Magnetic and magnetoelectric properties of lithium orthophosphates

Ellen Fogh

Research output: Book/ReportPh.D. thesis

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Understandingtheinterplaybetweenmagneticandelectricpropertiesincondensed matter is of importance in both fundamental science and in applications such as datastorage. Inthisthesis, magneticandmagnetoelectricpropertiesofthelithium orthophosphates were studied by means of magnetization, pyrocurrent and heat capacity measurements as well as neutron diffraction, inelastic neutron scattering and Monte Carlo simulations. The magnetic phase diagram of LiCoPO4 was established up to 25.9T applied along the easy b-axis and several magnetic phases were characterized. For fields in the interval 11.9−20.5T, a magnetized elliptic cycloid structure was identified. In the hysteresis region just below 11.9T, supplementing magnetic phases with longer periods coexist and a simple model was proposed in an endeavour to explain the observed behavior. For 20.5−21.0T, the periodicity of the magnetic structure stays the same as for the cycloid but a spin re-orientation takes place. For fields greater than 21.0T, a commensurate magnetic structure which supports the magnetoelectric effect resides. Furthermore, for magnetic fields applied along a, the induced ferromagnetic moment couples via the Dzyaloshinskii-Moriya interaction to yield a field-induced spin canting. This canting is speculated to be linked to the magnetoelectric effect in the low-field commensurate phase of LiCoPO4. Using both DC and pulsed magnetic fields together with time-of-flight Laue neutron diffraction, magnetic structures of LiNiPO4 were directly probed up to a staggering 42T applied along the easy c-axis. For fields in the interval 20.9−37.6T, magnetic structure refinements indicate that a spiral is formed. Like for LiCoPO4, there is then a short interval, 37.6−39.4T, where the period of the magnetic structure remains constant but the spins re-orient. Above 39.4T, a commensurate phase which displays the magnetoelectric effect is established and the effect was encapsulated by a microscopic model. The discovery of yet another commensurate magnetoelectric phase in LiNiPO4 thus consolidates the tie between commensurability and magnetoelectricity in the lithium orthophosphates in general. Subtituting Fe on the magnetic site in LiNiPO4 creates interesting new magnetic and magnetoelectric phases in the mixed compounds, LiNi1−xFexPO4. At low Fe contents, the magnetic behavior of the parent compound, LiNiPO4, is mostly preserved. However, the manner and number of discrepancies from the stoichiometric material only increase when the Fe content is raised. In case of LiNi0.8Fe0.2PO4, the major spin component in the ground state is along a as opposed to along c for LiNiPO4 anda long b for LiFePO4. Moreover, for a temperature interval in between the ground state and paramagnetic state, another commensurate antiferromagnetic phase with spins along b exists. Upon applying a magnetic field along a at low temperatures, there is a broad phase transition at ∼8T where the magnetic structure remains commensurate but the spin orientation changes. Abnormal behavior of the energy gap in the spin-wave dispersion was observed at this field-induced transition. All observed magnetic phases in LiNi0.8Fe0.2PO4 are commensurate and as with the other lithium orthophosphates, they also support the magnetoelectric effect. The temperature and field dependencies of the magnetoelectric tensor elements prove extremely complex. Furthermore, more finite tensor elements were observed in LiNi0.8Fe0.2PO4 than in the parent compounds, LiNiPO4 and LiFePO4. Monte Carlo simulations were performed on the general LiNi1−xFexPO4 system. The observed zero-field magnetic structures were reproduced, the understanding of the various phases extended and the importance of competing anisotropies in the system illuminated. Competing exchange couplings and single-ion anisotropies lead to a plethora of magnetic phases in the lithium orthophosphates. Finding the optimal spin configuration is not trivial in such systems and introducing different ions on the magnetic site only intensifies the frustration. Although the relationship between magnetism and magnetoelectricity in the lithium orthophosphates is not yet completely clearcut, one thing is clear: all commensurate phases are also magnetoelectric.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages201
Publication statusPublished - 2018

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