Neel model

In 2-D and 1-D systems even in non-uniaxial materials, the Neel model can still be used since the anisotropic field in the plane where the moments can rotate is small compared with the exchange field in the perpendicular direction. After first considering the particular case of CsNiF3, we will described several 2-D and 1-D Heisenberg fluorides showing spin-flop behavior. 

Spinu L, Stancu A (1998) Modelling magnetic relaxation phenomena in fine particles systems with a Preisach-Neel model. J Magnet Magnetic Mater 189 106-114 Srivastava KKP, Jones DH (1988) Toward a microscope description of superparamagnetism. Hyperfine Interactions 42 1047-1050... 

In order to estimate the strengths of the three basically different types of interactions the Neel model has been successfully applied in a large number of cases. The model involves a localized picture of magnetic moments on one or a number of sublattices. The total magnetization for two sublattices is given by Af = Ma Mb where M = M((0)S/,(x). Here we use the notation / = A or B. The molecular fields on each sublattice are Ha = Ma aa + Mb ab. Hb =... 

The Neel model was shown to be successful for obtaining insight into the order of magnitude of the various interactions in the R-3d compounds. The localization of the lanthanide moments is well established. However the 3d moments tend to show both localized and itinerant behaviour and therefore a pure localized picture as used in the original Neel model cannot be expected to describe the magnetic properties in detail. 

The Neel model for ferrimagnets also gives a non-linear (1/y, T) behaviour above Tq for localized A and B moments. This behaviour becomes linear at high temperatures. Burzo (1972b) presented an interpretation, different from the Bloch-Lemaire model, in which he analysed the paramagnetic data according to this Neel model where the paramagnetic susceptibility is expressed by... 

Room-temperature Mossbauer data revealed that fast electron transfer (ti, < 10 s) is retained throughout the compositional range 0 < x < 0.8. Samples with x = 0.4 and 0.8 exhibited room-temperature neutron-diffraction data consistent with collinear FeA -ion spins below T, but with a significantly reduced moment (ca. 1 Hb vis a vis a saturated spin-only value) on the B sites. However, at 4.2 K, the spontaneous magnetization could be interpreted with a Neel ferrimagnetic model ... 

Ramirez et al (1970) discussed a metal-insulator transition as the temperature rises, which is first order with no crystal distortion. The essence of the model is—in our terminology—that a lower Hubbard band (or localized states) lies just below a conduction band. Then, as electrons are excited into the conduction band, their coupling with the moments lowers the Neel temperature. Thus the disordering of the spins with consequent increase of entropy is accelerated. Ramirez et al showed that a first-order transition to a degenerate gas in the conduction band, together with disordering of the moments, is possible. The entropy comes from the random direction of the moments, and the random positions of such atoms as have lost an electron. The results of Menth et al (1969) on the conductivity of SmB6 are discussed in these terms. 

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... 

Thermal stability is a function of the grain size and grain size distribution [24], To estimate the minimal thermally stable grain core size Dp, one needs to make assumptions about the magnetization reversal and thermal decay mechanisms. Thermal decay is described by an exponential Arrhenius law, which relates the time constant z for storage to the ratio of a reversal energy barrier EB and thermal energy kBT (T = absolute temperature in Kelvin) (Neel-Brown model [25-27]) ... 

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