Magnetic Domain Control of Exchange Biased Magneto-Electric Thin Film Composites

Magneto-electric composites constitute a promising class of materials for the application as highly sensitive magnetic field sensors. Their compatibility to miniaturization, their passive nature and their large dynamic range are just some of the numerous advantages of these composites compared to alternative magnetic field sensors, as e.g. flux-gates or magneto-resistive sensors. However, the highest sensitivities are observed in the presence of well-defined magnetic bias fields that are of significant disadvantage in terms of miniaturization and noise performance.

Self-biased magneto-electric composites, which are fabricated using the exchange bias effect [1], exhibit an increased total anisotropy in comparison to systems without exchange bias. As a consequence, small exchange bias fields are favourable because of a minor reduction of the  magneto-electric voltage coefficient. However, weakly biased magneto-electric composites might loose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer a maximum voltage coefficient α_ME ≈ 430 V/cmOe was found for a magnetostrictive multilayer of 3 x (5 nm Ta / 3 nm Cu / 8 nm Mn-Ir / 333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with a slightly reduced magneto-electric cofficient of α_ME ≈ 340 V/cmOe.  This slight reduction is overcompensated by the improved control of the magnetic domain pattern that is highly desirable for magnetic field sensors and of special importance when frequency conversion techniques are applied [2].

[1] Lage, E., Kirchhof, C., Hrkac, V., Kienle, L., Jahns, R., Knöchel, R., Quandt, E. & Meyners, D. Exchange biasing of magnetoelec¬tric composites. Nature materials 11, 523–9 (2012).

[2] Jahns, R., Greve H., Woltermann, E., Quandt, E. & Knöchel, R. Sensitivity enhancement of mag¬netoelectric sensors through frequency-conversion. Sensors and Actuators A: Physical 183, 16-21 (2012).

This work was funded by the German Research Foundation (DFG) as part of the Collaborative Research Center 855 “Magnetoelectric Composites–Future Biomagnetic Interfaces”