Effect of Dy substitution in the giant magnetocaloric properties of HoB₂

Figures & data

Table 1. Lattice parameters obtained from XRD patterns for Ho_1-xDy_xB_2.

Nominal Dy (x)	a (Å)	c (Å)	V (Å^3)	ρ (g/cm^3)
0	3.283(3)	3.815(8)	35.62(4)	8.696
0.3	3.285(6)	3.827(1)	35.77(8)	8.626
0.5	3.285(7)	3.832(5)	35.82(9)	8.591
0.7	3.287(8)	3.839(7)	35.94(7)	8.540
1.0	3.292(0)	3.850(6)	36.14(0)	8.461

Figure 1. Powder XRD patterns and lattice constant evolution for Ho_1-xDy_xB₂ alloys. (a) XRD patterns of the obtained alloys. The red lines show the calculated patterns from Rietveld refinement for the REB₂ main phase. The black square (■) marks an REB₄ impurity phase, while the black star (★) marks a RE impurity peak and the black diamond (♦) marks a RE₂O₃ impurity peak (RE = Ho, Dy). (b) The lattice parameters normalized by the value at x = 0, as a function of x. The black dashed line shows a guide based on Vegard’s law, given by $1-x+x\left(\frac{a_1,c_1}{a_0,c_0}\right)$ where (a,c)_0,1 is the lattice constants at x = 0 or 1

Figure 2. Isofield (M-T) ZFC-FC, Normalized temperature-dependent derivatives of the ZFC curves, and Isothermal (M-H) magnetization curves of Ho_1-xDy_xB₂ alloys. (a-e) ZFC and FC curves for all synthesized alloys for an applied field of µ₀H = 0.01 T. The lower panels show the derivatives of the ZFC curves normalized by the minimum of the derivative value. The two magnetic transitions T_C and T*, are marked by the arrows. (f-g) Isothermal magnetization at T = 5 K

Figure 3. Phase diagram between ordering temperature and doping amount (x) for Ho_1-xDy_xB₂. The blue filled circles show the evolution of T_C while the orange triangles show T*. While T_C increases monotonically with x, T* remains almost constant

Figure 4. M-T curves at a vast range of applied fields and obtained magnetic entropy change for Ho_1-xDy_xB₂ alloys. (a-e) The obtained M-T curves measured by ZFC process from µ₀H = 5 T to 0.01 T. (f-j) Magnetic entropy changes for Ho_1-xDy_xB₂ alloys for µ₀ΔH ranging from 1 to 5 T obtained from the M-T curves of (a-e). (k)-(o) Estimated adiabatic temperature change for Ho_1-xDy_xB₂ alloys for µ₀ΔH ranging from 1 to 5 T [21]. With the increase of Dy content, the maximum value of |ΔS_M| decreases from 0.35 J cm⁻³ K⁻¹ (x = 0) to 0.16 J cm⁻³ K⁻¹ (x = 1.0)

Figure 5. |ΔS_M| at µ₀ΔH = 5 T for Ho_1-xDy_xB₂ alloys

Figure 6. Maximum entropy and adiabatic temperature change and TEC values for Ho_1-xDy_xB₂ alloys and diverse compounds for µ₀ΔH = 5 T. (a) |ΔS_M^MAX| and (b) |ΔT_ad^MAX| as a function of the magnetic ordering temperature T_mag. (c) Values of TEC for ΔT_lift = 10 K and (d) for ΔT_lift = 20 K. (e) TEC as a function of ΔT_lift for few representative materials with high TEC 10 values. The normal and dashed lines guide for the eyes. The TEC values were obtained by using the reported entropy curves within each material reference

Note that here an assumption that the adiabatic route tracks the horizontal line in S(H,T) is made, and that might not be valid in the case of crossing two successive transitions.

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