Strain-relaxed chiral hybrid bismuth halides for spin selectivity and magnetoresistance

Abstract

Chiral hybrid metal halides offer a versatile platform for spintronic devices through spin-selective charge transport governed by the chirality-induced spin selectivity (CISS) effect. However, halide mixing, commonly used to tune optical and electronic properties, has been believed to deteriorate chiroptical and spin-selective responses, and Pb-free alternatives remain scarcely explored. Here, we investigate iodide incorporation in zero-dimensional (0D) bismuth-based chiral metal halides (R/S-MBA)4Bi2Br10(1- x)I10x (MBA = methylbenzylamine, x = 0, 0.2, 0.4, 0.6, and 0.8), and show that iodide incorporation is accompanied by a systematic reduction in lattice microstrain and a concurrent increase in spin selectivity. Upon full iodide substitution, a structural transition to a one-dimensional (1D) phase, (R/S-MBA)2Bi2I8, is observed, in which the connected bismuth-iodide framework exhibits further microstrain relaxation and a higher spin selectivity of up to 61 %. Based on these results, we demonstrate the first spin-valve devices based on Pb-free chiral metal halides, exhibiting enhanced magnetoresistance values of up to +/- 0.3 % at room temperature. In parallel, we observe that iodide incorporation leads to enhanced circular dichroism across the same composition series. Taken together, these results establish low-dimensional Pb-free chiral bismuth halides as a viable spin-functional material platform and highlight lattice microstrain as a key structural parameter correlated with spin-selective transport, offering a new direction for the design of hybrid spintronic semiconductors.