Magnetically Ordered Materials

9 Magnetically Ordered Materials. - In this section we shall review work where the interest is largely in the ordered magnetic phase of a material. (Many of the materials discussed in Section 2.8 become AFM at low temperature but are mainly of interest because of their unusual properties in a higher-temperature phase.) 

In pure LaMnOs no HFEPR signal was expected, because it should contain only Mn ions, but at 95 GHz a line with a width of greater than 1 T was observed in the paramagnetic phase near g = 2. Below the AFM ordering temperature of 139 K the line disappeared due to the opening of a large AFM energy gap. The line was attributed to residual Mn + ions. 

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Magnetic contributions to the Gibbs energy due to an internal magnetic field are present in all magnetically ordered materials. An additional energetic contribution... 

Porous vycor glass Surface-bound, octahedrally coordinated Fe3+ species obtained in the photolysis of Fe(CO)5 physisorbed onto the glass Magnetically ordered materials were obtained which exhibited magnetic hyperfine fields of 370 and 425 kG 798... 

For magnetically ordered materials, photoemitted electrons have a characteristic spin polarization that reflects the electron spin orientation occurring in the sample before the photoemission process. Recently, techniques have been developed to measure this photoelectron spin polarization (photo ESP) (21). 

In the magnetically ordered materials, the additional contribution f(Cm/T) AT, expressed in eq. (2.7), reflects the amount of the entropy change associated with the ordering. In the case of localized moments this expression should be equal to RlnM where M is the multiplicity of the ground state, but in the itinerant systems strongly reduced values are found. 

The magnetically ordered materials include the numerous class of the ferromagnetics (spin magnetic moments of unpaired electrons aligned parallel below Tc) and the antiferromagnetics (an antiparallel alignment of... 

There are several other classes of magnetically ordered materials where the spatial distribution of the microscopic magnetic moments is more complex. 

As an example, we show in fig. 85 the level scheme involved with the 25.65 keV gamma transition in Dy. It can be seen from the right-hand part of fig. 85 that the Dy Mbssbauer spectrum of magnetically ordered materials will in general be composed of 16 hnes. 

Since the di,scovery more than 40 years ago, Mossbauer spectroscopy has become an extremely powerful analytical tool for the investigation of various types of materials. In most cases, only two parameters are needed, viz. the isomer shift and the quadrupole splitting, to identify a specific sample. In case of magnetically ordered materials, the magnetic dipole interaction is a further helpful parameter for characterization. In the following... 

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