Magnetic Properties of One-, Two-, and Three-dimensional Crystal Structures built of Manganese (III) Cluster-based Coordination Polymers

Transcription

1 Magnetic Properties of One-, Two-, and Three-dimensional Crystal Structures built of Manganese (III) Cluster-based Coordination Polymers Kevin J. Little * Department of Physics, University of Florida Gainesville, FL August, 006 Abstract Magnetization studies, using a SQUID magnetometer operating down to K and up to 7 T, were performed on one-, two-, and three-dimensional crystal structures built with Mn 3+ cluster-based coordination polymers. All three structures were found to exhibit strictly paramagnetic behavior with no signs of long-range ordering down to K. The magnetic properties of the D and 3D compounds were found to be a result of the Mn 3+ ions with quantum spin value S = present in each compound; however, the 1D compound exhibited anomalous magnetic properties. *Permanent address: Department of Physics, Taylor University, Upland, IN

2 Introduction Interest in the synthesis of one-, two- and three-dimensional (1D, D, and 3D) crystal structures using coordination polymers, which are chains of metal ion nodes and ligand linkers, has increased in recent years [1]. Some coordination polymer structures have novel magnetic and electrical properties and may have broad potential for applications in developing technologies. When a new chemical compound is fabricated, the magnetic properties of the material can be an essential element to understanding the atomic properties, intramolecular interactions, and usefulness of the material. Three new compounds using Mn 3+ Schiff-base complexes as the metal ion nodes (Figure 1) were created by the Department of Chemistry at Wake Forest University (WFU) and were sent to the Departments of Physics and Chemistry at the University of Florida (UF) for magnetic analysis. The low-temperature magnetic properties of the compounds, such as the type of magnetism, evidence for long-range ordering, and evidence for any novel magnetic properties, were determined. SQUID Magnetometer The magnetic moment of each sample was measured under temperatures ranging from K to 300 K and under external magnetic fields ranging from 0 to 7 T by a Quantum Design MPMS (Magnetic Properties Measurement System) SQUID (Superconducting QUantum Interference Device) magnetometer. The MPMS SQUID magnetometer is an automated system that contains a temperature control system, a superconducting magnet with a maximum field of 7 T, a sample transport system, and a SQUID amplifier system [3]. A sample is connected to a driving mechanism by a rod and is moved through a second-derivative system of four

3 superconducting coils. A magnetic moment is induced in the sample by the external field H created by the superconducting magnet. As the sample is moved through the coiled wire, the magnetic flux Φ through the coils due to the sample s magnetic moment changes. An electric field E v and a current in the coils are created in accordance with Faraday s Law of Induction, v v dφ E ds =, (1) dt where ds v is an infinitesimal displacement along the boundaries of the line integral and dt is the infinitesimal of time. The induced current is inductively coupled to the SQUID detector, which acts as a exceptionally sensitive current to voltage converter due to a quantum tunneling effect [3]. The voltage is proportional to the magnetic moment of the sample, and this value is processed and recorded by a computer. SQUID Measurements Method Three pairs of empty gel cap tops were prepared. Each gel cap pair was weighed with a digital scale, placed in the configuration in which it would hold the sample, and mounted in a drinking straw. The straw was attached to a transport rod which was placed in the SQUID magnetometer. The gel cap was cooled to K, and its magnetic moment was measured under external fields ranging from 0 to 7 T. The external magnetic field H was then held constant at 0.1 T, and the magnetic moment was measured as the temperature was swept from K to 300 K. A sample of each of the three compounds was carefully placed into one of the measured gel caps and was weighed. The tightly packed gel cap was then mounted in a drinking straw. The straw was attached to a transport rod and lowered into the bore of the SQUID magnetometer. The sample was zero-field cooled (), i.e., cooled to K without the application of an external magnetic field. At K, a field of 0.1 T was applied. The MPMS 3

4 SQUID magnetometer then took measurements at temperatures ranging from K to 300 K. The sample was then cooled in the presence of an external field of 0.1 T, i.e., field cooled (). Measurements were again made in a sweep from K to 300 K. The temperature was then cooled to K, and measurements of the sample s magnetic moment were made in external fields ranging from 0 to 7 T. Calculations and Results The SQUID magnetometer measures the magnetic moment for the entire sample, and this moment can be used to find the molar magnetic susceptibility χ m by χ = M m HN, () where M is the magnetic moment of the sample, H is the applied external field, and N is the number of moles of the material in the sample. A positive χ m value is paramagnetic and a negative value is diamagnetic. The χ m data have been calculated from the magnetic moment data recorded by the SQUID and adjusted for an intrinsic diamagnetism given in Table I. The results shown in Figure display similar paramagnetic behavior for all three materials. A significant difference in the χ m values for field cooled () data and zero-field cooled () data would be an indicator of long-range ordering. Long-range ordering occurs in ferromagnetic, antiferromagnetic, and ferrimagnetic materials when dipole moments are aligned throughout relatively distant areas of a material s structure. The difference between and data is displayed in the inset graphs of Figure. No significant difference within experimental uncertainties was found for any of the materials. A plot of χ m T vs. T reveals important information about the magnetic properties of a material. A plot that is a relatively straight, flat line indicates paramagnetism. A plot that curves 4

5 upward indicates ferromagnetism, and a plot that curves downward at low temperatures indicates antiferromagnetism. Figure 3 displays χ m T vs. T data for all three compounds. While the plots trend downward at around 50 K, the spins of the Mn 3+ ions are most likely not exhibiting antiferromagnetic coupling. The local molecular structure around the Mn 3+ ions suggests that anisotropy, the tendency for an atom to share its electrons along a preferential axis leading to distortions in the electron orbitals, may be occurring at low temperatures. The Curie constant C is equal to χ m T and is proportional to the effective magnetic moment µ eff. The Curie constant C is defined by the equation ( S + 1) N Ag µ B S χ mt = C =, (3) 3k B where N A is Avogadro s number, g is the Landé g-factor or the spectroscopic splitting factor, µ B is the Bohr magneton, k B is the Boltzmann constant, and S is the spin value. By rearranging Eq. (3), we obtain a method of solving for possible spin values: ( χ T ) 3 k S( S + 1) =. (4) N µ The structural symmetry of the ligands and Nb 6 clusters in the three compounds suggested that the dominant magnetic properties of each compound would be a result of the S = Mn 3+ ions. So, the values of χ m T were normalized by the number of Mn ions in the formula unit of each compound. With the starting value of g = used as an estimate for g, the value of χ m T at 300 K was used to calculate a possible spin. This S value was rounded to a possible quantum spin value (a multiple of ½) and used to calculate a value for g. The results for each compound are shown in Table II. The plots of χ -1 vs. T displayed in the inset graphs of Figure 3 exhibit a linear behavior over the entire range of data. This behavior is characteristic of paramagnetism. The intercept of m Ag B B 5

6 all three plots is near the origin, indicating Curie paramagnetism [3]. The data were fit to a Curie-Weiss Law, χ 1 m = ( T θ ) C (5) where C is the Curie constant and θ is the Curie-Weiss temperature, which may indicate a ferromagnetic or antiferromagnetic transition when non-zero. The results for C and θ are displayed in Table III. The relation 3k C B µ eff = (6) N A can be used to find the effective magnetic moment. The results are displayed in Table III. The magnetization per mole of material over a range of external magnetic fields is an effective indicator of a compound s magnetic characteristics. The M vs. H data, displayed in Figure 4, can be fit to the Brillouin function for paramagnetism, M = nn A ( S 1) + gµ BH ( ) 1 g BH gµ B S + ctnh ctnh µ 1, (7) kbt kbt where M is the molar magnetization, n is the number of interacting spins per formula unit, and ctnh(x) is the hyperbolic cotangent function [5]. The Brillouin function for paramagnetism is a model for non-interacting spins with no anisotropy, so the M vs. H data in Figure 4 suggests some spin interaction or anisotropy occurring in the materials at K. Discussion and Conclusion No long-range ordering was observed in the three compounds. The χ m -1 vs. T and M vs. H plots are characteristic of a paramagnetic material. Deviations from a simple Brillouin function for paramagnetism suggest that the anisotropy suggested by the χ m T data is an important 6

7 factor in spin behavior at low temperatures. The magnitudes of the values of θ from Curie-Weiss law fits of χ -1 m vs. T data are sufficiently near zero, suggesting paramagnetism. If ordering does occur, it does so at temperatures that are not within the range of the SQUID magnetometer. The data from the D and 3D compounds suggest that the Mn 3+ with S = ions in each compound are indeed the principal cause of the paramagnetic behavior of the materials. When the χ m T data are normalized for the number of Mn 3+ ions per formula unit, the χ m T / Mn ion values are extremely similar (Figure 5). The same is true for the D and 3D M vs. H data (Figure 6). The calculated spins from the 300 K χ m T values strongly support the Mn 3+ ion with S = and g as the factor responsible for the compounds magnetic properties. Corroborating evidence is offered by the Brillouin function fits of the M vs. H data. For the D compound, the parameters S =, g, and n provide a satisfactory fit. For the 3D compound, the parameters S =, g, and n 3 provide a similarly satisfactory fit. The µ eff value for the D data found from the Curie-Weiss law Curie constant (7.38µ B ) is close to the 7.34µ B value of [Me 4 N] {[Mn(salen)] [Nb 6 Cl 1 (CN) 6 ]}, a similar material []. The µ eff value calculated for the 3D data (8.98µ B ) is near the 8.84µ B value of a similar material, Na{[Mn(salen)] 3 [Re 6 Se 8 (CN) 6 ]} []. While the 1D and D compounds display a :3 relationship for M vs. H and χ m T vs. T, the data of the 1D compound appear anomalous. Since the 1D compound contains only one Mn 3+ ion, its molar magnetization and χ m T values should be similar to the normalized D and 3D data if all significant magnetism is due to the S = Mn 3+ ion. However, these values are too large by a factor of two to support this conclusion. For a more complete analysis, the cause of this anomaly needs to be investigated. A likely possibility is that excess Mn ions were present in the sample. The compound may have 7

8 contained excess or residued Mn ions, or a structure containing two Mn 3+ ions instead of one may have been created. Acknowledgements I would like to thank Mark W. Meisel for his instruction and patience this summer. I would like to thank Norman Anderson and Daniel Pajerowski for their enlightening discussions and coaching. I would also like to thank James Ch. Davis for his initial data measurements and instruction. Funding was provided by the National Science Foundation grant DMR through the Physics Research Experience for Undergraduates program at the University of Florida and by DMR (MWM). Samples were created by Huajun Zhou and Abdessadek Lachgar at the Wake Forest University Department of Chemistry under grant DMR References [1] K. Uemura, R. Matsuda, and S. Kitagawa, J. Solid State Chem. 178, 40 (005). [] H. Zhou, et al., One-, Two-, and Three-dimensional frameworks built of Octahedral Metal clusters and manganese(iii) complexes, preprint (006). [3] M. McElfresh, Fundamentals of Magnetism and Magnetic Measurements Featuring Quantum Design s Magnetic Properties Measurement System (Quantum Design, Inc., San Diego, CA, 1994). [4] H. Zhou, correspondence, 15 July 006. The following corrections were made: diamagnetic contribution and temperature independent paramagnetic contribution of the Nb 6 cluster using J.G. Converse and R.E. McCarley, Inorg. Chem. 9, 1361 (1970); and 8

9 diamagnetic contributions of inner chlorides and cyanide ligands from the cluster units and ligands using O. Kahn, Molecular Magnetism (Wiley-VCH, New York, 1993). [5] C. Kittel, Introduction to Solid State Physics, 5 th Ed. (John Wiley & Sons, New York, 1976). 9

10 Molecular Formula Molar Mass (g/mol) Sample Mass (mg) Intrinsic Diamagnetism (emu/mol) [4] 1D [Me 4N] 3{{Mn(L1)}{Nb 6Cl 1(CN) 6]} x 10-6 D [Me 4N] {[Mn(L)] [Nb 6Cl 1(CN) 6]}.0 MeOH x D [Me 4N]{[Mn(L3)] 3[Nb 6Cl 1(CN) 6]} 0.6 MeOH x 10-6 (L1 = 5-MeO-salen = C 18H 18N O 4; L = 7-Me-salen = C 18H 18N O ; and L3 = acacen = C 1N O H 18) TABLE I. Properties of measured materials. Possible spin calculated g D.98 D.1 3D.11 TABLE II. Rounded estimated spins and g values for compound / Mn ion. C (emu K/mol) (±0.0) θ (K) (±0.) µ eff (µ B ) 1D D D TABLE III. Curie constant and Curie-Weiss temperature found from fit of Curie-Weiss law to χ m -1 vs. T. Calculated µ eff given in Bohr magnetons. 10

11 (a) (b) FIG. 1. (a) 1D, (b) D, and (c) 3D crystal structures []. Blue: Nb; Magenta: Mn; Green: Cl; Cyan: N; Red: O; Grey: C. Molecular formulas are given in Table I. (c) 11

12 χ m (emu/mol) H = 0.1 T χ (emu x 10 - /mol) D χ m vs. T χ m (emu/mol) H = 0.1 T χ (emu x 10 - /mol) D χ m vs. T (a) 0.0 (b) 3D χ m vs. T χ m (emu/mol) H = 0.1 T χ (emu x 10 - /mol) FIG.. χ m vs. temperature with χ vs. temperature inset. χ = χ χ with uncertainties propagated from the standard deviations of SQUID measurements and an estimated mass uncertainty of 0.1 mg. All measurements performed at 0.1 T. (a) 1D, (b) D, (c) 3D (c) 1

13 χ m T (emuk/mol) H = 1 kg χ -1 (mol/emu) D χ m T vs. T H = 1 kg χ -1 vs. T Curie-Weiss Fit χ -1 = (T-θ)/C χ m T (emuk/mol) H = 1 kg χ -1 (mol/emu) D χ m T vs. T H = 1 kg χ -1 vs. T Curie-Weiss Fit χ -1 = (T-θ)/C (a) (b) 3D χ m T vs. T 10 H = 1 kg χ m T (emuk/mol) 9 8 χ -1 (mol/emu) H = 1 kg χ -1 vs. T Curie-Weiss Fit χ -1 = (T-θ)/C FIG. 3. χ m T vs. temperature with χ m -1 vs. temperature inset. A Brillouin function was fit to the χ m T vs. T data. A Curie-Weiss Law was fit to the χ m -1 vs. T data. (a) 1D, (b) D, (c) 3D (c) 13

14 5 1D M vs. H D M vs. H 4 4 M (emug x 10 4 /mol) 3 1 T = K UP DOWN Brillouin Fit S ±0 N ± g ±0 M (emug x 10 4 /mol) 3 1 T = K UP DOWN Brillouin Fit S ±0 n ± g.119 ± H (Tesla) (a) 3D M vs. H H (Tesla) (b) 6 5 M (emug x 10 4 /mol) T = K UP DOWN Brillouin Fit S ±0 N ±0.106 g ± FIG. 4. Molar magnetization vs. external magnetic field with a Brillouin function fit to the data. (a) 1D, (b) D, (c) 3D H (Tesla) (c) 14

15 χ m T / Mn ion (emuk/mol) Comparison of χ m T / Mn ion vs. T 1D D 3D FIG. 5. χ m T vs. T field cooled data normalized for the number of Mn 3+ ions per formula unit in each compound. M / Mn ion (emug x 10 4 /mol) Comparison of M / Mn ion vs. H 1D D 3D H (Tesla) FIG. 6. M vs. H data normalized for the number of Mn 3+ ions per formula unit in each compound. 15