Neel vector

To account for quantum mechanical effects, an approximate quantum model that reproduces the findings of the two classical spin-based approaches was constructed in a next step.37 One foundation of this model was the finding that several (nonfmstrated) molecular antiferromagnets of N spin centers 5 (which can be decomposed into two sublattices) have as their lowest excitations the rotation of the Neel vector, that is, a series of states characterized by a total spin quantum number S that runs from 0 to N x 5. In plots of these magnetic levels as a function of S, these lowest S states form rotational (parabolic) bands with eigenvalues proportional to S(S +1). While this feature is most evident for nonfmstrated systems, the idea of rotational bands can be... 

In the paramagnetic regime, the evolution of the EPR line width and g value show the presence of two transitions, observed at 142 and 61 K in the Mo salt, and at 222 and 46 K in the W salt. Based on detailed X-ray diffraction experiments performed on the Mo salt, the high temperature transition has been attributed to a structural second-order phase transition to a triclinic unit cell with apparition of a superstructure with a modulation vector q = (0,1/2, 1/2). Because of a twinning of the crystals at this transition, it has not been possible to determine the microscopic features of the transition, which is probably associated to an ordering of the anions, which are disordered at room temperature, an original feature for such centrosymmetric anions. This superstructure remains present down to the Neel... 

Another process responsible for a fluctuation of the local magnetic field is Neel relaxation. It corresponds to the flip of the crystal magnetization vector from one easy direction of anisotropy to another. The correlation time of this... 

Longitudinal fluctuations of a Neel-ordered state are usually strongly damped and not included in a spin-wave approximation of its excitation spectrum. Until now similar modes have been only observed in neutron scattering on spin chain systems as broadened maxima [44 46]. The present narrow linewidth is probably related to the small scattering vector involved in light scattering experiments. 

Therefore, for the internal (Neel) relaxation the parameter, r m plays the same role as the fluid viscosity r in the mechanism of the external (Brownian) diffusion. Note that the density of the anisotropy energy K is not included in x. This means that xD can be considered as the internal relaxation time of the magnetic moment only for magnetically isotropic particles (where K = a = 0). The sum of the rotations—thus allowing for both the diffusion of the magnetic moment with respect to the particle and for the diffusion of the particle body relative to the liquid matrix—determines the angle ft of spontaneous rotation of the vector p at the time moment t ... 

In our study we treat the two mechanisms separately. For each we assume the dominance of that mechanism. This means that for Debye relaxation we assume that the magnetization vector is fixed to the particle, that is the Neel relaxation is blocked or frozen due to an insurmountable energy barrier preventing its operation. On the other hand for the Neel relaxation, we assume that the particle is fixed in space. 

Domain wall structures in thin films and small particles can be different from those in massive samples, because some energy contributions may become significant when sample dimensions are decreased. In thin films, the magnetisation vector tends to remain parallel to the film plane to avoid any contribution to the magnetostatic energy. The spins within a domain wall also rotate within the film plane, which leads to Neel walls. Fig. 4.31. Neel walls appear in thin films below a critical thickness limit. 

Neutron diffraction measurements performed on TbRu2Ge2 indicate that the magnetic structure of the compound is sine modulated below 32 K (the Neel point). It becomes square modulated at 4.2 K with the terbium magnetic moment amounting to 9.06 /Hg and the propagation vector k = (0.2331, 0, 0). The magnetic moment is directed along the c-axis (Yakinthos 1986, Szytula et al. 1987). 

The holmium ions order antiferromagnetically in the HoRu2Ge2 compound at the Neel temperature T = 20K. This compound exhibits a square modulated structure with a propagation vector k = (0.2216, 0.0111, 0) and a holmium magnetic moment of 6.6 parallel to the c-axis (Yakinthos and Roudaut 1987). 

CeRh2Si2 exhibit the magnetic structure described by the wave vector k = (z.O 2) (see AFIII in fig. 23) (Quezel et al. 1984). A different type of magnetic structure is observed for CeRh2Si2 by Grier et al. (1984). Below the Neel point (Tff = 39K) it exhibits another second order transition at 27K it changes to a complex commensurate structure with modulated moments. 

Neutron diffraction measurements performed on a polycrystalline sample of TmCUjSij indicate that the compound is antiferromagnetic below the Neel temperature of 3.6 K. The magnetic structure is cosinusoidally modulated along the [110] direction with the propagation vector k = (0.147, 0.147, 0). The thulium magnetic moment of 5.1 makes an angle of 33(3)° with the c-axis (Allain et al. 1988). 

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