Takeshi Anayama

Magnetocrystalline Anisotropy of Co and Co-Ni Alloys

The magnetocrystalline anisotropy constants K u1 and K u2 of Co–Ni alloys containing Ni up to 30% were measured at temperatures between 77 K and 700 K by means of a torque magnetometer. For the alloys containing Ni less than about 7%, the sign of K u1 was found to change from positive to negative in the h.c.p. phase. The temperature at which K u1 becomes zero was found to increase from 510 K for Co to 620 K for 5% Ni–Co alloy. The temperature dependences of K u1 and K u2 are discussed on the basis of the theory developed by Zener and Carr. It is clarified that the temperature dependences of K u1 and K u2 cannot be explained by the power law of the reduced magnetization. The contributions of the magnetoelastic coupling energy to K u1 and K u2 are discussed. The obtained value of Δ K u2 for Co is one-tenth or one-hundredth as small as the value of K u2 .

Superconductive Transition Temperatures of Reactively Sputtered Niobium Films

Niobium films have been examined with respect to their electrical and superconductive properties. The films were deposited by dc and asymmetric ac sputtering techniques in argon with addition of a small amount of nitrogen gas. The films obtained had a wide range of resistivities and transition temperatures depending on the partial nitrogen pressure in the argon atomosphere. The maximum transition temperature of films obtained was 15.3°K, which very close to those reported for bulk NbN. It was found that the asymmetric ac sputtering technique is useful to increase the transition temperature of niobium films sputtered reactively in the presence of a nitrogen gas.

Resistance Anomaly of Nb–Si Thin Films

A resistance anomaly of very thin Nb–Si films is reported. The anomaly is characterized by a sudden drop in the resistance by a factor of ~103 at a temperature (TD) of ~180K without any application of pressure. The lower resistance-state (~zero Ω) is constant to 4.2K so far as we measured. TDdepends on measuring current and on magnetic fields. The anomaly has properties very similar to those of CuCl under high pressure.

Angular dependence of critical currents and transition fields of sputter‐deposited superconducting films

Anisotropies in critical currents and transition fields have been measured for sputter‐deposited superconducting films. For thicker films, the peak at θ = 90° in the Ic‐θ characteristics is strongly marked. As the film thickness decreases, the peak at θ = 90° tends to reduce, and finally disappears in the film thinner than the critical thickness. Furthermore, the transition fields Ht have been found to be anisotropic with respect to the applied field direction θ. The marked peak at θ = 90° in the Ht‐θ characteristics is found for films thicker than the critical thickness.

Simple Voltage-to-Time Converter With High Linearity

A simple voltage-to-time converter having a high linearity of time/voltage relationship is described. This circuit consists of one monostable multivibrator with a constant current source. The circuit does not have a ramp generator and a voltage comparator. The linearity error of the time/voltage relationship is less than ??0.05 percent for a voltage variation from 0.5 to 14 V.

THE UPPER CRITICAL FIELD Hc2 OF NbN FILM PREPARED BY REACTIVE SPUTTERING

The upper critical field Hc2 of about 200 kOe was obtained for niobium nitride films prepared by reactive sputtering. Electron microscopy was used to determine the crystal structure. It was shown that the electron diffraction patterns of these films were that of the NbN with the NaCl crystal structure.

CRITICAL CURRENT DENSITIES IN SUPERCONDUCTING NIOBIUM FILMS SPUTTERED REACTIVELY IN THE PRESENCE OF NITROGEN GAS.

A critical current density of about 2 × 105 A/cm2 in a transverse external field of 73 kOe, parallel to the film plane, was obtained at liquid‐helium temperature for niobium films deposited on glass substrates by sputtering technique in an argon atmosphere with a dilute addition of nitrogen. This critical current density is larger than any other reported values for refractory metal carbides and nitrides with the NaCl structure.

The effects of the substrate temperature on sputter-deposited niobium films

The superconducting transition temperatures and the structural properties of Nb films prepared by sputtering have been studied over a deposition temperature range of −130 to 700°C. Niobium films show a preferred orientation in the <110> direction perpendicular to the substrate surface. The transition temperatures of Nb films deposited onto cryogenic substrates were greatly reduced. A close correspondence between thed spacingd110 and superconductingTc has been found. TheTc of Nb films decreases rapidly with increasing spacingd110.

Critical Current Density in Superconducting Nb3Ge and NbN Films at Fields up to 30 T

The superconducting critical current densities Jc of Nb3Ge and NbN films prepared by sputtering have been studied in fields up to 30 T for the perpendicular field direction H⊥ to the film surface and up to 23 T for the parallel field direction H//. The Nb3Ge film exhibits Jc exceeding 1 ×105(1 ×104) A/cm2 in fields below 21(30) T for H⊥ and below 23 T for H//. The Jc of the NbN film for H⊥ is higher than 1 ×105 A/cm2 in fields below 21 T, while that for H// is much reduced. Their pinning properties and upper critical fields Hc2 are presented.

Superconducting Tc's of Sputter-Deposited Nb–Ge Films and Tc Nonuniformity within Films

The superconducting transition temperature Tc and nonuniformity of Nb–Ge films prepared on sapphire substraies from compound targets by dc sputtering have been studied. The relationships between the transition temperature, the electrical resistivity ratio and the film thickness have been studied using targets with different compositions, and the conditions for preparing high-Tc. Nb–Ge films below ~2000 A in thickness have been determined. The Tc values for Nb–Ge films are appreciably improved in films thinner than ~1000 A with a Ge-richer target (Nb/Ge=2.8) and onset Tcs greater than 20 K are obtained in films with thicknesses of ~500 A. The nonuniformity of Tc in the films has been measured. The high Tc arises from the film surface layer and the Tc variation along the film thickness is greatly affected by changes in the substrate temperature during deposition.