Annealing-induced changes of the magnetic anisotropy of (Ga,Mn)As epilayers

The dependence of the magnetic anisotropy of As-capped (Ga,Mn)As epilayers on the annealing parameters - temperature and time - has been investigated. A uniaxial magnetic anisotropy is evidenced, whose orientation with respect to the crystallographic axes changes upon annealing from [-110] for the as-grown samples to [110] for the annealed samples. Both cubic an uniaxial anisotropies are tightly linked to the concentration of charge carriers, the magnitude of which is controlled by the annealing process.Comment: 3 pages, 3 figure

Critical dynamics of an interacting magnetic nanoparticle system

Effects of dipole-dipole interactions on the magnetic relaxation have been investigated for three Fe-C nanoparticle samples with volume concentrations of 0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective behavior due to dipolar interactions, only the 17 vol% sample displays critical behavior close to its transition temperature. The behaviour of the 5 vol% sample can be attributed to a mixture of collective and single particle dynamics.Comment: 19 pages, 8 figure

Spin glass like transition in a highly concentrated Fe-C nanoparticle system

A highly concentrated (17 vol.%) Fe-C nano-particle system, with a narrow size distribution $d = 5.4\pm 0.4$ nm, has been investigated using magnetic ac susceptibility measurements covering a wide range of frequencies (17 mHz - 170 Hz). A dynamic scaling analysis gives evidence for a phase transition to a low temperature spin-glass-like phase. The critical exponents associated with the transition are $z\nu = 10.5 \pm 2$ and $\beta = 1.1 \pm 0.2$. The reason why the scaling analysis works for this sample, while it may not work for other samples exhibiting collective behavior as evidenced by aging phenomena, is that the single particle contribution to $\chi''$ is vanishingly small for $T>T_g$ and hence all slow dynamics is due to collective behavior. This criterion can only be fulfilled for a highly concentrated nano-particle sample with a narrow size distribution.Comment: 2 pages, 3 figures, Proceeding for ICM200

Influence of annealing parameters on the ferromagnetic properties of optimally passivated (Ga,Mn)As epilayers

The influence of annealing parameters - temperature and time - on the magnetic properties of As-capped (Ga,Mn)As epitaxial thin films have been investigated. The dependence of the transition temperature (Tc) on annealing time marks out two regions. The Tc peak behavior, characteristic of the first region, is more pronounced for thick samples, while for the second (`saturated') region the effect of the annealing time is more pronounced for thin samples. A right choice of the passivation medium, growth conditions along with optimal annealing parameters routinely yield Tc-values of ~ 150 K and above, regardless of the thickness of the epilayers.Comment: 5 pages, 3 figure

Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices

Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The structures contain magnetic (Ga,Mn)As layers, separated by thin layers of non-magnetic GaAs spacer. The short period Ga$_{0.93}$Mn$_{0.07}$As/GaAs superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to 60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For Ga$_{0.96}$Mn$_{0.04}$As/GaAs superlattices of similar dimensions, the Curie temperature associated with the ferromagnetic transition is found to oscillate with the thickness of non magnetic spacer. The observed oscillations are related to an interlayer exchange interaction mediated by the polarized holes of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure

Magnetic properties of GaMnAs single layers and GaInMnAs superlattices investigated at low temperature and high magnetic field

Magnetotransport properties of GaMnAs single layers and InGaMnAs/InGaAs superlattice structures were investigated at temperatures from 4 K to 300 K and magnetic fields up to 23 T to study the influence of carriers confinement through different structures. Both single layers and superlattice structures show paramagnetic-to-ferromagnetic phase transition. In GaMnAs/InGaAs superlattice beside the Curie temperature (Tc ~ 40 K), a new phase transition is observed close to 13 K.Comment: 8 pages, 5 figures, Proceedings of the XXXII International School on the Physics of Semiconducting Compounds, Jaszowiec 2003, Polan

Magnetotransport in a bi-crystal film of La_0.7Sr_0.3MnO_3

Transport properties of an epitaxial film of La_0.7Sr_0.3MnO_3 (LSMO), deposited epitaxially on a LaAlO_3 bi-crystal substrate having a misorientation angle of 9.2 deg., have been studied. The film was patterned into a meander containing 100 grain boundaries. The resistivity of the sample exhibits two components; one originating from the grain boundary regions, and one from the LSMO elements in the meander; the latter contribution is similar to the resistivity of a reference epitaxial LSMO film. The low (<0.5 T) and high (up to 6 T) field magnetoresistance was also studied. The meander show a large low field magnetoresistance, increasing with decreasing temperature, and a constant high field slope of the magnetoconductance, results that are well explained by a two-step spin polarized tunneling model.Comment: ICM2000 contribution - 6 pages, 3 figure

Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study

The dynamical behavior of the magnetism of diluted magnetic semiconductors (DMS) has been investigated by means of atomistic spin dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low concentration limit, although all simulations are done for 5% Mn-doped GaAs with various concentrations of As antisite defects. The magnetization curve, $M(T)$, and the Curie temperature $T_C$ have been calculated, and are found to be in good correspondence to results from Monte Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium behavior of the magnetic pair correlation function have been extracted. The dynamics of DMS systems reveals a substantial short ranged magnetic order even at temperatures at or above the ordering temperature, with a non-vanishing pair correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short ranged anti-ferromagnetic coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin-correlation develops very fast ($\sim$ 1ps) extending up to 15 atomic shells. Above $\sim$ 10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets like bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behaviour, for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet like bcc Fe.Comment: 24 pages, 15 figure

Fragility of the spin-glass-like collective state to a magnetic field in an interacting Fe-C nanoparticle system

The effect of applied magnetic fields on the collective nonequilibrium dynamics of a strongly interacting Fe-C nanoparticle system has been investigated. It is experimentally shown that the magnetic aging diminishes to finally disappear for fields of moderate strength. The field needed to remove the observable aging behavior increases with decreasing temperature. The same qualitative behavior is observed in an amorphous metallic spin glass (Fe_{0.15}Ni_{0.85})_{75}P_{16}B_6Al_3.Comment: 5 pages, 3 figure