Magnetic and electronic properties of layered antiferromagnets probed by NMR
Zusammenfassung der Projektergebnisse
The primary subject of the project was the test of the Berezinskii-Kosterlitz-Thouless (BKT) transition, i.e. the freezing of antiferromagnetic vortices at the Kosterlitz-Thouless temparture TBKT . For this purpose the magnetic properties of the quasi two-dimensional XY antiferromagnet BaNi2 V2 O8 were studied. The key quantity is the temperature and magnetic field dependence of the magnetic correlation length ξ, which can be determined by measuring the nuclear relaxation times e.g. T−1 ∝ ξ 2 and the line width of the exchange 1 narrowed EPR signal ∆B ∝ ξ 3. The static magnetic susceptibility, the X-band EPR and the 51 V NMR signal of BaNi2 V2 O8 was measured in a wide temperature range from room temperature down to liquid He temperatures. In addition various concentrations of the doped samples BaNi2−x Mgx V2 O8 were investigated in order to determine model independent the magnetic correlation length via satellite lines and line broadenings. This approach turned out to be not successful. The magnetic-field dependent NMR spectra of BaNi2 V2 O8 at 2 K with the magnetic field applied parallel and perpendicular to the NiO6 planes confirm the XY character of the magnetic structure and the influence of magnetostriction. Measurements of the transversal relaxation rate T2^−1 show echo oscillations at low temperatures due to a very tiny quadrupole interaction, allowing the determination of the strength of the quadrupole interaction. Due to these echo oscillations the short relaxation time T2 < 377 µs cannot be determined in the paramagnetic phase for temperatures above TN. The NMR spectra and the longitudinal relaxation time T1 can be measured in the whole temperature range, even for temperature where the longrange order starts to develop. Due to the line splitting caused by the onset of antiferromagnetic order it is possible to determine the transition temperature TN with the high relative accuracy of 0.2 %. This enables an accurate analysis of the critical behavior in the paramagnetic phase. T1^−1 measured above TN reveals the temperature dependence of ξBKT expected for the motion of free vortices and all measurements, including the EPR line width, can be described by just one set of parameters for ξBKT confirming the theoretically expected values. Also the ratio of T1^−1 measured for parallel and perpendicular field orientation confirms the long wavelength character of excitations expected for magnetic vortices. The BKT behavior can be suppressed by applying of a strong magnetic field in the NiO6 plane forcing the magnetic moments in a spin flop like configuration reducing the XY symmetry even in the paramagnetic phase. Consequently the T1 rate displays the usual power law behavior expected for the transition from the paramagnetic phase to the spin-flop phase.
Projektbezogene Publikationen (Auswahl)
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Der Berezinskii-Kosterlitz-Thouless Übergang von BaNi2 V2 O8 untersucht mit magnetischer Resonanz. 2014
Dirk Waibel
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Determining the Berezinskii-Kosterlitz-Thouless coherence length in BaNi2V2O8 by 51V NMR. Physical Review B, Vol. 91, Iss. 21 — 1 June 2015, 214412
D. Waibel, G. Fischer, Th. Wolf, H. von Löhneysen and B. Pilawa
