${\text{Cu}}_2\text{O}$ as a nonmagnetic semiconductor for spin transport in crystalline oxide electronics

We probe spin transport in ${\text{Cu}}_2\text{O}$ by measuring spin-valve effect in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3/{\text{Cu}}_2\text{O}/\text{Co}$ and ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3/{\text{Cu}}_2\text{O}/{\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$ epitaxial heterostructures. In ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3/{\text{Cu}}_2\text{O}/\text{Co}$ systems, we find that a fraction of out-of-equilibrium spin-polarized carrier actually travel across the ${\text{Cu}}_2\text{O}$ layer up to distances of almost 100 nm at low temperature. The corresponding spin-diffusion length $d_{\text{spin}}$ is estimated around 40 nm. Furthermore, we find that the insertion of a ${\text{SrTiO}}_3$ tunneling barrier does not improve spin injection, likely due to the matching of resistances at the interfaces. Our result on $d_{\text{spin}}$ may be likely improved, both in terms of ${\text{Cu}}_2\text{O}$ crystalline quality and submicrometric morphology and in terms of device geometry, indicating that ${\text{Cu}}_2\text{O}$ is a potential material for efficient spin transport in devices based on crystalline oxides.

Figure 1

X-ray pattern of a $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{LSMO}$ trilayer with $1.2\text{−}\mu \text{m}$-thick ${\text{Cu}}_2\text{O}$.

Figure 2

(Color online) (a) $5\mu \text{m}\times 5\mu \text{m}$ AFM image of a 200-nm-thick LSMO film; the surface is atomically smooth and atomic terraces are clearly seen. (b) $5\mu \text{m}\times 5\mu \text{m}$ AFM image of a LSMO film deposited on the top of a ${\text{Cu}}_2\text{O}$ several hundreds nanometer thick film, exhibiting a granular structure and a surface roughness of 20 nm rms. (c) $5\mu \text{m}\times 5\mu \text{m}$ AFM image of a 25-nm-thick ${\text{Cu}}_2\text{O}$ film, exhibiting a surface roughness of 3 nm rms.

Figure 3

Resistivity and carrier concentration measured by Hall effect in a 150-nm-thick ${\text{Cu}}_2\text{O}$ film, patterned as a $50\text{−}\mu \text{m}$-wide Hall bar.

Figure 4

Tentative sketch of band alignment at the ${\text{Cu}}_2\text{O}/\text{LSMO}$ interface. $\Phi$, $E_{gap}$, $E_{\text{f}}$, c.b., and v.b. indicate the work function, band gap, Fermi level, conduction band, and valence band, respectively. The energy barrier for diffusion of holes from LSMO to ${\text{Cu}}_2\text{O}$, supposed to be around 0.1–0.2 eV, is also indicated.

Figure 5

Left: resistance versus field curves of a $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{LSMO}$ trilayer with ${\text{Cu}}_2\text{O}$ thickness $1.6\mu \text{m}$, measured at 10 K (upper panel) and 300 K (lower panel). A sketch of the measurement configuration is shown in the inset. Right: resistance versus field curves of the bottom (upper panel) and top (lower panel) LSMO electrodes. The arrows indicate the direction of the magnetic field sweep.

Figure 6

Upper left panel: resistance versus temperature across a $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ trilayer with ${\text{Cu}}_2\text{O}$ thickness 50 nm. Other panels: resistance versus magnetic field across the same trilayer, measured at different temperatures $T=10$, 50, 100, 200, and 300 K. The arrows indicate the direction of the magnetic field sweep. In the top left panel, the directions of magnetizations of the lower LSMO and upper Co electrodes are also schematically indicated.

Figure 7

Magnetization measurements of a $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ trilayer at 10 K (upper panel) and 300 K (lower panel). In the inset, a zoom of the low-field regime at 10 K is shown, showing a coercive field of the Co layer of around 600 Oe. The linear slope due to the diamagnetic contribution of the substrate has been subtracted. The arrows indicate the direction of the magnetic field sweep.

Figure 8

Resistance versus magnetic field across $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ trilayers with different ${\text{Cu}}_2\text{O}$ thicknesses, measured at $T=10\text{K}$. The arrows indicate the direction of the magnetic field sweep. Our definition of the hysteretic contribution $\Delta R$ is indicated in the top panel curve. The horizontal axis of the top panel is zoomed to better emphasize the hysteresis; indeed, being the ${\text{Cu}}_2\text{O}$ spacer only 5 nm thick, the residual magnetic coupling between the top and bottom electrodes causes the hysteresis to be visible only in a smaller range of magnetic fields.

Figure 9

Upper panel: hysteretic contribution $\Delta R/R_0=\left(R^{+}-R^{-}\right)/R_0$ to magnetoresistivity of $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ heterostructures with different ${\text{Cu}}_2\text{O}$ thicknesses, where $R^{+}$ and $R^{-}$ the resistances of the decreasing-$\left|H\right|$ and increasing-$\left|H\right|$ branches, respectively, and $R_0$ their average value. Lower left panel: extracted spin-diffusion length in ${\text{Cu}}_2\text{O}$ inside $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ heterostructures as a function of temperature. Lower right panel: exponential fit of $\frac{\Delta R}{R_0}/{\phantom{|}\frac{\Delta R}{R_0}|}_{ref}$ data at $T=10\text{K}$ as a function of $\left(t_{{\text{Cu}}_2\text{O}}-t_{{\text{Cu}}_2\text{O}}^{ref}\right)$.

Figure 10

(Color online) Upper panel: resistance versus magnetic field across $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ and $\text{LSMO}/{\text{SrTiO}}_3\left(5\text{nm}\right)/{\text{Cu}}_2\text{O}/\text{Co}$ trilayers with $t_{{\text{Cu}}_2\text{O}}=50\text{nm}$, measured at $T=100\text{K}$ and normalized to the average resistance value $R_0=\left(R^{+}+R^{-}\right)/2$ (see text for definition of $R^{+}$ and $R^{-}$). In both cases, a comparable value $\Delta R/R_0\approx 9\times {10}^{-4}$ is observed. Lower panel: same as above but for trilayers with $t_{{\text{Cu}}_2\text{O}}=5\text{nm}$ and different $t_{{\text{SrTiO}}_3}=10\text{nm}$ and 15 nm. In both panels, the arrows indicate the direction of the magnetic field sweep. In the inset of the lower panel, the $\Delta R/R_0$ data extracted from the curves in the main panels and normalized to the corresponding ${\text{SrTiO}}_3$-free sample values are plotted. The device layer sequences $\text{LSMO}/{\text{Cu}}_2\text{O}/\text{Co}$ and $\text{LSMO}/{\text{SrTiO}}_3/{\text{Cu}}_2\text{O}/\text{Co}$ are sketched in the insets of the upper panel, close to the corresponding symbols of the data plots in both upper and lower panels.