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andrey.sidorenko:au_sto_lsmo_sto_heterostructures

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andrey.sidorenko:au_sto_lsmo_sto_heterostructures [Y/m/d H:i]
andrey.sidorenko:au_sto_lsmo_sto_heterostructures [Y/m/d H:i] (versione attuale)
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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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