Spin pumping in magnetic trilayer structures with an MgO barrier

Abstract

We present a study of the interaction mechanisms in magnetic trilayer structures with an MgO barrier grown by molecular beam epitaxy. The interlayer exchange coupling, Aex, is determined using SQUID magnetometry and ferromagnetic resonance (FMR), displaying an unexpected oscillatory behaviour as the thickness, tMgO, is increased from 1 to 4 nm. Transmission electron microscopy confirms the continuity and quality of the tunnelling barrier, eliminating the prospect of exchange arising from direct contact between the two ferromagnetic layers. The Gilbert damping is found to be almost independent of the MgO thickness, suggesting the suppression of spin pumping. The element-specific technique of x-ray detected FMR reveals a small dynamic exchange interaction, acting in concert with the static interaction to induce coupled precession across the multilayer stack. These results highlight the potential of spin pumping and spin transfer torque for device applications in magnetic tunnel junctions relying on commonly used MgO barriers.

Evidence of Pure Spin-Current Generated by Spin Pumping in Interface-Localized States in Hybrid Metal–Silicon–Metal Vertical Structures

“…The possibility of an enhancement of the indirect exchange via a resonant coupling between localized states and a FM contact has been already addressed for the issues of magnetic exchange [ 48 ,, 49 ] and spin-pumping effect between a FM insulator and a non-magnetic metal [50,51] . The scenario (iv) is also supported by the recent observations of the existence of an interlayer exchange coupling in relatively thick FM/MgO/FM junctions (1-3nm) revealed by a minimum value around 2nm [52] . This conclusion is reinforced by our results in the MgO thickness series samples showing up an increase of the ISHE signal with larger MgO barriers.…”

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

“…Moreover, the presence of oxygen through iron oxide electrodes [11,25] or interface oxidation [12,18] influences bilinear and/or biquadratic coupling. 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.…”

“…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).…”

Spin current propagation through ultra-thin insulating layers in multilayered ferromagnetic systems

“…Studies of both Si and oxide semiconductors 13 , have also shown some suppression of spin pumping and suggest that the carriers may continue to allow spin diffusion through the barrier. Baker et al 14 15 reported a rapid decrease in the damping due to reduced spin pumping with the addition of an ultra-thin MgO barrier layer between Fe and Pt, from which it was concluded that spin current can tunnel through a few monolayers of an insulating oxide barrier. The work was supported by transmission electron microscopy (TEM) imaging, which is limited to sampling very small areas and provides a projection of a thin 3D sample volume that may not show pinhole defects, and any defects present may be difficult to directly image 16 .…”