Mn5Ge3−xAlx ribbons with x=0,0.5, and 1 were prepared by single roller melt-spinning method. Textured structures were observed in these ribbons as a result of fast cooling, with highly ordered columnar grains perpendicular to the surface. The magnetocaloric effect and critical behavior were investigated by dc magnetization measurement. Curie temperatures decrease from 297 K to 283 K as the Al content increases, and the peak entropy changes also decrease from 4.92 J/kg K to 3.0 J/kg K under an external magnetic field change of 3 T. Critical exponents were obtained by modified Arrott plots technique, Kouvel-Fisher method, and critical isothermal analysis. Similar values of exponents of β∼0.37, γ∼1, and δ∼3.8 were obtained for all of the compositions. Moreover, rescaled entropy data for all compositions collapse into a universal curve, revealing universal behavior for the magnetocaloric effect in this series of compounds.
The crystal structure, magnetic properties and magnetostriction of polycrystalline magnetostrictive alloys Nd(Fe1−xCox)1.9 were investigated. It was found that the single cubic Laves phase can only be obtained in the samples with x>0.4 by a traditional annealing method. In contrast, the cubic Laves phase exists over the entire composition range studied in the samples annealed under high pressure. Both the Curie temperature and saturation magnetization increase with increasing Co concentration to a maximum at x = 0.2 and then decrease with further increasing x, which could be understood in the frame of rigid band model. The spin phase diagram for Nd(Fe1−xCox)1.9 is constructed to illustrate the arrangement for the easy magnetization direction and crystal structure. The magnetostriction of Nd(Fe1−xCox)1.9 increases with increasing Co concentration and peaks at x = 0.1 and then presents an anomalous behavior when 0.2≤x≤0.6. It is suggested that the external-field-induced lattice strain should be responsible for the anomalous magnetostrictive behavior.
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