John Bowlan

Metastability of free cobalt and iron clusters: a possible precursor to bulk ferromagnetism.

The cobalt and iron clusters CoN, FeN (20<N<150) measured in a cryogenic molecular beam are found to be bistable with magnetic moments per atom both {\mu}N/N 2{\mu}B in the ground states and {\mu}N */N {\mu}B in the metastable excited states (for iron clusters, {\mu}N ~3N{\mu}B and {\mu}N* N{\mu}B). This energy gap between the two states vanish for large clusters, which explains the rapid convergence of the magnetic moments to the bulk value and suggests that ground state for the bulk involves a superposition of the two, in line with the fluctuating local orders in the bulk itinerant ferromagnetism.

Nonclassical dipoles in cold niobium clusters

Electric deflections of niobium clusters in molecular beams show that they have permanent electric dipole moments at cryogenic temperatures but not higher temperatures, indicating that they are ferroelectric. Detailed analysis shows that the deflections cannot be explained in terms of a rotating classical dipole, as claimed by Andersen et al.. The shapes of the deflected beam profiles and their field and temperature dependence indicates that the clusters can exist in two states, one with a dipole and the other without. Cluster with dipoles occupy lower energy states. Excitations from the lower states to the higher states can be induced by low fluence laser excitation. This causes the dipole to vanish.

Size dependent magnetic moments and electric polarizabilities of free Tb, Ho, and Tm clusters

Stern–Gerlach deflection measurements have been performed on rare earth clusters TbN, HoN, and TmN (N≤40) at cryogenic temperatures (T≤77 K). TbN and HoN share a common size dependence in their magnetic moments. They both exhibit common “magic number” sizes which show reduced net magnetic moments, similar to previous observations for Gd and Dy clusters. TmN have smaller magnetic moments that do not differ significantly between cluster sizes. The reduced net magnetic moments are evidence that the atomic moments are canceled by a canted or antiferromagnetic alignment. Electric deflection experiments reveal that TmN have electric dipole moments and show an enhanced response to an electric field compared to TbN and HoN.

The effect of oxygen doping on the magnetism of Tb and Pr clusters

The magnetic moments and electric dipoles of Tb and Pr clusters are investigated using the Stern–Gerlach deflection technique. The addition of a single oxygen atom induces an increase in the electric dipole of TbN clusters, however the magnetic moment is largely not affected. In Pr neither the magnetic moment nor the electric dipole is affected. This raises questions as to the role of conduction electrons in the exchange interaction of rare earth clusters, and puts into doubt the validity of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism in small systems.

Manipulating multiple order parameters via oxygen vacancies: The case of Eu0.5Ba0.5TiO3−δ

Controlling functionalities, such as magnetism or ferroelectricity, by means of oxygen vacancies (V-O) is a key issue for the future development of transition-metal oxides. Progress in this field is currently addressed through V-O variations and their impact on mainly one order parameter. Here we reveal a mechanism for tuning bothmagnetism and ferroelectricity simultaneously by using V-O. Combining experimental and density-functional theory studies of Eu0.5Ba0.5TiO3-delta , we demonstrate that oxygen vacancies create Ti3+ 3d(1) defect states, mediating the ferromagnetic coupling between the localized Eu 4f(7) spins, and increase an off-center displacement of Ti ions, enhancing the ferroelectric Curie temperature. The dual function of Ti sites also promises a magnetoelectric coupling in the Eu0.5Ba0.5TiO3-delta.

Effects of biaxial strain on the improper multiferroicity in h−LuFeO3 films studied using the restrained thermal expansion method

Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which have however been elusive due to the experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of h-LuFeO3 (001) films. The results indicate that a compressive biaxial strain significantly enhances the ferrodistortion, and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced ferrodistortion together, cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO3, according to our first principle calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO3. The experimental elucidation of the strain effect in h-LuFeO3 films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other epitaxial thin film materials.