This thesis is devoted to the investigation of static and dynamic properties of
two different sets of quantum magnets with neutron scattering techniques and
the help of linear spin wave theory.
Both systems are copper-based with spin-1/2, which makes them ideal to
study the interplay between purely quantum and semi-classical effects.

I start with the analysis of the antiferromagnet SeCuO3, which has a canted spins
structure. Through careful
inelastic neutron scattering experiments on thermal and cold triple-axis spectrometers,
I demonstrate that this compound exhibits three primary types of excitations that are
intrinsically opposite : spin waves (magnons), singlet to triplet excitations
(triplons), and fractional spins excitations (spinons).
Such a strong coexistence and interdependence of these collective excitations has not
been observed yet, thereby the quantification and description of the excitations in
SeCuO3 leads the way to further theoretical work on multi-excitation spin systems,
as well as the existence of quantum effects in high dimensional systems.\

My second project is on the extraction of the magnetic structure of three members of the
A(BO)Cu4(PO4)4 chiral family, namely (A; B) = (Ba; Ti), (Sr; Ti) and (Pb; Ti),
from spherical neutron polarimetry measurements. I prove that the first two compounds exhibit
a highly non-collinear magnetic structure, with the Cu spins forming clusters of 'two-in--two-out'
arrangements on each structural unit. This structure is stabilised by the presence of a strong
Dzyaloshinskii-Moriya interaction, and explains the observation of magnetoelectric effects as
emerging from quadrupole moments. The analysis of the latter compound did not lead to the confirmation
of its magnetic structure due to strong nuclear-magnetic interference.

I conclude this thesis by the investigation of the magnetic excitation spectrum of some members
of the (A; B) family, probed by inelastic neutron scattering measurements. Indeed, its particular
crystallographic structure makes it an ideal playground to study tetramerisation effects on the
two dimensional square lattice.
Additionally, the aforementioned Dzyaloshinskii-Moriya interaction ensures the presence of a structural
gap, which competes with the quantum one emerging from tetramerisation effects. Using linear spin wave
theory, I describe (Ba; Ti) as a chequerboard system with almost equal intra- and inter-plaquette
couplings, with weak quantum effects. I also provide a qualitative description of (Pb; Ti), which exhibits
similar physics, and conclude by presenting the first results on the highly symmetric compound (K; Nb),
which shows hints of a strong quantum behaviour.