Unraveling the physical properties and superparamagnetism in anti-site disorder controlled Fe₂TiSn

Abstract

With an aim to control the anti-site disorder between Fe and Ti atoms in the full Heusler alloy, FeTiSn, we substitute a small percentage of Ti at Fe site to form the FeTiSn () series. Using the incident x-rays tuned to the Fe K-edge absorption energy, we record the high resolution synchrotron x-ray diffraction profiles and unambiguously show the reduction in anti-site disorder. In particular, the Fe–Ti anti-site disorder decreases up to an excess Ti content of 0.07; further increase of Ti content leads to disorder between Ti–Sn sites. Detailed characterization vis-á-vis the excess Ti content has been carried out in terms of its thermal and electrical transport, and magnetic properties. Signatures of strong spin fluctuation are seen in all the physical properties reported here. The much disputed high value of the Sommerfeld constant has been shown to be a resultant of such strong spin fluctuations, thus ruling out the long standing controversy of heavy fermionic nature of FeTiSn. Magnetization and the Seebeck coefficient show clear dependence on the disorder. Both dc and ac magnetic measurements reveal the low temperature superparamagnetic nature of this system, comprising of large magnetic clusters 3 nm in size.

Transport properties of Heusler compounds and alloys

“…The zero-field T variation of ρ of CoFeVGa shows a semiconducting character (see figure 35(a)) at least down to 90 K associated with a broad hump-like feature and resistivity drops with further lowering of T. The low-T data below AFM-like transition at T N = 20 K is associated with an upturn that follows an activated behaviour, (ρ ∼ exp(E g /T)) (figure 35(c)). A similar low-T upturn is also discussed earlier in the case of Fe 2 TiSn [121] but the origin is different in these two cases. For Fe 2 TiSn, it is attributed to weak localization effect (obeying a T 1/2 dependence) but for CoFeVGa, the upturn follows an exponential nature and is insensitive to the magnetic field that rules out the weak localization theory.…”

“…This is called the weak localization effect, where the constructive interference of the backscattered electrons from the disorder sites localizes the carriers and an insulating state emerges [123,124]. The low-T upturn in the ρ(T ) data follows a T 1/2 dependence and it disappears under an applied magnetic field supporting the weak localization model in Fe 2 TiSn [121]. It is to be noted that ab initio electronic structure calculation on ordered Fe 2 TiSn indicates a non-magnetic ground state with a small pseudo-gap of about 0.07 eV at the Fermi level [119].…”

“…The value of ρ in Fe 2 TiSn is significantly higher as compared to normal metals. ρ is 421 μΩ cm at room temperature and it decreases to 340 μΩ cm at 2 K. Reitveld analysis of the x-ray diffraction pattern indicates the presence of antisite disorder in Fe 2 TiSn [119][120][121], and the compound is considered to be a dirty metal. The weak ferromagnetism in Fe 2 TiSn may be induced by the disorder present in the system [119].…”

“…Chaudhuri et al [121] and Lue et al [120] addressed the antisite disorder between Fe and Ti by studying Fe 2−x Ti 1+x Sn (x < 0.1), where a small amount of excess Ti is doped at the Fe site as shown in figure 10. It is found that the magnitude of ρ(T ) decreases drastically as Ti concentration is increased in the alloy Fe 2−x Ti 1+x Sn due to the increased number of holes.…”

Itinerant Magnetism in Ga‐Rich Ga–Mn–Co Full Heusler Alloys

“…A similar straightforward analysis considering the ratio of (111)/(220) peak in terms of Y–Z disorder will be unreliable as the (111) reflection would be adversely affected due to the similar X‐ray scattering factors for Mn and Co atoms. To circumvent the problem, in future, an anomalous XRD technique [ 27,28 ] using a synchrotron source or neutron diffraction measurement [ 29 ] can be adopted.…”