Nd 2 Fe 14 B/α-Fe HARD MAGNETIC NANOCOMPOSITE. PERFORMANCES AND LIMITS WILHELM KAPPEL, MIRELA-MARIA CODESCU Institute for Electrical Engineering – Advanced Research ICPE-CA, Splaiul Unirii 313, sector 3, Bucuresti, Romania Phone/fax: + 40 - 021 – 346.72.83, E-mail: kappel@icpe-ca.ro Received May 28, 2004 The Nd 2 Fe 14 B/α-Fe nanocomposites were obtained by recrystallisation from an amorphous phase, prepared by melt spinning. For all Fe contents, between 83 and 89 at.%, we have obtained the hardening of the α-Fe phase by exchange interactions between the hard Nd 2 Fe 14 B phase and the soft α-Fe phase. For all isotropic permanent magnets obtained from the prepared nanocomposites, we measured remanence ratio higher than 0.6, which confirm the existence of the mentioned exchange interactions. The impossibility to control the recrystallisation performs to a large distribution of the grain sizes of the α-Fe phase and determines a significant decrease of the magnetic performances. The analysis of the interactions between the hard and the soft phases shows that this decrease is impossible to be avoided in our NdFeB nanocomposites. 1. INTRODUCTION Kneller and Hawig [1] recognized the possibility to harden through exchange interactions two-phase nanostructures, composed by a hard magnetic phase and a soft ferromagnetic phase. Such isotropic nanocomposites show a behaviour like anisotropic permanent magnets, having the relative remanence for this so-called spring magnets, M r / M S > 0.5. Shomski [2, 3] has showed that such aligned nanocomposites from the type Nd 2 Fe 14 /α-Fe can have very high energy densities. Schrefl et al. [4, 5] analyzed through numerical computation the dependence of the magnetic properties of Nd 2 Fe 14 B/α-Fe nanocomposites from the dimensions and concentration of α-Fe inclusions. They have found a decrease of the coercivity with the α-Fe content and a sharp deterioration of the magnetic properties, if the grain sizes of the α-Fe phase exceed 20 nm. Zern et al. [6] show, that the intrinsic coercivity, done by M H C (T) = α K α ex H A – N eff M S , (1) Rom. Journ. Phys., Vol. 49, Nos. 9– 1 0 , P. 733–741, Bucharest, 2004