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We have carried out the 11 B NMR experiments on single crystal of Ce0.75La0.25B6 in order to investigate the magnetic structure of the antiferromagnetic (AFM) phase III. The NMR spectrum can be well fitted by the theoretical model, which reproduces the spectrum in phase III of CeB6. It suggests that the magnetic structure in phase III of Ce0.75La0.25B6 is similar to that of CeB6. Also, from the analysis of the spectrum in phase III, it is shown that the amplitude of the AFM moment is large enough even just below the phase IV–III transition point, which suggests that the AFM moment may grow up well in phase IV.

Dynamical properties of the ceramic YBa 2 Cu 3 O 7− δ have been investigated by magnetic measurements. We measured ac-susceptibilities at various frequency under zero external field and carried out dynamic scaling analysis near inter-grain transition temperature. The results exhibit scaling behavior for the critical exponents zν =8 and β =0.5. These values are fairly close to the ones for the chiral-glass estimated by Monte Carlo simulations.

Proton NMR measurements have been performed to investigate random-bond effects in a spin-ladder system. We synthesized a random-bond ladder Cu2(C5H12N2)2(Cl1−xBrx)4 and measured the temperature dependence of 1 H spin-lattice relaxation time. The spin-gap energy for a random mixture compound is smaller than those for pure systems. The result suggests that the ground state of the random-bond ladder approaches to a gapless state.

We carried out the 11 B NMR experiments of CeB2C2 and determined the parameters of the electric field gradient and the components of the hyperfine coupling tensors at 11 B site in order to elucidate the magnetic properties. The nuclear quadrupole frequency and the asymmetry parameter are estimated to be ν Q =0.78 MHz and η=0.22. The hyperfine coupling tensors are highly anisotropic and are deduced to be A X =+0.39 kOe / μ B and A Y =−0.51 kOe / μ B , where the anisotropic hyperfine coupling can be explained by the atomic dipolar field from the 4f electrons.

We report the results of 125Te and 195Pt NMR measurements on (Ir,Pt)Te2 in order to elucidate the characteristic electronic states. For PtTe2, the NMR spectrum exhibits a sharp line, which shows the uniaxially symmetric powder pattern due to the anisotropic Knight shift. The Knight shift is almost independent of temperature and is larger than that for IrTe2. Also, the nuclear spin-lattice relaxation rate 1/T1 of PtTe2 is proportional to the temperature in a wide temperature range, that is, obeys the Korringa relation as expected for simple metallic systems. From the analyses of the Knight shift and 1/T1, it is suggested that the antiferromagnetic correlations slightly exist.

We report the results of the 121,123 Sb-nuclear quadrupole resonance (NQR) measurements on the filled skutterudite R Fe 4 Sb 12 ( R = La, Ce) synthesized under high pressure (HP), and compare them with those for the sample synthesized under ambient pressure (AP) in order to understand the effects of the R -site filling fraction on magnetic properties. The NQR spectra for the HP samples are much sharper than those for the AP samples, suggesting that the R -site filling fraction is high close to unity and the electronic states around Sb nuclei become uniform. In LaFe 4 Sb 12 -HP, the nuclear spin–lattice relaxation rate divided by temperature, 1/ T 1 T , exhibits the Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with the Weiss temperature θ=12 K above 30 K, indicating the development of ferromagnetic fluctuations. The magnitude of θ for the HP sample is smaller than that for the AP sample, which suggests that the system becomes close to the vicinity of ferromagnetic instability as the La-site fil...

We have measured 125 Te NMR of IrTe2 in order to elucidate the origin of the anomalous behaviors in electrical and magnetic properties around 270 K. In high-temperature region, the NMR spectrum exhibits a sharp line. On the other hand, in low-temperature region, the spectrum shifts to higher magnetic field and splits into three lines. Also, the nuclear spin-lattice relaxation rate, 1/T1, is proportional to the temperature in both temperature sides; Korringa-like behavior which is characteristic of a metallic state. From the T dependences of the spectrum and 1/T1 around 270 K, it is suggested that these anomalous behaviors may not be due to the charge density wave formation but be caused by a kind of lattice distortion at low temperature.

Intergrain ordering nature of weak-sintered ceramic of YBa2Cu4O8 (Y124) is studied experimentally by ultra precise electric resistivity observation together with magnetic measurements for linear and nonlinear susceptibilities. For this purpose, a new method to detect pure linear resistivity down to the order of 1 nΩm has been developed with the help of combination technique of pulse delta method and FFT treatment. Making use of this experimental system for several weak-sintered Y124 samples, it has been revealed that linear resistivity first drops toward zero steeply with decreasing temperature, and then turns abruptly upward to forms faint maximum at lower temperature. Such peaking of electric resistivity has been found to occur below the intergrain ordering temperature that was estimated by the magnetic behavior. This newfound phenomenon, to say 'revival of resistivity', is being regarded as general nature for the weak-sintered ceramics, and considered to be an intrinsic frustration effect for actual d-wave type ceramics of irregular network structure.

Superconducting behaviours in reduction-treated mixtures of fine Pr124 and Pr123 ceramics synthesized using citrate pyrolysis precursor method are examined by electrical resistivity and magnetic measurements. With increasing the ratio of Pr123, the temperature dependence of ρ(T) changes to be semiconductor-like behaviour hugely, and there exist no broad maximum for as-sintered Pr124/Pr123 (1:1.5) and (1:2) samples contrary with the case of (1:1). By reduction treatment, the ρ(T) curve shows the maximum according to the increase of carrier concentration in the CuO double chain in Pr 124 grains. By subtracting the Pr3+ paramagnetic contribution from the temperature dependence of magnetization, the remnant magnetization shows weak diamagnetism below ~17 K, indicating small Meissner effect. The diamagnetic susceptibility is independent of magnetic field in the measured field range below 20 Oe.

The ceramic YBa2Cu4O8 composed of sub-micron size grains is considered as random Josephson-coupled networks of 0 and π junctions and shows successive phase transitions. The upper transition occurs inside each grain at Tc1 and the lower transition occurs among the grains at Tc2 (< Tc1), where a negative divergence of nonlinear susceptibility is found. This critical phenomenon at Tc2 suggests the onset of the chiral-glass phase predicted by Kawamura and Li. We investigated the magnetic memory phenomena in the temperature dependence of thermoremanent magnetization (TRM). The memory effects are observed in TRM measured on heating after the cooling process with a halt at the temperature Ts below Tc2. The results indicate the existence of a glassy phase such as the chiral-glass one.

The d-wave ceramic YBa2Cu4O8 superconductor composed of sub-micron size grains is considered as random Josephson-coupled network of 0 and π junctions and shows successive phase transitions. The upper transition occurs inside each grain at Tc1 = 82 K and the lower transition occurs among the grains at Tc2 = 66 K. We measured the temperature dependence of the current-voltage characteristics of the ceramic YBa2Cu4O8 and derived the linear and nonlinear resistivity. The nonlinear resistivity ρ2 and ρ4 have finite values between Tc1 and Tc2 and have the peak at the same temperature Tp = 70 K above Tc2. The result agrees with the theoretical one obtained by Li and Dominguez. They interpreted Tp as the crossover temperature from the normal state phase to a chiral paramagnetic one.

We report on the results of the 73 Ge-NQR measurements for M Pt 4 Ge 12 ( M = Sr, Ba) to clarify the microscopic electronic states and the relationship between superconductivity and rattling. In the normal state, 1/ T 1 is proportional to temperature, showing a Korringa relation 1/ T 1 T = 0.020 and 0.016 (sK) -1 for SrPt 4 Ge 12 and BaPt 4 Ge 12 , respectively. Also, in the superconducting state, 1/ T 1 shows a distinct coherence peak just below T C , which suggests a conventional BCS superconductor.

Thermoelectric power are measured on high-T c oxide superconductors such as quenched YBa2Cu3O7−y (Y-(123)), Bi2Sr2CaCu2O8+y (Bi-(2212)) and Bi2Sr2Ca2Cu3 O1 0+y (Bi-(2223)), and Pr- or Ca-doped YBa2Cu 4 O8 (Y-(124)). There exists a universal relationship between T c and Seebeck coefficient (S) on the above superconductors. The T c shows its maximum at S∼ 0. The S is considered to be related with the carrier concentration which has influence upon the superconductivity. In the quenched Y-(123) and Bi-(2223), however, carrier concentration in the different type Cu-O structures (chain Cu-O and plane Cu-O without apical oxygen, respectively) is also considered to change in the higher S region.

Ca doping effects in YBa2Cu4O8 (124) phase are investigated by the citrate pyrolysis method under both atmospheric and high oxygen pressure. The decomposition temperature of (124) to the Y2Ba4Cu7O15−δ (247) and the YBa2Cu3O7−δ (123) phases decreases with Ca doping. The onset Tc measured by DC susceptibility increases linearly from 81K to 89K with increasing Ca content, x, from x=0 to x=0. 15 in nominal compositions Y1−xCaxBa2Cu4O8. The

YBa2Cu3O7−δ ceramics of small-size grains have been synthesized, and their intergrain orderings are studied by DC magnetizations and by AC magnetic and electric responses at zero DC field. Characteristic temperature dependences of linear resistivity, nonlinear resistivity and nonlinear susceptibility are found near the intergrain ordering point TC2, and the overall feature qualitatively agrees with that of YBa2Cu4O8 ceramics studied previously.

The temperature dependence of 13C spin-lattice relaxation time T1 in YNi2−xPtxB2C (x=0 and 0.8) has been measured. With decreasing temperature the relaxation rate 1/T1 forms a Hebel–Slichter peak just below Tc and after that it decreases exponentially with the gap parameter 2Δ 0 =4.8 k B T c . Below about 4 K it starts to vary linearly with T, where the values of 1/T1T were proportional to H0, the observing magnetic field. This means that the spin diffusion to normal cores is important for the relaxation. These results support the model of the phonon-mediated superconductivity in this system.

11 B NMR in the antiferromagnetic III and III′ phases has been measured for a CeB 6 single crystal which is thought to be more stoichiometric than the sample used previously. The spectra obtained are well resolved as compared with those obtained previously. The spectra for H ||[1 1 1] are interpreted using a model in which a hyperfine interaction of the 11 B nucleus with the octupolar moment of a Ce atom is taken into account. The parameters such as the amplitude of the octupolar moment are obtained.

Ceramic high-Tc superconductor has intra-grain superconducting transition at TC1 and the inter-grain one at TC2. We investigated the effects of pressure on the ceramic superconductor YBa2Cu3O7-y in the pressure region up to 5.3 GPa, through the AC susceptibility measurements using a miniature diamond anvil cell (mDAC). The TC1 increases against the initial pressure, whereas it turns to decrease at around 2.5 GPa. The TC2 also exhibits similar pressure dependence. The up-and-down change of TC1 has been already observed in the previous single crystal experiment, whereas that of Tc2 was observed through the present study for the first time. From the viewpoint of measurement technique, we also point out some problems on the high pressure AC susceptibility measurements using a SQUID magnetometer.

Ceramic YBa2Cu4O8 samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions, and they show successive phase transitions. The first transition occurs inside each grain at Tc1 and the second transition occurs among the grains at Tc2 (> Tc1), where a negative divergence of nonlinear susceptibility is found. This critical phenomenon at Tc2 suggests the onset of the chiral-glass phase, as predicted by Kawamura and Li. We measured the temperature dependencies of the current-voltage characteristics of the samples and derived the linear and nonlinear resistivities. With decreases in temperature, linear resistivity decreased monotonously and remained at a finite value at temperatures less than Tc2, while nonlinear resistivity diminished continuously for temperatures moving towards Tc2. These results are consistent with the theoretical predictions., Proceedings of the 25th International Conference on Low Temperature Physics (LT 25), August 6-13, 2008, Amsterdam, Netherlands

Superconductive characteristics of Pr2Ba4Cu7O15-δ (Pr247) ceramics with crystalline phase inhomogeneity for the stacking structures is examined experimentally, using reference observations for multi-phased ceramic sample consists simply of PrBa2Cu4O8 (Pr124) and PrBa2Cu3O7-δ (Pr123). After reduction treatment by vacuum-heating, the reference multiphased sample shows onset of abrupt electric resistivity dropping and also weak Meissner magnetization below ~20 K. The results suggest that superconductivity at CuO double chains in Pr124 grains is caused by charge transfer from neighbouring Pr123 grains. Such a charge transfer effect is thought to occur also in Pr247 sample including phase inhomogeneity.

We have synthesized the polycrystalline samples of Ge-based filled skutterudites R Pt 4 Ge 12 ( R =La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt 4 Ge 12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt 4 Ge 12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt 4 Ge 12 and NdPt 4 Ge 12 show the well-localized nature of 4 f -electrons. The NMR measurements in CePt 4 Ge 12 revealed that the nuclear spin–lattice relaxation rate 1/ T 1 shows an activation-type temperature dependence 1/ T 1 ∝exp (-Δ/ k B T ) with an energy gap Δ/ k B =150 K above 100 K, whereas it is proportional to the temperature belo...

13 C and 11 B NMR studies have been made on the quaternary superconductor Y(Ni1−xPtx)2B2C with x=0 and 0.4. Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel–Slichter peak in case of 13 C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor

The plane-site Cu(2) and chain-site Cu(1) NQR spectra in Y 1− x Pr x Ba 2 Cu 3 O 7−δ have been measured in order to study the mechanism of the suppression of T c by Pr doping from the microscopic view point. It is clearly seen that the NQR spectra become broader and the peak NQR frequencies decrease with increasing x for both Cu(2) and Cu(1), which means that both Cu(2) and Cu(1) on-site holes decrease with increasing x according to Ohta-Maekawa's equation. Another interesting point is that the spectra show fine structures with increasing x , which may concern the randomness observed by neutron scattering.

Linear and nonlinear electric response to the external AC current is observed in superconductive ceramics of a stoichiometric compound, YBa 2 Cu 4 O 8 . A broad negative anomaly of a third-harmonic in-phase component V′ 3ω is found around the inter-grain ordering temperature T C2 determined by previous magnetic measurements. The anomaly gets remarkable with decreasing AC current keeping the maximum position constant. From these results, a negative divergent anomaly of nonlinear electric response is suggested as an intrinsic critical phenomenon of inter-grain ordering in this system.

13 C NMR study has been made of the quaternary superconductor YNi 2 B 2 C. Temperature dependence of T 1 and K in the normal state showed normal metal behavior. Temperature dependence of T 1 in the superconducting state strongly suggests that the quaternary borocarbide is a conventional BCS superconductor.

We report the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic point of view. The nuclear spin–lattice relaxation time T 1 of Sb nuclei deviates from the relation T 1 T = constant above 4.2 K, where 1/ T 1 T has a Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with θ∼30 K. The temperature dependence of the Knight shift of 139 La nuclei, which is related to the susceptibility at q = 0, is scaled to that of 1/ T 1 T above 40 K. This relation strongly suggests that ferromagnetic fluctuations are predominant in LaFe 4 Sb 12 . We also point out that LaFe 4 Sb 12 is situated close to the ferromagnetic instability due to the small Weiss temperature in the Curie–Weiss behavior of 1/ T 1 T and the Knight shift.

The Pr contentx dependence of the superconducting transition temperatureT c is examined for various oxygen concentrations in Ln1−x Pr x Ba2Cu3O7−y , where Ln is Nd, Gd, Ho, Y and Yb. In the case of Ln≠Nd, ionic radii of which are smaller than that of Pr, the initial rate of decrease ofT c by Pr substitution (-dT c /dx) decreases with decreasing oxygen concentration for 6.6

Pulsed NMR study of13C has been carried out in the quaternary borocarbide superconductor YNi2B2C. The13C nuclear spin-lattice relaxation rate, 1/T 1, in an unoriented powder sample has been measured between 20 and 300K in the normal states. TheT 1 T=constant behavior is found between 20 and 100K which suggests the existence of density of states at Fermi level at carbon site. Furthermore, an anormalous decrease in 1/T 1 T with increasing temperature similar to11B NMR is found above 100K.

Cu(2) NQR spectrum and spin-lattice relaxation rate have been measured in Y 1−x Ba 2 Cu 3 O 7 for x=0.1 and 0.2. The spectrum is different from that reported for oxygen deficient YBa 2 Cu 3 O 6+x . The relaxation rate behavior indicates that the antiferromagnetic fluctuations are suppressed because of the spin gap opening.

Successive superconductive transitions in single phase YBa 2 Cu 4 O 8 ceramics composed of homogeneous grains are examined by AC susceptibility at zero DC magnetic field. In a series of the ceramics with different sintering temperatures intrinsic critical behaviors of X ′ and X ″ are derived from the limits of X ∗ − T behavior when AC field amplitude tends to zero. Compared with the previous DC magnetization measurements, the inter-grain coupling intensity is concluded to change the location of the lower critical point but does not the essential ordering characteristic at all.

The oxygen concentration dependence of T c is examined in Y 1−x Pr x Ba 2 Cu 3 O 7− y . The T c decreases with decreasing the oxygen concentration 7− y in not only Ortho I (6.75≦7− y ≦7.0) but also the region between Ortho II and Tetra (7− y ≦6.6) for the Pr contents of 0≦ x T c increases conversely with decrease of 7− y for 0.3≦ x y ≦6.75). The Pr content dependence of T c for Ortho II is quite different from that for Ortho I, and is preferably similar to that in the stoichiometric oxide superconductor Y 1−x Pr x Ba 2 Cu 4 O 8 .

Superconducting properties are examined in Y 1−x Pr x Ba 2 Cu 4 O 8 synthesized by the citrate pyrolysis method followed by sintering in a one atm oxygen flow. Lattice constants increase linearly, while the orthorhombicity decreases linearly with increasing x. Meissner effect measurements indicate that the onset T c decrease monotonically with the Pr substitution, and disappears at x =0.75. This is caused by the change in the hole concentration with the substitution of Pr for Y.

The citrate pyrolysis method is used to investigate the synthesis process and the superconducting transitions of the YBa 2 Cu 4 O 8 (124) phase. DC susceptibility measurements show that the intrinsic Tc, both for the one bar sintering route and the 90 bar treatment, is close to 82 K. In the former case, however, the reaction temperature must not exceed 800°C, and χ ec and resistive transitions are severely degraded due to the extremely small grain size.

The annealing and quenching effects were examined on Nd 2−x Ce x CuO 4−δ after it is annealed under Ar atmosphere. The quenched samples of x=0.15 show zero resistivity at about 13K. However, zero resistance does not appear in the furnace-cooled samples. On the contrary, dc susceptibility shows that the Meissner volume fraction is nearly the same for both samples. The results indicate that superconductive connection between grains is much weaker in the furnace-cooled sample than in the quenched one.

Structure and electrical property are studied in the quenched Bi 2 Sr 2 Ca 1-x Nd x Cu 2 O 8 + δ system. The quenched samples with 0≦x≦1 have a single phase of the Bi 2 Sr 2 CaCu 2 O 8 + δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 in the chemical formula upon quenching from 800°C into liquid nitrogen. The T c remains a constant (∼90K) for x≦0.15, and decreases linearly with x in the range of 0.15≦x≦0.45, and superconductivity disappears for x≧0.45. The result for the quenched systems, if x is shifted by +0.1 in it, agrees very well with that for the furnace-cooled Bi 2 Sr 2 Ca 1-x Nd x Cu 2 O 8 + δ and Bi 2 Sr 2-x Nd x CaCu 2 O 8 + δ systems. The relationship between the T c and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi 2 Sr 2 CaCu 2 O 8 + δ systems.

Structure, resistivity and susceptibility have been measured in the Bi2Sr2-x RE x CaCu2O8+δ (RE=Nd and Ce) system. The samples with 0≤x≤1 (RE=Nd) and 0≤x≤0.25 (RE=Ce) have almost single phases of the Bi2Sr2CaCu2O8+δ structure. The x-dependence on T c in the Ce-substituted system is almost twice as strong as that in the Nd-substituted one. The valences of Nd and Ce ions in the samples were determined from susceptibility measurement as trivalent and tetravalent, respectively. The hole concentration was determined from the Hall effect measurement. The results confirm that the hole concentration in the Ce-substituted system becomes twice as strong as that in the Nd-substituted one, as expected from the universal relationship between the T c and the hole concentration in the Bi2Sr2CaCu2O8+δ system.

XPS core level and valence band spectra of sintered Bi2Sr2Ca1-x Nd x Cu2O y which manifests a superconducting to semiconducting phase change around x=0.5 have been measured. The fraction of Cu1+ to the total Cu ions evaluated from Cu2p spectra amounts to 7% in Bi2Sr2CaCu2O y , but decreases to 0% with increasing Nd content. From Ca2p, Sr3d, Nd3d, Bi4f and O1s spectra, it is inferred that Ca2+ and Sr2+ are mixed and distributed over both Ca and SrO layers, whereas Nd3+ and Sr2+ are rather separate. From the valence band spectra, a finite density of states at E F is deduced for Ca-rich oxides, but it diminishes in Bi2Sr2NdCu2O y . The analysis of ionic configurations based on the mixed valencies of copper and oxygen ions gives a consistent account of the above spectra and the superconductor to semiconductor transition.

Samples of the Bi2-x Pb x Sr2CaCu2O8+δ system are prepared to be almost a single phase of the Bi2Sr2CaCu2O8+δ structure for a region of 0≦x≦0.15. Substitution of Pb for Bi ions is observed by EPMA at grain level. The T c decreases linearly with x in this region. Comparison with the results for the Bi2Sr2Ca1-x Nd x Cu2O8+δ shows that the hole concentration in [Cu-O] plays a most significant role in determining the T c in the Bi2Sr2CaCu2O8+δ system. The universal relationship between the hole concentration in [Cu-O] and T c is determined in a wide range of concentration in this system.

Structure, resistivity and susceptibility are studied in the quenched Bi2Sr2Ca1-x Nd x Cu2O8+δ system. The quenched samples with 0≦x≦1 have a single phase of the Bi2Sr2CaCu2O8+δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 upon quenching from 800°C to liquid nitrogen temperature. The T c remains a constant (∼90 K) for x≦0.15, and decreases linearly with x for 0.15≦x≦0.45, and superconductivity disappears in the samples with x≧0.45. Referring to the previous results for the furnace-cooled Bi2Sr2Ca1-x Nd x Cu2O8+δ system, the relationship between the T c and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi2Sr2Ca1-x Nd x Cu2O8+δ systems.

Spontaneous subsystem magnetizations in a heterogeneous system Mn(HCOO) 2 ·2D 2 O are separately determined by observing and analyzing the proton NMR at zero field. The temperature depenaence of spontaneous staggered magnetization L A of A subsystem consisting of antiferromagnetically coupled quadratic Heisenberg (100) planes follows to two distinct exponential laws. The critical index β=0.22±0.02 for 2×10 -2 < e

The change in electronic structure caused by the transformation from cubic to tetragonal phase has been investigated in near-equiatomic V-Ru and V-M-Ru (M=Ti, Ta) alloys through measurements of electrical resistivity, Hall coefficient and Knight shift of 51V. The absolute values of these properties in a specimen increase with the transformation, which means a decrease in the electronic density of states. The values in all the specimens are universally dependent on the electron-atom ratio Z in both cubic and tetragonal phases. This universal dependence on Z implies the validity of the rigid-band approximation. The increase in Z lowers the values in the cubic phase, while raising them in the tetragonal phase. These results suggest that the transformation is caused by the band Jahn-Teller effect.

A simultaneous measurement of susceptibility and spontaneous magnetization of a S =5/2 quadratic Heisenberg antiferromagnet with weak canting interaction, Mn(HCOO) 2 ·2H 2 O has been made at exact zero field in the immediate neighbourhood of T N . The critical exponents γ of the staggered susceptibility above T N and β of the spontaneous staggered magnetization below T N are derived to be 1.7±0.1 and 0.31±0.02, respectively in the same temperature range \(5\times10^{-4}{\lesssim}{\varepsilon}({=}|T/T_{\text{N}}{-}1|){\lesssim}1\times10^{-2}\). Such characteristic critical phenomena asymmetric against T N look apparently inconsistent with the scaling law and the universality in the conventional sense.

The fully mapped T - H (temperature-magnetic field) phase diagram of a quasi-two-dimensional (2d) antiferromagnet Mn(HCOO) 2 ·2H 2 O ( T N =3.69 K) has been determined by the measurements of magnetic heat capacity and susceptibility. The critical field has been estimated to be 105±5 kOe at T =0 K, which gives the intra-layer exchange constant | J |/ k B =0.35±0.02 K. By applying external field, an anomalous increase of the Neel temperature T N ( H ) has been found ( T N ( H )≈1.10 T N (0) at H =35 kOe) for the first time in this 2d compound as in the case of quasi-1d compounds.

The superconductivity and structural transformation from cubic to tetragonal phase have been investigated in near-equiatomic M-Ru (M = V, Nb and Ta) alloys through the measurements of X-ray diffraction, electrical resistivity and magnetic susceptibility. The transformation is possibly a second-order phase transition. The electronic specific heat coefficient estimated from the temperature dependence of upper critical field for a transformed sample is smaller than that for a non-transformed sample. The superconducting transition temperature and magnetic susceptibility at 77 K for V-Ru show a peak at the boundary composition between the cubic and tetragonal phases, while the values for Nb-Ru and Ta-Ru vary monotonically without a peak at the boundary composition. Using the McMillan's formula, it is found that the electronic density of states for V-Ru is about 2–4 times larger than the values for Nb-Ru and Ta-Ru, while the electron-phonon interaction is smaller.

Superconductive films composed of Ba2YCu3O7−δ were prepared by fume pyrolysis with a temperature programmed electric furnace. Fumes generated from an aqueous solution of Ba, Y, and Cu nitrates with an aid of supersonicant were introduced into the furnace with flowing O2. Fine particles consisting of Ba, Y, and Cu ions were formed in the furnace and deposited on the surface of a YSZ substrate placed at the top of the furnace. Films thus prepared and annealed at 1273 K showed superconductive properties below 90 K and zero resistivity was observed at 80 K with 100 μA of the current.

Proton NMR of a layer structure antiferromagnet Cu(HCOO) 2 ·2CO(ND 2 ) 2 ·2D 2 O is investigated in detail near the transition temperature T N . The temperature dependence of spontaneous staggered magnetization L 0 is found to be expressed by a single exponential law with an index β=0.22±0.02 in the measured temperature range \(4{\cdot}10^{-4}{\lesssim}\varepsilon{\lesssim}4{\cdot}10^{-1}\). The very weak Ising-type anisotropy is predicted to be essential to the novel critical index. The origin for such a phase transition is discussed, taking the possible canting interaction into account.

Effects of compression and magnetic dilution on the first and second order phase transitions have been examined with Mn(HCOO) 2 ·2H 2 O, which exhibits both of the spin axis reorientation at T R =1.70 K and the antiferromagnetic transition at T N = 3.69 K. The application of hydrostatic pressure ( p ) makes a clear shift of T N ( p ) (d T N ( p )/d p = +0.022 T N ( p 0 ) K·kbar -1 ; p 0 ≃0 kbar) preserving a sharp heat capacity peak. However, the transition at T R ( p 0 ) = 1.70 K becomes quite obscure in the pressure with a broadening of the heat capacity peak around T R ( p )(d T R ( p )/d p = +0.078 T R ( p 0 ) K·kbar -1 ). These phenomena are compared with the phase transitions in the magnetically diluted system with Zn 2+ ions.