Wave Vector Difference of Magnetic Bragg Reflections and Low Energy Magnetic Excitations in Charges-tripe Ordered La2NiO4.11

We report on the magnetism of charge-stripe ordered La2NiO4.11 +/- 0.01 by neutron scattering and mu SR. On going towards zero energy transfer there is an observed wave vector offset in the centring of the magnetic excitations and magnetic Bragg reflections, meaning the excitations cannot be described as Goldstone modes of the magnetic order. Weak transverse field mu SR measurements determine the magnetically order volume fraction is 87% from the two stripe twins, and the temperature evolution of the magnetic excitations is consistent with the low energy excitations coming from the magnetically ordered volume of the material. We will discuss how these results contrast with the proposed origin of a similar wave vector offset recently observed in a La-based cuprate, and possible origins of this effect in La2NiO4.11.

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Artem Korshunov, Irina Safiulina

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

2020

Magnetic model in multiferroic NdFe3(BO3)(4) investigated by inelastic neutron scattering

Henrik Moodysson Rønnow

We performed inelastic neutron scattering measurements on single crystals of NdFe3((BO3)-B-11)(4) to explore the magnetic excitations, to establish the underlying Hamiltonian, and to reveal the detailed nature of hybridization between the 4f and 3d magnetism. The observed spectra exhibiting a couple of key features, i.e., anticrossing of Nd and Fe excitations and anisotropy gap at the antiferromagnetic zone center, are explained by the magnetic model including spin interaction in the framework of weakly coupled Fe3+ chains, interaction between the Fe3+ and Nd3+ moments, and single-ion anisotropy derived from the Nd3+ crystal field. The combination of the measurements and calculations reveals that the hybridization between 4f and 3d magnetism propagates the local magnetic anisotropy of the Nd3+ moment to the Fe3+ network, leading to the determination of the bulk structure of both electric polarization and magnetic moment in the multiferroics of the spin-dependent metal-ligand hybridization type.

2015

Neutron spectroscopy in the layered quantum magnet SrCu₂(BO₃)₂ and in transition metal phosphorus trisulfides (MPS₃)

Diane Virginie Marie Lançon

Collective 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.

EPFL2017