Magnetoelastic effects and random magnetic anisotropy in highly strained ultrathin Ni nanowires epitaxied in a SrTiO3 matrix

Abstract

We analyze the magnetic anisotropy of Ni nanowires with diameters smaller than 5 nm. The nanowires are vertically epitaxied in a SrTiO 3 (001) matrix which generates huge tensile strains up to 3.6% along the nanowire axis. This leads to an unusual anisotropy, characterized by an easy magnetization plane perpendicular to the nanowire axis. Hysteresis cycles M(H) unveil an overall in-plane isotropy, while an opening of the M(H) cycles and thermal activation measurements indicate the presence of local energy barriers inside the nanowires. Surprisingly, the coercive field H c (T) decays exponentially with increasing temperature, for both the easy plane and the hard axis. Based on these findings, we provide an analysis of magnetoelastic effects in the nanowires. By considering global averaging over the anisotropy distribution and local averaging according to the Random Magnetic Anisotropy model, we find that the global anisotropy, with its hard axis and isotropic easy plane, is related to the mean strain, while coercivity arises from local strain variations. We evidence that a thermally activated anisotropy softening occurs in the nanowires, in addition to Sharrock's law of thermal reduction of coercivity. Possible mechanisms responsible for this thermal softening of anisotropy are proposed and discussed. Our study eventually allows to identify two major competing effects at play in the present system: an increasing magnetic anisotropy with increasing strain and a reduction of the anisotropy with increasing local strain fluctuations.

Strain-engineering of magnetic anisotropy in CoxNi1−x−SrTiO3/SrTiO3Weng1, Hennes2, Juhin3 et al. 2022Phys. Rev. Materials3010View full textAdd to dashboardCite

“…60) and the bulk value of m s , the estimated value of K (1) u using eq. ( 8) is in agreement with the one estimated by cyclic magnetometry [12] if α is taken equal to 0.016. Interestingly, such a value is smaller than the ones obtained using the same approach in thin films with typical α values reported so far varying between 0.05 and 1 [61][62][63].…”

“…In earlier studies, we have evidenced large magnetoelastic effects in Ni-SrTiO 3 VANs [11,12]. From a quantitative perspective, the uniaxial magnetic anisotropy has two sources in this system: (i) a magnetostatic term, or shape anisotropy, given by K ms = K (1) ms sin 2 α with α being the angle between the axis of the wires and the magnetization and K…”

“…The fact that M r /M s is maximal for γ=0 and the cos γ-like variation are characteristic of a uniaxial magnetic anisotropy with easy axis along the [001] direction. In the case of VANs hosting pure Ni nanowires, the [001] direction was found to be the hard axis [11,12]. This was traced back to the negative uniaxial magnetoelastic anisotropy arising from the large axial strain in the wires.…”

“…Co x Ni 1−x (CoNi) nanowires embedded in a SrTiO 3 matrix were grown by sequential pulsed laser deposition (PLD) of cobalt, nickel and SrTiO 3 (STO) on Nbdoped SrTiO 3 (001) and SrTiO 3 (001) substrates, following procedures established previously to synthesize Co, Ni and CoNi nanowires in CeO 2 /SrTiO 3 , (Sr,Ba)TiO 3 and SrTiO 3 [11,12,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. A quadrupled Nd:YAG laser (wavelength 266 nm) operating at 10 Hz and a fluence in the 1-3 J/cm 2 range was used.…”

“…In this equation, c ij are elastic constants, ϵ zz is the strain along [001] (axial strain), ϵ rr is the radial strain, and λ 001 is the magnetostriction coefficient. As λ 001 is negative in pure Ni, large axial strains eventually lead to a negative value of K (1) ms + K (1) me and the axis of the wires becomes a magnetic hard axis [12].…”