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A double-resonance experiment of $^{133}\mathrm{Cs}$ NMR was performed in the three-dimensionally ordered phase of a triangular-lattice Ising-like antiferromagnet ${\mathrm{CsCoCl}}_{3}$. The spin inversion in the ${\mathrm{Co}}^{2+}$ chain was detected by observing the change of the internal magnetic field at the Cs site. The inversion times, which are of the order of ${10}^{2}$ s at liquid helium temperatures, were obtained from an analysis by the rate equations. This spin inversion is a slow dynamics and is interpreted as a result of the sublattice switching arisen from the movement of the ferrimagnetic domain wall in the c plane.

The spin-lattice relaxation time ${\mathit{T}}_{1}$ of $^{133}\mathrm{Cs}$ in ${\mathrm{CsNiBr}}_{3}$ has been measured to study the spin fluctuation in the triangular-lattice Heisenberg antiferromagnet with a small easy-axis anisotropy. This compound shows the successive phase transitions at ${\mathit{T}}_{\mathit{N}1}$=14.06 K and ${\mathit{T}}_{\mathit{N}2}$=11.51 K. The relaxation rate ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ in the paramagnetic phase is governed by the paramagnetic modulation of the exchange interaction. It is well explained by using the observed static susceptibility down to ${\mathit{T}}_{\mathit{N}1}$. The relaxation rate changes drastically at ${\mathit{T}}_{\mathit{N}1}$, and below this temperature the rates at inequivalent Cs sites become different from each other. The relaxation rate in the low-temperature phase below ${\mathit{T}}_{\mathit{N}2}$ has a strong temperature dependence, while it has no field dependence. This relaxation rate is well explained by the two-magnon (Raman) process due to the swinging fluctuation of the triangular spin structure in the zx plane.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6%. © 2013 American Physical Society., The present work has been partly funded trough the Spanish MINECO (Grant No. MAT2012-38318-C03) and the Gobierno de Aragón (Project MOLCHIP).

The organic triangular-lattice 1/2-spin system EtMe${}_{3}$Sb[Pd(dmit)${}_{2}$]${}_{2}$ does not undergo classical antiferromagnetic ordering and is a strong candidate for a quantum spin liquid. It is currently an open issue whether or not the ground state of this system has a broken-symmetry and/or topological structure, and what kind of excitations the ground state has. We have performed NMR measurements on the inner carbons of the Pd(dmit)${}_{2}$ molecule in order to get reliable information on this issue. By comparing the inner-carbon NMR data with the previously obtained outer-carbon data, we have reached a robust conclusion that these NMR data are not affected by cation molecular motion and definitely reflect the spin state of this system. The temperature dependencies of the spin-lattice relaxation rates suggest that the spin state undergoes an abrupt change at around 1 K, and, thus, the ground state is likely to have a broken-symmetry and/or topological structure. The rates also strongly indicate that the imaginary part of the $q$-integrated dynamic susceptibility (${\mathrm{lim}}_{\ensuremath{\omega}\ensuremath{\rightarrow}0}{\ensuremath{\sum}}_{q}{\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(q,\ensuremath{\omega})/\ensuremath{\omega}$) disappears as $T\ensuremath{\rightarrow}0$.

We develop a general theory of the spin dynamics of Heisenberg antiferromagnetic rings (HAFRs) that explains the mechanism of NMR spin-lattice relaxation at low temperatures. In HAFRs, the imaginary parts of the q-summed dynamic spin susceptibilities parallel and perpendicular to an applied static field, χ ′′ sum‖(ω) and χ ′′ sum⊥(ω), are composed of the sum of many slightly broadened δ-functional modes at many frequencies. The NMR relaxation is caused by the quasielastic mode in χ ′′ sum‖(ω) at around zero frequency. This quasielastic mode is characterized by two physical quantities, intensity P0‖ and frequency width 0‖. Although P0‖ has to date been assumed to be identical to the uniform static susceptibility, we point out that the two quantities are not identical. Without making this unreliable assumption for P0‖, we demonstrate experimentally how P0‖ and 0‖ behave, by analyzing the NMR relaxation rates of two different nuclei, 1H and 13C, in a real HAFR. This analysis is more rigorous and thus can be used to estimate 0‖ and P0‖ more precisely than previously possible. We find that the temperature dependence of P0‖ exhibits activation-type behavior reflecting the first excitation gap. We also find that 0‖ decreases monotonically on cooling but saturates to a nonzero value at zero temperature. This strongly suggests that 0‖ is dominated not only by the electron-phonon interactions but also by internanomagnet dipole interactions, which have been neglected to date.

4 páginas, 4 figuras.-- PACS numbers: 75.50.Xx, 73.43.Nq, 75.40.Gb, 75.45.+j.-- et al., We show that a crystal of mesoscopic Fe8 single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class., The present work was partly funded by Spanish MICINN Grant No. MAT2009-13977-C03 (MOLCHIP) and Consolider-Ingenio on Molecular Nanoscience.

The nuclear spin-lattice relaxation time T 1 of 1 H nuclei in the S=1 quasi-one-dimensional Heisenberg antiferromagnet Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ) (NENP) was measured at temperatures above 1.4 K in the field range below 12 T. The field dependence of the relaxation rate T 1 -1 exhibited a maximum at 9,5 T, which corresponds to the critical field, for the field parallel to the chain at low temperatures below 4 K. The experimental results were analyzed by treating the magnetic excitation as free fermions. The 1 H NMR spectra were also measured for various fields and temperatures

The transverse relaxation time T2 of 133Cs NMR was measured in the 3D-ordered phase of the quasi-one-dimensional S = 1 2 Ising-like antiferromagnet CsCoCl3. A pulse-like fluctuation due to spin inversion in the Co2+ chains well explains the experimental results. This strongly suggests a domain-wall pairing in the chain.

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.

The nuclear spin-lattice relaxation time ${\mathit{T}}_{1}$ of the proton in the s=1 Haldane-gap system Ni(${\mathrm{C}}_{2}$${\mathrm{H}}_{8}$${\mathrm{N}}_{2}$${)}_{2}$${\mathrm{NO}}_{2}$${\mathrm{ClO}}_{4}$ (NENP) has been measured at low temperatures in high magnetic fields (4 TH8 T) below the critical field. From the experimental results below 4 K, the field dependence of the energy gap for the lowest-energy mode of the three excitation modes, which has been predicted theoretically in NENP, are obtained. The results of ${\mathit{T}}_{1}$ below 4 K are well explained by treating the magnetic excitations as free fermions rather than free bosons.

We investigate the effect of dynamic nuclear spin fluctuation on quantum tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing the nuclei temperature using radio frequency (RF) pulses before the hysteresis loop measurements. The RF pulses do not change the electrons spin temperature. Independently we show that the nuclear spin-spin relaxation time T2 has strong temperature dependence. Nevertheless, we found no effect of the nuclear spin temperature on the tunneling probability. This suggests that in our experimental conditions only the hyperfine field strength is relevant for QTM. We demonstrate theoretically how this can occur., 4 pages, 4 figures

The family of layered organic salts $X{[\mathrm{Pd}{(\text{dmit})}_{2}]}_{2}$ are Mott insulators and form scalene-triangular spin-$1∕2$ systems. Among them, $\mathrm{Et}{\mathrm{Me}}_{3}\mathrm{Sb}{[\mathrm{Pd}{(\text{dmit})}_{2}]}_{2}$ has a nearly regular-triangular lattice. We have investigated the spin state of this salt by $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$ and static susceptibility measurements. The temperature dependence of the susceptibility is described as that of a regular-triangular antiferromagnetic spin-$1∕2$ system with an exchange interaction $J=220\ensuremath{-}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$ measurements reveal that there is no indication of either spin ordering/freezing or an appreciable spin gap down to $1.37\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is lower than 1% of $J$. This result strongly suggests that this system is in the quantum spin-liquid state with no appreciable spin gap, which has been long sought after.

$^{27}\mathrm{Al}$ NMR measurements have been performed to investigate the ordering mechanism and spin fluctuations in a geometrically frustrated heavy-fermion antiferromagnet on the Kagome-like lattice CePdAl. This compound shows a partially ordered state below ${T}_{\mathrm{N}}$ $(2.7\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, in which $1∕3$ of the Ce moments remain paramagnetic. Whether the remaining paramagnetic moments order at lower temperature or not has been controversial. Our NMR measurements show that the nuclear magnetic relaxation rate ${T\phantom{\rule{0.2em}{0ex}}}_{1}^{\ensuremath{-}1}$ is proportional to the temperature below $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and there is no other phase transition down to $30\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The simulation of the spin echo spectra shows that the observed spectra are compatible with the partially ordered state and an incommensurate $k$ vector along the $c$ axis down to the lowest temperature. These results demonstrate that the paramagnetic Ce moments in the partially ordered state are in a heavy-fermion state and consequently the partially ordered state is considered to be stable down to $0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of ${T\phantom{\rule{0.2em}{0ex}}}_{1}^{\ensuremath{-}1}$ also shows that antiferromagnetic correlations develop from much higher temperatures above ${T}_{\mathrm{N}}$.

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.

We developed a method to detect the quantum nature of high spin molecules using muon spin rotation, and a three-step field cycle ending always with the same field. We use this method to demonstrate that the Fe8 molecule can remember 6 (possibly 8) different histories (bits). A wide range of fields can be used to write a particular bit, and the information is stored in discrete states. Therefore, Fe8 can be used as a model compound for Multi-bit Magnetic Memory. Our experiment also paves the way for magnetic quantum tunneling detection in films., Comment: 11 pages, 5 figures

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.

NQR studies of 27Al in CePd2Al3 of powdered single crystals and polycrystals were carried out to investigate the structural instabilities in CePd2Al3. NQR spectra show sample dependence and secular change. Furthermore NQR spectra show a temperature dependence above 4.2 K around crystal field splittings. These results suggest that the local symmetry of Al atoms has strong instabilities. The sample dependence of various physical properties are possibly related to the structural instabilities.

The spin dynamics of the Heisenberg kagom\'e lattice antiferromagnet, potassium jarosite ${\mathrm{KFe}}_{3}(\mathrm{OH}{)}_{6}({\mathrm{SO}}_{4}{)}_{2},$ have been investigated by means of NMR. The NMR spectra confirm the long-range magnetic ordering below 65 K and the $\mathbf{q}=0$ type $120\ifmmode^\circ\else\textdegree\fi{}$ spin structure with positive chirality in the ordered phase. Though the Heisenberg kagom\'e lattice antiferromagnet is considered theoretically to remain disordered down to zero temperature due to the continuous degeneracy of the ground state, the long-range magnetic ordering at the finite temperature is realized in this compound due to the weak anisotropy. The spin-lattice relaxation rate, ${1/T}_{1},$ in the ordered phase decreases sharply with lowering temperature. The experimental rate is well explained by the two-magnon process of the spin waves having an energy gap of 15 K. The temperature dependence of the sublattice magnetization also supports the existence of the spin wave. These are the first experimental evidence that the low-energy excitation in the frustrated classical kagom\'e lattice antiferromagnet is described by the spin wave. We calculate the spin wave in the $\mathbf{q}=0$ type $120\ifmmode^\circ\else\textdegree\fi{}$ spin structure with weak single-ion-type anisotropy, and discuss the characteristics of the spin fluctuations in the Heisenberg kagom\'e lattice antiferromagnet.

An NMR study of 121Sb in the heavy-fermion compound YbSb has been performed to investigate the unusual phase transition around 5 K. A distinct broadening of spin echo spectra below 5 K is observed. The spin echo spectra below the transition temperature are slightly asymmetric. The hyperfine coupling constant is estimated from the K-χ plot.

The 75 As nuclear spin-lattice relaxation rate ( T −1 1 ) of a heavy-fermion compound YbAs has been measured between 0.52 K and 100 K. The relaxation process seems to consist of two components, although As atoms have cubic local symmetry and have no quadrupolar field. The relaxation rate for the shorter T 1 decreases gradually below 1.5 K with decreasing temperature. This behavior is quite in contrast to that of YbP, which has a sudden increase at T N . This difference seems to be consistent with the difference in the temperature dependence of the order parameter between YbAs and YbP.

The spin-lattice relaxation rate $T_{1}^{-1}$ and NMR spectra of $^1$H in single crystal molecular magnets of Fe8 have been measured down to 15 mK. The relaxation rate $T_1^{-1}$ shows a strong temperature dependence down to 400 mK. The relaxation is well explained in terms of the thermal transition of the iron state between the discreet energy levels of the total spin S=10. The relaxation time $T_1$ becomes temperature independent below 300 mK and is longer than 100 s. In this temperature region stepwise recovery of the $^1$H-NMR signal after saturation was observed depending on the return field of the sweep field. This phenomenon is attributed to resonant quantum tunneling at the fields where levels cross and is discussed in terms of the Landau-Zener transition., 13 pages, 5 figures

Measurements of the magnetic susceptibility and neutron powder diffraction revealed that the kagom\'e lattice antiferromagnet ${\mathrm{KCr}}_{3}{(\mathrm{OD})}_{6}{(\mathrm{S}\mathrm{O}}_{4}{)}_{2}$ exhibits clear long-range magnetic order at ${T}_{N}=4.0\mathrm{K}$ in contrast to previous reports. The ordered magnetic structure is found to be the so-called $q=0$ type 120\ifmmode^\circ\else\textdegree\fi{} structure on the kagom\'e lattice. A model which satisfies the observed magnetic intensity is, however, that the direction of the ordered moments, which form a nearly coplanar 120\ifmmode^\circ\else\textdegree\fi{} ground state, distribute uniformly in the ab plane. We propose a $q=0$ structure fluctuating slowly around the c axis as a ground state. This explanation may give a reasonable account for the obtained magnetic structure and the sample dependence.

Inelastic neutron scattering measurements were performed to study the excitation spectra of a quadrupolar ordering compound YbSb. We observed the three peaks (12.5, 17 and 21 meV) around Γ 6 – Γ 8 excitations far above the quadrupolar ordering temperature ( T Q ). This result suggests that the magnetic excitations have a dispersion rather than the splitting of Γ 8 states at high temperatures. We also observed the strong q dependence in q scans at energy = 1 meV above T Q . The observed q dependence above T Q suggests that the theoretical approach beyond the molecular field approximation is required in order to understand the magnetic properties of Yb monopnictides.

Magnetization and 1 H-NMR measurements of the s = 1 2 kagome lattice antiferromagnet, [Cu 3 (titmb) 2 (CH 3 CO 2 ) 6 ]·H 2 O, have been performed between 1.5 and 300 K to study the magnetic properties in the quantum spin frustrated magnet. The measured susceptibility obeys the Curie-Weiss law and no magnetic phase transition was observed down to 1.5 K. The spin-lattice relaxation rate T 1 −1 shows that the ground state is singlet and the first excited state is triplet separated by the energy gap.

Jarosite compounds, RFe 3 ( OH ) 6 ( SO 4 ) 2 [ R = K, Na, NH 4 ] , of the kagome lattice antiferromagnets show the magnetic ordering below 65 K (T N1 ) . In addition to the transition at TN1, some jarosite samples indicate the second transition at a lower temperature TN2. The nature of the transitions was studied by the susceptibility and NMR experiments. The NMR results suggest the existences of the partial substitution of R+ by H3O+ and the deficiency of Fe3+ accompanied with the formation of H2O. The hierarchical orderings are considered to occur due to the effects of the substitutions, deficiencies, and the weak interplane interaction, that is, the two-dimensional ordering occurs at TN1 in the domains formed by the substitutions and the three-dimensional ordering occurs at TN2 between the kagome planes.

Magnetization and NMR measurements have been performed for single crystals of molecular magnet Fe8. The field and temperature dependences of magnetization below 25 K are well described in terms of the isolated clusters with the total spin S=10. The stepwise recoveries of 1 H-NMR signals at the level crossing fields caused by the resonant quantum tunneling of magnetization were observed below 400 mK . The recovery of the NMR signals are explained by the fluctuation caused by the transition between the energy states of Fe magnetizations governed by Landau–Zener quantum transitions.

Nuclear magnetic resonance have been performed to study hyperfine interactions in CePdAl. The temperature dependence of Knight shift(K) and susceptibility(χ) is highly anisotropic as expected from its crystal structure. Non-linearity of K–χ plot was observed only when the magnetic field was perpendicular to c-axis. Two-dimensional short-range correlations are discussed.

A molecular nanomagnet Fe8 with a total spin S=10 in the ground state attracts much attention as a substance which exhibits the quantum tunneling of magnetization below 300 mK . We performed 1 H NMR measurements for a single crystal of Fe8 in temperature range between 20 and 800 mK . The spectra below 300 mK strongly depend on the sequence of the applied field and those in the positive and negative fields are not symmetric about zero field, while they are symmetric above 300 mK . We discuss the origin of this hysteresis phenomenon, relating to the initial spin state of molecules, the resonant quantum tunneling and the nuclear spin relaxation process.

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...

We found by 13C-NMR experiment that a quasi-two-dimensional organic antiferromagnet with spin 1/2 on the triangular lattice, EtMe3Sb[Pd(dmit)2]2, has no classical magnetic ordering down to 19.4mK, although the magnetic exchange interaction J is 220-250K. The quantum spin liquid state is realized in this material due to the significant spin frustration and quantum fluctuation. The 13C nuclear spin-lattice relaxation rate T1−1 under an external magnetic field of 7.65T indicates that the excitations have no spin gap in this spin liquid phase above 1.0K. Then the relaxation rate exhibits a clear kink at 1.0K, and shows a steep decrease below this temperature proportional to T2. This clearly shows a continuous phase transition at 1.0K and the steep decrease of the spin fluctuation could show the appearance of the excitation gap in the lower spin liquid phase. The power law but not exponential law of the temperature dependence may imply a nodal gap rather than a full gap.

The organic Mott insulator EtMe3Sb[Pd(dmit)2]2 is a 1/2-spin system on a nearly-regular triangular lattice. It does not show classical antiferromagnetic ordering in spite of strong antiferromagnetic exchange interactions J = 220?250 K, and thus it is regarded as a spin liquid material. We have found that (T1T)?1 of this spin liquid state shows a peak at around 1 K and after that (T1T)?1 decreases on cooling. This is not attributed to slowing down of the spin dynamics because T2?1 does not show any enhancement down to low temperature limit. Therefore, the decrease of (T1T)?1 is attributed to disappearance of spin excitations, suggesting that magnetic excitations acquire an energy gap at least in some portion of q-space. This indicates that the ground state of the present spin-liquid material is not a state with spinon Fermi surface, where magnetic excitations are fully gapless and thus (T1T)?1 does not decrease down to low temperature limit.

We have performed 63,65Cu-NQR measurements of a quantum antiferromagnet on a deformed kagome lattice, Rb2Cu3SnF12. The spin-lattice relaxation rate T1−1 of 63Cu decreases steeply below 50 K. The rate shows no magnetic ordering down to 1.3 K and the existence of an energy gap. The temperature dependence of T1−1 between 10 and 50 K is expressed by an Arrhenius-type function, with which the gap energy was estimated to be 50 K. This value is rather large compared to the value of 21 K obtained from the magnetic susceptibility and magnetization measurements. This difference can be ascribed to the dispersion of triplet excitations due to Dzyaloshinsky-Moriya (DM) interaction and four different antiferromagnetic (AF) interactions. A large state density in the triplet state with the dispersion gives a significant effect on the spin fluctuation. Eight peaks were observed between 45 and 55 MHz in 63,65Cu-NQR spectra at 5 K. These peaks correspond to two isotopes and four crystallographic sites of Cu. This result implies that a crystal structure with two Cu sites at room temperature changes to a structure that has four Cu sites below the structural phase transition at 215 K.

The melanothallite Cu2OCl2 is a new example of pyrochlore-like antiferromagnet, which is composed of 3d transition metal electrons. We performed Cu- and Cl-NMR experiments on powder samples of Cu2OCl2 below transition temperature TN = 70 K and we observed six resonant peaks of Cu nuclei, which are composed of three symmetric peaks corresponding to 63Cu and three corresponding to 65Cu. The Cu nuclei feel the strong hyperfine fields because of ordered magnetic moments and the electric field gradients. We determined the spin structure by analyzing the Cu-NMR spectra. The melanothallite has an all-in-all-out spin structure. The spin lattice relaxation rates T1−1 of Cu- and Cl-NMR in the ordered phase are proportional to the temperature; This suggests that although long-range ordering occurs at rather high temperature, the large spin fluctuations caused by the geometrical frustration still remain.

A rare-earth compound CePdAl is a good example of the Kondo-screening partially-ordered antiferromagnet below TN = 2.7 K. In this compound, the formation of the Kondo-singlet state is realized as a new route of the relief of the frustration. The spin dynamics of CePdAl are, however, still controversial. In order to further investigate the spin dynamics of CePdAl, we have performed 27Al-NMR and specific heat single crystal measurements and reanalysed published powder neutron diffraction data. NMR intensities decrease with decreasing temperature in the ordered state suggesting the slow spin fluctuations below TN = 2.7 K, while the intensity of magnetic Bragg peak measured by neutron diffraction increases monotonously in the same temperature range. The critical exponents derived from the neutron diffraction and the specific heat measurements are not coincident with those of known universality classes. An unconventional magnetic order parameter or a charge degree of freedom originated from the coexistence of frustration and the Kondo effect may influence the critical behavior.