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Register a new userMagnetic Circular Dichroism spectroscopy in epitaxial La0.7Sr0.3MnO3 thin films
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T. K. Nath
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Magneto optic measurements are a very powerful tool for investigating the polarization of a conduction band as a function of temperature and are used here to study the polarization of the mobile electrons in 50nm LSMO (x=0.3) strained thin films grown epitaxially on single crystalline (001) LaAlO3 (LAO) and (001) lattice matched substrate (LSAT). The magnetic circular dichroism (MCD) has been investigated in magnetic fields up to 0.5 T and over a temperature range (10 to 450 K). The MCD spectra of both the films show a peak at the band gap at around 3 eV and the peak is found to be shifted towards lower energy side with the increase of temperature. A separate polaron peak (well known in insulating samples) appears at lower energy (about 1.8 eV) with the increase of temperature in all these metallic films. The rapid decrease in conduction band polarization in the film on LAO has strong implications for the use of these manganites in room temperature spintronics.
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The difference in the transmission for left and right circularly polarised light though thin films on substrates in a magnetic field is used to obtain the magnetic circular dichroism of the film. However there are reflections at all the interfaces and these are also different for the two polarisations and generate the polar Kerr signal. In this paper the contribution to the differences to the total transmission from the transmission across interfaces as well as the differences in absorption in the film and the substrate are calculated. This gives a guide to when it is necessary to evaluate these corrections in order to obtain the real MCD from a measure of the differential transmission due to differential absorption in the film.
We have detected strong dichroism in the Ni $L_{2,3}$ x-ray absorption spectra of monolayer NiO films. The dichroic signal appears to be very similar to the magnetic linear dichroism observed for thicker antiferromagnetic NiO films. A detailed experimental and theoretical analysis reveals, however, that the dichroism is caused by crystal field effects in the monolayer films, which is a non trivial effect because the high spin Ni $3d^{8}$ ground state is not split by low symmetry crystal fields. We present a practical experimental method for identifying the independent magnetic and crystal field contributions to the linear dichroic signal in spectra of NiO films with arbitrary thicknesses and lattice strains. Our findings are also directly relevant for high spin $3d^{5}$ and $3d^{3}$ systems such as LaFeO$_{3}$, Fe$_{2}$O$_{3}$, VO, LaCrO$_{3}$, Cr$_{2}$O$_{3}$, and Mn$^{4+}$ manganate thin films.
The Channel-Spark method was used for deposition of highly oriented ferromagnetic La0.7Sr0.3MnO3 films on NdGaO3 substrates. It was found that additional oxygen decreases the film quality suppressing the Curie temperature and metal-insulator transition below the room temperature. To achieve the best quality of the films the samples were either annealed in high vacuum at deposition temperature or even deposited in argon atmosphere with no oxygen annealing. For such films the resistive measurements showed a metallic behaviour in the interval 10-300 K in accordance with the high Curie point (Tc 350 K). Micro-Raman analysis indicate that the La0.7Sr0.3MnO3 films are well ordered, while some outgrowths show stoichiometrical deviations.
Scanning tunnelling spectroscopy measurements were performed on La0.7Sr0.3MnO3 thin films both at room temperature and liquid nitrogen temperature. While no inhomogeneities were recorded at liquid nitrogen temperature on any sample, a clear evidence of spectroscopic inhomogeneities was evident in tunnelling conductance maps collected at room temperature. The investigated films exhibit a transition from a ferromagnetic-metallic to a paramagnetic-insulating state around room temperature, so that the observed spectroscopic features can be interpreted within a phase separation scenario. A quantitative analysis of the observed spectroscopic features is reported pointing out the occurrence of phase modulation and its possible correlation with the properties of the system.
Imaging the magnetic configuration of thin-films has been a long-standing area of research. Since a few years, the emergence of two-dimensional ferromagnetic materials calls for innovation in the field of magnetic imaging. As the magnetic moments are extremely small, standard techniques like SQUID, torque magnetometry, magnetic force microscopy and Kerr effect microscopy are challenging and often lead to the detection of parasitic magnetic contributions or spurious effects. In this work, we report a new magnetic microscopy technique based on the combination of magnetic circular dichroism and Seebeck effect in semiconductor/ferromagnet bilayers. We implement this method with perpendicularly magnetized (Co/Pt) multilayers sputtered on Ge (111). We further show that the electrical detection of MCD is more sensitive than the Kerr magnetometry, especially in the ultra-thin film regime, which makes it particularly promising for the study of emergent two-dimensional ferromagnetic materials.
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