Additive-enhanced hydrogen- and hydroxide-based magneto-ionic control in Ni films

Magneto-ionic control of metals is a promising approach for energy-efficient and voltage-programmable magnetic devices. In this work, we demonstrate a dual hydrogen- and hydroxide-ion-based mechanism to control the magnetic properties of nickel films in an alkaline electrolyte. Upon application of reduction potentials, reversible electrochemical hydrogen absorption into a nickel film electrode is found to decrease its overall magnetic moment, while an increase in the overall magnetic moment at more negative potentials can be explained by the reduction of nickel hydroxide on the surface. Furthermore, coercivity is also decreased in the reduced state. This effect is reversible over 50 reduction-oxidation cycles and reaches up to 30% change at maximum. The addition of thiourea to the electrolyte amplifies the magnitude of the magneto-ionic control of the magnetic moment by a factor of 2 and promotes the energy efficiency of the magneto-ionic effect by decreasing the required overpotentials by <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>∼</a:mo> <a:mn>0.25</a:mn> <a:mspace width="0.16em"/> <a:mi mathvariant="normal">V</a:mi> </a:mrow> </a:math> . Raman spectroscopy reveals the formation of an <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mrow> <d:mi>α</d:mi> <d:mtext>−</d:mtext> <d:mi>Ni</d:mi> </d:mrow> <d:msub> <d:mrow> <d:mo>(</d:mo> <d:mi>OH</d:mi> <d:mo>)</d:mo> </d:mrow> <d:mn>2</d:mn> </d:msub> </d:mrow> </d:math> layer on the Ni surface in the presence of thiourea, which may be the cause for the improved magneto-ionic effect. Our study not only introduces Ni as a versatile magneto-ionic material, but also demonstrates how electrolyte additives can be used to boost magneto-ionic effects in terms of effect strength and energy efficiency.