Abstract
Many experimental and theoretical studies suggest that it is difficult to approach ferromagnetic (FM) quantum critical point (QCP) in real materials. Instead, a variety of escape routes have been observed, notably the occurrence of a first order transition or superconductivity. The bulk properties of the C14 Laves phase Nb_{(1y)}Fe_{(2+y)} present a third scenario: marginal Fermi liquid behaviour as expected of a ferromagnetic quantum critical point (FM QCP) [1], but also masking of the FM QCP itself by a spin density wave (SDW) order [2]. Polarised neutron diffraction measurements in the FM state have shown that the direction of the magnetic moment is along c. The ordering wave vector QSDW of the SDW as well as the temperature T and composition dependence y of QSDW have been directly observed by elastic unpolarised neutron scattering on several singlecrystalline samples [3].
We also collected and analysed comprehensive inelastic neutron scattering data which reveals the position in the reciprocal space q, T and y dependence of lowenergy excitations in a range covering the paramagnetic (PM), the SDW and the FM states. Those results show softening of the low energy magnetic excitations in a broad q range and a divergence of the inverse linewidth in energy in a considerable region of the phase diagram near the SDW phase. The observed excitation pattern reflects the simultaneous proximity of the Nb_{(1y)}Fe_{(2+y)} system to two types of magnetic order, which makes this a candidate system for SDW order emerging from a FM QCP. We use the models given by the spin fluctuation theory to discuss our observations.
[1] M Brando, WJ Duncan, D MoroniKlementowicz, C Albrecht, D Grüner, Rafik Ballou, and FM Grosche. Logarithmic fermiliquid breakdown in NbFe2. Physical review letters, 101(2):026401, 2008.
[2] D Rauch, M Kraken, FJ Litterst, S Süllow, H Luetkens, M Brando, T Förster, J Sichelschmidt, A Neubauer, C Pfleiderer, et al. Spectroscopic study of metallic magnetism in singlecrystalline Nb1yFe2+y. Physical Review B, 91(17):174404, 2015.
[3] P. G. Niklowitz, M. Hirschberger, M. Lucas, P. Cermak, A. Schneidewind, E. Faulhaber, J.M. Mignot, W. J. Duncan, A. Neubauer, C. Pfleiderer, and F. M. Grosche. Ultrasmall moment incommensurate spin density wave order masking a ferromagnetic quantum critical point in NbFe2. Phys. Rev. Lett., 123:247203, Dec 2019.
We also collected and analysed comprehensive inelastic neutron scattering data which reveals the position in the reciprocal space q, T and y dependence of lowenergy excitations in a range covering the paramagnetic (PM), the SDW and the FM states. Those results show softening of the low energy magnetic excitations in a broad q range and a divergence of the inverse linewidth in energy in a considerable region of the phase diagram near the SDW phase. The observed excitation pattern reflects the simultaneous proximity of the Nb_{(1y)}Fe_{(2+y)} system to two types of magnetic order, which makes this a candidate system for SDW order emerging from a FM QCP. We use the models given by the spin fluctuation theory to discuss our observations.
[1] M Brando, WJ Duncan, D MoroniKlementowicz, C Albrecht, D Grüner, Rafik Ballou, and FM Grosche. Logarithmic fermiliquid breakdown in NbFe2. Physical review letters, 101(2):026401, 2008.
[2] D Rauch, M Kraken, FJ Litterst, S Süllow, H Luetkens, M Brando, T Förster, J Sichelschmidt, A Neubauer, C Pfleiderer, et al. Spectroscopic study of metallic magnetism in singlecrystalline Nb1yFe2+y. Physical Review B, 91(17):174404, 2015.
[3] P. G. Niklowitz, M. Hirschberger, M. Lucas, P. Cermak, A. Schneidewind, E. Faulhaber, J.M. Mignot, W. J. Duncan, A. Neubauer, C. Pfleiderer, and F. M. Grosche. Ultrasmall moment incommensurate spin density wave order masking a ferromagnetic quantum critical point in NbFe2. Phys. Rev. Lett., 123:247203, Dec 2019.
Original language  English 

Qualification  Ph.D. 
Awarding Institution 

Supervisors/Advisors 

Award date  1 Feb 2020 
Publication status  Unpublished  20 Dec 2019 
Keywords
 ferromagnetism
 quantum criticality
 neutron scattering
 polarised neutron diffraction
 spin density wave
 damped harmonic oscillator
 spin fluctuations
 NbFe2
 doping
 single crystal