Carolina Redondo

Magnetization reversal and exchange bias effects in hard/soft ferromagnetic bilayers with orthogonal anisotropies

The magnetization reversal processes are discussed for exchange- coupled ferromagnetic hard/soft bilayers made from Co0.66Cr0.22Pt0.12 (10 and 20nm)/Ni (from 0 to 40nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect,

IR/UV and UV/UV double-resonance study of guaiacol and eugenol dimers

Guaiacol (2-methoxyphenol) and eugenol (4-allyl-2-methoxyphenol) molecules are biologically active phenol derivatives with an intramolecular -OH...OCH3 hydrogen bond (H bond). Pulsed supersonic expansions of mixtures of either of the two molecules with He yield weakly bound homodimers as well as other higher-order complexes. A number of complementary and powerful laser spectroscopic techniques, including UV-UV and IR-UV double resonances, have been employed to interrogate the species formed in the expansion in order to get information on their structures and spectroscopic properties. The interpretation of the spectra of eugenol dimer is complex and required a previous investigation on a similar but simpler molecule both to gain insight into the possible structures and support the conclusions. Guaiacol (2-methoxyphenol) has been used for that purpose. The combination of the broad laser study combined with ab initio calculations at the Becke 3 Lee-Yang-Parr/6-31+Gd level has provided the isomer structures, the potential-energy wells, and shed light on the inter- and intramolecular interactions involved. Guaiacol homodimer has been shown to have a single isomer whereas eugenol dimer has at least two. The comparison between the computed geometries of the dimers, their respective energies, and the vibrational normal modes permits the identification of the spectra.

Photophysics of 1-Aminonaphthalene: A Theoretical and Time-Resolved Experimental Study

The photophysics of 1-aminonaphthalene (1-napthylamine, AMN) has been investigated on the basis of a constructive experimental-theoretical interplay derived from time-resolved measurements and high-level quantum-chemical ab initio CASPT2//CASSCF calculations. Transient ionization signals at femtosecond resolution were collected for AMN cold isolated molecules following excitation from the vibrationless ground level to a number of vibrational states (within the pump resolution) in the lowest accessible excited state and further multiphoton ionization probing at 500, 800, and 1300 nm. Theory predicts two pipi* states, (1)L(b) and (1)L(a), as the lowest singlet electronic excitations, with adiabatic transitions from S(0) at 3.50 and 3.69 eV, respectively. Since the associated oscillator strength for the lowest transition is exceedingly small, the (1)L(b) state is not expected to become populated significantly and the (1)L(a) state appears as the main protagonist of the AMN photophysics. Though calculations foresee a surface crossing between (1)L(a) and the lower (1)L(b) states, no dynamical signature of it is observed in the time-dependent measurements. In the relaxation of (1)L(a), the radiant emission competes with the intersystem crossing and internal conversion channels. The rates of these mechanisms have been determined at different excitation energies. The internal conversion is mediated by a (1)L(a)/S(0) conical intersection located 0.7 eV above the (1)L(a) minimum. The relaxation of a higher-lying singlet excited state, observed above 40 000 cm(-1) (4.96 eV) and calculated at 5.18 eV, has been also explored.

Absolute electron-impact total ionization cross sections of chlorofluoromethanes.

An experimental study is reported on the electron-impact total ionization cross sections (TICSs) of CCl4, CCl3F, CCl2F2, and CClF3 molecules. The kinetic energy of the colliding electrons was in the 10-85 eV range. TICSs were obtained as the sum of the partial ionization cross sections of all fragment ions, measured and identified in a linear double focusing time-of-flight mass spectrometer. The resulting TICS profiles--as a function of the electron-impact energy--have been compared both with those computed by ab initio and (semi)empirical methods and with the available experimental data. The computational methods used include the binary-encounter-Bethe (BEB) modified to include atoms with principal quantum numbers n> or =3, the Deutsch and Märk (DM) formalism, and the modified additivity rule (MAR). It is concluded that both modified BEB and DM methods fit the experimental TICS for (CF4), CClF3, CCl2F2, CCl3F, and CCl4 to a high accuracy, in contrast with the poor accord of the MAR method. A discussion on the factors influencing the discrepancies of the fittings is presented.

Collisionally induced intramultiplet mixing of metastable states by Ne, Kr and Xe atoms

Abstract An investigation is presented of the spin–orbit mixing within the Sr [5 s5p ( 3 P 0,1,2 )] manifold on collision with Ne, Kr and Xe at elevated temperatures following pulsed dye-laser generation of Sr ( 3 P 1 ) . The population profiles of all three states were monitored individually by laser-induced fluorescence and fitted to a kinetic model involving six collisional rates connected by detailed balance. The model also includes mixing by Sr ( 1 S 0 ) itself. The resulting rate constants, together with those for Sr ( 1 S 0 ) , Ar and He reported previously, are considered using a standard model employing the potential wells of the reactants and also a collision-complex model.

Collisionally induced intramultiplet mixing of Sr(53PJ) metastable states by He, Ar and Sr ground state atoms

Abstract Intramultiplet collisionally induced mixing within the Sr[5s5p(3P0,1,2)] manifold is investigated by time-resolved laser-induced fluorescence (LIF) following the pulsed dye-laser generation of Sr(3P1) of the electronic ground state {Sr[5s5p(3P0,1,2)]←Sr[5s2(1S0)], λ=689.26 nm} at elevated temperatures. The population profiles of the three spin–orbit states were individually monitored by LIF as well as that of Sr(3P1) by spontaneous emission at the resonance wavelength. A kinetic model is employed that enables the process of spontaneous emission from Sr(3P1) to be isolated initially and characterised by experiment. Particular emphasis is placed on the modelling procedure itself in which the separate kinetic component due to spontaneous emission and the positions of the maxima in the 3P0 and 3P2 population profiles constitute severe constraints on the model. The collisional components within the model are reduced to three rate constants where pairs of J states are connected in this context by detailed balance. Thus k10 and k12, and, by detailed balance, k01 and k21 are quantified at various temperatures and pressures to yield the absolute value of these collision properties for Sr(3PJ) with He and Ar. Rate data for collisionally induced intramultiplet mixing in Sr(3PJ) by Sr(1S0) itself is also reported found to proceed at close to unit collisional efficiencies in all cases. Thus, at elevated temperatures, variations in atomic profiles are dominated by the differing vapour pressures of atomic strontium. Estimates of the activation energies associated with k10 and k12 for the noble gases observed against this large competing background are found to be of the order of the spin–orbit splittings. The model overall is found to be insensitive to k02 and k20 whose magnitudes are small by comparison with those for the collisional rate data connecting adjacent J states. Whilst collisional processes for He are some two orders of magnitude more efficient than those for Ar, all of these are seen to be ‘adiabatic’, in contrast with the gas kinetic rate constants of ground Sr, considered to be ‘sudden’ in character. The results are compared with analogous data derived by atomic resonance absorption spectroscopy following pulsed generation of Sr(3P1) and are considered in the context of theoretical calculations employing quantum close coupling calculations.

Anisotropy Field in Ni Nanostripe Arrays

Anisotropy fields of nanostripe arrays can be investigated by various techniques. One of these techniques is the ferromagnetic resonance spectroscopy due to the fact that the resonance field depends directly on the anisotropy field strength and its angular spread. In order to understand the dynamic fenomena in these samples, nanostripe arrays of Ni have been prepared by interference lithography. The film thickness is 45 nm for a lattice period of 2.7 μm and the stripe width equals 1500 and 750 nm. Ferromagnetic resonance measurements have been carried out at a frequency of 9.75 GHz as a funcion of H. The resonance field of the absorption peak increases upon changing the angle from parallel (0°) to perpendicular (90° ) to the film plane. This behaviour can be explained by the relation between resonance field, frequency and anisotropy field determined by demagnetizing factors and magnetization angle with respect to the sample easy axis direction after energy minimization.

Collective switching of single-layer and exchange bias coupled nanomagnet arrays

The combined influence of shape anisotropy, dipolar interactions and exchange bias (EB) on the magnetization reversal of arrays of NiFe and NiFe/IrMn sub-micrometre elliptical magnetic elements has been investigated using magneto-optical Kerr effect microscopy and micromagnetic modelling. An applied field during film growth resulted in exchange fields around 50 Oe for the unpatterned films, micrometre sized stripes and nanomagnet arrays. High aspect ratio single-layer ellipses reverse via a single-domain-like switching while low aspect ratio structures reverse through the movement of a vortex core. The vortex formation and annihilation fields for closely spaced elliptical elements depend on the inter-element spacing and the aspect ratio. In particular, the stability range of the vortex state is lower for more strongly interacting or higher aspect ratio structures. On the other hand, a modest EB can significantly increase the stability of the vortex states and promote single-domain states that lead to different remanent configurations. A combination of EB, shape and inter-element spacing can therefore be used to tailor the behaviour of coupled nanomagnet arrays.

Engineering magnetic nanostructures with inverse hysteresis loops

Top-down lithography techniques allow the fabrication of nanostructured elements with novel spin configurations, which provide a new route to engineer and manipulate the magnetic response of sensors and electronic devices and understand the role of fundamental interactions in materials science. In this study, shallow nanostructure-patterned thin films were designed to present inverse magnetization curves, i.e., an anomalous magnetic mechanism characterized by a negative coercivity and negative remanence. This procedure involved a method for manipulating the spin configuration that yielded a negative coercivity after the patterning of a single material layer. Patterned NiFe thin films with trench depths between 15%–25% of the total film thickness exhibited inverse hysteresis loops for a wide angular range of the applied field and the trench axis. A model based on two exchange-coupled subsystems accounts for the experimental results and thus predicts the conditions for the appearance of this magnetic behavior. The findings of the study not only advance our understanding of patterning effects and confined magnetic systems but also enable the local design and control of the magnetic response of thin materials with potential use in sensor engineering.

Wideband Ferromagnetic Resonance in Nanostructured Arrays

Wideband ferromagnetic resonance at room temperature has been measured in a set of nanostructured samples: arrays of lines and antidots for Ni and permalloy-based samples. This technique allows us to determine both saturation magnetization and anisotropy constant providing valuable information about the final properties of the nanostructured material.