Tim Böhnert

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-thermopower and magnetoresistance of single Co-Ni alloy nanowires

The magneto-thermopower is measured and correlated to the anisotropic magnetoresistance of Co-Ni alloyed nanowires with varying composition. The highest absolute and relative variation of the Seebeck coefficient in perpendicularly applied magnetic fields at room temperature is determined to be 1.5 μVK−1 for Co0.24Ni0.76 and 8.1% for Co0.39Ni0.61 nanowires. Power factors of 3.7 mW/mK2 have been achieved, which is competitive with common thermoelectric materials like Bi2Te3. For Co-Ni nanowires containing up to 39% Co, a linear relationship between the magnetic field dependent change of the Seebeck coefficient and the electrical conductivity is found.

Thermoelectric power factor of ternary single-crystalline Sb2Te3- and Bi2Te3-based nanowires

Nanowires of bismuth antimony telluride and bismuth telluride selenide (Bi15Sb29Te56 and Bi38Te55Se7) were grown by template-based pulsed electrodeposition. The composition and the crystallinity of the nanowires were determined by high-resolution transmission electron microscopy. The thermoelectric properties (Seebeck coefficient and electrical conductivity) of single p- and n-type nanowires, with diameter 80 nm and 200 nm, respectively, were determined as a function of temperature before and during heating in a helium atmosphere up to 300 K along the growth direction of the nanowires. After additional annealing in a tellurium atmosphere at 525 K, significantly enhanced transport properties are observed. Bulk-like power factors are achieved. In Bi38Te55Se7 nanowires, the Seebeck coefficients increase to -115 μV K(-1) and the thermoelectric power factors increase to 2820 μW K(-2) m(-1) at room temperature. In Bi15Sb29Te56 nanowires, Seebeck coefficients of up to S = +156 μV K(-1) and power factors of up to 1750 μW K(-2) m(-1) are obtained at room temperature.

Itinerant and localized magnetic moments in ferrimagnetic Mn(2)CoGa thin films probed by x-ray magnetic linear dichroism: Experiment and ab initio theory

Epitaxial thin films of the half-metallic Xa compound Mn(2)CoGa (Hg(2)CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5) mu(B) per unit cell were obtained. The L(3,2) x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.

Thermopower engineering of Bi2Te3 without alloying: the interplay between nanostructuring and defect activation

We report on the interplay between nanostructuring and defect activation in dense polycrystalline Bi2Te3 thin films in terms of the thermopower engineering. The Bi2Te3 thin films were prepared at relatively low temperatures (100–160 °C) by atomic layer deposition and their grains showed different sizes in the range of 50–200 nm according to the deposition temperatures. We monitored the conductivity, Seebeck coefficient, and power factor of all samples from the temperature of 50–400 K. By increasing the growth temperature, remarkably, we observed the gradual defect activation from the nominal p-type to n-type in our binary end compound, Bi2Te3 without any alloying. The present results give us an insight on the optimization of thermoelectric materials not only by nanostructuring (i.e., phonon engineering) but also by controlled defect activation (i.e., electron engineering).

Magnetothermopower and magnetoresistance of single Co-Ni/Cu multilayered nanowires

The magnetothermopower and the magnetoresistance of single Co-Ni/Cu multilayered nanowires with various thicknesses of the Cu spacer are investigated. Both kinds of measurement are performed as a function of temperature (50–325 K) and under applied magnetic fields perpendicular to the nanowire axis, with magnitudes up to −15% at room temperature. A linear relation between thermopower S and electrical conductivity σ of the nanowires is found, with the magnetic field as an implicit variable. Combining the linear behavior of the S vs σ relation and the Mott formula, the energy derivative of the resistivity is determined. In order to extract the true nanowire materials parameters from the measured thermopower, a simple model based on the Mott formula is employed to distinguish the individual thermopower contributions of the sample. By assuming that the nondiffusive thermopower contributions of the nanowire can be neglected, it is found that the magnetic-field-induced changes of thermopower and resistivity are equivalent. The emphasis in the present paper is on the comparison of the magnetoresistance and magnetothermopower results and it is found that the same correlation is valid between the two sets of data for all samples, irrespective of the relative importance of the giant magnetoresistance or anisotropic magnetoresistance contributions in the various individual nanowires.

Insights into the electronic structure of Co2FeSi from x-ray magnetic linear dichroism

Experimental evidence both for and against a half-metallic ground state of the Heusler compound Co2FeSi has been published. Density-functional-theory-based calculations suggest a non-half-metallic ground state. It has been argued that on-site Coulomb interaction of the d electrons has to be taken into account via the LDA+U method, which predicts a half-metallic ground state for U approximate to 2.5,...,4.5 eV. X-ray magnetic linear dichroism (XMLD) can be used as a tool to assess the appropriateness of the LDA+U approach: The calculated spectra within the LDA+U or GGA+U schemes are different from those within the LDA or GGA. Due to its ability to separate different orbital symmetries, XMLD allows us to distinguish between different models of the electronic structure of Co2FeSi. In this article we discuss experimental XMLD spectra and compare them with detailed first-principles calculations. Our findings give evidence for the inadequacy of the LDA+U or GGA+U band structures, whereas constrained calculations with the GGA and a fixed spin moment of 6 mu(B) give better overall agreement between experiment and theory.

Magnetic characterization and electrical field-induced switching of magnetite thin films synthesized by atomic layer deposition and subsequent thermal reduction

Magnetite (Fe3O4) of high quality was prepared by combining atomic layer deposition (ALD) with a subsequent thermal reduction process. The reduction process in hydrogen atmosphere was investigated by in situ x-ray diffraction studies as a function of temperature. A complete reduction to Fe3O4 was confirmed within a narrow temperature window during the thermal treatment. Magnetic characterization of magnetite thin films as a function of temperature, applied magnetic field and magnetic field orientation were performed. The highly stoichiometry- and impurity-sensitive Verwey transition was observed in magnetic and electrical measurements. Moreover, the isotropic point at which the magnetocrystalline anisotropy of magnetite vanishes was unveiled. Both findings prove, first, the formation of the magnetite phase against the undesired maghemite and, second, the quality of the ALD thin films to be comparable with samples grown by molecular beam epitaxy. The magnetic easy and hard axis could be found to be in-plane and out-of-plane, respectively. Consistent with angular-dependent studies of the coercive field, additionally performed first-order reversal curve measurements revealed a complex micromagnetic structure with different magnetization reversal paths for both configurations. Finally, electric field-induced resistive switching was studied in detail being in perfect agreement with results of single-crystalline samples. The presented data and its analysis support the assumption of previous works of the magnetization reversal in magnetite nanotubes, suggest improvement for future magnetization studies of nanostructures by exploiting the isotropic point and might open new paths for low-cost resistive switching devices.

Statistical magnetometry on isolated NiCo nanowires and nanowire arrays: a comparative study

The first-order reversal curve (FORC) method can be used to extract information about the interaction and switching field distribution of ferromagnetic nanowire arrays, yet it remains challenging to acquire reliable values. Within ordered pores of anodic alumina templates we electrochemically synthesize eight different Ni x Co1?x samples with x varying between 0.05 and 1. FORC diagrams are acquired using vibrating sample magnetometry. By dissolving the template and using the magneto-optical Kerr effect, we measure the hysteresis loops of up to 100 different and isolated nanowires for each sample to gain precise information about the intrinsic switching field distribution. Values of the interaction field are extracted from a deshearing of the major hysteresis loop. We present a comparative study between all methods in order to evaluate and reinforce current FORC theory with experimental findings.

Magnetothermoelectric figure of merit of Co/Cu multilayers

The switching of the magnetic configuration of giant magnetoresistance multilayers not only changes the electrical and thermal conductivities but also the thermopower. We study the magnetotransport and the magnetothermoelectric properties of Co/Cu multilayer devices in a lateral thermal gradient. The Seebeck coefficient reaches values up to −18 μV/K at room temperature and shows a magnetic field dependence up to 28.6% upon spin reversal. In combination with thermal conductivity data of the same Co/Cu stack, we find a magnetothermoelectric figure of merit of up to 65%. Furthermore, a magneto-power factor of up to 110% is derived.