Phase changing materials have long been a central theme in condensed matter physics. A representative example is an iron-rhodium (FeRh) which undergoes a first-order phase transition from antiferromagnetic to ferromagnetic phase near room temperature. Along with the change of magnetic ordering, the phase transition also accompanies a significant change of electrical conductivity of about 40% 1 as well as a small expansion of the unit cell of about 1% 2. Furthermore, the transition is modulated not only by temperature but also by magnetic field or doping 3, which makes FeRh-based materials very attractive for future applications, such as heat-mediated antiferromagnetic memory 4, magnetic refrigeration 5, and magnetic sensors 6. Despite the intensive researches, however, our understanding of phase transition of FeRh is still far from satisfactory.
The crux of this transition is a large change in the electrical conductivity, but its origin is still under debate. Such large resistance change cannot be understood solely by the change in magnetization, i.e., magnetic scattering mechanism between antiferromagnetic and ferromagnetic spin alignments. Recently, we obtained a high-quality FeRh films with a sharp phase transition, which shows no residual magnetic moment in the antiferromagnetic phase. In this study, we investigate the fundamental electrical transport of FeRh by using terahertz time-domain-spectroscopy (THz-TDS). Based on Drude model, the extrinsic (momentum scattering time, Ï„) and the intrinsic (charge density/effective mass, n/m) contributions of electrical conductivity was quantified independently. We found that, only n/m changes abruptly during the phase transition, contrary to monotonic decrease of Ï„ with increasing temperature. Therefore, our result strongly supports that the origin of the currently controversial FeRh phase transition is a change in band structure. Our work could be an important piece for the complete understanding of magnetic phase transition of FeRh.

References
1 I. Sujuki et al. J. Appl. Phys. 109, 07C717 (2011)
2 M. Ibarra et al. Phys. Rev. B 50, 4196 (1994)
3 R. Barua et al. Appl. Phys. Lett. 103, 102407 (2013)
4 X. Marti et al. Nat. Mater. 13, 367 (2014), T. Moriyama et al. Appl. Phys. Lett. 107, 122403 (2015)
5 M. P. Annaorazov et al. J. Appl. Phys. 79, 1689 (1996), V. K. Pecharsky et al. Phys. Rev. B 64, 144406 (2001)
6 J. Van Driel et al. J. Appl. Phys. 85, 1026 (1999), V. K. Pecharsk et al. J. Magn. Magn. Mater. 321, 3541 (2009)