Proximity in hybrid superconductor/ ferromagnetic structures

The coexistence of superconductivity and ferromagnetic orders has been the subject of study for many years. It well known that these materials possess two competing order parameters; however the two order parameters can coexist under special circumstances inducing interesting physical phenomena. In recent years the demand of ultra-low-power, high density cryogenic memories has brought considerable interest to integrate superconducting and magnetic thin films in one structure to produce novel memory elements. The operation of the device depends on the unusual electronic properties associated with the Superconductor (S) /Ferromagnetic (F) proximity effect.

Niobium (Nb) based Josephson junction devices were fabricated with barriers containing two ferromagnetic layers separated by a normal metal space layer. In device operation, electrons in the superconductor are injected into the ferromagnets, causing the superconductor wavefunction to shift its phase and decay in amplitude. Such devices have two different states that depend on the relative magnetization of their ferromagnetic barrier layers, parallel or antiparallel. In these different states, the junctions have different phase shifts and critical currents. Superconducting circuits containing these devices can be designed to operate as memory cells using either one of these outputs.

To quantify the shift in phase and amplitude decay of the wavefunction through a common ferromagnet, permalloy, a series of Nb/permalloy/Nb Josephson junctions with varying ferromagnetic layer thicknesses were fabricated. Data have shown that the optimal thickness of a fixed layer composed of permalloy is 2.4 nm, as it shifts the wavefunction phase to π/2, its “pivot point.” If set to precisely this value, the free layer in SFNF'S junctions will switch the junction into either the 0 or π state depending on its magnetic orientation. To minimize the free-layer switching energy dilute Cu-permalloy alloy [Cu0.7(Ni80Fe20)0.3] with a low magnetic saturation (Ms of ~80 emu/cm3) was used as the free layer. These devices exhibit switching energies at small magnetic fields, demonstrating their potential use for low power non-volatile memory for superconductor circuits.

Lastly, to study the proximity effect using other potentially-useful ferromagnetic layers, measurements were performed on Nb/F bilayers and Nb/F/AlOx/Al tunnel junctions with ferromagnets Ni8Fe19, Ni65Fe15Co20, and Pd1-xNix. The dependence of the critical temperature of the bilayers and density of states that propagated through the ferromagnetic layer were studied as a function of thickness. From this study, crucial magnetic and electrical parameters like magnetic coherence lengths (ξF), exchange energy (Eex), and the rate of shift in the wavefunction’s phase and amplitude as a function of thickness were determined.