Untangling the contributions of cerium and iron to the magnetism of Ce-doped yttrium iron garnet

The remarkable magnetic properties of yttrium iron garnets (YIGs) underpin the use of these materials in a broad scope of spintronic and photonic applications. In particular, the addition of rare earth metals in the structure enhances to a great extent the magneto-optical activity, which is beneficial for the development of nonreciprocal optical devices. Exploiting the wavelength selectivity of magneto-optics, we have identified a range of frequencies at which one can unravel the individual contributions to the magnetism and gyrotropic response arising from cerium and iron. We envision that this outcome may pave the way to further experiments to assess quantitatively the effect on the optical properties of rare earth incorporation into YIG.This work was supported by the Spanish Government by the MAT2011-29269-C03, MAT2014-56063-C2-1-R, and the Severo Ochoa SEV-2015-0496 Projects and the Generalitat de Catalunya (2014 SGR 734 Project). B.C. acknowledges his grant FPI BES-2012-059023 and R.C. acknowledges his fellowship from CNPq - Brazil. S.G., M.O., and R.G. thank T. Brenninger and E. Zamburg for technical support. We also acknowledge the contribution of C. Rubio in preliminary structural and magnetic characterization of thin films as well as Dr. F. Sanchez for scientific guiding and discussions on materials issues.Peer Reviewe

Dynamically Tunable Optical Cavities with Embedded Nematic Liquid Crystalline Networks

Tunable metal–insulator–metal (MIM) Fabry–P\ue9rot (FP) cavities that can dynamically control light enable novel sensing, imaging\ua0and display applications. However, the realization of dynamic cavities incorporating stimuli-responsive materials poses a significant engineering challenge. Current approaches rely on refractive index modulation and suffer from low dynamic tunability, high losses, and limited spectral ranges, and require liquid and hazardous materials for operation. To overcome these challenges, a new tuning mechanism employing reversible mechanical adaptations of a polymer network is proposed, and dynamic tuning of optical resonances is demonstrated. Solid-state temperature-responsive optical coatings are developed by preparing a monodomain nematic liquid crystalline network (LCN) and are incorporated between metallic mirrors to form active optical microcavities. LCN microcavities offer large, reversible and highly linear spectral tuning of FP resonances reaching wavelength-shifts up to 40\ua0nm via thermomechanical actuation while featuring outstanding repeatability and precision over more than 100 heating–cooling cycles. This degree of tunability allows for reversible switching between the reflective and the absorbing states of the device over the entire visible and near-infrared spectral regions, reaching large changes in reflectance with modulation efficiency ΔR\ua0=\ua079%

Generation and Imaging of Magnetoacoustic Waves over Millimeter Distances

Using hybrid piezoelectric-magnetic systems we have generated large amplitude magnetization waves mediated by magnetoelasticity with up to 25 degrees variation in the magnetization orientation. We present direct imaging and quantification of both standing and propagating acoustomagnetic waves with different wavelengths, over large distances up to several millimeters in a nickel thin film

Giant Optical Polarization Rotation Induced by Spin-Orbit Coupling in Polarons

We have uncovered a giant gyrotropic magneto-optical response for doped ferromagnetic manganite La2/3Ca1/3MnO3 around the near room-temperature paramagnetic-to-ferromagnetic transition. At odds with current wisdom, where this response is usually assumed to be fundamentally fixed by the electronic band structure, we point to the presence of small polarons as the driving force for this unexpected phenomenon. We explain the observed properties by the intricate interplay of mobility, Jahn-Teller effect and spin-orbit coupling of small polarons. As magnetic polarons are ubiquitously inherent to many strongly correlated systems, our results provide an original, general pathway towards the generation of gigantic gyrotropic responses that can be harnessed for nonreciprocal devices that exploit the polarization of light

Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe75Al25 (at. %) thin film on top of a (110)-oriented Pb(Mg1/3Nb2/3)O3-32PbTiO3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe75Al25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe75Al25, leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures

Giant topological Hall effect in correlated oxide thin films

Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca,Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density as a function of magnetic field peaks near the THE maximum. The THE critically depends on carrier concentration and diverges at low doping, near the metal–insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.The authors thank V. Cros, V. Dobrosavljevic, J. Iñiguez, J.-V. Kim, D. Maccariello, J. Matsuno, I. Mertig, N. Nagaosa and N. Reyren for useful discussions, J.-Y. Chauleau and M. Viret for second harmonic generation experiments, N. Jaouen for resonant magnetic X-ray diffraction, J. Varignon for preparing Fig. 1a and J.-M. George for his help with some magnetotransport measurements. This research received financial support from the ERC Consolidator grant ‘MINT’ (contract no. 615759) and ANR project ‘FERROMON’. This work was also supported by a public grant overseen by the ANR as part of the ‘Investissement d’Avenir’ programme (LABEX NanoSaclay, ref. ANR-10-LABX-0035) through projects ‘FERROMOTT’ and ‘AXION’ and by the Spanish Government through project no. MAT2014-56063-C2-1-R and MAT2017-85232-R (AEI/FEDER, UE), and Severo Ochoa SEV-2015-0496 and the Generalitat de Catalunya (2014SGR 734 project). B.C. acknowledges grant no. FPI BES-2012-059023, R.C. acknowledges support from CNPq-Brazil, and J.S. thanks the University Paris-Saclay (D’Alembert programme) and CNRS for financing his stay at CNRS/Thales. Work at Rutgers was supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, US Department of Energy under award no. DE-SC0018153. H.K. is supported by JSPS KAKENHI grants nos. 25400339, 15H05702 and 17H02929. K.N. is supported by a Grant-in-Aid for JSPS Research Fellow grant no. 16J05516, and by a Program for Leading Graduate Schools ‘Integrative Graduate Education and Research in Green Natural Sciences’.Peer reviewe

Uso de curvas de remanência magnética em sistemas que apresentam exchange-bias

O presente estudo discute a adaptação e a elaboração de uma técnica que usa medidas de remanência magnética em sistemas que apresentam interação de troca, visando a extração de resultados referentes a interações magnéticas presentes nas amostras. Tal adaptação consiste, basicamente, na translação da origem (magnetização contra campo magnético) do sistema. O primeiro passo foi a elaboração de um software de aquisição de dados que possibilite a aplicação da técnica para esse tipo de sistema. Uma série composta por filmes finos de NiCu/IrMn foi utilizada; devido ao fato de não apresentarem deslocamento em campo, esses filmes serviram como base para verificação da rotina. Posteriormente, foram utilizados filmes de Co/Cu/IrMn as quais apresentam deslocamento em campo (EB) e, em alguns casos, assimetria em suas curvas de histerese magnética. Concluindo os passos precedentes, foi possível então avaliar interações presentes em ambos os sistemas utilizando os gráficos de Henkel e δM. Para o sistema NiCu/IrMn que apresenta laços de histerese simétricos e não-deslocados em campo, obtêm-se as curvas Mr e Md usando a t´técnica de remanência em sua forma tradicional tornando possível, de uma maneira direta, a construção dos gráficos de remanência. Devido a` diferença entre os sentidos dos campos magnéticos aplicados durante as aquisições das curvas remanência da série Co/Cu/IrMn, foi necessária a elaboração de uma nova notação. Além disso, a aplicação da t´técnica em tais amostras possibilitou a fácil visualização da assimetria direcional causada pela interação de troca. Também foi possível desenvolver novas representações gráficas, facilitando a visualização das interações magnéticas presentes.The present study discusses on the adaptation and development of a technique that uses magnetic remanence of exchange-coupled systems aiming to extract information for the mag- netic interactions present. Such adaptation basically consists in the translation of the origin of the (magnetization versus magnetic field) system. Our first step was the preparation of data- acquisition software enabling the application of the technique for this type of systems. A series composed by NiCu/IrMn thin films was used; due to the fact that these films do not present displacement in field, they served as the basis for verification of the routine. Subsequently, we used samples composed by Co/Cu/IrMn films that show field displacement and, in some cases, asymmetry in their magnetic hysteresis curves. Having done the a bove steps, one can then assess interactions present in both type of systems using Henkel and δM plots. For the NiCu/IrMn series that shows symmetrical, unshifted (in-field) hysteresis loops, we obtained the Mr and Md curves using the technique in its traditional form thus making possible, in a very straightforward manner, the construction of the respective remanence plots. Due to the difference in the directions of the magnetic fields applied during the measurements of remanence curves of the Co/Cu/IrMn series, it was necessary to adopt a new curves’ notations. In addition, the application of the remanence technique for such samples allowed an easy visualization of the loops’ asymmetry caused by the exchange interactions. It was also possible to develop new graphical representations facilitating the visualization of the effects of the magnetic interactions

Análise micromagnética e desenvolvimento computacional aplicados a sistemas que apresentam acoplamento de troca

O presente estudo aborda a evolução temporal de filmes finos compostos por materiais ferroantiferromagnéticos que apresentam interação de troca, visando a melhor compreensão da origem desse comportamento. Técnicas como a de irradiação iônica e tratamento térmico foram utilizadas para a modificação dos sistemas magnéticos em questão. Após submetidos a essas ferramentas de indução do efeito de exchange bias (viés de troca), as amostras foram expostas a campos magnéticos durante determinado intervalo de tempo `a temperatura ambiente. Após realizada a exposição, a medida de histerese magnética era feita, seus os campos de interesse foram computados, e os resultados são expostos frente ao tempo de exposição há campo magnético. As amostras apresentaram comportamentos distintos dependentes das técnicas de indução. Após irradiação iônica, o estado magnético permanece instável, devido a configuração energética imposta pela irradiação; desse modo efeitos térmicos de baixa amplitude podem fornecer energia para que a camada ferromagnética mude as orientações das magnetizações de grãos interfaciais. Por outro lado, o tratamento térmico promoveu ao sistema de interesse uma configuração energeticamente estável, eliminando ou reduzindo os efeitos térmicos induzidos pós-tratamento. Além disso, o desenvolvimento de uma ferramenta computacional foi necessário na tentativa de obtenção dos resultados obtidos experimentalmente. Este software usa como base a bem conhecida teoria do micromagnetismo a qual, auxiliada pela equação de Landau-Lifshitz-Gilbert, é uma ótima ferramenta tanto para prever como para reproduzir o comportamento de sistemas magnéticos. Todos os procedimentos para o desenvolvimento desse software encontram-se definidos nas seções seguintes, incluindo um método de soma em memória compartilhada, o qual visa melhorar o desempenho computacional para o cálculo dos efeitos provinientes de interações dipolares. Para finalizar, resultados experimentais e computacionais são expostos, mostrando a meta-instabilidade presente nos sistemas magnéticos abordados.The present study examines the temporal evolution of thin magnetic films composed by ferro and antiferromagnetic materials that present exchange interactions, aiming a better understanding of the origin of their behavior. Techniques such as ion irradiation and magnetic annealing were used for modification of the systems studied. Once used such tools for initialization of the exchange-bias effect, the samples were exposed to magnetic fields during a certain time interval at room temperature. Subsequently, room temperature magnetization hysteresis loops were measured and the corresponding fields of interest were extracted. Depending on the different exchange-bias induction techniques, the samples showed distinct behaviors. After light ion irradiation, the magnetic state remained unstable, most probably due to thermal fluctuations that can alter the magnetic orientation of interface grains. The annealing, from other side, enabled a favorable energy configuration of the system, eliminating or reducing post-treatment thermally-induced effects in our the systems. Moreover, the development of a computational tool turned to be necessary when attempting to reproduce the experimental results. This software built is based on the well-established micromagnetic theory which, by additionally utilizing by the Landau-Lifshitz-Gilbert equation, represents a powerful tool used to predict and/or reproduce the behavior of magnetic systems. All procedures for the development of this software are defined in the following sections, including a shared memory programming, developed in order to increase the computational performance when reproducing dipolar interaction effects. Finally, experimental and simulation results are presented, demonstrating the metastable states of the magnetic systems studied here