Neutron diffraction study of the itinerant-electron metamagnetic Hf0.825Ta0.175Fe2 compound

• Distinct behaviour of Fe-2a magnetic moment in the FM and AFM states.
• d ln(TFM–AFM)/dV is one order of magnitude larger than dln(MS)/dV.
• Large volume change of about ΔV/V=0.80% at the FM–AFM transition is observed
• Compressibility value of κ=0.57×10−2 GPa−1 is determined.

The temperature and pressure dependence of the crystal and magnetic structures in the Hf0.825Ta0.175Fe2 have been studied by neutron powder diffraction experiments. It is revealed that Hf0.825Ta0.175Fe2 itinerant-electron system exhibits temperature-induced transitions from ferromagnetic (FM) to antiferromagnetic (AFM) order and then to the paramagnetic (PM) state upon heating, at ambient pressure. In the ferromagnetic state, magnetic moments have been observed on the two inequivalent Fe crystal sites (2a and 6h), a magnetic moment slightly lower being refined on the Fe-2a position. In the antiferromagnetic arrangement, the magnetic moments of the Fe-6h atoms are ferromagnetically coupled within a given layer (same z coordinate) and antiferromagnetically between successive Fe6h layers; the Fe-2a atoms carry no ordered moment. The disappearance of the ferromagnetic order is accompanied by a large volume shrinkage of about ΔV/V=0.80%. The application of an external pressure leads to the progressive decrease of the unit cell parameters and a compressibility value of κ=0.57×10−2 GPa−1 has been determined, which is in good agreement with the value deduced from the theoretical calculations.