Effect of piezoelectric strain on the magnetic properties of Pt/Co thin films

The effect of strain from piezoelectric transducers on the perpendicular magnetic anisotropy (PMA), magnetic domain wall energy and domain wall creep motion of Pt/Co thin films has been investigated. Thin films of Pt/Co/X, where X is Pt, Ir/Pt or Ir, were deposited by sputtering onto thin glass substrates, which were bonded to piezoelectric transducers. Applying a voltage of up to 150 V to the transducers caused tensile out-of-plane strain in the Pt/Co/X thin films for the case of biaxial transducers, and a uniaxial in-plane tensile strain in the case of uniaxial transducers. Measurements of magnetic anisotropy showed that tensile out-of-plane strain lowered the PMA of Pt/Co/Pt, while uniaxial in-plane strain induced an in-plane magnetic anisotropy.
To understand the effect of strain on the magnetic domain walls in Pt/Co/X thin films, the Dzyaloshinskii-Moriya interaction (DMI) field was measured. The DMI field varied between Pt/Co/X films, from close to zero in some Pt/Co/Pt films, indicating domain walls with a Bloch structure, to values large enough to give domain walls a strong Néel component. The DMI field did not change significantly under tensile out-of-plane strain, meaning changes in the domain wall energy were due only to the change in PMA.
Magnetic hysteresis loops of Pt/Co/X thin films showed that the coercive field was reduced by the change in PMA under strain. The magnetisation reversal was investigated further by measurements of the velocity of magnetic domain walls in the creep regime. The domain wall velocity increased by between 10 and 90 % under tensile out-of-plane strain. No measurable changes were seen in the pinning energy of the films, showing that while the height of the barrier to magnetisation reversal is reduced, the anisotropy energy landscape is not distorted significantly. The highest changes in domain wall velocity were seen in the Pt/Co/Pt films with very low DMI fields, from which it is concluded that the creep velocity of Bloch domain walls is more sensitive to strain than that of walls with a Néel component.