Electron Spin Resonance in Triangular Antiferromagnets

Abstract

We give an overview of our studies on the electron spin resonance (ESR) in the hexagonal triangular antiferromagnets of ABX 3 type using millimeter and submillimeter waves and static and pulsed high magnetic fields. Novel ESR modes, which cannot be understood by the conventional theory of the antiferromagnetic resonance, were observed. Examples of this include that there exists the gapless excitation even for the easy-axis anisotropy. The ESR modes can be classified by the signs of the exchange interaction J 0 along the c-axis and the anisotropy. The ESR modes were calculated within the framework of the mean-field approximation using threesublattice and six-sublattice models for the ferromagnetic and antiferromagnetic J 0 interaction, respectively. The experimental results were well described by the present calculations.KEYWORDS: ESR, triangular antiferromagnet, spin wave, antiferromagnetic resonance, frustration, hexagonal ABX 3 antiferromagnet §1. IntroductionSpin waves are beautiful collective excitations in magnetic systems. From the dispersion relations, we can know the exchange interactions, the magnetic anisotropies, the dimensionality of the system and so on. Neutron inelastic scattering and electron spin resonance (ESR) are powerful tools to observe the spin waves. Neutron inelastic scattering can excite the spin waves with various wave number Q, while the ESR can excite only Q = 0 modes. However, the ESR has the much higher resolution. Furthermore, using the millimeter and submillimeter waves ESR combined with pulsed high magnetic field, can observe the evolution of spin wave energies with magnetic field in the wide field range.The antiferromagnetic resonance (AFMR) is the ESR in the antiferromagnets, in which the spin structure can be described by two sublattices. The theory of AFMR has been established many years ago, 1-10) and were confirmed experimentally in many antiferromagnets. Figure 1 illustrates the frequency versus field diagrams of the AFMR modes for the uniaxial and biaxial anisotropies. For the uniaxial easy-axis and biaxial anisotropies, there is finite zero-field gap, while no zero-field gap exists for the uniaxial easy-plane anisotropy.For triangular antiferromagnets (TAF), little was known on the ESR modes.11) This seems because there was little suitable model substance and there was no standard theory. The comprehensive studies of the ESR * E-mail address: tanaka@lee.phys.titech.ac.jp. modes in the TAF started in the late 1980's on the hexagonal antiferromagnets of ABX 3 type with the CsNiCl 3 structure. 12-14)In the hexagonal antiferromagnets of ABX 3 -type, magnetic B 2+ ions form infinite chains along the crystallographic c-axis and triangular lattices in the basal c-plane, as shown in Fig. 2. Figure 3 shows the exchange interactions in the hexagonal antiferromagnets of ABX 3 -type. The intrachain interaction J 0 is stronger than the others. Thus, the present system exhibits the one-dimensional character at high temperatures. However, they behave as TAF at low temperatures, becau...

Spin excitations in the antiferromagnetNaNiO2

“…The comparison with other triangular magnets having antiferromagnetic coupling within the layers is intriguing: (including CsNiBr 3 , RbNiBr 3 , CsMnI 3 , CsMnBr 3 CsCuCl 3 , RbCuCl 3 and RbFe(MoO 4 ) 2 ). There, typically three or six ESR modes [16,17,18] are observed. This is related to the umbrellalike spin order with, for a given triangle, each spin pointing at 120…”

Magnetic field induced polar phase in the chiral magnetCsCuCl3

“…The emergence of the electric polarization is explained by the cooperation of the local electric polarization on the chiral solitonic spin arrangement and the paramagnetoelectric effect. Hence, we interpret this novel multiferroic phase in CsCuCl3 as a polar solitonic phase.PACS numbers: 75.30.Kz, 75.85.+t Various triangular lattice antiferromagnets (TAFM) have long been studied with the focus on their geometric frustration from the antiferromagnetic coupling of the neighboring spins on a triangular lattice [1][2][3]. Among the TAFM systems, a hexagonal CsCuCl 3 has received sustained interest because of its distinctive chiral crystal structure and its resultant magnetism under applied magnetic fields.…”

“…PACS numbers: 75.30.Kz, 75.85.+t Various triangular lattice antiferromagnets (TAFM) have long been studied with the focus on their geometric frustration from the antiferromagnetic coupling of the neighboring spins on a triangular lattice [1][2][3]. Among the TAFM systems, a hexagonal CsCuCl 3 has received sustained interest because of its distinctive chiral crystal structure and its resultant magnetism under applied magnetic fields.…”

“…In addition to the effect of quantum fluctuation in CsCuCl 3 , we are interested in the aspect of helical spinarrangements. Chiral magnets have recently attracted much attention because of novel electronic properties arising from their novel spin textures.…”