Field Induced Magnetic Ordering in CsFeBr₃Studied by High Field ESR

Abstract

High field ESR technique was applied to the study of the singlet ground state system CsFeBr 3 , to study the energy structure and the field induced magnetic ordering state, which occurs at low temperature and in a field with the configuration parallel to the c-axis. In low field paramagnetic regime, the complicated spectra were observed and the obtained frequency-field diagram at low temperatures can not be explained within the simple singlet-doublet energy scheme asserted from previous neutron experimental results, indicating that the exchange interaction, which brings a dispersive character on the excited doublet levels, may play an important role to understand the origin of observed ESR modes. One of the modes observed at 2.0 K altered its slope to almost twice one at the field of about 2.9 T, which corresponds to the level crossing field between singlet and the lower branch of doublet states. This behavior is reasonably explained as the direct transition between doublets due to the mixing around level crossing field and is consistent with recently developed theoretical prediction proposed by Lolezhuk and Mikeska. Further, in the high field regime, two broad absorption lines with each line width of about a few tesla were also observed. These two ESR modes depend remarkably on the frequency in such a way that both resonance fields coincide at about 160 GHz and separate linearly with large g-value of 4.8 from each other as the frequency is increased. We present these experimental results in detail and give some possible explanations.KEYWORDS: High field ESR, singlet ground state, field induced magnetic ordering §1. IntroductionThe macroscopic quantum coherence state appears as the long range magnetic ordering by the application of field to the gapped spin system at low temperature. It gives a good opportunity to study the most exotic phenomena, Bose-Einstein condensation (BEC), within an easily accessible experimental condition. Recent successful understanding of magnetic behaviors in the spin system within a context of BEC 1) has stimulated us to investigate again the topic of field induced magnetic ordering, which had been started with the experimental pioneer work on Cu(NO 3 ) 2 2.5H 2 O by Diederix et al. represented by the well known ABX 3 type structure crystallizes in hexagonal P 6 3 /mmc space group. A one dimensional chain along the hexagonal c-axis forms triangular lattice coupled within the c-plane. Consequently, the spin frustration comes in as an important concept to characterize the system, if an interaction is antiferromagntic one within a triangular plane.Like most other AFeX 3 (A=Rb, Cs and X=Cl, Br) type compounds, the magnetic state of AFeX 3 in lower energy part is described by the fictitious S=1 spin due to strong spin orbit coupling. The crystalline field with * E-mail address: inagaki@phys.sci.kobe-u.ac.jp the trigonal symmetry lifts the triplet degeneracy into the non-magnetic S z =0 ground state and the magnetic doublet S z =±1 states with a finite energy gap as a single ion state.5, 6)...