In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped Cr$_2$B, which, starting from the paramagnetic parent, orders ferromagnetically for Fe-doping concentrations $x$ larger than $x_{\rm c}=2.5$\%. In parent Cr$_2$B, $^{11}$B nuclear magnetic resonance data reveal the presence of both ferromagnetic and antiferromagnetic fluctuations. The latter are suppressed with Fe-doping, before the ferromagnetic ones finally prevail for $x>x_{\rm c}$. Indications for non-Fermi liquid behavior, usually associated with the proximity of a quantum critical point, were found for all samples, including undoped Cr$_2$B. The sharpness of the ferromagnetic-like transition changes on moving away from $x_{\rm c}$, indicating significant changes in the nature of the magnetic transitions in the vicinity of the quantum critical point. Our data provide constraints for understanding quantum phase transitions in itinerant ferromagnets in the limit of weak quenched disorder.
Path dependent resistivity study across field induced paramagnetic to ferromagnetic transition in Se doped CoS$_2$
对CoS $_{1.76}$ Se $_{0.24}$的热磁不可逆性进行了系统的研究。研究表明，在不同的磁场中冷却可以调节低温下的电阻率，并且顺磁（PM）到铁磁（FM）转变所需的临界磁场随温度非单调变化。在5 K时，场致PM到FM的转变导致了约160%的巨正磁电阻（MR）。在CHUF（不等磁场中冷却和加热）方案下的测量结果显示，在较高磁场（比冷却期间施加的磁场）下加热时出现折返转变。这表明该体系的类玻璃行为可以用一级相变动力学停滞的框架来解释。在越来越多的显示玻璃状被捕磁态的不同系统中，本系统是第一个观察到无序（此处为PM）到有序（此处为FM）一级跃迁的动力学被捕的例子。
Double exchange magnets: Spin-dynamics in the paramagnetic phase
采用有限温度对角化方法，在考虑铁磁Hund耦合和$t_{2g}$ -自旋间反铁磁交换的Kondo晶格模型下，研究了钙钛矿锰氧化物的电子结构.动态结构因子的结果与最近的双层锰氧化物La $_{1.2}$ Sr $_{1.8}$ Mn $_2$ O $_7$的中子散射实验是一致的。磁化率显示居里-外斯行为，并用于推导相图。在顺磁相载流子的特点是铁磁极化子在反铁磁自旋液体。
Thickness-dependent ferromagnetic metal to paramagnetic insulator transition in La$_{0.6}$Sr$_{0.4}$MnO$_3$ thin films studied by x-ray magnetic circular dichroism
金属过渡金属氧化物在临界厚度的几个单层（ML）上发生金属-绝缘体转变（MIT），但其驱动机制仍然存在争议。用X射线吸收光谱和X射线磁圆二色性研究了铁磁金属La $_{0.6}$ Sr $_{0.4}$ MnO $_3$的MIT随厚度的变化。当膜厚度在MIT的临界厚度（6 - 8 ML）上减小时，观察到铁磁信号逐渐减小并且伴随着顺磁信号的增加，而Mn价态朝向Mn $^{3+}$突然减小。这些观察结果表明，铁磁相逐渐和最有可能不均匀地变成顺磁相，两个阶段突然成为绝缘的临界厚度。
We have studied the magnetic torque in uranium monosulfide (US) single crystals to explore the magnetic anisotropy in this material. Uranium monosulfide crystallizes in cubic, NaCl-type of crystal structure and exhibits the largest magneto-crystalline anisotropy observed in cubic systems. By performing detailed torque measurements we observe a strongly anisotropic behavior in the paramagnetic phase due to crystal defects and quadrupolar pair interactions. Our studies also confirm the presence of a large anisotropy in the ferromagnetic state of the US system with the <100>, <111>, and <110> dir…
Effects of electron correlations on Anderson insulators have been one of the central themes for recent two decades, suggesting that the Anderson insulating phase turns into a novel insulating state referred to as many body localization (MBL). However, the role of spin degrees of freedom in this dynamical phase transition still remains unclarified as a function of the interaction strength. In this study, we perform real-space spin-resolved Hartree-Fock-Anderson simulations to investigate metal-insulator transitions above a critical disorder strength in three spatial dimensions, where all single…
Influence of magnetic field on paramagnetic-ferromagnetic transition in La$_{1-x}$Ca$_{x}$MnO$_{3}$ ($x\approx 0.25$) crystal: ultrasonic and transport studies
Quantum fluctuations from frustration can trigger quantum spin liquids (QSLs) at zero temperature. However, it is unclear how thermal fluctuations affect a QSL. We employ state-of-the-art tensor network-based methods to explore the ground state and thermodynamic properties of the spin-1/2 kagome Heisenberg antiferromagnet (KHA). Its ground state is shown to be consistent with a gapless QSL by observing the absence of zero-magnetization plateau as well as the algebraic behaviors of susceptibility and specific heat at low temperatures, respectively. We show that there exists an \textit{algebraic…
Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd$_2$As$_2$. We attribute this effect to the itinerant electrons experiencing quasistatic and quasi-long-range ferromagnetic fluctuations. Moreover, the spin nondegenerate band structure harbors a pair of…
We study the coupled-top model with three large spins located on a triangle. Depending on the coupling strength, there exist three phases: disordered paramagnetic phase, ferromagnetic phase, and frustrated antiferromagnetic phase, which can be distinguished by the mean-field approach. The paramagnetic-ferromagnetic phase transition is accompanied by the breaking of the global $Z_2$ symmetry, whereas the paramagnetic-antiferromagnetic phase transition is accompanied by the breaking of both the global $Z_2$ symmetry and the translational symmetry. Exact analytical results of higher-order quantum…
Different metamagnetism between paramagnetic Ce and Yb isomorphs
为了解决重费米子系统中的变磁现象之谜，我们比较了顺磁同质结构Ce和Yb系统，CeNi $_2$ Ge $_2$和YbNi $_2$ Ge $_2$，它们都位于磁不稳定性附近。最有趣的结果是在脉冲磁场中的磁化测量发现了两个系统中的变磁性转变。这是Ce和Yb类质同象顺磁系统的变磁性的首次观测。类似于其他的变磁体，两种化合物的变磁性转变场都很好地由温度$T_χ^{\rm max}$来缩放，在该温度下磁化率显示出最大值。此外，CeNi $_2$ Ge $_2$的非线性磁化率$χ_3$的峰值出现在$T_χ^{\rm max}/2$附近，与其它重费米子超磁体一样。相反，YbNi $_2$ Ge $_2$在$T_χ^{\rm max}$处仅显示$χ_3$的符号变化，如在位于铁磁临界点附近的巡游变磁体中所观察到的。CeNi $_2$ Ge $_2$的变磁性对应于典型的近藤晶格系统，而YbNi $_2$ Ge $_2$的变磁性类似于近铁磁巡游系统。本文还讨论了YbNi $_2$ Ge $_2$变磁行为的其它可能性。
Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector $\mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $μ\parallel\mathbf{a}$, with a sine-wave modulation of wavevector $\mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder stru…
