Anomalous scaling of spin accumulation in ferromagnetic tunnel devices with silicon and germanium

Abstract

Anomalous scaling of spin accumulation in ferromagnetic tunnel devices with silicon and germanium Sharma, S.; Spiesser, A.; Dash, S.P.; Iba, S.; Watanabe, S.; Wees, B.J. van; Saito, H.; Yuasa, S.; Jansen, R. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. The magnitude of spin accumulation created in semiconductors by electrical injection of spin-polarized electrons from a ferromagnetic tunnel contact is investigated, focusing on how the spin signal detected in a Hanle measurement varies with the thickness of the tunnel oxide. An extensive set of spin-transport data for Si and Ge magnetic tunnel devices reveals a scaling with the tunnel resistance that violates the core feature of available theories, namely, the linear proportionality of the spin voltage to the injected spin current density. Instead, an anomalous scaling of the spin signal with the tunnel resistance is observed, following a power law with an exponent between 0.75 and 1 over 6 decades. The scaling extends to tunnel resistance values larger than 10 9 μm 2 , far beyond the regime where the classical impedance mismatch or back flow into the ferromagnet play a role. This scaling is incompatible with existing theory for direct tunnel injection of spins into the semiconductor. It also demonstrates conclusively that the large spin signal does not originate from two-step tunneling via localized states near the oxide/semiconductor interface. Control experiments show that spin accumulation in localized states within the tunnel barrier or artifacts are also not responsible. Altogether, the scaling results suggest that, contrary to all existing descriptions, the spin signal is proportional to the applied bias voltage, rather than the (spin) current.

Spin transport and precession in graphene measured by nonlocal and three-terminal methods

“…On the other hand, spin lifetimes in metals obtained by 3T measurements are significantly longer than theoretically predicted 17 . Several experiments also proposed an enhancement 15,18 or inversion 8 of the 3T spin signal, whereas multiple control experiments rule out any enhancement due to interface states 6,16,19 . Therefore, it is required to understand the contributing factors in both techniques by performing experiments in the NL and 3T configuration using the same magnetic tunnel contacts.…”

Spin transport and Hanle effect in silicon nanowires using graphene tunnel barriers

“…As noted earlier, previous work has suggested that anomalously large 3T spin signals may result from spin accumulation in localized interface states rather than in the semiconductor channel itself 52 , anomalous scaling 53 or magnetic field-modulated resonant tunnelling 54 . Further work is necessary to elucidate the origin of these large signals.…”

“…The large discrepancy may be attributed to leakage currents, an overestimate of the actual tunnel contact area due to inhomogeneous tunnelling or an overestimate of the tunnelling spin polarization. Previous work has suggested that spin signals much larger than those predicted by theory may be due to spurious effects such as spin accumulation in localized interface states rather than in the semiconductor channel itself 52 , anomalous scaling 53 or magnetic field-modulated resonant tunnelling 54 . The fact that our experimental value is smaller than that predicted provides some evidence that such effects do not play a significant role in our devices.…”

Spin Heat Accumulation Induced by Tunneling from a Ferromagnet

“…However, the resulting enhancement of the Hanle signal is not sufficiently large to explain recent experiments in which Hanle signals that scale with the tunnel resistance and are many orders of magnitude larger than predicted by theory are observed [38][39][40][41][42][43][44], as recently reviewed [27,45]. It would require α ≪ 1 by several orders of magnitude.…”