Felix Haering

Towards quantitative magnetic force microscopy: theory and experiment

We introduce a simple and effective model of a commercial magnetic thin-film sensor for magnetic force microscopy (MFM), and we test the model employing buried magnetic dipoles. The model can be solved analytically in the half-space in front of the sensor tip, leading to a simple 1/R dependence of the magnetic stray field projected to the symmetry axis. The model resolves the earlier issue as to why the magnetic sensors cannot be described reasonably by a restricted multipole expansion as in the point pole approximation: the point pole model must be extended to incorporate a ?lower-order? pole, which we term ?pseudo-pole?. The near-field dependence (?R?1) turns into the well-known and frequently used dipole behavior (?R?3) if the separation, R, exceeds the height of the sensor. Using magnetic nanoparticles (average diameter 18?nm) embedded in a SiO cover as dipolar point probes, we show that the force gradient?distance curves and magnetic images fit almost perfectly to the proposed model. The easy axis of magnetization of single nanoparticles is successfully deduced from these magnetic images. Our model paves the way for quantitative MFM, at least if the sensor and the sample are independent.

Perpendicular magnetisation from in-plane fields in nano-scaled antidot lattices

Investigations of geometric frustrations in magnetic antidot lattices have led to the observation of interesting phenomena like spin-ice and magnetic monopoles. By using highly focused magneto-optical Kerr effect measurements and x-ray microscopy with magnetic contrast we deduce that geometrical frustration in these nanostructured thin film systems also leads to an out-of-plane magnetization from a purely in-plane applied magnetic field. For certain orientations of the antidot lattice, formation of perpendicular magnetic domains has been found with a size of several μm that may be used for an in-plane/out-of-plane transducer.

Magnetic switching of nanoscale antidot lattices

Summary We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices.

Role of developing L10 chemical order on the (0?0?1)-texture formation of (Fe1 ? xCux)Pt films grown on amorphous substrates

We study the technologically important (0 0 1)-texture formation in 10 nm thick (Fe0.9Cu0.1)52Pt48 and, as reference, Fe49Pt51 alloy films. The samples are grown on SiO2(200 nm)/Si(0 0 1) substrates at ambient temperature by pulsed laser deposition. Subsequent rapid thermal processing (RTP) at 650 °C for various time steps drives the initially nanocrystalline and chemically disordered films into the tetragonal L10 phase accompanied by a strong (0 0 1)-texture leading to perpendicular magnetic anisotropy. The fundamental role of the chemical order during short-time annealing as an additional source of strain in the films is experimentally addressed. The structural and magnetic results indicate selective grain growth leading to the (0 0 1)-texture. Strongly prolonged annealing, however, leads to a reorientation of grains towards the (1 1 1)-texture pointing to the increasing importance of surface energies when the initial strain has released.

Tuning the properties of magnetic thin films by interaction with periodic nanostructures.

Summary The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

Combined FORC and X-ray microscopy study of magnetisation reversal in antidot lattices

Magnetic nanostructures, that are patterned on the length scale of the dipole and exchange interaction, have gained significant scientific interest in the past years. These nanostructures have great potential for technological applications in data processing and storage, and spintronics. Magnonic crystals are a class of such nanostructures and are metamaterials with periodically alternating magnetic properties - similar to photonic crystals. This periodic variation is achieved by creating holes in a magnetic host material to form a so-called antidot lattice. The introduction of the artificial antidot lattice changes the spin wave dispersion in the material and can be used to form a spin wave guide or filter. To tune the spin wave dispersion, understanding the magnetisation states and the static magnetic properties is of great importance. These static properties like the anisotropy, the coercivity and the orientation of the easy axes are determined by the hole size and distance, the antidot lattice symmetry and its orientation, and the magnetic host material. Here, we present new insights into the magnetisation reversal behaviour of nanoscaled hexagonal antidot lattices, patterned both in in-plane (Fe) and out-of-plane (GdFe) magnetised thin films. The antidots were prepared by polystyrene self-organisation lithography or FIB milling of the magnetic materials. An approach combining first-order reversal curve (FORC) measurements and x-ray microscopy (XM) with magnetic contrast was used to identify irreversible processes and to subsequently image their microscopic origin. Using a fast laser magneto-optical Kerr effect (MOKE) based FORC technique, it was possible to individually measure specific sample areas (spatial resolution <;2 μm) and to compare a large number of samples. Subsequent XM investigations allowed to reproduce, localise, and quantify the magnetic states involved in the reversal processes.