npsm 새물리 New Physics : Sae Mulli

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Research Paper

New Phys.: Sae Mulli 2023; 73: 1149-1154

Published online December 31, 2023 https://doi.org/10.3938/NPSM.73.1149

Copyright © New Physics: Sae Mulli.

Electrical and Magnetic Properties of FeGa3 with Antisite Disorder

C. Kaufmann Ribeiro1, A. Faé Rabello1, V. Martelli1, I. Zivkovic2, E. Fogh2, H. M. Rønnow2, D. Cornejo3, J. Larrea Jiménez1*

1Laboratory for Quantum Matter under Extreme Conditions, Institute of Physics, University of São Paulo, São Paulo 05508-090, Brazil
2Laboratory for Quantum Magnetism, Institute of Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
3Institute of Physics, University of São Paulo, São Paulo 05508-090, Brazil

Correspondence to:*larrea@if.usp.br

Received: October 1, 2023; Revised: November 7, 2023; Accepted: November 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

We investigated the electronic and magnetic ground states of the cage compound FeGa3 under the presence of Fe antisite disorder (namely as Fe0.258Ga0.742) by x-ray diffraction (XRD), Mössbauer spectroscopy (MS), magnetization and electrical resistivity. Our results show that approximately 4.5 (5)% Fe antisite occupying one of the two non-equivalent Ga sites changes significantly the electronic and magnetic ground states of the pristine compound. We uncover that Fe0.258Ga0.742 undergoes two magnetic transitions, T1 ≈ 60 K and T2 ≈ 15 K, the latter associated with the suppression of semiconducting behavior. We focus our discussion on the role that in-gap impurity states and spin disorder play across semiconducting-metal and paramagnetic-magnetic transitions.

Keywords: Condensed Matter Physics, Strongly correlated electron systems, Narrow-band semiconductors

Correlated electron systems host interactions involving electron and spin degrees of freedom and are ideal platforms to carefully examine the quantum many-body physics and novel quantum states under different non-thermal tuning control parameter (e.g. chemical substitution, pressure, magnetic field)[1]. The oddity of quantum states, as for instance, strange metal behavior and superconductivity without phonon mediation, persuades the investigation of underlying low-lying energy scales inside a realistic local environment of materials, i.e., considering the presence of ubiquitous atomic disorder.

In the context of the synthesis of correlated quantum materials, enormous effort has been devoted to achieve samples that feature pristine compounds or with negligible influence of disorder, albeit with a scarcity of examples so far[2]. Inside the approach of pristine compounds, the absence of disorder favors the scenario for the loss of long-range orders and fermionic quasiparticles responsible for the strange metal behavior seen as T- linear resistivity[3]. Most recently, the presence of spatial disorder in a two-dimensional correlated metal has been highlighted as mechanism to account for disordered scattering of electrons which reveals both T- linear resistivity and Tln(1/T) specific heat behaviors[4]. Last but not least, the influence of random disorder might induce non-canonical quantum critical phase diagrams mostly reported in metals than in semiconductors[5]. Among rare cases, we can find the non-magnetic narrow-gap semiconductor FeGa3, which is tuned by random substitution of Ge in Ga atomic position in FeGa3-yGey to a ferromagnetic instability at yC 0.13. Above yC, an unusual coexistence between long-range and short-range ferromagnetic orders is observed[6]. This finding calls for more careful investigation of the influence of disorder in FeGa3 system. Moreover, recent DFT calculations show that the presence of intrinsic defects, such as Fe replacing Ga positions, are responsible for the formation of local magnetic moments in Fe atomic positions[7]. This motivates the investigation of FeGa3 under the presence of Fe antisite disorder and its influence in the formation of magnetic orders and localized/delocalized electron band structures.

In this communication, we present an investigation of the half-filled semiconductor FeGa3 with presence of Fe antisite disorder by spectroscopy and bulk physical properties. Our aim is to reveal the evolution of magnetic and electronic ground states originate from the influence of known atomic defect position.

Polycrystalline samples Fe0.258Ga0.742 were synthesized as reported in our previous work [8], using a solid-state reaction in an arc-melt furnace, followed by annealing treatment. High-purity Ga and Fe were melted in a copper crucible under argon at ambient pressure. Powder as-cast samples were ground, size-homogenized, and annealed in a vacuum-sealed quartz tube at 650 °C for 24 hours, cooling at 0.4 °C/min. 57Fe Mössbauer spectra were taken at 295 K, in a transmission geometry with an active 57Co:Rh source (25 mCi) moving with triangular reference signal. The obtained isomer shift values (δ) in our spectra are referred to Fe foil (α - Fe). As in our previous work [8], energy dispersive x-ray spectroscopy (EDS) confirms the expected Fe off-stoichiometric composition, in agreement with the Rietveld refinement analysis of XRD and Mössbauer spectroscopy experiments, at least within 5% of uncertainty. The synthesized powder samples were compressed at nominal pressure of 10(1) kbar in order to measure their bulk physical properties. The temperature dependence of electrical resistivity was measured using a four-points standard method, inside a cryofree ICEOXFORD system that cools the sample down to 1.5 K. Gold contacts were sputtered onto the sample and subsequently annealed to optimize the contact resistance. Magnetization measurements were performed in a MPMS magnetometer from Quantum Design.

Figure 1(a) shows the XRD pattern of Fe0.258Ga0.742 at two different conditions, powder and compressed. For both cases, all reflecting planes point out the presence of a single phase associated with symmetry space group P42/mnm[9]. However, our Rietveld refinements (solid lines) reveals the presence of antisite disorder, where 4.5 (1)% excess of Fe atoms occupy one of the non-equivalent Ga at Wickoff position 8j (Fe8j in Fig. 2(a)). This is different from pristine compound FeGa3 where atoms occupy positions of Fe in 4f (Fe4f), Ga1 in 4c (Ga4c) and Ga2 in 8j (Ga8j) (see Fig. 2(a)). The presence of Fe antisite is also confirmed by the Q-vector dependence of the structure factor (S(Q)) of Fe0.258Ga0.742 which was calculated from the occupancy number obtained in our Rietveld refinement (RR) analysis in previous work [8]. Figure 1(b) shows the difference between structure factors of our sample and that expected for a pristine compound (Sp(Q)). The deviation from Sp(Q) and the oscillatory patterns suggest the presence of Fe antisite disorder which remains similar in our powder and compressed samples.

Figure 1. (Color online) (a) Experimental (open symbols) and Rietveld refinement (solid lines) XRD patterns of polycrystalline Fe0.258Ga0.742 at two different conditions: powder (ambient pressure) and compressed (10 kbar). (b) Structure factor difference between our sample Fe0.258Ga0.742 at different conditions (S(Q)) and that expected for the pristine FeGa3 (Sp(Q)) [8].

Figure 2. (Color online) (a) Crystal structure of Fe0.258Ga0.742 depicts Fe atoms occupying two different Wickoff positions at: (upper panel) 4f sites as in pristine symmetry and, (lower panel) additional 8j sites in Fe antisite disorder (with cross symbols). (b) Mössbauer spectrum of Fe0.258Ga0.742 at room temperature and the fit (solid line) with two subspectra associated with Fe at two different environments: pristine (FeGa3) and Fe antisite disorder (Antisite).

Another indication of Fe antisite disorder is inferred from Mössbauer spectroscopy. Figure 2(b) depicts the Mössbauer spectrum of powder Fe0.258Ga0.742 at ambient conditions. The observed asymmetric doublet in polycrystalline samples suggests the presence of more than one Fe environment. The best fit of the spectrum provides hyperfine parameters (see Table 1 [10]) that reveal Fe atoms in two different surroundings, i.e., in one environment similar to what is found in a pristine compound and another associated with Fe antisite, likely at Wickoff site 8j. The relative concentration of the Fe antisite (around 4%) agrees with our RR analysis. In addition, hyperfine values of δ, quadrupolar splitting (Δ) and linewidth (Γ 0.25(2) mm/s) also indicate that all non-equivalent Fe sites remain non-magnetic and at the same valence state Fe3+, at least, at room temperature.

Table 1 . Comparison between hyperfine parameters and occupancy numbers of Fe sites in Fe0.258Ga0.742 (this work) and FeGa3 (pristine) [10].

Fe siteArea (%)δ (mm/s)Δ (mm/s)Occ
Fe4f [this work]96(1)0.27(1)0.31(1)0.25
Fe8j [this work]4(1)0.05(2)0.81(9)0.02(1)
Fe4f [pristine [10]1000.280.310.25
Fe8j [pristine [10]0---


We now turn on the influence of Fe antisite disorder on the magnetic and electronic physical properties. Looking at the temperature dependence of magnetization (M(T)) in Fig. 3(a), signatures of magnetic order can be distinguished. Decreasing the temperature, M(T) at low fields (= 1 kOe) steadily increases below 65 K but it does not saturate at very low temperature. Instead, a subsequent decrease well below 15 K is observed. This suggests the onset of two different magnetic transitions, T1 65 K and T2 15 K, which can be better determined from the first derivative of the magnetization, dM(T)/dT (Fig. 3(b)). The application of higher magnetic field (= 60 kOe) smears out the transitions and increases T1. This behavior is reminiscent of short-range ferromagnetic (SRF) order in the presence of spin disorder[11]. The observation of T1 and T2 suggests two magnetic transitions for which the spin couplings can be consequence of two magnetic subsystems with local moments formed at two different Fe site environments, the latter supported by our Mössbauer experiments at room temperature. Besides that, Fig. 3(d) shows the absence of hysteresis in the MH curve at 1.8 K, at least, inside the resolution of our instrument. This behavior might suggest the formation of a very small magnetic moment in the SRF order or in a more complex magnetic structure, the latter not possible to be resolved in our measurements. On the other hand, the paramagnetic regime is described by the presence of an intrinsic semiconducting activation gap (Ei) and a Curie-Weiss contribution following the ansatz:

Figure 3. (Color online) (a) Isofield magnetizations as function of temperature of compressed Fe0.258Ga0.742 measured at zero field cooling (ZFC). (b) First derivative of magnetization dM(T)/dT. Arrows show minima associated with the onset of magnetic orders at T1 and T2. (c) Magnetization in the paramagnetic regime. Solid lines are fits to obtain the direct in-gap excitation.

MH=AT1/2exp(Ei2kBT)+CTΘ

The fit using Eq. (1) (solid lines in Fig. 3(c)) provides values C= 0.0368 and 0.0493 emu K/g, Θ=477 and -1,087 K, A = 6.7797 × 10-5 and 1.6654 × 10-5 emu K-1/2/g and Ei= 0.2324 and 0.2321 eV, for 1 kOe and 60 kOe, respectively. It is noteworthy that Ei 0.23 eV are quite close to what is reported for single crystal FeGa3 where no magnetic transition is reported [10]. This support that our compound Fe0.258Ga0.742 can still be classified as a strongly correlated narrow-gap semiconductor where the gap opens around the Fermi energy (EF) due to the strong Fe-3d and Ga-4p hybridization.

In addition to spin disorder, Fe antisite might also lead the formation of in-gap impurity states inside the intrinsic gap which dictates the electronic ground state at low temperatures. The electrical resistivity shown in Fig. 4(a) shows the transition from semiconducting to metallic behavior at temperatures well below T2, the latter also determined from the minimum in first derivative of the electrical resistivity dρ/dT (Fig. 4(b)). The semiconducting behavior below 150 K is, however, dominated by an extrinsic impurity band structure well below the conduction band whose separation is characterized by an activation energy Eg. Like other semiconductors, the determination of Eg by Arrhenius law ρ=Aexp(Eg/2kBT) can give arbitrary values[12]. However, it is noteworthy that the maximum of Eg 18(1) meV obtained from the fit with Arrhenius of our ρ(T) data in intervals of 5 K (dashed line in Fig. 4(c)) agrees with the value for extrinsic gap reported in single crystals FeGa3[9, 10].

Figure 4. (Color online) (a) Electrical resistivity as a function of temperature of Fe0.248Ga0.752 compressed at 10 kbar. (b) First derivative of electrical resistivity dρ/dT. Arrows show the onset of magnetic orders at T1 and T2. (c) Temperature dependence of the extrinsic in-gap excitation (Eg) obtained by Arrhenius law. Dashed line depicts the maximum in Eg(T).

In order to reconcile signatures of magnetic order with semiconducting-metal transition, we can recall the scenario of spin disorder-driven semiconducting state[13]. Based on recent DFT calculations, the formation of small magnetic moments is possible in Fe antisite positions[7]. This turns out for the emergence of spin correlations which might change the mobility edge energy inside the intrinsic gap. If the mobility edge is near the conduction band then decreasing the temperature drives electrons to be delocalized into the conduction band and thus a metallic state forms. Such metallic state is mainly favored by the enhancement of the spin-spin correlation length that accounts for the magnetic order observed well below T2[13].

We have investigated the electronic and magnetic ground states of FeGa3 polycrystals with the presence of Fe antisite disorder. The substitution of 4.5(5)% Ga atoms at 8j positions by Fe atoms leads to the formation of a narrow-gap semiconductor which hosts in-gap impurity bands and local magnetic moments at low temperatures, the latter is responsible for the onset of two magnetic transitions. In our present case, Fe antisite-driven spin correlations can account for the mobility edge position with respect to the conduction or valence band and thus determining the metallic and semiconducting behavior. Our work triggers further investigation using pressure or fine inclusion of Fe antisite disorder as tuning parameters to understand the interplay between localized/non-localized electrons and spin correlations which can be responsible for correlation-driven metal-insulator transitions.

J. L. J acknowledges grants 2018/08845-3 and 2022/14202-3, São Paulo Research Foundation (FAPESP) and CNPq 310065/2021-6. C. K. R., A. F. R. and V. M. also acknwoledge grants 2019/24522-2, 2020/01377-4 and 2018/19420-3, FAPESP. H. M. R and E. F. acknowledge financial support of the European Research Council through the Synergy network HERO (Grant No. 810451). J. L. J and C. K. R. acknowledge J. D. Thompson, J. Palmstrom and S. M. Thomas for discussion and support in the magnetic measurements at Los Alamos National Laboratory. Authors acknowledge M.C.A. Fantini, C. Nagamine, R. Cohen, S. Romero for the access to LCr, Mössbauer and LMM-IFUSP laboratories.

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