  Magnetic ordering in the two-dimensional limit has been one of the most
important issues in condensed matter physics for the past several decades. The
recent discovery of new magnetic van der Waals materials heralds a much-needed
easy route for the studies of two-dimensional magnetism: the thickness
dependence of the magnetic ordering has been examined by using Ising- and
XXZ-type magnetic van der Waals materials. Here, we investigated the magnetic
ordering of MnPS3, a two-dimensional antiferromagnetic material of
Heisenberg-type, by Raman spectroscopy from bulk all the way down to bilayer.
The phonon modes that involve the vibrations of Mn ions exhibit characteristic
changes as the temperature gets lowered through the N\'eel temperature. In bulk
MnPS3, the Raman peak at ~155 cm-1 becomes considerably broadened near the
N\'eel temperature and upon further cooling is subsequently red-shifted. The
measured peak positions and polarization dependences of the Raman spectra are
in excellent agreement with our first-principles calculations. In few-layer
MnPS3, the peak at ~155 cm-1 exhibits the characteristic red-shift at low
temperatures down to the bilayer, indicating that the magnetic ordering is
surprisingly stable at such a thin limit. Our work sheds light on the hitherto
unexplored magnetic ordering in the Heisenberg-type antiferromagnetic systems
in the atomic-layer limit.
