# Ultra-small moment incommensurate spin density wave order masking a ferromagnetic quantum critical point in NbFe$_2$

## TL;DR

This study reveals a long-wavelength spin density wave state in NbFe$_2$ that masks a ferromagnetic quantum critical point, with the SDW wavevector depending on composition and temperature, challenging simple band structure explanations.

## Contribution

First characterization of a SDW state in NbFe$_2$ that conceals a ferromagnetic quantum critical point, with detailed phase diagram and Landau theory analysis.

## Key findings

- SDW order depends on Fe excess y and temperature
- SDW wavevector decreases approaching ferromagnetism
- Results suggest a common origin for SDW and ferromagnetic order

## Abstract

In the metallic magnet Nb$_{1-y}$Fe$_{2+y}$, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess $y$ within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of magnetic order from Fe-rich, ferromagnetic Nb$_{0.981}$Fe$_{2.019}$ to approximately stoichiometric NbFe$_2$, in which we can, for the first time, characterise a long-wavelength spin density wave state burying a ferromagnetic quantum critical point. The associated ordering wavevector $\mathbf{q}_{\rm SDW}=$(0,0,$l_{\rm SDW}$) is found to depend significantly on $y$ and $T$, staying finite but decreasing as the ferromagnetic state is approached. The phase diagram follows a two order-parameter Landau theory, for which all the coefficients can now be determined. Our findings suggest that the emergence of SDW order cannot be attributed to band structure effects alone. They indicate a common microscopic origin of both types of magnetic order and provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder.