Magnetic Anisotropy of Two Cyclic Hexanuclear Fe(III) Clusters Entrapping Alkaline Ions

Waldmann, Oliver; Schülein, J.; Koch, R. J.; Müller, Paul; Bernt, Ingo; Saalfrank, Rolf W.; Andres, Hanspeter; Güdel, and H. U.; Allenspach, P.

doi:10.1021/ic9906510

Inorg. Chem.

1999

DOI: 10.1021/ic9906510

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Magnetic Anisotropy of Two Cyclic Hexanuclear Fe(III) Clusters Entrapping Alkaline Ions

Oliver Waldmann

J. Schülein

R. J. Koch

et al.

Abstract: The magnetic anisotropy of the two cyclic hexanuclear Fe(III) clusters [Li⊂Fe 6 L 6 ]Cl‚6CHCl 3 and [Na⊂Fe 6 L 6 ]-Cl‚6CHCl 3 , L ) N(CH 2 CH 2 O) 3 , was investigated. Based on a spin Hamiltonian formalism, the magnetic anisotropy was calculated exactly to first order, i.e., in the strong exchange limit, using Bloch's perturbational approach and irreducible tensor operator techniques. Experimentally, the magnetic anisotropy was investigated by magnetic susceptibility and high-field torque magnetometry of sing… Show more

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Cited by 95 publications

(113 citation statements)

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“…This agrees with findings for other planar molecules [4,5,8,9,10], but was also checked experimentally. The in-plane anisotropy of the grid was found to be at best 15% of the main ||-⊥ anisotropy, i.e.…”

supporting

confidence: 91%

“…This is immediately evident from Fig. 3(a) as the magnitude of the torque is related to the magnetic anisotropy [8,10]. These observations actually represent the main result of this work.…”

supporting

confidence: 77%

“…The increase of signal at low fields demonstrates a ground state with S > 0. At intermediate fields a clear torque step is seen indicative of a level-crossing at a field B * ≈ 7.5 T [8,10]. From B * the excitation energy of the excited state involved in the level-crossing is estimated to ∆ 1 ≈ 10 K. The anisotropy of the ground state is too weak to yield such excitation energies (see below).…”

mentioning

confidence: 99%

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Field-Dependent Anisotropy Change in a Supramolecular Mn(II)-[3×3]Grid

2002

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The magnetic anisotropy of a novel Mn(II)-[3×3] grid complex was investigated by means of high-field torque magnetometry. Torque vs. field curves at low temperatures demonstrate a ground state with S > 0 and exhibit a torque step due to a field induced level-crossing at B * ≈ 7.5 T, accompanied by an abrupt change of magnetic anisotropy from easy-axis to hard-axis type. These observations are discussed in terms of a spin Hamiltonian formalism.The design of clusters of magnetic metal centers with novel magnetic properties has become a major goal in the area of nanoscale materials. A particularly interesting class of compounds is that of the supramolecular [N × N ] grid structures [1]. The essentially flat, squarematrix-like arrangement of exactly N 2 metal ions realized in these complexes is not only aesthetically pleasant but also suggests applications as, e.g., molecular storage devices [1]. From the magnetic perspective, the grids may be regarded as molecular model systems for magnets with extended interactions on a square lattice which are, e.g., relevant in the context of high-temperature superconductors.Numerous [2×2] grids containing magnetic metal ions have been created, and magnetic studies have shown a remarkable variety of their magnetic properties [2,3,4,5]. For a [2×2] square of spins the distinction between a "two-dimensional" grid and a "one-dimensional" ring is really semantic, but for a [3×3] grid it is truly "twodimensional". However, the first [3×3] grid complexes with magnetic ions could be synthesized only very recently [6,7].In this work, we investigated the Mn-[3×3] grid cluster [Mn 9 (2POAP-2H) 2 ] 6+ (Fig. 1) by means of magnetization and high-field torque measurements, focusing on its anisotropic properties. At low temperatures an unusual field dependence of the magnetic anisotropy was observed: With increasing field, the torque signal exhibits a clear step where the magnetic anisotropy changes its sign abruptly from easy-axis to hard-axis type.

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supporting

confidence: 91%

“…This is immediately evident from Fig. 3(a) as the magnitude of the torque is related to the magnetic anisotropy [8,10]. These observations actually represent the main result of this work.…”

supporting

confidence: 77%

mentioning

confidence: 99%

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Field-Dependent Anisotropy Change in a Supramolecular Mn(II)-[3×3]Grid

2002

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“…The first term, being proportional to δm, leads to a broadened torque step. These magnetization and torque steps, as mentioned already, are well known [14,15,16,24]. The second term, however, whose field dependence is controlled by G(b, ϕ) only, leads to a peak-like contribution centered at the LC field b c .…”

mentioning

confidence: 58%

Quantum Magneto-Oscillations in a Supramolecular Mn(II)-[3×3] Grid

et al. 2004

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The magnetic grid molecule Mn(II)- [3 × 3] has been studied by high-field torque magnetometry at 3 He temperatures. At fields above 5 T, the torque vs. field curves exhibit an unprecedented oscillatory behavior. A model is proposed which describes these magneto oscillations well.PACS numbers: 33.15. Kr, 75.10.Jm Among the known magneto-oscillatory effects, the de Haas-van Alphen (dHvA) effect in metals (and the group of effects related to it) is the most prominent example, having had a formative influence on our modern picture of solid state physics [1]. The dHvA effect originates from a quantization of closed electron orbits into Landau levels in a magnetic field, so that the density of states at the Fermi energy exhibits a markedly oscillatory evolution as a function of magnetic field. Currently, it finds application in a wide range of materials, e.g. heavy fermion compounds [2], low-dimensional organic metals [3], hightemperature superconductors [4], two-dimensional electron gases [5], or in the recently discovered superconductor MgB 2 [6].In this work, we report on a new magnetooscillatory effect, discovered in the molecular nanomagnet [Mn 9 (2POAP-2H) 6 ](ClO 4 ) 6 · 3.57 MeCN · H 2 O, the so called Mn-[3 × 3] grid. Molecular nanomagnets are compounds with many magnetic metal ions linked by organic ligands to form well defined magnetic nanoclusters. They have attracted much interest since they can exhibit fascinating quantum effects at the mesoscopic scale. For instance, quantum tunneling of the magnetization has been observed in the clusters Mn 12 or Fe 8 [7]. We have performed high-field torque magnetometry on the Mn-[3 × 3] grids at 3 He temperatures and observed striking magneto oscillations in the torque signal. We show that they arise from the interplay between antiferromagnetic interactions within a molecule and Zeeman splitting on the one hand and the magnetic anisotropy on the other hand.In the Mn-[3 × 3] grid molecules, nine spin-5/2 Mn(II) ions occupy the positions of a regular 3 × 3 matrix, held in place by a lattice of organic ligands [inset of Fig. 1]. These grids were characterized recently by magnetization and torque measurements and found to exhibit unusual magnetic properties [8]. The Mn ions within a molecule experience an antiferromagnetic interaction leading to a total spin S = 5/2 ground state at zero field. At a field of about 7 T, the ground state changes abruptly to a new state, a S = 7/2 level, accompanied by a change of the sign of the magnetic anisotropy. Notably, intermolecular magnetic interactions are at best on the order of few 10 mK. This is evident from the crystal structure (the smallest separation between two molecules in the crystal is > 8Å), but has been checked also experimentally [9]. Accordingly, even at 3 He temperatures the magnetism of a macroscopic crystal sample reflects that of a single molecule.Single crystals of Mn-[3 × 3] were prepared as reported [10]. They crystallize in the space group C 2 /c. The cation [Mn 9 (2POAP-2H) 6 ] 6+ exhibits a slightly dis...

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“…In a field the Zeeman splitting lifts the degeneracy of the S = 1 level, such that a sufficiently strong field induces a level crossing (LC), where the ground state of the molecule changes from S = 0, M = 0 to S = 1, M = −1 [inset to Fig. 1(b)] [2,4,5]. Due to the quasi degeneracy near the LC, the magnetism of the molecule is sensitive to weak interactions with the environment, such as the lattice vibrations or the surrounding magnetic molecules.…”

mentioning

confidence: 99%

Huge Transverse Magnetic Polarization in the Field-Induced Phase of the Antiferromagnetic Molecular WheelCsFe8

Schnelzer¹,

Waldmann²,

Horvatić³

et al. 2007

Phys. Rev. Lett.

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The 1 H-NMR spectrum and nuclear relaxation rate T −1 1 in the antiferromagnetic wheel CsFe8 were measured to characterize the previously observed magnetic field-induced low-temperature phase around the level crossing at 8 T. The data show that the phase is characterized by a huge staggered transverse polarization of the electronic Fe spins, and the opening of a gap, providing microscopic evidence for the interpretation of the phase as a field-induced magneto-elastic instability. . Due to the quasi degeneracy near the LC, the magnetism of the molecule is sensitive to weak interactions with the environment, such as the lattice vibrations or the surrounding magnetic molecules. Studying the LCs in AFM wheels is hence of broad interest as it provides experimental insight into the interplay of a mesoscopic quantum system with its environment.The molecule [CsFe 8 {N(CH 2 CH 2 O) 3 } 8 ]Cl, or CsFe 8 in short [3], which realizes a ring of eight spin-5/2 Fe(III) ions, is particularly interesting as it is the only AFM wheel system in which magnetic torque studies revealed a phase transition below a temperature T c of about 0.7 K and in a field range of about ±1.5 T around the LC, between fields B c1 and B c2 [the phase diagram is sketched in the inset to Fig.

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Magnetic Anisotropy of Two Cyclic Hexanuclear Fe(III) Clusters Entrapping Alkaline Ions

Cited by 95 publications

References 27 publications

Field-Dependent Anisotropy Change in a Supramolecular Mn(II)-[3×3]Grid

Field-Dependent Anisotropy Change in a Supramolecular Mn(II)-[3×3]Grid

Quantum Magneto-Oscillations in a Supramolecular Mn(II)-[3×3] Grid

Huge Transverse Magnetic Polarization in the Field-Induced Phase of the Antiferromagnetic Molecular WheelCsFe8

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