It is shown that, within a Gutzwiller type of approach to correlated electrons, the effective mass of quasiparticles composing an almost-localized Fermi liquid is spin dependent and varies strongly with magnetic field. The magnetization of such a system saturates in physically accessible fields. The results are used to explain the field dependence of both the effective mass and of the specific heat in the heavyfermion systems at low temperature.

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We analyze the properties of a diluted ferromagnetic semiconductor Pb Sn& "~Mn~Te for x =0.20 and y =0. 08, for the two concentrations of holes p =2.3X10" cm and p =2.7X10 cm '. We determine from the field dependence of the magnetization the magnon contribution to the spontaneous magnetization; this contribution is used to calculate the magnitude of the effective exchange integral J, =4.8 K between nearest neighbors for the sample with higher p. The paramagnetic Curie temperature determined theoretically from the magnon contribution to the magnetization agrees within 30% with that obtained from measurements in the paramagnetic regime for the sample with higher p. The magnon contribution to the specific heat is estimated and subtracted from the corresponding data. The large value of the linear specific-heat coefticient y =73 mJ/molK, as well as the spin reduction AS=0.2+0.05 of the atomic spin S = -' of the Mn + ion indicate an antiferromagnetic coupling of the Kondo type between the valence holes and the magnetic ions.

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