Antonio Ruotolo

Phase-locking of magnetic vortices mediated by antivortices

Synchronized spin-valve oscillators may lead to nanosized microwave generators that do not require discrete elements such as capacitors or inductors. Uniformly magnetized oscillators have been synchronized, but offer low power. Gyrating magnetic vortices offer greater power, but vortex synchronization has yet to be demonstrated. Here we find that vortices can interact with each other through the mediation of antivortices, leading to synchronization when they are closely spaced. The synchronization does not require a magnetic field, making the system attractive for electronic device integration. Also, because each vortex is a topological soliton, this work presents a model experimental system for the study of interacting solitons.

Effect of the magnetic order on the room-temperature band-gap of Mn-doped ZnO thin films

Exchange interaction between localized magnetic moments mediated by free charge carriers is responsible for a non-monotonic dependence of the low-temperature energy band-gap in dilute magnetic semiconductors. We found that in weakly doped Mn-ZnO films, increasing the exchange interaction by increasing the concentration of free charge carriers results in a red-shift of the near-band-edge emission peak at room temperature. An increase of Mn concentration widens the band gap, and a blue-shift prevails. Exchange interaction can be used to tune the room-temperature optical properties of the wide-band gap semiconductor ZnO.

Non-volatile, reversible switching of the magnetic moment in Mn-doped ZnO films

We report on the observation of a non-volatile, reversible switching of the magnetic moment in Mn-doped ZnO thin films. The system is a typical oxide memristor based on an oxygen-deficient semiconductor oxide. In the present study, the oxide semiconductor is ferromagnetic at room temperature. We found that the bistable switching of the resistive state was accompanied by a bistable switching of the magnetic moment at room temperature. Our results support the hypothesis that ferromagnetism in Mn-doped ZnO is mediated by oxygen-vacancies.

Non-volatile, electric control of magnetism in Mn-substituted ZnO

We show that the magnetic properties of a dilute semiconductor oxide can be altered in a reversible and non-volatile manner by the application of an electric field. The selected ferromagnetic oxide was manganese-substituted zinc oxide. Bipolar resistive memory switching was induced in the film sandwiched between two metallic electrodes. The bistable switching of the resistive state was accompanied by a bistable switching of the magnetic moment. The scalability of the system was investigated by fabricating devices with lateral size down to 400 nm.

Magnetism as a probe of the origin of memristive switching in p-type antiferromagnetic NiO

We induced bipolar resistive switching in p-type nickel oxide. By probing the magnetic properties of the films, we proved that bipolar resistive switching in this antiferromagnetic oxide was due to the formation and rupture of oxygen-vacancy filaments, rather than electrochemical growth and dissolution of nickel-ion filaments. In the low resistive state, oxygen-mediated super-exchange interaction was suppressed along the conductive paths. This led to a reduction of the saturation moment but not the appearance of a ferromagnetic phase, excluding the formation of nickel filaments.

Perpendicular magnetic anisotropy and structural properties of NiCu/Cu multilayers

Perpendicular magnetic anisotropy (PMA) was studied at low temperature (T=30 K) in dc-magnetron sputtered Ni60Cu40/Cu multilayers. PMA has been observed in many multilayer structures for ferromagnetic layer thicknesses less than a certain thickness t⊥. In general cases t⊥ is less than a few nanometers, making such structures unsuitable for low-cost fabrication techniques. Our results show a strong perpendicular easy direction of magnetization for NiCu layer thickness between 4.2 nm and 34 nm. The thickness t⊥ at which the multilayers change the preferential orientation from perpendicular to in-plane is estimated to be 55 nm. Structural studies show that the low magnetostatic energy density is likely to be the main reason for the large t⊥ value obtained in this system.

Chemical states and ferromagnetism in heavily Mn-substituted zinc oxide thin films

A concentration of Manganese as high as 8% was successfully diluted into Zinc Oxide epitaxial films deposited by pulsed laser deposition. The films showed strong ferromagnetism with a large coercivity. Low temperature X-ray absorption spectroscopy measurements indicated that all the Manganese ions substitute for Zinc sites of the wurtzite lattice in the valency of +2. Photoluminescence measurements excluded the presence of Zinc vacancies, as well as Zn interstitials. All the magnetic moments measured were to ascribe to the formation of bound magnetic polarons, with no other contribution due to Manganese-secondary phases or Zinc vacancy-mediated double exchange interaction.

Giant negative magnetoresistance in Manganese-substituted Zinc Oxide

We report a large negative magnetoresistance in Manganese-substituted Zinc Oxide thin films. This anomalous effect was found to appear in oxygen-deficient films and to increase with the concentration of Manganese. By combining magnetoresistive measurements with magneto-photoluminescence, we demonstrate that the effect can be explained as the result of a magnetically induced transition from hopping to band conduction where the activation energy is caused by the sp-d exchange interaction.

Ferromagnetism in Ti-doped ZnO thin films

We report our study on the origin of ferromagnetism in Ti-doped ZnO. A series of Ti doped ZnO films with increasing concentration of Ti dopant were grown and characterized in terms of structural, electrical, and magnetic properties. We found that Ti has a low solubility in the ZnO wurtzite structure. This favors stabilization of a large number of Zn vacancies, and theoretical calculations have shown that they can carry substantial magnetic moment. A carrier mediated exchange interaction between Zn vacancies is at the origin of the surprisingly high magnetic moment we measure in this compound.

Magnetic-Polaron-Induced Enhancement of Surface Raman Scattering

The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications.