Thin film quantum materials

Abstract:

Manganese arsenide is a promising candidate for new spintronics applications since it is ferromagnetic at room temperature and can be grown with high epitaxial quality on semiconductors. However, the transition temperature of ∼40 °C is a limiting factor for device applications. Since the coupled magnetic and structural transition is of first order, it is in principle possible to shift the transition temperature by changing external parameters. Here we show that by either applying an external magnetic field or by growing the MnAs films on GaAs(1 1 1), i.e., by changing the epitaxial constraints which are equivalent to external pressure, ferromagnetic order can be stabilized well above the bulk-phase transition temperature. © 2004 Elsevier B.V. All rights reserved.

Abstract:

We provide the first direct evidence of interactions in a self-organized pattern of coexisting ferromagnetic and paramagnetic MnAs stripe domains as a function of the coupling strength between the stripes (inter) and within one stripe (intra). The value of the intra exchange constant A = 17.7 × 1010 erg/cm is determined unambiguously from spin wave resonance. Additionally, in-plane ferromagnetic resonance measurements reveal the presence of a strong coupling between the neighboring stripes, which can dominate the weak intra-stripe interaction.

Abstract:

High power applications of Surface Acoustic Wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress will be shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Abstract:

The Si-doped n-type AIN/GaN distributed bragg reflectors grown on 6H-SiC(0001) were discussed. It was observed that the structures were crack-free and have a stopband centered around 450 nm with a full width at half maximum between 40 and 50 nm. It was also observed that the maximum measured reflectance was ≥99%. It was found that the vertical conductance measurements at room temperature on the samples shown on ohmic I-V behavior in the entire measurement range.