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Mapping Local Manifestations of the Strain Mediated Magnetoelectric Effect in Composites

Trivedi, Harshkumar

The magnetoelectric effect has gained immense impetus in the last century, mainly due to its interesting potential applications into the technologies of the era. The fact that the two important space/matter permeating fields, the electric and the magnetic field, could be in a material, gives rise to key applications like high sensitivity magnetic field sensors, and voltage controlled non-volatile memory storage. As the various materials exhibiting magnetoelectric effect are being continually explored, the composites with combination of a ferroelectric and a ferro/ferrimagnetic material have emerged as favorite candidates with exceptional performance under ambient conditions. Quite often the magnetoelectric effect in composite is mediated via strain, requiring a complex constitutive model for their representation. Very recently, attempts have been made to construct robust models for understanding the strain mediated magnetoelectric effect in composites with various morphologies. However, such models lack the necessary experimental support. In general, there is a lack of proper understanding about the behavior of the strain mediation in the vicinity of the interface between the constituent phases. The present study is aimed to tackle the lack of information surrounding the local manifestations of the magnetoelectric effect in composites, with a focus on pushing the boundaries of microscopic techniques like Scanning Probe Microscopy (SPM), and Confocal Raman Microscopy. Various SPM modes like Piezoresponse Force Microscopy (PFM), Magnetic Force Microscopy (MFM), and Kelvin-probe Force Microscopy (KPFM) were synergistically utilized. As a subject of study, bulk polycrystalline composites based on combinations of ferroelectric BaTiO3 and various ferrimagnetic ferrites (viz. BaFe12O19, SrFe12O19, CoFe2O4, and NiFe2O4) were investigated. The choices of the magnetic components served as a wide spectrum of materials with different magnetocrystalline symmetries. In addition, the constituent morphologies favorably allowed longitudinal investigations in the vicinity of the ferroelectric-ferrimagnetic interface. Both, direct and converse magnetoelectric effects were locally studied using in-situ MFM and PFM respectively. As a part of the converse effect the application of instantaneous electric poling under the SPM tip resulted in interesting micromagnetic fluctuations which were observed in MFM, and later analyzed using a self-developed image processing algorithm. The results suggest an intricate role of defects and microstructure in inducing the observed changes in magnetic domain configurations. As a part of the direct magnetoelectric effect, the in-situ application of magnetic field during PFM imaging results in a corresponding variation in PFM amplitude. This was rationalized as modulation of the local electromechanical coupling in BaTiO3 by the magnetoelectrically induced stress via the magnetic phase. This was further studied in a systematical manner, using Principal Component Analysis (PCA) of PFM image sequences. The results revealed interesting magnetic field modulations of PFM amplitude, the extent of which followed a distinct spatial distribution. These patterns and their corresponding spatial distributions, indicated existence of various stress regimes which was mainly attributed tothe constituent morphologies. The distributions of the effect also suggested certain degrees of closeness to the classical micromechanical problem of inclusion in a matrix (Eshelby‟s solution). Later, the localized ferroelectric switching using the Switching Spectroscopy PFM (SSPFM) mode was also studied under and applied in-situ magnetic field. Maps of nucleation biases of the local switching were generated, suggesting presence of in-built electric fields localized at the interfaces. In addition, the application of magnetic field apparently modulated these fields, which could be attributed to the appearance of magnetoelectrically induced fields that could influence the balance of fields during the switching process. In order to corroborate the indirect observations of the effects of induced stress, a direct observation of the stress was carried out by the means of in-situ Confocal Raman Microscopy. A self-developed algorithm, based on a conjunction of PCA and Self Modeling Curve Resolution (SMCR) was utilized to generate 2D distributions of least-square fitted peak positions of BaTiO3 Raman modes. On application of magnetic field the Raman modes manifest spatially consistent frequency shifts, which were considered as a direct consequence of the magnetoelectrically induced strain in the BaTiO3 phase. The outcomes, in totality, suggest a greater role of microstructure, as compared to that of the constituent material characteristics, in determining the persistence of the stress and hence the magnetoelectric effect within the composite.

Der magnetoelektrische Effekt hat grosses Gewicht um vergangene Jahrhundert gewonnen, hauptsächlich durch seine interessantes Potential in technologischen Anwendungen. Die Tatsache, dass elektrische und magnetische Felder durch magnetoelektrische Effekte gekoppelt werden können, eröffnet die Möglichkeit für Anwendungen wie hochsensitive Magnetfeldsensoren und Spannungskontrolle in nicht flüchtigen Schreibprozessen für Datenspeicher. Kompositmaterialien mit getrennten ferroelektrischen und magnetischen Phasen sind für Anwendungen vielversprechender. Normalerweise, ist der Kopplungsmechanismus zwischen diesen Phasen spannungsvermittelt und benötigt komplexe Modelle um beschrieben zu werden. Neuerdings wurden robuste theoretische Modelle erstellt mit verschiedenen Morphologien. Diese benötigen jedoch weitere experimentelle Unterstützung. Es gibt generelle einen Bedarf an genauem Verständnis über die Spannungsvermittlung an Grenzflächen. Die vorliegende Arbeit, zielt darauf das Verständnis auf diesem Gebiet auszuweiten, mit dem Fokus, die Grenzen mit Techniken wie Rasterkraftmikroskopie und konfokaler Raman Mikroskopie zu erweitern. Ausserdem sollen piezoresponse force microscopy (PFM), magnetic force microscopy (MFM) und Kelvin-probe force microscopy (KPFM) synergetisch eingesetzt werden. Untersucht werden insbesondere polikrsitalline Systeme aus farroelektiscfhem BaTiO3 und verschiedenen ferrimagntischen ferrites (BaFe12O19, SrFe12O19, CoFe2O4 und NiFe2O4). Sowohl direkter als auch der konverse magnetoelektrische Effekt wurde lokal durch in-situ PFM und MFM untersucht. Durch die Effekte konnten interessante mikromagnetische Fluktuationen bzw. Variationen in den PFM Amplituden beobachtet werden und mit selbst entwickelten Algorithmen, teilweise auf Basis von Principal Component Analysis (PCA) untersucht werden. Die Ergebnisse enthüllten interessante Muster der Magnetfeldmodulation der PFM Amplitude einhergehende mit einer speziellen räumlichen Verteilung. Später kam switching spectroscopy zum Einsatz unter in-situ Magnetfeld um Karten von lokalen Keimbildungsspannungen aufzunehmen. Diese wiesen auf intrinische elektrische Felder an den Grenzflächen hin, die durch Magnetfelder moduliert werden konnten. Diese wurden wieder mithilfe von Algorithmen analysiert und mit den Raman Moden aus Raman Mikroskopiemessungen verglichen. Durch Einschalten des Magnetfelds, wurden diese Moden konsistent freqeunzverschoben was auf die magnetoelektrische Spannungseinwirkung zurückzuführen ist. Die Ergebnisse weisen im Endeffekt auf eine größere Rolle der Mikrostruktur hin, als es bei den Einzelkomponenten der Fall wäre.

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Trivedi, Harshkumar: Mapping Local Manifestations of the Strain Mediated Magnetoelectric Effect in Composites. 2016.

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