Hong-wei 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.

Magnetostructural Transformation and Magnetoresponsive Properties of

The martensitic and magnetic phase transformations in MnNiGe_1-xSn_x (0 ≤ <i>x</i> ≤ 0.200) alloys were investigated using X-ray diffraction, differential thermal analysis, and magnetization measurements. Results indicate that increasing the Sn substitution in MnNiGe_1-xSn_x results in 1) a decrease of the martensitic transformation temperature from 460 to 100 K and 2) a conversion of spiral antiferromagnetic (AFM) to antiparallel AFM structure in martensite. Based on these features, a remarkable magnetic-field-induced paramagnetic/spiral AFM and FM/AFM magnetostructural transformations are found, and large positive and negative magnetocaloric effects are obtained. The magnetoresponsive effects of MnNiGe_1-xSn_x alloys are enhanced by Sn substitution. A structural and magnetic phase diagram of MnNiGe_1-xSn_x alloys has been proposed.

{\rm MnNiGe}_{1-x}{\rm Sn}_{x}

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.

Alloys

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

The phase evolution and magnetic properties of melt-spun Pr9Fe86-xCoxB5 (x=0,6,10,12,14,16) nanocomposites have been investigated. It was found that substitution of Co for Fe obviously raises the magnetization (M, H=6.5T), from 1.50 T for x=0 to 1.80 T for x=12. At the same time, the remanence (M-r), intrinsic coercivity (H-c) and maximum energy product (BHmax) changes from 0.96 T, 5.69 kOe, 11.8 MGOe for x=0 to 1.15T, 6.33 kOe, 17.3 MGOe for x=12, respectively. Further substitution of Co for Fe results in a decrease of M-r and BHmax because of the presence of minor amounts of 1:5 and 2:17 phases. Thermomagnetic analysis shows that the Co atoms enter into both the magnetically hard and soft phases. The Curie temperature of the hard magnetic phase increases linearly by increasing Go-substitution at the rate of DeltaT(c)=9 degreesC/at% Pr

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

We have succeeded in preparing magnetically anisotropic SmCo5 ribbons with high permanent performance by single-roller melt spinning at low wheel velocity. The anisotropy is associated with a crystallographic texture formed during melt-spinning process, with the c-axis parallel to the longitudinal direction of the ribbons. The formation of the crystallographic texture is attributed to a directional solidification process resulting from a thermal gradient. A remanence of 9.1 kG, remanence ratio of 0.9, intrinsic coercivity of 16.2 kOe and energy product of 18.2 MGOe at room temperature are obtained in the melt-spun and subsequently annealed SmCo5 ribbons prepared at 5 m/s

Effect of the substitution of Co for Fe on phase components and magnetic properties of melt-spun Pr9Fe85B5 nanocomposites

The correlation between initial permeability and temperature (mu (i)-T curve) for Fe-Cu-Nb-Si-B, Fe=Cu=Nb-V-Si-B and Fe-Cu-Mo-Si-B alloys annealed at 300-610 degreesC was investigated. Three types of mu (i)-T curve were observed. The corresponding phase structure of alloy for the three types of mu (i)-T curve was analyzed and the magnetic coupling between nanocrystalline grains versus mu (i)-T curve as well as the reasons far the changes of mu (i)=T with temperature were also discussed

Melt-spun magnetically anisotropic SmCo5 ribbons with high permanent performance

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.

Temperature dependence of permeability for Fe-Cu-M-Si-B alloys

Topological Dirac semimetal is a newly discovered class of materials and has attracted intense attentions. This material can be viewed as a three-dimensional (3D) analogue of graphene and has linear energy dispersion in bulk, leading to a range of exotic transport properties. Here we report direct quantum transport evidence of 3D Dirac semimetal phase of layered material ZrTe5 by angular dependent magnetoresistance measurements under high magnetic fields up to 31 Tesla. We observed very clear negative longitudinal magnetoresistance induced by chiral anomaly under the condition of the magnetic field aligned only along the current direction. Pronounced Shubnikov-de Hass (SdH) quantum oscillations in both longitudinal magnetoresistance and transverse Hall resistance were observed, revealing anisotropic light cyclotron masses and high mobility of the system. In particular, a nontrivial {\pi}-Berry phase in the SdH gives clear evidence for 3D Dirac semimetal phase. Furthermore, we observed clear Landau Level splitting under high magnetic field, suggesting possible splitting of Dirac point into Weyl points due to broken time reversal symmetry. Our results indicate that ZrTe5 is an ideal platform to study 3D massless Dirac and Weyl fermions in a layered compound.

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

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.