Magnetism in two-dimensional materials is not only of fundamental scientific interest but also a promising candidate for numerous applications. However, studies so far, especially the experimental ones, have been mostly limited to the magnetism arising from defects, vacancies, edges, or chemical dopants which are all extrinsic effects. Here, we report on the observation of intrinsic antiferromagnetic ordering in the two-dimensional limit. By monitoring the Raman peaks that arise from zone folding due to antiferromagnetic ordering at the transition temperature, we demonstrate that FePS3 exhibits an Ising-type antiferromagnetic ordering down to the monolayer limit, in good agreement with the Onsager solution for two-dimensional order–disorder transition. The transition temperature remains almost independent of the thickness from bulk to the monolayer limit with TN ∼ 118 K, indicating that the weak interlayer interaction has little effect on the antiferromagnetic ordering.

Ising-Type Magnetic Ordering in Atomically Thin FePS3.

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Raman spectroscopy of atomically thin two-dimensional magnetic iron phosphorus trisulfide (FePS3) crystals

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New Frontiers on van der Waals Layered Metal Phosphorous Trichalcogenides

A temperature‐dependent study of the four Raman‐active phonons in MnF2 and FeF2 has shown that their Raman frequencies and intensities are affected to varying extents by the antiferromagnetic ordering. A theory for spin‐dependent effects in rutile structure antiferromagnets has been developed to calculate the anomalous variation in the phonon frequency and to estimate the corresponding spin‐phonon coupling coefficients.

The spin‐phonon interaction in FeF2 and MnF2 studied by Raman spectroscopy

Coherent magnons and phonons are excited by subpicosecond laser pulses in the weak ferromagnet ${\text{FeBO}}_{3}$. Impulsive stimulated Raman scattering (ISRS) is proven to be the microscopic mechanism of the excitation. It is shown that coherent magnons can be excited by both linearly and circularly polarized laser pulses where the efficiency of the process depends on the mutual orientation of the magnetic and crystallographic axes and the light propagation direction. The strong ellipticity of the ferromagnetic magnon mode is demonstrated, both experimentally and theoretically, to be essential for the excitation and observation of such coherent magnons. Because of this ellipticity, the amplitude of the coherent magnons excited by linearly polarized light may exceed by 2 orders of magnitude the amplitude of those excited by circularly polarized light. The primary difference between the excitation of coherent magnons by linearly polarized pulses via ISRS and via the earlier reported process of photoinduced magnetic anisotropy is discussed. Furthermore, the ISRS process is found to be responsible for the excitation of two optical phonon branches (8.4 and 12.1 THz) observed in our experiments. A coherent excitation, with a temperature-independent frequency of 0.7 THz, has also been observed in the magnetically ordered phase but could not be assigned to any optical phonon modes known in ${\text{FeBO}}_{3}$. The well-pronounced dependence of the amplitude of this mode on temperature suggests that this mode of nonmagnetic origin becomes Raman active only in the magnetically ordered phase and, therefore, can be excited and observed only below the N\'eel temperature.

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Impulsive excitation of coherent magnons and phonons by subpicosecond laser pulses in the weak ferromagnet FeBO3

A general theory of the spin-dependent phonon Raman scattering in magnetic crystals is developed. The integrated Raman intensity as a function of temperature is expressed as S ( T )=( n 0 +1)| R + M | 2 , where the first term represents the spin-independent part and the second one the spin-dependent part which is proportional to the nearest neighbor spin correlation function . Three types of temperature variation of intensity are predicted in ferromagnetic and antiferromagnetic cases, depending on values of R / M . Two microscopic mechanisms, variation of the d electron transfer with lattice vibrations and that of the non-diagonal exchange, contribute to the spin-dependent part, and it is shown that the former is essentially important in producing the large temperature variation. The theory is applied to explaining the observed temperature variation of various Raman active phonon modes in CdCr 2 S 4 and the experimental results are explained successfully.

Naoshi Suzuki

Papers

Theory of Spin-Dependent Phonon Raman Scattering in Magnetic Crystals

Theory of phonon Raman scattering in transition metal compounds

Theory of infrared absorption by two magnon excitations in two-dimensional antiferromagnets