Co-Doped $(\mathrm{La},\mathrm{Sr}){\mathrm{TiO}}_{3-\delta }$: A High Curie Temperature Diluted Magnetic System with Large Spin Polarization

We report on tunneling magnetoresistance (TMR) experiments that demonstrate the existence of a significant spin polarization in Co-doped $(\mathrm{La},\mathrm{Sr}){\mathrm{TiO}}_{3-\delta }$ (Co-LSTO), a ferromagnetic diluted magnetic oxide system (DMOS) with high Curie temperature. These TMR experiments have been performed on magnetic tunnel junctions associating Co-LSTO and Co electrodes. Extensive structural analysis of Co-LSTO combining high-resolution transmission electron microscopy and Auger electron spectroscopy excluded the presence of Co clusters in the Co-LSTO layer and thus, the measured ferromagnetism and high spin polarization are intrinsic properties of this DMOS. Our results argue for the DMOS approach with complex oxide materials in spintronics.

Figure 1

Auger intensity for several elements, measured as a function of the sample etching time, for a Co-LSTO film of thickness $t=130\mathrm{nm}$ grown at $P_{{\mathrm{O}}_2}=6\times {10}^{-7}\mathrm{mbar}$. Inset: AES mapping of Co in the same film.

Figure 2

(a) HRTEM image of a $\mathrm{LSTO}/\mathrm{STO}$ system ($t=100\mathrm{nm}$, $P_{{\mathrm{O}}_2}=6\times {10}^{-7}\mathrm{mbar}$) in cross section in $[100]$ zone. (b)–(d) EELS element-selective images. Grey levels are proportional to the $[\mathrm{\text{element}}]/[\mathrm{\text{oxygen}}]$ ratio. Black is zero; white is 50% in Ti and La images, 1% in Co image. The pixel size is 12 nm. (e) Experimental and simulated EELS spectra at the $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{L}}_{2,3}$ edge, together with reference spectra for ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Ti}}^{4+}$ [adapted from Ref. 17 ].

Figure 3

(a) Magnetization hysteresis cycle measured with an alternating gradient force magnetometer (AGFM) for a 130 nm Co-LSTO film grown at $P_{{\mathrm{O}}_2}=6\times {10}^{-7}\mathrm{mbar}$, with the magnetic field applied in plane. Inset: same $M(H)$ up to saturation. (b) I-V curve of a $\mathrm{Co}\mathrm{\text{−}}\mathrm{LSTO}/\mathrm{LAO}/\mathrm{Co}$ tunnel junction measured at 4 K and in a field $H=6000\mathrm{Oe}$. (c) Field dependence of the resistance of a $\mathrm{Co}\mathrm{\text{−}}\mathrm{LSTO}/\mathrm{LAO}/\mathrm{Co}$ magnetic tunnel junction (size $64\mu {\mathrm{m}}^2$), at 4 K and bias voltage of 50 mV.

Figure 4

(a) TMR bias dependence of a $64\mu {\mathrm{m}}^2$ $\mathrm{Co}\mathrm{\text{−}}\mathrm{LSTO}/\mathrm{LAO}/\mathrm{Co}$ MTJ. The solid line corresponds to the fit obtained taking into account a Lorentzian damping function with a characteristic value $V_{1/2}=150\mathrm{mV}$ and the following parameter values: $P_L=-0.3$, $P_R=-0.78$, ${\Gamma }_L=({\Gamma }_{L\uparrow }+{\Gamma }_{L\downarrow })/2=0.88\mathrm{meV}$, ${\Gamma }_R=({\Gamma }_{R\uparrow }+{\Gamma }_{R\downarrow })/2=5.7\mathrm{meV}$, ${\epsilon }_C=1.65\mathrm{meV}$. Optimal fittings were obtained by assuming a narrow energy band ($W=0$), corresponding to the case of impurities occupying the same energy level (i.e., the mean spatial separation of impurities might be too large to induce any overlapping and hybridization between two neighboring defects). (b) Temperature dependence of a $\mathrm{Co}\mathrm{\text{−}}\mathrm{LSTO}/\mathrm{LAO}/\mathrm{Co}$ junction (size: $12\mu {\mathrm{m}}^2$) at bias $V=20\mathrm{mV}$.