Thermally Programmable 90° Easy–Axis Rotation in Graded FeGa–B/LiNbO ₃ Magnetoelastic Heterostructures With a Tunable Crossover

ABSTRACT This work demonstrates giant Temperature Controlled Magnetic Anisotropy (TCMA) in compositionally graded FeGa–B thin films integrated on YZ –cut LiNbO 3 (LNO) substrates. By programming the boron concentration gradient during DC magnetron sputtering, we engineered two inverted architectures with comparable overall composition: Sample A with a FeGa–rich interfacial layer (increasing B‐gradient toward the surface) and Sample B with a boron–rich interfacial layer (decreasing B‐gradient away from the substrate). Temperature–dependent magnetometry, magneto–optical Kerr microscopy, and broadband ferromagnetic resonance (FMR) collectively reveal a robust, thermally driven 90° in–plane anisotropy reorientation. The crossover temperature ( T crossover ) is interface–tunable, shifting from 306 K (FeGa–rich interface) to 282 K (boron–rich interface), consistent with gradient–dependent interface clamping and residual stress. We quantify a record TCMA up to 40% K − 1 , approximately 15 × higher than that of comparable magnetoelastic systems, corresponding to a net anisotropy modulation approaching ∼ 2500%. This exceptional thermal leverage arises from the synergistic coupling between the film's large positive magnetostriction (λ S ∼ 41−48 ppm) and the anisotropic thermal–expansion mismatch of the LNO substrate. These findings provide a framework for high–sensitivity thermal magnetic sensors and energy–efficient spintronic devices.