Ferromagnet spontaneous magnetization

Below the critical (Curie) temperature, Tc a ferromagnet spontaneously magnetizes to a permanent magnet whose magnitude and direction is denoted by the magnetization vector, M(T). The experimental order parameter is defined by ... 

An analysis of the magnetization-vs-temperature curve, as shown in Fig. 1, indicates a Neel temperature of approximately 217 K. This value is the highest among those previously reported for difluorides.. Above the Neel temperature, PdFj is paramagnetic and the inverse of the molar susceptibility varies linearly with T (see Fig. 1). For 7 <217 K a ferromagnetic spontaneous magnetization appears, superimposed upon a susceptibility term, linear in the field, which can be represented by the relation... 

Ferromagnetics Spontaneous magnetization Magnetic field Fe304... 

Equation (A2.5.20) is the Curie-Weiss law, and the temperature at which the magnetic susceptibility becomes infinite, is the Curie temperature. Below this temperature the substance shows spontaneous magnetization and is ferromagnetic. Nonnally the Curie temperature lies between 1 and 10 K. However, typical ferromagnetic materials like iron have very much larger values for quantum-mechanical reasons that will not be pursued here. 

In the case of very weak ferromagnets, like UNi2 and UPt , the anisotropy is very small but problems are connected with M(H). There is of course no saturation of the magnetization due to the very large superimposed susceptibility in the ordered state. Spontaneous magnetization may be conveniently obtained from Arrott plots (linear versus H/M for small M values) and extrapolation to H = 0. Such results for UNi2 are shown on Fig. 2. 

Spontaneous magnetization of ferromagnets, therefore, decreases with increasing temperature and it disappears at temperatures above Tc. Above To ferromagnets become paramagnets and obey the Curie-Weiss law. Metals such as iron, cobalt, and nickel and their alloys, and y-Fe203 are examples of ferromagnetic materials. 

Ferromagnets have their spins aligned parallel and exhibit large positive (about 102) magnetic susceptibilities. They possess a macroscopic spontaneous magnetization at zero applied field below the Curie temperature, Tc, and have a characteristic saturation moment, Ms, in a finite applied field (see Fig. 128). 

Spontaneous magnetization in ferromagnetic semiconductors gives rise to spin splitting of the conduction and valence bands due to the presence of exchange interaction. This spin... 

Intriguing forms of competition between superconductivity and ferromagnetism have recently been reported for the elements carbon and iron, where the two cooperative phenomena are related to different crystallographic structures. As discussed in section 1.1 pristine C60 consisting of dominantly van der Waals bounded molecules becomes superconducting if doped by electrons or holes. On the other hand, under sufficiently high pressure and temperature, a layered rhombohedral structure of Cft) forms where, within the layers, the C60 molecules are covalently bound. This phase is metastable at room temperature and ambient pressure. It shows a spontaneous magnetization, which is assumed to be based on unpaired electrons created by structure defects (Makarova et al. 2001 Xu and Scuseria 1995). 

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