Ferran Macia - Academia.edu (original) (raw)

Papers by Ferran Macia

Spin waves in ferromagnets are coherent dispersive waves, with typical frequencies in the low GHz... more Spin waves in ferromagnets are coherent dispersive waves, with typical frequencies in the low GHz regime and wavelengths from hundreds of nanometers to a few micrometers. Recent interest for spin waves is motivated by the possibility of their integration in nano-scale devices for high-speed and low-power signal processing. However, generation of spin waves with high amplitudes---and their detection---is challenging due to the mismatch of wavelengths with electromagnetic waves in free space, which is of the order of several centimeters. Using hybrid piezoelectric/magnetic systems we have generated large amplitude spin waves mediated by magneto-elasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating spin waves with different wavelengths, over large distances up to several millimeters, orders of magnitude longer than previously achieved.

Physical Review B, 2019

Droplet solitons are large amplitude localized spin-wave excitations that can be created in perpe... more Droplet solitons are large amplitude localized spin-wave excitations that can be created in perpendicularly magnetized thin films by a spin-polarized current flowing through an electrical nanocontact. Here, we report a low temperature (4 K) experimental study that shows there are multiple and, under certain conditions, combinations of droplet modes, each mode with a distinct high-frequency spin precession (tens of GHz). Low frequency (1 GHz) voltage noise is used to assess the stability of droplet modes. It is found that droplets are stable only in a limited range of applied field and currents, typically near the current and field at which they nucleate, in agreement with recent predictions. Applied fields in the film plane favor multiple droplet modes, whereas fields perpendicular to the film plane tend to stabilize a single droplet mode. Micromagnetic simulations are used to show that spatial variation in the energy landscape in the nanocontact region (e.g. spatial variation of magnetic anisotropy or magnetic field) can lead to quantized droplet modes and low frequency mode modulation, characteristics observed in our experiments.

Nanotechnology, 2018

A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscil... more A spin-polarized current in a nanocontact to a magnetic film can create collective magnetic oscillations by compensating the magnetic damping. In particular, in materials with uniaxial magnetic anisotropy, droplet solitons have been observed a self-localized excitation consisting of partially reversed magnetization that precesses coherently in the nanocontact region. It is also possible to generate topological droplet solitons, known as dynamical skyrmions. Here we study the conditions that promote either droplet or dynamical skyrmion formation and describe their stability in magnetic films without Dzyaloshinskii-Moriya interactions. We show that Oersted fields from the applied current as well as the initial magnetization state can determine whether a droplet or dynamical skyrmion forms. Dynamical skyrmions are found to be more stable than droplets. We also discuss electrical characteristics that can be used distinguish these magnetic objects.

Journal of Physics: Condensed Matter, 2019

Journal of Synchrotron Radiation, 2018

The quantification of surface acoustic waves (SAWs) in LiNbO3 piezoelectric crystals by strobosco... more The quantification of surface acoustic waves (SAWs) in LiNbO3 piezoelectric crystals by stroboscopic X-ray photoemission electron microscopy (XPEEM), with a temporal smearing below 80 ps and a spatial resolution below 100 nm, is reported. The contrast mechanism is the varying piezoelectric surface potential associated with the SAW phase. Thus, kinetic energy spectra of photoemitted secondary electrons measure directly the SAW electrical amplitude and allow for the quantification of the associated strain. The stroboscopic imaging combined with a deliberate detuning allows resolving and quantifying the respective standing and propagating components of SAWs from a superposition of waves. Furthermore, standing-wave components can also be imaged by low-energy electron microscopy (LEEM). Our method opens the door to studies that quantitatively correlate SAWs excitation with a variety of sample electronic, magnetic and chemical properties.

Subnanosecond magnetization dynamics driven by strain waves

MRS Bulletin, 2018

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Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2015

Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitti... more Until recently the important role that spin-physics (‘spintronics’) plays in organic light-emitting devices and photovoltaic cells was not sufficiently recognized. This attitude has begun to change. We review our recent work that shows that spatially rapidly varying local magnetic fields that may be present in the organic layer dramatically affect electronic transport properties and electroluminescence efficiency. Competition between spin-dynamics due to these spatially varying fields and an applied, spatially homogeneous magnetic field leads to large magnetoresistance, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. Spatially rapidly varying local magnetic fields are naturally present in many organic materials in the form of nuclear hyperfine fields, but we will also review a second method of controlling the electrical conductivity/electroluminescence, using the spatially varying magnetic fringe f...

New Journal of Physics, 2017

Scientific Reports, 2016

The collective dynamics in populations of magnetic spin torque oscillators (STO) is an intensely ... more The collective dynamics in populations of magnetic spin torque oscillators (STO) is an intensely studied topic in modern magnetism. Here, we show that arrays of STO coupled via dipolar fields can be modeled using a variant of the Kuramoto model, a well-known mathematical model in non-linear dynamics. By investigating the collective dynamics in arrays of STO we find that the synchronization in such systems is a finite size effect and show that the critical coupling—for a complete synchronized state—scales with the number of oscillators. Using realistic values of the dipolar coupling strength between STO we show that this imposes an upper limit for the maximum number of oscillators that can be synchronized. Further, we show that the lack of long range order is associated with the formation of topological defects in the phase field similar to the two-dimensional XY model of ferromagnetism. Our results shed new light on the synchronization of STO, where controlling the mutual synchroniz...

Nature Communications, 2017

The magnetoelastic effect—the change of magnetic properties caused by the elastic deformation of ... more The magnetoelastic effect—the change of magnetic properties caused by the elastic deformation of a magnetic material—has been proposed as an alternative approach to magnetic fields for the low-power control of magnetization states of nanoelements since it avoids charge currents, which entail ohmic losses. Here, we have studied the effect of dynamic strain accompanying a surface acoustic wave on magnetic nanostructures in thermal equilibrium. We have developed an experimental technique based on stroboscopic X-ray microscopy that provides a pathway to the quantitative study of strain waves and magnetization at the nanoscale. We have simultaneously imaged the evolution of both strain and magnetization dynamics of nanostructures at the picosecond time scale and found that magnetization modes have a delayed response to the strain modes, adjustable by the magnetic domain configuration. Our results provide fundamental insight into magnetoelastic coupling in nanostructures and have implicat...

Journal of Applied Physics, 2015

We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnet-normal... more We investigate the effect of eddy currents on ferromagnetic resonance (FMR) in ferromagnet-normal metal (FM/NM) bilayer structures. Eddy-current effects are usually neglected for NM layer thicknesses below the microwave (MW) skin depth (≃800 nm for Au at 10 GHz). However, we show that in much thinner NM layers (10–100 nm of Au or Cu) they induce a phase shift in the FMR excitation when the MW driving field has a component perpendicular to the sample plane. This results in a strong asymmetry of the measured absorption lines. In contrast to typical eddy-current effects, the asymmetry is larger for thinner NM layers and is tunable through changing the sample geometry and the NM layer thickness.

Low frequency dynamics of Magnetic Droplet Solitons in Spin Transfer Nanocontacts

In this work we describe experiments in which we have used surface acoustic waves to induce contr... more In this work we describe experiments in which we have used surface acoustic waves to induce controlled magnetic avalanches in (La, Pr)-based manganites. The avalanches propagate inside the sample following the law of the magnetic deflagration, and occur at well determined values of the temperature and the applied magnetic field, that depend on the phase separation fraction. Another important point

Revista de física, 2007

Bros (Sabadell, 1980) és llicenciat en matemàtiques i enginyer en telecomunicacions. Està estudia... more Bros (Sabadell, 1980) és llicenciat en matemàtiques i enginyer en telecomunicacions. Està estudiant el doctorat a la Universitat de Barcelona.

Physical Review B, 2013

Random hyperfine fields are essential to mechanisms of low-field magnetoresistance in organic sem... more Random hyperfine fields are essential to mechanisms of low-field magnetoresistance in organic semiconductors. Recent experiments have shown that another type of random field-fringe fields due to a nearby ferromagnet-can also dramatically affect the magnetoresistance. A theoretical analysis of the effect of these fringe fields is challenging, as the fringe field magnitudes and their correlation lengths are orders of magnitude larger than that of the hyperfine couplings. We extend a recent theory of organic magnetoresistance to calculate the magnetoresistance with both hyperfine and fringe fields present. This theory describes several key features of the experimental fringe-field magnetoresistance, including the applied fields where the magnetoresistance reaches extrema, the applied field range of large magnetoresistance effects from the fringe fields, and the sign of the effect.

Nature Communications, 2014

Magnetic and spin-based technologies for data storage and processing provide unique challenges fo... more Magnetic and spin-based technologies for data storage and processing provide unique challenges for information transduction to light because of magnetic metals' optical loss, and the inefficiency and resistivity of semiconductor spin-based emitters at room temperature. Transduction between magnetic and optical information in typical organic semiconductors poses additional challenges, as the spin-orbit interaction is weak and spin injection from magnetic electrodes has been limited to low temperature and low polarization efficiency. Here we demonstrate room temperature information transduction between a magnet and an organic light-emitting diode that does not require electrical current, based on control via the magnet's remanent field of the exciton recombination process in the organic semiconductor. This demonstration is explained quantitatively within a theory of spin-dependent exciton recombination in the organic semiconductor, driven primarily by gradients in the remanent fringe fields of a few nanometre-thick magnetic film.

Nanotechnology, 2014

Spin torque nano oscillators (STNO) are nano-scale devices that can convert a direct current into... more Spin torque nano oscillators (STNO) are nano-scale devices that can convert a direct current into short wavelength spin-wave excitations in a ferromagnetic layer. We show that arrays of STNO can be used to create directional spin-wave radiation similar to electromagnetic antennas. Combining STNO excitations with planar spin waves also creates interference patterns. We show that these interference patterns are static and have information on the wavelength and phase of the spin waves emitted from the STNO. We describe means of actively controlling spin-wave radiation patterns with the direct current flowing through STNO, which is useful in on-chip communication and information processing and could be a promising technique for studying short wavelength spin waves in different materials.

Journal of Physics: Condensed Matter, 2008

The structure and magnetic properties of the compounds Zn 1−x Cu x Cr 2 O 4 (ZCCO) are investigat... more The structure and magnetic properties of the compounds Zn 1−x Cu x Cr 2 O 4 (ZCCO) are investigated systematically. A structural phase transition from space-group symmetry Fd3m to I 4 1 /amd is observed in ZCCO. The critical value of the doping, x, appears at 0.58-0.62 through the appearance of a splitting of diffraction peaks at room temperature. Measurements of dc magnetization, ac susceptibility, memory effect and exchange-bias-like (EB-like) effect have been performed to reveal the glassy magnetic behaviors of ZCCO. The system with x 0.50 suggests a spin glass-like (SG-like) magnetic characterization whereas doping values of 0.58 x 0.90 define the system as 'cluster-glass-like' (CG-like) with unidirectional anisotropy. The Cu content suppresses the geometrical frustration of ZnCr 2 O 4 , which may correlate with the pinning effect of the Cu sublattice on the Cr sublattice to a preferential direction.