Bulk electronic states of pyrite-type CuS2 and CuSe2 have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES). In the valence-band HAXPES spectra, the Cu 3d subshell is basically fully occupied in CuS2 and CuSe2, and the partially occupied anion p orbitals are responsible for the metallic behavior. This situation is opposite to the various conducting transition-metal compounds with open shell transition-metal cations and closed shell anions. The Cu 2p core-level HAXPES spectra exhibit satellite peaks, which indicate the presence of charge fluctuations mediated by inverse charge transfer from Cu 3d to S 3p/Se 4p.

The divalent Eu intermetallic compound EuAl _4 with the BaAl _4 -type structure (space group: I4/mmm) orders antiferromagnetically at 16 K. Al atoms have two crystallographically inequivalent sites, denoted as Al(I) and Al(II). Three ^27 Al zero-field NMR spectra in the antiferromagnetic state of EuAl _4 at 4.2 K are detected in the vicinity of 12.8, 19.7 and 41.6 MHz, corresponding to the ^27 Al internal magnetic fields of 1.2, 1.8 and 3.7 T, respectively. The ^27 Al spectra at 12.8 and 19.7 MHz split into five resonance-lines because of the nuclear quadrupole interaction by I = 5/2. However, the ^27 Al spectrum at 41.6 MHz shows a broad line-shape. From analysis of these spectra, the ^27 Al spectra at 12.8 and 19.7 MHz correspond to those of the Al(II) site and the spectrum at 41.6 MHz belongs to that of the Al(I) site.

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.

The EuRu2P2 single crystal was investigated by means of magnetic, transport and thermodynamic studies at ambient and hydrostatic pressures. A small magnetocrystalline anisotropy with crystallographic [100] direction as an easy magnetization direction was found by experimental measurements and confirmed by first-principles calculations. We connect a previously reported change in the compressibility observed at room temperature to a rapid change of ordering temperature under applied hydrostatic pressure. (C) 2021 Elsevier B.V. All rights reserved.

We report the results of muon spin relaxation (mu SR) and Si-29 NMR measurements carried out on the cubic chiral magnet EuPtSi. This compound exhibits a helimagnetic transition at T-N( )= 4 K, and hosts a unique skyrmion phase characterized by a short modulation period and strong anisotropy under magnetic fields. Our zero-field mu SR experiments revealed the development of critical slowing down of Eu spin fluctuations over a relatively wide critical region (T - T-N )/T-N < 5 above T-N. We also found from Si-29 NMR that the spin fluctuations are strongly suppressed by magnetic field in the paramagnetic state above 20 K. These characteristic spin dynamics observed over a wide region of temperature and magnetic field suggest the presence of magnetic frustration in the spin system. Such frustration would underlie the mechanism stabilizing the short-period skyrmion lattice observed in this compound.

We have succeed in growing single crystals of EuCu and EuAu with the hexagonal structure by the Bridgman method. Both compounds are known to be ferromagnets with Curie temperatures and 13 K, respectively, and the Eu-4f magnetic moments of EuCu are known to orient along the hexagonal c-axis or the [0001] direction by the Mossbauer experiment. The magnetisations at 2 K in EuCu and EuAu in the present experiment saturate at an extremely low field of 2 kOe for , with a Eu-moment of . On the other hand, the hard-axis magnetisations for [0001] saturate at a high field of 40 kOe. It is also found from the electrical resistivty measurement that the electrical resistivities along the current [0001] in EuCu and EuAu are about at room temperature, which are compared with the resistivities of 30-40 for [0001]. The quasi-one dimensional conductivities are also characteristic, which were clarified from the de Haas-van Alphen (dHvA) experiment and FLAPW energy band calculation for EuAu, revealing the existence of a large plate-like Fermi surface.

We grew high-quality single crystals of AuSb2 with the pyrite (FeS2)-type cubic structure by the Bridgman method and studied the Fermi surface properties by the de Haas-van Alphen (dHvA) experiment and the full potential LAPW band calculation. The Fermi surfaces of AuSb2 are found to be similar to those of NiSbS and PdBiSe with the ullmannite (NiSbS)-type cubic chiral structure because the crystal structures are similar each other and the number of valence electrons is the same between two different compounds. Note that each Fermi surface splits into two Fermi surfaces in NiSbS and PdBiSe, reflecting the non-centrosymmetric crystal structure.

$^{59}\mathrm{Co}$ and $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) measurements in external magnetic and zero magnetic fields have been performed to investigate the magnetic properties of the A-type antiferromagnetic (AFM) ${\mathrm{CaCo}}_{2}{\mathrm{P}}_{2}$. NMR data, especially the nuclear spin lattice relaxation rates $1/{T}_{1}$ exhibiting a clear peak, provide clear evidence for the AFM transition at a N\'eel temperature of ${T}_{\mathrm{N}}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The magnetic fluctuations in the paramagnetic state were found to be three-dimensional ferromagnetic, suggesting ferromagnetic interaction between Co spins in the $\mathit{ab}$ plane characterizes the spin correlations in the paramagnetic state. In the AFM state below ${T}_{\mathrm{N}}$, we have observed $^{59}\mathrm{Co}$ and $^{31}\mathrm{P}$ NMR signals under zero magnetic field. From $^{59}\mathrm{Co}$ NMR data, the ordered magnetic moments of Co are found to be in $ab$ plane and are estimated to be 0.35 ${\ensuremath{\mu}}_{\mathrm{B}}$ at 4.2 K. Furthermore, the external field dependence of $^{59}\mathrm{Co}$ NMR spectrum in the AFM state suggests a very weak magnetic anisotropy of the Co ions and also provides microscopic evidence of canting the Co-ordered moments along the external magnetic field directions. The magnetic state of the Co ions in ${\mathrm{CaCo}}_{2}{\mathrm{P}}_{2}$ is well explained by the local-moment picture in the AFM state, although the system is metallic, as seen by $1/{T}_{1}T=\mathrm{constant}$ behavior.

EuCo2Ge2 with the tetragonal structure is a Eu-divalent antiferromagnet with the Néel temperature TN=23 K. The magnetic easy-axis corresponds to the [100] direction (a-axis), while the [001] direction (c-axis) is a hard-axis. The magnetization for H∥[100] indicates a metamagnetic transition at 25 kOe and saturates above 75 kOe. On the other hand, the hard-axis magnetization increases approximately linearly and saturates above 110 kOe. The magnetic phase diagram was constructed. A characteristic feature in EuCo2Ge2 is known as a valence transition under pressure, from Eu 2+δ to Eu 3−δ′(δ, δ′<1). We also clarified the valence transition by measuring the electrical resistivity under pressure. The valence transition occurs at 3 GPa, with a hysteresis, and terminates at about 4.5 GPa. Further increasing pressure, the electronic state is changed into a moderate heavy fermion state and approaches the nearly trivalent electronic state.

In EuCo2P2, 4f electron spins of Eu2+ ions order antiferromagnetically below a Néel temperature TN=66.5K. The magnetic structure below TN was reported to be helical with the helix axis along the c-axis from the neutron diffraction study. We report the results of 153Eu, 59Co and 31P nuclear magnetic resonance (NMR) measurements on EuCo2P2 using a single crystal and a powdered sample. In the antiferromagnetic (AFM) state, we succeeded in observing 153Eu, 59Co and 31P NMR spectra in zero magnetic field. The sharp 153Eu zero field NMR (ZF NMR) lines indicate homogeneous Eu ordered moment. The 59Co and 31P ZF NMR spectra showed an asymmetric spectral shape, indicating a distribution of the internal magnetic induction at each nuclear position. The AFM propagation vector k characterizing the helical AFM state can be determined from the internal magnetic induction at Co site. We have determined the model-independent value of the AFM propagation vector k distributed from (0, 0, 0.86)2π/c to (0, 0, 0.73)2π/c, where c is the lattice parameter.

We grew high-quality single crystals of AuAl2, AuGa2, and AuIn2 with the fluorite (CaF2)-type cubic structure and determined the Fermi surface properties by the de Haas-van Alphen (dHvA) experiments using full-potential LAPW bad calculations. The Fermi surface and optical properties for three compounds were once studied from an interest of colors because AuAl2 has a striking bright reddish-purple color, whereas AuGa2 and AuIn2 are, respectively, neutral and bluish. The detected dHvA frequencies in the present study are found to be in a wide range of (0.1–13)×107 Oe. The main dHvA branches for three compounds are in excellent agreement with the theoretical ones, but some dHvA branches with small dHvA frequencies are slightly deviated from the theoretical ones, especially in AuGa2 and AuIn2.

We succeeded in growing high-quality single crystals of pyrite-type cubic compounds CoSe2 and CoS2 using a transport agent of CoBr2 and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and the de Haas–van Alphen (dHvA) effect. We confirmed that CoSe2 is an exchange-enhanced paramagnet revealing a broad maximum at around 50 K in the temperature dependence of the magnetic susceptibility. The electronic specific heat coefficient is moderately large, γ = 18 mJ/(K2·mol). On the other hand, CoS2 is a ferromagnet with a Curie temperature TC = 122 K and an ordered moment μs = 0.93 μB/Co. The γ value of 21 mJ/(K2·mol) in CoS2 is slightly larger than that of CoSe2. A large ordered moment, together with a large γ value, is a characteristic feature in CoS2 because CoS2 is a half-metallic spin state in the ferromagnetic state. Correspondingly, we detected a main dHvA branch with a large cyclotron effective mass of 13m0 in the dHvA experiments. The detected dHvA branches in CoS2 and CoSe2 are discussed on the basis of the results of energy band calculations, revealing a broken four-fold-symmetry in the angular dependence of the dHvA frequency.

We have conducted 63,65Cu nuclear quadrupole resonance (NQR) measurements on A-site ordered perovskite compounds LaCu3Ru4O12 and NdCu3Ru4O12 to investigate their ground state and spin fluctuations. While there is only one Cu site in the crystal structure, multiple NQR resonance lines were observed. This is presumed to be due to the presence of slight distortion and lattice defects in the samples. The nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, for LaCu3Ru4O12 showed almost constant value indicating the Fermi-liquid state. A remarkable increase in 1/T1T due to spin fluctuations was observed in NdCu3Ru4O12. Furthermore, an evident magnetic phase transition at TM=0.6K was revealed from the distinct peak of 1/T1T and the broadening of the NQR spectrum.

An extremely large magnetoresistance of PtSn4 has been recently observed and discussed from a viewpoint of de Haas-van Alphen (dHvA) oscillations and theoretical small Fermi surfaces. We have studied precisely the Fermi surfaces by measuring angular dependences of dHvA frequencies and have also carried out the full potential LAPW band calculation. Furthermore, small Fermi surfaces have been detected in another Pt-based compound of Pt3In7 with the cubic structure.

Most of the Eu compounds are in the divalent (Eu2+) electronic state and order magnetically. On the other hand, the Eu-trivalent (Eu3+) compounds exist but are small in number. An energy difference between the Eu2+ and Eu3+ states is, however, not extremely large. Therefore, the valence transition occurs in some Eu compounds. We present the characteristic properties of the Eu compounds from several viewpoints: a canting magnetisation in the Eu2+-antiferromagnets, the Fermi surface property in the Eu3+ compounds of EuPd3 and EuCo2Si2, the heavy fermion state in EuNi2P2, the temperature-induced valence transition in EuPd2Si2, the pressure-induced valence transition in EuRh2Si2 and EuRu2P2, and the heavy fermion state approaching to the quantum critical point with increasing pressure in Eu2Ni3Ge5.

We grew single crystals of Eu-divalent antiferromagnets EuTIn4 (T: Ni, Pd, Pt, Au) by the In-self flux method, with the constitution of Eu:T:In = 1.1:1:30. Single crystals are characteristic in shape, being long along the orthorhombic [100] direction. We measured the electrical resistivity, specific heat, magnetic susceptibility, and de Haas-van Alphen effect for these compounds, together with the electrical resistivity under pressure for EuTIn4 (T: Ni, Pd, Pt). Under pressure, the Eu-divalent electronic state is often changed into the Eu-trivalent state, revealing the valence transition. In the present experiments, the valence transition was, however, not observed even at high pressures up to 8 GPa for these compounds, but a sharp resistivity drop was observed just below the Néel temperature under pressure. This is most likely due to a change of the magnetic structure.

Electrical resistivity ρ and thermopower S of the pseudo-binary compounds of Y1-xPrxCo2 have been measured in the temperature range between 2 and 300 K under magnetic fields up to 10 T, together with the pressure measurements of ρ and S in Y0.4Pr0.6Co2. The Curie temperature decreases with decreasing x, and vanishes at the critical composition xc ≈ 0.4, where the residual resistivity attains a maximum value. The Curie temperature and the residual resistivity of Y0.4Pr0.6Co2 show the same pressure dependence as those of the heavy-rare-earth based compounds. These behaviors of ρ and S indicate the inhomogeneous distribution of the Co 3d magnetization. The magnetoresistance of the light-rare earth Y1-xPrxCo2 system is negative in the whole range of x, except for x = 0 and 1, which is a characteristic behavior related with magnetic state and magnitude of the effective field acting on the Co 3d subsystem.

In order to microscopically investigate the properties in SrGa4, the Ga NMR measurements of a powder sample were carried out. The Ga NMR spectra corresponding to Ga(I) and Ga(II) sites are obtained. The NMR spectra of 69&71Ga (a nuclear spin I = 3/2) in the powder sample of SrGa4 do not take a typical powder pattern caused by the NQR interaction, but take the spectra consisting of three well resolved resonance-lines, which indicates that the nonuniform distribution of crystal orientation in the powder sample occurs because of the magnetic anisotropy. From the analysis of the Ga NMR spectrum, it is found that the ab-plane of the crystal is parallel to the external magnetic field, which would be attributed to the anisotropy of the magnetic susceptibility with the easy axis parallel to the ab-plane. This result is also confirmed by the 69Ga NQR in SrGa4. The Knight shifts of the 69Ga(I) and 69Ga(II) shift slightly to the negative side with decreasing temperature due to the core polarization of the d-electrons. The values of the Knight shift of the 69Ga(I) and 69Ga(II) are 0.01 and –0.11 % at 4.2 K, and 0.09 and –0.08 % at 300 K, respectively. The values of the 1/ T 1 T of the NMR of both 69Ga(I) and 69Ga(II) are almost constant between 4.2 and 100 K, whose values are 1.5 s −1 K −1 at 69Ga(I) and 0.12 s −1 K −1 at 69Ga(II), while the 1/ T 1 T slightly increase above 100K with increasing temperature. The value of T 1 of 69Ga(I) is one order of magnitude less than that of 69Ga(II).