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The transition-metal based alloy MnFe 4 Si 3 not only is a potential candidate for room temperature magnetocaloric applications, but also shows a large magnetic anisotropy forming an interesting case study in the search for rare-earth free permanent magnets. However, former polycrystalline and single crystal studies led to major disagreements about the order of the magnetic transition and the magnetocrystalline anisotropy scheme, which are two essential points for the understanding of this alloy. Here, magnetic, magnetocaloric properties and the magnetic anisotropy of MnFe 4 Si 3 (Mn~0 .86 Fe~4 .24 Si~2 .90) are investigated on a high quality single crystal grown by flux method, and compared to polycrystalline materials. Using the recently proposed criterion of field dependence of the magnetocaloric effect, we show that the ferromagnetic transition is more likely to be of second order, which is fully compatible with the absence of thermal hysteresis at the ferromagnetic transition in the present MnFe 4 Si 3 crystal. The c axis is confirmed to be the hard magnetic axis, both in single crystal and polycrystalline MnFe 4 Si 3 , and a large, dominant, K 1 anisotropy constant (~−2.5 MJ m −3) is found at low temperatures.

Mn compounds presenting magneto-structural phase transitions are currently intensively studied for their giant magnetocaloric effect; nevertheless, several parameters remain to be further optimized. Here, we explore the Mn(Fe,Ni)(Si,Al) series, which presents two advantages. The Mn content is fixed to unity ensuring a large saturation magnetization, and it is based on non-critical Si and Al elements instead of the more commonly employed Ge. Structural and magnetic properties of MnFe0.6Ni0.4Si1-xAlx compounds are investigated using powder X-ray diffraction, SEM, EDX, DSC, and magnetic measurements. We demonstrate that a magneto-structural coupling leading to transformation from ferromagnetic with orthorhombic TiNiSi-type structure to a paramagnetic hexagonal Ni2In-type phase can be realized for 0.06 < x ≤ 0.08. Unfortunately, the first-order transition is relatively broad and incomplete, likely as the result of insufficient sample homogeneity. A comparison between samples synthesized in different conditions (as-cast, quenched from 900 °C, or quenched from 1100 °C) reveals that Mn(Fe,Ni)(Si,Al) samples decompose into a Mn5Si3-type phase at intermediate temperatures, preventing the synthesis of high-quality samples by conventional methods such as arc-melting followed by solid-state reaction. By identifying promising MnFe0.6Ni0.4Si1-xAlx compositions, this study paves the way toward the realization of a giant magnetocaloric effect in these compounds using alternative synthesis techniques.

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