  The mechanism of spontaneous exchange bias (SEB) and the dominant factor of
its blocking temperature are still unclear in Heusler alloys. Here, the related
investigations are performed in Mn2Ni1.5Al0.5 Heusler alloys with SEB. The
results of both magnetic measurements and first-principles calculations
confirmed that spin frustrated and unfrustrated antiferromagnetic (AFM) states
coexist there and they have different magnetic anisotropies, which are
essential for SEB. Based on a series of measurement strategies, we demonstrate
that the frustrated AFM state undergoes a first-order magnetic transition to
the superferromagnet (SFM) state with the help of an external magnetic field,
and SFM is retained due to the first-order property of the magnetic transition.
SEB originates from the interface coupling of multiple sublattices between the
unfrustrated AFM state and SFM state. By analyzing the Arrott plot using the
Landau model, we found that the internal field of the system dominates the
blocking temperature of SEB, which paves the way for improving the blocking
temperature.
