Študij magnetizma spinskih verig v spojini $\beta$-TeVO$_4$ z jedrsko magnetno resonanco

Novak, Špela (author), Zorko, Andrej (mentor)

V zaključni nalogi bom predstavila spojino $\beta$-TeVO$_4$ ter njene strukturne in magnetne lastnosti. Ta spojina spada med spinske verige s feromagnetno izmenjalno sklopitvijo med najbližjimi sosedi in antiferomagnetno sklopitvijo med naslednjimi najbližjimi sosedi. Zaradi geometrijske frustracije obstajajo tri različne magnetno urejene faze, v katerih so magnetni momenti drugače urejeni. To so vektorsko kiralno (VC) osnovno stanje v magnetnem polju pod 5 T, stanje spinskega gostotnega vala (SDW) pri višjih poljih in temperaturah ter stanje spinskih prog med stanjem VC in SDW. Pri merjenju z jedrsko magnetno resonanco zato za vsako fazo pričakujemo drugačen spekter. Predhodne magnetne meritve so nadalje pokazale prisotnost nenavadne spinske dinamike v fazi spinskih prog. Namen zaključne naloge je ugotoviti, kako se spektrer NMR in relaksacijski časi spreminjajo v odvisnosti od temperature, ko prečkamo posamezne magnetne prehode in s tem okarakterizirati spinsko dinamiko posameznih magnetnih faz. V ta namen sem $^{17}$O spekter NMR pomerila pri temperaturah med 2K in 50 K. Pri višjih temperaturah dobimo v spektru 20 spektralnih črt zaradi štirih različnih kristalografskih okolic iona O$^{2-}$ ter jedrskega spina I = 5/2 jeder $^{17}$O. Z nižanjem temperature pa večina črt izgine, saj ostane le še 5 spektralnih črt od kisikovega jedra, ki je najšibkeje magnetno sklopljen s sosednjimi ioni v spojini. Pri prehodu iz neurejene paramagnetne faze v urejeno amplitudno modulirano fazo se spektru spremeni oblika. Spektralne črte razpadejo v krivulje oblike U zaradi dodatnih notranjih polj. Ob prehodu v progasto fazo spektralne črte povsem izginejo. Meritev relaksacijskih časov pokaže, da se pri temperaturi prehoda iz amplitudno modulirane faze v progasto fazo spinsko-mrežni in spinsko-spinski relaksacijska časa močno zmanjšata, zaradi česar signala tam ne moremo zaznati. Po prehodu signal spet zaznamo, spekter pa dobi drugačno obliko. Ponovno vidimo več kot 5 spektralnih črt, kar je verjetno posledica prispevkov preostalih kisikovih ionov. Meritve NMR tako kažejo na zelo različno spinsko dinamiko v posameznih magnetnih fazah spojine $\beta$-TeVO$_4$.

• Language: Slovenian
• Keywords: spinske verige, nizkodimenzionalen magnetizem, geometrijska frustracija, spinske proge, telurjev vanadat $\beta$-TeVO$, jedrska magnetna resonanca (NMR), spisnki odmev, spinsko-mrežna relaksacija, spinsko-spinska relaksacija
• Work type: Final paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-102872
• COBISS.SI-ID: 3230308
• Publication date in RUL: 09.09.2018

Secondary language

• Language: English
• Title: Nuclear magnetic resonance study of magnetism of spin chains in the $\beta$-TeVO$_4$ compound

Abstract

In this thesis, I will present the $\beta$-TeVO$_4$ compound and its structural and magnetic properties. It exhibits spin chain behavior with ferromagnetic exchange coupling between nearest neighbors and antiferromagnetic coupling between next nearest neighbors. Geometric frustration induces various arrangements of magnetic moments in different phases of this compound. They consist of a vector-chiral (VC) groundstate at magnetic fields under 5 T, a spin-density-wave (SDW) at higher fields and a spin stripe phase in between the VC and SDW. Therefore, we expect distinct NMR spectrum for each of the phases. Previous magnetic measurements have demonstrated the presence of unusual spin dynamics in the spin stripe phase The objective of this thesis is finding how the NMR spectrum and relaxation times change with temperature across different magnetic phase transitions and thereby characterize the spin dynamics of each phase. I measured $^{17}$O spectrum in the temperature region between 2 and 50 K. At higher temperatures, we observe 20 spectral lines due to four different crystallographic sites of the O$^{2-}$ ions and the nuclear spin of $^{17}$O I = 5/2. With decreasing temperature, only 5 spectral lines remain, corresponding to the oxygen ion that is least coupled to the neighbouring ions. At the transition from the disordered paramagnetic phase to the spin-density-wave phase, the shape of the spectrum changes. Spectral lines split into U-shaped curves due to additional internal fields. At the transition into the stripe phase, spectral lines disappear. Relaxation-time measurements show a decrease of the spin-lattice and spin-spin relaxation times at the transition temperature from the spin-density-wave phase phase to the stripe phase, which is why the signal cannot be detected. Below the transition, the signal reappears with a different spectrum of more than 5 spectral lines, most likely corresponding to the remaining oxygen ions. Therefore, NMR measurements clearly show very distinct spin dynamics in each of the magnetic phases of $\beta$-TeVO$_4$.

• Keywords: spin chains, low-dimensional magnetism, geometrical frustration, spin stripes, tellurium vanadate $\beta$-TeVO$, nuclear magnetic resonance (NMR), spin echo, spin-lattice relaxation, spin-spin relaxation