Electrical detection of spin pumping: dc voltage generated by ferromagnetic resonance at ferromagnet/nonmagnet contact

We describe electrical detection of spin pumping in metallic nanostructures. In the spin pumping effect, a precessing ferromagnet attached to a normal-metal acts as a pump of spin-polarized current, giving rise to a spin accumulation. The resulting spin accumulation induces a backflow of spin current into the ferromagnet and generates a dc voltage due to the spin dependent conductivities of the ferromagnet. The magnitude of such voltage is proportional to the spin-relaxation properties of the normal-metal. By using platinum as a contact material we observe, in agreement with theory, that the voltage is significantly reduced as compared to the case when aluminum was used. Furtheremore, the effects of rectification between the circulating rf currents and the magnetization precession of the ferromagnet are examined. Most significantly, we show that using an improved layout device geometry these effects can be minimized.Comment: 9 pages, 11 figure

Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2

A key aspect of unconventional pairing by the antiferromagnetic spin-fluctuation mechanism is that the superconducting energy gap must have opposite sign on different parts of the Fermi surface. Recent observations of non-nodal gap structure in the heavy-fermion superconductor CeCu$_2$Si$_2$ were then very surprising, given that this material has long been considered a prototypical example of a superconductor where the Cooper pairing is magnetically mediated. Here we present a study of the effect of controlled point defects, introduced by electron irradiation, on the temperature-dependent magnetic penetration depth $\lambda(T)$ in CeCu$_2$Si$_2$. We find that the fully-gapped state is robust against disorder, demonstrating that low-energy bound states, expected for sign-changing gap structures, are not induced by nonmagnetic impurities. This provides bulk evidence for $s_{++}$-wave superconductivity without sign reversal.Comment: 5 pages, 4 figures + Supplemental Material (1 page, 1 figure). Will appear in Phys. Rev. Let

Nonlocal magnetization dynamics in ferromagnetic heterostructures

Two complementary effects modify the GHz magnetization dynamics of nanoscale heterostructures of ferromagnetic and normal materials relative to those of the isolated magnetic constituents: On the one hand, a time-dependent ferromagnetic magnetization pumps a spin angular-momentum flow into adjacent materials and, on the other hand, spin angular momentum is transferred between ferromagnets by an applied bias, causing mutual torques on the magnetizations. These phenomena are manifestly nonlocal: they are governed by the entire spin-coherent region that is limited in size by spin-flip relaxation processes. We review recent progress in understanding the magnetization dynamics in ferromagnetic heterostructures from first principles, focusing on the role of spin pumping in layered structures. The main body of the theory is semiclassical and based on a mean-field Stoner or spin-density--functional picture, but quantum-size effects and the role of electron-electron correlations are also discussed. A growing number of experiments support the theoretical predictions. The formalism should be useful to understand the physics and to engineer the characteristics of small devices such as magnetic random-access memory elements.Comment: 48 pages, 21 figures (3 in color

Os casos de ensino na formação-investigação de professores dos anos iniciais

Este estudo se insere nos processos de formação que investigou as possibilidades dos casos de ensino serem propulsores de processos reflexivos sobre os conhecimentos da docência. Os casos de ensino são narrativas de episódios extraídos geralmente de dados reais do cotidiano escolar e que dão visibilidade às vivências dos professores, os quais, ao analisá–los ou construí-los, articulam e revivem experiências, estabelecendo processos reflexivos e relações entre a teoria e a prática. Para a realização da pesquisa, de abordagem qualitativa, objetivou-se investigar e avaliar as potencialidades dos casos de ensino, enquanto narrativa de episódios escolares, quando utilizados com professores alfabetizadores, ao lidarem com o ensino e a aprendizagem da leitura e da escrita dos alunos, auxiliando-os a reconstruir suas bases de conhecimento para o ensino. Para tanto foram trabalhados casos que retratavam eventos relacionados ao cotidiano das alfabetizadoras que atuam no anos iniciais da educação básica. O processo de formação-investigação foi realizado num ambiente virtual de aprendizagem, na perspetiva de formação continuada de docentes dos anos iniciais. As respostas dos professores participantes apontaram que a adoção de casos de ensino no desenvolvimento profissional dos professores possibilitou que as reflexões se fizessem muito presentes, posto que, ao analisarem as narrativas dos conhecimentos profissionais de colegas e seus, refletiam suas concepções, num movimento reflexivo possibilitador de aprendizagens na/da docência.This study falls in the field of the processes of formation that researched the possibilities of teaching cases being propellers of reflexive processes on knowledge in teaching. Teaching cases are narratives of episodes usually drawn from real daily school life, that bring to light teachers' experiences, establishing reflexive processes and relations between theory and practice. In order to realize this research, with a qualitative approach, it was established the objective of investigating and evaluating the potentiality of the teaching cases as narratives of school episodes, when used with primary school teachers, dealing with the teaching and learning of their students' reading and writing, helping them the re-construe their knowledge foundations in their teaching role. To attain this goal, some cases that pictured events related to the daily work of the primary teachers who play their educational role in the beginning years of basic education. The process of training-research was realized in a virtual learning environment, with a view to an ongoing training of the teachers of the first basic education years. The answers provided by the teachers who participated in the research suggest that the adoption of teaching cases in the professional development of teachers made it possible for the reflexions to become very actual, since, while analysing their colleagues' and their own narratives of professional knowledge, they were reflecting their own ideals, in a reflexive moment that fostered learning in/of teaching.CIEC - Centro de Investigação em Estudos da Criança, IE, UMinho (UI 317 da FCT

Gilbert Damping in Magnetic Multilayers

We study the enhancement of the ferromagnetic relaxation rate in thin films due to the adjacent normal metal layers. Using linear response theory, we derive the dissipative torque produced by the s-d exchange interaction at the ferromagnet-normal metal interface. For a slow precession, the enhancement of Gilbert damping constant is proportional to the square of the s-d exchange constant times the zero-frequency limit of the frequency derivative of the local dynamic spin susceptibility of the normal metal at the interface. Electron-electron interactions increase the relaxation rate by the Stoner factor squared. We attribute the large anisotropic enhancements of the relaxation rate observed recently in multilayers containing palladium to this mechanism. For free electrons, the present theory compares favorably with recent spin-pumping result of Tserkovnyak et al. [Phys. Rev. Lett. \textbf{88},117601 (2002)].Comment: 1 figure, 5page

Parity-controlled spin-wave excitations in synthetic antiferromagnets

We report in this study the current-induced-torque excitation of acoustic and optical modes in Ta/NiFe/Ru/NiFe/Ta synthetic antiferromagnet stacks grown on SiO2/Si substrates. The two Ta layers serve as spin torque sources with the opposite polarisations both in spin currents and Oersted fields acting on their adjacent NiFe layers. This can create the odd symmetry of spatial spin torque distribution across the growth direction, allowing us to observe different spin-wave excitation efficiency from synthetic antiferromagnets excited by homogeneous torques. We analyse the torque symmetry by in-plane angular dependence of symmetric and anti-symmetric lineshape amplitudes for their resonance and confirm that the parallel (perpendicular) pumping nature for the acoustic (optical) modes in our devices, which is in stark difference from the modes excited by spatially homogeneous torques. We also present our macrospin model for this particular spin-torque excitation geometry, which excellently supports our experimental observation. Our results offer capability of controlling spin-wave excitations by local spin-torque sources and we can explore further spin-wave control schemes based on this concept.Comment: 31 pages, 12 figure

Dynamic exchange coupling and Gilbert damping in magnetic multilayers

We theoretically study dynamic properties of thin ferromagnetic films in contact with normal metals. Moving magnetizations cause a flow of spins into adjacent conductors, which relax by spin flip, scatter back into the ferromagnet, or are absorbed by another ferromagnet. Relaxation of spins outside the moving magnetization enhances the overall damping of the magnetization dynamics in accordance with the Gilbert phenomenology. Transfer of spins between different ferromagnets by these nonequilibrium spin currents leads to a long-ranged dynamic exchange interaction and novel collective excitation modes. Our predictions agree well with recent ferromagnetic-resonance experiments on ultrathin magnetic films.Comment: 15 pages, 3 figures, for MMM'02 proceeding