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The magnetic phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ([Formula: see text]SR). The weak pressure dependence for the [Formula: see text] compounds suggests that the rich phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of x at ambient pressure may not solely be attributed to chemical pressure effects. The [Formula: see text] compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at [Formula: see text] kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of magnetic domains is formed above [Formula: see text] kbar where they co-exist with a magnetically disordered state. These domains are likely to be ferromagnetic islands (FMI) and consist of an high- (FMI-[Formula: see text]) and low-temperature (FMI-[Formula: see text]) region, respectively, separated by a phase boundary at [Formula: see text] K. This kind of co-existence is unusual and is originating from a coupling between lattice and magnetic degrees of freedoms.

Novel amorphous iron hydrides (AIHs) are synthesized for the first time under ambient conditions by employing novel “quiescent reaction”, without stirring for mixing solutions, during a conventional aqueous reduction-precipitation process. The kind and morphology of AIHs are dependent on the processing condition, where two types are found, with one form consisting of a tangle of uniform nanowires and the other being granular in nature. Both AIHs undergo transformation to crystalline α-Fe by heat treatment at 600 °C. The nanowire AIH exhibits the hydrogen content of 0.10 wt%, while the granular AIH of 0.22 wt%. Their magnetic and thermal properties are accordingly different, and the non-diffusive hydrogen contributes to stability of AIHs. It is strongly suggested that, by use of quiescent reaction, iron-hydrogen clusters are formed and preserved at an early stage of precipitation reaction, and subsequently aggregated into novel AIHs, preventing α-Fe crystallization. Hence, the AIHs would be categorized as metastable hydrides stabilized with iron-hydrogen clusters. In addition, newly discovered quiescent reaction in aqueous solution, from which unprecedented AIHs are derived, sheds new light on fundamental and essential aqueous reaction.

The magnetic phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ($\mu^+$SR). The weak pressure dependence for the $x\neq1$ compounds suggests that the rich phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of $x$ at ambient pressure may not only be attributed to solely chemical pressure effects. The $x=1$ compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at $p_{\rm c2}\approx9.8$~kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of dilute ferromagnetic islands (FMI) is formed above $p_{\rm c1}\approx8$~kbar where they co-exist with a magnetically disordered state. Moreover, such FMI phase seems to consist of an high- (FMI-\textcircled{\small{1}}) and low-temperature (FMI-\textcircled{\small{2}}) region, respectively, separated by a phase boundary at $T_{\rm i}\approx20$~K., Comment: 10 pages, 8 figures

The ferromagnetic (FM) nature of the metallic LaCo$_2$P$_2$ was investigated with the positive muon spin rotation, relaxation and resonance ($\mu^+$SR) technique. Transverse and zero field $\mu^+$SR measurements revealed that the compound enters a long range FM ground state at $T_{\rm C}=130.91(65)$~K, consistent with previous studies. Based on the reported FM structure, the internal magnetic field was computed at the muon sites, which were predicted with first principles calculations. The computed result agree well with the experimental data. Moreover, although LaCo$_2$P$_2$ is a paramagnet at higher temperatures $T>160$~K, it enters a short range ordered (SRO) magnetic phase for $T_{\rm C}, Comment: 9 pages, 7 figures

Anomalous successive structural transitions in layered 1T-CrSe2 with an unusual Cr4+ valency were investigated by synchrotron X-ray diffraction. 1T-CrSe2 exhibits dramatic structural changes in in- ...

An $A$-type antiferromagnet, ${\mathrm{NaNiO}}_{2}$, was examined by means of positive muon spin rotation and relaxation (${\ensuremath{\mu}}^{+}\mathrm{SR}$) measurements and first-principles calculations based on a density functional theory (DFT). Below ${T}_{\mathrm{N}}=20$ K, a clear muon spin precession signal was observed in the ${\ensuremath{\mu}}^{+}\mathrm{SR}$ time spectrum recorded under zero field, due to the formation of a static internal magnetic field. The microscopic origin of such an internal field was computed as a sum of dipolar and hyperfine contact fields at the site (0.624, 0, 0.854), where both the muon site and the local spin density at such a site were predicted with DFT calculations. While the computed values were consistent with experimentally obtained ones, in both the antiferromagnetic and the paramagnetic states, the contribution of the hyperfine contact field was shown to be insignificant even below ${T}_{\mathrm{N}}$. Finally, measurements at higher temperatures signified thermally activated Na-ion diffusion with ${E}_{\mathrm{a}}=50(20)$ meV and ${D}_{\mathrm{Na}}(300\phantom{\rule{0.16em}{0ex}}\mathrm{K})=8.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}$/s, commonly observed in layered-type compounds.

Significance A unifying mechanism for the origin of ferromagnetism which takes into account both well-known and polar extreme theories, namely, the localized- and itinerant-electron regimes, remains a longstanding mystery. Here, we study a particularly interesting system of FeGa 3− y Ge y , within which the magnetic orderings can be adjusted by the Ge-electron filling control, turning from itinerancy to adequate localized through a ferromagnetic quantum critical transition with increasing y . By involving the effects of the spin fluctuations in general, the theoretical estimations of the magnetic properties at the intermediate state of magnetic orderings formulated in terms of the correlation between itinerancy electrons and the spin fluctuations are in quantitative agreements with experimental observations. Our analysis shows a potential universality to all itinerant ferromagnets.

The physical properties of novel cluster-based triangular lattice antiferromagnets Na3 A2 (MoO4 )2 Mo3 O8 (A=In, Sc), synthesized through a topochemical Na-intercalation to nonmagnetic Na2 A2 (MoO4 )2 Mo3 O8 , are reported. The S=1/2 [Mo3 ]11+ clusters form a regular triangular lattice, which gives the magnetic system a strong geometrical spin frustration effect. Despite the strong antiferromagnetic couplings among [Mo3 ]11+ clusters, they show no long-range magnetic orderings down to 0.5 K with the finite residual magnetic entropy. The ground states of Na3 A2 (MoO4 )2 Mo3 O8 have been characterized as a quantum spin liquid, owing to the strong spin frustration of cluster spins on the triangular lattice.

Recently, intercalation compounds with various intercalants between hexagonal boron nitride (h-BN) layers have been studied, where lithium BN intercalation compound (Li-BNIC) is one of such compounds successfully synthesized. They are expected to exhibit similar properties to lithium graphite intercalation compounds (Li-GICs) that are known as the anode material for lithium ion batteries. It is difficult, however, to apply Li-BNIC for the batteries due to its returning to an insulator when Li is deintercalated. In this study a Li–BN–graphite ternary system has been focused because it is reported that graphite-like BC2N is a promising material for rechargeable Li batteries. The primary purpose of this study is thus to investigate combined reactivity of BN and graphite with Li through milling and heating processes, and possible intercalation of Li into the matrix: h-BN, graphite or B C N. The pieces of lithium metal, h-BN and graphite powders were ball-milled using a vibratory ball-mill machine and heat-treated at 700 °C for 2 h under argon atmosphere. The samples were then characterized by X-ray diffractometry, X-ray photoelectron spectroscopy, 7Li nuclear magnetic resonance and Differential thermal analysis study. Li-GICs were mainly produced by milling, while post-annealing caused their eliminations and instead produced Li-BNICs with small amount of other lithium compounds. In terms of thermal stability, Li-BNIC is more stable than Li-GICs. In a Li–BN–graphite system, an activation energy of Li-BNIC was estimated to be 119.6 kJ/mol, which is higher than reported activation energies for Li-GICs.

We grew large single crystals of the cluster magnet Nb3Cl8 with a magnetic triangular lattice and investigated its magnetic properties and crystal structure. In Nb3Cl8, the [Nb3]8+ cluster has a single unpaired spin, making it an S = 1/2 triangular lattice anti-ferromagnet. At low temperatures, Nb3Cl8 exhibits a magnetic-nonmagnetic phase transition driven by a charge disproportionation, in which the paramagnetic [Nb3]8+ clusters transform into alternating layers of nonmagnetic [Nb3]7+ and [Nb3]9+ clusters. The observed exotic phenomenon with the strong correlation between the magnetism and structure are based on the nature of the cluster magnetism.

We investigated the magnetic properties of the ilmenite-type manganates $\mathrm{MgMn}{\mathrm{O}}_{3}$ and $\mathrm{ZnMn}{\mathrm{O}}_{3}$, both of which are composed of a honeycomb lattice of magnetic $\mathrm{M}{\mathrm{n}}^{4+}$ ions. Both compounds show antiferromagnetic order with weak ferromagnetic moments. In particular, $\mathrm{MgMn}{\mathrm{O}}_{3}$ exhibits a magnetization ``reversal'' behavior which can be described by the $N$-type ferrimagnetism in the N\'eel's classification. The relationship between the magnetic properties and the crystal and magnetic structures probed by the neutron diffraction experiments indicates that the two honeycomb lattice magnets have different ${J}_{1}\ensuremath{-}{J}_{2}$ parameter sets, placing them in the distinct regions in the phase diagram; both nearest neighbor (NN) and next nearest neighbor (NNN) exchange interactions are antiferromagnetic in $\mathrm{MgMn}{\mathrm{O}}_{3}$, while NN and NNN interactions become ferromagnetic and antiferromagnet, respectively, in $\mathrm{ZnMn}{\mathrm{O}}_{3}$.

We synthesized a kagome fluoride ${\mathrm{Cs}}_{2}{\mathrm{LiTi}}_{3}{\mathrm{F}}_{12}$ with $S=1/2$ spins, and studied magnetic properties of the compound. The temperature dependence of the magnetic susceptibility indicates that it has dominant antiferromagnetic interactions and that it has no magnetic order down to 2 K. We found two magnetization plateaus in its magnetization process approximately at 1/3 and 0.8 ${\ensuremath{\mu}}_{\mathrm{B}}$ per Ti. The monoclinic crystal structure gives four inequivalent nearest-neighbor exchange interactions. Our density functional theory calculations suggest that three of them are antiferromagnetic and one of them is weakly ferromagnetic, resulting in a magnetic system composed of antiferromagnetically coupled linear chains and $\mathrm{\ensuremath{\Delta}}$ chains. This explains the observed suppression of magnetic order. Numerical diagonalization gives a magnetization curve in good agreement with the experimental results.

Anomalous successive structural transitions in layered 1

Hexagonal boron nitride (h-BN) exhibits a layered solid structure such as graphite, and the h-BN intercalation compounds (BNICs) have been actively studied as with graphite intercalation compounds (GICs). It is difficult, however, to synthesize BNICs compared with GICs, where the synthesis of BNICs requires high temperature and/or pressure. In this study, the Li-BNIC is synthesized by ball milling and heat treatment, and the samples were characterized by X-ray diffractomety (XRD), differential thermal analysis (DTA) and 7 Li NMR (nuclear magnetic resonance) study. New XRD peaks corresponding to Li-BNICs were observed from the milled sample after heat treating at 700 °C for 2 h. The shifted peaks of h-BN to the lower angles imply an expanded BN lattice through Li insertion. The exothermic DTA peaks observed between 200 and 500 °C might be the synthesis temperatures of Li-BNIC. The NMR spectra of the Li-BNIC are modified according to the ratio of Li to BN, although the corresponding XRD patterns are not changed.

To specify preferential occupation sites of Co substituents and to clarify charge and spin states of Co ions in (La, Co)-cosubstituted hexagonal magnetoplumbite-type (M-type) Sr ferrite ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Fe}}_{12\ensuremath{-}y}{\mathrm{Co}}_{y}{\mathrm{O}}_{19}$ ($x,y\ensuremath{\le}0.4$), $^{57}\mathrm{Fe}$ and $^{59}\mathrm{Co}$ nuclear magnetic resonance (NMR) spectra are measured under zero and external magnetic fields using powdered and single crystalline specimens. For comparison, NMR investigations of nondoped and La- or Co-doped M-type Sr ferrites are also performed. Ferrimagnetic M-type Sr ferrite contains the following five crystallographic Fe sites: the majority spin sites $12k$, $2a$, and $2b$, and the minority spin sites $4{f}_{1}$ and $4{f}_{2}$. Based on $^{57}\mathrm{Fe}$ and $^{59}\mathrm{Co}$ NMR, a plausible model of (La, Co)-codoped Sr ferrite is deduced. To a considerable degree, the charge compensation between ${\mathrm{La}}^{3+}$ and ${\mathrm{Co}}^{2+}$ works in the equal (La, Co)-codoped case, where more than half of the Co ions are considered to be present in the minority spin $4{f}_{1}$ sites at the center of the oxygen tetrahedra, with the $S=3/2$ state carrying a small orbital moment owing to spin-orbit interaction. The remaining small number of high-spin ${\mathrm{Co}}^{2+}$ ($S=3/2$, $L=1)$ ions with unquenched orbital moments would be distributed to the other octahedral $12k$, $2a$, and $4{f}_{2}$ sites.

We have investigated the structural and magnetic properties of $S=3/2\phantom{\rule{4pt}{0ex}}{\mathrm{Cr}}^{3+}$ Heisenberg-like kagome lattice antiferromagnets ${\mathrm{Cs}}_{2}{\mathrm{K}}_{}{\mathrm{Cr}}_{3}{\mathrm{F}}_{12}$ and ${\mathrm{Cs}}_{2}{\mathrm{Na}}_{}{\mathrm{Cr}}_{3}{\mathrm{F}}_{12}$ using single crystals. Each compound has a slightly distorted kagome lattice, although the space groups differ for the two compounds. The magnetic data suggest that each compound exhibits the formation of antiferromagnetic ordering with in-plane-spin canting approximately below 6 K. The in-plane coplanar structures are clearly different from disordered ground states of the isostructural ${\mathrm{Ti}}^{3+}$ compounds and out-of-plane coplanar structures of the ${\mathrm{V}}^{3+}$ compounds, which is mainly caused by the differences in the strength of quantum fluctuation and the strength of magnetic anisotropy for these three spin systems. In contrast, as well as the ${\mathrm{Ti}}^{3+}$ and ${\mathrm{V}}^{3+}$ compounds, the magnetization curves show 1/3-magnetization-plateaulike anomalies.

We have successfully synthesized the novel antiferromagnets with Ir$^{4+}$ honeycomb lattice ZnIrO$_3$ and MgIrO$_3$ and investigated their magnetic and thermodynamic properties. The two iridates are isomorphic but exhibit qualitatively different magnetic properties. ZnIrO$_3$ shows antiferromagnetic ordering below 46.6 K, whereas MgIrO$_3$ displays weak ferromagnetic behavior below 31.8 K owing to formation of a canted antiferromagnetic ordering. The measurement of magnetic susceptibility with using an oriented powder sample revealed the presence of an XY-like magnetic anisotropy and a tilting magnetic structure which is possibly stabilized by the Kitaev interaction. Moreover, magnetization curves of MgIrO3 and ZnIrO3 up to 60 T show different behaviors, demonstrating that each magnetic ground state is different with each other. We discuss the difference in the ground state between MgIrO$_3$ and ZnIrO$_3$ from the viewpoint a magnetic model consisting of the Kitaev and Dzyaloshinskii-Moriya interactions with the spin frustration effect on the honeycomb lattice.

The physical properties of the mixed-valent iron oxide $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{NaFe}}_{2}{\mathrm{O}}_{3}$ were investigated by means of synchrotron radiation x-ray diffraction, magnetization, electrical resistivity, differential scanning calorimetry, $^{23}\mathrm{Na}$ NMR, and $^{57}\mathrm{Fe}\phantom{\rule{4pt}{0ex}}\mathrm{M}\stackrel{\ifmmode \ddot{}\else \"{}\fi{}}{\mathrm{o}}\mathrm{ssbauer}$ measurements. This compound has double triangular layers consisting of almost perfect regular ${\mathrm{Fe}}_{4}$ tetrahedra, which suggests geometrical frustration. We found that this compound exhibits an electrostatically unstable double-stripe-type charge ordering, which is stabilized by the cooperative compression of ${\mathrm{Fe}}^{3+}{\mathrm{O}}_{6}$ octahedra, owing to a valence change and ${\mathrm{Fe}}^{2+}{\mathrm{O}}_{6}$ octahedra due to Jahn-Teller distortion. Our results indicate the importance of electron-phonon coupling for charge ordering in the region of strong charge frustration.

A unifying mechanism for the origin of ferromagnetism which takes into account both well-known and polar extreme theories, namely, the localized- and itinerant-electron regimes, remains a longstanding mystery. Here, we study a particularly interesting system of FeGa3−yGey, within which the magnetic orderings can be adjusted by the Ge-electron filling control, turning from itinerancy to adequate localized through a ferromagnetic quantum critical transition with increasing y. By involving the effects of the spin fluctuations in general, the theoretical estimations of the magnetic properties at the intermediate state of magnetic orderings formulated in terms of the correlation between itinerancy electrons and the spin fluctuations are in quantitative agreements with experimental observations. Our analysis shows a potential universality to all itinerant ferromagnets.

QC 20211215

QC 20211124

The magnetic properties of a melt-spun Cu 60 Au 35 Fe 5 alloy are examined by X-ray diffraction (XRD), magnetic measurements, and transmission electron microscopy (TEM). The XRD pattern of an as-spun sample indicates the formation of a solid solution of Cu and Au with no Fe phase, whereas that of the annealed sample (773 K × 36 ks) shows a small amount of the α-Fe phase because the Fe atoms precipitate during annealing. TEM observations and EDX analysis of the as-spun sample show no distinct Fe-rich domains, implying that some Fe atoms are dissolved in the matrix, and others form very fine magnetic clusters. Moreover, magnetic measurements indicate the presence of magnetic clusters and isolated Fe atoms. The AC magnetic susceptibilities show that the as-spun sample has a sharp cusp around 20 K, which is characteristic of spin glass behavior. The simulations of the M – H curves of the as-spun sample measured at 77 and 300 K, show that most Fe atoms are dissolved in the matrix as isolated atoms, not as clusters. Additionally, the calculated rate of clustering in the as-spun sample tends to decrease with temperature, suggesting that the magnetic exchange interactions are weak.

The magnetic properties of a melt-spun Fe-dilute Cu 60 Ag 35 Fe 5 alloy are examined by X-ray diffraction, magnetic measurements, and transmission electron microscopy (TEM). The X-ray diffraction patterns show that the as-spun and annealed (773 K×36 ks) samples contain Cu and Ag phases and no Fe phases; thus, most Fe atoms are dispersed as clusters. Magnetic measurements indicate that the as-spun and annealed samples exhibit superparamagnetic behavior at 300 K, whereas ferromagnetic and superparamagnetic behaviors coexist at 4.2 K. The magnetic moments of small clusters at 300 K are determined by the nonlinear least squares method as 5148 and 4671 μ B for as-spun and annealed samples, respectively, whereas those at 300 K are experimentally determined as 3500 and 3200 μ B . This decrease in magnetic moments may imply the formation of anti-ferromagnetic coupling by annealing. TEM observation of the melt-spun sample suggests that there are three regions with different compositions: Cu-rich, Ag-rich, and Fe-rich with no precipitation in the matrix. In addition, these regions have obscure interfaces. The magnetic clusters are attributed to the Fe-rich regions.

In order to elucidate the dependence of the magnetic ground state on the Ca content (x) in Sr1-xCaxCo2P2 (0 ≤x ≤1, ThCr2Si2-type structure), we have performed muon spin rotation− and relaxation (μ+SR) experiments on Sr1-xCax Co2P2 powder samples mainly in a zero applied field. The end member compound, SrCo2P2, is found to be paramagnetic down to 19 mK. As x increases, such a paramagnetic ground state is observed down to 1.8K until x = 0.45. Then, as x increases further, a short-range antiferromagnetic (AF) ordered phase appears at low temperatures for 0.48 ≤ x ≤ 0.75, and finally, a long-range AF ordered phase is stabilized for x > 0.75. The internal magnetic field of the other end member compound, CaCo2P2, is well consistent with that of the A-type AF order state, which was proposed from neutron scattering experiments. The phase diagram determined with μ+SR is different from that proposed by macroscopic measurements. For an isostructural compound, LaCo2P2, static magnetic order is found to be formed below

The ThCr2Si2-type layered compound SrCo2P2 shows an itinerant-electron metamagnetic transition at high magnetic field. To investigate substitution effects on the itinerant metamagnetic transition, we synthesized Sr[1-x]LaxCo2P2 and SrCo2(P[1-x]Gex)2 and measured their magnetic properties, including magnetizations under pulsed high magnetic fields. We have revealed a strong x dependence of the metamagnetic transition which is consistent with the band theory describing the physical properties in metals., 20th International Conference on Magnetism, ICM 2015

The physical properties of novel cluster-based triangular lattice antiferromagnets Na

We have systematically investigated the magnetic properties of ${\mathrm{La}}_{}$- and ${\mathrm{Co}}_{}$-substituted ${\mathrm{Sr}}_{}{\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ using single crystals. By utilizing the traveling solvent floating zone technique, we have grown single crystals over a wide range of substitution content, and examined their saturation moments and magnetocrystalline anisotropy. With increasing content of ${\mathrm{La}}_{}$ and ${\mathrm{Co}}_{}$, the saturation moment at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$ monotonically increases. The increment of magnetocrystalline anisotropy is almost proportional to the content of Co. In addition, further La substitution also enhances magnetocrystalline anisotropy, which demonstrates that ${\mathrm{Fe}}^{2+}$ plays an important role to enhance magnetocrystalline anisotropy. We have analyzed magnetocrystalline anisotropy energy as a function of substitution content, and elucidate the effects of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Fe}}^{2+}$.

We have investigated crystal structure and magnetic properties of three $S=1\phantom{\rule{4pt}{0ex}}{\mathrm{V}}^{3+}$ Ising-like anisotropic kagome lattice antiferromagnets, ${\mathrm{Cs}}_{2}\mathrm{K}{\mathrm{V}}_{3}{\mathrm{F}}_{12},{\mathrm{Cs}}_{2}\mathrm{Na}{\mathrm{V}}_{3}{\mathrm{F}}_{12}$, and ${\mathrm{Rb}}_{2}\mathrm{Na}{\mathrm{V}}_{3}{\mathrm{F}}_{12}$, using single crystals. Each compound crystallizes in a monoclinic system and has a slightly distorted kagome lattice. Although the magnetic properties are similar, the details depend on the magnitude of magnetic anisotropy. The magnetization and magnetic susceptibility are highly anisotropic owing to anisotropic $g$ factors and Ising-like single ion anisotropy originating from partially unquenched orbital moments of ${\mathrm{V}}^{3+}$, and the easy axis of Ising-like anisotropy is perpendicular to the kagome plane. In contrast to disordered ground states in the isostructural ${\mathrm{Ti}}^{3+}$ $(S=1/2)$ compounds, the ${\mathrm{V}}^{3+}$ $(S=1)$ compounds exhibit antiferromagnetic ordering with distorted ${120}^{\ensuremath{\circ}}$ structure or other nearly coplanar magnetic structure whose basal spin plane includes the easy axis, which is mainly due to large magnetic anisotropy and decrease of quantum fluctuation. Moreover, when magnetic fields are applied perpendicular to the kagome plane, magnetization curves of them show distinct 1/3 and 2/3 magnetization plateaus, which are stabilized by Ising-like single ion anisotropy. In particular, the 2/3 magnetization plateaus are notable magnetic phenomena in kagome lattice antiferromagnets and are due to the combination of spin frustration, Ising-like single ion anisotropy, and the small monoclinic distortion of kagome lattices.

We grew large single crystals of the cluster magnet Nb

We took $^{31}\text{P}$ NMR measurements of mainly paramagnetic phase ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Co}}_{2}{\mathrm{P}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.5$) to reveal the itinerant-electron metamagnetic transition, and of its magnetically ordered phase ($0.7\ensuremath{\le}x\ensuremath{\le}1$), and characterized their spin fluctuations by estimating the spin fluctuation parameter ${T}_{0}$ corresponding to the width of the spin fluctuation in the spectrum in frequency space. ${\mathrm{SrCo}}_{2}{\mathrm{P}}_{2}$ has a quasi-two-dimensional uncollapsed tetragonal (ucT) cell without interlayer P-P bonds, whereas ${\mathrm{CaCo}}_{2}{\mathrm{P}}_{2}$ has a three-dimensional collapsed tetragonal (cT) cell with P-P bonds. The $ab$-in-plane component of ${T}_{0}$ is much larger than the out-of-plane component in ${\mathrm{SrCo}}_{2}{\mathrm{P}}_{2}$. As $x$ increases from 0 to 0.5, the in-plane component of ${T}_{0}$ decreases proportionally with the metamagnetic transition field. In the antiferromagnetic cT phase ($0.7\ensuremath{\le}x\ensuremath{\le}1$), ${T}_{0}$ is constant and spin fluctuations show an isotropic character in contrast to their behavior in the paramagnetic ucT phase ($0\ensuremath{\le}x\ensuremath{\le}0.5$). These results indicate that the in-plane spin fluctuations due to the quasi-two-dimensional crystal structure play a significant role in the metamagnetic transition of this system.

We have investigated the crystal structure and magnetic properties of three kagome lattice antiferromagnets, ${\mathrm{Rb}}_{2}\mathrm{Na}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}, {\mathrm{Cs}}_{2}\mathrm{Na}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$, and ${\mathrm{Cs}}_{2}\mathrm{K}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$, using single crystals. These compounds represent a $S=1/2$ kagome system consisting of magnetic ${\mathrm{Ti}}^{3+}$ ions, which is expected to have negligibly small Dzyaloshinsky-Moriya interaction. The structural analyses revealed that each of the three compounds has a slightly distorted kagome lattice. The distortion of the kagome lattice becomes small as the ionic radii of constituent alkali metals increase. All three compounds have nearly the same Weiss temperature of $\ensuremath{-}45$ K, and the ground states are disordered and strongly depend on the distortion. The ground states of ${\mathrm{Rb}}_{2}\mathrm{Na}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}, {\mathrm{Cs}}_{2}\mathrm{Na}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$, and ${\mathrm{Cs}}_{2}\mathrm{K}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$ are found to be a two-component state including approximately 1/3 nearly free spins, a gapless disordered state, and a gapped disordered state, respectively. Our experimental results suggest that the ground state of the ideal $S=1/2$ Heisenberg kagome lattice antiferromagnet is gapped. In addition, the magnetization curves of ${\mathrm{Cs}}_{2}\mathrm{Na}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$ and ${\mathrm{Cs}}_{2}\mathrm{K}{\mathrm{Ti}}_{3}{\mathrm{F}}_{12}$ show anomalies at approximately 1/3 of the full magnetic moment of ${\mathrm{Ti}}^{3+}$, which are a notable observation of signs of the theoretically proposed 1/3 magnetization plateau in $S=1/2$ kagome antiferromagnets.

Physical properties of new S = 3/2 triangular-lattice compounds LiCrSe2, LiCrTe2, and NaCrTe2 have been investigated by X-ray diffraction and magnetic measurements. These compounds crystallize in the ordered NiAs-type structure, where alkali metal ions and Cr atoms stack alternately. Despite their isomorphic structures, magnetic properties of these three compounds are different; NaCrTe2 has an A-type spin structure with ferromagnetic layers, LiCrTe2 is likely to exhibit a helical spin structure, and LiCrSe2 shows a first-order-like phase transition from the paramagnetic trigonal phase to the antiferromagnetic monoclinic phase. In these compounds and the other chromium chalcogenides with a triangular lattice, we found a general relationship between the Curie-Weiss temperature and magnetic structures. This relation indicates that the competition between the antiferromagnetic direct d-d exchange interaction and the ferromagnetic superexchange interaction plays an important role in determining the ground state of chromium chalcogenides.

We synthesized polycrystalline samples of the fluorine-substituted pyrochlore rhenates Cd2Re2O7-x F x , and investigated their magnetic, transport and structural properties. The transition temperature T s1, where each Re4 tetrahedron in the Re pyrochlore network alternately expands and contracts, decreases with increasing x from 200 K at x = 0 to 100 K at x = 0.5. The strong x dependence of the magnetic and transport properties at the low-temperature phase indicates that the driving force of structural phase transition is fluctuations of the tetramer spin singlet formation in order to release the spin frustration in the pyrochlore lattice. Furthermore, we found unconventional superconducting properties in Cd2Re2O7-x F x . It was found that the superconducting phase transition temperature T c markedly decreases with increasing x, suggesting that the addition of imperfection suppresses a condensation of Cooper-pair. In addition, the estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit and increases with increasing x in spite of the reduction of T c. Hence, Cd2Re2O7-x F x is suggested to be an exotic superconductor realized in the itinerant electron systems on a spin frustrated lattice.

The magnetic nature of a metallic two-dimensional triangular compound, ${\mathrm{CrSe}}_{2}$, has been investigated by muon spin rotation and relaxation ($\ensuremath{\mu}^{+}\mathrm{SR}$) measurements using both powder and single crystal samples. It is found that ${\mathrm{CrSe}}_{2}$ enters into a static antiferromagnetic (AF) ordered state below 157 K ($={T}_{\mathrm{N}}$). Furthermore, the AF state is slightly changed below around 20 K ($={T}_{\mathrm{N}2}$). Based on the analysis of the internal magnetic fields at the muon sites predicted with DFT calculations, collinear AF and helical ${120}^{\ensuremath{\circ}}$ AF are clearly eliminated for the ground state of ${\mathrm{CrSe}}_{2}$. The most probable one is an incommensurate spin density wave order.

The Curie temperature TC of the weakly ferromagnetic system FeGa3−yGey is investigated by means of isothermal magnetization experiments. The critical temperature of the ferromagnetic phase transition is estimated by the so-called M4 plots rather than the Arrott plots technique. The M4 plots are exploited from the sixth power term of the expanded free energy, which is neglected by the Arrott plots. The magnetic properties are analyzed by taking into account the effects of spin fluctuations in terms of related spin-fluctuation theories.

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

We synthesized the novel $S=1/2{\mathrm{Mo}}_{3}$ cluster magnet ${\mathrm{Li}}_{2}{\mathrm{ScMo}}_{3}{\mathrm{O}}_{8}$ as well as the isomorphic compound ${\mathrm{Li}}_{2}{\mathrm{InMo}}_{3}{\mathrm{O}}_{8}$, and investigated their macroscopic and microscopic physical properties. ${\mathrm{Li}}_{2}{\mathrm{InMo}}_{3}{\mathrm{O}}_{8}$ showed magnetic ordering at ${T}_{N}\ensuremath{\sim}12$ K with a ${120}^{\ensuremath{\circ}}$ structure, while no magnetic ordering is observed down to 0.5 K in ${\mathrm{Li}}_{2}{\mathrm{ScMo}}_{3}{\mathrm{O}}_{8}$ despite the strong antiferromagnetic interaction among clusters probed by the Weiss temperature. Results of the $^{7}\mathrm{Li}$ nuclear magnetic resonance spectra and the nuclear spin-lattice relaxation rate $(1/{T}_{1})$ measurement show direct microscopic proof of the spin-disordered ground state. We discuss the difference in the ground state between ${\mathrm{Li}}_{2}{\mathrm{ScMo}}_{3}{\mathrm{O}}_{8}$ and ${\mathrm{Li}}_{2}{\mathrm{InMo}}_{3}{\mathrm{O}}_{8}$ using the description of localized magnetism based on spin frustration within the triangular lattice and that of inter- and intracluster charge fluctuations.

We synthesized single crystals of an itinerant ferromagnet ${\mathrm{LaCo}}_{2}{\mathrm{P}}_{2}$ with ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure and studied their magnetism by magnetization and ${}^{31}\mathrm{P}$ NMR measurements. We measured Knight shift $K$ and spin-lattice relaxation rate divided by temperature $1/{T}_{1}T$ with the applied fields parallel to the $a$ and $c$ axes, and estimated spin fluctuations in the $ab$ plane and $c$. In addition, we evaluated spin fluctuations from the result of magnetization data with a three-dimensional ferromagnetic model. There is little anisotropy in evaluated spin fluctuations in the $ab$ plane and $c$. Spin fluctuations of ${\mathrm{LaCo}}_{2}{\mathrm{P}}_{2}$ have a three-dimensional character and can be understood in the framework of the self-consistent renormalization theory of spin fluctuations.

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

We investigated macroscopic and microscopic physical properties of S = 1/2 Mo3 cluster magnets Li2ScMo3O8 and Li2InMo3O8. Li2InMo3O8 shows magnetic ordering at TN 12 K, while no magnetic ordering is observed down to 0.5 K in Li2ScMo3O8 in spite of the strong antiferromagnetic interaction among clusters probed by the Weiss temperature. 115In and 45Sc NMR measurements demonstrate that there should be difference in charge fluctuation in clusters between Li2InMo3O8 and Li2ScMo3O8. Our result suggests that the origin of different ground states realized in Li2InMo3O8 and Li2ScMo3O8 is different condition in charge and local structure as one of characteristics of the cluster magnet.