Magnetómetros de alta sensibilidad implementados con micro-osciladores mecánicos / High sensitivity magnetometers implemented with mechanical micro-oscillators

Antonio, Darío (2009) Magnetómetros de alta sensibilidad implementados con micro-osciladores mecánicos / High sensitivity magnetometers implemented with mechanical micro-oscillators. PhD in Physics, Universidad Nacional de Cuyo, Instituto Balseiro.

PDF (Tesis)

Abstract in Spanish

En esta tesis describimos el diseño, la fabricación y distintas aplicaciones de micromagnetometros para la medición de propiedades magnéticas en muestras micro- y nano- métricas. Estos dispositivos están compuestos por un microoscilador mecánico de alto Q, sensible a las variaciones de campo y momento magnético, y por la electrónica que acondiciona y amplifica las señales que este produce. En el Capítulo 4 presentamos los distintos osciladores fabricados, sus características principales y los métodos utilizados para su excitación y detección. A continuación en el Capítulo 5 detallamos el proceso de diseño y fabricación del amplificador criogénico de transimpedancia integrado que utilizamos para la detección de las oscilaciones mecánicas. El alto Q del oscilador y la posibilidad de detectar cambios en su frecuencia de resonancia de aproximadamente 10mHz nos permiten alcanzar sensibilidades del orden de 10-¹⁸ Nm en torque, y del orden de 10-¹⁶Am"2 en momento magnético, cuando se aplica un campo magnético del orden de 10 kOe. Finalmente en el Capítulo 6 completamos la presentación de los osciladores con el desarrollo de un modelo analítico que describe correctamente su dinámica en los rangos de operación lineal y no-lineal. En cuanto a la implementación de los micromagnetometros, en el Capítulo 7 mostramos como se pueden aplicar a la detección de campos magnéticos externos, utilizándolos en el rango no-lineal y sin necesidad de incorporarle materiales magnéticos al oscilador. Luego en el Capítulo 8 presentamos mediciones de la respuesta magnética de dos nanotubos de manganita con campo magnético aplicado en la dirección perpendicular al eje fácil de magnetización. A partir de ellas obtenemos resultados cuantitativos para la magnetización de saturación, la constante de anisótropa y la susceptibilidad de campos bajos. Además observamos dos regímenes de magnetización diferentes en función de campo magnético, separados por una transición abrupta que esta acompañada por un pico de disipación. Finalmente, en el Capítulo 9 mostramos las mediciones de propiedades magnéticas realizadas en un lm de Nb superconductor de 20x20 μm"2 de superficie y con un espesor de 110 nm. En las curvas de momento magnético en función de campo aplicado y temperatura observamos discontinuidades o picos característicos del régimen mesoscópico. Este comportamiento es cualitativamente diferente al observado en muestras macroscópicas y esta asociado a eventos discretos de entrada de vórtices en el material. Su detección confirma a los micro magnetómetros como herramientas apropiadas para la medición de propiedades magnéticas en la micro- y nano- escala.

Abstract in English

The study of magnetism in micro- and nano- structures has attracted a lot of interest in both basic physics and technological applications research. This relevance is due to the dramatic modication of the physical properties as sizes are reduced below certain characteristic lengths and by a large array of applications such as ultra-high density magnetic recording, spin electronics, and magnetic force microscopy, among others. Manipulating and sensing these type of samples is a challenging task which often requires the development of new tools and technologies. In this thesis we present the design, fabrication and implementation of high sensitivity magnetometers based on micromechanical oscillators. These devices, fabricated with MEMS technology, extend all the advantages of high-Q mechanical oscillators to the microscopic scale oering the benets of reduced size, low cost, high sensitivity, direct integration with electronics, and low power. First, we describe the dierent fabricated designs, their main characteristics, and the methods used for their actuation and detection. Next, we describe the design and fabrication of the cryogenic electronics for the sensor, developed in order to investigate samples at low temperatures and with maximum sensitivity. The device consists of a transimpedance amplier that works at a broad range of temperatures, from room temperature down to 2K and was realized with a standard complementary metal oxide semiconductor 1.5 m process. Measurements of current-voltage characteristics, open-loop gain, input referred noise current, and power consumption are presented as a function of temperature. The integration of the high-Q oscillator with this cryogenic electronics results in a micromagnetometer that allows detection of magnetic moments as small as 10-¹³ emu. To complete the presentation of the micromagnetometer we describe our analytical model for the dynamics of an electrostatically actuated torsional oscillator, and its experimental validation. While at low excitations the system is well described by a damped linear oscillator, at higher oscillation amplitudes a non linear regime is observed. Nonlinearity is originated exclusively by the electrostatic driving and detection, and can be tuned by modifying the excitation or detection bias voltages. The parameters of the analytical model are obtained from the device dimensions and material properties. No empirical or tting parameters are needed except for the quality factor, which is extracted from the linear resonance curve. The proposed model can be a valuable and straightforward tool for the design and analysis of many other similar devices based on electrostatically actuated mechanical resonators. After this exhaustive presentation, the remaining of the thesis is dedicated to the description of three dierent implementations of the micromagnetometer. First, we use the device operating in its nonlinear regime to measure external magnetic eld. When the resonator is driven into resonance and a magnetic eld is applied, the induced currents originate damping in the mechanical system. If the resonator is nonlinear this damping produces signicant variations in the peak frequency that can be measured with good sensitivity due to its high Q. Thus, the sensor does not require incorporation of magnetic materials and is appropriate for fabrication with standard micromachining processes. Additionally we introduce an analytical model that correctly describes the device in the linear and nonlinear regimes. The model takes in account the electrostatic excitation and detection, and incorporates the operation dependence with the applied magnetic field. Second, we study both experimentally and theoretically the magnetic behavior of two isolated ferromagnetic nanotubes of perovskite La_0:67Ca_0:33MnO_3.We investigate the specic conguration where a magnetic field H is applied perpendicular to the magnetic easy axis of an isolated nanotube characterized by an uniaxial anisotropy constant K. In this situation, the magnetization M reduces the effective elastic constant of the resonator. This softening of the mechanical system is opposed to the hardening effect of M observed in a previous work, where H was applied parallel to the easy axis. Moreover, in this magnetic field conguration two distinct magnetization regimes are manifested, depending on the magnitude of H. For H»2K M the magnetization is almost parallel to the applied magnetic field and for H«2K M it is almost parallel to the easy axis of the nanotube. At a certain value of H there is a sharp transition from one regime to the other, accompanied by a peak in the energy dissipation. Finally, we measure the magnetic response of a 110nm lm of superconducting Nb with dimensions 20x20 μm"2. The magnetization vs. temperature and applied magnetic eld curves show discontinuities or peaks, related to the entrance of vortex in the sample. This behaviour is due to the interaction between the conned vortex inside the sample and the surface currents. The discontinuities are typical of the mesoscopic scale and disappear in macroscopic samples. The possibility of measuring these small signals conrm the micromagnetometers as suitable tools for the measurement of magnetic properties at the micro- and nanoscales.

Item Type:Thesis (PhD in Physics)
Keywords:Magnetic properties; Propiedades magnéticas; [Micromagnetometers; Micromagnetómetros; Oscillators; Osciladores]
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Subjects:Physics > Magnetismo
Divisions:Gcia. de área de Investigación y aplicaciones no nucleares > Gcia. de Física > Materia condensada > Bajas temperaturas
ID Code:1098
Deposited By:Tamara Cárcamo
Deposited On:07 Sep 2022 15:24
Last Modified:07 Sep 2022 15:24

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