3 FIG. 3: (Color online) Temperature dependence of the magnetization for different applied fields, between 50mT and 0.22T. Solid curves the respective T C and T M field dependencies FIG. 4: (Color online) Isothermal field dependence of mag- netization up to 0.3T for the intermediate phase ( T C < T < T M ). Between 4K and 5K the measurements were carried every 0.1K, but represented every 0.2 K for clarity. The marked area centered at at B = 0 . 12 T is discussed in the text. sis is strongly weakened in a field of 0.2 T in agreement with the specific heat results, see Fig. 2b. This means that the first order transition vanishes above that field. The M ( T ) dependence of was measured under dif- ferent applied fields, starting from B = 0 . 05 T. Be- tween 0.1 T and 0.22 T the isodyna curves M ( T ) |$_{B}$ were measured with steps of 0.02 T as shown in Fig. 3. This study shows how the FM phase overcomes the modulated one, being the boundary of the former determined by the maximum slope of M ( T ) (i.e. dM/dT |$_{max}$ ) and that of the later by the cusp at M ( T ) at ( T$_{M}$ ). Both phase boundaries are depicted in the phase diagram of Fig. 6 by respective curves which join at a critical point. Isothermal M ( B ) measurements were performed within the 2 ≤ T ≤ 5 K range of temperature up to B = 1 T. In Fig. 4 we present the low field mag- netization curves, with the typical FM dependence at T = 2 K, including a weak hysteresis (not shown for clarity) between increasing and decreasing field. Above the transition (i.e. T ≥ 2 . 5 K), M ( B ) shows an S-shape dependence indicating that a re-arrangement of the magnetic structure occurs as a broad field in- duced meta-magnetic transformation. Such a broad- ening is attributed to the randomly distribution of the magnetic directions in this poly-crystalline and strongly anisotropic compound. Interestingly, M ( B ) curves start to intersect each other between 4 K and 4.7 K, see the remarked region in the figure around 0.12 T. This suggests that some anomalous variation of the magnetization occurs close to the critical field at that temperature range. III. DISCUSSION A simple thermodynamic analysis of the field effect on the thermal dependence of the specific heat allows Those results are presented in Fig.5b. Within that range of temperature one may distinguish between a main slope of ∂M/∂B |$_{T}$ , practically fixed at B ≈ 0 . 11 T, and a second contribution which arises up to Also in Fig.5a one can see well defined maxima of ∂M/∂B isotherms as a function of field, whose in- tensity decreases as the temperature increases. Ap- proaching the critical point, each maximum splits into two maxima hardly distinguish in the 4.5 K isotherm. To elucidate the real existence of a structure within that narrow range of field and temperature, we have performed detailed measurements of M ( B ) between 3.8 K and 4.7 K up to B = 0 . 3 T. Since the specific heat jump is progressively smeared by the mentioned random orientation of indi- vidual crystals and the maximum slope of ∂M/∂T |$_{B}$ is not properly defined, to determine the phase bound- ary between the magnetic phases for a phase diagram construction requires to use a precise procedure. For such a purpose we have analyzed the field dependence of the isothermal ∂M/∂B |$_{T}$ derivative between 2 K and 5 K. From Fig.5a one may extract the maximum slope of this derivative, which is related with the max- imum curvature of the M ( B ) dependence (see Fig. 4) and provides a better approach for determining the field at which the induced FM phase sets in. to extract some trends concerning the magnetic be- havior of the system. From Fig. 1, one sees that at low field ( B ≤ 0 . 2 T) C$_{m}$ ( B ) /T |$_{T}$ slightly increases below T$_{M}$ and consequently S$_{m}$ ( B ) |$_{T}$ computed as ∫ C$_{m}$ ( T ) /TdT . From Maxwell relations, such increase of S$_{m}$ ( B ) |$_{T}$ corresponds to the observed increase of M ( T ) as T → T$_{M}$ since ∂S$_{m}$/∂B |$_{T}$ = ∂M/∂T |$_{H}$ > 0. On the contrary, at higher fields (i.e. B ≥ 0 . 5 T) an in- duced FM behavior is reflected in a strong ∂S$_{m}$/∂B |$_{T}$ decrease. This change of behavior reveals that the apparent AF character of the intermediate phase is overcome by a FM behavior around 0.2 T.