| 其他摘要 | Nd-Fe-C and Nd-Co-B-C alloys were prepared by mechanical-alloying plus annealing in vacuum. NdFeB/α-Fe multilayer films were prepared by magneto-sputtering. (Mn, Al)As, MnAsCx and Mn(As, Si) alloys were prepared by mechanical-alloying and vacuum annealing. Surface, interface and microstructure were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectra (XPS), atomic force microscope (AFM) and magnetic force microscopy (MFM). Magnetic properties were characterized by superconducting quantum interference device (SQUID).
In Nd16Fe84-xCx (6 ≤ x ≤ 12) alloys, it is found that a re-milling process can significantly increase the 2:14:1-type hard-magnetic phase ratio and their coercivities. The largest coercivity is obtained at x = 8. 2:14:1-type structure can be formed when x ≤ 7 in mechanical-alloyed Nd16Co76B8-xCx and their hydrides. The Nd-sublattice magnetocrystalline anisotropy of 2:14:1-type phases and their hydrides are found to be much enhanced by C substitution for B, consequently leading to higher spin-reorientation temperature. The effects of C substitution for B and hydrogen interstitial on the magnetocrystalline anisotropy of the Nd-sublattice are independent.
Textured NdFeB/α-Fe coupled multilayer films are prepared by depositing onto heated Si substrates with Mo spacer-layers. It is found that interfacial interdifussion can be effectively inhibited by Mo spacer-layer, which is important to realize the textured structure in double-phased films. The maximum values for coercivity and maximum energy product are 0.8 T and 200kJ/m3(25 MGOe) respectively. Due to the existence of spacer-layer, the exchange-coupling between soft- and hard-magnetic phases is proved to be indirect.
Coercivity mechanism in multilayer films with different thickness of Mo spacer- layer and repeats of NdFeB layers is investigated. It is found that reverse-domain nucleation is dominated in multilayer films with different thickness of Mo spacer- layer. With increased repeats, more interfaces leads to domain-pinning as the dominating mechanism. The pinning centers are the thin inhomogeneous regions.
The effects of non-magnetic spacer-layer on the exchange-coupling between soft- and hard-magnetic phases are investigated. The effective critical-correlation length shows a non-linear dependence on the thickness of effective correlation length, independent with spacer-layer materials, direction of applied field, measuring temperature, hard-magnetic materials and thickness. Under certain thickness of spacer-layer, the values of effective critical-correlation length are decreased, due to the enhanced magnetocrystalline anisotropy, shape anisotropy or the reduced thickness of hard-magnetic phases.
Time-consuming sintering in the preparation of MnAs alloys can be simplified by mechanical-alloying. After 1.5 % Al substitution for Mn, the maximum of magnetic entropy change is increased from 48.2 Jkg-1K-1 to 65.7 J kg-1K-1. And the Curie temperature in Mn0.985Al0.015As is 271 K. With increasing Al content to 3%, magnetic entropy change is greatly lowered. Under the magnetic field change of 5 T and 2 T, the refrigeration capacities are increased from 340 J kg-1 and 130 J kg-1 in MnAs to 420 J kg-1 and 160 J kg-1.
Due to the sensitivity of MnAs lattice to the pressure, the lattice strains can also be induced by the interstitial atoms. The Curie temperature gradually decreases with more interstitial carbon contents from 0.015 to 0.05. The maximum of entropy change and the magnetic refrigeration capacity for the field change of 5 T are 56.3 Jkg-1K-1 and 460 Jkg-1 in MnAsC0.03, indicating that interstitial effect can also be used to tune the MnAs metamagnetic transition and magneto-caloric effect.
Si substitution for As is beneficial to eliminate the thermal hysteresis when silicon substitution content is larger than 6%. With increasing Si substitution, the Curie temperature is correspondingly shifted to near room temperature. The maximum entropy change for a field change of 5 T is also decreased to be within 10 – 15 J kg-1K-1, which is insensitive to more silicon content. According to the requirement of active refrigeration, after adjusting the ratio of MnAs0.97Si0.03、MnAs0.94Si0.06 and MnAs0.91Si0.09, the refrigeration region as wide as 30 – 35 K and entropy change of 5 – 6 Jkg-1K-1 are obtained under a field change of 5 T. |
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