Magnetic-Field-Induced Quantum Phase Transitions in a van der Waals Magnet

Abstract

Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of two-dimensional (2D) magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field-and temperature-dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of chromium triiodide (CrI 3) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its threedimensional (3D) bulk. However, we reveal a novel mixed state of layered AFM and FM in 3D CrI 3 bulk crystals where the layered AFM survives in the surface layers, and the FM appears in deeper bulk layers. We then show that the surface-layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few-layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from C 3i (rhombohedral) to C 2h (monoclinic). Our result not only unveils the complex single-magnon behavior in 3D CrI 3 , but it also settles the puzzle of how CrI 3 transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI 3 thin layers, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-tomonoclinic phase transitions. These findings further open opportunities for future 2D magnet-based magnetomechanical devices.

Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

“…2a. The 1st-order single-phonon peaks appear in the relatively low frequency range of 50-150 cm −1 , and are assigned to be of either A g or E g symmetries under the C 3i point group (see Supplementary Note 1), which is consistent with earlier work 34,[37][38][39][40][41][42][43][44] and proves the high quality of our samples. The 2nd-order two-phonon and the 3rd-order three-phonon modes show up in slightly higher frequency ranges of 190-290 cm −1 and 310-410 cm −1 , respectively, and show decreasing mode intensities at higher-order processes, same as typical multiphonon overtones under harmonic approximation 45 or cascade model 46 .…”

“…Figure 2a displays a representative Raman spectrum acquired in the crossed linear polarization channel at 40 K (slightly below T C = 45 K). Note that this spectrum covers a much wider frequency range than earlier Raman studies on CrI 3 34,[37][38][39][40][41][42][43][44] . The multiphonon scattering is visible up to the 3rd order, and their zoom-in Raman spectra are shown in the inset of Fig.…”

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

“…A thin flake of CrI 3 (≈7 nm from atomic force microscopy, or ≈10 layers) was encapsulated between two 20 nm and 30 nm flakes of hBN using the dry transfer technique 22 , 23 . In the ab plane, the Cr 3+ atoms are arranged in a honeycomb lattice, where each chromium atom is bonded with six iodine atoms to form a distorted octahedron (see Fig.…”

“…Raman spectroscopy is a powerful technique to study a variety of phenomena in 2D quantum materials, including effects of strain 16 , electron–phonon coupling 17 , phase transitions 18 , spin-phonon coupling 19 , and magnetic excitations 20 – 22 . In addition, the diffraction-limited spot size allows for the investigation of atomically thin samples and heterostructures using a non-contact probe.…”

Tunable layered-magnetism–assisted magneto-Raman effect in a two-dimensional magnet CrI 3

“…It undergoes a layered-AFM to ferromagnetic (FM) phase transition upon applying a moderate magnetic field (1-7), or electric field (8)(9)(10), or electrostatic doping (11), or hydrostatic pressure (12,13). The strong coupling between spin and charge degrees of freedom in 2D CrI 3 allows magneto-optical effects manifested in a variety of ways including large magneto-optical Kerr effect (5) and magnetic circular dichroism (8)(9)(10)(11)(12)(13), spontaneous helical photoluminescence (14), giant nonreciprocal second harmonic generation (15), and anomalous magneto-optical Raman effect (7,(16)(17)(18)(19). All of these magneto-optical effects can be tuned across the layered-AFM to FM phase transition, making 2D CrI 3 a promising candidate for applications in magnetic sensors, optical modulation, and data storage.…”