Strongly Correlated Molecular Magnet with Curie Temperature above 60 K

Abstract

•2D superconducting FeSe for the growth of strongly correlated molecular magnet•The ferromagnetic ordering temperature above 60 K•Tunable magnetodielectric coupling by magnetic field•Conducting state and charge crystallization induced by photoirradiation Two-dimensional (2D) molecular magnets are intensively needed for the development of magnetic, optical, and electrical devices in strongly correlated quantum matter fields. The stimuli-induced magnetic characteristics of molecular magnets are particularly critical to realization of such promising potential. However, the insulating nature and thermal diminished magnetism of molecular magnets limit its broad applications. The strongly correlated molecular magnet presented in this work from a 2D superconducting FeSe template demonstrates the charge-spin-lattice coupling. The magnetodielectric coupling effect and photoinduced conductivity shown here would benefit the multifunctional devices based on molecular magnets and could help reach a “holy grail” in the field of molecular conducting magnets. Two-dimensional crystals with ferromagnetic ordering would lead to unprecedented possibilities for magnetic, magneto-optic, and magnetoelectric applications. However, long-range ferromagnetic ordering in two-dimensional van der Waals materials is strongly diminished by thermal fluctuation, while three-dimensional systems show the stiffness of magnetic ordering temperature to magnetic fields. Here, we report the discovery of intrinsic ferromagnetic ordering temperature of 60 K in an iron/tetracyanoquinodimethane layer via superconducting precursor. In such a molecular ferromagnet, an unprecedented control of ferromagnetic fluctuation temperature by ∼200% improvement is realized through magnetic fields. We demonstrate magnetodielectric coupling with a concomitant transition in magnetic ordering. A small magnetic field of 0.01 T enables an effective route to realize a sizable transition shift in magnetodielectric states. Stimulated by photoirradiation, it manifests a hidden metallic conducting state, declaring a metallic ferromagnet. The molecular ferromagnet and photoirradiation-induced hidden metallic phase present a “holy grail” in the field of molecular conducting magnets. Two-dimensional crystals with ferromagnetic ordering would lead to unprecedented possibilities for magnetic, magneto-optic, and magnetoelectric applications. However, long-range ferromagnetic ordering in two-dimensional van der Waals materials is strongly diminished by thermal fluctuation, while three-dimensional systems show the stiffness of magnetic ordering temperature to magnetic fields. Here, we report the discovery of intrinsic ferromagnetic ordering temperature of 60 K in an iron/tetracyanoquinodimethane layer via superconducting precursor. In such a molecular ferromagnet, an unprecedented control of ferromagnetic fluctuation temperature by ∼200% improvement is realized through magnetic fields. We demonstrate magnetodielectric coupling with a concomitant transition in magnetic ordering. A small magnetic field of 0.01 T enables an effective route to realize a sizable transition shift in magnetodielectric states. Stimulated by photoirradiation, it manifests a hidden metallic conducting state, declaring a metallic ferromagnet. The molecular ferromagnet and photoirradiation-induced hidden metallic phase present a “holy grail” in the field of molecular conducting magnets. 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The molecular-based strategy through materials design and supramolecular chemistry allows for the creation of 2D molecular magnets that cannot otherwise be obtained, for example, strongly correlated 2D molecular magnets that combine two or more degrees of freedom in the same compound or multistability under fundamental excitations. In addition, the 2D conductive magnet is a “holy grail” in the field of molecular magnets, which attracts extensive attention.6Gibertini M. Koperski M. Morpurgo A.F. Novoselov K.S. Magnetic 2D materials and heterostructures.Nat. Nanotechnol. 2019; 14: 408-419Crossref PubMed Scopus (625) Google Scholar,23Gong S.J. Gong C. Sun Y.Y. Tong W.Y. Duan C.G. Chu J.H. Zhang X. Electrically induced 2D half-metallic antiferromagnets and spin field effect transistors.Proc. Natl. Acad. Sci. U S A. 2018; 115: 8511-8516Crossref PubMed Scopus (99) Google Scholar,24Vickers E.B. Giles I.D. Miller J.S. M[TCNQ]gamma-based magnets (M = Mn, Fe, Co, Ni; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane).Chem. Mater. 2005; 17: 1667-1672Crossref Scopus (88) Google Scholar In our quest for 2D atomically thin crystals as building blocks for molecular magnets with new types of structural features, we discovered that a superconducting 2D FeSe layer is capable of serving the reaction framework for 2D molecular structures that order ferromagnetically above 60 K. Here, we demonstrate a 2D iron-tetracyanoquinodimethane (FeTCNQ) molecular magnet with dynamic control of magnetism and conductive states. The magnetic fluctuation in the Fe network can arise prior to the ordering temperature and be tuned by magnetic field. We observe an unprecedented control of ferromagnetic fluctuation at a temperature of ∼200% improvement realized through magnetic fields. Comprehensive charge-spin-lattice coupling in the 2D FeTCNQ magnet activates local dipole moments that mediate spin reorientation and magnetodielectric multistability. The charge modulation by electromagnetic excitation (e.g., photoirradiation) initiates the cooperative control of a long-lived low-resistivity state. The FeTCNQ molecular ferromagnet is obtained from solvothermal reaction of 2D FeSe layers and TCNQ molecules. As illustrated in Figure 1A, FeSe4-tetrahedra stacking layer by layer transform to the coplanar coordination of Fe and TCNQ25Zhang X. Saber M.R. Prosvirin A.P. Reibenspies J.H. Sun L. Ballesteros-Rivas M. Zhao H. Dunbar K.R. Magnetic ordering in TCNQ-based metal–organic frameworks with host–guest interactions.Inorg. Chem. Front. 2015; 2: 904-911Crossref Google Scholar, 26Ma Y. Dai Y. Wei W. Yu L. Huang B. Novel two-dimensional tetragonal monolayer: metal-TCNQ networks.J. Phys. Chem. 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The 2D FeSe precursor with a superconducting diamagnetic transition at 8 K possesses a van der Waals layered structure (Figure 1C and the inset transmission electron microscopy [TEM] image). Due to the strong electronegativity of -C≡N, the Se atoms in 2D FeSe block can be substituted by TCNQ to form FeTCNQ, maintaining the 2D morphology of FeSe (Figure S1). The calculated structure of FeTCNQ matches with the X-ray diffraction (XRD) pattern (Figures 1D, S2, and S3), verifying the replacement reaction and 2D nature of FeTCNQ. Through the four -C≡N bridges of one TCNQ molecule, Fe cations stabilize in a spin configuration of ferromagnetism that is strongly coupled and of higher Tc when a stronger exchange interaction of Fe and TCNQ occurs. Fine-tuning of reaction temperature presents a variable Tc of FeTCNQ ferromagnet from 15 K to 60 K (Figure 1E and Table S1) while maintaining its 2D morphology (inset of Figure 1E). This variable Tc of FeTCNQ may relate to the complexity of their crystal structures that can be glimpsed from the evolution of their morphologies and XRD patterns (Figures S1 andS2). The FeTCNQ samples are thermally stable at almost 490 K from the thermal weight analysis (Figures S4–S6). Here we only discuss magnetic order, magnetodielectric coupling, and light-induced conducting state in the FeTCNQ sample #1, which has the highest Tc. The existence of TCNQ in these samples is confirmed by Fourier transform infrared spectra (Figure S7) and Raman spectra. Ferromagnetic ordering in FeTCNQ is shown through temperature-dependent magnetic susceptibility and magnetic hysteresis loops (Figures 2 and S8–S13). Figure 2A shows the magnetization of sample #1 with Tc of 60 K under different magnetic fields measured from 8 K to 300 K. At magnetic field of 0.1 T, a rapid increase in magnetization below 60 K indicates the emergence of a magnetic ordering. Due to magnetic-field effects,29Janssen P. Cox M. Wouters S.H. Kemerink M. Wienk M.M. Koopmans B. Tuning organic magnetoresistance in polymer-fullerene blends by controlling spin reaction pathways.Nat. Commun. 2013; 4: 2286Crossref PubMed Scopus (95) Google Scholar magnetic fluctuation arises at higher temperature than Tc under higher magnetic fields, suggesting the potential magnetoelastic and spin-flip behavior from strong anisotropy of 2D molecular magnets.30dela Cruz C.R. Lorenz B. Sun Y.Y. Chu C.W. Park S. Cheong S.W. Magnetoelastic effects and the magnetic phase diagram of multiferroic DyMn2O5.Phys. Rev. B. 2006; 74: 180402(R)Crossref Scopus (33) Google Scholar The decreased magnetization and susceptibility at 50 K and 22 K under magnetic field of 10 mT and 0.1 T indicates the development of a diamagnetic behavior, the details of which are discussed in Figure S8. In addition, a Verwey-like transition at 130 K results from charge hopping between iron cations in 2D FeTCNQ, which can be suppressed by magnetic field above 1 T (Figure S9). Inverse magnetic susceptibility (Figure 2B) also confirms a ferromagnetic ordering temperature (Tc) of 60 K from the extrapolation of the linear part. Even though Tc is around 60 K, the magnetic coupling interaction can occur at a higher temperature (∼100 K), which is evident from the susceptibility and temperature product χT shown in Figure 2C. With the decrease in temperature, the χT gradually decreases to a minimum valley at Tsc, then increases to a maximum at a peak temperature Tp. The characteristic temperature Tsc, revealing the spin-coupling strength,31Bovo L. Twengstrom M. Petrenko O.A. Fennell T. Gingras M.J.P. Bramwell S.T. Henelius P. Special temperatures in frustrated ferromagnets.Nat. Commun. 2018; 9: 1999Crossref PubMed Scopus (17) Google Scholar increases from 137 K to 194 K with the increase in magnetic field from 1 T to 7 T that is consistent with the magnetic-field-dependent transition temperature shown in Figure 2A. Correspondingly, the χT peak temperature (Tp) increases from 34 K to 52 K with the decrease in peak magnitude, indicating the onset of hidden antiferromagnetic interaction that is pervasive in frustrated ferromagnets.31Bovo L. Twengstrom M. Petrenko O.A. Fennell T. Gingras M.J.P. Bramwell S.T. Henelius P. Special temperatures in frustrated ferromagnets.Nat. Commun. 2018; 9: 1999Crossref PubMed Scopus (17) Google Scholar Both characteristic temperatures of Tsc and Tp can be enhanced by magnetic fields (Figure S14). Magnetic hysteresis loops at different temperatures (Figures 2D and S10) further confirm ferromagnetism in 2D FeTCNQ. The magnetic hysteresis at 10 K presents a ferromagnetic nature in 2D FeTCNQ with a coercivity of 0.12 T and a saturation magnetization of 12.3 emu g−1 at 7 T (Figure S10A). As the temperature is increased to 70 K (Tc ∼ 60 K), a linear magnetization hysteresis loop is obtained, indicating its paramagnetic state (Figures 2D and S10). The strongly correlated nature of 2D molecular magnets induces the spontaneous intercorrelation of lattice symmetry and magnetic ordering, which activates the magnetodielectric coupling in 2D FeTCNQ controlled by magnetic-field effects. Magnetic ordering in 2D FeTCNQ is closely linked to the dipole response, involving the anisotropy-induced spin flip between spin singlet (S) and triplet (T) states (Figure 3A) under magnetic fields, which give rise to polarons and triplet excitons. Figure 3B shows the temperature-dependent dielectric constant at 1 kHz and 10 kHz under magnetic field. At low temperature, 2D FeTCNQ undergoes a magnetic ordering transition with a concomitant jump in dipole moment at Tc (Inset of Figure 3B), where the dielectric constant decreases with temperature and abruptly changes at ∼60 K (1 kHz). The abrupt jump in dielectric constant near Curie temperature indicates magnetoelastic coupling between spin and lattice dipoles.30dela Cruz C.R. Lorenz B. Sun Y.Y. Chu C.W. Park S. Cheong S.W. Magnetoelastic effects and the magnetic phase diagram of multiferroic DyMn2O5.Phys. Rev. B. 2006; 74: 180402(R)Crossref Scopus (33) Google Scholar Furthermore, the dipole transition temperature increases with magnetic field (Figures 3B and S15A), implying a potential magnetoelastic phenomenon in 2D FeTCNQ. At a higher frequency of 10 kHz, a jump in dielectric constant occurs at a higher temperature of 140 K (Figures 3B and S15B) near the Verwey-like transition in 2D FeTCNQ. The magnetic-field-dependent magnetocapacitance is shown in Figure 3C, and the change in dielectric constant saturates above 5 T where the magnetization saturates in 2D FeTCNQ (Figure S10). The enhancement in magnetocapacitance at 2 T can be ascribed to magnetic-field effects through spin flip, as illustrated in Figure 3A, while the relative permittivity increases with magnetic field, confirming the magnetodielectric coupling (Figures 3C and S15C). The magnetic-field-dependent dielectric transition temperature in 2D FeTCNQ is shown in Figure 3D. For the low frequency (1 kHz), the lattice dipoles couple with ferromagnetic ordering at Tc, contributing to a magnetoelastic effect and magnetic-field-dependent transition temperature, and possible ferroelectric performance (Figure S16). With regard to a higher frequency, the magnetoelastic effect is activated at higher temperature where the incipient Verwey-like transition arises. From the magnetodielectric coupling at ferromagnetic ordering and Verwey-like transition, it is evident that the spontaneous intercorrelation of lattice symmetry, magnetic ordering, and dipole moment occurs in the strongly correlated 2D FeTCNQ magnet. Charge crystallization and charge-lattice coupling dominate electron transport in 2D FeTCNQ layers. The temperature-dependent resistivity of 2D FeTCNQ undergoes a sharp semiconductor-to-insulator (SI) transition (Figure 4A), following a thermally activated model with a small activation energy of 3.3 meV. If 2D FeTCNQ is cooled at a higher rate of 10 and 20 K min−1 (Figure 4A), such transition is shifted toward a lower temperature, which is ascribed to the development of electron crystallization.32Sasaki S. Hashimoto K. Kobayashi R. Itoh K. Iguchi S. Nishio Y. Ikemoto Y. Moriwaki T. Yoneyama N. Watanabe M. et al.Crystallization and vitrification of electrons in a glass-forming charge liquid.Science. 2017; 357: 1381-1385Crossref PubMed Scopus (28) Google Scholar,33Sato T. Kitai K. Miyagawa K. Tamura M. Ueda A. Mori H. Kanoda K. Strange metal from a frustration-driven charge order instability.Nat. Mater. 2019; 18: 229-233Crossref PubMed Scopus (6) Google Scholar Upon application of magnetic field, the SI transition temperature changes from 197 K toward 228 K and 232 K, respectively, under magnetic field of 50 and 150 mT (Figure 4A). Such a phenomenon can be rationalized from the charge-spin-lattice coupling in such a 2D correlated molecular magnet, while the applied magnetic fields activate electron crystallization at a higher transition temperature. Raman spectroscopy provides the information of molecular vibration in the 2D FeTCNQ layer with the control of cooperative insulating transition throughout electron crystallization. The three vibrational modes, C=CH bending (v5 ∼ 1,204 cm−1), C–CN (v4 ∼ 1,376 cm−1), and C=C ring stretching (v3 ∼ 1,602 cm−1), as labeled on the TCNQ molecule in Figure 4B, are responsible for the dynamic process of electric dipoles and charge-lattice coupling on TCNQ, which is crucial to the electron crystallization. The Raman shifts for these vibration frequencies are summarized in Figure 4B. As the temperature decreases, v5 mode tends to lower its vibrational frequency below 200 K (Figure 4B) due to the transformation from electron glassy state34Clerac R. O'Kane S. Cowen J. Ouyang X. Heintz R. Zhao H.H. Bazile M.J. Dunbar K.R. Glassy magnets composed of metals coordinated to 7,7,8,8-tetracyanoquinodimethane: M(TCNQ)(2) (M = Mn, Fe, Co, Ni).Chem. Mater. 2003; 15: 1840-1850Crossref Scopus (123) Google Scholar to crystalline state.35Hassan N. Cunningham S. Mourigal M. Zhilyaeva E.I. Torunova S.A. Lyubovskaya R.N. Schlueter J.A. Drichko N. Evidence for a quantum dipole liquid state in an organic quasi-two-dimensional material.Science. 2018; 360: 1101-1104Crossref PubMed Scopus (34) Google Scholar The v4 and v3 modes, correlated with the negative charge accepted from Fe cations, reveal the charge redistribution during the electron crystallization. Figures 4C and 4D show the temperature-dependent Raman shift spectra of those three vibrational modes, while a new peak shoulder at 1,192 cm−1 emerges below 200 K (Figure 4C). Raman vibrational modes v4 of 1,376 cm−1 and v3 of 1,602 cm−1 shift to a higher frequency with decreasing temperature and stabilize below 130 K, bearing the characteristics of charge crystallization at low temperature. A strongly correlated FeTCNQ ferromagnet manifests itself as a dynamic electronic crystal solid in which its low-resistivity state can be cooperatively controlled by photoexcitation. Under UV irradiation (Figures 5A and S17), the charge in the 2D FeTCNQ layer is induced and coupled with a distorted crystal lattice for polaron formation. Instead of an insulating state in darkness after the SI transition (Figure 5B), a low-resistivity state in 2D FeTCNQ with a dramatic resistance reduction can be directly induced by photoirradiation at 10 K (Figure 5B). Moreover, the low-resistivity state can remain at room temperature without the SI transition by heating up under photoirradiation (inset of Figure 5B). Figure 5C shows the time trace of the resistance at 140 K after 2D FeTCNQ transitions to a pronounced insulating state (Figure S18). During a short UV irradiation (∼2 s), the resistance decreases and recovers to the initial value after removing the irradiation, showing the reversible capability (inset of Figures 5C and S19). During a long UV irradiation (∼120 s), the resistivity decreases drastically and can persist even after the irradiation is removed (Figure 5C). The resistivity switching in 2D FeTCNQ under UV irradiation can be repeated, with negligible attenuation. The photothermal effect can be ruled out by conducting the control experiments on 2D FeTCNQ magnets at 80 K whereby the low-resistivity state is much more pronounced (Figure S20). In summary, our molecular FeTCNQ ferromagnet presents a Curie temperature Tc of 60 K with a concomitant magnetodielectric coupling and undergoes magnetic frustration at higher temperature through magnetic-field effects. The ferromagnetism in FeTCNQ is ascribed to the spin coupling among the unpaired electrons (Figure S21) in Fe networks mediated by exchange interaction between Fe and TCNQ building blocks. Electron crystallization verified by Raman vibration spectra bears the magnetodielectric coupling in the correlated FeTCNQ magnet. The ground state nature of ferromagnetic coupling is revealed by first-principle calculations (Figure S22), which also contribute toward elucidating the charge distribution and polarization (Figures S23–S26) in the process of magnetodielectric coupling and electron crystallization. These results reveal that emergent magnetic and electric properties in FeTCNQ ferromagnets enable a promising platform from which to study molecular magnetodielectrics. In addition, we demonstrate the cooperative photoirradiation control of the low-resistivity transport behavior and stabilize the low-resistivity state in FeTCNQ magnet as a manifestation of unusually large coupling of charge to lattice and orbital degrees of freedom, promoting an FeTCNQ ferromagnet well suited for strongly correlated molecular solids.