Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares

Abstract

We investigated the magnetic properties of two isostructural Ni(II) metal complexes [Ni4LIn each molecule the four Ni(II) centers form almost perfect regular squares. Magnetic coupling and anisotropy of single crystals were examined by magnetization measurements and in particular by high-field torque magnetometry at low temperatures. The data were analyzed in terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both compounds, we found a weak intramolecular ferromagnetic coupling of the four Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The coupling strengths are roughly identical for both compounds, whereas the zero-field-splitting parameters are significantly different. Possible reasons for this observation are discussed.

Observation of field-dependent magnetic parameters in the magnetic molecule{Ni4Mo12}

“…In particular, from (2) it follows that the level crossing fields are uniformly spaced. 25,26 In stark contrast to the expectation of uniformly spaced crossing fields, the experimental magnetization M(B) curve of {Ni 4 Mo 12 } features nonequidistant steps at 4.5, 8.9, 20.1, and 32 T. Even assuming an anisotropic Hamiltonian 27 with two exchange couplings and biquadratic terms as done for other Ni IIcompounds, 16,18 we were unable to account for the specific sequence of steps in the low-temperature magnetization of {Ni 4 Mo 12 }. In order to provide a comprehensive picture of the unusual high-field magnetic behavior of {Ni 4 Mo 12 }, we have been led to invoke field-dependent exchange and anisotropy parameters.…”

“…14,15 Small magnetic molecules, with their simpler chemical and magnetic structure, can therefore provide an important opportunity to understand the dependence of magnetic observables on model parameters. 16,17,18 In this work we report our joint experimental and theoretical efforts to understand the behavior of [Mo V 12 O 30 (µ 2 -OH) 10 H 2 {Ni II (H 2 O) 3 } 4 ], henceforth abbreviated as {Ni 4 Mo 12 }, a magnetic molecule which is comprised of Ni II centers positioned at the nucleophilic sites of an ǫ-Keggin cluster forming an almost ideal tetrahedron. 19 In contrast to several other nickel compounds which exhibit ferromagnetic 16,20,21,22,23 or mixed coupling, 24 the Ni centers of this molecule are antiferromagnetically coupled, as is also the case for certain Ni-2 × 2-grid molecules.…”

Efimov effect in quantum magnets

“…Therefore, it is possible to find a compound with spin S ≥ 1 whose D/J ratio is already close to the critical value. For example, there is an S = 1 ferromagnetic compound based on molecular Ni 2+ squares with D/J 3.0(5) [30], which is not far from the critical value 4.76874. Furthermore, the exchange coupling J can be tuned with pressure [31] to bring the system near the two-magnon resonance and realize the Efimov effect.…”

Magnetic molecular wheels and grids – the need for novel concepts in “zero-dimensional” magnetism

“…The magnetism of both complexes 2 and 3 were investigated in detail by magnetization and high-field torque magnetometry on single crystals [18]. Employing a novel data analysis scheme, this allowed a precise determination of all the parameters in Eq.…”