  Weyl (WSMs) evolve from Dirac semimetals in the presence of broken
time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in
TaAs-class materials and photonic crystals are due to the loss of
space-inversion symmetry. For TRS-breaking WSMs, despite numerous theoretical
and experimental efforts, few examples have been reported. In this Article, we
report a new type of magnetic semimetal Sr1-yMn1-zSb2 (y,z<0.1) with nearly
massless relativistic fermion behaviour (m*=0.04-0.05m0, where m0 is the free
electron mass). This material exhibits a ferromagnetic order for 304K < T <
565K, but a canted antiferromagnetic order with a ferromagnetic component for T
< 304K. The combination of relativistic fermion behaviour and ferromagnetism in
Sr1-yMn1-zSb2 offers a rare opportunity to investigate the interplay between
relativistic fermions and spontaneous TRS breaking.
