Nonvolatile modulation of electronic structure and correlative magnetism of L10-FePt films using significant strain induced by shape memory substrates

Abstract

Tuning the lattice strain (εL) is a novel approach to manipulate the magnetic, electronic, and transport properties of spintronic materials. Achievable εL in thin film samples induced by traditional ferroelectric or flexible substrates is usually volatile and well below 1%. Such limits in the tuning capability cannot meet the requirements for nonvolatile applications of spintronic materials. This study answers to the challenge of introducing significant amount of elastic strain in deposited thin films so that noticeable tuning of the spintronic characteristics can be realized. Based on subtle elastic strain engineering of depositing L10-FePt films on pre-stretched NiTi(Nb) shape memory alloy substrates, steerable and nonvolatile lattice strain up to 2.18% has been achieved in the L10-FePt films by thermally controlling the shape memory effect of the substrates. Introduced strains at this level significantly modify the electronic density of state, orbital overlap, and spin-orbit coupling (SOC) strength in the FePt film, leading to nonvolatile modulation of magnetic anisotropy and magnetization reversal characteristics. This finding not only opens an efficient avenue for the nonvolatile tuning of SOC based magnetism and spintronic effects, but also helps to clarify the physical nature of pure strain effect.

Figure 1. Schematics of sample preparation and corresponding atomic structure change during sample deformation.

(a) Pre-treatment of NiTi(Nb) substrate: A NiTi(Nb) sheet was pre-stretched to induces reorientation of the martensitic phase. Then, the surface of the NiTi(Nb) sheet was cleaned and polished to achieve a surface roughness around 1 nm. (b) Film deposition: the FePt film was deposited on the polished SMA substrate by magnetron sputtering. (c) Sample annealing: the as-deposited sample was annealed to introduce both the L1₀ ordering in the FePt film and the shape recovery in the substrate through an inverse martensitic phase transformation, resulting in a nonvolatile lattice strain in the L1₀-FePt film. (d) Sample cooling: the compressive strain maintained in the film even after cooling down to room temperature.

Figure 2. Microstructure and strain evolution in L1₀-FePt(10 nm) films grown on shape memory alloy substrates with different amount of pre-deformation.

(a) XRD patterns of the samples with different macro-strain (ε_M) in the SMA substrates. (b)Variations of the FePt(111) d-spacing and the in-plane compressive lattice strain (ε_L) with ε_M. Cross-sectional HRTEM images of the samples: (c,d) ε_M = 0%; (e,f) ε_M = −3.5%. (g–k) Elemental mapping obtained from EDX scan of the area marked by the red square in Fig. 2f.

Figure 3. Magnetic property tunability induced by lattice strain in the L1₀-FePt (t = 10, 15, 20 nm) films.

(a) K_eff dependence on ε_L (from 0% to −2.18%). (b) Dependence of K_eff*t on t as a function of strain. (c,d) Modification of bulk anisotropy (K_v) and interfacial anisotropy (K_i) by ε_L, where K_v and K_i are obtained from the slope and the y axis interception of the curves in Fig. 3b, respectively.

Figure 4. Effect of elastic strain on the interfacial electronic structure of the L1₀-FePt film.

(a,c) High resolution XPS spectra of characteristic Fe2p and Pt4f electrons at the NiTi(Nb)/FePt interface of the NiTi(Nb)-SMA/L1₀-FePt (3 nm)/Ta(5 nm) sample; and (b,d) Binding energy evolutions of Fe2p_3/2 and Pt4f_7/2 electrons with different ε_L.

Figure 5. First-principles calculations on modifications of interfacial electronic structure and related physical properties due to the lattice strain in L1₀-FePt.

(a) Unstrained unit cell of L1₀-FePt with lattice constants of a = b = 3.86 Å and c/a = 0.98. (b) Representative DOS of the spin-up (↑) and spin-down (↓) electrons in L1₀-FePt films with three different strains. (c,d) Comparison of the PDOS of L1₀-FePt films with and without a compressive lattice strain (ε_L = −3%). (e) Calculated SOC strengths (ξ) for different orbitals in L1₀-FePt with a ε_L of −3%. (f) Dependences of total SOC strength (ξ _total) and MAE of L1₀-FePt on the ε_L.