Masahide Nishiyama

Observation of Successive Phase Transitions in Kagomé Lattice Antiferromagnets RFe3(OH)6(SO4)2[R=NH4, Na, K]

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.

Magnetization of new kagomé lattice antiferromagnets: Cr-jarosites, ACr3(OH)6(SO4)2 [A = Na, K, Rb, NH4]

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...

Spin dynamics in classical and quantum Kagome lattice magnets studied by NMR

Jarosite, KFe 3 (OH) 6 (SO 4 ) 2 , is a typical example of a classical frustrated antiferromagnet with s = 5/2 Fe 3+ ions on the Kagome lattice, and a metallic complex, [Cu 3 (titmb) 2 (CH 3 CO 2 ) 6 ]·H 2 O, abbreviated as Cu-titmb, is considered to be a candidate material for a quantum frustrated magnet with s = 1/2 on the Kagome lattice. Jarosite has a Weiss temperature of -530 K and shows magnetic ordering below 65 K due to weak anisotropy. The magnetic structure in the ordered phase has been determined, using neutron scattering and NMR, to be a q = 0 type 120° spin structure with + chirality. The nuclear spin-lattice relaxation rate T -1 1 and the spontaneous magnetization indicate the existence of the spin wave in the ordered phase. A small number of substitutions or deficiencies induce successive phase transitions, which represent two-dimensional ordering on the Kagome planes and three-dimensional ordering between the planes. For the quantum spin magnet, Cu-titmb, the nuclear spin relaxation at low temperatures shows the existence of anomalous low-lying excited states in the magnetic field. The relaxation rates at high temperatures give much smaller exchange interaction compared to the value estimated from the specific heat. Several interesting properties, such as double peaks of the specific heat, a 1/3 magnetization plateau for the pulse field and a ferromagnetic transition at 56 mK have been reported. We review the experimental results of Cu-titmb.

Spin dynamics in 3d electron pyrochlore-like systems

1H NMR measurements of 3d electron three-dimensional frustrated systems, Co2(OH)3Cl with s = 3/2 and Cu2(OH)3Cl with s = 1/2 have been performed. The relaxation rates T-11 of both compounds suggest the development of spin correlations at much higher temperatures compared with the magnetic transition temperatures. The temperature dependence of T-11 below 6 K of both compounds indicates the magnetic ordering and the existence of magnon excitations. The estimated magnon energy gaps are 13 K and 27 K for Cu2(OH)3Cl and Co2(OH)3Cl, respectively, which are consistent with the inelastic neutron experiments. The intermediate phase between TN1 = 18.1 K and TN2 = 6.4 K for Cu2(OH)3Cl shows the conflicting behaviors which are the coexistence of the freezing of the moments and the paramagnetic spin dynamics. The partial freezing and the time scale of spin dynamics should be crucial to understand these frustrated systems.

NMR study of pyrochlore lattice antiferromagnet, melanothallite Cu2OCl2

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.

Critical behavior in a Kondo-screening partially-ordered antiferromagnet CePdAl

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.

Hierarchical ordering in kagomé lattice antiferromagnets, jarosites

Abstract 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.

An exotic ordered phase in the pyrochlore-like antiferromagnet Ni2(OH)3Cl studied by NMR

We investigated a three-dimensional frustrated 3d-electron magnet on a pyrochlore-like lattice Ni2(OH)3Cl with s = 1 by means of 1H-NMR measurements. Previous specific heat measurements reported that this material shows a phase transition at 4.0 K and also has an anomaly at around 20 K. Our 1H-NMR measurement in 0.73 T has demonstrated that both the spin-lattice relaxation rates T1−1 and the spectra show clear evidences of a magnetic long-range ordering below 4.0 K. However, they do not show any anomalies at 20 K. By contrast the width of the 1H-NMR spectra in 4.3 T starts to deviate from linearity to the magnetization below 20 K, which suggests that the magnetic anisotropy is induced by the magnetic field. In addition, T1−1 starts to decrease below 20 K, which indicates the suppression of spin fluctuation. We discuss the unusual spin state below 20K.

Temperature-dependent hyperfine interactions in CePdAl

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.