We observed superconductivity ($T_{c}$ $simeq$2-3 K) in Li$_{x}$RhB$_{y}$ intermetallics wherein $x$ and $y$ vary over a wide compositional range. The crystal structure consists of cubic unit-cell ($a$ $simeq$ 12.1 AA ) with centro-symmetric space gr
oup $Pnbar{3}n$. A weak but positive pressure-induced increase of $T_{c}$ was observed. The correlations between the composition and each of the followings were followed over a wide range of $x$ and $y$: the unit-cell dimensions, $T_{c}$ , Sommerfeld coefficient $gamma$, Debye temperature $theta_{D}$, and critical fields $H$$_{c1}$ and $H$$_{c2}$. The thermal evolution of the electronic specific heat within the superconducting phase was observed to follow a quadratic-in-$T$ behavior. In addition, a paramagnetic Meissner Effect (PME) is manifested during a low-field-cooled magnetization cycle. This manifestation of quadratic-in-$T$ behavior and PME feature will be discussed.
The magnetic properties of polycrystalline Tb(Co_{x}Ni_{1-x})_{2}B_{2}C (x=0.2,0.4,0.6,0.8) samples were probed by magnetization, specific heat, ac susceptibility, and resistivity techniques. For x{ eq}0.4, the obtained curves are consistent with the
features expected for the corresponding magnetic modes, namely k_{1}=(0.55,0,0) at x=0; k_{2}=([nicefrac] icefrac{1}{2}</LaTeX>,0,[nicefrac]<LaTeX> icefrac{1}{2}) at x= 0.2; k_{3}=(0,0,[nicefrac] icefrac{1}{3}) at x= 0.6, and k_{4}=(0,0,0) at x= 0.8 and 1. For x=0.4, even though the neutron diffraction indicates a k_{2} mode, but with a reduced magnetic moment, the magnetization, the ac susceptibility, and resistivity indicate two magnetic events; furthermore, deviation from Curie-Weiss behavior is observed below 150 K for this sample. These features, together with the evolution of both magnetic moment and critical temperature, are attributed to an interplay between competing magnetic couplings; for the particular x=0.4 case, additional factors such as crystalline electric field effects may be in operation.
Neutron diffraction and thermodynamics techniques were used to probe the evolution of the magnetic properties of Tb(Co_{x}Ni_{1-x})_{2}B_{2}C. A succession of magnetic modes was observed as x is varied: the longitudinal modulated k=(0.55,0,0) state a
t x=0 is transformed into a collinear k=([nicefrac]<LaTeX> icefrac{1}{2}</LaTeX>,0,[nicefrac]<LaTeX> icefrac{1}{2}</LaTeX>) antiferromagnetic state at x= 0.2, 0.4; then into a transverse c-axis modulated k=(0,0,[nicefrac]<LaTeX> icefrac{1}{3}</LaTeX>) mode at x= 0.6, and finally into a simple ferromagnetic structure at x= 0.8 and 1. Concomitantly, the low-temperature orthorhombic distortion of the tetragonal unit cell at x=0 is reduced smoothly such that for x >= 0.4 only a tetragonal unit cell is manifested. Though predicted theoretically earlier, this is the first observation of the k=(0,0,[nicefrac]<LaTeX> icefrac{1}{3}</LaTeX>) mode in borocarbides; our findings of a succession of magnetic modes upon increasing x also find support from a recently proposed theoretical model. The implication of these findings and their interpretation on the magnetic structure of the RM_{2}B_{2}C series are also discussed.
In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 cdot yH_2O, Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on thre
e different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c-axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate c-axis length and the NQR frequency.
The magnetic structures of the title compounds have been studied by neutron diffraction. In contrast to the isomorphous RNi2B2C compounds wherein a variety of exotic incommensurate modulated structures has been observed, the magnetic structure of ErC
o2B2C is found to be collinear antiferromagnet with k=((1/2),0,(1/2)) while that of HoCo2B2C and DyCo2B2C are observed to be simple ferromagnets. For all studied compounds, the moments are found to be confined within the basal plane and their magnitudes are in good agreement with the values obtained from the low-temperature isothermal magnetization measurements. The absence of modulated magnetic structures in the RCo2B2C series (for ErCo2B2C, verified down to 50 mK) is attributed to the quenching of the Fermi surface nesting features.
The borocarbides RNi2B2C (R=Gd, Ho, Er) exhibit a large variety of magnetic states and as a consequence rich phase diagrams. We have analyzed the nature of these states by specific heat investigations. The data were measured down to 0.5 K and up to 8
0 kOe. The overall evolution of each Cm(T,H) curve is observed to reflect faithfully the features of the corresponding H-T phase diagram. Within the lower ranges of temperature and fields, the calculations based on linearized field-dependent spin-wave theory are found to reproduce satisfactorily the measured Cm(T,H) curves: accordingly, within these ranges, the thermodynamical properties of these compounds can be rationalized in terms of only two parameters: the spin-wave energy gap and the stiffness coefficient. For the intermediate fields ranges (H1<H<Hsat) wherein successive field-induced metamagnetic modes are stabilized, the evolution of Cm(T,H) is discussed in terms of the Maxwell relation (dCm/dH)T=T(d^2M/dT^2)H. For the particular case of GdNi2B2C wherein the anisotropy is dictated by the classical dipole interaction, Cm(T,H) across the whole ordered state is numerically evaluated within the model of Jensen and Rotter [PRB 77 (2008) 134408].
We have performed $^{69,71}$Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) and muon spin rotation/resonance on the quasi two-dimensional antiferromagnet (AFM) NiGa$_2$S$_4$, in order to investigate its spin dynamics and ma
gnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa$_2$S$_4$, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the novel spin freezing occurring at $T_{rm f}$, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate $1/T_{1}$ and nuclear spin-spin relaxation rate $1/T_{2}$ measured by Ga-NQR as well as the muon spin relaxation rate $lambda$. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in NiGa$_2$S$_4$. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards $T_{rm f}$, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multi-component $T_1$ were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and $T_{rm f}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below $T_{rm f}$, but keep fluctuating down to 2 K with the MHz frequency range.
The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in bo
th the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.
