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Thin-Film Deposition of an Organic Magnet Based on Vanadium Methyl Tricyanoethylenecarboxylate Yu Lu, Megan Harberts, Chi-Yueh Kao, Howard Yu, Ezekiel Johnston-Halperin,* and Arthur J. Epstein* Organic-based magnets of the form M[Acceptor]x (M = transition metal, x ≈ 2) have attracted extensive interest due to the potential for chemical tailoring of magnetic interactions through the careful choice of transition metal ions and organic ligands, as well as for their easy integration with existing organic electronics.[1–3] Thin films of vanadium tetracyanoethylene (V[TCNE]2) grown via low temperature chemical vapor deposition (CVD) are both the most robust of these materials, demonstrating substantially less air sensitivity than solution synthesized powders and are predicted to have unique half semiconductor properties with fully spin polarized conduction and valence bands.[4–6] These films show magnetic ordering at room temperature, opening the door to applications in organic spintronics including all-organic spin valves and spin-based light emitting diodes (spin-LEDs).[6–8] An extensive array of organic-based analogues in the M[TCNE]x family have been explored using high vacuum/temperature CVD or physical vapor deposition (PVD), including M = Co, Ni, Cr, Fe, Nb, and Mo;[9–14] however, the relatively harsh film deposition conditions and lack of room temperature magnetic ordering limit their use in spintronics devices. As a result, parallel efforts have explored replacing TCNE with other organic single electron acceptors as an alternate route to synthesizing organic magnets with tunable magnetic properties and room-temperature magnetic ordering.[15–19] Here, we show the preparation of new organic-based magnetic thin films that replace TCNE with methyl tricyanoethylenecarboxylate (MeTCEC) to form V[MeTCEC]x. These films are grown via low temperature CVD and exhibit room temperature magnetic ordering and semiconducting transport similar to the V[TCNE]2 parent compound, establishing V[MeTCEC]x as a new thin film organic-based magnet with potential applications as an organic based spin polarizer for hybrid and all-organic spin-based electronics. Methyl tricyanoethylenecarboxylate (Figure 1b) is a structural intermediate between tetracyanoethylene (TCNE) and dimethydicyanofumarate (DMDCF) with one cyano group substituted
Y. Lu, Dr. C.-Y. Kao, Prof. A. J. Epstein Department of Chemistry and Biochemistry The Ohio State University Columbus, Ohio 43210-1173, USA E-mail: [email protected] M. Harberts, H. Yu, Prof. E. Johnston-Halperin, Prof. A. J. Epstein Department of Physics The Ohio State University Columbus, Ohio 43210-1173, USA E-mail: [email protected]
DOI: 10.1002/adma.201403834
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by an ester. The ester substitution is attractive because it has little effect on the electronegativity, which allows for a single electron transfer reaction between the MeTCEC and the V(CO)6 to form V[MeTCEC]x. Additionally, this replacement does not disrupt the π network over which the unpaired spin is distributed, maintaining the exchange coupling that is essential for magnetism in these organic-based materials.[20] The intermediate structure and electrochemical properties make MeTCEC a natural choice to substitute for TCNE,[3,20] and in fact, previous reports of the reaction of V(CO)6 and MeTCEC in dichloromethane (CH2Cl2) yielded magnetically soft powders of V[MeTCEC]2 with a magnetic-ordering temperature of about 275 K.[17] We extend this work to thin films through the growth of V[MeTCEC]x via the reaction of V(CO)6 with MeTCEC in a customized low temperature CVD furnace.[4] The films are deposited on various substrates including silicon wafers, polytetrafluoroethylene (PTFE) (Teflon), KBr, glass and quartz with substantially similar magnetic properties and thickness. The typical thickness of the films, as measured by profilometry, is ca. 400 nm for a four-hour deposition. The air sensitivity of these materials presents a significant challenge to detailed structural characterization. However, based on previous results in analogue organic magnetic thin films,[4,17,21,22] we speculate that V[MeTCEC]2 exhibits short-range structural ordering, e.g., on the length scale of the magnetic exchange interaction, but is disordered at longer length scales. We verify the chemical structure and stoichiometry of the V[MeTCEC]x films using a combination of X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. Figure 1a shows the XPS spectra of the thin film after etching the surface by in-situ argon sputtering (removing a surface layer of ca. 3 nm). Elemental analysis indicates the chemical composition of the thin film is VC15.6N4.3O2.5, which gives a V to N ratio of 1:4.3 and suggests roughly about one and a half MeTCEC (C7N3O2H3) molecules per vanadium ion. The excess carbon is likely due to adventitious carbon. The stoichiometric composition of V[MeTCEC]1.5 is slightly different from the ideal composition V[MeTCEC]2. In analogy with thin films of V[TCNE]x,[4,22] where x has been found to vary from 0.6 to 2.2, it is likely that this discrepancy arises from a difference between the stoichiometry at the surface of the film and the bulk. In Figure 1a, the V 2p spectra are split into two bands, V 2p3/2 and V 2p1/2, ranging from 510 eV to 528 eV due to spinorbit splitting.[4] Each band has two components representing different oxidation states. The 515.1 eV and 522.3 eV peaks are characteristic of V2+, while the higher binding energy peaks at 517.5 eV and 526.4 eV correspond to V5+, which is attributed to minor residual oxidation of the film. The N 1s main peak is located at 398.8 eV and has a weak shake-up satellite on the
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COMMUNICATION Figure 1. a) XPS spectra of V2p, O1s and N1s electrons of the V[MeTCEC]x film on a Si substrate. The elemental analysis gives a V to N ratio of 1:4.3 suggesting a stoichiometry of V[MeTCEC]x ≈ 1.5. (The curves are fits to the data: blue is for the overall regions of V, O and N; red and green are for components of V2+ and V5+ respectively; pink and black are for the components of N in MeTCEC− and MeTCEC respectively, see text.) b) The IR spectrum of V[MeTCEC]x film deposited on a KBr plate. (The inset is the molecular structure of MeTCEC).
high-binding-energy side.[23] Two components at 398.9 eV and 400.7 eV are revealed via Gaussian deconvolution, similar to the N 1s band in CVD prepared V[TCNE]2 thin films.[4] The former can be assigned to the reduced [MeTCEC]− while the latter is attributed to the neutral MeTCEC. The IR spectrum of the V[MeTCEC]x film deposited on a KBr plate (Figure 1b) matches the data from solution-prepared V[MeTCEC]2 reported in the literature.[17] Absorption of the C≡N and C=O stretches (νCN and νCO) at 2209, 2143, and 1755 cm−1 are close to peaks 2201, 2134, and 1754 cm−1 observed in bulk, solution-synthesized V[MeTCEC]2, suggesting both the nitrile and the carbonyl groups in the MeTCEC are coordinated to vanadium. The slight blue shift of νCN indicates a stronger bonding between V and CN (the M–N≡C bonding causes a blue shift in general),[24] which might play an important role in the increasing magnetic ordering temperature observed in the V[MeTCEC]x thin films as compared to bulk powders.[24] Next, we explore the magnetic ordering of the V[MeTCEC]x films through temperature-, field-, and frequency-dependent measurements of the magnetization. The temperature dependence of the magnetization, M(T), shows an extrapolated magnetic ordering temperature of 305 K, indicating room-temperature magnetization (Figure S2, Supporting Information). The field-cooled (FC) and zero-field-cooled (ZFC) magnetizations are measured between 5 K and 300 K. Both curves (Figure 2a) rise sharply below 300 K and increase gradually upon cooling. They reach a broad maximum and display a characteristic freezing temperature, Tf, indicating the onset of irreversibility below ca. 190 K. Below Tf, M(T)FC and M(T)ZFC decrease almost linearly, consistent with disorder-induced random magnetic
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anisotropy, as observed in other organic-based magnets.[25–27] The temperature dependence of the magnetization in different applied fields for V[MeTCEC]x reveals the field dependence of the freezing temperature (Figure S3, Supporting Information), which shifts towards lower temperature with increasing field and vanishes above 200 Oe. The temperature dependences of in-phase (χ′) and out-ofphase (χ″) components of the AC susceptibility are measured at different frequencies and representative scans are shown in Figure 2b,c. The in-phase component, χ′, resembles the FC curves at low external field (Figure 2a, Hdc = 25 Oe) and has a magnetic ordering temperature of 300 K. It reaches a cusp at 190 K and has a shoulder at 205 K. The out-of-phase component, χ″, is distinct from χ′ and has a sharp rise at 198 K. The behavior of χ″ is suggestive of a transition into a phase with longer spin relaxation time in V[MeTCEC]x,[25,26,28] which is consistent with the freezing behavior observed in the FC and ZFC curves. There is a frequency dependence to the magnitude of χ′(T) and χ″(T), but no obvious frequency dependent shift of Tmax (the temperature of the maximum in χ′(T) and χ″(T), insets of Figure 2b,c). This behavior is distinct from V[MeTCEC]2 powder synthesized in solution, which does show a frequency dependence in Tmax that was interpreted as a direct transition from paramagnetic to spin glass behavior,[24] but similar to V[TCNE]2 CVD grown films.[29] The lack of frequency dependence in Tmax in CVD grown films based on both TCNE and MeTCEC suggests that the low-temperature phase is not a true spin glass, but may in fact retain some magnetic ordering. Recent theory suggests that this may in fact represent a sperimagnetic phase.[25,26,28] The term
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Figure 3. The field dependence of magnetization at 300 K (ⵧ) and 5 K (䊊). (Inset: magnified view of the same data.)
sperimagnet refers to a ferrimagnet with random magnetic anisotropy, i.e., a system with two sub-networks of spins where the moments of one or both sub-networks are frozen in random directions.[28,30] Similar to other organic magnets,[25,26,28] the random magnetic anisotropy in V[MeTCEC]x thin films may arise from the rotational freedom in biconnected MeTCEC ligands. Based on this model, we speculate that the transition at 300 K is a paramagnetic to ferrimagnetic transition, as evidenced by the sharp rise in the ZFC/FC curves and the inphase AC susceptibility, χ′, and the absence of a comparable transition in the out-of-phase AC susceptibility, χ″.[25,26] In this case, the peak in the ZFC/FC curves may indicate a re-entrant transition from a ferrimagnetic to a sperimagnetic state. Further support for this model can be found in the field dependence of the magnetization, M(H), which is characteristic of a magnetically ordered state. It exhibits hysteresis with a small coercive field, ca. 10 Oe at 5 K and ca. 20 Oe at 300 K (Figure 3). The small coercive field suggests V[MeTCEC]x is a soft magnet, likely due to its disordered structure. However, this coercive field is still larger than its solution-prepared analogue, which shows no hysteresis down to fields of ca. 1 Oe.[17] The magnetization approaches saturation quickly with applied field and is close to the saturation value, Ms(T), at 1 kOe. The temperature dependence of the saturation magnetization, Ms(T) (M(H) at 1 kOe) is plotted in the inset of Figure 2a ⎛ ⎝
s
⎞ ⎠
and can be fit to the Bloch law Ms (T ) = Ms ( 0 ) ⎜ 1 − BT 2 ⎟ , with
Figure 2. a) Temperature dependence of zero-field-cooled (ⵧ) and fieldcooled (䊊) magnetization of V[MeTCEC]x on a Si substrate in a 25 Oe external field. (Inset: saturated magnetization Ms(T). The solid line represents a fit to the Bloch spin-wave theory, see text.) b,c) Temperature dependence of in-phase, χ′, and out-of-phase, χ″, components of the AC magnetic susceptibility of V[MeTCEC]x thin film on Si at 100 Hz (ⵧ), 3333 Hz (䊊) and 10000 Hz (Δ). (Insets: magnified view of the peaks.)
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B = 1.77 × 10−4 K−3/2, which yields an extrapolated Curie temperature of ca. 317 K.[31] This is consistent with the observation that the magnetization vanishes at 305 K. The fact that Ms(T) fits to the Bloch law over the entire temperature range is typical behavior for amorphous magnets and is similar to the solution prepared V[MeTCEC]2 powder.[4] Finally, we consider charge transport in our V[MeTCEC]x thin films. Samples are grown on glass substrates with top contacts (30 nm Al/40 nm Au) deposited via thermal evaporation. Current–voltage (I–V) characteristics are measured in the two probe geometry and reveal an Ohmic response
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Figure 4. a) Temperature dependence of resistance of the V[MeTCEC]x film (Inset: I–V curves at 175 K and 275 K). b) The values of R(T) (T = 175–275 K) are fitted to an Arrhenius plot in the left panel revealing an activation energy of ca. 0.56 eV. In the right panel Log Ln(R) vs Log 1000/T is plotted to extract a hopping parameter of m = 0.78 (see text). c) Magnetoresistance of a V[MeTCEC]x film at 140 K and 200 K reveals a positive slope.
at all temperatures (Figure 4a, inset). The resistance increases monotonically with decreasing temperature (Figure 4a), similar to that which has been seen previously for V[TCNE]2.[5] At temperatures above 275 K the resistance increases rapidly, likely due to a combination of residual air contamination in the transport apparatus and current annealing of the sample (Figure S4, Supporting Information). This increase is not correlated with the electronic properties, therefore we consider transport data only for temperatures below 275 K. Since this class of organic semiconductor exists at the boundary between a low-mobility band conductor and high-mobility hopping, we fit the data to both the Arrhenius equation and a hopping model. Fitting the data to thermally activated transport Arrhenius equation: R = R0 exp( −E a / kBT ), we extract an activation energy of Ea ≈ 0.56 eV (Figure 4b left panel). This value of Ea is similar to V[TCNE]2 (Ea ≈ 0.50 eV) suggesting a similar electronic structure.[5] For hopping transport, we choose a simple m model, R = R0 exp (T0 / T ) , where R0 is characteristic resistance, T0 is the characteristic temperature and m has a value between 0 and 1, which in principle determines the dimensionality of the transport and the hopping mechanism.[24] Our fit (Figure 4b right panel) yields an extracted value m = 0.78. This value is not explicitly consistent with either nearest neighbor hopping (m = 1), or Efros Shklovskii variable range hopping (VRH, m = 0.5); however, we note the available temperature range is limited to less than a decade due to the competing constraints of carrier freeze out (T ≈ 190 K) and sample degradation (T ≈ 275 K), limiting the precision of this determination. Magnetoresistance (MR) measurements are performed at temperatures above and below the blocking temperature (Tb ≈ 190 K). These results, shown in Figure 4c, reveal an anomalous positive magnetoresistance that increases with increasing temperature, as previously observed in V[TCNE]2.[5,32] The MR (R (H ) − R (0 )) , and gives a slope of is defined as MR % = 100 × R (0 ) 0.16% per Tesla below Tf (140 K) and 0.39% per Tesla above it (200 K). This anomalous linear behavior and increasing MR for temperatures close to TC are consistent with what has been previously observed in V[TCNE]2.[5,32,33] These results are inconsistent with the predicted H2 dependence expected for Mott VRH in non-magnetic systems.[34] In summary, we have reported the preparation and characterization of thin films of V[MeTCEC]x with low-temperature CVD. The thin film is a structurally disordered magnetic material with magnetic ordering above room temperature. Electronic-transport measurements show linear I–V characteristics and an activation energy of ca. 0.56 eV. The low-temperature CVD synthesis is promising as it will enable straightforward incorporation of V[MeTCEC]x magnetic thin films into multilayered devices for hybrid and all organic spintronics applications. These new films are an important addition to the family of thin-film organic-based magnets and may provide a route to tuning magnetic properties while maintaining room-temperature magnetic ordering through ligand modification.
Experimental Section All the experiments were performed under an inert argon atmosphere (
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Thin-Film Deposition of an Organic Magnet Based on Vanadium Methyl Tricyanoethylenecarboxylate Yu Lu, Megan Harberts, Chi-Yueh Kao, Howard Yu, Ezekiel Johnston-Halperin,* and Arthur J. Epstein* Organic-based magnets of the form M[Acceptor]x (M = transition metal, x ≈ 2) have attracted extensive interest due to the potential for chemical tailoring of magnetic interactions through the careful choice of transition metal ions and organic ligands, as well as for their easy integration with existing organic electronics.[1–3] Thin films of vanadium tetracyanoethylene (V[TCNE]2) grown via low temperature chemical vapor deposition (CVD) are both the most robust of these materials, demonstrating substantially less air sensitivity than solution synthesized powders and are predicted to have unique half semiconductor properties with fully spin polarized conduction and valence bands.[4–6] These films show magnetic ordering at room temperature, opening the door to applications in organic spintronics including all-organic spin valves and spin-based light emitting diodes (spin-LEDs).[6–8] An extensive array of organic-based analogues in the M[TCNE]x family have been explored using high vacuum/temperature CVD or physical vapor deposition (PVD), including M = Co, Ni, Cr, Fe, Nb, and Mo;[9–14] however, the relatively harsh film deposition conditions and lack of room temperature magnetic ordering limit their use in spintronics devices. As a result, parallel efforts have explored replacing TCNE with other organic single electron acceptors as an alternate route to synthesizing organic magnets with tunable magnetic properties and room-temperature magnetic ordering.[15–19] Here, we show the preparation of new organic-based magnetic thin films that replace TCNE with methyl tricyanoethylenecarboxylate (MeTCEC) to form V[MeTCEC]x. These films are grown via low temperature CVD and exhibit room temperature magnetic ordering and semiconducting transport similar to the V[TCNE]2 parent compound, establishing V[MeTCEC]x as a new thin film organic-based magnet with potential applications as an organic based spin polarizer for hybrid and all-organic spin-based electronics. Methyl tricyanoethylenecarboxylate (Figure 1b) is a structural intermediate between tetracyanoethylene (TCNE) and dimethydicyanofumarate (DMDCF) with one cyano group substituted
Y. Lu, Dr. C.-Y. Kao, Prof. A. J. Epstein Department of Chemistry and Biochemistry The Ohio State University Columbus, Ohio 43210-1173, USA E-mail: [email protected] M. Harberts, H. Yu, Prof. E. Johnston-Halperin, Prof. A. J. Epstein Department of Physics The Ohio State University Columbus, Ohio 43210-1173, USA E-mail: [email protected]
DOI: 10.1002/adma.201403834
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by an ester. The ester substitution is attractive because it has little effect on the electronegativity, which allows for a single electron transfer reaction between the MeTCEC and the V(CO)6 to form V[MeTCEC]x. Additionally, this replacement does not disrupt the π network over which the unpaired spin is distributed, maintaining the exchange coupling that is essential for magnetism in these organic-based materials.[20] The intermediate structure and electrochemical properties make MeTCEC a natural choice to substitute for TCNE,[3,20] and in fact, previous reports of the reaction of V(CO)6 and MeTCEC in dichloromethane (CH2Cl2) yielded magnetically soft powders of V[MeTCEC]2 with a magnetic-ordering temperature of about 275 K.[17] We extend this work to thin films through the growth of V[MeTCEC]x via the reaction of V(CO)6 with MeTCEC in a customized low temperature CVD furnace.[4] The films are deposited on various substrates including silicon wafers, polytetrafluoroethylene (PTFE) (Teflon), KBr, glass and quartz with substantially similar magnetic properties and thickness. The typical thickness of the films, as measured by profilometry, is ca. 400 nm for a four-hour deposition. The air sensitivity of these materials presents a significant challenge to detailed structural characterization. However, based on previous results in analogue organic magnetic thin films,[4,17,21,22] we speculate that V[MeTCEC]2 exhibits short-range structural ordering, e.g., on the length scale of the magnetic exchange interaction, but is disordered at longer length scales. We verify the chemical structure and stoichiometry of the V[MeTCEC]x films using a combination of X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. Figure 1a shows the XPS spectra of the thin film after etching the surface by in-situ argon sputtering (removing a surface layer of ca. 3 nm). Elemental analysis indicates the chemical composition of the thin film is VC15.6N4.3O2.5, which gives a V to N ratio of 1:4.3 and suggests roughly about one and a half MeTCEC (C7N3O2H3) molecules per vanadium ion. The excess carbon is likely due to adventitious carbon. The stoichiometric composition of V[MeTCEC]1.5 is slightly different from the ideal composition V[MeTCEC]2. In analogy with thin films of V[TCNE]x,[4,22] where x has been found to vary from 0.6 to 2.2, it is likely that this discrepancy arises from a difference between the stoichiometry at the surface of the film and the bulk. In Figure 1a, the V 2p spectra are split into two bands, V 2p3/2 and V 2p1/2, ranging from 510 eV to 528 eV due to spinorbit splitting.[4] Each band has two components representing different oxidation states. The 515.1 eV and 522.3 eV peaks are characteristic of V2+, while the higher binding energy peaks at 517.5 eV and 526.4 eV correspond to V5+, which is attributed to minor residual oxidation of the film. The N 1s main peak is located at 398.8 eV and has a weak shake-up satellite on the
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COMMUNICATION Figure 1. a) XPS spectra of V2p, O1s and N1s electrons of the V[MeTCEC]x film on a Si substrate. The elemental analysis gives a V to N ratio of 1:4.3 suggesting a stoichiometry of V[MeTCEC]x ≈ 1.5. (The curves are fits to the data: blue is for the overall regions of V, O and N; red and green are for components of V2+ and V5+ respectively; pink and black are for the components of N in MeTCEC− and MeTCEC respectively, see text.) b) The IR spectrum of V[MeTCEC]x film deposited on a KBr plate. (The inset is the molecular structure of MeTCEC).
high-binding-energy side.[23] Two components at 398.9 eV and 400.7 eV are revealed via Gaussian deconvolution, similar to the N 1s band in CVD prepared V[TCNE]2 thin films.[4] The former can be assigned to the reduced [MeTCEC]− while the latter is attributed to the neutral MeTCEC. The IR spectrum of the V[MeTCEC]x film deposited on a KBr plate (Figure 1b) matches the data from solution-prepared V[MeTCEC]2 reported in the literature.[17] Absorption of the C≡N and C=O stretches (νCN and νCO) at 2209, 2143, and 1755 cm−1 are close to peaks 2201, 2134, and 1754 cm−1 observed in bulk, solution-synthesized V[MeTCEC]2, suggesting both the nitrile and the carbonyl groups in the MeTCEC are coordinated to vanadium. The slight blue shift of νCN indicates a stronger bonding between V and CN (the M–N≡C bonding causes a blue shift in general),[24] which might play an important role in the increasing magnetic ordering temperature observed in the V[MeTCEC]x thin films as compared to bulk powders.[24] Next, we explore the magnetic ordering of the V[MeTCEC]x films through temperature-, field-, and frequency-dependent measurements of the magnetization. The temperature dependence of the magnetization, M(T), shows an extrapolated magnetic ordering temperature of 305 K, indicating room-temperature magnetization (Figure S2, Supporting Information). The field-cooled (FC) and zero-field-cooled (ZFC) magnetizations are measured between 5 K and 300 K. Both curves (Figure 2a) rise sharply below 300 K and increase gradually upon cooling. They reach a broad maximum and display a characteristic freezing temperature, Tf, indicating the onset of irreversibility below ca. 190 K. Below Tf, M(T)FC and M(T)ZFC decrease almost linearly, consistent with disorder-induced random magnetic
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anisotropy, as observed in other organic-based magnets.[25–27] The temperature dependence of the magnetization in different applied fields for V[MeTCEC]x reveals the field dependence of the freezing temperature (Figure S3, Supporting Information), which shifts towards lower temperature with increasing field and vanishes above 200 Oe. The temperature dependences of in-phase (χ′) and out-ofphase (χ″) components of the AC susceptibility are measured at different frequencies and representative scans are shown in Figure 2b,c. The in-phase component, χ′, resembles the FC curves at low external field (Figure 2a, Hdc = 25 Oe) and has a magnetic ordering temperature of 300 K. It reaches a cusp at 190 K and has a shoulder at 205 K. The out-of-phase component, χ″, is distinct from χ′ and has a sharp rise at 198 K. The behavior of χ″ is suggestive of a transition into a phase with longer spin relaxation time in V[MeTCEC]x,[25,26,28] which is consistent with the freezing behavior observed in the FC and ZFC curves. There is a frequency dependence to the magnitude of χ′(T) and χ″(T), but no obvious frequency dependent shift of Tmax (the temperature of the maximum in χ′(T) and χ″(T), insets of Figure 2b,c). This behavior is distinct from V[MeTCEC]2 powder synthesized in solution, which does show a frequency dependence in Tmax that was interpreted as a direct transition from paramagnetic to spin glass behavior,[24] but similar to V[TCNE]2 CVD grown films.[29] The lack of frequency dependence in Tmax in CVD grown films based on both TCNE and MeTCEC suggests that the low-temperature phase is not a true spin glass, but may in fact retain some magnetic ordering. Recent theory suggests that this may in fact represent a sperimagnetic phase.[25,26,28] The term
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Figure 3. The field dependence of magnetization at 300 K (ⵧ) and 5 K (䊊). (Inset: magnified view of the same data.)
sperimagnet refers to a ferrimagnet with random magnetic anisotropy, i.e., a system with two sub-networks of spins where the moments of one or both sub-networks are frozen in random directions.[28,30] Similar to other organic magnets,[25,26,28] the random magnetic anisotropy in V[MeTCEC]x thin films may arise from the rotational freedom in biconnected MeTCEC ligands. Based on this model, we speculate that the transition at 300 K is a paramagnetic to ferrimagnetic transition, as evidenced by the sharp rise in the ZFC/FC curves and the inphase AC susceptibility, χ′, and the absence of a comparable transition in the out-of-phase AC susceptibility, χ″.[25,26] In this case, the peak in the ZFC/FC curves may indicate a re-entrant transition from a ferrimagnetic to a sperimagnetic state. Further support for this model can be found in the field dependence of the magnetization, M(H), which is characteristic of a magnetically ordered state. It exhibits hysteresis with a small coercive field, ca. 10 Oe at 5 K and ca. 20 Oe at 300 K (Figure 3). The small coercive field suggests V[MeTCEC]x is a soft magnet, likely due to its disordered structure. However, this coercive field is still larger than its solution-prepared analogue, which shows no hysteresis down to fields of ca. 1 Oe.[17] The magnetization approaches saturation quickly with applied field and is close to the saturation value, Ms(T), at 1 kOe. The temperature dependence of the saturation magnetization, Ms(T) (M(H) at 1 kOe) is plotted in the inset of Figure 2a ⎛ ⎝
s
⎞ ⎠
and can be fit to the Bloch law Ms (T ) = Ms ( 0 ) ⎜ 1 − BT 2 ⎟ , with
Figure 2. a) Temperature dependence of zero-field-cooled (ⵧ) and fieldcooled (䊊) magnetization of V[MeTCEC]x on a Si substrate in a 25 Oe external field. (Inset: saturated magnetization Ms(T). The solid line represents a fit to the Bloch spin-wave theory, see text.) b,c) Temperature dependence of in-phase, χ′, and out-of-phase, χ″, components of the AC magnetic susceptibility of V[MeTCEC]x thin film on Si at 100 Hz (ⵧ), 3333 Hz (䊊) and 10000 Hz (Δ). (Insets: magnified view of the peaks.)
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B = 1.77 × 10−4 K−3/2, which yields an extrapolated Curie temperature of ca. 317 K.[31] This is consistent with the observation that the magnetization vanishes at 305 K. The fact that Ms(T) fits to the Bloch law over the entire temperature range is typical behavior for amorphous magnets and is similar to the solution prepared V[MeTCEC]2 powder.[4] Finally, we consider charge transport in our V[MeTCEC]x thin films. Samples are grown on glass substrates with top contacts (30 nm Al/40 nm Au) deposited via thermal evaporation. Current–voltage (I–V) characteristics are measured in the two probe geometry and reveal an Ohmic response
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COMMUNICATION
Figure 4. a) Temperature dependence of resistance of the V[MeTCEC]x film (Inset: I–V curves at 175 K and 275 K). b) The values of R(T) (T = 175–275 K) are fitted to an Arrhenius plot in the left panel revealing an activation energy of ca. 0.56 eV. In the right panel Log Ln(R) vs Log 1000/T is plotted to extract a hopping parameter of m = 0.78 (see text). c) Magnetoresistance of a V[MeTCEC]x film at 140 K and 200 K reveals a positive slope.
at all temperatures (Figure 4a, inset). The resistance increases monotonically with decreasing temperature (Figure 4a), similar to that which has been seen previously for V[TCNE]2.[5] At temperatures above 275 K the resistance increases rapidly, likely due to a combination of residual air contamination in the transport apparatus and current annealing of the sample (Figure S4, Supporting Information). This increase is not correlated with the electronic properties, therefore we consider transport data only for temperatures below 275 K. Since this class of organic semiconductor exists at the boundary between a low-mobility band conductor and high-mobility hopping, we fit the data to both the Arrhenius equation and a hopping model. Fitting the data to thermally activated transport Arrhenius equation: R = R0 exp( −E a / kBT ), we extract an activation energy of Ea ≈ 0.56 eV (Figure 4b left panel). This value of Ea is similar to V[TCNE]2 (Ea ≈ 0.50 eV) suggesting a similar electronic structure.[5] For hopping transport, we choose a simple m model, R = R0 exp (T0 / T ) , where R0 is characteristic resistance, T0 is the characteristic temperature and m has a value between 0 and 1, which in principle determines the dimensionality of the transport and the hopping mechanism.[24] Our fit (Figure 4b right panel) yields an extracted value m = 0.78. This value is not explicitly consistent with either nearest neighbor hopping (m = 1), or Efros Shklovskii variable range hopping (VRH, m = 0.5); however, we note the available temperature range is limited to less than a decade due to the competing constraints of carrier freeze out (T ≈ 190 K) and sample degradation (T ≈ 275 K), limiting the precision of this determination. Magnetoresistance (MR) measurements are performed at temperatures above and below the blocking temperature (Tb ≈ 190 K). These results, shown in Figure 4c, reveal an anomalous positive magnetoresistance that increases with increasing temperature, as previously observed in V[TCNE]2.[5,32] The MR (R (H ) − R (0 )) , and gives a slope of is defined as MR % = 100 × R (0 ) 0.16% per Tesla below Tf (140 K) and 0.39% per Tesla above it (200 K). This anomalous linear behavior and increasing MR for temperatures close to TC are consistent with what has been previously observed in V[TCNE]2.[5,32,33] These results are inconsistent with the predicted H2 dependence expected for Mott VRH in non-magnetic systems.[34] In summary, we have reported the preparation and characterization of thin films of V[MeTCEC]x with low-temperature CVD. The thin film is a structurally disordered magnetic material with magnetic ordering above room temperature. Electronic-transport measurements show linear I–V characteristics and an activation energy of ca. 0.56 eV. The low-temperature CVD synthesis is promising as it will enable straightforward incorporation of V[MeTCEC]x magnetic thin films into multilayered devices for hybrid and all organic spintronics applications. These new films are an important addition to the family of thin-film organic-based magnets and may provide a route to tuning magnetic properties while maintaining room-temperature magnetic ordering through ligand modification.
Experimental Section All the experiments were performed under an inert argon atmosphere (
