The recent discovery of 2D magnets has revealed various intriguing phenomena due to the coupling between spin and other degree of freedoms (such as helical photoluminescence, nonreciprocal SHG). Previous research on the spin-phonon coupling effect mainly focuses on the renormalization of phonon frequency. Here we demonstrate that the Raman polarization selection rules of optical phonons can be greatly modified by the magnetic ordering in 2D magnet CrI 3 . For monolayer samples, the dominant A 1g peak shows abnormally high intensity in the cross polarization channel at low temperature, which is forbidden by the selection rule based on the lattice symmetry. While for bilayer, this peak is absent in the cross polarization channel for the layered antiferromagnetic (AFM) state and reappears when it is tuned to the ferromagnetic (FM) state by an external magnetic field. Our findings shed light on exploring the emergent magneto-optical effects in 2D magnets.
Circularly
polarized light carries light spin angular momentum,
which may lead helicity-resolved Raman scattering to be sensitive
to the electronic spin configuration in magnetic materials. Here,
we demonstrate that all Raman modes in the 2D ferromagnet VI3 show different scattering intensities to left and right circularly
polarized light at low temperatures, which gives direct evidence of
the time-reversal symmetry breaking. By measuring the circular polarization
of the dominant Raman mode with respect to the temperature and magnetic
field, the ferromagnetic (FM) phase transition and hysteresis behavior
can be clearly resolved. Besides the lattice excitations, quasielastic
scattering is detected in the paramagnetic phase, and it gradually
evolves into the acoustic magnon mode at 18.5 cm–1 in the FM state, which gives the spin wave gap that results from
large magnetic anisotropy. Our findings demonstrate that helicity-resolved
Raman spectroscopy is an effective tool to directly probe the ferromagnetism
in 2D magnets.
We present a detailed study on the structural phase transition in α-TiBr3, which is deeply connected with the lattice and orbital degree of freedoms. A chemical vapor transport method is adopted to synthesize the α-TiBr3 single crystal samples, and the structural phase transition at about 180 K is characterized by x-ray diffraction (XRD), magnetic susceptibility, and specific heat capacity. To further the understanding in the physical nature of this phase transition, a systematic Raman spectroscopic study is performed on α-TiBr3 crystals. With temperature decreasing, a large frequency blue shift and peak width narrowing are observed in the vibrational mode associated with Ti in-plane relative movement, which indicates the formation of Ti–Ti bonding and orbital-fluctuation freezing at low temperatures. These results are fully consistent with magnetic–nonmagnetic phase transition resolved by the measurement of magnetic susceptibility and lattice changes by XRD.
Layered ferromagnets with strong
magnetic anisotropy energy (MAE)
have special applications in nanoscale memory elements in electronic
circuits. Here, we report a strain tunability of perpendicular magnetic
anisotropy in van der Waals (vdW) ferromagnets VI3 using
magnetic circular dichroism measurements. For an unstrained flake,
the M–H curve shows a rectangular-shaped
hysteresis loop with a large coercivity (1.775 T at 10 K) and remanent
magnetization. Furthermore, the coercivity can be enhanced to a maximum
of 2.6 T under a 3.8% external in-plane tensile strain. Our DFT calculations
show that the increased MAE under strain contributes to the enhancement
of coercivity. Meanwhile, the strain tunability on the coercivity
of CrI3, with a similar crystal structure, is limited.
The main reason is the strong spin–orbit coupling in V3+ in VI6 octahedra in comparison with that in Cr3+. The strain tunability of coercivity in VI3 flakes
highlights its potential for integration into vdW heterostructures.
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