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The effects of Si substitution for As on the magnetic and magnetocaloric properties of MnFeP0.5As0.5-xSix compounds have been investigated. The Si-substituted compounds MnFeP0.5As0.5-xSix with x=0, 0.1, 0.2, and 0.3 have been prepared by high-energy ball milling and solid-state reaction. X-ray diffraction shows that all the MnFeP0.5As0.5-xSix compounds crystallize in the Fe2P-type structure. Magnetic measurements show that the compounds are ferromagnetic with a nonlinear dependence of TC on Si content. All compounds exhibit a field-induced first-order magnetic phase transition. The magnetic-entropy change is derived from the magnetization data by using the Maxwell relations. The maximum magnetic-entropy change observed in this system reaches about 32 J/kg K for a field change from 0 to 2 T for x=0.2, at temperature around 294 K. [ABSTRACT FROM AUTHOR]

We have studied the crystal structure, magnetic structure and magnetic properties of the tetragonal compounds HoTiGe and ErTiGe by means of neutron diffraction and magnetic measurements. The neutron-diffraction measurements were made in the range from 1.7 K to temperatures above the corresponding magnetic-ordering temperatures (TN=88 and 41 K, respectively). Both compounds have a simple antiferromagnetic structure with a propagation vector q=(0, 0, 0.5). The Ho moments have a preferred moment direction along the tetragonal c-axis at all temperatures in the magnetically ordered regime, while the direction is perpendicular to the c-axis for the Er moments. This difference in easy moment direction has been discussed in terms of a crystal-field-induced rare-earth-sublattice anisotropy. The magnetic measurements showed that both compounds give rise to metamagnetic transitions in their magnetically ordered states. This field-induced transition is particularly pronounced in HoTiGe where it is accompanied by a strong hysteresis in the lower-temperature range. [Copyright &y& Elsevier]

The structural and magnetic properties of MnFe1-xCox Ge compounds with x = 0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.85,0.9, and 1.0 were investigated by means of X-ray diffraction (XRD) and magnetization measurements. XRD shows that the MnFe1-x Cox Ge compounds crystallize in the hexagonal Ni2In-type crystal structure for x ≤ 0.8 and in the orthorhombic NiTiSi-type structure for x > 0.8. The magnetization measurements show that the MnFe1-x Cox Ge compounds exhibit a complex magnetic behavior. The Curie temperature increases with increasing of x. The saturation magnetization of the compounds with Ni2In type structure increase with increasing of x and the saturation of the magnetization in the NiTiSi-type structure also increases with increasing x. We investigated the magnetocaloric effects in these compounds by means of magnetization measurements. The maximum magnetic-entropy change observed in these compounds reaches 9 J/kgK for x = 0.8 in a field change from 0 to 5 T at around 289 K. [ABSTRACT FROM AUTHOR]

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 MnFe 0.95 P 0.575 Si 0.36 B 0.065 compounds prepared by powder metallurgy, resulting in an enhancement of the isothermal entropy change (Δ S) in 1 T up to ∼40 %. This outcome is unexpected as Δ S 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. [Display omitted] • Magnetocaloric performances of MnFe(P,Si,B) increase with higher green compaction. • Final density not affected by green pressure with common 24 h high temperature sintering. • Increase in ΔS is not related to secondary phase content but rather to its distribution. [ABSTRACT FROM AUTHOR]

The structural and magnetic properties of Sm2Co17-xCrx (0≤x≤3.0) compounds have been investigated by means of X-ray diffraction and magnetization measurements. The powder X-ray diffraction patterns show that all samples crystallize as a single phase with the rhombohedral Th2Zn17-type structure. The lattice parameters a and V increase monotonically with increasing Cr content, but the lattice parameter c increases very slowly with increasing Cr content. The X-ray diffraction patterns of the aligned powder of the samples have confirmed that all the compounds investigated have uniaxial anisotropy. The Curie temperature of the compounds decreases rapidly the spontaneous magnetization M0, the anisotropy field Ba, and the anisotropy constant K1 of Sm2Co17-xCrx (0≤x≤3.0) compounds decrease strongly with increasing Cr content. [Copyright &y& Elsevier]

The magnetic phase transitions and the magnetocaloric properties of the pseudo binary system Mn5−xFexSi3 with x=0, 1, 2, 3, 4, 5 have been investigated by measuring the magnetisation as a function of temperature and magnetic field. With increasing Fe content, the antiferromagnetic ordering temperature shifts to higher temperatures. The compound Mn5Si3 is an antiferromagnet with a field-induced transition. At 4.2 K, the Mn5−xFexSi3 compounds with x=1 and 2 display antiferromagnetic behavior up to 38 T. For the compounds with x=4 and 5 we find ferromagnetic order. The magnetic-entropy changes in this system are derived from the temperature and field dependence of the magnetization by means of the thermodynamic Maxwell relation. For the MnFe4Si3 compound, enhanced but modest magnetic-entropy changes are observed. [Copyright &y& Elsevier]

We have studied the magnetic properties of Mn5Ge3−xSbx compounds with x=0, 0.1, 0.2 and 0.3 by means of magnetisation measurements at different temperatures and fields up to 5 T. The compounds crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm. The compounds show enhanced Curie temperatures but decreasing average magnetic Mn moments with increasing Sb content. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetisation using the thermodynamic Maxwell relation. Mn5Ge3 exhibits considerably large magnetic entropy changes that are comparable with that of Gd metal. The Sb substitution has two kinds of effects on the magnetocaloric effect (MCE) of Mn5Ge3−xSbx. One is the magnetic entropy change that decreases with increasing Sb content, the other is the MCE peak that becomes broadened. [Copyright &y& Elsevier]

We explore the crystal structure and magnetic properties of quaternary materials deriving from the hexagonal Fe 2 P, 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- z Co z P 1- x B x and Fe 2- z Co z P 1- y Si 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-z Co z P 1- x B 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- z Co z P or Fe 2 P 1- y Si y ternaries, simultaneous Co for Fe and Si for P substitutions in Fe 2- z Co z P 1- y Si y are found to significantly expand the range of stability of the Fe 2 P-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- z Co z P 1- y Si y materials are prepared and exhibit an appreciable magnetic anisotropy with K 1 up to approximately 0.93 MJm−3 at room temperature. This study experimentally demonstrates that Fe 2 P-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. Image 1 • A sizable magnetic anisotropy is obtained in Fe 2 P-quaternaries at room temperature. • Phase diagrams and structure-properties relations of Fe 2 P-quaternaries are explored. • Fe 2- z Co z P 1- y Si y alloys preserve an Fe 2 P-type crystal structure up to y ∼0.6. • Fe 2- z Co z P 1- y Si y alloys present high Curie temperature and uniaxial anisotropy. • Fe 1.8 Co 0.2 P 1- x B x alloys are synthesized, but B content is limited to x ∼0.12 [ABSTRACT FROM AUTHOR]

Herein, we report the synthesis of nanocrystalline La 1-x Ca x B 6 (0 ≤ x ≤ 1) hexaboride powders by solid-state reaction and their subsequent consolidation via spark plasma sintering. The structural, magnetic and thermionic emission properties of La 1-x Ca x B 6 hexaboride are investigated. All of the synthesized nanocrystalline hexaboride powders are single phase with the CsCl -type structure and no ferromagnetic impurity phases have been detected from X-ray diffraction. Magnetic measurements show that weak ferromagnetism at room temperature is found in nanocrystalline La 1-x Ca x B 6 hexaboride powders, and the magnetism was attributed to the presence of the intrinsic defects, based on the data of the HRTEM. Thermionic emission measurements indicate that the maximum emission intensity for bulk La 0.4 Ca 0.6 B 6 at 1873 K reached 20.02 A/cm 2 , which is more than three times higher as compared to bulk CaB 6 (∼6.04 A/cm 2 ). When the La doping was increased to 40 at%, the work function of CaB 6 decreased from 2.95 to 2.76 eV, indicating an improvement in the thermionic emission performance. Therefore, the quasibinary La 1-x Ca x B 6 hexaboride may have an application as a promising cathode. [ABSTRACT FROM AUTHOR]

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 Fe 2 P-type structure is observed. In Fe 1.93- x Co x P 1- y Si 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 Fe 2 P unit cell reaches a critical volume of about 102.4 Å3, the samples crystallize in a Co 2 P-type orthorhombic structure. Within the Fe 2 P-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 Fe 2 (P,Si) and (Fe,Co) 2 P. 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. [Display omitted] • Structural and magnetic phase diagrams of (Fe,Co) 1.93 (P,Si) compounds are built. • Relationships between the three crystal structure types are established. • Ferromagnetic ordering temperature surprisingly not correlated to the c/a structural ratio. • Composition range desirable for permanent magnetic properties is determined. [ABSTRACT FROM AUTHOR]

Abstract: Recently, we found a large magnetocaloric effect (MCE) and favourable magnetic properties in low cost and nontoxic MnFe(P,Si,Ge) compounds [D.T. Cam Thanh, E. Brück, O. Tegus, J.C.P. Klaasse, T.J. Gortenmulder, K.H.J. Buschow, J. Appl. Phys. 99 (2006) 08Q107]. These compounds are promising for magnetic refrigeration applications. One of the interesting points in these compounds is a nonlinear dependence of the Curie temperature (T C) on Si concentration. This dependence is associated with the change in the lattice parameters a and c, and their ratio c/a. Compounds with larger a parameter and smaller c/a ratio have higher T C. It is clear that Si and Ge atoms play an important role in the magnetic and magnetocaloric properties in the MnFe(P,Si,Ge) compounds. In this paper, we study the effect of Si and Ge on the magnetic phase transition in these materials. Our study shows that the temperature of the phase transition, from paramagnetic to ferromagnetic, can be tuned in the room temperature range without losing giant magnetocaloric properties. [Copyright &y& Elsevier]

Abstract: An accurate method to determine the magnetic entropy change in materials with hysteretic first-order transitions is presented, which is needed to estimate their potential for applications. We have investigated the effect of the maximal entropy change derived from magnetization measurements performed in different measurement processes. The results show that the isothermal entropy change can be derived from the Maxwell relations even for samples with large thermal hysteresis. In the temperature region with hysteresis, overestimating the entropy change can be avoided by measuring the isothermal magnetization of the sample after it is cooled from the paramagnetic state to the measurement temperature. In this way the so-called peak effect is not observed as shown here for a few compounds. [Copyright &y& Elsevier]

Abstract: The effects of substitution of Co for Fe on the magnetic and magnetocaloric properties of La0.8Ce0.2Fe11.4− x Co x Si1.6 (0, 0.2, 0.4, 0.6, 0.8 and 1.0) compounds have been investigated. X-ray diffraction shows that all compounds crystallize in the NaZn13-type structure. Magnetic measurements show that the Curie temperature (T C) can be tuned between 184 and 294K by changing the Co content from 0 to 1. A field-induced methamagnetic transition occurs in samples with x=0, 0.2 and 0.4. The magnetic entropy changes of the compounds have been determined from the isothermal magnetization measurements by using the Maxwell relation. [Copyright &y& Elsevier]

In this work, a novel method of producing maghemite (γ-Fe2O3) nanopowders has been developed, which can be performed by the direct thermal decomposition of an Fe–urea complex ([Fe(CON2H4)6](NO3)3) in a single step. The reaction mechanism, particle morphology, and the magnetic properties of the γ-Fe2O3 nanopowders have been studied by using thermogravimetric (TG), differential scanning calorimetry (DSC), fourier transformed infrared (FTIR) spectroscopy, elemental analysis, X-ray powder diffraction (XRD), transmission electron micrograph (TEM) observations, and magnetic measurements. Thermal analyses together with the results of XRD show that the formation of γ-Fe2O3 occurs at ~200 °C through a two-stage thermal decomposition of the [Fe(CON2H4)6](NO3)3 complex. The resulting iron oxide phases (i.e., γ-Fe2O3 and α-Fe2O3) are strongly dependent on the synthesis conditions of the [Fe(CON2H4)6](NO3)3. When the molar ratio of Fe(NO3)3 · 9H2O to CON2H4 that is used for the synthesis of [Fe(CON2H4)6](NO3)3 is 1:6 (i.e., molar ratio in stoichiometry), a mixed phase of γ-Fe2O3 and α-Fe2O3 is formed. When the molar ratio is 1:6.2 (i.e., using an excess CON2H4), on the other hand, a pure γ-Fe2O3 is obtained. Magnetic measurements show that resulting nanopowders exhibit a ferromagnetic characteristic and their maximum saturation magnetization increases from 47.2 to 67.4 emu/g with an increase in the molar ratio of Fe(NO3)3 · 9H2O to CON2H4 from 1:6 to 1:6.2. [ABSTRACT FROM AUTHOR]

Abstract: DyAl2 nanoparticles have been prepared by means of arc discharge in a mixture of argon and hydrogen gas. The structure of DyAl2 nanoparticles is studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy shows that the as-prepared DyAl2 nanoparticles are coated with a layer of Al2O3 phase on the surface, and their sizes vary from 20nm to about 100nm. The DyAl2 nanoparticles exhibit ferromagnetic properties that are different from bulk DyAl2 compound. The gradual decrease of the magnetization with increasing temperature in a wide temperature range reveals the size distribution of the DyAl2 nanoparticles. The magnetic-entropy changes are derived from the isothermal magnetization curves measured at different temperatures. The magnetic-entropy change of the DyAl2 nanoparticles is lower than that of the bulk DyAl2 material but has a broadened peak. [Copyright &y& Elsevier]