Scrutinizing the sharp magnetoelastic transition and kinetic arrest in Fe49Rh51 alloy using neutron thermo-diffraction

Abstract

The Fe₄₉Rh₅₁ bulk alloy undergoes a sharp first-order magnetostructural transition from the antiferromagnetic (AFM) to ferromagnetic (FM) state around 332 K, accompanied by a drastic change of around 0.8% in the unit cell volume. Neutron thermo-diffraction experiments have been carried out to investigate the concomitant coupling between spin and lattice degrees of freedom in detail. Although it seems that the alloy entirely changes from the AFM to FM order in a very narrow temperature range (with a hysteresis of about 6 K), evidence of AFM order persists even 70 K above the first-order phase transition, suggesting a kinetic arrest of the AFM phase during both heating and cooling procedures. The estimated value for the Fe magnetic moment in the AFM phase at room temperature, around μ_Fe ≈ 3.4μ_B, agrees with those already reported and reaches 3.8μ_B at T = 10 K. However, in the FM phase, μ_Fe decreases to ≈ 2.3μ_B, while Rh acquires a magnetic moment of around 0.9μ_B. The use of temperature first-order reverse curves of neutron thermo-diffraction gives additional information about the magnetostructural coupling within the transition. Time-resolved neutron diffraction patterns collected at selected temperatures show that the alloy fully relaxes above the transition temperature, with both the magnetic and structural transformations occurring at the same temperature and with similar relaxation times.