Structure, magnetic properties and magnetic interaction have been studied for (Nd,Pr)_2 (Fe,Co)_(14)B compounds with X-ray diffraction, AC-Susceptibility and high pulse magnetic field tech. etc. From theseexperiments one can realized that at the spin-reorienation temperature of Nd_2Fe_(14)B compound, X-ray diffraction strength show an abnormal which can be associated with abnormal thermal expansition of lattice of Nd_2Fe_(14)B at low temperature. Partial substitution of Pr for Nd makes magnetic behavior of (Nd_(1-x)Pr_x)_2Fe_(14)B compounds very complicated, and this phenomenon has been attributed to the valence instability of Pr ions in these compounds. Due to the reason mentionedabove, the contrinution to the magnetocrystalline anisotropy from 4f sublattice of the compounds may deviated from Single Ion Model. From experimental results of the high field magnetization, it brings to a conclusion that first order magnetization process (FOMP) exists in all (Nd_(1-x)Pr_x)_2Co_(14)B compounds at T = 4.2 K, and as a consequence, the magnetocrystalline anisotropy field of Pr_2Co_(14)B compound can be only about 49T at T = 4.2K, which is much lower than the published result 75T. It was also found that in (Nd,Pr)_2 (Fe,Co)_(14)B compounds, Partial substitution of Pr for Nd led todecrease of 4f-3d exchange interaction; substitution of Co for Fe made the difference of 4f-3d exchange interaction of Pr and Nd sublattice lower, meanwhile it made the difference of the magnetocrystalline anisotropy of the two sublattice go up.
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