Shao-ying Zhang

Investigation on intergrain exchange coupling of nanocrystalline permanent magnets by Henkel plot

In a real magnet, the relation between isothermal remanence Jr(H) and dc demagnetization remanence Jd(H) is expressed as δm(H)=[Jd(H)−Jr(∞)+2Jr(H)]/J(∞). It is believed that nonzero δm is due to the interactions between particles in the magnet. Using Pr2Fe14B as a sample, the relation is examined by the micromagnetic finite element method. The positive value of δm is primarily caused by intergrain exchange coupling. The decrease of intergrain exchange coupling results in the drop of the maximum value of δm. However, the variation of anisotropy in grain boundaries produces no change in the maximum value of δm. A Henkel plot is suggested to be effective for checking intergrain exchange coupling in magnets.

Structure, magnetic properties, and coercivity mechanism of nanocomposite SmCo5/α-Fe magnets prepared by mechanical milling

Nanocomposite SmCo5+x wt %α-Fe (x=0, 20, 30, and 35) powders were prepared by mechanical milling and subsequent annealing. X-ray analyses show that hard phases can be 1:7, 1:5, or 2:7 phase with the increase of α-Fe content in as-milled powders annealed at 550 °C for 30 min. The high remanence and maximum energy product (BH)max were obtained by this method. The single-phase behavior of some powders was discussed according to the exchange spring model of Kneller and Hawig. For as-milled SmCo5+x wt %α-Fe (x=0, 20, 30, and 35) powders annealed at 550 °C for 30 min, analyses of their initial magnetization curves and the coercivities of minor hysteresis loops in dependence on the applied field reveal that coercivities of these powders are all controlled mainly by domain wall pinning. The addition of α-Fe can not change the coercivity mechanism of powders, although it affects strongly their magnetic properties.

Superconductivity of Mg(B1-xCx)(2) ternary compounds

The structural properties and superconductivity of Mg(B1−xCx)2 compounds were investigated by means of powder x-ray diffraction (XRD) and magnetization measurements. Powder XRD Rietveld analysis indicates that the samples crystallize in a hexagonal AlB2-type structure. The lattice parameter a decreases slightly with increasing carbon content, while c remains unchanged. The addition of carbon results in a decrease of TC and an increase in the superconducting transition width, while the critical current density Jc data indicate that MgB1.8C0.2 manifests comparable superconducting properties with MgB2.

Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd1-xPrx)(9)Fe86B5 (x = 0-1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (H-i(c)) and the maximum magnetic energy product ((BH),,,) increase from 414kA/m and 124kJ/m(3) for x = 0 to 493 kA/m and 152 kJ/m(3) for x = 0.6, respectively. Further substituting Nd by Pr (x >0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases

Structure and magnetic properties of SmxCo5/α-Fe (x=0.65–1.3) prepared by mechanical milling and subsequent annealing

Powder mixtures of SmxCo5 (x=0.65–1.3)+20 wt % α-Fe were mechanically milled. Annealing these as-milled powders results in the formation of a mixture of the hard phase Sm–(Co, Fe) and the soft phase Fe–Co. For the as-milled Sm1Co5+20 wt % α-Fe powder, the hard phase changes with the increase of annealing temperatures. The optimal maximum energy product (BH)max is obtained in the powder annealed at 550 °C for 30 min. Depending on the Sm content in the as-milled SmxCo5 (x=0.65–1.3)+20 wt % α-Fe powders, the hard phases can be 1:7, 1:5, or 2:7 phase after a heat treatment at 550 °C for 30 min. The coercivity of 6.5 kOe and maximum energy product of 17.8 MGOe is achieved for the powder with x=1.0. The highest coercivity of 9.67 kOe is achieved for the powder with x=1.2. From the measurements of the coercivity obtained from minor hysteresis loops, it is concluded that the coercivities of this type of magnets are controlled mainly by the domain wall pinning.

Investigation of hard magnetic properties of nanocomposite Fe-Pt magnets by micromagnetic simulation

Micromagnetic finite element method is used to simulate magnetic properties of FePt∕Fe3Pt exchange-coupled nanocomposites. Numerical results show that the maximum energy product (BH)max about 34.6 MGOe can be obtained for the 3-nm-scale isotropic magnets with the volume fraction of soft Fe3Pt phase vs=15%. The appearance of touched soft grains causes the decrease of reduced remanence mr when vs exceeds 20%. Coercivity decreases with the simultaneous decrease of Ds and Dh, where Ds and Dh are the grain sizes of soft and hard phases, respectively. Only under the prerequisite of sufficiently large Dh, the coercivity can be improved remarkably by decreasing Ds below twice the domain wall width of hard phase.

The bond ionicity in (R = Pr, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm)

(R = Pr, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm) has been studied using complex chemical bond theory. The results indicated that with the decreasing of R radius, the ionicities for all considered types of bond decrease. This is in good agreement with the experimental fact that decreases with the decreasing of R radius. with no Ba-site Pr in this calculation is also predicted to be a superconductor. This supports the conclusion obtained by Blackstead et al. The ionicity for each bond obeys the following order: Ba - O > R - O > Cu(2) - O(1) > Cu(2) - O(2,3) > Cu(1) - O(4) Cu(1) - O(1).

The magnetic properties of Gd2Co17−xGax compounds

X-ray diffraction (XRD) and magnetization measurements were performed to investigate the effect of Ga substitution for Co on the structural and magnetic properties of Gd2Co17−xGax compounds (x=0, 1, 2, 3, 4, 5, 6, 7, and 8). Crystal-structure studies indicate that all samples are single phase with the rhombohedral Th2Zn17-type structure except for the samples with x=7 and 8, which contain a small amount of Co. The Ga substitution for Co in Gd2Co17 compounds leads to a monotonic increase in the unit cell volume and an approximately linear decrease in the saturation magnetization. The Curie temperature TC is found to decrease monotonically from 1210 K for x=0 to 30 K for x=8. The compensation points are observed for the Gd2Co17−xGax samples with x=5 and 6. The value of the compensation temperature, determined from the temperature dependence of the magnetization measured in a magnetic field of 1000 Oe, is 120 and 152 K for x=5 and 6, respectively. It is noteworthy that the substitution of Ga for Co in the Gd...

Investigation of magnetization reversal in Sm-Fe-Cu(Zr)-Ga-C nanocomposite magnets

A comparative study has been performed concerning the microstructure and the magnetization reversal behavior of nearly single-phase Sm2Fe14.5Cu0.5Ga2C2 and two-phase exchange-coupled Sm2Fe14.5Cu0.5Ga2C2/α-Fe and Sm2Fe14.25Zr0.25Cu0.5Ga2C2/α-Fe synthesized by melt spinning and annealing. A few percent of Zr substitution proved to be effective in refining the microstructure and enhancing exchange-spring behavior after melt spinning and annealing. All samples show remanence enhancement, the value of Mr/Ms being the largest for the sample containing Zr. Measurements of the reversible susceptibility show a large reversible magnetization in the two-phase materials. For the Zr-containing sample, a more detailed analysis of the temperature dependence of the exchange-spring behavior was made.

GA-CONCENTRATION DEPENDENCE OF MAGNETOCRYSTALLINE ANISOTROPY IN GD2FE17-XGAX COMPOUNDS

The structure and magnetic properties of arc‐melted Gd2Fe17−xGax compounds were studied by means of x‐ray diffraction and magnetization measurements. X‐ray diffraction patterns demonstrate that all samples have a rhombohedral Th2Zn17‐type structure and the substitution of Ga leads to an approximately linear increase in the unit cell volumes. The Curie temperature is found to first increase and then decrease with increasing Ga concentration. It is noteworthy that the substitution of Ga has a significant effect on the magnetic anisotropy of iron sublattice. The easy magnetization direction of Gd2Fe17−xGax compounds changes from basal plane to c axis with increasing Ga concentration. The sample with x=7 exhibits a uniaxial anisotropy.