Direct Evidence of Non-Oxidative Mechanism in Oxygen Based Magnetoionics

The convergence of microelectronics and neuroscience in research on artificial synapses as base element for the fabrication of artificial neural networks mimicking the brain activity, opens the potential for magnetoionics, where the magnetic anisotropy of the ferromagnet layer is modulated by the ion migration from an adjacent oxide layer by voltage application [1, 2, 3]. In this respect, the understanding of the chemistry at the oxide/ferromagnet interface is the key for a direct evidence of the mechanism at the root of magnetic state change.<br/>Here we report an advanced chemical characterization based on time-of-flight-secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) of the magnetic stack Ta(5)/Co₄₀Fe₄₀B₂₀(1)/Pt(0.09)/MgO(2)/ HfO₂(3) (nominal thickness in nm) on top of Si/SiO₂(300) substrate analyzed as-grown and after the exposure to different voltages for several seconds.<br/>A careful analysis of ToF-SIMS depth profiles evidences that, when a voltage of -3.5 V is applied for 360 s, a decrease of the OH^- inside the MgO layer is revealed, implying a depletion of oxydrilic groups, or hydrogen, from pristine MgO. It is worth noting that, concomitantly, 18O^- and Mg^- intensity remains unchanged. High resolution XPS analysis corroborates this finding by showing a shift in the binding energy of the Mg(2s) edge compatible with an initial Mg(OH)₂ reverting to MgO upon voltage application [4]. Such change in the MgO layer is observed to have implication on the magnetic anisotropy, moving from in-plane anisotropy to out-of-plane anisotropy, as seen with anomalous Hall effect (AHE) and magneto-optic Kerr microscopy (MOKE), thus proving that is feasible to change the magnetic state in ultrathin ferromagnets by the application of a controlled external voltage assisted by ion migration.