2022-11, Szyjka, Thomas, Baumgarten, Lutz, Rehm, Oliver, Richter, Claudia, Matveyev, Yury, Schlueter, Christoph, Mikolajick, Thomas, Schroeder, Uwe, Müller, Martina

Lanthanum (La) doping is considered as a promising route to overcome reliability issues of switchable ferroelectric HfO₂-based devices. This study links the local chemistry at the La lattice sites with local and collective electronic properties of the La:HfO₂ matrix using hard X-ray photoelectron spectroscopy. The satellite structure of the La 3d core level, the plasmonic excitation energies, and core-level rigid binding energy shifts are investigated for La:HfO₂ samples with a wide range of La doping, ranging from 3.5% to 33%, i.e., from the doping to dilution level. The emerging chemical phases and electronic properties are discussed as a function of La content. From the evolution of the plasmon excitation energies and rigid binding energy shifts, it is concluded that electronic charge compensation by oxygen vacancies occurs for increasing La content.

2022, Heider, Tristan, Gerber, Timm, Köksal, Okan, Eschbach, Markus, Młyńczak, Ewa, Lömker, Patrick, Gospodaric, Pika, Gehlmann, Mathias, Plötzing, Moritz, Pentcheva, Rossitza, Plucinski, Lukasz, Schneider, Claus M., Müller, Martina

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly dispersive nearly fully polarized Eu 4f bands, as well as O 2p levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrate the exchange splitting in O 2p and its vanishing at the Curie temperature. Our calculations with a Hubbard U term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of indirect exchange in the O 2p levels by analyzing the orbital resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu 4f band manifold. The analysis of the shape of spin-resolved spectra in the Eu 4f spectral region reveals signatures of hybridization with O 2p states, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature, and demonstrate that they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.

2021-05, Szyjka, Thomas, Baumgarten, Lutz, Mittmann, Terence, Matveyev, Yury, Schlueter, Christoph, Mikolajick, Thomas, Schroeder, Uwe, Müller, Martina

2021, Rosenberger, Paul, Opel, Matthias, Geprägs, Stephan, Huebl, Hans, Gross, Rudolf, Müller, Martina, Althammer, Matthias

The spin Hall magnetoresistance (SMR) allows to investigate the magnetic textures of magnetically ordered insulators in heterostructures with normal metals by magnetotransport experiments. We here report the observation of the SMR in in situ prepared ferromagnetic EuO/W thin film bilayers with magnetically and chemically well-defined interfaces. We characterize the magnetoresistance effects utilizing angle-dependent and field-dependent magnetotransport measurements as a function of temperature. Applying the established SMR model, we derive and quantify the real and imaginary parts of the complex spin mixing interface conductance. We find that the imaginary part is by one order of magnitude larger than the real part. Both decrease with increasing temperature. This reduction is in agreement with thermal fluctuations in the ferromagnet.

2022, Müller, Martina, Lömker, Patrick, Rosenberger, Paul, Hussein Hamed, Mai, Mueller, David N., Heinen, Ronja A., Szyjka, Thomas, Baumgarten, Lutz

The tunability of the oxygen content in complex oxides and heterostructures has emerged as a key to designing their physical functionalities. Controlling the interface reactivity by redox reactions provides a powerful means to deliberately set distinct oxide phases and emerging properties. We present routes on how to control oxygen-driven redox mechanisms in ultrathin ferro(i)magnetic and ferroelectric oxide films and across oxide interfaces. We address the growth and control of metastable EuO oxide phases, the control of phase transitions of binary Fe oxides by oxygen migration, the in operando determination of NiFe₂O₄/SrTiO₃ interface band alignments, as well as the role of interfacial oxide exchange in ferroelectric HfO₂ -based capacitors—uncovered by the unique capabilities of photoelectron spectroscopy and, in particular, using hard x-rays.

2022, Rosenberger, Paul, Müller, Martina

Interfacial oxygen exchange at oxide interfaces bears huge potential in stabilizing metastable or novel phases of functional oxides down to the monolayer limit. By taking advantage of active oxygen supply of the substrate material, waiving any external oxygen dosage, high-quality, crystalline ultrathin films of the Heisenberg ferromagnet europium monoxide (EuO) were stabilized on YSZ (001). This so-called redox-assisted growth mode (or, vice versa, the extreme case of a distillation growth) was monitored end to end by in situ x-ray photoelectron emission spectroscopy and electron diffraction techniques. The evolution of Eu 3d core levels allows us to disentangle the processes of interfacial oxygen diffusion and vacancy formation in stabilizing the very first monolayers of EuO on YSZ (001). An expedient background correction analysis is presented, which allows us to quantify the critical Eu³⁺/Eu²⁺ ratio in the ultrathin film regime. We concluded on the key mechanisms of redox-assisted EuO/YSZ (001) thin film synthesis, which merge in a universal three-process growth model that may serve as guideline for redox-assisted synthesis of metastable low-dimensional oxides.

2021, Hamed, Mai Hussein, Mueller, David N., Müller, Martina

The bulk and emerging interface properties of magnetite Fe₃O₄ /YSZ(001) heterostructures grown by pulsed laser deposition are investigated. Fe₃O₄ thin films (4–38 nm) grow epitaxially in (111) orientation and undergo a Verwey transition at max. = 1170.6K. Surprisingly, the formation of interfacial Fe₂O₃ phase is observed albeit the quasi-inert properties of yttrium-stabilized zirconia (YSZ). Possible mechanisms include either (i) thermodynamically induced interfacial redox reaction at the YSZ substrate surfaces or (ii) oxygen diffusion from the outside atmosphere, as YSZ is a very good oxygen ion conductor. Hence, substrate-assisted oxygen supply may enable the control of emerging interface functionalities.

2022, Schöffmann, Patrick, Sarkar, Anirban, Hussein Hamed, Mai, Bhatnagar-Schöffmann, Tanvi, Pütter, Sabine, Kirby, Brian J., Grutter, Alexander J., Barthel, Juri, Kentzinger, Emmanuel, Stellhorn, Annika, Müller, Martina

2021-05, Mittmann, Terence, Szyjka, Thomas, Alex, Hsain, Istrate, Marian Cosmin, Lomenzo, Patrick D., Baumgarten, Lutz, Müller, Martina, Jones, Jacob L., Pintilie, Lucian, Schroeder, Uwe

Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O₂, and N₂ annealing atmospheres significantly change the ferroelectric performance. The use of oxygen‐deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake‐up effect. It is found that oxygen‐rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.

2021, Baumgarten, Lutz, Szyjka, Thomas, Mittmann, Terence, Materano, Monica, Matveyev, Yury, Schlueter, Christoph, Mikolajick, Thomas, Schroeder, Uwe, Müller, Martina

We investigate the emerging chemical states of TiN/HfO2/TiN capacitors and focus especially on the identification of vacancies and impurities in the ferroelectric HfO2 layers, which are produced either by physical vapor deposition (PVD) or atomic layer deposition (ALD). Depending on the specific growth conditions, we identify different mechanisms of oxygen vacancy formation. Corresponding spectral features are consistently observed for all HfO2- and TiN-related core levels by hard x-ray photoelectron spectroscopy (HAXPES). In ALDgrown samples, we find spectral signatures for the electronic interaction between oxygen vacancies and nitrogen impurities. By linking the HAXPES results to electric field cycling experiments on the TiN/HfO2/TiN capacitors, we discuss possible formation mechanisms and stabilization of the ferroelectric HfO2 phase directly related to specific PVD or ALD conditions.