H.W. Chang

Effect of initial stress/strain state on order-disorder transformation of FePt thin films

Initial stress (σi) of a room-temperature deposited FePt films was manipulated to study the order-disorder transformation. We observed that, while σi was increased from −1.01 (compressive) to 0.18 GPa (tensile), the phase transformation activation energy decreased from 0.387 to 0.23 eV/atom. This causes a reduction in ordering temperature of about 100 °C. We also found that densification induces an increase in tensile stress of about 1 GPa before ordering. In the films with small σi, strong tension facilitates the nucleation of L10 FePt; however, if highly compressive σi cancels the densification tension, ordering is retarded causing higher ordering temperature.

Structural and nanomechanical properties of BiFeO3 thin films deposited by radio frequency magnetron sputtering

The nanomechanical properties of BiFeO3 (BFO) thin films are subjected to nanoindentation evaluation. BFO thin films are grown on the Pt/Ti/SiO2/Si substrates by using radio frequency magnetron sputtering with various deposition temperatures. The structure was analyzed by X-ray diffraction, and the results confirmed the presence of BFO phases. Atomic force microscopy revealed that the average film surface roughness increased with increasing of the deposition temperature. A Berkovich nanoindenter operated with the continuous contact stiffness measurement option indicated that the hardness decreases from 10.6 to 6.8 GPa for films deposited at 350°C and 450°C, respectively. In contrast, Youngs modulus for the former is 170.8 GPa as compared to a value of 131.4 GPa for the latter. The relationship between the hardness and film grain size appears to follow closely with the Hall–Petch equation.

Alloying effect on the magnetic properties of RFeB-type bulk magnets

Composition dependence on the magnetic properties of directly casted cylindrical RFeBM-type magnets with diameters from 0.7 mm to 1.5 mm has been studied. In order to refine the microstructure for improving the magnetic properties of bulk magnets with various sizes, the following ways were adopted in the alloys: (1) changing the boron concentration, (2) substituting different kinds of refractory elements for Fe, (3) adding a slight carbon and finally, (4) adopting multi-component refractory elements. The experimental results showed that the Nd9.5Fe72.5Ti3B15 bulk magnets with a diameter of 0.7 mm demonstrated the optimal magnetic properties of Br = 6.5 kG, iHc = 10.3 kOe and (BH)max = 8.7 MG Oe, while the highest iHc of 16.2 kOe was achieved for the Pr9.5Fe71.5Nb4B15 magnet. Meanwhile, the diameter of the magnets could be increased to 0.9 mm preserving the attractive magnetic properties by the substitution of 0.5 at% Zr for Ti in the Nd9.5Fe72.5Ti3B15 magnet. Finally, the directly casted Nd9.5Fe72.5−yTi2.5Zr0.5CryB15−xCx (x = 0.25–1; y = 0–3) magnets with a larger diameter of 1.3 mm exhibited attractive magnetic properties of Br = 5.3–6.1 kG, iHc = 7.2–12.5 kOe and (BH)max = 6.5–7.2 MG Oe.

Sputter-prepared (001) BiFeO3 thin films with ferromagnetic L10-FePt(001) electrode on glass substrates

Highly textured BiFeO3(001) films were formed on L10-FePt(001) bottom electrodes on glass substrates by sputtering at reduced temperature of 400°C. Good electric polarization 2Pr = 80 and 95 μC/cm2, comparable to that of the reported epitaxial films, and coercivity Ec = 415 and 435 kV/cm are achieved in the samples with 20-nm- and 30-nm-thick electrodes. The BiFeO3(001) films show different degrees of compressive strain. The relation between the variations of strain and 2Pr suggests that the enhancement of 2Pr resulted from the strain-induced rotation of spontaneous polarization. The presented results open possibilities for the applications based on electric-magnetic interactions.

Hard magnetic property enhancement of Co7Hf-based ribbons by boron doping

Hard magnetic property enhancement of melt spun Co88Hf12 ribbons by boron doping is demonstrated. B-doping could not only remarkably enhance the magnetic properties from energy product ((BH)max) of 2.6 MGOe and intrinsic coercivity (iHc) of 1.5 kOe for B-free Co88Hf12 ribbons to (BH)max = 7.7 MGOe and iHc = 3.1 kOe for Co85Hf12B3 ribbons but also improve the Curie temperature (TC) of 7:1 phase. The (BH)max value achieved in Co85Hf12B3 ribbons is the highest in Co-Hf alloy ribbons ever reported, which is about 15% higher than that of Co11Hf2B ribbons spun at 16 m/s [M. A. McGuire, O. Rios, N. J. Ghimire, and M. Koehler, Appl. Phys. Lett. 101, 202401 (2012)]. The structural analysis confirms that B enters the orthorhombic Co7Hf (7:1) crystal structure as interstitial atoms, forming Co7HfBx, in the as-spun state. Yet B may diffuse out from the 7:1 phase after post-annealing, leading to the reduction of Curie temperature and the magnetic properties. The uniformly refined microstructure with B-doping results i...

Study on strengthening and toughening of sintered rare-earth permanent magnets

In recent years, we have a systematic investigation on the mechanical properties, fracture behavior and micromechanism, strengthening and toughening, and impact stability of sintered rare-earth permanent magnets (being abbreviated to “REPM”) at the aim of improving the workability and increasing the dynamic applications of REPM. A scanning electron microscope was used to study the fracture behavior and micromechanism of the experimental magnets. The impact stability of REPM was investigated using a falling-weight impact tester and an impact-acceleration standard device based on laser interference technique. The brittleness index of B=Hv∕KC was used to appraise the workability of REPM. It shows that the fracture mechanism of sintered NdFeB magnets mainly appears intergranular fracture. Sintered Sm-Co magnets tend to cleavage fracture. The fracture behavior REPM obviously exhibit anisotropy. The mechanical strength of the experimental magnets prepared by dual-alloy sintering method surpassed 390MPa without ...

Magnetic properties and microstructure of bulk Nd–Fe–B magnets solidified in magnetic field

The Nd–Fe–B bulk magnets with a slab shape of 0.9 × 4 × 15 mm3 were prepared by injection casting into a copper mold. The effects of applying a magnetic field during the casting process on the magnetic properties and microstructure of Nd9.5Fe71.5Ti2.5Zr0.5Cr1B14.5C0.5 alloy have been studied. The results show that the sample cast with magnetic field has a stronger (00L) texture of Nd2Fe14B phase with the c-axis perpendicular to the slab plane than the sample cast without magnetic field. The intensity of the texture weakens from surface to inner region of the bulk magnets. Applying a magnetic field during the casting process is helpful to refine the grain size effectively. As a result, the magnetic properties are improved from Br = 5.8 kG, iHc = 6.5 kOe, and (BH)max = 5.9 MGOe for thesample cast without magnetic field to Br = 6.1 kG, iHc = 10.3 kOe, and (BH)max = 7.3 MGOe for the sample cast with a 3.7 kOe magnetic field.

High Magnetic Anisotropy Field in CoZr Thin Films

Magnetic properties and high frequency characteristics of Co_100-xZr_x (x=5-17) thin films have been investigated. The experimental results show that with the increase of Zr content, the saturation magnetization, 4πM_s, decreases from 12.3 kG for x=5 to 7.3 kG for x=17; induced anisotropic field, H_k, of the films increases from 232 Oe for x=5 at first and reaches a maximum value of 266 Oe for x=9; coercivive force along hard and easy direction, H_ch and H_ce, ranges 1.6-7.1 Oe and 5.4-10.9 Oe, respectively. The highest ferromagnetic resonance frequency, <i>f</i>_FMR of 4.9 GHz could be achieved for the films with x=9. In addition, H_ch and H_ce are reduced by adopting Cu underlayer or Ni₂₀Fe₈₀ underlayer to the studied CoZr films. The optimal magnetic properties of 4πM_s=12 kG , H_ce=5.4 Oe, H_ch=0.7 Oe, H_k=258 Oe , ρ = 233 μΩ-cm and <i>f</i>_FMR=5 GHz are obtained for Co₉₁Zr₉(110 nm)/Ni₂₀Fe₈₀(15 nm) bilayer.

Domain wall pinning on strain relaxation defects (stacking faults) in nanoscale FePd (001)/MgO thin films

FePd (001) films, prepared by an electron beam deposition system on MgO(100), exhibit a perpendicular magnetic anisotropy (1.7 × 107 erg/cc) with a high order parameter (0.92). The relation between stacking faults induced by the strain relaxation, which act as strong domain wall pinning sites, and the perpendicular coercivity of (001) oriented L10 FePd films prepared at different temperatures have been investigated. Perpendicular coercivity can be apparently enhanced by raising the stacking fault densities, which can be elevated by climbing dissociation of total dislocation. The increased stacking fault densities (1.22 nm−2) with large perpendicular coercivity (6000 Oe) are obtained for samples prepared at 650 °C. This present work shows through controlling stacking fault density in FePd film, the coercivity can be manipulated, which can be applied in future magnetic devices.

Magnetocaloric effect of the Gd-LRE-Al-(Co, Fe) (LRE=La, Ce, Pr, Nd) glassy ribbons in the intermediate temperature

The magnetic properties and mgnetocaloric effect (MCE) of the Gd65Al15Co20−xFex(x = 0 − 20) and Gd65−yLREyAl15Fe20 (LRE = La, Ce, Pr, Nd; y = 0 − 20) glassy ribbons have been investigated. The Curie temperature (Tc) of the Gd-based Gd65Al15Co20−xFex glassy ribbons can be greatly increased by moderate Fe substitution. Furthermore, it is found that Ce atom among the other light-rare-earth elements is more prefer one for adjusting the Tc of the Gd65−yLREyAl15Fe20 glassy ribbons. The low materials cost, improved MCE properties make Gd65−yCeyAl15Fe20 glassy ribbons attractive candidates for magnetic refrigeration application in the intermediate temperatures of 80–220 K.