Abstract
Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (Pout) in the range of 1–40 nW. Here, Pout values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (JSTT) was observed, without any further change in the magnetic configuration.
Highlights
The spin transfer torque (STT) effect[1,2,3,4,5,6,7,8] allows the effective and selective manipulation of the magnetization of nano-magnets using local spin polarized electrical currents. It has been suggested as a key mechanism enabling a large number of spintronic devices, including magnetic random access memories (MRAM)[9], domain wall based storage[10] or spin transfer torque nano-oscillators (STNOs)[11,12,13,14,15,16,17,18,19]
The STNOs with the largest reported integrated matched output power (Pout) are fabricated starting from magnetic tunnel junction (MTJ) stacks based on CoFeB/MgO/CoFeB which benefit from their high tunnel magnetoresistance ratio (TMR)[11, 24, 25, 28, 29]
Despite the large effort of the STNOs community to work in the lowest possible R × A range, this work presents a large set of consistent data showing that thicker MgO barriers increase the Pout of these oscillators
Summary
Two MTJ stacks incorporating MgO wedges were deposited over 200 mm Si 〈100〉 wafers in a Timaris Singulus tool, leading to a variable R × A over the wafer from below 1 Ωμm[2] up to 40 Ωμm[2] (corresponding to MgO thicknesses from ~0.6 to ~0.9 nm). This is attributed to the existence of intrinsic defects in the MgO layer The conclusion is clear: the nanofabrication process was successful in preventing the formation of redeposited material shorting the tunnelling current through the MgO layer, but below the 10 Ωμm[2] value, the thin MgO barrier contains intrinsic defects that partially de-polarize the current that crosses it[37]. The TMR values achieved in sample S2 were larger than those achieved in sample S1 This is unexpected, since the free layer of sample S2 is thicker, and it is likely due to small variations in the nanofabrication process.
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