  The recent discovery of two-dimensional (2D) magnets offers unique
opportunities for the experimental exploration of low-dimensional magnetism4
and the magnetic proximity effects, and for the development of novel
magnetoelectric, magnetooptic and spintronic devices. These advancements call
for 2D materials with diverse magnetic structures as well as effective probes
for their magnetic symmetries, which is key to understanding intralayer
magnetic order and interlayer magnetic coupling. Here we apply second harmonic
generation (SHG), a technique acutely sensitive to symmetry breaking, to probe
the magnetic structure of a new 2D magnetic semiconductor, CrSBr. We find that
CrSBr monolayers are ferromagnetically ordered below 146 K, an observation
enabled by the discovery of a giant magnetic dipole SHG effect in the
centrosymmetric 2D structure. In multilayers, the ferromagnetic monolayers are
coupled antiferromagnetically, with the N\'eel temperature notably increasing
with decreasing layer number. The magnetic structure of CrSBr, comprising spins
co-aligned in-plane with rectangular unit cell, differs markedly from the
prototypical 2D hexagonal magnets CrI3 and Cr2Ge2Te6 with out-of-plane moments.
Moreover, our SHG analysis suggests that the order parameters of the
ferromagnetic monolayer and the antiferromagnetic bilayer are the magnetic
dipole and the magnetic toroidal moments, respectively. These findings
establish CrSBr as an exciting 2D magnetic semiconductor and SHG as a powerful
tool to probe 2D magnetic symmetry, opening the door to the exploration of
coupling between magnetic order and excitonic/electronic properties, as well as
the magnetic toroidal moment, in a broad range of applications.
