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The phases RR′Fe17−xGaxCy [ ( RR ′)=( Tm , Pr ) , (Nd, Sm), (Sm, Pr); x=0–7; y=0 and x=0, 1, 2 ; y=1] were synthesized by arc melting technique. The substitution of larger Ga atoms for smaller Fe leads to an expansion of the lattice. Substitution of Ga atoms increase the Curie temperature in the initial Ga concentration range (x⩽4). The increase in TC with Ga content is attributed to the increase in the Fe–Fe exchange coupling as a result of increasing interatomic distance, thereby reducing the effect of negative exchange interactions between Fe–Fe pairs. The primary effect of the interstitial carbon on the magnetic properties is related to volume expansion. The magnetization of all compounds increases with carbon and is probably due to the competition between band narrowing effect as a result of lattice dilation and chemical effect due to charge transfer mechanism. Concepts of spin fluctuation theory are invoked to understand variation of TC with carbon insertion. X-ray diffraction measurements on magnetically oriented powder samples show that the substitution of Ga for Fe in RR′Fe17 and further modification with interstitial carbon has an exceptionally important effect on their magnetocrystalline anisotropy. The EMD lies along the c-axis in the compounds NdSmFe17−xGaxC ( x=0, 1 and 2) and SmPrFe17−xGaxC (x=1 and 2) and is probably due to the increase in the magnitude of the A20 which increases the positive contribution of rare earth sublattice to total anisotropy. |
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