X. B. Liu

Structure and magnetic properties of bulk nanocrystalline SmCo6.6Nb0.4 permanent magnets

The crystal structure and magnetic properties were studied for the bulk nanostructural permanent magnet SmCo6.6Nb0.4 prepared by spark plasma sintering method. The magnet crystallizes in a single phase with a hexagonal TbCu7-type crystal structure after the sintering process. Rietveld fitting results show that the alloying element of Nb prefers to occupy the 3g site. Microstructure observation indicates that the average crystal grain size is about 30nm. The coercivity decreases almost linearly from 2.8to0.5T with increasing temperature from 300to773K. A remanence enhancement resulting from intergrain exchange coupling among the fine grains was also observed.

Microstructure and magnetic properties of isotropic bulk NdxFe94−xB6 (x=6,8,10) nanocomposite magnets prepared by spark plasma sintering

Nd2Fe14B∕α‐Fe isotropic bulk nanocomposite magnets were prepared by spark plasma sintering (SPS) technique using melt-spun powders with a nominated composition of NdxFe94−xB6, with x=6, 8, and 10. It was found that higher sintering temperature improved the densification of the magnets, while it deteriorated their magnetic properties simultaneously due to the excess crystal grain growth. An increased compressive pressure led to better magnetic properties and higher density for the SPS magnets. An increase in the Nd amount resulted in a gradual increase in intrinsic coercivity and an obvious reduction of the remanence of the magnets simultaneously. A magnet with the composition of Nd8Fe86B6 possessed a Br of 0.99T, a Hci of 386kA∕m, and a (BH)max of 101kJ∕m3 under the optimal sintering condition. In addition, microstructure observation using transmission electron microscopy showed that compared with the starting powders the full-density magnets nearly maintain the morphology, indicating that there was no si...

Phase formation and magnetocaloric effect in rapidly quenched La(Fe1-xCox)11.4Si1.6

The effect of Co content on the phase formation, structure, and magnetocaloric effect of rapidly quenched La(Fe1−xCox)11.4Si1.6 with x=0 to 0.12 were investigated by x-ray diffractometry and magnetic measurements. Rietveld analyses indicate that with increasing Co content, the amount of the 1:13 phase increases from about 22 to 62 wt. % in the rapidly quenched alloys. A very short time annealing (1273K∕20min) of the as-quenched alloy is sufficient for the formation of a single phase 1:13 structure. The peak value of the magnetic entropy change is 20.5,11.8,9.9J∕kgK for the annealed rapidly quenched compounds with x=0, 0.04, and 0.08, respectively. Increasing Co content drives the order of magnetic transition, near TC, from first order to second order and eliminates the occurrence of the field-induced magnetic transition above TC, which is responsible for the decrease in the magnetocaloric effect.

Structure and magnetic properties of magnetically isotropic and anisotropic Nd–Fe–B permanent magnets prepared by spark plasma sintering technology

Spark plasma sintering technique had been applied to prepare bulk isotropic and anisotropic nanostructured Nd–Fe–B permanent magnets via hot pressing and subsequent hot deformation process. Influences of processing conditions and deformation height reduction on the structure and magnetic properties of the magnets were investigated. For the hot deformed magnet with 80% height reduction, XRD patterns of the anisotropic magnets show dominant (00l) diffraction peaks indicating evident c-axis crystallographic alignment in the magnet. Under the optimal processing conditions, the anisotropic magnet with 80% height reduction exhibits excellent magnetic properties as remanence (Br) of 1.492 T, coercive force (Hci) of 1004 kA/m, and the maximum energy product [(BH)max] of 400 kJ/m3, which are among the highest reported magnetic properties of nanostructured Nd–Fe–B permanent magnets.

Crystal structure and magnetic transition of MnFePGe compound prepared by spark plasma sintering

The crystal structure and magnetic transition were studied for the bulk Mn1.1Fe0.9P0.8Ge0.2 compound prepared by a simple blending and subsequent spark plasma sintering route. X-ray diffraction analysis and refinement show that the compound crystallizes in the hexagonal Fe2P-type structure, in which the Mn atoms occupy all the 3g sites and some 3f sites, the Fe atoms occupy the rest of the 3f sites, and P and the Ge atoms randomly occupy the 2c and 1b sites. Magnetic measurement indicates that the Curie temperature TC is at 253K and the thermal hysteresis of M-T curves at TC upon heating and cooling, a signature of a first-order magnetic phase transition, is about 15K. The maximum magnetic entropy change of the compound reaches as high as 49.2J∕kgK in a field change from 0to5T at 253K, while the adiabatic temperature change of the compound reaches only 1.2K in a field change from 0to1.5T at the same temperature.

Magnetocaloric effect in Mn5Ge3−xSix pseudobinary compounds

Structure and magnetic properties of Mn5Ge3−xSix pseudobinary compounds with x=0–1.5 were studied by x-ray diffraction and magnetic measurements. The compounds retain the hexagonal D88-type structure with a space group P63∕mcm, and their lattice parameters decrease from a=7.204(2)A and c=5.028(2)A to a=7.084(2)A and c=4.945(3)A with increasing Si content. Wigner-Seitz cell volume calculations indicate that Mn site volumes and Mn-Mn atomic distances decrease with increasing x, which influences the exchange interaction. All the compounds are ferromagnetic and their Curie temperatures, TC, decrease from 298 to 252K as the Si content is increased. In addition, the average atomic magnetic moment of Mn (at 5K) decreases from 2.64 to 2.35μB with increasing x from 0 to 1.5. The magnetocaloric effect is evaluated by measuring the isothermal magnetic entropy change, ΔSm, using the appropriate thermodynamic Maxwell relation. The peak value of ΔSm ranges from about 9.0to6.3J∕kgK for x=0–1.5 near their TC under an ext...

A first-principles study on the magnetocaloric compound MnFeP2∕3Si1∕3

The electronic structure and magnetic properties for MnFeP2∕3Si1∕3 with a hexagonal Fe2P-type structure have been studied by a first-principle density functional theory calculation. The calculated magnetic moments for Fe and Mn are 1.35 and 2.89μB, respectively, leading to a total magnetization of 4.15μB per formula unit due to the small negative moments of P and Si atoms. The total energy calculations show that the Si atoms prefer to occupy the 2c site rather than the 1b site and increase the moment of Fe while decreasing the moment of Mn. The nearest Mn–Fe exchange coupling interaction (JMn–Fe=1.33mRy) is much stronger than for Fe–Fe (JFe–Fe=−0.16mRy) and Mn–Mn atomic pair (JFe–Fe=−0.53mRy) interactions. The competed exchange interactions are responsible for the field induced first order magnetic transition and the large magnetocaloric effect.

Structure and magnetic properties of La(Fe0.88Al0.12)13Cx interstitial compounds

Structure and magnetic properties of La(Fe0.88Al0.12)13Cx interstitial compounds with x=0–0.8, were studied by x-ray diffraction and magnetic measurements. The interstitial compounds retain the NaZn13-type structure and the lattice constant increases with carbon content. The Wigner–Seitz cell volume and void calculations show there are seven different voids in the NaZn13 structure with the void at the 24d site (0, 0.25, 0.25) having the largest volume and the largest number of La neighbors. It is highly likely that the carbon atoms enter the 24d site. The small carbon doping collapses the antiferromagnetic state in La(Fe0.88Al0.12)13 and forms a ferromagnetic state. The magnetic transition temperature increases from 198 K to 280 K and the spontaneous magnetization (5 K) decreases from 22.9 μB to 21.3 μB per formula unit with increasing carbon content from x=0 to 0.8. The change in magnetic properties on carbon addition is attributed to cell volume expansion and Fe–Fe distance variations.

Exchange interaction in GdT2 (T=Fe,Co,Ni) from first-principles

The intersites exchange coupling parameters Jij of the Heisenberg model have been studied from a first-principles density functional plus Hubbard U approach calculation and a linear-response method for GdT2 Laves phase with T=Co, Fe, and Ni. The calculated magnetic moments of Gd are well localized and range from 7.3μB to 7.8μB, while the moments of Fe, Co, and Ni are −1.96μB, −1.19μB, and −0.11μB, respectively. The calculated effective exchange interaction parameters J0 of Gd sublattices are 2.62, 1.95, and 1.00 mRy, while those of T sublattices are 6.92, 4.37, and 0.00 mRy for T=Fe, Co, and Ni, respectively. The calculation results indicate that the magnetic ordering is dominated by the Gd sublattice in GdNi2, while the magnetic ordering is mainly determined by the Fe-sublattice in GdFe2.

Magnetocaloric effect in layer structural Gd5(SixGe1−x)4∕Gd composite material

A series of layer structural composite magnetic refrigerant materials with composition of (Gd5Si2Ge2)x(Gd)y(Gd5Si1.85Ge2.15)z was synthesized via spark plasma sintering technique. The adiabatic temperature change (ΔTad) of the composites and of the three individual components was measured directly from 245to315K with a magnetic field change of 1.5T using a homemade magnetocaloric effect measuring apparatus. For all composites with different x:y:z ratios, their resultant ΔTad uniformly peaks at 246, 276, and 296K, respectively, corresponding to the magnetic transition temperature of the three components. In addition, proper x:y:z ratio can improve both the value and the constancy of ΔTad of the composite. Compared with the three components, the value of ΔTad of the composites exhibits a more constant tendency, which is more suitable for practical application in room temperature refrigeration, when compared to those of the three components.