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Development of a nanofabricated magnetometer at very low temperature (30 mK) and high magnetic field (16 T) : new magnetic states in frustrated nanomagnets

Abstract : The objective of this work was to develop a Faraday force magnetometer to measureabsolute values of the magnetization at very low temperatures (30 mK) andhigh applied magnetic fields (16 T) with a high sensitivity (10-5 emu). This magnetometer will be especially dedicated to the study of the field induced properties of frustrated magnets.In a first part, I present the development of this magnetometer. It involves the optimization of the dilution refrigerator and field gradient coils, and the development of a capacitive detection with a cold amplifier stage to improve the measurement sensitivity. Preliminary magnetic measurements are shown. The changes in the design required to make the magnetometer operational are discussed.In a second part, I focus on experimental studies of frustrated magnets: Gadolinium garnets, described by classical spins, and Copper based frustrated molecular compounds, to probe quantum effects. These measurements were performed from 70 mK to 300 K, with the existing magnetometers at the Institut Neel.In Gd3Ga5O12, we have complemented the H-T phase diagram. This phase diagram was proven to be robust by our study on the isomorphous compound, Gd3Al5O12. We evidence the convergence of all the observed phases to a unique point in both samples.In quantum systems, we performed preliminary studies on Cu44 clusters with tetrahedral motives, and on a triangular system Cu3. Although promising, these studies were not pursued due to sample problems.
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Ovidiu Florea. Development of a nanofabricated magnetometer at very low temperature (30 mK) and high magnetic field (16 T) : new magnetic states in frustrated nanomagnets. Condensed Matter [cond-mat]. Université Grenoble Alpes, 2015. English. ⟨NNT : 2015GREAY061⟩. ⟨tel-01686561⟩

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