Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators

We study the magnetization dynamics in a nanocontact magnetic vortex oscillators as function of temperature. Low temperature experiments reveal that the dynamics at low and high currents differ qualitatively. At low currents, we excite a temperature independent standard oscillation mode, consisting in the gyrotropic motion of a free layer vortex about the nanocontact. Above a critical current, a sudden jump of the frequency is observed, concomitant with a substantial increase of the frequency versus current slope factor. Using micromagnetic simulation and analytical modeling, we associate this new regime to the creation of a vortex-antivortex pair in the pinned layer of the spin valve. The vortex-antivortex distance depends on the Oersted field which favors a separation, and on the exchange bias field, which favors pair merging. The pair in the pinned layer provides an additional spin torque altering the dynamics of the free layer vortex, which can be quantitatively accounted for by an analytical model

Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio

We study in-plane magnetic tunnel junctions with additional perpendicular polarizer for subnanosecond-current-induced switching memories. The spin-transfer-torque switching dynamics was studied as a function of the cell aspect ratio both experimentally and by numerical simulations using the macrospin model. We show that the anisotropy field plays a significant role in the dynamics, along with the relative amplitude of the two spin-torque contributions. This was confirmed by micromagnetic simulations. Real-time measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratios, a precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was studied. For high aspect ratios, we observed magnetization reversals in less than 1 ns for high enough current densities, the final state being controlled by the current direction in the magnetic tunnel junction cell.Comment: 6 pages, 7 figure

La infecunditat a Espanya : tic-tac, tic-tac, tic-tac!!!

Manca menys d'una dècada per a que les dones nascudes al 1975 assoleixin el mig segle d'existència i esdevinguin les més infecundes de totes les generacions nascudes a Espanya en els darrers 130 anys, període per al que disposem d'informació estadística. Molt probablement, una de cada quatre dones nascudes al 1975 no tindran fills quan arribin als 50 anys d'edat, moment en que des de la demografia es fa balanç de la descendència final de les generacions. Part d'aquesta infecunditat tindrà el seu origen a la frustració del projecte reproductiu d'aquestes dones i eventualment de les seves parelles. Aquesta dada, poc coneguda, s'ha anat covant any rere any per causes teòricament sabudes però difícils de discernir i quantificar. A societats, com la nostra, tan capficades per les conseqüències que l'envelliment de la població tindrà sobre la sostenibilitat de les pensions, la viabilitat d'un sistema de salut universal, l'augment de la dependència i les seves repercussions en la desigualtat de gènere o en la minva de la població en edat de treballar, sorprèn el poc debat que genera que una de cada quatre dones acabi per no tenir cap fill, en contra de les seves pròpies expectatives i desitjos. Per aquesta raó, el primer número de Perspectives Demogràfiques, una publicació científica sobre demografia destinada a tots els públics, està dedicat a les tendències i possibles causes de la infecunditat a Espanya.In less than a decade, women born in 1975 will be reaching half a century of existence as members of the most infertile of all cohorts born in Spain in the past 130 years, the period for which we have statistical information. In all likelihood, one in four women born in 1975 won't have had children when she turns fifty, the cut-off point at which demography appraises the completed fertility of a birth cohort. Part of this childlessness is due to frustration of the reproductive project of these women and, eventually, that of their partners. This little-known datum has been envisaged year after year because of causes that are known in theory but difficult to discern and quantify. In societies like ours, which are so concerned about population ageing and its consequences for sustainability of pensions, the viability of a universal health system, increased dependence and its repercussions on gender inequality, or the decline of the working-age population, it's surprising that there is so little discussion of the fact that, contrary to her own expectations and wishes, one in every four women will be childless. This is why the first number of Perspectives Demogràfiques, a scientific publication on demography addressed to the general public, is devoted to the trends and possible causes of infertility in Spain

Interrelaciones entre migración interna y fecundidad en España en la segunda mitad del siglo XX

Una versió d'aquest article es publicarà a la revista Quaderni del Dipartimento per lo Studio delle Società Mediterranee.Utilizando los datos de la Encuesta Sociodemográfica de 1991 del INE, se analizan los comportamientos de fecundidad en relación con las migraciones y, en concreto: a) se examina, para las personas que han realizado un movimiento migratorio entre regiones expulsoras y regiones de atracción, los cambios en su comportamiento de fecundidad ligados a la migración, des de una perspectiva de ciclo de vida; b) se compara el comportamiento de fecundidad de los migrantes con los sedentarios, tanto los de las regiones de origen como las de destino. Esto permite, entre otras cosas, determinar si hay efectos de selección en la emigración y si existen o no procesos de convergencia del comportamiento de fecundidad de los inmigrantes en relación con los sedentarios de las regiones de destino.Utilitzant les dades de l'Enquesta Sociodemogràfica de 1991 de l'INE, s'analitzen els comportaments de fecunditat en relació amb les migracions i, en concret: a) s'examina, per a les persones que han realitzat un moviment migratori entre regions d'origens i regions de destinació, els canvis en el comportament de fecunditat lligats a la migració, des d'una perspectiva de cicle de vida; b) es compara el comportament de fecunditat dels migrants amb els sedentaris, tant els de les regions d'origen com les de destinació. Això permet, entre altres coses, determinar si hi ha efectes de selecció en l'emigració i si hi ha o no processos de convergència del comportament de fecunditat dels immigrants en relació amb els sedentaris de les regions de destinació.Using data from the INE (National Institute of Statistics) 1991 Sociodemographic Survey, different kinds of fertility behaviour are examined in relation with migratory movements. Particular attention is given to a) changes in migration-linked fertility among people who have migrated between sending and receiving regions, as seen from a life-cycle perspective; and b) comparing the fertility behaviour of migrant and sedentary populations, both in the region of origin and in the receiving region. This makes it possible, inter alia, to determine whether there are selection effects in emigration, and whether or not there are processes of convergence in the fertility behaviour of immigrants in relation with the sedentary population in the receiving regions

Fecundidad diferencial y potencial de reagrupación familiar de la población extranjera de la Comunidad de Madrid a partir del padrón continuo a 1/1/1999

En la presente comunicación realizamos un análisis exploratorio del uso del Padrón continuo como fuente para el estudio diferencial de la fecundidad y también, algo más novedoso, del potencial de reagrupación familiar de la población de nacionalidad extranjera de la Comunidad de Madrid. Nuestro propósito es doble: llegar a un mejor conocimiento de los comportamientos y las características familiares de la población extranjera residente en esta comunidad, pero también evaluar las limitaciones del Padrón continuo de población para este tipo de estudios. Para el calculo de indicadores de fecundidad utilizamos el método de los hijos propios y para calcular el potencial de reagrupación familiar aplicamos un modelo basado en el examen de las pautas familiares por nacionalidad. La fuente principal analizada es el fichero informático del Padrón continuo a 1/1/1999.En la present comunicació es realitza un anàlisi exploratori de la utilitat del Padró continu com a font per a l'estudi diferencial de la fecunditat i com a base per avaluar el potencial de reagrupament familiar de la població de nacionalitat estrangera de la Comunitat de Madrid. El nostre objectiu és doble: arribar a un millor coneixement dels comportament i de les característiques familiars de la població estrangera resident a aquesta Comunitat, però també avaluar les limitacions, per aquest tipus d'estudis, del Padró continu. Per al càlcul dels indicadors de fecunditat utilitzem el mètode dels fills propis i per calcular el potencial de reagrupament familiar apliquem un model basat en l'examen de les pautes familiars per nacionalitat. La font principal és el fitxer informàtic del Padró continu a 1/1/1999.This is an exploratory analysis of the new Spanish permanent population register ("Padrón continuo") as a dataset useful for the study of differential fertility, and something less classical, of the family regrouping potential of the foreign population in the Madrid region. We have two aims: first measure as accurately as possible the fertility level and the family characteristics of foreign populations (total and by large groups of nationalities) living in that region and second discover what limitations the continuous population registers present for this kind of analysis. For the fertility analysis, our indicators are calculated using the own-children method, and for the family regrouping potential, we apply a model based on family pattern differences between the foreign and the Spanish population. The main dataset used in this study is a computer file from the population register on January 1 1999 for the Madrid region

Fast Primal-Dual Gradient Method for Strongly Convex Minimization Problems with Linear Constraints

In this paper we consider a class of optimization problems with a strongly convex objective function and the feasible set given by an intersection of a simple convex set with a set given by a number of linear equality and inequality constraints. A number of optimization problems in applications can be stated in this form, examples being the entropy-linear programming, the ridge regression, the elastic net, the regularized optimal transport, etc. We extend the Fast Gradient Method applied to the dual problem in order to make it primal-dual so that it allows not only to solve the dual problem, but also to construct nearly optimal and nearly feasible solution of the primal problem. We also prove a theorem about the convergence rate for the proposed algorithm in terms of the objective function and the linear constraints infeasibility.Comment: Submitted for DOOR 201

Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.Comment: submitted to Phys Rev.

Thermally nucleated magnetic reversal in CoFeB/MgO nanodots

Power consumption is the main limitation in the development of new high performance random access memory for portable electronic devices. Magnetic RAM (MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising candidate for reducing the power consumption given its non-volatile nature while achieving high performance. The dynamic properties and switching mechanisms of MTJs are critical to understanding device operation and to enable scaling of devices below 30 nm in diameter. Here we show that the magnetic reversal mechanism is incoherent and that the switching is thermally nucleated at device operating temperatures. Moreover, we find an intrinsic thermal switching field distribution arising on the sub-nanosecond time-scale even in the absence of size and anisotropy distributions or material defects. These features represent the characteristic signature of the dynamic properties in MTJs and give an intrinsic limit to reversal reliability in small magnetic nanodevices