the inflexion field $H_{c}$ (Fig. 6b). It has been proved that at $H_{c}(T_{c})$ a discontinuity of the magnetization is provided by the sudden vanishing of the chemical potential $\mu$ [@7; @20; @25; @26; @28].

The magnetization was measured using a vibrating sample magnetometer, VSM, in temperatures above 0.4 K at the High Magnetic Field Laboratory facility of the University of São Paulo. The sample was immersed in a $^{3}$He bath, and the temperature was measured by vapor pressure and carbon-glass or cernox thermometer. The magnetic field was supplied by a Nb$_{3}$Sn superconductor. Measurement at $T = 0.019$ K was made using the gradient field method in the dilution refrigerator. The transitions were determined in the magnetization versus field curves by locating anomalies in the second derivative of the magnetization [@33].

![\[fig6\] a) Curves of magnetization taken at several fields. The minimum indicates the transition temperature $T_c$. b) Representative curve of $dM/dH$. The sharp bending indicates the transition field $H_c$.](figure6){width="7.5cm"}

4.2. Specific Heat
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All measurements were made on crystals with the external field $H$ provided by the 17-20 T superconducting magnet in a dilution refrigerator system at the National High Magnetic Field Laboratory in Los Alamos. The magnetic field was 