Dr Benjamin Huddart

Abstract:

Skyrmions are particlelike vortices of magnetization with nontrivial topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions (DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd- and Eu-based skyrmion-lattice (SL) hosts with zero DMI, where both the SL stabilization mechanisms and magnetic ground states remain controversial. We address these here by investigating both the static and dynamical spin properties of the centrosymmetric SL host Gd₂⁢PdSi₃ using muon spectroscopy. We find that spin fluctuations in the noncoplanar SL phase are highly anisotropic, implying that spin anisotropy plays a prominent role in stabilizing this phase. We also observe strongly anisotropic spin dynamics in the ground-state (IC-1) incommensurate magnetic phase of the material, indicating that it hosts a meronlike multi-q structure. In contrast, the higher-field, coplanar IC-2 phase is found to be single q with nearly isotropic spin dynamics.

Abstract:

We present the results of microscopic investigations of antiferromagnetic CeNiGe3 using neutron powder diffraction (NPD), inelastic neutron scattering (INS), and muon spin relaxation (𝜇⁢SR) measurements. CeNiGe3 crystallizes in a centrosymmetric orthorhombic crystal structure (space group 𝐶⁢𝑚⁢𝑚⁢𝑚) and undergoes antiferromagnetic (AFM) ordering. The occurrence of long-range AFM ordering at 𝑇N≃5.2K is confirmed by magnetic susceptibility, heat capacity, neutron diffraction, and 𝜇⁢SR measurements. The NPD data characterize the AFM state with an incommensurate helical magnetic structure having a propagation vector k = (0, 0.41, 1/2). In addition, INS measurements at 10 K identified two crystal electric field (CEF) excitations at 9.17 meV and 18.42 meV. We analyzed the INS data using a CEF model for an orthorhombic environment of Ce3+ (𝐽=5/2) and determined the CEF parameters and ground state wave functions of CeNiGe3. Moreover, zero-field 𝜇⁢SR data for CeNiGe3 at 𝑇<𝑇N show long-range AFM ordering with three distinct oscillation frequencies corresponding to three different internal fields at the muon sites. The internal fields at the muon-stopping sites have been further investigated using density functional theory calculations.