  Dimerized quantum magnets are exotic crystalline materials where
Bose-Einstein condensation of magnetic excitations can happen. However, known
dimerized quantum magnets are limited to only a few oxides and halides. Here,
we unveil 9 dimerized quantum magnets and 11 conventional antiferromagnets in
ternary metal borides MTB$_4$ (M = Sc, Y, La, Ce, Lu, Mg, Ca, Al; T = V, Cr,
Mn, Fe, Co, Ni). In this type of structure, 3d transition-metal atoms T are
arranged in dimers. Quantum magnetism in these compounds is dominated by strong
antiferromagnetic interactions between Cr (both Cr and Mn for M = Mg and Ca)
atoms within the structural dimers, with much weaker interactions between the
dimers. These systems are proposed to be close to a quantum critical point
between a disordered singlet spin-dimer phase, with a spin gap, and the ordered
conventional N\'eel antiferromagnetic phase. This new family of dimerized
quantum magnets greatly enriches the materials inventory that allows
investigations of the spin-gap phase. All the quantum-, conventionally-, and
non-magnetic systems identified, together with experimental synthesis methods
of a phase suitable for characterization, provide a platform with abundant
possibilities to tune the magnetic exchange coupling by doping and study this
unconventional type of quantum phase transition. This work opens up new avenues
for studying the quantum magnetism of spin dimers in borides and establishes a
theoretical workflow for future searches for dimerized quantum magnets in other
families or types of materials.
