  We show that metastable phases of an antiferromagnetic spin-1 condensate in a
simple model with pure contact interactions can exhibit a rotonlike minimum in
the excitation spectrum. The introduction of magnetic field gives rise to the
instability of roton modes, which can lead to spontaneous emergence of regular
periodic, polygonal, polyhedral or crystalline patterns, as shown in numerical
simulations within the truncated Wigner approximation. An explanation of the
occurrence of rotonlike instability is given based on the energy and spin
conservation laws.
