Ferromagnetic measurements

The study of ferromagnetic components in catalysts calls for a field sufficient to saturate the sample, and for appropriate temperature control. The fields used should be variable over at least one or two thousand gauss the temperature region so far extensively used is from room temperature to about 700 . 

The apparatus to be described is very similar to those described elsewhere (Hofer el cd., 11 Buehl and Wulff, 12). It differs from these in that it automatically records specific magnetization as a function of time 

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 field of about 1500 gauss is supplied by a moderate-sized controlled electromagnet. A uniform field gradient may be obtained either 

The sample is suspended on a torsion arm so that it has a few millimeters of free lateral motion in the re on of uniform field gradient. The torsion arm is heavily damped by vanes extending into an oil pot. 

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Other magnetic measurements of catalysts include electron paramagnetic resonance and magnetic susceptibility. Although those are not as common as NMR, they can be used to look at the properties of paramagnetic and ferromagnetic samples. Examples of these applications can be found in the literature [87. 

Eor steel and other ferromagnetic materials, property deterrnination is more difficult. Other tests are made to measure the continuity of protective metallic coatings. Residual stresses induced in welded stmctures and in components in service owing to chemical attack may contribute to early failure. 

The instrumentation required to measure the hysteresis loop of a ferromagnetic surface with MOKE can be very simple. Figure 4a shows one such implementation of the experimental setup for analysis of ultrathin film samples maintained in a... 

The work on iron-nickel alloys has described shock-compression measurements of the compressibility of fee 28.5-at. % Ni Fe that show a well defined, pressure-induced, second-order ferromagnetic to paramagnetic transition. From these measurements, a complete description is obtained of the thermodynamic variables that change at the transition. The results provide a more complete description of the thermodynamic effects of the change in the magnetic interactions with pressure than has been previously available. The work demonstrates how shock compression can be used as an explicit, quantitative tool for the study of pressure sensitive magnetic interactions. 

As shown in Fig. 7.6, the Mossbauer data show a reduction in Morin transition temperature with increasing shock severity. At temperatures below the transition, increasing shock severity causes greater retention of the higher temperature, weak ferromagnetic contribution. The measure of weak ferromagnetic (WF) fraction (the high temperature form) is a sensitive indication of shock modification. 

Fig. 7.6. The weak ferromagnetic (WF) fraction (high temperature form) of hematite provides a sensitive measure of shock modification. Sample 31G836 is an 8 GPa experiment. Sample 29G836 is a 17 GPa experiment, while 17G846 is a 27 GPa sample (after Williamson et al. [86W03]).

The limited magnetic measurements of very mixed -metal clusters are summarized in Table XIII. The magnetic behavior of some anti-ferromagnetic very mixed -metal carbonyl clusters (Fig. 82) has been studied by Pasynskii and eo-workers. Temperature dependences of the magnetic susceptibilities of Cr2Co(/t3-S)3(/i-SBu )(CO)2() -C3H4R)2l (R = H. Me) have been determined us-... 

Figure 5b. Variation in the magnetic properties of metal clusters are investigated by measuring the depletion of a highly collimated cluster beam by an inhomogeneous magnetic field. Fe clusters and their oxides (FexO and Fex02) at several applied fields. The uniform depletion of Fe clusters indicates that their magnetic moments increase approximately linearly with number of atoms, as would be anticipated for incipient ferromagnetic iron. Unexpected, however, is the much larger depletion of iron oxide clusters.

Fig. 7.73 Mossbauer spectra of Pt (99 keV) in ferromagnetic alloys measured in backscattering geometry with the scatterer kept at 29K. The count rate at infinite velocity is normalized to 1 (from [330])...

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