Josephson junctions incorporating a conical magnetic holmium interlayer

Josephson junctions incorporating a conical magnetic spacer layer of Ho were measured as a function of spacer layer thickness and an applied magnetic field. The characteristic voltage decreased as a function of layer thickness without the obvious presence of any $\pi$ phase shifts, although the decay was somewhat fragmented. The decay is exponential with a decay coherence length of ${\xi }_f\approx 4.34$ nm and the Ho mean-free path is calculated to be $l_{\text{Ho}}=0.28$–$0.87$ nm, which suggests the system is in the dirty limit. The dependence of the junction critical current on the magnetic field is nonmonotonic with interlayer thickness, but is explicable in terms of the Ho long-wavelength magnetic structure.

Figure 1

High-angle x-ray diffraction data for a Nb/Ho bilayer film with a thin Nb capping layer. The peaks of interest are highlighted and labeled. The sharp peak originates from the Si substrate. Inset: (left) An idealized representation of the Ho magnetic structure at low temperature ($<$20 K). In this regime $\alpha ={80}^{\circ }$ and on average $\beta ={30}^{\circ }$. (right) A representation of the 101 plane set, where $\gamma ={61}^{\circ }$.

Figure 2

Saturation magnetization vs Ho layer thickness at 10 K. Inset: Corresponding magnetization loops for the samples shown in the main panel.

Figure 3

The product of device resistance and area plotted against device thickness at 4.2 K. Linear fits to the data give both the resistivity of the Ho interlayer and the interfacial resistance. Inset: A typical example of an $IV$ curve and the RSJ model fit.

Figure 4

$I_c{R}_n$ as a function of Ho thickness $d$ at 4.2 K. Shown also are solutions to the Usadel equation for a conical magnetic interlayer, discussed in the text.

Figure 5

The critical current $I_c$ as a function of applied magnetic field ${\mu }_0\mathrm{H}$. The characteristic Fraunhofer pattern is shown for two devices with the same Ho thickness $d$, but differing areas. Also shown are theoretical fits to the data.

Figure 6

A function of the applied magnetic field required to minimize the $I_c$ and the junction lateral dimension plotted against the junction Ho thickness $d$. The fits to the plot are discussed in the text.