The evolution of multiferroics

A simple technique for size-controllable nanostructure array formation has been developed, using self-assembled polystyrene beads whose diameters can be arbitrarily reduced by reactive ion etching. We have produced a hole array of 83 and 157 nm diameter with 200 nm pitch on Si substrate. This technique can find potential applications in many areas of science and technology.

Nanostructure array fabrication with a size-controllable natural lithography

A new method for observing magnetic domain structures with a scanning electron microscope has been developed in which the image is the result of spin polarization of secondary electrons. With this method the domain structures on an iron (001) surface have been observed. At present, spatial resolution is not less than 10 µm, but can be expected to be reduced to far less than that of both conventional electron microscopic and optical reflection methods.

https://iopscience.iop.org/article/10.1143/JJAP.23.L187/pdf

Scanning Electron Microscope Observation of Magnetic Domains Using Spin-Polarized Secondary Electrons

Perpendicular magnetic anisotropy (PMA) of cobalt-ferrite CoxFe3-xO4 (x = 0.75 and 1.0) epitaxial thin films grown on MgO (001) by a reactive magnetron sputtering technique was investigated. The saturation magnetization was found to be 430 emu/cm3 for x = 0.75, which is comparable to that of bulk CoFe2O4 (425 emu/cm3). Torque measurements afforded PMA constants of Kueff=9.0 Merg/cm3 (Ku=10.0 Merg/cm3) and Kueff=9.7 Merg/cm3 for x = 0.75 and 1.0, respectively. The value of Kueff extrapolated using Miyajima's plot was as high as 14.7 Merg/cm3 for x = 1.0. The in-plane four-fold magnetic anisotropy was evaluated to be 1.6 Merg/cm3 for x = 0.75. X-ray diffraction measurement revealed our films to be pseudomorphically strained on MgO (001) with a Poisson ratio of 0.4, leading to a considerable in-plane tensile strain by which the extraordinarily large PMA could be accounted for.

Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite CoxFe3−xO4(001) (x = 0.75, 1.0) thin films

By using electron beam lithography, a continuous CoCrPt film with a perpendicular crystal magnetic anisotropy has been patterned into a magnetic nanoparticle array of 29 Gdot/in.2 with a 150 nm period, an 80 nm diameter, and a 44 nm height. Studies of magnetic properties using a magnetic force microscope and a vibrating sample magnetometer show that this patterning increases the remanent-to-saturation magnetization ratio from 0.2 of the continuous film to 1 of the particles, and that each particle has a single magnetic domain with perpendicular anisotropy. The application of this array to future high density magnetic recording media is discussed.

Magnetic nanoparticle array with perpendicular crystal magnetic anisotropy

We revealed the micromagnetic structure of an Fe thin film exchange interacting with the spins of a fully compensated (001) surface of antiferromagnetic NiO. The interface exchange interaction causes the Fe domains to follow the NiO domains. The Fe spin polarization is in plane and the spin polarization in each domain is roughly perpendicular to an easy-spin axis of the NiO. These results agree with numerically calculated spin directions. Our numerical results also show that the NiO spins at the interface cant from the easy-spin axis towards the Fe spin because of exchange coupling.

Kazuyuki Koike

Papers

Nanostructure array fabrication with a size-controllable natural lithography

Scanning Electron Microscope Observation of Magnetic Domains Using Spin-Polarized Secondary Electrons

Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite CoxFe3−xO4(001) (x = 0.75, 1.0) thin films

Magnetic nanoparticle array with perpendicular crystal magnetic anisotropy

Microscopic imaging of Fe magnetic domains exchange coupled with those in a NiO(001) surface.