Abstract
The influence of quantum-well (QW) states formed in a Ru spacer layer on tunneling magnetoresistance (TMR) was investigated. A magnetic tunnel junction (MTJ) with a Ru spacer inserted between an $\mathrm{Al}\mathrm{O}x$ tunnel barrier and one of the ferromagnetic ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}$ layers was deposited on a single crystal MgO(110) substrate using an ultrahigh vacuum sputtering system. We clearly observe an oscillation of TMR ratios as a function of Ru thickness up to $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ with the same phase previously observed in an interlayer exchange coupling. The TMR ratios also oscillate and change their signs under both positive and negative bias voltage applications for the Ru thickness more than $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. These periodical oscillations are caused by the oscillation of spin polarization derived from the contribution of QW states of up spin electrons confined in the Ru spacer layer to the spin-dependent electron transport.
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