Ultrasmall Moment Incommensurate Spin Density Wave Order Masking a Ferromagnetic Quantum Critical Point in NbFe_{2}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Magnetism

Background:

• The metallic magnet Nb_{1-y}Fe_{2+y} exhibits complex magnetic behavior near stoichiometry.
• Investigating the low-temperature threshold of ferromagnetism requires precise control over composition.

Purpose of the Study:

• To characterize the magnetic order in Nb_{1-y}Fe_{2+y} across a range of iron excess (y).
• To elucidate the relationship between ferromagnetism and spin density wave (SDW) states.
• To provide experimental constraints for theoretical models of magnetic quantum criticality.

Main Methods:

• Elastic neutron scattering was employed to probe magnetic structures.
• Compositional variation (y) was used to tune the material towards and away from stoichiometry.
• Temperature dependence was studied to map magnetic phase transitions.

Main Results:

• A long-wavelength spin density wave (SDW) state was identified in approximately stoichiometric NbFe_{2}.
• The SDW ordering wave vector (q_{SDW}) showed significant dependence on composition (y) and temperature (T).
• The observed phase diagram is consistent with a two-order-parameter Landau theory.

Conclusions:

• The emergence of SDW order is not solely due to band structure effects.
• A common microscopic origin for both ferromagnetism and SDW order is suggested.
• Findings constrain theoretical models, including those involving quantum order by disorder.