Magnetic properties and magnetocaloric effect of DyCo₉Si₄

Polycrystalline samples of DyCo9Si4 with the tetragonal LaFe9Si4-type structure have been synthesized by arc melting and high-temperature annealing. Results of the magnetization M and specific heat C-p measurements show the existence of two magnetic anomalies at T-C = 40 K and T-m = 20 K. The transition at T-C = 40 K is attributed to the ferromagnetic transition of the Co sublattice. The magnetic moments of Dy3+ appear to align gradually below T-C, and almost fixed below T-m, at which a broad maximum in C-p has been observed. Field-dependent M at T = 2 K up to H = 7 T suggested that the magnetic moment of Dy3+ is coupled antiferromagnetically with those of Co, forming a ferrimagnetic structure. Magnetization measurements up to 50 T at low temperatures using a pulse magnet demonstrated a spin-flip transition around H = 20 T, above which the magnetic moments of Co and Dy are in parallel. The spin-flip transition becomes broad with increasing temperature but still remains at T-C = 40 K, indicating a strong antiparallel coupling between Co and Dy moments even in the paramagnetic state. The magnetocaloric effect was evaluated from C-p and M, and also from the sample-temperature variation in the quasi-adiabatic process with the pulsed magnetic field. The maximum entropy change for the field change from 5 to 0 T was observed at around T = T-m with the value Delta S = 5 J/kg K. The maximum temperature of Delta S shifts to higher temperatures in high magnetic fields, resulting in the large value of Delta S = 24 J/kg K at T similar to T-C by the field change from 50 to 0 T. The present results demonstrate that the pulsed-field measurement is a powerful method to evaluate the magnetocaloric effect of materials.