Trabajo presentado al "The 23rd IIR International Congress of Refrigeration" celebrado del 21 al 26 de Agosto del 2011 en Praga.-- Dedicated to Alfred Werner on the 100th Anniversary of his Nobel prize in Chemistry in 1913.We study the magnetothermal properties of magnetically isotropic high-spin molecular nanomagnets containing 17 Fe3+ ions per molecule linked via oxide and hydroxide ions, packed in a crystallographic cubic symmetry. Low-temperature magnetization and heat capacity experiments reveal that each molecular unit carries a net spin ground state as large as S = 35/2 and a magnetic anisotropy as small as D = −0.023 K, while no magnetic order, purely driven by dipolar interactions, is to be expected down to very-low temperatures. These characteristics suggest that the Fe17 molecular nanomagnet can potentially be employed as a sub-Kelvin magnetic refrigerant.This work has been partially supported by Spanish MINECO through grants MAT2009-13977-C03 and PIE201060I012, the EPSRC and The Leverhulme Trust (UK).Peer Reviewe

Brechin, Euan K.

Evangelisti, Marco

Gass, Ian A.

We study the magnetothermal properties of magnetically isotropic high-spin molecular nanomagnets containing 17 Fe3+ ions per molecule linked via oxide and hydroxide ions, packed in a crystallographic cubic symmetry. Low-temperature magnetization and heat capacity experiments reveal that each molecular unit carries a net spin ground state as large as S = 35/2 and a magnetic anisotropy as small as D = -0.023 K, while no magnetic order, purely driven by dipolar interactions, is to be expected down to very-low temperatures. These characteristics suggest that the Fe-17 molecular nanomagnet can potentially be employed as a sub-Kelvin magnetic refrigerant.</p

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     Edinburgh Research Explorer                                      Cryogenic magnetocaloric effect in the Fe17 molecularnanomagnetCitation for published version:Gass, IA, Brechin, EK & Evangelisti, M 2013, 'Cryogenic magnetocaloric effect in the Fe17 molecularnanomagnet' Polyhedron, vol 52, pp. 1177-1180. DOI: 10.1016/j.poly.2012.06.049Digital Object Identifier (DOI):10.1016/j.poly.2012.06.049Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Peer reviewed versionPublished In:PolyhedronPublisher Rights Statement:Copyright © 2012 Elsevier Ltd. All right reserved.General rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 28. Apr. 2017 Cryogenic magnetocaloric effect in the Fe17 molecular nanomagnet** Ian A. Gass,1 Euan K. Brechin,1 Marco Evangelisti2,*  [1]EaStCHEM, School of Chemistry, Joseph Black Building, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK. [2]Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, Departamento de Física de la Materia Condensada, 50009 Zaragoza, Spain. [*]Corresponding author; e-mail: evange@unizar.es, WWW address: http://molchip.unizar.es/ [**]This work has been partially supported by the Spanish Ministry for for Economy and Competitiveness through grants MAT2009-13977 and CSD2007-00010 and by the EPSRC and The Leverhulme Trust (UK). Graphical abstract:   Keywords: iron; magnetite; magnetocaloric effect; magnetic refrigeration   This is the peer-reviewed author’s version of a work that was accepted for publication in Polyhedron. Changes resulting from the publishing process, such as editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version is available at: http://dx.doi.org/10.1016/j.poly.2012.06.049  Cite as: Gass, I. A., Brechin, E. K., & Evangelisti, M. (2013). Cryogenic magnetocaloric effect in the Fe17 molecular nanomagnet. Polyhedron, 52, 1177-1180.  Article published: 28/06/2012 Page 1 of 8 Abstract We study the magnetothermal properties of a magnetically isotropic high-spin molecular nanomagnet containing 17 Fe3+ ions per molecule linked via oxide and hydroxide ions. Low-temperature magnetization and heat capacity experiments reveal that each molecular unit carries a net spin ground state as large as S = 35/2 and a magnetic anisotropy as small as D = 0.023 K. These characteristics result in an increase of the magnetic entropy change when the material is cooled down to 1020 K, reaching a maximum at 23 K, suggesting that the Fe17 molecular nanomagnet can potentially be employed as a low-temperature magnetic refrigerant.  1. Introduction The topic of magnetic refrigeration constitutes one of the most promising applications envisioned for molecule-based materials, specifically molecular nanomagnets[1]. The refrigeration process is based on the magnetocaloric effect (MCE), i.e. the change of the magnetic entropy and related adiabatic temperature upon the change of an applied magnetic field. All magnetic materials intrinsically show MCE, although the intensity of the effect depends on the properties of each material. Besides the fundamental interest on related magnetic and magnetothermal properties of novel materials, MCE is of great technological importance since it can be used for cooling applications[2] according to a process known as adiabatic demagnetization[3]. This technique is particularly promising for refrigeration at very low temperature, beyond the reach of liquid helium-4, providing, e.g., a valid alternative to the use of helium-3 which is quickly becoming rare and expensive. In molecular nanomagnets, a net magnetic moment (spin) can be defined for each individual molecule as a result of dominant intramolecular magnetic interactions. If one targets a large MCE, it is easy to demonstrate that the molecular nanomagnet should have a high spin state, in addition to a minimal anisotropy[4-6]. This is because: (i) the higher the spin value the larger the density of spin levels and thus the larger the magnetic entropy content; (ii) a negligible anisotropy permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength. These two pre-requisites therefore dictate the synthetic strategy for obtaining the molecular nanomagnets that can be potentially exploited for magnetic refrigeration at the molecular level. The synthesis of the Fe17 molecular nanomagnet, containing 17 Fe3+ ions per molecule linked via oxygen atoms derived from oxide and hydroxide ions (Fig. 1) was reported by some of us[7]. Magnetic studies on this and related molecules belonging to the same family, have shown that the spin ground state is as large as S = 35/2, whilst the anisotropy D is uniaxial, although extremely small with typical values of the order of 10-2 K[8-10]. It seems therefore logical to investigate further the title compound in order to ascertain whether Page 2 of 8 this material represents the excellent candidate for magnetic refrigeration as its properties seem to promise. Herein, we present our first MCE study of the Fe17 molecular nanomagnet.   Figure 1. The molecular structure of [Fe17O16(OH)12(pyr)12Br4]Br3 (1); colour scheme, Fe = yellow, O = red, N = blue; Br = green, C = grey. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)  2. Experimental methods In order to prepare the samples, the synthesis is simple – dissolution of anhydrous FeBr3 in a coordinating base, e.g., pyridine (pyr) with stirring leads to a dark red solution which, after approximately one hour, is then filtered and allowed to evaporate. Dark red crystals [Fe17O16(OH)12(pyr)12Br4]Br3 (hereafter denoted as 1) form within three days. The yield can be improved by adding a co-solvent of crystallization such as isopropylalcohol, resulting in a crystallographic arrangement of Fe17 molecules with the cubic space group symmetry Pa-3 with a = b = c = 29.285(3) Å[8]. The yield is approximately 30%. Complex 1 contains a central tetrahedral Fe3+ ion linked via 4-oxo bridges to 12 outer octahedral Fe3+ ions – forming a truncated tetrahedron (Fig. 1). These ions are linked to each other via a combination of 3- and 4-oxides, and 2-hydroxide ligands. The hexagonal faces of the truncated tetrahedron are capped by four further Fe3+ ions each linked via three 3-oxo ligands. The inner Fe3+ ion and the four outer Fe3+ ions sit in the tetrahedral Page 3 of 8 sites of the ‘lattice’, with the others occupying the octahedral sites. The four bromide ions cap the outer tetrahedral Fe3+ ions with the pyr molecules capping the octahedral Fe3+ ions. The Fe–O–Fe bridges fall into two clear categories[7]: those that connect the tetrahedral Fe3+ ions to the octahedral Fe3+ ions are all characterized by angles in the range ~121–127°, whilst those that bridge solely between octahedral Fe3+ ions are characterized by angles in the range ~93–100°. The oxidation states of both Fe and O centres were confirmed by bond length and charge balance considerations, and bond-valence-sum calculations[7].  Measurements of magnetization down to 2 K and heat capacity down to ~0.3 K were carried out for the 0 < B0 < 7 T magnetic field range. Since all experiments were performed on powder samples, the calculated fits were obtained taking into account spin random orientations.  0 50 100 150 200 250 300120130140150160170180  mT / cm3 K mol-1T / KB0 = 0.1 T Figure 2. Experimental temperature (T) dependence of the χMT product, where χM is the molar susceptibility, collected for an applied field B0 = 0.1 T.  3. Magnetic and magnetothermal properties  Variable-temperature magnetic susceptibility data were collected on 1 in the temperature range 3005 K for an applied field of 0.1 T (Fig. 2). The room-temperature χMT value of approximately 120 cm3 K mol-1 rises constantly as temperature is decreased to a maximum value of approximately 180 cm3 K mol-1 at 5 K. The spin-only (g = 2.0) value for an uncoupled [Fe3+17] unit is approximately Page 4 of 8 74 cm3 K mol-1. This behaviour is indicative of dominant antiferromagnetic exchange between the metal centres with the low-temperature (5 K) maximum indicating an S ≈ 35/2 spin ground state.  In order to determine the spin ground state for 1, magnetisation data were collected in the ranges 0 – 7 T and 2 – 20 K and these are plotted in Figure 3. It can be seen that saturation occurs at a value of approximately 36 B, in agreement with the spin ground state estimated above on basis of the susceptibility experiment. For a precise determination, we fitted the magnetization data by a matrix-diagonalization method to a model that includes the Zeeman term and axial zero-field splitting. Although smaller anisotropy components could be present, the data do not justify a more sophisticated fitting. The corresponding Hamiltonian is given by:                                                                           (1) The best fit gave S = 35/2, g = 2.06, and D = 0.023 K. The ground state can be rationalized by assuming an antiferromagnetic interaction between the tetrahedral and octahedral Fe3+ sites – consistent with the two distinct categories of Fe-O-Fe bridging angles present in the complex[7].  0 1 2 3 4 5 6 70102030 experimental magnetization calculated magnetizationT = 20 KT = 5 KT = 2 K  M / NBB0 / T Figure 3. Isothermal molecular magnetization of 1 collected for T = 2, 5 and 20 K. Solid lines are the results of the fit (see text), yielding net molecular spin S = 35/2 and axial D = 0.023 K.  The magnetothermal properties of 1 were studied by means of heat capacity C(T,B0) experiments. Figure 4 shows the collected C(T,B0) data as a function of temperature for several applied fields. Because of the small anisotropy (D = 0.023 K), it is expected that the magnetic contribution to ,2SBgDSH Bz Page 5 of 8 C(T,B0) for B0 ≥ 1 T is due to Schottky-like Zeeman splitting of the otherwise nearly degenerate energy spin states[11]. Indeed, the calculated Schottky curves (solid lines in Fig. 4) arising from the field-split levels, and assuming g = 2.0, account very well for the experimental data. From the zero-field heat capacity, we further notice that no onset of phase transition exists, at least down to the minimum temperature of ~0.3 K, suggesting that the Fe17 molecules are magnetically isolated from each other, as also suggested by the large intermolecular distances[7]. Not even the unavoidable magnetic dipolar intermolecular interactions play any role in this temperature range, since the cubic crystal symmetry essentially cancels them out[8]. We estimate the lattice contribution (dashed line in Fig. 4) by fitting to a model given by the sum of a Debye term for the acoustic low-energy phonon modes plus an Einstein term that likely arises from intramolecular vibrational modes. The best fit provides the values of D23 K and  for the Debye and Einstein temperatures, respectively. The so-obtained lattice contribution allows us to estimate the magnetic entropy as a function of temperature by using the relation:     ,d)/()(/)(0 Tm TRTTCRTS        (2) where Cm(T) is the magnetic contribution obtained from C(T) after subtraction of the respective lattice contribution. The so-obtained S(T) is depicted in the inset of Figure 4 for several applied field changes. It can be seen that total entropy content tends to the expected value for a fully occupied spin state S = 35/2, i.e. Rln(2S+1)3.6.  0 10 20 30012341 1010-210-1100101 experimental heat capacity Schottky latticeC / RT / KB0 = 0B 0 = 1 TB 0 = 3 TB 0 = 5 TB 0 = 7 T7 T5 T3 T1 T  entropy / RT / KB0 = 0  ← Figure 4. Temperature-dependence of the heat capacity of 1 normalized to the gas constant R for several applied fields, as labelled. Solid curves are explained in the main text. Inset: Temperature-dependence of the magnetic entropy normalized to the gas constant R for several applied fields, as obtained from the heat capacity data. Page 6 of 8 We next evaluate the MCE, specifically the magnetic entropy change Sm(T,B0), which can straightforwardly be estimated from the entropy curves plotted in the inset of Figure 4. Figure 5 shows that 1 has the maximum Sm(T,B0) of 8.9 J kg-1 K-1 at T = 2.7 K for the applied-field change B0 = (7 – 0) T. This value for the entropy change is equivalent to ~3.3 R, which is not too distant from the total available entropy of the system (3.6 R associated with the spin ground state S = 35/2, see above), to achieve which an applied field change B0 of about 73.6/3.37.6 T should likely be needed. We finally notice that the very large value of the spin ground state has also a further result which is that of promoting relatively large applied-field splittings of the S = 35/2 multiplet. This implies that the entropy change maintains its relatively large value over a wide temperature range, for instance for B0 = (7 – 0) T, Sm reaches the half value of its maximum only at T24.5 K, i.e. a temperature of more than 9 times larger than that at which the maximum is observed (Figure 5). 0 5 10 15 20 25 300246810  Sm  / J kg-1 K-1T / K B0 = (1  0) T B0 = (3  0) T B0 = (5  0) T B0 = (7  0) T Figure 5. Temperature-dependence of the magnetic entropy change Sm of 1, as obtained from heat capacity data (Fig. 4) for the indicated applied-field changes B0.  4. Conclusions In conclusion, a polynuclear Fe3+-based molecular complex has been studied for its potential application in magnetic refrigeration for very low temperatures. The combination of high spin and low magnetic anisotropy was the main attraction of this material for the determination of its magnetocaloric effect, which we have evaluated from heat capacity experiments. We have observed an increase of the magnetic entropy change when the material is cooled down to 1020 K, below which Sm reaches a maximum at 23 K, suggesting this to be the starting temperature for an adiabatic demagnetization process. This temperature range is of considerable technological interest because it is easily reachable by pumping liquid helium-4.  Page 7 of 8 References [1] For recent overviews, see, (a) M. Evangelisti, E. K. Brechin, Dalton Trans. 39 (2010) 4672, and references therein; (b) R. Sessoli, Angew. Chem. Int.-Ed. 51 (2012) 43, and references therein. [2] (a) C. Zimm, A. Jastrab, A. Sternberg, V. K. Pecharsky, K. A. Gschneidner Jr., M. Osborne, I. Anderson, Adv. Cryog. Eng. 43 (1998) 1759; (b) V. K. Pecharsky, K. A. Gschneidner Jr., J. Magn. Magn. Mater. 200 (1999) 44; (c) K. A. Gschneidner Jr., A. O. Pecharsky, V. K. Pecharsky, in R. S. Ross Jr. (Ed.), Cryocoolers 11, Kluwer Academic/Plenum Press, New York, 2001. [3] (a) P. Debye, Ann. Phys. 81 (1926) 1154; (b) W. F. Giauque, J. Am. Chem. Soc. 49 (1927) 1864. [4] Yu. I. Spichkin, A. K. Zvezdin, S. P. Gubin, A. S. Mischenko, A. M. Tishin, J. Phys. D: Appl. Phys. 34 (2001) 1162. [5] M. Evangelisti, A. Candini, A. Ghirri, M. Affronte, E. K. Brechin, E. J. L. McInnes, Appl. Phys. Lett. 87 (2005) 072504. [6] M. Manoli, R. D. L. Johnstone, S. Parsons, M. Murrie, M. Affronte, M. Evangelisti, E. K. Brechin, Angew. Chem. Int.-Ed. 46 (2007) 4456. [7] G. W. Powell, H. N. Lancashire, E. K. Brechin, D. Collison, S. L. Heath, T. Mallah, W. Wernsdorfer, Angew. Chem. Int.-Ed. 43 (2004) 5772. [8] M. Evangelisti, A. Candini, A. Ghirri, M. Affronte, G. W. Powell, I. A. Gass, P. A. Wood, S. Parsons, E. K. Brechin, D. Collison, S. L. Heath, Phys. Rev. Lett. 97 (2006) 167202. [9] I. A. Gass, C. J. Milios, M. Evangelisti, S. L. Heath, D. Collison, S. Parsons, E. K. Brechin, Polyhedron 26 (2007) 1835. [10] C. Vecchini, D. H. Ryan, L. M. D. Cranswick, M. Evangelisti, W. Kockelmann, P. G. Radaelli, A. Candini, M. Affronte, I. A. Gass, E. K. Brechin, O. Moze, Phys. Rev. B 77 (2008) 224403. [11] See, e.g., M. Evangelisti, F. Luis, L. J. de Jongh, M. Affronte, J. Mater. Chem. 16 (2006) 2534. 

Cryogenic magnetocaloric effect in the Fe17 molecular nanomagnet

We study the magnetothermal properties of magnetically isotropic high-spin molecular nanomagnets containing 17 Fe 3+ ions per molecule linked via oxide and hydroxide ions, packed in a crystallographic cubic symmetry. Low-temperature magnetization and heat capacity experiments reveal that each molecular unit carries a net spin ground state as large as S = 35/2 and a magnetic anisotropy as small as D = 0.023 K, while no magnetic order, purely driven by dipolar interactions, is to be expected down to very-low temperatures. These characteristics suggest that the Fe17 molecular nanomagnet can potentially be employed as a sub-Kelvin magnetic refrigerant

Cryogenic magnetocaloric effect in the Fe17 molecular nanomagnet

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