Non-coplanar helimagnetism in the layered van-der-Waals metal DyTe3.

Van-der-Waals magnetic materials can be exfoliated to realize ultrathin sheets or interfaces with highly controllable optical or spintronics responses. In majority, these are collinear ferro-, ferri-, or antiferromagnets, with a particular scarcity of lattice-incommensurate helimagnets of defined left- or right-handed rotation sense, or helicity. Here, we report polarized neutron scattering experiments on DyTe3, whose layered structure has highly metallic tellurium layers separated by double-slabs of dysprosium square nets. We reveal cycloidal (conical) magnetic textures, with coupled commensurate and incommensurate order parameters, and probe the evolution of this ground state in a magnetic field. The observations are well explained by a one-dimensional spin model, with an off-diagonal on-site term that is spatially modulated by DyTe3's unconventional charge density wave (CDW) order. The CDW-driven term couples to antiferromagnetism, or to the net magnetization in an applied magnetic field, and creates a complex magnetic phase diagram indicative of competing interactions in this easily cleavable van-der-Waals helimagnet.

Enhanced emergent electromagnetic inductance in Tb5Sb3 due to highly disordered helimagnetism.

In helimagnetic metals, ac current-driven spin motions can generate emergent electric fields acting on conduction electrons, leading to emergent electromagnetic induction (EEMI). Recent experiments reveal the EEMI signal generally shows a strongly current-nonlinear response. In this study, we investigate the EEMI of Tb5Sb3, a short-period helimagnet. Using small angle neutron scattering we show that Tb5Sb3 hosts highly disordered helimagnetism with a distribution of spin-helix periodicity. The current-nonlinear dynamics of the disordered spin helix in Tb5Sb3 indeed shows up as the nonlinear electrical resistivity (real part of ac resistivity), and even more clearly as a nonlinear and huge EEMI (imaginary part of ac resistivity) response. The magnitude of the EEMI reaches as large as several tens of µH for Tb5Sb3 devices on the scale of several tens of µm, originating to noncollinear spin textures possibly even without long-range helimagnetic order.

Transition between distinct hybrid skyrmion textures through their hexagonal-to-square crystal transformation in a polar magnet.

Magnetic skyrmions, topological vortex-like spin textures, garner significant interest due to their unique properties and potential applications in nanotechnology. While they typically form a hexagonal crystal with distinct internal magnetisation textures known as Bloch- or Néel-type, recent theories suggest the possibility for direct transitions between skyrmion crystals of different lattice structures and internal textures. To date however, experimental evidence for these potentially useful phenomena have remained scarce. Here, we discover the polar tetragonal magnet EuNiGe3 to host two hybrid skyrmion phases, each with distinct internal textures characterised by anisotropic combinations of Bloch- and Néel-type windings. Variation of the magnetic field drives a direct transition between the two phases, with the modification of the hybrid texture concomitant with a hexagonal-to-square skyrmion crystal transformation. We explain these observations with a theory that includes the key ingredients of momentum-resolved Ruderman-Kittel-Kasuya-Yosida and Dzyaloshinskii-Moriya interactions that compete at the observed low symmetry magnetic skyrmion crystal wavevectors. Our findings underscore the potential of polar magnets with rich interaction schemes as promising for discovering new topological magnetic phases.

Square and rhombic lattices of magnetic skyrmions in a centrosymmetric binary compound.

Magnetic skyrmions are topologically stable swirling spin textures with particle-like character, and have been intensively studied as a candidate of high-density information bit. While magnetic skyrmions were originally discovered in noncentrosymmetric systems with Dzyaloshinskii-Moriya interaction, recently a nanometric skyrmion lattice has also been reported for centrosymmetric rare-earth compounds, such as Gd2PdSi3 and GdRu2Si2. For the latter systems, a distinct skyrmion formation mechanism mediated by itinerant electrons has been proposed, and the search of a simpler model system allowing for a better understanding of their intricate magnetic phase diagram is highly demanded. Here, we report the discovery of square and rhombic lattices of nanometric skyrmions in a centrosymmetric binary compound EuAl4, by performing small-angle neutron and resonant elastic X-ray scattering experiments. Unlike previously reported centrosymmetric skyrmion-hosting materials, EuAl4 shows multiple-step reorientation of the fundamental magnetic modulation vector as a function of magnetic field, probably reflecting a delicate balance of associated itinerant-electron-mediated interactions. The present results demonstrate that a variety of distinctive skyrmion orders can be derived even in a simple centrosymmetric binary compound, which highlights rare-earth intermetallic systems as a promising platform to realize/control the competition of multiple topological magnetic phases in a single material.

Chirality-Induced Spin Polarization over Macroscopic Distances in Chiral Disilicide Crystals.

A spin-polarized state is examined under charge current at room temperature without magnetic fields in chiral disilicide crystals NbSi_{2} and TaSi_{2}. We found that a long-range spin transport occurs over ten micrometers in these inorganic crystals. A distribution of crystalline grains of different handedness is obtained via location-sensitive electrical transport measurements. The sum rule holds in the conversion coefficient in the current-voltage characteristics. A diamagnetic nature of the crystals supports that the spin polarization is not due to localized electron spins but due to itinerant electron spins. A large difference in the strength of antisymmetric spin-orbit interaction associated with 4d electrons in Nb and 5d ones in Ta is oppositely correlated with that of the spin polarization. A robust protection of the spin polarization occurs over long distances in chiral crystals.

Linear magnetoresistance in the low-field limit in density-wave materials.

The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects, unusual topological band structures, and inhomogeneities that lead to wandering current paths can induce a cross-over from quadratic to linear MR with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear MR. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semiclassical narrative for a broad category of materials with partially gapped Fermi surfaces due to density waves.

Quadrupole effects in tetragonal crystals PrCu2Si2 and DyCu2Si2.

We have investigated quadrupole effects in tetragonal crystals of PrCu2Si2 and DyCu2Si2 by means of low-temperature ultrasonic measurements. The elastic constant C44 of PrCu2Si2 exhibits pronounced softening below 70 K down to a Néel temperature TN = 20 K, which is described in terms of a quadrupole susceptibility for a Γ5 doublet ground state and a Γ3 singlet first excited state located at 15.6 K in the crystalline electric field scheme. The C44 and C66 of DyCu2Si2 also show softening below 70 K down to TN1 = 9.7 K. A low-lying pseudo-sextet state consisting of three Kramers doublets of Γ6⊕2Γ7 brings about softening of C44 and C66 in DyCu2Si2.

Miniature ceramic-anvil high-pressure cell for magnetic measurements in a commercial superconducting quantum interference device magnetometer.

A miniature opposed-anvil high-pressure cell has been developed for magnetic measurement in a commercial superconducting quantum interference device magnetometer. Non-magnetic anvils made of composite ceramic material were used to generate high-pressure with a Cu-Be gasket. We have examined anvils with different culet sizes (1.8, 1.6, 1.4, 1.2, 1.0, 0.8, and 0.6 mm). The pressure generated at low temperature was determined by the pressure dependence of the superconducting transition of lead (Pb). The maximum pressure P(max) depends on the culet size of the anvil: the values of P(max) are 2.4 and 7.6 GPa for 1.8 and 0.6 mm culet anvils, respectively. We revealed that the composite ceramic anvil has potential to generate high-pressure above 5 GPa. The background magnetization of the Cu-Be gasket is generally two orders of magnitude smaller than the Ni-Cr-Al gasket for the indenter cell. The present cell can be used not only with ferromagnetic and superconducting materials with large magnetization but also with antiferromagnetic compounds with smaller magnetization. The production cost of the present pressure cell is about one tenth of that of a diamond anvil cell. The anvil alignment mechanism is not necessary in the present pressure cell because of the strong fracture toughness (6.5 MPa m(1∕2)) of the composite ceramic anvil. The simplified pressure cell is easy-to-use for researchers who are not familiar with high-pressure technology. Representative results on the magnetization of superconducting MgB(2) and antiferromagnet CePd(5)Al(2) are reported.

5d and 4f electron configuration of CeB6 at 340 and 535 K.

X-ray atomic orbital (XAO) analysis revealed that at both temperatures the electrons are transferred from B 2p(x)(= p(y)) to Ce 5d and 4f orbitals. At 340 K 5d(j = 5/2)Gamma(8) orbitals are occupied partially, but 4f(j = 5/2)Gamma(8) orbitals are more populated than 4f(j = 5/2)Gamma(7) orbitals, in contrast to our observation at 430 K [Makita et al. (2007). Acta Cryst. B63, 683-692]. At 535 K the XAO analysis revealed clearly that the order of the energy levels of 4f(j = 5/2)Gamma(8) and Gamma(7) states reversed again and is the same as that at room temperature. It also limited the possible 5d configurations to three models among the nine possible ones. However, the XAO analysis could not decide which of the three models was the best with the present accuracy of the measurement. Two of them have partially and fully occupied 5d(j = 5/2)Gamma(7) orbitals and the remaining one has a fully occupied 5d(j = 3/2)Gamma(8) orbital. Since the lobes of 5d(j = 3/2)Gamma(8) or 5d(j = 5/2)Gamma(7) orbitals do not overlap with the 4f(j = 5/2)Gamma(8) orbitals as well as the 5d(j = 5/2)Gamma(8) orbitals, the order of the energy levels of the 4f(j = 5/2) orbitals became the same as that at room temperature. These results indicate that the crystal field varies with temperature due to the electron transfer from B 2p to Ce 5d orbitals. The difference densities after the spherical-atom refinement at the three temperatures clearly revealed the different combinations of 4f and 5d orbitals which are occupied. In the present study positive peaks due to the 4f electrons appear near the Ce nucleus and those due to 5d orbitals are found in the area outside the 4f peaks. Between the two areas there is a negative area distributed spherically at 340 K. The negative area produced by the contraction of 4f(j = 5/2)Gamma(8) orbitals seems to reduce the electron repulsion of the 5d(j = 5/2)Gamma(8) orbitals and helps the 4f(j = 5/2)Gamma(8) orbitals to remain as the ground state.

Inversion of 4f-states in CeB6 thermally excited at 430 K.

The 4f states of Ce in a typical Kondo crystal, CeB(6), are split into an excited state Gamma(7) and the ground state Gamma(8), with an excitation energy at 560 K. The electron-density distribution of the thermally excited state was measured at 430 K using a four-circle diffractometer equipped with a small furnace. In contrast to the previous results at lower temperature, electrons are transferred from B(6) to Ce at 430 K. X-ray atomic-orbital analysis revealed that the 5d-Gamma(8) orbitals (the energy level of which is similar to that of the B-2p orbitals) are fully occupied and the 4f-Gamma(7) orbitals are more populated than the 4f-Gamma(8) orbitals. Fully occupied 5d-Gamma(8) makes the 4f-Gamma(8) states unstable and the energy levels of 4f-Gamma(7) and 4f-Gamma(8) are inverted.

Enhancing the superconducting transition temperature of CeRh 1-x IrxIn5 due to the strong-coupling effects of antiferromagnetic spin fluctuations: an 115In nuclear quadrupole resonance study.

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh(1-x)Ir(x)In(5) via an (115)In nuclear-quadrupole-resonance experiment. The nuclear spin-lattice relaxation rate 1/T(1) has revealed the marked development of AFM spin fluctuations as approaching an AFM ordered state. Concomitantly, the superconducting transition temperature T(c) and the energy gap Delta0 increase drastically from T(c)= 0.4K and 2Delta0/k(B)T(c)=5 in CeIrIn(5) up to T(c) =1.2K and 2Delta0/k(B)T(c) =8.3 in CeRh(0.3)Ir(0.7)In5 , respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the strong-coupling unconventional SC in HF compounds.

Enhancing the superconducting transition temperature of the heavy fermion compound CeIrIn5 in the absence of spin correlations.

We report on a pressure- (P-)induced evolution of superconductivity and spin correlations in CeIrIn(5) via the (115)In nuclear-spin-lattice-relaxation rate measurements. We find that applying pressure suppresses dramatically the antiferromagnetic fluctuations that are strong at ambient pressure. At P = 2.1 GPa, T(c) increases to T(c) = 0.8 K, which is twice T(c) (P = 0 GPa), in the background of Fermi-liquid state. This is in sharp contrast to the previous case in which a negative, chemical pressure (replacing Ir with Rh) enhances magnetic interaction and increases T(c). Our results suggest that multiple mechanisms work to produce superconductivity in the same compound CeIrIn(5).

Observation of 4f electron transfer from Ce to B6 in the Kondo crystal CeB6 and its mechanism by multi-temperature X-ray diffraction.

Electron density distributions (EDD) in CeB(6) were measured by X-ray diffraction at 100, 165, 230 and 298 K. Analysis with a weak-field model, in which the spin-orbit interaction dominates the energy splitting of the 4f levels, revealed that more 4f electrons were donated from Ce to B(6) at the lower temperature. Donated electrons localize around the B-B bonds connecting B(6) octahedra. The localized electrons and an expansion of the outermost 5p orbitals change the effective atomic potentials and enhance the anharmonic vibration (AHV) of constituent atoms at lower temperature. Enhanced AHV increases the entropy and makes the electron donation inevitable. Changes in crystal structure, EDD, electron configuration and AHV are found to be closely correlated with one another and the mechanism of the electron transfer in the Kondo crystal CeB(6) in the studied temperature range was elucidated. This is, to the authors' knowledge, the first multi-temperature measurement of EDD that elucidates a mechanism of change from the temperature dependence of the EDD. Parameters change consistently at all the temperatures except 298 K, at which the excited states Gamma(7) of the Ce 4f states have significant electron population. The thermal excitation to Gamma(7) levels expands the B(6) octahedra, since Gamma(7) has main lobes along <111> or from Ce to the centre of B(6) octahedra. The energy gap between the ground state Gamma(8) and Gamma(7) was calculated to be 470 K from the ratio of electron populations of both states. The present experiment opens the door to accurate X-ray EDD analyses of rare earth complexes.