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We adopted sodium silicate to prepare composites of La(Fe, Si)13 hydrides in this work. A homogenous and continuous matrix of sodium silicate was formed at a relatively low solidification temperature. No counter diffusion between the matrix and the grains of La(Fe, Si)13 hydrides was observed. The magnetocaloric effect of pure La(Fe, Si)13 hydrides is largely preserved in the composites, showing a high tolerance to sodium silicate. These composites possess high thermal conductivity and decent compressive strength compared with the epoxy resin bonded samples. These results suggest that sodium silicate is a promising binder material for producing bulk La(Fe, Si)13 hydrides for their application in magnetic refrigeration.

As a new evolutive grain boundary diffusion technology, rotating diffusion technology exhibits many advantages such as easy to achieve mass production and reutilization of heavy rare earth materials for diffusion. In this study, the magnetic properties and microstructure of sintered Nd-Fe-B magnets with enhanced coercivity by rotating diffusion using Dy strips were studied. The optimal rotating diffusion parameters was obtained at 800 °C for 8 h, and the coercivity of diffused magnet increased from 12.51 kOe to 17.38 kOe. A (Nd,Dy) 2 Fe 14 B core-shell microstructure was observed in the surface region of the Nd 2 Fe 14 B grains after the rotating diffusion. In order to evaluate the reutilization rate of heavy rare earth materials, multiple reutilization experiments of Dy strips by rotating diffusion were implemented. The coercivity enhancement declines slightly with the increase of rotating diffusion times. Due to the evaporation of Dy strips, the diffusion process results in uneven, rough surface of Dy strips with many small concaves, which reduces the saturated vapor pressure of Dy strips, leading to less Dy evaporation and diffusion, and in turn to lower coercivity. But it is worth to mention that the coercivity of the fifth rotating diffusion magnet still reaches to 16.67 kOe, which is 33% higher than that of the original magnet. It is therefore concluded that the reutilization of Dy strips in rotating diffusion for sintered Nd-Fe-B magnets with enhanced coercivity is feasible and rotating diffusion technology could realize the mass production.

Permanent magnet materials based on the MM2Fe14B (MM = Misch-metal) are widely noted. However, as a room temperature paramagnetic phase, the existence of CeFe2 phase in MM2Fe14B alloy is unavoidable. In order to eliminate the CeFe2 phase, the La substitution on MM in MM2Fe14B alloys was described. Alloys with nominal composition of (LaxMM1−x)33Fe66B (x = 0, 0.025, 0.05, 0.075, 0.1, 0.15) were prepared, and the crystal structure and magnetization of the (LaxMM1−x)33Fe66B alloys were investigated. XRD results reveal that the lattice parameter a, c, unit-cell volume V, and c/a ratio of (LaxMM1−x)33Fe66B alloys increase linearly with the La increase, indicating that La atoms enter into the main phase. Key to this study is the finding that CeFe2 phase disappears with the La content above 0.075. It is therefore concluded that La substitution could inhibit formation of CeFe2 phase. This is consistent with the observation from SEM-EDS. Another interesting finding is that La substitution would cause the change of Nd, Pr, La and Ce ratio in main phase, which means the ratios of four rare earth elements change with the La substitution. With the increase of x, the (Pr + Nd)/(La + Ce) ratio in main phase of (LaxMM1−x)33Fe66B alloys decreases first, showing the lowest point at x = 0.075, and then increases. As a result, the saturation magnetization (Ms) of (LaxMM1−x)33Fe66B alloys has the same tendency, which decreases from 11.78 kG (x = 0) to 11.35 kG (x = 0.075) first, and then increases to 11.96 kG (x = 0.15). And the Curie temperature (Tc) displays the same trend compared to the Ms. These facts show that the MM has a great potential to prepare the low cost sintered permanent magnets.

The structural and magnetic properties of La1−yTbyFe11.5Si1.5 compounds have been investigated. The substituted 5 percent of Tb has remarkably increased the maximum entropy change (−ΔSM) to a value of 25.2 J/kg·K. However, a further increase of Tb leads to a monotonous decrease in the entropy change, mainly due to phase separation. The Tb substitution also can lower the thermal and magnetic hysteresis loss of the system. Another feature of this replacement is that, with Tb content higher than 10 percent, the Curie temperature (TC) starts to increase and reaches 197 K when 30 percent of La is substituted. These results indicate that La1−yTbyFe11.5Si1.5 may be a promising candidate for magnetic refrigeration material in certain temperature range.

The rapid development of Ce-based magnets has promoted the utilization of rare earth (RE) resources, yet the performance of Ce-based magnets is often undermined due to the easy formation of CeFe2 phase with a cubic MgCu2-type structure. In order to eliminate the CeFe2 phase, the La substitution on Ce in (LaxCe1-x)33Fe66B (x = 0,0.1,0.2,0.3,0.4,0.5) alloys is performed in this paper. It is found that partial replacement of Ce by La suppresses the emergence of the CeFe2 laves phase and promotes the formation of (La, Ce)2Fe14B phase. The inhibition mechanism is ascribed to the reduced phase stability of (Ce, La)Fe2 phase due to the partial replacement Ce by La, which is confirmed by our experiments and DFT (density functional theory) calculation results. Moreover, the La atom is prone to enter the lattice of (La, Ce)2Fe14B phase and expands the unit cell. Both the Curie temperature (Tc) and the saturation magnetization (Ms) of the alloy show a noticeable improvement because of this volume effect, which is consistent with our DFT calculations.

In this study, new all-d-metal Heusler alloys Fe50−xCr25+xV25 (x = 0–25) were investigated on the behaviors of atomic configuration, lattice constant change, and ferromagnetism from the aspects of theoretical calculations and experiments. The theoretical calculations found that both end-member compounds Fe50Cr25V25 (x = 0) and Cr50Fe25V25 (x = 25) prefer to crystallize in Hg2CuTi-type (space group: 216) structure rather than Cu2MnAl-type (space group: 225) one. During the whole substitution process, the structure of Fe50−xCr25+xV25 alloys remains Hg2CuTi-type structure, taking the path from 216 to 216, but with different degrees of BC-site disorder. Both the experimental and theoretical results reveal that the lattice constant increases first, and then abnormally decreases, showing a maximum at Cr40, due to the enhanced covalent d-d hybridizations between the transition metals. The magnetic moment and Curie temperature tuned by Cr substitution vary remarkably between 3.3 and 1.15 μB/f.u. and between 700 and 170 K, respectively, which indicates that the Cr substitution for Fe alters the spin polarizations and exchange interactions in the system. These results provide new insights into the atomic configuration, magnetism and bonding effect in all-d-metal Heusler alloys based on the d-d hybridizations.

A series of Co ferrites (Co x Fe 3− x O 4 ( x =0−1)) were prepared using solid-state method in this work. The aging effect of their structures and constrictions of hysteresis loops under low magnetic field were investigated. It was found that during the aging process, the migration of trivalent (bivalent) ions between tetrahedral (A-site) and octahedral (B-site) coordination induced a shrinking of the lattice, which would expand again due to the precipitation of Fe 3+ after a much longer aging time. The first process caused a pronounced constriction of the loops, due to the uniaxial anisotropy led by this migration. The depression of constriction could attribute to both the expansion of lattice and the change of ionic ratios as a result of the second-phase-precipitation. The impacts of Co content, aging time and temperature upon the constriction were also discussed.

A series of rare-earth doped BiFeO3 samples, Bi1−xRxFeO3 (x=0–1, R=La, Nd, Sm, Eu and Tb), were prepared in this work. X-ray diffraction analysis showed that the structure of rare-earth doped BiFeO3 was transformed from rhombohedral lattice to orthorhombic one by increasing x. The lattice constants and unit-cell volume decreased with the increasing of the doping content, while both the Neel temperature and magnetization were enhanced. A magnetic phase transition was observed at about 35 K for BiFeO3. The variation of the magnetization with temperature depended on applied field strength and magnetizing history, which was explained according to the antiferromagnetic exchange interaction between Fe and R sites in Bi1−xRxFeO3(x>0). The magnetocrystalline anisotropy contributed by Fe sublattice gave rise to a large coercivity in BixNd1−xFeO3 with an orthorhombic structure.

The initial permeability in composite ferrites consisting of low-temperature sintering (Ni 0.15 Cu 0.25 Zn 0.60 )Fe 2 O 4 ferrite and Co 2 Z (Ba 3 Co 2 Fe 24 O 41 ) hexaferrite matrix has been measured from 1 to 1000 MHz. The results reveal that frequency dependence of initial permeability in the composite ferrites is much different from that of pure (Ni 0.15 Cu 0.25 Zn 0.60 )Fe 2 O 4 spinel or Co 2 Z hexagonal ferrite. A simple quantitative estimation of permeability for the composite ferrite materials is proposed and possible magnetic mechanisms generating this kind of difference are discussed.

The magnetic properties after sintering of NiZnCu ferrite from self-propagated powders were investigated. Results from frequency spectra showed very high initial permeability, high cutoff frequencies and stable quality factor values for the sintered ferrites. The reactivity, sintering temperature and microstructure were correlated to the permeability. The magnetizing mechanism of the materials mainly resulted from domain walls motion.

A nitrate–citrate gel was prepared from metal nitrates and citric acid by sol–gel process, in order to synthesize Ni 0.25 Cu 0.25 Zn 0.50 Fe 2 O 4 ferrite. The thermal decomposition process was investigated by DTA-TG, IR and XRD techniques. The results revealed that the nitrate–citrate gel exhibits self-propagating combustion behavior. After combustion, the gel directly transformed into single-phase, nano-sized NiCuZn ferrite particles with spinel crystal structure. The synthesized powder can be densified at a temperature lower than 900°C. The sintered body possesses fine-grained microstructure, good frequency stability and high-quality factor compared to the sample prepared by conventional ceramic route.