L. M. C. Pereira

Searching for room temperature ferromagnetism in transition metal implanted ZnO and GaN

Significant progress in the field of wide-gap dilute magnetic semiconductors (DMS) depends on the discovery of a material system which not only shows high-temperature ferromagnetism but is also simple to prepare and thus easy to reproduce. In this context, ion implantation is an attractive doping method, being both relatively simple and highly reproducible. Here, we report on the search for high-temperature ferromagnetism in ZnO and GaN implanted with Mn, Fe, and Co, prepared under a wide range of implantation and post-processing conditions. We focused on the low concentration regime (∼0.3−4%) in order to avoid phase segregation and applied strict experimental procedures to avoid ferromagnetic contamination. Despite the wide range of materials, implantation and post-processing conditions, none of the DMS systems showed room-temperature ferromagnetism. These results support the view that dilute transition-metal moments do not order ferromagnetically in ZnO and GaN.

Relaxor Ferroelectricity and Magnetoelectric Coupling in ZnO–Co Nanocomposite Thin Films: Beyond Multiferroic Composites

ZnO-Co nanocomposite thin films are synthesized by combination of pulsed laser deposition of ZnO and Co ion implantation. Both superparamagnetism and relaxor ferroelectricity as well as magnetoelectric coupling in the nanocomposites have been demonstrated. The unexpected relaxor ferroelectricity is believed to be the result of the local lattice distortion induced by the incorporation of the Co nanoparticles. Magnetoelectric coupling can be attributed to the interaction between the electric dipole moments and the magnetic moments, which are both induced by the incorporation of Co. The introduced ZnO-Co nanocomposite thin films are different from conventional strain-mediated multiferroic composites.

Paramagnetism and antiferromagnetic interactions in single-phase Fe-implanted ZnO

As the intrinsic origin of the high-temperature ferromagnetism often observed in wide-gap dilute magnetic semiconductors becomes increasingly debated, there is a growing need for comprehensive studies on the single-phase region of the phase diagram of these materials. Here we report on the magnetic and structural properties of Fe-doped ZnO prepared by ion implantation of ZnO single crystals. A detailed structural characterization shows that the Fe impurities substitute for Zn in ZnO in a wurtzite Zn(1-x)Fe(x)O phase which is coherent with the ZnO host. In addition, the density of beam-induced defects is progressively decreased by thermal annealing up to 900 ° C, from highly disordered after implantation to highly crystalline upon subsequent annealing. Based on a detailed analysis of the magnetometry data, we demonstrate that isolated Fe impurities occupying Zn-substitutional sites behave as localized paramagnetic moments down to 2 K, irrespective of the Fe concentration and the density of beam-induced defects. With increasing local concentration of Zn-substitutional Fe, strong nearest-cation-neighbor antiferromagnetic interactions favor the antiparallel alignment of the Fe moments.

Anisotropic magnetism and spin-dependent transport in Co nanoparticle embedded ZnO thin films

Oriented Co nanoparticles were obtained by Co ion implantation in crystalline ZnO thin films grown by pulsed laser deposition. Transmission electron microscopy revealed the presence of elliptically shaped Co precipitates with nanometer size, which are embedded in the ZnO thin films, resulting in anisotropic magnetic behavior. The low-temperature resistance of the Co-implanted ZnO thin films follows the Efros-Shklovskii type variable-range-hopping. Large negative magnetoresistance (MR) exceeding 10% is observed in a magnetic field of 1 T at 2.5 K and the negative MR survives up to 250 K (0.3%). The negative MR reveals hysteresis as well as anisotropy that correlate well with the magnetic properties, clearly demonstrating the presence of spin-dependent transport.

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As.

Precise lattice location of substitutional and interstitial Mg in AlN

The lattice site location of radioactive 27Mg implanted in AlN was determined by means of emission channeling. The majority of the 27Mg was found to substitute for Al, yet significant fractions (up to 33%) were also identified close to the octahedral interstitial site. The activation energy for interstitial Mg diffusion is estimated to be between 1.1 eV and 1.7 eV. Substitutional Mg is shown to occupy ideal Al sites within a 0.1 A experimental uncertainty. We discuss the absence of significant displacements from ideal Al sites, in the context of the current debate, on Mg doped nitride semiconductors.

The 68mCu/68Cu isotope as a new probe for hyperfine studies: The nuclear moments

Time Differential Perturbed Angular Correlation of γ-rays (TDPAC) experiments were performed for the first time in the decay of 68m Cu produced at the ISOLDE facility at CERN. Due to the short half-life of the source isotope, the measurements were carried out online. The intermediate state offers the unique opportunity to study the electromagnetic fields acting at a copper probe in condensed matter via hyperfine interactions. The present work allowed determination of the nuclear moments for this state. The electric quadrupole moment was obtained from an experiment performed in Cu2O and the magnetic dipole moment from measurements in cobalt and nickel foils. The results are discussed in the framework of shell model calculations and the additivity rule for nuclear moments with respect to the robustness of the N = 40 sub-shell.

Multipurpose setup for low-temperature conversion electron Mössbauer spectroscopy

We describe an experimental setup for conversion electron Mössbauer spectroscopy (CEMS) at low temperature. The setup is composed of a continuous flow cryostat (temperature range of 4.2-500 K), detector housing, three channel electron multipliers, and corresponding electronics. We demonstrate the capabilities of the setup with CEMS measurements performed on a sample consisting of a thin enriched 57Fe film, with a thickness of 20 nm, deposited on a silicon substrate. We also describe exchangeable adaptations (lid and sample holder) which extend the applicability of the setup to emission Mössbauer spectroscopy as well as measurements under an applied magnetic field.

Emission Channeling with Short-Lived Isotopes (EC-SLI) at CERN’s ISOLDE Facility

We give an overview on the historical development and current program for lattice location studies at CERN’s ISOLDE facility, where the EC-SLI (Emission Channeling with Short-Lived Isotopes) collaboration maintains several setups for this type of experiments. We illustrate that the three most decisive factors for the success of the technique are access to facilities producing radioactive isotopes, position-sensitive detectors for the emitted decay particles, and reliable simulation codes which allow for quantitative analysis.