Origin of the Unusual Ground-State Spin S = 9 in a Cr₁₀ Single-Molecule Magnet

Abstract

The molecular wheel [Cr10(OMe)20(O2CCMe3)10], abbreviated {Cr10}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/k B = −0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state (S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr10}. For powder samples, the temperature dependence of the susceptibility plotted as χT(T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M(H) did not saturate at the expected 30 μB/fu for 10 FM coupled 3/2 spin Cr3+ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 μB/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr10} wheel is formed by two semiwheels, each with four Cr3+ spins FM coupled (J FM/k B = 2.0 K), separated by two Cr3+ ions AF coupled asymmetrically (J 23/k B = J 78/k B = −2.0 K; J 34/k B = J 89/k B = −0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/k B = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr10} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr–O–Cr alkoxy bridges coupling the Cr3+ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr3+ ion in the wheel.

Surface Deposition Induced Reduction of the Ground State Spin in Cr10 Wheel

“…interactions were taken as identical and FM ( J FM / k B = 2.0 K), except around two Cr 3+ ions on opposite sides of the wheel, denoted Cr3 and Cr8 (see Figure 9a for numbering), which were considered AF and asymmetric ( J 23 / k B = J 78 / k B = − 2.0 K ≠ J 34 / k B = J 89 / k B = − 0.25 K, D i / k B = − 0.31 K). [ 76 ] The same model and exchange interaction constants used for the bulk were tested against the experimental data for the 1ML/Au at 3.4 K (see Figure 9b). The failure of such a model is obvious.…”

“…The significant observed changes in the M(H) and 𝜒T(T) curves must be related to the accommodation of the {Cr 10 } molecules onto the substrate and the subsequent modification of Cr-O-Cr angles on which exchange coupling constants are very dependent. [76,85] Moreover, this change seems to be progressive as additional layers are deposited, with an apparent tendency toward the magnetic behavior of the molecules in the bulk crystalline form. As it will be shown below, the low values of the magnetization at low and intermediate magnetic fields with respect to the single crystal results can be explained consistently as a slight change in the exchange interactions giving rise to a lower spin ground state.…”

“…However, slight changes in the Cr-O-Cr angles and distances may provoke important modifications in the exchange coupling constants. [76,85] In contrast, the single-ion anisotropies at the Cr 3+ ions are expected to be much less affected, as they are mainly determined by the full octahedral cage.…”

“…Very recently, we unveiled the origin of this intermediate S = 9 ground state in {Cr 10 }. [76] Its magnetic properties could be rationalized within a model where {Cr 10 } is formed by two semiwheels, each with four Cr 3+ spins FM coupled, separated by two Cr 3+ ions AF coupled asymmetrically (vide infra, Figure 9a Remarkably, some of us succeeded in depositing {Cr 10 } on a Ag(110) single-crystal [20] by UHV sublimation, paving the way for studying the magnetic properties of these interesting wheels selfassembled onto different substrates. In this work, we present the magneto-structural properties of UHV-sublimated {Cr 10 } wheels on Cu(111) and Au(111) metallic substrates, investigated combining scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Xray natural linear dichroism (XNLD) and X-ray magnetic circular dichroism (XMCD).…”

“…We compare the magnetic behavior of the films with that of bulk {Cr 10 } . [74,76] Monte Carlo simulations have been used to rationalize the modified magnetic properties of surface-deposited monolayer samples. 20 (O 2 CCMe 3 ) 10 } (from now on {Cr 10 }) single crystals were prepared by a solvothermal reaction using a method previously described.…”