M. Farle

Ferromagnetic resonance of ultrathin metallic layers

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

Magnetic and optical tunable microspheres with a magnetite/gold nanoparticle shell

Multifunctional core-shell microspheres consisting of a polystyrene 640 nm diameter core covered with a selectable number of layers of magnetite (12 nm) and silica-coated gold (15 nm) nanoparticles have been fabricated using the layer-by-layer (LbL) technique in aqueous solution. By varying the shell thickness and layer composition the magnetic and optical properties and the diameter of the colloids can be controlled independently. Optical spectra of the core-shell colloidal microspheres show a well-resolved Au surface plasmon peak in the visible which red-shifts with the number of adsorbed Au nanoparticle layers. The magnetic moment per sphere increases nearly linearly with the number of adsorbed magnetite nanoparticle layers, and the spheres can be assembled into chains of up to 1 mm length by deposition in a magnetic field.

Nanostructuring of the Cu(001) surface by ion bombardment: a STM study

Abstract A detailed investigation of the effects of ion bombardment and thermal treatment on the topography of Cu(001) surfaces is presented. By means of 400 eV Ar + bombardment (flux = 6 × 10 12 ions/cm 2 s) at 300 K rectangularly shaped vacancy islands are created. These wells are on average 5 atomic layers deep, typically 20 nm wide at the top and regularly spaced about 27 nm apart. The dependence of this periodic well structure on the ion beam parameters and thermal treatment is discussed. Annealing the crystal to 370 K completely flattens this structure and the smooth topography of the clean unbombarded Cu(001) surface is recovered. We also present evidence that pinning of step flow at crystal defects or contamination on the order of 10 −4 leads to much larger terrace sizes than would be expected from the miscut of the surface.

Hysteresis effects in the inverse magnetocaloric effect in martensitic Ni-Mn-In and Ni-Mn-Sn

The presence of a large inverse magnetocaloric effect around the martensitic transformation in Ni-Mn-Sn and Ni-Mn-In alloys is expected to lead to substantial cooling on applying a magnetic field. However, the occurrence of hysteresis around the transition causes limitations on adiabatic temperature-changes. We study the adiabatic temperature-change in both systems in relation to the hysteresis effects. Ni-Mn-In, having a relatively narrower hysteresis and a greater shift of the characteristic transition temperatures with applied field with respect to Ni-Mn-Sn, shows reversibility in the adiabatic-temperature change related to the inverse magnetocaloric effect when the state of the system is cycled within a minor transitional hysteresis loop. Ni-Mn-Sn does not show reversibility in the inverse magnetocaloric effect under cycling-fields up to 5 T. The reversibility in the adiabatic temperature-change is directly related to the reversibility in the relative amount of austenite and martensite in the sample w...

Room-Temperature Ferromagnetism in Antiferromagnetic Cobalt Oxide Nanooctahedra

Cobalt oxide octahedra were synthesized by thermal decomposition. Each octahedron-shaped nanoparticle consists of an antiferromagnetic CoO core enclosed by eight {111} facets interfaced to a thin (∼ 4 nm) surface layer of strained Co3O4. The nearly perfectly octahedral shaped particles with 20, 40, and 85 nm edge length show a weak room-temperature ferromagnetism that can be attributed to ferromagnetic correlations appearing due to strained lattice configurations at the CoO/Co3O4 interface.

Ferromagnetic resonance of monodisperse Co particles

Two-dimensional arrays of monodisperse nanosized Co particles are prepared on carbon and glass substrates by a magnetophoretic deposition technique from colloidal suspensions. Transmission electron microscopy (TEM) reveals a complicated cubic crystalline structure of the particles and hexagonal ordering over several micrometers squared, if the colloidal suspension is dried in magnetic fields of up to 0.8 T. Angular-dependent ferromagnetic resonance (FMR) spectra of 4-, 5-, 9-, and 12-nm-diameter particles at 297 K show that the easy axis of magnetization is in plane and that only the 12 nm particles are measured below the blocking temperature estimated to be 656 K. The resonance linewidth is on the order of 0.1 T, indicating a much larger magnetic inhomogeneity of the particles than the small geometric and size distribution (<10%) observed by TEM suggests. Characteristic differences of the FMR spectra for different substrates and deposition parameters are observed.

Pseudomorphic growth of Ni films on Cu(001): a quantitative LEED analysis

Abstract We present LEED structure determinations of ultra-thin epitaxial Ni films on Cu(001) for coverages of 3, 5 and 11 ML. From full dynamical intensity analyses, a tetragonal distortion of all films can be deduced, in good accordance with FMR results. The structural parameters of the 5 and 11 ML films are practically identical. So, obviously there are no structural changes at about 7 ML, where the orientation of the magnetization is reported to switch from in-plane to out-of-plane. The film growth is pseudomorphic with an in-plane lattice parameter a p = 2.53 A at coverages of 3 and 5 ML. This value, which is slightly reduced with respect to the copper bulk, was recently also found to produce the best fit for a similarly prepared clean Cu(100) surface. With further increasing coverage, a p tends to approach the value of the nickel bulk (2.49 A) but does not fully reach it even at 11 ML ( a p = 2.51 A ). The films at 5 and 11 ML (and probably also in the regime between) show considerable tetragonal distortions. The top Ni layer shows an outward relaxation of 8% relative to the distances between deeper layers, which, due to the tetragonal distortion, are reduced compared to the ideal value of the nickel bulk.

Mastering hysteresis in magnetocaloric materials.

Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La–Fe–Si-, Heusler- and Fe2P-type compounds. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

Stearate-Based Cu Colloids in Methanol Synthesis: Structural Changes Driven by Strong Metal-Support Interactions

Metal stearate‐stabilized Cu nanoparticles, synthesized by an efficient one‐step process, were applied in the continuous liquid‐phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu–Zn colloids, compared to the conventional ternary Cu/ZnO/Al2O3 catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H2, spherical, well‐separated 5–10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high‐pressure conditions Zn stearate was hydrolyzed forming ZnO. High‐resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnOx species, which diffuse onto the Cu particles and promote the catalytic activity.

A guideline for atomistic design and understanding of ultrahard nanomagnets.

Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments.