Interface Magnetism in Co/CoO core-shell nanoparticles and their transformation to pure metallic nanoparticles

Monodisperse magnetic nanoparticles have generated huge interest in applied research within the last years. The control of their monodispersity and surface properties leads to a variety of nanotechnological applications. The use as non-volatile data storage media and sensor applications are in the focus of industry. Moreover, magnetic nanoparticles are close to be employed in tumor therapy, bio-labelling or contrast agents in magnetic imaging. To an increasing extend these are studied in fundamental research as well, since nanoparticles with diameters of a few nm bridge the gap between atomic and solid state physics. In particular, the intrinsic magnetic properties of fine particles such as the magnetocrystalline anisotropy, the saturation magnetization and the Curie temperature are affected by the reduction of their size. The subject of this thesis is the investigation of magnetic Co/CoO core-shell nanoparticles and metallic Co nanoparticles with diameters in the range of 9-14 nm. The nanoparticles have been prepared by means of organometallic synthesis, and they exhibit a high degree of monodispersity (σ < 10%). The colloidal Co/CoO nanoparticles consist of a fcc ferromagnetic Co core covered with a naturally formed antiferromagnetic CoO shell. The main purpose of this study is the direct correlation of structure and magnetism in the particles. For better understanding of the influence of the antiferromagnetic shell and the Co/CoO interface on the magnetism of the particles pure metallic Co nanoparticles were studied. These have been repared from Co/CoO particles by reduction of the oxidic shell with a reactive plasma treatment. A erromagnetic/antiferromagnetic exchange coupled system shows an additional unidirectional contribution to the total magnetic anisotropy energy which can be measured by the so-called exchange bias. A general description of exchange anisotropy at ferromagnetic/antiferromagnetic interfaces is still lacking, since exchange bias strongly depends on the interface conditions. To address the interface magnetism explicitly, techniques have to be applied which give direct access to the interface. For direct comparison to metallic nanoparticles it is desirable to have control on the oxide shell without any loss of Co atoms. The system Co/CoO with typical layer thicknesses of 2-3 nm of the antiferromagnet CoO is a prototype for exchange bias. As shown in this thesis, the nanoparticles present a very strong exchange bias of µ0HEB = 0.4 T at 10 K compared to Co/CoO thin films. The microscopic quantities which govern the exchange bias are (i) the magnetic moments of the core and the shell Co atoms, (ii) the number of contributing interface moments, and (iii) the magnetic anisotropies of Co and CoO. The magnetic moment and its orbital contribution are quantitatively measured to understand the microscopic mechanisms which determine the exchange bias and the magnetic anisotropy energy. Two techniques which provide the necessary information with submonolayer sensitivity are Ferromagnetic Resonance (FMR) and X-ray Magnetic Circular Dichroism (XMCD). Explicitly, the ratio of orbital-to-spin magnetic moment can be measured. FMR probes the ferromagnetic core, only, while XMCD in the total electron yield mode is surface sensitive and - as demonstrated in this thesis - can be employed to extract the contribution of buried interfaces. The unique combination of both techniques yields a fcc bulk-like Co magnetic moment of a ferromagnetically ordered Co core (≤5 nm) and uncompensated Co2+ moments at the Co/CoO interface carrying a large orbital moment. XMCD studies at two different oxide shell thicknesses show that the interface moments are coupled parallel to the core. The effective magnetic anisotropy energy of Co/CoO particles has been determined by FMR to Keff = 9 µeV/atom at T = 15 K which is much smaller than the hcp bulk value of 65 µeV/atom at T = 0 K and surprisingly matches the fcc bulk Co value of 8.5 µeV/atom. By frequency-dependent FMR in ambient conditions in a 18 months period of time after sample preparation, it has been measured that directly after the deposition of the particles on a substrate a 2-2.5 nm thick CoO shell forms within hours. After three weeks the shell grows to about 3 nm. From this point of time on the CoO shell acts as a self-passivating layer for each individual particle. The oxidation slows down to a few atoms per day. This experiment demonstrates that small structural modifications can be addressed by FMR investigations. In order to verify and to compare the results of Co/CoO particles to pure metallic particles, an in-situ reactive plasma etching process has been employed. By successive oxygen and hydrogen plasma exposure it was possible (i) to remove the organic ligands and (ii) to reduce the entire CoO to metallic Co. This method allowed the controlled modification of the surface without any agglomeration or movement of the particles on the substrate for coverages up to one monolayer. Slightly larger coverages admitted to partial sintering of particles in a top layer while particles in the bottom layer remained fixed to the substrate. The removal of the organic ligands led to the formation of a double layer system which shows exchange coupling between particles perpendicular to the substrate. Using detailed structural and morphological studies as input parameters for theory, Landau-Lifshitz-Gilbert type of simulations have been performed to simulate element-specific hysteresis loops. The calculations have shown that (i) the metallic particles exhibit a low effective magnetic anisotropy energy density Keff = 1.5 µeV/atom at T = 15 K which is less than 20% of the fcc bulk Co anisotropy constant. (ii) The exchange coupling strength suggests that in medium only four atomic pairs of Co atoms are exchange coupled at the interface between top and bottom layer particles. The magnetic moment of the metallic particles has been determined to be 1.56 µB/atom by means of XMCD investigations on monolayer samples. The reduction compared to the bulk magnetic moment of 1.72 µB/atom has been assigned to the hydrogen-load of particles during the hydrogen plasma exposure. Annealing at T = 950 K successfully dissociates cobalt hydride and forms pure Co nanoparticles and a total magnetic moment of 1.83 µB/atom has been found. The enhancement compared to the bulk magnetic moment can be explained by the contribution of about 10% surface atoms in reduced symmetry which is manifested in an enhanced orbital moment. In future studies the surface anisotropy can be measured in pure metallic nanoparticles. Compared to ultrathin films the nanoparticle approach has the big advantage that the contact area to the substrate is very small and thus the influence of the substrate/ferromagnet interface is negligible. To correlate the magnetic properties to the structure of the nanoparticles detailed transmission electron microscopy (TEM) investigations have been performed. High-resolution TEM has shown that the Co/CoO particles consist of multiply twinned fcc Co core and a multigrain fcc CoO shell. The thickness of the CoO shell has been determined to be 2-2.5 nm by energy-filtered TEM. The passivating CoO shell encases a metallic Co core of 5-9 nm depending on the total particle diameter. With these results the low effective magnetic anisotropy energy density Keff = 1.5 µB/atom found for metallic Co nanoparticles can be explained by the multiply twinned structure of the ferromagnet. Individual grains with randomly oriented anisotropy axes reduce Keff remarkably and produce an effective uniaxial anisotropy. In the case of Co/CoO nanoparticles the oxide shell increases Keff which results in a magnetic hardening

Magnetic switching of nanoscale antidot lattices

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.Web of Science775073

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

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

Magnetic hardening of Fe30Co70nanowires

3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of FeConanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips.We acknowledge funding from the European Community's Seventh Framework Programme (FP7-NMP) under grant agreement no. 280670 (REFREEPERMAG)

2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-mu m MXene multilayer for different organic vapors and humidity at 10(1)-10(4) ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2O down to 10 ppm. Moreover, humidity suppresses the response of Mo2CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10(-2)-10(6) Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2C surface interaction with organic analytes and H2O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance

Preparation, properties and applications of magnetic nanoparticles

License and terms: see end of document. Though studies on small particles of various materials go way back before Nanoscience was emerging, this new interdiscipli-nary branch of science not only re-termed them into nanoparti-cles (NPs), but also lead to a dramatically enhanced interest in this type of nanoscaled material with an often given though arbitrary upper diameter limit of 100 nm [1]. As a natural consequence of this worldwide growing interest, the toolbox for preparing NPs has been amazingly broadened including now both, physics and chemistry related approaches. Furthermore, the meanwhile widely accepted distinction between top down and bottom up preparational methods can be applied to the fabrication of NPs as well. Examples for top down approaches are sculpting NPs out of a previously deposited thin film by e.g. Focused Ion Beam techniques [2] or evaporating/sputtering

Manipulation of the Size and Phase Composition of Yttrium Iron Garnet Nanoparticles by Pulsed Laser Post-Processing in Liquid

Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties

Structural and thermoelectric properties of TMGa3 (TM = Fe, Co) thin films

Based on chemically synthesized powders of FeGa3, CoGa3, as well as of a Fe0.75Co0.25Ga3 solid solution, thin films (typical thickness 40 nm) were fabricated by flash evaporation onto various substrates held at ambient temperature. In this way, the chemical composition of the powders could be transferred one-to-one to the films as demonstrated by Rutherford backscattering experiments. The relatively low deposition temperature necessary for conserving the composition leads, however, to ‘X-ray amorphous’ film structures with immediate consequences on their transport properties: A practically temperature-independent electrical resistivity of ρ = 200 μΩ·cm for CoGa3 and an electrical resistivity of about 600 μΩ·cm with a small negative temperature dependence for FeGa3. The observed values and temperature dependencies are typical of high-resistivity metallic glasses. This is especially surprising in the case of FeGa3, which as crystalline bulk material exhibits a semiconducting behavior, though with a small gap of 0.3 eV. Also the thermoelectric performance complies with that of metallic glasses: Small negative Seebeck coefficients of the order of −6 μV/K at 300 K with almost linear temperature dependence in the range 10 K ≤ T ≤ 300 K

Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): effect of thermal treatments and influence of substrate and particle size

Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: “small” NPs with diameters in the range of 2–3 nm and “large” ones in the range of 5–8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600–650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min