Emergent Spin Filter at the Interface between Ferromagnetic and Insulating Layered Oxides

We report a strong effect of interface-induced magnetization on the transport properties of magnetic tunnel junctions consisting of ferromagnetic manganite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and insulating cuprate ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$. Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits a sudden anomalous decrease at low temperatures. Interestingly, this anomalous behavior can be attributed to the competition between the positive spin polarization of the manganite contacts and the negative spin-filter effect from the interface-induced Cu magnetization.

Figure 1

(a) Temperature dependence of the TMR of the MTJs consisting of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ electrodes and ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ barriers wth different barrier thicknesses, exhibiting an anomalous suppression in TMR at low temperatures. The TMR is scaled for different junctions for comparison. (b) The reduced TMR peak temperature, with respect to the TMR onset temperature, slightly decreases as the PBCO thickness increases. The error bar comes from the uncertainty of the onset temperature, which is $\sim 5\mathrm{K}$.

Figure 2

(a) Polarized neutron reflectivity in saturation ($H=5\mathrm{kOe}$) at 12 and 80 K. The best-fit curve (line) is overlaid on the data (circles). (b) Depth profiles of the neutron nuclear scattering length density and the saturation magnetization at 12 and 80 K.

Figure 3

Temperature dependences of the XMCD peak intensities at the Mn $L_3$ edge (circles at 641.8 eV) and the Cu $L_3$ edge (diamonds at 947.8 eV), respectively. The data were collected in the remnant states after applying a 1 kOe in-plane field along the [100] direction. The solid line is the best fit to the Mn signal using the modified Bloch law. Also shown is the magnetization acquired with a SQUID magnetometer during cooling in a 100 Oe field.

Figure 4

(a) The interface Cu magnetization gives rise to a negative spin-filter effect due to lifted spin degeneracy; thus, the majority-spin electrons (spin-up, labeled by red) of the LCMO electrode experience a higher barrier than the minority-spin electrons (spin-down, labeled by blue) when tunneling through the interfacial PBCO region. The lines in the top panel illustrate the spin-dependent wave functions of the charge carriers. In the bottom panel, the colored thick lines illustrate the spin-dependent conduction bands in the interfacial PBCO region, and the size of the arrows shows the populations of the spin-up (spin-down) charge carriers. (b) Effective spin polarization as a function of the spin polarization of the LCMO electrode $p$ and the zero-temperature exchange splitting ${\Delta }_{\mathrm{ex}}(0)$ in the interfacial PBCO region. (c) Calculated TMR as a function of the reduced temperature for different ${\Delta }_{\mathrm{ex}}(0)$. The TMR displays complex temperature dependences for high ${\Delta }_{\mathrm{ex}}(0)$.