63/65 Cu-and 35/37 Cl-NMR experiments were performed to investigate triplet localization in the S = 1/2 dimer compound NH 4 CuCl 3 , which shows magnetization plateaus at one-quarter and three-quarters of the saturation magnetization. In 63/65 Cu-NMR experiments, signal from only the singlet Cu site was observed, because that from the triplet Cu site was invisible due to the strong spin fluctuation of onsite 3d-spins. We found that the temperature dependence of the shift of 63/65 Cu-NMR spectra at the singlet Cu site deviated from that of macroscopic magnetization below T = 6 K. This deviation is interpreted as the triplet localization in this system. From the 35/37 Cl-NMR experiments at the 1/4-plateau phase, we found the two different temperature dependences of Cl-shift, namely the temperature dependence of one deviates below T = 6 K from that of the macroscopic magnetization as observed in the 63/65 Cu-NMR experiments, whereas the other corresponds well with that of the macroscopic magnetization in the entire experimental temperature region. We interpreted these dependences as reflecting the transferred hyperfine field at the Cl site located at a singlet site and at a triplet site, respectively. This result also indicates that the triplets are localized at low temperatures. 63/65 Cu-NMR experiments performed at high magnetic fields between the one-quarter and three-quarters magnetization plateaus have revealed that the two differently oriented dimers in the unit cell are equally occupied by triplets, the fact of which limits the theoretical model on the periodic structure of the localized triplets.
Cu-NMR study under high-field up to 17 T has been performed on the three-dimensional spin dimer system NH4CuCl3, which shows the two-stepped magnetization plateaus. The local magnetization of Cu-site measured by NMR showed a significant deviation from the macroscopic magnetization at low temperatures, suggesting the localization of the fieldinduced triplons. In the high field region around 16 T, two inequivalent Cu sites are observed. Only one of the two sites shows a split in the resonance line below TN 3.2 K, indicating the existence of the field-induced magnetic order, to which a part of spins in the system participate. §1. Introduction NH 4 CuCl 3 is a three-dimensional spin-dimer magnet with the ladder-like crystal structure 1) isomorphic with TlCuCl 3 and KCuCl 3 . The latter two have a gapped ground state at zero-field. 2) The field-induced triplet dimers move around in the crystal as nearly free bosons to show the Bose-Einstein condensation at low temperatures. 3)-5) The title compound NH 4 CuCl 3 does not have a spin excitation gap at zero field. With increasing the magnetic field, gap is switched to be open and closed to show the magnetization plateaus at 5.2-13.7 T and 18.5-26.1 T (H b-axis). 1) Recent specific heat 6) and neutron 7) experiments by Kurniawan and Ruegg have reported that there exists magnetic order in the low-field-gapless region of Phase-I (0-5.2 T, H b-axis), and that its transition temperature strongly depends on the applied field. From kinks in the M -H curves at low temperatures, Kurniawan 6) suggests the existence of the field-induced order at the higher-field-gapless regions (Phase-II, 13.7-18.5 T; Phase-III, 26.1-30.4 T). He also predicts that the quarter of spins order in Phase-I, the half in Phase-II, and finally the remaining quarter in Phase-III. This view is supported by recent theoretical investigations by Matsumoto,8) who predicts that the existence of the three inequivalent dimers can be the origin both for the field-induced magnetic orders and the magnetization plateaus.The purpose of this work is to investigate directly by Cu-NMR the microscopic spin state in high field region, 9) where the detailed spin state or the existence of the phase transition have not been confirmed. First, by comparing the Knight shift and the macroscopic magnetization in the paramagnetic state, we show that the field-induced triplons in NH 4 system are localized. Next, from the splitting of the
The high-field magnetization process in the randomly diluted metamagnet Fe1−xMgxTiO3 has been investigated in single crystal samples with x≤0.3 by using dc magnetic fields of up to 300 kOe generated by a hybrid magnet. In contrast to the sharp metamagnetic transition along the hexagonal c axis observed in FeTiO3, the diluted compounds show a considerable smearing in the transition with increasing Mg concentration. When the field is applied along the a axis at 4.2 K, a sudden jump in magnetization occurs at 235 kOe in FeTiO3, while no anomaly is observed up to 300 kOe along the a* axis which is taken to be perpendicular to both c and a axes. The jump field is lower in the diluted compounds. For the sample with x=0.185 we have also observed an anomalous increase in magnetization along the a* axis at 270 kOe.
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