Defect-assisted ion transport in magneto-ionic nitrides probed by positrons

Magneto-ionics deals with a class of spintronic materials where the external electrical field induces ion migration and leads to a raise of magnetization, a consequence of magnetic species local segregations or increased magnetic interactions between them. Since this ion transport is activated by the voltage actuation, no large electrical currents are required and heat dissipation processes are mostly negligible. In addition, simply reversing the direction of the voltage bias, the generated ferromagnetic state returns to its original magnetic configuration, which realizes the magnetic switch concept. Using magnetometry and electron microscopy supported with positron annihilation spectroscopy techniques different nitrides (CoN, FeN, NiN) have been investigated. CoN and FeN are promising candidates for fast magneto-ionic switching, whereas NiN clearly underperforms. Positron annihilation spectroscopy provides a unique probe of open volume defects, e.g. dislocations, vacancies and their agglomerations at grain boundaries, and it was successfully utilized to study the defect nanostructure here. As a reference, we first present electrolyte-gated and defect-mediated oxygen migration in single-layer, paramagnetic Co3O4 at room temperature, which allows voltage-controlled ON-OFF magnetic switching via internal reduction/oxidation processes [1]. Here, the bias-induced motion of oxygen ions was caused by dominant vacancy clusters, with oxygen motion promoted at grain boundaries and assisted by the development of O-rich diffusion channels and Co-rich grain inner regions. In the case of nitrides, on the other hand, nitrogen transport is found to occur uniformly throughout the film, creating a plane-wave-like migration front (Fig. 1), without assistance of diffusion channels [2,3]. Using positrons as a probe, we will show that the initial average open volume is larger in nitrides compared to oxides, which likely governs the migration process and allows for enhanced switching rates and cyclability as well as lowers threshold voltages. We will try to propose factors playing a role in case of hindered ionic migration in NiN, too.

Figure 1: Depth profile of the S-parameter as a function of increasing electrical field.

[1] A. Quintana, E. Menéndez, M. O. Liedke et al., ACS Nano, 12, 10291 (2018)
[2] J. de Rojas, A. Quintana, A. Lopeandía et al., Nature Communications, 11, 5871 (2020)
[3] J. de Rojas, J. Salguero, F. Ibrahim et al. ACS Appl. Mater. Interfaces, 13, 30826 (2021)