Slide 1

Report
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.
1
Bingham ,
1
Phan ,
M.H.
H.
1
Srikanth
M.A.
2
Torija
and C.
2
Leighton
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
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.
• If the system is under isothermal and
isobaric conditions.
• Entropy decreases when |H| > 0.
• Entropy increases when applied field
is removed.
(3)
RC    DS M (T )dT
T1
Tco
66
55
Tc
44
50 kOe
33
22
11
10 kOe
50
100 150 200 250 300
•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.
• 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.
• 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
•
•
•
•
45
30
15
T=300 K
0
75
T=164K
60
•
TFWHM
0
T1
T2
-2
-4
RC at TCO
-6
45
0
30
15
T=65K
0
0
50
100 150 200 250 300
Temperature (K)
10
20
30
40
50
• RC was calculated using equation (3), we see that when the hysteretic effects are subtracted the refrigerant
capacity is significantly larger around Tc.
60
Conclusion
References
•
RC at Tc
D0H = 2.4 T
2
Applied Field (kOe)
•
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)?
T=164 K
60
Magnetization (emu/g)
• The change in entropy is calculated using
thermodynamic Maxwell relations.
 M (T , H ) 
DS M T , DH   S M (T , H 0 ) S M (T ,0)   
 dH
T
H
0
T2
77
• Transition at Tc~250K is a SOMT
associated with PM/FM transition.
• SOMT around Tc progressively
broadens as field is increased.
75
H0
(2)
Magnetism in Pr0.5Sr0.5MnO3
Temperature (K)
Credit: Talbott, NIST
(1)
High quality Pr0.5Sr0.5MnO3 polycrytalline sample was made from Pr2O3,
SrCO3, and MnO using standard solid-state reaction method.
0
0
• If the system is adiabatic and isobaric .
• Temperature increases when |H| > 0.
• Temperature decreases when applied
field is removed.
 S 
 M 

  

 H T
 T  H
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.
Magnetization (emu/g)
Magnetocaloric Effect
Magnetocaloric effect in Pr0.5Sr0.5MnO3
-DSmax(J/kg.K)
Introduction
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.
IEEE Magnetics Society Summer School
•
•
Nanjing University, China 2009
•
•
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.
Work at USF supported by the US Department of Energy through Grant No. DE-FG02-07ER46438.

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