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In this work, I start by introducing a relatively recently innovated thin film architecture which offers a new direction in strain control, the vertically aligned nanocomposite (VAN). I first present the literature in the field, explaining the advantages and unique novel properties stemming from VAN structures. Next, I introduce the work I did to examine the unique strain states of Ba0.6Sr0.4TiO3–Sm2O3 VAN structures. It was found that the strain states in the functional Ba0.6Sr0.4TiO3 phase are unconventional compared to those in planar thin films. 3-dimensional strain was found to be acting on the Ba0.6Sr0.4TiO3 phase in the VAN structure. The origin of the strain was explained using a simple model which takes into account thermal expansion mismatch as well as lattice mismatch and elastic coefficients. The ferroelectric properties of the films were presented in relation to the observed strain states. I next present the work I did on the influence of strain on the magnetic properties in VAN film of Sm0.34Sr0.66MnO3–Sm2O3. Ferromagnetism was achieved in an otherwise antiferromagnetic Sm0.34Sr0.66MnO3. The effect was explained by a strain induced transition from super-exchange to double exchange coupling in the material. Last but not least, the potential of scalability of VAN films was explored by using sputtering to grow VAN structures instead of the commonly-used PLD growth. BaTiO3–Sm2O3 was used as a primary study material due to its well reported VAN properties. Preliminary results showing indications of a VAN structure. Some basic physical property characterization is also presented and compared to the properties of PLD-grown films in the literature. Limitations and challenges that arise due to the fundamental differences between sputtering and PLD are also described.

Disertacijoje nagrinėjamos įvairios galimybės panaudoti feromagnetinių oksidų – manganitų sluoksnius bei jų darinius naujų spintronikos prietaisų gaminimui. Šio darbo pagrindiniu tyrimo objektu pasirinktos sandūros, sudarytos tarp dvivalenčiais (Ca, Ba, Sr) ir keturvalenčiais (Ce) jonais legiruotų lantano mangano oksidų plonųjų sluoksnių, o taip pat tarp manganitų ir elektroninio laidumo SrTiO3<Nb> (STON) bei n - Si padėklų. Darbe pateiktas išsamus minėtų manganitų sluoksnių ir jų darinių gaminimo magnetroninio dulkinimo ir impulsinio lazerinio garinimo būdais aprašymas. Disertacijoje pateikti plonųjų manganitų sluoksnių, užaugintų ant skirtingų padėklų, kristalinės struktūros bei paviršiaus kokybės tyrimo duomenys, aprašytas magnetiniu lauku valdomų diodinių darinių formavimas, jų elektrinių bei magnetinių savybių tyrimai, įvertinti svarbiausi atskirų manganitų sluoksnių bei jų diodinių darinių elektrofiziniai parametrai. Atlikus kompleksinius keturvalenčiais Ce4+ jonais legiruotų lantano manganito sluoksnių kristalinės sandaros bei elektrinių savybių tyrimus nustatyta, kad šie sluoksniai pasižymi ne elektroniniu, kaip buvo skelbta anksčiau, o skyliniu elektriniu laidumu. Skylinis junginio elektrinis laidumas paaiškintas nežymiu šalutinės CeO2 fazės ir katijonų vakansijų susidarymu auginamuose sluoksniuose. Pateikti Ca, Ba, Sr ir Ce jonais legiruotų manganitų įvairialyčių darinių palyginamieji tyrimai, įvertinta padėklo įtaką kristalinės manganitų sandaros tobulumui... [toliau žr. visą tekstą] / In this dissertation application of the lanthanum manganite films and their heterostructures for fabrication of new spintronic devices is discussed. The main subjects of this work are the junctions between lanthanum manganite oxide thin films doped by divalent (Ca, Ba, Sr) and tetravalent (Ce) ions as well heterojunctions formed between lanthanum manganites and n-type SrTiO3<Nb> (STON) or n - Si substrates. The influence of doping and substrate influence on crystalline quality of manganite film structures, interface roughness as well as their electrical and magnetic properties has been estimated in this dissertation. After performing complex investigations it has been determined that tetravalent Ce ion doped lanthanum manganite films have not the electron but hole-type conductivity on the contrary to that has been reported earlier. Forming of magnetic filed dependent “manganite / (STON, n - Si)“ diode structures has been described, comparative studies of electrical and magnetic properties have been presented, and major electro-physical parameters have been estimated in this work. Possible reasons of the origin of positive and negative magnetoresistance have been pointed out. In addition, structural stabilization problems of BiFeO3 compound, exhibiting at the same time magnetic as well as ferroelectric properties and possibilities of application of its unique properties in various lanthanum manganite structures for the development of new magnetic and electrical filed... [to full text]

Šiai dienai ypač populiarūs ferroelektrikai susidedantys iš kelių feroiškai aktyvių subgardelių, kurių persitvarkymas fazinio virsmo temperatūroje atskleidžia naujų, dar neaprašytų reiškinių. Prie šių medžiagų priskiriami antiferoelektrikai, ferielektrikai ir multiferoikai. Šiame darbe buvo tiriama nauja medžiagų šeimos MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se ), kurios pasižymi ferielektrinėmis bei multiferoinėmis savybėmis, ir kurių dielektrines ir elektrines savybes galima efektyviai keisti įvedant priemaišas. Minėtų medžiagų dielektrinės ir elektrinės savybės buvo tiriamos dielektrinės spektroskopijos metodais, kurie leidžia tirti kristalų kolektyvinius reiškinius susijusius su tvarkos – netvarkos bei poslinkio tipo faziniais virsmais, jonų migracija bei dipolių užšalimu (stiklėjimu) plačiame dažnių (10-5 Hz iki 3 GHz) bei temperatūrų (25 K iki 500 K) intervaluose. Įvedus 10% Ag jonų vietoj Cu jonų ferielektriniame kristale CuInP2S6 fazinio virsmo temperatūra pasislenka į žemesnias temperatūras, o padidinus indžio koncentraciją fazinio virsmo temperatūra pasislenka į aukštesnes temperatūras. Minėtų kristalų fazinių virsmų temperatūrų skirtumas 50 K. Sumaišius skirtingomis proporcijomis feroelektriką (CuInP2S6) su antiferoelektriku (CuCrP2S6) stebima dipolinio stiklo fazė. Iš dielektrinių matavimų stiklo fazėje buvo paskaičiuota relaksacijos trukmių pasiskirstymo funkcija, kurios aprašymas dvigubos potencialinės duobės modeliu leido susieti mikroskopinius kristalo... [toliau žr. visą tekstą] / Nowadays the ferroelectrics containing of several feroically active sublattices are very attractive, because interactions between these sublattices can caused novel phenomena. Antiferroelectrics, ferrielectrics and multiferoics belong to these materials. In this work new crystalline materials MNP2X6 (M = Cu, Ag; N=In, Cr, Bi; X=S, Se) were investigated, which have ferrielectric and multiferoic properties. The dielectric and electric properties of above mentioned materials have been investigated by broadband dielectric spectroscopy methods, which allows to analyze the collective processes related to order – disorder and displacive phase transitions, ions migration and freezing of dipoles (glassy state) in wide temperature (25 K - 500 K) and frequency (10-5 HZ - 3 GHz) ranges. By substitution or doping it becomes possible to tailor the ferroelectric materials to different properties. In this work is determined that the substitution of 10% Cu ions by Ag ions shifts the phase transition temperature of CuInP2S6 crystal toward lower temperatures while the addition of In ions shifts the phase transitions temperature toward the higher ones. The phase transition temperature difference is about 50 K for mentioned crystals. If the ferroelectric crystal CuInP2S6 is mixed with the antiferroelectric CuCrP2S6 the dipole glass phase occupies the middle of the phase diagram. The distribution of relaxation times has been calculated from the broadband dielectric spectra of dipolar glasses. The... [to full text]

With recent trends in miniaturization in the electronics sector, ferroelectrics have gained popularity due to their applications in non-volatile RAM. Taking one step further researchers are now exploring multiferroic devices that overcome the drawbacks of ferroelectric (FE) and ferromagnetic (FM) RAM’s while retaining the advantages of both. The work presented in this dissertation focuses on the growth of FE and FM thin film structures. The primary goals of this work include, (1) optimization of the parameters in the pulsed laser deposition (PLD) of FE and FM films and their heterostructures, (2) development of a structure-property relation that leads to enhancements in electric and magnetic polarizations of these structures, (3) investigation of doping on further enhancement of polarizations and coupling between the FE and FM layers. The materials of choice are La0.7Sr0.3MnO3 (LSMO) as the ferromagnetic and PbZr0.52Ti0.48O3 (PZT) as the ferroelectric component. Epitaxial thin film capacitors were grown using PLD. The work starts with the establishment of the optimum deposition conditions for PZT and goes on to describe results of attempts at performance enhancement and tuning using two methods. It is demonstrated that ferroelectric and ferromagnetic properties can be tuned by inserting a ferromagnetic buffer layer of CoFe2O4 (CFO) between PZT and LSMO. One of the key findings of this work was the anomalously high ferroelectric polarizations produced by lanthanum (La) doped PZT films. This work attempts to shine light on a possible mechanism that leads to such high enhancements in polarization.

With the overriding goal of developing functional multiferroic systems with technological potential, this thesis focuses on the role of orbital and charge ordering in coupling magnetism and ferroelectricity in synthetic quadruple perovskites. Using x-ray diffraction as the primary characterisation tool, modulations to crystal ordering have been interpreted in terms of orbital occupation and charge variation. Expanding on previous magnetic structure studies and polarisation measurements, structural analysis of CaMn₇O₁₂ has led to the experimental realisation of a new mechanism for multiferroicity, resulting from a "magneto-orbital helix". Motivated by the idea of tuning multiferroic properties through varying manganese valence, the doped system CaCu_xMn_7-xO₁₂ has been studied. Structural models considering the possibility of domain formation and multiple coexisting modulations have been tested against x-ray diffraction data. Finally, motivated by theoretical predictions of ferroelectric phases and multiferroicity in doped, simple, manganite perovskites, a structural model for the low temperature phase of NaMn₇O₁₂ has been developed, based upon theoretical predictions for orbital ordering and the experimentally determined magnetic structure. This model has been tested against previously measured neutron diffraction data. The importance of understanding crystal formation and domain structures when applying theoretical models has been highlighted, and has prompted the consideration of future work involving viewing and manipulating twin formation.