N.S. Bingham 1 , M.H. Phan 1 , H. Srikanth 1 M.A. Torija 2 and C. Leighton 2

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. Bingham1, M.H. Phan1, H. Srikanth1 M.A. Torija2 and C. Leighton2 1Department of Physics, University of South Florida, Tampa, FL, USA 2Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, USA Experimental Details Introduction Magnetocaloric effect in Pr0.5Sr0.5MnO3 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 R0.5M0.5MnO3 (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 (Tco) would be more useful for magnetic refrigeration than those around ferromagnetic Curie temperature (Tc). 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 Pr0.5Sr0.5MnO3. These results are of practical importance in assessing the potential use of magnetic refrigerant materials for advanced magnetic refrigerators. 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 Pr0.5Sr0.5MnO3 polycrytalline sample was made from Pr2O3, SrCO3, and MnO using standard solid-state reaction method. Magnetism in Pr0.5Sr0.5MnO3 • Transition at Tc~250K is a SOMT associated with PM/FM transition. • SOMT around Tc progressively broadens as field is increased. • Transition at Tco~150K is associated with charge ordering, antiferromagnetic ordering and structural transition. This transition remains sharp under high fields revealing strong coupling between these parameters. Magnetocaloric Effect Tco • 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. Tc • Positive -DSM at Tc is consistent with the PM to FM transition. • Negative -DSM at Tco is consistent with the FM/AFM transition. • The large -DSM peak at Tco appears desirable, however it occurs over a small temperature range. • What about refrigerant capacity (RC)? Credit: Talbott, NIST • Isothermal magnetization curves were used to evaluate MCE. • There is significantly more change in the magnetization around Tco giving rise to larger magnetic entropy change (DSM) • The saturation magnetization strongly decreases as the temperature is lowered from Tco • The change in entropy is calculated using thermodynamic Maxwell relations. • The refrigerant capacity (RC) is defined as the heat transferred from the cold end (at T1) to the hot end (at T2) of a refrigerator in an ideal thermodynamic cycle. • RC was calculated using equation (3), we see that when the hysteretic effects are subtracted the refrigerant capacity is significantly larger around Tc. References 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 Ni50Mn34In16” J. Phys. D: Appl. Phys. 40, 1869 2007. • We have studied the influence of first- and second-order magnetic phase transitions on the MCE and RC of Pr0.5Sr0.5MnO3. • We show that while the FOMT at TCO results in a larger MCE in terms of magnitude, the peak is confined to a narrow temperature region. The SOMT at TC yields a smaller MCE with a broader peak spanning a wider temperature range. This results in a larger value of the RC around TC, which is more useful for practical applications. • Hysteretic losses accompanying the FOMT are very large below TCO and therefore detrimental to the RC, whereas they are negligible below TC 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. IEEE Magnetics Society Summer School Nanjing University, China 2009 Work at USF supported by the US Department of Energy through Grant No. DE-FG02-07ER46438.