本文解释了立方螺旋磁体（$M n S i,F e G e$等）中A相的起源。结果表明，它的上界是自旋波不稳定性在$H_\perp>\sqrt{8/3}Δ=H_{A2}$的结果，其中$H_\perp$是垂直于螺旋轴的磁场，$\m k$和$Δ$分别是自旋波带隙.如果考虑到Dzyaloshinskiii-Moriya相互作用在不同自旋之间的作用，最后一个出现是由于自旋波相互作用。在$H_\perp\to H_{A2}$的磁能的1/S系列中的红外发散（IRD）负责较低的A相边界$H_{A1}$。结果表明，在所有的$T<T_C$上都存在A相，但在$T\ll T_C$上A相很窄，无法观察到。而在$T_C$之下的临界区，其宽度强烈增加。初步估计表明与现有的实验数据半定量协议。自旋波带隙的存在是我们考虑的一个关键点。在附录A中，我们以比以前的出版物更透明的形式介绍了它的推导。
英文摘要：
A-phase origin in cubic helimagnets ($M n S i,F e G e$ etc) is explained. It is shown that its upper bound is a result of the spin-wave instability at $H_\perp>\sqrt{8/3}Δ=H_{A2}$ where $H_\perp$ are the magnetic field perpendicular to the helix axis $\m k$ and $Δ$ the spin-wave gap respectively. The last appears due to the spin-wave interaction if one takes into account that the Dzyaloshinskii-Moriya interaction acts between different spins. The infra-red divergences (IRD) in the 1/S series for the magnetic energy at $H_\perp\to H_{A2}$ are responsible for the lower A-phase boundary $H_{A1}$. It is shown that the A-phase exists at all $T<T_C$ but if $T\ll T_C$ it is very narrow and can not be observed. However in the critical region just below $T_C$ its width increasing strongly. Preliminary estimations demonstrate semi-quantitative agreement with the existing experimental data.
The existence of the spin-wave gap is a crucial point of our consideration. In Appendix A we present its derivation in more transparent form than in previous publication.
We report studies of thermal conductivity as functions of magnetic field and temperature in the helimagnetic insulator Cu$_2$OSeO$_3$ that reveal novel features of the spin-phase transitions as probed by magnon heat conduction. The tilted conical spiral and low-temperature skyrmion phases, recently identified in small-angle neutron scattering studies, are clearly identified by sharp signatures in the magnon thermal conductivity. Magnon scattering associated with the presence of domain boundaries in the tilted conical phase and regions of skyrmion and conical-phase coexistence are identified.
The stability of the four known stationary points of the cubic helimagnet energy functional: the ferromagnetic state, the conical helix, the conical helicoid, and the skyrmion lattice, is studied by solving the corresponding spectral problem. The only stable points are the ferromagnetic state at high magnetic field and the conical helix at low field, and there is no metastable state. Thermal fluctuations around the stationary point, included to quadratic order in the saddle point expansion, destabilize the conical helix in a region where the ferromagnetic state is unstable. Thus, a new interme…
We present a detailed study of the ground state phase diagram of the classical frustrated Heisenberg model on the face-centered-cubic lattice. By considering exchange interactions up till third nearest neighbors, we find commensurate, helimagnetic, as well as noncollinear multi-{\bf Q} orders which include noncoplanar and chiral spin structures. We reveal the presence of subextensively degenerate manifolds that appear at triple points and certain phase boundaries in the phase diagram. Within these manifolds, the spin Hamiltonian can be recast as a complete square of spins on finite motifs, per…
The effect of thermal fluctuations on the phase structure of monoaxial helimagnets with external magnetic field parallel to the chiral axis is analyzed by means of a saddle point expansion of the free energy. The phase transition that separates the conical and forced ferromagnetic phases is changed to first order by the thermal fluctuations. In a purely monoaxial system the pitch of the conical state remains independent of temperature and magnetic field, as in mean field theory, even when fluctuations are taken into account. However, in presence of weak Dzyaloshinskii-Moriya interactions in th…
We investigate the spin $S=1/2$ Heisenberg model on the body centered cubic lattice in the presence of ferromagnetic and antiferromagnetic nearest-neighbor $J_{1}$, second-neighbor $J_{2}$, and third-neighbor $J_{3}$ exchange interactions. The classical ground state phase diagram obtained by a Luttinger-Tisza analysis is shown to host six different (noncollinear) helimagnetic orders in addition to ferromagnetic, Néel, stripe and planar antiferromagnetic orders. Employing the pseudofermion functional renormalization group (PFFRG) method for quantum spins ($S=1/2$) we find an extended nonmagneti…
The phase diagram of cubic helimagnets near the critical temperature is obtained from a Landau-Ginzburg model, including fluctuations to gaussian level. The free energy is evaluated via a saddle point expansion around the local minima of the Landau-Ginzburg functional. The local minima are computed by solving the Euler-Lagrange equations with appropriate boundary conditions, preserving manifestly the full nonlinearity that is characteristic of skyrmion states. It is shown that the fluctuations stabilize the skyrmion lattice in a region of the phase diagram close to the critical temperature, wh…
Single helicity of the triple-$q$ triangular skyrmion lattice state in cubic chiral helimagnet EuPtSi
Hiroshima University（广岛大学）；Japan Atomic Energy Agency（日本原子能机构）；Institute of Materials Structure Science, High Energy Accelerator Research Organization（高能加速器研究机构物质结构研究所）；University of the Ryukyus（琉球大学）；RIKEN Center for Emergent Matter Science（理化学研究所 新兴物质科学中心）
Spin waves and the origin of commensurate magnetism in Ba2CoGe2O7
本文用中子衍射和非弹性散射研究了正方晶格反铁磁体Ba_2CoGe_2O_7。这种材料是同构的著名的Dzyaloshinskiii-Moriya helimagnet Ba 2CuGe 2 O 7，但在低温下表现出相称的长程尼尔秩序。自旋波色散关系的测量揭示了强的面内各向异性，这是抑制helimagnetism的可能原因。
The phase diagram of the monoaxial chiral helimagnet as a function of temperature (T ) and magnetic field with components perpendicular (H x ) and parallel (H z ) to the chiral axis is theoretically studied via the variational mean field approach in the continuum limit. A phase transition surface in the three dimensional thermodynamic space separates a chiral spatially modulated phase from a homogeneous forced ferromagnetic phase. The phase boundary is divided into three parts: two surfaces of second order transitions of instability and nucleation type, in De Gennes terminology, are separated …
We discuss theoretically frustrated Heisenberg spiral magnets in magnetic field $\bf H$. We demonstrate that small anisotropic spin interactions (single-ion biaxial anisotropy or dipolar forces) select the plane in which spins rotate (spiral plane) and can lead to the spiral plane flop upon in-plane field increasing. Expressions for the critical fields $H_{flop}$ are derived. It is shown that measuring of $H_{flop}$ is an efficient and simple method of quantifying the anisotropy in the system (as the measurement of spin-flop fields in collinear magnets with axial anisotropy). Corresponding rec…
Comparative study of helimagnets MnSi and Cu2OSeO3 at high pressures
本文用交流量热法研究了高压下螺旋磁体Cu_2OSeO_3和MnSi的热容。尽管它们的磁矩性质不同，但Cu2OSeO3和MnSi在相变时的磁性和热力学性质的行为表现出惊人的相似性。如果作为温度的函数进行分析，则两种物质（峰和肩）在相变时的热容的两个特征在高压下也以类似的方式表现。这可能意味着像MnSi这样的弱巡回磁体的典型纵向自旋波动对相变的贡献很小。热容量曲线的肩部随着温度的降低而收缩，这表明它们来自经典涨落。在MnSi的情况下，尖锐的峰和肩部的热容同时消失，可能意味着存在一个三临界点，并确认在MnSi以及在Cu 2 OSe O 3的一级相变的波动性质。
Magnetic skyrmions are nano-sized topological spin textures stabilized by a delicate balance of magnetic energy terms. The chemical substitution of the underlying crystal structure of skyrmion-hosting materials offers a route to manipulate these energy contributions, but also introduces additional effects such as disorder and pinning. While the effects of doping and disorder have been well studied in B20 metallic materials such as Fe$_{1-x}$Co$_x$Si and Mn$_{1-x}$Fe$_x$Si, the consequences of chemical substitution in the magnetoelectric insulator Cu$_2$OSeO$_3$ have not been fully explored. In…
We study magnetoelastic resonance phenomena in a mono-axial chiral helimagnet belonging to hexagonal crystal class. By computing the spectrum of coupled elastic wave and spin wave, it is demonstrated how hybridization occurs depending on their chirality. Specific features of the magnetoelastic resonance are discussed for the conical phase and the soliton lattice phase stabilized in the mono-axial chiral helimagnet. The former phase exhibits appreciable non-reciprocity of the spectrum, the latter is characterized by a multi-resonance behavior. We propose that the non-reciprocal spin wave around…
Successive phase transitions in a rare-earth monoaxial chiral magnet DyNi$_3$Ga$_{9}$ have been investigated by resonant x-ray diffraction. Magnetic dipole and electric quadrupole degrees of freedom arising from the large angular moment of $J=15/2$, in combination with the symmetric and antisymmetric exchange interactions and the crystal field anisotropy, give rise to competing ordered phases. We show that the antiferromagnetically coupled Dy moments in the $ab$-plane form an incommensurate helimagnetic order with $q\sim(0, 0, 0.43)$ just below $T_{\text{N}}=10$ K, which further exhibits succe…
SANS and magnetometry study of the magnetic phase diagram of the B20 helimagnet FeRhSi
Saint Petersburg State University（圣彼得堡国立大学）；Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences（俄罗斯科学院韦列夏金高压物理研究所）；Mozhaisky Military Space Academy（莫扎伊斯基军事航天学院）
Magnetoelastic mapping of the high-field phase diagram in the topological cubic helimagnet SrFeO$_{3}$
University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所物质结构科学研究中心）；Hiroshima University（广岛大学）；High Energy Accelerator Research Organization（高能加速器研究机构）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）
CrAs is a well-known helimagnet with the double-helix structure originating from the competition between the Dzyaloshinskii-Moriya interaction (DMI) and antiferromagnetic exchange interaction $J$. By resonant soft X-ray scattering (RSXS), we observe the magnetic peak (0~0~$q_m$) that emerges at the helical transition with $T_S$ $\approx$ 267.5 K. Intriguingly, the helimagnetic domains significantly shrink on cooling below $\sim$255 K, opposite to the conventional thermal effect. The weakening of DMI on cooling is found to play a critical role here. It causes the helical wave vector to vary, or…
Effect of uniaxial stress on helimagnetic phases in the square-lattice itinerant magnet EuAl$_{4}$
Institute for Solid State Physics, University of Tokyo（东京大学固体物理研究所）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；Department of Physics, Tokyo Metropolitan University（东京都立大学物理学系）；Institute of Materials Structure Science, High Energy Accelerator Research Organization（高能加速器研究机构材料结构科学研究所）；Department of Advanced Materials Science, The University of Tokyo（东京大学先进材料科学系）；Research Center for Advanced Science and Technology, University of Tokyo（东京大学先进科学技术研究中心）；Department of Applied Physics, The University of Tokyo（东京大学应用物理学系）
