Nonmagnetic Species Research Articles

Ultrafast Switching of Sliding Polarization and Dynamical Magnetic Field in van der Waals Bilayers Induced by Light.

Sliding ferroelectricity is a unique type of polarity recently observed in van der Waals bilayers with a suitable stacking. However, electric-field control of sliding ferroelectricity is hard and could induce large coercive electric fields and serious leakage currents that corrode the ferroelectricity and electronic properties, which are essential for modern two-dimensional electronics and optoelectronics. Here, we proposed laser-pulse deterministic control of sliding polarization in bilayer hexagonal boron nitride by first principles and molecular dynamics simulation with machine-learned force fields. The laser pulses excite shear modes that exhibit certain directional movements of lateral sliding between bilayers. The vibration of excited modes under laser pulses is predicted to overcome the energy barrier and achieve the switching of sliding polarization. Furthermore, it is found that three possible sliding transitions-between AB (BA) and BA (AB) stacking-can lead to the occurrence of dynamical magnetic fields along three different directions. Remarkably, the magnetic fields are generated by the simple linear motion of nonmagnetic species, without any need for more exotic (circular, spiral) pathways. Such predictions of deterministic control of sliding polarization and multistates of dynamical magnetic field thus expand the potential applications of sliding ferroelectricity in memory and electronic devices.

Recovery of Catapleiite and Eudialyte from Non-Magnetic Fraction of Eudialyte ore Processing of Norra Kärr Deposit

Eudialyte ores from Norra Kärr (Sweden) and Kringlerne (Greenland) are considered a potential source of rare-earth elements (REE) for the development of a sustainable REE industry outside China. Magnetic separation is successfully applicated to recover eudialyte as a magnetic fraction. In the case of the Norra Kärr deposit, up to 20% of the REE and up to 40% of the Zr are lost during mineral processing in the non-magnetic fraction. Zr and REE are associated with non-magnetic minerals such as catapleiite, low- or non-magnetic eudialyte species, and both their intergrowths. Besides zirconosilicates such as catapleiite and eudialyte, the non-magnetic fraction has valuable and already-liberated minerals such as alkali feldspars and nepheline, which should not be considered as tailings. In this investigation, a possible way to recover REE bearing zirconosilicates from the non-magnetic fraction using flotation is presented. First, a low-grade eudialyte concentrate (1.8% Zr, 0.94% REE) from ground ore was obtained using magnetic separation. The non-magnetic fraction was then treated using froth flotation, and a Zr-REE bearing product (9% Zr, 1.5% REE) was obtained as froth product. For this purpose, phosphoric acid esters were used as selective collectors for zirconosilicates at a pH between 3.5 and 4.5. The reagent regime could be proposed not only to recover Zr- and REE-bearing minerals, but also simultaneously to remove Fe, Ti, and other colored impurities from the nepheline-feldspar product and to minimize the tailings volume.

Magnetism and site occupancy in epitaxial Y-rich yttrium iron garnet films

Y-rich yttrium iron garnet (Y-YIG) thin films were grown on gadolinium gallium garnet substrates using pulsed laser deposition from a ${\mathrm{YFeO}}_{3}$ target. The films are epitaxial on the substrate, and the unit cell of (001)- and (111)-oriented Y-YIG undergoes a tetragonal or rhombohedral distortion, respectively. The Y-YIG has lower room temperature magnetization but higher low-temperature magnetization than bulk YIG, and its Curie temperature was in the range of 330--400 K, depending on the growth conditions. First-principles calculations predict that ${\mathrm{Y}}_{\mathrm{Fe}}$ antisite defects are more stable in octahedral than tetrahedral Fe sites. Temperature-dependent magnetization measurements together with a superexchange dilution model and composition determined from x-ray photoelectron spectroscopy indicate that nonmagnetic species (${\mathrm{Y}}^{3+}$ and vacancies) are predominantly accommodated in the octahedral sites, but there is a significant occupancy of tetrahedral sites.

Dynamic thermal trapping enables cross-species smart nanoparticle swarms.

Bioinspired nano/microswarm enables fascinating collective controllability beyond the abilities of the constituent individuals, yet almost invariably, the composed units are of single species. Advancing such swarm technologies poses a grand challenge in synchronous mass manipulation of multimaterials that hold different physiochemical identities. Here, we present a dynamic thermal trapping strategy using thermoresponsive-based magnetic smart nanoparticles as host species to reversibly trap and couple given nonmagnetic entities in aqueous surroundings, enabling cross-species smart nanoparticle swarms (SMARS). Such trapping process endows unaddressable nonmagnetic species with efficient thermo-switchable magnetic response, which determines SMARS' cross-species synchronized maneuverability. Benefiting from collective merits of hybrid components, SMARS can be configured into specific smart modules spanning from chain, vesicle, droplet, to ionic module, which can implement localized or distributed functions that are single-species unachievable. Our methodology allows dynamic multimaterials integration despite the odds of their intrinsic identities to conceive distinctive structures and functions.

Exchange interactions and long-range magnetic order in the (Mg,Co,Cu,Ni,Zn)O entropy-stabilized oxide: A theoretical investigation

The magnetic structure of the entropy-stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O has been investigated using first-principles methods in combination with Monte Carlo (MC) simulations. Similar to other transition metal oxides with the rock salt structure, such as CoO and NiO, the dominant interaction in this entropic oxide is the antiferromagnetic (AFM) superexchange interaction that takes place between second nearest neighbor cations. This superexchange interaction is responsible for the long-range type-II antiferromagnetic order observed in the material, with ferromagnetic (111) planes coupled antiferromagnetically in the (111) direction. The Néel temperature (TN) is evaluated via MC simulation, where the entropic oxide is modeled by a lattice of randomly distributed strengths of magnetic exchanges obtained from the binary and ternary oxides. The composition dependence of TN suggests that the material becomes paramagnetic when the concentration of nonmagnetic species exceeds 84%. The comparison between the theoretical results and the available experimental data indicates that the magnetic interactions in the entropic oxide can be predicted from magnetic exchange parameters calculated in the binary and ternary oxides.

Modeling of mass transfer enhancement in a magnetofluidic micromixer

The use of magnetism for various microfluidic functions such as separation, mixing, and pumping has been attracting great interest from the research community as this concept is simple, effective, and of low cost. Magnetic control avoids common problems of active microfluidic manipulation such as heat, surface charge, and high ionic concentration. The majority of past works on micromagnetofluidic devices were experimental, and a comprehensive numerical model to simulate the fundamental transport phenomena in these devices is still lacking. The present study aims to develop a numerical model to simulate transport phenomena in microfluidic devices with ferrofluid and fluorescent dye induced by a nonuniform magnetic field. The numerical results were validated by experimental data from our previous work, indicating a significant increase in mass transfer. The model shows a reasonable agreement with experimental data for the concentration distribution of both magnetic and nonmagnetic species. Magnetoconvective secondary flow enhances the transport of nonmagnetic fluorescent dye. A subsequent parametric analysis investigated the effect of the magnetic field strength and nanoparticle size on the mass transfer process. Mass transport of the fluorescent dye is enhanced with increasing field strength and size of magnetic particles.

Using Magnetic Ions to Probe and Induce Magnetism of Pyrophosphates, Bacteria, and Mammalian Cells

Magnetic isolation using magnetic nanoparticles (MNPs) as trapping probes have been widely used in sample pretreatment to shorten analysis time. Nevertheless, to generate MNPs is time-consuming. Furthermore, the generated MNPs have to be further functionalized to gain the capability of recognizing their target species. Thus, an alternative approach that can impose magnetism to nonmagnetic species by simply using magnetic ions as the probes is developed in this study. That is, we employ magnetic ions (Fe3+, Co2+, and Ni2+) that can interact with nonmagnetic species containing oxygen-containing functional groups as the probes. Pyrophosphate (PPi), bacteria, and mammalian cells were selected as the model samples. Our results show that the as-prepared magnetic ion-PPi conjugates gain sufficient magnetism and can be readily aggregated by applying an external magnetic field. Moreover, the magnetic trapping is reversible. The PPi-containing conjugates can lose their magnetic property simply using ethylenediaminetetraacetic acid or aluminum ions as competing agents to remove or to replace, respectively, the conjugated magnetic ions. In addition, bacteria and mammalian cells that possess abundant oxygen-containing functional groups on their cell surfaces can be selectively probed by magnetic ions and gain sufficient magnetism for magnetic isolation from complex serum samples.

Copper Nanospheres Self‐Propelled into Continuums of Iron Nanoclusters to Fabricate and Engineer Two‐Dimensional Heterometallic Arrays under an External Magnetic Field

AbstractControlled dislocation/arrangement metals in a metallic network with nonmagnetic species presents a challenge to fabricate/engineer structured heterometallic compounds. Heterometallic structures have dangling bonds at the shell of neighboring metallic particles, and therefore present a strong strain field in the vicinity of these particles. These structures can be used as a suitable bed with high surface energy to catalyze organic reactions. Here, we show a method for the production of two‐dimensional (2‐D) arrays of Cu nanospheres self‐propelled into continuums of Fe nanoclusters. The synthesis was carried out under magnetic‐field induction (MFI) with adjusting intensity. Electromagnetic (EM) results show that Cu species trapping themselves at the Fe species interface under the influence of magnetic momentum of Fe particles and MFI to rearrange periodic particle arrays of Cu and Fe. The metallic regions in 2‐D composites appear to be efficient catalytic centers in Csp−S cross‐coupling reactions. The synergistic effect of metallic species is a key support for the high activity of 2‐D Cu/Fe nanocompounds in coupling reactions. Nanocompounds are very stable after multiple reaction cycles.

Magnetic behavior of superatomic-fullerene assemblies.

It has recently been possible to synthesize ordered assemblies composed of magnetic superatomic clusters Ni9Te6(PEt3)8 separated by C60 and study their magnetic behavior. We have carried out theoretical studies on model systems consisting of magnetic superatoms separated by non-magnetic species to examine the evolution in magnetic response as the nature of the magnetic superatom (directions of spin quantization), the strength of isotropic and anisotropic interactions, the magnetic anisotropy energy, and the size of the assembly are varied. We have examined square planar configurations consisting of 16, 24 and 48 sites with 8, 12 and 24 magnetic superatoms respectively. The magnetic atoms are allowed 2 or 5 orientations. The model Hamiltonian includes isotropic exchange interactions with second nearest neighbor ferromagnetic and nearest neighbor antiferromagnetic couplings and anisotropic Dzyaloshinskii-Moriya interactions. It is shown that the inclusion of Dzyaloshinskii-Moriya interaction that cause spin canting is necessary to get qualitative response as observed in experiments.

Influence of Cooling Rate on Coercivity and Microstructures in Hot-Deformed Nd-Fe-B Magnets By Nd-Cu Electrochemical/Electroless Deposition

In order to produce a high performing Dy free NdFeB permanent magnet for use at high temperature, control of grain size and interfaces is a significant factor: fabrication of magnetic domain-sized grains aligned in a specific direction and well-separated interfaces by non-magnetic materials. In this study, we used a melt spun powder of several tens of nanometer to produce domain-sized grains and then hot-pressing and dye-upsetting processes were performed to form anisotropic-grained NdFeB magnets. Now, we focused on to coat thin layers in between grains to produce well-separated interfaces by non-magnetic species. For that purpose, we used a well-known grain boundary diffusion process, GBDP. Nd-Cu system was chosen because of making lower melting eutectic alloys, and electrochemical and electroless deposition methods were adopted for mass production. Nd and Cu were deposited by electrochemical and electroless methods, respectively followed by heat treatment in vacuum at 600°C for 25 min. And then they were cooled down in three different ways; furnace cooling, oil cooling at room temperature, and frozen oil cooling. By controlling the microstructures of the interfaces, we can restrict the reverse of magnetic domain mostly generated by defects and grain growth of the NdFeB raw materials. Interfacial microstructures could be controlled by heat treatment especially on cooling rate after GBDP. As a result, it was found that the Nd-rich phases were well distributed in the grain boundaries with the cooling rate. It was confirmed that the change of interfacial microstructure influenced on the magnetic properties, e.g. coercivity, remanence. In case of room temperature oil cooling, coercivity was increased 15.65kOe to16.37kOe, while the coercivity was reduced by frozen oil cooling. Presumably, α-Fe phase decreased with increasing cooling rate which induced increase of coercivity. On the other hand, main phase of NdFeB might be transformed to amorphous phase with further increase of cooling rate that caused decrease of coercivity. In order to identify the phases during GBDP and cooling process, an intensive TEM work will be presented in this presentation.

In-plane/out-of-plane disorder influence on the magnetic anisotropy of Fe1−yMnyPt-L1 bulk alloy

The random substitution of a non-magnetic species instead of Fe atoms in FePt-L10 bulk alloy will permit to tune the magnetic anisotropy energy of this material. We have performed by means of first principles calculations a study of Fe1−yMnyPt-L10 (y = 0.0, 0.08, 0.12, 0.17, 0.22, and 0.25) bulk alloy for a fixed Pt concentration when the Mn species have ferro-/antiferromagnetic (FM,AFM) alignment at the same(different) atomic plane(s). This substitution will promote several in-plane lattice values for a fixed amount of Mn. Charge hybridization will change compared to the FePt-L10 bulk due to this lattice variation leading to a site resolved magnetic moment modification. We demonstrate that this translates into a total magnetic anisotropy reduction for the AFM phase and an enhancement for the FM alignment. Several geometric configurations were taken into account for a fixed Mn concentration because of different possible Mn positions in the simulation cell.

Iridium containing honeycomb Delafossites by topotactic cation exchange.

We report the structure and magnetic properties of two new iridium-based honeycomb Delafossite compounds, Cu3NaIr2O6 and Cu3LiIr2O6, formed by a topotactic cation exchange reaction. The starting materials Na2IrO3 and Li2IrO3, which are based on layers of IrO6 octahedra in a honeycomb lattice separated by layers of alkali ions, are transformed to the title compounds by a topotactic exchange reaction through heating with CuCl below 450 °C; higher temperature reactions cause decomposition. The new compounds display dramatically different magnetic behavior from their parent compounds - Cu3NaIr2O6 has a ferromagnetic like magnetic transition at 10 K, while Cu3LiIr2O6 retains the antiferromagnetic transition temperature of its parent compound but displays significantly stronger dominance of antiferromagnetic coupling between spins. These results reveal that a surprising difference in the magnetic interactions between the magnetic Ir ions has been induced by a change in the non-magnetic interlayer species. A combination of neutron and X-ray powder diffraction is used for the structure refinement of Cu3NaIr2O6 and both compounds are compared to their parent materials.

Magnetic properties of the In-doped MnWO4-type solid solutions Mn1−3xIn2x□xWO4 [□=vacancy; 0≤x≤0.11

Polycrystalline Mn1−3xIn2x□xWO4 (□=vacancy; 0≤x≤0.11) solid solutions synthesized via solid state reactions were characterized by means of magnetic ac susceptibility (χAC) and specific heat (C). The effect of this substitution on the magnetic transition temperatures give rise to three consequences: (1) the disappearance of the basic antiferromagnetic (AFM) phase AF1 when exceeding the doping concentration x=0.03; (2) the phase transition of the paramagnetic phase (PM) to the sinusoidal AFM phase AF3 at the Neel temperature (TN) is shifted toward lower temperatures with respect to pure MnWO4. This is valid also for the transition of AF3 to the cycloidal AFM phase AF2; (3) a systematic lowering of the effective magnetic moment with increasing In-doping. These substantial changes are attributed to the weakened overall magnetic interaction and a strong spin–lattice coupling due to the static disorder of Mn2+ with the nonmagnetic species, i.e. In3+ and vacancy.

Spin Hall effects in metallic antiferromagnets.

We investigate four CuAu-I-type metallic antiferromagnets for their potential as spin current detectors using spin pumping and inverse spin Hall effect. Nontrivial spin Hall effects were observed for FeMn, PdMn, and IrMn while a much higher effect was obtained for PtMn. Using thickness-dependent measurements, we determined the spin diffusion lengths of these materials to be short, on the order of 1nm. The estimated spin Hall angles of the four materials follow the relationship PtMn>IrMn>PdMn>FeMn, highlighting the correlation between the spin-orbit coupling of nonmagnetic species and the magnitude of the spin Hall effect in their antiferromagnetic alloys. These experiments are compared with first-principles calculations. Engineering the properties of the antiferromagnets as well as their interfaces can pave the way for manipulation of the spin dependent transport properties in antiferromagnet-based spintronics.

Magnetic anisotropy of Fe1−yXyPt-L1 [X = Cr, Mn, Co, Ni, Cu] bulk alloys

We demonstrate by means of fully relativistic first principles calculations that, by substitution of Fe by Cr, Mn, Co, Ni, or Cu in FePt-L10 bulk alloys, with fixed Pt content, it is possible to tune the magnetocrystalline anisotropy energy by adjusting the content of the non-magnetic species in the material. The changes in the geometry due to the inclusion of each element induces different values of the tetragonality and hence changes in the magnetic anisotropy and in the net magnetic moment. The site resolved magnetic moments of Fe increase with the X content while those of Pt and X are simultaneously reduced. The calculations are in good quantitative agreement with experimental data and demonstrate that models with fixed band structure but varying numbers of electrons per unit cell are insufficient to describe the experimental data for doped FePt-L10 alloys.

First-order and tricritical wetting transitions in the two-dimensional Ising model caused by interfacial pinning at a defect line.

We present a study of the critical behavior of the Blume-Capel model with three spin states (S=±1,0) confined between parallel walls separated by a distance L where competitive surface magnetic fields act. By properly choosing the crystal field (D), which regulates the density of nonmagnetic species (S=0), such that those impurities are excluded from the bulk (where D=-∞) except in the middle of the sample [where D(M)(L/2)≠-∞], we are able to control the presence of a defect line in the middle of the sample and study its influence on the interface between domains of different spin orientations. So essentially we study an Ising model with a defect line but, unlike previous work where defect lines in Ising models were defined via weakened bonds, in the present case the defect line is due to mobile vacancies and hence involves additional entropy. In this way, by drawing phase diagrams, i.e., plots of the wetting critical temperature (T(w)) versus the magnitude of the crystal field at the middle of the sample (D(M)), we observe curves of (first-) second-order wetting transitions for (small) high values of D(M). Theses lines meet in tricritical wetting points, i.e., (T(w)(tc),D(M)(tc)), which also depend on the magnitude of the surface magnetic fields. It is found that second-order wetting transitions satisfy the scaling theory for short-range interactions, while first-order ones do not exhibit hysteresis, provided that small samples are used, since fluctuations wash out hysteretic effects. Since hysteresis is observed in large samples, we performed extensive thermodynamic integrations in order to accurately locate the first-order transition points, and a rather good agreement is found by comparing such results with those obtained just by observing the jump of the order parameter in small samples.

Spontaneous Tensor Properties for Multiferroic Phases

We have constructed dichromatic matrices of property coefficients for all 1601 Aizu species. This involves 122 non-magnetic Aizu species and extends the work to the 773 species of phase transitions from disordered magnetic prototypic (parent) phases and the 616 species from ordered magnetic prototypic phases. In addition to coefficients describing the non-magnetic effects we have included the coefficients of pyromagnetic, magnetoelectric, piezomagnetic effects, and magnetic susceptibility. All components of these property tensors are displayed in 13 by 13 matrices; non-zero components of the prototypic phase are given in black and the spontaneous coefficients, that are non-zero in the ferroic phase and zero in the prototypic phase, are given in red.

Printing Nearly-Discrete Magnetic Patterns Using Chemical Disorder Induced Ferromagnetism

Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼ 40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools.

Electrical and Magnetic Transport Properties of Nanocrystalline Fe73.5Si13.5B9Cu1Nb3 Alloy

Electrical and magnetic transport properties of Fe73.5Si13.5B9Cu1Nb3 (FINEMET) metallic ribbons prepared by standard melt-spinning technique have been investigated through dc and ac magnetic as well as electrical properties. Fe73.5Si13.5B9Cu1Nb3 shows conventional anisotropic magnetoresistance (MR) which is found to vary from 1 to 15 %. The dynamic behaviour of the sample vibrating at a constant frequency with the help of an external transducer in an external magnetic field is investigated. It is observed that a small self-induced ac voltage is superimposed on the dc response of the sample. This small ac signal is observed to be periodic in nature and may be attributed to the presence of non-magnetic metallic species in the ribbon. The effect of temperature (ranging from 30 to 550 °C) on the resistivity of the sample is measured and the glass transition temperature (T g) of the alloy is estimated from the dc resistivity anomaly observed at the elevated temperature. The frequency dependent responses of permeability, quality factor and dissipation factor are studied by Wayne Kerr impedance analyser with frequency range of 100 Hz–100 MHz. The observed electrical and magnetic properties of the material indicate that the alloy in its ribbon form is suitable for its potential use in electrical and magnetic switching devices.

Spinodal decomposition of a binary magnetic fluid confined to a surface

In our previous work [J. Chem. Phys. 136, 024502 (2012)], we reported a demixing phase transition of a quasi-two-dimensional, binary Heisenberg fluid mixture driven by the ferromagnetic interactions of the magnetic species. Here, we present a theoretical study for the time-dependent coarsening occurring within the two-phase region in the density-concentration plane, also known as spinodal decomposition. Our investigations are based on dynamical density functional theory (DDFT). The particles in the mixture are modeled as Gaussian soft spheres on a two-dimensional surface, where one component carries a classical spin of Heisenberg type. To investigate the two-phase region, we first present a linear stability analysis with respect to small, harmonic density perturbations. Second, to capture nonlinear effects, we calculate time-dependent structure factors by combining DDFT with Percus' test particle method. For the growth of the average domain size l during spinodal decomposition with time t, we observe a power-law behavior l∝t^{δ_{α}} with δ_{m}≃0.333 for the magnetic species and δ_{n}≃0.323 for the nonmagnetic species.