| Ferromagnetic bulk amorphous alloys usually show soft magnetic properties atroom temperature due to the microstructure of long-range disorder and short-rangeorder. However, the ferromagnetic Nd-based bulk amorphous alloys show hardmagnetic properties at room temperature. Therefore, the mechanisms responsible forcoercivity in the Nd-based bulk amorphous alloys have attracted much attention.When the Nd-based bulk amorphous alloys crystallize completely by annealingtreatment, the former hard magnetism will disappear and exhibit paramagnetism. Inthis work, the magnetic properties, phase composition and microstructure ofNd-Pr-Fe-Al bulk amorphous alloys are investigated during the crystallizationprocess. The correlations between the microstructure and the magnetic properties ofthe Nd-Pr-Fe-Al bulk amorphous alloys during this process are studied in detail. Thebroad aim of this study is to better understand coercivity mechanisms in Nd-basedbulk amorphous alloys. Some of the main experimental results are listed as follows:Based on the SEM and EDS results, the crystallization process of the(Nd0.7Pr0.3)60Fe30Al10bulk amorphous alloys (rod, Φ=3mm) can be divided into threestages.(1) Preparation stage. The alloys are composed of mainly amorphous phaseand a small volume fraction of Fe-rich precipitated phases with various compositions.(2) Main crystallizing stage. The volume fraction of the amorphous phase decreasedgradually while the vast amount of the black lath-shaped Fe-rich phase and whiteNd(Pr)-rich crystalline phase precipitated at this stage. A spot of black rod metastable phase appeared and then disappeared at higher annealing temperature.(3)Crystallizing completed stage. The amorphous alloy was fully crystallized andconverted to the above lath-shaped Fe-rich phase and white Nd(Pr)-rich crystallinephase.The magnetic properties for the (Nd0.7Pr0.3)60Fe30Al10bulk amorphous alloys (rod,Φ=3mm) during the crystallization process was investigated. It is indicated that thepresence of amorphous phase is responsible for the appearing of the hard magnetismin this alloy. With increasing the annealed temperature, the values of saturationmagnetization Msand remanence Mrdecrease gradually. The hard magnetismdisappeared after the alloy crystallized completely. The value of coercivity alsodeclined during the same process. With annealed at773K, the Msand Mrdropped to5.4Am2/kg and0.8Am2/kg, respectively, while the coercivityiHcdecreased to176.8kA/m. Obviously, the coercivity mechanism in this alloy is quite distinct.The existence of magnetic interaction in as-cast sample is confirmed by theHenkel plot. The temperature dependence of the coercivity in as-cast sampleagreement agreed to strong pinning model. Hence, both magnetic interaction andstrong pinning of domain walls were used to explain coercivity mechanism in bulkamorphous (Nd0.7Pr0.3)60Fe30Al10alloy. With increasing the annealed temperature, theexchange coupling effect decreased and vanished while the strong pinningmechanism still existed and maintained the high coercivity of the annealed alloy. The (Nd0.7Pr0.3)60-xFe30Al10Cux(x=0~4) and (Nd0.7Pr0.3)60-xFe30Al10Dyx(x=0~2) bulkamorphous alloys (rod, Φ=2mm) were fabricated and investigated. It was found thatthe coercivity mechanism in these alloys can be also attributed to the effect ofexchange coupling and strong pinning effect.The magnetic viscosity behavior in the Nd-based bulk amorphous alloys wasstudied by means of the sweep rate method (the dependence of the coercivity on thesweep rate of the applied magnetic field). Investigation of this phenomenon wasfound to be useful in obtaining the related magnetic microstructure size andcalculating out the maximum restoring force per pin, f. It was found that thecoercivity were closely related to the values of f for the above Nd-based bulkamorphous alloys. |
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