  Twist engineering, or the alignment of two-dimensional (2D) crystalline
layers with desired orientations, has led to tremendous success in modulating
the charge degree of freedom in hetero- and homo-structures, in particular, in
achieving novel correlated and topological electronic phases in moir\'e
electronic crystals. However, although pioneering theoretical efforts have
predicted nontrivial magnetism and magnons out of twisting 2D magnets,
experimental realization of twist engineering spin degree of freedom remains
elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide
(CrI3) to fabricate twisted double bilayer homostructures with tunable twist
angles and demonstrate the successful twist engineering of 2D magnetism in
them. Using linear and circular polarization-resolved Raman spectroscopy, we
identify magneto-Raman signatures of a new magnetic ground state that is
sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3.
With careful magnetic field and twist angle dependence, we reveal that, for a
very small twist angle (~ 0.5 degree), this emergent magnetism can be
well-approximated by a weighted linear superposition of those of 2L and 4L CI3
whereas, for a relatively large twist angle (~ 5 degree), it mostly resembles
that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1
degree), its magnetism cannot be simply inferred from the 2L and 4L cases,
because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3
and features a dramatic Raman circular dichroism that is absent in natural 2L
and 4L ones. Our results demonstrate the possibility of designing and
controlling the spin degree of freedom in 2D magnets using twist engineering.
