Kimio Adachi

Helical Magnetic Structure in CsCuCl3

Neutron diffraction measurements were made on single crystals of CsCuCl 3 . The sense of Cu 2+ ion helix was found from nuclear reflections to be predominantly right-handed. The magnetic reflection data could be interpreted satisfactorily by considering a triangular spin structure of helically modulated ferromagnetic chains: The magnetic moments lie in the c-plane and rotate by 5.1±0.1° along the chain or the c-axis, while the moments rotate by 120° in the c-plane. The magnitude of the magnetic moment is 0.58±0.01 µ B at 4.2 K. The critical exponent of the magnetization, β was found to be 0.358±0.015 around the Neel temperature, 10.70±0.05 K. We present a model that the helical modulation along the c-axis is caused by the antisymmetric exchange (Dzyaloshinsky-Moriya) interaction characteristic of the crystal symmetry of CsCuCl 3 . The model predicts a right-handed magnetic helix in the present case.

Magnetic Ordering in a Partially Frustrated Triangular Antiferromagnet RbFeBr3

Specific heat of quasi-one-dimensional RbFeBr 3 has been measured in the temperature range 1.1∼10 K in applied magnetic fields. Two anomalies with slight field dependences have been found at T N =5.61±0.02 K and T t =2.00±0.04 K in zero field. Below 120 K, the spin frustration is partially released by a lattice distorsion. Analysis by molecular field approximation for magnetic ordering process in the triangular spin ( S =1) system with frustration partially relased by lattice modulation is presented. The partial release of the frustration enables us to predict from the analysis that the high temperature anomaly corresponds to the transition from paramagnetic phase with mutually uncorrelated magnetic chains to a partially disordered phase in which one third of magnetic chains remains uncorrelated with the others and the low temperature one to the transition from the partially disorded phase to the triangular phase in the c -plane.

ESR Study of Quasi-One-Dimensional Ising-Like Antiferromagnet CsCoCl3

Electron spin resonance was measured at 9.40 GHz as a function of temperature in a quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 . Observed resonance in the partially disordered phase, \(T_{N2} \lesssim T<t_{N1}\) and its neighborhood is interpreted in terms of ESR due to spins at propagating domain walls in one-dimensional Ising-like antiferromagnet. The theory developed by Shiba and the present author gives a more consistent interpretation of the temperature dependence of the absorption intensity and the angular dependences of the resonance field and the linewidth than the existing theories. Discussion is given in connection with spin-cluster resonance.

ESR Due to Propagating Domain Walls in One-Dimensional Ising-Like Antiferromagnets

A theory is presented on the ESR due to spins at thermally activated domain walls in antiferromagnetic Ising-like chains. Several features of the resonance are pointed out, including a notable asymmetric line shape for the magnetic field tilted from the Ising axis. A study is made also on the effect of interchain molecular field on the temperature dependence of intensity.

Magnetic Phase Transition of Organic Free Radical p-Cl-BDPA

The heat capacity of polycrystalline organic free radical, p -Cl-BDPA, has been measured between 1.5 and 20 K. The broad maximum near 5 K in the magnetic heat capacity corresponds the short range magnetic ordering of the unpaired electron. At 3.25 K a sharp peak was observed, which indicates paramagnetic-antiferromagnetic phase transition. The magnetic entropy of this transition yields 101.5% of the expected Rln2. Below T N the magnetic heat capacity was in good agreement with the three-dimensional spin wave theory of antiferromagnet.

Spin frustration in CsCoCl3

Abstract In connection with spin frustration, spin dynamics was studied in magnetically ordered phases of CsCoCl 3 by neutron scattering, ESR and NMR measurements. It was concluded that in the high temperature phase the domain walls travel in the frustrated magnetic chains whereas they come to a standstill in the low temperature phase.

133Cs NMR Study in Quasi-One-Dimensional Ising-Like Antiferromagnet CsCoCl3

The magnetic phase transition and the spin dynamics in quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 were investigated by means of nuclear magnetic resonance of 133 Cs. The observed angular dependence of the resonance field measured at 4.2 K and 20.4 K is consistent with the spin configuration determined by neutron diffraction experiments. The nuclear spin-lattice relaxation time T 1 was found to be dominated by two-phonon Raman process via magnon-phonon interaction at low temperatures. Relaxation in the higher temperature region is discussed in terms of Villains theory of propagative relaxation.

Nuclear Spin-Lattice Relaxation Mechanism of Singlet-Ground-State Magnet with Exchange Coupling: Application to CsFeCl3

A model is proposed on the exchange coupled singlet-ground-state magnet around the level crossing field. The model starts from the energy level of the isolated single spin in paramagnetic spin system. The effect of the exchange coupling is taken into account by modifying the single ion level scheme to the broadened continuum band. On the basis of this model is discussed a mechanism of the nuclear spin-lattice relaxation inherent in the level crossing, due to the direct process between the nuclear spin and the exchange coupled electron spin system. A drastic increase of the nuclear spin-lattice relaxation rate of 133 Cs in CsFeCl 3 observed at 4.2 K around the level crossing field at 7.5 T is well explained by this model. The magnetization as a function of external magnetic field is also discussed and compared with the result of CsFeCl 3 up to 12 T.

Nuclear Spin-Lattice Relaxation Due to Domain Wall Solitons in CsCoCl3

Nuclear spin-lattice relaxation of 133 Cs has been studied in the frustrated antiferromagnet, CsCoCl 3 . A pronounced anomaly in the relaxation rate was observed at the lower transition temperature, T N2 =10 K, whereas a weak change in the relaxation rate was observed at the upper transition temperature, T N1 =21 K. The results are discussed in terms of the characteristic dynamics of domain wall solitons which propagate in disordered magnetic chains which feel no molecular field from the neighboring chains.

Temperature Dependence of Spin Cluster Resonance Intensity

A consideration is given to understand the temperature dependence of spin cluster resonance intensity by simple extention of the Date and Motokawa model. The calculated result shows that the intensity increases sharply with increasing temperature, and decreases at higher temperatures after reaching a maximum at a certain temperature. The experimental check is mainly done by the measurements in a quasi-one-dimensional Ising ferromagnet, [(CH 3 ) 3 NH]CoCl 3 ·2H 2 O. The proposed model gives a good agreement with the observed behavior over the whole temperature range studied.