Antiferromagnetic Barkhausen noise induced by weak random-field disorder
Jožef Stefan Institute（约热·斯特凡研究所）；Complexity Science Hub（复杂科学中心）；Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia（塞尔维亚贝尔格莱德普雷格雷维察街118号物理研究所）
This study numerically investigates magnetisation reversal processes driven by an external magnetic field in three-dimensional antiferromagnetic spin models with weak random field disorder. Considering an extremely weak disorder and low temperature, we observe a step-wise hysteresis loop and the appearance of short magnetisation bursts of a characteristic triangular shape; the number of bursts increases with disorder, indicative of Barkhausen-type noise. These phenomena are attributed to the simultaneous reversal at a given external field of segments composed of spins with identical neighbourhoods. A local random field orients one or more spin neighbours, resulting in small, ferromagnetic-like clusters distributed throughout the system. As disorder increases, these clusters may merge to form a labyrinthine structure within the antiferromagnetic background, facilitating brief avalanche propagation. The results demonstrate that, compared with familiar random-field ferromagnets, the observed antiferromagnetic Barkhausen noise and the related avalanche sequence have a profoundly different structure, organised into peaks associated with the transition between magnetisation plateaus. They exhibit prominent cyclical trends and disorder-dependent multifractal fluctuations, with the singularity spectrum quantifying the degree of disorder. The activity avalanches exhibit scale invariance resembling that recently found in experiments with disordered ferr\textit{i}magnets and martensites, as well as in quantum Barkhausen noise, which are associated with active geometric regions rather than individual-spin dynamics. The observed scaling behaviour is interpreted in terms of self-organised critical dynamics.
Barkhausen noise in the Random Field Ising Magnet Nd$_2$Fe$_{14}$B
在烧结针排列和磁场横向于其易磁化轴的情况下，稀土铁磁体Nd $_2$ Fe $_{14}$ B成为随机场伊辛模型的室温实现。横向场以连续的方式调谐磁畴的钉扎电势。我们使用巴克豪森噪声技术研究了液氦和室温之间的磁畴反转和雪崩动力学。雪崩的大小和能量分布遵循幂律的行为与切断依赖于拨在由横向场的钉扎强度，与理论预测巴克豪森雪崩在无序材料。标度分析揭示了两个制度的行为：一个在低温和高横向场，其中的动态是由随机性，和第二个在高温和低横向场的热波动占主导地位的动态。
Noise in disordered systems: The power spectrum and dynamic exponents in avalanche models
长期以来，人们已经知道巴克豪森噪声的功率谱在高频处具有幂律衰减。到目前为止，对这种衰变的理论预测是不正确的，或者只适用于一小部分模型。在本文中，我们描述了一个仔细的推导雪崩模型的功率谱指数，特别是在零温度随机场伊辛模型的变化。我们发现，天真的指数，（3-tau）/（σ nu z），这已经在其他几篇论文中推导出来，一般是不正确的小τ，当大雪崩是常见的。(tau是描述雪崩大小分布的指数，（σ nu z）是描述雪崩大小和雪崩持续时间之间关系的指数。我们发现，对于一个大类的雪崩模型，包括巴克豪森噪声的几个模型，正确的指数τ <2是1/（sigma nu z）。我们明确地推导出平均场指数2。在这个过程中，我们计算的平均雪崩形状的雪崩的固定持续时间和缩放形式的一些物理性质。
Strong Disorder Renormalization Group Method for Bond Disordered Antiferromagnetic Quantum Spin Chains with Long Range Interactions: Ground State Properties
Strong Disorder Renormalization Group Method for Bond Disordered Antiferromagnetic Quantum Spin Chains with Long Range Interactions: Excited States and Finite Temperature Properties
Max Planck Institute for the Physics of Complex Systems（马克斯·普朗克复杂系统物理研究所）；Constructor University（Constructor大学）
Thermodynamic Spin Glass Phase Induced by Weak Random Exchange Disorder in a Classical Spin Liquid: the Case of the Pyrochlore Heisenberg Antiferromagnet
在烧绿石磁体Y2 Mo 2 O 7中观察到的玻璃态行为是一类独特的自旋玻璃系统，其中磁性Mo^{4+}离子主要通过各向同性最近邻反铁磁交换相互作用，可能还有额外的弱无序。描述这种自旋玻璃行为的一个模型是海森堡模型，它是在具有随机但严格反铁磁交换无序的烧绿石晶格上提出的。在本文中，我们提供了令人信服的数值证据，从广泛的蒙特卡罗模拟表明，该模型表现出有限的温度自旋玻璃转变，因此是一个实现的自旋玻璃诱导的随机弱无序自旋液体。结果表明，Y_2Mo_2O_7的自旋玻璃态是由有效强无序驱动的。
Adaptive Density-Matrix Renormalization-Group study of the disordered antiferromagnetic spin-1/2 Heisenberg chain
利用最近提出的自适应密度矩阵重整化群方法，我们研究了具有随机耦合常数的自旋- $1/2$反铁磁海森堡链的多种自旋-自旋关联，即体关联和端-端关联的平均值、体关联的典型值以及体关联的分布.我们的结果与强无序重整化群方法的预言惊人地一致。我们在Shu等人最近报道的整体平均相关性[Phys. Rev. B 94，174442（2016）]和端到端平均相关性中均未发现对数校正的任何暗示。我们报告计算的对数校正的存在性的端到端的相关性的清洁链。最后，我们已经确定，当适当地重新调整相关的李雅普诺夫指数，是一个狭窄的和普遍的（无序无关）的概率函数的散装相关性的分布。
