Spin-dependent quantum oscillations in magnetic tunnel junctions with Ru quantum wells

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 $1.7\mathrm{nm}$ 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 $1.3\mathrm{nm}$. 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.

Figure 1

(a) Ru thickness dependence of the TMR ratios under the bias voltage application of $V=+15\mathrm{mV}$ in the Ru thickness range of $0.3⩽t⩽1.7\mathrm{nm}$. The inset shows the entire Ru thickness dependence of the TMR ratios in $0⩽t⩽1.7\mathrm{nm}$. The solid line is the calculation result of the proposed model described in the text. (b) Representative TMR curves of the MTJs with $t=0$, 0.37, and $1.4\mathrm{nm}$. The curves for $t=0.37$ and $1.4\mathrm{nm}$ are magnified by 10 and 100 times, respectively, in order to show details clearly.

Figure 2

Ru thickness dependence of the saturation fields in the ${\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}\left(15\mathrm{nm}\right)∕\mathrm{Ru}\left(t\right)∕{\mathrm{Co}}_{90}{\mathrm{Fe}}_{10}\left(5\mathrm{nm}\right)$ sandwich structures on the MgO(110) substrates measured by VSM.

Figure 3

Bias voltage dependence of the TMR ratios in the Ru thickness range of (a) $0.3⩽t⩽0.7\mathrm{nm}$ and (b) $1.3⩽t⩽1.7\mathrm{nm}$.

Figure 4

Schematic diagrams of the electron tunneling processes under (a) positive and (b) negative bias voltage applications for a MTJ with an ultrathin NM (energy states for only up spins are shown), where ${\mathrm{FM}}_i$ ($i=1$ and 2) are ferromagnetic layers, NM is a nonmagnetic layer and I is an insulator. DOS for both up and down spins is also schematically shown in (c).