Spin Structure Changes Research Articles

Magneto-electric effect in Y1-xRxFeO3 (R = Sm and Er) ceramics

In the present work, we synthesize a series of Y1-xRxFeO3R=Er,Sm&0.≤x≤1 polycrystalline compound by solid-state reaction method to study the A-site substitution effects on physical properties in YFeO3. The Rietveld refinement of X-ray diffraction analysis indicates that with Er3+ substitution, the lattice parameters have decreased, and more structural distortion has been shown compared to Sm3+ substitution. The obtained magnetic hysteresis loops reveal that Er3+ ions have induced an antiferromagnetic behavior in YFeO3 because of a reduction in the Dzyloshinskii-Moriya interaction. The Y1-xSmxFeO3 family, however, shows a weak ferromagnetic behavior. Furthermore, due to the spin reorientation (SR) transition, a distinct anomaly has been detected in the temperature dependence of Ac susceptibility and dielectric constant curves. With substituting Sm at the Y-site in the YFeO3 compound, SR transition happens at a temperature, TSR, in which the spin structure changes high-temperature Γ4 to Γ2 configuration, and therefore the starting and ending of the SR region depend on Sm concentration. Interestingly, we have observed a gigantic dielectric constant in Y0.7Sm0.3FeO3, Y0.5Er0.5FeO3, and Y0.25Er0.75FeO3. It, however, has demonstrated that Y0.7Sm0.3FeO3 is the only compound that shows a relaxor ferroelectric behavior. The observed peak in the temperature dependence of the dielectric constant has also described a magnetic transition temperature indicating the magneto-dielectric effect in our studied samples.

Observation of magnetic field-induced second magnetic ordering and peculiar ferroelectric polarization in L-type ferrimagnetic Fe2(MoO4)3

${\mathrm{Fe}}_{2}{({\mathrm{MoO}}_{4})}_{3}$ was synthesized and characterized as a prototype of L-type ferrimagnets (L-FiM) with an ordering temperature ${T}_{\mathrm{N}1}\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ using magnetic susceptibility $(\ensuremath{\chi})$, specific heat $({C}_{\mathrm{p}})$, and dielectric $({\ensuremath{\varepsilon}}^{\ensuremath{'}})$ anomaly. Two remarkable findings are magnetic field $(H)$ induced (i) an additional magnetic phase transition at ${T}_{\mathrm{N}2}$ below ${T}_{\mathrm{N}1}$; and (ii) the emergence of flexible ferroelectric polarization $(P)$ with the tuning parameters $H$ and $T$ below ${T}_{\mathrm{N}2}$. Thus, the H-T phase diagram for spin-induced type-II multiferroics was established. In contrast to some known multiferroics with a critical field-induced spin-flip $P$ below ${T}_{\mathrm{N}1}$, the observed multiferroic nature is exotic. The origin of the profound multiferroic nature may be hidden in the complex $T$-and $H$-dependent spin and lattice structures. Consequently, the schematic picture of $H$-induced possible change of lattice symmetry and conical spin structure has been proposed. More experimental and theoretical works providing solid evidence and interpretations have been suggested to explore these peculiar multiferroic phenomena.

Magnetic Properties and Magnetodielectric Effect in (C6H5CH2NH3)2[CuCl4] with Organic-Inorganic Hybrid Layered Structure

Metal Organic Frameworks (MOFs) are a group of multiferroic candidates that exhibit various optical, electrical, magnetic, and structural properties depending on the type of organic cations, transition metals and halogen elements, and their molar ratio. (C6H5CH2NH3)2[CuCl4] (BZACC), one of the MOF materials, has a special hybrid layered structure of organic and inorganic layers. Magnetic property measurement showed that BZACC exhibits weak in-plane ferromagnetism below the Curie point (TC) of 10.25 K and the saturated magnetic moment of 1 µB per formula unit at 2 K. When the magnetic field is less than 0.1 T along the c axis, it exhibits weak antiferromagnetism below 8 K. From the measurement of the dielectric constant with external magnetic field, we observed a maximum magnetodielectric (MD) effect of 0.4% at 2 K. Furthermore, it was found that the magnetic-field dependence of the MD effect is similar to that of the second derivative of magnetization with respect to magnetic field. We suggest that the observed MD effect is the consequence of the magnetic-field-induced spin structure change in the inorganic layer, which induces the change in the interlayer magnetic coupling and, as a result, induces the change in the dielectric constant by the applied magnetic field.

Fermionic CFTs and classifying algebras

We study fermionic conformal field theories on surfaces with spin structure in the presence of boundaries, defects, and interfaces. We obtain the relevant crossing relations, taking particular care with parity signs and signs arising from the change of spin structure in different limits. We define fermionic classifying algebras for boundaries, defects, and interfaces, which allow one to read off the elementary boundary conditions, etc.As examples, we define fermionic extensions of Virasoro minimal models and give explicit solutions for the spectrum and bulk structure constants. We show how the A- and D-type fermionic Virasoro minimal models are related by a parity-shift operation which we define in general. We study the boundaries, defects, and interfaces in several examples, in particular in the fermionic Ising model, i.e. the free fermion, in the fermionic tri-critical Ising model, i.e. the first unitary N = 1 superconformal minimal model, and in the supersymmetric Lee-Yang model, of which there are two distinct versions that are related by parity-shift.

High-pressure magnetism of the double perovskite Sr2FeOsO6 studied by synchrotron Fe57 Mössbauer spectroscopy

The pressure dependence of the magnetism of the double perovskite ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ was investigated by temperature- and magnetic-field-dependent synchrotron $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy in the energy domain up to $\ensuremath{\sim}50$ GPa. ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ is known to feature antiferromagnetic ordering below ${T}_{\mathrm{N}}\ensuremath{\sim}140$ K and a change in spin structure from AF1 to AF2 near 70 K at ambient pressure. Previous Os ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) spectra [Veiga et al., Phys. Rev. B 91, 235135 (2015)] indicated a pressure-driven change from the antiferromagnetic to the ferrimagnetic state. Raman spectra of ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ collected at room temperature and up to 34 GPa are in agreement with previous x-ray-diffraction studies which verified that the tetragonal ambient pressure crystal structure is retained at high pressure. The M\ossbauer investigations show that the Fe ions remain in the +3 high-spin $({{t}_{2g}}^{3}{e}_{g}^{2})$ state over the whole pressure range. The magnetic ordering is strongly stabilized by high pressure and for $pg30$ GPa the ordering temperature is increased to above room temperature. Broad magnetic hyperfine patterns as well as coexistence of magnetic and paramagnetic signals at high pressures indicate the formation of inhomogeneous magnetic states. The shapes of applied-field M\ossbauer spectra at low temperature are similar at 2 and 33 GPa and do not allow resolution of distinct antiferromagnetic and ferrimagnetic components. Pressure-induced spin canting in a basically antiferromagnetic spin structure or magnetic phase separation involving a pressure-induced minority ferrimagnetic phase may be the origin for the ferromagnetic XMCD signal.

Ferromagnetism at Room Temperature Induced by Spin Structure Change in BiFe1-x Cox O3 Thin Films.

The coexistence and coupling of ferromagnetic and ferroelectric orders in a single material is crucial for realizing next-generation multifunctional applications. The coexistence of such orders is confirmed at room temperature in epitaxial thin films of BiFe1-x Cox O3 (x ≤ 0.15), which manifests a spin structure change from a low-temperature cycloidal one to a high-temperature collinear one with canted ferromagnetism.

Spin Structure Change in Co-Substituted BiFeO3

The spin structure in BiFe1−xCoxO3 (\(x = 0.05,0.10,0.15,0.20\)) was investigated as a function of Co substitution, temperature, and magnetic field. It was found that the cycloidal spin structure of BiFeO3 changed to a collinear one with spin canting and composition-independent spontaneous magnetization of ∼0.25 μB/f.u. for the Co substituted samples on heating. The collinear phase was stabilized under magnetic fields. The spin structure change was clarified also by the temperature dependence of 57Fe Mossbauer spectra, and the results indicated the first order nature of this transition.

Field-Induced Ferromagnetism of Fe4+-Perovskite System, Sr1-xBaxFeO3 (0 ≤x≤1)

A series of cubic perovskite-type oxides, Sr1-xBaxFe4+O3 (\(0 \leq x \leq 1\)), have been studied through measurements of neutron diffraction and magnetization. It has been found that their helical spin structure changes from the G-type to the A-type with increasing Ba content at \(x \sim 0.85\). The application of an external magnetic field induces ferromagnetism with \({\sim}3.5\) µB/Fe, and the strength of the critical field decreases markedly from 39 T for SrFeO3 to 0.3 T for BaFeO3. The transition process can be categorized into three: SrFeO3, Sr0.4Ba0.6FeO3, and BaFeO3 types.

Correlation between spin structure oscillations and domain wall velocities

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape.

Spin reorientation and magnetization reversal in the perovskite oxides, YFe1−xMnxO3 (0≤x≤0.45): A neutron diffraction study

Members of the YFe1−xMnxO3 (0≤x≤0.45) family crystallize in the GdFeO3 type orthorhombic perovskite structure (space group Pnma) where the Fe and Mn ions are disordered at the 4b crystallographic site. Upon substitution of Mn at the Fe-site in the canted antiferromagnetic YFeO3 (TN=640K), a first-order spin-reorientation transition occurs at a temperature, TSR, where the magnetic structure changes from the canted to a collinear state. With increasing Mn-concentration, TSR increases whereas TN decreases. Neutron diffraction studies on the x=0.4 sample reveal that the spin structure changes from Γ4 to Γ1 below TSR. Intriguingly, when x=0.4 and 0.45, a temperature-induced magnetization reversal is observed below a compensation temperature T⁎ (T⁎<TSR<TN). The reversal is explained on the basis of a ferrimagnetic ground state resulting from antiferromagnetic coupling of the canted moments of Fe–O–Fe and Mn–O–Mn with that of Fe–O–Mn ordering.

New Optical Route to Magnetic State Control

Devices that harness the spin of the electron (rather than solely its charge) to process and store information are the basis of spintronics. Currently, static forces are used to switch the magnetization in a spintronics device, but the vision for future devices is to be able to switch them at high speeds with short-pulsed laser light [1]. So far, pulsed lasers have mainly been a tool for heating a sample to a temperature where it is in a different magnetic phase. Laser heating a sample can modify the magnetization on a time scale of picoseconds (ps) [2]; however, in the absence of an external magnetic field, the heated regions will form multiple domains with an equal mixture of ↑ and ↓ magnetization. A better solution is to find a way to optically switch individual domains into a definite magnetic state. Now, Johan de Jong, at Radboud University in the Netherlands, and colleagues report in Physical Review Letters that they have been able to accomplish this sort of magnetic state control in the insulator Sm1/2Pr1/2FeO3 [3]. With short pulses of circularly polarized laser light, they flip the magnetization of a single magnetic domain within ∼ 5 ps. de Jong et al. achieve this by taking advantage of the interaction between light of a certain helicity and a spin reorientation transition unique to the material they studied. The present finding is an important step for the complete light control of magnetic domains in a variety of magnets, and highlights an important light-matter interaction in magnets. Sm1/2Pr1/2FeO3 is a member of the class of materials called orthoferrites. Orthoferrites have a distorted perovskite structure, and exhibit a rich variety of thermally induced magnetic transitions [4]. In the case of Sm1/2Pr1/2FeO3, there are two magnetic phase transitions that are relevant to de Jong et al.’s work. The first occurs at a temperature T2 ∼ 130 K, when the Fe3+ ions have a weak ferromagnetic moment along the c axis. The second occurs at a temperature T1 ∼ 98 K, when this ferromagnetic moment rotates to the a axis. (The iron magnetic moments are actually antiferromagnetically ordered, but a small symmetry breaking interaction causes them to cant in the same direction, which is why they have a weak ferromagnetic moment.) de Jong et al. have confirmed that a linearly polarized pulse of light can heat an orthoferrite that is below T1 to above the phase transition in of order a picosecond, a time scale that is mainly determined by the characteristic time for spins to exchange heat with the surrounding lattice. In this nonequilibrium state, the spin structure changes in the same manner as the thermally induced spin reorientation transition: the magnetization flips from being aligned along the a axis to being aligned along the c axis. However, if there is no magnetic field, the magnetization vector is equally likely to be oriented ↑ or ↓ along the c axis, so multiple domains form. To have a better control over the final state of the domains, de Jong et al. use circularly polarized light to perform the switching. The electric field vector associated with circularly polarized light rotates clockwise or counterclockwise around the light’s direction of propagation. Circularly polarized photons therefore carry angular momentum, which makes it possible for these photons to flip a spin. de Jong et al.’s idea is that the helicity of light can selectively drive a spin excitation in a solid via a coherent optical process, an effect that the group reported several years ago [5]. In orthoferrites, the spin precession is known to occur at gigahertz to terahertz frequencies [6], a speed that is accessible for commercially available pulsed lasers. The group uses a state-of-the-art optical pump and probe method to control domains in a roughly 100micrometer-thick plate of Sm1/2Pr1/2FeO3. First, a circularly polarized pulse acts as a “pump,” in that it ex-

Spin state and structural changes at the metal-insulator transition in YBaCo2O5.5 by synchrotron x-rays

Synchrotron x-ray diffraction and absorption, together with magnetic and thermoelectric measurements, have been used to investigate the mechanism driving the metal to insulator transition in YBaCo2O5.50. Results show a structural and magnetic transition concomitant with the metal to insulator transition. Absorption spectroscopy at the Co L3 edge as a function of temperature reveals minor changes across the transition. The set of experimental results are hardly conciliative with a high to low spin state change of Co ions at octahedral sites in this cobaltite.

Magnetodielectric effect in Z-type hexaferrite

The authors report on the magnetodielectric (MD) effect of Z-type hexaferrite Sr3Co2Fe24O41 at various temperatures and frequencies. A fairly large negative MD effect was observed with a peak near room temperature and a maximum at low frequencies. Analysis suggests that the MD effect shows a quadratic dependence on magnetization. The results were discussed by considering the magnetic field induced change of transverse conical spin structure and spin-phonon coupling. This work is helpful for understanding the MD effect in materials with complicated spin structures.

Site directed spin labeling studies of Escherichia coli dihydroorotate dehydrogenase N-terminal extension

Dihydroorotate dehydrogenases (DHODHs) are enzymes that catalyze the fourth step of the de novo synthesis of pyrimidine nucleotides. In this reaction, DHODH converts dihydroorotate to orotate, using a flavine mononucleotide as a cofactor. Since the synthesis of nucleotides has different pathways in mammals as compared to parasites, DHODH has gained much attention as a promising target for drug design. Escherichia coli DHODH (EcDHODH) is a family 2 DHODH that interacts with cell membranes in order to promote catalysis. The membrane association is supposedly made via an extension found in the enzyme’s N-terminal. In the present work, we used site directed spin labeling (SDSL) to specifically place a magnetic probe at positions 2, 5, 19, and 21 within the N-terminal and thus monitor, by using Electron Spin Resonance (ESR), dynamics and structural changes in this region in the presence of a membrane model system. Overall, our ESR spectra show that the N-terminal indeed binds to membranes and that it experiences a somewhat high flexibility that could be related to the role of this region as a molecular lid controlling the entrance of the enzyme’s active site and thus allowing the enzyme to give access to quinones that are dispersed in the membrane and that are necessary for the catalysis.

Relationship between Nonadiabaticity and Damping in Permalloy Studied by Current Induced Spin Structure Transformations

By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter beta does not equal the damping alpha, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core.

Pulsed EPR spectrometer with injection-locked UCSB free-electron laser

We describe the first free-electron laser (FEL)-based pulsed electron paramagnetic resonance (EPR) system designed to study spin dynamics and structure changes of proteins in aqueous solution with nano-second of time resolution. This novel approach opens up the possibility for high-power sub-THz and THz pulsed EPR spectroscopy.

Magnetic structure of MnSi under an applied field probed by polarized small-angle neutron scattering

The magnetic structure of a single crystal MnSi under applied field has been studied by small angle diffraction with polarized neutrons below ${T}_{C}=28.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Experiments have shown that in zero field the magnetic structure of MnSi consists of four left-handed spiral domains oriented along four ⟨111⟩ axes. The magnetic field, applied along one of the ⟨111⟩ axes, produces a single domain helix oriented along the field at ${H}_{C1}\ensuremath{\approx}80\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ at low temperatures. The magnetic mosaic of the spin structure changes with the magnetic field and has a maximum at ${H}_{C1}$. The integral intensity of the Bragg reflection shows a sharp minimum at ${H}_{\mathit{in}}\ensuremath{\approx}160\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$ attributed to an instability of the helix structure. When the field has a component perpendicular to the helix wave vector $\mathbf{k}$, it rotates toward the field direction in the field range $H&lt;{H}_{\mathit{in}}$. Additionally, a second harmonic of the helix structure is induced by the perpendicular magnetic field for $H&lt;{H}_{\mathit{in}}$. These three features are well explained accounting for the presence of a spin wave gap $\ensuremath{\Delta}\ensuremath{\sim}g{\ensuremath{\mu}}_{B}{H}_{\mathit{in}}∕\sqrt{2}\ensuremath{\simeq}12\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}$, which provides the stability of the spin wave spectrum with respect to the perpendicular magnetic field. Further increase of the field leads to a magnetic phase transition from conical to a ferromagnetic state near ${H}_{C2}\ensuremath{\approx}600\phantom{\rule{0.3em}{0ex}}\mathrm{mT}$. The critical field ${H}_{C2}$ is related to the spin wave stiffness $A$ as $g{\ensuremath{\mu}}_{B}{H}_{C2}=A{k}^{2}$. Our findings are in agreement with the recently developed theory [Phys. Rev. B 73, 174402 (2006)] for cubic magnets with Dzyaloshinskii-Moriya interaction, which relates the major parameters of the spin wave spectrum (such as the spin wave stiffness and the gap) with the features of the spin structure of MnSi being observed under applied magnetic field.

PH Controls the Rate and Mechanism of Nitrosylation of Water-Soluble FeIII Porphyrin Complexes

In-depth kinetic and mechanistic studies on the reversible binding of NO to water-soluble iron(III) porphyrins as a function of pH revealed unexpected reaction kinetics for monohydroxo-ligated (P)Fe(III)(OH) species formed by deprotonation of coordinated water in diaqua-ligated (P)Fe(III)(H(2)O)(2). The observed significant decrease in the rate of NO binding to (P)Fe(OH) as compared to that of (P)Fe(H(2)O)(2) does not conform with expectations based on previous mechanistic work on NO-heme interactions, which would point to a diffusion-limited reaction for the five-coordinate Fe(III) center in (P)Fe(OH). The decrease in rate and an associatively activated mode of NO binding observed at high pH is ascribed to an increase in the activation barrier related to spin state and structural changes accompanying NO coordination to the high-spin (P)Fe(III)(OH) complex. The existence of such a barrier has previously been observed in the reactions of five-coordinate iron(II) hemes with CO and is evidenced for the first time for the process involving coordination of NO to the iron heme complex. The observed reactivity pattern, relevant in the context of studies on NO interactions with synthetic and biologically important hemes (in particular, hemoproteins), is reported here for an example of a simple water-soluble iron(III) porphyrin [meso-tetrakis(sulfonatomesityl)porphinato]-iron(III), (TMPS)Fe(III).

Direct Observation of Domain-Wall Configurations Transformed by Spin Currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

NMR evidence for a two-step phase separation in Nd1.85Ce0.15CuO4-delta.

By Cu NMR we studied the spin and charge structure in Nd(2-x)Ce(x)CuO(4-delta). For x=0.15, starting from a superconducting sample, the low temperature magnetic order in the sample reoxygenated under 1 bar oxygen at 900 degrees C reveals a peculiar modulation of the internal field, indicative of a phase characterized by large charge droplets ("blob" phase). By prolonged reoxygenation at 4 bars the blobs break up and the spin structure changes to that of an ordered antiferromagnet. We conclude that the superconductivity in the n-type systems competes with a genuine type I Mott-insulating state.