Field tuned ferromagnetic quantum criticality in Fe-rich NbFe2 itinerant transition metal system

Giri Mani

Research output: ThesisDoctoral Thesis

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Quantum criticality plays an important role in many areas of condensed matter physics. These areas include unconventional and high-temperature superconductivity and heavy fermion physics. The simplest example for quantum criticality is the ferromagnetic one. However, nature tends to avoid ferromagnetic quantum critical points. When suppressing ferromagnetic second-order phase transitions the transitions become either first-order or modulated order emerges. The system NbFe2 is a prime candidate of the latter. Additionally, large single crystals exist which have allowed neutron scattering to study the order and excitations across the composition temperature phase diagrams. In this work, the evolution of the NbFe2 system with the field has been explored. The Fe-rich samples studied contained a ferromagnetic (FM) ground state and spin density wave (SDW) and paramagnetic phase at higher temperatures. Longitudinal fields H∥c and transverse fields H∥a have been applied.

With magnetic neutron diffraction in longitudinal fields, the location of a tricritical point (TCP) has directly been observed at Htr=53 mT and Ttr = 26.5 K. In magnetic neutron diffraction in transverse field suppression of SDW order has been observed but it was not possible to follow the unmasked FM-PM to low temperature. With inelastic neutron scattering in longitudinal field the evolution of spin fluctuations across the TCP has been observed. The TCP has been found to feature simultaneously enhanced and soft FM and SDW spin fluctuations. With inelastic neutron scattering in transverse field the ferromagnetic low energy excitations have been observed to show softening and an enhancement that indicate existence of a field-induced unmasked ferromagnetic quantum critical point.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Niklowitz, Philipp, Supervisor
  • Refson, Keith, Advisor
Award date1 May 2023
Publication statusUnpublished - 17 Mar 2023


  • Quantum phase transitions
  • Ferromagetic quantum criticality
  • Magnetic excitations
  • Neutron scattering
  • Tricritical points
  • Magnetic phase transitions
  • Spin density wave
  • Spin fluctuations
  • Transition metals
  • Magnetic phase diagrams
  • NbFe2
  • single crystal

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