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横田 光史
Journal of the Physical Society of Japan, 91(6), p.064003_1 - 064003_7, 2022/06
被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)Heisenberg antiferromagnet on a face-centered cubic (fcc) lattice with a nearest-neighbor exchange interaction is known to order antiferromagnetically with type-I spin structure. On the other hand, Heisenberg magnet on a fcc lattice with only magnetic dipole-dipole interactions orders ferromagnetically. Both the antiferromagnetic nearest-neighbor exchange interaction and the dipole-dipole interactions introduce frustration into the Heisenberg magnets but differently. Here the Heisenberg fcc antiferromagnets, in which both interactions are included are investigated by numerically solving the Landau-Lifshitz (LL) equation. Various ordered states which include multi-K structures are obtained depending on the relative strength of the two kinds of interactions.
横田 光史
Journal of the Physical Society of Japan, 90(10), p.104701_1 - 104701_5, 2021/10
被引用回数：2 パーセンタイル：30.35(Physics, Multidisciplinary)Skyrmion strings in a layered Heisenberg model with dipole-dipole interactions are investigated by solving the Landau-Lifshitz equation numerically. Twisted skyrmion strings appear in some cases, for which the helicity of the skyrmion for the top surface is opposite to that for the bottom surface. The skyrmion near the middle layer is a N
el skyrmion with inward spin directions for the twisted skyrmion strings.
横田 光史
Physical Review E, 103(1), p.012128_1 - 012128_6, 2021/01
被引用回数：1 パーセンタイル：17.64(Physics, Fluids & Plasmas)Ground state phase diagram for a three-layered Heisenberg ferromagnet with the magnetic anisotropy and the dipole-dipole interactions are inferred from the linear analysis of the fastest growing mode for the time-dependent Ginzburg-Landau equation. It is also investigated numerically by solving the Landau-Lifshitz equation for the model. The phase diagrams obtained from the two methods resemble each other. There are in-plane ferromagnetic state and states with out-of-plane stripe orders. Although the phase diagram is similar to that for the corresponding two-dimensional model qualitatively, the region of the in-plane ferromagnetic state shrinks considerably compared to that for the two-dimensional model.
横田 光史
Journal of the Physical Society of Japan, 89(1), p.014706_1 - 014706_6, 2020/01
被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)Magnetic skyrmions are known to exist in ferromagnetic films with dipole-dipole interactions. A two-dimensional Heisenberg ferromagnet with magnetic anisotropy and dipole-dipole interactions is investigated numerically. There exist various topological spin textures. Antiskyrmions coalesce to form spin textures with 
and 
, where 
is the topological number. Skyrmion molecules and antiskyrmion molecules as well as donut-shaped spin textures with 
appear. The appearance of these (meta)stable topological spin textures depends on the values of temperature and (or) external magnetic field.
横田 光史
Journal of the Physical Society of Japan, 88(8), p.084702_1 - 084702_6, 2019/08
被引用回数：4 パーセンタイル：36.66(Physics, Multidisciplinary)Landau-Lifshitz equation for a 2-dimensional anisotropic Heisenberg model with dipole-dipole interactions is solved numerically to investigate magnetic bubbles. Type I magnetic bubbles (magnetic skyrmions) with both left-handed and right-handed rotations appear. There is a minimal anisotropy necessary for the existence of magnetic bubbles. Magnetic skyrmions appear only in a limited region of the parameter space. The region is approximately the same as the one where mixed states with both in-plane ferromagnetic order and out-of- plane stripe pattern coexist at zero magnetic field. The size and the domain wall width of the magnetic skyrmions decrease as the anisotropy increases. The size depends also on the strength of the magnetic field. It decreases as the magnetic field increases. There may be a critical magnetic field above which the magnetic skyrmions do not exist and the size of the magnetic skyrmions is minimum at the critical field. The domain wall width of the magnetic skyrmions depends a little on the magnetic field.
横田 光史
Journal of the Physical Society of Japan, 87(9), p.094701_1 - 094701_5, 2018/09
被引用回数：5 パーセンタイル：41.65(Physics, Multidisciplinary)In the two-dimensional Heisenberg model with dipole-dipole interactions and uniaxial anisotropy, the ordered state is known to be an in-plane ferromagnetic state or a state with an out-of-plane stripe domain pattern. The ground-state phase diagram is approximately obtained from the fastest growing mode for the time-dependent Ginzburg-Landau equation in a continuous space approximation. A more detailed description of the ordered states is obtained by solving the Landau-Lifshitz equation numerically in the continuous space approximation. Mixed states with both in-plane ferromagnetic order and an out-of-plane stripe pattern appear between the region of the in-plane ferromagnetic state and the region of the out-of-plane stripe states in the phase diagram.
横田 光史
Physica A, 490, p.458 - 464, 2018/01
被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)Ising antiferromagnet on tetrahedron cactus lattices with randomness in the exchange interactions is studied. Instability line of the paramagnetic state, beyond which glassy or antiferromagnetic state is (meta)stable is obtained. The model is investigated by the replica method. Instability toward antiferromagnetic state does not occur for 
where 
is the number of corner sharing tetrahedra for the cactus lattices. Instability toward glassy state occurs at as weak randomness as 
, and 
for 
, and 
, respectively, where 
and 
are the mean and variance of the Gaussian random exchange interaction, respectively.
横田 光史
Journal of Magnetism and Magnetic Materials, 432, p.532 - 538, 2017/06
被引用回数：1 パーセンタイル：5.94(Materials Science, Multidisciplinary)Surface magnetic domain structure can be different from internal one. Both of them are influenced by structural randomness. Three-dimensional numerical simulations are performed using a model with random exchange interactions and dipolar interactions on a lattice with the periodic boundary condition for the horizontal directions to simulate infinite system in the directions and with the free boundary condition for vertical direction (z) to represent the existence of surface. The lattice sizes are 
for 
and 
for 
. To simulate highly anisotropic materials, time dependent Ginzburg-Landau equation using a model with Ising symmetry is solved numerically. Dependence of magnetic domain patterns on thickness of the system and the degree of disorder is investigated for systems with small thickness. Magnetic structure factor for the surface layer is compared with that for the internal layer. The inverse of the wave number that gives the maximum value of the magnetic structure factor depends on the thickness of the system 
as 
approximately. The distribution of the local magnetization depends on the disorder rather differently for different thicknesses of the system.
横田 光史
Physica A, 447, p.270 - 275, 2016/00
被引用回数：2 パーセンタイル：20.23(Physics, Multidisciplinary)Phase diagrams for a random Ising antiferromagnet on Bethe-like lattices with triangular loops are obtained. Triangular loops cause strong geometrical frustration for the Ising antiferromagnet. Spin glass states appear by introducing randomness in the interaction between Ising spins. The random Ising antiferromagnet is studied by the replica method using global order parameter. The phase diagrams are compared with those for the corresponding random Ising ferromagnet to see the effects of the geometrical frustration. Antiferromagnetic phase does not appear for where is the number of the corner sharing triangles on the Bethe-like lattices. In these cases, spin glass phase appears with a reentrant behavior. Spin glass phase in the random antiferromagnet appears for much weaker randomness than that in the corresponding random ferromagnet.
横田 光史
Journal of Magnetism and Magnetic Materials, 385, p.119 - 123, 2015/07
被引用回数：1 パーセンタイル：4.82(Materials Science, Multidisciplinary)Magnetic domain pattern formation is known to be influenced by structural disorder. We perform numerical simulations using a model of two-dimensional magnetic domain pattern formation focusing on effects of exchange randomness. The model is a scalar model with exchange and dipolar interactions, which includes disorder in the exchange interactions. Morphological change of the patterns with the increase in the disorder is observed. The distribution of the local magnetization becomes broader with the increase of the disorder. For a definite realization of the randomness, many patterns can appear.
横田 光史
Physical Review E, 89(1), p.012128_1 - 012128_5, 2014/01
被引用回数：10 パーセンタイル：55.34(Physics, Fluids & Plasmas)Randomness and frustration are important for the spin glass transitions. Ising antiferromagnet on a stacked triangular lattice is a highly frustrated system. Phase diagram is obtained within a cluster approximation for the system with randomness in the interaction between Ising spins. To see the frustration effects, the phase diagram of the antiferromagnet is compared with the corresponding one of the ferromagnet. Spin glass phase of the antiferromagnet appears much more easily by the randomness than that of the ferromagnet.
横田 光史
Physics Letters A, 377(3-4), p.139 - 144, 2013/01
被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)Networklike structures are observed in various materials and nature. The networklike structures in viscoelastic phase separation of polymer mixtures have been explained by a model with a couple of hydrodynamic equations. On the other hand, the large-scale structure of the universe has been explained by a multi-dimensional extension of the Burgers equation. The networklike structures of the two very different systems have some morphological similarities. To see origins of the similarities and differences, the Burgers equation is derived from the model of the polymer mixtures using several approximations. The meaning of the approximations is discussed in connection with the morphology.
横田 光史
Journal of the Physical Society of Japan, 80(1), p.014708_1 - 014708_6, 2011/01
被引用回数：0 パーセンタイル：0(Physics, Multidisciplinary)局所状態が2種類あるポッツ模型を導入し、レプリカ法を用いた平均場理論で調べた。この模型には長距離の強磁性秩序を妨げるようなフラストレーションの効果が含まれている。2種類の混合グラス相間の転移線が現れることを示した。2種類の混合グラス相の出現は2種類の局所状態の存在と関係している。
横田 光史
Progress of Theoretical Physics, 121(3), p.603 - 625, 2009/03
被引用回数：3 パーセンタイル：26.3(Physics, Multidisciplinary)スピングラス転移温度近傍でのレプリカ対称性の破れに焦点を当てて、カクタス格子上のイジングスピングラスを調べた。小さなループがもたらすフラストレーションの効果を見るために、スピングラス秩序関数と小さな磁場中のAT線を求めた。現実の格子上における短距離相互作用モデルの近似として、それらの結果を小さなループのないベーテ格子上のモデルにおける結果と比較した。三角,四面体,正方形カクタス格子上のモデルを取り扱った。スピングラス秩序関数の傾きに関しては、対応するベーテ格子上のモデルと比べて、カクタス格子上のモデルにおける値が小さくなるという結論を得た。また、磁場中のレプリカ対称性の破れる領域の広さに関しては、三角及び四面体カクタスでループ数が最小の場合を除いて、対応するベーテ格子上のモデルに比べて、カクタス格子上のモデルでは広くなるという結論を得た。これらの結論を得るために、レプリカ法でスピングラス秩序関数間に成り立つ方程式を用いた。この方程式とレプリカ法を用いないで導かれる方程式との関係を明らかにした。
横田 光史
Physica A, 387(14), p.3495 - 3502, 2008/06
被引用回数：17 パーセンタイル：65.76(Physics, Multidisciplinary)小さなループを持つ場合と持たない場合のベーテ的格子上におけるイジングスピングラス模型において、スピングラス秩序に対する方程式を調べた。それぞれの格子について、レプリカ法を用いた場合と用いない場合に方程式を求めた。レプリカ対称近似では、両方の方法で求めた方程式は一致する。小さなループとレプリカ対称性の破れの影響をみるために、転移点近くの温度において、ある種のスピングラス秩序変数を格子点における接続数の関数として調べた。レプリカ対称性の破れは、小さなループが存在することによって大きくなることが示された。
横田 光史
Physica A, 379(2), p.534 - 544, 2007/06
被引用回数：23 パーセンタイル：71.06(Physics, Multidisciplinary)磁場中でのベーテ的格子(カクタス格子)上のイジングスピングラス模型をレプリカ法で調べた。スピングラス転移点近傍でのParisi秩序関数と磁場中でのレプリカ対称性の破れを示すAT線を求めた。ループの影響を見るために、結果をループのないベーテ格子の結果と比べた。どちらの格子に対するParisi秩序関数も平均場理論であるSK模型の結果に比べて、傾きが小さくなることがわかった。一方、AT線の位置はカクタス格子の場合とベーテ格子の場合では、それぞれSK模型におけるAT線の位置よりも上と下になることがわかった。
横田 光史
Journal of Magnetism and Magnetic Materials, 310(2, Part2), p.e518 - e519, 2007/03
イジングスピングラスをスピングラス転移温度近傍で調べた。さまざまなクラスター近似を用いて、秩序関数であるパリシ関数と、磁場中でのレプリカ対称性の破れを表すAT線を求めた。バビエラらによるクラスター近似,ベーテ近似、及びカクタス近似を用いて、レプリカ対称性に対する揺らぎの影響を調べた。揺らぎは、秩序関数の傾きを小さくして、レプリカ対称性の破れを大きくすることを示した。クラスター近似で求めたAT線の位置は、平均場近似から変化するが、その変化の仕方は、クラスターにループが含まれるかどうかに依存していることを示した。
横田 光史
Physics Letters A, 356(6), p.439 - 445, 2006/08
被引用回数：6 パーセンタイル：41.83(Physics, Multidisciplinary)磁場中のイジングスピングラス模型をベーテ近似及びそれに関係するクラスター近似で調べた。レプリカ法を用いて、スピングラス転移温度近傍でのスピングラス秩序関数とAT線を求めた。ベーテ近似での結果は、平均場理論であるSK模型での結果から大きくずれることを示した。
横田 光史
Physica A, 363(2), p.161 - 170, 2006/03
被引用回数：6 パーセンタイル：41.83(Physics, Multidisciplinary)スピングラス転移温度近傍におけるイジングスピングラス模型をクラスター近似を用いて調べた。スピングラス秩序パラメーター関数及び磁場中においてレプリカ対称性が破れるAT線を求めた。
横田 光史
Physics Letters A, 344(2-4), p.211 - 219, 2005/09
被引用回数：0 パーセンタイル：0.01(Physics, Multidisciplinary)外場中のポッツグラス模型に対する平均場理論において、外場方向の秩序変数に対するレプリカ対称解の安定性を調べた。ポッツ成分の数が4以上のときに、外場と垂直方向のグラス秩序が形成されるよりも高い温度でレプリカ対称性の破れが起こっていることを示した。レプリカ対称性の破れ方も求めた。
