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Negative Colossal Magnetoresistance Driven by Carrier Type in the Ferromagnetic Mott Insulator GaV4S8 Etienne Janod,*,‡ Eugen Dorolti,§ Benoit Corraze,‡ Vincent Guiot,‡ Sabrina Salmon,‡ Viorel Pop,§ Frédéric Christien,‡ and Laurent Cario*,‡ ‡

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP32229, 44322 Nantes, France Faculty of Physics, Babes-Bolyai University, Mihail Kogalniceanu, Nr. 1, 400084 Cluj-Napoca, Romania

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S Supporting Information *

ABSTRACT: We report here a study on the evolution of structural and electronic properties of the lacunar spinel compounds GaV4S8 with charge doping. In this ferromagnetic (FM) Mott insulator, the heterovalent substitutions of Ga3+ by Zn2+ or Ge4+ allow induction of charge doping either by holes or by electrons. We show that electron-doped GaV4S8 displays a bulk, negative, and colossal magnetoresistance (CMR) with a relative drop of resistivity reaching −80% at 7 T in the vicinity of the Curie temperature. Conversely, hole-doped GaV4S8 does not display any negative CMR but a classical positive magnetoresistance. This asymmetric electron−hole doping effect challenges the common view stating that CMR effects in doped FM Mott insulators depends only on the density of carrier and not on their electron/hole nature. We show that a simple model based on multiorbital effects and Hund’s rule is able to capture the presence (absence) of negative CMR in electron- (hole-) doped GaV4S8.

1. INTRODUCTION Mott insulators represent a large class of materials with halffilled d or f orbitals that should be metallic according to conventional band theory, but are actually insulators because of the on-site electron−electron (Coulomb) repulsion U, not included in the conventional models. An interesting characteristic of Mott insulators is that deviation from half filling caused by charge carriers doping, either with electrons or holes induces insulator-to-metal transitions (IMTs). Outstanding electronics properties are often associated with these IMTs,1 such as superconductivity in copper oxides2 and iron pnictides/ chalcogenides,3,4 as well as colossal magnetoresistance and full spin polarization in manganites.5 These properties are not only interesting from a fundamental point of view but also promising for applications.6 The study of doped Mott insulator is therefore a vast field of research on which thousands of articles were published during the last 2 decades. Despite this huge interest, only a few experimental studies address the issue of electron−hole asymmetry, i.e., the possibility to obtain very different properties between electron-doped (n-doped) and hole-doped (p-doped) Mott insulators. The particular class of ferromagnetic (FM) Mott insulators is potentially well-suited to check the existence of such asymmetry. Experimental studies have indeed established that a negative colossal magnetoresistance (CMR) emerges in lightly doped ferromagnetic Mott insulators.7,8 In these compounds, a possible n−p asymmetry, such as the presence of negative CMR for one type of carrier and its absence for the other, is indeed very easy to detect. However, the scarcity of © XXXX American Chemical Society

ferromagnetic Mott insulators which are ambipolar, i.e., able to be doped both by electrons and holes, have prevented this issue from being tackled thus far. In this context, only the study of new ambipolar FM Mott insulators is likely to clarify this continuing problem. We report here a study on the s = 1/2 ferromagnetic Mott insulator GaV4S8 that we have successfully doped by both electrons and holes. Our study reveals the existence of a negative colossal magnetoresistance reaching −80% for electron doping, whereas hole doping only induces a conventionnal positive magnetoresistance in this compound. This study therefore reveals that, beyond the common view that carrier density is the only parameter driving CMR in doped ferromagnetic Mott insulators,9 the nature of carriers is fully relevant to achieve negative CMR in FM Mott insulators.

2. EXPERIMENTAL DETAILS Synthesis. Single crystals of GaV 4S 8 , Ga 1−xGe xV 4 S 8 , and Ga1−xZnxV4S8 were prepared using stoichiometric mixtures of elemental gallium, zinc, germanium, vanadium, and sulfur (purities >99.5%). These mixtures were loaded in evacuated sealed quartz ampules, heated up at 300 °C/h to 970−1000 °C and then held at this temperature for 1 h. The quartz tubes were subsequently cooled down, first slowly to 700−750 °C at 1−2 °C/h and then more quickly (300 °C/h) to room temperature. These syntheses yielded black powders Received: March 30, 2015 Revised: May 14, 2015

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DOI: 10.1021/acs.chemmater.5b01168 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials containing a high yield of GaV4S8, Ga1−xGexV4S8, or Ga1−xZnxV4S8 metallic-gray cubic and tetrahedral crystals. The maximal crystal’s size varies from one batch to another and ranges from 30 to 300 μm. The synthesis protocol used to obtain Ga1−xMxV4S8 (M = Zn, Ge) sintered pellets is similar to the one described in ref 10. Chemical Analysis. The samples’ compositions were checked either with energy-dispersive X-ray (EDX) spectroscopy (JEOL JSM5800LV Scanning Electron Microscope equipped with a PGT-Ge detector) or with wavelength-dispersive spectroscopy (WDS; Carl Zeiss Merlin Scanning Electron Microscope with an Oxford Instruments INCAWave 500 spectrometer). X-ray Diffraction. Analysis of Ga1−xMxV4S8 (M = Zn, Ge) powder samples were performed on a Bruker D8 Advance Diffractometer using Cu Kα1 radiation (λ = 1.540598 Å). Data were collected at room temperature in the 2θ range of 10−120° with acquisition time of 12 h. The cell parameters and the crystal structures were refined by the Rietveld method using the Jana Chain Program.11 All refinements were performed with the fundamental approach.12 The structure was refined by starting with the atomic coordinates found for GaV4S8 in ref 10. The ratio Ga/Zn or Ga/Ge was fixed at the value of the nominal composition. When necessary, we used a multiple-phase procedure to take into account a small amount of V2O3 impurity (proportion 0.1. This is a clear indication of a limit in the Ga/Zn solid solution, which is further confirmed by the wavelength-dispersive spectroscopy chemical analysis

3. RESULTS AND DISCUSSION The AM4Q8 (A = Ga, Ge; M = Ti, V, Nb, Ta, Mo; Q = S, Se, Te)13−17 compounds exhibit a lacunar spinel structure containing M4 tetrahedral clusters (see Figure 1a−d). These compounds represent an interesting family of canonical Mott insulators18,19 that display exceptional electronic properties, such as superconductivity under pressure,14,15 multiferroicity,20 resistive switching induced by electric field,21−28 or extrinsic magnetoresistance associated with a half-metal ferromagnetic state in GaTi3VS8.10 GaV4S8 is also a very interesting member of this family of Mott insulators as it becomes ferromagnetic below TC ≈ 13 K. In GaV4S8, the relevant electronic units are the V4 tetrahedral clusters rather than the vanadium ions.10 Each V4 cluster contains seven 3d electrons. One of them is unpaired and occupies, at room temperature, threefold degenerate t2 molecular orbitals of the tetrahedral clusters (see Figure 1c). Thanks to the substantial repulsion between electrons within the V4 site, GaV4S8 is a paramagnetic Mott insulator at room temperature.29 A structural transition occurs at low temperature (TS = 43 K) from the F4̅3m to the R3m space group.30 This transition lifts the orbital degeneracy by distorting the V4 tetrahedra, defining two inequivalent V1 and V2 vanadium within the cluster (see Figure 1b,d). Neutrons diffraction and band structure calculations indicate that, below TS, the unpaired electron occupies an a1 molecular orbital of the distorted V4 cluster, mainly localized on the vanadium V1 (see Figure 1b,d).29 Conversely, the empty e orbitals are located B


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Chemistry of Materials

by our preliminary thermopower measurements. A positive Seebeck coefficient is indeed evidenced at room temperature in hole-doped Ga0.94Zn0.06V4S8. In the same conditions, we detected a negative Seebeck coefficient in electron-doped Ga0.875Ge0.125V4S8, in agreement with the results obtained in ref 34. Magnetic Properties. Figure 3 shows the main magnetic properties of undoped, n-doped, and p-doped GaV4S8. All these

Figure 2. (a) Result of the Rietveld refinement of the powder X-ray diffraction pattern of Ga0.95Zn0.05V4S8. The red dots represent the experimental intensities. The black lines represent the intensities calculated by the Rietveld method. The blue curve at the bottom represents the difference between the experimental and calculated intensities. The small marks indicate the Bragg peak positions. (b) Measured Zn content vs nominal composition for both Zn and Ge substitutions. (c) Evolution of the cubic unit cell parameter of Ga1−xMxV4S8 (M = Zn, Ge) with the substitution rate x. (d) Evolution of V−V distances, within and between V4 tetrahedra. The errors bars in (c) and (d) are lower than the symbol’s size.

performed on crystals from batches targeting Ga1−xZnxV4S8 compositions with x = 0.1, 0.2, 0.3. As illustrated in Figure 2b, a maximum of 6 ± 1% of Zn can be substituted for Ga. This limit in the Ga/Zn substitution contrasts with the Ga/Ge series, for which our study confirms the existence of a full solid solution for 0 ≤ x ≤ 1. Figure 2c,d displays the main results of the Rietveld refinements performed on Zn- and Ge-doped GaV4S8 (see Table S1 for more details). First, we found that the substitutions of Ga3+ by either Zn2+ or Ge4+ have a very small effect on the cubic cell parameter. A very small decrease, lower than 0.01 Å, is observed along the whole series of compounds from Ga0.95Zn0.05V4S8 to GeV4S8. In comparison, the structural analysis reveals that both substitutions have a much stronger impact on the V−V distance within the cluster. This distance decreases along the whole series of compounds from the lightly doped Zn compound to the full Ge compound. Concomitantly, the intercluster distances increases along the series. Both features are clear signatures of the effect of an electronic doping on the t2 molecular orbital of the tetrahedral V4 cluster, which has a V−V bonding character.30 The 0.07 Å decrease of the intracluster V−V distance shown in Figure 2d thus results from the continuous increase of the number of electrons lying on the t2 levels along the series going from lightly Zn-doped GaV4S8 to GeV4S8. This synthesis and chemical characterization work therefore establishes that the ferromagnetic Mott insulator GaV4S8 can be lightly doped both by holes (Ga1−xZnxV4S8) and electrons (Ga1−xGexV4S8). This ambipolar character is further confirmed

Figure 3. Magnetic properties of undoped and doped GaV4S8 measured on sintered pellets. (a) Inverse magnetic susceptibility χ−1 vs temperature of undoped, p-doped, and n-doped GaV4S8. (b) Magnetization vs magnetic field of undoped, p-doped, and n-doped GaV4S8 in the ferromagnetic state at 2 K.

compounds are paramagnetic above 50 K and their magnetic susceptibility χ(T) follow essentially a Curie−Weiss law χ(T) = C/(T − θ). Figure 3a shows that the Weiss temperature θ is negative for undoped and doped GaV4S8, indicating the existence of antiferromagnetic interactions at high temperature. In undoped GaV4S8, a sudden change in χ(T) occurs at the structural transition temperature TS = 43 K. Below TS, GaV4S8 remains paramagnetic, but the Weiss temperature θ becomes positive, indicating a change in magnetic interactions from antiferromagnetic above TS to ferromagnetic below. Both electron and hole doping lead to a decrease of the structural temperature TS, which remains well-defined up to xGe = 0.25 for Ga/Ge substitution. Figure 3a also clearly shows that the ferromagnetic transition temperature TC decreases upon both p and n doping (TC = 13 K, TC ≈ 12 K, and TC ≈ 10 K in GaV4S8, Ga0.95Zn0.05V4S8, and Ga0.88Ge0.12V4S8, respectively). The persistence of ferromagnetism in lightly doped GaV4S8 is further confirmed by field dependencies of magnetization M(H) shown in Figure 3b. Even if both electron and hole doping tend to decrease the magnetization at 50 kG, hysteresis cycles clearly appear at low field for both electron and hole C


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Chemistry of Materials doping, at least for substitution rates below 25%. To sum up, these magnetic studies demonstrate the persistence of ferromagnetism in lightly p- or n-doped Mott insulator GaV4S8. Electrical Transport Properties. Figure 4 displays the resistivities measured on crystals of the Ga1−xZnxV4S8 and

Figure 4. Resistivity vs temperature measured on single crystals for the series Ga1−xGexV4S8 (a) and Ga1−xZnxV4S8 (b). The chemical compositions of all these crystals were determined by EDX or WDS analyses.

Ga1−xGexV4S8 series. The undoped compounds GaV4S8 and GeV4S8 behave as typical Mott insulators, i.e., with rather large resistivities increasing as temperature decreases. Figure 4a shows that, for all the compositions within the Ga1−xGexV4S8 solid solution, resistivity is considerably reduced compared to that of the undoped end members, at least by a factor of 107 at 50 K. The resistivity of doped compounds, especially for Ge doping, is loosely temperature-dependent above 50 K, and can be either semiconducting (dρ/dT < 0) or metallic-like (dρ/dT > 0). This behavior may be ascribed to a slight localization by disorder. Figure 4a,b confirms that the strong resistivity drop appears also for small (x < 0.10) substitution rates, both in the n-doped Ga1−xGexV4S8 series and in the p-doped Ga1−xZnxV4S8 series. This corresponds to a filling-controlled insulator-tometal (IMT) transition, a typical fingerprint of doped Mott insulators.1 Figure 5 summarizes the main information gathered from magnetic and transport measurements for lightly doped GaV4S8. Taken together, these measurements demonstrate that ferromagnetism is retained on both sides of the fillingcontrolled IMT occurring in doped GaV4S8. Magnetotransport Properties. As GaV4S8 is a ferromagnetic Mott insulator, one might expect to induce colossal magnetoresistance in this compound after doping.9 Moreover, the possibility of doping GaV4S8 both n and p provides a unique opportunity to look for possible electron−hole asymmetry of CMR. We have therefore investigated the magnetotransport properties of doped and undoped GaV4S8 single crystals. Figure 6a displays magnetotransport data measured below 50 K in undoped GaV4S8 and in n-doped Ga0.98Ge0.02V4S8. The resistivity ρ(T) of GaV4S8 shows a small anomaly of resistivity around the structural transition at TS = 43 K. Unfortunately, no reliable data were obtained around the FM transition (TC ≈ 13 K) as the sample resistance was too high to be measured (≈1010 Ohm at 20 K). The resistivity ρ(T) of Ga0.98Ge0.02V4S8 also displays a clear signature of the structural transition, with a kink at TS = 40 K (see Figure 6a). In addition, a small but clearly visible local maximum of ρ(T) appears at the ferromagnetic transition around TC = 12.3 K in

Figure 5. (a) Dependence of resistivity at 50 K with Zn/Ga and Ge/ Ga substitution, based on resistivity measurements on single crystals. The black line is a guide for the eyes. (b) Temperature dependence of the structural and ferromagnetic transitions with Ge and Zn doping in GaV4S8, based on magnetic measurements on polycrystals.

Figure 6. Magnetotransport properties of undoped and doped GaV4S8 single crystals. (a) Resistivity vs temperature in undoped GaV4S8 (H = 0 T) and Ga0.98Ge0.02V4S8 (H = 0 and 8 T). Magnetoresistance MR = [ρ(H,T) − ρ(0,T)]/ρ(0,T) vs magnetic field (b) and vs temperature (c) in Ga0.98Ge0.02V4S8.

Ga0.98Ge0.02V4S8. Figure 6a shows that the application of a magnetic field on this n-doped sample strongly decreases ρ(T) in a wide temperature range around TC and suppresses the resistivity peak. This behavior corresponds to a negative magnetoresistance, which is maximum at the ferromagnetic transition (see Figure 6b,c), but also exists far above TC in the paramagnetic regime. The amplitude of this negative magnetoD


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Chemistry of Materials resistance Δρ/ρ(0) is unusually large and reaches −80% at TC ≈ 12.3 K under 8 T for Ga0.98Ge0.02V4S8. At this stage, it is interesting to establish whether such a negative CMR also appears in p-doped GaV4S8. Figure 7 shows

Figure 8. Temperature dependencies of the magnetoresistance MR = [ρ(H,T) − ρ(0,T)]/ρ(0,T) for several n (Ge) and p (Zn) doping levels.

most likely a bulk and intrinsic magnetoresistance, usually referred to as colossal magnetoresistance (CMR). The presence of a zero-field resistivity peak at the FM transition of n-doped GaV4S8 further supports this idea. A peak at TC indeed also exists in other ferromagnetic compounds that exhibit bulk CMR effects, such as pyrochlore compounds,38,39 Cr-based chalcogenides spinels,7,40 or europium oxide/chalcogenide.41 Interestingly, Majumdar and Littlewood9 have proposed a unified theoretical view explaining both the resistivity peak and the negative CMR in such doped FM compounds. According to their model, electrical resistance can be substantially increased, near and above the Curie temperature, by magnetic fluctuations. These fluctuations, which are maximum at TC and thus give rise to the resistivity peak, can be suppressed by a magnetic field, leading to a negative magnetoresistance. A key prediction of this model is that magnetoresistance MR = [ρ(H)− ρ(0)]/ρ(0) should behave as x−2/3 (where x is the number of carriers per magnetic unit cell), i.e. to increase when the number of carriers decreases. Figure 8 shows that this trend clearly exists in n-doped Ga1−xGexV4S8 since the largest MR values above TC are obtained for the smallest doping rate xGe. Hence, the negative CMR in n-doped GaV4S8 seems to be well captured by the mechanism of resistance driven by magnetic fluctuation proposed by Majumdar and Littlewood. However, the main message of the Majumdar−Littlewood model, which is now widely accepted within the materials science community, is that negative CMR in doped FM systems depends only on one parameter, the carrier concentration x. This model does not mention the possibility of a strong asymmetry of CMR with carriers type, electrons, or holes. The absence of negative CMR in p-doped Ga1−xZnxV4S8 might mean that this view is at least incomplete. To gain insight into this asymmetry of CMR, a closer look at the local electronic structure of GaV4S8 is necessary. As depicted in Figure 1d, the relevant electronic sites in GaV4S8 are the V4 tetrahedra. In (lightly) n-doped GaV4S8, most V4 tetrahedra have only one electron on their a1 orbital (black circles in Figure 9) and a few, the “doped” sites (pink circles in Figure 9), host a second electron on the e orbitals, coupled to the first electron of the same site via the Hund’s rule. Qualitatively, the hopping of one electron from one site to its neighbors is favored if the spins of both sites are parallels. In the paramagnetic state just above TC, the disorder between spins leads to a poor hopping probability, which contributes to increased electrical resistance. The application of magnetic field

Figure 7. (a) Resistivity ρ(T) of Ga0.94Zn0.06V4S8 (p-doped GaV4S8) at H = 0 and 7 T close to its ferromagnetic transition at TC = 12 K. Magnetoresistance MR = [ρ(H,T) − ρ(0,T)]/ρ(0,T) vs magnetic field (b) and vs temperature (c) in Ga0.94Zn0.06V4S8.

the resistivity ρ(T) measured on Ga0.94Zn0.06V4S8 single crystals. In this compound, the temperature of the structural transition TS is lowered compared to that of undoped GaV4S8 (TS = 37 vs 43 K in GaV4S8). But surprisingly, no traces of resistivity peak nor of any anomaly can be detected in Ga0.94Zn0.06V4S8 at the Curie temperature TC ≈ 10 K. Moreover, Figure 7a−c shows that this hole-doped GaV4S8 does not present any negative magnetoresistance in the vicinity of TC. Conversely, a positive MR, comparable with the one of classic metals such as platinum at low temperature,36 exists below 40 K in Ga0.94Zn0.06V4S8 (see Figure 7b,c). To sum up, the n-doped FM Mott insulator Ga0.98Ge0.02V4S8 presents a resistivity peak at TC and negative CMR around TC. Conversely, p-doped Ga0.94Zn0.06V4S8 does not show any of these features. Figure 8 demonstrates that these trends are not restricted to specific doping rates. All Ge-doped GaV4S8 indeed display negative CMR as long as ferromagnetism is maintained (i.e., up to xGe = 0.25), whereas all our available Zn-doped samples (up to xZn = 0.06) show a positive MR without any peak at TC. These measurements clearly demonstrate the role of the carrier’s type in the appearance of the negative CMR around the ferromagnetic transition in the Mott insulator GaV4S8. This property of negative magnetoresistance which exists only for one type of carrier is puzzling and raises the question of the microscopic origin of such an asymmetry. Let us first discuss the possible mechanism explaining the negative MR in n-doped GaV4S8. We note that the presence of this negative MR above TC discards an extrinsic mechanism related to grain boundaries or interface effects for which the negative MR appears only below TC.37 Hence, n-doped GaV4S8 presents E


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doping.40 On the other hand, it is not straightforward to check the existence of asymmetry of CMR in known FM Mott insulators materials with only one electron per site since they are extremely rare. We can however note that the electrondoped pyrochlore compound Lu2V2O7,39 a good example of FM Mott insulator with one electron per site,42 displays a large negative CMR. Unfortunatly, the absence of CMR could not be ascertained as p-doping was not achieved so far for this material. In that respect, an experimental and theoretical challenge ahead is to tackle the issue of absence and presence of CMR in, respectively, p- and n-doped ferromagnetic Mott insulators with a single electron per site.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01168.

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Figure 9. (a) Orbital schemes of neighboring sites in n-doped and pdoped GaV4S8. (b) Comparison of the local electronic state in GaV4S8 and in the manganite perovskites showing negative colossal magnetoresistance.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

tends to align the spins, which increases the hopping probability and thus lowers the resistance. This minimum model explains qualitatively the negative magnetoresistance of n-doped GaV4S8. We note that this simple mechanism is the same as the one explaining CMR above TC in manganite perovskites,5 with the t2g−eg orbitals of manganites playing the same role as the a1−e orbitals of V4 clusters, as shown in Figure 9b. Similar arguments might also explain the absence of negative CMR in the p-doped GaV4S8. In contrast to electron doping, hole doping leads to empty doped sites. Figure 9a shows that, in this context, carrier transport occurs through hopping via empty sites. As these sites own no spin, the hopping probability will be neither disfavored by magnetic fluctuations nor favored by an external magnetic field. In that respect, p-doped Ga1−xZnxV4S8 samples are expected to display neither a peak at TC nor a negative magnetoresistance in the vicinity of TC.

Funding

Financial support was provided by the Pays de la Loire Region (postdoctoral fellowship of E.D.) and by the Agence Nationale de la Recherche (grants ANR-05-JCJC-0123-01, NV-CER, and ANR-09-Blan-0154-01, Nano-Mott. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank N. Stephant for his help with EDXS measurements.

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REFERENCES

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4. CONCLUSION Overall, our work demonstrates that, beyond the common view stating that carriers density is the only relevant parameter, the nature of these carriers, electron or holes, plays a crucial role in colossal magnetoresistance in doped ferromagnetic Mott insulators. The model described above provides simple ideas that might be included in a complete theoretical model able to predict the effects of both carriers density and carriers type on colossal magnetoresistance. An important consequence of this simple model is that such an electron−hole asymmetry might exist only for s = 1/2, i.e., with only one unpaired electron per site. If each site contains at least two unpaired electrons, hole doping will not create empty sites: colossal magnetoresistance should emerge for both hole and electron doping in such context. A quick examination of other FM Mott insulators seems to confirm these views. For example, in the ferromagnetic compound CdCr2Se4, which contains three unpaired electrons per Cr3+(d3), negative colossal magnetoresistance can indeed appear both after electron and hole F


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