The discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) crystals has generated widespread interest. Making further progress in this area requires quantitative knowledge of the magnetic properties of vdW magnets at the nanoscale. We used scanning single-spin magnetometry based on diamond nitrogen-vacancy centers to image the magnetization, localized defects, and magnetic domains of atomically thin crystals of the vdW magnet chromium(III) iodide (CrI3). We determined the magnetization of CrI3 monolayers to be ≈16 Bohr magnetons per square nanometer, with comparable values in samples with odd numbers of layers; however, the magnetization vanishes when the number of layers is even. We also found that structural modifications can induce switching between ferromagnetic and antiferromagnetic interlayer ordering. These results demonstrate the benefit of using single-spin scanning magnetometry to study the magnetism of 2D vdW magnets.

https://science.sciencemag.org/content/sci/early/2019/04/24/science.aav6926.full.pdf

Probing magnetism in 2D materials at the nanoscale with single-spin microscopy.

Organized multilayers of nanoparticles (9-, 20-, and 45-nm-diameter silica or 12-nm magnetite) and glucose oxidase (GOx) were assembled in alternation with oppositely charged polyelectrolytes on 420-nm latex particles. Stepwise growth of the multilayer films on latex was confirmed by microelectrophoresis and transmission electron microscopy. The inclusion of silica layers on latex yields a higher surface area, resulting in greater GOx adsorption and thereby increasing the catalytic activity of the bioreactor. The bioactivity was proportional to the core surface area and also to the number of GOx layers in the shells. Also, the presence of magnetic nanoparticles allows self-stirring of the nanoreactors with a rotating magnetic field and enhances its productivity. The ensemble of GOx and fluorescent dyes in the shells demonstrated the correlation between Ru-bpy fluorescence and glucose concentration in solution.

Magnetic bio/nanoreactor with multilayer shells of glucose oxidase and inorganic nanoparticles

The development of magnetic field sensors for biomedical applications primarily focuses on equivalent magnetic noise reduction or overall design improvement in order to make them smaller and cheaper while keeping the required values of a limit of detection. One of the cutting-edge topics today is the use of magnetic field sensors for applications such as magnetocardiography, magnetotomography, magnetomyography, magnetoneurography, or their application in point-of-care devices. This introductory review focuses on modern magnetic field sensors suitable for biomedicine applications from a physical point of view and provides an overview of recent studies in this field. Types of magnetic field sensors include direct current superconducting quantum interference devices, search coil, fluxgate, magnetoelectric, giant magneto-impedance, anisotropic/giant/tunneling magnetoresistance, optically pumped, cavity optomechanical, Hall effect, magnetoelastic, spin wave interferometry, and those based on the behavior of nitrogen-vacancy centers in the atomic lattice of diamond.

Ultrasensitive Magnetic Field Sensors for Biomedical Applications.

We report the fabrication of solution-processable organic thin-film transistors (OTFTs) based on poly(9,9′-dioctylfluorene-alt-bithiophene) (F8T2) doped with an electron-acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The 8% doping of the F8T2 film was found to result in an increased hole mobility, up to 0.01 cm2 V−1 s−1, and in an improved on/off ratio, as well as in a low off-current, of the order of 10−11 A. The doped F8T2 device was also found to exhibit a better threshold voltage and reduced hysteresis behavior. These improvements in performance are attributed to the formation of the F8T2-F4TCNQ complex, which results in better hole injection and improved device stability.

Doping effect of solution-processed thin-film transistors based on polyfluorene

The microwave damping mechanisms in magnetic inhomogeneous systems have displayed a richness of phenomenology that has attracted widespread interest over the years. Motivated by recent experiments, we report an extensive experimental study of the Gilbert damping parameter of multicomponent metal oxides micro- and nanophases. We label the former by $M$ samples, and the latter by $N$ samples. The main thrust of this examination is the magnetization dynamics in systems composed of mixtures of magnetic $(\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ and nonmagnetic (ZnO and epoxy resin) materials fabricated via powder processing. Detailed ferromagnetic resonance (FMR) measurements on $N$ and $M$ samples are described so to determine changes in the microwave absorption over the $6--18\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ frequency range as a function of composition and static magnetic field. The FMR linewidth and the field dependent resonance were measured for the $M$ and $N$ samples, at a given volume fraction of the magnetic phase. The asymmetry in the form and change in the linewidth for the $M$ samples are caused by the orientation distribution of the local anisotropy fields, whereas the results for the $N$ samples suggest that the linewidth is very sensitive to details of the spatial magnetic inhomogeneities. For $N$ samples, the peak-to-peak linewidth increases continuously with the volume content of magnetic material. The influence of the volume fraction of the magnetic phase on the static internal field was also investigated. Furthermore, important insights are gleaned through analysis of the interrelationship between effective permeability and Gilbert damping constant. Different mechanisms have been considered to explain the FMR linewidth: the intrinsic Gilbert damping, the broadening induced by the magnetic inhomogeneities, and the extrinsic magnetic relaxation. We observed that the effective Gilbert damping constant of the series of $N$ samples are found to be substantially smaller in comparison to $M$ samples. This effect is attributed to the surface anisotropy contribution to the anisotropy of ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles. From these measurements, the characteristic intrinsic damping dependent on the selected material and the damping due to surface/interface effects and interparticle interaction were estimated. The inhomogeneous linewidth (damping) due to surface/interface effects decreases with diminishing particle size, whereas the homogeneous linewidth (damping) due to interactions increases with increasing volume fraction of magnetic particles (i.e., reducing the separation between neighboring magnetic phases) in the composite.

Magnetization damping in two-component metal oxide micropowder and nanopowder compacts by broadband ferromagnetic resonance measurements

A series of (Co50Fe50)x-(Al2O3)1−x cermet granular thin films deposited on glass substrates by dual electron beam evaporation was studied for their structural, magnetotransport, and magnetic properties. Upon varying the magnetic volume fraction, x, from 0.07 to 0.52 the percolation threshold (xc) was determined from resistivity measurements to be ∼0.17. This value agrees well with the theoretical prediction for a three-dimensional system of spherical particles. Values of the isotropic tunneling magnetoresistance (TMR) as high as 10% at room temperature were found for films with x<0.16. The relationship between magnetotransport (maximum TMR) and the granular film topology (the percolation threshold) is discussed.

Room temperature tunneling magnetoresistance of electron beam deposited (Co50Fe50)x(Al2O3)1−x cermet granular films

Optical properties of ferromagnetic half-metallic full-Heusler Co$_{2}$FeGe alloy are investigated experimentally and theoretically. Co$_{2}$FeGe thin films were obtained by DC magnetron sputtering and show the saturation magnetization at $T$=10 K of $m\approx$5.6 $\mu_{B}$/f.u., close to the value predicted by the Slater-Pauling rule. First-principles calculations of the electronic structure and the dielectric tensor are performed using the full-potential linearized-augmented-plane-wave method in the generalized gradient (GGA) and GGA+U approximations. The measured interband optical conductivity spectrum for the alloy exhibits a strong absorption band in the 1 - 4 eV energy range with pronounced fine structure, which agrees well with the calculated half-metallic spectrum of the system, suggesting a near perfect spin-polarization in the material.

Electronic structure, optical and magnetic properties of Co$_{2}$FeGe Heusler alloy films

Optical and magneto-optical properties of metal-insulator (CoFe)x(Al2O3)1−x granular films have been investigated. The results have been compared with the experimental data of the magnetorefractive effect in the IR region. The optical and magneto-optical spectra of the films depend strongly on the volume fraction of ferromagnetic particles. It was found that the Kerr effect is substantially increased in the spectral region of the plasma frequency. It was revealed that appearance peculiarities observed in the magnetoreflection spectra at 7.5–9.5μm are associated with the excitation of longitudinal phonon modes in the Al2O3 dielectric matrix. It has been shown that both intraband and interband electron transitions contribute to the magnetoresistivity as well as to the IR magnetoreflection. The optical and magneto-optical properties of the (CoFe)x(Al2O3)1−x granular films can be interpreted in the frame of the effective-medium approximation. The magnetorefractive effect can be explained in terms of the modif...

Optical and magneto-optical properties and magnetorefractive effect in metal-insulator CoFe-Al2O3 granular films

Granular cermet films (Fe50Co50)x–(Al2O3)1−x fabricated using the electron-beam coevaporation technique at oblique incidence of FeCo and alumina atom fluxes have been found to exhibit both oblique and in-plane uniaxial magnetic anisotropy. This anisotropy first appears just below the percolation threshold due to a magnetic coupling of particles taking place at a certain stage of their growth and coalescence. The FeCo content x varied from 0.07 to 0.49. A simple model of the film microstructure is presented based on the results of magnetization measurements and ferromagnetic resonance at intermediate (9.4 GHz) and high (94 GHz) frequencies. At 94 GHz the concentration dependence of the effective anisotropy field follows the solid solution law, since then the magnetic field is sufficient to magnetize the films close to saturation. The 9.4 GHz data points deviate from the solid solution line below the percolation threshold due to both modification of the resonance fields by intergranular interactions in nons...

A. F. Kravets

Papers

Room temperature tunneling magnetoresistance of electron beam deposited (Co50Fe50)x(Al2O3)1−x cermet granular films

Electronic structure, optical and magnetic properties of Co$_{2}$FeGe Heusler alloy films

Optical and magneto-optical properties and magnetorefractive effect in metal-insulator CoFe-Al2O3 granular films

Ferromagnetic resonance experiments in an obliquely deposited FeCo-Al2O3 film system

Tunneling magnetoresistance in granular cermet films with particle size distribution