The Faraday Method

This method differs from that of Gouy in that the sample may be much smaller, and is placed in a field of relatively high gradient as shown in Fig. 3. The sample tends to move along the axis of the field gradient, the force being given by 

The force may be measured by a balance, but more frequently the apparatus is arranged so that the force is exerted horizontally, and is measured by a torsion arm. Many modifications have been used. 

The Faraday method appears to have some definite advantages over the Gouy method for work on powdered samples. 

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Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

Magnetic Susceptibility. Magnetization as a function of applied field was determined at various temperatures according to the Faraday method. A Cahn model RG microbalance and an Alpha model 4800 magnet were used In this respect. A detailed description of the technique used can be found in reference 4. 

Infrared spectra were recorded on a Perkin Elmer model 680 spectrophotometer as mulls in nujol or fluorolube. The magnetic susceptibility of the copper complexes was measured from 4.2 to 300 K by the Faraday method . X-ray photoelectron spectroscopy (XPS) was performed with a Perkin Elmer hemispherical spectrometer. 

Magnetic Susceptibility Measurements. Magnetic susceptibilities were measured by the Faraday method and were accurate to 2%. Samples, usually as powders, were placed in a cylindrical Teflon boat having an internal volume of 0.30 cc. Measurements were made at 2963, 4357, 5724, 7077, and 8243 G, and at temperatures 77°-295°K. It was possible to correct for ferromagnetic impurities from any field dependent effects. 

Magnetic susceptibility and magnetization measurements are made by a variety of techniques. These include the Gouy method, the Faraday method, and change in flux methods. Excellent descriptions of the methods as well as their advantages and limitations are available. " ... 

There are several other techniques to measure magnetic susceptibility, including nuclear magnetic resonance and the Faraday method using an unsymmetrical magnetic field. ... 

Although for p-azoxyanisole (PAA) a number of measurements in electric fields have been reported (I, 5), only Foex ( ) reported measurements in a high magnetic field. In this paper we report the results of magnetic susceptibility measurements on PAA, which have been obtained by using the Faraday method with a sensitive balance described by Poulis et al. (6). The sample consisted of 0.2841 gram of PAA held in an evacuated spherical quartz container. Measurements were made at fields of 2550, 1050, 275, and 200 oersteds at a number of temperatures in the liquid crystal and liquid phase. The absolute accuracy of the susceptibility measurements was about 3%, determined by the irreproducibility in the adjustment of... 

Magnetic Susceptibility Measurements. The magnetic susceptibility of the solids was measured by the Faraday method with Hg[Co(SCN)4] used as the calibrant. Corrections for diamagnetism were made using Pascals constants. Solution measurements were made in CHCI3 by NMR techniques (17),... 

In the Faraday method, that end of the tube which contains the P is heated to 450-530°C, while that containing the metal is heated first to 750°C and then to 950 to 1100°C. 

The principle of the method is that of the Faraday method except that as ff, the specific ms etization, is being measured, the force exerted is simply,... 

The total, uniform static susceptibility, Xwtai of Eq. (3.4), has been measured along the coexistence curve up to the region of the critical point for sealed samples of rubidium and cesium (Freyland, 1979, 1980). The Faraday method and sample cells used for these measurements are described in chapter 7. The temperature dependence of the total mass susceptibility, Xg < the susceptibility per gram, is shown in Fig. 3.1 for liquid rubidium and cesium. The mass susceptibilities of the two metals exhibit similar behavior as the temperature is raised, namely, weak temperature dependence at low temperatures followed by a striking rise at higher temperatures. In the case of cesium, the mass susceptibility reaches a maximum at a temperature near, but clearly below, the critical temperature... 

Despite the importance of the magnetic properties of expanded fluid mercury, experimental difficulties related to the high critical pressure have prohibited measurements of the static susceptibility except close to r j. As discussed in Sec. 3.2.2, the Faraday method is not readily adapted to use with internally heated autoclaves and the high critical pressure prevents the use of sealed sample cells. Thus measurement of the static, uniform susceptibility of mercury under conditions close to the critical point remains an open challenge to experimentalists. 

Fig. 7.3. Internally pressured cell for magnetic susceptibility measurements of fluid selenium by the Faraday method (Freyland and Cutler, 1980). The sealed quartz capsule containing the sample is mechanically supported by the surrounding molybdenum pressure cell.

Niedzielski and Troc (1984) studied the magnetic susceptibility of a polycrystalline sample NdP in the temperature range 4.2-800 K and // < 7kE using the Faraday method and found antiferromagnetic order at Tn = 11 K. Ren and Meng (1985) found NdP to be a semiconductor with a bandgap of 1.15 eV... 

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