Three Er(iii) single-ion magnets which differ in the peripheral ligand sphere but exhibit similar first coordination spheres show profoundly different inelastic neutron scattering spectra and magnetic properties.

Research efforts in the quest for new single-molecule magnets (SMMs) have increasingly focused on systems either based on or else incorporating 4f ions. [1] For most pure 3d systems, and especially those containing the Mn III ion such as the original Mn 12 -Ac coordination cluster, spin reorientation is blocked when the ground state spin (S) combines with uniaxial magnetic anisotropy (D) to give an energy barrier to magnetic relaxation with the superexchange interactions between the metal centers leading to a molecular spin ground state and a molecular anisotropy. [2,3] The resultant exchange-based blocking of magnetization can be analyzed using a giant spin model. [4] In systems incorporating highly anisotropic 4f ions [1] it has become clear that magnetic interactions between 4f ions are weak and generally dipolar in nature. Here the single-ion spin and anisotropy become of greater relevance. For example, recent calculations on a Dy 2 SMM showed that the blocking mechanism largely arises from the individual Dy III ions with exchange-based behavior only seen at very low temperatures. [5] In systems combining 3d and 4f ions the aim is to embed highly anisotropic 4f ions into an exchangecoupled molecular 3d system, since 3d-4f interactions can be intermediate in magnitude between 3d-3d and 4f-4f. How-ever, analysis of the origins of the blocking mechanism in such systems is not straightforward and can generally only be achieved through detailed ab initio calculations, such as we recently reported for a Cr 4 Dy 4 SMM. [6] We now present a SMM comprising two Co II and two Dy III ions for which we can demonstrate the novel situation of single-ion blocking of the Dy III ions at higher temperatures with a crossover to molecular exchanged-based blocking at low temperatures.Reaction of Dy(NO 3 ) 3 ·6 H 2 O, Co(NO 3 ) 2 ·6 H 2 O, H 2 L and Et 3 N in the molar ratio 1:1:2:4 in MeOH gives crystalline red powder which was recrystallized from THF giving pink crystals of [Co 2 Dy 2 (L) 4 (NO 3 ) 2 (THF) 2 ]·4 THF (1) in 75% yield. H 2 L is the Schiff-base we previously described [7] resulting from condensation of o-vanillin and 2-aminophenol to give a "pocket ligand" capable of binding two different types of metal ion (see Figure S1 in the Supporting Information).Compound 1 crystallizes in the triclinic space group P " 1 with Z = 1. Within the core of the centrosymmetric complex, the metal ions are linked by four (L) 2À ligands in the butterfly (or defect-dicubane) topology (Figure 1). One of the two crystallographically independent ligands chelates Dy(1) through its imine nitrogen and the two phenolate oxygens O(1) and O(3) (corresponding to pocket I, see Figure S1 in the Supporting Information). Co(1) and Co(1') are linked Figure 1. Molecular structure of the Co 2 Dy 2 complex in 1.

Ferromagnetic interactions in an Fe4Dy4 single‐molecule magnet were studied using a combination of magnetic susceptibility measurements (see diagram; inset: cluster core) and 57Fe Mössbauer spectroscopy.

In recent years polynuclear transition metal molecules have been synthesized and proposed for example as magnetic storage units or qubits in quantum computers. They are known as molecular nanomagnets and belong in the class of mesoscopic systems, which are large enough to display many-body effects but small enough to be away from the finite-size scaling regime. It is a challenge for physicists to understand their magnetic properties, and for synthetic chemists to efficiently tailor them by assembling fundamental units. They are complementary to artificially engineered spin systems for surface deposition, as they support a wider variety of complex states in their low energy spectrum. Here a few characteristic examples of molecular nanomagnets showcasing unusual many-body effects are presented. Antiferromagnetic wheels and chains can be described in classical terms for small sizes and large spins to a great extent, even though their wavefunctions do not significantly overlap with semiclassical configurations. Hence, surprisingly, for them the transition from the classical to the quantum regime is blurred. A specific example is the Fe18 wheel, which displays quantum phase interference by allowing Néel vector tunneling in a magnetic field. Finally, the Co5Cl single-molecule magnet is shown to have an unusual anisotropic response to a magnetic field.

Relaxen auf zweierlei Weisen: Ein Einzelmolekülmagnet mit einem defekten Co2Dy2‐Dicuban als Kern zeigt langsame Relaxation der Magnetisierung bei einer Blocking‐Temperatur von 22 K (bei 1500 Hz), die für einen 3d‐4f‐Einzelmolekülmagneten die höchste berichtete ist. Eine Analyse der Relaxation ergibt zwei Bereiche an Blocking‐Temperaturen, von denen einer intraionisch an den DyIII‐Ionen lokalisiert und der andere austauschbasiert ist.