Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been studied using ͑i͒ a singleimpurity based generalized Ruderman-Kittel-Kasuya-Yosida ͑RKKY͒ approach which goes beyond linear response theory, and ͑ii͒ a mean-field-plus-spin-fluctuation approach within a ͑purely fermionic͒ Hubbard-model representation of the magnetic impurities, which incorporates dynamical effects associated with finite frequency spin correlations in the ordered state. Due to a competition between the magnitude of the carrier spin polarization and its oscillation length scale, the ferromagnetic spin coupling is found to be optimized with respect to both hole doping concentration and impurity-carrier spin coupling energy J ͑or equivalently U). The ferromagnetic transition temperature T c , deteremined within the spin-fluctuation theory, corresponds closely with the observed T c values. Positional disorder of magnetic impurities causes significant stiffening of the high-energy spin-wave modes. We also explicitly study the stability/instability of the mean-field ferromagnetic state, which highlights the role of competing antiferromagnetic interactions causing spin twisting and noncollinear ferromagnetic ordering.

The interplay of disorder and competing interactions is investigated in the carrier-induced ferromagnetic state of the Kondo lattice model within a numerical finite-size study in which disorder is treated exactly. Competition between impurity spin couplings, stability of the ferromagnetic state, and magnetic transition temperature are quantitatively investigated in terms of magnon properties for different models including dilution, disorder, and weakly-coupled spins. A strong optimization is obtained for Tc at hole doping p << x, highlighting the importance of compensation in diluted magnetic semiconductors. The estimated Tc is in good agreement with experimental results for Ga1−xMnxAs for corresponding impurity concentration, hole bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively studied within a locally self-consistent magnon renormalization scheme, which yields a substantial enhancement in Tc due to spin clustering, and highlights the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large enhancement in density of low-energy magnetic excitations due to disorder and competing interactions results in a strong thermal decay of magnetization, which fits well with the Bloch form M0(1 − BT 3/2 ) at low temperature, with B of same order of magnitude as obtained in recent squid magnetization measurements on Ga1−xMnxAs samples.

The spontaneous magnetization as a function of temperature has been investigated for a number of ͑Ga,Mn͒As layers grown on GaAs ͑001͒ using superconduction quantum interference device magnetometry. The measurements have been performed with a focus on the low-temperature behavior of the magnetization. The temperature dependence of the spontaneous magnetization M S for T = 0.5T C is found to be well described by Bloch's T 3/2 law for all measured samples. However, we observe an extraordinary enhancement of the spin-wave parameter compared to conventional 3d ferromagnets such as Fe, which implies an enhanced low-temperature decay of the magnetic order. For a thorough understanding of this result, we have theoretically investigated the effect of dilution ͑disorder͒ on the finite-temperature spin dynamics and on the spin-wave parameter. It is shown that, due to the preferential accumulation of holes in spin clusters, which weakens the majority bulk spin coupling and results in low-energy magnon softening, there is a strong enhancement in the low-temperature decay of the magnetization with dilution. We find excellent agreement between the calculated and measured values for the spin-wave parameter.

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zoneboundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Among the various methods employed in the synthesis of nanostructures, those involving high operating temperature and sharp thermal gradients often lead to the establishment of new exotic properties. Herein, we report on the formation of Cu-Ni metallic alloy nanoparticles with greatly enhanced stiffness achieved through direct-current transferred arc-thermal plasma assisted vapour-phase condensation. High pressure synchrotron X-ray powder diffraction (XRPD) at ambient temperature as well as XRPD in the temperature range 180 to 920 K, show that the thermal arc-plasma route resulted in alloy nanoparticles with much enhanced bulk modulus compared to their bulk counterparts. Such a behaviour may find an explanation in the sudden quenching assisted by the retention of a large amount of local strain due to alloying, combined with the perfect miscibility of the elemental components during the thermal plasma synthesis process.