Spin injection at remanence into III-V spin light-emitting diodes using (Co/Pt) ferromagnetic injectors

We have studied the perpendicular magnetic anisotropy of Co/Pt multilayers and the electron spin injection efficiency by optical spectroscopy from a [Co(0.6 nm)/Pt(1 nm)]${}_4$/Fe(0.3 nm)/MgO perpendicular tunnel spin injector grown on AlGaAs/GaAs semiconductor light-emitting diodes. We observe a 2.5% circular polarization at low temperature close to the magnetic remanence when the 0.3 nm Fe film of the ferromagnetic injector is sufficiently thin to maintain the magnetization out of plane. The acquired squared magnetization cycle is explained by the remaining interlayer exchange coupling existing between Fe and the (Co/Pt) multilayer through Pt or possible perpendicular magnetic anisotropy at the MgO/Fe interface. The corresponding spin polarization of the current is then estimated as 7%, measured by photoluminescence techniques, after the necessary up-renormalization, taking into account the electron spin-flip rate in the quantum well. In contrast, no circular polarization is observed when the thin Fe layer is removed and despite the rather high magnetic polarizability of the 5$d$${}^9$ electronic open shell of Pt at the interface with MgO. This emphasizes the reduced size of tunneling branching of wave functions at the interface, of the order of the atomic plane unit.

Figure 1

(a) Cross-sectional TEM image of sample SiO${}_2$//Pt (20 nm)/[Co (0.6 nm)/Pt (1.0 nm)]${}_5$/Au (10 nm). (b) Electron intensity profile obtained from selected area shown in (a), revealing the stacking of alternate Co (peak) and Pt (valley) layers. (c) AGFM hysteresis loops for the same sample with magnetic field applied respectively in the out-of-plane and in-plane directions.

Figure 2

(a) Reflection high-energy electron diffraction (RHEED) pattern acquired on a 20-nm-thick Pt buffer grown on SiO${}_2$, highlighting the (111)-growth zone axis. The in-plane atomic parameter corresponds to a (111) growth of fcc Pt. (b),(c) RHEED pattern acquired after the respective deposition of a 0.6 nm Co layer (b) and a 1 nm Pt layer (c), showing the same (111)-growth zone axis. Note that, in each case, the in-plane atomic parameter is the same as in (a). (d) Large-angle x-ray diffraction spectrum of SiO${}_2$//Pt (20 nm)/[Co (0.6 nm)/Pt (1 nm)]${}_3$/Pt (2 nm) displaying the two diffraction peaks corresponding, respectively, to Pt(111) and Co(111) (left and right peaks). The central peak matches the zero-order peak of the multilayer.

Figure 3

Sample series A: (a) Hysteresis loops measured at 300 K for an out-of-plane magnetic field for SiO${}_2$//Pt(20 nm)/[Co(0.6nm)/Pt($X$ nm)]${}_5$/Au(10 nm) with $X=0.2$, 0.4, 0.6, and 0.8 nm. The $X=1$, 1.2, 1.4, 1.6, 1.8, 2, and 2.6 nm samples (not shown) display square hysteresis cycles. Inset: Coercive field of (Co/Pt) multilayers vs Pt thickness displaying oscillations obtained on a different sample series.

Figure 4

Sample series A: Magnetization at saturation ($M_S$) measured with a superconducting quantum interference device (SQUID) at 300 K of [Co(0.6 nm)/Pt($X$ nm)]${}_5$ multilayers normalized to the Co volume vs the Pt thickness. Note the large enhancement of the magnetization for the thinnest Pt spacers due to Co-Pt interdiffusion and alloying. The average value of 1700 emu/cm${}^3$ measured for the thicker Pt spacer exhibits an enhancement of the Co atomic magnetic moment together with a partial polarization of the Pt atoms at the (Co/Pt) interfaces (see the text).

Figure 5

Sample series B series: $T=20$ K. (a) Polarization-resolved electroluminescence (EL) spectra for $V=2.5$ V under a saturating magnetic field of 5 kOe for sample B with $X_{\text{Pt}}=0.6$ nm. Respective intensities $I^{{\sigma }^{+}}$ and $I^{{\sigma }^{-}}$ of the right- (red thick line) and left-circularly-polarized (black thick line) EL components vs the emission wavelength. Upper inset: zoom around 806.5 nm; right axis, EL circular polarization (open circles). Lower inset: EL circular polarization measured at the peak emission vs the applied bias. (b) Out-of-plane (solid line) and in-plane (open squares) magnetization cycles.

Figure 6

Sample C: $T=20$ K. (a) Polarization-resolved electroluminescence (EL) spectra for $V=2.4$ V under a magnetic field of 0.5 kOe for sample C with $X_{\text{Pt}}=0.6$ nm and $t_{\text{Fe}}=0.3$ nm. Respective intensities $I^{{\sigma }^{+}}$ and $I^{{\sigma }^{-}}$ of the right- (red thick line) and left-circularly-polarized (black thick line) EL components vs the emission wavelength. Upper inset: zoom around 800 nm; right axis, EL circular polarization (open circles). Lower inset: EL circular polarization measured at the peak emission vs. the applied bias. (b) Time- and polarization-resolved photoluminescence performed; left axis, $I^{{\sigma }^{+}}$ (red thick line) and $I^{{\sigma }^{-}}$ (black thin line) photoluminescence (PL) components vs time; right axis, corresponding PL circular polarization (open circles).

Figure 7

Sample C: $T=20$ K. Full squares: EL circular polarization vs the applied magnetic field $H$ for sample C with $X_{\text{Pt}}=0.6$ nm and $t_{\text{Fe}}=0.3$ nm ($V=2.4$ V). Open circles: Photoluminescence circular polarization under excitation with a linearly polarized He-Ne laser ($E_{\text{exc}}=1.96$ eV, excitation power $P=300$ $\mu$W. Inset: Out-of-plane (solid line) and in-plane (open circles) magnetization cycles. The characteristic coercive field of about 280 Oe demonstrates the out-of-plane overall magnetization in the (Co/Pt)-Fe system.