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 (TC 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 TC, making the latter quaternary alloys most favorable for room temperature applications. Submicron-sized particles of Fe1.75Co0.20P0.75Si0.25 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.@en

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 Fe1.93-xCoxP1-ySiy, 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 Å3, 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, TC increases with Co for Fe substitutions. For y = 0.2, the evolution is not significant, while at high Si content TC 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.@en

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 MnFe0.6 Ni0.4 Si1-x Alx 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 Ni2 In-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 Mn5 Si3-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 MnFe0.6 Ni0.4 Si1-x Alx compositions, this study paves the way toward the realization of a giant magnetocaloric effect in these compounds using alternative synthesis techniques.@en

After almost 20 years of intensive research on magnetocaloric effects near room temperature, magnetic refrigeration with first-order magnetocaloric materials has come close to real-life applications. Many materials have been discussed as potential candidates to be used in multicaloric devices. However, phase transitions in ferroic materials are often hysteretic and a metric is needed to estimate the detrimental effects of this hysteresis. We propose the coefficient of refrigerant performance, which compares the net work in a reversible cycle with the positive work on the refrigerant, as a universal metric for ferroic materials. Here, we concentrate on examples from magnetocaloric materials and only consider one barocaloric experiment. This is mainly due to lack of data on electrocaloric materials. It appears that adjusting the field-induced transitions and the hysteresis effects can minimize the losses in first-order materials. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.@en

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 Fe2-zCozP1-xBx and Fe2-zCozP1-ySiy 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 Fe2-zCozP1-xBx, 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 Fe2-zCozP or Fe2P1-ySiy ternaries, simultaneous Co for Fe and Si for P substitutions in Fe2-zCozP1-ySiy 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 Fe1.95-zCozP1-ySiy 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.@en

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 Fe2-zCozP1-xBx and Fe2-zCozP1-ySiy 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 Fe2-zCozP1-xBx, 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 Fe2-zCozP or Fe2P1-ySiy ternaries, simultaneous Co for Fe and Si for P substitutions in Fe2-zCozP1-ySiy 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 Fe1.95-zCozP1-ySiy 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.@en

The field dependence of the magnetocaloric effect (MCE) in Mn1.22Fe0.73P0.47Si0.53 is studied in terms of the entropy change (Δ S) and the temperature change (Δ T) for applied magnetic fields up to 5 and 14 T, respectively. The magnetic fields required to saturate the MCE in this system are ∼ 1.7 and 4-5 T for Δ S and Δ T, respectively. The MCE field dependence is compared with the two approaches of the literature: 1) latent heat model and 2) the power law evolution predicted from the universal analysis of the MCE. It turns out that both of these methods are unsuitable to describe the MCE field evolution in MnFe(P,Si) materials.@en

The field dependence of the magnetocaloric effect (MCE) in Mn1.22Fe0.73P0.47Si0.53 is studied in terms of the entropy change (Δ S) and the temperature change (Δ T) for applied magnetic fields up to 5 and 14 T, respectively. The magnetic fields required to saturate the MCE in this system are ∼ 1.7 and 4-5 T for Δ S and Δ T, respectively. The MCE field dependence is compared with the two approaches of the literature: 1) latent heat model and 2) the power law evolution predicted from the universal analysis of the MCE. It turns out that both of these methods are unsuitable to describe the MCE field evolution in MnFe(P,Si) materials.@en

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential step for new and improved magnetic refrigerants.@en