Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field

The reference chiral helimagnet MnSi is the first system where skyrmion lattice correlations have been reported. At zero magnetic field the transition at $T_C$ to the helimagnetic state is of first order. Above $T_C$, in a region dominated by precursor phenomena, neutron scattering shows the build up of strong chiral fluctuating correlations over the surface of a sphere with radius $2\pi/\ell$, where $\ell$ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above $T_C$, both of general relevance to chiral magnetism, remain an open question

Spin Glass Correlation Length: a Caliper for Temperature Chaos

The spin glass correlation length is used as a caliper for the onset of temperature chaos in a Cu$_{0.94}$Mn$_{0.06}$ single crystal sample. From the values of the correlation length at different temperatures, we are able to calibrate the onset of the transition from reversible to chaotic behavior. We find that temperature chaos sets in abruptly as the chaos length scale $L^*$ becomes comparable to the spin glass correlation length $\xi$. We find the chaotic exponent for temperature chaos, $\zeta$, to be the order of unity assuming either fractal or compact glassy domains, in good agreement with previous theoretical analyses and numerical simulations

Short-range magnetic correlations in Tb5Ge4

We present a single crystal neutron diffraction study of the magnetic short-range correlations in Tb$_5$Ge$_4$ which orders antiferromagnetically below the Neel temperature $T_N$ $\approx$ 92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above $T_N$. The antiferromagnetic ordering in Tb$_5$Ge$_4$ can be described as strongly coupled ferromagnetic block layers in the $ac$-plane that stack along the b-axis with weak antiferromagnetic inter-layer coupling. Diffuse scattering was observed along both $a^*$ and $b^*$ directions indicating three-dimensional short-range correlations. Moreover, the $q$-dependence of the diffuse scattering is Squared-Lorentzian in form suggesting a strongly clustered magnetic state that may be related to the proposed Griffiths-like phase in Gd$_5$Ge$_4$.Comment: 6 pages, 5 figure

Neutron diffraction studies of the magnetoelastic compounds Tb5SixGe4−x (x=2.2 and 2.5)

We report the results of a neutron diffraction study, carried out on both single crystalline and polycrystalline samples of Tb5Si2.2Ge1.8 and polycrystalline Tb5Si2.5Ge1.5. On cooling, at approximately 120 K, the Tb5Si2.2Ge1.8 system undergoes a magnetoelastic transition from a high-temperature monoclinic-paramagnetic to a low-temperature orthorhombic-ferromagnetic structure. Between 120 K and 75 K, the magnetic structure has a net ferromagnetic component along the a axis direction. The moments are slightly canted with respect to the a axis, while the components along the b and c axes are ordered antiferromagnetically. A second magnetic transition occurs at approximately 75 K. Below this temperature, the magnetic structure consists of ferromagnetically aligned μx and μz projections of the magnetic moments and an antiferromagnetic arrangement of theμy moment components. Magnetic structures of Tb5Si2.5Ge1.5 are nearly identical to those of Tb5Si2.2Ge1.8

Scaling Law Describes the Spin-Glass Response in Theory, Experiments, and Simulations

The correlation length ξ, a key quantity in glassy dynamics, can now be precisely measured for spin glasses both in experiments and in simulations. However, known analysis methods lead to discrepancies either for large external fields or close to the glass temperature. We solve this problem by introducing a scaling law that takes into account both the magnetic field and the time-dependent spin-glass correlation length. The scaling law is successfully tested against experimental measurements in a CuMn single crystal and against large-scale simulations on the Janus II dedicated computer

Hydrostatic pressure control of the magnetostructural phase transition in Gd5Si2Ge2 single crystals

Magnetic and structural properties of single crystalline Gd5Si2Ge2 under hydrostatic pressure have been characterized by using magnetization, linear thermal expansion, and compressibility measurements. A strong dependence of Curie temperature on pressure, dTC∕dP=+4.8 K∕kbar, is observed in contrast with the smaller values of about 3 K∕kbar found in polycrystalline specimens. This difference reflects the role the microstructure may play in pressure-induced magnetic-crystallographic phase changes, likely related to stress relaxation at the grain boundaries, domain pinning and/or nucleation of defects. The pressure dependence of the critical magnetic field, d(dHC∕dT)∕dP, drops at the rate −0.122(5)kOe∕K kbar, which points to an enhancement of the magnetoelastic coupling with pressure. The latter affects the magnetocaloric behavior of the material at the rate d(ΔSM)∕dP≅1.8 J∕kg K kbar. The linear thermal expansion confirms the strongly anisotropic change of the lattice parameters through the orthorhombic to monoclinic crystallographic transformation with Δa∕a=+0.94%, Δb∕b=−0.13%, and Δc∕c=−0.22%. The structural transition temperature varies with pressure synchronously with the Curie temperature, and the size and shape of the strain anomalies remain nearly unaffected by the hydrostatic pressure, indicating, respectively, that the structural and magnetic transformations remain coupled, and the anisotropic behavior of the lattice is preserved as pressure increases. The room temperature linear compressibility data show that the magnetostructural transformation can be triggered isothermally at ∼6 kbar and that the compressibility is anisotropic