Search Results (6 total)

In this paper, we report on field-effect experiments in La_0.7Sr_0.3MnO₃ side-gate channels patterned on ultrathin epitaxial films having thickness ranging from 12 to 4 unit cells. Transport mechanisms and competition between phases, under the effect of electric and magnetic fields, as well as of other perturbations such as disorder and proximity to the interface with substrate are explored. We observe, in a 7 unit cells thick sample, a shift of the metal-insulator transition temperature as high as 43 K and a resistivity modulation up to 250% at low temperatures. In striking contrast, the 6–4 unit cells thick samples result to be insulating and almost insensitive to field-effect modulation. Such a finding indicates that for films thinner than 7 unit cells, a strong localization mechanism develops, which cannot be healed by band refilling. On the other hand, our results are compatible with a Mn e_g orbital rearrangement driven by the broken translational symmetry at the surface and/or interface, which suppresses the double-exchange mechanism and localizes the carriers.

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

We explore the possibility of tracking magnetization orientation in artificial structures patterned in La_0.7Sr_0.3MnO₃ epitaxial films by magnetotransport measurements, exploiting the anisotropic magnetoresistance effect. We perform resistance measurements as a function of temperature, magnetic field, and angle between the applied in-plane magnetic field and the channel or crystalline axes in micrometric channels of different widths. We analyze quantitatively our results and extract information about magnetization easy axes, crystalline anisotropy, domain wall resistance, anisotropic magnetoresistance, energy of magnetic domain pinning, and magnetic reversal mechanisms. For channel widths larger than a few micrometers, the magnetization direction at low field (≤200  Oe) is determined by magnetocrystalline easy axes, whereas for channel widths of ∼1  μm or smaller, the shape anisotropy forces the magnetization to align along the channel axis. This gives a precise indication on how artificial patterning can be used to force the magnetization direction in manganite based spintronic devices. Values of magnetocrystalline constant up to 8000  J∕m³ and anisotropic magnetoresistance values between 0.1% and 0.6% are found. Our data also indicate that, in the low field (≤200  Oe) hysteretic regime, magnetization reversal occurs by thermal activated hopping of domain walls, with a characteristic hopping distance of 4–5  nm.

By carrying out differential resistance measurements in oxygen deficient La_0.67Ba_0.33MnO_3−δ thin films at different magnetic fields, in submicrometric constricted regions patterned by focused ion beam, we find evidence of hysteretic resistance behavior as a function of both the external magnetic field and dc bias current. The resistance curves exhibit a marked asymmetry with respect to the polarity of the current. We suggest that the spin-polarized injected current exerts a torque on magnetic domains, whose rotation accounts for the hysteretic resistance changes. The memory effect of such constrictions is potentially interesting both for studying micromagnetic effects and in view of spintronics devices applications.

We perform field-effect experiments on side-gate devices made of underdoped La_0.7Sr_0.3MnO_3−δ epitaxial thin films. The resistance modulation is small due to the high carrier concentration in this compound; however, it is significantly enhanced by an external magnetic field. We discuss our results in the framework of a double-exchange mechanism and phase separation and suggest that our experimental data together with other complementary data of the literature are a straightforward consequence of the percolative nature of transport in the phase-separation regime of manganites. We demonstrate the possibility of driving the percolation process by field effect.