R. Buder

New antiferromagnetic properties of manganese gallium sulfides MnGa2S4-α and MnGa2S4-β

Abstract The α and β phases of MnGa 2 S 4 -α and β have been prepared and characterized. Single crystals of the α-phase have been grown by the vapor phase transport method ; their structure found monoclinic, space group C 2/c, has been refined by the X-ray technique and the positional and thermal parameters are reported. Both compounds are antiferromagnetic with Neel temperatures of 23.5 K and 11 K, for α and β respectively. EPR spectra show a single line, exchange-narrowed at 300 K, with a strong critical broadening above T N . These results are discussed in relation with the crystal structure and the possible superexchange interactions between the Mn 2+ .

Magnetic and transport properties of some Ln-Ba-Cu-O (Ln=Yb, Tm, Sm) superconducting oxides

Abstract Powders of Yb-Ba-Cu-O, Tm-Ba-Cu-O and Sm-Ba-Cu-O have been synthetized. Electrical resistivity and magnetic susceptibility measurements show that they are superconducting with onset transition temperatures in the range 91 K – 85 K. The thermopower, in these oxides as well as in Sr x La 2−x CuO 4−δ and Ba 2 YCu 3 O 7−δ , is found to be positive between T c and 300 K. Hysteresis of the diamagnetic magnetization is also reported.

Short time study of the fishtail effect in Ba0.6K0.4BiO3 single crystals

Abstract A magnetization study using three techniques with different time scales down to 10 −4 s has been performed on single crystals of the three-dimensional high- T c superconductor Ba 0.6 K 0.4 BiO 3 . A fishtail effect (second peak in M ( B )) is observed in a large temperature range down to the shortest time scale. At high temperature ( T = 0.74 T c ) the peak is still visible for the shortest time scale but disappears with increasing waiting time showing that it is not related to flux creep phenomena. The origin of the fishtail effect in high- T c superconductors is discussed.

Magnetic properties and 151Eu Mössbauer spectroscopy of a new molybdenum bronze:Eu0.08MoO3

Abstract The magnetic properties, the Mossbauer resonance between 2 K and 300 K, the low temperature specific heat have been studied on Eu 0.08 MoO 3 : a new molybdenum bronze. The magnetic transition which takes place at 4.3 K is due to the ordering of the Mo V spin and not to the europium which stays in a trivalent state in the explored temperature range.

EPR studies of the charge density wave state of A0.3MoO3 (A = K, Rb)

Abstract We have performed EPR measurements between 4.2 K and 100 K at 9.4 GHz on A0.3MoO3 (A = K, Rb) single crystals. The EPR spectra, due to Mo5+ sites arising from non stoechiometry, are found to be strongly dependent on the thermal history of the samples. We suggest that this is the result of changes in the spatial configuration of charge density wave structural defects coupled to point lattice defects.

Field modulated microwave absorption and irreversibility line in YBa2Cu3O7 thin films

Abstract Measurements of the field modulated microwave absorption of YBa2Cu3O7 thin films as a function of temperature and magnetic field are presented. The Fourier spectrum of the reflected microwave power below Tc reveals a peak at the fundamental frequency of the modulation field and its even harmonics. The amplitude of the even harmonics decreases rapidly when the dc field superimposed on the modulation field is increased, then shows a maximum and vanishes above a characteristic field B ∗ . We show that this maximum is related to the full penetration of the modulation field and that the B ∗ (T) curve can be viewed as an irreversibility line which is consistent with that obtained from irreversible magnetization measurements.

Field modulated microwave absorption in YBa2Cu3O7−X thin films

Abstract We report investigations on the field modulated non resonant microwave absorption in the superconducting state of textured YBa 2 Cu 3 O 7−x thin films. The temperature dependence of the modulated signal shows irregular structures below T c which are shifted towards lower temperature with increasing the amplitude of the modulation field. The modulated signal shows, as a function of the applied field, a rather regular pattern of narrow lines. These results are ascribed to a distribution of weak intergrain Josephson links.

Paramagnetic resonance and microwave studies of YBa2Cu3O7−δ and SmBa2Cu3O7−δ

Electron paramagnetic resonance and microwave absorption in thin films and bulk samples of RBa 2 Cu 3 O 7−δ (R = Y, Sm) is reported. A single line with g2.05 is observed at 300 K. Below T c , a strong low field absorption signal, peaked around 40 G in bulk samples and 300 G in thin films, appears accompanied with large fluctuations. The EPR signal is ascribed to Cu 2+ ions in non-superconducting regions and the low field one to flux penetration.

Magnetic properties of YBa2Cu3O7 superconducting thin films

Abstract The magnetic properties of thin films of YBa 2 Cu 3 O 7−x prepared by reactive radiofrequency sputtering on single crystalzirconia substrates have been measured. In the best samples, the perpendicular low field susceptibility (B⊥ film plane) is found giant at low temperature, as expected from a large demagnetizing field. Critical currents have been estimated from the hysteresis loop. Preliminary measurements of the upper critical field are also reported.