Article pubs.acs.org/JPCC
Antiferromagnetic Ordering of MII(TCNE)[C4(CN)8]1/2 (M = Mn, Fe; TCNE = Tetracyanoethylene) Amber C. McConnell, Endrit Shurdha, Joshua D. Bell,† and Joel S. Miller* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112-0850, United States ABSTRACT: The magnetic behaviors of MII(TCNE)[C4(CN)8]1/2 (M = Mn, Fe) prepared from a new synthetic route yielding purer magnetic materials are reported. Detailed DC magnetization and AC magnetic susceptibility measurements reveal that both compounds exhibit magnetic ordering with antiferromagnetic ground states arising from antiferromagnetically coupled ferrimagnetic layers with ordering temperatures of 69 and 84 K for polycrystalline M = Mn (Mn) and Fe (Fe), respectively. Because of the presence of two spin sites, these antiferromagnets are best described as compensated ferrimagnets. A spin flop transition between the antiferromagnetically coupled ferrimagnetic layers occurs at 19 500 Oe at 5 K for Mn, whereas Fe is a metamagnet. Fe also exhibits a constricted hysteretic behavior with a 5 K critical field of 12 600 Oe, and a coercive field and remanent magnetization of 4800 Oe and 1850 emu Oe/mol, respectively.
■
INTRODUCTION
The discovery that [FeIII(C5Me5)2]•+(TCNE)•− (TCNE = tetracyanoethylene) magnetically ordered as a bulk ferromagnet with a critical temperature, Tc, of 4.8 K,1,2 led to numerous additional organic-based magnets.3−5 Several other TCNE6based magnets, including 1-D [MnIIITPP]+(TCNE)•−,7 2-D Mn(TCNE)I(OH2),8 [FeII(TCNE)-(NCMe)2][FeCl4],9 and 3-D Mn(TCNE)3/2I3/210 as well as MII(TCNE)2·zCH2Cl2 (M = Mn,10 Fe11), have been structurally characterized. The roomtemperature V(TCNE)x magnet (Tc ∼ 400 K),12 however, is amorphous, and its structure has yet to be elucidated.13 The reaction of acetonitrile solvates of MI2 with TCNE in CH2Cl2 forms M(TCNE)2·zCH2Cl2 (M = Fe, Mn, Co, Ni) with Tc’s reported as high as 100 K, and coercive fields as high as 6500 Oe.14 Additionally, Fe(TCNE)2·zCH2Cl2 was prepared via the reaction of Fe(CO)5 and TCNE in CH2Cl2.15 Previous detailed studies on the Mn16a and Fe16b analogues reveal that they magnetically order as ferrimagnets; however, in retrospect, the materials were not pure and exhibited multiple magnetic phases with sample-to-sample variations. Recently, the reaction of MII(NCS)2(OCMe2)2 (M = Fe,17 Mn,10,17 Co17) and (NBu4)(TCNE) in CH2Cl2 resulted in the discovery of a new synthetic route for M(TCNE)2·zCH2Cl2 that produced materials with less magnetic impurities, based on the coincidence of the zero-field-cooled, MZFC(T), and fieldcooled, MFC(T), magnetization data, no out-of-phase χ″(T) AC susceptibility, as well as only one magnetic feature (vide infra). The structures of M(TCNE)2·zCH2Cl2 [M = Fe11 (Fe), Mn10 (Mn), Co17] are isostructural and have corrugated layers of MII bonded to four S = 1/2 μ4-[TCNE]•−, and these layers are bridged by the diamagnetic (S = 0) μ4-[C4(CN)8]2− dimer (Figure 1) and is best formulated as MII[μ4-[TCNE]•−{[μ4C4(CN)8]2−}1/2. © 2012 American Chemical Society
Figure 1. Extended 3-D network structure of MII (TCNE)[C4(CN)8]1/2·zCH2Cl2 (M = Mn,10 Fe11) possessing μ4-[TCNE]•− in 2-D, in which these layers are bridged by μ4-[C4(CN)8]2− to form an extended 3-D lattice. M is gold, N is blue, and C is black. The disordered CH2Cl2 solvent fills the channels.
Received: June 5, 2012 Revised: July 17, 2012 Published: August 27, 2012 18952
dx.doi.org/10.1021/jp305523t | J. Phys. Chem. C 2012, 116, 18952−18957
The Journal of Physical Chemistry C
Article
Table 1. Summary of the Average Magnetic Data for Mn and Fe Mn(TCNE)[C4(CN)8]1/2 χT (RT), emu K/mol χT, spin-only, emu K/mol θ, K Tmax, χ(T), K Tc = Tmax, d(χT)/dT, K Tmax, χ′(T), K Tmax, χT(T), K Tmax, M(T, 5 Oe), K Ms, emu Oe/mol (9 T, 5 K) Ms, emu Oe/mol (calcd) Mr, emu Oe/mol (5 K) Hcr, Oe (5 K) Hc, Oe (5 K) Hspin flop, Oe (5 K) frustration, f{= |θ|/Tc}
II
With the availability of magnetically purer M (TCNE)[C4(CN)8]1/2 (M = Fe,17 Mn,10), we have reinvestigated their magnetic behavior and report herein that they have an antiferromagnetic ground state and, for iron, a more complex field-dependent magnetic behavior.
■
Fe(TCNE)[C4(CN)8]1/2
3.95 4.75
3.12 3.375
87 68 69
102 90 84
70 71 69 20 100
86 90 82 16 500
22 340
16 755
0
1850
0
4800 12 600
EXPERIMENTAL SECTION M(TCNE)[C 4(CN)8 ]1/2·zCH2Cl2 (M = Fe, Mn) were prepared as previously reported,10,17 and, in addition, for M = Mn, via the reaction of MnI 2 (THF) 3 1 8 a and NBu4[TCNE].18b,19 Magnetic susceptibilities were measured in an 1000 Oe applied field between 5 and 300 K on a Quantum Design MPMS superconducting quantum interference device (SQUID) equipped with a reciprocating sample measurement system, low field option, and continuous lowtemperature control with enhanced thermometry features, as previously described.20 Polycrystalline samples for magnetic measurements were loaded in gelatin capsules, and protected from the atmosphere. The temperature dependence of the magnetization, M(T), was obtained by cooling in zero field and collecting the data upon warming. The remanent magnetization was taken in zero applied field upon warming after cooling in a 5 Oe field. Hysteresis and AC susceptibilities measured at 33, 100, and 1000 Hz were executed in a Quantum Design 9 T PPMS instrument, as previously described.20 In addition to correcting for the diamagnetic contribution from the sample holder, the core diamagnetic corrections of −134 (Mn), −135 (Fe), and −46.6 (CH2Cl2) × 10−6 emu/mol were used. Thermogravimetric analyses (TGA) were performed using a TGA 2050 TA Instruments located in a Vacuum Atmospheres DriLab under nitrogen to protect air- and moisture-sensitive samples. Samples were placed in an aluminum pan and heated at 5 °C/min under a continuous 10 mL/min nitrogen flow. The values of z ranged from 0.65 to 1.08 for Mn and from 0.60 to 0.72 for Fe with an average value determined to be 0.86 and 0.66 for Mn and Fe, respectively.
Figure 2. χ(T) of Mn (blue ●) and Fe (red ▲) in a 1000 Oe applied field.
RESULTS AND DISCUSSION DC magnetization and AC susceptibility measurements were performed on several samples (ca. 10 each) of both Mn and Fe between 5 and 300 K, but only data in a restricted temperature range are reported. Data from representative Mn and Fe samples are reported, and deviations are discussed. Table 1 summarizes the average magnetic data. The magnetic susceptibility, χ(T), of Mn increases gradually with decreasing temperature until ∼125 K, when it has a more rapid rise, reaching a maximum at ∼72 K (Figure 2). The magnitude and cusp shape of χ(T) is characteristic of antiferromagnetic ordering for polycrystalline materials.21 The temperature at which the maximum in χ(T) occurs lies above the true Tc,22,23 and Tc is best determined as the temperature at which the maximum in the heat capacity occurs. However, Fisher showed that Tc can be determined from the temperature at which the maximum in d(χT)/dT occurs, subsequently, d(χT)/dT is referred to as the “Fisher heat capacity” and is
commonly used to determine Tc.24,25 The d(χT)/dT data for Mn lack a peak, as is typically observed,24,25 due to the polycrystalline nature of the sample, and the rapid drop in the χT(T) data below 71 K (Figure 3), but it is estimated to occur at 67 K for the data in Figure 4. Tc's that were determined from the maximum in d(χT)/dT varied from 67 to 70 K with an average of 69 K. Hence, Tc is 69 K for Mn. Mn has a room-temperature χT value of 3.95 emu K/mol that is less than the expected spin-only value of 4.75 emu K/ mol for one high-spin, S = 5/2, MnII and one S = 1/2 [TCNE]•−. This is attributed to antiferromagnetic coupling. χT(T) does not exhibit a minimum below 300 K,26 as anticipated for a ferrimagnet, but this probably occurs at higher temperature. χT(T) increases slightly with decreasing temperature until ∼140 K, when it starts to increase more rapidly, reaching a maximum at 71 K, before decreasing toward zero. The sample-to-sample variation in the χT(T) maximum ranges from 70 to 75 K with an average value of 71 K.
■
18953
19 500 1.26
1.21
dx.doi.org/10.1021/jp305523t | J. Phys. Chem. C 2012, 116, 18952−18957
The Journal of Physical Chemistry C
Article
Figure 3. χT(T) (blue ●, red▲) and 1/χ(T) (blue ○, red Δ) for Mn (blue ●, blue ○) and Fe (red ▲, red Δ) in a 1 kOe applied field.
Figure 5. 5 Oe MZFC(T) (▼) and MFC(T) (▲) for Mn (blue ▼, blue ▲) and Fe (red ▼, red ▲).
bifurcation temperature along with the insignificant remanent magnetization further support bulk antiferromagnetic ordering. The 5 K field-dependent magnetization, M(H), for Mn does not have an observable hysteresis and linearly increases up to ∼16 000 Oe, and exceeds the expected values for a system with g = 2, S = 5/2 spin antiferromagnetically coupled to an S = 1/2 spin (total S = 2) based on the Brillouin function. This is attributed to antiferromagnetic correlations, and at ∼19 500 Oe, Mn undergoes a transition and begins to approach saturation, reaching a magnetization value of 19 700 emu Oe/ mol at 9 T (Figure 6). This is attributed to a spin flop between
Figure 4. d(χT)/dT for Mn (blue ●) and Fe (red
▲).
The peak in both χ(T) and χT(T) indicates magnetic ordering. χ−1(T) is linear above 200 K, and it can be extrapolated to a Curie−Weiss intercept, θ, of 89 K, arising from increasing uncompensated moments until 3-D interactions lead to an ordered state (Figure 3).27 θ ranges from 76 to 94 K and averages 87 K. Also, minimal deviations from linearity were observed in χ−1(T) for a few samples, indicating incomplete compensation between ferrimagnetic layers. The zero-field-cooled, MZFC(T), and field-cooled, MFC(T), magnetizations were measured in an applied field of 5 Oe between 5 and 150 K (Figure 5). The MZFC(T) and MFC(T) data are coincident with no irreversibility and thus lack a bifurcation temperature. Although all samples of Mn lacked irreversibility in their MZFC(T) and MFC(T) data, the lowtemperature region for each sample showed moderate variations that suggest that the solvent content or the presence of impurities may affect the low-temperature interactions. This is in accord with previous studies, albeit to a reduced extent.16a The temperature-dependent remanent magnetization, Mr(T), was measured in zero applied field after cooling the sample in an applied field of 5 Oe; however, the data are essentially zero throughout the entire temperature range. The lack of a
Figure 6. 5 K M(H) for Mn (blue ●) and Fe (red
▲).
the antiferromagnetically coupled ferrimagnetic layers. This spin flop transition is typical for isotropic systems in which there are rotations of the local spin directions. This is in contrast to a metamagnet, in which the “spin flop” transition is characterized by a simple reversal of spins (vide infra). The saturation value, however, varied between samples with a range 18954
dx.doi.org/10.1021/jp305523t | J. Phys. Chem. C 2012, 116, 18952−18957
The Journal of Physical Chemistry C
Article
a slight irreversibility at ∼86 K. In addition, a small, finite remanent magnetization reaching a plateau at ∼78 K was noted for some samples. Contrary to the χ(T) data, this indicates that a ferro- or ferrimagnetic transition occurs. However, it is attributed to an impurity that causes incomplete cancellation of moments or spin canting that varies from sample to sample, and therefore, it is not representative of the true ground state. The 5 K M(H) for Fe exhibits a constricted-shaped hysteresis, as previously reported.14 The M(H) data for Fe increases linearly until 1100 Oe, after which it rapidly increases in fields up to 25 000 Oe when it begins to saturate (Figure 6). The magnetization at 90 000 Oe is 16 500 emu Oe/mol, which is comparable to previously reported samples.11,14 This approaches the spin-only expected value of 16 755 emu Oe/ mol for the antiferromagnetic coupling of Fe (S = 5/2) and μ4[TCNE]•− (S = 1/2); however, saturation is not yet achieved. Because of the anisotropic FeII site,28 higher values are expected as g should exceed 2. The sigmoidal shape of the initial M(H) (Figure 6) is indicative of a metamagnetic transition, which is not unexpected due to the single-ion anisotropy of high-spin FeII.28 Below 8000 Oe, Fe exhibits a linear increase in M(H), as expected for an antiferromagnetic ground state. Above 8000 Oe, the M(H) increases more rapidly, indicating the fieldinduced transition to a high-moment state, that is, metamagnetic behavior. The dM/dH versus H data reveal a critical field, Hc, which averages 12 600 Oe at 5 K. The value of Hc varied slightly from sample to sample with a range of 10 900− 14 300 Oe. This transition is attributed to the weak antiferromagnetic coupling between the layers in Fe that is exceeded by the relatively small applied field, which locks the system into a high moment state once the critical field, Hc is reached. Contrary to typical metamagnets,28,29 however, Fe exhibits hysteretic behavior. Nonetheless, several previously reported organic-based metamagnetic materials have also been reported to exhibit hysteresis.30 Although the general sigmoidal shape of the hysteresis is always observed,14,16b sample-to-sample variations in the coercive field, Hcr, ranging from 700 to 4800 Oe as well as the remanent magnetization, Mr, from 900 to 1850 emu Oe/ mol occur, with the higher values being associated with purer samples. The isothermal field-dependent magnetization data were collected at various temperatures for a representative sample of Fe (Figure 8). At lower temperatures, the M(H) rises with increasing applied field, as described above, but at lower temperatures, the M(H) increases linearly, indicating two different regions, namely, a first-order and, at higher temperature, a continuous, second-order antiferromagnetic-to-ferrimagnetic transition. Oriented single-crystal studies are needed, however, to establish the magnetic phase diagram and determine the temperature at which the first- and secondorder transition meet, that is, the tricritical temperature.31 The AC susceptibility data are frequency-independent and have a peak for χ′(T), but not for χ″(T). As the temperature is decreased, χ′(T) increases until reaching a maximum at 86 K (Figure 7), in accord with Fe exhibiting an antiferromagnetic ground state. A response in χ″(T) indicates an irreversibility due to dissipative properties that can arise from spin glass phenomena or domain walls in ferri- or ferromagnetic systems. Therefore, a lack of response in χ″(T) is important for determining an antiferromagnetic ground state. Slight variations in the χ″(T) data were observed from sample to sample in
from 17 500 to 22 500 emu Oe/mol with an average value of 20 100 emu Oe/mol. This is lower than 22 340 emu Oe/mol expected for antiferromagnetic coupling, but the data are still increasing with applied field. In addition, the spin flop transition field ranges from 17 700 to 22 800 Oe with an average value of 19 500 Oe. Hysteresis was not observed for any of the Mn samples. The AC susceptibility data are frequency independent and confirmed antiferromagnetic ordering for Mn, in which a peak from the real, in-phase, χ′(T), was observed, but not for the imaginary, out-of-phase χ″(T) data (Figure 7). The temper-
Figure 7. χ′(T) and χ″(T) for Mn (blue ●) and Fe (red
▲).
ature at which the peak in χ′(T) occurs is in agreement with the DC susceptibility data and occurs at 69 K for the sample reported. The peak in χ′(T) has a small variation between samples with an average value of 70 K and has a similar temperature range as the DC susceptibility data. The χ(T) of Fe increases gradually with decreasing temperature until about 125 K, when it exhibits a considerable rise, reaching a maximum at 90 K (Figure 2). As occurs for Mn, the cusp shape of χ(T) for Fe is indicative of antiferromagnetic ordering. Like for Mn, the d(χT)/dT data for Fe lack a peak due to the polycrystalline nature of the sample, and due to the rapid drop in the χT(T) data below 90 K (Figure 3), but the peak is estimated to occur at 84 ± 1 K (Figure 4). Fe has a room-temperature χT value of 3.12 emu K/mol that is a little less than the expected spin-only value of 3.375 emu K/ mol for one high-spin, S = 2, FeII and one S = 1/2 [TCNE]•−, due to antiferromagnetic coupling. χT(T) increases slightly with decreasing temperature until ∼140 K, when it starts to increase more rapidly, reaching a maximum at 90 K, before decreasing toward zero. χT(T) does not exhibit a minimum below 300 K,26 as expected for a ferrimagnet, but this probably occurs at higher temperature. The peak in both χ(T) and χT(T) suggests magnetic ordering. χ−1(T) is linear above 150 K, and the linear region can be extrapolated to a θ of 102 K, also attributed to increasing uncompensated moments until 3D interactions lead to an ordered state. As observed for Mn, the shape of χ−1(T) near θ differs from that expected for a Néel ferrimagnet that arises from antiferromagnetic interactions between the inequivalent moments.21 The MZFC(T) and MFC(T) data for Fe are coincident, as occurs for Mn, again indicating an antiferromagnetic ground state (Figure 5). Note, however, that several samples exhibited 18955
dx.doi.org/10.1021/jp305523t | J. Phys. Chem. C 2012, 116, 18952−18957
The Journal of Physical Chemistry C
Article
one spin site (or lattice) whose adjacent identical spins are antiferromagnetically coupled, which compensate, and thus cancel.27,34 Systems with two spins sites are associated with ferrimagnets, whereby the adjacent spins differ in magnitude, but still antiferromagnetically couple, but cannot cancel, and results in a net moment. MII(TCNE)[C4(CN)8]1/2 has two different spin sites akin to a ferrimagnet, but has an antiferromagnetic ground state, and perhaps is best described as a compensated ferrimagnet. A similar behavior is observed for [NEt4]MnII3(CN)7.35
■
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest. † Deceased May 7, 2012. http://www.meaningfulfunerals.net/ fh/obituaries/obituary.cfm?o_id=1476970&fh_id=13348.
■
Figure 8. Isothermal field M(H) for polycrystalline Fe at various temperatures.
ACKNOWLEDGMENTS We appreciate the helpful discussions with R. F. Fishman (ORNL), J. L. Manson (Eastern Washington University), F. Palacio (University of Zaragoza), and O. Starykh (University of Utah), and continued partial support by the Department of Energy, Division of Material Science (Grant No. DE-FG0393ER45504).
which a weak feature was sometimes evident. This would indicate bulk ferro- or ferrimagnetic ordering, again, which is attributed to spin canting or incomplete cancellation of spin arising from defects or impurities. Magnetic frustration can be assessed from the ratio of |θ|/Tc,32 and is ∼1.2, indicating a minimal level of magnetic frustration.32,33
■
■
REFERENCES
(1) Miller, J. S.; Calabrese, J. C.; Epstein, A. J.; Bigelow, R. W.; Zhang, J. H.; Reiff, W. M. J. Chem. Soc., Chem. Commun. 1986, 1026− 1028. (2) Miller, J. S.; Calabrese, J. C.; Rommelmann, H.; Chittipeddi, S. R.; Zhang, J. H.; Reiff, W. M.; Epstein, A. J. J. Am. Chem. Soc. 1987, 109, 769−781. (3) Ovcharenko, V. I.; Sagdeev, R. Z. Russ. Chem. Rev. 1999, 68, 345−363. (4) Blundell, S. J.; Pratt, F. L. J. Phys.: Condens. Matter 2004, 16, R771−R828. (5) Miller, J. S. Chem. Soc. Rev. 2011, 40, 3266−3296. (6) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508−2525. (7) Miller, J. S.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. Adv. Mater. 1992, 4, 498−501. (8) Lapidus, S. H.; McConnell, A. C.; Stephens, P. W.; Miller, J. S. Chem. Commun. 2011, 47, 7602−7604. (9) Pokhodnya, K. I.; Bonner, M.; Her, J.-H.; Stephens, P. W.; Miller, J. S. J. Am. Chem. Soc. 2006, 126, 15592−15593. (10) Stone, K. H.; Stephens, P. W.; McConnell, A. C.; Shurdha, E.; Pokhodnya, K. I.; Miller, J. S. Adv. Mater. 2010, 22, 2514−2519. (11) Her, J.-H.; Stephens, P. W.; Pokhodnya, K. I.; Bonner, M.; Miller, J. S. Angew. Chem., Int. Ed. 2007, 46, 1521−1525. (12) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415−1417. (13) Miller, J. S. Polyhedron 2009, 28, 1596−1605. (14) Zhang, J.; Ensling, J.; Ksenofontov, V.; Gütlich, P.; Epstein, A. J.; Miller, J. S. Angew. Chem., Int. Ed. 1998, 37, 657−660. (15) Pokhodnya, K. I.; Petersen, N.; Miller, J. S. Inorg. Chem. 2002, 41, 1996−1997. (16) (a) Wynn, C. M.; Girtu, M. A.; Zhang, J.; Miller, J. S.; Epstein, A. J. Phys. Rev. B 1998, 58, 8508−8514. (b) Pejakovic, D.; Kitamura, C.; Miller, J. S.; Epstein, A. J. Phys. Rev. Lett. 2002, 88, 57202/1. (c) Pejakovic, D.; Epstein, A. J.; Kitamura, C.; Miller, J. S. J. Appl. Phys. 2002, 91, 7176−7178. (d) Girtu, M. A.; Wynn, C. M.; Zhang, J.; Miller, J. S.; Epstein, A. J. Phys. Rev. B 2000, 61, 492−500. (17) Shurdha, E.; Lapidus, S. H.; Stephens, P. W.; Moore, C. E.; Rheingold, A. L.; Miller, J. S. Manuscript in preparation. (18) (a) Stolz, P.; Pohl, S. Z. Naturforsch., B 1988, 43, 175−181. (b) Webster, O.; Mahler, W.; Benson, R. E. J. Org. Chem. 1960, 25, 1470−1470.
CONCLUSION Mn and Fe are attributed to having direct, antiferromagnetic exchange coupling between the MII (S = 5/2 for Mn; S = 2 for Fe) and the S = 1/2 μ4-[TCNE]•− spin sites, leading to 2-D ferrimagnetic layers. These ferrimagnetic layers are antiferromagnetically coupled via five-atom, conjugated −NC−C− CN− superexchange pathways leading to a bulk antiferromagnet, an unusual arrangement for an antiferromagnet. However, both Mn and Fe do not appear to be simple collinear antiferromagnets as their magnetization data are more complex. This is evident in the rising magnetization at low temperatures (
