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Tunable Layered-Magnetism–Assisted Magneto-Raman Effect in a Two Tunable layered-magnetism–assisted magneto-Raman effect in a two-dimensional magnet CrI3 Wencan Jina,1, Zhipeng Yeb, Xiangpeng Luoa, Bowen Yangc,d, Gaihua Yeb, Fangzhou Yinc,d, Hyun Ho Kimc,d,2, Laura Rojasb, Shangjie Tiane,f, Yang Fue,f, Shaohua Yane,f, Hechang Leie,f, Kai Suna, Adam W. Tsenc,d, Rui Heb,3, and Liuyan Zhaoa,3 aDepartment of Physics, University of Michigan, Ann Arbor, MI 48109; bDepartment of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409; cInstitute for Quantum Computing, Department of Physics and Astronomy, University of Waterloo, ON N2L 3G1, Canada; dDepartment of Chemistry, University of Waterloo, ON N2L 3G1, Canada; eDepartment of Physics, Renmin University of China, 100872 Beijing, China; and fBeijing Key Laboratory of Opto-electronic Functional Materials & Micro-Nano Devices, Renmin University of China, 100872 Beijing, China Edited by S. Lance Cooper, University of Illinois, Urbana, IL, and accepted by Editorial Board Member Zachary Fisk August 14, 2020 (received for review June 22, 2020) We used a combination of polarized Raman spectroscopy experi- the linear crossed polarization channel across the layered-AFM to ment and model magnetism–phonon coupling calculations to study FM transition (7, 16–18). However, the physical origin of the the rich magneto-Raman effect in the two-dimensional (2D) magnet magneto-optical Raman effect remains elusive with diverse pro- CrI . We reveal a layered-magnetism–assisted phonon scattering 3 posals ranging from Davydov split for bilayer CrI3 (16, 18), to mechanism below the magnetic onset temperature, whose Raman zone-folded phonon for few-layer CrI3 (17), to coupled magnetism– excitation breaks time-reversal symmetry, has an antisymmetric phonon scattering for bulk CrI3 (7), none of which can be trivially Raman tensor, and follows the magnetic phase transitions across generalized to explain CrI3 of arbitrary thickness. critical magnetic fields, on top of the presence of the conventional In this work, we systematically examine the magneto-optical phonon scattering with symmetric Raman tensors in N-layer CrI . 3 Raman effect for N-layer CrI (N = 1to4)byperformingpo- We resolve in data and by calculations that the first-order A pho- 3 g larization, temperature, and magnetic field-dependent micro- non of the monolayer splits into an N-fold multiplet in N-layer CrI3 due to the interlayer coupling (N≥2) and that the phonons within Raman spectroscopy measurements and unambiguously identify – the multiplet show distinct magnetic field dependence because of the layered-magnetism assisted phonon scattering as the origin APPLIED PHYSICAL SCIENCES their different layered-magnetism–phonon coupling. We further applicable for CrI3 of any thickness. N-layer CrI3 crystalline flakes find that such a layered-magnetism–phonon coupled Raman scatter- were exfoliated from high-quality CrI3 single crystals, sandwiched ing mechanism extends beyond first-order to higher-order multi- between hexagonal boron nitride (hBN) thin flakes, and then phonon scattering processes. Our results on the magneto-Raman effect of the first-order phonons in the multiplet and the higher- Significance order multiphonons in N-layer CrI3 demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and The two-dimensional (2D) magnetic semiconductor CrI hosts a – 3 underline the unique opportunities of spin phonon physics in van variety of strong and tunable magneto-optical effects and al- der Waals layered magnets. lows for the development of novel magneto-optical devices. While the elastic magneto-optical effects in CrI3 are well un- two-dimensional layered magnetism | magneto-Raman effect | Raman derstood, its recently discovered inelastic magneto-Raman ef- spectroscopy fect remains to have case-specific interpretations varying upon the thickness of CrI3. We perform comprehensive Raman wo-dimensional (2D) CrI3 of few-layer form features a measurements on 2D CrI3 with polarization, temperature, layer Tlayered-antiferromagnetic (AFM) order where the spins align number, and magnetic field dependence. We resolve a Davydov- along the out-of-plane direction ferromagnetically within each split–induced N-fold multiplet in N-layer CrI3 and reveal the dis- layer and antiferromagnetically between adjacent layers (1–5). It tinct magneto-Raman behaviors of individual phonons within undergoes a layered-AFM to ferromagnetic (FM) phase transition the multiplet. Our results discover a layered-magnetism–coupled upon applying a moderate magnetic field (1–7), or electric field phonon scattering mechanism that explains the rich magneto- (8–10), or electrostatic doping (11), or hydrostatic pressure (12, Raman effect in CrI3 of arbitrary thickness and elucidates the 13). The strong coupling between spin and charge degrees of spin–phonon coupling physics in layered magnets. freedom in 2D CrI3 allows magneto-optical effects manifested in a variety of ways including large magneto-optical Kerr effect (5) and Author contributions: W.J., R.H., and L.Z. designed research; W.J., Z.Y., X.L., B.Y., G.Y., F.Y., H.H.K., L.R., S.T., Y.F., S.Y., H.L., K.S., A.W.T., R.H., and L.Z. performed research; W.J., magnetic circular dichroism (8–13), spontaneous helical photo- X.L., K.S., R.H., and L.Z. analyzed data; W.J., X.L., R.H., and L.Z. wrote the paper; B.Y., F.Y., luminescence (14), giant nonreciprocal second harmonic genera- H.H.K., and A.W.T. fabricated the sample; and S.T., Y.F., S.Y., and H.L. grew the tion (15), and anomalous magneto-optical Raman effect (7, single crystal. 16–19). All of these magneto-optical effects can be tuned across The authors declare no competing interest. This article is a PNAS Direct Submission. S.L.C. is a guest editor invited by the the layered-AFM to FM phase transition, making 2D CrI3 a promising candidate for applications in magnetic sensors, optical Editorial Board. modulation, and data storage. This open access article is distributed under Creative Commons Attribution-NonCommercial- NoDerivatives License 4.0 (CC BY-NC-ND). Among all magneto-optical effects in CrI3, the magneto-optical 1Present address: Department of Physics, Auburn University, Auburn, AL 36849. Raman effect is of particular interest for two reasons. First, among 2Present address: School of Materials Science and Engineering, Kumoh National Institute all known magnets, the largest magnetism-induced optical rotation of Technology, Gumi, Gyeongbuk 39177, Korea. is observed for the linearly polarized, inelastically scattered light 3 −1 To whom correspondence may be addressed. Email: [email protected] or lyzhao@ off the Ag phonon mode (∼129 cm ) in the FM phase of CrI3 umich.edu. (16). Second, different phonon modes exhibit distinct magneto- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ optical behavior that the Ag mode emerges whereas its neigh- doi:10.1073/pnas.2012980117/-/DCSupplemental. − boring strongest antisymmetric mode (∼127 cm 1)disappearsin www.pnas.org/cgi/doi/10.1073/pnas.2012980117 PNAS Latest Articles | 1of6 Downloaded by guest on September 28, 2021 placed onto SiO2/Si substrates inside a high-purity (>99.999%) between√̅̅̅̅̅̅̅̅̅ adjacent layers, equivalent to a coupling frequency nitrogen-filled glovebox. Micro-Raman spectroscopy measure- ω = k=m. Diagonalizing H leads to N nondegenerate eigenfre- ~ ments in the backscattering geometry were carried out with a quencies Ωi(ω0, ω) and their corresponding eigenmodes Ui (i = 1, 633-nm excitation laser resonant with the charge-transfer transi- 2, ..., N) (i.e., Davydov splitting), with i = 1 being the highest- frequency mode and i = N being the lowest-frequency mode. By tion (14), inside a vacuum cryostat at a base pressure lower than − − −7 ω = ± 1 ω = ± 1 7 × 10 mbar, and under an out-of-plane magnetic field (B⊥) up choosing 0 129.10 0.10 cm and 15.98 0.55 cm , Ω to 2.2 T. the calculated frequencies i (solid lines with open diamonds) We start with resolving in N-layer CrI3 the interlayer coupling- match well with all of the experimental values (red solid squares induced split of the A mode of monolayer CrI (20, 21). Fig. 1A and blue solid circles), as highlighted by the fan diagram in g 3 Ω ~ shows Raman spectra in both parallel and crossed linear polar- Fig. 1B. See detailed calculations of i and Ui in SI Appendix, ization channels taken at T = 10 K and B⊥ = 0 T on 1L to 4L section II and Tables S1 and S2. Because N-layer CrI3 is structurally centrosymmetric, its CrI3 (see full-range spectra in both channels and comparison to off-resonance 532-nm excitations in SI Appendix, section I and N-calculated eigenmodes have alternating parities, with the highest-frequency mode always parity even as a result of Figs. S1 and S2, respectively). It has been established for CrI3 ~ = that the modes in the crossed channel in Fig. 1A correspond to equal, in-phase√̅̅̅̅ atomic displacement between layers (i.e., U1 (1,1, ...,1)= N). In the parallel channel where only modes with antisymmetric Raman tensor (RAS) whereas those in the parallel even parity and symmetric Raman tensor RS can be detected, we channel are for symmetric Raman tensor of Ag symmetry (RS) (19). We highlight three key observations that have not been expect to see every other mode starting with the highest- frequency one (i = 1, 3, 5, ...). This expectation is indeed con- reported in previous work (7, 16–19, 22, 23) and summarize them sistent with our data that U for N = 1 and 2 and U for N = 3 in Fig. 1B with fitted mode frequencies vs. N. First, the number 1 1,3 and 4 are observed in the linear parallel channel in Fig. 1 A and of modes increases proportional to the number of layers (with an B.
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