Ferromagnetic Curie points

Nickel is ferromagnetic, but less markedly so that either iron or cobalt and its Curie point (375°C) is also lower. 

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 15.2 Curie-point pyrolysis Temperature profile and Curie points of typical ferromagnetic alloys used.

Pure iron is a fairly soft silver/white ductile and malleable moderately dense (7.87 gcm ) metal melting at 1,535 °C. It exists in three allotropic forms body-centered cubic (alpha), face-centered cubic (gamma), and a high temperature body-centered cubic (delta). The average value for the lattice constant at 20 °C is 2.86638(19)A. The physical properties of iron markedly depend on the presence of low levels of carbon or silicon. The magnetic properties are sensitive to the presence of low levels of these elements, and at room temperature pure iron is ferromagnetic, but above the Curie point (768 °C), it is paramagnetic. 

Molten iron Density 7.00 g/cm at 1,564°C vapor pressure 0.06 torr at 1,600° C, and 1 torr at 1,850°C, respectively viscosity 4.45 centipoise at 1,743°C surface tension 1,835-1,965 dynes/cm electrical resistivity 139 microhm-cm at the melting point. Magnetic properties attracted by magnets rapidly loses its magnetism ferromagnetic at ordinary temperature becomes paramagnetic when heated to its Curie point, 768°C. 

NpAs2 is antiferromagnetic below T = 52 K and becomes ferromagnetic below Tc = 18 K . Under field, the Curie point rises rapidly, until the antiferromagnetic phase is suppressed by 30 KOe (Fig. 8). 

If the material is ferromagnetic then the entropy, susceptibility and resistance at temperatures just above the Curie point are to be calculated in much the same way (i.e. in terms of spin fluctuations). A treatment of this problem starting from the Stoner-Wohlfarth model is due to Moriya and Kawabata (1973). 

CURIE, PIERRE (1859-1906) CURIE, MARIE (1867-1934). Pierre Cume was bom and raised in Paris. With his brother. Jacques, he studied crystals and in 1880 discovered piezoelectricity. Piezoelectricity is the production of an electric charge by pressure on certain crystals. Pierre became director at the School of Industrial Physics and Chemistry in Paris where he worked for 22 years. His doctoral thesis on magnetism led lo his discovery, the Curie point, a temperature at which ferromagnetic substances lose their magnetism. 

CURIE POINT (or Curie Temperature). Ferromagnetic materials lose their permanent or spontaneous magnetization above a critical temperature (different for different substances). This critical temperature is called the Curie point. Similarly, ferroelectric materials lose their spontaneous polarization above a critical temperature. For some such materials, this lemperaLure is called the "upper Curie point." for there is also a "lower Curie point." below which the ferroelectric property disappears. See also Ferromagnetism. 

CURIE-WEISS LAW. The transition from ferromagnetic to paramagnetic properties, which occurs in iron and other ferromagnetic substances at the Curie point, is accompanied by a change in the relationship of Ihe magnetic susceptibility lo the temperature. P. Curie stated in 1895 that above this point the susceptibility varies inversely as the absolute temperature. But this was found in be not generally true, and was modified in 1907 by P. Weiss to stare that the susceptibility uf a paramagnetic substance above the Curie point varies inversely as the excess of the temperature above that point. At or below the Curie point, the Curie-Weiss law does not hold. 

Few of the many known ferromagnetic solids are suitable as catalysts for the nondissociative ortho-parahydrogen conversion. This is especially true if measurements are needed in the neighborhood of the magnetic phase transition, Tc. The reasons for this are threefold the solid may decompose at the temperature necessary to free the surface from contaminants, the Curie point may be so low that the experimental difficulties are formidable, and many such solids show strong dissociative conversion activity near Tc. Of the three solids named above none is very satisfactory. 

For the CDA experiments described below a PERKIN-ELMER TGS-1 system was used with a cylindrical furnace on alumina ceramic, 6 mm i.d., with bifilar Pt windings as sketched in Fig. lb. The sample, 4x4 mm and 1.5-3 mm thick, was suspended from a fused silica hang-down wire and rested on a fused silica loop below the rim of the furnace. A Ni electrode 3 x 5 mm was introduced sidewise above the furnace, parallel to the sample surface at a distance of <0.5 mm. The temperature (T) was calibrated by means of the Curie points of ferromagnetic alloys. During the runs the balance was... 

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