Nobuo Wada

An organic ferromagnet: α-phase crystal of 2-(2′,5′-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide (α-HQNN)

The α-phase crystal of 2-(2′,5′-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide (α-HQNN), which consists of one-dimensional arrays formed by intra- and inter-molecular hydrogen bonds, showed a ferromagnetic phase transition at 0.5 K.

Incommensurate Magnetic Phase Transitions in the Triangular XY-Like Antiferromagnet RbFeCl3

RbFeCl 3 is composed of one-dimensional chains in which the spins interact ferromagnetically with XY-like anisotropy. These chains are coupled antiferromagnetically in the hexagonal basal plane. It undergoes three transitions at T N1 =2.5 K, T N2 =2.35 K and T N3 =1.95 K. In order to understand the nature of these transitions, we performed a neutron scattering study. Besides the previously determined 120°-structure observed at T < T N 3 , we found two different incommensurate structures in the basal plane at T N3 < T < T N2 and T N2 < T < T N1 . Only, the transition at T N3 is the first order, but the other two are the second order. The effect of the field applied parallel to the easy plane has also been studied.

Observation of Spin-Gap State in Two-Dimensional Spin-1 Kagomé Antiferromagnet m-MPYNN·BF4

Low temperature properties of the organic spin-1 Kagome antiferromagnet, m -MPYNN·BF 4 , have been studied by measuring heat capacity and magnetic susceptibility down to 35 mK. The heat capacity maximum due to a magnetic short-range order was observed at 1.4 K, which is about half of the antiferromagnetic interaction 2| J |/ k B =3.1 K in the Kagome lattice. As temperature decreases, the susceptibility begins to decrease below 0.24 K. The susceptibilities both parallel and perpendicular to the Kagome lattice collapse to almost zero at the lowest temperature, which indicates a nonmagnetic ground state. The temperature dependence suggests that the gap energy of the magnetic state is about 0.25 K.

Magnetic Properties of a Heavy Fermion Substance CeInCu2 with a Cubic Structure

We have grown a single crystal of CeInCu 2 and have confirmed that it is the first known heavy fermion substance with a cubic structure in Ce compounds. The specific heat coefficient is about 1.2 J/mol·K 2 below 1 K.

Superconducting, electronic and magnetic anomalies due to structural transition around x=0.12 in La2−xBaxCuO4

Abstract From anomalies of the heat capacity and the thermal expansion, the structural phase boundary of the LTT-LTO transition is determined in the narrow range around x=0.12 of La 2−x Ba x CuO 4 , where superconductivity is suppressed remarkably. The widely-spread CuNQR spectra and the decay of the asymmetry parameter of μSR are indicative of the magnetic order of Cu moments in the LTT phase where the induced charge transfer between Cu and O atoms causes the localization of the electronic state and the increase of magnetic moments at the Cu site.

Some Anomalies Due to the Low-Temperature Structural Transition around x=0.12 in La2-xBaxCuO4

Anomalies of heat capacity and thermal expansion have been observed near x =0.12 in La 2- x Ba x CuO 4 . The anomalies are ascribed to the low-temperature structural transition. We determine accurately the phase baundary as a function of x . Widely broadened Cu-NQR spectra for x <0.14 show that the local charged states of the CuO 2 plane are largely distributed in the low-temperature phase. The phase diagram and its relation with the suppression of superconductivity around x =0.12 are briefly discussed.

The Magnetic Aftereffect in the Triangular XY-Like Antiferromagnet RbFeCl3

The triangular XY-like antiferromagnet RbFeCl 3 shows successive incommensurate-incommensurate-commensurate phase transitions with decreasing temperature in zero field. The T - H phase diagram in the easy plane is determined by the susceptibility measurement. Two new ordered states are found in the magnetic field region between 5 and 22 kOe. Magnetic aftereffects are observed in the commensurate phase at the lowest temperatures in zero field. The susceptibility just after the demagnetization is larger than that before the field application and decreases with a logarithmic time decay.

Metamagnetic Behavior in NdCu6

We have found metamagnetic behavior of orthorhombic single crystals NdCu 6 . Magnetization curve in the field along the b -axis shows four steps, whereas that along the c -axis suggests a canted spin structure. These results suggest the existence of four canted sublattices.

Magnetic properties of antiferromagnetic GdCu6

We report magnetic susceptibility, high-field magnetization, specific heat and electrical resistivity measurements on the orthorhombic single crystal GdCu 6 . These results show GdCu 6 to be antiferromagnet which exhibits plane-type anisotropy below 16 K. The specific heat data show evidence of a low-temperature multiple phase below 16 K.

Ferromagnetic Transition of Pyrochlore Compound Yb2Ti2O7

R2Ti2O7 (R = Y and various rare earth elements) has pyrochlore type structure, which consists of two kinds of three-dimensional networks individually formed by the corner-sharing R4and Ti4-tetrahedra, respectively. Due to this structural characteristic, magnetic moments at the R sites are expected to be frustrated, if their nearest neighbor interaction is antiferromagnetic. For R = Tb, the value of Weiss temperature w is 19K, indicating the Tb3þ moments have antiferromagnetic nearest neighbor interaction and the system does not exhibit clear evidence for magnetic ordering. Previously, we investigated both the dynamical and static magnetic properties of Tb2Ti2O7 by means of neutron scattering on a single crystal. Based on results of measurements of the specific heat C and ac magnetic susceptibility , low temperature state of Tb2Ti2O7 was discussed in ref. 4, too. The frustration is also expected even for the system with the ferromagnetic nearest neighbor interaction, if the moments have strong uniaxial anisotropy, where each moment lies along the local principal axis corresponding to the line which connects the site with the center of gravity of the tetrahedron. (There are four principal axes along [111] and other crystallographically equivalent directions.) Such the situation can be found in R2Ti2O7 with R = Dy 5) and Ho, which are called ‘‘spin ice’’. For Yb2Ti2O7, w is equal to be 0:53K as shown in the inset of Fig. 1, indicating the nearest neighbor interaction between the Yb3þ moments is ferromagnetic. The electronic ground state of Yb3þ ion was reported to be a Kramers doublet with relatively small planar anisotropy, g? 1⁄4 4:27 and gk 1⁄4 1:70, where g? and gk are the g-values perpendicular to and along the local principal axis, respectively. Sengupta et al. reported that the system has uniaxial anisotropy, g? 1⁄4 0 and gk 1⁄4 3:4. A sharp peak of C–T curve was reported at 0.24K, indicating the existence of the phase transition. However, Hodges et al. did not observe magnetic reflection except the small angle diffuse scattring. In order to identify the specific heat anomaly at 0.24K, we have carried out neutron diffraction and other magnetic measurements on a single crystal of Yb2Ti2O7 down to 0.03K by using dilution refrigerator. Here, we report that the system exhibits ferromagnetic transition at TC 1⁄4 0:24K. We also discuss the low temperature behavior of the moments. A single crystal of Yb2Ti2O7 was grown by a floating zone (FZ) method. The magnetization M was measured by using a SQUID magnetometer. The method of the ac magnetic susceptibility is described in ref. 4. Neutron measurements were carried out by using the triple axis spectrometer HQR (T1-1) installed at the thermal guide of JRR-3 of JAERI in Tokai. The crystal was oriented with [hh0] and [00l] axes in the scattering plane. Figure 1 shows the M–H curves of Yb2Ti2O7 obtained at 5K with the magnetic fields along [001], [110] and [111], where the anisotropy of the curves is found to be relatively small. We calculated the M–H curves considering an anisotropic Kramers doublet and using a molecular field treatment of the ferromagnetic interaction with a coupling constant . By fitting the calculated curve to the data, the parameters are estimated to be 1⁄4 0:64 0:10T/ B, g? 1⁄4 3:9 0:2 and gk 1⁄4 2:6 0:4. Judging from the rather small differences among the M–H curves, the deduced anisotropy is surprisingly large, but at least much smaller than that reported in refs. 7 and 8. This can be understood as follows. Because Yb sites are divided into four sets with different local principal axes along [111] and three equivalent directions, the averaged anisotropy of the moments over these sets becomes very small, even though the anisotropy is rather large within each set of the Yb moments. The value of saturation magnetization estimated from the M–H curves is 1:8 B/Yb. The T-dependence of the real part of the ac magnetic susceptibility 0 of Yb2Ti2O7 measured with increasing T is shown in the inset of Fig. 2, where a clear anomaly has been observed in the T-dependence at 0.24K. The value of 0 at 0.24K is found to agree with the value of 1=4 N within the experimental error bar, where N is the demagnetization coefficient of the used sample. Because M=H is described as 0=ð1þ 4 0NÞ, which approaches 1=4 N as 0 ! 1, the result indicates that the system exhibits a ferromagnetic transition at TC 1⁄4 0:24K. At T 1⁄4 0:03K (