2009-12-01, Müller, Martina, Miao, Guo-Xing, Moodera, Jagadeesh S.

We report on transport measurements in EuO-based tunnel junctions, studying the characteristic features of spin filter tunneling in magnetic insulators. Current-voltage (I-V) curves show a unique voltage dependence that reveals the interplay between direct and spin-selective Fowler-Nordheim (FN) tunneling. Successive onsets of FN tunneling give direct evidence for a spin filtering effect, without relying on the use of external magnetic fields for spin detection. The variation of the effective tunnel barrier height below the Curie temperature was extracted from temperature-dependent I-V characteristics, and systematically correlates with the spontaneous magnetization of EuO. The magnitude of the exchange splitting was deduced and allows the evaluation of spin filter efficiency of EuO spin filter tunnel barriers by fully electrical means.

2009-02-20, Miao, Guo-Xing, Müller, Martina, Moodera, Jagadeesh S.

Spin filtering happens due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetic barrier and can result in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to large magnetoresistance without the necessity of magnetic electrodes. We demonstrate the realization of such unconventional tunnel junctions using double EuS spin filter barriers with Al electrodes. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations.

2007-10-01, Müller, Martina, Matthes, Frank, Schneider, Claus M.

The electronic structure of ferromagnet-insulator interfaces plays a key role in spin-dependent transport processes by determining the spin polarization of the tunneling charge carriers. Employing spin-polarized, angle-resolved photoelectron spectroscopy we studied the spin-resolved electronic structure in the model system MgO/Fe(001) and observed a surprisingly multifaceted influence of the MgO stoichiometry. In particular, oxygen-deficient MgO barriers are found to strongly enhance the interfacial spin polarization. The results highlight the important role of the specific nature of the oxygen bonding and its relevance for the tunnel magnetoresistance effect.

2006-06-01, Tong, Liu-Niu, Deng, Cai-Lian, Matthes, Frank, Müller, Martina, Schneider, Claus M., Lee, Chan-Gyu

The spin polarization of the valence band electronic states of strained bcc Co(001) and MgO/Co(001) thin films grown onto a bcc Fe(001) seed layer on GaAs(001) are investigated by employing spin-resolved photoemission spectroscopy. The experimental results are compared with the calculated energy band structure of bcc and bct Co(001), and discussed in the framework of the interband transition model, which allows one to ascribe the observed spectral features to bands of given spin and spatial symmetry. In contrast to the positive spin polarization observed at the MgO/Fe(001) interface, a large negative spin polarization of the electronic states at the Fermi level is observed for the MgO/Co/Fe/GaAs(001) system. Such a large negative spin polarization is attributed to a change in the energy band structure at the bct Co/bcc Fe(001) interface.

2009-10, Schneider, Christian M., Krug, Ingo, Müller, Martina, Matthes, Frank, Kaiser, Alexander, Wiemann, Carsten, Cramm, Stefan, Elmers, Hans-Joachim, Wegelin, Frederik, Krasyuk, Alexander

Spintronics is a research field involving a wide variety of different magnetic materials. Synchrotron radiation in the VUV and soft X-ray regime is ideally suited to investigate the relationships between magnetic properties and electronic structure of spintronics thin film stacks. Complex layered structures and nanomagnets are the main building blocks for current and future spintronics applications. In this contribution we describe the study of spintronics model systems with respect to the static and dynamic behavior with an emphasis on interfaces.

2009, Müller, Martina

2007-05, Müller, Martina, Matthes, Frank, Schneider, Claus M.

The nature of interface bonding essentially determines the spin polarization of tunneling currents through ferromagnet/insulator interfaces. We focus on the influence of ultrathin off-stoichiometric MgO coverages on the spin-polarized electronic structure of the (001) surface of bcc-Fe. By applying a combination of low-energy spin-resolved and core-level photoelectron spectroscopy, we show that the MgO stoichiometry has a direct impact on the interfacial spin polarization. The results underline the importance of controlling the chemical composition and bonding at ferromagnet/oxide interfaces to further optimize spin-dependent tunneling devices.

2009-04, Müller, Martina, Miao, Guo-Xing, Moodera, Jagadeesh S.

We have studied the thickness dependence of the magnetic and transport properties of thin EuO films in the range of 10–60 Å. The ferromagnetic phase transition shows a systematic dependence of the critical temperature T_c with decreasing EuO film thickness. This behavior has been attributed to the interface layers which play a major role by reducing the number of average magnetic neighbors; we find the effect of interface intermixing becoming relevant in low thickness regime. In addition, we could identify a clear dependence of the onset of the metal-to-insulator transition on the ferromagnetic ordering of thin EuO films.

2008-02-19, Tong, Liu-Niu, Matthes, Frank, Müller, Martina, Schneider, Claus M., Lee, Chan-Gyu

2007, Müller, Martina