Biesenkamp, S.

2022

ISBN: http://nbn-resolving.de/urn:nbn:de:hbz:38-633471

Abstract: This thesis reports thorough studies on multiferroic and frustrated systems, whereby specific focus was put on the investigation of multiferroic domain dynamics. For the respective experiments mainly neutron scattering techniques ranging from conventional diffraction, inelastic scattering, neutron polarization analysis to Larmor diffraction were utilized. The well-known multiferroic tungstate MnWO4 and the structurally related doubletungstate system NaFe(WO4)2 both exhibit competing commensurate and incommensurate order at low temperature. Within the course of this thesis, it was possible to observe a structural dimerization in the respective commensurate spin up-up-down-down phases, which is caused by a bond-angle modulation due to the alternating ferro- and antiferromagnetic exchange along the Fe3+, respectively Mn2+ chain. A structural refinement yielded a bond-angle modulation of about ±1.15(16)° in the ac-plane for NaFe(WO4)2, which is however estimated to be of similar size in MnWO4. For MnWO4 it was further seen that second and third harmonic reflections evolve in the incommensurate multiferroic phase, whereby the odd harmonic reflection diverges in contrast to the even one, when approaching the lower transition to commensurate spin up-up-down-down ordering. The developing third harmonic and the related squaring up of spins can hence be seen as precursor of the lower commensurate phase and refers to tiny commensurate fragments that evolve already in the multiferroic phase. They are capable to effectively depin multiferroic domains and in view of the investigation of the multiferroic relaxation behavior it was thus seen beneficial to study multiferroic spiral type-II systems that exhibit simple phase diagrams, which do not exhibit an interference of incommensurate and commensurate ordering. A potential candidate was considered to be LiFe(WO4)2, which is so far the only multiferroic double-tungstate system. In this thesis, a thorough single-crystal study and the respective characterization of both low-temperature phases are presented. The magnetic refinements yielded that the system first develops below T = 22.2K a spin-density wave with moments that are lying within the ac-plane. Further, the refinement of a low-temperature data set showed that below T = 19K the magnetic arrangement in the multiferroic phase transforms into a spiral-spin structure, which exhibits an additional b-component. Hence, LiFe(WO4)2 possesses a typical and simple sequence of magnetic phases for a spiral type-II multiferroic, whose ferroelectric polarization can be well described in the framework of the Dzyaloshinskii-Moriya interaction. Temperature dependent measurements of the chiral ratio by neutron polarization analysis further revealed a finite value of it at low temperature, which thus implies an imbalanced multiferroic domain distribution even in zero-field. Another multiferroic system that exhibits an equivalent simple phase diagram is NaFeGe2O6. This thesis reports studies on the magnetic structure and the multiferroic domain dynamics. The respective investigation showed that the system does not possess solely magnetic components in the ac-plane but simultaneous to the lock-in of the incommensurate modulation it develops an additional b-component in the multiferroic phase. The time-resolved analysis of the multiferroic domain inversion revealed that the relaxation behavior can be well described by a combined Arrhenius-Merz law over a broad temperature and field range. Even at very low temperature the relaxation behavior follows the description for thermally activated domain-wall motion, whereby no indication of emerging quantum driven domain wall tunneling processes was seen. Unpolarized inelastic neutron scattering experiments further showed that the fasted speed of domain inversion, respectively of domain-wall motion at the multiferroic transition temperature is comparable to the spin-wave velocity. In the further course of this thesis also the relaxation behavior of other multiferroic systems is reported. In this context it was possible to observe that the molecular system (NH4)2[FeCl5 (H2O)], the Kagomé staircase material Ni3V2O8 and the high-temperature multiferroic compound CuO also follow the combined Arrhenius-Merz law for thermally activated multiferroic domain inversion over a finite field and temperature range. The investigations of relaxation times in the course of this thesis were performed by time-resolved studies of the domain population, whose magnitude is encoded in the chiral ratio. Complementary, the time-resolved domain size development can be deduced from the correlation length, which can be deduced from the respective peak widths. The observation of increasing domain sizes during a multiferroic inversion process is however commonly hindered by the finite Q-space resolution at conventional diffraction instruments. This thesis documents time-resolved Larmor diffraction experiments, which represent a different approach to sense the correlation length. Here, the respective scattering information is not encoded in the scattering angle but in the Larmor precession phase of the neutron spin, when it transits magnetic-field regions before and behind the sample. This makes the demanded scattering information independent of a finite beam collimation or a finite wavelength spread, which thus tremendously enhances the achievable resolution in the course of scattering experiments. However, it turned out that the alteration of the beam polarization by chiral scattering processes enormously impacts the Larmor diffraction signal and thus necessitates appropriate corrections. Apart from the studies on multiferroic systems, this thesis reports also on a detailed investigation of the magnetic structure in the Ising-like spin-1/2 chain material BaCo2V2O8, when a transverse magnetic-field is applied along [1 1 0] direction. In this context, it was possible to observe a finite peak splitting of the commensurate magnetic reflection at finite field, whereby the onset of this high-field incommensurate order is discussed within the framework of a Tomonaga-Luttinger phase, which is potentially driven by longitudinal staggered fields along the easy-axis.

Keyword(s): Magnetic Materials (1st) ; Condensed Matter Physics (2nd) ; Magnetism (2nd)

1. KOMPASS (KOMPASS)
2. HEIDI (HEIDI)

1. KOMPASS: Cold three axes spectrometer with polarization analysis (NL1)