The influence of Nb substitution on the structure, magnetoelastic transition and magnetocaloric properties has been investigated for the Mn_1.1Fe_0.85-xNb_xP_0.43Si_0.57 alloys. The substitution for Fe by merely 4.7 at.% Nb (i.e. x = 0.04) significantly diminishes the thermal hysteresis from 10 to 1 K due to the reduced structural discontinuity crossing the magnetoelastic transition. This also improves the mechanical stability. The Curie temperature of the magnetoelastic transition is lowered by approximately 11.6 K per at.% of the Nb substitution, originating from the enhanced covalent bonding that favors the paramagnetic state. The giant magnetocaloric effect is still retained in the Nb-substituted alloys.

We explore the crystal structure and magnetic properties of quaternary materials deriving from the hexagonal Fe₂P, an iron binary known to present a particularly large magnetocrystalline anisotropy potentially interesting for permanent magnets, but with unfortunately a Curie temperature far too low for applications. Using simultaneous metal and metalloid substitutions in Fe_2-zCo_zP_1-xB_x and Fe_2-zCo_zP_1-ySi_y quaternaries, we found it is possible to increase the Curie temperature up to at least 640 K while maintaining a c axis uniaxial magnetocrystalline anisotropy, leading to an appreciable magnetic anisotropy at room temperature. In Fe_2-zCo_zP_1-xB_x, though boron is appropriate to increase the Curie temperature, its amount is limited as it also favors the formation of secondary phases. Contrary to what could be anticipated from the phase diagrams of Fe_2-zCo_zP or Fe₂P_1-ySi_y ternaries, simultaneous Co for Fe and Si for P substitutions in Fe_2-zCo_zP_1-ySi_y are found to significantly expand the range of stability of the Fe₂P-type crystal structure. X-ray diffraction patterns on powders oriented in magnetic field indicate a c axis uniaxial magnetic anisotropy. Appropriate heat-treatments and slight metal deficiency are found suitable to limit the formation of unwanted secondary phase with 3:1 metal:metalloid ratio. As a result, nearly single-phase Fe_1.95-zCo_zP_1-ySi_y materials are prepared and exhibit an appreciable magnetic anisotropy with K₁ up to approximately 0.93 MJm⁻³ at room temperature. This study experimentally demonstrates that Fe₂P-type transition metal quaternaries can present an interest not only for their first-order magnetic transition, but also for their magnetic anisotropy making them potential candidates for magnetostatic applications.