Spin-induced negative thermal expansion and spin–phonon coupling in van der Waals material CrBr3

Abstract

The two-dimensional van der Waals (vdW) magnets retaining magnetic order in atomically thin limit demonstrate challenging physical phenomena and they are considered as prospective building blocks for construction of advanced spintronics and nanoelectronics devices. Here, we present experimental evidence for negative thermal expansion of lattice volume and vdW layers and strong spin–phonon coupling effects, caused by formation of the long-range ferromagnetic order in the vdW material CrBr3. The neutron and X-ray diffraction measurements revealed anomalous temperature variation of lattice parameters and interatomic distances and angles in the vicinity of Curie temperature (TC). A pronounced rise of the frequencies of the most of the observed vibrational modes and unusual reversal broadening of their full widths at half maximum below TC was found from Raman spectroscopy measurements.

Gapless Dirac magnons in CrCl$_{3}$

“…S3 in the Supplement. A similar lattice constant anomaly has been observed in other quasi-2D honeycomb magnets, namely CrBr 3 [26] and Cr 2 Ge 2 Te 6 [27], though in those materials the anomaly occurs near the magnetic ordering temperatures. The ilmenite compound NiTiO 3 , which shares the quasi-2D magnetic structure of CrCl 3 but with 3D structural bonding, also exhibits an anomaly in its lattice constants [28].…”

“…The onset of spin waves occurs on the same temperature scale as the in-plane FM spin correlations, as indicated via the anomalies in the a-axis lattice constant and the phonon feature; these behaviors all occur gradually on cooling below ∼50 K, 10s of K higher than T N . In contrast, CrBr 3 exhibits abrupt changes at the Curie temperature in the spinphonon coupling and the lattice constants [26], and these differences may be due to differing magnetic anisotropy. Anisotropy can play an important role in the ordering behavior of 2D systems; the 2D isotropic Heisenberg model has no long-range order above T > 0 for short-range interactions [34], in contrast to the transition present in the 2D Ising model [35].…”

Chiral Phonons and Giant Magneto‐Optical Effect in CrBr3 2D Magnet

“…Similarly, the temperature dependence of the phonon linewidths also reveals a clear anomaly below ≈50 K, as demonstrated by Figure 3b,d. This anomalous behavior of linewidths below ≈50 K can be attributed to several factors including spin‐phonon coupling, spin‐induced negative thermal expansion [ 45 ] and magnetic ordering in the sample. We have performed the magnetic susceptibility measurement (Figure 3e) and fitted the χ –1 versus T plot (inset) by using Curie‐Weiss law to get the θ CW = 47 K, which is consistent with the FM interaction in CrBr 3 and agrees well with the predicated T c point from our temperature‐dependent Raman results.…”

A Combined First Principles Study of the Structural, Magnetic, and Phonon Properties of Monolayer CrI$_{3}$

“…Spinphonon couplings in the range of 0.27 − 0.4 cm −1 have subsequently been observed in other transition-metal trihalides, again arising from the E g modes. 76 In our phonon calculations, we initially fixed CrI 3 's lattice parameter to its bulk value. Considering AFM ordering as one specific realization of the paramagnetic phase, we find that the largest change in phonon frequency between the AFM and FM phases is for the E g symmetric mode, and is ∼4 cm −1 (see Table II, modes 8 and 9).…”