Y. Nakazawa

Bulk ferromagnetism in the β-phase crystal of the p-nitrophenyl nitronyl nitroxide radical

Transition to a ferromagnetic long-range ordered state was found at 0.60 K in the orthorhombic β-phase crystal of p-nitrophenyl nitronyl nitroxide (or 2-(4′-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-N-oxide in the IUPAC nomenclature). This finding follows our recent discovery of the first organic bulk ferromagnet in the triclinic γ-phase crystal of the same compound.

First observation of an antiferromagnetic phase in the Y1Ba2Cu3Ox system

The magnetic properties of four different phases of the YBa2Cu3Ox system, including the famous 90 K superconductor, have been probed by the positive muon spin relaxation method (µ+SR). The oxygen-deficient tetragonal insulating phase (x~6.2) was found to be magnetically ordered, thus most likely antiferromagnetic, near room temperature.

Effects of oxygen stoichiometry and ordering in Ba2YCu3Oy (6≤y≤7)

Abstract We undertook a careful investigation on the effects of oxygen stoichiometry and ordering in BYCO samples prepared by an improved technique and found conspicuous effects of oxygen ordering in various properties. The present results of resistivity, magnetic susceptibility and specific heat experiments provide strong evidences for decisive role of the ordering of Cuue5f8O chain around Cu(1) sites. From low temperature specific heat measurements, we found an almost constant residual λ ∗ value in the whole range of y, which is tentatively interpreted as due to low energy excitations of the spins of Cu(2).

μ+SR Studies of Magnetic Properties of the YBa2Cu3Ox System

Samples of YBa 2 Cu 3 O x in four different phases have been probed by positive muon spin relaxation (µ + SR) method. Each phase was found to show different magnetic behavior. In the famous orthorhombic 90K superconductor ( x ∼6.9), neither magnetic ordering nor magnetic fluctuations were observed. However, in another orthorhombic superconducting phase ( x ∼6.4, T c ≈60 K) a magnetic fluctuation was observed below 7 K via muon spin-nuclear spin double relaxation. Its magnetic ordering temperature was expected to be below 2.4 K. Similar behavior was observed in an oxygen-rich ( x ∼6.5) tetragonal insulating phase, whereas the oxygen-deficient ( x ∼6.2) tetragonal insulating phase was found to be magnetically ordered, most likely antiferromagnetic, near room temperature. This is the first observation of a magnetically ordered state in the YBa 2 Cu 3 O x system.

Magnetic properties of the S=1/2 antiferromagnetic triangular lattice LiNiO2

The authors measured susceptibility, resistivity and specific heat of the S=1/2 anti-ferromagnetic triangular lattice LiNiO2. In addition to the formerly recognized characteristic temperatures TN1( approximately 210 K) and TN2( approximately 20 K), they found another characteristic temperature TN3( approximately 70 K) from the susceptibility measurement, at which three-dimensional short-range order presumably sets in at the expense of the planar frustration. In the range between TN1 and TN3, an exponentially increasing susceptibility with decreasing temperature was disclosed, which is believed to be one of the characteristic features of the planar frustration in this system. The specific heat results revealed that the magnetic entropy gradually decreases from R In 2 without any pronounced anomalies corresponding to a long-range magnetic order down to 0.4 K. The nature of the ordering process of LiNiO2 is discussed by referring these characteristic temperatures.

Low-temperature specific heat of orthorhombic and tetragonal phases of Ba2(RE)Cu3O7−δ (RE=Gd, Dy, Ho, ErandTm)

Abstract From the specific heat measurements on the superconducting (orthorhombic) and the non-superconducting (tetragonal) phases of Ba 2 (RE)Cu 3 O 7-δ , we found a magnetic ordering for RE=Gd, Dy and Er, and no long-range ordering for RE=Ho and Tm in the temperature range studied (0.4–6.0 K). The orthorhombic phase of the former group revealed a phase transition of two-dimensional Ising type, while the tetragonal phase of Dy and Er compounds are of two-dimensional XY -type. However the difference in ordering temperature of these two phases is very small. This point is briefly discussed in relation to the superconductivity in these oxides.

Magnetic and superconducting behaviour of the oxides, Pr1-xGdxBa2Cu3Oy

Abstract The results of our investigation on the superconducting behaviour of the oxides Pr 1− x Gd x Ba 2 Cu 3 O y by resistivity and AC susceptibility measurements suggest that the superconducting transition temperature ( T c ) increases monotonically with x for x ≥0.6. The concentration of Pr required to depress T c in this Gd series appears to be smaller than in the corresponding Y series, possibly due to volume dependent hole trapping effects. Specific heat measurements were performed in the temperature range 0.4–20 K in order to understand the magnetism of the rare-earth ions. Interestingly, Pr and Gd ions undergo magnetic ordering independently though both are presumably present in the same magnetic sub-lattice and the observed ordering temperatures rule out any direct intersite Pr-Gd magnetic interaction in this series of compounds.

Specific heat study on Ba2YCu3O7 samples with a double superconducting transition

Abstract We report the results of high and low temperature specific heat study on sintered samples of Ba 2 YCu 3 O 7 . We found that some samples exhibit two distinct peaks of specific heat around 90 K. The result provides strong evidence for the existence of another superconducting phase akin to the well-known orthorhombic phase. At low temperatures these samples revealed a small value of γ, about 4.3 mJ/mol-f.u.-K 2 and no more of the anomalous upturn of C p /T. These new features in the specific heat are discussed in relation to other physical properties.

Transition from valence-fluctuating state with a gap to magnetic Kondo state in CeNi1−xCuxSn

Abstract Magnetic, thermal and transport measurements on the pseudo-ternary system CeNi1−xCuxSn (0 ≤ x ≤ 0.5) demonstrate that a valence-fluctuating state with a pseudo-gap for x = 0 evolves to an antiferromagnetic Kondo state for x ≥ 0.13 through a heavy-fermion region. The energy gap for x = 0 was found to be smeared out not only by the partial substitution of about 10% of Cu but also by magnetic fields higher than 20 T.

Characterization of the high-tc phase and its metamorphic phases, Ba2YCu3O7−δ

Abstract Sintered samples of good quality of the four different phases, Ba 2 YCu 3 O 7-δ , were reinvestigated by means of resistivity, susceptibility and specific heat experiments. The specific heat of the 90 K superconducting phase revealed a distinct jump at T c of Δ C p / T c = 53 mJ/molK 2 and in the superconducting state a small but finite value of γ of about 8 mJ/molK 2 was discerned at low temperatures. Variation of these parameters in its metamorphic phases with different oxygen content is discussed in relation to other physical properties and sample quality.