Magnetic And Magnetocaloric Effect Of Geometrically Frustrated Magnets RE₃BWO₉ And RE₂SiBe₂O₇(RE=Gd-Ho)

Geometrically frustrated magnets(GFMs)have attracted intensive attentions in condensed matter physics and materials science,especially in the past several decades.After the first proposal by PW Anderson in 1973 of a resonating valence bond(RVB)ground state,a wide variety of novel quantum states and emergent phenomena have been discovered in GFMs,such as quantum spin liquid,spin ice,quantum spin-dimer,topological Weyl magnets,et al.To identify the novel quantum magnetic states,the discovery of new spin-frustrated materials are critical.Until now,more than 1 thousands of GFM compounds have been discovered including the materials extended from3d/4d-transition metal compounds to the 5d-/4f-based materials.Among many material systems,Rare-earth-based geometric magnets have become the focus of research on novel magnetic quantum states in recent years due to their varying spin configuration,strong spin-orbit coupling,spin-lattice coupling,unique crystal field effect and complex magnetic anisotropy.In experiment,various exotic magnetic phases such as Kiteav QSL,quantum spin ice and multipolar order have been discovered.From application viewpoint,RE-based frustrated magnets can be used as magnetic refrigeration materials in liquid helium temperature regime,where enhanced magnetocaloric effect can be obtained because the large magnetic entropy(?S_m)arising from highly degenerate magnetic ground states related to highly geometric frustration.Moreover,the variety of spin types,large moments and magnetic anisotropy of RE³⁺ions make them flexible for designing magnetocaloric materials working at different field and temperature regions.In this respect,the magnetic and magnetocaloric study on new RE-based materials are quite important from both fundamental science and potential applications.Here,we perform the study on two series of RE-based frustrated magnets,Kagome lattice RE₃BWO₉and Shastry-Sutherland-lattice RE₂SiBe₂O₇(RE=Gd,Tb,Dy,Ho),where RE ions have relative larger magnetic moments in the RE family members.The synthesis,structure,magnetic and magnetocaloric effect have been systematically studied.The main content and results are as follows:1.In 1^st chapter,we firstly discuss the concept,typical materials and physical phenomena on geometrically frustrated magnets,then we review the recent material development and emergent physical phenomena on Rare-earch-based frustrated magnets based on the triangular,pyrochlore and Kagome-lattice as typical examples.We introduce the principle of magnetocaloric effect and typical low-temperature MC materials.Finally,we provide the motivation of present thesis.2.We present the synthesis details and structure characterizations for the RE₃BWO₉and RE₂SiBe₂O₇(RE=Gd,Tb,Dy,Ho)compounds used in this thesis.The magnetic measurement and method on calculating magnetocaloric effect are briefly discussed.3.The structural analysis on the serial RE₃BWO₉(RE=Gd-Ho)boratotungstates show that all compounds crystallize in hexagonal coordinated structure with space group P6₃(No.173),where magnetic RE³⁺ions are located on the distorted Kagome lattice connections within the ab plane.The inverse magnetic susceptibility data analysis of RE₃BWO₉(RE=Gd-Ho)results show that there is an antiferromagnetic interaction between RE³⁺ions and without magnetic ordering down to 2K,and different magnetic anisotropic effect are proposed.The Gd₃BWO₉ forms long range magnetic order below T_N=1.0K,the ferromagnetic and antiferromagnetic interactions coexist below T_N.The further magnetic analysis show that RE₃BWO₉(RE=Gd-Ho)exhibit large magnetocaloric effect(MCE)in liquid helium temperature regimes.At T=2.3K,Gd₃BWO₉shows maximum–?S_m up to 63.27 J·kg^-1·K^-1 at?H=7 T around 1.6 times large of commercial Gd₃Ga₅O₁₂(GGG)materials and–?S_m=31.1 J·kg^-1·K^-1 at?H=2T around 2.1 times of large of GGG,both values are quite competitive in the reported available MCE materials.4.The structure analysis on a family of melilite-type RE₂SiBe₂O₇ (RE=Gd-Ho) compounds crystallized in a tetragonal P(?)₁m structure,where magnetic RE³⁺ ions lay out on Shastry-Sutherland lattice(SSL)within ab-plane and are well separated by nonmagnetic[SiBe₂O₇]^6-polyhedrons along c-axis.The susceptibilities and isothermal magnetization measurements reveal that most RE₂SiBe₂O₇ compounds except RE=Tb show no magnetic ordering down to 2K despite the dominant antiferromagnetic(AFM)interactions,where Tb₂SiBe₂O₇ undergoes AFM transition with Neel temperature T_N?3.0K and field-induced spin flop behaviors(T<T_N).In addition,the calculated magnetic entropy change–?S_mfrom the isothermal M(H)curves reveal viable magnetocaloric effect(MCE)for RE₂SiBe₂O₇(RE=Gd,Dy)in liquid helium temperature regimes,Gd₂SiBe₂O₇ shows maximum–?S_m up to–?S_m?55.92 J·kg^-1·K^-1 at ?H=7T and 26 J·kg^-1·K^-1 at ?H=2T in this family,both values are better than commercial GGG materials,enables is an promising MCE materials as potential applications.