1. A Comparative Study of the Influence of First and Second Order Transitions on the Magnetocaloric Effect and Refrigerant Capacity in Half-doped Manganites N.S. Bingham 1, M.H. Phan 1, H. Srikanth 1 M.A. Torija 2 and C. Leighton 2 1 Department of Physics, University of South Florida, Tampa, FL, USA 2 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, USA Magnetic refrigeration has many promising practical applications. The low cost of materials, high efficiency, and the elimination of green house gases can revolutionize cooling technologies. Half-doped R 0.5 M 0.5 MnO 3 (R=Pr, La; M=Ca, Sr) manganites that exhibit a giant magnetic entropy change in the vicinity of the charge-ordered transition have attracted attention. This leads to a general expectation that the consistently larger values of the change in entropy around the charge-ordered temperature (T co ) would be more useful for magnetic refrigeration than those around ferromagnetic Curie temperature (T c ). To address this issue, we conducted a comparative study of the influence of the first- and second-order magnetic transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr 0.5 Sr 0.5 MnO 3. These results are of practical importance in assessing the potential use of magnetic refrigerant materials for advanced magnetic refrigerators. Introduction Magnetocaloric Effect Work at USF supported by the US Department of Energy through Grant No. DE-FG02-07ER46438. Magnetocaloric effect in Pr 0.5 Sr 0.5 MnO 3 Experimental Details Positive -  S M at T c is consistent with the PM to FM transition. Negative -  S M at T co is consistent with the FM/AFM transition. The large -  S M peak at T co appears desirable, however it occurs over a small temperature range. What about refrigerant capacity (RC)? Conclusion N. S. Bingham, et. al. “Magnetocaloric effect and refrigerant capacity in charge-ordered manganites” J. Appl. Phys. 106, 023909 (2009) V. K. Pecharsky and K. A. Gschneidner, Jr., “Some common misconceptions concerning magnetic refrigerant materials” J. Appl. Phys. 90, 4614 (2001) V. K. Sharma, M. K. Chattopadhyay, and S. B. Roy, “Large inverse magnetocaloric effect in Ni 50 Mn 34 In 16 ” J. Phys. D: Appl. Phys. 40, 1869 2007. References If the system is under isothermal and isobaric conditions. Entropy decreases when |H| > 0. Entropy increases when applied field is removed. If the system is adiabatic and isobaric. Temperature increases when |H| > 0. Temperature decreases when applied field is removed. Magnetic measurements were performed using a commercial Physical Property Measurement System (PPMS) in the temperature range of 5–300 K at applied fields up to 7 T. The magnetization isotherms were measured with a field step of 0.05 mT in the range of 0–5 T and with a temperature interval of 3 K over a temperature range of 5–300 K. High quality Pr 0.5 Sr 0.5 MnO 3 polycrytalline sample was made from Pr 2 O 3, SrCO 3, and MnO using standard solid-state reaction method. Magnetism in Pr 0.5 Sr 0.5 MnO 3 The change in entropy is calculated using thermodynamic Maxwell relations. Transition at T c ~250K is a SOMT associated with PM/FM transition. SOMT around T c progressively broadens as field is increased. Transition at T co ~150K is associated with charge ordering, antiferromagnetic ordering and structural transition. This transition remains sharp under high fields revealing strong coupling between these parameters. Isothermal magnetization curves were used to evaluate MCE. There is significantly more change in the magnetization around T co giving rise to larger magnetic entropy change (  S M ) The saturation magnetization strongly decreases as the temperature is lowered from T co The refrigerant capacity (RC) is defined as the heat transferred from the cold end (at T 1 ) to the hot end (at T 2 ) of a refrigerator in an ideal thermodynamic cycle. We have studied the influence of first- and second-order magnetic phase transitions on the MCE and RC of Pr 0.5 Sr 0.5 MnO 3. We show that while the FOMT at T CO results in a larger MCE in terms of magnitude, the peak is confined to a narrow temperature region. The SOMT at T C yields a smaller MCE with a broader peak spanning a wider temperature range. This results in a larger value of the RC around T C, which is more useful for practical applications. Hysteretic losses accompanying the FOMT are very large below T CO and therefore detrimental to the RC, whereas they are negligible below T C due to the nature of the SOMT. A proper comparison between magnetocaloric materials should be made with the use of RC, paying attention to the fact that magnetic hysteretic losses must be estimated and subtracted from the RC calculation. RC was calculated using equation (3), we see that when the hysteretic effects are subtracted the refrigerant capacity is significantly larger around T c. Credit: Talbott, NIST TcTc T co IEEE Magnetics Society Summer School Nanjing University, China 2009