Thin film synthesis of the organic-based magnet vanadium ethyl tricyanoethylenecarboxylate

Abstract

We report the preparation and characterization of a new thin film organic-based magnet V[ETCEC]_x, with T_C of 161 ± 10 K, via low temperature chemical vapor deposition (CVD; T = 55 °C). X-ray photoelectron spectroscopy (XPS) indicates the ratio of V to ETCEC is 1 : 1.4. In addition, analysis of Fourier-transform infrared (FTIR) spectroscopy suggests a similar physical structure to V[ETCEC]_x powder, with all nitrile and carbonyl oxygen coordinating to V(II) sites. Temperature-, field-, and frequency-dependent measurements of the magnetization as well as ferromagnetic resonance (FMR) measurements reveal complex magnetic behavior with a magnetic transition to a more disordered state at a freezing temperature just below the Curie temperature. The development of this magnetic thin film enables the direct incorporation of V[ETCEC]_x into existing organic spintronics devices and opens up the potential exploration of all-organic magnetic heterostructures.

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Introduction

Organic-based magnets have been investigated for many years. During this time many advances have been made, including the preparation of magnets with magnetic ordering above room temperature^1–3 and the discovery of new magnetic phenomena.^4–7 These materials, in the form of M[acceptor]_x, are interesting due to the possibility of creating custom magnetic systems by varying the building blocks of metal ions or organic ligands.^8–11 For example, the magnetic coercivity can be tuned from a few Oersted to hundreds of Oersted by varying the composition in the V_xCo_1−x[TCNE]_y system.¹² By using different organic ligands, such as porphyrin^13,14 and cyclopentadienyl,¹⁵ the structure of the organic-based magnet can change from one-dimensional to three-dimensional, with corresponding changes to the type of magnetic interaction. In addition, organic-based magnets also carry the advantages of organic materials: being light weight, flexible, and easy to process. The development of thin film deposition techniques for V[TCNE]_x has enabled potential applications in electronics and spintronics.^16–19 The preliminary results have demonstrated the functionality of V[TCNE]_x as a spin polarizer in organic spin valve^20–22 and spin light emitting diodes (LEDs).²³ Moreover, ferromagnetic resonance (FMR) studies show that V[TCNE]_x thin films have extremely narrow FMR linewidth, which is promising for use in high frequency magnetoelectronics.²⁴ In addition to V[TCNE]_x, an extensive array of analogues in the M[TCNE]_x family have been explored using high vacuum/temperature chemical vapor deposition (CVD) or physical vapor deposition (PVD), including M = Co, Ni, Cr, Fe, Nb, Mo.^25–29 However, the harsh deposition conditions and metal defects limit their applications in heterostructures. As a result, parallel efforts have explored replacement of TCNE with other single electron acceptors, such as methyl tricyanoethylene carboxylate (MeTCEC).¹⁷ Here, we report the preparation of a new organic-based magnetic thin film based on vanadium ethyl tricyanoethylenecarboxylate (V[ETCEC]_x) via low temperature CVD. This thin film has similar chemical structure and composition as its powder analogue prepared in solution,³⁰ but as has been shown in related compounds^16,17 the thin film growth mode reveals distinct magnetic properties. It exhibits magnetic homogeneity with a magnetic ordering temperature of 161 ± 10 K, establishing V[ETCEC]_x as a new thin film organic-based magnet with potential application as a spin-polarizer for hybrid and all-organic spin-based electronics. Moreover, the structural similarity to thin films of V[TCNE]_x and V[MeTCEC]_x enables investigation of the relationship between physical structure and emergent properties for developing new organic-based magnets.

Experimental

Materials

Due to the air and moisture sensitivity of the materials studied, all manipulations and reactions are performed in an argon glove-box (<1 ppm O₂ and <1 ppm H₂O). [(Et)₄N]⁺[V(CO)₆]⁻¹ and subsequently V(CO)₆ are prepared according to the literature.³¹ Tetracyanoethylene (TCNE) is purchased from Sigma-Aldrich and purified by sublimation at 80 °C and 20 mTorr. Ethyl cyanoacetate is purchased from Sigma-Aldrich and used as received. Dichloromethane and THF are distilled from CaH₂ and degassed with dry nitrogen prior to use. Ethyl tricyanoethylene carboxylate (ETCEC) is synthesized according to the literature³² and verified pure by ¹H NMR and ¹³C NMR.

Deposition

Thin films of V[ETCEC]_x are prepared via chemical vapor deposition (CVD) using a customized chamber (Fig. 1). The two precursors V(CO)₆ and ETCEC are synthesized as described above. The evaporation temperatures for V(CO)₆ and ETCEC are 55 °C and 10 °C respectively. The substrates are placed in the reaction zone at 55 °C. The pressure during deposition is 25 mmHg. The flux of argon for V(CO)₆ and ETCEC is 40 and 60 sccm respectively. Films can be easily deposited on a variety of substrates including Si wafers, glass, gold, quartz, KBr, Teflon, etc. The thickness is typically ∼200 nm for a three-hour deposition as measured by profilometry. The film growth is much slower than its analogue V[TCNE]_x, which can be attributed to the low reaction tendency between V(CO)₆ and ETCEC. Scanning electron microscopy (SEM; Fig. S1†) shows the film has a smooth and featureless surface, which is consistent with previous reports of thin film V[TCNE]_x.³³

Fig. 1 Schematic indication of customized CVD furnace and chemical reaction between two precursors V(CO)₆ and ETCEC.

Characterization

Samples were deposited on Si or sapphire substrates and sealed in quartz electron paramagnetic resonance (EPR) tubes to avoid air exposure during measurement. Temperature- and field-dependence of the magnetization was determined using a Quantum Design MPMS SQUID magnetometer. Measurements of zero field-cooled magnetization as a function of temperature were obtained by cooling in zero field, and data was collected on warming in 15, 25, 100 and 1000 Oe external magnetic fields. After each zero filed-cooled measurement, the sample was cooled to 5 K again and field-cooled magnetization data were also collected on warming from 5 K to 300 K at the same applied fields. Measurements of the magnetization as a function of the applied magnetic field were also performed at 5 K and 150 K. AC magnetic susceptibility measurements were performed using a Quantum Design physical properties measurement system (PPMS). The samples were measured in an 8 Oe AC field (zero DC applied field) at 333, 3333, and 10 000 Hz. The FMR measurement was performed on a Bruker EMXPlus EPR spectrometer. Transmission FTIR spectra were acquired using a Perkin-Elmer Spectrum 100 FT-IR spectrometer. For these measurements the film was deposited directly on a KBr pellet and sealed in an airtight cell prior to measurement. X-ray photoelectron spectra were measured by a Kratos Axis Ultra XPS. Films were deposited on Si substrates and transferred to the chamber of the spectrometer using an airtight transfer tool to reduce air exposure.

Results and discussion

The chemical structure and stoichiometry of the V[ETCEC]_x films are verified via a combination of XPS and infrared spectroscopy (FTIR). Fig. 2 shows the XPS spectra of V[ETCEC]_x before and after etching with an argon ion beam. The analysis of the XPS spectra indicates the V_2p peak is split into two bands ranging from 510 eV to 528 eV, V_2p3/2 and V_2p1/2, due to spin–orbit splitting. Each band has two components representing different oxidation states. The 514.5 eV and 522.1 eV peaks are characteristic of V²⁺, while the higher binding energy peaks at 516.9 eV and 524.3 eV correspond to V⁵⁺. Before surface etching, the V_2p peaks are dominated by the V⁵⁺ component, indicating surface oxidation of the film. This is also supported by the higher oxygen content. After surface etching (∼5 nm), the V²⁺ components dominate the V_2p peaks and the oxygen content drops dramatically, which suggests the oxidation mainly happens in the surface layer and may occur during transport to the XPS chamber. The N_1s main peak is located at 399 eV. Gaussian deconvolution reveals two components at 398.9 eV and 400.4 eV, similar to the N_1s band in CVD-prepared V[TCNE]_x thin films.¹⁶ The former can be assigned to the reduced [ETCEC]⁻ while the latter is attributed to the neutral ETCEC. Elemental analysis shows a chemical composition of VC_16.6N_4.2O_1.9 (after surface etching), which gives a V : N ratio of 1 : 4.2 and suggests approximately 1.4 ETCEC molecules per vanadium ion. Without considering the adventitious carbon (generally located at 284.8 eV), the ratio between carbon and nitrogen is 2.60 : 1, close to that of ETCEC (2.66 : 1).

Fig. 2 XPS spectra of V_2p, O_1s and N_1s electrons of thin film V[ETCEC]_x on Si. Curves are fits to the data: blue is for overall regions of V and N; red and green are for components of V²⁺ and V⁵⁺ respectively; violet and orange are for the components of N in ETCEC⁻ and ETCEC respectively (see text).

The IR spectrum of V[ETCEC]_x grown on a KBr pellet is similar to that of solution synthesized V[ETCEC]_x with a slight blue shift for the cyano group (Fig. 3). Absorption peaks at 2152 and 2134 cm⁻¹ are attributed to the nitrile groups in ETCEC due to a delicate balance between the σ-type donation (blue shift) and π*-backbonding (red shift) in the V–CN bond. Absorption peaks at 1756 and 1674 cm⁻¹ are attributed to carbonyl in μ4-[ETCEC]⁻ bridged V²⁺ and trans-μ-[ETCEC]⁻ bridged V²⁺ respectively.

Fig. 3 The IR spectrum of ETCEC.

The magnetic properties of V[ETCEC]_x thin films are explored by temperature, field and frequency dependent measurements of magnetization as well as ferromagnetic resonance (FMR). The field-cooled (FC) and zero-field-cooled (ZFC) magnetization of V[ETCEC]_x are measured between 5 K to 300 K as shown in Fig. 4a. The Curie temperature, T_C, is determined by a phenomenological estimation of the temperature at which M reaches zero. Using sample to sample variation in this value the uncertainty in T_C is estimated to be 161 ± 10 K. Below T_C, both curves rise sharply and reach a broad maximum, indicating a characteristic freezing temperature, T_f, of 90 K at 15 Oe applied field. Below T_f, M_ZFC(T) and M_FC(T) decrease with decreasing temperature, which is consistent with disorder-induced random magnetic anisotropy as observed in other organic-based magnets.^3,5,6 It is also noted that T_f shifts to lower temperature with increasing applied external field, suggesting a competition with the random anisotropy field in this material below the freezing temperature.

Fig. 4 (a) Temperature dependence of zero-field-cooled and field-cooled magnetization of V[ETCEC]_x at 15, 25, and 200 Oe external field. (b) Hysteresis of V[ETCEC]_x at 5 K (red) and 100 K(black). Inset: magnified view of the same data.

The field dependence of the magnetization, M(H), is characteristic of a magnetically ordered state as shown in Fig. 4b. The hysteresis loop has a small coercive field, ∼6 Oe at 5 K, which increases to ∼25 Oe at 100 K. The small coercive field suggests that V[ETCEC]_x is a soft magnet, which could be attributed to the weak ion anisotropy of V(II) and the amorphous structure of the film. The magnetization approaches saturation quickly with applied field.

The temperature dependences of the in-phase (χ′) and out-of-phase (χ′′) components of the AC susceptibility at different frequencies are shown in Fig. 5. The in-phase component has a shape similar to the FC magnetization curves at low external field (Fig. 4a) with an onset transition at ∼142 K, suggesting magnetic ordering at this temperature. It reaches a broad peak at ∼85 K, consistent with the freezing temperature T_f observed in the M(T) curve. Conversely, χ′′ has a sharp rise at ∼94 K. This behavior suggests a transition into a phase with longer spin relaxation time in the thin film.¹⁷ There is no obvious frequency dependent shift of T_max (the temperature of the peaks in χ′ and χ′′), which is similar to analogous V[TCNE]_x and V[MeTCEC]_x thin films.^17,34 This indicates that the film is not a true spin glass but may in fact maintain some magnetic ordering.³⁵

Fig. 5 Temperature dependence of in-phase, χ′, and out-of-phase, χ′′, components of the AC magnetic susceptibility of V[ETCEC]_x at 333 Hz, 3333 Hz, and 10 000 Hz.

The FMR measurements are performed at a temperature of 5 K with an applied microwave frequency of 9.388 GHz and the static magnetic field applied in the sample plane (Fig. 6). The spectrum is cleanly single-peaked, indicating a high degree of magnetic homogeneity and lack of domains, and is fit well by a Lorentzian derivative. The Lorentzian fit finds a linewidth of 10.68 ± 0.04 G, comparable with permalloy films of similar thickness³⁶ but considerably broader than what has been observed²⁴ in V[TCNE]_x, and a resonance field of 3360.08 ± 0.08 G. The FMR response at this temperature does not appear to change significantly when the applied field rotates (Fig. S2†), which is consistent with SQUID data that indicates a low saturation magnetization.

Fig. 6 FMR spectrum measured in an EPR spectrometer of a 200 nm V[ETCEC]_x film at 5 K with H in-plane.

Conclusion

In summary, we have reported the preparation of thin films of organic-based magnet V[ETCEC]_1.4 with T_C of 161 ± 10 K via low temperature CVD. This film is a structurally disordered but magnetically homogenous material with a smooth and featureless surface. Analysis of the FTIR spectrum suggests a similar structure to its analogues with all nitriles and carbonyl oxygen coordinated to V(II) sites.³ The combination of temperature-, field-, and frequency-dependent measurements of the magnetization indicates complex magnetic behavior with a transition to a more disordered with a freezing temperature below the Curie temperature. This low temperature CVD-prepared magnetic thin film is an important addition to the family of thin film organic-based magnets and is promising as it will enable direct incorporation of V[ETCEC]_1.4 magnetic thin films into all-organic magnetic heterostructures.^16,17

Acknowledgements

The authors thank the NanoSystems Laboratory and Surface Analysis Laboratory at the Ohio State University. This work was supported in part by the National Science Foundation Division of Materials Research DMR-1507775 and the Center for Emergent Materials (an NSFMRSEC; Award Number DMR-1420451) at The Ohio State University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16699c

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