Research ArticleCONDENSED MATTER PHYSICS

Voltage-induced ferromagnetism in a diamagnet

See allHide authors and affiliations

Science Advances  29 Jul 2020:
Vol. 6, no. 31, eabb7721
DOI: 10.1126/sciadv.abb7721

Abstract

Increasingly impressive demonstrations of voltage-controlled magnetism have been achieved recently, highlighting potential for low-power data processing and storage. Magnetoionic approaches appear particularly promising, electrolytes and ionic conductors being capable of on/off control of ferromagnetism and tuning of magnetic anisotropy. A clear limitation, however, is that these devices either electrically tune a known ferromagnet or electrically induce ferromagnetism from another magnetic state, e.g., antiferromagnetic. Here, we demonstrate that ferromagnetism can be voltage-induced even from a diamagnetic (zero-spin) state suggesting that useful magnetic phases could be electrically induced in “nonmagnetic” materials. We use ionic liquid–gated diamagnetic FeS2 as a model system, showing that as little as 1 V induces a reversible insulator-metal transition by electrostatic surface inversion. Anomalous Hall measurements then reveal electrically tunable surface ferromagnetism at up to 25 K. Density functional theory–based modeling explains this in terms of Stoner ferromagnetism induced via filling of a narrow eg band.

INTRODUCTION

Magnetic materials have been linchpins of high technology for decades. Contemporary examples include hard disk drives and magnetic random access memories, although ongoing advances in spintronics have created myriad additional possibilities, in sensing, data storage, and processing (1). One such advance is the capability to control ferromagnetic magnetization with electrical currents, in addition to magnetic fields, via spin-transfer (2) and spin-orbit torques (3, 4). Field- and current-based manipulation of ferromagnetism intrinsically involves power consumption, however, which is a critical limiting factor in data storage and processing (1, 5). This situation has focused great attention on voltage-based (i.e., electric field–based) control of magnetism, with potential for much lower power consumption (1, 57).

While multiple approaches to voltage-controlled magnetism exist, magnetoionics, where electric field–induced ion motion is used to manipulate magnetism, appears particularly promising (68). The electric double-layer transistor (EDLT; Fig. 1A) is foundational to this effort, as it has proven highly effective in voltage control of insulator-metal and superconducting transitions and is now being used in magnetoionics (69). An EDLT is formed by replacing the dielectric in a field-effect transistor with an electrolyte, often an ionic liquid (IL). When a gate voltage (Vg) is applied between the magnetic material and a metallic gate, IL cations or anions (depending on the Vg polarity) drift to the surface of the magnet, where they accumulate (Fig. 1A, right). The electric field created is then screened by induction of a two-dimensional (2D) sheet of electrons/holes in the magnetic material, forming an electric double layer (EDL). This EDL is essentially a nanoscale capacitor, resulting in tens of microfarad per square centimeter capacitances and electron/hole densities up to 1015 cm−2 at just a few volts (810). This is sufficient to induce and control electronic phase transitions and order, including magnetism (810).

Fig. 1 Measurement schematic and reversible voltage-induced surface insulator-metal transition.

(A) Schematic of the ionic liquid (IL) gating device (left) and FeS2 surface electric double layer (EDL) (right). Vg, V, and I are the gate voltage, measurement voltage, and excitation current, respectively. The FeS2 single crystal, gold contacts, metallized gate electrode/liquid container, the IL, the IL cations and anions, and the induced electrons at the FeS2 surface are shown. (B) Temperature (T) dependence of the FeS2 sheet resistance (Rs) at multiple Vg between 0 and +3.0 V (0, +1.0, +1.1, +1.2, +1.3, +1.4, +1.6, +1.8, +2.0, and +3.0 V) for sample 1. The bulk to surface conduction crossover at Vg = 0 is labeled. (C) Rs(T) for sample 2 at Vg = 0 and +3.0 V through the two consecutive Vg cycles indicated in the legend. EMI/TFSI [1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl) imide] was used for all data in this figure.

EDLT-based voltage control of ferromagnetism is advancing rapidly. Control of the Curie temperature (TC) of ferromagnets, for example, has progressed from Vg-induced shifts of ~30 K in IL-gated perovskite manganites (11) to 110 K in ultrathin Co (12) to >150 to 200 K in perovskite cobaltites (1316). The reversible voltage induction of ferromagnetism from nonferromagnetic states, such as antiferromagnetic LaMnO3 (17) or SrCoO2.5 (14), has also been demonstrated. The gating mechanisms in these EDLTs are often not simply electrostatic, as in Fig. 1A, but involve electrochemistry (8). Oxide ionic gating effects based on creation and annihilation of oxygen vacancies (1315, 1820) and insertion and extraction of hydrogen (14, 21, 22) are now known, for example. This has spawned expansion from IL-based EDLTs to devices based on ion gels, solid electrolytes, ionic conductors, etc. (68, 10), applied beyond insulators and semiconductors to thin film ferromagnetic metals, for example (12, 2329). Notable achievements include sizable voltage-based modulation of TC, magnetization, coercivity, and magnetic anisotropy (68, 1117, 2329), in addition to encouraging progress with reversibility and speed (27, 30).

While impressive, these advances are subject to a clear limitation. Specifically, they involve either voltage control of a known ferromagnet (11, 12, 2329) or voltage-induced ferromagnetism from some other magnetic state (13, 14, 16, 17, 31), be it ordered (e.g., antiferromagnetic) or disordered (paramagnetic). This raises a simple question with deep implications: Can ferromagnetism be voltage-induced from a “nonmagnetic” state, i.e., a diamagnet, with a zero-spin electronic configuration? We address this here using IL-gated single crystals of diamagnetic FeS2, i.e., Fool’s Gold. This is a pyrite structure 0.95-eV gap semiconductor with t2g6eg0 electronic configuration and well-established zero-spin diamagnetism (3234). What makes FeS2 ideal for such studies is that alloying with ferromagnetic CoS2 (electronic configuration, t2g6eg1) to produce Fe1−xCoxS2 induces ferromagnetism at as little as 1% Co (35, 36), eventually reaching a state with TC ≈ 150 K in Fe0.2Co0.8S2 (3739). Interfacial charge transfer in pyrite-containing heterostructures has also been predicted to induce ferromagnetism in FeS2 (40). While likely not unique, diamagnetic FeS2 thus exists in unusual proximity to a ferromagnetic instability, rendering it ideal for investigation of the plausibility of voltage-induced ferromagnetism in a diamagnetic system.

RESULTS AND DISCUSSION

As described in Materials and Methods, EDLTs based on pyrite FeS2 single crystals (see fig. S1 and table S1 for structural and chemical characterization details) were fabricated using a metallized cylinder as both the gate electrode and IL container (Fig. 1A). Figure 1B shows the resulting temperature (T) dependence of the sheet resistance (Rs) of a representative EDLT (sample 1) at Vg values between 0 and +3 V (i.e., in electron accumulation mode). At Vg = 0, typical single-crystal FeS2 behavior is observed, semiconducting transport (dRs/dT < 0) occurring down to ~150 K, where Rs(T) flattens before increasing at low T. This arises due to the surface conduction that is now established in FeS2 crystals, where the semiconducting interior, which is lightly n-doped (e.g., 1016 cm−3 at 300 K) by S vacancies, is shunted at low T by a heavily doped (e.g., 1020 cm−3 to 1021 cm−3) p-type surface layer (see fig. S2 for further details) (4144). The surface Fermi level pinning and band bending required for this likely arise from surface states, with important implications for pyrite photovoltaics (41, 42, 44). More important in the current context, application of Vg up to +3 V is seen in Fig. 1B to result in marked decreases of the low T resistance (by ~104-fold at 30 K, extrapolating to much more at lower T). Rs falls beneath h/e2 ≈ 26 kilohm, Rs(T) becoming flat by +3 V, aside from a weak low T upturn. As discussed below, detailed Rs(T, H) data and analyses (where H is the applied magnetic field) establish that this occurs due to a transition from Efros-Shklovskii variable-range hopping (45) to weak localization (46) at ≥1.2 V. Note that the 300 K (bulk) resistance in Fig. 1B is barely affected by Vg, meaning that this strongly localized to essentially metallic transition occurs on the FeS2 surface. This occurs at positive Vg, implying inversion of the initially p-type surface (44) to an n-type metal, as confirmed below.

The reversibility of the observed Vg-induced surface insulator-metal transition is explored in Fig. 1C using a second sample (sample 2). This sample has lower bulk n-doping (see Materials and Methods), resulting in higher Rs (300 K) and thus a higher bulk-to-surface crossover temperature (>300 K). The solid black and red lines in Fig. 1C depict the 0- and +3 V behavior during the initial Vg cycle, where the 30 K resistance drops by a factor of ~105. Returning Vg to zero (dashed black line, cycle 2) then results in Rs(T) essentially identical to the initial state, reapplication of +3 V (dashed red line, cycle 2) inducing very similar Rs(T) to initial gating. This extraordinary level of reversibility, in a system where even minor surface structural or chemical modifications result in large changes in surface conduction (41, 42), provides strong evidence of a reversible, predominantly electrostatic gating mechanism (Fig. 1A), as opposed to the electrochemical mechanisms frequently encountered (8).

The data of Fig. 1B replotted to permit Zabrodskii analysis are shown in Fig. 2A. In this approach, lnW is plotted versus lnT, where W, the reduced activation energy, is defined as W = −dlnRs/dlnT. This linearizes Rs = R0exp(T0/T)m, where T0 is a characteristic temperature, R0 is Rs(T → ∞), and the exponent m provides insight into conduction mechanisms. The rapid decrease in lnW on initial cooling in Fig. 2A reflects the bulk to surface conduction crossover, which is followed by a clear progression with Vg in the low T limit. At Vg = 0 and +1 V, positive lnW values that increase linearly with decreasing lnT are seen in Fig. 2A, with a slope m ≈ 0.5 (blue solid line). As in prior work on ungated FeS2 crystals (42, 47), this indicates Efros-Shklovskii variable-range hopping (45), i.e., Rs = R0exp(T0/T)1/2, on the insulating side of the insulator-metal transition. As Vg is increased above 1.1 V, however, lnW becomes negative and T independent, indicating a crossover to nonactivated transport, i.e., effectively metallic behavior. Figure 2B further emphasizes this by plotting (RsRs,100K) versus lnT (where Rs,100K is the value of Rs at 100 K) at each Vg. The rapid increase in Rs on cooling at low Vg is seen to give way to a weak, approximately lnT dependence at high Vg. This behavior is expected for weakly localized transport in a 2D surface layer (46), which we show below to be the case in these EDLTs.

Fig. 2 Details of the voltage-induced insulator-metal transition.

(A) Zabrodskii analysis of the temperature (T) dependence of the sheet resistance (Rs) for the sample 1 data shown in Fig. 1B. lnW is plotted versus lnT, where W = −dlnRs/dlnT, at gate voltages (Vg) between 0 and +3.0 V (0, +1.0, +1.1, +1.2, +1.3, +1.4, +1.6, +1.8, +2.0, and +3.0 V). The solid blue line corresponds to m = 0.5, i.e., Efros-Shklovskii variable-range hopping. (B) (RsRs,100 K) versus lnT, where Rs,100K is Rs(T = 100 K), for the same Rs(T, Vg) curves in (A). (C) Magnetoresistance (MR) versus perpendicular magnetic field (H) at 30 K for the same Vg shown in (A) and (B). (D) MR(H) at Vg = 3.0 V for T between 300 and 1.8 K (300, 240, 180, 120, 60, 30, 10, 5, and 1.8 K). All data are for sample 1 using EMI/TFSI.

Further support for these conclusions is provided by magnetoresistance (MR) measurements (with H perpendicular to the crystal surface), as in Fig. 2C at 30 K. At Vg = 0, parabolic positive MR occurs, consistent with the Efros-Shklovskii variable-range hopping (45) deduced from Fig. 2A. As Vg is increased, the MR switches to negative, taking on a form characteristic of weak localization, again consistent with Fig. 2 (A and B). Figure 2D then illustrates MR(T) at fixed Vg = 3 V, which is also qualitatively consistent with our analysis of Rs(T). Specifically, positive parabolic MR at high T (due to the ordinary MR effect when bulk semiconducting conduction dominates) crosses over to negative MR at low T (when weakly localized surface conduction dominates). Figure 2 thus evidences a positive-Vg–induced transition in the low T surface-dominated conduction from strongly localized variable-range hopping to effectively metallic/weakly localized transport at Vg ≥ 1.2 V.

The magnetic behavior in the Vg-induced effectively metallic FeS2 surface state is explored in Fig. 3 via anomalous Hall effect measurements. The 30 K Vg dependence of the transverse (i.e., Hall) conductance versus perpendicular magnetic field (H) is shown in Fig. 3A. This is defined as Gxy = Rxy/Rs, 02, where Rxy is the transverse resistance and Rs,0 is the zero field sheet resistance. The data in Fig. 3A were acquired on sample 1 at the same Vg values as in Figs. 1B and 2, the inset showing a low H view. Gxy is negligible at Vg = 0, as the Hall effect in this sample is suppressed at low T by hopping conduction (fig. S2) (42, 47). Increasing Vg to +1.6 V induces a clear Hall effect, however, with both negative slope and distinct curvature (Fig. 3A, inset). The negative slope is important, as it suggests that at this Vg (which is effectively metallic from Figs. 1B and 2), inversion of the p-type surface to n-type has occurred. Further increasing Vg to +1.8, +2, and +3 V (Fig. 3A, main panel) then leads to the emergence of a very different state with a much larger, positive slope, nonlinear Hall effect. Gxy(H) is sigmoidal, immediately suggestive of the anomalous Hall effect, the positive slope being consistent with the anomalous Hall coefficient in ferromagnetic Fe1−xCoxS2 (36).

Fig. 3 Voltage-induced ferromagnetism at the FeS2 surface.

Transverse (Hall) conductance (Gxy) versus applied magnetic field (H) for sample 1 at multiple gate voltages (Vg); data are shown at (A) 30 K (0, +1.0, +1.1, +1.2, +1.3, +1.4, +1.6, +1.8, +2.0, and +3.0 V) and (B) 1.8 K (+1.1, +1.2, +1.3, +1.4, +1.6, +1.8, +2.0, and +3.0 V). Note the low H insets. (C) Temperature (T) dependence of the coercivity (Hc, log10 scale) in the ferromagnetic state of samples 1, 2, and 3 at multiple Vg. (D) H dependence of the transverse (Hall) resistance (Rxy) of sample 1 at Vg = 3.0 V (open circle symbols), at multiple T, along with fits to the Langevin-based model described in the text (lines). Data are shown at 300, 280, 200, 100, 20, and 1.8 K. EMI/TFSI was used in all data in this figure.

To probe for possible hysteresis, Fig. 3B shows Vg-dependent (from +1.1 to +3 V) Gxy(H) in the same sample at 1.8 K, with the low-field behavior highlighted in the inset. Clear coercivity (Hc = 700 Oe) and remanence occur at the highest Vg (see inset). The data of Figs. 1 to 3 thus establish gate induction of an effectively metallic FeS2 surface state with a strong, sigmoidal Gxy(H), a sign consistent with the anomalous Hall coefficient of Fe1−xCoxS2 and both coercivity and remanence. We take these observations as strong evidence for electric field–induced ferromagnetism in this diamagnet. The data of Fig. 3C establish repeatability, demonstrating finite low temperature Hc at high Vg in anomalous Hall data on three samples (samples 1, 2, and 3). Note that the cutoff Hc value in Fig. 3C (which has a log10 scale) is 7 Oe, the minimum value detectable in our superconducting magnet-based measurements. We thus take the point where Hc falls to ≤7 Oe as a good estimate of TC. At a high Vg of +3 V, the Hc (1.8 K) and TC values vary somewhat from sample to sample (from 400 to 2300 Oe and 10 to 24 K), although both increase monotonically with Vg in a single sample: Fig. 3C shows sample 3 at +2, +3, and +4 V, for example, where TC is 10, 12, and 15 K, respectively. The crystal-to-crystal variations in the gate-induced surface ferromagnetic parameters may be related to variations in surface conduction in FeS2 single crystals, which are highly sensitive to the surface structure and chemistry (42); this is a topic worthy of further study. The highest TC achieved was 24 K in sample 2 at 3 V, where Hc (1.8 K) reached 2300 Oe.

To further explore the temperature evolution of the high Vg ferromagnetism, Fig. 3D plots Rxy(H) in sample 1 from 1.8 (blue open circles) to 300 K (red open circles). The large positive slope anomalous Hall effect at low T is seen to eventually give way to a linear negative slope ordinary Hall effect at 300 K. However, two effects are convoluted here: the loss of surface magnetism and the crossover to conduction dominated by the n-type interior. What is remarkable in Fig. 3D is that, while TC = 16 K, the anomalous Hall effect remains dominant to 200 K and is detectable (on the ordinary Hall background) at 280 K. While Hc vanishes at 16 K, clear contributions to anomalous Hall thus exist at up to 280 K (i.e., 20 times TC), suggesting that ferromagnetic correlations persist to unusually high T. This is highlighted in Fig. 4 (A and B), which shows the T dependence of |dRxy/dH|H → 0 and RxyF. The first of these quantities is simply the low-field Hall slope, while the second is the Rxy obtained by linearly extrapolating high H data (>70 kOe) to H = 0; both thus probe the nonlinear magnetic contribution to Rxy(H). Both quantities fall rapidly at TC (see the dashed vertical line in Fig. 4, A and B) but exhibit long, near-linear high T tails, reaching zero only at 300 K, where bulk conduction dominates. This T dependence is in stark contrast to standard Curie-Weiss behavior in an interacting paramagnet, as illustrated by the solid line in Fig. 4A of the form C/(T − 16 K) (where C is a constant).

Fig. 4 Analysis of temperature dependence of the anomalous Hall effect.

Temperature (T) dependence of (A) [dRxy/dH]H = 0 (the field derivative of the transverse resistance as H → 0) and (B) RxyF [i.e., the transverse resistance obtained by extrapolating the high H Rxy(H) curve to H = 0] in sample 1 at Vg = 3.0 V. The solid green curve in (A) illustrates Curie-Weiss behavior. (C and D) T dependence of the parameters extracted from the Langevin-based fitting described in the text, including the parameter k2 (C) and the cluster magnetic moment (μc) and deduced diameter (D) (D, left and right axis, respectively). As described in the text, k2 reflects a paramagnetic background and the negative bulk ordinary Hall effect at low and high T, respectively. D is deduced from μc assuming 0.2 μB per Fe and an accumulation layer with a thickness of 2 nm, resulting in disk-shaped clusters. The vertical dashed line indicates the Curie temperature (16 K). EMI/TFSI was used in all data in this figure.

One possible interpretation of Fig. 4 (A and B) is that the induced FeS2 surface ferromagnetism occurs close to the 2D limit, thus suppressing TC despite strong ferromagnetic correlations. While the cubic magnetocrystalline anisotropy of Fe1−xCoxS2 is unknown at low x, at x = 1 (i.e., CoS2), it is parameterized by low T anisotropy constants K1 and K2 of only −2.5 × 104 erg cm−3 and ~0 erg cm−3. (48) This is an order of magnitude lower than even face-centered cubic Ni. (49) The ferromagnetism electrically induced in FeS2 could thus be expected to be weakly anisotropic, i.e., of Heisenberg type (50). Mermin-Wagner physics (51) would then apply, limiting the 2D TC despite strong ferromagnetic interactions. In this context, we note that electrostatic calculations (fig. S3) indicate that 90% of the induced electrons in these EDLTs are confined within (at most) ~2 nm (i.e., four unit cells) of the surface. In terms of direct extraction of information on magnetic anisotropy from our data, some weak perpendicular magnetic anisotropy apparently occurs based on the finite HC and remanence in Fig. 3B. Simple estimates using HC ≈ 1 kOe (Fig. 3C) and saturation magnetization ~0.2 μB per Fe (see below, Fig. 5) yield low effective uniaxial anisotropy values, ~104 erg cm−3. Further work will be needed to fully characterize magnetic anisotropy in the voltage-induced ferromagnetic state, including understanding the origin of the perpendicular magnetic anisotropy. We note, in this context, that the high-field curvature in Fig. 3B may indicate an additional paramagnetic contribution, rather than ferromagnetic anisotropy.

Fig. 5 Theoretical results on voltage-induced ferromagnetism at the FeS2 surface.

Calculated induced ferromagnetic magnetization (M) versus added 2D projected electron density, n2D. The top axis shows the conversion to electrons per Fe ion (n3D); this axis is not linear due to cell volume changes that take place with electron addition (the structure is relaxed at each density). The data points are color-coded to the solid curves in the inset, which show the wave vector (k) dependence of the susceptibility (χ) at each calculated electron density. Γ, X, M, and R correspond to k = [0, 0, 0], [1, 0, 0], [1, 1, 0], and [1, 1, 1] in momentum space, respectively. A peak emerges at the Γ point, corresponding to long-range–ordered ferromagnetism (k = 0). Note that n2D values in the main panel assume an accumulation thickness of one unit cell. Surface n2D values above 3 × 1014 cm−2 result in estimated accumulation layer thicknesses at the unit cell level (see fig. S3), rendering experiment and Fig. 5 comparable.

An alternative interpretation of the temperature dependence in Fig. 3D involves superparamagnetism. The zero-coercivity sigmoidal-shaped curves seen above 16 K in Fig. 3D can be well fit to Rxy = k1Lcμ0H/kBT) + k2μ0H, where L is the Langevin function, μc is a cluster magnetic moment, μ0 is the vacuum permeability, and kB is Boltzmann’s constant. This describes the H and T dependence of the magnetization with a Langevin function (as expected in a simple superparamagnet above its blocking temperature), k1 being related to the anomalous Hall coefficient and k2 capturing a paramagnetic background and the ordinary Hall effect. As shown by the solid lines in Fig. 3D, good fits can be obtained, indicating that Rxy(H) at any T > 16 K can be described as a Langevin superparamagnet. Figure 4C further illustrates that the extracted k2(T) behaves rationally, crossing from positive at low T (likely a paramagnetic background) to negative at high T (due to the bulk negative ordinary Hall coefficient). The extracted μc values (Fig. 4D) of ~200 to 1000 μB are also plausible. Using again an approximate magnetization of 0.2 μB per Fe (see below, Fig. 5) and an accumulation layer thickness of 2 nm, we deduce superparamagnetic volumes corresponding to disk-shaped clusters of diameter ~4 to 12 nm (Fig. 4D, right axis). Inhomogeneities in the EDL or at the FeS2 crystal surface could be responsible for this nanoscopic magnetic inhomogeneity, rendering the 16 K scale a superparamagnetic blocking temperature. However, there are at least two issues with this picture. First, as shown in Fig. 4D, the deduced μc(T) has non-negligible T dependence, meaning that the expected H/T scaling of a Langevin function is not exactly obeyed. Second, in this interpretation, the true TC would be as high as ~280 K, around double the maximum TC of bulk Fe1−xCoxS2, which is also only reached at high x. We thus view it as unclear whether the high T behavior in Fig. 3D is best viewed as a paramagnet with strong ferromagnetic correlations persisting to high T or as a superparamagnet. Vitally, however, regardless of interpretation of Figs. 3 and 4 in terms of a ferromagnetic to paramagnetic transition or a ferromagnetic (blocked) to superparamagnetic (unblocked) transition, voltage-induced ferromagnetism in diamagnetic FeS2 remains clear.

In terms of rationalization of the Vg-induced ferromagnetism in FeS2 EDLTs, we first note that quantitative comparison to Fe1−xCoxS2 supports feasibility of induction of a ferromagnetic state. Prior work on Fe1−xCoxS2 crystals found ferromagnetism to emerge with TC ≈ 1 to 2 K at a doped electron density of ~1 × 1020 cm−3 (35). This is equivalent to a 2D surface density of ~2 × 1013 cm−2, assuming a four unit cell accumulation layer (see fig. S3). These 2D densities are easily achievable in EDLTs (810), although quantitative comparison is frustrated by the fact that the anomalous Hall effect overwhelms the ordinary Hall effect at high Vg (consider Fig. 3, A, B, and D where there is no indication of a negative high H slope at low T), precluding quantification of induced electron densities.

Further insight is provided by first-principles electronic structure calculations, which have been previously applied to Fe1−xCoxS2 (52). Hubbard U–corrected density functional theory (DFT + U) using approaches described in Materials and Methods (53, 54) was used to first reproduce the known crystal and electronic structures of FeS2. Electrostatic gating was then simulated by artificially adding electrons to the FeS2 unit cell to the 3D and projected 2D densities shown in Fig. 5 (top and bottom axes, respectively). The magnetization (M) was then extracted versus electron density, resulting in the colored points in the main panel. Ferromagnetic magnetization is found to emerge above ~3 × 1014 cm−2, which, critically, is again within realistic capabilities of IL gating (810). Note that the magnetization eventually reaches 1 μB per Fe, consistent with the half-metallicity, or at least high spin polarization, in Co1−xFexS2 (3739, 52). Additional understanding of the origin of the induced ferromagnetism was obtained from Wannier function–based (55) tight-binding parameterization of the DFT-calculated FeS2 conduction band structure, followed by calculation of the filling-dependent susceptibility χ(k) (see Materials and Methods). The resulting χ(k) is shown in the inset in Fig. 5, color-coded to the electron densities in the main panel. As the density is increased, a single peak in χ(k) emerges at the Γ point (k = 0), demonstrating instability toward long-range ferromagnetism, apparently of Stoner type. The threshold filling for the onset of ferromagnetism corresponds to the point at which the Fermi level moves from sulfur p states in the lower region of the conduction band to Fe eg states. These calculations thus confirm the feasibility of ferromagnetism in FeS2 induced solely by filling of a narrow eg band, strongly supporting the experimental findings. More detailed calculations will be reported elsewhere, clarifying the different mechanisms and doping thresholds for ferromagnetism in Co-doped and electrostatically doped FeS2 (56).

Additional insight into the Vg-induced insulator-metal and diamagnet-ferromagnet transition is provided by Vg sweep data (Fig. 6). The resistance (R) during a 5 mV s−1 ± 4 V cycle at 240 K is shown in Fig. 6A; this temperature was chosen to be as low as possible (to maximize the R modulation) while maintaining tolerable ion mobility in the IL (see Materials and Methods). Unlike the data in Figs. 1 to 4, which were acquired after a 30 min saturating Vg application (see Materials and Methods), these data reveal gating dynamics. Substantial hysteresis is apparent in Fig. 6A, with a consequent sweep rate dependence (fig. S4). Given the evidence of predominantly electrostatic gating, this hysteresis is ascribed not to electrochemistry at the FeS2 surface but rather to limited ion mobility in the IL, as expected in a macroscopic side-gate device at this T (10, 57). Beginning at Vg = 0, R is seen to rapidly increase with increasing Vg, reaching a peak at 1.5 V, at about twice the initial R. Further increasing Vg then induces a precipitous drop to ~1.5 kilohm, which is maintained as Vg is decreased back to 0 V. R is thus bistable at Vg = 0 due to hysteresis associated with IL dynamics. Negative Vg then increases R to a small, broad peak at −2 V and ~24 kilohm, R remaining relatively constant out to −4 V and back to −1 V. Simply, we interpret the peak at small positive Vg as a characteristic signature of the previously deduced surface inversion. Specifically, R first increases with positive Vg due to depletion of the initially p-type surface, reaching a peak at the compensation point, limited only by bulk conduction. Further increasing Vg then accumulates electrons, rapidly decreasing R. Under the slow gating in Fig. 1B (see Materials and Methods), this inversion is already complete in the +1 V Rs(T), highlighting the complementary information in Fig. 6A. As shown in fig. S5, resistance anisotropy also peaks at the +1 V compensation point in Fig. 6A, likely due to gating inhomogeneity at the highly sensitive compensation point; these factors will also play a role in the more subtle inversion from n- to p-type on reversing Vg in Fig. 6A.

Fig. 6 Hysteretic gate voltage response at 240 K.

Gate voltage (Vg) dependence of (A) the sample resistance (R) (in one van der Pauw orientation) and (B) the gate current (Ig) during a sweep (5 mV/s) at 240 K for sample 2. At this temperature, the IL is in the liquid state, with workable ion mobility. Panels (C) to (J) show transverse (Hall) resistance (Rxy) versus applied magnetic field (H) plots at the gate voltages indicated by the colored solid points in (A); the Hall data are color-coded to these points. Note that the Hall data were acquired after cooling to 180 K, not at 240 K, to minimize measurement drift by freezing the IL. The vertical dashed lines in (A) and (B) highlight the simultaneity of the large changes in R with the Ig peaks. DEME/TFSI was used in all data in this figure.

Further supporting the above, Hall data at the indicated Vg are shown in Fig. 6 (C to J) (color coded to the points in Fig. 6A), taken by pausing loops such as Fig. 6A and then rapidly cooling to 180 K. This cooling freezes the IL, thus achieving sufficiently stable resistances to permit Hall measurements. At Vg = 0 (Fig. 6C), Rxy(H) is near linear with a negative slope due to the small (hopping-suppressed) surface contribution but finite n-type bulk contribution. The situation is similar at +1 V (Fig. 6D), although the increased surface resistance (Fig. 6A) now drives additional bulk current, increasing the magnitude of the Hall slope. However, marked changes occur at +2 V (Fig. 6E), where Rxy(H) transforms to a nonlinear sigmoidal shape with positive slope. As already discussed, this occurs due to a positive anomalous Hall coefficient, the anomalous Hall effect arising from ferromagnetic correlations that extend far above TC at high positive Vg. Figure 6E is thus consistent with Fig. 6A, where R is falling precipitously at +2 V, indicating transformation to the n-type ferromagnetic metallic state. Also consistent with Fig. 6A, the positive slope sigmoidal Rxy(H) is maintained at +4, +2, and 0 V (Fig. 6, F to H) due to persistence of the magnetic state. This is then lost on going from 0 to −4 V (compare Fig. 6, H and I), panels (I) and (J) in Fig. 6 being similar to panels (C) and (D) in Fig. 6. Last, we note that the corresponding gate current (Ig) sweep in Fig. 6B is also consistent with the above. The overall form is typical of electrostatically functioning EDLTs, ascending and descending Vg sweeps inducing positive and negative Ig, respectively, due to charging and discharging. These currents are small for such a macroscopic device (<0.5 μA), except at ∣Vg∣ > 3 V, which we attribute to the electrochemical stability window of the IL. The only additional features are small peaks on the ascending Vg sweep at −1 and 2 V, which (see the vertical dashed lines) correspond to the onset and completion of surface inversion.

CONCLUSIONS

Detailed experimental evidence has been presented establishing that semiconducting diamagnetic FeS2 with a zero-spin electronic configuration can be electrolyte-gated into a ferromagnetic metallic state in single-crystal EDLTs. A predominantly electrostatic gating mechanism is concluded, driving inversion of the initially p-type FeS2 surface to an n-type ferromagnetic metal with voltage-tunable TC up to 25 K. Computational and analytical theory support this, demonstrating feasibility of inducing ferromagnetism by band filling alone. State-of-the-art electrolyte gating is thus capable of inducing surface ferromagnetism in even nonmagnetic materials, with deep implications. Future work could focus on theoretical identification and experimental verification of other diamagnets in which ferromagnetism can be induced, extension to thin films, further exploration of the nature of the voltage-induced ferromagnetism (including the ferromagnetic anisotropy and the issue of paramagnetism versus superparamagnetism), enhancement of the induced TC, and demonstration of important device function. The latter could include generation of voltage-controlled highly spin-polarized surface currents for spin injection into diamagnetic interiors or overlying heterostructures, representing a small fraction of the possibilities opened up by this work.

MATERIALS AND METHODS

Crystal and device preparation

Pyrite FeS2 single crystals were grown using chemical vapor transport, as described earlier (42, 43, 47). Precursor FeS2 powder was first synthesized by reacting Fe (99.998% purity; Alfa Aesar) and S (99.9995% purity; Alfa Aesar) powders in evacuated quartz ampoules at 500°C for 6 days, followed by repeated grinding and reaction until phase-pure pyrite FeS2 was obtained by powder x-ray diffraction. Quartz ampoules were then loaded with 2.2 g of this precursor FeS2, 100 mg of FeBr2 transport agent (99.999% purity; Sigma-Aldrich), and additional S powder (0.06 g for sample 1 and 0.34 g for samples 2 and 3), followed by evacuation to ~10−6 torr and flame sealing. The ampoule was then placed in a two-zone furnace for a 17-day growth with the source and growth zones at 670° and 590°C, respectively [after a 3-day inversion period (42, 43, 47)]. Detailed characterization was presented earlier (42, 43, 47), indicating phase purity, total metals-basis impurity concentrations of <40 parts per million, and unit cell–level surface roughness. Select structural and chemical characterization data are provided in fig. S1 and table S1. As described in prior work (43), lower S loading (as for sample 1) leads to increased S vacancy density and n-type doping and thus lower bulk resistivity.

Crystals were prepared for transport by polishing from the opposite side of a large facet (with SiC grinding paper and diamond slurries down to 3 μm) to thicknesses of ~200 μm, followed by sequential ultrasonication in acetone, methanol, and isopropanol. Au contacts (~50 nm thick) were then sputtered on the pristine (i.e., unpolished) facet. Crystals were then placed on an Al2O3 wafer with an Au-coated quartz cylinder, which served as both a high-area gate electrode and liquid container. Two ILs were used: 1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl) imide (EMI/TFSI) and diethylmethyl(2-methoxyethyl)ammonium TFSI (DEME/TFSI).

Electronic transport measurements

Four-terminal van der Pauw resistance measurements [as well as gate voltage (Vg) application and gate current measurement] were achieved with Keithley 2400 source-measure units in a commercial cryostat from 1.4 to 300 K in perpendicular magnetic fields to 9 T. In Figs. 1 to 4, Vg was applied at 300 K for ~30 min, followed by cooling to the base temperature or the temperature shown before warming or sweeping field. EMI/TFSI was the IL for these measurements. In contrast, the Vg sweep in Fig. 6 (A and B) was collected by cooling to 240 K at Vg = 0, followed by sweeping Vg from 0 to 4 V to −4 to 0 V at 5 mV s−1 using DEME/TFSI as the IL. This IL was selected in this case as its lower melting point than EMI/TFSI-enabled Vg sweeps at lower temperature, where surface conduction is more dominant, and the dynamic range in sample resistance is thus higher. The 0 to 4 V to −4 to 0 V voltage sequence was repeated until no qualitative features of the hysteresis loop changed in subsequent loops (the loop in Fig. 6 was the third taken). A subsequent and final Vg loop was then performed to collect the Hall data in Fig. 6 (C to J). At each Vg shown in Fig. 6 (C to J), the 240 K Vg sweep was paused, the sample was cooled to 180 K at ~10 K min−1, Hall data were collected, the sample was returned to 240 K, and the Vg sweep was continued. The cooling to 180 K (below the melting point of DEME/TFSI) was done to minimize drift during the Hall sweep.

Theoretical methods

First principles calculations were performed with DFT with a Hubbard U correction (i.e., DFT + U) using the Vienna ab-initio simulation package implementation of the projector-augmented-wave (PAW) approach (53, 54). The PBEsol-generalized gradient approximation was used to approximate the exchange-correlation functional. A Γ-centered 8 × 8 × 8 k-point grid and a plane wave cutoff of 500 eV were used. As discussed elsewhere (56), U = 5 eV was used for these calculations, achieving agreement with FeS2 lattice constants and S─S bond lengths to within <1 and <2.5%, respectively. Electrostatic gating was then simulated by varying the total number of electrons per unit cell, with the highest level considered being 0.5 added electrons per Fe ion (see Fig. 5). The ion positions and lattice vectors were allowed to relax at each electron concentration. The magnetization per Fe ion was then calculated by integrating the magnetization density over the whole cell and dividing by the number of Fe ions. Following this, a Wannier function–based (55), tight-binding model was built using nonspin-polarized DFT calculations as an input, and the electronic susceptibility was then calculated using the Lindhard function. More detailed theoretical results will be reported elsewhere (56).

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/31/eabb7721/DC1

https://creativecommons.org/licenses/by-nc/4.0/

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We gratefully acknowledge E. S. Aydil for his role in shaping our understanding of electronic transport in FeS2 crystals, forming a foundation for this work. Funding: This work was primarily supported by the NSF through the University of Minnesota (UMN) MRSEC under DMR-1420013. Parts of the work were carried out in the Characterization Facility, UMN, which receives partial support from NSF through the MRSEC program. Portions of this work were also conducted in the Minnesota Nano Center, which is supported by the NSF through the National Nano Coordinated Infrastructure Network under NNCI-1542202. The Minnesota Supercomputing Institute is acknowledged for providing resources that contributed to the research results reported. Author contributions: C.L. and J.W. conceived the study. J.W. and B.V. performed the measurements, assisted by K.H., on single crystals grown and characterized by B.V. and J.W. Supervision of all aspects of the experimental work was provided by C.L. Calculations were performed by E.D.-R. and supervised by T.B. and R.M.F. The paper was written by J.W. and C.L. with input from all authors. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.
View Abstract

Stay Connected to Science Advances

Navigate This Article