High Field Magnetization And ESR Studies Of Triangular Lattice Magnets Cu₂（OH）₃X（X=Cl,Br,I,CHO₂）

Geometrically frustrated magnets have attracted widespread attention due to the emergence of novel physical phenomena,such as spin ice,spin liquids,and non-collinear ground states.Triangular lattice magnet is the representative one of geometrically frustrated magnets.Meanwhile,it is considered as an ideal platform for studying novel magnetic properties and quantum effects because of its simple structure.In this dissertation,we present a systematical study for the magnetic properties of the triangular lattice magnet Cu₂（OH）₃X（X=Cl,Br,I,CHO₂）by using high-field magnetization,electron spin resonance（ESR）and conventional magnetic measurements.The results show that this series of compounds have a half-saturated magnetization plateau and magnetic decoupling effect.The main contents are as follows:Firstly,we studied the angle-dependent magnetization behaviors and magnetic excitation in Cu₂（OH）₃Br in detail.The results of magnetization measurements show that the easy magnetization axis is the diagonal direction of the ac plane（i.e.the orientation of Cu1 spins）,the spin-flop axis is the b axis and the hard magnetization axis is perpendicular to the easy magnetization axis and the b axis.Meanwhile,it was found that only the antiferromagnetic（AFM）coupled Cu1 ferromagnetic（FM）chains are responsible for the spin-flop transition.A phenomenological model is proposed to describe the angle-dependent spin-flop transition,which suggests that compared with the Cu1 spins,the Cu2 spins are insensitive to the external magnetic field,showing partial decoupling.Correspondingly,the ESR modes are originated from the Cu1 FM chains with the AFM coupling.In addition,by using the mean field theory,the magnetization curves of the single crystal were well simulated and the AFM resonance modes were qualitatively described.Further,the magnetic anisotropy parameters are obtained.As a result,the compound Cu₂（OH）₃Br exhibits biaxial anisotropy.Secondly,using Cu₂（OH）₃Br as a reference,the magnetic properties of isostructural compounds Cu₂（OH）₃X（X=Cl,I,CHO₂）were studied by high-field magnetization and ESR measurements with powder samples.Among them,Cu₂（OH）₃Cl and Cu₂（OH）₃CHO₂undergo spin-flip transition;Cu₂（OH）₃I undergoes a spin-flop transition similar to Cu₂（OH）₃Br.The different types of the field-induced magnetic transition suggest that the magnetic anisotropy of the three compounds exists significant differences.High-field magnetization curves show a half-saturated magnetization process,suggesting that the Cu1FM chains and Cu2 AFM chains are decoupled in magnetism.Corresponding to magnetic decoupling,the magnetic excitations of Cu1 and Cu2 spins also are independent of each other,and it is observed that ESR modes originate from the Cu1 spins.The magnetic anisotropy parameters of these compounds were obtained from the analysis of the ESR modes.As a result,Cu₂（OH）₃Cl exists biaxial anisotropy while Cu₂（OH）₃I and Cu₂（OH）₃CHO₂ have uniaxial anisotropy.The magnetization behaviors and magnetic excitations are strongly associated with the Cu1 FM chains with the AFM coupling for Cu₂（OH）₃X（X=Cl,Br,I,CHO₂）.With the increase of X anion mass,the Curie-Wiess temperatures decrease,indicating that the contribution of AFM interactions gradually gets enhanced in comparison to that of FM interaction;the Néel temperatures and exchange fields also increase gradually,which suggested that the interactions between Cu1 FM chains increase.In particular,Cu₂（OH）₃CHO₂ and Cu₂（OH）₃Cl have spin-flip transitions while Cu₂（OH）₃Br and Cu₂（OH）₃I undergo spin-flop transitions,indicating that the anisotropic energy is decreasing compared to the increasing interactions between Cu1 FM chains.The magnetic anisotropy mainly comes from the anisotropy of intra-chain interactions of Cu1 FM chains.Different X anions lead to changes in the interaction between magnetic ions,especially the interactions between Cu1 spins.In conclusion,the triangular lattice magnets Cu₂（OH）₃X（X=Cl,Br,I,CHO₂）can be considered as candidates for the study of magnetic decoupling effect and field-induced magnetic transitions in triangular lattices.