The authors fabricated high quality Nb∕Al2O3∕Ni0.6Cu0.4∕Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11K. The critical current density in the π state was up to 5A∕cm2 at T=2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.

High quality ferromagnetic 0 and π Josephson tunnel junctions

Publisher Summary This chapter discusses the transport properties of ferromagnets. The focus is on transport properties of the transition ferromagnets Fe, Co, and Ni and their alloys. In pure ferromagnetic metals, the “internal magnetoresistance” enhances the resistivity that is no longer proportional to the concentration of impurities. This effect is particularly important for Fe and a high transverse magnetoresistance. For demagnetized Fe polycrystals, it was pointed out that the apparent residual resistivity ratio would never increase beyond about 300, however pure the sample. It is now standard practice to measure the low temperature resistivity of Fe samples in a saturating longitudinal magnetic field to eliminate transverse magnetoresistance. This can reduce the apparent resistivity by a factor of five or more. In principle, a correction should still be made for the longitudinal magnetoresistance. In Ni samples, the enhancement of the residual resistivity by the internal magnetoresistance is less important than in Fe but still significant. For non-magnetic alloys, the low temperature magnetoresistance behavior generally follows Kohler's rule. Schwerer and Silcox showed by a careful study of dilute Ni alloy samples that for a given series of alloys the ordinary magnetoresistance follows a Kohler's rule, but that the Kohler function varied considerably with the type of scatterer.

Chapter 9 Transport properties of ferromagnets

Pd-Ni alloys are of special interest as a system where a critical composition for the paramagnetic to ferromagnetic transition exists. Interpretation of the magnetization as a function of the field, temperature and concentration for this system is not quite straightforward and previous measurements have not examined the low field behaviour in detail. New data on this system have been obtained and careful analysis with the help of Arrott plots yield Curie temperatures T0 which extrapolate to the same value for the critical concentration as that found previously from the resistivity measurements. Existing susceptibility results for some paramagnetic Pd-Ni alloys appear to conform with this value for the critical concentration and in particular the specific heat data, where disagreement has been cited previously, are reanalysed and shown to be in agreement also with the authors value of 2.3+or-0.05 at% Ni.

https://iopscience.iop.org/article/10.1088/0305-4608/7/11/020/pdf

Critical concentration for the onset of ferromagnetism in Pd-Ni alloys

Electronic and magnetic states in metallic compound—II: Electron transport and magnetic susceptibility in NiAl and FeAl

Superconductor—ferromagnet—superconductor (SFS) π- junctions1 were investigated experimentally. The transitions to π-state and back to the conventional 0-state were found to result from temperature decrease or ferromagnetic interlayer thickness increase. Thickness dependence of the critical current was studied and nonmonotonic behavior was observed. The SF-proximity effect was also examined on SF-bilayers. It is shown that, due to the spatial oscillations of the induced superconducting order parameter in the ferromagnet, the critical temperature of the SF-bilayer has a minimum when the thickness of the ferromagnetic layer is close to a quarter of the oscillation period. PACS numbers: 74.50.+r, 85.25.Cp.

Superconductor—Ferromagnet—Superconductor π-junctions

Anomalous Electrical Resistivity and the Existence of Giant Magnetic Moments in Ni-Cu Alloys

Electrical resistivity and magnetoresistance of paramagnetic Ni-Cu giant-moment alloys

Electrical resistivity of giant moment systems: NiRh

We have measured the resistivity of a series of Ni–Cu alloys as a function of temperature between 1.5° and 300°K, and as a function of field from 0 to 50 kG at 4.2°K. Experiments have been completed on nine paramagnetic alloys ranging in composition from 30 to 44 at.% Ni. A minimum occurs at low temperatures1 and the depth of the minimum increases with Ni content. At higher temperatures, the slope of the resistivity‐temperature curve decreases with increasing temperature, and in fact, becomes negative for high nickel content. The high‐field magnetoresistance is negative in all cases and is proportional to the concentration of giant polarization clouds2 as a function of Ni–Cu composition. These properties may be attributed to the scattering of conduction electrons from polarization clouds acting as large superparamagnetic moments. Results will also be presented for alloys beyond the critical composition for ferromagnetism.

R. W. Houghton

Papers

Anomalous Electrical Resistivity and the Existence of Giant Magnetic Moments in Ni-Cu Alloys

Electrical resistivity and magnetoresistance of paramagnetic Ni-Cu giant-moment alloys

Electrical resistivity of giant moment systems: NiRh

Electrical Resistivity of the Ni–Cu Giant Moment System