The (Mn,Fe)₂(P,Si) compounds are one of the rare materials systems that exhibit an isostructural first-order ferromagnetic transition (FOMT) near ambient temperature. Since the discovery of its giant magnetocaloric effect (GMCE), this system is garnering ongoing interest, both for its promising performances for applications and for the scientific interest in uncovering the fundamental mechanisms driving the FOMT. This study examines the evolution of the structure, the microstructure, the thermal and magnetic properties in Mn_0.60+x Fe_1.3-x P_0.66-y Si_0.34+y (0 ≤ x ≤ 0.08, x = 2y ) compounds prepared by the melt-spun technique. The simultaneous increase in Mn and Si concentrations leads to a 40 % enhancement in the isothermal entropy change (|Δ S _max|) compared to parent compound. Furthermore, we propose a method to separate the latent heat ( L ) from the reversible specific heat. This allows us to establish a convincing correlation between two intrinsic quantities, the latent heat ( L ) and the elastic strain energy ( U _e). Our results demonstrate that both latent heat ( L ) and thermal hysteresis (Δ T _hys) are proportionally linked and vanish simultaneously at a critical end point.

First-order magnetoelastic transitions usually involve mechanisms unique to each family of materials. For (Mn,Fe)₂(P,Si) compounds, it is generally predicted that the unit cell distortion occurring at the ferromagnetic transition leads to a strong electronic reconstruction of the Fe d states accompanied by a notable change in magnetic moment. However, there is no experimental consensus on this mechanism. Here, we use x-ray emission spectroscopy (XES) complemented by first-principles calculations, high-energy resolution fluorescence detected x-ray absorption (HERFD-XAS), and resonant inelastic x-ray scattering (RIXS) experiments, to clarify the nature of the first-order transition in a Mn_0.74Fe_1.23P0.71Si_0.32 crystal. HERFD-XAS and RIXS data show a minor evolution of the spectral features in the upper part of the K-edge for Mn and Fe, consistent with the calculated 4p density of states and fingerprinting the transition. In contrast, no significant evolution of the XES spectra is observed when the transition is crossed. In Fe-rich compositions, the calculations indicate that Fe at the 3g site develops a magnetic moment (2.43 μ_B) that is smaller than that of Mn at the 3g site (2.93 μ_B), but larger than that of Fe at the 3f site (1.48 μB). Quantitative XES analysis using the IAD method gives a reasonable agreement with the magnetic moments for different (Mn,Fe)₂(P,Si) compositions. However, the reduction of the Fe moment predicted by theory (approx. −0.6μB) is not observed around the transition. This study indicates that the Fe moment collapse at the transition may be weaker than the theoretically predicted value or more gradual in temperature, suggesting a secondary role for this moment instability in the giant magnetocaloric effect of (Mn,Fe)₂(P,Si) compounds.

In view of the interest that (Fe,Co)₂(P,Si) compounds have as potential permanent magnets, their structural and magnetic phase diagrams are explored focusing on establishing the range where the hexagonal Fe₂P-type structure is observed. In Fe_1.93-xCo_xP_1-ySi_y, the highest Si content prior entering a mixed phase domain is y ≈ 0.5. At high Si content but low Co for Fe substitutions, a structural distortion leading to a body-centered orthorhombic structure occurs. At high Co contents, when the Fe₂P unit cell reaches a critical volume of about 102.4 ų, the samples crystallize in a Co₂P-type orthorhombic structure. Within the Fe₂P-type structural range, the evolution of the unit-cell volume appears to follow the Vegard's law, but this hides strongly anisotropic changes. Simultaneous Co for Fe and Si for P substitutions increase the range where the hexagonal structure is observed in comparison to ternary Fe₂(P,Si) and (Fe,Co)₂P. The samples are ferromagnetic, but with Curie temperatures showing an unusual evolution, uncorrelated to the c/a ratio of the lattice parameters. At low Si content, T_C increases with Co for Fe substitutions. For y = 0.2, the evolution is not significant, while at high Si content T_C systematically decreases with the increase in Co. Large Si and Co substitutions lead to a swift weakening of the magnetocrystalline anisotropy until the easy axis anisotropy turns from the c axis toward the a-b plane. This study guides future investigations by restricting the range where desirable properties for permanent magnetic applications can be expected to 0.1 ≲ x ≲ 0.3 and 0.1 ≲ y ≲ 0.3.

While rare-earth magnets exhibit unchallenged hard-magnetic properties, looking for alternatives based on inexpensive elements of non-critical supply remains of utmost interest. Here, we demonstrate that (Fe,Co)₂(P,Si) single crystals combine a large magnetocrystalline anisotropy (K₁ ≈ 0.9 MJ m⁻³ at 300 K), high Curie temperatures (T_C up to 560 K) and an appreciable saturation specific magnetization (101 A m² kg⁻¹) leading to a theoretical |BH|_max ≈ 165 kJ m^-3, making them promising candidate materials as rare-earth-free permanent magnets. Our comparison between (Fe,Co)₂P and (Fe,Co)₂(P,Si) single crystals highlights that Si substitution reduces the low-temperature magnetocrystalline anisotropy, but strongly enhances T_C, making the latter quaternary alloys most favorable for room temperature applications. Submicron-sized particles of Fe_1.75Co_0.20P_0.75Si_0.25 were prepared by a top-down ball-milling approach. While the energy products of bonded particles are to this point modest, they demonstrate that permanent magnetic properties can be achieved in (Fe,Co)₂(P,Si) quaternary alloys. This work correlates the development of permanent magnetic properties to a control of the microstructure. It paves the way toward the realization of permanent magnetic properties in (Fe,Co)₂(P,Si) alloys made of economically competitive Fe, P and Si elements, making these materials desirable for applications.

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.