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Enhanced Magnetic Order and Reversed Magnetization Induced by Strong Antiferromagnetic Coupling at Hybrid Ferromagnetic−Organic Heterojunctions
Ming-Wei Lin (林銘偉), Po-Hong Chen(陳柏宏), Li-Chung Yu(余立中), Hung-Wei Shiu(許紘瑋), Yu-Ling Lai(賴玉鈴), Su-Ling Cheng(鄭淑齡), Jeng-Han Wang,*(王禎翰) Der-Hsin Wei(魏德新), Hong-Ji Lin(林宏基), Yi-Ying Chin(秦伊瑩), and Yao-Jane Hsu*(許瑤真)
Organic-molecular magnets based on a metal−organic framework with chemically tuned electronic and magnetic properties have been attracting tremendous attention due to their promising applications in molecular magnetic sensors, magnetic particle medicines, molecular spintronics, etc. Here, we investigated the magnetic behavior of a heterojunction comprising a ferromagnetic nickel (Ni) film and an organic semiconductor (OSC) 2,3,5,6-tetrafluoro-7,7,8,8-etracyanoquinodimethane (F4-TCNQ) layer. Through the magneto-optical Kerr effect (MOKE), a photoemission electron microscopy (PEEM), X-ray magnetic circular dichroism (XMCD), and X-ray photoelectron spectroscopy (XPS), we found that the adsorption of F4-TCNQ on Cu(100)/Ni not only reverses the in-plane magnetization direction originally exhibited by the Ni layer but also results in enhanced magnetic ordering. Furthermore, the cyano group (CN) in adsorbed F4-TCNQ was found spin-polarized along with conspicuous charge transfer with Ni. The density functional theory (DFT) calculations suggest that the experimentally found spin polarization originates from hybridization between the CN group’s π orbitals and Ni’s d band. These findings signify that the hybrid states at the organic−ferromagnet interface play a key role in tailoring the magnetic behavior of interfaces. For the case of the F4-TCNQ and Ni heterojunction reported here, interface coupling is an antiferromagnetic one.