Neel order

Recent muon spin rotation (//-SR) experiments evidence the existence of internal fields. Hence, a weak Neel ordering is assumed [40], The substitution of Br by Cl reduces the unit cell volume by 7% and increases the transition... 

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. 

A long-range or even a mesoscopic AFM order (Neel ordered) encumbers the hole motion. (As a hole moves to the nearest-neighbouring site, the spin at that site concomitantly moves to the site previously occupied by the hole). A succession of such moves shift-registers a line of spins, which become laterally misaligned with respect to the spins on the sides (Figure 4). The broken AFM bonds shown by the dashed lines create a linearly rising potential that impedes free hole motion. A compact... 

Figure 4. a) A hole at the initial site in a Neel-ordered antiferromagnet b) a hole... 

Fig. 10.16 Mossbauer spectra at 4-2 K of (a) NiFc204 and O ) NiFeo.3Cr1.7O4. Note the absence of Aw = 0 lines in a 70-kG field in the former (Neel ordering) but not in the latter (Yafet-Kittel ordering). [Ref. 86, Fig. 1]...

Prr Hamiltonian for f-f exchange interaction 7 n Neel ordering temperature... 

The linearized equations of motion for transverse spin deviations from antiferromagnetic Neel order are expressed in the unit of 2S/h = 1 by... 

Figure 4.13 shows the heat capacity of NiCE as a function of temperature.12 The heat capacity peak at 52 K occurs at the Neel point. Below this temperature, NiCl2 becomes antiferromagnetic and the electron spins become increasingly ordered, so that at 0 Kelvin, So = 0. Busey and Giauque13 were able... 

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... 

Figure 2.6 The initial dT/dp slope of selected second-order transitions relative to the transition temperature at p = 1 bar a- to /J-quartz (SiC>2) [8], the Neel temperature (T j) of Fei yO [9] and the Neel temperature of Lao.7Cao.3Mn03 [10].

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

The very low Neel temperature (77 K) of lepidocrocite ensures that this compound is paramagnetic at RT. The Mossbauer spectrum at RT consists of a doublet with a AEq of 0.53 mm s . Magnetically ordered lepidocrocite shows a sextet with a Bhf of 45-46 T at 4K that is lower than that of any other Fe oxide (Johnson, 1969 Murad Schwertmann, 1984). 

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