Emerging order and excitations at the border of metallic magnetism in NbFe2

Magnetically correlated itinerant-electron systems exhibit a rich array of unconventional
metallic behaviors and emergent electronic phases. More recently, it has become clear
that ferromagnetic quantum critical points are often avoided, giving way to unconven-
tional phase diagrams and the emergence of modulated (e.g., spatially inhomogeneous)
phases. Against this backdrop, this thesis examines the magnetic behavior of the in-
termetallic compound NbFe2, a material poised at the verge between ferromagnetism
and paramagnetism. Using spherical neutron polarimetry, unpolarized neutron diffrac-
tion, and triple-axis neutron spectroscopy, it delivers a comprehensive picture of the low-
temperature magnetic ground state and its excitations. The central finding is a longi-
tudinal spin-density wave (SDW) in stoichiometric NbFe2, with moments aligned along
the easy c-axis. The SDW consists of weakly ferrimagnetic sheets stacked antiparallel
and exhibits an exceptionally small ordered moment of ∼ 0.039 μB per atom—among the
weakest complex magnetic structures reliably refined to date.
Inelastic neutron scattering further uncovers a nuanced excitation spectrum featur-
ing overdamped magnetic modes. These reflect strong fluctuations tied to the nearby
quantum critical point and are captured within a renormalized dynamical susceptibility
framework. The results reveal the coexistence and competition of ferromagnetic and SDW
fluctuations, a continuous phase transition at an applied field of 3.3 Tesla but without
critical fluctuations before more conventional Zeeman like induced fluctuations for higher
applied fields, underscoring how itinerant magnetism and spin fluctuations shape the
ground state of NbFe2. More broadly, the work advances our understanding of quantum
phase transitions in itinerant electron systems and sets a benchmark for probing fragile
magnetic orders with state-of-the-art neutron techniques.