Atomic domain magnetic nanoalloys: interplay between molecular structure and temperature dependent magnetic and dielectric properties in manganese doped tin clusters.

We present extensive temperature dependent (16-70 K) magnetic and electric molecular beam deflection measurements on neutral manganese doped tin clusters Mn/SnN (N = 9-18). Cluster geometries are identified by comparison of electric deflection profiles and quantum chemical data obtained from DFT calculations. Most clusters adopt endohedral cage structures and all clusters exhibit non-vanishing magnetic dipole moments. In the high temperature regime all species show exclusively high field seeking magnetic response and the magnetic dipole moments are extracted from the shift of the molecular beam. At low nozzle temperatures, some of the clusters show considerably broadened beam profiles due to non-uniform deflection in the magnetic field. The results reflect the influence of the chemical environment on the magnetic properties of the transition metal in atomic domain magnetic nanoalloys. Different ground state spin multiplicities and coupling of rotational and vibrational degrees of freedom with the spin angular momentum of isolated clusters of different size apparently cause these variations of spin orientation. This is discussed by taking electronic and molecular structure data into account.

Stern-Gerlach experiments on Mn@Sn12: identification of a paramagnetic superatom and vibrationally induced spin orientation.

Beam deflection experiments in inhomogeneous magnetic fields reveal a new limiting case of the magnetization distribution of isolated clusters. Endohedrally doped clusters are produced in a temperature controlled, cryogenically cooled laser ablation source. Temperature dependent experiments indicate a crucial contribution of molecular vibrations to the spin dynamics of Mn@Sn12. In its vibrational ground state the cluster behaves magnetically like a paramagnetic atom, with quantized spin states. However, excited molecular vibrations induce spin orientation in the magnetic field.

Breakdown of the pseudopotential approximation for magnetizabilities and electric multipole moments: test calculations for Au, AuF, and Sn(n) cluster (n ≤ 20).

The response of the electronic wavefunction to an external electric or magnetic field is widely considered to be a typical valence property and should, therefore, be adequately described by accurately adjusted pseudopotentials, especially if a small-core definition is used within this approximation. In this paper we show for atomic Au and Au(+), as well as for the molecule AuF and tin clusters, that in contrast to the case of the static electric dipole polarizability or the electric dipole moment, core contributions to the static magnetizability are non-negligible, and can therefore lead to erroneous results within the pseudopotential approximation. This error increases with increasing size of the core chosen. For tin clusters, which are of interest in ongoing molecular beam experiments currently carried out by the Darmstadt group, the diamagnetic and paramagnetic isotropic components of the magnetizability tensor almost cancel out and large-core pseudopotentials do not even predict the correct sign for this property due to erroneous results in both the diamagnetic and (more importantly) the paramagnetic terms. Hence, all-electron calculations or pseudopotentials with very small cores are required to adequately predict magnetizabilities for atoms, molecules and the solid state, making it computationally more difficult to obtain this quantity for future investigations in heavy atom containing molecules or clusters. We also demonstrate for this property that all-electron density functional calculations are quite robust and give results close to wavefunction based methods for the atoms and molecules studied here.

Size- and temperature-dependent magnetic response of molecular cage clusters: manganese-doped tin clusters.

Endohedral clusters, formed by incorporating a single Mn atom into a cage of tin atoms, have been generated in the gas phase. Mass spectrometry reveals that a cage size of 10 tin atoms is necessary for the efficient incorporation of one Mn atom. Some of the cluster compounds with one Mn atom attached to the tin clusters display large intensities compared to the pure tin clusters, indicating that the compound clusters are particularly stable. The manganese-doped tin cluster assemblies Mn@Sn12, Mn@Sn13, and Mn@Sn15 have been further analyzed within a molecular beam magnetic deflection experiment. Interestingly, although the effect of the magnetic field on the behavior of Mn@Sn12 is quite different from that of Mn@Sn13 and Mn@Sn15, the magnetic dipole moments are the same within the uncertainty of the measurements. Magnetic dipole moments have been found in close agreement with the spin quantum number S = 5/2 predicted by theory for Mn@Sn12, indicating that the magnetic moment of the Mn atom is not quenched. This supports the idea that within a tin cluster cage a single Mn atom can be encapsulated, resulting in the formation of endohedral clusters consisting of a central Mn2+ ion surrounded by a particularly stable Zintl-ion cage Sn(N)(2-). The observed molecular beam profiles indicate that at a nozzle temperature of 55 K the magnetic moment is strongly locked to the molecular framework of Mn@Sn12; in contrast, the magnetic moment of Mn@Sn13 and Mn@Sn15 tends to align with the magnetic field. The experiments therefore demonstrate that the size of a presumably nonmagnetic cluster cage might have a fundamental influence on the magnetization dynamics of paramagnetic species. The influence of vibrational excitation on the Stern-Gerlach profile of Mn@Sn12 is further analyzed, and it is shown that the behavior of Mn@Sn12 at elevated nozzle temperatures changes continuously toward a nonlocked moment, pointing to size- and temperature-dependent magnetization dynamics.