Strain‐Gradient‐Induced Unidirectional Magnetic Anisotropy in Nanocrystalline Thin Permalloy Films

Abstract

We investigate the effect of inhomogeneous elastic deformation on the magnetic anisotropy of Ni71.5Fe28.5 wt.% nanocrystalline film. The in-plane controlled strains are induced in the film by bending of a glass substrate which has a thickness step in the middle. Ferromagnetic resonance measurements reveal the existence of the in-plane unidirectional magnetic anisotropy. The anisotropy behavior directly correlates with the calculated strain gradients. We show that this correlation is well explained by the flexomagnetic effect, which establishes the relation between the magnetization and the inhomogeneous strains. The experimental value of the flexomagnetic coefficient for the thin Ni71.5Fe28.5 wt.% film is 1.5×10 -3 T•m.The flexoeffects are electromagnetomechanical effects in which the electric polarization or magnetization exhibits a linear response to inhomogeneous mechanical impact (elastic strain or stress gradient). Among these effects, the most studied one is flexoelectricity. The coupling between electric polarization and strain gradient in crystals was theoretically investigated as early as the 1960s by

Flexomagnetism and vertically graded Néel temperature of antiferromagnetic Cr2O3 thin films

“…Flexomagnetism can be found in magnets of 69 out of 90 bulk symmetry classes 14 . This includes the lowest symmetry groups 1 and (54 nontrivial independent tensor components out of 81), which makes flexomagnetism to be readily expected even in Permalloy 10 . A higher symmetry of the lattice reduces number of independent components of the flexomagnetic tensor, e.g., leaving 10 independent components among 37 non-trivial ones for crystals of symmetry, or 5 independent components among non-trivial 18 ones for crystals (e.g., ThMn 2 ).…”