Iron Curie temperature

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. 

Fig. 3. Effect of siHcon on properties of iron (10). = Curie temperature = magnetocrystaUine anisotropy constant. To convert T to G, multiply by...

The detrimental effects of Si addition are (/) Si iacreases the yield strength and decreases the ductiHty of iron such that commercial-grade materials are limited to ca 4% Si, and (2) as shown ia Eigure 3, the saturation iaduction and Curie temperature are decreased with increasing siHcon content. 

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

The polymorphism of certain metals, iron the most important, was after centuries of study perceived to be the key to the hardening of steel. In the process of studying iron polymorphism, several decades were devoted to a red herring, as it proved this was the P-iron controversy. P-iron was for a long time regarded as a phase distinct from at-iron (Smith 1965) but eventually found to be merely the ferromagnetic form of ot-iron thus the supposed transition from P to a-iron was simply the Curie temperature, p-iron has disappeared from the iron-carbon phase diagram and all transformations are between a and y. 

The ferroelectricity usually disappears above a certain transition temperature (often called a Curie temperature) above which the crystal is said to be paraelectric this is because thermal motion has destroyed the ferroelectric order. Occasionally the crystal melts or decomposes before the paraelectric state is reached. There are thus some analogies to ferromagnetic and paramagnetic compounds though it should be noted that there is no iron in ferroelectric compounds. Some typical examples, together with their transition temperatures and spontaneous permanent electric polarization P, are given in the Table. 

The calculated energy of interaction of an atomic moment and the Weiss field (0.26 uncoupled conduction electrons per atom) for magnetic saturation is 0.135 ev, or 3070 cal. mole-1. According to the Weiss theory the Curie temperature is equal to this energy of interaction divided by 3k, where k is Boltzmann s constant. The effect of spatial quantization of the atomic moment, with spin quantum number S, is to introduce the factor (S + 1)/S that is, the Curie temperature is equal to nt S + l)/3Sk. For iron, with 5 = 1, the predicted value for the Curie constant is 1350°K, in rough agreement with the experimental value, 1043°K. 

For alloys of iron, cobalt, nickel, and copper the calculated values of saturation magnetic moments agree closely with the observed values in particular, the maximum value of about 2.48 magnetons at electron number about 26.3 is reproduced by the theory. There is, however, only rough agreement between the observed and calculated values of the Curie temperature. 

Above a specific temperature, the Curie temperature, a ferroelectric substance becomes paraelectric since the thermal vibrations counteract the orientation of the dipoles. The coordinated orientation of the dipoles taking place during the ferroelectric polarization is a cooperative phenomenon. This behavior is similar to that of ferromagnetic substances, which is the reason for its name the effect has to do nothing with iron (it is also called seignette or rochelle electricity). 

Iron is a material whose ferromagnetic properties have been applied for a long time. It becomes paramagnetic when heated above the Curie temperature of 766 °C this does... 

The simplest example of an order-disorder transformation in which only one element is involved is the ferro- to diamagnetic transformation of b.c.c. a-iron, when the magnetic properties change over a range of temperature, the completion of the transformation being at the Curie temperature. Since this transformation only requires a randomization of electron spins without atomic diffusion, the process is very rapid, and the degree of spin disorder closely follows the thermodynamic model as the temperature of the solid is brought up to the Curie temperature. 

In the system Mo,Fe3 x04, there is a regular variation with x, from Fe304 to MoFe204, in the lattice parameter (ao), the magnetization (Ms), the Curie temperature (Tc) and the electrical conductivity (a, AH ) apparently substitution of Fe by Mo leads to a mixed iron valence on both sites for 0 < x < 1. It is worth noting that AH varies smoothly with x from a AH < kT for x = 0 in the temperature interval 300 < T < 600 K to 0.027 eV for x = 1.0, where the narrow minority-spin band is maximally perturbed and filled by the introduction of substitutional Mo ... 

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