Регулярный семинар аспирантов и молодых ученых КФТИ Вторник 15. 0 0

Регулярный семинар аспирантов и молодых ученых КФТИ Вторник 15.00

1.Цели и задачи 2.Структура докладов 3.Пожелания к презентациям 4.Обязательная информация при подготовке к докладу 5.Слушатели

1.Цели и задачи • Научиться выступать с докладами и делать презентации • 2.Получить информацию о проблемах, объектах и методах • 3.«Обкатать» выступления на конференциях, защитах и т.п. • 4.Учебные лекции сотрудников института и гостей

2.Структура докладов • Заголовок, автор • План • Краткий обзор проблемы, мотивация • Описание исследуемой системы. • Используемый метод • Полученные результаты • Выводы • Заключение, планы, благодарности, список соавторов

Leibniz Institute for Solid State and materials research Dresden Electron Spin Resonance in Vanadium Oxide Nanotubes (VOx-NT) Yulieth Cristina Arango, IFW-Dresden

Outline ● Motivation ● Why VOx-NTs are interesting? ● Introduction ● Morphology and structure of VOx-NT ● Basic principles on ESR ● Experimental data and discussion ● VOx-NTs – Temperature dependence in ESR and comparison with magnetization data ● Influence of Lithium doping - ESR measurements ● Conclusions & Outlook

Why VOx-NT becomes interesting Nanoscale magnetic materials novel aspects in magnetic structure and dynamic on the nanometer scale Mixed valence Nanoscale materials based on coexistence of two valence states transition-metal (TM) oxide low-D structural units and V5+ V4+ 3d0 Configurations of a partially filled d-shell Strong electronic correlations in the spin and charge sectors 3d1 ferromagnetism by doping with either electrons or holes VOx-NT and its character of mixed valence vanadium oxide

TEM Synthesis &Morphology Hydrothermal procedure Multiwall NT + scroll TEM image of VOx-NT VOx-layer Outer diameter: Range from 50 to 100 nm Organic molecules Dodecylamine, C12H25NH2 Inner diameters: Between 20 and 30 nm

V V O O Structure side view Electron diffraction pattern Two dimensional quasi-tetragonal cell Distorted octahedron tetrahedron Similar structure in BaV7O16x nH2O V7O16 layer edge-sharing VO-Octahedra coupled in zig- zag chains isolated VO-Tetrahedra

VO2 V4+ V4.4+ V 2p V5+ Vanadium valence Binding energy in-between mixed valency V2O5 Increase in the number of the 3d electrons downshift the binding energy Expected: 60% V4+ [3d1] , magnetic (S = ½) 40% V5+ [3d0] , non-magnetic (S = 0) Core level photoemission(XPS) data from the V 2p levels of VOx-NT Found:(Magnetization and NMR measurements) 14% free spinsTetrahedral coordination + 28% spins coupled in dimers + 18% spins coupled in trimers Octahedral coordination }

Energy RF-induced transition H0 Increasing magnetic field B Electron Spin Resonance ESR Absorption: P dP/dH Electron magnetic moment Zeeman Energy: Describes the Interaction between the spin and the field. Hres: g-factor The energy separation of the two spin states: Line- width: Spin-spin interaction Spin angular momentum Area: Spin susceptibility

b1t b2t a1t t2g eg et 2L+1 = 5 free Vanadium O O V V V x2-y2 x2-y2 3z2-r2 3d Energy 3z2-r2 xy xy xz yz xz yz Effective magnetic field experienced by each electron Zeeman interaction Electron-electron interaction Spin-orbit coupling Hiperfine Coupling Crystal field V4+ in cubic symmetry V4+ in tetragonal symmetry [3d1], S=1/2 V: [Ar] 3d3 4s2

ESR Signal from VOx-Nanotubes X-band ESR Broad line 9.52 GHz Field = 0 G to 9000 G Two Lorentzian lines Two nonequivalent magnetic V4+ (S = 1/2) sites Narrow line Line 1:Individual V spins in tetrahedral positions Line 2:spin trimers andthermally excite spin dimers in distorted octahedral positions

» Quasi-free spins in V(3)-sites 14 % Dimers in the chains High temperature 28 % Trimers in the chains 18 %  = 4.4 K Dimers (thermally activated) Dimers (thermally activated)

Conclusions • Undoped VOx-NTs: • Different spin species identified by ESR • - Individual spins Equal amount • - Trimers • - Dimers Thermally activated above 100 K • Good agreement with Magnetization measurements and NMR • Li-doped VOx-NTs: • Focus on the Ferromagnetic sample Li = 0.1 • - Individual spins and trimers present • - Dimers do not survive • - Additional narrow line Ferromagnetic resonance ?? }

Outlook • Determine Li position (X-rays) • Spin dynamics vs. Lithium concentration by ESR and NMR • Confront ferromagnetism • ESR on newly prepared samples • High field ESR (Good spectral resolution)

Acknowledgements B. Büchner, V. Kataev Christian, Ferenc, Uwe and Mohammed from the ESR group Christine Täschner, Andreia Ioana Popa, Rüdiger Klingeler, Ingo Hellmann, Jenia Vavilova Financial support by the Alban Office

3.Пожелания к презентациям 1.Больше наглядной информации 2.Выделяйте главное 3.Не увлекайтесь подробностями и деталями, малопонятными неспециалисту. 4.Объясняйте «то, что всем известно» На Вашем докладе Вы – лектор, мы – студенты, расскажите так, чтоб было понятно студенту.

ESR Signal from VOx-Nanotubes

ESR Signal from VOx-Nanotubes X-band ESR 9.52 GHz Field = 0 G to 9000 G Two Lorentzian lines Two nonequivalent magnetic V4+ (S = 1/2) sites Line 1: g=2.0 Individual V spins in tetrahedral positions Line 2:g=1.97 spin trimers andthermally excite spin dimers in distorted octahedral positions

ESR Signal from VOx-Nanotubes X-band ESR Broad line 9.52 GHz Field = 0 G to 9000 G Two Lorentzian lines Two nonequivalent magnetic V4+ (S = 1/2) sites Narrow line Line 1:Individual V spins in tetrahedral positions Line 2:spin trimers andthermally excite spin dimers in distorted octahedral positions

4.Обязательная информация при подготовке к докладу Не позже, чем за неделю до семинара: 1.Название доклада 2.Лаборатория, отдел, группа 3.Фамилия, координаты руководителя 4.Кандидаты в зрители  5. Краткое CV pishi_jenia@rambler.ru petrushkin@inbox.ru sufia@mail.ru

5.Слушатели • Во время доклада – только технические вопросы. • После доклада – максимум вопросов! Вы здесь, чтоб узнать новое! • После вопросов и ответов – замечания, мнения и пожелания по поводу представления доклада.

5.Слушатели 30-45 минут!!!

Спасибо за внимание! До встречи 07.10 в 15:30

Регулярный семинар аспирантов и молодых ученых КФТИ Вторник 15. 0 0

Регулярный семинар аспирантов и молодых ученых КФТИ Вторник 15. 0 0

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