Spin filtering on demand via localized states in an atomic-scale resonant tunneling magnetic tunnel junction

Abstract

Spin filtering and its back-action spin transfer torque (STT) are key ingredients of latest spintronic devices based on magnetic tunnel junctions (MTJs). Resonant tunneling (RT), implemented by design or occurring as parasitic effects, is known to crucially affect macroscopic device performance, but direct experimental access to its individual microscopic processes has remained difficult. Here we apply the RT scheme from MTJs to spin-polarized scanning tunneling microscopy (SP-STM) for ultimate miniaturization obtained by addressing distinct sites on individual nanomagnets. Combined with energy selectivity, our experimental model set-up enables to study the spin filtering capabilities of RT through an individual spin-split vacuum resonance state and of the corresponding STT exerted on the nanomagnet. We find, that the sign and magnitude of the STT follow the effective spin-polarization of the resonance state, which, as we show, can by tailored on demand either by adjusting the applied bias or the current injection point on the nanostructure. We anticipate, that our atomic-scale RT-MTJ approach and the discovery of a versatile tunable spin-filter at smallest scale will prove invaluable for studying and designing next generation MTJs potentially based on recently discovered 2D van-der-Waals magnets or altermagnets.

Figures (4)

• Figure 1: Resonant tunneling magnetic tunneling junction (RT-MTJ).(a) Typical pentalayer design, composed of a central layer hosting quantum well resonances confined by adjacent insulating layers, sandwiched by ferromagnetic leads. (b) The resonant tunneling scheme in an STM set up. Upon selecting an appropriate bias voltage $U$, electrons from the probe tip tunnel resonantly through a vacuum resonance trapped in the potential well in front of the surface. ($\phi_{\rm t, s}$: tip and sample work function, $E_{\rm F,t, s}$: tip and sample Fermi level.) (c) Atomic-scale analogue of an RT-MTJ. Due to the atomic-scale dimensions of the magnetic SP-STM tip apex, spatially localized spin-polarized electrons tunnel resonantly through a spin-dependent vacuum resonance into a nanomagnet on the substrate.
• Figure 2: Spin-polarization of a nanomagnet's center and rim vacuum resonance state.(a) STM topography of an ML Fe/W(110) nanomagnet ($U=0.1$ V). (b) d$I$/d$U$ maps recoded at $U=4.9$ V and $5.1$ V, respectively, revealing the center state (left) and the rim state (right). Spatial extent of the nanoisland as determined from (a) is indicated. The bright region (marked by arrows) in the right map reveals the spatial extent of the rim state. (c) Top: Spin-resolved d$I$/d$U(U)$ curves, taken on the center (left) and the rim (right) as indicated in (a), in parallel $\uparrow\uparrow$ (blue) and antiparallel $\uparrow\downarrow$ (red) alignment of tip and island magnetization, respectively. Bottom: Spin-polarization $SP(U)$ of the hot-electron gas on the center (left) and the rim (right), respectively. (d)$SP$ map at $U=5.01$ V (bias voltage marked by dashed lines in (c)). (e) Top: Schematic of the nanomagnet with rim and center $SP$, as observable in (d). Bottom: Corresponding schematic of the spin-dependent tunneling at the island center and rim, respectively. ($I=2$ nA, $f=3980$ Hz, $U_{\rm {Pk-Pk}}=120$ mV, $T=36$ K.)
• Figure 3: Energy dependence and spatial modulation of a nanomagnet's hot-electron RS spin-polarization.(a) STM topography of a ML Co/Ir(111) nanomagnet ($U=-0.5$ V). (b) d$I$/d$U$ maps recoded at $U=4.8$ V and $4.95$ V, respectively, revealing the center state (left) and the rim state (right). Spatial extent of the nanoisland as determined from (a) is indicated. (c) Top: Spin-resolved d$I$/d$U(U)$ curves, taken on the center and the rim as indicated in (a), in parallel $\uparrow\uparrow$ (blue) and antiparallel $\uparrow\downarrow$ (red) alignment of tip and island magnetization, respectively. Bottom: Spin-polarization $SP(U)$ of the hot-electron gas on the center and the rim, respectively. (d) Top: Bias-dependent series of SP maps (with schematic of the rim and center SP in the inset). Bottom: Schematic of the spin-dependent tunneling as a function of electron energy e$U$. ($I=0.1$ nA, $f=3777$ Hz, $U_{\rm {Pk-Pk}}=10$ mV, $T=6.5$ K, $B=\pm1$ T.)
• Figure 4: Tailoring the spin-transfer torque (STT) by tunneling through spin-polarized vacuum resonances.(a) Inset: Constant current image of a thermally switching Fe/W(110) nanomagnet ($U=-0.2$ V). Scale bar is $2$ nm. Top: Spin-resolved d$I$/d$U(U)$ curves, taken on the nanomagnet's center, revealing the spin-split RS. The signal jumps between the curves for parallel $\uparrow\uparrow$ (blue) and antiparallel $\uparrow\downarrow$ (red) alignment of tip and island magnetization, due to frequent switching events. Bottom: Corresponding spin-polarization $SP(U)$ of the hot-electron gas on the center (dark grey) with measurement uncertainty (light grey). The crossing point of d$I$/d$U(U)_{\uparrow\uparrow}$ and d$I$/d$U(U)_{\uparrow\downarrow}$ and the respective inversion of SP is marked by the dashed line. Current-induced asymmetry $a_{\rm H}$ (green) between the two magnetization states of the island as a function of bias $U$. (b) Telegraph noise, recorded on the nanomagnet's center within the first RS ($U=4.7$ V). (c) State histogram, as determined from the telegraphic noise recorded at $4.7$ V and (d)$U=4.9$ V, respectively. Schematic sketch: Upon sign inversion of $SP(U)$, the STT reverses accordingly, favoring a parallel (antiparallel) alignment of tip and sample magnetization for positive (negative) SP, as deduced from $a_{\rm H}$. ($I=2$ nA, $f=4333$ Hz, $U_{\rm {Pk-Pk}}=120$ mV, $T=39,5$ K.)