Pt and CoB trilayer Josephson [Formula: see text] junctions with perpendicular magnetic anisotropy

Abstract

We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co[Formula: see text]B[Formula: see text]/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co[Formula: see text]B[Formula: see text] interfaces, allowing us to study barriers with ultra-thin Co[Formula: see text]B[Formula: see text]. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co[Formula: see text]B[Formula: see text] strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to [Formula: see text]. The multiple oscillations in the thickness range [Formula: see text] nm suggests that we have access to the first zero-[Formula: see text] and [Formula: see text]-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions.

Figure 1

Magnetic characterisation of the sheet film samples S-Pt(10)-Co${}_{68}$B${}_{32}$(d${}_{\text{CoB}}$)-Pt(5)-S. (a,b) Magnetic hysteresis loops of the magnetic moment (m) per area acquired at a temperature of 10 K with the applied field oriented out-of-plane for (a) d${}_{\text{CoB}}=0.6$ nm and (b) d${}_{\text{CoB}}=1.4$ nm. The diamagnetic contribution from the substrate has been subtracted. (c) Collated saturation moment per area versus nominal thickness of Co${}_{68}$B${}_{32}$. To model the lower m/area of the thinnest samples in the study, we construct a partial layer coverage model detailed in the text and shown in the inset. The result of fitting the model over the entire data range is shown by the solid line. The extracted magnetisation $M=760\pm 90$ emu/cm${}^3$. The dashed line shows an extrapolation of the linear part of the model to the intercepts. Values of m/area are calculated from the measured total magnetic moments and areas of the samples. The uncertainty in each point is dominated by the area measurements, and is less than 5%.

Figure 2

(a) Schematic cross section of the S-F-S Josephson junction device (not to scale). The thickness of each layer is given in nanometers. The Co${}_{68}$B${}_{32}$ layer thickness, d${}_{\text{CoB}}$, is ranged from 0.2 to 1.4 nm. (b) Product of critical Josephson current times normal-state resistance versus applied magnetic field for the device depicted in (a) with d${}_{\text{CoB}}=0.6$ nm at 1.8 K. $I_c$ is determined from the measured I–V characteristic at each field value and $R_N$ is the average normal state resistance across all measured fields. The uncertainty in determining $I_c{R}_N$ is smaller than the data points. The data are fit with Eqs. (2) and (3). Inset: The I–V characteristic at zero applied field with fit to Eq. (1).

Figure 3

Top: Product of critical Josephson current times normal-state resistance versus nominal Co${}_{68}$B${}_{32}$ thickness for ferromagnetic Josephson junctions of the form S-Pt(10)-Co${}_{68}$B${}_{32}$(d${}_{\text{CoB}}$)-Pt(5)-S at 1.8 K. Each data point represents one Josephson junction and the uncertainty in determining $I_c{R}_N$ is smaller than the data points. The data are fit to Eqs. (4) and (5). The best fit parameters for Eq. (4) corresponds to ${\xi }_F=0.28\pm 0.01$ nm, and for Eq. (5) to ${\xi }_{F1}=0.28\pm 0.02$ nm and ${\xi }_{F2}=0.20\pm 0.02$ nm. The first minimum at $0.30\pm 0.05$ nm indicates a transition between the zero and $\pi$-phase states. Bottom: Product of the area times normal-state resistance for the same junctions. The scatter in $A{R}_N$ is most likely sample-to-sample variation in A.