Spatially Modulated Tunnel Magnetoresistance on the Nanoscale

Abstract

We investigate the local tunnel magnetoresistance (TMR) effect within a single Co nanoisland using spin-polarized scanning tunneling microscopy. We observe a clear spatial modulation of the TMR ratio with an amplitude of ~20% and a spacing of ~1.3 nm between maxima and minima around the Fermi level. This result can be ascribed to a spatially modulated spin polarization within the Co island due to spin-dependent quantum interference. Our combined experimental and theoretical study reveals that spin-dependent electron confinement affects all transport properties such as differential conductance, conductance, and TMR. We demonstrate that the TMR within a nanostructured magnetic tunnel junction can be controlled on a length scale of 1 nm through spin-dependent quantum interference.

Spin-polarized quantum confinement in nanostructures: Scanning tunneling microscopy

“…This would induce a spatial modulation of transport properties, such as the TMR. This effect has been observed experimentally and described by theory (Oka et al, 2011). It is reviewed in the following.…”

“…The overall agreement in the modulation pattern of the central region of the island at different energies identifies the dominant role of the energydependent spin-resolved DOS for change of contrast. The spatial variation of the spin polarization influences also the tunnel magnetoresistance, where it leads to a pronounced spatial oscillation (Oka et al, 2011). This is reviewed in Sec.…”

“…Spinpolarized STM (SP-STM) offers insight into the spindependent electronic structures with high spatial resolution (Bode, 2003;Wulfhekel and Kirschner, 2007;Wiesendanger, 2009). Combined experimental and theoretical studies reveal the decisive role of spin-dependent electron confinement in magnetic nanostructures for spin polarization and spindependent transport properties on the nanoscale Oka et al, 2010Oka et al, , 2011. Magnetic adatoms on metal surfaces induce oscillations of the spin polarization in nonmagnetic metals due to spin-dependent scattering of surface-state electrons.…”

Quantum interference effects in molecular spin hybrids

“…[79] for the rim of the islands. Since the wave function of the sp surface state cannot vanish abruptly at the edges of the TPT molecule, but rather extends into the molecules, a superposition of the spin polarization of TPT and of the modulated sp surface state is detected above the TPT molecules in the spin-resolved STM measurements.…”

Quantum spin holography with surface state electrons

“…Instead of using magnetic statterers, we switch to a system with an inherently spin-polarized surface state -Co nanoislands or multilayers on Cu(111). 5 This system is well studied both experimentally and theoretically 5,8,13 and provides us exactly with what we need -the spindependence of scattering parameters α and δ as well as the spin-dependence of the electron wave vector k. However, Co nanoislands do themselves scatter the surface state electrons 5 and would thus slightly interfere with the encoding of the hologram. To avoid that, and at the same time protect the system from additional contamination and intermixing, the Cu surface with islands grown on top can be covered with a few additional layers of copper thus sealing the island within the surface and reducing their effect on the scattering of surface state electrons while still retaining the spin-polarized character of the surface state.…”