Magnetoresistance in Double Spin Filter Tunnel Junctions with Nonmagnetic Electrodes and its Unconventional Bias Dependence

Spin filtering happens due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetic barrier and can result in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to large magnetoresistance without the necessity of magnetic electrodes. We demonstrate the realization of such unconventional tunnel junctions using double EuS spin filter barriers with Al electrodes. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations.

Figure 1

(Top panel) schematic illustration of the SF mechanism in double SF tunnel junctions across $T_C$. Dotted lines roughly indicate the barriers that the spin-up and spin-down electrons have to cross. (Bottom panel) calculated MR bias dependence. The label indicates the $\mathrm{EuS}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{EuS}$ barrier layers’ thicknesses (in nm) from bottom to top. The inset is a schematic illustration of the calculation method: the dc conductance is an integration of the tunnel probabilities from all the available states for a given applied bias [Eq. (2)]. It also approximately shows the Fermi level alignment for the peak MR which is circled out on the calculated bias curve [21].

Figure 2

MR in a SF tunnel junction with (in nm): $10\mathrm{Al}/1.5\mathrm{EuS}/0.6{\mathrm{Al}}_2{\mathrm{O}}_3/3\mathrm{EuS}/10\mathrm{Al}$. (a) Bias dependence at 4.2 K (closed circles) and 1 K (open circles). Shaded part indicates the regions below the onset of FN tunneling and is much noisier due to the extremely high impedance. (b) MR loop at 4.2 K (closed circles) and 1 K (open circles) with $+1.2\mathrm{V}$ bias voltages. (c) Normalized junction resistance as a function of temperature, measured in P state under $+1.2\mathrm{V}$ bias voltage.

Figure 3

(a) Measured bias dependence for the SF tunnel junctions. The label indicates the $\mathrm{EuS}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{EuS}$ barrier layers’ thicknesses (in nm) from bottom to top. Shaded part indicates the regions below the onset of FN tunneling and is much noisier due to the extremely high impedance. (b) The magnitude asymmetry in resistance and MR between $\pm 1.2\mathrm{V}$. The top EuS layer and the spacer ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer thickness are fixed at 2.25 and 0.6 nm, respectively.