<p style="text-align: justify;">Kroz istoriju, otkrivanje novih materijala i njihovog dizajna dovodilo je do tehnoloških revolucija. U prošlom veku, novi materijali naprednih svojstava uveli su elektronske uređaje u svakodnevni život čoveka. Industrija mikročipova predstavlja ogroman deo svetskog tržišta, i traži neprestan razvoj da bi pružila bolje proizvode potrošačima. Početkom ovog veka, nova grupa materijala, pod nazivom multiferoici, privukla je pažnju naučno-istraživačkog društva u svetu. Ovi materijali poseduju jedinstvenu karakteristiku da istovremeno ispoljavaju više od jedne feroične osobine (feroelektričnost, feromagnetizam, feroelastičnost), a što je još važnije, mogu da ostvare interakciju među njima. Naime, magnetizacija multiferoika se može postići primenom spoljašnjeg električnog polja, a takođe se mogu i polarisati primenom spoljašnjeg magnetnog polja. Ovo vrlo interesantno svojstvo otvara potencijlanu primenu u oblasti hibridne računarske memorije, senzora, aktuatora, i dr. Sredinom prošlog 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 potrošnje električne energije, malog utroška meterijala i malih dimenzija, već i zbog dobre magnetoelektrične interakcije. Glavni cilj ove doktorske disertacije je bio određivanje optimalnog procesa sinteze/depozicije, i vršenje strukturne i funkcionalne karakterizacije multiferoičnih višeslojnih tankih filmova, sačinjenih od naizmenično deponovanih feroelektričnih BaTiO<sub>3</sub> i fero/ferimagnetnih NiFe<sub>2</sub>O<sub>4</sub> slojeva (uglavnom na silicijumskim supstratima sa slojem platine). Različite strukture slojeva dizajnirane su u cilju određivanja optimalne, koja bi dala najviše vrednosti magnetoelektričnog efekta. U prvom koraku, sintetisani su stabilni solovi/rastvori prekursora, veličine čestica od nekoliko nanometara, reoloških karakteristika pogodnih za depoziciju tehnikama iz tečne faze. Višeslojni filmovi su dobijeni "spin" procesom nanošenja, pri čemu je termički tretman svakog sloja na 500 °C bio neophodan radi potpunog otparavanja zaostalog rastvarača. Filmovi bez pukotina, ukupne debljine ispod 1 μm, uniformne debljine sloja (60 nm sloj BaTiO<sub>3</sub> i 40 nm sloj NiFe<sub>2</sub>O<sub>4</sub>) i ravne površine mogu biti dobijeni sinterovanjem u temperaturnom opsegu od 750 do 900 °C. Strukturna karakterizacija je potvrdila sistem bez prisustva sekundarnih faza, sačinjen od perovskitnog BaTiO<sub>3</sub> i spinelnog NiFe<sub>2</sub>O<sub>4</sub>. 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 povišenim temperaturama usled termičke aktivacije nosilaca naelektrisanja u feritnoj fazi. Samo čist BaTiO<sub>3</sub> film je pokazao slab feroelektrični histerezis nepotpune saturacije, malo više polarizacije filma sinterovanog na 900 °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 NiFe<sub>2</sub>O<sub>4</sub> filmove i višeslojne filmove. Analizom višeslojnih 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 površina, 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 °C was necessary in order to completely evaporate traces of residual solvents. Crack free films with overall thickness below 1 μm, 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 °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 °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>
| Identifer | oai:union.ndltd.org:uns.ac.rs/oai:CRISUNS:(BISIS)104952 |
| Date | 06 November 2017 |
| Creators | Bajac Branimir |
| Contributors | Srdić Vladimir, Cvejić Željka, Stojanović Goran, Milanović Marija, Stijepović Ivan |
| Publisher | Univerzitet u Novom Sadu, Tehnološki fakultet Novi Sad, University of Novi Sad, Faculty of Technology at Novi Sad |
| Source Sets | University of Novi Sad |
| Language | Serbian |
| Detected Language | English |
| Type | PhD thesis |
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