Induction Mapping of the 3D-Modulated Spin Texture of Skyrmions in Thin Helimagnets.

Envisaged applications of Skyrmions in magnetic memory and logic devices crucially depend on the stability and mobility of these topologically nontrivial magnetic textures in thin films. We present for the first time quantitative maps of the magnetic induction that provide evidence for a 3D modulation of the Skyrmionic spin texture. The projected in-plane magnetic induction maps as determined from in-line and off-axis electron holography carry the clear signature of Bloch Skyrmions. However, the magnitude of this induction is much smaller than the values expected for homogeneous Bloch Skyrmions that extend throughout the thickness of the film. This finding can only be understood if the underlying spin textures are modulated along the out-of-plane z direction. The projection of (the in-plane magnetic induction of) helices is further found to exhibit thickness-dependent lateral shifts, which show that this z modulation is accompanied by an (in-plane) modulation along the x and y directions.

Modulations in martensitic Heusler alloys originate from nanotwin ordering.

Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite.

Advanced platform for the in-plane ZT measurement of thin films.

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging, but important to understand their structure and transport mechanism, and to improve current industrial products and production processes. We report on the 2nd generation of a measurement chip, which allows for a simplified sample preparation process, and the measurement of samples deposited from the liquid phase using techniques like spin coating and drop casting. The new design enables us to apply much higher temperature gradients for the Seebeck coefficient measurement in a shorter time, without influencing the sample holder's temperature distribution. Furthermore, a two membrane correction method for the 3ω thermal conductivity measurement will be presented, which takes the heat loss due to radiation into account and increases the accuracy of the measurement results significantly. Errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our platform, we present temperature-dependent measurements of the thermoelectric properties of an 84 nm Bi87Sb13 thin film and a 15 µm PEDOT:PSS thin film.

Influence of surface states and size effects on the Seebeck coefficient and electrical resistance of Bi1-xSbx nanowire arrays.

The Seebeck coefficient and electrical resistance of Bi1-xSbx nanowire arrays electrodeposited in etched ion-track membranes have been investigated as a function of wire diameter (40-750 nm) and composition (0 ≤ x ≤ 1). The experimental data reveal a non-monotonic dependence between thermopower and wire diameter for three different compositions. Thus, the thermopower values decrease with decreasing wire diameter, exhibiting a minimum around ∼60 nm. This non-monotonic dependence of the Seebeck coefficient is attributed to the interplay of surface and bulk states. On the one hand, the metallic properties of the surface states can contribute to decreasing the thermopower of the nanostructure with increasing surface-to-volume ratio. On the other hand, for wires thinner than ∼60 nm, the relative increase of the thermopower can be tentatively attributed to the presence of quantum-size effects on both surface and bulk states. These measurements contribute to a better understanding of the interplay between bulk and surface states in nanostructures, and indicate that the decrease of Seebeck coefficient with decreasing diameter caused by the presence of surfaces states can possibly be overcome for even thinner nanowires.

Magnetic and electrical characterization of nickel-rich NiFe thin films synthesized by atomic layer deposition and subsequent thermal reduction.

Nickel-rich NiFe thin films (Ni92Fe8, Ni89Fe11 and Ni83Fe17) were prepared by combining atomic layer deposition (ALD) with a subsequent thermal reduction process. In order to obtain Ni x Fe1-x O y films, one ALD supercycle was performed according to the following sequence: m NiCp2/O3, with m = 1, 2 or 3, followed by one FeCp2/O3 cycle. The supercycle was repeated n times. The thermal reduction process in hydrogen atmosphere was investigated by in situ x-ray diffraction studies as a function of temperature. The metallic nickel iron alloy thin films were investigated and characterized with respect to crystallinity, morphology, resistivity, and magnetism. As proof-of-concept magnetic properties of an array of Ni83Fe17, close to the perfect Permalloy stoichiometry, nanotubes and an isolated tube were investigated.

Tailoring the nucleation of domain walls along multi-segmented cylindrical nanoelements.

The magnetization reversal of three-segment cylindrical nanoelements comprising alternating nanowire and nanotube sections is investigated by means of Monte Carlo simulations. Such nanoelements may feature a three-state behaviour with an intermediate plateau in the hysteresis curve due to a metastable pinning of the domain walls (DWs) at the wire-tube interfaces. It turns out that vortex as well as transverse DWs contribute to the magnetization reversal. By varying the geometric parameters, the sequence, or the material of the elements the nucleation location of DWs, as well as their nucleation field, can be tailored. Especially interesting is the novel possibility to drive DWs coherently in the same or in opposite directions by changing the geometry of the hybrid nanoelement. This important feature provides additional flexibility to the construction of logical devices based on DW movement. Another prominent outcome is that DWs can be nucleated near the centre of the element and then traverse to the outer tips of the cylindrical structure when the applied field is increased, which also opens the possibility to use these three-segment nanoelements for the field-induced delivery of DWs as substitutes for large nucleation pads.

Electrochemical synthesis and magnetic characterization of periodically modulated Co nanowires.

The synthesis of templates with modulated pore channels by combined mild and hard anodization processes is described. The hard anodization pulses, implemented during anodization, are controlled not only in time length and amplitude, but also in shape: square and exponential signals have been applied. Electrodeposition of Co is subsequently performed to obtain uniform and modulated diameter nanowire arrays. Square and exponential modulated diameter nanowires are imaged by scanning electron microscopy and hcp hexagonal polycrystalline structure is confirmed in all Co nanowires. Magnetic behavior strongly depends on nanowire shape and is interpreted considering the modification of magnetostatic interactions between wires induced by local stray fields from magnetic charges at the ends of the wider segments in modulated wires. As a consequence, magnetization processes under parallel and perpendicular field configurations denote the contribution of both thin and wide segments.

Tuning the magnetic anisotropy of Co-Ni nanowires: comparison between single nanowires and nanowire arrays in hard-anodic aluminum oxide membranes.

Co(x)Ni(1-x) alloy nanowires with varying Co content (0 ≤ x ≤ 0.95), having a diameter of 130 nm and length of around 20 µm, are synthesized by template-assisted electrodeposition into the nanopores of SiO(2) conformal coated hard-anodic aluminum oxide membranes. The magneto-structural properties of both single isolated nanowires and hexagonally ordered nanowire arrays of Co-Ni alloys are systematically studied by means of magneto-optical Kerr effect magnetometry and vibrating sample magnetometry, respectively, allowing us to compare different alloy compositions and to distinguish between the magnetostatic and magnetocrystalline contributions to the effective magnetic anisotropy for each system. The excellent tunable soft magnetic properties and magnetic bistability exhibited by low Co content Co-Ni nanowires indicate that they might become the material of choice for the development of nanostructured magnetic systems and devices as an alternative to Fe-Ni alloy based systems, being chemically more robust. Furthermore, Co contents higher than 51 at.% allow us to modify the magnetic behavior of Co-rich nanowires by developing well controlled magnetocrystalline anisotropy, which is desirable for data storage applications.

Magneto-optical properties of core-shell magneto-plasmonic Au-Co(x)Fe(3 - x)O4 nanowires.

The magneto-optical properties of Au-Co(x)Fe(3 - x)O(4) core-shell nanowires embedded in porous alumina membranes are studied. The structures were obtained by depositing Co(x)Fe(3 - x)O(4) on the pore walls of alumina membranes by atomic layer deposition and filling the resulting nanotube with gold by electrodeposition. The effect of plasmon resonance excitation on the magneto-optical activity is clearly observed as a modification of the spectral line shape of the Kerr rotation signal.

Thermal conductivity measurements using 1ω and 3ω methods revisited for voltage-driven setups.

1ω and 3ω methods are widely established transient measurement techniques for the characterization of thermal transport in bulk-materials, thin films, and 1D nano-objects. These methods are based on resistance oscillations of a heater caused by Joule-heating from a sinusoidal current at frequency 1ω which lead to changes in the 1ω voltage and produce a voltage component at 3ω. Although the usual formalism for analyzing the measurement data assumes an ideal current source, voltage-driven measurement setups are employed in many cases. In this context, we find that there has been lack of clarity if a correction generally has to be considered when analyzing the measurement data from voltage driven setups. In this work, Fourier-analysis is employed to show that a correction is not required for 1ω methods and for 3ω measurements that use common-mode-subtraction. Experimental results are presented for a line heater on a fused silica substrate with known thermal properties, and for an individual nickel wire with diameter of 150 nm.

Synthesis and magnetic characterization of MnAs nanoparticles via nanoparticle conversion.

We report on the synthesis of ferromagnetic manganese arsenide (MnAs) nanoparticles via the conversion of primary Mn particles which are generated in an aerosol process in a spark discharge generator. After sintering and size selection in an aerosol setup, the particles are deposited on GaAs(100)B and Si(111) substrates. Subsequent conversion to MnAs particles takes place in an annealing process under a hydrogen atmosphere with an arsine background pressure. The magnetic properties are studied using a SQUID magnetometer. The annealed MnAs particles exhibit hexagonal facets and show anisotropic magnetic behaviour on GaAs(100)B substrates, whereas on Si(111) they remain spherical and show isotropic magnetic behaviour. Scanning transmission electron microscopy studies are used to confirm the conversion from Mn to MnAs.

Atomic layer deposition of ZnS nanotubes.

We report on the growth of high-aspect-ratio (approximately > 300) zinc sulfide nanotubes with variable, precisely tunable, wall thicknesses and tube diameters into highly ordered pores of anodic alumina templates by atomic layer deposition (ALD) at temperatures as low as 75 degrees C. Various characterization techniques are employed to gain information on the composition, morphology and crystal structure of the synthesized samples. Besides practical applications, the ALD-grown tubes could be envisaged as model systems for the study of a certain class of size-dependent quantum and classical phenomena.

Spin-wave quantization in ferromagnetic nickel nanowires.

The dynamical properties of uniform two-dimensional arrays of nickel nanowires have been investigated by inelastic light scattering. Multiple spin waves are observed that are in accordance with dipole-exchange theory predictions for the quantization of bulk spin waves. This first study of the spin-wave dynamics in ferromagnetic nanowire arrays reveals strong mode quantization effects and indications of a subtle magnetic interplay between nanowires. The results show that it is important to take proper account of these effects for the fundamental physics and future technological developments of magnetic nanowires.