Metamagnetism and Slow Relaxation of the Magnetization in the 2D

Communication pubs.acs.org/crystal

Metamagnetism and Slow Relaxation of the Magnetization in the 2D Coordination Polymer: [Co(NCSe)2(1,2-bis(4-pyridyl)ethylene)]n Susanne Wöhlert,† Uwe Ruschewitz,§,∥ and Christian Naẗ her*,† †

Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Strasse 2, 24118 Kiel, Germany Department für Chemie, Universität zu Köln, Greinstraße 6, 50939 Köln, Germany

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S Supporting Information *

ABSTRACT: Reaction of Co(NCSe)2 with 1,2-bis(4pyridyl)ethylene (bpe) leads to the formation of [Co(NCSe)2(1,2-bis(4-pyridyl)ethylene)]n, in which Co(NCSe)2 chains are linked by the bpe ligands into a two-dimensional (2D) coordination network. This compound shows metamagnetic behavior with slow relaxation of the magnetization above the critical field (HC) and dominating antiferromagnetic exchange below HC. This is a very rare phenomenon which was never observed before in a 2D selenocyanato coordination polymer.

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this compound metamagnetism and a slow relaxation of the magnetization coexist, which is still a very rare phenomenon and which has never been observed for a thiocyanato coordination compound before. Unfortunately, as for most SCM compounds, the ordering temperature is relatively low, and therefore, we investigated if a similar compound can be prepared on the basis of selenocyanato anions. In this context, the question is raised if it will still show slow relaxation of the magnetization and if this will be accompanied by metamagnetic behavior. To prepare such a compound, Co(NO3)2·6H2O was reacted in different molar ratios with KNCSe and bpe in acetonitrile. XRPD investigations reveal that in contrast to the thiocyanato compound, only one crystalline phase occurred that might be isotypic to the NCS− analogue. Elemental analysis clearly proved that a compound of composition [Co(NCSe)2(bpe)]n (1) was obtained, and IR-spectroscopy revealed that μ-1,3 bridging ligands are present (Supporting Information Figure S1).9 Consequently, compound 1 might show the same coordination topology as its thiocyanato analogue. Unfortunately, all attempts to prepare single crystals failed and the solids prepared in solution were of poor crystallinity. However, because some similarities are found in the powder pattern of [Co(NCS)2(bpe)]n and that of 1, the lattice parameters of the former were systematically varied, until the positions of the reflections fit to those of the NCSe analogue. The resulting unit cell parameters are as follows: a = 9.483(1) Å, b = 9.580(2) Å, c = 10.589(2) Å, α = 112.88(1)°, β = 111.62(1)°, γ = 92.02(1)°, V = 805.8(2) Å3 (P1̅, Z = 2). On the basis of these parameters, a Rietveld refinement was performed. Comparing the

ecently, investigation of the magnetic properties of coordination polymers has become an active field of chemical research.1 In this context, compounds that exhibit a slow relaxation of the magnetization, such as single chain magnets (SCMs), are of current interest because of their possible future applications in, for example, quantum computing.2 Therefore, the number of papers that reports on this behavior has continuously increased in the past few years, and there are some excellent reviews in this field.3 For the preparation of such compounds, cations with a large uniaxial magnetic anisotropy, such as Co2+, Mn3+, Ni2+, and Fe2+ must be connected into chains that exhibit strong intrachain interactions between the magnetic units. To establish strong intrachain interactions, the paramagnetic metal centers must be connected by small-sized anionic ligands, and therefore, several SCM compounds are based on, for example, azido anions.4 Moreover, practically no interchain interactions should be present because otherwise 3D ferromagnetic ordering is observed, which is the case for several 1D compounds.5 It is noted that SCM behavior can also be found in cobalt(II) thioor selenocyanato compounds that contain additional monodentate coligands, as we have shown for the first time in our previous work.6 However, as mentioned above, SCM behavior usually occurs only in 1D coordination polymers, but a slow relaxation of the magnetization can also be observed in 2D coordination networks if the chains are separated by ligands that effectively prevent interchain interactions.7 In this context, we have reported on the synthesis and magnetic properties of a 2D coordination compound based on cobalt(II) thiocyanate and 1,2-bis(4-pyridyl)ethylene (bpe).8 In this compound the metal cations are linked into chains by μ-1,3 bridging thiocyanato anions, which are further connected into layers by the bidentate bpe ligands. Surprisingly, magnetic measurements reveal that in © 2012 American Chemical Society

Received: December 15, 2011 Revised: March 22, 2012 Published: April 12, 2012 2715

dx.doi.org/10.1021/cg201659v | Cryst. Growth Des. 2012, 12, 2715−2718

Crystal Growth & Design

Communication

data using a Curie−Weiss law leads to an effective magnetic moment of 4.90 μB and a Weiss constant of 6.06 K (Supporting Information Table S1). Saturation magnetization experiments at 4.2 K showed a step in the hysteresis loop which is indicative for metamagnetic behavior and a really small hysteresis (Figure 3).10 An initial curve was also measured, but even at 9 T, no full

experimental pattern of 1 with that calculated from the structural data obtained from this refinement clearly shows that compound 1 is isotypic to [Co(NCS)2(bpe)]n and is obtained as a phase pure crystalline material (Figures 1 and 2 and Supporting Information Figure S2).

Figure 3. Magnetization curve for [Co(NCSe)2(bpe)]n (1) at 4.2 K. The solid lines are only guides for the eyes.

saturation is reached, which can be traced back to the magnetic anisotropy of the system (Supporting Information Figure S4).11 The critical field HC was determined from the first derivative of dM/dH to be 1.05 kOe (Supporting Information Figure S5). Additional dc measurements were performed above HC at HDC = 3 kOe and reveal ferromagnetic behavior (Supporting Information Figure S6), which is also confirmed by ZFC-FC measurements at HDC = 1.5 kOe (Supporting Information Figure S7). From the first derivative of the χM versus T curves, an ordering temperature of 6.9 K is determined, which is significantly higher than that in the thiocyanato analogue of 4.3 K (Supporting Information Figure S8). To further characterize the ferromagnetic phase, ac magnetic susceptibility measurements were performed at HDC = 1.5 kOe and HAC = 5 Oe. These measurements show broad maxima in the χM′ (in-phase) and χM″ (out-of-phase) curves, which are frequency dependent (Figure 4; Supporting Information Table S2). Therefore, a 3D ferromagnetic ordering above HC can be excluded and a slow relaxation of the magnetization is observed.3c,b However, ac measurements below HC also show frequency dependent maxima, which indicates that the slow relaxation of the magnetization is still present and not fully suppressed by the antiferromagnetic ordering (Supporting Information Figure S9). Analysis of the frequency dependence of the ac data above HC leads to a Mydosh-parameter (φ = (ΔTP/TP)/Δ(log f)) of φ = 0.16, which is in agreement with that value expected for superparamagnetic behavior (0.1 < φ > 0.3) and which is identical with the value calculated for the thiocyanato compound.8 The χM″ versus T data were fitted to the Arrhenius law (τ = τ0 exp[Δ/kBT]; τ, relaxation time; Δ, effective energy barrier; kB, Boltzmann constant; T, temperature), leading to an effective energy barrier −Ueff/kB of 56.9 K and a τ0 = 1.88 × 10−11 s, which is higher than that of [Co(NCS)2(bpe)]n of 52.9 K (Figure 5).8 In this context, it is noted that both compounds show metamagnetic behavior, and therefore, the title compound was measured at a higher field. However, it should

Figure 1. Packing arrangement of [Co(NCSe)2(bpe)]n (bpe = 1,2bis(4-pyridyl)ethylene) (1) (green = cobalt, blue = nitrogen, orange = selenium, gray = carbon, white = hydrogen).

Figure 2. Experimental X-ray powder pattern for compound 1 (top) and that calculated on the basis of the Rietveld data (bottom).

Magnetic dc measurements of compound 1 at HDC = 1 kOe show antiferromagnetic ordering at TN = 5.3 K (Supporting Information Figure S3). The room-temperature value of 4.93 μB is larger than that expected (μB for S = 3/2 = 3.87), which is frequently observed in Co(II) compounds because of the very large spin−orbit coupling of Co(II) and consequently a g-factor which deviates from the ideal value. Analysis of the magnetic 2716



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Figure 6. Cole−Cole plot: χM″ as a function of χM′ at 3.2 K for compound 1 (red lines represent best fit to the experimental data).

analogue (Supporting Information Figure S10, and Table S3 and S4).13 In summary, we have presented the first selenocyanato coordination compound in which metamagnetism and slow relaxation of the magnetization coexist. Therefore, this unusual magnetic behavior is still present if the thiocyanato ligand is exchanged by selenocyanato anions. Interestingly, this modification leads to an significant increase of the ordering temperature and the critical field. Moreover, in comparison to the thiocyanato analogue, also the effective energy barrier for spin reversal slightly increases. The values obtained for the intrachain interactions obtained from the ln(χMT versus T −1) data are slightly higher than those of the thiocyanato analogue but strongly depend on the nature of the fit. Therefore, for a deeper understanding of the differences between the anionic ligands, the magnetic behavior must be investigated in more detail, which will include, for example, field dependent measurements as well as investigation of the magnetic anisotropy. This will be the subject of further investigations. However, in any case, our investigations indicate that even in the 2D compounds the different chemical components can be exchanged to some extent without loosing the slow relaxation of the magnetization. This would offer the opportunity to study the influence of the different components on those parameters that describe the performance of such materials in more detail. This will be helpful for a deeper understanding and is absolutely needed for the optimization of such materials.

Figure 4. Temperature dependence of χM′ (top) and of χM″ (bottom) ac magnetic susceptibility (HDC = 1.5 kOe and HAC = 5 Oe; 50/250/ 500/1000/2500/5000/7500 Hz).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, and characterization information for 1. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Arrhenius plot of relaxation rate as a function of reciprocal temperature for 1 (red lines represent best fit to experimental data).

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be kept in mind that the energy barrier decreases with increasing field, and therefore, the differences in −Ueff/kB would become larger.12 To investigate the distribution of the relaxation times, the isothermal frequency dependence of χM′ and χM″ at 3.2 K was analyzed (Figure 6). The Cole−Cole plot of χM″ versus χM′ can be fitted to a general Debye model in the range 50−10000 Hz, which leads to a Cole exponent of α = 0.04, indicating an infinitely narrow distribution of the relaxation time.3b,2a Finally, the creation energy for a domain wall along the chain was determined from a linear fit of ln(χMT versus T −1), leading to values which are significant higher than those in the thiocyanato

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49 431 880 1520; Tel: +49 431 880 2092. Notes

The authors declare no competing financial interest. ∥ Fax: +49 221-470-4899. Tel: +49 221-470-3285.

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ACKNOWLEDGMENTS We acknowledge financial support by the DFG (Project No. NA 720/3-1) and the State of Schleswig-Holstein, and we 2717



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thank Professor Dr. W. Bensch for access to his experimental facilities.

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REFERENCES

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Metamagnetism and Slow Relaxation of the Magnetization in the 2D

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