Ferromagnetism and Spin-Polarized Luminescence in Lead-Free CsEuCl₃ Perovskite Nanocrystals and Thin Films

Abstract

The emergence of next-generation spintronic and spin-photonic technologies will be aided by the development of materials showing strongly coupled magnetic, electronic, and optical properties. Through a combination of magneto-photoluminescence and magnetic circular dichroism spectroscopies we demonstrate strong magneto-optical responses from CsEuCl3 perovskite nanocrystals and thin films in the near-UV/visible region, stemming from the f–d transitions centered at the B-site Eu2+ cations. We show that this material undergoes a ferromagnetic phase transition at ∼3 K in both the nanocrystal and thin-film samples, resulting in complete spin alignment and indicating intrinsic ferromagnetism. We also report the observation of spin-polarized photoluminescence in the presence of a magnetic field at cryogenic temperatures, saturating with a large polarization ratio (ΔI/I = (I L – I R)/(I L + I R)) of nearly 30% at modest magnetic fields (∼2 T). These results highlight CsEuCl3 as an intrinsically ferromagnetic, luminescent metal-halide perovskite with potentially interesting implications for future spin-based technologies using perovskites.

Antiferromagnetic Order in the Rare-Earth Halide Perovskites CsEuBr3 and CsEuCl3

“…This is the first report of antiferromagnetic ordering in a pure-phase halide perovskite with a rare-earth element on the B-site, demonstrating that rare-earth halide perovskites can be considered for use in spintronic devices. Furthermore, our finding of antiferromagnetism in bulk CsEuCl 3 , in contrast to the previous report of ferromagnetism in nanocrystals and thin films, shows that confinement and/or strain can be used to tune magnetic interactions in halide perovskites from antiferromagnetic to ferromagnetic. This conclusion is also supported by our calculations because both the antiferromagnetic and ferromagnetic configurations of CsEuCl 3 are more stable than a hypothetical nonmagnetic configuration. , …”

“…Furthermore, density-functional theory calculations show that the antiferromagnetic and ferromagnetic configurations of CsEuBr 3 and CsEuCl 3 are nearly equally stable, and both the ferromagnetic and antiferromagnetic configurations of CsEuCl 3 provide stabilizing interactions over a nonmagnetic analog. Our calculations combined with the previous observation of ferromagnetism in nanocrystals and thin films of CsEuCl 3 demonstrate that the type and degree of magnetic coupling can be tuned from antiferromagnetic to ferromagnetic through quantum confinement or substrate–film interactions . The observation of magnetic ordering in bulk CsEuBr 3 and CsEuCl 3 expands the possible applications of chloride, bromide, and iodide halide perovskites beyond optical and electronic devices to now include spintronic devices where both antiferromagnetic and ferromagnetic devices can be made out of the same material.…”

“…Our calculations combined with the previous observation of ferromagnetism in nanocrystals and thin films of CsEuCl 3 demonstrate that the type and degree of magnetic coupling can be tuned from antiferromagnetic to ferromagnetic through quantum confinement or substrate− film interactions. 25 The observation of magnetic ordering in bulk CsEuBr 3 and CsEuCl 3 expands the possible applications of chloride, bromide, and iodide halide perovskites beyond optical and electronic devices to now include spintronic devices where both antiferromagnetic and ferromagnetic devices can be made out of the same material.…”

“…35 A previous report determined that nanocrystals and thermally evaporated thin films of CsEuCl 3 are ferromagnetic with Curie temperatures of 2.5−3.0 K using magnetic circular dichroism measurements. 25 Nanocrystalline and thin film samples may behave differently than bulk materials because of differences in preparation, interactions with the substrate, or the formation of Eu 3+ in addition to Eu 2+ to compensate for the presence of excess Cl − or deficiency of Cs + . The oxide perovskite LaMnO 3 , for example, is antiferromagnetic when prepared as a bulk material, but thin films of LaMnO 3 can be ferromagnetic.…”

“…23 The magnetic behavior of bulk CsEuCl 3 and CsEuI 3 has not been reported. Thin films and nanocrystals of CsEuCl 3 were recently reported to be ferromagnetic with Curie temperatures of 2.5−3.0 K, 25 but the magnetism of nanocrystalline and thin film samples can differ from bulk samples due to substrate-induced strain or surface reconstruction and large surface-to-volume ratios resulting in multiple metal oxidation states. 26,27 We demonstrate that CsEuBr 3 and CsEuCl 3 are magnetic semiconductors that order antiferromagnetically at Neél temperatures T N of 2.0 K and 1.0 K, respectively.…”