Search

In prototype ferromagnet-antiferromagnet interfaces we demonstrate that surface acoustic waves can be used to identify complex magnetic phases arising upon evolution of exchange springs in an applied field. Applying sub-GHz surface acoustic waves to study the domain structure of the ferromagnetic layer in exchange-biased bilayers of Ir20Mn80-Co60Fe20B20, we are able to associate the magnetoelastic resonance with the presence of the exchange spin-spirals in both the ferromagnetic and antiferromagnetic layer. Our findings offer a complementary, integrative insight into emergent magnetic materials for applications of noncollinear spin textures in view of low-energy-consumption spintronic devices.

Magnetic nanoparticles can be functionalized in many ways for biomedical applications. Here, we combine four advantageous features in a novel Fe-Pt-Yb 2 O 3 core-shell nanoparticle. (a) The nanoparticles have a size of 10 nm allowing them to diffuse through neuronal tissue. (b) The particles are superparamagnetic after synthesis and ferromagnetic after annealing, enabling directional control by magnetic fields, enhance NMRI contrast, and hyperthermia treatment. (c) After neutron-activation of the shell, they carry low-energetic, short half-life β-radiation from 175 Yb, 177 Yb, and 177 Lu. (d) Additionally, the particles can be optically visualized by plasmonic excitation and luminescence. To demonstrate the potential of the particles for cancer treatment, we exposed cultured human glioblastoma cells (LN-18) to non-activated and activated particles to confirm that the particles are internalized, and that the β-radiation of the radioisotopes incorporated in the neutron-activated shell of the nanoparticles kills more than 98% of the LN-18 cancer cells, promising for future anti-cancer applications., Competing Interests: The authors declare no competing interests., (© 2023 The Author(s).)

Magnetic nanoparticles are highly desirable for biomedical research and treatment of cancer especially when combined with hyperthermia. The efficacy of nanoparticle-based therapies could be improved by generating radioactive nanoparticles with a convenient decay time and which simultaneously have the capability to be used for locally confined heating. The core-shell morphology of such novel nanoparticles presented in this work involves a polysilico-tungstate molecule of the polyoxometalate family as a precursor coating material, which transforms into an amorphous tungsten oxide coating upon annealing of the FePt core-shell nanoparticles. The content of tungsten atoms in the nanoparticle shell is neutron activated using cold neutrons at the Heinz Maier-Leibnitz (FRMII) neutron facility and thereby transformed into the radioisotope W-187. The sizeable natural abundance of 28% for the W-186 precursor isotope, a radiopharmaceutically advantageous gamma-beta ratio of γβ≈30% and a range of approximately 1mm in biological tissue for the 1.3MeV β-radiation are promising features of the nanoparticles' potential for cancer therapy. Moreover, a high temperature annealing treatment enhances the magnetic moment of nanoparticles in such a way that a magnetic heating effect of several degrees Celsius in liquid suspension - a prerequisite for hyperthermia treatment of cancer - was observed. A rise in temperature of approximately 3°C in aqueous suspension is shown for a moderate nanoparticle concentration of 0.5mg/ml after 15min in an 831kHz high-frequency alternating magnetic field of 250Gauss field strength (25mT). The biocompatibility based on a low cytotoxicity in the non-neutron-activated state in combination with the hydrophilic nature of the tungsten oxide shell makes the coated magnetic FePt nanoparticles ideal candidates for advanced radiopharmaceutical applications., (Copyright © 2014 Elsevier B.V. All rights reserved.)

We present magnetic FePt nanoparticles with a hydrophilic, inert, and biocompatible silico-tungsten oxide shell. The particles can be functionalized, optically detected, and optically manipulated. To show the functionalization the fluorescent dye NOPS was bound to the FePt core-shell nanoparticles with propyl-triethoxy-silane linkers and fluorescence of the labeled particles were observed in ethanol (EtOH). In aqueous dispersion the NOPS fluorescence is quenched making them invisible using 1-photon excitation. However, we observe bright luminescence of labeled and even unlabeled magnetic core-shell nanoparticles with multi-photon excitation. Luminescence can be detected in the near ultraviolet and the full visible spectral range by near infrared multi-photon excitation. For optical manipulation, we were able to drag clusters of particles, and maybe also single particles, by a focused laser beam that acts as optical tweezers by inducing an electric dipole in the insulated metal nanoparticles. In a first application, we show that the luminescence of the core-shell nanoparticles is bright enough for in vivo multi-photon imaging in the mouse neocortex down to cortical layer 5.

Magnetic Fe2Pt core-shell nanoparticles with 2 nm cores were synthesized with a monolayer coating of silicotungstate Keggin clusters. The core-shell composition is substantiated by structural analysis performed using high-resolution scanning transmission electron microscopy (HR-STEM) and small angle X-ray scattering (SAXS) in a liquid suspension. The molecular metal oxide cluster shell introduces an enhanced dispersibility of the magnetic Fe-Pt core-shell nanoparticles in aqueous media and thereby opens up new routes to nanoparticle bio-functionalization, for example, using pre-functionalized polyoxometalates.

The pH is one of the key parameters governing protein conformation and activity. In protein crystals, however, the pH is so far not accessible by experiment. Here, we report on the optical detection of the pH in a lysozyme crystal employing the pH-sensitive fluorescent dyes SNARF-1 and SNARF-4F. The molecular probes were loaded into the crystal by diffusion. Two-dimensional fluorescence spectra of the labeled protein crystal were recorded, and the average pH of the crystal at different bath pH's was determined by calibrating fluorescence peak ratios. In addition, we used two-photon microscopy to spatially resolve the pH inside a lysozyme crystal three-dimensionally and to follow pH changes in response to a pH change of the bath over time. At equilibrium at bath pH between 5.5 and 8.0, we found a pH in the water-filled crystal channels that was ΔpH = -0.3 to -1.0 lower than that of the bath. This corresponds to a 2- to 10-fold higher proton concentration in the crystal channels than in the bath. The lower pH at equilibrium in the crystal channels can be explained by slower proton diffusion in the channels than in the bath and a resulting proton accumulation in the crystal for conservation of mass and so an equilibrium of proton flux.

We analyze the origin of the electrical resistance arising in domain walls of perpendicularly magnetized materials by considering a superposition of anisotropic magnetoresistance and the resistance implied by the magnetization chirality. The domain wall profiles of L1(0)-FePd and L1(0)-FePt are determined by micromagnetic simulations based on which we perform first-principles calculations to quantify electron transport through the core and closure region of the walls. The wall resistance, being twice as high in L1(0)-FePd than in L1(0)-FePt, is found to be clearly dominated in both cases by a high gradient of magnetization rotation, which agrees well with experimental observations.

An angle dependent analysis of the planar Hall effect (PHE) in nanocrystalline single-domain Co(60)Fe(20)B(20) thin films is reported. In a combined experimental and theoretical study we show that the transverse resistivity of the PHE is entirely driven by anisotropic magnetoresistance (AMR). Our results for Co(60)Fe(20)B(20) obtained from first principles theory in conjunction with a Boltzmann transport model take into account the nanocrystallinity and the presence of 20 at. % boron. The ab initio AMR ratio of 0.12% agrees well with the experimental value of 0.22%. Furthermore, we experimentally demonstrate that the anomalous Hall effect contributes negligibly in the present case., (© 2011 American Physical Society)

We determine the composition of intrinsic as well as extrinsic contributions to the anomalous Hall effect (AHE) in the isoelectronic L1_{0} FePd and FePt alloys. We show that the AHE signal in our 30 nm thick epitaxially deposited films of FePd is mainly due to an extrinsic side jump, while in the epitaxial FePt films of the same thickness and degree of order the intrinsic contribution is dominating over the extrinsic mechanisms of the AHE. We relate this crossover to the difference in spin-orbit strength of Pt and Pd atoms and suggest that this phenomenon can be used for tuning the origins of the AHE in complex alloys.

Point contact Andreev reflection (PCAR) spectroscopy is a common technique for determining the spin polarization of a ferromagnetic sample. The polarization is extracted by measuring the bias dependence of the conductance of a metallic/superconducting point contact. Under ideal conditions, the conductance is dominated by Andreev reflection and the Blonder-Tinkham-Klapwijk (BTK) model can be used to extract a value for the polarization. However, PCAR spectra often exhibit unwanted features in the conductance that cannot be appropriately modelled with the BTK theory. In this paper we isolate some of these unwanted features and show that any further extraction of the spin polarization from these non-ideal spectra proves unreliable. Understanding the origin of these features provides an objective criterion for rejection of PCAR spectra unsuitable for fitting with the modified BTK model.