Realizing Room‐Temperature Resonant Tunnel Magnetoresistance in Cr/Fe/MgAl₂O₄ Quasi‐Quantum Well Structures

Abstract

The quantum well (QW) realizes new functionalities due to the discrete electronic energy levels formed in the well‐shaped potential. Magnetic tunnel junctions (MTJs) combined with a quasi‐QW structure of Cr/ultrathin‐Fe/MgAl2O4(001)/Fe, in which the Cr quasi‐barrier layer confines Δ 1 up‐spin electrons to the Fe well, are prepared with perfectly lattice‐matched interfaces and atomic layer number control. Resonant peaks are clearly observed in the differential conductance of the MTJs due to the formation of QWs. Furthermore, enhanced tunnel magnetoresistance (TMR) peaks at the resonant bias voltages are realized for the MTJs at room temperature, i.e., it is observed that TMR ratios at specific and even high bias‐voltages (V bias) are larger than zero‐bias TMR ratios for the MTJs with odd Fe atomic layers, in contrast to the earlier experimental studies. In addition, a new finding in this study is unique sign changes in the temperature coefficient of resistance (TCR) depending on the Fe thickness and V bias, which is interpreted as a signature of the QW formation of Δ1 symmetry electronic states. The present study suggests that the spin‐dependent resonant tunneling via the QWs formed in Cr/ultrathin‐Fe/MgAl2O4/Fe structures should open a new pathway to achieve a large TMR at practically high V bias.

Boosting room temperature tunnel magnetoresistance in hybrid magnetic tunnel junctions under electric bias

“…Spin-dependent resonant tunneling through quantumwell 14,15 and interface 12 states investigated within line (ii) showed that devices are either operative at low temperatures only or involve technically challenging symmetry confinement in a few mono-layers of ferromagnetic material 16 . Other similar 17 approaches providing TMR modulation are also not robust enough to build energy efficient 18 room temperature spintronic devices.…”

Quantum-well tunneling anisotropic magnetoresistance above room temperature

“…On the other hand, metallic QWs can incorporate the large exchange splitting on d electrons in spin-polarized QW devices [3][4][5][6][7]. In more recent technological advances, spinpolarized d-band QWs were demonstrated in magnetic tunnel junctions (MTJs), where the confinement potential is produced both by band-gap and band-symmetry mismatches [8][9][10][11].…”

“…A dislocation-free Fe/MgAl 2 O 4 interface can be realized [27,28], which significantly enhances the phase coherence in QW structures [10,11]. We measure the QW-TAMR effect in epitaxial stacks of Cr/Fe/MgAl 2 O 4 , with the geometry and coordinates depicted in Fig.…”

“…Further details on the preparation and TMR properties are provided in Ref. [11]. As a comparison sample, we use a high-TMR non-QW sample epitaxially grown by sputtering with thick Fe electrodes [28], with the film stack of: MgO-(0 0 1) substrate/Cr (40)/Fe (100)/MgAl 2 O 4 (2.17)/Fe (7)/IrMn (12)/Ru (10) (thicknesses in nm), which has a TMR ratio of 401 % at 5 K, and 224 % at 300 K.…”

“…3(e)]. On the other hand, when t Fe = 5 ML and the QWRS level is at 0.6-eV away from the Fermi level, QW-TAMR magnitude decreases at zero bias, while QW-TAMR has a small magnitude at the QWRS level of −0.6 V. The quantization levels form either close to or far from zero bias depending on the parity of the number of Fe MLs, due to the Fe electronic structure [9,11,26]. The QW-TAMR magnitude correspondingly shows oscillations in t Fe with a 2-ML period (Sec.…”