The spinel compound CuV _2 S _4 exhibits two-step anomalies at about 92 K ( T_t1 ) and 56 K ( T_t2 ), indicating a charge density wave (CDW) state. While previous studies mainly used ^51 V-NMR to investigate the Cu(Ir _1-x V _x ) _2 S _4 system, the present work employs ^63 Cu-NMR to study the local electronic state and spin dynamics from the copper sites. Measurements were performed on powdered samples of Cu(Ir _1-x V _x ) _2 S _4 ( x = 1.00, 0.98, and 0.95) at temperatures between 4.2 K and 300 K. The results for the ^63 Cu Knight shift (K) and spin-lattice relaxation rate divided by temperature ( 1/T_1T ) show distinct differences compared to the V sites. In CuV _2 S _4 , K follows the magnetic susceptibility, similar to V-site observations. Notably, the Cu-site Knight shift clearly captures a thermal hysteresis at T_t2 , which was not clear in prior ^51 V-NMR studies. Above T_t1 , 1/T_1T of CuV _2 S _4 follows the Korringa law, with an enhancement observed near T_t1 . Below T_t1 , both K and 1/T_1T decrease rapidly, reflecting a reduction in the density of states at the Fermi level N(E_F) due to the CDW transition. For K, this reduction is more pronounced in x = 1.00 than in x = 0.98 and 0.95. In contrast, 1/T_1T shows a similar large decrease for all samples. Below T_t2 , the decrease in 1/T_1T becomes gradual, and it follows the Korringa law below 10 K. Furthermore, for x = 0.98 and 0.95, an enhancement in 1/T_1T was observed around 130 K, which is above T_t1 . These behaviors of 1/T_1T at the Cu site are different from those observed in ^51 V-NMR. These findings suggest that the Cu site is highly sensitive to local electronic correlations and structural distortions that are difficult to detect at the V site.

The magnetic structure of EuRhGe_3, a noncentrosymmetric body-centered tetragonal magnet with the space group I4mm, has been investigated by resonant X-ray diffraction. Below T_N=12 K, EuRhGe_3 undergoes a helical magnetic ordering with an incommensurate propagation vector q=(0, 0, 0.809), in which the magnetic moments lie in the ab plane and rotate by a constant turn angle of 145.8^∘ between adjacent layers. When a magnetic field is applied along the a axis at 2 K, a second-harmonic 2q peak develops, indicating that the circular helix is gradually distorted into a helimagnetic soliton-lattice state, which eventually undergoes a lock-in transition to the commensurate structure with q=0.8 at 3.8 T. Above the subsequent phase boundary at 5 T, the helicity is lost, and a spin-flop xyz-fan (elliptic conical) state is realized, in which the moments oscillate predominantly along the b axis but are accompanied by a small c-axis component. At higher fields, the system enters a conventional planar xy-fan phase without a c-axis component. EuRhGe_3 provides a prototypical example of a helimagnet that exhibits a full sequence of field-induced structures, evolving from a circular helix to a spin-flop xyz-fan (elliptic conical), and finally to a planar xy-fan structure, which has been theoretically predicted.

Magnetic skyrmions are particle-like spin-swirling objects ubiquitously realized in magnets. They are topologically stable chiral kink structures composed of multiple modulation waves of spiral spin structures. The helicity of each spiral is usually determined by antisymmetric exchange interactions in noncentrosymmetric crystals. We report an experimental observation of a distorted triangular lattice of skyrmions in the polar tetragonal magnet EuNiGe3 in a magnetic field, reflecting a strong coupling with the lattice. Moreover, through circularly polarized resonant X-ray diffraction on Bragg peaks from four separate magnetic domains, we discovered a change in the magnetic helicity of the original spiral at zero field when the skyrmion lattice is formed with a unified helicity of the three q components. This implies that the energy gain provided by the skyrmion lattice formation is larger than the antisymmetric exchange interaction.

We investigated the magnetic helicity of the triple-q magnetic structure of the triangular skyrmion lattice in the A phase of EuPtSi for a magnetic field along the [111] axis by resonant x-ray diffraction using a circularly polarized beam. We show that all three Fourier components of the triple-q structure are perpendicular to the respective q vectors and have the same helicity. They are connected by the rotation operations about the [111] axis. The helicity is the same as that of the single-q helimagnetic phase at low fields, suggesting that the antisymmetric exchange interaction inherent in the chiral structure supports the formation of the triangular skyrmion lattice. We also observe that the helical plane in the helimagnetic phase is tilted to the magnetic field to form a conical structure before the first-order transition to the skyrmion lattice phase.

The magnetic helicities of the cycloidal ordering in EuIrGe$_3$, with a noncentrosymmetric tetragonal structure, have been studied by circularly polarized resonant X-ray diffraction. It is shown that the helicity of each cycloidal domain is uniquely determined and satisfies the symmetry relations of the $C_{4v}$ point group of the crystal structure. The result shows that the cycloidal helicity is determined by the Dzyaloshinskii-Moriya type antisymmetric exchange interaction. The domain selection and the phase transition by the external magnetic field along [100] and [110] have also been studied. It is shown that the cycloidal plane prefers to be perpendicular to the field and the transverse conical state is realized.

Magnetic skyrmions are particle-like spin-swirling objects ubiquitously realized in magnets. They are topologically stable chiral kinks composed of multiple modulation waves of spiral spin structures, where the helicity of each spiral is usually selected by antisymmetric exchange interactions in noncentrosymmetric crystals. We report an experimental observation of a distorted triangular lattice of skyrmions in the polar tetragonal magnet EuNiGe$_3$, reflecting a strong coupling with the lattice. Moreover, through resonant x-ray diffraction, we find that the magnetic helicity of the original spiral at zero field is reversed when the skyrmion lattice is formed in a magnetic field. This means that the energy gain provided by the skyrmion lattice formation is larger than the antisymmetric exchange interaction. Our findings will lead us to a further understanding of emergent magnetic states.

Successive magnetic phase transitions at $T_{\text{N}}$=12.2 K, $T_{\text{N}}^{\;\prime}$=7.0 K, and $T_{\text{N}}^{\;*}$=5.0 K in EuIrGe$_3$, an intermetallic compound with a body centered tetragonal lattice belonging to a polar space group $I4mm$, has been investigated by neutron diffraction and resonant X-ray diffraction. It is shown that EuIrGe$_3$ exhibits an incommensurate longitudinal sinusoidal order with $q\sim (0, 0, 0.792)$ and $m_{q} \parallel c\text{-axis}$ in the high temperature phase ($T_{\text{N}}^{\;\prime}<T<T_{\text{N}}$), which changes to a cycloidal order with $q=(\delta', 0, 0.8)$ ($\delta'\sim 0.017$) and $m_{q} \parallel ac\text{-plane}$ in the intermediate phase ($T_{\text{N}}^{\;*}<T<T_{\text{N}}^{\;\prime}$). In the low temperature phase ($T<T_{\text{N}}^{\;*}$), the cycloidal plane rotates by $45^{\circ}$ to have $q=(\delta, \delta, 0.8)$ ($\delta\sim 0.012$). It is also pointed out that the X-ray scattering amplitude from odd-parity magnetic quadrupole due to the polar environment interfere with that from normal even-parity magnetic dipole in the magnetic ordered phase.

The 4f-electrons in the rare-earth compounds are generally localized and order magnetically mediated by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The electronic states or the Fermi surface properties in the Ce- and Eu-based compounds can be changed by decreasing temperature and/or applying pressure. Antiferromagnets CeRhIn $$_5$$ and EuCu $$_2$$ Ge $$_2$$ are characteristic, revealing the first-order phase transition under pressure, which is based on a combined phenomenon between the Kondo effect and the sharp valence crossover. Other antiferromagnets, CeIrSi $$_3$$ with Rashba-type tetragonal structure and EuPtSi with chiral cubic structure, revealed huge upper critical fields in superconductivity for $$H \parallel $$ [001] and the magnetic skyrmion lattice for $$H \parallel \langle 111 \rangle $$ , respectively. The noncentrosymmetric crystal structure also brings distinctive properties to the electronic states.

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

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.

We grew single crystals of EuCo2P2 and EuT2Ge2 (T: Co, Rh, Ir, Ni, and Pd) by the Bridgman method and Sn- and In-flux methods, and studied their magnetic properties by measuring the specific heat, ...

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound EuNi2P2. Both the Eu2+ and Eu3+ components arising from the 4f(6) and 4f(5) final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4f reference compounds SrNi2P2 and YNi2P2. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of SrNi2P2 rather than that of YNi2P2. The heavy-fermion bands in EuNi2P2 are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for SrNi2P2, and the Eu2+ components located at the very vicinity of the Fermi level.

In this study, we measure the magnetic field and temperature variations of the magnetization M(H, T) of the cubic chiral compound EuPtSi and extensively investigate the magnetic phase transitions in a wide temperature range down to 60 mK. The incommensurate-commensurate transition at T-N* similar or equal to 2.5 K in the helical state, which has not been observed in transport or thermodynamic measurements, is exclusively detected in M(T) for H parallel to < 111 >. T-N* shows a rapid decrease in H. For H parallel to [100] and [111], M(H) at T < 100 mK showed that the helical q vectors are depinned from the preferred orientations at H greater than or similar to 10 kOe and gradually rotate toward the field directions as H increases. The skyrmion lattice phase (A-phase) field-induced in H parallel to < 111 > only above 0.4 K in equilibrium conditions can be created at 60 mK under field cooling, with a sharp tilted plateau structure developing in M(H) of the A-phase. A similar metastable state is also created below 100 mK for the A' phase induced by H parallel to < 100 > at T greater than or similar to 0.3 K in equilibrium conditions. The magnetic phase diagrams established for the three directions [111], [110], and [100] are highly anisotropic and discussed herein on the basis of the q vectors rather strongly pinned to the lattice and transform under the lattice symmetry operations.

Multiple transition phenomena in divalent Eu compound EuAl4 with the tetragonal structure were investigated via the single-crystal time-of-flight neutron Laue technique. At 30.0 K below a charge-density-wave (CDW) transition temperature of T-CDW = 140 K, superlattice peaks emerge near nuclear Bragg peaks described by an ordering vector q(CDW) = (0, 0, delta(c)) with delta(c) similar to 0.19. In contrast, magnetic peaks appear at q(2) = (delta(2), delta(2), 0) with delta(2) = 0.085 in a magnetic-ordered phase at 13.5 K below T-N1 = 15.4 K. By further cooling to below T-N3 = 12.2 K, the magnetic ordering vector changes into q(1) = (delta(1), 0, 0) with delta(1) = 0.17 at 11.5 K and slightly shifts to delta(1) = 0.194 at 4.3 K. No distinct change in the magnetic Bragg peak was detected at T-N2 = 13.2 K and T-N4 = 10.0 K. The structural modulation below T-CDW with q(CDW) is characterized by the absence of the superlattice peak in the (0 0 l) axis. As a similar CDW transition was observed in SrAl4, the structural modulation with q(CDW) could be mainly ascribed to the displacement of Al ions within the tetragonal ab-plane. Complex magnetic transitions are in stark contrast to a simple collinear magnetic structure in isovalent EuGa4. This could stem from different electronic structures with the CDW transition between two compounds.