PI 2 -ICMA. Título del proyecto: Understanding the magnetic anisotropy in Single- Molecule-Magnets for future molecular spintronic applications

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PI 2 -ICMA Título del proyecto: Understanding the magnetic anisotropy in Single- Molecule-Magnets for future molecular spintronic applications Nombre del supervisor: Javier Campo / Javier Luzón

1 PI 2 -ICMA Título del proyecto: Understanding the magnetic anisotropy in Single- Molecule-Magnets for future molecular spintronic applications Nombre del supervisor: Javier Campo / Javier Luzón Memoria científica BACKGROUND AND CURRENT STATUS Single Molecule Magnets. Magnetic materials play an important role in current information technology. The storage density of magnetic hard drives doubles every year: for instance, recently, TDK has announced a new technology that will push this storage density beyond 1.5 Tb per square inch. However, the miniaturization of magnetic recording devices, which store information in magnetic grains or bits, was supposed to be constrained by the so called superparamagnetic limit: when grains are too small, thermal fluctuations can easily flip the direction of magnetization in each bit, causing permanent loss of information. Therefore, the discovery that a cluster of manganese ions, Mn 12 acetate retains its magnetization [1] (first example of Single Molecule Magnet, SMM) in the absence of magnetic field has added a new dimension to research in magnetic materials, because it established that discrete molecules could act as nano magnets [2]. These materials, SMMs, are based on molecules whose two lowlying electronic states, with opposing magnetic moments, are separated by an energy barrier. This barrier gives rise to a slow interchange between the two states, which causes the molecules to behave as nano magnets, with potential applications in areas including spintronic, nano scale memory, quantum computing [3] or in magnetic refrigeration, among others [4]. Because of that, SMMs are the kind of materials in which the Seventh Framework Program of the European Union has placed a great emphasis with the research theme NMP ('Nanosciences, nanotechnologies, materials and new production technologies): New advanced materials based on increased knowledge and experience with matter at the nano scale, with new functionalities and improved performance. The design of these materials requires a deep understanding of their fundamental properties in order to improve and optimize their functionalities and applications. It is in this context that we frame the content and global objective of this project. The main aim in the research of SMMs is the quest for molecules with higher energy barriers since they determine the blocking temperatures bellow which the SMMs behave as nano magnets. All the first examples of SMMs were 3d transition metal clusters in which the energy barrier is given by DS 2, where D is an uniaxial anisotropy coefficient and S the total spin. Despite the success in increasing the total spin in the previous expression up to S=83/2[5] in a Mn 19 complex, the highest blocking temperature for a 3d transition metal cluster has remained of only 4.5 K [6].

2 Indeed, nowadays, it is clear that, due to the correlation between the single ion magnetic anisotropy and the magnetic interactions, an increase in the total spin also involves a decrease in D making really hard to produce a significant improvement in the energy barrier. Because of that, in the last decade other approaches have appeared in order to go beyond the blocking temperatures of the 3d transition metal based SMMs: 1 D and 2 D SMMs arrangements. One of these approaches is to increase the dimensionality of the magnetic systems by arranging the SMMs in 1 D or 2 D magnetic structures in which the energy barrier is enhanced by the collective magnetic behavior. In particular, there is a great interest in the 1 D systems, the socalled Single Chain Magnets (SCM) [7], both from the point of view of fundamental physics and for their potential applications, for example as nanowires in information storage [8]. Because of their capacity of generating extended magnetic networks a promising family of compounds for this research line are the so called cubane clusters, in which four metal atoms occupy alternate vertices of a slightly distorted cube, whose other four vertices are occupied by oxygen atoms. The first cobalt containing cubanes to be identified as SMM were [Co 4 (hmp) 4 (MeOH) 4 Cl 4 ][9] and [(NMe 4 ) 3 Na{Co 4 (cit) 4 [Co(H2O) 5 ] 2 }] 11H 2 O[10] In addition, a threedimensional compound formed by cobalt citrate cubanes [Co 4 (citr) 4 ], and behaving as a SMM, has been described recently [11]. Our group has also reported results obtained from a symmetrical 2 D array of cobalt cubane SMM[12]. The special feature of this SMM is that the square net of cubanes is interlaced with a square net of bridging Co(II) centers, which have their own magnetic response. This two dimensional net, which is anionic, opens a broad array of new possibilities for studying collective magnetic behavior, since it can be crystallized with an essentially unlimited number of cations to give different crystalline environments. Lanthanide based SMMs. Another approach for increasing the blocking temperatures of SMMs is the use of lanthanide ions since their usual large magnetic anisotropy and large magnetic moments, the two main ingredients in order to produce a large magnetic energy barrier. These are the same reasons because lanthanides are widely used in magnet technology. Some examples of this are provided by the SmCo 5 and Nd 2 Fe 14 B permanent magnets, which have found large market use in the past few years. After the discovering of the first lanthanide molecule behaving as a SMM[13] a few number of mononuclear, oligonuclear and polynuclear lanthanide based clusters showing slow relaxation of the magnetization have been reported, in some cases with energy barriers much higher than in 3d based SMMs, indicating very promising possibilities for this research line. The observation of slow relaxation even in mononuclear lanthanides complexes indicates that the relaxation mechanism is different from that of the transition metal cluster SMMs. However, due to the complexity of the energy level structure of lanthanide ions, it is still not clear the factors which control the magnetic energy barrier and the mechanism of the magnetic relaxation. Molecular spintronics Once the operating temperature of SMMs will reach the room temperature order, one of the most promising applications of SMMs would be in molecular nano spintronic [14], especially in the search for molecular magnetic qubits [15], since transport experiments have revealed a strong interaction between the current and the magnetic state of the molecules. To date, all of the

3 studies on molecular spintronic have focused on systems whose magnetism is only weakly anisotropic, in which the magnetization is aligned along a unique axis of anisotropy [16]. However, there is now more interest in a non collinear regime, in which the magnetic anisotropy of the ion is dominant [17]. In this regime the magnetic molecules can be prepared in degenerate states characterized by non dipolar magnetic moments, like that predicted and found for molecular magnetic wheels [18]. There are two arguments for promoting interest in the spintronic of noncollinear magnets. The first arises from studies of spin transport across mesoscopic rings with non collinear internal magnetic fields, which have predicted spin switching effects [19]. The second argument is related to the use of non collinear states to implement molecular qubits. GOAL OF THE PROJECT As can be deduced, understanding and controlling magnetic anisotropy at the atomic (single ion) level, as well as at the molecular level (clusters), or even at a more extended level (1 D or 2 D networks), will become a basic prerequisite for designing SMMs for spintronic devices. Thus, in this project we will focus on fundamental aspects of magnetic anisotropy in molecular magnets, both theoretical and experimental points of view, whose control will eventually be a necessary prerequisite for the development of molecular spintronic devices METHODOLOGY Magnetic anisotropy is one of the main factors establishing the properties in SMMs. Whereas in most transition metal clusters the magnetic anisotropy can be modeled with only a few spin Hamiltonian parameters, in Co(II) clusters and lanthanide molecules the magnetic anisotropy is not so simple since the large spin orbit coupling effect produces a complex electronic structure. But, precisely it is the large spin orbit coupling effect what makes these compounds so interesting in the search of SMM with improved properties. Because of that, we propose in this project the study of the magnetic anisotropy in such compounds by combining relativistic ab initio calculations with neutron techniques, which, in our opinion, can be complementary: on one hand, relativistic ab initio methods are promising in the determination of the electronic structure of systems in which the spin orbit coupling effect is important [20] but, due to its novelty, calculations must be confronted with experimental data, on the other hand, neutron techniques are very powerful tools for the study of the energy level structure (inelastic scattering) and of the magnetic anisotropy (e.g. with polarized neutrons to measure susceptibility tensors). In this research project we propose to study of the magnetic anisotropy of two types of magnetic molecular materials related with SMMs and their application in molecular spintronic, including the different types of materials commented in the introduction: 2 D SMMs networks, lanthanidebased SMMs and molecules with non collinear magnetic moments. 1. Internal spin ordering and energy levels in new Co 4 and Co 2 Ln 2 SMM As already mentioned in the introduction, our group is working in the study of the collective magnetic behaviour effect on the SMM in networks of cobalt cubanes. In this task we propose a study of the ordering of the magnetic centers within the molecules, and of their magnetic interactions, by means of polarized neutron diffraction, inelastic neutron scattering and relativistic ab initio calculations. The polarized neutron diffraction study permits the accurate location of the spin density as well as the magnetic susceptibility tensors of the independent magnetic centers whereas inelastic neutron scattering data yield information on the molecular

4 energy levels. This information, in combination with relativistic ab initio calculations, would allow determining the principal parameters of the molecular spin Hamiltonian. From this spin Hamiltonian the analysis/fit of both static and dynamic behavior would be overtaken through theoretical modelization. New batches of all the compounds required for this task will be synthesized in the laboratory of Dr. M. Tomás of the Materials Science Institute of Aragón (ICMA). Dr. M. Tomás is also working in the synthesis of Co 2 Ln 2 SMMs in order to profit of the large magnetic anisotropy of the lanthanide ions. 2. Anisotropy Axes and energy levels in isolated lanthanoid ions Molecular magnets displaying magnetic tunneling and magnetic relaxation in which the magnetism arises from a single atom of a rare earth element have been discovered recently. In the present task we will study two different families of such compounds: (i) Polyoxometallates [21] such as [Ln(W 5 O 18 ) 2 ] 9 and [Ln(β 2 SiW 11 O 39 )2] 13, Ln III = Tb, Dy, Ho, Er, Tm, y Yb; and (ii) rare earth phthalocyanines [22]. In the first family of compounds, the rare earth center is located at the vertices of square based pyramids and in the second case the pyramids surrounding the lanthanide ion are triangular. A detailed study of the macroscopic magnetism of these families of compounds will be done at ICMA. For the proposed project, we would like to complement the magnetic study by using inelastic neutron scattering in order to determine the molecular energy levels at the IN5 instrument (7 days). This would enable a direct, unambiguous calculation of the parameters of the Hamiltonian in each case. Moreover, we propose an experimental determination of the axes of anisotropy of the rare earth atoms using polarized neutron diffraction (D9 and D3). In both cases, energy levels and anisotropy axes, the neutron experiments will be complemented by relativistic ab initio calculations. Dr. Javier Campo and Dr. Javier Luzón will supervise this research work at the local institution (ICMA). Whereas Dr. Campo will be the responsible for the experimental part, Dr. Luzón will be in charge of the supervision of the computational ab initio calculations of the research project. PREVIOUS ACTIVITIES AND ACHIEVEMENTS OF THE HOSTING GROUP IN THE FIELD OF THE PROJECT The hosting research group is composed of recognized experts in physical studies on molecular magnetism. This group includes four permanent researchers from the CSIC (Spanish Research Council), Prof. F. Palacio, Dr J Campo, Dr. M Tomás and Prof. L Falvello, one tenured professor at the University Defense Annex, posted to ICMA (J. Luzón), two non permament Ph.D. researchers contracted, and three graduate student. The members of this group also belong to a "Consolidated Research Group" as defined by the regional government of Aragón ( and also participate in the Spanish Molecular Nanoscience network (under the "Consolider Ingenio" program) and in the European Institute of Molecular Magnetism (EIMM). The latter two entities are administrative structures that group the leading research laboratories in molecular magnetism at the Spanish and European levels, respectively. REFERENCES: [1] R. Sessoli, D. Gatteschi, A. Camescjo, M. A. Novak. Nature 1993, 365, 141 [2] J. R. Friedman, M. P. Sarachik, J. Tejada, R. Ziolo. Phys. Rev. Lett. 1996, 76, 3830; L. Thomas. et al. Nature 1996, 383,

5 145; W. Wernsdorfer, R. Sesoli, Science 1999, 284, 133 [3] Lehmann et al. J. Mater. Chem. 2009, 19, 1672 [4] C. J. Milios, R. Inglis, A. Vinslava, R. Bagai, W. Wernsdorfer, S. Parsons, S. P.perlepes, G. Chrisotu and E. K. Brenchin. J. Am. Chem. Soc. 2007, 129, 12505; W. H. Jarman, T. D. Harris, D. E. Freedman, H. Fong, A. Chang, J. D. Rinehart, A. Ozarowski, M. T. Sougrati, F. Grandjean, G. J. Long, J. R. Long and C. J. Chang, J. Am. Chem. Soc. 2010, 132, [5] A.M. Ako, I.J. Hewitt, V. Mereacre, R. Clerac, W. Wernsdorfer, C.E. Anson, A.K. Powell. Angew. Chem., Int. Ed. Engl., 2006, 45, 4926 [6] A.J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K.A. Abboud, G. Christou. Angew. Chem., Int. Ed. Engl., 2004, 43, 2117 [7], P. H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wermsdprfer, M. Murugesu, Angew. Chem. Int. Ed. 2009, 48, 9489 [8] Luis et al. Phys. Rev. B 2010, 82, [9] G. L. Christou y colb. J. Appl. Phys, 2002, 91, 7382 [10] H. U. Güdel, Angew. Chem. Int. Ed. 2003, 42, 4653 [11] M. Murrie, Chem. Soc. Rev. 2010, 39, 1986 [12] E. Burzuri, J. Campo, L. R. Falvello, E. Forcén Vázquez, F. Luis, I. Mayoral, F. Palacio, C. Sáenz de Pipaón, M. Tómas, Chem. Eur. J, [13] N. Ishikawa et al. J. Phys. Chem. B., 2004, 108, 11265; [14] A. R. Rocha et al., Nat. Mater. 2005, 4, 335 and L. Bogani and W. Wernsdorfer, Nat. Mater. 2008, 7, 17. [15] G. A. Timco et al., Nat. Nanotechnol , 4, 173 [16] F. Elste and C. Timm, Phys. Rev. B 2005, 71, ; C. Romeike, M. R. Wegewijs, W. Hofstetter, and H. Schoeller, Phys. Rev. Lett. 2006, 96, ;. G. Gonzalez and M. N. Leuenberger, Phys. Rev. Lett. 2007, 98, [17] A. Soncini and L. F. Chibotaru, Phys. Rev. B, 2008, 77, [18] L. F. Chibotaru, L. Ungur, and A. Soncini, Angew. Chem., Int. Ed. 2008, 47, 4126; J. Luzon et al., Phys. Rev. Lett. 2008, 100, [19] D. Frustaglia, M. Hentschel, and K. Richter, Phys. Rev. Lett. 2001, 87, [20] J. Luzon and R. Sessoli, Dalton Trans, 2012, 41, [21] AlDamen et al. Inorganic chemistry, 2009, 48, 3467 [22] Gonidec et al. Angew. Chem. Int. Ed.,2010, 49, [23] Bertaina, S. et al., Nature 2008, 453, 203 [24] Aguilà, D. et al., Inorg. Chem. 2010, 49, 6784

Quantum tunneling of magnetization in lanthanide single-molecule. magnets, bis(phthalocyaninato)terbium and bis(phthalocyaninato)-

Quantum tunneling of magnetization in lanthanide single-molecule magnets, bis(phthalocyaninato)terbium and bis(phthalocyaninato)- dysprosium anions** Naoto Ishikawa, * Miki Sugita and Wolfgang Wernsdorfer