Multiferroic materials with a coupled ordering of electric and magnetic moments could be used to build energy-efficient, magnetic computer memory that is written with an electrical field. To understand the interaction between the magnetic and electric ordering in such materials, two examples, namely yttrium manganate YMn2O5 and neodymium iron borate NdFe3(BO3)4, are studied by means of resonant x-ray diffraction. The important role of a pure electronic contribution to the ferroelectric polarization is shown in YMn2O5. Furthermore, substitution of Fe can change the magnetic order of YMn2O5 from antiferromagnetic into ferrimagnetic, allowing the storage of easily readable magnetic information. Therefore the change of the magnetic structure upon small Fe substitution is studied. Although most of the magnetic structure of the parent compound is kept, the Fe moments have larger components along the c-direction. In NdFe3(BO3)4 the microscopic origin of the magnetoelectric coupling is addressed as the consequence of the frustration of the Fe and Nd magnetic sublattices. The application of an electrical field shifts the balance from the helical to the collinear magnetic domains, revealing again the strong magnetoelectric coupling. / Multiferroische Materialien mit einer starken magnetoelektrischen Kopplung können als energieeffizienter, magnetischer Speicher benutzt werden, welcher mit einem elektrischen Feld geschrieben wird. Um die Wechselwirkung der elektrischen mit der magnetischen Ordnung in solchen Materialien zu verstehen, werden hier zwei Beispiele, nämlich Yttriummanganat YMn2O5 und Neodymeisenborat NdFe3(BO3)4, mit resonanter Röntgenbeugung untersucht. In YMn2O5 wird die wichtige Rolle eines rein elektronischen Beitrags zur ferroelektrischen Polarisation gezeigt. Um die magnetische Struktur von YMn2O5 von antiferromagnetisch zu ferrimagnetisch zu verändern, kann Fe substituiert werden. Dies ermöglicht es, leicht zu lesende, magnetische Informationen zu speichern. Daher wurde die Änderung der magnetischen Struktur bei leichter Fe Substituierung untersucht. Auch wenn die magnetische Struktur von Fe im wesentlichen der magnetischen Struktur von Mn folgt, haben die Fe Momente größere Komponenten entlang der c-Richtung. In NdFe3(BO3)4 wird der Ursprung der starken magnetischen Kopplung als Folge der Frustration des Nd und Fe Untergitters erklärt. Das Anlegen eines elektrischen Feldes führt zur Verschiebung des Gleichgewichts von den helikalen zu den kollinearen magnetischen Domänen, welches wieder die starke magnetoelektrische Kopplung veranschaulicht.

For over half a century, the technological promise of spins manipulable by a small voltage has captivated the interest of experimental and theoretical researchers alike. However, if thin-film multiferroics are to be incorporated into future data storage devices, a much greater understanding of their behaviour and how they differ from their bulk counterparts is required. In this thesis, we probe the fundamental multiferroic properties of BiFeO₃ films through a combination of state-of-the-art diffraction and microscopy techniques. We investigate the coupling between magnetic, ferroelectric, and structural order, with a focus on domains, and how the domain structure may be manipulated in order to tailor the multiferroic properties of the material. Using non-resonant magnetic x-ray scattering (NXMS) and neutron diffraction, we study the magnetic and structural properties of (111)_pc-oriented BiFeO₃ films. Contrary to the general belief that to they grow as a rhombohedral monodomain, we find that they comprise a sub-micron texture of monoclinic domains. The magnetic structure is found to be intimately coupled to the structure, resulting in the propagation vector being locked to the monoclinic b-axis. This magnetoelastic coupling opens up a route to strain-engineer the magnetic domains via epitaxial strain. By growing BiFeO₃ films on a lower-symmetry, TbScO₃ substrate, we are able to engineer a magnetic, structural and ferroelectric monodomain, coherent over the entire film, constituting an increase in the domain size by over five orders of magnitude. We directly demonstrate the coupling between ferroelectric and magnetic order parameters of the cycloidal magnetic structure. Using NXMS polarimetry to measure directly the magnetic polarity, we show that upon switching the ferroelectric polarisation, the magnetic polarity switches accordingly---a major rearrangement of the magnetic structure, with each spin rotating by 90 degrees on average. This goes counter to idea that magnetic and ferroelectric order parameters are only weakly coupled in type-I multiferroics. Finally, using photoemission electron microscopy we are able to directly image the sub-micron magnetostructural domain structure. We further show that there is a strong interfacial coupling between the magnetostructural domains of BiFeO₃ with a ferromagnetic overlayer. The BiFeO₃ domains are found to impose a uniaxial anisotropy in the overlayer, opening up a route to control ferromagnetic domains.

L'avenir de la spintronique repose sur le développement de matériaux ayant des propriétés magnétiques remarquables. L'objectif de cette thèse est de comprendre la physique des deux matériaux fonctionnels proposés pour des applications spintroniques qui utilisent des simulations de la densité fonctionnelle.Nous nous sommes intéressés dans une première partie au ferrite de gallium pour lequel il a été montré que les propriétés dépendaient de la concentration de fer.Les spectres optiques ont été calculés et comparés aux spectres expérimentaux suggérant des niveaux élevés de désordre. Dans la deuxième partie, nous avons montré une polarisation de spin à l’interface hybride formée entre la phthalocyanine de manganèse et la surface de cobalt,en accord avec les expériences de photoémission.La formation de la spinterface a été expliquée par différents mécanismes d'hybridation dans chaque canal de spin.Cette polarisation de spin est coordonnée avec des moments magnétiques induits sur les sites moléculaires. / The future of the spintronics technology requires developing functional materials with remarkable magnetic properties. The aim of this thesis is to understand the physics of functional materials proposed for spintronic applications using ab-initio density functional simulations. We investigated the properties of two different functional materials. We first studied the magnetoelectric gallium ferrite GFO. The dependence of the different properties on the iron concentration has been demonstrated and discussed. The optical spectra were calculated and compared to the experimental once suggesting high levels of iron disorder. In the second part, we demonstrated a highly spin polarized hybrid interface formed between manganese phthalocyanine and cobalt surface in agreement with photoemission experiments. The formation of this spinterface was described by different hybridization mechanisms in each spin channel. This high spin polarization is coordinated with induced magnetic moments on the molecular sites.

<p style="text-align: justify;">Kroz istoriju, otkrivanje novih materijala i njihovog dizajna dovodilo je do tehnolo&scaron;kih revolucija. U pro&scaron;lom veku, novi materijali naprednih svojstava uveli su elektronske uređaje u svakodnevni život čoveka. Industrija mikročipova predstavlja ogroman deo svetskog trži&scaron;ta, i traži neprestan razvoj da bi pružila bolje proizvode potro&scaron;ačima. Početkom ovog veka, nova grupa materijala, pod nazivom multiferoici, privukla je pažnju naučno-istraživačkog dru&scaron;tva u svetu. Ovi materijali poseduju jedinstvenu karakteristiku da istovremeno ispoljavaju vi&scaron;e od jedne feroične osobine (feroelektričnost, feromagnetizam, feroelastičnost), a &scaron;to je jo&scaron; važnije, mogu da ostvare interakciju među njima. Naime, magnetizacija multiferoika se može postići primenom spolja&scaron;njeg električnog polja, a takođe se mogu i polarisati primenom spolja&scaron;njeg magnetnog polja. Ovo vrlo interesantno svojstvo otvara potencijlanu primenu u oblasti hibridne računarske memorije, senzora, aktuatora, i dr. Sredinom pro&scaron;log veka, jednofazni multiferoici su prvi privukli pažnju, ali poseldnjih godina, kompozitni multiferoici su pokazali bolje rezultate u pogledu funkcionalnih karakteristika. Trend minijaturizacije je takođe prisutan u ovoj oblasti, stoga su multiferoični tanki filmovi vrlo atraktivni u istraživačkih krugovima ne samo zbog niske potro&scaron;nje električne energije, malog utro&scaron;ka meterijala i malih dimenzija, već i zbog dobre magnetoelektrične interakcije.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Glavni cilj ove doktorske disertacije je bio određivanje optimalnog procesa sinteze/depozicije, i vr&scaron;enje strukturne i funkcionalne karakterizacije multiferoičnih vi&scaron;eslojnih tankih filmova, sačinjenih od naizmenično deponovanih feroelektričnih BaTiO₃ i fero/ferimagnetnih NiFe₂O₄ slojeva (uglavnom na silicijumskim supstratima sa slojem platine). Različite strukture slojeva dizajnirane su u cilju određivanja optimalne, koja bi dala najvi&scaron;e vrednosti magnetoelektričnog efekta.&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; U prvom koraku, sintetisani su stabilni solovi/rastvori prekursora, veličine čestica od nekoliko nanometara, reolo&scaron;kih karakteristika pogodnih za depoziciju tehnikama iz tečne faze. Vi&scaron;eslojni filmovi su dobijeni &quot;spin&quot; procesom nano&scaron;enja, pri čemu je termički tretman svakog sloja na 500 &deg;C bio neophodan radi potpunog otparavanja zaostalog rastvarača. Filmovi bez pukotina, ukupne debljine ispod 1 &mu;m, uniformne debljine sloja (60 nm sloj BaTiO₃ i 40 nm sloj NiFe₂O₄) i ravne povr&scaron;ine mogu biti dobijeni sinterovanjem u temperaturnom opsegu od 750 do 900 &deg;C. Strukturna karakterizacija je potvrdila sistem bez prisustva sekundarnih faza, sačinjen od perovskitnog BaTiO₃ i spinelnog NiFe₂O₄. Dielektrična merenja su bila u saglasnosti sa mikrostrukturnom analizom, i vrednostima dielektrične konstante tipične za nanostrukturni sistem, niske vrednosti dielektričnih gubitaka i male provodljivosti. Uticaj međuslojne polarizacije, koja nalikuje Debajeovoj relaksaciji, izražena kroz povećanje dielektrične konstante uspod 100 kHz, bio je jači u nižoj frekventnoj regiji na povi&scaron;enim temperaturama usled termičke aktivacije nosilaca naelektrisanja u feritnoj fazi. Samo čist BaTiO₃ film je pokazao slab feroelektrični histerezis nepotpune saturacije, malo vi&scaron;e polarizacije filma sinterovanog na 900 &deg;C usled ogrubljivanja strukture. Meuđuslojni efekti su takođe primećeni kod feroelektričnih merenja na sobnoj temperaturi, sa izraženijim prisustvom kada se primeni jače električno polje. Na osnovu dielektričnih i feroelektričnih merenja, zaključeno je da film sa debljim titanatnim i tanjim feritnim slojevima ima najverovatnije najbolji dizajn slojeva. Magnetne histerezisne petlje su snimljene na sobnoj temperaturi za čiste NiFe₂O₄ filmove i vi&scaron;eslojne filmove. Analizom vi&scaron;eslojnih filmova različitog dizajna slojeva, pretpostavljeno je da zatezanje nastalo mehaničkom interakcijom između titanatnih i feritnih slojeva jeste prisutno, i da raste sa povećanjem broja kontaktnih povr&scaron;ina, stoga film sa tanjim titanatnim i feritnim slojevima verovatno predstavlja najbolji izbor sa aspekta megnetnih osobina.</p> / <p>Through history, discovery of new materials and material design have led to technological revolutions. In the last century, new materials with advanced properties have introduced electronic devices in our everyday lives. Microchip industry represents one huge part of world market, and needs constant development to provide better products to consumers. In the beginning of this century, a novel group of materials, called multiferroics, have attracted close attention of research society around the world. These materials have a unique property to express more than one ferroic property simultaneously (feroelectricity, ferromagneticity, ferroelasticity), and more important, to achieve coupling between them. Namely, magnetization of multieferroic may be changed with application of external electric field, and they can be polarized with application of the external magnetic field. This is a very interesting property that opens the potential applications in fields of hybrid computer memory, sensors, actuators, etc. In the middle of last century, single phase multiferroics were the first to trigger interest in this special property, but in recent years, composite multiferroics have shown more promising results in terms of functional properties. The trend of miniaturization is also present in this field, so multiferroic thin films are very attractive for research not only because of low power and material consumption or small size, but also because of strong magnetoelectric coupling.<br />The main goal of this thesis was to determine optimal synthesis/deposition process, and perform structural and functional characterization of multiferroic multilayer thin films, composed of ferroelectric BaTiO3 and ferro/ferrimagnetic NiFe2O4 layers in alternating order (mostly on platinum coated silicon substrates). Different layer structures were designed in order to find optimal one which could show the strongest magnetoelectric effect.<br />In the first step, stable precursor sols/solutions were synthesized, with particle size of a few nanometes, and rheological properties suitable for solution deposition. The multilayered thin films were obtained by spin coating and thermal treatment of each layer on 500 &deg;C was necessary in order to completely evaporate traces of residual solvents. Crack free films with overall thickness below 1 &mu;m, uniform single layer thickness (60 nm of BaTiO3 layer and 40 nm of NiFe2O4) and flat surface can be obtained by sintering in temperature range from 750 to 900 &deg;C. Structural characterization confirmed that secondary phase free system with microstructure on nanometer scale was obtained, composed of perovskite BaTiO3 and spinel NiFe2O4. Dielectric measurements were in agreement with microstructural characterization, showing the values of dielectric constant typical for nanostructured system, low values of dielectric losses and low conductivity. The influence of interfacial polarization, resembling Debye behavior, expressed as a rise of dielectric constant below 100 kHz, was stronger in lower frequency range on higher temperatures due to thermal activation of mobile charge carriers in ferrite phase. Only the pure BaTiO3 films showed weak unsaturated ferroelectric hysteresis loops, with slightly higher polarization of films sintered on 900 &deg;C due to coarsening of the structure. The interface effects were also detected in ferroelectric measurements on room temperatures, showing increased presence when higher field is applied. Regarding dielectric and ferroelectric characterization, it was concluded that the multilayered films with thick titanate and thin ferrite layers may probably have the most promising layer design. Magnetic hysteresis loops were recorded on room temperatures for the pure NiFe2O4 and multilayers films. By analysis of different layer design of multilayers, it was assumed that mechanical straining between the ferrite and titanate layers may be present, and increases with the number of contact surfaces, thus the films with thinner titanate and ferrite layers may probably have the best layer design from aspect of magnetic properties.</p>

Es werden die Verbindungen YMn2O5 und YFeMnO5 diskutiert. Die erstere zeigt unterhalb von TN = 45 K Ferromagnetismus und in der magnetischen Phase schwache Ferroelektrizität. Die elektrische Polarisation wird mit Symmetriebrechung durch die Spinstruktur erklärt, die zur Aufhebung der Inversionssymmetrie führt (sog. unechtes Ferroelektrikum). Isostrukturelle Ersetzung von Mn durch Fe führt zu YFeMnO5, einer Verbindung, die bei T<165 K ferrimagnetisch, jedoch nicht ferroelektrisch ist. Die Spin-Strukturen beider Verbindungen sind bereits eingehend untersucht und zeigen charakteristische Unterschiede. Für Verbindungen der Zusammensetzung YFe(x)Mn(2-x)O5 wurden Röntgenbeugungs-und Absorptionsfeinstruktur-Experimente zur Bestimmung der Kristallstrukur in Abhängigkeit vom Fe-Anteil x durchgeführt und ausgewertet. Die Ergebnisse zeigen, daß die Strukturparameter einen nahezu linearen Verlauf zwischen den aus der Literatur bekannten Grenzfällen YFeMnO5 und YMn2O5 nehmen. Fe ersetzt dabei das Mn auf der fünffach koordinierten Lage innerhalb einer Sauerstoff-Pyramide. Besonders markant ist die unterschiedliche Position von Mn bzw. Fe in dieser Umgebung. Mit Hilfe der Strukturdaten wurden kollineare DFT-Rechnungen im LSDA+U-Formalismus in skalar-relativistischer Näherung durchgeführt. Für YFeMnO5 konnte der experimentell bekannte Grundzustand im Rahmen der Näherung reproduziert werden, obgleich eine Bandlücke nur in Abhängigkeit von der U-Korrektur auftritt. Der berechnete Grundzustand von YMn2O5 gibt die komplizierte magnetische Struktur dieser Verbindung nicht wieder, weil die gewählte Elementarzelle des Gitters dafür zu klein ist. Statt dessen ist der berechnete Grundzustand hier sehr ähnlich zu dem von YFeMnO5. Eine ausführliche Untersuchung der projizierten Zustandsdichten der Metallatome ermöglicht die Diskussion der Kristallfeldaufspaltung im Zusammenhang mit deren Position innerhalb der Sauerstoffpolyeder. Durch Berechnung mehrerer Spinstrukturen in einer kristallographischen Elementarzelle mit erniedrigter Symmetrie konnten die Austauschparameter eines Heisenberg-Modells zwischen den lokalisierten Spins der Metallatome berechnet werden. Die Größenverhältisse dieser Parameter können mit den aus der Literatur bekannten Spinstrukturen in Einklang gebracht werden. Die Wechselwirkungen sind überwiegend antiferromagnetisch, in Übereinstimmung mit den GKA-Regeln für den Superaustausch. Bei YMn2O5 wird insbesondere eine der schwächeren Kopplungen in der magnetischen Struktur periodisch frustriert. Man geht davon aus, daß dies eine mögliche Ursache für das Auftreten von Ferroelektrizität in der magnetischen Phase ist. Bei YFeMnO5 ist der berechnete Wert dieser Kopplung wesentlich größer und die magnetische Struktur beinhaltet keine Frustration. Dies ist eine mögliche Erklärung für die Abwesenheit der magnetisch induzierten Ferroelektrizität in YFeMnO5. Im zweiten Teil stehen das in Perowskitstruktur kristallisierende SrTiO3 und die durch Hinzufügen von SrO daraus hervorgehenden Kristallstrukturen der sog. Ruddlesden-Popper-Phasen (RP) im Mittelpunkt. Die Daten von Nahkanten-Elektronenenergieverlustspektren (ELNES) an der Sauerstoff K-Kante in SrTiO3, SrO und einer RP-Phase wurden ausgewertet und mit DFT-berechneten projizierten Zustandsdichten (PDOS) der 2p-Orbitale der O-Atome in diesen Verbindungen verglichen. Bei ELNES-Nahkantenspektren ist ein solcher Vergleich mit Experimenten im Bereich hoher Elektronenenergien möglich, weil die Auswahlregel auch für die inelastischen Elektronenstöße zutrifft. Die Spektren zeigen für jede Verbindung charakteristische Maxima, deren Ursache die unterschiedliche nähere Umgebung der Sauerstoffatome ist. Weiterhin wurden Experimente an SrTiO3-Einkristallen unter Einfluß elektrischer Gleichströme und -felder durchgeführt. Bei Experimenten an Einkristall-Wafern waren Hinweise auf lokale Veränderungen der Kristallstruktur unter diesen Bedingungen gefunden worden. Ergänzend dazu wurden mikroskopische einkristalline Proben untersucht. Bei geringen Stromstärken zeigte sich dabei das bereits bekannte Degradationsverhalten des elektrischen Widerstands. Bei hohen Stromstärken kommt es zum elektrischen Durchbruch und dauerhafter Erniedrigung des Widerstands. Röntgenbeugungsmessungen ergaben keine Hinweise auf Veränderungen an der Kristallstruktur oder in Form von Zwillings-oder Bruchstückbildung. Im dritten Teil werden Röntgenbeugungsmessungen an CeCu2Si2-Einkristallen diskutiert. Bei der Auswertung älterer Messungen fielen nach der Strukturbestimmung charakteristische Maxima der Restelektronendichte auf, deren Ursprung nicht erklärt werden konnte und die bei mehreren Kristallen beobachtet wurden. Mit erneuten Messungen und Simulationen konnte nun gezeigt werden, daß diese Maxima von einer fehlerhaften Auswertungsmethode verursacht wurden.

The structure and magnetism of selected compounds of the pnictides iron based superconductors with chemical formula LnO{1-x}FeAsFx (Ln = La,Sm and Ce), commonly known as 1111, and of rare earth iron borates RFe3(BO3)4 (R = Tb, Gd, Nd and Y), were studied by means of hard x-ray diffraction. For the 1111 pnictides compounds, Rietveld refinement of powder x-ray diffraction measurements at room temperature reveals, that the ionic substitution of O by F has no effect on the structure of the FeAs layers of tetrahedra, whereas the major changes takes place in the LnO layer. These changes are reflected as a shrinkage of the crystal lattice, specially in the c direction. Additionally, a study of the temperature dependent structure of the Sm and Ce-1111 compounds was performed and an estimation of the the structural transition temperature was obtained. The results of the structural measurements, combined with electrical resistivity and µSR, were used to construct the Sm and Ce-1111 phase diagrams. These phase diagrams are characterized by two regions, consisting of a spin density wave (SDW) state and a superconducting state, which are sharply separated upon doping. Considering the different Ln ion, upon F doping the transition temperatures are more efficiently suppressed in Ce-1111 as compared to Sm-1111. More intriguingly, for the Ce case, a coexistence region between static magnetism and superconductivity without an orthorhombic distortion has been observed. Further analysis of the width of the Bragg peaks reveals strong lattice fluctuations towards phase transitions, which are reflected in magnetic and transport properties. Moreover, a strong damping of the lattice fluctuations is observed at Tc for superconducting Sm-1111 samples, giving experimental evidence of competing orders towards phase transitions in the iron pnictides. Regarding the iron borates, non-resonant x-ray scattering studies have shown several new diffraction features, from the appearance of additional reflections that violate the reflection conditions for the low temperature crystal structure, to the emerging of commensurate superlattice peaks that appear below TN. A detailed analysis of the structure factors and q dependencies of the earlier reflections, demonstrate their magnetic nature. Additional resonant x-ray magnetic scattering experiments on NdFe3(BO3)4 were performed at the Nd L2,3 and Fe K edges. The results show that the magnetization behavior is different for the Nd and for the Fe sublattices. Moreover, we find that the magnetization of the Nd sublattice is induced by the Fe magnetization. The temperature dependent measurements also show a commensurate to incommensurate transition where the magnetic structure changes from a commensurate collinear structure, where both Nd and Fe moments align in the hexagonal basal plane, to an incommensurate spin helix structure that propagates along c. When a magnetic field is applied, the spin helix is destroyed and a collinear structure is formed where the moments align in a direction perpendicular to the applied magnetic field. Moreover, the critical field at which the spin helix is destroyed is the same field at which the magnetic induced electric polarization is maximum, thus, showing that the spin helix is not at the origin of the electric polarization.

The effects of strain on the magnetic phases in perovskites are of interest in the highly active research field of multiferroics. A Monte Carlo program is written to investigate the influence of strain on the low– temperature magnetic phase diagram of the manganese perovskites, RMnO3, where R is a cation in the lanthanide series. A Metropolis simulation scheme is implemented together with parallel tempering to perform computations in a two–dimensional geometry using a conventional nearest–neighbor and next–nearest–neighbor Heisenberg Hamiltonian, extended to include spin–lattice couplings and single–ion anisotropies. The latter two are important to account for structural distortions such as octahedral tilting and the Jahn–Teller effect. It is shown that even weak single–ion anisotropies render incommensurability in the otherwise structurally commensurate E–type ordering, and that the Dzyaloshinskii–Moriya interaction, in combination with single–ion anisotropies, is crucial for the stabilization of previously experimentally observed incommensurate spin spirals. Simulations performed to account for strain in the crystallographic ab–plane show that tensile strain may improve stability of E–type ordering for R elements with small atomic radii and that compressive strain drives the magnetic ordering toward the incommensurate spiral states. / Spänningsinverkan på de magnetiska faserna i perovskiter är av intresse inom den just nu högaktiva forskningen om multiferroiska material. Ett Monte Carlo-program har skrivits för att undersöka effekterna av spän- ning på de magnetiska lågtemperaturfaserna i multiferroiska manganitpe- rovskiter, RMnO3, där R är en katjon i lantanoidserien. En kombination av Metropolisalgoritmen och parallelltemperering har använts för att utföra beräkningar i tvådimensionell geometri med en konventionell Heisenberghamiltonian, utökad till att även inkludera spinn–gitterkopplingar och enkeljonsanisotropier. De senare har visats vara viktiga för att ta i beaktande den strukturella distortion i materialet som följer av t.ex. syreoktahederförskjutning och Jahn–Tellereffekten. Det visas att även svaga anisotropier orsakar inkommensurabilitet i den i övrigt kommensurabla E–typsfasen, och att Dzyaloshinskii-Moriyainteraktionen, i kombination med anisotropitermerna, är avgörande för att kunna stabilisera de sedan tidigare experimentellt bekräftade inkommensurabla spinnspiralsfaserna. Simuleringar som modellerar spänning i materialets kristallografiska ab–plan visar att dragspänning kan förbättra stabiliteten hos E–typsfasen för R–atomer med liten radie och att tryckspänning leder den magnetiska ordningen mot inkommensurabla spiraltillstånd.

This cumulative Habilitation thesis summarizes several examples related to the application of Raman spectroscopy for the investigation of coupling phenomena induced by epitaxially, mechanically or piezoeletrically applied strain. Methods for quantitative determination of strain by Raman spectroscopy are proposed for some materials such as BiFeO3 or strained Si. Raman spectroscopy was also used for understanding temperature induced phase transitions or orbital ordering, which are intimately related to specific phonon modes, as in the case of BiCrO3 or LaVO3, respectively. A method based on the Raman tensor formalism, which allows an assignment of the BiFeO3 Raman modes of pure as well as mixed character/symmetries is proposed. Relying on this assignment it is shown that Raman spectroscopy is a powerful tool for the investigation of ferroelastic domain formation in multiferoic materials, being able to probe the tilt of the domain walls.

Nanomagnetic logic, incorporating logic bits in the magnetization orientations of single-domain nanomagnets, has garnered attention as an alternative to transistor-based logic due to its non-volatility and unprecedented energy-efficiency. The energy efficiency of this scheme is determined by the method used to flip the magnetization orientations of the nanomagnets in response to one or more inputs and produce the desired output. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque inhibit the promised energy-efficiency. Another technique offering superior energy efficiency, “straintronics”, involves the application of a voltage to a piezoelectric layer to generate a strain which is transferred to an elastically coupled magnetrostrictive layer, causing magnetization rotation. The functionality of this scheme can be enhanced further by introducing magnetocrystalline anisotropy in the magnetostrictive layer, thereby generating four stable magnetization states (instead of the two stable directions produced by shape anisotropy in ellipsoidal nanomagnets). Numerical simulations were performed to implement a low-power universal logic gate (NOR) using such 4-state magnetostrictive/piezoelectric nanomagnets (Ni/PZT) by clocking the piezoelectric layer with a small electrostatic potential (~0.2 V) to switch the magnetization of the magnetic layer. Unidirectional and reliable logic propagation in this system was also demonstrated theoretically. Besides doubling the logic density (4-state versus 2-state) for logic applications, these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing. Experimental work in strain-based switching has been limited to magnets that are multi-domain or magnets where strain moves domain walls. In this work, we also demonstrate strain-based switching in 2-state single-domain ellipsoidal magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate (PMN-PT) and studied using Magnetic Force Microscopy (MFM). A nanomagnetic Boolean NOT gate and unidirectional bit information propagation through a finite chain of dipole-coupled nanomagnets are also shown through strain-based "clocking". This is the first experimental demonstration of strain-based switching in nanomagnets and clocking of nanomagnetic logic (Boolean NOT gate), as well as logic propagation in an array of nanomagnets.

Ce travail porte sur la synthèse et la caractérisation de nouveaux matériaux multiferroïques basés sur le concept très récent de la Ferroélectricité Hybride Impropre.Deux classes de matériaux ont été envisagées : les oxydes de type Ruddlesden-Popper NaRMO4 (R=Y, La; M= Mn, Cr) et les doubles pérovskites ordonnées NaLnCoWO6 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb). Les essais de synthèse sur la première classe n’ont pas permis d’obtenir les composés visés. Pour la seconde classe, l’ensemble des composés ont pu être obtenus par synthèse par voie solide à haute température. Les composés NaLnCoWO6 (Ln=La, Pr, Nd) ont été synthétisés à pression ambiante. L’usage des techniques de Hautes Pressions – Hautes Températures (HP-HT) a permis de stabiliser les composés contenant des terres rares plus petites et d’obtenir ainsi neuf nouveaux composés aux propriétés inédites.L’utilisation combinée de la diffraction sur poudre des rayons X au synchrotron et des neutrons a permis une étude structurale fine de la famille des doubles pérovskites ordonnées NaLnCoWO6. Les groupes d’espace ont été déterminés grâce aux affinements Rietveld des diffractogrammes de Rayons X sur poudre haute résolution. Les composés NaLnCoWO6 (Ln=La, Pr, Nd) cristallisent dans le groupe d’espace centrosymétrique C2/m tandis que les 9 nouveaux composés (Ln= Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) cristallisent dans le groupe d’espace polaire P21. Des mesures de génération de seconde harmonique confirment la structure non-centrosymétrique des nouveaux composés. L’analyse adaptée des modes de symétrie des composés cristallisant dans la structure polaire montre que l’amplitude du mode polaire induit augmente avec la diminution de la taille du cation Ln3+. La polarisation estimée à partir des affinements de la structure pourrait atteindre jusqu’à ~20µC/cm2.Une transition de phase présentant une très large hystérésis en température (~150K) a été observée par diffraction des neutrons pour le composé NaLaCoWO6. De plus, les images obtenues en Microscopie Electronique en Transmission révèlent la présence de bandes dans la phase haute température. Cette superstructure présente une périodicité de 12ap selon la direction [100]p ou [010]p . On a pu montrer à l’aide d’observations en STEM (microscopie à balayage en transmission) combinées avec des mesures en EELS (spectroscopie de pertes d’énergie des électrons) que ce contraste de bandes n’est pas lié à une variation de composition mais bien à une variation structurale. Différents modèles de rotations des octaèdres d’oxygènes ont été élaborés pour valider les données expérimentales obtenues par diffraction des rayons X et de neutrons. Le schéma de rotation qui décrit au mieux les données est a-a-c0. Concernant la phase basse température le groupe d’espace attribué est le groupe polaire P21.Les caractérisations magnétiques ont été réalisées pour toutes ces phases. Tous les composés NaLnCoWO6 s’ordonnent dans une configuration antiferromagnétique. Les températures de Néel varient entre 4 et 13K en fonction de la nature de la terre-rare. Les moments effectifs déterminés par la loi de Curie-Weiss sont en accord avec les moments théoriques attendus. Toutes les températures de Weiss sont négatives traduisant le fait que les interactions antiferromagnétiques sont prépondérantes dans ces systèmes. Les structures magnétiques ont été déterminées pour les composés Ln= Y, La, Tb, and Ho. Pour ces mêmes composés, des mesures diélectriques en fonction de la température et du champ magnétique ont permis de mettre en évidence un couplage magnéto-diélectrique conséquent pour Ln=Y and Ho. Les mesures de courant pyroélectrique autour de la transition magnétique montrent qu’il existe une polarisation induite par l’ordre magnétique dans le composé NaYCoWO6.. C’est la première mise en évidence expérimentale d’un couplage magnéto-électrique dans la famille des doubles pérovskites ordonnées AA’BB’O6. / In this study, new magneto-electric materials were synthesized on the basis of the very recently recognized ferroelectric inducing mechanism, hybrid improper ferroelectricity, and structural and physical properties characterizations were carried out on these new phases.Two classes of materials were focused on: the Ruddlesden-Popper oxides NaRMO4 (R=Y, La; M= Mn, Cr) and the doubly ordered perovskites NaLnCoWO6 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb). Attempts to synthesize the former class failed to give the target phases. All compounds in the latter class were prepared successfully by solid-state reactions at high temperature, among which the compounds NaLnCoWO6 (Ln=La, Pr, Nd) were prepared at ambient pressure while the other nine compounds were synthesized at high pressure.The structural study of the doubly ordered perovskite family NaLnCoWO6 was performed by synchrotron X-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD). Based on the Rietveld refinement of the SXRPD patterns, the space groups were assigned. NaLnCoWO6 (Ln=La, Pr, Nd) compounds crystallize in the centrosymmetric C2/m symmetry, whereas the other nine new compounds crystallize in the polar space group P21. Second harmonic generation measurements on powder confirmed the non-centrosymmetric structure of the new compounds. Symmetry mode analysis demonstrates that the amplitude of the induced polar mode increases with a decreasing Ln cation size. The amplitude of the polarization was estimated from the refined structures, and can be as large as ~20µC/cm2.A structural phase transition was observed by NPD in NaLaCoWO6 with a large temperature hysteresis of ~150K. In addition, stripes were observed on the high-resolution transmission electron microscopy (TEM) images in the high temperature phase. The periodicity of this superstructure is 12ap along either the [100]p or [010]p direction. Further investigations by scanning TEM and electron energy loss spectroscopy revealed that the contrast of the stripes is due to a structural modulation rather than a compositional variation. Octahedral tilt twinning models were built with different tilting schemes to fit the observed SXRPD and NPD patterns. The tilting scheme a-a-c0 describes successfully the data. The low temperature phase was unambiguously determined to possess the polar space group P21.Magnetic and electric properties were experimentally characterized. All NaLnCoWO6 compounds order antiferromagnetically below TN which is between 4 and 13K. Curie-Weill fits were performed for all compounds, yielding reasonable effective magnetic moments compared to the theoretical ones. Weiss temperatures were all determined to be negative further indicating that antiferromagnetic interactions are dominant in these systems. Magnetic structures were determined for four NaLnCoWO6 (Ln= Y, La, Tb, and Ho) compounds, of which two have non-magnetic Ln cations (Y and La) and two have magnetic ones (Tb and Ho). Magneto-dielectric coupling was experimentally observed in compounds NaLnCoWO6 (Ln=Y, Tb, Ho) by dielectric measurements as a function of temperature and magnetic field. Polarization was derived for the Y and Ho compounds from pyroelectric current measurement, however, only the NaYCoWO6 compound demonstrates a polar behavior which cannot be switched. This is the first evidence that electric polarization can be induced by the magnetic ordering in the AA’BB’O6 class materials.