Metamagnetic transition and magnetocaloric effect in charge-ordered Pr0.68Ca0.32−xSrxMnO3 (x=0, 0.1, 0.18, 0.26 and 0.32) compounds

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 427–433

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Metamagnetic transition and magnetocaloric effect in charge-ordered Pr0.68Ca0.32  xSrxMnO3 (x =0, 0.1, 0.18, 0.26 and 0.32) compounds V.S. Kolat, T. Izgi, A.O. Kaya, N. Bayri, H. Gencer, S. Atalay  Inonu University, Science and Arts Faculty, Physics Department, 44069 Malatya, Turkey

a r t i c l e in fo

abstract

Article history: Received 12 May 2009 Received in revised form 20 September 2009 Available online 26 September 2009

In this study, magnetic and magnetocaloric properties of Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) compounds were investigated. X-ray results indicated that all the samples have a single phase of orthorhombic symmetry. The orthorhombic unit cell parameters increase with the increase in Sr content. Large negative magnetic entropy changes (  26.2 J/kg K at 38 K and 5 T for x=0 and  6.5 J/kg K at 83 K and 6 T for x=0.1) were attributed to ultrasharp metamagnetic transitions. The peak value of DSm decreased from  4.1 J/kg K for x=0.18 sample to  2.4 J/kg K for x= 0.32 at 1 T magnetic field. & 2009 Elsevier B.V. All rights reserved.

PACS: 75.30.Sg 75.47.Lx 75.30.Kz Keywords: Manganites Magnetocaloric effect Magnetic entropy change Metamagnetic transition

1. Introduction Mixed-valent perovskite manganites with the general formula R1  xAxMnO3 (R =rare-earth cation, A= alkali-metal or alkalineearth cation) have been studied in great detail due to the discovery of colossal magnetoresistance (CMR) and magnetocaloric (MC) effect in these compounds [1–8]. Earlier experimental and theoretical studies have shown that the mixed-valent manganites have a great deal of fascinating properties arising from the strong interplay between the charges, spin, orbital and lattice degrees of freedom, which leads to a variety of phases with different physical properties such as a ferromagnetic metal (FM-M), antiferromagnetic insulator (AFM-I), ferromagnetic insulator (FMI), orbital ordered (OO) and charge-ordered (CO) insulator ground states [9–11]. The origin of ferromagnetism and antiferromagnetism as well as the transport properties in these systems has been well-explained by double exchange (DE) and super exchange (SE) mechanisms. While DE interactions promote ferromagnetic spin ordering, SE interactions favor an antiferromagnetic spin arrangement. In many studies, it has been shown that the strength of DE and SE interactions in manganites is very sensitive to the variation of Mn–O bond length and Mn–O– Mn bond angle (controlled by the average ionic radius of the A- or

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Mn-site ions) and the carrier density (controlled by the Mn3 + / Mn4 + ratio) [9–11]. It is believed that the magnetic and transport properties of these systems can be determined by competition between DE and SE interactions. Another interesting property of mixed-valent manganites is the existence of a phase-separated state [12–14]. Interestingly, a chemically homogeneous material forms a magnetically inhomogeneous system with spatially coexisting regions with distinct magnetic and electronic properties, such as simultaneous coexistence of submicrometer FM metallic and AFM CO insulating regions. The competition between both phases is resolved in a nanoscale length, giving rise to real-space inhomogeneities in the material. Also now it is well-understood that phase-separated state in manganites is closely related with the competition between antiferromagnetic CO insulating and FM metallic phases (the DE interaction prefers the FM metallic phase and CO results in AFM insulating phase via the SE interaction). At low temperatures the neutron diffraction measurements [15] clearly showed that the antiferromagnetic CO and FM metallic phases coexist with the populations of the two phases being strongly influenced by temperature and magnetic field. Recently, another surprising result found mostly in manganites is the appearance of ultrasharp magnetization steps at low temperatures [16–21]. This ultrasharp magnetic transition is included in the category of metamagnetic transitions, which is defined as the field induced transition of the entire compound from one phase to the other coexisting states [19,20,22]. However,

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2. Experimental The polycrystalline Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) compounds were prepared by the conventional solidstate reaction using high-purity powders Pr6O11, SrCO3, CaCO3 and MnO. The powders were mixed in stoichiometric ratio. Thoroughly mixed powders were grounded and calcined in air at 800 1C for 10 h. After grinding, the mixed powders were pressed into a disk-shape with a diameter of 13 mm and a thickness of about 2 mm. The disk samples were first sintered at 1200 1C for 24 h in air. For good crystallization, this sintering, regrinding and pelleting process was repeated three times. Final sintering was performed at 1350 1C for 24 h in air. All the samples were cooled to room temperature at a cooling rate of 3 1C/min. The X-ray diffractograms were recorded with a power diffractometer at room temperature using Cu-Ka radiation. Grain structure was observed using a LEO-EVO-40 scanning electron microscope. The magnetic measurements were performed using a Q-3398 (Cryogenic) magnetometer in a temperature range from 5 to 300 K and 6 T maximum magnetic field was applied. The magnetic entropy, which is associated with the magnetocaloric effect, can be calculated from the isothermal magnetization curves under the influence of a magnetic field.

3. Results and discussion Fig. 1 shows the XRD patterns of the Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) samples. The results of X-ray diffraction indicate that all the samples have a single phase of orthorhombic symmetry. As shown in Fig. 1, only the diffraction lines of the orthorhombic perovskite are obtained. Based on the

3000 Pr0.68(Ca0.32-xSrx)MnO3

2500

Intensity (a.u)

unlike the broad continuous transitions expected for inhomogeneous granular systems, in this case transition occurs in an extremely narrow magnetic field range and at low temperature. These ultrasharp metamagnetic transitions were observed in single crystal, polycrystalline and thin film forms of manganites, indicating that it is mainly an intrinsic nature of the samples [8,22]. It is not related to a particular microstructure of the material and crystalline anisotropy [21]. Among various mixed valence manganites studied so far, Pr-based manganites are of great interest for the study of the competition between the CO and FM states and consequently the metamagnetic transition [8,16–21]. The systems such as Pr0.68Ca0.32  xSrxMnO3 provide an opportunity to study these phase competition phenomena and ultrasharp metamagnetic phase transition. Due to the coexistence of antiferromagnetic CO and FM metallic phases in Pr-based manganites, the magnetic and magnetotransport properties of these compounds were partly studied. Recently, Gomez et al. [23] investigated magnetocaloric properties of Pr1  xCaxMnO3 (0.3 rxr0.45) manganites and reported Pr0.68Ca0.32MnO3 compound shows large positive and negative magnetic entropy changes at 8 and 31 K temperatures, respectively. In another work, Pr0.63Sr0.37MnO3 single crystal sample was investigated by Phan et al. [9,24], who found largest magnetic entropy change of 8.52 J/kg K for an applied field change of 50 kOe around 300 K. In all the previous studies on Pr0.68Ca0.32  xSrxMnO3 manganites, only magnetocaloric properties of PrCaMnO (without Sr) [8,23] and PrSrMnO (without Ca) [8,24] manganites have been reported. However, there has not been any report about the magnetocaloric (MC) properties for x= 0.1, 0.18, 0.26 and 0.32. We therefore report magnetic and magnetocaloric properties of Pr0.68Ca0.32  xSrxMnO3 (x =0, 0.1, 0.18, 0.26 and 0.32) compounds in details in this article.

x = 0.32

2000 x = 0.26

1500 1000

x = 0.18

500

x = 0.1 x=0

0 20

30

40

50 2 Theta (°)

60

70

80

Fig. 1. XRD patterns of the Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) compounds.

5.46 Lattice parameter (10-8 cm)

428

a b c

Pr0.68(Ca0.32-xSrx)MnO3

5.44

5.42

0.00

0.05

0.10

0.15 X

0.20

0.25

0.30

Fig. 2. The variation of lattice parameters versus x of the Pr0.68Ca0.32  xSrxMnO3.

XRD pattern, the unit cell parameters were calculated for Pr0.68Ca0.32  xSrxMnO3 samples. The variation of lattice parameters (a, b and c) with increasing Sr concentration (x) is plotted in Fig. 2. It is clear from Fig. 2 that all the orthorhombic unit cell parameters a, b and c increase slightly as the A-site cation ˚ to Sr2 + (1.31 A). ˚ The observed increase in varies from Ca2 + (1.18 A) unit cell parameters is concluded as the substitution of a large ion (Sr2 + ), which expands the unit cell in all the three directions (a, b and c). Fig. 3 shows typical SEM micrograph for Pr0.68 Ca0.22Sr0.1MnO3 sample. The SEM image reflects a smooth polycrystalline structure with the grain size 20–60 mm. Fig. 4 shows the temperature dependence of magnetization of Pr0.68Ca0.32  xSrxMnO3 sample for different Sr-doping levels (x= 0, 0.1, 0.18, 0.26 and 0.32). The data were taken under an applied magnetic field of 0.1 T. The magnetizations curves show a weak maximum around 200 K for x= 0 and 185 K for x =0.1, which are attributed to the onset of transition into the antiferromagnetic CO state. As the temperature is lowered, the sample changes from a paramagnetic to a CO antiferromagnetic phase at TCO. With the increase in Sr content, CO transition temperature (TCO) shifted towards the lower temperature. For the further Sr doping (x= 0.18, 0.26 and 0.32), the peak represents the charge-ordering that has not been observed. As can be seen from Fig. 4, at low temperature, the magnetization increase is too small for x= 0 sample (0–5 emu/

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the temperature is increased from 5 K the magnetization exhibits a small increase between 30 and 45 K (Fig. 4 for x= 0.1). Although this feature is not entirely understood, in many studies it has been interpreted as due to the thermally induced increase in FM phase fraction. The sample is blocked in a metastable phase-separated state due to the disorder [26,27] or the strains between the FM and CO antiferromagnetic states [28]. With the increase in temperature, the system becomes unblocked due to the thermal fluctuation and then magnetization shows a small increase. The application of a 6 T magnetic field increased the Curie temperature from 83 to 173 K, which is not given here. This clearly illustrates that the applied magnetic field favors melting of CO antiferromagnetic state. The disappearance of weak maximum observed at 175 K shows that the CO antiferromagnetic phase suppressed by FM phase at 6 T magnetic field. Interestingly, for the samples x = 0.18, 0.26 and 0.33, the magnetization curves (Fig. 4) show a typical full FM-like behavior. While the saturation magnetization is nearly same for all three samples, the Curie temperature increases with the increase in Sr (x) content (TC =228 K for x =0.18, 264 K for x = 0.26 and 284 K for x= 0.32). One widely used method of classifying perovskite structures is to use the tolerance factor t, defined as pffiffiffi t ¼ ðrA þ rO Þ= 2ðrMn þ rO Þ

Fig. 3. The typical SEM photograph for Pr0.68Ca0.22Sr0.1MnO3 sample.

45 40 35 5 x = 0.00 x = 0.10 x = 0.18 x = 0.26 x = 0.32

25 20 15

M (emu/g)

M (emu/g)

30

4 3 2

x = 0.1 TCO

1 0

10

100 150 200 250 300 T (K)

5 0 0

50

100

150

200

250

429

300

T (K) Fig. 4. The temperature dependence of magnetization for Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) samples. The inset presents the charge-order transition temperature.

g), which indicated that the magnetization increase may not be associated with a common FM ordering. In earlier studies for x= 0 sample (Pr0.68Ca0.32MnO3) [25], it has been shown that this alloy displays a particularly rich set of phenomena. Upon cooling from high temperatures in low fields, this material undergoes a CO transition and then an AFM transition. At lower temperature the system enters a different state called as canted AFM state, which is defined as actually an inhomogeneous mixed phase. At lower temperatures, application of a field induces a transition from canted AFM to a FM metallic state. For x =0.1 sample, the magnetization displays an upturn corresponding to a rapid growth of FM phase below the TCO. The magnetization increase is considerably large (0–41 emu/g) due to the FM transition. The results have indicated that FM and CO antiferromagnetic phases coexist in the sample and FM phase develops in the CO antiferromagnetic matrix. A more pronounced ferromagnetic phase appears at TC = 83 K for H= 0.1 T magnetic field. The magnetization increases rapidly and then saturates near 45 K. As

where rA, rO and rMn are the ionic radius at A-, O- and Mn-site, respectively. When A ions are substituted by an ion, which has smaller or larger ionic radius, the tolerance factor reflects the degree of mismatch between the equilibrium A–O and Mn–O bond lengths (or mismatch between the mean radii of the A- and Mn-site) in AMnO3. When t =1, the compound has a perfect cubic structure with equal Mn–O bonds and straight Mn–O–Mn angles (1801). When t o1, the Mn–O and A–O bonds are subjected to stresses. In order to relieve the stress, MnO6 octahedral will cooperatively rotate or tilt. This decreases the Mn–O–Mn bond angle from 1801, which plays an important role in the doubleexchange (DE) mechanism. The above-mentioned distortions also decrease the symmetry of the structure resulting in the hexagonal, tetragonal, orthorhombic, monoclinic and even triclinic structures, depending on the value of t. It is known that the structure of manganites is perovskite if tolerance factor value lies in the interval of 0.75ot o1, and the perovskite structure of manganites assumed to be stable. The calculated values of t vary from 0.917 for x =0 to 0.932 for x= 0.32 in Pr0.68Ca0.32  xSrxMnO3 samples, which indicated that all the samples correspond to the stable perovskite structure. The increase of t with the increase in x indicates that the lattice structure becomes more ordered when the Sr content is increased in the Pr0.68Ca0.32  xSrxMnO3 com˚ ions are substituted by Sr2 + (1.31 A) ˚ pounds. When Ca2 + (1.18 A) ions, which have larger ionic radius, the average ionic radius of A-site orA 4 increases with the increase in Sr concentration from 1.179 A˚ for x= 0 to 1.221 A˚ for x =0.32 in Pr0.68Ca0.32  xSrxMnO3 alloy. Due to the increase in orA 4 , the Mn–O–Mn bond angle distortion decreases and accordingly the ferromagnetic DE interaction increases and consequently Tc is increased. Figs. 5a–c show the magnetization measurement as a function of applied magnetic field of Pr0.68Ca0.32  xSrxMnO3 alloys for x= 0, 0.1 and 0.26, respectively, at various temperatures. For all samples (x = 0, 0.1 and 0.26), after the field is turned on, the magnetization increases as the field aligns the FM phase in the samples. A sudden increase of magnetization in the form of a sharp step at a threshold field, which is called as metamagnetic transition, was only observed for the samples x = 0 and 0.1. The appearance of a metamagnetic transition for the samples x =0 and 0.1 is a clear signature of the coexistence of FM and AFM-CO phases. The origin of this steplike transition is still a matter of controversy. Different

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100

120

90

M (emu/g)

70 60 50 40 x=0

30

100 80 M (emu/g)

80

5K 60 K 70 K 75 K 80 K 85 K 90 K 95 K 120 K 130 K 150 K 160 K 170 K 180 K 190 K 210 K 230 K 250 K 272 K 292 K

x = 0.1

10 K 15 K 20 K 25 K 30 K 32.5 K 35 K 0K 45 K 50 K 5K 65 K 100 K

60 40

20

20

10 0

0 0

2

4 H (T)

6

8

0

1

2

3

5

6

7

8

211 K 220 K 225 K 235 K 240 K 245 K 250 K 255 K 260 K 265 K 270 K 275 K 280 K

70 60 50 M (emu/g)

4 H (T)

40 30 20 x = 0.26

10 0 0

1

2

3

4

5

6

7

8

H (T) Fig. 5. Isothermal magnetization curves at various temperatures for (a) x =0, (b) x= 0.1 and (c) x= 0.26 samples.

interpretations have been proposed [15–28]. In most of interpretations the steplike transition in manganites is based on the scenario of phase separation, in which the CO antiferromagnetic and FM phases coexist in the sample and FM phase develops in the CO antiferromagnetic matrix. There is a considerable competition between these two phases. The slightly different cell parameter of the FM and CO phases generates strain at the interface region. At low temperature, the sample gets into a strongly blocked regime, in which the FM cluster cannot grow against the CO background. If the temperature reaches up the critical value then the system becomes highly unstable. These unstable CO regions now can be easily transformed to FM states. As the field is large enough for the deriving force action on the spins to overcome the elastic constrains, the local stress is destabilized, which causes a sudden jump in magnetization. At the end, all the regions behave as CO antiferromagnetic matrix and are converted from CO antiferromagnetic to FM phase by applied magnetic field. The threshold field at which the metamagnetic transition appears depends on composition and decrease with increase in Sr-doping level. The metamagnetic transition starts to develop at vicinity of 4 T for x = 0 and then decreases to 1 T for x= 0.1 sample. For the further Sr content (x40.1), the metamagnetic transition disappear due to the stability of FM phase. As mentioned above, the increase of Sr content in the compound causes an increase in DE interaction and

hence volume fraction of FM phase is increased. The only FM-like behavior of isothermal magnetization curve for the sample x= 0.26 is an evidence stability of FM ground state. The magnetic entropy, which is associated with the MC effect, can be calculated from the isothermal magnetization curves (Fig. 5) under the influence of a magnetic field. According to the classical thermodynamical theory, the magnetic entropy change DSm produced by the variation of a magnetic field from 0 to Hmax is given by

DSm ðT; HÞ ¼

Z

Hmax  0

@M @T



dH

ð1Þ

H

To evaluate the magnetic entropy change DSm numerical approximation of the integral in Eq. (1) is required. The usual method is to use isothermal magnetization measurement at small discrete field intervals and then DSm can be approximated from Eq. (1) by jDSm j ¼

X Mi  Mi þ 1 i

Ti þ 1  Ti

DH

ð2Þ

where Mi and Mi + 1 are the experimental values of the magnetization at Ti and Ti + 1, respectively. Using Eq. (2) and experimental M–H curves at various temperatures, the magnetic entropy change with the magnetic field variation can be calculated.

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Fig. 6 shows the magnetic entropy change as a function of temperature at various magnetic field for x= 0 sample. The presented result is in a good agreement with the previous study [23]. At low temperatures (near 38 K) we find extremely large value of DSm. As can be observed in Fig. 4 for x= 0 sample, the presence of FM regions dispersed in the AFM matrix leads to a magnetization increase with decrease in temperature just below 130 K and then reaches the maximum value at a lower temperature. On increasing the magnetic field, it tends to align the FM domains up to a maximum field value, which is at the onset of metamagnetic transition. At this critical magnetic field value some AFM domains become FM. As a consequent of metamagnetic transition, the magnetic entropy change (DSm) reaches  26.2 J/kg K at 38 K and 5 T magnetic field. At still lower temperature DSm becomes positive. Fig. 7 shows the magnetic entropy change at various magnetic fields for Pr0.68Ca0.22Sr0.1MnO3 sample (x= 0.1). For this sample an anomalous magnetic entropy change is observed just below the charge-ordering temperature (TCO). A positive DSm peak develops nearly at charge-ordering temperature. In many studies it has been reported that this uncommon effect is attributed to

60 0.5 T 1T 2T 3T 4T 5T 6T

50 40

Δ S (J/kg.K)

30 20 10 0 -10 -20 -30 10

15

20

25

30

35

40

45

T (K) Fig. 6. Magnetic entropy change of Pr0.68Ca0.32MnO3 sample at various magnetic fields.

431

CO transition and causes a positive magnetic entropy change [29]. The peak value of entropy change related to CO transition increases with the increase in magnetic field. For clarity, the enlarged curves are shown in the inset of Fig. 7 for x =0.1 sample. The peak value of DSm reaches up to 0.45 J/kg K for 3 T magnetic field. However, for further magnetic fields, the entropy peak has been disappeared. For 6 T magnetic field, the disappearance of the entropy peak is interpreted as the CO antiferromagnetic phase completely suppressed by FM phase at 6 T magnetic field. As in the case of x = 0 sample, at lower temperature (nearly at 83 K) for x= 0.1 sample, we find extremely large negative entropy change at a narrow temperature range ( 6.5 J/kg K at 6 T magnetic field). However, the peak temperatures do not change with the increase in magnetic field. At first, the peak value of DSm increases with the increase in magnetic field and then becomes stable above 2 T magnetic fields. This large magnetic entropy change is also attributed to ultrasharp metamagnetic transitions. The negative entropy change related to common FM–PM phase transition is observed at near the Curie temperature. It is clearly seen that the Curie temperature shifts to higher temperature and reaches up to the CO transition temperature at 6 T magnetic fields. Fig. 8 shows magnetic entropy change as a function of temperature of Pr0.68Ca0.32  xSrxMnO3 alloys for x =0.18, 0.26 and 0.32 at 1 and 6 T magnetic fields. However, above mentioned anomalous magnetic entropy change has not been observed for further Sr concentration (x 40.18). The entropy changes look like as in a typical full-FM material. For all the samples, the magnetic entropy change exhibits a peak in the vicinity of Curie temperature. The peak temperatures shift to higher temperature with increase in Sr content from 203 K for x =0.18 to 267 K for x= 0.32. The peak value of DSm decreases from  4.1 J/kg K for x= 0.18 sample to  2.4 J/kg K for x =0.32 at 1 T magnetic field. The increase in the peak temperature at which the FM–PM phase transition occurs with increase in Sr content was interpreted ˚ ions are substituted by Sr2 + (1.31 A) ˚ ions, as when Ca2 + (1.18 A) which have larger ionic radius, the average ionic radius of A-site orA 4 increases in Pr0.68Ca0.32  xSrxMnO3 alloys. Due to the increase in orA 4 , the Mn–O–Mn bond angle distortion decreases and accordingly the ferromagnetic DE interaction increases and consequently Tc increased. As can be seen in Fig. 8, the magnetocaloric peak of x= 0.18 sample is sharp and intense. With the increase in Sr content the entropy peaks become broader with a lower maximum value. The decreasing and broadening of magnetocaloric effect with increasing Sr content

1

0

0

-1 -2

-3

ΔS (J/kg.K)

-2 0.6 0.4

-4

0.5 T 1T 2T 3T 6T

-5 -6

ΔS (J/kg.K)

ΔS (J/kg.K)

-1

0.2 0.0

-0.6 100

50

75

-4 -5 -6

-0.2

x= 0.18 x = 0.26 x =0.32 H=1T H=6T

-7

-0.4

-7

-3

125

150

175 200 T (K)

225

250

100 125 150 175 200 225 250 275 300 T (K)

Fig. 7. Magnetic entropy change of Pr0.68Ca0.22Sr0.1MnO3 sample at various magnetic fields. The inset presents the peak values of entropy change related to CO.

-8 -9 180

195

210

225

240 T (K)

255

270

285

300

Fig. 8. Magnetic entropy change of Pr0.68Ca0.32  xSrxMnO3 (x = 0.18, 0.26 and 0.32) compounds at 1 and 6 T magnetic fields.

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was interpreted as the change of nature of magnetic phase transition with Sr content. Generally, the materials with secondorder magnetic phase transition exhibit very small MC effect than the materials with first-order magnetic phase transition [30,31]. The magnetic free energy F(M,T) can be expanded as a Landau expansion in powers of magnetization M neglecting higher order parts as FðM; TÞ ¼

c1 ðTÞ 2 c3 ðTÞ 4 M þ M  MH 2 4

ð3Þ

Here, c1(T) and c3(T) are called as Landau coefficients and the sign of the c3(T) of M4 term determines the type of the magnetic phase transition. The first-order phase transition is expected in the case c3(T)o0 and the second-order phase transition in the case c3(T)40. From the condition of equilibrium qF(M,T)/qM=0, we obtain H ¼ c1 ðTÞM þc3 ðTÞM 3

ð4aÞ

H ¼ c1 ðTÞ þ c3 ðTÞM 2 M

ð4bÞ

In order to get a deeper insight in the nature of magnetic phase transition, M2 versus H/M, which is called as Arrott plot, was plotted in a critical temperature interval as shown in Fig. 9(a) and (b) for x=0.18 and 0.32, respectively. The slope of the resulting curves denotes whether a magnetic transition is of first- or second-order. It can be deduced that if all the curves have a positive slope, the

magnetic transition is second order. On the other hand, if some of the curve shows a negative slope at some point, then magnetic transition is first order. A negative slope in the isotherm plots of M2 versus H/M is a clear indication of a first-order phase transition from FM to PM for x=0.18 sample. For x=0.32 sample, the M2 versus H/M curves exhibit the positive slope indicating that the transition between FM and PM phases is of the second order. The Landau coefficient c3(T) could be determined by fitting Eq. (4a) to the magnetic field, H against magnetization M (isothermal magnetization curves) or fitting Eq. (4b) to the Arrott plot (Fig. 9). Our calculation has showed that c3(T) is negative (  9.19  10  6 Tgr3/emu3 at 211 K) for x= 0.18 sample and positive (5.32  10  5 Tgr3/emu3 at 275 K) for x= 0.32 sample. According to the theory mentioned above, the change of sign of c3(T) coefficient from negative to positive for x = 0.32 sample suggests the phase transition in x =0.32 sample is the second order. The nature of phase transition (second order) in Pr0.68Sr0.32MnO3 is another evidence of small magnetic entropy change. Although there have been some works on magnetocaloric properties of Pr1 xCaxMnO3 and Pr1 xCaxMnO3 [8,23,24], no previous systematic study has presented substitution of Sr for Ca in PrCaMnO3 manganites. Our present study first time indicates that the substitution of Sr in PrCaMnO3 manganites has a great influence on lattice parameters, Curie temperature, saturation magnetization and magnitude of magnetic entropy change. 4. Conclusions

H/M (Tg/emu)

0.18

x =0.18

177 K 181 K 186 K 191 K 196 K 201 K 206 K 211 K 216 K 221 K 226 K 231 K 236 K 241 K 246 K 250 K

0.12

0.06

0.00 0

1000

2000

3000

4000

5000

6000

7000

M2 (emu2/g2) 0.16 x = 0.32 239 K 245 K 250 K 255 K 260 K 265 K 270 K 275 K 279 K 285 K 290 K 294 K

H/M (Tg/emu)

0.12

0.08

0.04

0.00 0

1000

2000 M2

3000

4000

5000

(emu2/g2)

Fig. 9. Arrot plots of (a) x= 0.18 and (b) x= 0.32 samples at various temperatures.

In this work, magnetic field induced steplike metamagnetic transition and magnetocaloric properties of charge-ordered Pr0.68Ca0.32  xSrxMnO3 (x= 0, 0.1, 0.18, 0.26 and 0.32) compounds were investigated. The polycrystalline compounds were prepared by the conventional solid-state reaction method. X-ray results indicated that all the samples have a single phase of orthorhombic symmetry. All the orthorhombic unit cell parameters increase as the A-site cation varies from Ca2 + to Sr2 + , which has larger ionic radius. For lower Sr concentrations (x =0 and 0.1), the magnetization curves showed a weak maximum around 185 K, which was a signature of the charge-ordered (CO) transition. For the further Sr doping (x= 0.18, 0.26 and 0.32), the peak represents the CO that was not observed. The magnetic isotherms showed that a sharp steplikemetamagnetic transition observed only in x= 0 and 0.1 samples. For lower Sr concentration, two anomalous magnetic entropy changes were observed just below and above the traditional Curie temperature. A positive DSm peak developed above the Tc (0.45 J/ kg K at 3 T for x= 0.1 sample) was attributed to CO transition. At lower temperatures, extremely large negative magnetic entropy change ( 26.2 J/kg K at 38 K and 5 T for x= 0 and  6.5 J/kg K at 83 K and 6 T for x= 0.1) observed was attributed to ultrasharp metamagnetic transitions. For the further Sr concentration, the negative entropy change related to common FM–PM phase transition was observed near the Curie temperature. The peak temperatures of entropy change shifted to higher temperature with the increase in Sr content from 203 K for x= 0.18 to 267 K for x= 0.32. The peak value of DSm decreased from -4.1 J/kg K for x= 0.18 sample to 2.4 J/kg K for x =0.32 at 1 T magnetic field. The decrease of magnetocaloric effect with increase in Sr content was interpreted as the change of nature of magnetic phase transition from first order to second order with the increase in Sr content. Acknowledgements This work was supported by Inonu University Research fund with the project number 2008/08.

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