Spin-density wave of ferrimagnetic building blocks masking the ferromagnetic quantum-critical point in NbFe2

Abstract

In the metallic magnet NbFe2, the low temperature threshold of ferromagnetism can be investigated by varying the Fe concentration within a narrow homogeneity range. NbFe2 is one of a number of compounds where modulated order is found to mask the ferromagnetic quantum critical point. However, here we report the rare case where the masking modulated magnetic order has been fully refined. Spherical neutron polarimetry and high-intensity single-crystal neutron diffraction reveal the first case of a longitudinal spin-density wave masking the ferromagnetic quantum critical point. The spin-density wave is characterised by a large-wavelength incommensurate modulation of its low average moment. It is formed from ferrimagnetic building blocks with antiparallel ferromagnetic sheets. The existence of ferromagnetic sheets and cancellation of the magnetisation only over mesoscopic length scales show local similarity between the spin-density wave and the ferromagnetic parent phase and indicate the spin-density wave's unconventional nature as emerging from underlying ferromagnetic quantum criticality.

Figures (3)

• Figure 1: Phase diagram of Nb$_{1-y}$Fe$_{2+y}$ with results for bulk $T_{\rm C}$ (squares) and $T_{\rm N}$ (diamonds) from previous single-crystal (filled symbols) niklowitz2019ultrasmall and polycrystal (empty symbols) mor09a studies. Vertical solid lines indicate the $T$ range of previous cold neutron diffraction measurements. 'A' and 'B' denote samples also studied with thermal neutron diffraction reported here. Of the two ferromagnetic (FM) phases, the one on the more Fe-rich side is separated from the paramagnetic (PM) state by a spin-density wave (SDW) at low temperatures, where non-Fermi liquid (NFL) behaviour is found as well. $T_{\rm 0}$, the FM phase boundary buried by the SDW phase (dashed line) is an extrapolation of $T_{\rm 0}$ values (circles) measured or calculated for the single crystals. The inset shows the relevant reciprocal-space region, which was accessible during previous neutron diffraction experiments. Circles show the presence and crosses the absence of SDW peaks.
• Figure 2: Refinement of SDW Bragg peaks from unpolarised ND by the $\Gamma_2$ representation that leads to the best agreement (see fit parameters in Table \ref{['refinement_result_pars']}).
• Figure 3: Six-unit-cell SDW structure with highlighted region (left) and zoomed-in single-unit-cell model (right), including Cartesian axes, of the ferrimagnetic building block of the incommensurate long-wavelength SDW structure of $NbFe_2$ from refinement of unpolarised ND data. Fe atoms (orange) and Nb atoms (blue) each form ferrimagnetic sublattices.