Metamagnetism in Nanosheets of CoII-MOF with TN at 26 K and a

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Metamagnetism in Nanosheets of CoII-MOF with TN at 26 K and a Giant Hysteretic Effect at 5 K Kriti Gupta,† Arun Dadwal,‡ Shammi Rana,† Plawan Kumar Jha,† Anil Jain,§,⊥ S. M. Yusuf,§,⊥ Pattayil A. Joy,‡ and Nirmalya Ballav*,†

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Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pune 411008, India ‡ Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India § Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ⊥ Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India S Supporting Information *

instance, exfoliation of two-dimensional (2D) hematene nanosheets from three-dimensional (3D) natural iron ore hematite (α-Fe2O3) resulted in a remarkable change in the long-range magnetic order: from antiferromagnetic to ferromagnetic.17 Herein, we have synthesized 2D nanosheets of a CoII-MOF [Co2(OH)2BDC] revealing a metamagnetic transition at fields higher than HCr with a notable coercive field of ∼10 kOe at 5 K. Solutions of cobalt(II) chloride and 1,4-benzenedicarboxylic acid (H 2 BDC) were mixed to get a pink precipitate [Co2(OH)2BDC] upon the addition of triethylamine at room temperature, here named as Co-BDC (Figure 1a).15 Powder Xray diffraction (PXRD) pattern of the synthesized Co-BDC matched well with the simulated pattern, extracted from the literature.15 A reduced crystallinity could be due to random

ABSTRACT: Herein, we have synthesized at roomtemperature two-dimensional nanosheets of a MOF comprised of cobalt(II) ion with benzenedicarboxylic acid ligand, which exhibited unusual magnetic properties. Direct-current magnetic susceptibility revealed an antiferromagnetic (AFM) transition at 26 K (Néel temperature, TN) followed by a canting of the spin moments along with the concomitant appearance of a sigmoidalshaped magnetization versus field (M−H) curve at 15 K. Such a canted AFM ordering led to nonzero remnant magnetization with a remarkably high coercive field of ∼10 kOe at 5 K. Metamagnetism was further substantiated by the alternating-current magnetic susceptibility measurements.

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etal−organic frameworks (MOFs) comprised of metal ions and organic ligands are celebrated materials for their numerous application promises such as the separation and storage of gases and liquids, catalysis, sensing, and recent emergence in the domain of energy research.1−5 A combination of well-defined metal−ligand coordination in three dimensions and porosity permitted long-range structural order in MOFs.6 However, cooperativity of electron spins is, most of the time, inadequate to bring long-range magnetic order to the MOFs. Recent developments suggest that the incorporation of specific organic ligands along with spin-bearing metal ions could strongly modulate the magnetic properties in MOFs covering antiferromagnetism, canted antiferromagnetism, ferrimagnetism, and ferromagnetism.7−10 Tuning of the magnetic property upon application of an external magnetic field (critical magnetic field, HCr), the so-called metamagnetism,11 is usually observed in molecular magnets12 but is rare in the class of MOFs .13 In metamagnetism, it is the competition between the applied external magnetic field trying to align the spins and the reluctant magnetocrystalline anisotropy, which is known to be high for cobalt(II) among the transition-metal ions, bringing a metamagnetic phase and justifying the presence of cobalt(II) in most of the metamagnetic MOFs (Table S1).13,14 Apart from the chemical structure, the physical dimensions of the material can also affect the long-range magnetic order.16 For © XXXX American Chemical Society

Figure 1. (a) Scheme illustrating the Co-BDC synthesis at room temperature, with Co-BDC powder. (b) Crystal structure of Co-BDC along the crystallographic b axis (color code: purple, cobalt; red, oxygen; gray, carbon; white, hydrogen).15 (c) Crystal structure of CoBDC visualizing the two different cobalt(II) octahedral environments shaded in yellow and blue. Received: October 30, 2018

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DOI: 10.1021/acs.inorgchem.8b03064 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

XPS spectrum confirmed the presence of cobalt(II) in Co-BDC (Figure 2d).24,25 The high-spin state of cobalt(II) (S = 3/2) in Co-BDC was realized by the appearance of a characteristic satellite feature in the Co 2p XPS spectrum (Figure 2d).26 N 1s and the survey XPS spectrum confirmed the absence of any impurity from the base and solvent that could potentially affect the magnetic ordering of the framework (Figure S5b,c).27 The Co 2p XPS spectrum also excludes the possibility of the presence of cobalt(0) and complements the PXRD pattern, whereby characteristic diffraction peaks for metallic cobalt were absent (Figure S1d).28,29 Three distinct magnetically ordered states could be identified in Co-BDC: paramagnetic, antiferromagnetic, and canted antiferromagnetic by the dc magnetic susceptibility measurements in the field-cooled (FC) and zero-field-cooled (ZFC) modes (Figure 3b). The ZFC plot could be fitted by the Curie−

orientations and disordered stacking of layers of Co-BDC, an observation similar to that reported earlier (Figure S1a).15 The linkage of cobalt polyhedral layers with BDC linkers can be clearly visualized from the crystallographic b axis, where the distance between adjacent cobalt ions in a single layer lies in the range of 2.3−2.5 Å and 9−10 Å in adjacent layers linked through BDC (Figures 1b,c and S2). The Tyndall effect was captured in the dispersion of Co-BDC powder in methanol (Figure 2a, inset). A field-emission

Figure 2. (a-c) FESEM image (inset: capturing the Tyndall effect) (a), TEM image (b), and AFM image (c) of Co-BDC nanosheets. (d) Co 2p XPS spectrum of Co-BDC. The raw data points are fitted (green and pink lines). The satellite feature is highlighted with gray shading.

scanning electron microscopy (FESEM) image revealed nanosheet formation of Co-BDC, which was also supported by transmission electron microscopy (TEM; Figure 2a,b). Additionally, an atomic force microscopy (AFM) image showed the presence of stacked 2D nanosheets of Co-BDC (Figure 2c). The Brunauer−Emmett−Teller surface area of Co-BDC was estimated to be ∼52.8 m2·g−1. Also, the adsorption−desorption profiles of N2 gas at 77 K were found to be reversible without any hysteresis, which suggested that there was no emergence of open-metal sites upon thermal activation (Figure S3). The synthesized Co-BDC was observed to be stable at high temperature (Figure S1b,c), unlike similar MOFs undergoing structural phase transitions due to partial/complete solvent loss, which could alter the overall magnetic response of the MOF.18 The electrical conductivity value of Co-BDC in a pressed pellet was found to be ∼10−8 S·cm−1 through both alternating-current (ac) and direct-current (dc) conductivity measurements, and such a value is similar to those reported for various carboxylatebased MOFs (Figure S4).19−21 X-ray photoelectron spectroscopy (XPS) was used and the C 1s XPS spectrum showed two peaks at binding energies of 284.7 and 288.4 eV corresponding to the carbon atoms of the benzene ring in BDC and of the carboxylate (COO − ) ions, respectively22,23 (Figure S5a). The almost exclusive presence of carboxylate (COO−) ions was also evidenced from the Fourier transform infrared (FTIR) spectrum of Co-BDC, exhibiting characteristic peaks of νas(COO−) and νs(COO−) at 1585 and 1365 cm−1, respectively (Figure S6). The characteristic 2p3/2 peak at ∼781.1 eV (and the 2p1/2 peak at ∼796.3 eV with a spin−orbit coupling of ∼15.2 eV) in the Co 2p

Figure 3. (a) χ and χ−1 versus temperature plots, measured at 1000 Oe and fitted with the Curie−Weiss equation. (b) ZFC plot at 1000 Oe and FC plots at different fields from 100 to 10000 Oe. C

Weiss equation χ = T − θ , where the Curie constant and paramagnetic Curie temperature were found to be C = 6.18 emu· Oe−1·K per formula unit [two cobalt(II) ions] and θ = −50.3 K, respectively (Figure 3a). The effective magnetic moment per cobalt(II) was estimated as ∼4.97 μB, which is higher than the spin-only value of ∼3.87 μB for the cobalt(II) (high-spin) ion and could be assigned to a significant contribution of the orbital moment.30 A large negative θ value suggested an overall antiferromagnetic interaction among the cobalt(II) ions. The ZFC curve at 1000 Oe manifested a peak at 26 K (TN), indicating antiferromagnetic order, followed by a halt at 15 K B

DOI: 10.1021/acs.inorgchem.8b03064 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

cobalt(II) that can compete with the external applied magnetic field and results in canting of the spin moments. This anisotropy, along with strong inter- and intralayer magnetic interactions mediated via bridged ligands, could open the loop in the M−H plot of Co-BDC nanosheets. The transition at 26 K may not be like classical antiferromagnets because of the presence of shortrange magnetic ordering, as reflected by a sharp increase in the magnetization curve below 50 K (indicated by the arrow in Figure 3a,b) as well as the appearance of an S-shaped M−H curve at 30 K (Figure S9). Figure 4b depicts the temperature variation of the ac magnetic susceptibility (χac′) at different frequencies under zero applied dc magnetic field, where a frequency-independent peak was observed. The peak is centered at 27 K, corroborating the inflection point in the dc magnetic susceptibility at 26 K. Thus, combined ac and dc susceptibility measurements confirmed the existence of antiferromagnetic coupling in Co-BDC (Figure 4b). The shoulder at 15 K suggests canting of the spin moments and/ or ferromagnetic-like interactions. In summary, we have demonstrated metamagnetism in nanosheets of a CoII-MOF synthesized at room temperature. Three distinctive magnetic states were identified in Co-BDC: paramagnetic (26 K < T < 300 K) with short-range ordering below 50 K, antiferromagnetic (15 K < T < 26 K), and canted antiferromagnetic (5 K < T < 15 K) with giant hysteresis. Notably, both the Néel temperature (26 K) and value of the coercive field (∼10 kOe) surpasses those available in the literature for metamagnetic MOFs and coordination polymers/ networks.

and a subsequent fall in the magnetization to 5 K (Figure 3). Such a feature in the magnetic susceptibility plot of Co-BDC nanosheets is similar to that reported for other bridged homospin compounds, reflecting a transition from antiferromagnetic to spin-canted antiferromagnetic order at lower temperature.31 However, with increasing applied external magnetic field, peak maxima gradually shift to lower temperatures, as shown in the H−T phase diagram (Figure S7). At sufficiently higher magnetic fields (>HCr), spins were getting aligned to the external magnetic field by overcoming the antiferromagnetic interaction. Thus, manifestation of a single peak in the magnetic susceptibility plot of our Co-BDC nanosheets is characteristic of a field-induced metamagnetic transition (Figure 3b). The metamagnetic critical field (HCr) was realized to be ∼3500 Oe from the first derivative of the M− H plot (Figure S8). In light of our present study, the salient features of various metamagnetic MOFs and coordination polymers/networks are summarized in Table S1. Field-dependent magnetization (M−H) was recorded for CoBDC at various temperatures (Figures 4a and S9). At 300 K, a

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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.8b03064. Experimental section and additional data on the characterization of Co-BDC, PXRD and high-temperature PXRD, gas adsorption−desorption, ac and dc conductivity, XPS, FTIR, magnetic data, and a table of metamagnetic MOFs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kriti Gupta: 0000-0003-1707-8237 Pattayil A. Joy: 0000-0003-2125-0976 Nirmalya Ballav: 0000-0002-7916-7334

Figure 4. (a) M−H plots at 300 K, 15 K and 5K . (b) Real part of the ac magnetic susceptibility (χac′) measured at 98, 987, 1987, and 2987 Hz (shoulder at 15 K highlighted in gray).

Notes

The authors declare no competing financial interest.

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typical M−H curve corresponding to paramagnetic behavior was observed. At 15 K, the M−H curve adopted a sigmoidal shape with steplike features characteristic of a metamagnet.12 The M− H loop at 5 K exhibited a remarkably large coercive field (HC) of ∼10 kOe, which is a record value as far as metamagnetic MOFs are concerned (Table S1). Upon lowering of the temperature below 15 K, it was the magnetocrystalline anisotropy of

ACKNOWLEDGMENTS Financial support from SERB (Grant EMR/2016/001404), MHRD-FAST (India, Project CORESUM), and IISER Pune is thankfully acknowledged. K.G., S.R., and P.K.J. thank IISER Pune for fellowships. N.B. thanks Dr. Vivek K. Malik, IIT Roorkee, for support in magnetic measurements. C

DOI: 10.1021/acs.inorgchem.8b03064 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03064 Inorg. Chem. XXXX, XXX, XXX−XXX