Magnetic heat capacity ferromagnet

EuRu4Sbn is metallic and becomes ferromagnetic for temperatures below 3.3 K (Takeda and Ishikawa, 2000b). The low temperature saturation moment is about 6.2/xb, 89% of the Eu+2 value. Low temperature heat capacity measurements indicate that the magnetic entropy removed due to magnetic order is also only about 90% of its expected value (Rln8). It is likely that the lanthanide site is not completely filled in this compound although mixed valence behavior can not be ruled out with the available data. 

In Fig. 13 the result of the field dependent calculation of the heat capacity using Landau theory for fields within the tetragonal plane is shown. In agreement with the experiment (Fig. 5), the magnetic field broadens the anomaly observed at the Neel temperature TN and lowers the transition point T of the commensurate to incommensurate phase transition. The increase of the calculated heat capacity while lowering the temperature below T, is a consequence of the approach to the temperature T of the proper instability of the ferromagnetic subsystem. As such, the same increase is observed in the experimental data too and it is possible to assume that the broad maximum at low temperature is connected with the subsequent phase transition in the magnetic subsystem of copper metaborate. 

An extended two-particle cluster approximation involving nearest-and next-nearest neighbor exchange for para- and ferromagnets (with particular reference to EuO, EuS, EuSe, and EuTe) has been developed (22). Heat capacity curves. Curie temperatures, magnetization curves. 

Heat capacities taken over the range 1.3°-20°K. 19) show an extremely narrow peak at 4.58 0.03 °K. at the EuSe ferromagnetic Curie temperature and one at 9.64 0.06 °K. in EuTe identified with the antiferromagnetic transition. The magnetic transition entropy increments are 4 and 3 cal./(mole °K.). 

The heat capacities of sintered specimens of LaN and NdN were measured from 1.2°K. to 45°K. by Veyssie, et al. 185). LaN shows a slight anomaly attributed to magnetic impurities NdN has a peak at 27.6 zh 0.1°K. corresponding to a (ferromagnetic) Curie transition. Debye 0 values are 300° and — 360 °K., respectively, but the data do not cover an adequate range to permit evaluation of the thermodynamic properties. 

Roth ( ) observed a maximum in the magnetic susceptibility of Co O at 40 K, while Mossbauer studies by Kundig et al. (15) indicated a Neel temperature of 33.0 1.0 K. With the assumption that Co O is a normal 2-3 spinel (16), this transition can be associated with the anti ferromagnetic ordering of the Co ion spin moments. Therefore, the entropy is based on S (51 K) 1.36 + 2.75 4.11 cal K mol , where 1.36 is a lattice contribution and 2.75 is the magnetic entropy. In assigning the magnetic entropy, it is assumed that all of the contribution remains to be extracted below the minmum temperature (54 K) of the heat capacity measurements. 

Considerable work has been done recently to determine the details of the CFI in ErAl2 compound. Inoue et al (54) utilized calorimetric results to evaluate the CFI parameters in this material. Heat capacity measurements reported by Inoue (55) revealed that this material forms a cooperative magnetic phase at 10.2 K. From the earlier magnetization study it is clear that the material orders ferromagnetically. 

The magnetic term in the heat capacity is what would be expected from the ferromagnetic spiral (i.e. the cone) structure of erbium (Kaplan 1961) and has been supported by the discovery of a linear spin-wave dispersion law along the c-axis in neutron scattering experiments (Nicklow et al. 1971). 

Fig. 9. Temperature dependence of the magnetic susceptibility in paramagnets, ferromagnets and antiferro-magnets. Below the Neel temperature of an antiferromagnet the spins have antiparallel orientations the susceptibility attains its maximum value at where there is a well-defined kink in the curve of / vs. T. The transition is also marked by peaks in the heat capacity and the thermal expansion coefficient.

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