Thomas Fix

Effects of strain relaxation on the electronic properties of epitaxial Sr2FeMoO6 grown by pulsed laser deposition on SrTiO3 (001)

Thin films of Sr2FeMoO6 (SFMO) are grown epitaxially on (001) SrTiO3 substrates by pulsed laser deposition. The in-plane and out-of-plane SFMO lattice parameters are determined for different thicknesses, from 13 to 600 nm. Samples below a critical thickness of around 34 nm are fully strained, samples between 34 and 80 nm are relaxing, and samples between 80 and 600 nm are fully relaxed. Transmission electron microscopy reveals that the relaxation is done by stacking the faults perpendicular to the substrate. Ab initio calculations show first that the variation of SFMO lattice parameters due to strain has a limited impact on the magnetization, which is also observed experimentally. Second, ab initio calculations indicate that SFMO is half metallic only for thicknesses above 44 nm.

Strong room temperature exchange bias in self-assembled BiFeO3–Fe3O4 nanocomposite heteroepitaxial films

Self-assembled, nanocomposite heteroepitaxial films of BiFeO3 + Fe3O4 (x BiFeO3 + (1 − x) Fe3O4), where x = 0.5 or 0.9, were grown on (011) SrTiO3. Depending on the value of x and on the film thickness, either exchange bias or exchange enhancement of coercivity was demonstrated. In epitaxially and highly strained (7%) films of 250 nm thickness, and for x = 0.9, exchange bias (HEB) values of 40 Oe and HEB/HC ratios of 0.5 were achieved. Most crucially, these effects were measured at room temperature, showing the high potential of chemically compatible BiFeO3 + Fe3O4 for achieving room temperature magnetoelectricity.

Delta-doped LaAlO3/SrTiO3 interfaces

The quasi-two-dimensional electron gases formed at the interface between insulators such as SrTiO3/LaAlO3 are predicted to arise from an interfacial electronic reconstruction, but the extent to which oxygen vacancies within the substrate contribute to the conduction remains unclear. We show that delta-doping the interface with transition metal cations dramatically alters the properties and supports the model of a highly two-dimensional charge sheet.

Electric-Field Control of Ferromagnetism in a Nanocomposite via a ZnO Phase

La2CoMnO6 (LcmO)-ZnO nanocomposite thin films grown on SrTiO3 and Nb-SrTiO3 (001) are investigated. The films grow in the form of self-assembled epitaxial vertically aligned structures. We show that, at 120 K, an electric field applied across the nanocomposite reversibly alters magnetic properties of LcmO. The effect is consistent with charge-mediated coupling between magnetism and an electric field that can be induced by changes in ion valences.

Structural defects in Sr2FeMoO6 double perovskite: Experimental versus theoretical approach

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about −7.6μB per Mo–Fe antisite pair as in other previous experiments, where the theoretical calculation predicts −6.56μB per anti...

Optimized transport properties of LaAlO3/SrTiO3 heterointerfaces by variation of pulsed laser fluence

We show the influence of pulsed laser deposition fluence on the transport properties of the LaAlO(3)/SrTiO(3) (LAO/STO) heterointerface. Structural characterization by x-ray diffraction and medium energy ion spectrometry enables us to deduce that the electronic behaviour is extremely sensitive to the stoichiometry of the LAO layer as well as the structural quality of the STO surface. An optimum balance of these two quantities is demonstrated for an intermediate laser fluence.

Coexistence of strong ferromagnetism and polar switching at room temperature in Fe3O4–BiFeO3 nanocomposite thin films

10% Fe3O4–90% BiFeO3 nanocomposite thin films of 180 nm thickness were grown by pulsed laser deposition on SrTiO3 (011) single crystals. A 3–4 nm nanolamella structure of Fe3O4 and BiFeO3 was formed. While BiFeO3 has the expected epitaxial relationship with the substrate, Fe3O4 grew epitaxially and highly strained (7%). Compared to pure Fe3O4 films of similar thickness, the nanolamella structure of Fe3O4 gives rise to a greatly enhanced saturation magnetization of 900 emu/cc, and, after field cooling, an enhanced coercivity of 450 Oe. Piezoresponse force microscopy measurements show similar polar switching properties between the nanocomposite and pure BiFeO3 films.

Strain dependent defect mediated ferromagnetism in Mn-doped and undoped ZnO thin films

The structural and magnetic properties of pulsed laser deposited zinc oxide thin films have been investigated. Room temperature ferromagnetism is present in undoped as well as Mn-doped films. The saturation magnetization of the thin films reveals a dependence on both the composition and the out of plane lattice parameter. X-ray magnetic circular dichroism down to 2 K reveals a purely paramagnetic contribution from the Mn in Mn:ZnO films. We conclude that the observed ferromagnetism arises entirely from intrinsic defects in the ZnO which can be varied by manipulation of the lattice parameter.

Pressure effect on the magnetization of Sr2FeMoO6 thin films grown by pulsed laser deposition

Thin films of Sr2FeMoO6 (SFMO) are grown on SrTiO3 (001) substrates by pulsed laser deposition. The best films provide 3.2μB∕f.u. at 5K, a Curie temperature above 400K, low roughness, high crystallinity, and low splashing. Therefore, the use of such SFMO electrodes in magnetic tunnel junctions patterned with conventional lithography is promising. Pseudomorphic epitaxial growth is obtained for thicknesses under 50nm. Above this thickness the films do not relax homogeneously. A coherent and systematic variation of the magnetization with the deposition conditions is obtained, which highlights a high reproducibility. Under a reasonable O2 partial pressure to avoid parasite phases, the limiting factor for high magnetization is the total pressure or the deposition rate. Therefore, the deposition rate is suspected to have a strong influence on the Fe∕Mo ordering. Highly magnetic samples are obtained under a low gas flow of either a 20% O2+N2 or a 0.3% O2+Ar.

Room Temperature Ferrimagnetism and Ferroelectricity in Strained, Thin Films of BiFe0.5Mn0.5O3

Highly strained films of BiFe0.5Mn0.5O3 (BFMO) grown at very low rates by pulsed laser deposition were demonstrated to exhibit both ferrimagnetism and ferroelectricity at room temperature and above. Magnetisation measurements demonstrated ferrimagnetism (TC ∼ 600K), with a room temperature saturation moment (MS) of up to 90 emu/cc (∼ 0.58 μB/f.u) on high quality (001) SrTiO3. X-ray magnetic circular dichroism showed that the ferrimagnetism arose from antiferromagnetically coupled Fe3+ and Mn3+. While scanning transmission electron microscope studies showed there was no long range ordering of Fe and Mn, the magnetic properties were found to be strongly dependent on the strain state in the films. The magnetism is explained to arise from one of three possible mechanisms with Bi polarization playing a key role. A signature of room temperature ferroelectricity in the films was measured by piezoresponse force microscopy and was confirmed using angular dark field scanning transmission electron microscopy. The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.