Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

Abstract

Current induced magnetization switching and interlayer exchange coupling (IEC) in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin (0.96–0.62 nm) MgO wedge barrier is investigated. The IEC is found to be ferromagnetic for all samples and the associated energy increases exponentially down to a barrier thickness of 0.7 nm. Nanopillars with resistance area product ranging from 1.8 to 10 Ω μm2 and sizes of 0.13 μm2 down to 0.03 μm2 and tunneling magnetoresistance values of up to 170% were prepared. We found, that the critical current density increases with decreasing MgO barrier thickness. The experimental data and theoretical estimations show that the barrier thickness dependence of the spin transfer torque can largely be explained by a reduction in the tunnel current polarization at very small barrier thickness.

Metalliclike behavior of the exchange coupling in (001) Fe/MgO/Fe junctions

“…We point out that a weaker ferromagnetic coupling (<0.03 erg/cm 2 ) has also been * christine.bellouard@univ-lorraine.fr reported for MgO thicknesses close to 1 nm [9,10,24]. In the case of sputtered junctions, a ferromagnetic coupling has been observed in the planar magnetic configuration with an exponential MgO thickness dependence [13,17], and has therefore been attributed to a cooperative electronic effect. Nevertheless, an antiferromagnetic coupling (<0.1 erg/cm 2 ) has been observed recently with a sputtered planar multilayer with a quite large MgO thickness (1.6 nm) [23].…”

“…On the contrary, for a semiconducting or insulating spacer [3][4][5][6][7][8], magnetic properties seem to depend deeply on growth conditions and then on structural specifications. Among them, the MgO spacer [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] appears unique in presenting a large variety of experimental results. They concern either the MgO thickness range and the coupling sign and strength, or the junction magnetic geometry (planar or perpendicular magnetization).…”

Influence of MgO tunnel barrier thickness on spin-transfer ferromagnetic resonance and torque in magnetic tunnel junctions

“…The high TMR ratio of 170% for a 1.01 nm thick barrier and the exponential decrease in RA product with decreasing MgO thickness confirm good tunnel barrier quality. 11 Similar TMR ratios and RA products were measured on full wafers using a current in-plane tunneling technique before patterning. 12 The overall offset field (H S ) is shifted approximately 30-40 Oe with respect to the wafer-level measurements due to dipolar magnetostatic stray-field coupling in the nanopillar junctions.…”

“…The deposition process was similar to the one used in our previous studies. 11,12 After thin-film deposition, three different parts of the sample were selected for patterning into nanometer size pillars (referred to as S1, S2, and S3; see Table I for details). Using a three-step electron beam lithography process, which included ion beam milling, lift-off and oxide and metallic layer deposition steps, nanopillars with an elliptical cross section of 250 × 150 nm were fabricated.…”

Microscopic theory of the Coulomb based exchange coupling in magnetic tunnel junctions

“…This problem requires a fundamental research and understanding of spin currents and interlayer exchange coupling (IEC) mechanisms in MTJ. The IEC was studied theoretically [7][8][9][10][11][12][13][14][15] and experimentally [12,[16][17][18][19][20][21][22] in numerous papers. These works show that there are several phenomena responsible for magnetic coupling between FM layers in MTJ.…”