Neel point

If a paramagnetic salt is cooled, eventually a temperature is reached where the magnetic moments of the electrons line up. This temperature is known as the Curie point or the Neel point, depending upon whether the spins line up parallel below this temperature as in (b), with the moments pointing in the same direction to reinforce one another and produce ferromagnetism, or the moments line up in opposite directions as in (c) so that they cancel and antiferromagnetism occurs. 

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... 

Neel point, 27 38—46, 49 Neighboring atoms, effect in X-ray absorption spectroscopy, 35 22-23 Neodymia, conversion rates, 27 37 Neohexane... 

Curium metal was found to be antiferromagnetic Its Curie-WeiB behaviour above the Neel point is in good agreement with the localized picture (pett = 8 Pb as in Gd metal). In the case of Bk, again the Curie-WeiB law gives the expected effective moment value ... 

It is believed that a length such as must tend to infinity at a second-order transition, just as the relevant lengths do at a critical or Neel point. 

Above the Curie or Neel point, a spin polaron will move by a diffusive process. A moment on the periphery of the polaron will reverse its direction in a time t (the relaxation time for a spin wave). Each time it does so, the polaron can be thought to diffuse a distance (a/R)3R, so the diffusion coefficient is... 

Next, we discuss the magnetic behaviour of the materials described here as B/U is varied. The moment at T=0 in the antiferromagnetic lattice and Goodenough s (1971) conjecture for the Neel temperature are shown in Fig. 4.12. There exists at present no good theoretical description of the way TN drops as we go towards the value of B/U where the moments vanish. Near this value we do not expect an entropy /cBln 2 on going through the Neel point. With small values of there is a quantum-mechanical resonance between the two directions of the moment, and the moments on a given atom are nearly equal, so < z> is small at TN they become equal. The susceptibility should appear as in Fig. 4.13. 

Fig, 6.21 Magnetic susceptibility for the system Ni(S/Se)2. Note that the maximum in % occurs well above the Neel point for the compositions 0-25 < x < 0-45 (which are all metals... 

The Neel point of cobalt monoxide is 291 K. For obvious reasons this oxide cannot be heated in hydrogen without reduction to the metal. A sample of surface 5.8 m2 g 1 was pretreated in situ by heating it in purified helium at 823 K. The catalyst was then cooled to near room temperature before the admission of hydrogen. Figure 20 shows A k for CoO as a function of extrinsic field up to 18 kOe at 301 K. At this temperature ka was about 7.2 /u.mol m-2 s l. Figure 21 shows results for the same sample at 275 K. Figure 22 shows k0 and AkH at 17.3 kOe over the temperature range 275-300 K. 

At same time the low-temperature broadening of resonance line at T < 15 K is reproduced completely in powder case. For single crystal this broadening was related to the short-range magnetic order when temperature is approached to Neel point. 

FeF2, CoF2, and NiF2 also have body-centered ordering of the first kind, as is predicted since the t2o orbitals are more than half filled. Because of the low Neel points, no crystallographic studies of the spin-orbit distortions about the octahedral Fe2+ and Co2+ ions have been reported. 

The existence of a critical point in the pressure-volume-temperature (PVT) diagram (actually, a point in the planar PV projection, but a critical line in a three-dimensional representation), a critical point (Curie temperature) in ferromagnetism, a critical point (Neel point) in antiferromagnetism, a critical temperature in superconductivity, and a critical point (lambda point) in liquid 2He4 are physical descriptions of the onset of a sudden macroscopic collective transition. If one approaches the critical point very closely, dimensionless parameters, defined to describe this approach, are common to all these disparate phenomena the approach to criticality, or to a phase transition, are really the same. 

For most of the hydrides in the region of the dihydride the susceptibility rises to maximum 40° and 50° K. (Figures 1, 2, and 3). For TbH2i08 there is in addition a minimum at about 20° K. The maximum implies the onset of some type of antiferromagnetic ordering and the temperature at which this begins is the Neel point, Tn. The values of TN are also given in Table II. 

The foregoing remarks do not hold, of course, for the dihydrides of the triva-lent lanthanides. They exhibit metallic conduction (10), as would be expected, since their conduction band is only somewhat depleted. One would expect them to display a tendency to order at low temperatures, but it seems not unreasonable to expect that this tendency would be weaker than the corresponding element, in view of the decreased electron concentration, and the ordering would hence occur at lower temperatures. This was in fact observed for HoH2, which exhibits (6) a Neel point at 8° K., as coippared to 135° K. for Ho. It is also observed in the present work for the terbium dihydrides, whose Neel points are 40° to 50° K., whereas that for the element is 241° K. These properties are compatible with the notion that hydrogen in all the lanthanide hydrides is anionic. On this basis the dihydrides appear as an intermediate form between the truly metallic elements on the one hand and the truly ionic or saline trihydrides on the other. 

The heat capacity of EuS was measured to test the predictions of spin-wave theory from 1° to 38°K. by McCollum and Callaway (137) and independently from 10° to 35°K. by Moruzzi and Teaney (145). A sharp Neel peak was found at 16.2 °K. Magnetic and lattice contributions to the heat capacities were resolved on the assumption of a dependence for the lattice and a T dependence for the magnetic contribution at temperatures above the Neel point. A plot of CT vs. yields a straight line between 21° and 31 °K. and a Debye temperature of 208 °K. 

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