The effect of the thermal decomposition reaction on the mechanical and magnetocaloric properties of La(Fe,Si,Co)13

We report on the influence of the Co content in the magnetocaloric system La(Fe,Si,Co)13 on the thermal decomposition (TD) reaction, and subsequently on the magnetocaloric properties. In the course of the TD reaction, the magnetocaloric La(Fe,Si,Co)13 phase reversibly decomposes into α-Fe(Co,Si) and the intermetallic LaFeSi phase, thus enhancing the mechanical properties and therefore the machinability of the compound. The addition of Co significantly speeds up the reaction kinetics. The optimum temperature range for the TD reaction was determined to be 973–1073 K, with the lower and upper limit at 873 K and 1173 K, respectively. With electron microscopy a lamellar microstructure has been found in the decomposed state, indicating a eutectoid-type phase reaction. The width of the lamellae is ∼26 nm in LaFe12Si and decreases with increasing Co content. Three-dimensional atom probe (3DAP) measurements show the enrichment of Co and Si in LaFeSi lamellae. We conclude that the addition of Co somehow decreases the lamellar spacing, which is the main reason for the enhanced TD kinetics. Finally, it is interesting to note that the highly ordered nano-scale mixture of strongly ferromagnetic α-Fe(Co) with the non-ferromagnetic phase induces a significant increase in coercivity, Hc. The shape anisotropy of the thin α-Fe(Co) lamellae yields a semi-hard permanent magnet with a coercivity of ∼100 A cm−1.