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NMR measurements of 121 Sb and 123 Sb nuclei have been performed in a heavy-fermion compound YbSb to investigate a phase transition around 5 K from the microscopic point of view. The nuclear magnetic relaxation rate ( T 1 -1 ) shows a distinct increase around the transition temperature. The isotope ratio 123 ( T 1 -1 ) / 121 ( T 1 -1 ) shows that the increase of the relaxation rate is mainly ascribed to fluctuations of the electric field gradient at Sb sites. This supports the proposal that the phase transition around 5 K is due to quadrupolar ordering. A large broadening of the NMR spectra is observed below the transition temperature, furthermore, the width continues to increase anomalously when the temperature is decreased, in contrast with usual magnetic orderings. This anomalous temperature dependence of the width is well explained by calculations using the molecular field approximation including the crystal field potential and intersite quadrupolar interactions.

The jarosite-type compounds, KFe 3 (OH) 6 (SO 4 ) 2 and KCr 3 (OH) 6 (SO 4 ) 2 , are model materials of the frustrated antiferromagnet on the kagome lattice. We have synthesized new Cr-jarosites, A Cr 3 (OH) 6 (SO 4 ) 2 , with other monovalent cations A + [ A = Rb, NH 4 , Na] and have measured the magnetization between 2 K and 300 K. The measured susceptibilities above 20 K agree well with the calculation based on the high-temperature expansion and with the Monte Carlo simulation for the two-dimensional Heisenberg antiferromagnet on the kagome lattice, which indicates that the new Cr-jarosites are good examples of the Heisenberg kagome lattice antiferromagnets with S = 3/2. The exchange interactions for these Cr-jarosites are estimated to be about 5 K. The susceptibilities of the Cr-jarosites with K, Rb, and NH 4 increase steeply below about 4 or 8 K, although they are antiferromagnets. The susceptibilities after zero-field-cooling and those after field-cooling are different below this temperature, and th...

Nuclear magnetic resonance and relaxation studies on 75 As in an Yb-monopnictide YbAs have been carried out at temperatures down to 20 mK to clarify the magnetic properties of this compound from a microscopic point of view. A notable result is that the spin-lattice relaxation rate 1/ T 1 shows Fermi-liquid-like behavior given as ( T 1 T ) -1 =2.9 (s K) -1 below 0.2 K. The constant value of ( T 1 T ) -1 in YbAs is discussed using the Korringa relation for a single impurity model to compare with those in other heavy Fermion compounds. The NMR spectra between 0.4 K and 0.6 K indicate that the paramagnetic and the antiferromagnetically ordered regions coexist. From these results, it is found that YbAs is a heavy Fermion compound which undergoes an antiferromagnetic transition.

The ordering process in the Ising-like Heisenberg antiferromagnet on the hexagonal lattice has been studied through the 133 Cs NMR in CsNiBr 3 . The observed local field at 133 Cs was 3.5 times as large as the calculated field from the point dipoles at the Ni 2+ sites. The limited applicability of the conventional point dipole model has been overcome by treating the symmetry property at the 133 Cs site. Under the applied field of 4.6 T, we have found that the Ni 2+ spin component parallel to the c -axis orders below T N1 =15.0 K, whereas the perpendicular component orders below T N2 =12.5 K to form the triangular three-sublattices in the c-plane: one spin is parallel to the c -axis and the other two are canted ± 38° from the c -axis. The comparison is made with the existing data of CsNiCl 3 with an emphasis on the dynamical behavior of the triangular spin arrangement.

Proton spin-lattice relaxation time T 1 in the Haldane-gap system with S =1, Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ), was measured at temperatures between 1.4 K and 77 K. The relaxation rate T 1 -1 decreased over more than three orders of magnitude with decreasing temperature, thus suggesting scarce development of the antiferromagnetic short-range order. Monotonous decrease of T 1 -1 above 6 K was analyzed on the basis of the theory for insulating paramagnets using static susceptibility data. To explain the behavior of T 1 -1 below 6 K, which accompanies a rounded peak, we consider the effect of local-field fluctuation of the order of the nuclear Larmor period, which is associated with magnetic excitation from the nonmagnetic ground state.

Successive magnetic phase transitions were found in the Kagome lattice antiferromagnets RFe 3 (OH) 6 (SO 4 ) 2 [ R=NH 4 , Na, K]. The temperature dependence of the susceptibility has two cusps around 60 K for each compound. The X-ray diffraction patterns and the quadrupole resonance spectra show that the transitions are not caused by a structural change. The presence of these transitions was confirmed by NMR experiments. The NMR spectra show the development of magnetic ordering below the upper transition temperature. In the low-temperature phase the nuclear magnetization does not relax exponentially, which may suggest a domain structure of 120° spin structure due to degeneracy on the Kagome lattice. The successive transitions may be caused by a small Ising-like anisotropy in the Heisenberg antiferromagnet.

We measured the nuclear magnetic spin–lattice ( T 1 -1 ) and spin–spin ( T 2 -1 ) relaxation rates of 119 Sn nuclei of the Ising pyrochlore magnet Dy 2 Sn 2 O 7 and the Heisenberg pyrochlore magnet Gd 2 Sn 2 O 7 at high temperatures (120–294 K for Dy 2 Sn 2 O 7 and 20–284 K for Gd 2 Sn 2 O 7 ) to investigate their high-temperature 4 f -electron spin dynamics. We found the following two results, which reveal remarkable differences between the spin dynamics of Dy 2 Sn 2 O 7 and Gd 2 Sn 2 O 7 . First, the ratio T 2 -1 / T 1 -1 for powder samples is almost unity in Dy 2 Sn 2 O 7 , whereas it is about 6 in Gd 2 Sn 2 O 7 . Second, these relaxation rates increase markedly on cooling in Dy 2 Sn 2 O 7 , reflecting thermal activation with a gap of ∼600 K, whereas they are almost constant with temperature in Gd 2 Sn 2 O 7 . The former result indicates that spin fluctuations of Dy 2 Sn 2 O 7 are completely restricted to the direction, whereas Gd spins fluctuate three-dimensionally and the component of the fluc...

A pyrochlore lattice is a three-dimensional corner-sharing network of tetrahedra. Spin systems on pyrochlore lattices exhibit several exotic magnetic properties, such as spin ice, spin liquid and order by disorder, due to macroscopic degeneracy caused by geometrical frustration, depending on whether the interactions are ferromagnetic or antiferromagnetic, whether the spin system is classical or quantum, the symmetry of the spin space and the kinds of additional interactions. We survey the magnetic behavior of spin systems on pyrochlore lattices and present nuclear-magnetic-resonance (NMR) studies on the quantum Heisenberg antiferromagnets consisting of magnetic 3d transition metal ions Cu 2 (OH) 3 Cl and Ni 2 (OH) 3 Cl with spin 1/2 and 1, respectively. Cu 2 (OH) 3 Cl exhibits successive phase transitions at 18.1 and 6.4 K. The NMR spectra and spin–lattice relaxation rates indicate curious static and dynamic properties that appear to conflict. The coexistence of freezing and quantum fluctuation of spins a...

Spin dynamics in the antiferromagnetic ring Fe10 has been studied by means of magnetic susceptibility and 1 H-NMR experiments. The spin–lattice relaxation rate 1/ T 1 above 40 K reflects the spin fluctuations in the continuous energy levels of the spin states, while the rate below 40 K indicates the development of antiferromagnetic correlations and discrete energy levels. Especially the experimental rates below 4 K can be expressed by an exponential function of the temperature with an energy gap between the singlet ground state and the triplet excited state. The field dependence of the low-lying energy levels was clearly revealed by the 1/ T 1 measurements. On the other hand, the spin–spin relaxation rate 1/ T 2 is dominated by the temperature-independent fluctuations of the nuclear dipolar fields and a small contribution proportional to 1/ T 1 , and is consistent with the spectral widths.

The nuclear spin-lattice relaxation time T 1 of 87 Rb in the singlet-ground-state easy-plane antiferromagnet RbFeCl 3 has been measured at the temperatures above and below T N ≈2.5 K with the external field applied both along and perpendicularly to the hard c -axis. From the analysis based on the dynamical correlated-effective-field approximation, it is found that the trend of the divergence of T 1 -1 in the paramagnetic phase around T N is closely related to the remarkable softening of the magnetic excitation for the K-point mode (\(\bm{q}=\bm{q}_{0}\)) via the in-plane component of the static susceptibility χ (\(\bm{q}_{0}\)).

The successive magnetic phase transitions ( T N1 and T N2 ) and magnetic properties of CsNiCl 3 , a Heisenberg antiferromagnet on the triangular lattice, have been investigated by means of the nuclear magnetic resonance of 133 Cs. In the low temperature phase ( T < T N2 ), the 120° structure of Ni 2+ spins was confirmed by the angular dependence of NMR spectra. In the intermediate phase ( T N2 < T < T N1 ), the observed angular dependences of NMR spectra can not be explained by the simple component disorder model which has been proposed previously. We interpret the NMR results in the intermediate phase by the component disorder model with the quasi-degeneracy. It was found that the quasi-degeneracy due to the freedom of rotation of the 120° spin structure in the a c -plane survives beyond T N2 down to about 3 K.