Nonmagnetic Host Research Articles

Toward understanding the dimensional crossover of canonical spin-glass thin films

Spin-glass thin films exhibit many features different from the bulk. The freezing temperatures of spin-glass films are suppressed for reduced thickness and follow the Kenning relation. The dynamics are altered near the vacuum interface. These phenomena are closely related to the lower critical dimension of spin glasses, the spin-glass correlation length, and the dimensional crossover from d = 3 to d = 2. In this article, we review the experimental facts and theoretical perspectives for spin-glass thin films. We focus on canonical spin-glass systems with the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between magnetic impurities in a nonmagnetic host. Open questions to be addressed are emphasized.

Impact of evolution of structural defects on the ferromagnetic and electrical properties of laser deposited Indium-doped SnO2 thin films

Impact of non-magnetic cationic-substitution on the evolution of structural defects and correlated ferromagnetic and electrical properties are investigated in series of pulsed laser deposited Sn1-xInxO2 (0.0 ≤ x ≤ 0.12) thin films. Beyond the nominal doping concentration of 2 at.% (i.e. x > 0.02), Indium (In)-doped SnO2 film switches to exhibit from n-type to p-type electrical conductivity and simultaneously the magnetization (MS) as well as the Curie temperature (TC) of the films increase significantly. Estimated values of ‘MS’ and ‘TC’ are found to achieve as large as 15.21 emu/cm3 and 540 K respectively when In-doping concentration approaches towards x = 0.08 and afterwards tend to decrease abruptly. Various spectroscopic techniques including Positron Annihilation Lifetime Spectroscopy (PALS) have detected the existence of Sn vacancy (VSn) defects within Sn1-xInxO2 films arise as the effect of In-substitution at Sn site (InSn) under O-rich atmosphere. The estimated positron lifetimes and the increase of line-shape S-parameter confirm the rise of VSn defects which serve as the major source of magnetic moments in non-magnetic host SnO2. Besides, InSn defects introduce excess holes within SnO2 lattice and thereby the magnetic spin-spin RKKY interaction between near-by VSn defects are mediated ferromagetically through the localized holes. For x > 0.08, stabilization of various donor-type defects such as Sn interstitial (Sni), indium interstitial (Ini) actually compensates the acceptors that leads to reduce the effective hole density consequently diminishing the strength of ferromagnetism within SnO2. Hence, tuning of such ferromagnetic and semiconducting properties through non-magnetic cationic substitution in transparent conducting oxides can be very promising in the field of next-generation spintronics.

Room temperature d 0 ferromagnetism in carbon doped LaH3: insights from density functional theory simulations

Employing the state-of-the-art density functional theory with both generalized gradient approximation and the hybrid HSE06 functional, along with the incorporation of spin–orbit coupling, we have engineered stable room temperature ferromagnetism (FM) (RTF) in nonmagnetic LaH3 through C substitution at octahedral and tetrahedral H sites where the induced magnetic moment is mostly contributed by the 2p orbital of the C atom. It is interesting that the magnetic signature is switched on with an impurity concentration as low as 1.04 at% with a magnetic moment of ∼1.0 µ B per impurity, where the localized behavior of the 2p states of C, along with significant exchange splitting energy, can be attributed as the origin of the induced magnetic moment. The verification of the Stoner criterion in the material further confirmed the onset of FM in the system, and the computed Curie temperature is found to be well above room temperature. Reduced formation energy and the requirement of lower impurity concentration ensure practical feasibility towards a spintronic device where RTF is established from the nonmagnetic host and the dopant.

Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi2Te4 and its nonmagnetic counterpart Bi2Te3 host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi2Te4 and MnBi4Te7 following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi2Te4 and Bi2Te3 surface terminations of MnBi4Te7, revealing an instantaneous occupation of states associated with the Bi2Te3 surface layer followed by carrier extraction into the adjacent MnBi2Te4 layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi2Te4 septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi2Te4-based van der Waals compounds.

Toward a systematic discovery of artificial functional magnetic materials

Although ferromagnets are found in all kinds of technological applications, only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high energy Ne ions and own experiments on low energy Ar ion irradiation of TiO$_2$ hosts. In the high energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states.

Image of Dynamic Local Exchange Interactions in the dc Magnetoresistance of Spin-Polarized Current through a Dopant.

We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spin-polarized electrical contact through a magnetic dopant in a nonmagnetic host. Using the stochastic Liouville formalism we calculate clearly defined resonances in the dc magnetoresistance when the applied magnetic field matches the exchange interaction with a nearby spin. At these resonances spin precession in the applied magnetic field is canceled by spin evolution in the exchange field, preserving a dynamic bottleneck for spin transport through the dopant. Similar features emerge when the dopant spin is coupled to nearby nuclei through the hyperfine interaction. These features provide a precise means of measuring exchange or hyperfine couplings between localized spins near a surface using spin-polarized scanning tunneling microscopy, without any ac electric or magnetic fields, even when the exchange or hyperfine energy is orders of magnitude smaller than the thermal energy.

Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.

Structural, electronic, magnetic and optical properties of AB2O4 (A = Ge, Co and B = Ga, Co) spinel oxides

Structural, magnetic, optical and electronic properties of GeGa2O4, CoGa2O4, GeCo2O4 and CoCo2O4 spinel oxides have been studied. Spinel oxides are studied by performing theoretical calculations based on density functional theory. In non-magnetic host GeGa2O4 spinel oxide, significant effects of Co occupancy (at A, B and/or both sites) on the electronic, magnetic and optical behavior have been observed. Among the spinel oxides, the CoGa2O4, GeCo2O4 and CoCo2O4 spinel oxides show 100% spin-polarization and exhibit stable cubic structure in ferromagnetic phase. The calculated structural and magnetic parameters predict the cation distribution at A and B sites as follows: (Ge2+)[Ga23+]O4−2, (Co2+)[Ga23+]O4−2, (Ge4+)[Co22+]O4−2 and (Co+2)[Co23+]O4−2 for GeGa2O4, CoGa2O4, GeCo2O4 and CoCo2O4 spinel oxides, respectively. The electronic band gap is found to be decreased from 1.605 eV to 0.762 eV for spin↑ state while it is decreased from 1.578 eV to 0.000 eV for spin↓ state with Co occupancy in host GeGa2O4 lattice. Lattice constant decreases (8.7877 Å to 8.1390 Å) as Co occupies B site and/or both (A and B) sites. For GeCo2O4 spinel oxide, the calculated value of total magnetic moment (mtot = 6.12 μB/f.u) and saturation magnetization (MS = 134.22 emu/g) is found to maximum. Optical properties reveal that the CoGa2O4, GeCo2O4 and CoCo2O4 spinel oxides have less threshold electronic transition energy relative to host GeGa2O4 spinel oxide. Electronic, magnetic and optical characteristics show potential integration of these spinel oxides in magnetic-semiconducting devices.

Optimized process for the fabrication of PDMS membranes integrating permanent micro-magnet arrays

Here we report on the fabrication of micro-magnet arrays by powder agglomeration in a polymer matrix. The NdFeB@polydimethylsiloxane (PDMS) inner microstructure and the generated magnetic forces were studied, when prepared under two different magnetic field configurations. The initial process uses the classical crosslinking of PDMS mixed with NdFeB powder under a low magnetic field gradient provided by a permanent magnet (LG set-up for low gradient). In contrast, the optimized process uses an intermediate layer, composed of iron microstructures in a PDMS matrix that amplifies and focuses the magnetic field gradient given by the permanent magnet (HG set-up for high gradient). Both processes result in a heterogeneous material that can be described as an array of permanent micro-magnets diluted in a non-magnetic host matrix. The NdFeB@PDMS microstructure was characterized by X-ray tomography and optical microscopy. The magnetic properties were also measured by magnetometry and colloidal probe AFM. Results showed that the HG set-up leads to an array of micro-magnets localized at the surface, with higher compactness and density, resulting in stronger magnetic performances compared to the LG set-up. This technology only implies easy-to-handle and cheap fabrication processes, paving the way for the development of low-cost lab-on-chip devices integrating magnetophoretic trapping functionality.

Prospect for detecting magnetism of a single impurity atom using electron magnetic chiral dichroism

Dopants, even single atoms, can influence the electrical and magnetic properties of materials. Here we demonstrate the opportunity for detecting the magnetic response of an embedded magnetic impurity in a nonmagnetic host material. We combine a depth sectioning approach with electron magnetic circular dichroism in scanning transmission electron microscopy to compute the depth-resolved magnetic inelastic-scattering cross section of single Co impurity buried in the host crystal of GaAs. Our calculations suggest that the magnetic dichroic signal intensity is sensitive to the depth and lateral position of the electron probe relative to the magnetic impurity. Additionally, a more precise dichroic signal localization can be achieved via choosing higher-collection-angle ($\ensuremath{\beta}$) apertures. Quantitative evaluation of the inelastic-scattering cross section and signal-to-noise ratio indicates that the magnetic signal from a single Co atom is on the verge of being detectable with today's state-of-the-art instrumentation.

High-temperature ferromagnetism in new n-type Fe-doped ferromagnetic semiconductor (In,Fe)Sb

Over the past two decades, intensive studies on various ferromagnetic semiconductor (FMS) materials have failed to realize reliable FMSs that have a high Curie temperature (TC > 300 K), good compatibility with semiconductor electronics, and characteristics superior to those of their nonmagnetic host semiconductors. Here, we demonstrate a new n-type Fe-doped narrow-gap III–V FMS, (In1−x,Fex)Sb. Its TC is unexpectedly high, reaching ∼335 K at a modest Fe concentration (x) of 16%. The anomalous Hall effect and magnetic circular dichroism (MCD) spectroscopy indicate that the high-temperature ferromagnetism in (In,Fe)Sb thin films is intrinsic and originates from the zinc-blende (In,Fe)Sb alloy semiconductor.

Quasiparticle interference from magnetic impurities

Fourier transform scanning tunneling spectroscopy (FT-STS) measures the scattering of conduction electrons from impurities and defects, giving information about the electronic structure of both the host material and adsorbed impurities. We interpret such FT-STS measurements in terms of the quasiparticle interference (QPI), here investigating in detail the QPI due to single magnetic impurities adsorbed on a range of representative nonmagnetic host surfaces, and contrasting with the case of a simple scalar impurity or point defect. We demonstrate how the electronic correlations present for magnetic impurities markedly affect the QPI, showing, e.g., a large intensity enhancement due to the Kondo effect, and universality at low temperatures/scanning energies. The commonly used joint density of states interpretation of FT-STS measurements is also considered, and shown to be insufficient in many cases, including that of magnetic impurities.

Specific heat of a localized magnetic impurity in a non-magnetic host: A spectral density method for the Anderson–Holstein model

The effect of electron–phonon interaction on the spectral function of a magnetic impurity in a non-magnetic host metal is studied within the framework of the Anderson–Holstein model using a spectral density method. The impurity contribution to the specific heat of the host metal is also calculated.

Three-dimensional distribution of igneous rocks near the Pebble porphyry Cu-Au-Mo deposit in southwestern Alaska: Constraints from regional-scale aeromagnetic data

Aeromagnetic data helped us to understand the 3D distribution of plutonic rocks near the Pebble porphyry copper deposit in southwestern Alaska, USA. Magnetic susceptibility measurements showed that rocks in the Pebble district are more magnetic than rocks of comparable compositions in the Pike Creek–Stuyahok Hills volcano-plutonic complex. The reduced-to-pole transformation of the aeromagnetic data demonstrated that the older rocks in the Pebble district produce strong magnetic anomaly highs. The tilt derivative transformation highlighted northeast-trending lineaments attributed to Tertiary volcanic rocks. Multiscale edge detection delineated near-surface magnetic sources that are mostly outward dipping and coalesce at depth in the Pebble district. The total horizontal gradient of the 10-km upward-continued magnetic data showed an oval, deep magnetic contact along which porphyry deposits occur. Forward and inverse magnetic modeling showed that the magnetic rocks in the Pebble district extend to depths greater than 9 km. Magnetic inversion was constrained by a near-surface, 3D geologic model that is attributed with measured magnetic susceptibilities from various rock types in the region. The inversion results indicated that several near-surface magnetic sources with moderate susceptibilities converge with depth into magnetic bodies with higher susceptibilities. This deep magnetic source appeared to rise toward the surface in several areas. An isosurface value of 0.02 SI was used to depict the magnetic contact between outcropping granodiorite and nonmagnetic sedimentary host rocks. The contact was shown to be outward dipping. At depths around 5 km, nearly the entire model exceeded the isosurface value indicating the limits of nonmagnetic host material. The inversion results showed the presence of a relatively deep, northeast-trending magnetic low that parallels lineaments mapped by the tilt derivative. This deep low represents a strand of the Lake Clark fault.

Broadband effective magnetic response of inorganic dielectric resonator-based metamaterial for microwave applications

A single-sized dielectric cylinder-based metamaterial is fabricated from TiO2 nanoparticles, using a bottom-up approach. The sub-elements constituting the metalayer are embedded in a nonmagnetic transparent host matrix in the microwave regime and arranged in a square lattice. We demonstrate numerically and experimentally a broadband magnetic activity. The key feature to achieve this performance remains in the high aspect ratio of the metamaterial building blocks. This is a very promising step towards complex electromagnetic functions, involving low-cost metamaterials with simple fabrication.

Efficient implementation of the continuous-time hybridization expansion quantum impurity solver

Strongly correlated quantum impurity problems appear in a wide variety of contexts ranging from nanoscience and surface physics to material science and the theory of strongly correlated lattice models, where they appear as auxiliary systems within dynamical mean-field theory. Accurate and unbiased solutions must usually be obtained numerically, and continuous-time quantum Monte Carlo algorithms, a family of algorithms based on the stochastic sampling of partition function expansions, perform well for such systems. With the present paper we provide an efficient and generic implementation of the hybridization expansion quantum impurity solver, based on the segment representation. We provide a complete implementation featuring most of the recently developed extensions and optimizations. Our implementation allows one to treat retarded interactions and provides generalized measurement routines based on improved estimators for the self-energy and for vertex functions. The solver is embedded in the ALPS-DMFT application package. Program summaryProgram title: ct-hybCatalogue identifier: AEOL_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOL_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Use of the hybridization expansion impurity solvers requires citation of this paper. Use of any ALPS program requires citation of the ALPS [1] paper.No. of lines in distributed program, including test data, etc.: 650044No. of bytes in distributed program, including test data, etc.: 20553265Distribution format: tar.gzProgramming language: C++/Python.Computer: Desktop PC, high-performance computers.Operating system: Unix, Linux, OSX, Windows.Has the code been vectorized or parallelized?: Yes, MPI parallelized.RAM: 1 GBClassification: 7.3.External routines: ALPS [1, 2, 3], BLAS [4, 5], LAPACK [6], HDF5 [7]Nature of problem:Quantum impurity models were originally introduced to describe a magnetic transition metal ion in a non-magnetic host metal. They are widely used today. In nanoscience they serve as representations of quantum dots and molecular conductors. In condensed matter physics, they are playing an increasingly important role in the description of strongly correlated electron materials, where the complicated many-body problem is mapped onto an auxiliary quantum impurity model in the context of dynamical mean-field theory, and its cluster and diagrammatic extensions. They still constitutes a non-trivial many-body problem, which takes into account the (possibly retarded) interaction between electrons occupying the impurity site. Electrons are allowed to dynamically hop on and off the impurity site, which is described by a time-dependent hybridization function.Solution method:The quantum impurity model is solved using a continuous-time quantum Monte Carlo algorithm which is based on a perturbation expansion of the partition function in the impurity–bath hybridization. Monte Carlo configurations are represented as segments on the imaginary time interval and individual terms correspond to Feynman diagrams which are stochastically sampled to all orders using a Metropolis algorithm. For a detailed review on the method, we refer the reader to [8].Running time:1–8 h.

Nonradiative energy transfer from Mn2+ to Eu3+ in K2CaP2O7:Mn2+,Eu3+ phosphor

A series of color tunable phosphors K2Ca1−x−yP2O7:xMn2+, yEu3+ are synthesized by solid state reaction method. The energy transfer phenomenon from Mn2+ to Eu3+ has been observed in the Mn2+/Eu3+ codoped non-magnetic K2CaP2O7 host, which was confirmed by PL spectra and decay curves. The Mn2+→Eu3+ energy transfer is controlled by quadrupole–quadrupole interaction between sensitizer and activator. The maximum efficiency of energy transfer is estimated to be 33% with x=0.125 and y=0.03 in K2Ca1−x−yP2O7:xMn2+, yEu3+ phosphor. The phosphors can emit light from green to yellow and eventually to orange under 400nm excitation by changing the Mn2+/Eu3+ content ratio, indicating that K2CaP2O7: Mn2+, Eu3+ would be potential candidates for use in lighting and displays applications.

Spin-polarized half-metallic state of a ferromagneticδlayer in a semiconductor host

We discuss a model which is apt to describe the appearance of a spin-polarized half-metallic state around a single δ layer of a magnetic transition metal embedded into a nonmagnetic semiconductor host. We show that ferromagnetism in this system can be attributed to the intrinsic physical properties of the δ layer. The relevant physicaleffectsdescribedbyourmodelarethehybridizationoftheelectronstatesofthemetalandsemiconductor atoms, the charge redistribution around the δ layer, and the electron-electron correlation on a metal atom, which is the driving force of ferromagnetism. We obtain the mean-field phase diagram of the model at zero and finite temperature, both in the case when the chemical potential is fixed, and when the density of particles on the δ layer isfixed.Therelevanceofourresultsinconnectionwithnumericalandexperimentalresultsontheso-calleddigital magnetic heterostructures, in the absence and in the presence of a quantum-well carrier channel, is eventually discussed.

Theory of oscillations in STM conductance caused by subsurface defects (Review Article)

In this review we discuss recent theoretical studies of single subsurface defects by means of a scanning tunneling microscope (STM). These investigations are based on quantum interference effects between the electron partial waves that are directly transmitted through the contact and the partial waves scattered by a defect. In particular, we demonstrate the feasibility of imaging the position of a defect below a metal surface by means of STM. Different types of subsurface defects are discussed: point-like magnetic and nonmagnetic defects, magnetic clusters in a nonmagnetic host metal, and nonmagnetic defects in an s-wave superconductor. The effect of Fermi surface anisotropy is analyzed. Studies of the effect of high magnetic fields on the STM conductance of tunnel point contacts in the presence of a single defect are also discussed.

Magneto-orientation and quantum size effects in spin-polarized STM conductance in the presence of a subsurface magnetic cluster

The influence of a single magnetic cluster in a non-magnetic host metal on the spin current $\mathbf{j}^{(s)}$ and the charge current $\mathbf{j}$ in the vicinity of a ferromagnetic STM tip is studied theoretically. Spin-flip processes due to electron interaction with the cluster are taken into account. We show that quantum interference between the partial waves injected from the STM tip and those scattered by the cluster results in the appearance of components perpendicular to the initial polarization of the spin current $\mathbf{j}^{(s)}$, which obtain a strongly inhomogeneous spatial distribution. This interference produces oscillations of the conductance as a function of the distance between the contact and the cluster center. The oscillation amplitude depends on the current polarization. We predict a strong magneto-orientational effect: the conductance oscillations may grow, shrink, or even vanish for rotation of the cluster magnetic moment $\mathbf{\mu}_{\mathrm{eff}}$ by an external magnetic field. These results can be used for the determination of the $ \mathbf{\mu}_{\mathrm{eff}}$ for magnetic clusters below a metal surface.