Formation of metastable Pr2Fe23B3 phase and its effect on magnetic properties in rapidly quenched Pr9Fe91-xBx nanocomposites

The formation of metastable Pr₂Fe₂₃B₃ phase and its effect on magnetic properties of melt-spun Pr₉Fe_91-xB_x (x = 4-12) have been investigated. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies show that the samples consist of Pr₂Fe₁₄B and α-Fe phases at low B content, and Pr₂Fe₁₄B, α-Fe and Fe₃B at high B content. The metastable Pr₂Fe₂₃B₃ phase appears in the intermediate B content (8-12 at%). This metastable Pr₂Fe₂₃B₃ phase results in a poor squareness in the demagnetization curve and is detrimental to the hard magnetic properties. Transmission electron microscopy (TEM) studies reveal larger average grain sizes when the B content is at 8.5 at% or above. It is further found that this metastable phase can be transformed into Pr₂Fe₁₄B, α-Fe, and Fe₃B by annealing the sample at temperature of 750°C or above, leading to a more square demagnetization curve. However, B_r of the nanocomposite decreases slightly due to the excessive grain growth during high-temperature annealing.