Giant Magnetic Moment Research Articles

Giant effective magnetic moments of chiral phonons from orbit-lattice coupling

Giant effective magnetic moments of chiral phonons from orbit-lattice coupling

Giant orbital magnetic moments and paramagnetic shift in artificial relativistic atoms and molecules.

Materials such as graphene and topological insulators host massless Dirac fermions that enable the study of relativistic quantum phenomena. Single quantum dots and coupled quantum dots formed with massless Dirac fermions can be viewed as artificial relativistic atoms and molecules, respectively. Such structures offer a unique testbed to study atomic and molecular physics in the ultrarelativistic regime (particle speed close to the speed of light). Here we use a scanning tunnelling microscope to create and probe single and coupled electrostatically defined graphene quantum dots to unravel the magnetic-field responses of artificial relativistic nanostructures. We observe a giant orbital Zeeman splitting and orbital magnetic moment up to ~70 meV T-1 and ~600μB (μB, Bohr magneton) in single graphene quantum dots. For coupled graphene quantum dots, Aharonov-Bohm oscillations and a strong Van Vleck paramagnetic shift of ~20 meV T-2 are observed. Our findings provide fundamental insights into relativistic quantum dot states, which can be potentially leveraged for use in quantum information science.

Spin Hall effect driven by the spin magnetic moment current in Dirac materials

The spin Hall effect of a Dirac Hamiltonian system is studied using semiclassical analyses and the Kubo formula. In this system, the spin Hall conductivity is dependent on the definition of spin current. All components of the spin Hall conductivity vanish when spin current is defined as the flow of spin angular momentum. In contrast, the off-diagonal components of the spin Hall conductivity are non-zero and scale with the carrier velocity (and the effective $g$-factor) when spin current consists of the flow of spin magnetic moment. We derive analytical formula of the conductivity, carrier mobility and the spin Hall conductivity to compare with experiments. In experiments, we use Bi as a model system that can be characterized by the Dirac Hamiltonian. Te and Sn are doped into Bi to vary the electron and hole concentration, respectively. We find the spin Hall conductivity ($\sigma_\mathrm{SH}$) takes a maximum near the Dirac point and decreases with increasing carrier density ($n$). The sign of $\sigma_\mathrm{SH}$ is the same regardless of the majority carrier type. The spin Hall mobility, proportional to $\sigma_\mathrm{SH}/n$, increases with increasing carrier mobility with a scaling coefficient of $\sim$1.4. These features can be accounted for quantitatively using the derived analytical formula. The results demonstrate that the giant spin magnetic moment, with an effective $g$-factor that approaches 100, is responsible for the spin Hall effect in Bi.

Control of Giant Topological Magnetic Moment and Valley Splitting in Trilayer Graphene.

Bloch states of electrons in honeycomb two-dimensional crystals with multivalley band structure and broken inversion symmetry have orbital magnetic moments of a topological nature. In crystals with two degenerate valleys, a perpendicular magnetic field lifts the valley degeneracy via a Zeeman effect due to these magnetic moments, leading to magnetoelectric effects which can be leveraged for creating valleytronic devices. In this work, we demonstrate that trilayer graphene with Bernal stacking (ABA TLG), hosts topological magnetic moments with a large and widely tunable valley g factor (g_{ν}), reaching a value g_{ν}∼1050 at the extreme of the studied parametric range. The reported experiment consists in sublattice-resolved scanning tunneling spectroscopy under perpendicular electric and magnetic fields that control the TLG bands. The tunneling spectra agree very well with the results of theoretical modeling that includes the full details of the TLG tight-binding model and accounts for a quantum-dot-like potential profile formed electrostatically under the scanning tunneling microscope tip.

Studies on the morphological, structural, optical and electrical properties of Fe-doped ZnO magnetic nano-crystal thin films

In this work, the effect of substrate material on the morphological, structural, optical and electrical properties of Fe-doped ZnO magnetic nano-crystal thin films was investigated. Fe-doped ZnO is a magnetic material and shows ferromagnetism at room temperature. FeO phases were detected in x-ray diffraction patterns. The surfaces of the films are fully occupied-spherical-like nano crystalline. The Fe/Zn ratios were calculated 1.6 and 5.9%. The optical transmittance value of the film is approximately 40%. The band gap value was shifted toward to lower value and the value was found as 3.15 eV. The maximum magnetic moments were obtained at 4000 and 2500 Oe for the film deposited onto glass and Si substrates, respectively. The electrical properties were determined at the maximum magnetic moment situation for the nano crystallites. It was found that diluted Fe-doped ZnO magnetic semiconductors have giant magnetic moment and electrical conductivity.

Control of Mooij correlations at the nanoscale in the disordered metallic Ta\u2013nanoisland FeNi multilayers

Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10^3–10^5 Bohr magnetons (mu _{mathrm{B}}). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer lead to a decrease in the Drude contribution of free charge carriers and the appearance of the low-energy electronic excitations in the 1–2 eV spectral range characteristic of electronic correlations, which may accompany the formation of electronic inhomogeneities. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated localised electronic states. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc conductivity falls down below the MIR limit by about 60%. The discovered effect leading to the dc conductivity fall below the MIR limit can be associated with non-ergodicity and purely quantum (many-body) localisation phenomena, which need to be challenged further.

First-principles study of a Mn-doped In2Se3 monolayer: Coexistence of ferromagnetism and ferroelectricity with robust half-metallicity and enhanced polarization

Two-dimensional (2D) materials with coexistence of ferromagnetism (FM) and ferroelectricity (FE) are rare. By using first-principles modeling, here we report that doping of magnetic transition metal (TM) atoms in FE monolayer In2Se3 could introduce giant local FM magnetic moments (4μB per Mn atom). The exchange splitting energy, and the C3ν crystal field together with the hybridization of the Mn−d and Se−p orbitals result in spin-polarized states near the Fermi surface. More interestingly, the asymmetric charge distribution of the Jahn-Teller ion (Mn3+d4) further lift the twofold degeneracy of the Mn-d e1∗ states. This affects the band structure and enhances the FE polarization in monolayer In2Se3. Our work paves the way for tailoring FM in 2D FE via doping magnetic moments from TM atoms. The half-metallicity combined with the enhanced FE polarization make Mn-doped In2Se3 a candidate for potential applications in information technology and spintronic devices.

Films of rhombohedral graphite as two-dimensional topological semimetals

Topologically non-trivial states appear in a number of materials ranging from spin-orbit-coupling driven topological insulators to graphene. In multivalley conductors, such as mono- and bilayer graphene, despite a zero total Chern number for the entire Brillouin zone, Berry curvature with different signs concentrated in different valleys can affect the material’s transport characteristics. Here we consider thin films of rhombohedral graphite, which appear to retain truly two-dimensional properties up to tens of layers of thickness and host two-dimensional electron states with a large Berry curvature, accompanied by a giant intrinsic magnetic moment carried by electrons. The size of Berry curvature and magnetization in the vicinity of each valley can be controlled by electrostatic gating leading to a tuneable anomalous Hall effect and a peculiar structure of the two-dimensional Landau level spectrum.

Structural Characterization, Magnetic Properties, and Heating Power of Nickel Ferrite Nanoparticles

Magnetic nanoparticles of nickel ferrite were synthesized by using the high-temperature thermal decomposition method. The crystal structure and the size of the nanoparticles were determined. The magnetic properties of the nanoparticles were studied in detail, as well as the heating power of a magnetic fluid prepared with the synthesized nanoparticles. X-ray diffraction revealed that the nanoparticles crystallized in the cubic spinel structure. Transmission electron microscopy showed two populations of dispersed nanoparticles. The mean particle size of the most abundant population is approximately 13 nm. Magnetization measurements showed that the nanocompound is in the superparamagnetic regime at room temperature. It was found that nanoparticles have giant magnetic moments of more than 16 $\text{k}\mu _{\mathrm {B}}$ . Self-heating experiments carried out in ac magnetic fields indicate that the nanoparticles have high efficiency as heating agents. Specific power absorption (SPA) and intrinsic loss power (ILP) values as large as 997 W/g and 3.6 nH $\cdot \text{m}^{2}$ /kg, respectively, were obtained.

Magnetic and electronic properties of diamond-shaped graphene-boron nitride nanoribbons and nanoflakes

New and interesting physical phenomena arise in ultra-small materials, such as nanoribbons and nanoflakes. Such structures have properties that are significantly modified by the width, type and shape of the edge, size and composition. These diverse possibilities to control them, makes some types of nanoribbons and nanoflakes are not fully understood so far. Here, we study diamond-shaped graphene-boron nitride nanoribbons and nanoflakes using density functional theory. We investigate the structural, magnetic and electronic properties as functions of variable widths along nanoribbon and the lateral size of the nanoflakes. We find that boron nitride structures with predominantly zigzag edges are more stable than graphene systems with such edges. Magnetic moment in graphene-boron nitride nanoflakes is directly proportional to the difference between the number of carbon atoms in each sublattice. We observed giant magnetic moments by supercell (approximately 5 μB). The n×n supercell of the diamond-shaped graphene-boron nitride nanoflakes has a magnetic moment nμB. Variable width, 1-D and 2-D quantum confinement can adjust the properties of nanoribbons and nanoflakes.

First-principles study of the complex magnetism in Fe16N2

Magnetic exchange interactions in pure and vanadium (V)-doped Fe16N2 are studied within the framework of density functional theory (DFT). The Curie temperatures were obtained via both mean field approximation (MFA) and Monte Carlo (MC) calculations based on interactions that were obtained through DFT. The Curie temperature (TC) for pure Fe16N2 that was obtained under MFA is substantially larger than the experimental value, suggesting the importance of thermal fluctuations. At zero field, the calculated magnetic susceptibility shows a sharp peak at T = TC that corresponds to the presence of localized d-states. From the nature of the exchange interactions, we have determined the reason for the occurrence of the giant magnetic moment in this material, which remained a mystery for decades. Finally, we posit that Fe16N2 can also act as a satisfactory spin injector for III–V semiconductors, in addition to its application as a permanent magnet, since it has very high spin polarization (compared to elemental ferromagnets) and smaller lattice mismatch (compared to half-metallic Heusler alloys) with conventional III–V semiconductors such as GaAs and InGaAs. We demonstrate this application in the case of Fe16N2(001)/InGaAs(001) hetero-structures, which exhibit substantial spin polarization in the semiconductor (InGaAs) region. PACS number: 82.65.My, 82.20.Pm, 82.30.Lp, 82.65.Jv.

Local magnetism and spin fluctuation behavior of extremely dilute Fe impurity in Pd-U binary alloys: An experimental and theoretical investigation

We report the magnetic behavior of isolated Fe impurity atoms in Pd1−xUx (0 ≤ x ≤ 0.12) alloys studied through measurements of the local susceptibility (χloc) and spin relaxation time for 54Fe nuclear probe using the time differential perturbed angular distribution (TDPAD) technique. With an increase of the U concentration in Pd-U alloys, the magnetic response of Fe, exhibiting Curie-Weiss type χloc(T), reflects a steady decrease of the giant magnetic moment arising from ferromagnetic polarization of host Pd-4d band electrons. Concomitantly, we find an exponential rise in the spin fluctuation temperature TSF. The experimental observation of U induced suppression of giant magnetic moment of Fe in Pd is supported by ab intio electronic structure calculations based on density functional theory. The results presented here support the idea that sign and strength of spin polarization of host conduction band electrons play a crucial role on the spin fluctuation rate and hence moment stability.

Interaction-Driven Giant Orbital Magnetic Moments in Carbon Nanotubes.

Carbon nanotubes continue to be model systems for studies of confinement and interactions. This is particularly true in the case of so-called "ultraclean" carbon nanotube devices offering the study of quantum dots with extremely low disorder. The quality of such systems, however, has increasingly revealed glaring discrepancies between experiment and theory. Here, we address the outstanding anomaly of exceptionally large orbital magnetic moments in carbon nanotube quantum dots. We perform low temperature magnetotransport measurements of the orbital magnetic moment and find it is up to 7 times larger than expected from the conventional semiclassical model. Moreover, the magnitude of the magnetic moment monotonically drops with the addition of each electron to the quantum dot directly contradicting the widely accepted shell filling picture of single-particle levels. We carry out quasiparticle calculations, both from first principles and within the effective-mass approximation, and find the giant magnetic moments can only be captured by considering a self-energy correction to the electronic band structure due to electron-electron interactions.

First principles investigation on the electronic structure and magnetism of dilute Fe impurity in [formula omitted][formula omitted] alloys

Employing the all electron augmented plane wave + local orbital (APW + lo) method based on the density function theory (DFT), we have performed ab initio calculations of electronic structure and magnetic properties of dilute Fe impurity in PdHx alloys (0⩽x⩽1.0). Here we present our results for the H induced changes in the density of state (DOS), local magnetic moment and the macroscopic magnetic properties. The calculations performed for PdHx without the Fe impurity reveal H concentration dependent lattice expansion and Pd-H bonding which results in large change in electronic band structure and the DOS at the Fermi energy. The nonmagnetic DOS calculated for Fe doped PdHx is analyzed within Stoner model and the condition for local moment formation is found to hold for all concentrations of H. With an increase of the H content in the Pd matrix, the local moment of Fe declines from 3.43μB at x = 0 to 2.72μB at x = 1.0. More significantly, the giant magnetic moment, associated with the Fe impurity in Pd, progressively decreases with H concentration as the positive spin polarization of Pd-4d band electrons diminish and crossover to negative sign for x > 0.75. We show that H induced changes in s-d and d-d hybridization plays a crucial role on the magnetic properties of Fe doped PdHx alloys. In particular, we suggest that the rapid decline of the giant moment in PdHx alloys results from the weakening of ferromagnetic d-d exchange interaction between Fe-3d and Pd-d band electrons caused by changes in density of states arising from H induced filling up of the Pd-4d band.

Thermal stability of Fe16N2 thin film on GaAs (0 0 1) substrate

There is a major need for a high performance magnetic system for high temperature applications. One propitious low cost permanent magnet candidate is Fe16N2, with its giant magnetic moment predicted to be above other materials from conventional first principles calculations. Here we report on a comprehensive study of the thermal stability of Fe16N2 thin films on GaAs (0 0 1) substrate. Using polarized neutron reflectometry (PNR), the saturation magnetization depth profile (Ms) of the films and its modification with temperature is directly measured at various thermal conditions. The structural modifications probed by x-ray diffraction (XRD) unravel that above 250 °C the Fe16N2 thin films decompose into α-Fe and γ′-Fe4N phases. An influence of the strain effect is investigated by grazing incidence x-ray diffraction (GIXRD). We reveal that despite a large Fe16N2 in-plane lattice constant of ~5.88 Å and different strains from substrate or seed layer (up to 2.8% tensile strain), Fe16N2 thin films have the same thermal stability. Our results demonstrate that the high thermal stability of partially order Fe16N2 thin films makes them very promising candidates for spintronics and permanent magnet applications.

Effect of Ar+ ion post-irradiation on crystal structure, magnetic behavior and optical band gap of Co-implanted ZnO wafers

Single crystals wurtzite ZnO with (001) orientation were implanted with Co+ ions at room temperature (RT). To tune their magnetic behavior as well as the band gap of the implanted wafers, Ar+ ion post-irradiation (PI) was performed using the calculated energy and ion dose. The formed Co clusters present in the high dose Co-implanted ZnO wafer were observed to be absent after the PI, which is quite different from the low dose doped one. It is found that all the implanted samples showed a giant magnetic moment and a narrowing optical band gap, and that the post-irradiated ones exhibited an even further redshifted absorption edge and ferromagnetic behavior but with saturation magnetization (M-S) drastically decreased.

Observation of room temperature ferromagnetism with giant magnetic moment based on Zn1−xCrxO thin films grown on Si(111) substrate via 1,2-dihydroxyethane modified sol-gel dip-coating technique

Dilute magnetic semiconductors have shown a great encouragement from the technological point of view owing to their potential application in multiple spintronics, such as spin light-emitting diode, spin-valve transistor, logic device, nonvolatile memory and ultrafast optical switches. Here, we report on the synthesis of Zn1−xCrxO (0.01≤x≤0.09) thin films grown on Si (111) substrate via 1,2-dihydroxyethane modified sol-gel dip-coating technique for spintronic applications. The influence of partial substitution of Cr ions into the Zn sites on the chemical composition, morphology, crystal structure, and magnetic properties of the prepared films was investigated by the X-ray diffraction (XRD), atomic force microscopy (AFM), high resolution transmission electron microscopy (HR-TEM), dispersive X-ray spectroscopy (EDS), electron diffraction (SAED), X-ray electron spectroscopy (XPS) and vibrating magnetometer (VSM). A single phase and highly crystalline films are obtained. These films showed room temperature ferromagnetism at all Cr ions concentration. The Zn0.95Cr0.05O film showed a remarkable giant magnetic moment of 2.17μB at room temperature. This is the highest magnetic moment value ever achieved among all derived transition elements doped ZnO based dilute magnetic semiconductors till the date.

Phase Concentration Determination of Fe16N2 Using State of the Art Neutron Scattering Techniques

Due to limitations on the availability of rare earth elements it is imperative that new high energy product rare earth free permanent magnet materials are developed for the next generation of energy systems. One promising low cost permanent magnet candidate for a high energy magnet is α″-Fe16N2, whose giant magnetic moment has been predicted to be well above any other from conventional first principles calculations. Despite its great promise, the α″ phase is metastable; making synthesis of the pure phase difficult, resulting in less than ideal magnetic characteristics. This instability gives way to a slew of possible secondary phases (i.e. α-Fe, Fe2O3, Fe8N, Fe4N, etc.) whose concentrations are difficult to detect by conventional x-ray diffraction. Here we show how high resolution neutron diffraction and polarized neutron reflectometry can be used to extract the phase concentration ratio of the giant magnetic phase from ultra-small powder sample sizes (~0.1 g) and thin films. These studies have led to the discovery of promising fabrication methods for both homogeneous thin films, and nanopowders containing the highest reported to date (>95%) phase concentrations of room temperature stable α″-Fe16N2.

The investigation of giant magnetic moment in ultrathin Fe3O4 films

The magnetic and transport properties of Fe3O4 films with a series of thicknesses are investigated. For the films with thickness below 15 nm, the saturation magnetization (Ms) increases and the coercivity decreases with the decrease in films’ thickness. The Ms of 3 nm Fe3O4 film is dramatically increased to 1017 emu/cm3. As for films’ thickness more than 15 nm, Ms is tending to be close to the Fe3O4 bulk value. Furthermore, the Verwey transition temperature (Tv) is visible for all the films, but suppressed for 3 nm film. We also find that the ρ of 3 nm film is the highest of all the films. The suppressed Tv and high ρ may be related to the islands morphology in 3 nm film. To study the structure, magnetic, and transport properties of the Fe3O4 films, we propose that the giant magnetic moment most likely comes from the spin of Fe ions in the tetrahedron site switching parallel to the Fe ions in the octahedron site at the surface, interface, and grain boundaries. The above results are of great significance and also provide a promising future for either device applications or fundamental research.

Enhanced magnetism of Cu(n) clusters capped with N and endohedrally doped with Cr.

The focus of our work is on the production of highly magnetic materials out of Cu clusters. We have studied the relative effects of N-capping as well as N mono-doping on the structural stability and electronic properties of the small Cu clusters using first principles density functional theory based electronic structure calculations. We find that the N-capped clusters are more promising in producing giant magnetic moments, such as 14 μB for the Cu6N6 cluster and 29 μB for the icosahedral Cu13N12 cluster. This is accompanied by a substantial enhancement in their stability. We suggest that these giant magnetic moments of the capped Cun clusters have relevance to the observed room temperature ferromagnetism of Cu doped GaN. For cage-like hollow Cu-clusters, an endohedral Cr-doping together with the N-capping appears as the most promising means to produce stable giant magnetic moments in the copper clusters.