  Half-metals and spin gapless semiconductors are promising candidates for
spintronic applications due to the complete (100%) spin polarization of
electrons around the Fermi level. Based on recent experimental and theoretical
findings of graphene-like monolayer transition metal carbides and nitrides
(also known as MXenes), we demonstrate from first-principles calculations that
monolayer Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the
magnetic moments of 1.91 and 1.00 UB per formula unit, respectively, while
monolayer V2C is a metal with instable antiferromagnetism, and monolayer V2N is
a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase
transition from nearly half-metal to truly half-metal, spin gapless
semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly
half-metal under a proper biaxial strain. Large magnetic moments can be induced
by the biaxial tensile and compressive strains for monolayer V2C and V2N,
respectively. We also show that the structures of these four monolayer MXenes
are stable according to the calculated formation energy and phonon spectrum.
Our investigations suggest that, unlike monolayer graphene, monolayer MXenes
Ti2C and Ti2N without vacancy, doping or external electric field exhibit
intrinsic magnetism, especially the half-metallic ferromagnetism and spin
gapless semiconductivity, which will stimulate further studies on possible
spintronic applications for new two-dimensional materials of MXenes.
