Spiral spin structures and skyrmions in multiferroics

2019 ◽  
Vol 5 (1) ◽  
Author(s):  
Takashi Kurumaji
Keyword(s):  
Generalized Inverse ◽  
Material Design ◽  
Theoretical Models ◽  
Frustrated Magnets ◽  
Type Ii ◽  
Spin Structures ◽  
Microscopic Models ◽  
Basic Ideas ◽  
Magnetic Skyrmions ◽  
Spiral Spin

Abstract In this article, we focus on (1) type-II multiferroics driven by spiral spin orderings and (2) magnetoelectric couplings in multiferroic skyrmion-hosting materials. We present both phenomenological understanding and microscopic mechanisms for spiral spin state, which is one of the essential starting points for type-II multiferroics and magnetic skyrmions. Two distinct mechanisms of spiral spin states (frustration and Dzyaloshinskii–Moriya [DM] interaction) are discussed in the context of the lattice symmetry. We also discuss the spin-induced ferroelectricity on the basis of the symmetry and microscopic atomic configurations. We compare two well-known microscopic models: the generalized inverse DM mechanism and the metal-ligand d-p hybridization mechanism. As a test for these models, we summarize the multiferroic properties of a family of triangular-lattice antiferromagnets. We also give a brief review of the magnetic skyrmions. Three types of known skyrmion-hosting materials with multiferroicity are discussed from the view point of crystal structure, magnetism, and origins of the magnetoelectric couplings. For exploration of new skyrmion-hosting materials, we also discuss the theoretical models for stabilizing skyrmions by magnetic frustration in centrosymmetric system. Several basic ideas for material design are given, which are successfully demonstrated by the recent experimental evidences for the skyrmion formation in centrosymmetric frustrated magnets.

scholarly journals Temperature dependence of hardness prediction for high-temperature structural ceramics and their composites

2021 ◽  
Vol 10 (1) ◽  
pp. 586-595
Author(s):  
Ruzhuan Wang ◽  
Dingyu Li ◽  
Weiguo Li
Keyword(s):  
High Temperature ◽  
Temperature Dependence ◽  
Material Design ◽  
Theoretical Models ◽  
Ceramic Composites ◽  
Basic Material ◽  
Experimental Measurements ◽  
Control Mechanisms ◽  
Oxide Ceramics ◽  
Structural Ceramics

Abstract Hardness is one of the important mechanical properties of high-temperature structural ceramics and their composites. In spite of the extensive use of the materials in high-temperature applications, there are few theoretical models for analyzing their temperature-dependent hardness. To fill this gap in the available literature, this work is focused on developing novel theoretical models for the temperature dependence of the hardness of the ceramics and their composites. The proposed model is just expressed in terms of some basic material parameters including Young’s modulus, melting points, and critical damage size corresponding to plastic deformation, which has no fitting parameters, thereby being simple for materials scientists and engineers to use in the material design. The model predictions for the temperature dependence of hardness of some oxide ceramics, non-oxide ceramics, ceramic–ceramic composites, diamond–ceramic composites, and ceramic-based cermet are presented, and excellent agreements with the experimental measurements are shown. Compared with the experimental measurements, the developed model can effectively save the cost when applied in the material design, which could be used to predict at any targeted temperature. Furthermore, the models could be used to determine the underlying control mechanisms of the temperature dependence of the hardness of the materials.

scholarly journals Giant Dzyaloshinskii-Moriya interaction in Rashba superlattices

2020 ◽  
Author(s):  
Woo-Seung Ham ◽  
Abdul-Muizz Pradipto ◽  
Kay Yakushiji ◽  
Kwangsu Kim ◽  
Sonny Rhim ◽  
...  
Keyword(s):  
Heavy Metal ◽  
Material Design ◽  
Orbit Coupling ◽  
Inversion Symmetry ◽  
Spin Orbit Coupling ◽  
Spin Orbit ◽  
Band Formation ◽  
Stacking Order ◽  
Magnetic Skyrmions ◽  
Moriya Interaction

Abstract Dzyaloshinskii-Moriya interaction (DMI) is considered as one of the most important energy for specific chiral texture such as magnetic skyrmions. The key of generating DMI is absence of structural inversion symmetry and exchange energy with spin-orbit coupling. Therefore, a vast majority of researches about DMI is mainly limited to heavy metal/ferromagnet bilayer systems, only focusing on their interfaces. Here, we report that asymmetric band formation in an artificial superlattice arises from inversion symmetry breaking in stacking order of atomic layers, resulting in bulk DMI. Such bulk DMI is more than 300% larger than simple sum of interfacial contribution. Moreover, the asymmetric band is largely affected by strong spin-orbit coupling, showing crucial role of a heavy metal even in the non-interfacial origin of DMI. Such Rashba superlattices can be a new class of material design for spintronics applications.

scholarly journals The low-luminosity Type II SN 2016aqf: a well-monitored spectral evolution of the Ni/Fe abundance ratio

2020 ◽  
Vol 497 (1) ◽  
pp. 361-377
Author(s):  
Tomás E Müller-Bravo ◽  
Claudia P Gutiérrez ◽  
Mark Sullivan ◽  
Anders Jerkstrand ◽  
Joseph P Anderson ◽  
...  
Keyword(s):  
Spectral Synthesis ◽  
Theoretical Models ◽  
Abundance Ratio ◽  
Late Time ◽  
Type Ii ◽  
Spectral Evolution ◽  
Spectral Parameters ◽  
Bolometric Luminosity ◽  
V Band ◽  
Explosion Mechanism

ABSTRACT Low-luminosity Type II supernovae (LL SNe II) make up the low explosion energy end of core-collapse SNe, but their study and physical understanding remain limited. We present SN 2016aqf, an LL SN II with extensive spectral and photometric coverage. We measure a V-band peak magnitude of −14.58 mag, a plateau duration of ∼100 d, and an inferred 56Ni mass of 0.008 ± 0.002 M⊙. The peak bolometric luminosity, Lbol ≈ 1041.4 erg s−1, and its spectral evolution are typical of other SNe in the class. Using our late-time spectra, we measure the [O i] λλ6300, 6364 lines, which we compare against SN II spectral synthesis models to constrain the progenitor zero-age main-sequence mass. We find this to be 12 ± 3 M⊙. Our extensive late-time spectral coverage of the [Fe ii] λ7155 and [Ni ii] λ7378 lines permits a measurement of the Ni/Fe abundance ratio, a parameter sensitive to the inner progenitor structure and explosion mechanism dynamics. We measure a constant abundance ratio evolution of $0.081^{+0.009}_{-0.010}$ and argue that the best epochs to measure the ratio are at ∼200–300 d after explosion. We place this measurement in the context of a large sample of SNe II and compare against various physical, light-curve, and spectral parameters, in search of trends that might allow indirect ways of constraining this ratio. We do not find correlations predicted by theoretical models; however, this may be the result of the exact choice of parameters and explosion mechanism in the models, the simplicity of them, and/or primordial contamination in the measured abundance ratio.

scholarly journals Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Danila Amoroso ◽  
Paolo Barone ◽  
Silvia Picozzi
Keyword(s):  
Data Storage ◽  
First Principles ◽  
Short Range ◽  
Magnetic Frustration ◽  
Two Dimensional ◽  
Spin Structures ◽  
Long Range Interactions ◽  
Magnetic Skyrmions ◽  
Chiral Interaction ◽  
Inherent Stability

AbstractTopological spin structures, such as magnetic skyrmions, hold great promises for data storage applications, thanks to their inherent stability. In most cases, skyrmions are stabilized by magnetic fields in non-centrosymmetric systems displaying the chiral Dzyaloshinskii-Moriya exchange interaction, while spontaneous skyrmion lattices have been reported in centrosymmetric itinerant magnets with long-range interactions. Here, a spontaneous anti-biskyrmion lattice with unique topology and chirality is predicted in the monolayer of a semiconducting and centrosymmetric metal halide, NiI2. Our first-principles and Monte Carlo simulations reveal that the anisotropies of the short-range symmetric exchange, when combined with magnetic frustration, can lead to an emergent chiral interaction that is responsible for the predicted topological spin structures. The proposed mechanism finds a prototypical manifestation in two-dimensional magnets, thus broadening the class of materials that can host spontaneous skyrmionic states.

DESCRIPTION OF HEAVY-NUCLEI MASSES BY MACROSCOPIC–MICROSCOPIC MODELS

2012 ◽  
Vol 21 (05) ◽  
pp. 1250038 ◽  
Author(s):  
YU. A. LITVINOV ◽  
A. SOBICZEWSKI ◽  
A. PARKHOMENKO ◽  
E. A. CHEREPANOV
Keyword(s):  
Empirical Model ◽  
Predictive Power ◽  
Theoretical Models ◽  
Neutron Number ◽  
Heavy Nuclei ◽  
Local Type ◽  
Microscopic Models ◽  
The Masses ◽  
Semi Empirical

The quality of the description of masses of heavy nuclei by theoretical models is analyzed. Five of widely used macroscopic–microscopic models and one semi-empirical model are considered. All measured masses of nuclei with proton number Z≥82 and neutron number N≥126 are used in the analysis. Besides the accuracy of the description of the masses, the predictive power of the models in this description is also examined. It is obtained that the best accuracy is reached for the semi-empirical model (which is of a local type, specially adjusted to heavy nuclei). This model does not show, however, the best predictive power.

Spectral analysis of body waves from the earthquake of February 18, 1956

1964 ◽  
Vol 54 (6A) ◽  
pp. 2017-2035 ◽  
Author(s):  
Tomowo Hirasawa ◽  
William Stauder
Keyword(s):  
Amplitude Ratio ◽  
Source Region ◽  
Focal Depth ◽  
Mean Amplitude ◽  
Theoretical Models ◽  
Body Waves ◽  
P Wave ◽  
Type I ◽  
Type Ii ◽  
S Wave

abstract The earthquake which occurred south of Honshu, Japan, on February 18, 1956 is studied by means of Fourier analysis. The focal depth of the shock is about 450 km and the magnitude is 714 to 712. Three theoretical models of the source mechanism, that is, Type Ia, Type Ib, and Type II, are examined by the observed amplitude spectra of S and ScS waves. It is found that the observed amplitude ratios of the Fourier components between two horizontal components of the S wave and of the ScS wave, respectively, agree well with the theoretical ratios for a Type II source. Under the assumption that spectral structures should be the same at all observing points, the scattering from the mean amplitude is calculated. The result shows that the Type II model is preferable to either of the Type I models. Assuming Honda's volume model, whose radiation pattern corresponds to that of a Type II point source, the radius of the source region is estimated by making use of the amplitude ratio of the Fourier component of the S wave to that of the P wave. The radius of the source is found to be 11 km ± 2 km.

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