Č. Šimkevičius

AGEING EFFECTS ON ELECTRICAL RESISTIVITY AND MAGNETORESISTANCE OF NANOSTRUCTURED MANGANITE FILMS

The long-term stability of electrical resistance and magnetoresistance in nanostructured La1-xSrxMnO3 (x = 0.17) manganite thin films grown on lucalox (Al2O3 + MgO) substrate by the MOCVD method was investigated. It was found that the storage of up to 3 months of the free surfaces of these films in normal atmosphere (air) conditions increases their resistivity by almost two times, while the annealing of the films in an Ar atmosphere at 450 °C decreases their resistivity only by 15%. It was concluded that the final increase of resistivity is determined by a long-term relaxation of the grain boundaries in the nanostructured films. The magnetoresistance of the films does not change significantly, which produces an advantage for magnetic field sensor applications. The passive protective coating of the free surfaces of the films stabilizes their electrical and magnetic properties. The results were analysed using various electron scattering mechanisms when the films were in a ferromagnetic state, and the Mott’s variable range hopping model when they were in a paramagnetic insulating state.

Manganite Based Strong Magnetic Field Sensors Used for Magnetocumulative Generators

The effects of strong pulsed magnetic field and shock waves on the electrical properties of La 0.67 Ca 0.33 MnO 3 and La 0.7 Sr 0.3 MnO 3 thin films were investigated. It was demonstrated that these films could be used to measure the magnetic fields produced by magnetocumulative generators. Possible ways to design fast magnetic field sensors operating at room temperature are also discussed.

Magnetoresistance anisotropy of ultrathin epitaxial La0.83Sr0.17MnO3 films

We present the study of temperature dependence of resistivity (ρ), magnetoresistance (MR), and magnetoresistance anisotropy (AMR) of thin epitaxial La0.83Sr0.17MnO3 films. The films with thickness from 4 nm to 140 nm were grown on an NdGaO3 (001) substrate by a pulsed injection metal organic chemical vapor deposition technique. We demonstrate that the resistivity of these films significantly increases and the temperature Tm of the resistivity maximum in ρ(T) dependence decreases with the decrease of film thickness. The anisotropy of ρ(T) dependence with respect to the electrical current direction along the [100] or [010] crystallographic axis of the film is found for ultrathin films (4–8 nm) at temperatures close to Tm. Both MR and AMR, measured in magnetic fields up to 0.7 T applied in the film plane parallel and perpendicular to the current direction, have shown strong dependence on the film thickness. It was also found that the anisotropy of magnetoresistance could change its sign from positive (thicke...

Multistep Accelerated Aging of Magnetic Field Sensors Based on Nanostructured La–Sr–Mn–O Thin Films

We present results of investigation of accelerated aging influence on electrical resistivity and magnetoresistance (MR) of nanostructured La–Sr–Mn–O films used for magnetic field sensors, which could be applied in advanced scientific and industrial devices. The 400-nm-thick manganite films were grown by metal organic chemical vapor deposition technique. To investigate the effectiveness of accelerated aging, the three methods were tested: 1) thermal treatment of the sensor samples at 100 °C temperature in Ar atmosphere when film surface was uncoated; 2) thermal treatment of the samples coated prior to treatment by polymer; and 3) multistep aging including both previous methods: thermal treatment of the samples with uncoated surface in Ar atmosphere and additional thermal treatment of the coated samples. The sensor samples were studied without and in pulsed magnetic field up to 20 T. To describe the underlying processes, the kinetics of resistivity change during accelerated aging was analyzed using a stretched exponent relaxation function. It was found that the best long-term stability of resistivity in time demonstrated samples treated by a multistep accelerated aging method. All three aging methods resulted in minor influence (<1%) on MR of the films in magnetic fields up to 20 T.

Fast damaging processes in the TaN thin film absorbers under action of nanosecond electrical pulses

Purpose The purpose of this paper is to investigate damaging processes in TaN thin film absorbers under action of high-voltage electrical pulse of nanosecond duration. Despite having mechanical origin of crack opening, estimation based on the readings from oscillograms shows uncharacteristically high velocities of the crack propagation. Design/methodology/approach Microscopic images of damaged absorbers showing the final result of the damaging process provided initial information about its geometrical peculiarities. Then, to clarify the dynamics of the process, the authors create the model of the crack, having elements of self-similarities and multiple stage opening. The influence of heating induced by current concentration at crack tip and of magnetic stress of this concentrated current are both included in the model. Findings Using physical parameters of TaN layers with flowing current and performing calculations the authors define the conditions required to initiate the damaging process and to sustain it. Danger of such damage is relevant for high-Tc superconducting thin films after their switching to normal state which is induced by the high-voltage pulse. Practical implications There were made recommendations to manufactures aiming to improve electrical durability of the absorbers in an effort to prevent the damaging influence of power nanosecond electrical pulses. Originality/value Three stage opening model implies the appearance of zone of high-energy dissipation that can lead to detonation-like destruction of the film and, therefore, explain the high velocities of crack propagation.

Current Sensing System for Protection of High Power Frequency Converters

The design and test results of a current-sensing system used for the protection of frequency converters, which are used for controlling the speeds of ac induction motors, are presented. The proposed pulse current-sensing system consists of a magnetic field sensor, which is made of thin polycrystalline La-Sr-Mn-O manganite film exhibiting the colossal magnetoresistance (CMR) effect, and a microcontroller with a 12-bit A/D converter that is used for recording the fault current signal. The magnetic field generated by the high power current cable is fed using a flux concentrator to the manganite magnetic field sensor (CMR-resistor). The design of the contactless current sensor, consisting of two manganite resistors, is developed and investigated. The system is tested in the ambient temperature range of 0-45°C. The possibilities of using this method in other pulsed power systems are discussed.