Spin-Dependent Resonant Tunneling through Quantum-Well States in Magnetic Metallic Thin Films

Abstract

Quantum-well (QW) states in nonmagnetic metal layers contained in magnetic multilayers are known to be important in spin-dependent transport, but the role of QW states in magnetic layers remains elusive. Here we identify the conditions and mechanisms for resonant tunneling through QW states in magnetic layers and determine candidate structures. We report first-principles calculations of spin-dependent transport in epitaxial Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr tunnel junctions. We demonstrate the formation of sharp QW states in the Fe layer and show discrete conductance jumps as the QW states enter the transport window with increasing bias. At resonance, the current increases by one to two orders of magnitude. The tunneling magnetoresistance ratio is several times larger than in simple spin tunnel junctions and is positive (negative) for majority-(minority-) spin resonances, with a large asymmetry between positive and negative biases.The results can serve as the basis for novel spintronic devices.

Correlation between symmetry-selective transport and spin-dependent resonant tunneling in fully epitaxial Cr/ultrathin-Fe/MgO/Fe(001) magnetic tunnel junctions

“…10 On the other hand, even in a (001)Cr/ultrathin Fe/ MgO/Fe "single" insulating barrier MTJ, symmetry-selective tunneling makes the metallic Cr(001) layer a second potential barrier thanks to the large offset of its D 1 band against that of majority spin in Fe(001), so that spin-dependent resonant tunneling limited to D 1 Bloch states occurs via the ultrathin Fe layer grown on Cr(001). 11,12 The resonant peaks of conductance and TMR were clearly observed, and the con-ductance peak positions are in good agreement with the prediction of ab-initio electronic structure calculations. However, physical understanding of spin-dependent resonant tunneling in pseudo double-barrier MTJs is still insufficient since the role of each layer is not individually clarified in abinitio electronic structure calculations.…”

Long-Range Phase Coherence in Double-Barrier Magnetic Tunnel Junctions with a Large Thick Metallic Quantum Well

“…[7]. An important parameter Φ inf is taken account for the additional phase shift at interfaces due to other effects such as interface roughness, chemical disorder, impurities and strain inhomogeneity, etc..To qualitatively compare our results with such an analysis, 6 we first set Φ inf =0, whichwill be discussed below. As displayed in Fig.2 oscillation.…”

Tunneling spectroscopy in core/shell structured Fe/MgO nanospheres

“…16 We surmise that the small dip at zero bias probably results from the magnon and/or phonon assisted tunnelling. 8 Interestingly, the feature observed at 0.3 V was previously interpreted as a surface roughness associated transition. 10 Indeed, studies on magnetic tunnel junctions with a MgO insulating barrier show a characteristic conductance curve with a dip at 0.3 eV, 17 likely due to the d z 2 -like spin polarized surfaces that boost up low frequency noise.…”

“…5 In addition, the spatial confinement of electrons at the nanoscale has been shown to have important consequences on the magnetoresistive response of thin Fe͑001͒ films and double tunnel junction structures due to the formation of quantum well states. [6][7][8][9][10] In these cases, tunneling spectroscopy reflects the energy levels of the quantum well states instead of the intrinsic density of states ͑DOS͒ of the ultrathin layer. Likewise, spectroscopic measurements in single metal particles also reflect effects generated by the spatial confinement of the electrons.…”