Magnetism and site exchange in CuFeAs and CuFeSb: A microscopic and theoretical investigation

Abstract

We have investigated the magnetic ground state of CuFeAs and CuFeSb by means of Fe-57-Mossbauer spectroscopy, muon spin rotation/ relaxation (mu SR), neutron diffraction, and electronic structure calculations. Both materials share the 111-LiFeAs crystal structure and are closely related to the class of iron-based superconductors. In both materials there is a considerable occupancy of the Cu site by Fe, which leads to ferromagnetic moments, which are magnetically strongly coupled to the regular Fe site magnetism. Our study shows that CuFeAs is close to an antiferromagnetic instability, whereas a ferromagnetic ground state is observed in CuFeSb, supporting theoretical models of anion height driven magnetism

Breakdown of Hund's rule for CuFeAs

“…If only FM, NAFM, CAFM, and BAFM states are taken into account in the classic limit of Heisenberg J 1 − J 2 − J 3 model, CAFM is energetically favorable over other magnetic configurations when J 2 > −J 1 /2 and J 2 > 2J 3 . The conditions are satisfied when h As is smaller than h c , resulting in an agreement between our first-principles results and recent experimental observations [23,25]. However, when h As is greater than h c , the FM state becomes the ground state owing to J 1 < −2J 2 and J 1 < −J 2 − 2J 3 .…”

“…As can been seen in Fig. 1 (a) where various As heights reported in different experiments are shown, at h As less than h c , our theoretical calculations point to a CAFM ground state, which is mainly consistent with the experimental results where either long-range [23] or short-range [25] AF order was observed, though the magnetic structure has not been determined ex-perimentally yet. But, in the cases of h As greater than h c , while the theoretical ground state is strongly FM, it was inferred from experiments [22,24,26] that CuFeAs should be an antiferromagnet probably with ferromagnetic components.…”

“…This can account for various experimental observations in CuFeAs, where all magnetic susceptibility measurements at low external magnetic fields [22,23,26] suggest this material is in an antiferromagnetic state for 1.57 A ≤ h As ≤ 1.80Å. At h As = 1.53Å, the short-range CAFM order was observed by the Mössbauer spectroscpy and muon spin resonance experiments [25], which is also qualitatively consistent with our results (shown in Fig. 1 (a)) since the quantum fluctuations are completely frozen in the DFT calculations.…”

“…CuFeAs is isostructural to 111-type iron-pnictide superconductor parent compounds LiFeAs [30] and NaFeAs [31]. It is characterized by large h As in comparison to other iron pnictides, varying from h Kam =1.53Å [25] to h Li =1.57Å [23] and h Thakur =1.74Å [22], finally to h Zou =1.80Å [26]. Moreover, it was reported to be nonstoichiometric [22][23][24][25][26], namely the Cu vacancies are always present.…”

“…The magnetic susceptibility measurements showed that it is antiferromagnetic with Néel temperature T N of around 9 K [22,23]. The antiferromagnetism was further demonstrated by neutron diffraction experiments [24,25], where either an unusual G-type antiferromagnetic order or proximity to an antiferromagnetic instability was proposed. Though ferromagnetism was also reported in the literature [26,27], it was pointed out that the weak ferromagnetism probably comes from a ferromagnetic component of a canted antiferromagnetic state [24,26].…”