arXiv subjects

Masashi Takigawa

Publications and source records attributed to Masashi Takigawa.

33 records · Page 2Linked to original sources

arXiv2014

Novel Phase Transitions in the Breathing Pyrochlore Lattice: 7Li-NMR on LiInCr4O8 and LiGaCr4O8

We report 7Li-NMR studies on LiInCr4O8 and LiGaCr4O8, in which Cr3+ ions with spin 3/2 form a breathing pyrochlore lattice, a network of tetrahedra with alternating sizes. In LiInCr4O8 with large alternation, the nuclear relaxation rate 1/T1 shows an activated temperature (T) dependence down to 18 K, indicating a singlet ground state with a spin gap. This behavior, however, is disrupted by an antiferromagnetic (AF) transition at 13 K, which is preceded by another, most likely structural, transition at 16 K. In contrast, LiGaCr4O8 with small alternation shows no spin gap but exhibits a first-order AF transition over a distributed T-range 13-20 K. Nevertheless, 1/T1 of the paramagnetic phase diverges toward 13 K, indicating proximity to a second-order transition. The results indicate that LiGaCr4O8 is located in the vicinity of a tricritical point in the phase diagram.

cond-mat.str-el↗

arXiv2013

Anisotropic spin fluctuations in the quasi one-dimensional frustrated magnet LiCuVO_4

We report results of NMR experiments on a single crystal of the quasi one-dimensional frustrated magnet LiCuVO_4. The NMR spectra of ^7Li and ^{51}V nuclei indicate a helical spin order in a magnetic field of 4 T with the helical spin plane perpendicular to the field and a spin-density-wave (SDW) order at 10 T with modulation in the magnitude of the moments aligned along the field, in agreement with earlier reports. The nuclear spin-lattice relaxation rate 1/T_1 at ^{51}V nuclei, which is selectively coupled to the transverse spin fluctuations perpendicular to the field, shows a pronounced peak near the helical ordering temperature in the field of 4 T applied along the a-axis. In the field of 10 T, however, such a peak is absent. Instead 1/T_1 at ^7Li nuclei probing longitudinal spin fluctuations shows divergent behavior towards the SDW ordering temperature. These results are qualitatively consistent with the theoretical description that the SDW correlation is due to bound magnon pairs, which produce an energy gap in the transverse spin excitation spectrum.

cond-mat.str-el↗

arXiv2013

Incomplete devil's staircase in the magnetization curve of SrCu2(BO3)2

We report on NMR and torque measurements on the frustrated quasi-two-dimensional spin-dimer system SrCu2(BO3)2 in magnetic fields up to 34 T that reveal a sequence of magnetization plateaus at 1/8, 2/15, 1/6, and 1/4 of the saturation and two incommensurate phases below and above the 1/6 plateau. The magnetic structures determined by NMR involve a stripe order of triplets in all plateaus, suggesting that the incommensurate phases originate from proliferation of domain walls. We propose that the magnetization process of SrCu2(BO3)2 is best described as an incomplete devil's staircase.

cond-mat.str-el↗

arXiv2012

Magnetic Order in the Spin-1/2 Kagome Antiferromagnet Vesignieite

We report results of magnetic susceptibility and 51V- and 63,65Cu-NMR measurements on a high-quality powder sample of vesignieite BaCu3V2O8(OH)2, a candidate for the spin-1/2 kagome antiferromagnet. We observed a twostep magnetic transition: the appearance of spatially inhomogeneous static moments below 13 K and a long-range order below 9 K. The NMR data indicate a Q = 0 magnetic structure at 1.4 K with the in-plane spin components of three sublattices oriented at nearly 120 degrees to each other and the magnitude of the ordered moments of at least 0.6 muB

cond-mat.str-el↗

arXiv2012

High-field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2/2H2O

We report results of 51V NMR experiments on a high-quality powder sample of volborthite Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome lattice. Following the previous experiments in magnetic fields $B$ below 12 T, the NMR measurements have been extended to higher fields up to 31 T. In addition to the two already known ordered phases (phases I and II), we found a new high-field phase (phase III) above 25 T, at which a second magnetization step has been observed. The transition from the paramagnetic phase to the antiferromagnetic phase III occurs at 26 K, which is much higher than the transition temperatures from the paramagnetic to the lower field phases I (B < 4.5 T) and II (4.5 < B < 25 T). At low temperatures, two types of the V sites are observed with different relaxation rates and line shapes in phase III as well as in phase II. Our results indicate that both phases II and III exhibit a heterogeneous spin state consisting of two spatially alternating Cu spin systems, one of which exhibits anomalous spin fluctuations contrasting with the other showing a conventional static order. The magnetization of the latter system exhibits a sudden increase upon entering into phase III, resulting in the second magnetization step at 26 T.We discuss the possible spin structure in phase III.

cond-mat.str-el↗

arXiv2011

Metal-Insulator Transition and Magnetic Order in the Pyrochlore Oxide Hg2Ru2O7

We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the pyrochlore structure, which exhibits a metal-insulator transition at TMI = 107 K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate 1/T1 and the Knight shift at the Hg sites follow the Korringa relation, indicating the absence of substantial spatial spin correlation. At low temperatures in the insulating phase, 99,101Ru-NMR signals are observed at zero magnetic field, providing evidence for a commensurate antiferromagnetic order. The estimated ordered moment is about 1 muB per Ru, much smaller than 3 muB expected for the ionic (4d)3plus configuration of Ru5plus. Thus the localized spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also discuss possible antiferromagnetic spin structures.

cond-mat.str-el↗

arXiv2010

Crossover from commensurate to incommensurate antiferromagnetism in stoichiometric NaFeAs revealed by single-crystal 23Na,75As-NMR experiments

We report results of 23Na and 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of stoichiometric NaFeAs. The NMR spectra revealed a tetragonal to twinned-orthorhombic structural phase transition at T_O = 57 K and an antiferromagnetic (AF) transition at T_AF = 45 K. The divergent behavior of nuclear relaxation rate near T_AF shows significant anisotropy, indicating that the critical slowing down of stripe-type AF fluctuations are strongly anisotropic in spin space. The NMR spectra at low enough temperatures consist of sharp peaks showing a commensurate stripe AF order with a small moment \sim 0.3 muB. However, the spectra just below T_AF exhibits highly asymmetric broadening pointing to an incommensurate modulation. The commensurate-incommensurate crossover in NaFeAs shows a certain similarity to the behavior of SrFe2As2 under high pressure.

cond-mat.str-el↗

arXiv2009

Space efficient opposed-anvil high-pressure cell and its application to optical and NMR measurements up to 9 GPa

We have developed a new type of opposed-anvil high pressure cell with substantially improved space efficiency. The clamp cell and the gasket are made of non-magnetic Ni-Cr-Al alloy. Non-magnetic tungsten carbide (NMWC) is used for the anvils. The assembled cell with the dimension ϕ29mm \times 41mm is capable of generating pressure up to 9 GPa over a relatively large volume of 7 mm3. Our cell is particularly suitable for those experiments which require large sample space to achieve good signal-to-noise ratio, such as the nuclear magnetic resonance (NMR) experiment. Argon is used as the pressure transmitting medium to obtain good hydrostaticity. The pressure was calibrated in situ by measuring the fluorescence from ruby through a transparent moissanite (6H-SiC) window. We have measured the pressure and temperature dependences of the 63Cu nuclear-quadrupole-resonance (NQR) frequency of Cu2O, the in-plane Knight shift of metallic tin, and the Knight shift of platinum. These quantities can be used as reliable manometers to determine the pressure values in situ during the NMR/NQR experiments up to 9 GPa.

cond-mat.mtrl-sci↗

arXiv2009

Antiferromagnetism of SrFe2As2 studied by Single-Crystal 75As-NMR

We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown high-quality single crystal of SrFe2As2. The NMR spectra clearly show sharp first-order antiferromagnetic (AF) and structural transitions occurring simultaneously. The behavior in the vicinity of the transition is compared with our previous study on BaFe2As2. No significant difference was observed in the temperature dependence of the static quantities such as the AF splitting and electric quadrupole splitting. However, the results of the NMR relaxation rate revealed difference in the dynamical spin fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear to be more anisotropic in BaFe2As2 than in SrFe2As2.

cond-mat.str-el↗

arXiv2009

Magnetization "Steps" on a Kagome Lattice in Volborthite

Magnetic properties of the spin-1/2 kagome-like compound volborthite are studied using a high-quality polycrystalline sample. It is evidenced from magnetization and specific heat measurements that the spins on the kagome lattice still fluctuate at low temperature, down to T = 60 mK that corresponds to 1/1500 of the nearest-neighbor antiferromagnetic interaction, exhibiting neither a conventional long-range order nor a spin gap. In contrast, 51V NMR experiments revealed a sharp peak at 1 K in relaxation rate, which indicates that a certain exotic order occurs. Surprisingly, we have observed three "steps" in magnetization as a function of magnetic field, suggesting that at least four liquid-like or other quantum states exist under magnetic fields.

cond-mat.str-el↗

arXiv2008

Commensurate Itinerant Antiferromagnetism in BaFe2As2: 75As-NMR Studies on a Self-Flux Grown Single Crystal

We report results of 75As nuclear magnetic resonance (NMR) experiments on a self-flux grown single crystal of BaFe2As2. A first-order antiferromagnetic (AF) transition near 135 K was detected by the splitting of NMR lines, which is accompanied by simultaneous structural transition as evidenced by a sudden large change of the electric field gradient tensor at the As site. The NMR results lead almost uniquely to the stripe spin structure in the AF phase. The data of spin-lattice relaxation rate indicate development of anisotropic spin fluctuations of the stripe-type with decreasing temperature in the paramagnetic phase.

cond-mat.supr-con↗

arXiv2008

Band Jahn-Teller Instability and Formation of Valence Bond Solid in a Mixed-Valent Spinel Oxide LiRh2O4

We have synthesized a new spinel oxide LiRh2O4 with a mixed-valent configuration of Rh3+ and Rh4+. At room temperature it is a paramagnetic metal, but on cooling, a metal-insulator transition occurs and a valence bond solid state is formed below 170 K. We argue that the formation of valence bond solid is promoted by a band Jahn-Teller transition at 230 K and the resultant confinement of t2g holes within the xy band. The band Jahn-Teller instability is also responsible for the observed enhanced thermoelectric power in the orbital disordered phase above 230 K.

cond-mat.str-el↗

arXiv2008

A Nuclear Magnetic Resonance Study on Rubrene-cobalt Nano-composites

We implemented a nuclear magnetic resonance (NMR) study on rubrene(C42H28)-Co nano-composites that exhibit an enhanced magnetoresistance (MR) ratio of 80%. The 59Co NMR spin echo experiment enabled clarification of the hyperfine field of Co at the interface between the ferromagnet and the molecules, which has not been investigated for molecular spintronics. An enhanced hyperfine field of the Co was observed in the rubrene-Co nano-composites, which may be related to the enhancement of the MR ratio. This study demonstrates the importance of microscopic investigation of the interface between molecules and ferromagnets that governs spin-dependent transport in molecular spin devices.

cond-mat.mtrl-sci↗

arXiv2007

A Novel Ordered Phase in SrCu2(BO3)2 under High Pressure

We report results of 11B NMR and susceptibility measurements on the quasi 2D frustrated dimer spin system SrCu2(BO3)2 under high pressure. At 2.4 GPa and in a magnetic field of 7 T, NMR lines split with decreasing temperature in two steps. A gradual splitting below T=30 K breaking the four-fold symmetry of magnetic response is followed by a further sudden splitting below 3.6 K. The latter indicates a magnetic phase transition, which is also marked by a kink in the susceptibility at 1.44 GPa. From the magnetic hyperfine shift data, we conclude that the low-T phase has a doubled unit cell containing two types of dimers, one in a nearly singlet state and the other with a finite magnetization down to T=0.

cond-mat.str-el↗

arXiv2005

Structural Transitions and Magnetic Structure in NH4CuCl3 via 14N-NMR

We report results of 14N-NMR experiments on NH4CuCl3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole splitting parameter $ν_{z}$ splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both $ν_{z}$ and the magnetic hyperfine shift $K_{z}$ are resolved in the NMR spectrum for general field directions. We then conlude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Ruegg et al. [Phys. Rev. Lett. 93 (2004) 037207], we propose that triplet dimers in the 1/4-plateau is formed not between the nearest neighbor pairs but over different chains.

cond-mat.str-el↗