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Magnetic and Charge Order in Lufe2o4 and Ybfe2o4 Multiferroics

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Magnetic and Charge Order in Lufe2o4 and Ybfe2o4 Multiferroics Magnetic and Charge Order in LuFe2O4 and YbFe2O4 Multiferroics Hailey L Williamson The University of Warwick Department of Physics A thesis submitted for the degree of Masters by Research 25th January 2012 Abstract Multiferroicity through the years has gained increasing interest based on a deeper understanding of the various types of ferroic coupling which exist, particulary in the rare earth oxides. Despite many years of research and investigation into the `primary candidate' charge ordered multiferroic mate- rials LuFe2O4 and YbFe2O4, a true understanding of their magnetism and charge order have only recently come to light. The following thesis presents detailed studies on the magnetism and charge order of LuFe2O4 and the potential multiferroic properties of YbFe2O4 in single crystal form. The research performed over the last 20 years on the rare earth oxides has highlighted one main afflicting factor, governing the magnetic and charge order effects, especially within the RFe2O4 series (R= Y, Ho, Er, Tm, Yb and Lu), which stems from a sensitivity to oxygen stoichiometry. Isostruc- tural LuFe2O4 and YbFe2O4 both exhibit a coupling between magnetism and electric polarization, but the true origin is still unclear. LuFe2O4, once clarified as a material which exhibited ferroelectricity though charge or- dering, has, through extensive neutron scattering, X-ray magnetic circular dichroism (XMCD) as well as wide range of macroscopic measurements, now been established as non-polar. Paying careful attention to the oxygen defi- ciency and its effects on crystal quality through basic magnetization mea- surements, LuFe2O4 can be characterized into four different qualities (poor to excellent). Therefore by tuning the stoichiometry, through a partial pres- sure CO:CO2 atmospheric environment during crystal growth, via floating zone technique, an optimal single crystal of LuFe2O4−δ can be grown. Magnetization measurements on LuFe2O4−δ grown in the CO:CO2=1:5, exhibit two large transitions, one present at 235 K and representative of the paramagnetic to ferrimagnetic phase, followed by a smaller but greatly defined transition at 175 K, indicative of spin glass behavior previously re- ported. Identical measurements performed on the LuFe2O4−δ crystal grown in the CO:CO2=1:3 gas atmosphere presented only one broad transition at 202 K, which does not correspond to either the 235 K N´eeltransition or the spin glass transition present at 175 K, but rather purely spin glass in nature based on poor oxygen stoichiometry. Specific heat data provided initial insight into the presence of charge order at 314 K. Single crystal x- ray diffraction highlighted the appearance of 2D diffuse scattering and 3D charge order peaks along the (1/3, 1/3, l) line, present in the LuFe2O4 crys- tals grown in CO:CO2=1:3 and CO:CO2=1:5 gas atmospheres, respectively. The new understanding into the magnetism and charge order of LuFe2O4−δ and it's large sensitivity to oxygen stoichiometry has produced a surge of new interest within the remaining rare earth series. Single crystals of YbFe2O4−δ were grown in two different partial pressure oxygen environ- ments, CO:CO2=1:3 and CO:CO2=1:3.5. In order to investigate the mag- netism and charge order properties, macroscopic measurements of mag- netization, specific heat, single crystal x-ray diffraction, ac susceptibility and M¨ossbauerspectroscopy were performed. To investigate the type of magnetic ordering within a YbFe2O4−δ single crystal, oriented along c, Diffuse Neutron Scattering (DNS) was performed at DNS-FRM II. Mag- netization measurements of each single crystal grown in CO:CO2=1:3 and CO:CO2=1:3.5, provided very similar data curves with three main transi- tion points; the main ferrimagnetic transition at 240 K, followed by a small peak at 220 K. The third largest transition present on the magnetization data appears at 150 K, finalized by a small very broad feature at 30 K. The appearance of charge order at 305 K is seen clearly in the specific heat data supported by single crystal x-ray diffraction which exhibits strong diffuse scattering along (1/3, 1/3, l) line. iv To the most supportive parents; Christine and Neville...... Acknowledgements I would like to acknowledge the following people who were connected to both the experimental investigations and writing of this thesis, who without their help would not have been possible. Geetha Balakrishnan, Manuel Angst and Raphael Hermann,I would like to thank you all for co-supervising this project, providing me with invaluable knowledge regarding the experimental research into the realm of multiferroics. The depth of research conducted this year, focusing on the crystal growth of such interesting compounds complimented by a full sup- port in learning new techniques and expanding my previous knowledge in this area, enabled me to grow as a researcher. Without you all this would not have been possible and for this I am truly grateful. Martin Lees, Oleg Petrenko and Karen Friese, I thank you for sup- port with the necessary training on experimental instruments and software, providing me with the ability to ask questions on my research and guiding me when I reached certain obstacles along the way. Thomas Br¨uckel, I would like to thank you for giving me the opportunity to work in such a prestigious institute and be surrounded by a wonderful group of people in JCNS-2. J¨orgPersson, without your persistence and guidance with orientating sometimes difficult samples, I would not have been able to obtain the mirac- ulous results I have during the course of this masters. Yixi Su and Kirill Nemkovskiy, my time at DNS was made successful with the great support from you both, with respect to both the sample mounting and measurement time, as well as understanding and interpreting the results. I thank you greatly for this. Barbara K¨oppchen and Susan Tatlock, I could not have got through this year without you both, Barbara, you have not only been a complete saint when helping me with travel and administration work, but often been a sounding board then things were going wrong. Thank you. Joost deGroot, I do not think my time at J¨ulich and learning new ex- perimental equipment and software would have gone so smoothly without your support. You guided me through each step, showing me the ropes and allowed me to gain great results with your prior knowledge in this area. Thank you. Shilpa Adiga, Pankaj Thakuria, Thomas M¨uller, I am grateful that I was able to become a apart of the young investigates group and work with you all, especially our times at conferences and beam time experiments, which will stay fondly with me in my memories. Ronnie Simon, Marcus Herlitschke, Benedikt Klobes, Paula Bauer, Tania Claudia Weber, Thank you for giving me guidance with training on experimental equipment and software as well as many chats about research in the coffee room. Ravi Singh, Robert Cook, Tom Orton, Tom Hayes, Olga Young, Michael Smidman and Natalia Parzyk, I really enjoyed being apart of the Superconductivity and Magnetism group, you all, not only each pro- vided me with invaluable training during the course of my masters, but you were a great group of people to work and socialize with, making me feel completely welcome and at home. Thank you. Klaus Neumann, There are not many words that express how truly grate- ful I am to you. If you hadn't stood by me through my time at Loughbor- ough university, encouraging me to keep going even when I felt like giving up, I would not be in the position I am today. You provided me with invalu- able contacts and experiences in research over the last seven years, which has given me the drive and inspiration to hopefully one day become a good scientist. Thank you Klaus for everything. Family and friends as well as the people no longer here with me, Thank you to all those you have supported me through the ups and downs, because this year, although completely and utterly rewarding has seen it's difficulties. The five people I wish to thank are Chris, Mike and Paul who allowed me to become part of a family when living in Coventry and who I will keep close to my heart for the many years to come. I finally thank the last two people, the two most important people in my life, who have given me constant support through the good and bad times, providing me with sound advice as well as love and care to see me through.... Mum and Dad, thank you from the bottom of my heart, I hope I have made you proud because without you both I would not be the person I am today. I would also like to say a final thank you to the close people who have supported me and are no longer with us on this earth. Nan I did this for you. Contents List of Figures ix List of Tables xiii 1 Introduction 1 1.1 Multiferroics . 1 1.1.0.1 Proper and Improper Multiferroics . 5 1.1.0.2 Type I Multiferroics . 7 1.1.0.3 Type II Multiferroics . 11 1.2 RFe2O4 System . 14 1.2.1 Effects of Stoichiometry . 16 1.2.1.1 YFe2O4−δ .......................... 17 1.2.1.2 LuFe2O4−δ ......................... 20 1.2.1.3 YbFe2O4−δ ......................... 23 2 Experimental Methods 27 2.1 Sample Preparation . 27 2.2 Crystal Growth . 27 2.3 X-ray Diffraction . 29 2.3.1 Powder X-ray Diffraction . 29 2.3.2 Laue Diffraction . 31 2.3.3 Single Crystal X-ray Diffraction . 32 2.4 Magnetization . 33 2.4.1 MPMS (Magnetic Properties Measurement System) .
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