Influence of cold compaction pressure on intergranular secondary phase distribution and magnetocaloric/thermomagnetic performances of MnFe(P,Si,B) compounds

Abstract

Recent studies have pointed out that first-order magnetic transitions can be affected by defects. It however remains unclear which precise microstructural features are involved and how to pragmatically control their formation. Here, we show that a genuine technical parameter such as the compaction pressure employed prior to the sintering can induce subtle differences on magnetization data and yet correspond to a large evolution of the magnetocaloric performances in MnFe0.95P0.575Si0.36B0.065 compounds prepared by powder metallurgy, resulting in an enhancement of the isothermal entropy change (ΔS) in 1 T up to ∼40 %. This outcome is unexpected as ΔS normalized per mass should not be affected by what is at first glance a matter of compaction or density. Our detailed structural and microstructural investigations indicate that the sharpening of the transition for the highest compaction pressures originate from different distributions of the secondary phase at the grain boundaries. This work not only bring into light an interesting fundamental question how microstructural details can affect the development of a magnetic transition, it has also important practical consequences to ensure a reproducibility of large magnetocaloric or thermomagnetic effects in MnFe(P,Si,B) materials for possible applications.