K. Temst

Tuning Quantum Corrections and Magnetoresistance in ZnO Nanowires by Ion Implantation

Using ion implantation, the electrical as well as the magnetotransport properties of individual ZnO nanowires (NWs) can be tuned. The virgin NWs are configured as field-effect transistors which are in the enhancement mode. Al-implanted NWs reveal a three-dimensional metallic-like behavior, for which the magnetoresistance is well described by a semiempirical model that takes into account the presence of doping induced local magnetic moments and of two conduction bands. On the other hand, one-dimensional electron transport is observed in Co-implanted NWs. At low magnetic fields, the anisotropic magnetoresistance can be described in the framework of weak electron localization in the presence of strong spin-orbit scattering. From the weak localization, a large phase coherence length is inferred that reaches up to 800 nm at 2.5 K. The temperature-dependent dephasing is shown to result from a one-dimensional Nyquist noise-related mechanism. At the lowest temperatures, the phase coherence length becomes limited by magnetic scattering.

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.

Comparative study of the interface roughness of Ag/Au and Cu/Au multilayers with scanning tunneling microscopy and x-ray diffraction

The interface roughness is a very important parameter determining the performances of multilayered structures. Ag/Au and Cu/Au textured multilayers have been produced by thermal evaporation in an ultrahigh vacuum chamber. After deposition of the Au, Ag, or Cu layer, the surface roughness at different sampling lengths was probed in situ by scanning tunneling microscopy. After completion of the multilayers, the samples were taken from the ultrahigh vacuum chamber and measured in an x‐ray‐diffraction unit. Low‐angle as well as high‐angle specular x‐ray‐diffraction curves were analyzed using a refinement program which takes into account layer thickness fluctuations, interface roughness, and crystalline disorder. The average interface roughness, obtained from fitting the x‐ray‐diffraction profiles, is then compared to the roughness of the film growth front, which was probed with the scanning tunneling microscope. The roughness obtained from the x‐ray‐diffraction profiles corresponds to a lateral sampling scale...

Tuning the ferromagnetic-antiferromagnetic interfaces of granular Co-CoO exchange bias systems by annealing

The low-temperature magnetic behavior of granular Co-CoO exchange bias systems, prepared by oxygen ion implantation in Co thin films and subsequent annealing, is addressed. The thermal activation effects lead to an O migration which results in virtually pure Co areas embedded in a structurally relaxed and nearly stoichiometric CoO phase. This yields decreased training and exchange bias shifts, while the blocking temperature significantly increases, coming close to the Neel temperature of bulk CoO for samples implanted to a fluence above 1u2009×u20091017 ions/cm2 (15% O). The dependence of the exchange bias shift on the pristine O-implanted content is analogous to that of the antiferromagnetic thickness in most ferromagnetic/antiferromagnetic systems (i.e., an increase in the exchange bias shift up to a maximum followed by a decrease until a steady state is reached), suggesting that, after annealing, the enriched Co areas might be rather similar in size for samples implanted above 1u2009×u20091017 ions/cm2, whereas the co...

(Invited) Assessment of Ge1-xSnx Alloys for Strained Ge CMOS Devices

Strained Ge CMOS Devices S. Takeuchi, Y. Shimura, T. Nishimura, B. Vincent, G. Eneman, T. Clarysse, J. Demeulemeester, K. Temst, A. Vantomme, J. Dekoster, M. Caymax, R. Loo, O. Nakatsuka, A. Sakai, and S. Zaima Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Covalent Materials Co., Ltd., Higashikou, Seirou-machi, Kitakanbara-gun, Niigata 957-0197, Japan Imec, Kapeldreef 75, B-3001 Leuven, Belgium Instituut voor Kernen Stralingsfysica, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan

Fluence dependence of ion implantation-induced exchange bias in face centered cubic Co thin films

The fluence dependence of exchange bias induced by oxygen ion implantation has been studied in highly textured face centered cubic Co films. These films exhibit a strong magnetocrystalline anisotropy prior to implantation. Upon implantation, the crystalline order is strongly reduced, even for the lowest implantation fluence, as shown by an isotropic magnetic behavior. Detailed analysis of the structural changes shows that the crystallite size remains basically unaltered upon implantation, suggesting that CoxOy is formed at the Co grain boundaries. A large suppression of the magnetocrystalline anisotropy is observed after implantation. This anisotropy has no influence on the unidirectional anisotropy associated to the exchange bias effect. Our study identifies a narrow implantation fluence window in which exchange bias by oxygen ion implantation is established. With increasing oxygen fluence, an increase in the magnitude of the exchange bias effect for higher fluences and, finally, a saturation of the exch...

Tailoring Fe∕Ag superparamagnetic composites by multilayer deposition

Fe∕Ag granular multilayers were examined by magnetization and Mossbauer spectroscopy measurements. Very-thin (0.2 nm) discontinuous Fe layers show superparamagnetic properties that can be tailored by the thickness of both the magnetic and the spacer layers. Novel heterostructures, superparamagnetic and ferromagnetic layers stacked in different sequences, were prepared and the specific contribution of the ferromagnetic layers to the low-field magnetic susceptibility was identified.

In-plane vector magnetometry on rectangular Co dots using polarized neutron reflectivity

We have measured the off-specular polarized neutron reflectivity of a periodic array of rectangular polycrystalline magnetic Co dots, which were prepared by a combination of electron beam lithography and molecular beam deposition. The dots have strong shape anisotropy, imposed by a length-to-width ratio of 4:1. The intensity of the first-order off-specular satellite reflection was monitored as function of the magnetic field parallel to the rows of dots, allowing us to analyze the magnetization reversal process using the four spin-polarized scattering cross sections.

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 1u2009T at 2.5u2009K and the negative MR survives up to 250u2009K (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.