Y. Ajiro

Photo-Induced Phenomena in Isolated Selenium Chains

Photoacoustic spectroscopy, ESR and EXAFS measurements have been carried out on the Se chains isolated in the channels of mordenite crystal with 6.7 A diameter. The optical gap is estimated to be 2.3 eV at room temperature, which is much larger than that of trigonal Se. At low temperatures, illumination with the light having energy near or above the optical gap induces new absorption bands at 1.85 and 2.25 eV, which are associated with paramagnetic defects. Higher energy light (>2.9 eV) shifts the absorption edge to lower energy. The fluctuations in the covalent bond length increase with illumination, though the helical chain structure is preserved. These changes vanish by annealing at room temperature. The phenomena are reversible in the illumination and annealing cycle. The photo-induced absorption and the edge shift are reduced when the induced bands are excited optically.

Paramagnetic relaxation and adiabatic cooling in Cu(NO3)2 · 2.5H2O

Abstract Cross relaxation among Cu 2+ ion pairs in Cu(NO 3 ) 2 · 2.5H 2 O has been studied by high frequency susceptibility measurement up to 50 kG. A temperature minimum was observed at 36 kG due to cooling and heating in the course of adiabatic magnetization of the salt.

Magnetization measurements in very high pulsed fields produced by a single-turn coil system

Abstract Magnetization measurements are tried in megagauss fields produced by a single-turn coil system at temperatures down to 4.2 K. The measuring apparatus and techniques are described as well as the observed metamagnetic transition of the 1-D Ising antiferromagnet CsCoCl3. Application to other systems is also discussed.

Hysteresis loops and adiabatic Landau-Zener-Stückelberg transitions in the magnetic molecule {V6}.

We have observed hysteresis loops and abrupt magnetization steps in the magnetic molecule {V(6)}, where each molecule comprises a pair of identical spin triangles, in the temperature range 1-5 K for external magnetic fields B with sweep rates of several Tesla per millisecond executing a variety of closed cycles. The hysteresis loops are accurately reproduced using a generalization of the Bloch equation based on direct one-phonon transitions between the instantaneous Zeeman-split levels of the ground state (an S=1/2 doublet) of each spin triangle. The magnetization steps occur for B approximately 0, and they are explained in terms of adiabatic Landau-Zener-Stückelberg transitions between the lowest magnetic energy levels as modified by an intertriangle anisotropic exchange of order 0.4 K.

Single crystal susceptibility of the S=1 one-dimensional Heisenberg antiferromagnet AgVP2S6

Abstract A reliable single crystal susceptibility data is reported to reveal the ground state property of the S = 1 one-dimensional Heisenberg antiferromagnet AgVP2S6. The observed susceptibilities for the parallel and perpendicular direction to the chain axis fall down rapidly as the temperature is lowered down to about 50 K, revealing a non-magnetic singlet ground state nature of the system. The results are interpreted quantitatively within the framework of the Haldane prediction, giving the quantum gap of E g k B ≈ 250 K which should be compared with 320 K from neutron scattering.

Magnetisation measurements in ultra-high magnetic fields produced by a single-turn coil system

A technique has been developed for measuring magnetisation in very high magnetic fields (up to 90 T) produced by single-turn coils. The system is applied for measurements of high-field magnetisation of magnetic substances in a wide temperature range from room temperature down to 4.2 K. In a simple magnetic system, the one-dimensional Ising antiferromagnet CsCoCl3, the magnetisation curve was successfully obtained up to 90 T including a stepwise change around 40T at 4.2 K.

Spin frustration in CsCoCl3

Abstract In connection with spin frustration, spin dynamics was studied in magnetically ordered phases of CsCoCl 3 by neutron scattering, ESR and NMR measurements. It was concluded that in the high temperature phase the domain walls travel in the frustrated magnetic chains whereas they come to a standstill in the low temperature phase.

Sr2FeO3 with Stacked Infinite Chains of FeO4 Square Planes

The synthesis of Sr2FeO3 through a hydride reduction of the Ruddlesden-Popper layered perovskite Sr2FeO4 is reported. Rietveld refinements using synchrotron and neutron powder diffraction data revealed that the structure contains corner-shared FeO4 square-planar chains running along the [010] axis, being isostructural with Sr2CuO3 (Immm space group). Fairly strong Fe-O-Fe and Fe-Fe interactions along [010] and [100], respectively, make it an S = 2 quasi two-dimensional (2D) rectangular lattice antiferromagnet. This compound represents the end-member (n = 1) of the serial system Sr(n+1)FenO(2n+1), together with previously reported Sr3Fe2O5 (n = 2) and SrFeO2 (n = ∞), thus giving an opportunity to study the 2D-to-3D dimensional crossover. Neutron diffraction and Mössbauer spectroscopy show the occurrence of G-type antiferromagnetic order below 179 K, which is, because of dimensional reduction, significantly lower than those of the other members, 296 K in Sr3Fe2O5 and 468 K in SrFeO2. However, the temperature dependence of magnetic moment shows a universal behavior.

Non-magnetic impurity effect on the spin frustration in CsCoCl3

CsCoCl 3 containing various amount of Mg ++ or Zn ++ ions have been studied by elastic neutron diffraction and magnetic susceptibility measurements. With increasing impurity concentration, T N1 decreases gradually, whereas T N2 could not be found down to 1.6 K even in the crystal with 0.2% impurity. The observed phenomena were interpreted in terms of the random field due to impurity which destroys the long range ordering. One third of magnetic chain segments with odd number of Co ++ ions remains paramagnetic even below T N1 and frozen in the antiferromagnetic lattice at about 3 K irrespective of composition. The interchain interaction was evaluated from the freezing temperature to be 1.4 K. The present results afford another evidence for the distinguished spin frustration in CsCoCl 3 .

Bose–Einstein Condensation of Quasi-Two-Dimensional Frustrated Quantum Magnet (CuCl)LaNb2O7

A quasi-two-dimensional S = 1/2 Heisenberg square-lattice antiferromagnet (CuCl)LaNb 2 O 7 is studied by specific heat and magnetic susceptibility measurements in external fields up to 14 T. The experimental results in low fields verify the absence of the order–disorder transition, as reported previously. By further application of magnetic fields, we obtain direct evidence for the phase transition, which can be interpreted as the Bose–Einstein condensation (BEC) of magnons. However, the critical field of 10 T significantly deviates from that estimated from the zero-field gap, indicating that, unlike known experimental examples, the one-magnon mode is not a primary source to drive the BEC.