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We have investigated the electronic structure and magnetic properties of GdN as a function of unit cell volume. Based on the first-principles calculations of GdN, we observe that there is a transformation in conduction properties associated with the volume increase: first from halfmetallic to semi-metallic, then ultimately to semiconducting. We show that applying stress can alter the carrier concentration as well as mobility of the holes and electrons in the majority spin channel. In addition, we found that the exchange parameters depend strongly on lattice constant, thus the Curie temperature of this system can be enhanced by applying stress or doping impurities.

The exchange interaction parameters of Gd monopnictides are deduced from fitting the total energies of different magnetic configurations to those computed within the Heisenberg model. The magnetic structures predicted by first-principles calculations as well as the Curie (Néel) temperatures obtained from Monte Carlo simulations are both in good agreement with experiments. A detailed analysis of the exchange parameters suggests that the Ruderman-Kittel-Kasuya-Yosida-type indirect exchange interactions and antiferromagnetic superexchange interactions coexist in these compounds. The magnetic order changes from ferromagnetic in GdN to antiferromagnetic in other Gd pnictides as a result of the increased ionic radius of a pnictide in the latter.

The metastable Ni 3 C phase has been produced by mechanically alloying Ni and C. Ni 3 C particles of diameter 10 nm are produced after 90 h of mechanical alloying with no evidence of crystalline Ni in x ray or electron diffraction. Linear muffin-tin orbital band-structure calculations show that Ni 3 C is not expected to be ferromagnetic due to strong Ni-C hybridization in the ordered alloy; however, the introduction of even small amounts of disorder produces locally Ni-rich regions that can sustain magnetism. Mechanically alloyed Ni 3 C is ferromagnetic, with a room-temperature coercivity of 70 Oe and a magnetization of 0.8 emu/g at 5.5 T, although the hysteresis loop is not saturated. The theoretical prediction that interacting locally nickel-rich regions may be responsible for ferromagnetic behavior is supported by the observation of magnetically glassy behavior at low magnetic fields.

First-principle studies of magnons and magnon-phonon interactions are carried out in bcc and fcc iron in the adiabatic approximation. It is shown that the phonons have a minor effect on magnons in bcc Fe and thus the lattice vibrations make a small contribution to the Curie temperature. fcc Fe is unstable against magnon excitations but the phonons seem to reduce this instability. The magnonphonon interactions are analyzed in terms of the pair-exchange interaction variations as functions of the interatomic distances. The fcc results suggest that metastable fcc Fe in thin-film or nanostructure form should have interesting magnetic properties. PACS numbers: 75.10.Lp, 75.30.Ds Generally, it is expected that phonons do not have an appreciable effect on the magnetic properties of most materials. However, some iron-rich systems have low Curie temperature ͑T c ͒ which increases significantly with a small lattice expansion. For example, R 2 Fe 17 (R rare earth) compounds have T c 's around 350 K which increases to about 750 K with a volume expansion of ϳ6% [1]. Such systems, which are close to magnetic and/ or structural instability, should have reasonable magnonphonon interactions. Classic examples of such systems are metastable fcc and amorphous Fe. fcc Fe has been stabilized recently in thin films and as a precipitate in Cu matrix [2]. Theoretical studies show that fcc Fe is a weak magnet exhibiting magnetovolume instability and noncollinear magnetic structure under compression [3]. On the other hand, bcc Fe is a strong magnet with a high T c and stable magnetic structure. The very different magnetic properties of bcc and fcc iron should be reflected in their magnon dispersion curves without and with phonon excitations.Because of the complexity of the magnon-phonon interactions, only a few simple model studies have been reported so far [4][5][6]. Ab initio calculations of the magnon spectra in the absence of phonons in some transition metals have been reported only recently using a frozen-magnon [7] and linear-response scheme [8]. The former calculations are based on linear muffin-tin orbitals (LMTO) method in the atomic sphere approximation, while the latter use the full-potential LMTO method. The results for Fe by the two methods are similar for small wave vectors. We report here a first-principles study of the influence of lattice vibrations on the magnon excitations in bcc and fcc Fe using a frozen-magnon frozen-phonon scheme. The results show significant differences in the magnon spectra of magnetically stable (bcc) and metastable (fcc) structures.Starting with the Hamiltonian, H, a brief outline of the magnon-phonon theory is as follows:where the first term is the vibrational term in the harmonic approximation with u a ͑i͒ being the a component of the vector displacement of atom i from its equilibrium position and the second term is the Heisenberg exchange term with S͑i͒ as the unit vector. The distance dependent exchange parameter j͑ R i , R j ͒ is expanded in terms of the atomic displacements as follows:wher...