G. E. Sterbinsky

Beating the Stoner criterion using molecular interfaces.

Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the material’s bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal–molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7).

Strain-driven spin reorientation in magnetite/barium titanate heterostructures

We report spin reorientation transitions in a Fe3O4/BaTiO3 heterostructure driven by strain at the structural phase transitions of BaTiO3. These spin reorientations result from the emergence of an in-plane uniaxial magnetic anisotropy. The magnetoelastic response of Fe3O4 to the variations in epitaxial strain that occur at the BaTiO3 phase transitions gives rise to the uniaxial anisotropy. The anisotropy energies calculated from the in-plane strain are in quantitative agreement with a change in the Zeeman energy.

Investigation of heteroepitaxial growth of magnetite thin films

Epitaxial magnetite (Fe3O4) thin films were deposited by molecular beam epitaxy using molecular oxygen as the oxidant. Films deposited on (001) SrTiO3, (001) MgO, and (001) BaTiO3 surfaces are epitaxial with the film (001) parallel to the substrate (001) and the film ⟨100⟩ parallel to the substrate ⟨100⟩. X-ray magnetic circular dichroism was used to determine the relative Fe2+∕Fe3+ stoichiometry of the magnetite films, which was nearly independent of oxygen partial pressure over the range studied. All films show no in-plane magnetic anisotropy. Coercive fields ranged from 0.019to0.039T and depended on film roughness.

Synthesis, structural and magnetic properties of epitaxial MgFe2O4 thin films by molecular beam epitaxy

Epitaxial MgFe2O4 thin films were synthesized by oxide molecular beam epitaxy at 300°C. Reflection high energy electron diffraction and transmission electron microscopy showed that the films were epitaxial with a (001) orientation. Magneto-optic Kerr effect and superconducting quantum interference device magnetization measurements revealed that films were ferrimagnetic with a saturation ellipticity of 5mdeg, and saturation magnetization of 120emu∕cm3, respectively. The ferrimagnetism was attributed to the formation of a partially inverted spinel structure with a cation site distribution of [Mg0.22+Fe0.83+][Mg0.82+Fe1.23+]O4.

Antiferromagnetic phase of the gapless semiconductor V3Al

Discovering new antiferromagnetic (AF) compounds is at the forefront of developing future spintronic devices without fringing magnetic fields. TheAF gapless semiconducting D03 phase ofV3Alwas successfully synthesized via arc-melting and annealing. The AF properties were established through synchrotron measurements of the atom-specific magnetic moments, where the magnetic dichroism reveals large and oppositely oriented moments on individual V atoms. Density functional theory calculations confirmed the stability of a type G antiferromagnetism involving only two-thirds of the V atoms, while the remaining V atoms are nonmagnetic. Magnetization, x-ray diffraction, and transport measurements also support the antiferromagnetism. This archetypal gapl

Using the infrared magnetorefractive effect to compare the magnetoresistance in (100) and (111) oriented Fe3O4 films

The infrared magnetorefractive effect (MRE) is used to compare the magnetoresistance (MR) in epitaxial thin films of Fe3O4 grown on MgO with (100) and (111) crystal orientations. The smaller MRE detected in the (111) film is shown to correlate with the smaller electrically measured MR, its behavior consistent with a lower density of antiphase boundaries in the (111) film

Epitaxial Fe3O4 on SrTiO3 characterized by transmission electron microscopy

Epitaxial magnetite (Fe3O4) thin films were deposited on (001) SrTiO3 substrates by molecular beam epitaxy using molecular oxygen and characterized by transmission electron microscopy and magneto-optic Kerr effect measurements. The films are epitaxial with (001)f‖(001)s and [110]f‖[110]s and the film∕substrate interface is nearly atomically abrupt. Misfit dislocations and twins are present in the film due to the large mismatch between the film and substrate. Antiphase boundaries were also observed. The values for the coercive field of the epitaxial film are nearly the same as those of bulk magnetite.

Epitaxial Fe[sub 3]O[sub 4] on SrTiO[sub 3] characterized by transmission electron microscopy

Epitaxial magnetite (Fe3O4) thin films were deposited on (001) SrTiO3 substrates by molecular beam epitaxy using molecular oxygen and characterized by transmission electron microscopy and magneto-optic Kerr effect measurements. The films are epitaxial with (001)f‖(001)s and [110]f‖[110]s and the film∕substrate interface is nearly atomically abrupt. Misfit dislocations and twins are present in the film due to the large mismatch between the film and substrate. Antiphase boundaries were also observed. The values for the coercive field of the epitaxial film are nearly the same as those of bulk magnetite.

Defect and interfacial structure of heteroepitaxial Fe3O4/BaTiO3 bilayers.

The defect and interfacial structure in a Fe3O4/BaTiO3 heteroepitaxial bilayer was investigated by scanning transmission electron microscopy. The results show that the Fe3O4 film grew epitaxially on BaTiO3. The orientation relationship between Fe3O4, BaTiO3 and MgO is [100]Fe3O4//[100]BaTi3O//[100]MgO and (010)Fe3O4//(010)BaTiO3//(010)MgO. An initial interfacial nucleation layer was formed that partially accommodated the lattice mismatch strain between BaTiO3 and MgO. This investigation indicates that the formation of this buffer layer provides a high-quality BaTiO3 surface for subsequent Fe3O4 growth, resulting in a semicoherent interface. The Fe3O4 surface is nearly atomically abrupt (roughness Rrms = 0.78 nm). The Fe3O4 film exhibits magnetic domains with a diameter in the range of 0.4-2 microm.

Stoichiometry dependence of potential screening at La ( 1 - δ ) Al ( 1 + δ ) O 3 / SrTiO 3 interfaces

Hard x-ray photoelectron spectroscopy (HAXPES) and variable kinetic energy x-ray photoelectron spectroscopy (VKE-XPS) analyses have been performed on 10 unit cell La