Influence of shape and size variations on exchange bias and magnetocaloric effect in Ni₅₁Mn₃₄In₁₄Si₁ alloy

Recently, exchange bias (EB) and magneto-caloric effect (MCE) have been observed in off-stoichiometric Heusler compositions of Ni-Mn-X (X = Ga, Sb, Sn, In) ferromagnetic shape memory alloys (FSMAs). These observations have opened up the possibility of developing magnetic devices such as read heads for ultrahigh density magnetic recording and environment friendly magnetic refrigerators. Magnitude of exchange bias and MCE (in terms of adiabatic temperature change (ΔT_ad)) of Ni₅₁Mn₃₄In₁₄Si₁ ferromagnetic shape memory alloy in bulk, melt spun ribbon and powder forms have been evaluated using magnetometry and calorimetry techniques. EB has been observed in this alloy at low temperatures due to the coexistence of ferromagnetic and antiferromagnetic phases. At 50 K, the value of exchange bias exhibited by bulk > melt spun ribbon > powder. The highest peak ΔT_ad = -3.9 K is observed for 1.2 T field change in bulk alloy. But, the bulk sample shows a narrow operating temperature range (T_op = 2.4 K) as well as a comparatively small refrigeration capacity (RC = 35.9 Jkg^-1). These disadvantages have been overcome by obtaining the alloy as melt spun ribbon and micrometer sized ball milled powder. Maximum RC of 40.4 Jkg^-1 and Top of 4.6 K with peak ΔT_ad of -1.99 K are obtained for alloy powder with particle size ranging between 600 µm and 710 µm. The corresponding values displayed by melt spun ribbons prepared with a wheel speed of 16 ms^-1 are 37.3 Jkg^-1, 3.5 K and 2.6 K, respectively. Possibility of tailoring the value of exchange bias and the adiabatic temperature change of this alloy over a wide range is demonstrated by shape and size variations of the potential alloy composition Ni₅₁Mn₃₄In₁₄Si₁. (author)