| 其他摘要 | The structure, phase relationship, hyperfine interaction, magnetocrystalline anisotropy, first order magnetization and spin reorientation of Nd_2Fe_(14)B type pseudo-ternary compounds were studied by optical microscopy, differential thermal analysis, differential, scanning calorimetry, X-ray diffraction, Mossbauer spectroscopy, electron probe microanalysis, A.C. susceptibility and pulsed high field measurements. The Nd_2Fe_(14)B type psudo-ternary compound series studied were: (1) Nd_2Fe_(14-x)Ga_xB (x < 2.0), (2) Nd_2Fe_(11.5-x)GO_(2.5)Ga_xB (x < 2.0), (3) Nd_2Fe_(13-x)CoGa_xB (x < 2.0), (4) Nd_2Fe_(11.5-x-y)Co_(2.5)Nb_yB (x < 0.2; y < 0.3). The magnetocrystalline anisotropy and high field magnetization process of (Pr, R)_2Co_(14)B (R = Gd, Y) and R_2Co_(14-x)Al_xB (R = Pr, Gd) were also studied. Investigation of the macrostructure and intrinsic magnetic properties of Nd_2Fe_(14)B compound in which Fe was partially substituted by Ga, (Ga+Co) or (Ga+Co+Nb) respectively suggests that the remarkable increase of coercivity of magnets made from these pseudo-ternary compounds is probably not attributed to the improvement of the intrinsic magnetic properties of the hard Nd_2Fe_(14)B type matrix phase, rather it is probable that the alloying metals may dissolve in the phases present in the magnet resulting in the improvement of the inter-phase wettability especially that between the hard magnetic phase and the Nd-rich phase; and the fluidity of Nd-rich phase at high temperature. These effectively eliminate defects which usually act as nuclei for formation of reversed domains or render the Nd-rich particles finer and more uniformly distributed. All these will be responsible for the enhancement of coercivity of the magnet. Mossbauer effect study shows that Ga atoms preferentially occupy some Fe-sites in the Nd_2Fe_(14)B lattice, hower the behavior of Ga is controlled by Co atoms. When the Co-content is equal or less than one atom, Ga preferencially occupies the k_2-site which has the largest interatomic distance; whereas when the Co-content is 2 atoms, it tends to occupy the j_2-site which has the highest internal field. These results are used to elucidate with satisfaction the characteristic structure and magnetic properties of these pseudo-ternary compound series. Based on theoretical analysis, it is propored that spin reorientation temperature can be properly assessed by A.C. susceptibility measurement. The temperature corresponding to the initiation of the A.C. resceptibility peak was taken as the true spin reorientation temperature. Experimental results and crystal field analysis show that the decrease of spin reorientation temperature with increasing Ga-content in Nd_2(Fe,M)_(14)B (M being Co and /or Ca) is associated with the weakening of exchange interaction of the compound. By comparing the spin phase diagram of (R,R')_2Fe_(14)B (both R and R' are rare-earths) found by experiment with that by theoretical calculation, it is believed that the single ion theory can be used to describe the anisotropy of R_2Fe_(14)B compound with satisfaction. Finally the remarkable high anisotropy of Pr_2Co_(14)B was accounted for by the crystal field theory and the magnetization at high field of Gd_2Co_(14)B was tentatively proposed using two magnetic sub-lattice model. |
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