Epitaxial Multiferroic Thin Film Heterostructure of (SrTiO3/NiO)n/MgO For Use as a Future Negative Index Material
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Multiferroic materials are those that exhibit both magnetic polarization and electrical polarization in the same phase. A multiferroic thin film heterostructure consisting of antiferromagnetic NiO and dielectric SrTiO3 is interesting due to the possibility of achieving a negative index of refraction in the far infrared. If the ionic resonance frequency of SrTiO3 (~100cm-1) and the antiferromagnetic resonance of NiO (~36 cm-1) can be shifted to match at some frequency, the composite material should exhibit a negative index of refraction at that frequency. It should be possible to shift the SrTiO3 resonance to lower frequency by lowering the temperature or by doping with Ba and therefore raising the ferroelectric Curie temperature. The NiO antiferromagnetic resonance should shift to higher temperature by applying an external magnetic field or by doping with ions with higher anisotropy, such as Fe or Co. Pressed powder bulk composite samples of NiO/SrTiO3 have been fabricated and used to verify that NiO and SrTiO3 are compatible and non-reacting up to a temperature of 1550?C. FTIR measurements on these bulk samples verify the existence of the ionic and antiferromagnetic resonances of interest. An epitaxial multiferroic composite of (SrTiO3/NiO)n/MgO has also been fabricated using reactive off-axis rf sputtering with n = 1 or 2. Crystal quality has been verified using x-ray diffraction and ion channeling with Rutherford backscattering. The full width at half max for the SrTiO3 (100) diffraction rocking curve is only 1.3? for the composite with n = 2. Off-axis sputtering is a useful technique because it can be used to achieve a concentration gradient between constituents. This allows for a method of quickly determining the effects of Ba doping in SrTiO3 or Co, Fe doping in NiO. It should be possible to measure the frequency response of these films in the future with FTIR techniques with polarized radiation and/or a synchrotron high intensity source.