Oscillatory tunnel magnetoresistance in double barrier magnetic tunnel junctions

Abstract

We report an unconventional oscillatory tunnel magnetoresistance as a function of the applied bias in double barrier magnetic tunnel junctions that were made of two Al 2 O 3 barriers sandwiched by three ferromagnetic layers. When the center ferromagnetic layer is aligned antiparallel to the top and bottom magnetic layers, a distinct magnetoresistance oscillation appears with respect to the increase of the bias voltage at 4.2 K and at room temperature. The period of the oscillation is about 1.6 mV.

Enhancing spin-transfer torque through the proximity of quantum well states

“…The tunneling magnetoresistance ͑TMR͒ can be dramatically enhanced in collinear DBMTJs by the presence of quantum well states ͑QWSs͒ under appropriate resonant conditions. [13][14][15][16] Recently, the discrete energy spectrum of FM nanoparticles has been shown to enhance spin accumulation and to control the bias dependence of the TMR. 17 While the physics in collinear DBMTJs has been studied extensively, [13][14][15][16] the effect of spin-polarized QWSs ͑SPQWSs͒ on the spin torque in noncollinear DBMTJs remains an unexplored area thus far.…”

Oscillations of tunnel magnetoresistance induced by spin-wave excitations in ferromagnet-ferromagnet-ferromagnet double-barrier tunnel junctions

“…Since the number of the above self-consistent equations nonlinearly increases with increasing the number of wave vectors of electrons and the number of spin-wave modes in the middle FM region, which makes the calculations too complicated to perform, for the sake of simplicity but without losing the generality, we shall only consider the situations where both the numbers of k and q taken in the following calculations are not so large that the numerical calculations can be readily proceeded. This is plausible, because the magnon-assisted transport property mainly depends on the low-lying quantum well states of electrons in the middle FM, and only the lower modes of spin waves are easy to emit [6], leading to small energy levels of magnons [27]. Besides, considering that only those electrons near the Fermi surface participate in the tunneling process, we may take ε kσ ≈ ε k F − σM, denoted by ε ↑ and ε ↓ for spin up and down electrons, respectively.…”

“…presented in Ref. [27]. It has been unveiled that the average number of magnons oscillates with the bias, which could be the main reason for the oscillations of the conductance and TMR of the system.…”

“…Although single barrier MTJs already show abundant characteristics concerning the spindependent electrical transport, a double barrier magnetic tunnel junction (DBMTJ), in which the formation of quantum well states and the resonant tunneling phenomenon are theoretically anticipated, has also attracted much attention in recent years [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. In order to observe the coherent tunneling thru the DBMTJ, people have attempted to improve the junction quality to eliminate the influences from the interface roughness and impurity scattering, and remarkable advances have been achieved on this aspect.…”

“…Unlike the previous oscillatory tunnel magnetoresistance, this unusual phenomenon can neither be explained by Coulomb blockade effect since the middle FM layer is continuous, the charge effect should be equal in P and AP configurations and the charging energy is negligibly small, nor be attributed to the resonant tunneling, because the observed period of oscillation is too small to account for the energy level spacing of the quantum well states. Considering that the conductance oscillation is asymmetrical for P and AP configu-rations, and the energy level of the unusual phenomenon is the same as the typical energy of a magnon, one may speculate that the unusual oscillation behavior could be induced by the magnon-assisted tunneling [27]. This is because in the AP state, the nonequilibrium spin density, which is proportional to the applied bias, could be accumulated near the interfaces in the middle region to emit spin waves, and the magnon-assisted tunneling would contribute to the conductance, while in the P state the spin wave emission is forbidden due to the spin angular momentum conservation, as discussed previously [28].…”