Marco Liberati

Enhanced magnetism in epitaxial SrRuO3 thin films

We observed enhanced magnetization in epitaxial SrRuO3 thin films compared to previously reported bulk and thin film values. The enhancement is strongly dependent on the orientation of the lattice distortions imposed by (001), (110), and (111) oriented SrTiO3 substrates. A larger magnetization enhancement for coherently strained SrRuO3 films on (111) and (110) oriented SrTiO3 compared to films on (001) SrTiO3 confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. Moreover, SrRuO3 films on (111) SrTiO3 exhibit enhanced moments as high as 3.4 μB/Ru ion, suggesting the stabilization of a high-spin Ru4+ state.

Enhanced magnetization in epitaxial SrRuO3 thin films via substrate-induced strain

Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.Epitaxial SrRuO3 thin films were grown on SrTiO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and LaAlO3 substrates inducing different compressive strains. Coherently strained SrRuO3 films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1–1.6μB per formula unit. A comparison of (001) SrRuO3 films on each substrate indicates that strained films have consistently higher saturated moments than corresponding relaxed films, which exhibit bulk moments. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.

Ferromagnetism in tetragonally distorted LaCoO3 thin films

Thin films of epitaxial LaCoO{sub 3} were synthesized on SrTiO{sub 3} and (La, Sr)(Al, Ta)O{sub 3} substrates varying the oxygen background pressure in order to evaluate the impact of epitaxial growth as well as oxygen vacancies on the long range magnetic order. The epitaxial constraints from the substrate impose a tetragonal distortion compared to the bulk form. X-ray absorption and x-ray magnetic circular dichroism measurements confirmed that the ferromagnetism arises from the Co ions and persists through the entire thickness of the film. It was found that for the thin films to show ferromagnetic order they have to be grown under the higher oxygen pressures, since a decrease in oxygen deposition pressure alters the film structure and suppresses ferromagnetism in the LaCoO{sub 3} films. A correlation of the structure and magnetism suggests that the tetragonal distortions induce the ferromagnetism.

Effect of electrode and EuO thickness on EuO-electrode interface in tunneling spin filter

The effect of electrode material and EuO film thickness on the interface between the two was studied. Of the electrodes examined, yttrium was found to decrease the formation of the nonmagnetic oxide Eu2O3. By decomposing the x-ray absorption spectroscopy (XAS) spectra of the samples with different electrodes against the reference EuO and Eu2O3 spectra, the relative fraction of these two species was quantified. Multilayers with silver electrodes had the highest amount of Eu2O3, about 41%, whereas aluminum had 28% compared to the less than 5% observed for the multilayers with yttrium electrodes. A slight decrease in the fraction of Eu2O3 with increasing EuO thickness was found. Angle dependent XAS measurements, done on 80A EuO film, indicated the presence of Eu2O3 at high grazing angles which then decreased drastically with decreasing grazing angle. This indicated that the Eu2O3 was localized at the EuO-electrode interface.

Rotatable magnetic anisotropy of CoO/Fe/Ag(001) in ultrathin regime of the CoO layer

CoO/Fe thin films were grown epitaxially onto vicinal Ag(001) and investigated using magneto-optic Kerr effect, x-ray magnetic circular dichroism (XMCD), and x-ray magnetic linear dichroism (XMLD) techniques. We show that the CoO film in the ultrathin regime does not induce a uniaxial magnetic anisotropy but a coercivity enhancement. This result provides a mechanism for the microscopic origin of the rotatable magnetic anisotropy. XMLD measurement further reveals that the underlying mechanism is that the CoO spins are totally rotatable in the ultrathin regime to follow the Fe magnetization.

Role of magnetic anisotropy in spin-filter junctions

Role of magnetic anisotropy in spin-filter junctions R.V. Chopdekar, 1,2* Franklin Wong, 1,3 B.B. Nelson-Cheeseman, 1† M. Liberati, 4 E. Arenholz, 4 Y. Suzuki 1,3 Department of Materials Science and Engineering, UC Berkeley, Berkeley, CA 94720 2. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, 14853 3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT We have fabricated oxide based spin filter junctions in which we elucidate the role of magnetic anisotropy in the transport behavior of spin filter junctions. Until recently, spin filters have been largely comprised of polycrystalline materials where the spin filter barrier layer and one of the electrodes are ferromagnetic. While these spin filter junctions have relied on the weak magnetic coupling between one ferromagnetic electrode and a barrier layer or the insertion of a nonmagnetic insulating layer in between the spin filter barrier and electrode, we have demonstrated that by careful choice of the magnetic anisotropy of the ferromagnetic layers, we can tune the interface anisotropy and hence the junction magnetoresistance in epitaxial oxide based spin filter junctions. presently at Argonne National Laboratory, Argonne, IL, 60439.

Amorphous clusters in Co implanted ZnO induced by boron preimplantation

We demonstrate the formation of superparamagnetic/ferromagnetic regions within ZnO(0001) single crystals sequently implanted with B and Co. While the pre-implantation with B plays a minor role for the electrical transport properties, its presence leads to the formation of amorphous phases. Moreover, B acts strongly reducing on the implanted Co. Thus, the origin of the ferromagnetic ordering in local clusters with large Co concentration is itinerant d-electrons as in the case of metallic Co. The metallic amorphous phases are non-detectable by common X-ray diffraction.

Electronic structure of halogen doped CuCr2Se4

Electronic Structure of Halogen Doped CuCr 2 Se 4 M. Liberati 1,6 , J. R. Neulinger 2 , R.V. Chopdekar 3,1 , J.S. Bettinger 1 , E. Arenholz 4 , W.H. Butler 5 , A.M. Stacy 2 , Y.I. Idzerda 6 , Y. Suzuki 1 Materials Science and Engineering, UC Berkeley, Berkeley, California; Department of Chemistry, UC Berkeley, Berkeley, California; School of Applied Physics, Cornell University, Ithaca, New York; Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California; Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama. Department of Physics, Montana State University, Bozeman, Montana; ABSTRACT We have employed element and chemically sensitive X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) in order to address a long standing controversy regarding the electronic and magnetic state of CuCr 2 Se 4 via halogen doping of the Se anion site in CuCr 2 Se 4-x Y x (Y=Cl and Br). Long range magnetic order is observed above room temperature for all samples. The Cr L 2,3 XAS spectra show a prevalent 3+ valence for the Cr ions independent of doping concentration and doping agent. The Cu L 2,3 XAS spectra show a combination of 1+ and 2+ valence states for all samples. XMCD spectra indicate the presence of a magnetic moment associated with the Cu ions that is aligned antiparallel to the Cr moment.

New materials for spintronics: CuCr2Se4.

We have synthesized a new structural variation of CuCr₂Se₄ by chemical vapor transport technique (CVT) with SeCl₄. Both bulk and surface magnetic measurements are in agreement in measuring an enhancement in the saturation moment as the doping level is increased. On the other hand, a decrease in T_c is observed for increasing CI doping.