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Investigation of the Magnetic and Magnetocaloric Properties of Complex Lanthanide Oxides

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Investigation of the Magnetic and Magnetocaloric Properties of Complex Lanthanide Oxides Investigation of the magnetic and magnetocaloric properties of complex lanthanide oxides Paromita Mukherjee Department of Physics University of Cambridge This dissertation is submitted for the degree of Doctor of Philosophy King’s College April 2018 This thesis is dedicated to my parents. Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification in the University of Cambridge, or any other university. This dissertation is my own work and contains nothing which is the outcome of work done in collaboration, except as specified in the text and Acknowledgements. This dissertation contains fewer than 60,000 words including abstract, tables, footnotes and appendices. Some of the work described herein has been published as follows: Chapter 3 is an expanded version of: P. Mukherjee, A. C. Sackville Hamilton, H. F. J. Glass, S. E. Dutton, Sensitivity of magnetic properties to chemical pressure in lanthanide garnets Ln3A2X3O12, Ln = Gd, Tb, Dy, Ho, A = Ga, Sc, In, Te, X = Ga, Al, Li, Journal of Physics: Condensed Matter 29, 405808 (2017). Chapter 4 contains material from: P. Mukherjee, S. E. Dutton, Enhanced magnetocaloric effect from Cr substitution in Ising lanthanide gallium garnets Ln3CrGa4O12 (Ln = Tb, Dy, Ho), Advanced Functional Materials 27, 1701950 (2017). P. Mukherjee, H. F. J. Glass, E. Suard, and S. E. Dutton, Relieving the frustration through Mn3+ substitution in holmium gallium garnet, Physical Review B 96, 140412(R) (2017). Chapter 5 contains material from: P. Mukherjee, Y. Wu, G. I. Lampronti, and S. E. Dutton, Magnetic properties of monoclinic lanthanide orthoborates, LnBO3, Ln = Gd, Tb, Dy, Ho, Er, Yb, Materials Research Bulletin 98, 173 (2018). Chapter 6 contains material from: P. Mukherjee, E. Suard, S. E. Dutton, Magnetic properties of monoclinic lanthanide metaborates, Ln(BO2)3, Ln = Pr, Nd, Gd, Tb, Journal of Physics: Condensed Matter 29, 405807 (2017). Paromita Mukherjee April 2018 Acknowledgements I am very grateful to my supervisor Dr. Siân E. Dutton for giving me the opportunity to work in her group for my PhD. This work would not have been possible without her continuous support and guidance in all aspects of research. I am very thankful for her invaluable mentorship and for giving me the freedom to pursue my research interests. I thank all members of the Dutton group, past and present, for being such amazing colleagues and friends: Adrien Amigues, Hugh Glass and Hajime Shinohara who welcomed me into the group as well as Jack Hodkinson, Joshua Lewis and Dr. Joseph Paddison for all their friendly advice and making the office a wonderful place to work in. Many thanks to members of the Quantum Matter group for all the friendly discussions, help and encouragement: Philip Brown, Konstantin Semeniuk, Jiasheng Chen, Matthew Coak, Yu-Te Hsu, Mate Hartstein, Sophia Taylor, Hsu Liu and others. I am also very thankful to Dr. Patricia Alireza, Dr. Siddharth Saxena, Dr. Michael Carpenter and Dr. Nalin Patel for their guidance. Thanks to Doug Astill for his help with the SQUID and to Adam Brown and Dr. Oleg Ignatchik for their assistance with measurements on the new SQUID and PPMS. I am indebted to Dr. Emmanuelle Suard, ILL and Dr. Pascal Manuel, ISIS for their patience and guidance in all aspects of neutron diffraction experiments from proposal writing to data analysis. Thanks to Hugh Glass, Jack Hodkinson and Joshua Lewis for their support during the various neutron experiments. I thank Dr. Giulio Lampronti, Department of Earth Sciences, Cambridge for collecting X-Ray diffraction data on the lanthanide orthoborates dis- cussed in this thesis. Thanks also to Yutian Wu and Alice Sackville Hamilton for preliminary work on the lanthanide orthoborates and lanthanide garnets respectively prior to my PhD. I thank the Winton Programme for the Physics of Sustainability for my PhD funding. My mentors, friends and family at home have been a huge source of strength and inspiration. I thank all my teachers from school, college and university. Thanks to my friends for their good wishes and support, especially Anwesha Sengupta, Prianthi Roy, Jahnavi Jha, Sreya Banerjee, Amartya Mitra and Soumi De. Also to my relatives, well-wishers and family friends. I thank my mother Sumita Mukherjee and father Pradeep Kumar Mukherjee for their unwavering love, support and faith in me. Many thanks to Jack Hodkinson, Amartya Mitra and Philip Brown for their help and advice on writing the thesis in LaTeX and to Dr. Joseph Paddison, Philip Brown, Jack Hod- kinson, Joshua Lewis, Alexander Hickey, Keiron Murphy for proofreading thesis chapters. Abstract Complex lanthanide oxide systems are known to host novel phases of matter, while also providing functionality for practical applications. In this dissertation, the structural, magnetic and magnetocaloric properties of three families of lanthanide oxides have been studied with the dual aims of investigating the magnetic behaviour and identifying promising magnetic refrigerants for cooling to temperatures currently accessible using non-renewable liquid He. The thesis presents a two-part study of the magnetic and magnetocaloric properties of the geometrically frustrated lanthanide garnets, where the magnetic Ln3+ form corner-sharing triangles. First, the family of garnets Ln3A2X3O12, Ln = Gd, Tb, Dy, Ho, A = Ga, Sc, In, Te, X = Ga, Al, Li are investigated. Changes to the single-ion anisotropy of the magnetic ion as well as variations in the chemical pressure radically alters the nature of magnetic ordering, the degree of frustration and the magnetocaloric performance. In the second part, the garnets Ln3AGa4O12, Ln = Gd, Tb, Dy, Ho, A = Cr, Mn, are studied. Introducing additional spins significantly reduces the frustration in the garnet lattice. Low temperature 3+ 3+ powder neutron diffraction of Ho3MnGa4O12 reveals concomitant ordering of Ho and Mn moments below the ordering temperature, TN = 5.8 K. The magnetocaloric performance of Ln3CrGa4O12, Ln = Gd, Dy, Ho, greatly surpasses that of the parent Ln3Ga5O12 at T = 2 K. The final results chapters in the thesis describe the magnetism and magnetocaloric effect in the lanthanide orthoborates, LnBO3, Ln = Eu, Gd, Tb, Dy, Ho, Er, Yb and the 3+ lanthanide metaborates, Ln(BO2)3, Ln = Pr, Nd, Gd, Tb. The magnetic Ln form slightly distorted edge-sharing triangular layers in LnBO3. Unique magnetic features are observed, including short-range ordering and spin reorientation transitions depending on the single-ion 3+ anisotropy of the Ln . The LnBO3 are also efficient magnetocalorics in the liquid helium temperature range. The lanthanide metaborates contain one-dimensional chains of magnetic lanthanide ions. Bulk magnetic measurements show features consistent with low-dimensional magnetism, such as magnetisation plateaux at one-third of the saturation magnetisation for Nd(BO2)3 and Tb(BO2)3 in a field of 14 T. This thesis provides insight into the fundamental magnetic properties of complex lan- thanide oxide systems and also demonstrates strategies for identifying new magnetocaloric materials: both through chemical control of the structure of well-known magnetocalorics and by studying materials that have not been explored previously. The results pave the way for further in-depth investigations and finding new magnetic coolants based on complex lanthanide oxide systems. Table of contents 1 Introduction1 1.1 Magnetic moments in atoms . .2 1.2 Bulk magnetism: isolated moments . .6 1.2.1 Magnetic susceptibility . .6 1.2.2 Diamagnetism . .7 1.2.3 Paramagnetism . .7 1.2.4 van Vleck paramagnetism . .9 1.2.5 Magnetic entropy . 10 1.3 Crystal fields and magnetic interactions . 10 1.3.1 Crystal fields . 11 1.3.2 Dipolar interactions . 12 1.3.3 Exchange interactions . 13 1.3.4 Relative magnitude of dipolar and exchange interactions . 15 1.3.5 Heisenberg, Ising and XY models . 15 1.4 Bulk magnetism: interacting moments . 16 1.4.1 Ferromagnetism . 16 1.4.2 Antiferromagnetism . 17 1.4.3 General form of magnetic susceptibility . 18 1.4.4 Ferrimagnetism . 19 1.4.5 Field-induced magnetic transitions . 20 1.4.6 Spin glasses . 20 1.5 Geometrically frustrated magnets (GFMs) . 21 1.5.1 Ln2B2O7 ............................... 24 1.5.2 Ln3A2Sb3O14 ............................. 27 1.5.3 SrLn2O4 ............................... 29 1.5.4 Gd3Ga5O12 .............................. 31 1.5.5 YbMgGaO4 .............................. 33 xii Table of contents 1.6 Low temperature magnetic refrigeration . 35 1.7 Overview of thesis . 40 2 Experimental methods 42 2.1 Solid state synthesis . 42 2.2 Structural characterisation . 43 2.2.1 Crystal systems . 43 2.2.2 Crystallographic point groups and space groups . 45 2.2.3 Scattering experiments on crystals . 46 2.2.4 Diffraction . 48 2.2.5 Powder X-ray diffraction (PXRD) . 51 2.2.6 Powder neutron diffraction (PND) . 54 2.2.7 Comparison of PXRD and PND techniques . 57 2.2.8 Rietveld refinement . 60 2.2.9 Magnetic neutron scattering . 63 2.2.10 Magnetic structure determination . 65 2.3 Magnetic measurements . 68 2.3.1 Sample preparation for measurement . 68 2.3.2 Measurements using a Magnetic Properties Measurement System (MPMS) . 68 2.3.3 Measurements using a AC Measurement System (ACMS) on the Physical Properties Measurement System (PPMS) . 70 2.3.4 Quantification of the magnetocaloric effect from the isothermal mag- netisation data . 70 2.4 Heat capacity measurements . 71 2.4.1 Heat capacity . 71 2.4.2 Sample preparation for measurement . 73 2.4.3 Measurements using a PPMS . 73 2.4.4 Thermal models . 73 3 Magnetic properties of lanthanide garnets, Ln3A2X3O12: Part I 76 3.1 Background . 76 3.1.1 Gd3A2X3O12 ............................. 77 3.1.2 Ln3A2X3O12, Ln = Tb, Dy, Ho . 79 3.2 Sample preparation . 80 3.3 Structural characterisation . 81 3.4 Bulk magnetic measurements . 87 Table of contents xiii 3.4.1 Magnetic susceptibility .
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