  The recent discovery of two-dimensional (2D) magnetic materials which are
compound of transition metal (TM) with other elements, has opened new avenues
for basic research on low-dimensional magnetism and potential applications in
spintronics. To further explore new 2D magnets of pure TM is thus of an
interesting topic. Based on the first-principles calculations, here we propose
a strategy of obtaining monolayer TM magnets, i.e., depositing TM atoms on
graphyne (Gy) which has proper hexagonal hollow geometry. We find that TM
monolayer with perfect hexagonal geometry can be formed on Gy. The TM monolayer
exhibits a wealth of physical properties in dependence of TM species, such as
ferromagnetic and antiferromagnetic ground states, as well as intriguing
semimetal and half-metal characteristics. We also find that the half-metal
characteristics makes the monolayer TM have great potential applications in the
horizontal magnetic tunnel junction (MTJ) devices, where the tunneling
magnetoresistance can reach as high as 850000%. Our results provide a new
framework for obtaining 2D magnets with outstanding spintronic properties.
