Among them, the magnetic entropy change reaches a maximum value of 2.67 J/kg K at the applied field of 13.5 kOe for the Cu-doped sample, suggesting that this material would be a suitable
Trang 1phys stat sol (b) 241, No 7, 1744 – 1747 (2004) / DOI 10.1002/pssb.200304587
Large magnetocaloric effect in La0.845Sr0.155Mn1–xMxO3
(M = Mn, Cu, Co) perovskites
Manh-Huong Phan1
, The-Long Phan2
, Seong-Cho Yu*, 2
, Nguyen Duc Tho3
,
1
Department of Aerospace Engineering, Bristol University, Queen’s Building, University Walk,
Bristol, BS8 1TR, UK
2
Department of Physics, Chungbuk National University, Cheongju, 361-763, South Korea
3
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
Received 10 November 2003, revised 3 March 2004, accepted 8 March 2004
Published online 6 May 2004
PACS 75.30.Sg, 75.47.Lx
We present the results of an investigation on the magnetocaloric effect in the perovskites of
La0.845Sr0.155Mn1–xMxO3 (M = Mn, Cu, Co) It is found that there was a large magnetic entropy change, i.e a large magneto-caloric effect, in all these samples Among them, the magnetic entropy change reaches a maximum value of 2.67 J/kg K at the applied field of 13.5 kOe for the Cu-doped sample, suggesting that this material would be a suitable candidate for the advanced magnetic refrigeration technology The large magnetic entropy change produced by the abrupt reduction of magnetization is attributed to the strong coupling between spin and lattice that occurs in the vicinity of the ferromagnetic-paramagnetic transition
temperature (TC) – which is experimentally verified by electron paramagnetic resonance study
© 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
1 Introduction Recently, a large magnetocaloric (LMC) effect, which is defined as changes of the
entropy related to the temperature change of a magnetic substance associated with an external magnetic field change in an adiabatic process, discovered in such perovskite manganites has attracted considerable interest in the science community, owing to its excellent potential for magnetic refrigeration applications [1 – 4] Generally, there are two crucial factors for a magnetic material to possess a LMC One is a large enough spontaneous magnetization (belongs to a class of heavy rare-earth metals, for example, Gd metal) and the other is a sharp drop in magnetization with increasing temperature, associated with the ferromag-netic-paramagnetic transition at the Curie temperature (being found in such perovskite manganites) Apart from the above, it should be noted that most of the famous LMC materials were found to undergo
a first-order magnetic transition (FOMT) [5, 6] As reported in Ref [6], the magnitude of the magnetic entropy change, ∆SM , around the first-order transition is about three times larger than that obtained around the second-order transition in the same compound of Nd0.5Sr0.5MnO3 The large entropy change in
a FOMT originates from a difference in the degree of magnetic ordering between two adjacent magnetic phases The FOMT from a ferromagnetic state to paramagnetic one is therefore expected to show a LMC effect
By far, Tian et al [7] have reported a larger magnetic entropy change in a La0.7Ca0.3MnO3 single crys-tal than in its polycryscrys-talline material It is interesting to note that the ∆S M distribution in this single crys-tal has been found to be much more uniform than that of pure Gd mecrys-tal and polycryscrys-talline perovskite materials This feature is very desirable for an Ericson-cycle magnetic refrigerator Owing to its superior magnetocaloric properties, the lanthanum manganite single crystal is being expected to be one of the
* Corresponding author: e-mail: scyu@chungbuk.ac.kr, Phone: +82 43 261 2269, Fax: 82 43 275 6416
Trang 2phys stat sol (b) 241, No 7 (2004) / www.pss-b.com 1745
most active magnetic refrigerants as working substances in magnetic refrigerators [8] Nevertheless, further efforts to search for poly-crystalline perovskite materials, which exhibit a larger magnetocaloric effect, have been devoted, because of the fact that polycrystalline manganese oxide samples are easy to
be obtained by the conventional ceramic technique
In this context, a detaied study of the magneto-caloric effect in the perovskites of La0.845Sr0.155Mn1–xMxO3
(M = Mn, Cu, Co) has been made Results show that the presently investigated samples could be suitable
candidates as working materials in magnetic refrigerators
2 Experiment Ceramic polycrystalline samples: La0.845Sr0.155MnO3 (sample No 1),
La0.845Sr0.155Mn0.9Cu0.1O3 (sample No 2), and La0.845Sr0.155Mn0.98Co0.02O3 (sample No 3) were prepared by the conventional solid-state reaction technique using a stoichiometric mixture of 3N La2O3, SrCO3, MnCO3, CuO and CoO The samples were sintered at 1250 °C for 15 hours The quality of the samples was confirmed using a Bruker X-ray Diffractometer D5005 Results show that the three samples are of single phase with orthorhombic structure Magnetic measurements were carried out with a Vibrating Sample Magnetometer (VSM) DMS880 Digital Measurement Systems in the field up to 13.5 kOe
3 Results and discussion As shown in Fig 1, temperature dependences of the magnetization of a
representative sample of La0.845Sr0.155Mn0.9Cu0.1O3 (sample No 2) were measured in the fields of 50 Oe
and 10 kOe (in the insert of Fig 1) The Curie temperature is ~ 265 K and ~ 267 K at H = 50 Oe and
10 kOe, respectively The ferromagnetic ordering transition temperature TC, defined as the temperature at which the ∂M/∂T – T curve reaches a minimum, has been determined from the M–T curves It is found
that partial substitution of Mn by Cu or Co in the precursor of La0.845Sr0.155MnO3 (sample No 1) led to a
slight decrease in TC but enhanced magnetization Interestingly, at H = 10 kOe the TC is shifted to a
higher temperature (~ 267 K) meanwhile the shape of the M – T curve remains almost unchanged As
reported earlier in Ref [9] on the other hand, the LMC material MnAs0.9Sb0.1 indicated smooth
tempera-ture variation of the magnetization under high fields whereas the shape of the M – T curve for MnAs was almost unchanged, except the increase of magnetization and the shift of TC towards higher temperature
as usual Consequently, MnAs was found to exhibit a larger magneto-caloric effect than that in MnAs0.9Sb0.1 In the present case, a more abrupt jump in magnetization associated with the ferromagnetic
to paramagnetic transition at TC is observed for the two Cu- and Co-doped samples (samples No 2 and 3, respectively), as compared to sample No 1 Therefore, samples No 2 and 3 would be expected to show a larger magnetic entropy change than that observed in sample No 1 In Fig 2, the isothermal magnetiza-tion of sample No 2 was measured with a field step of 500 Oe in the field range of 0 – 13.5 kOe and a temperature step of 5 K in the temperature range of 100 – 300 K It is adequate to consider the magnetiza-tion curves to be isothermal for a sufficiently low sweeping rate of the magnetic field To ensure the readability of the figure, only several of them are presented in Fig 2 Obviously, there shows a drastic
100 150 200 250 300 350 400 450
0
1
2
3
4
5
6
110 165 220 275 0
20 40 60
TC= 265 K
H = 50 Oe
T (K)
T
C = 267 K
H = 10 kOe
Fig 1 Temperature dependence of the magnetization under
magnetic fields of H = 50 Oe and 10 kOe (in the inset) for
Sample No 2
Trang 31746 M.-H Phan et al.: Large magnetocaloric effect in La0.845Sr0.155Mn1–xMxO3 (M = Mn, Cu, Co) perovskites
0
10
20
30
40
50
60
70
220 K
230 K
240 K
245 K
250 K
255 K
260 K
265 K
270 K
280 K
300 K
H (kOe)
0.5 1.0 1.5 2.0 2.5 3.0
SM
T (K)
Sample No 1 Sample No 2 Sample No 3
change of the magnetization curves around the TC, indicating a large magnetic entropy change around the
TC It is worth noting that the main part of changes of the magnetization curves occurs in a relative
low-field range (< 13.5 kOe), which is very beneficial for the household application of the LMC materials
On the basis of the thermodynamic theory, the magnetic entropy change caused by the variation of the
external magnetic field from 0 to Hmax is given by
max
0
d
H M
T
S
H
∂
∆ =
∂
From the Maxwell’s thermodynamic relationship,
,
=
∂ ∂
Eq (1) can be rewritten as follows:
max
0
d
H M
H
M
T
∂
∆ =
∂
Numerical evaluation of the magnetic entropy change was carried out from formula (3) using isothermal
magnetization measurements In spite of magnetization measurements at small discrete field and
tem-perature intervals, ∆SM can be computed approximately from Eq (3) by
1 1
M
+ +
−
−
where M i and M i+1 are the magnetization values measured at temperatures T i and T i+1 in a field H,
respec-tively Finally the magnetic entropy changes, ∆S M , are calculated from M – H isotherms using Eq (4) As
shown in Fig 3, the magnetic entropy change is plotted as a function of temperature for the three
sam-ples at ∆H = 13.5 kOe Upon a 13.5 kOe applied field, La0.845Sr0.155Mn0.9Cu0.1O3 (sample No 2) exhibits
the largest ∆S M of ~ 2.76 J/kg K among the samples investigated (see Fig 3) The ∆S M values are
~ 1.72 J/kg K for sample No 1 and ~ 2.58 J/kg K for sample No 3 In terms of the experimental data, we
conclude that the partial replacement of Mn by Cu or Co in the precursor of La0.845Sr0.155MnO3 favored
the magnetocaloric effect of the sample Owing to their magnetocaloric properties, samples No 2 and 3
Fig 2 Isothermal magnetization for Sample
No 2 measured at different temperatures
around TC
Fig 3 Magnetic–entropy change plotted against temperature for the three samples, ∆H
= 13.5 kOe
Trang 4phys stat sol (b) 241, No 7 (2004) / www.pss-b.com 1747
could be effectively used as magnetic refrigerants in magnetic refrigerators Besides, sample No 1 could also be a suitable candidate for magnetic refrigeration above room temperature
In interpreting the large magnetic entropy changes in perovskite manganites, Guo et al [1] indicated that the large magnetic entropy change could originate from a strong spin-lattice coupling in the mag-netic ordering process Due to the strong coupling between spin and lattice, a significant change in the lattice accompanying the magnetic transition in perovskite manganites has been observed [1, 4] The lattice structural change in the 〈Mn–O〉 bond distance as well as 〈Mn–O–Mn〉 bond angle would in turn
favor the spin ordering Consequently, a more abrupt variation of magnetization near TC occurs, thereby leading to a large magnetic entropy change, i.e the LMC effect In the present case, the large magnetic entropy changes, in the Cu- and Co-doped manganites, could originate mainly from the abrupt reduce-tion of magnetizareduce-tion – which is associated with the magnetic phase transireduce-tion This might be attributed
to the strong spin-lattice coupling that occurs just at the Curie temperature It is the strong spin-lattice coupling that retains almost unchanged a sharp ferromagnetic-to-paramagnetic phase transition, even under high fields [7] A primary reason here is that the partial substitution of Cu or Co for Mn in
La0.845Sr0.155MnO3 probably favored a soft ferromagnetic character of the sample as well as the spin-lattice coupling To further understand the contribution of the spin-spin-lattice coupling to the change of en-tropy, we measured and analyzed electron paramagnetic resonance (EPR) spectra for all three samples The peak-to-peak EPR linewidth, ∆H, is estimated to be 674 Oe for sample No 1, 890 Oe for sample
No 2, and 750 Oe for sample No 3 As shown earlier in Ref [10], the larger the ∆H value, the stronger
the spin-lattice coupling Upon this basis, it is concluded that sample No 2 has the strongest spin-lattice coupling among the compounds investigated Therefore, the most abrupt jump in magnetization and the largest magnetic entropy change were observed in the case of sample No 2
4 Conclusions A detailed study of the magneto-caloric effect in the La0.845Sr0.155Mn1–xMxO3 (M = Mn,
Cu, Co) compounds has been made We have found the large magnetic entropy changes, i.e the large magneto-caloric effect, in these samples Among them, the magnetic entropy change reaches a maximum value of 2.76 J/kg K at the applied field of 13.5 kOe for the Cu-doped sample, suggesting that this mate-rial would be a suitable candidate for the advanced magnetic refrigeration technology The large mag-netic entropy change produced by the abrupt reduction of magnetization is attributed to the strong cou-pling between spin and lattice in the magnetic ordering process The EPR spectra allow us to get some insight into the nature of the contribution of the spin-lattice coupling to the entropy change
Acknowledgements Research at Chungbuk National University was supported by the Korea Science and
Engi-neering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National Univer-sity
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