Supporting Information for: Patterning-Induced Ferromagnetism of Fe 3 GeTe 2 van der Waals Materials beyond Room Temperature

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1 Supporting Information for: Patterning-Induced Ferromagnetism of Fe 3 GeTe 2 van der Waals Materials beyond Room Temperature Qian Li,, Mengmeng Yang,, Cheng Gong, Rajesh V. Chopdekar, Alpha T. N'Diaye, John Turner, Gong Chen, Andreas Scholl, Padraic Shafer, Elke Arenholz, Andreas K. Schmid, Sheng Wang, Kai Liu,, Nan Gao, Alemayehu S. Admasu, Sang-Wook Cheong, Chanyong Hwang, Jia Li, *,# Feng Wang, Xiang Zhang and Ziqiang Qiu *, Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA Nano-scale Science and Engineering Center (NSEC), 3112 Etcheverry Hall, University of California, Berkeley, California 94720, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA NCEM, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Physics, University of California, Davis, CA 95616, USA Department of Physics, Georgetown University, Washington, DC Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Korea Research Institute of Standards and Science, Yuseong, Daejeon , Republic of Korea # International Center for Quantum Materials, School of Physics, Peking University, Beijing , China. These authors contributed equally. * Corresponding Authors jiali83@pku.edu.cn. qiu@berkeley.edu.

2 1. Sample fabrication The bulk Fe3GeTe2 crystal was fabricated via chemical vapor transport. Thin Fe3GeTe2 flakes ( nm thick) were exfoliated on Si(111) substrates. To facilitate the Fe3GeTe2 exfoliation, the sample was heated to 50 C and was immediately (within 15 minutes) transferred into an ultrahigh vacuum chamber (~10-11 Torr). A 1nm thick Pd protection layer was grown on top of the sample using an e-beam evaporator. Microstructures were patterned by focused ion beam (FIB) milling with Ga + source at 30 kv and 100 pa. The thicknesses of Fe3GeTe2 flakes were determined by line scans using atomic force microscopy (AFM). 2. SQUID, XMCD and PEEM measurement During the SQUID measurement, the Fe3GeTe2 crystal was fixed with tape on a straw, which is a conventional method. No substrate was used in the experiment, so that no SQUID background was subtracted for hysteresis loops in Fig. 1. X-ray magnetic circular dichroism shown in Fig. 1e was measured at Beamline of the Advanced Light Source (ALS) in total electron yield (TEY) mode. During the XMCD measurement, x-ray absorption spectra at the Fe 2p level were recorded at normal incidence, i.e. the incoming circularly polarized x-ray beam parallel to the sample surface normal and collinear to the external magnetic field (0.7 T) applied parallel and antiparallel to the x-ray helicity. The difference of these two spectra is defined as the XMCD signal. PEEM measurements were carried out at Beamline of the ALS. The x-ray beam was circularly polarized and incident at an angle of 60o to the surface normal direction. Because the XMCD effect measures the projection of magnetization onto the x-ray beam direction, our measurements are sensitive to the in-plane as well as out-of-plane magnetization components. All samples were demagnetized using an in-plane ac magnetic field (~0-800 Oe) before being loaded into the PEEM chamber. We obtained magnetic domain images by calculating the ratio of images taken with left- and right-circular polarized x-rays at a photon energy of ev (Fe L3). All

3 images were captured at zero magnetic field because PEEM measurements are not compatible with externally applied fields due to interactions with the photoemitted electrons. 3. Supplementary data Fig. S1. Morphology (left), magnetic stripe domains (middle), and AFM line scans (right) for different Fe3GeTe2 flakes used in Fig. 3 of the main manuscript. The PEEM images were taken at 110 K and the AFM line scans were taken at room temperature.

4 Fig. S2. Curie temperature determination of a rectangular microstructure. The Fe3GeTe2 microstructure exhibits a magnetic multi-domain phase at temperature K, and a paramagnetic phase above 370 K, indicating a Curie temperature of ~370 K for this in-plane magnetized Fe3GeTe2 microstructure. Fig. S3. In-plane magnetic vortex state at room temperature from a FIB patterned 2 µm x 2 µm square microstructure. Noting that XMCD measures the projection of the magnetization onto the incidence direction of a circularly polarized x-ray beam, the corresponding changes of the four colored domain contrasts a before and b after changing the relative direction between the incident circularly polarized x-rays and the microstructure by 90 degrees confirm the in-plane magnetic vortex state.

5 Fig. S4. Micromagnetic simulation results of different domain configurations for a 2 µm x 2 µm square microstructure. The micromagnetic simulations were carried out using the Object Oriented MicroMagnetic Framework code 1 based on the Landau Lifshitz Gilbert equation. Saturation magnetization of 376 emu/cm 3 2 and different exchange coupling stiffnesses ranging from pure Fe (A = J/m) 3 to calculated Fe3GeTe2 (A = J/m) 2 were used in the simulation. The result shows that the critical thickness for vortex formation inside the 2 µm x 2 µm square microstructure is ~ nm for exchange coupling stiffness of ( ) J/m, suggesting that the in-plane magnetic vortex observed in this paper (Fig. 4 and Fig. S3) extends to at least the top ~50-nm thick region.

6 Fig. S5. Ga + distribution in patterned structures. a, Mapping of Ga intensity at photon energy of 1160 ev. b, Ga X-ray Absorption Spectra from the sputtered region A and the un-sputtered region B. The result shows clearly that most of the Ga atoms are located outside the microstructure. With the FIB parameters of 30-KeV voltage and 100-pA current for 2 minutes, the fluence of the Ga + in region A is estimated to be ~ ions/cm 2. Then from the Ga XAS intensity ratio between region A and B, we estimate an upper limit of ~ Ga ions/cm 2 = Ga ions/å 2 in the Fe3GeTe2 microstructure. Supplementary References: (1) Donahue, M. J.; Porter, D. G. OOMMF User s Guide, Version 1.0; National Institute of Standards and Technology: Gaithersburg, MD, (2) Leon-Brito, N.; Bauer, E. D.; Ronning, F.; Thompson, J. D.; Movshovich, R. J. Appl. Phys. 2016, 120, (3) You C.-Y. Appl. Phys. Express 2012, 5,