Epitaxial films have the potential to be used as model systems for fundamental investigations on the martensitic transformation in binary NiTi. In this paper, we discuss growth of binary NiTi thin films on single crystalline MgO substrates. Sputter deposition is used to grow NiTi films. Films prepared by complementary preparation routes (with different deposition temperatures and subsequent heat treatments) are investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electrical resistivity measurements, with the aim of optimizing film properties, particularly to obtain a well defined orientation of the austenitic unit cell and smooth surfaces. Our results show that deposition at elevated temperatures and carefully controlled subsequent heat treatments allow to produce epitaxially grown and smooth NiTi films that exhibit reversible one-or two-step martensitic transformations.

Magnetization fluctuations caused by thermal energies are poorly understood, yet they represent a fundamental limit for many magnetic technologies. Here, thermal fluctuations of the magnetization are quantitatively characterized. The model magnetic system is thin square magnetic dots (<10 nm) with side lengths down to 200 nm. The magnetic state and the fluctuations are determined in individual dots by measuring the anisotropic magnetoresistance. The magnetization exhibits random telegraph noise with a transition rate determined by the dot size, the applied magnetic field, and the sample temperature. An energy barrier model of the system explains the dependence of the magnetic fluctuations on these parameters and is supported by additional experiments unaffected by the magnetic fluctuations. This independent identification of the mechanism producing magnetic noise, and its agreement with micromagnetic simulations, demonstrates a microscopic understanding of noise extendable to more complex noise sources such as 1/f noise.

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We measure a shape dependent anisotropy energy in isolated square permalloy dots with characteristic dimensions as small as 200 nm. The magnetization behavior is characterized using the anisotropic magnetoresistance to determine the magnetic state of the sample. The behavior is consistent with the sample adopting a quasi-single-domain magnetization with a direction determined by an anisotropy intrinsic to the square geometry. We quantify the magnitude of this anisotropy through two independent measurements and confirm the observed magnetic behavior with micromagnetic simulations.

We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.