Research On Magnetism Measurement System Of Ferromagnetic Materials Under High Pressure

Recently, the high pressure science is an active research field, and it has gainedincreasing attentions. As an independent, safe and handy physical parameter, pressureis one of the most important methods to research the properties of materials in thelaboratory. With the increase of pressure on materials, interactions between atoms ofmaterials change obviously and then the structure and properties of materials transformaccordingly. Sometimes a phase transition occurs to materials under high pressure.Therefor the research ranges of physics, material science and geology have beenexpanded greatly.The diamond anvil cell (DAC) is the most powerful and pervasive high-pressuredevice, helping researchers discover many novel phenomena concerning the behaviorof materials under high pressure. With the development of DAC technology, DAC hasbeen able to combine with the most precision measurement instrument to measure thestructure and properties of sample under high pressure. Now the studies about structure,thermal properties, mechanical properties, optical properties, electrical properties underhigh pressure develop rapidly and achieve a lot of satisfactory results. However, themagnetism study of material under high pressure progress slowly due to the limitationof the size of sample and the sensitivity of the measurement equipment.Magnetism is the common and important properties of materials. All materials,even microscopic particles or the universe, all have magnetic properties. Theferromagnetic materials have the unique magnetic properties, which not only show ahigh susceptibility, but also display the hysteresis phenomenon under an externallyapplied magnetic field. Therefore, ferromagnetic materials are the most important andnotable objects of magnetic study. The ferromagnetic materials are the importantcomponent of the earth. In the earth’s crust there are a large number of ferromagnetic and ferromagnetic mineral. In the earth mantle there are massive ilmenite, chromite andmineral including Fe element. Iron and nickel are the chief component in earth core.Therefore, the study of magnetism of ferromagnetic materials under high pressure andhigh temperature is very important to understand the magnetism of materials in theearth.At present, the study of magnetism under high pressure mainly surroundsresearching the magnetism of sample under constant high pressure and metabolictemperature. We try to explore a novel method to measure the magnetism offerromagnetic materials under high pressure using DAC. The method probabilizesmeasuring the magnetic hysteresis loop of ferromagnetic materials as a function ofpressure at room temperature。 This work relates to high pressure technology,magnetism measurement and weak signal detection, and it is a comprehensiveinterdiscipline project. In the dissertation, the measurement system of magnetizationcurve of ferromagnetic sample under high pressure using DAC is designed, elaborateand analyze. The magnetization curves of iron as a function of pressure at roomtemperature are measured with a prototype measurement system.The measurement system is based on the Faraday’s law, and the means ofmeasurement is that alternating current magnetic field superimposes on direct currentmagnetic field. The system employs one exciting coil and two pick-up coil to measurethe incremental magnetic susceptibility of sample as a function of direct currentmagnetic field under different pressure. And the integral of the incremental magneticsusceptibility is the magnetization curve of sample. The information of magnetictransition can be gained through analyzing magnetization curve of sample underdifferent pressure.It is a challenge to measure the magnetization curve of sample of small sampledue to the poor signal-noise ratio. To improve the sensitivity and stability of themeasurement system, the principle of the measurement is elaborated and analyzed indetail. According to the principle, a theoretical model of the detection probe issummarized. The theoretical model analyzes alternating current magnetic field, theeffect of the magnetic susptibility of sample to the output signal, the effect of coil parameters to the output signal and the offset of background noise. At last a formulaincluding all the parameters which can affect the measurement is summarized. All theparameters in the formula are analyzed one by one to gain the methods and probabilityof improving the sensitivity and stability of the measurement system. A set of optimalparameters for our experimental conditions is presented and a common method ofconfirming the optimal parameters is mentioned tentatively.According to the optimal parameters, a measurement system of ferromagneticmaterials under high pressure using DAC is built, and the performance of the system isanalyzed. The magnetization curves of iron as a function of pressure at roomtemperature are measured with this system.The phase graph of iron under high temperature and high pressure and theâ†'phase transition of iron due to pressure is introduced. The reliability of the system isverified through the contrast between our experimental data and the result of otherliteratures.The experimental data imply that theâ†'phase transition of iron accompaniesby magnetic transition; the start and end of the magnetic transition are about11.5GPaand19.6GPa and the domain of magnetic transition is about8GPa. The shapes ofmagnetization curves suggest that-iron is either original ferromagnetic orparamagnetic.