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

Abstract

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.

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

“…A similar situation holds for D C and D R . J and D were determined such that the position of the first ground state level crossing and the zero-field splitting of the S = 5/2 ground state in the calculated energy spectrum match the experimental values of ≈ 7 T and ≈ 3 K, respectively [8]. The result is J = -5.0 K and D = -0.14 K, in agreement with INS experiments [20].…”

“…The average distance between the Mn(II) ions is 3.93Å, the smallest distance between clusters is larger than 8Å. Consistent with the planar structure, the Mn-[3 × 3] grid exhibits a practically uniaxial magnetic behavior, and uniaxial and molecular C 2 symmetry axes coincide [8]. Magnetic torque was measured with a homemade silicon cantilever device [8,11] inserted into the M10 magnet at the Grenoble High Magnetic Field Laboratory using a 3 He evaporation cryostat.…”

“…Consistent with the planar structure, the Mn-[3 × 3] grid exhibits a practically uniaxial magnetic behavior, and uniaxial and molecular C 2 symmetry axes coincide [8]. Magnetic torque was measured with a homemade silicon cantilever device [8,11] inserted into the M10 magnet at the Grenoble High Magnetic Field Laboratory using a 3 He evaporation cryostat. Resolution of the sensor was about 10 −11 Nm, the weight of the crystals investigated was about 20 µg, nonlinearity was less than 2%, and the accuracy of the in situ alignment of the C 2 crystal axis with respect to the magnetic field was 0.3 • .…”

“…In Ref. [8], |J C | ≪ |J R | was suggested. With this parameter regime, however, the experimental results could not be reproduced satisfactorily.…”

“…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.…”