UNIVERSITÀ DI PARMA
DIPARTIMENTO DI SCIENZE MATEMATICHE, FISICHE E INFORMATICHE

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====== Au/STO/LSMO/STO heterostructures ======
See also [[http://arxiv.org/abs/0805.1813|arXiv.org:0805.1813]], [[http://arxiv.org/abs/0706.2688|arXiv.org:0706.2688]]

===== NMR results =====


Zero-field <sup>55</sup>Mn [[NMR]] spectra were collected with the home-built broadband fast-averaging [[NMR]] spectrometer [[http://www.fis.unipr.it/infm/home/pmwiki.php?n=PaRMA.HyReSpect|HyReSpect]] on a tuned probe circuit at T=1.6 K. The <sup>55</sup>Mn NMR spectra were obtained by Fourier transforming two pulse sequence. The plotted spectra are always corrected for [[NMR]] sensitivity, rescaling their amplitudes by ω<sup>2</sup>.

{{ andrey.sidorenko:nmr_spec.png| Zero-field <sup>55</sup>Mn [[NMR]] in variant heterostructures measured at T = 2 K. RF field is applied in the film plane.}}

Figure (for t<sub>LSMO</sub>=4 nm solid lines represent a best two components fit) shows the <sup>55</sup>Mn [[NMR]] spectra that we collected from several heterostructures with and without the gold overlayer. In all spectra at least two distinct lines at f<sup>4+</sup>≈322 MHz and f<sup>DE</sup>≈376 MHz are observed and may be attributed to two different strain induced phases with the localized charges (Mn<sup>4+</sup> state) and with itinerant carriers (Mn<sup>DE</sup> state), respectively. As can be seen the intensity of f<sup>DE</sup> peak (the area under curve) scales with the film thickness, and it is expected to vanish for a LSMO thickness corresponding to the dead-layer, whereas the intensity of the peak at f<sup>4+</sup> is practically unvaried. //Note: the thickness of the dead-layer in LSMO/STO roughly coincides with the critical thickness of the order of 3 nm below which the manganite films appear to be insulating.//

The spectra of the LSMO films terminated with Au capping also reveal two resonance lines at the same frequencies f<sup>4+</sup> and f<sup>DE</sup> (red squares in Figure). If an oxygen deficiency took place in Au/LSMO heterostructures, the positions of these resonance lines would be different. An oxygen deficiency decreases the Mn<sup>4+</sup> concentration and affects the crystal lattice, reducing the transfer interaction of e<sub>g</sub> electrons and weakening the ferromagnetic double exchange interactions, and, hence, is expected to have an influence on the hyperfine interactions of manganese ions, which can be measured by [[NMR]]. The expected hole concentration (Mn<sup>4+</sup>) is given by simple relation c=0.33-2·δ so assuming the oxygen vacancies δ = 0.05 (it corresponds to ΔT<sub>c</sub> ≈ 60 K in the case of LSMO) we find that c is of the order of 0.2. However, such hole concentration gives rise to the different zero-field <sup>55</sup>Mn [[NMR]] spectra.

On the other hand, because the intensity of the [[NMR]] line is proportional to the number of the nuclei in the specific electronic and magnetic state, the changing of the Mn<sup>3+</sup>/Mn<sup>4+</sup> ratio due to the presence of the gold layer can be measured by [[NMR]] directly. As can be seen in Figure a marked reduction of the f<sup>DE</sup> signal is found, indicating that the capping layer has a direct influence on this specific electronic state (double exchange state) of the manganite film. The spectroscopic sensitivity of [[NMR]] allows us to detect quite a large effect already on the thickest 12 nm structures. The reduction is percentually larger for thinner films, as it would be expected in the case Au induces a constant finite thickness depletion layer in LSMO. For the case of the 4 nm thick LSMO film with the gold capping the zero-field [[NMR]] signal is beyond the sensitivity of our [[NMR]] spectrometer and was not detected revealing that major part of the LMSO film might be in the disordered state. 

===== Results of SQUID measurements=====

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UNIVERSITÀ DI PARMA
DIPARTIMENTO DI SCIENZE MATEMATICHE, FISICHE E INFORMATICHE