Room Temperature Multiferroicity in Transition Metal Dichalcogenide Bulk Single Crystals

The search for new and better multiferroicity is arduous. Ferroic orders such as ferroelectricity and ferromagnetism must coexist simultaneously ¹. Nonetheless, ferroelectrics tend to be insulators as a need to preserve electric polarization (free charges in metals screen this effect) while ferromagnets are metals in their majority ². Coupling among the multiple degrees of freedom allows that the order parameters of one state can be controlled by tuning parameters different from their conjugate variable. Historically, it has been found mainly in 3-dimensional complex oxides and perovskites, such as Cr2O₃ ³, YMnO₃ ⁴, BiFeO₃ ⁵, among others, or in heterostructures of ferroelectric/ferromagnetic thin-films ⁶. Recent advancements in the field of 2D-multiferroics have been done, as for example in the halide compound NiI₂ and Fe-doped In₂Se₃ ^7,8. However, this state has still been elusive for the most widely studied and characterized family of 2D compounds, the transition metal dichalcogenides (TMDs), in spite of theoretical predictions in this respect ^9,10. In this study, we report the first experimental realization of multiferroic states in TMDs via doping, at room temperature, in bulk single crystals. We observe a coexistence of ferromagnetism and ferroelectricity, revealed in the observation of magnetic hysteresis loops and piezoresponse force microscopy (PFM) measurements, resistive switching effects, Density functional theory (DFT) calculations, and substantial piezoelectricity with similar effective coefficients than its undoped monolayer counterparts¹¹. This work opens the possibility of building devices for new nanoelectronic and spintronic applications.<br/><br/><b>References</b><br/><br/>1. Spaldin, N. A., Cheong, S.-W. & Ramesh, R. Multiferroics: Past, present, and future. Phys. Today 63, 38–43 (2010).<br/>2. Hill, N. A. Why Are There so Few Magnetic Ferroelectrics? J. Phys. Chem. B 104, 6694–6709 (2000).<br/>3. Astrov, D. N. The magnetoelectric effect in antiferromagnetics. Sov. Phys. JETP 11, 708–709 (1960).<br/>4. Hanamura, E. & Tanabe, Y. Phase transitions and second-harmonics of ferroelectric and antiferromagnetic RMnO₃. Phase Transitions 79, 957–971 (2006).<br/>5. Kadomtseva, A. M. et al. Phase transitions in multiferroic BiFeO₃ crystals, thin-layers, and ceramics: enduring potential for a single phase, room-temperature magnetoelectric ‘holy grail’. Phase Transitions 79, 1019–1042 (2006).<br/>6. Ueda, K., Tabata, H. & Kawai, T. Coexistence of ferroelectricity and ferromagnetism in BiFeO₃–BaTiO₃ thin films at room temperature. Appl. Phys. Lett. 75, 555–557 (1999).<br/>7. Behera, B., Sutar, B. C. & Pradhan, N. R. Recent progress on 2D ferroelectric and multiferroic materials, challenges, and opportunity. Emergent Materials 4, 847–863 (2021).<br/>8. Song, Q. et al. Evidence for a single-layer van der Waals multiferroic. Nature 602, 601–605 (2022).<br/>9. Tu, Z. & Wu, M. 2D diluted multiferroic semiconductors upon intercalation. Adv. Electron. Mater. 5, 1800960 (2019).<br/>10. Zhong, T., Li, X., Wu, M. & Liu, J.-M. Room-temperature multiferroicity and diversified magnetoelectric couplings in 2D materials. Natl Sci Rev 7, 373–380 (2020).<br/>11. Duerloo, K.-A. N., Ong, M. T. & Reed, E. J. Intrinsic Piezoelectricity in Two-Dimensional Materials. J. Phys. Chem. Lett. 3, 2871–2876 (2012).