  Direct measurements of the magnetocaloric effect (MCE) in different materials
(Gd, Fe48Rh52, Ni43Mn37.9In12.1Co7 and Ni2.07Co0.09Mn0.84Ga) in alternating
magnetic fields with frequencies f < 22 Hz and an amplitude deltaH = 6.2 kOe
are carried out. The MCE in Gd shows inconsiderable changes with field
frequency. Near paramagnetic-ferromagnetic phase transition in
Ni43Mn37.9In12.1Co7 Heusler alloy a slight reduction of MCE with frequency is
observed. In weak alternating fields in materials with AFM-FM
magneto-structural phase transitions (Fe48Rh52, Ni43Mn37.9In12.1Co7) it is not
possible to get a structural contribution to overall MCE because of
irreversibility of the transitions in these fields. Near magneto-structural
phase transitions the MCE in these alloys has only magnetic contribution, and
does not show a significant dependence on the magnetic field frequency. In
Ni2.07Co0.09Mn0.84Ga Heusler alloy the MCE vanishes at frequencies about 20 Hz.
The obtained results show the increase of frequencies of operating cycles is
one of the powerful methods to improve the efficiency of magnetic refrigerators
in case of Gd as a refrigerant.
