Spin Structure Research Articles

Structural, Mossbauer spectra and magnetic properties of Ni(1-x) Cdx Fe2O4 by solid state Reaction method

The preparation of Ni-Cd ferrite having the chemical formula Ni(1-x)CdxFe2O4 where x varies from 0, 0.1, 0.2, 0.3, 0.4, and 0.5 were synthesized by solid state reaction method. The X-ray analysis shows the existence of single phase spinel structure with increase of lattice parameter and density with increasing Cd contents were as the porosity decreases. The particle size was calculated by X-ray broadening studies. The Vickers hardness of Ni-Cd ferrite system variation between 532 to 568.6. The increase in hardness of composites results from increased Cd-content. The Atomic Force Microscopy (AFM) showed that the average grain size was increases with the increase in Cd content. The variation of dielectric constant, loss tangent and AC conductivity as a function of frequency in frequency range 100Hz-5MHz was studied. The dielectric constants decreases with increasing frequency for all the samples and follow the Maxwell-Wagner's interfacial polarization. The Hall coefficient was found to be positive. It demonstrates that the majority of charge carriers of p-type. The magnetic properties of Ni(1-x)CdxFe2O4 ferrites were strongly affected by the Cd content. The Mossbaure spectra show canted spin structure for all the samples.

On classification of fermionic rational conformal field theories

We systematically study how the integrality of the conformal characters shapes the space of fermionic rational conformal field theories in two dimensions. The integrality suggests that conformal characters on torus with a given choice of spin structures should be invariant under a principal congruence subgroup of PSL(2, ℤ). The invariance strongly constrains the possible values of the central charge as well as the conformal weights in both Neveu-Schwarz and Ramond sectors, which improves the conventional holomorphic modular bootstrap method in a significant manner. This allows us to make much progress on the classification of fermionic rational conformal field theories with the number of independent characters less than five.

Observation of the skyrmion side-face state in a chiral magnet

We identify a three-dimensional skyrmion side-face state in chiral magnets that consists of a thin layer of modulated surface spirals and an array of phase-locked skyrmion screws. Such chiral spin structures lead to a characteristic X-shaped magnetic diffraction pattern in resonant elastic x-ray scattering, reminiscent of Photo 51 of the DNA double-helix diffraction. By measuring both thin plates and bulk ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ crystals in the field-in-plane geometry, we unambiguously identify the modulated skyrmion strings by retrieving their chirality and helix angle. The breaking of the translational symmetry along the side faces suppresses the bulk-favored conical state, providing a stabilization mechanism for the skyrmion lattice phase that has been overlooked so far.

A geometrical approach to nontrivial topology via exotic spinors

Exotic spinors arise in non-simply connected base manifolds due to the nonequivalent spinor structure. The dynamics of exotic spinors are endowed with an additional differential factor. In this work, we merge the exotic spinor scenario with Cartan’s spinor viewpoint, according to which a given spacetime point is understood as a kind of composition of spinor entries. As a result, we arrive at a geometrical setup in which the Minkowski metric is perturbed by elements reflecting the nontrivial topology. Such corrections shall be felt by any physical system studied with the resulting bilinear form. Within the flat spacetime context, we investigate quasinormal modes arising from the interference of nontrivial topology in the scalar field dispersion relation.

Trapping and manipulating skyrmions in two-dimensional films by surface acoustic waves

Skyrmions, topologically stable spin structures with particle-like properties, are promising for spintronics applications such as skyrmion racetrack memory. Though reliable control of skyrmion motion is essential for the operation of spintronics devices, the straight motion of skyrmions along the driving force is in general difficult due to an inevitable transverse force originating from their topology. Here, we propose a method of precise manipulation of skyrmions based on surface acoustic waves (SAWs) propagating in two dimensions. Using two standing SAWs, saddle-shape local potentials like quadrupole ion traps are created to trap skyrmions robustly. Furthermore, by tuning the frequencies of the SAWs, we show that trapped skyrmions not only move in straight lines but also move precisely in any direction in a two-dimensional thin film. These results could be helpful for the future design of spintronics devices based on skyrmions.

Multiple Dielectric Relaxations in CaMn2O4 Polycrystalline Compound

The dielectric response of the CaMn2O4 polycrystalline compound has been reported in this paper. The orthorhombic room-temperature structure (space group Pbcm) is confirmed by powder X-ray diffraction (XRD). The temperature dependent magnetization M(T) data reveal that antiferromagnetic ordering below T N = 220 K. Further decreasing the temperature magnetization shows increasing trend of canted spin structure. Dielectric anomalies are clearly observed at the onset of the magnetic ordering (∼220 K) and also at the onset of the canted structure (50 K) in CaMn2O4. Such type of dielectric anomaly around long range order and canted spin transition will correlate the influence of underlying magnetic structure with electric ordering.

Development of a laser-based angle-resolved-photoemission spectrometer with sub-micrometer spatial resolution and high-efficiency spin detection.

Angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution (μ-ARPES), has become a powerful tool for studying quantum materials. To achieve sub-micrometer or even nanometer-scale spatial resolution, it is important to focus the incident light beam (usually from synchrotron radiation) using x-ray optics, such as the zone plate or ellipsoidal capillary mirrors. Recently, we developed a laser-based μ-ARPES with spin-resolution (LMS-ARPES). The 177nm laser beam is achieved by frequency-doubling a 355nm beam using a KBBF crystal and subsequently focused using an optical lens with a focal length of about 16mm. By characterizing the focused spot size using different methods and performing spatial-scanning photoemission measurement, we confirm the sub-micron spatial resolution of the system. Compared with the μ-ARPES facilities based on the synchrotron radiation, our LMS-ARPES system is not only more economical and convenient, but also with higher photon flux (>5 × 1013 photons/s), thus enabling the high-resolution and high-statistics measurements. Moreover, the system is equipped with a two-dimensional spin detector based on exchange scattering at a surface-passivated iron film grown on a W(100) substrate. We investigate the spin structure of the prototype topological insulator Bi2Se3 and reveal a high spin-polarization rate, confirming its spin-momentum locking property. This lab-based LMS-ARPES will be a powerful research tool for studying the local fine electronic structures of different condensed matter systems, including topological quantum materials, mesoscopic materials and structures, and phase-separated materials.

Efimovian three-body potential from broad to narrow Feshbach resonances

We analyze the change in the hyperradial Efimovian three-body potential as the two-body interaction is tuned from the broad to narrow Feshbach resonance regime. Here, it is known from both theory and experiment that the three-body dissociation scattering length ${a}_{\ensuremath{-}}$ shifts away from the universal value of $\ensuremath{-}9.7\phantom{\rule{4pt}{0ex}}{r}_{\mathrm{vdW}}$, with ${r}_{\mathrm{vdW}}=\frac{1}{2}{(m{C}_{6}/{\ensuremath{\hbar}}^{2})}^{1/4}$ the two-body van der Waals range. We model the three-body system using a separable two-body interaction that takes into account the full zero-energy behavior of the multichannel wave function. We find that the short-range repulsive barrier in the three-body potential characteristic for single-channel models remains universal for narrow resonances, while the change in the three-body parameter originates from a strong decrease in the potential depth. From an analysis of the underlying spin structure we further attribute this behavior to the dominance of the two-body interaction in the resonant channel compared to other nonresonant interactions.

Photonic Pseudospin Skyrmion in Momentum Space

Magnetic Skyrmions are topologically protected spin structures with sophisticated swirls of electron spins. In recent times, photonic counterparts of the magnetic Skyrmions have been discovered with new deep-subwavelength characteristics promising for future applications in optical information storage and transfer. While the photonic Skyrmions in real space have been observed in diverse photonic systems, studies on the photonic Skyrmions in momentum space have rarely been done. Here we show different forms of pseudospin Skyrmion around the Dirac point in momentum space of photonic crystals with the Kagome and honeycomb lattice. The pseudospin Skyrmions in momentum space are directly related to the topologically nontrivial states and manipulated by the inter-cell and intra-cell interactions between cylinders of the photonic crystal. The observation of the pseudospin Skyrmion in momentum space is expected to be valuable for manipulation of the flows and polarizations of light in topologically robust ways.

Systematic study of Mn atoms, artificial dimers, and chains on superconducting Ta(110)

Magnetic adatoms coupled to an $s$-wave superconductor give rise to local bound states, so-called Yu-Shiba-Rusinov states. Focusing on the ultimate goal of tailoring chains of such adatoms into a topologically superconducting phase, we investigate basic building blocks - single Fe and Mn adatoms and Mn dimers on clean superconducting Ta(110) - using scanning tunneling microscopy and spectroscopy. We perform a systematic study of the hybridizations and splittings in dimers, and their dependence on the crystallographic directions and interatomic spacings, in order to identify potentially interesting chain geometries for this novel sample type. Subsequently, we study the spin structure as well as the length dependent Shiba band structure in Mn chains of those geometries using spin-resolved scanning tunneling spectroscopy. All results are compared to the according properties of structurally identical dimers and chains on the previously studied Nb(110), which has almost identical surface structure and electronic properties, but an about three times smaller spin-orbit interaction.

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in non-collinear or non-centrosymmetric spin structures. The rare-earth nickelate NdNiO3 is known to be a non-collinear antiferromagnet in which the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here we find that for low electron doping, the magnetic order on the nickel site is preserved, whereas electronically, a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of rare-earth nickelates and may enable spintronics applications in this family of correlated oxides. Films of the correlated oxide NdNiO3 form a metallic antiferromagnetic phase that can be identified using electrical currents, raising the prospect of applications in spintronics.

Acoustic Black Holes in a Spinning Beam

Abstract Through creating slow waves in structures, Acoustic Black Hole (ABH) shows promise for potential vibration control applications. However, it remains unclear whether such phenomena can still occur in a structure undergoing high-speed spinning, and if so, what is the interplay among various system parameters and what are the underpinning physical mechanisms. To address this issue, this work establishes a semi-analytical model for a spinning ABH beam based on Euler-Bernoulli beam theory under the energy framework. After its validation, the model is used to reveal a few important vibration features pertinent to the spinning ABH beam through examining its dynamics, modal properties and energy flow. It is shown that the spinning-induced centrifugal effects generate hardening effects inside the structure, thus increasing the overall structural stiffness and stretching the wavelength of the modal deformation of flexural waves as compared with its counterpart at still. Meanwhile, energy flow to the ABH portion of the beam is also adversely affected. As a result, the ABH-induced overall damping enhancement effect of the viscoelastic coating, as observed in conventional ABH beam at still, is impaired. Nevertheless, the study confirms that typical ABH features, in terms of wave compression, energy trapping and dissipation, though affected by the spinning effects, are still persistent in a high-speed spinning structure. This paves the way forward for the embodiment of ABH phenomena in the design of high performance rotating mechanical components such as turbine blades.

Evolution of anisotropic magnetic properties through helix-to-fan transition in helical antiferromagnetic EuCo2As2

A helimagnet comprises a noncollinear spin structure formed by competing exchange interactions. Recent advances in antiferromagnet-based functionalities have broadened the scope of target materials to include noncollinear antiferromagnets. However, a microscopic understanding of the magnetic anisotropy associated with the intricate evolution of noncollinear spin states has not yet been accomplished. Here, we have explored the anisotropic magnetic aspects in a layered helimagnet of EuCo2As2 by measuring the magnetic field and angle dependence of the magnetic torque. By adopting an easy-plane anisotropic spin model, we can visualize the detailed spin configurations that evolve in the presence of rotating magnetic fields. This is directly related to the two distinctive magnetic phases characterized by the reversal of the magnetic torque variation across the helix-to-fan transition. Our advanced approach provides an in-depth understanding of the anisotropic properties of noncollinear-type antiferromagnets and a useful guidance for potential applications in spin-processing functionalities.

Topological Hall Effect in Thin Films of an Antiferromagnetic Weyl Semimetal Integrated on Si.

Since the large room-temperature anomalous Hall effect was discovered in noncollinear antiferromagnets, Mn3Sn has received immense research interest as it exhibits abundant exotic physical properties including Weyl points and enormous potential for antiferromagnetic spintronic device applications. In this work, we report the emergence of the topological Hall effect in Mn3Sn films grown on Si that is the workhorse for the modern highly integrated information technology. Importantly, through a series of systematic comparative experiments, the intriguing topological Hall effect phenomenon related to the appearance of the noncoplanar chiral spin structure is found to be induced by the Mn3Sn/SiO2 interface. Furthermore, it was found that the current injection to a Pt/Mn3Sn bilayer Hall bar device can effectively manipulate the chiral spin structure of Mn3Sn, which demonstrates the feasibility of Si-based noncollinear antiferromagnetic spintronics.

Magnetic phases of the frustrated ferromagnetic spin-trimer system Gd3Ru4Al12 with a distorted kagome lattice structure

The magnetization and specific heat measurements have been performed on single-crystalline Gd_3_Ru_4_Al_12_ with a distorted Kagome lattice structure. This spin system is regarded as an antiferromagnetic triangular lattice of XY like Heisenberg model at low temperatures. The magnetic phase diagrams indicate the existence of frustration and Z_2_ degeneracy. The magnetization and specific heat imply the successive phase transitions with partial disorder and a T-shaped spin structure in the ground state.

Final-state interactions and spin structure in E1 breakup of Li11 in halo effective field theory

We calculate the $E1$ breakup of the $2n$ halo nucleus $^{11}\mathrm{Li}$ in halo effective field theory (Halo EFT) at leading order. In Halo EFT, $^{11}\mathrm{Li}$ is treated as a three-body system of a $^{9}\mathrm{Li}$ core and two neutrons. We present a detailed investigation of final-state interactions (FSIs) in the neutron-neutron $(nn)$ and neutron-core $(nc)$ channels. We employ M\o{}ller operators to formulate an expansion scheme that satisfies the non-energy-weighted cluster sum rule and successively includes higher-order terms in the multiple-scattering series for the FSI. Computing the $E1$ strength up to third order in this scheme, we observe apparent convergence and good agreement with experiment. The neutron-neutron FSI is by far the most important contribution and largely determines the maximum value of the $E1$ distribution. However, inclusion of $nc$ FSI does shift the peak position to slightly lower energies. Moreover, we investigate the sensitivity of the $E1$ response to the spin structure of the neutron-$^{9}\mathrm{Li}$ interaction. We contrast results for an interaction that is the same in the spin-1 and spin-2 channels with one that is only operative in the spin-2 channel, and find that good agreement with experimental data is only obtained if the interaction is present in both spin channels. The latter case is shown to be equivalent to a calculation in which the spin of $^{9}\mathrm{Li}$ is neglected.

Canted ferromagnetic order in nonsuperconducting Eu(Fe1−xNix)2As2

The magnetic order in a series of nonsuperconducting $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals is investigated in detail by polarized neutron diffraction. The Fe and Eu magnetic sublattices are found to be almost magnetically decoupled in this system. With the Ni doping, the Eu sublattice shows a dramatic change in its magnetic ground state, from the A-type antiferromagnetic order for $\mathit{x}\ensuremath{\le}0.02$, in which the spins lie in the $\mathit{ab}$ plane, to a pure canted ferromagnetic order for $0.04\ensuremath{\le}\mathit{x}\ensuremath{\le}0.20$, in which the spins are canted with a small angle off the $\mathit{c}$ axis. By comparison with the $\mathit{c}$-axis perfectly aligned ferromagnetic structure widely observed in other superconducting compounds, the lack of superconductivity in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{As}}_{2}$ might be associated with the formation of in-plane ferromagnetism. The doping-induced change of the lattice constants and accordingly the variation of the strength of indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments is speculated to be responsible for the dramatic change of the Eu spin structure. In addition, a spin reorientation of the ${\mathrm{Eu}}^{2+}$ moments in an intermediate temperature range is observed for $\mathit{x}=0.20$, in which some in-plane spin fluctuations persists above the long-range ordering temperature ${T}_{\mathrm{Eu}}$.

Ferrochiral, antiferrochiral, and ferrichiral skyrmion crystals in an itinerant honeycomb magnet

Topological spin textures, such as a skyrmion crystal, are a source of unusual physical phenomena owing to the interplay between magnetism and topology. Since physical phenomena depend on the topological property and the symmetry of underlying spin structures, the search for new topological spin textures and emergent phenomena is one of the challenges in condensed matter physics. In this letter, we theoretically explore new topological spin textures arising from the synergy between spin, charge, and sublattice degrees of freedom in an itinerant magnet. By performing simulated annealing for an effective spin model of the honeycomb Kondo lattice model, we find a plethora of skyrmion crystal instabilities at low temperatures, whose topological spin textures are classified into three types: ferrochiral, antiferrochiral, and ferrichiral skyrmion crystals. We show that the obtained skyrmion crystals are the consequence of the spin-orbit-coupling-free honeycomb structure. Our results reveal the potential for itinerant honeycomb magnets to host a wide variety of SkXs and emergent phenomena.

Comparative studies by X-ray diffraction, Raman, vibrating sample magnetometer and Mössbauer spectroscopy of pure, Sr doped and Sr, Co co-doped BiFeO3 ceramic synthesized via tartaric acid-assisted technique

Pure BiFeO3 (BFO), Sr-doped Bi0.97Sr0.03FeO3 (BSFO) and Sr, Co co-doped Bi0.97Sr0.03Fe0.8Co0.2O3 (BSFCO) samples have been prepared via a tartaric acid-assisted sol-gel technique. Investigation of the structure, lattice dynamics, magnetization and hyperfine interaction have been performed through X–ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy (RS), vibrating sample magnetometer (VSM) and Mössbauer spectroscopy (MS). The TEM images show that the particle sizes <D> of BFO, BSFO and BSFCO are respectively 11.4, 12.1 and 8.8 nm. The XRD analysis reflects a structural phase transition from rhombohedral R3c structure in case of BFO and BSFO samples to two phase coexistence (rhombohedral R3c and trigonal R–3m:R) in case of BSFCO sample. This structural phase transition has been strongly confirmed by the alterations in the vibration modes observed in Raman spectra (expansion, shift, merge, and reduced intensity). Second order vibration modes, corresponding to the two-phonon scattering in the range 650–1000 cm−1, have been recorded for all samples. Investigation of magnetic properties reveals that pure and Sr doped samples possess considerably high magnetic saturations (Ms = 5.61 and 5.79 emu/g respectively). Additionally, BSFCO sample shows enhancement in Ms (Ms rises to 8.26 emu/g). Suppression of spiral spin structure caused by the small nano-size has been assumed. The destruction of the spiral spin has been also suggested through M össbauer studies. The hyperfine parameters reveal that only Fe3+ is observed in two different nonequivalent trigonal distorted octahedral environments. The outcome of this work suggests the potentiality of Sr, Co co-doping to improve the structure and ferromagnetism of BiFeO3 ceramic making it feasible to be employed in enormous applications.

Computational study of switching properties in Mn and transition metal co-doped BFO

The coupling between electric polarization and magnetization in single-phase bismuth ferrite BiFeO3 (BFO) has a potential for low-power devices. To improve the switching ability of bismuth ferrite, the effect of lanthanum (La) at A-site, and manganese (Mn) at B-site as mono- and co-dopants on spin electronic properties, spin polarization, and spin magnetic properties of Bi0.75La 0.25Fe0.75Mn0.25O3 in cubic phase have been explored by using the first principles. An upsurge in the density of states is detected in the inclusion of Mn and La atoms, which modify the band gap in the spin-down state. Enhancement in the magnetic moment is observed on doping of Mn and La atoms in BFO as mono- and co-dopant due to the combined involvement of these impurities to terminate the cycloidal spin structure and activate the hidden spin. The switching properties such as linear Edelstein effect and spin polarization of co-doped BFO material enhanced to 1.612 × 10−9 m/V and 100% respectively whereas the high value of 2.161 × 10−11 (A/m2) spin current density, 3.600 × 10−2 (Sm−1) conductivity, and 9.200 × 10−11 (A/m2) charge current density have been detected in La-doped system.