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Register a new userMagneto-structural correlations in a systematically disordered B2 lattice
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Ferromagnetism in certain B2 ordered alloys such as Fe$_{60}$Al$_{40}$ can be switched on, and tuned, via antisite disordering of the atomic arrangement. The disordering is accompanied by a $sim$1 % increase in the lattice parameter. Here we performed a systematic disordering of B2 Fe$_{60}$Al$_{40}$ thin films, and obtained correlations between the order parameter ($S$), lattice parameter ($a_0$), and the induced saturation magnetization ($M_{s}$). As the lattice is gradually disordered, a critical point occurs at 1-$S$=0.6 and $a_0$=291 pm, where a sharp increase of the $M_{s}$ is observed. DFT calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, whereas above, it is largely the increase of $a_0$ in the disordered state that determines the $M_{s}$. The insights obtained here can be useful for achieving tailored magnetic properties in alloys through disordering.
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We propose a self-consistent approximate solution of the disordered Kondo-lattice model (KLM) to get the interconnected electronic and magnetic properties of local-moment systems like diluted ferromagnetic semiconductors. Aiming at $(A_{1-x}M_x)$ compounds, where magnetic (M) and non-magnetic (A) atoms distributed randomly over a crystal lattice, we present a theory which treats the subsystems of itinerant charge carriers and localized magnetic moments in a homologous manner. The coupling between the localized moments due to the itinerant electrons (holes) is treated by a modified RKKY-theory which maps the KLM onto an effective Heisenberg model. The exchange integrals turn out to be functionals of the electronic selfenergy guaranteeing selfconsistency of our theory. The disordered electronic and magnetic moment systems are both treated by CPA-type methods. We discuss in detail the dependencies of the key-terms such as the long range and oscillating effectice exchange integrals, the local-moment magnetization, the electron spin polarization, the Curie temperature as well as the electronic and magnonic quasiparticle densities of states on the concentration $x$ of magnetic ions, the carrier concentration $n$, the exchange coupling $J$, and the temperature. The shape and the effective range of the exchange integrals turn out to be strongly $x$-dependent. The disorder causes anomalies in the spin spectrum especially in the low-dilution regime, which are not observed in the mean field approximation.
We investigate the evolution of spin polarization, spontaneous Hall angle (SHA), saturation magnetization and Curie temperature of $B2$-ordered Fe$_{60}$Al$_{40}$ thin films under varying antisite disorder, induced by Ne$^{+}$-ion irradiation. The spin polarization increases monotonically as a function of ion fluence. A relatively high polarization of 46 % and the SHA of 3.1 % are achieved on 40 nm thick films irradiated with 2 $cdot$ 10$^{16}$ ions/cm$^2$ at 30 keV. An interesting divergence in the trends of the magnetization and SHA is observed for low disorder concentrations. The high spin polarization and its broad tunability range make ion-irradiated Fe$_{60}$Al$_{40}$ a promising material for application in spin electronic devices.
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