Slow Magnetic Relaxation in Weak Easy-Plane Anisotropy: the Case

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Slow Magnetic Relaxation in Weak Easy-Plane Anisotropy: the Case of a Combined Magnetic and HFEPR Study Yuan-Yuan Zhu,*,† Fang Liu,† Jia-Jia Liu,‡ Yin-Shan Meng,‡ Shang-Da Jiang,*,‡ Anne-Laure Barra,∥ Wolfgang Wernsdorfer,§ and Song Gao‡ †

School of Chemistry and Chemical Engineering, Hefei University of Technology and Key Laboratory of Advanced Functional Materials and Devices, Hefei 230009, Anhui Province, China ‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Institut Néel, CNRS & Université Grenoble Alpes, BP 166, 25 avenue des Martyrs, 38042 Grenoble Cedex 9, France ∥ LNCMI-CNRS, 25 rue des Martyrs BP 166, 38042 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: Two pseudotetrahedral cobalt(II) complexes exhibiting slow magnetic relaxation under an applied direct-current field are investigated. The weak easy-plane anisotropy is accurately determined by highfield/high-frequency electron paramagnetic resonance spectroscopy as D = 2.57 cm−1 and E = 0.82 cm−1 for 1 and D = 5.56 cm−1 and E = 1.05 cm−1 for 2. In addition, hysteresis loops are observed for the two compounds at very low temperatures. Figure 1. Molecular structures of (a) 1 and (b) 2.

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he slow magnetic relaxation phenomena have been frequently reported in mononuclear coordination complexes since its discovery by Ishikawa et al. in rare-earth systems.1,2 Even though the relaxation energy barriers and the blocking temperatures in the mononuclear rare-earth and transition-metal systems have exceedingly overcome the ones of the cluster-based single-molecule magnets (SMMs),3,4 the relaxation processes involved are far more complicated than expected, requiring a precise understanding of the fine electronic structures. More recently, the exploration of new mononuclear systems has been extended to mononuclear transition-metal complexes that exhibit field-induced slow relaxation of magnetization.5 Different from the traditional-metal SMMs, whose axial parameter of the zero-field-splitting (ZFS) D value is negative, several single-ion magnets (SIMs) of transition-metal ions exhibit positive D values.4c,5b−e,k,l,6 Herein we report two examples of mononuclear tetrahedral [Co(NCS)4]2−-type complexes that exhibit slow magnetic relaxation under an applied direct-current (dc) field. The relaxation dynamics of magnetization in these compounds were characterized in detail, and the axial and rhombic ZFS parameters D and E were determined accurately by high-field/high-frequency electron paramagnetic resonance (HFEPR) spectroscopy. Additionally, hysteresis loops were also observed on a micro-SQUID magnetometer at very low temperatures. In this work, two mononuclear tetrahedral cobalt(II) compounds, [K(C12 H 24 O 6 )] 2 [Co(NCS) 4 ] (1) and [Ba(C12H24O6)·3H2O][Co(NCS)4] (2), were facilely synthesized from the reaction of thiocyanate salts (Figure 1; KNCS for 1 and © 2016 American Chemical Society

Ba(NCS)2 for 2), CoCl2, and 18-crown-6 in deionized water (see the Supporting Information for details and Figure S1 for Fourier transform infrared spectra).7 Evaporation of the solution afforded dark-blue block-shaped crystals in several days. Singlecrystal X-ray structural analysis reveals that 1 crystallizes in the orthorhombic space group Pna21 with Z = 4. The sole CoII ion is a pseudotetrahedron coordinated with N atoms from four thiocyanate ions. The four Co−N bond lengths are 1.946, 1.951, 1.952, and 1.960 Å, respectively. Two K+ ions, which are coordinated with 18-crown-6, serve as the countercation of the Co(NCS)42− anion complex, located in its periphery. 2 crystallizes in the monoclinic space group P21/n with Z = 4. The sole CoII ion is in a similar coordination sphere, and the four Co−N bond lengths are 1.942, 1.948, 1.954, and 1.969 Å, respectively. The Ba2+ ion is coordinated with one 18-crown-6 and an additional three H2O solvent molecules. Overall, the CoN4 tetrahedra of the two complexes deviate from idealized Td symmetry. According to the calculation results of continuous shape measures,8 the deviation parameter values of the two CoII centers to the idealized Td symmetry are 0.036 and 0.103, indicating that distortion of the CoII coordination sphere is larger in 2 than in 1. In the lattices of the two complexes, the shortest Co···Co distances are 10.016 Å for 1 and 8.408 Å for 2, implying that the paramagnetic CoII centers are well isolated and Received: August 15, 2016 Published: December 22, 2016 697

DOI: 10.1021/acs.inorgchem.6b01972 Inorg. Chem. 2017, 56, 697−700

Communication

Inorganic Chemistry

frequency; one can therefore determine the ZFS parameters with a high precision from the HFEPR approach. Taking account of eq 1, all of the EPR spectra can be well reproduced with one set of parameters for each sample. The parameters for 1 are found to be D = 2.57(1) cm−1, E = 0.82(1) cm−1, gxx = gyy = 2.21(2), and gzz = 2.25(2), and those for 2 are D = 5.56(1) cm−1, E = 1.05(1) cm−1, gxx = gyy = 2.23(2), and gzz = 2.24(2), where we assume the g ̿ tensor to be axially symmetric and gxx = gyy ≠ gzz to avoid overparameterization. On the basis of the magnetization and EPR data, one is able to confidently claim that the two CoII ions in these pseudotetrahedron geometries are of weak easy-plane anisotropy. We tried to simulate the χMT data with these spinHamiltonian parameters. It was found that the simulations of HFEPR for the two compounds were well consistent with the experimental data (see Figures S3(a) and S4(a)). In the absence of a static magnetic field, the two compounds were found to display fast magnetic relaxation (see Figures S9 and S10). Then alternating-current (ac) magnetic susceptibility was investigated under an applied static field. When a 500 Oe static dc field is applied, 1 displays temperature- and frequencydependent ac signals, indicating the existence of slow magnetic relaxation (see Figures S11 and S12). When even larger static dc fields such as 1500 and 3500 Oe are applied, χM″ peaks appear in the relatively high temperature and frequency ranges (see Figures 3 and S13 and S14). 2 displays a similar field-induced slow

that no close intermolecular exchange pathways exist in the solid state (see Figure S2). Dc magnetic susceptibility data were collected in the range 2− 300 K using a Quantum Design MPMS-XL SQUID magnetometer for crystalline powder samples of the two complexes, revealing the characteristics of noninteracting mononuclear cobalt(II) complexes (see Figures S3 and S4). Because of the considerable excited-state contribution of CoII ions, there still exists a large unquenched orbital momentum, which will result in a large offset of χMT values at the high-temperature range. So, the experimental data have been corrected by deducting the temperature-independent paramagnetism contribution. The χMT values at 300 K for 1 and 2 are 2.37 and 2.41 cm3 mol−1 K, corresponding to an S = 3/2 spin center with g = 2.25 and 2.27, respectively. Upon cooling, the χMT values of both complexes decrease slowly first but drop quickly below 10 K, reaching 1.85 and 1.64 cm3 mol−1 K at 2 K, because of depopulation of the crystal-field split sublevels of the CoII ions. The field-dependent magnetizations were measured at low temperatures in temperature-sweep mode for 1 and in field-sweep mode for 2, respectively (see Figure S5). The spin Hamiltonian (eq 1) was utilized to describe the magnetic anisotropy of the CoII center quantitatively. ⎡ 2 S(S + 1) ⎤ 2 2 H = D⎢Sẑ − ⎥ + E(Sx̂ − Sŷ ) + μB Bg ̿ S ̂ ⎣ ⎦ 3

(1)

where the first and second terms represent the axial and rhombic ZFS Hamiltonian, respectively, and the third term describes the Zeeman interaction, where g ̿ is the matrix form of the Landé factor. The magnetization data of the two complexes at low temperatures were fitted to eq 1 (see Figure S5). The best fits gave D = 2.67 cm−1, E = 0.01 cm−1, and giso = 2.28 for 1 and D = 5.20 cm−1, E = 0.02 cm−1, and giso = 2.26 for 2. Similar to the other reported tetrahedral cobalt(II) compounds,9 the first-order orbital momentum is largely quenched because of the tetrahedral symmetry (see Figure S6). The relatively small D and E values were mainly caused by the asymmetric population of the electronic configuration in the T2g orbital and the contribution of excited states. It is necessary to note that the E value obtained by fitting the magnetization data to the spin Hamiltonian is not always precise, and a reliable result can be viewed from the electron paramagnetic resonance (EPR) measurement.10 High-field/ high-frequency EPR (HFEPR) measurements on pressed pellet of powder samples of 1 and 2 were performed at 5 and 15 K with various frequencies around 300 GHz. At 230 GHz, both samples feature more than three transitions with respect to an Seff = 1/2 description (see Figures 2 and S7 and S8). This is a clear indication that the ZFS energy scale is close to the microwave

Figure 3. Temperature-dependent (a and b) and frequency-dependent (c and d) ac susceptibilities of 1 under a 3500 Oe dc field.

magnetic relaxation as well. When a 2000 Oe static dc field is applied, temperature-dependent ac signals appear, but no χM″ peak can be observed (see Figure S15), indicating that the slow relaxation mainly occurs in temperatures below 2 K. To determine the distribution of relaxation time, the Cole−Cole plots were fitted by the generalized Debye model (see Figures S16−S18 and Tables S2−S4). The semicircular curves at low temperatures imply that the relaxation time is very small and only one relaxation process is present. Because the slow magnetic relaxation is obvious in these weak easy-plane anisotropic compounds, hysteresis loops should be observed at low temperature. Micro-SQUID measurements at temperatures in the range of 0.03−1.2 K and field-sweep rates varying in the range of 0.04−0.28 T s−1 were performed on single crystals (see Figures 4 and S19). The magnetic field was aligned along the easy axes of the single crystals. The observed hysteresis loops are strongly temperature- and sweep-rate-dependent. At zero dc field, a closed hysteresis loop is caused by fast quantum

Figure 2. Experimental and simulated HFEPR spectra of (a) 1 and (b) 2 powder samples at 5 and 15 K. The simulation was carried out using the Easyspin toolbox for MATLAB. 698

DOI: 10.1021/acs.inorgchem.6b01972 Inorg. Chem. 2017, 56, 697−700

Communication

Inorganic Chemistry Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Natural Science Foundation of China (Grants 21302035 and 21371043), the Fundamental Research Funds for the Central Universities (Grant 2014HGCH0009), and Young Elite Scientist Sponsorship Program by China Association of Science and Technology (Grant 2015QNRC001).

Figure 4. Field dependence of the normalized magnetization of (a) 1 and (b) 2 in the temperature range of 0.03−1.2 K (field-sweep rate: 0.12 T s−1).

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tunneling of magnetization. It can be found that the fast groundstate tunneling around zero field at the lowest temperature of 0.03 K is more pronounced. Above 1.0 K, hysteresis loops cannot be observed for both complexes. To investigate the influence of intermolecular interactions on the magnetic relaxation, two magnetically dilute samples were prepared by cocrystallization with the isostructural compound [Ba(18-crown-6)]2+·[Zn(SCN)4]2− to yield [Ba(18-crown-6)]2+[CoxZny(SCN)4]2− (3, x = 0.05, y = 0.95; 4, x = 0.02, y = 0.98). In diluted samples 3 and 4, the spin density decreases, but the phonon density is still unaffected. Compared with 2, the loop bifurcations in the two diluted samples become larger after dilution (see Figures S20 and S21). The influence of the hysteresis curve by dilution indicates that both intermolecular interactions and the phonon bottleneck effect may play roles in this relaxation process. In conclusion, we report the observations of slow magnetic relaxation and hysteresis loops for two tetrahedral [Co(NCS)4]2− compounds. The weak easy-plane anisotropy is accurately determined by HFEPR spectroscopy. It is an excellent example that slow magnetic relaxation, and even magnetic hysteresis, can be observed in easy-plane anisotropic molecules. We would not attribute the present two molecules as SMMs or SIMs because they do not show uniaxial anisotropy and the typical double-well potential does not apply to these systems. The observed field-induced slow relaxation belongs to a typical spin−lattice relaxation process.6b,11 We believe our research will enrich and deepen the study on mononuclear complexes with easy-plane anisotropy featuring field-induced slow magnetic relaxation. A further theoretical attempt on the relaxation mechanism and its potential application as a spin qubit is underway.12

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01972. Experimental procedures, structural, magnetic, and HFEPR characterization, and ac magnetic and microSQUID measurements (PDF) CIF files of 1 and 2 (CCDC 1498514 and 1498515) (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.-Y.Z.). *E-mail: [email protected] (S.-D.J.). ORCID

Yuan-Yuan Zhu: 0000-0002-3142-0396 Shang-Da Jiang: 0000-0003-0204-9601 699

DOI: 10.1021/acs.inorgchem.6b01972 Inorg. Chem. 2017, 56, 697−700

Communication

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DOI: 10.1021/acs.inorgchem.6b01972 Inorg. Chem. 2017, 56, 697−700