Angle-dependent magnetoresistance in epitaxially strain-engineered thin films

This thesis reports on magnetoresistance studies in strained thin films of La0.67Sr0.33MnO3 (LSMO) on LaAlO3 and SrTiO3. Strain-engineering thin films is a method to alter material properties such as the magnetization and electronic transport to alter them to a more desirable state. In this thesis, the magnetic anisotropy of the films by varying temperature, applied magnetic field and the angle between current and field in etched LSMO Hall bars is studied. Based on these studies three different magnetic phases when rotating the magnetic field in-plane. At temperatures between 10 to 125 K, a fourfold symmetry is observed, indicating a biaxial anisotropy parallel to the (110) and (1-10) directions . A second magnetic phase between 125 K to 250 K is observed and characterized by a twofold in-plane angle-dependent magnetoresistance signal. Increasing the temperature, leads to a paramagnetic insulating phase. The results show that the temperature variation of the angular dependence of the magnetoresistance is a reliable and alternative approach to study the different kinds of anisotropy induced in such strained oxide films and can complement torque-based magnetometry studies. In the second angle-dependent study (chapter 5) the aim was to find the strain at which the magnetic easy-axis would rotate out-of-plane. The strong (1.9%) compressive strain indeed caused the magnetic easy axis to point in the out-of-plane direction and remain present up to 80 K. The observed trend indicates a gradual change from in-plane to out-of-plane anisotropy with increasing compressive strain

Erratum:Surface anisotropy induced spin wave nonreciprocity in epitaxial La0.33Sr0.67MnO3film on SrTiO3substrate (Appl. Phys. Lett. (2020) 117 (232402)

In the original published article,1the concentrations of La and Sr are reversed. The correct concentration should be La0.67Sr0.33MnO3(which is in the ferromagnetic phase) rather than La0.33Sr0.67MnO3(which is in the antiferromagnetic phase) in the original published version. They were typos of the element concentrations. This misprint does not change the identification or conclusion presented in the original published paper

Stability and thermoelectric performance of doped higher manganese silicide materials solidi fied by RGS (ribbon growth on substrate) synthesis

Large scale deployment of thermoelectric devices requires that the thermoelectric materials have stable electrical, thermal and mechanical properties under the conditions of operation. In this study we examine the high temperature stability of higher manganese silicide (HMS) materials prepared by the RGS (ribbon growth on substrate) technique. In particular we characterize the effect of element substitution on the structural and electrical changes occurring at the hot side of temperatures of thermoelectric devices relevant to this material (600°C). Only by using suitable substitution (4% vanadium at the Mn site) can we obtain temperature-independent structural parameters in the range 20°C - 600°C, a condition that results in stable electrical properties. Additionally, we show that 4% vanadium substitution at the Mn site offers the best thermoelectric figure of merit among the different compositions reported here with ZTmax=0.52, a value comparable to the state of the art for HMS materials. Our analysis suggests that ionized impurity scattering is responsible for the better performance of this material

Temperature dependence of the magnetization of La0.67Sr0.33MnO3 thin films on LaAlO3

The authors report on the interplay between magnetically ordered phases with temperature and magnetic field across compressively strained interfaces of thin La0.67Sr0.33MnO3 films on LaAlO3 substrates. From the temperature dependence of the magnetization and resistivity studies, they find two distinct temperature regimes, where this interplay is clearly exhibited. They ascribe this to the strain induced Jahn-Teller-like distortion that favors the stabilization of the d(3z2) (r2) orbitals and enhances superexchange between adjoining Mn atoms. The temperature and field sweep of the magnetization and electronic transport lead to a hybridization between the closely spaced energy levels of d(3z2 r2) and d(x2) (y2) orbitals leading to the coexistence of ferromagnetic and antiferromagnetic phases. Such an observation, not reported earlier, offers new routes for the design and study of magnetic textures in variously strained interfaces between perovskite oxides. Published by the AVS

Surface anisotropy induced spin wave nonreciprocity in epitaxial La0.33 Sr0.67 MnO3 film on SrTiO3 substrate

Spin wave propagation in perovskite La0.33 Sr0.67 MnO3 films epitaxially grown on a SrTiO3 substrate of (001) orientation was investigated using an all electrical spin wave spectroscopy technique. The spin wave nonreciprocity in amplitude, resonance frequency, and group velocity of the transmission spectra were observed. The origin of the spin-wave nonreciprocity is attributed to the out-of-plane surface anisotropy, with a value of 1.3 mJ/m2 at the interface with the substrate, as extracted from the theoretical model. The magnetic field dependence of the frequency shift is attributed to the perpendicular surface anisotropy. The important role of the surface anisotropy in the spin wave nonreciprocity was further confirmed by the angle dependent measurements of the spin wave transmission spectra

Investigating the Electromechanical Behavior of Unconventionally Ferroelectric Hf0.5Zr0.5O2‐Based Capacitors Through Operando Nanobeam X‐Ray Diffraction

Abstract Understanding various aspects of ferroelectricity in hafnia‐based nanomaterials is of vital importance for the development of future nonvolatile memory and logic devices. Here, the unconventional and weak electromechanical response of epitaxial La0.67Sr0.33MnO3/Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 ferroelectric capacitors is investigated, via the sensitivity offered by nanobeam X‐ray diffraction experiments during application of electrical bias. It is shown that the pristine rhombohedral phase exhibits a linear piezoelectric effect with piezoelectric coefficient (|d33|) ≈ 0.5–0.8 pmV−1. It is found that the piezoelectric response is suppressed above the coercive voltage. For higher voltages, and with the onset of DC conductivity throughout the capacitor, a second‐order effect is observed. The work sheds light into the electromechanical response of rhombohedral Hf0.5Zr0.5O2 and suggests its (un)correlation with ferroelectric switching