Molecular Spin Memory
Magnetic Tunnel Junctions
(a) Majority and minority electron energy bands of the two FM electrodes. (b) Typical magnetoresistance curve in a magnetic tunnel junction.
In the past few years of MTJ development, the establishment of coherent tunneling has led to giant TMR of over 600% at room temperature. Thus MgO is a highly promising spin injection barrier for future semiconductor spintronics. Because of such technological importance, we study the epitaxial growth of MgO on (100)-Si by molecular beam epitaxy [1]. MgO matches Si with 4:3 cell ratio, which renders Fe to be 45° rotated relative to Si, in sharp contrast to the direct epitaxial growth of Fe on Si. It is observed that the compressive strain from Si leads to the formation of small angle grain boundaries in MgO below 5 nm, and also affects the transport characteristics of Fe/MgO/Fe magnetic tunnel junctions formed on top. In addition, we show that by introducing oxygen vacancies in the MgO barriers of epitaxial Fe/MgO/Fe MTJs, symmetry-breaking scatterings are introduced and hence open up channels for noncoherent tunneling processes that follow the normal WKB approximation [2]. The evanescent waves inside the MgO barrier thus experience two-step tunneling: the coherent process followed by the noncoherent process, and thus lead to lower tunnel magnetoresistance, higher junction resistance, as well as increased bias and temperature dependence. The characteristic length of the symmetry scattering process was determined to be about 1.6 nm.
[1] Miao, Chang, Veenhuizen, Thiel, Seibt, Eilers, Münzenberg, Moodera, APL 93, 142511 (2008).
[2] Miao, Park, Moodera, Seibt, Eilers, Münzenberg, PRL 100, 246803 (2008).
Spin Filtering: towards total spin polarization
(a) A double spin-filter. (b) The barrier height experienced by tunneling electrons depends on the mutual spin alignment of the two spin filters.
Spin filtering (SF) takes place due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetictunnel barrier and can result in highly spin polarized tunnel current. This is in contrast to junctions with conventional magnetic electrodes which give rise to the spin polarized tunnel current through a nonmagnetic barrier. Thus combining two SF barriers in a magnetic tunnel junction (MTJ) can lead to a large magnetoresistance, by tuning the magnetization direction of the two SF layers without the necessity of magnetic electrodes. To demonstrate this phenomenon we use the chalcogenides material EuS, which is a well-established SF material and its magnetic properties are shown to be tunable with deposition temperature [1]. In tunnel junctions consisting of double EuS SF barriers, different coercive fields of the two adjacent SFs are achieved by depositing one EuS layer at room temperature and quench-condensing the other at liquid nitrogen temperature. A thin Al2O3 spacer layer is introduced in between the two SF layersto prevent direct magnetic coupling. Tunnel magnetoresistance (TMR) up to 60% with well-defined spin-parallel and -antiparallel states is obtained this way, and the resistance change originates completely within the SF/I/SF composite tunnel barrier, with no ferromagnetic electrodes involved. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations [2].
[1] Miao, Moodera, APL 94, 182504 (2009).
[2] Miao, Müller, Moodera, PRL 102, 076601 (2009).
Tuning superconductivity with spin polarized current
(a) Superconducting spin valve effect in the structure (in nm): Si(100) / 10MgO / 6Fe / 40V / 6Fe / CoO. (b) Superconducting transition of the same sample in its spin P and AP configurations. Inset shows the thickness dependence of the SC spin valve effect, and an example of the MR loop with 50 nm V is also shown.
Ferromagnetism is known to suppress the conventional s-wave superconductivity through the proximity effect. This is because the ferromagnetic exchange splitting energy, which prefers spin parallel alignment, is typically ~three orders of magnitude larger than the Cooper pairing energy, which prefers spin antiparallel alignment. We show that the superconducting state is tunable by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator) [1]. The superconducting transition temperature (TC) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. For an opaque ballistic interface, the supercurrent is significantly reduced, and the TC shift is attributed to the leakage of nonequilibrium spin carriers from the ferromagnets into the superconductors. For a clean interface, the Cooper pair mediated proximity effect also prevails. Superconductivity in a fully epitaxial bcc-Fe/V/Fe hybrid spin valve structures is influenced by the spin currents and supercurrents as well as band symmetry [2]. The transition temperature is spin orientation dependent in the presence of the proximity effect. A unique feature in this system is the band symmetry filtering taking place at the Fe/V interface. The absence of Δ2 Bloch states at the Fermi level in the Fe spin majority channel leads to spin selectivity and reduced transparency at the interface. Infinite magnetoresistance with clear remanence states is obtained, and implies the potential for spintronic applications.
[1] Miao, Yoon, Santos, Moodera, PRL 98, 267001 (2007).
[2] Miao, Ramos, Moodera, PRL 101, 137001 (2008).