Cobalt monoxide is a transition metal oxide which has a Neel temperature TN = 291K. Above TN , it presents a rocksalt face centered cubic structure, as MnO, NiO, FeO. All these monoxides are type II antiferromagnets in their magnetic state. CoO is however peculiar since, while MnO, NiO, FeO distorts trigonally below their TN , its major distor- tion is tetragonal, along cube edges. The discrepancy between the magnetic symmetry and lattice symmetry is still under debate and not fully understood. The structural phase transition in CoO is accompanied by twin formation in the material. The capability to observe, identify and finally index those twins is a key element to better understand the phase transition. CoO sample has been analyzed using three different techniques. A preliminary 4-circle- diffraction analysis has been carried out at BM28 with a x-ray, copper-based, lab source. This analysis allowed to measure the evolution in temperature ( in the range [100 K, 300 K] ) of (002) Bragg reflection. The difference of the peak position has been related to temperature. Then Dark Field X-ray Microscopy measurements has been performed at ID06-HXM. Thanks to the analysis of these measurements, the formation of a triplet in reciprocal space (corresponding to lower symmetry) together with twin boundaries for- mation in direct space have been observed. A geometric procedure for twin identification has been proposed. Finally Scanning X-ray Diffraction measurements were carried out. Those measurements were rocking curves obtained using a 2D detector. Analysing the data also in this case the formation of a triplet in reciprocal space, below TN , has been observed, confirming the previous analysis. Thanks to the analysis and fit of the intensity versus d-spacing, a plot relating d-spacing and temperature has been produced.
Il monossido di cobalto è un ossido di un metallo di transizione, la cui temperatura di Neel è TN = 291K. Sopra TN , presenta un reticolo cubico a facce centrate, come MnO, FeO, NiO. Tutti questi monossidi sono antiferromagneti di tipo II, nel loro stato magnetico. L’ossido di cobalto è particolare visto che, mentre MnO, FeO, NiO subiscono una distorsione trigonale sotto la loro TN , esso viene distorto tetragonalmente, lungo gli spigoli del cubo. La discrepanza tra la simmetria magnetica e la simmetria del reticolo è ancora oggetto di dibattito e non completamente capita. La transizione di fase del reticolo, nell’ossido di cobalto, è accompagnata dalla formazione di twin boundaries. La capacità di osservare, identificare e indicizzare questi twins è cruciale per capire meglio questa transizione di fase. Il campione di CoO è stato analizzato utilizzando tre tecniche. Una misura preliminare di 4-circle-diffraction è stata condotta a BM28 utilizzando una sorgente a raggi x basata sul rame. Questa analisi ha permesso di misurare l’evoluzione in temperatura ( range [100 K, 300 K] ) della riflessione (002). La differenza nella posizione del picco è stata poi correlata con la temperatura. Le misure di Dark Field X-ray Microscopy sono state condotte a ID06- HXM. Grazie all’analisi di queste misure è stata osservata la formazione di un tripletto nello spazio reciproco, accompagnata dalla formazione di twin boundaries nello spazio reale. É stata inoltre proposta una procedura geometrica per l’identificazione dei twins. Le ultime misure sono state di Scanning X-ray Diffraction. Sono state, in particolare, acquisite misure di rocking curves usando un detector bidimensionale. Analizzando i dati ottenuti, anche in questo caso è stata registrata la presenza di un tripletto nello spazio reciproco, sotto TN , confermando la precedente analisi. Infine attraverso un fit dell’intensità rispetto alla distanza interplanare, è stato prodotto un grafico dell’evoluzione delle distanze interplanari rispetto alla temperatura
Observation by Dark Field X-ray Microscopy of structural phase transition in cobalt monoxide
Giovannelli, Nicolò
2021/2022
Abstract
Cobalt monoxide is a transition metal oxide which has a Neel temperature TN = 291K. Above TN , it presents a rocksalt face centered cubic structure, as MnO, NiO, FeO. All these monoxides are type II antiferromagnets in their magnetic state. CoO is however peculiar since, while MnO, NiO, FeO distorts trigonally below their TN , its major distor- tion is tetragonal, along cube edges. The discrepancy between the magnetic symmetry and lattice symmetry is still under debate and not fully understood. The structural phase transition in CoO is accompanied by twin formation in the material. The capability to observe, identify and finally index those twins is a key element to better understand the phase transition. CoO sample has been analyzed using three different techniques. A preliminary 4-circle- diffraction analysis has been carried out at BM28 with a x-ray, copper-based, lab source. This analysis allowed to measure the evolution in temperature ( in the range [100 K, 300 K] ) of (002) Bragg reflection. The difference of the peak position has been related to temperature. Then Dark Field X-ray Microscopy measurements has been performed at ID06-HXM. Thanks to the analysis of these measurements, the formation of a triplet in reciprocal space (corresponding to lower symmetry) together with twin boundaries for- mation in direct space have been observed. A geometric procedure for twin identification has been proposed. Finally Scanning X-ray Diffraction measurements were carried out. Those measurements were rocking curves obtained using a 2D detector. Analysing the data also in this case the formation of a triplet in reciprocal space, below TN , has been observed, confirming the previous analysis. Thanks to the analysis and fit of the intensity versus d-spacing, a plot relating d-spacing and temperature has been produced.File | Dimensione | Formato | |
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Executive Summary Giovannelli.pdf
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thesis giovannelli definitive.pdf
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https://hdl.handle.net/10589/211424