Evidence of the Anomalous Fluctuating Magnetic State by Pressure-Driven 4f Valence Change in EuNiGe3

In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu³⁺ atomic shell with J = 0, due to intra-atomic exchange and spin-orbital coupling interactions with the Eu²⁺ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in EuNiGe₃ single crystals is probed. Magnetic polarization is observed for both valence states of Eu²⁺ and Eu³⁺ across the entire pressure range. The anomalous magnetism is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor.

Modulations in martensitic Heusler alloys originate from nanotwin ordering

Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni₂MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite.

Pressure-Induced Ferromagnetism due to an Anisotropic Electronic Topological Transition in Fe_{1.08}Te

A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. We show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in Fe_{1.08}Te. Around 2 GPa, the orthorhombic and incommensurate antiferromagnetic ground state of Fe_{1.08}Te is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a "type-0" transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of Fe_{1.08}Te is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.

Signatures of a magnetic field-induced unconventional nematic liquid in the frustrated and anisotropic spin-chain cuprate LiCuSbO4

Modern theories of quantum magnetism predict exotic multipolar states in weakly interacting strongly frustrated spin-1/2 Heisenberg chains with ferromagnetic nearest neighbor (NN) inchain exchange in high magnetic fields. Experimentally these states remained elusive so far. Here we report strong indications of a magnetic field-induced nematic liquid arising above a field of ~13 T in the edge-sharing chain cuprate LiSbCuO₄ ≡ LiCuSbO₄. This interpretation is based on the observation of a field induced spin-gap in the measurements of the ⁷Li NMR spin relaxation rate T ₁^-1 as well as a contrasting field-dependent power-law behavior of T ₁^-1 vs. T and is further supported by static magnetization and ESR data. An underlying theoretical microscopic approach favoring a nematic scenario is based essentially on the NN XYZ exchange anisotropy within a model for frustrated spin-1/2 chains and is investigated by the DMRG technique. The employed exchange parameters are justified qualitatively by electronic structure calculations for LiCuSbO₄.

Theory of skyrmion states in liquid crystals

Within the Oseen-Frank theory we derive numerically exact solutions for axisymmetric localized states in chiral liquid crystal layers with homeotropic anchoring. These solutions describe recently observed two-dimensional skyrmions in confinement-frustrated chiral nematics [P. J. Ackerman et al., Phys. Rev. E 90, 012505 (2014)]. We stress that these solitonic states arise due to a fundamental stabilization mechanism responsible for the formation of skyrmions in cubic helimagnets and other noncentrosymmetric condensed-matter systems.

Large noncollinearity and spin reorientation in the novel Mn2RhSn Heusler magnet

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn(2)RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn(2)RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

Quantum order in the chiral magnet MnSi

Systems lacking inversion symmetry, such as selected three-dimensional compounds, multilayers and surfaces support Dzyaloshinsky-Moriya (DM) spin-orbit interactions. In recent years DM interactions have attracted great interest, because they may stabilize magnetic structures with a unique chirality and non-trivial topology. The inherent coupling between the various properties provided by DM interactions is potentially relevant for a variety of applications including, for instance, multiferroic and spintronic devices. The, perhaps, most extensively studied material in which DM interactions are important is the cubic B20 compound MnSi. We review the magnetic field and pressure dependence of the magnetic properties of MnSi. At ambient pressure this material displays helical order. Under hydrostatic pressure a non-Fermi liquid state emerges, where a partial magnetic order, reminiscent of liquid crystals, is observed in a small pocket. Recent experiments strongly suggest that the non-Fermi liquid state is not due to quantum criticality. Instead it may be the signature of spin textures and spin excitations with a non-trivial topology.

Phase transitions and rare-earth magnetism in hexagonal and orthorhombic DyMnO(3) single crystals

The floating-zone method with different growth ambiences has been used to selectively obtain hexagonal or orthorhombic DyMnO(3) single crystals. The crystals were characterized by x-ray powder diffraction of ground specimens and a structure refinement as well as electron diffraction. We report magnetic susceptibility, magnetization and specific heat studies of this multiferroic compound in both the hexagonal and the orthorhombic structure. The hexagonal DyMnO(3) shows magnetic ordering of Mn(3+) (S = 2) spins on a triangular Mn lattice at T(N)(Mn) = 57 K characterized by a cusp in the specific heat. This transition is not apparent in the magnetic susceptibility due to the frustration on the Mn triangular lattice and the dominating paramagnetic susceptibility of the Dy(3+) (S = 9/2) spins. At T(N)(Dy) = 3 K, a partial antiferromagnetic order of Dy moments has been observed. In comparison, the magnetic data for orthorhombic DyMnO(3) display three transitions. The data broadly agree with results from earlier neutron diffraction experiments, which allows for the following assignment: a transition from an incommensurate antiferromagnetic ordering of Mn(3+) spins at T(N)(Mn) = 39 K, a lock-in transition at T(lock-in) = 16 K and a second antiferromagnetic transition at T(N)(Dy) = 5 K due to the ordering of Dy moments. Both the hexagonal and the orthorhombic crystals show magnetic anisotropy and complex magnetic properties due to 4f-4f and 4f-3d couplings.

Investigations on the spin-glass state in Dy(0.5)Sr(0.5)MnO(3) single crystals through structural, magnetic and thermal properties

Single crystals of Dy(0.5)Sr(0.5)MnO(3) are grown using the optical floating zone technique, and their structural, magnetic, transport and thermal properties have been investigated. Magnetization measurements under field-cooled and zero-field-cooled conditions display irreversibility below 35 K. The magnetization does not saturate up to fields of 5 T in the temperature range 5-350 K. AC susceptibility shows a cusp around 32 K that shifts to higher temperature with increasing frequency. This frequency dependence of the peak temperature follows a critical slowing down with exponent zν = 3.6. Electrical resistivity shows insulating behavior, and the application of magnetic fields up to 10 T does not change this qualitative behavior. However, a marked negative magnetoresistance is observed in the paramagnetic phase reaching 80% at 70 K and 10 T. The observed resistivity behavior does not obey an activated type of conduction. These features are characteristic of spin-glass behavior in this half-doped insulating manganite. It is argued that the spin-glass-like state originates from the A-site disorder, which in turn results from the random distribution of cations with different ionic radii. Specific-heat measurements reveal a sizable linear contribution at low temperature that may be associated with the glassy magnetic ordering and a Schottky-like anomaly in a wide temperature range between 8 and 40 K. The distribution of Schottky levels is explained by the inhomogeneity of the molecular field in the spin-glass state that leads to variable splitting of the Kramers ground-state doublets in Dy(3+).