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Publication# High field magnetization of FePS3

Abstract

High field magnetization measurements in pulsed fields up to 65 T have been performed on FePS3, which is nominally a good example of a two-dimensional Ising-like antiferromagnet on a honeycomb lattice. Measurements with the field parallel to the moment direction confirm the presence of two first-order transitions above 35 T, to M/M-sat = 1/2 and M/M-sat = 1, respectively, at 4 K. The measurements are in contradiction with published estimates for the magnetic exchange parameters, but the contradiction can be resolved by allowing for anisotropic exchange parameters in the Hamiltonian. The magnetization with the field perpendicular to the moment direction is anisotropic, with no transitions observed for fields along the a axis while a cascade of first-order transitions is observed for fields above 50 T along the b axis, the latter case also showing a strong degradation of the sample after repeated pulses. The results indicate a strong magnetolattice coupling in FePS3. Temperature-dependent measurements hint at a possible tricritical point.

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Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the heavily hole-doped pnictide superconductor KFe2As2, was studied. Well-ordered VL scattering patterns were measured with fields applied in directions between B parallel to c and the basal plane, rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when B parallel to c. However, the scattering pattern distorted more strongly as the field was rotated away from the c axis. At low field, the arrangement of vortices is affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the distortion with the anisotropic London model, we obtain an estimate of similar to 3.4 for the anisotropy factor gamma between the in-plane and c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase transitions as a function of field direction. We discuss these transitions using the "hairy ball" theorem.

Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the heavily hole-doped pnictide superconductor KFe2As2, was studied. Well-ordered VL scattering patterns were measured with fields applied in directions between B∥c and the basal plane, rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when B∥c. However, the scattering pattern distorted more strongly as the field was rotated away from the c axis. At low field, the arrangement of vortices is affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the distortion with the anisotropic London model, we obtain an estimate of ∼3.4 for the anisotropy factor γ between the in-plane and c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase transitions as a function of field direction. We discuss these transitions using the “hairy ball” theorem.

2013Collective magnetic excitations are a fascinating aspect of condensed matter physics, where neutron scattering can provide valuable insight into the magnetic properties of physical realisations of model systems. This thesis focuses on the excitation spectra of layered quantum magnets in the case of the frustrated quantum magnet SrCu$_2$(BO$_3$)$_2$ and the family of quasi-2D antiferromagnets MPS$_3$, with M a transition metal. \begin{itemize}[topsep=2pt,itemsep=-0.7ex] \item SrCu$_2$(BO$_3$)$_2$ is a physical realisation of the two-dimensional Shastry-Sutherland theoretical model, constructed as orthogonal dimers with the product of singlets on the strong antiferromagnetic $J$ bond as an exact ground state. The spin interactions for such a particular geometry induces strong frustration which leads to unconventional magnetism and exotic phases of matter. This work is concerned with a series of aspects of the magnetic excitations in this compound. The excitation spectra as a function of field, temperature and pressure are measured using neutron time-of-flight spectroscopy. The experimental results show that correlations, bound magnons and finite temperature properties are highly unconventional and these results are compared with existing theories on frustrated model systems. In addition, predicted topological properties of SrCu$_2$(BO$_3$)$_2$ in an applied field are confirmed experimentally. \item The transition metal phosphorus trisulfides (MPS$_3$) are a family of quasi two-dimensional materials on a honeycomb lattice with weakly bound magnetic planes. This work focuses mainly on the exchange interactions and critical properties of FePS$_3$, which is largely anisotropic with the S=2 Fe$^{2+}$ moments pointing normal to the $(a,b)$ plane. Inelastic neutron scattering on single crystals is used to measure the spin wave dispersion, providing new insight on the strength of the coupling interactions and anisotropies and showing that FePS$_3$ is a good two-dimensional model antiferromagnet. Similar experiments on powdered samples of NiPS$_3$ show low-Q dispersive spin waves with a small spin-gap. Critical properties of FePS$_3$ close to the Néel temperature are further discussed, as the magnetic nature of the measured quasi-elastic scattering is confirmed. Based on magnetization measurements in high pulsed fields, a possible tricritical point in the 40-50T range is proposed. \end{itemize} The work presented in this thesis has been carried out in a collaboration between the Institut Laue Langevin in Grenoble and the Laboratory for Quantum Magnetism of the EPFL.