  We study the interatomic exchange interactions and Curie temperatures in
half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study
is performed within the framework of density functional theory. The calculation
of exchange parameters is based on the frozen-magnon approach. It is shown that
the exchange interactions in NiCrZ vary strongly depending on the Z
constituent. The Curie temperature, Tc, is calculated within the mean field and
random phase approximations. The difference between two estimations is related
to the properties of the exchange interactions. The predicted Curie
temperatures of all four systems are considerably higher than room temperature.
The relation between the half-metallicity and the value of the Curie
temperature is discussed. The combination of a high spin-polarization of charge
carriers and a high Curie temperature makes these Heusler alloys interesting
candidates for spintronics applications.
