11 research outputs found
Ferromagnetism in Co-doped ZnO films grown by molecular beam epitaxy: magnetic, electrical and microstructural studies
We studied structural, optical and magnetic properties of high-quality 5 and
15% Co-doped ZnO films grown by plasma-assisted molecular beam epitaxy (MBE) on
(0001)-sapphire substrates. Magnetic force microscopy (MFM) and magnetic
measurements with SQUID magnetometer show clear ferromagnetic behavior of the
films up to room temperature whereas they are antiferromagnetic below 200 K
approximately. Temperature dependence of the carrier mobility was determined
using Raman line shape analysis of the longitudinal-optical-phonon-plasmon
coupled modes. It shows that the microscopic mechanism for ferromagnetic
ordering is coupling mediated by free electrons between spins of Co atoms.
These results bring insight into a subtle interplay between charge carriers and
magnetism in MBE-grown Zn(1-x)CoxO films.Comment: 10 pages, 9 figures, 2 table
Disordered Correlated Kondo-lattice model
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 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 of
magnetic ions, the carrier concentration , the exchange coupling , and
the temperature. The shape and the effective range of the exchange integrals
turn out to be strongly -dependent. The disorder causes anomalies in the
spin spectrum especially in the low-dilution regime, which are not observed in
the mean field approximation.Comment: Accepted by JMM