Title Local probe investigation of trivalent rare-earth titanates Title (english) Istraživanje trovalentnih titanata rijetkih zemalja lokalnim probama Author Ana Najev Mentor Damjan Pelc (mentor)Committee member Miroslav Požek (predsjednik povjerenstva)Committee member Ivan Kupčić (član povjerenstva)Committee member Marina Ilakovac Kveder (član povjerenstva)Committee member Vladislav Kataev Committee member Damjan Pelc (član povjerenstva)Granter University of Zagreb Defense date and country 2023-06-05, Croatia Scientific / art field, NATURAL SCIENCES Universal decimal classificationUDC ) 53 - Physics Abstract This thesis contains an in-depth experimental investigation of trivalent rare-earth titanates. Titanates are a model system for exploring intricate interactions between spin, orbital, and lattice degrees of freedom. These compounds belong to a broader family of transition metal oxides. Their ground state can be easily manipulated by strain, chemical, and charge doping, offering a unique opportunity to explore the Mott insulator phase and metal-insulator transitions. One of the foremost open questions is the orbital state of titanates which we try to resolve with this thesis. We present a detailed nuclear magnetic resonance (NMR) and electron spin resonance (ESR) study of single-crystalline samples of (Y,Ca,La)TiO_3 in a wide range of isovalent substitution (La) and hole doping (Ca). 89^Y NMR demonstrates a clear discrepancy between the static and dynamic local magnetic susceptibilities, with deviations from Curie-Weiss behavior far above the Curie temperature 𝑇_𝐶. ESR of the unpaired Ti electron shows broad resonance lines at all temperatures and substitution/doping levels, which we find to be caused by short electron spin-lattice relaxation times. Modeling of the relaxation as an Orbach process revealed a small gap likely produced by Jahn-Teller splitting of the two lower Ti 𝑡_2𝑔 orbitals. We find that the value of the gap closely follows 𝑇_𝐶 and is consistent with the temperatures at which deviations from Curie-Weiss fluctuations are observed in NMR. These results indicate that full orbital degeneracy lifting is associated with ferromagnetic order. Ferromagnetic resonance with orientational dependence on the three primary YTiO_3 axes was successfully measured and modeled. Free energy description of the data revealed values for the parameters reflecting the inherent magnetocrystalline anisotropy. Furthermore, we present measurements of uniaxial strain on a range of (Y,Ca,La)TiO_3 single crystals and its influence on the bulk ferromagnetism. The results demonstrate direct, reversible and continuous control of ferromagnetism by influencing the TiO6 octahedral tilts and rotations. Ab initio calculations, provided by our collaborators, agree well with the experimental data and offer a better understanding of the complex interactions among structural, orbital and magnetic properties that drive the physics of rare-earth titanates.
Abstract (croatian) Rezultati ovog rada daju jedinstven i detaljan uvid u lokalne interakcije u titanatima rijetkih zemalja. Mjerenja lokalnih statičkih i dinamičkih susceptibilnosti s pomoću reix zonantnih tehnika upotpunjavaju fazni dijagram ne samo stehiometrijskog spoja YTiO_3, već i čvrstih otopina Y(Ca,La)TiO_3 na kojima do sada nije bilo eksperimentalnih rezultata. Pokazali smo da postoje anomalna ponašanja u paramagnetskoj fazi titanata, koja se posebno očituju u relaksacijskim mjerenjima detektiranim NMR tehnikom. Snažna divergencija relaksacije 1/𝑇_2 na temperaturama ≈ 70K nije objašnjena, što upućuje na potrebu boljeg razumijevanja važnosti mehanizma relaksacije spin-spin u titanatima i srodnim oksidima prijelaznih metala. Jedinstven uvid u orbitalno stanje titanata dobili smo mjerenjem elektronske spinske rezonancije. Potvrdili smo postojanje malog procjepa između najnižih nivoa 𝑡_2𝑔 orbitala te utvrdili jasnu vezu s feromagnetizmom ovih spojeva. Naši rezultati potvrđuju važnost blagih Jahn-Tellerovih deformacija oktaedara za bolje razumijevanje kompleksnih interakcija u jakokoreliranim elektronskim sustavima. Ovim radom ističemo važnost korištenja rezonantnih tehnika pri istraživanju oksida prijelaznih metala. Nadalje, originalnim eksperimentom uniaksijalnog naprezanja potvrdili smo teorijska predviđanja o utjecaju kontrolirane kompresije na magnetsko osnovno stanje. Naši rezultati, uz dobro slaganje s teorijskim modelom, dali su uvid u točne distorzije rešetke koje pospješuju FM i AFM vezanja u titanatima. Ovaj jedinstveni eksperiment daje snažan poticaj za mogućnost inženjerstva željenih svojstava u materijalima s pomoću naprezanja.
Keywords
Rare-earth titanates
Mott insulator
metal-insulator transition
spin and orbital physics
nuclear magnetic resonance
electron spin resonance
ferromagnetic resonance
uniaxial strain
Keywords (croatian)
Titanati rijetkih zemalja
Mottov izolator
prijelazi metal-izolator
spinska i orbitalna fizika
nuklearna magnetska rezonancija
elektronska spinska rezonancija
feromagnetska rezonancija
jednoosno naprezanje
Language english URN:NBN urn:nbn:hr:217:962282 Promotion 2023 Project Number: IP-2018-01-2970 Project Number: 3.4-1022249-HRV-IP Study programme Title: Physics Type of resource Text Extent 143 str. File origin Born digital Access conditions Embargoed access Terms of use Created on 2023-07-14 08:25:22
