Juan Pedro Cascales

Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films

The time dependent transient lateral photovoltaic effect has been studied with μs time resolution and with chopping frequencies in the kHz range, in lithographically patterned 21 nm thick, 5, 10, and 20 μm wide, and 1500 μm long Co lines grown over naturally passivated p-type Si (100). We have observed a nearly linear dependence of the transitorial response with the laser spot position. A transitorial response with a sign change in the laser-off stage has been corroborated by numerical simulations. A qualitative explanation suggests a modification of the drift-diffusion model by including the influence of a local inductance. Our findings indicate that the microstructuring of position sensitive detectors could improve their space-time resolution.

Controlling Shot Noise in Double-Barrier Magnetic Tunnel Junctions

We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic tunnel junctions is determined by the relative magnetic configuration of the junction and also by the asymmetry of the barriers. The proposed theoretical model, based on sequential tunneling through the system and including spin relaxation, successfully accounts for the experimental observations for bias voltages below 0.5 V, where the influence of quantum well states is negligible. A weak enhancement of conductance and shot noise, observed at some voltages (especially above 0.5 V), indicates the formation of quantum well states in the middle magnetic layer. The observed results open up new perspectives for a reliable magnetic control of the most fundamental noise in spintronic structures.

Magnetization reversal in sub-100 nm magnetic tunnel junctions with ultrathin MgO barrier biased along the hard axis

We report on room temperature magnetoresistance and low frequency noise in sub-100 nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin (0.9 nm) barriers. For magnetic fields applied along the hard axis, we observe current induced magnetization switching between the antiparallel and parallel alignments at dc current densities as low as 4 × 106 A/cm2. We attribute the low value of the critical current to the influence of localized reductions in the tunnel barrier, which affects the current distribution. The analysis of random telegraph noise, which appears in the field interval near a magnetization switch, provides an estimate to the dimension of the pseudo pinholes that trigger the magnetization switching via local spin torque. Micromagnetic simulations qualitatively and quantitatively reproduce the main experimental observations.

Superpoissonian shot noise in organic magnetic tunnel junctions

Organic molecules have recently revolutionized ways to create new spintronic devices. Despite intense studies, the statistics of tunneling electrons through organic barriers remains unclear. Here, we investigate conductance and shot noise in magnetic tunnel junctions with 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) barriers a few nm thick. For junctions in the electron tunneling regime, with magnetoresistance ratios between 10% and 40%, we observe superpoissonian shot noise. The Fano factor exceeds in 1.5–2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in tunneling. We explain our main findings in terms of a model which includes tunneling through a two level (or multilevel) system, originated from interfacial bonds of the PTCDA molecules. Our results suggest that interfaces play an important role in the control of shot noise when electrons tunnel through organic barriers.

Band structure of topological insulators from noise measurements in tunnel junctions

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al2O3/Co tunnel junctions with bottom TI electrodes of either Bi2Te3 or Bi2Se3. We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes.

Magnetic state dependent transient lateral photovoltaic effect in patterned ferromagnetic metal-oxide-semiconductor films

We investigate the influence of an external magnetic field on the magnitude and dephasing of the transient lateral photovoltaic effect (T-LPE) in lithographically patterned Co lines of widths of a few microns grown over naturally passivated p-type Si(100). The T-LPE peak-to-peak magnitude and dephasing, measured by lock-in or through the characteristic time of laser OFF exponential relaxation, exhibit a notable influence of the magnetization direction of the ferromagnetic overlayer. We show experimentally and by numerical simulations that the T-LPE magnitude is determined by the Co anisotropic magnetoresistance. On the other hand, the magnetic field dependence of the dephasing could be described by the influence of the Lorentz force acting perpendiculary to both the Co magnetization and the photocarrier drift directions. Our findings could stimulate the development of fast position sensitive detectors with magnetically tuned magnitude and phase responses.

Band-Edge Noise Spectroscopy of a Magnetic Tunnel Junction

We propose a conceptually new way to gather information on the electron bands of buried metal-(semiconductor-) insulator interfaces. The bias dependence of low frequency noise in Fe1-xVx/MgO/Fe(0 < x < 0.25) tunnel junctions shows clear anomalies at specific applied voltages, reflecting electron tunneling to the band edges of the magnetic electrodes. The change in magnitude of these noise anomalies with the magnetic state allows evaluating the degree of spin mixing between the spin polarized bands at the ferromagnet-insulator interface. Our results are in qualitative agreement with numerical calculations.

Transient lateral photovoltaic effect in patterned ferromagnetic metal-oxide-semiconductor films

The time dependent transient lateral photovoltaic effect (T-LPE) has been studied with microsecond time resolution and with chopping frequencies in the kHz range, in lithographically patterned 21 nm thick, 5, 10 and 20 micron wide and 1500 micron long Co lines grown over naturally passivated p-type Si (100). We have observed a nearly linear dependence of the LPE transient response with the laser spot position. An unusual T-LPE dynamic response with a sign change in the laser-off stage has also been corroborated by numerical simulations. A qualitative explanation suggests a modification of the drift-diffusion model by including the influence of a local inductance. In addition, influence of anisotropic magnetoresistance of the Co line structure on dynamic response on T-LPE has been investigated. Specifically, we have experimentally investigated influence of the direction of the external magnetic field respect to the drift velocity of the photogenerated carriers on the T-LPE. We have observed notable dependence of the T-LPE on the magnetic field in the small field range (below 100 Oe), compatible with anisotropic magnetoresistance values. The strong influence of the magnetization alignment on the dynamic response of photogenerated carriers has been also observed through a phase sensitive lock-in experiment. These findings indicate that the microstructuring of the ferromagnetic line based position sensitive detectors (PSD) could improve their space-time resolution and add capability of magnetic field tuning of the main PSD characteristics.

Electron transport and noise spectroscopy in organic magnetic tunnel junctions with PTCDA and Alq3 barriers

The possible influence of internal barrier dynamics on spin, charge transport and their fluctuations in organic spintronics remains poorly understood. Here we present investigation of the electron transport and low frequency noise at temperatures down to 0.3K in magnetic tunnel junctions with an organic PTCDA barriers with thickness up to 5 nm in the tunneling regime and with 200 nm thick Alq3 barrier in the hopping regime. We observed high tunneling magneto-resistance at low temperatures (15-40%) and spin dependent super-poissonian shot noise in organic magnetic tunnel junctions (OMTJs) with PTCDA. The Fano factor exceeds 1.5-2 values which could be caused by interfacial states controlled by spin dependent bunching in the tunneling events through the molecules.1 The bias dependence of the low frequency noise in OMTJs with PTCDA barriers which includes both 1/f and random telegraph noise activated at specific biases will also be discussed. On the other hand, the organic junctions with ferromagnetic electrodes and thick Alq3 barriers present sub-poissonian shot noise which depends on the temperature, indicative of variable range hopping.

Detection of spin torque magnetization dynamics through low frequency noise

We present a comparative study of high frequency dynamics and low frequency noise in elliptical magnetic tunnel junctions with lateral dimensions under 100 nm presenting current-switching phenomena. The analysis of the high frequency oscillation modes with respect to the current reveals the onset of a steady-state precession regime for negative bias currents above J=107A/cm2, when the magnetic field is applied along the easy axis of magnetization. By the study of low frequency noise for the same samples, we demonstrate the direct link between changes in the oscillation modes with the applied current and the normalised low frequency (1/f) noise as a function of the bias current. These findings prove that low frequency noise studies could be a simple and powerful technique to investigate spin-torque based magnetization dynamics.