Structural Magnetic Magnetocaloric and Critical Exponent Properties of La0.67Sr0.33MnO3 Powders Synthesized by Solid-State Reaction

DOI 10.1007/s10948-015-3212-5

ORIGINAL PAPER

Structural, Magnetic, Magnetocaloric, and Critical Exponent Properties of La _0.67 Sr _0.33 MnO ₃ Powders Synthesized by Solid-State Reaction

A. Ettayfi

· R. Moubah

· A. Boutahar

· E. K. Hlil

· H. Lassri

Received: 22 March 2015 / Accepted: 7 September 2015

© Springer Science+Business Media New York 2015

Abstract Polycrystalline La

Sr

MnO

(LSMO) pow- ders were prepared by solid-state reaction, and their struc- tural, magnetic, magnetocaloric, and critical exponent prop- erties have been investigated. X-ray diffraction has shown that the sample is polycrystalline with a rhombohedral (R- 3c) structure. Scanning electron microscopy has revealed that the surface of the sample is chemically homogeneous.

The samples undergo a second-order magnetic transition from a ferromagnetic to a paramagnetic state with a Curie temperature of 361 K. The maximum entropy change is determined to be 274 J/kg K for a magnetic field change of 0–5 T. For the same applied field, the adiabatic temperature change and relative cooling power are found to be 1.75 K and 242 J/kg, respectively. These values are compared with those reported in the literature. It is concluded that large grain sizes are mandatory in order to obtain high magnetic refrigeration efficiency in LSMO-based compounds. More- over, it is shown that the critical exponent values are in agreement with those of the long-range mean field model.

Keywords Manganite · Magnetocaloric effect · Solid-state reaction · X-ray diffraction · Curie temperature · Magnetic entropy change · Critical exponent

R. Moubah

reda.moubah@hotmail.fr

1 Facult´e des Sciences, LPMMAT, Universit´e Hassan II—Casablanca, B.P. 5366 Maˆarif, Morocco

2 Institut N´eel, CNRS, Universit´e Joseph Fourier, BP 166, 38042 Grenoble Cedex 9, France

1 Introduction

The magnetocaloric effect refers to the heating or cool- ing of a magnetic material due to the application of a magnetic field. This effect was discovered in 1881, and since then, it was mainly used in magnetic refrigeration in low-temperature physics [1]. In order to use magnetic refrigeration in everyday-life applications, intense research activities have been devoted in the last decades for find- ing new materials with a large magnetocaloric effect close to room temperature. Among these magnetocaloric materi- als, we may mention LaFe

Si [2], Gd

(Si

Ge

) [3], and MnCuCoGe [4]. Magnetic refrigeration is a highly desirable technology for replacing current conventional compression–

vaporization technology because of its environmentally friendly and energy-efficient features. Nevertheless, in order to probe the magnetic refrigeration effectiveness, isothermal magnetic entropy alone is, however, not sufficient. The adi- abatic temperature change (T

) is indeed a direct param- eter for assessing the usefulness of magnetic refrigeration.

As a consequence, magnetocaloric materials with both large isothermal magnetic entropy and T

are required [5].

The La

Sr

MnO

(LSMO) compound is a suitable

material for investigation due to its magnetocaloric prop-

erties. This system is well known in the literature and has

attracted a lot of interest in recent years owing to its large

colossal magnetoresistance and half-metal character with

high potential for various devices such as magnetic field

sensors and hard disk heads [6–8]. Here, we investigate

the structural, magnetic, magnetocaloric, and critical expo-

nent properties of LSMO powders prepared by solid-state

reaction.

20 30 40 50 60 70 80 0

200 400 600 800 1000

(218)(134)(036)(312)

(208)(220)

(214)(300)

(116)(122)

(024)

(006)(202)(113)

(104)(110)

(012)

). u . b r a( st n u o C

2

(°)

Fig. 1 X-ray diffraction pattern of La0.67Sr0.33MnO3powders

2 Experimental Section

Polycrystalline LSMO powders were synthesized by solid- state reaction using high-purity La

O

, SrCO

, and MnCO

reagents. The stoichiometric mixture was heated in air at 1200

C for 16 h. The resulting powder was ground, pressed into a disk form, and annealed at 1450

C for 16 h in air. The structural properties were investigated by X-ray diffraction (XRD) using a Siemens D-5000 diffractome- ter with a monochromatic Cu Kα1 incident beam (λ = 0.15406 nm). Quantitative elemental analysis of La, Sr, and Mn and the sample morphology was performed by energy- dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM), respectively, using a JEOL (JSM840) scanning electron microscope. The field and temperature dependences of magnetic properties were carried out using a superconducting quantum interference device (SQUID) magnetometer with an externally applied magnetic field up to 5 T.

3 Results and Discussions

Figure 1 displays the XRD pattern of LSMO powders.

Different diffraction peaks can be observed, indicating a polycrystalline character of the sample. All the diffrac- tion peaks can be indexed according to a rhombohedral (R-3c) LSMO structure, which is in agreement with the data found in the literature [9]. The absence of any addi- tional peak in the XRD pattern demonstrates that there are no spurious phases such as La

O

, SrO, or MnO

in the detection limit of XRD experiments. The average grain size is calculated using the Debye–Scherrer formula:

λθ D =

where, D is the crystallite size, λ is the wavelength of x-ray, and α is the full width at half maxi- mum (FWHM) of diffraction peaks. In order to determine

the FWHM, the peaks were fitted to Gaussian distribu- tion. We found an average grain size of around 30 nm.

Figure 2 shows SEM images obtained for LSMO powders.

The samples are fairly uniform and homogeneous. The EDX analysis showed only the presence of La, Sr, Mn, and O peaks. Chemical analyses of different regions of the sam- ple showed that the powders is chemically homogeneous and that the estimated La/Sr and La/Mn ratios are 1.98 and 0.53, respectively, which are close to the nominal expected values.

The temperature dependence of the magnetization M(T ) of LSMO powders is presented in Fig. 3. The measure- ments were performed in a constant applied field of 0.1 mT in zero-field cooling (ZFC) and field cooling (FC) modes.

These curves are superimposed and show that the LSMO powder is ferromagnetic with a second-order magnetic transition. The superimposition of the ZFC and FC curves is also an indication of the absence of parasitic phases such as MnO

(antiferromagnetic phase), which should induce a divergence in the ZFC and FC curves at low temperature. In order to determine precisely the Curie temperature, we plotted in the inset of Fig. 3 the

ver- sus T curve. We found a Curie temperature of around 361 K, which is deduced from the inflection point of the

dM dT

curve.

Isothermal magnetization curves have been measured at various temperatures in the vicinity of Curie temper- ature (Fig. 4). The gradual evolution of these curves to linear behavior characterizes a typical ferromagnetic–

paramagnetic transition in the vicinity of T

. From the M–H curves, the isothermal magnetic entropy change ( − S

) was numerically estimated from the thermodynamics theory using the Maxwell formula:

S

(T , H ) =

H1

∂M

∂T

dH (1)

5 µm

Fig. 2 Scanning electron microscopy image of a sample of La0.67Sr0.33MnO3powders

0 50 100 150 200 250 300 350 400 0

10 20 30 40 50

0 200 400

-1,0 -0,5 0,0

dM/dT (emu/ g K)

T (K)

H = 0.1 T FC ZFC

) g/ u m e( M

T (K)

Fig. 3 The temperature dependence of the magnetizationM(T )mea- sured for an applied magnetic field of 0.1 T for the La0.67Sr0.33MnO3

samples. Theinsetshows the dM/dTas a function of temperature

In the case of magnetization measurement at small dis- creet fields and temperature intervals, numerical approx- imation of the integral in (3) could be as follows [10, 11]:

SM(T , H )=Mi+1(T i+1, H i)−Mi(T i, H i) T i+1−T i dH (2)

where M

and M

are the experimental data of the mag- netization at T

and T

under a magnetic field H

The calculated magnetic entropy for different applied fields is plotted as a function of temperature in Fig. 5. In all cases, the ( − S

) curves exhibit a broad positive peak at around T

The caret-like shape of the curves indicates that the magnetic phase transition near T

is a second-order phase transition. We note that the peak magnitude increases with increasing H. The magnetic entropy changes are

0 1 2 3 4 5 6

0 30 60

)g/ume(M

H (T)

T= 250 K

T= 391 K

Fig. 4 Isothermal magnetization curves for La0.7Sr0.3MnO3powders recorded at different temperatures

250 275 300 325 350 375 400

0.0 0.5 1.0 1.5 2.0

2.5 0-1 T

0-2 T 0-3 T 0-4 T 0-5 T

-SM)K.gk/J(

T (K)

Fig. 5 Temperature dependence of magnetic entropy change under different applied magnetic field changes for La0.67Sr0.33MnO3

powders

0.69, 1.29, 1.82, 2.31, and 2.74 J/kg K corresponding to applied magnetic fields of 1, 2, 3, 4, and 5 T, respectively.

One can notice that the FWHM of the magnetic entropy change peak is relatively large, about 80 K at 5 T. This is an important parameter related to the relative cooling power (RCP), which can be expressed using the following formula:

RCP = | − S

| ∗ δT

(3) where |− S

| is the maximum magnetic entropy change and (δT

) is the FWHM of the | − S

| versus T curve. In order to study the variation of the RCP factor, we plotted in Fig. 6 the dependence of the RCP as a function of the applied magnetic field. A linear increase of the RCP factor with increasing magnetic field is observed, from 40

0 1 2 3 4 5 6

40 80 120 160 200

) g k/ J( r e w o p g nil o oc e vi t al e R

₀

H (T)

Fig. 6 Relative cooling power values (RCP) versus applied magnetic field for La0.67Sr0.33MnO3powders

1 2 3 4 5 0.4

0.8 1.2 1.6



T

(K)

₀

H (T)

Fig. 7 Field dependence of the maximum adiabatic temperature change for La0.67Sr0.33MnO3powders

to 240 J/kg at 1 to 5 T, respectively. The adiabatic temper- ature change (T

) is an important parameter to evaluate the MCE of magnetocaloric materials, and it was calculated from the M (T , H ) data by using the following equation [12]:

T

(T , H ) = − T

C

(T , H ) S

(T , H ) (4) where C

is the specific heat capacity. The C

values were extracted from the experimental data of La

Sr

MnO

samples reported in Ref. [13]

Figure 7 shows the dependence of T

as a func- tion of the magnetic field change determined at T

. One can notice that T

increases linearly with increasing magnetic field to reach a value of 1.75 K at 5 T.

In order to examine the usefulness of the LSMO sam- ples reported in this work, we made a comparative study of magnetocaloric properties of other polycrystalline mangan- ite with a close composition but different grain sizes. The comparison of − S

, T

, T

, and H is summarized

in Table 1. It should be noted that the − S

and T

of the present powders are smaller than those of LSMO sam- ples with larger grain sizes. It is important to mention that several authors [14–17] have reported a tunable dependence of grain size as a function of magnetocaloric properties in LSMO samples. As the grain size gets larger, − S

and T

get bigger. Hueso et al. [17] have shown that the increase of the grain size from 60 to 500 nm leads to a signif- icant increase of − S

from 1.75 to 5 J/kg K, respectively, under a field change of H = 1 T. From the comparison (Table 1), we note that the samples which have the largest entropy change ( − S

= 2.2 J/kg K for H = 1 T) [14] are prepared by a co-precipitation route. This is not surprising, if we consider the high purity and homogeneity of samples produced by this technique. This is due to the fact that for the co-precipitation route, the starting materi- als can be dissolved and highly mixed in a solution; thus, a homogeneous and uniformly sized mixture can be obtained.

However, for the solid-state samples, the grain sizes are smaller and have a much wider distribution of sizes than the samples prepared by co-precipitation. Therefore, the lower magnetic entropy change of our samples is due to the small grain size of the LSMO samples prepared by solid-state reaction.

Now, we describe the critical behavior of the present sam- ple. In order, to determine which model describes the best the critical behavior, we display in Fig. 8 the modified Arrott plots at different temperatures obtained using three different models: 3D-Ising, 3D-Heisenberg, and mean field. For all cases, quasi-straight lines in the high-field region can be seen. The positive slopes indicate that the LSMO powders undergo a second-order ferromagnetic to paramagnetic tran- sition, according to the Banerjee criterion [18]. However, we cannot conclude which model is the most appropriate to illustrate the experimental data, since all the different mod- els exhibit parallel straight lines in the high-field region. As a consequence, we calculated the relative slope (RS), which is defined at the critical point as RS = S(T )/S(T

). The temperature dependence of RS for the three different models is presented in Fig. 9. Only the mean field model presents

Table 1 Comparison of changes in Curie temperature, magnetic entropy, and adiabatic temperature at different field changes for some LSMO samples with different grain sizes reported in literature

Compounds TC(K) −SM(J/kg K) μ0H (T ) T_ad^max(K) Grain size (nm) Ref.

La0.67Sr0.33MnO3 361 0.69 1 0.42 30 This work

La_0.67Sr_0.33MnO₃ 361 1.29 2 0.75 30 This work

La0.7Sr0.3MnO3 358 2.2 1 – 42 [14]

La0.6Sr0.4MnO3 370 2 2 1.26 100 [15]

La0.6Sr0.4MnO3 365 1.34 2 0.76 40 [15]

La0.67Sr0.33MnO3 367 1.3 1.5 – 51 [16]

La0.67Sr0.33MnO3 369 1.74 1.5 – 85 [16]

Fig. 8 Modified Arrott plots of La0.67Sr0.33MnO3powders:

isotherms ofM^1/βversus (H/M)^1/γ for different models:

aIsing,bHeisenberg, andc mean field

0.1 0.2 0.3 0.4

10000 20000 30000

(H/M)^1/(T.g/emu)^1/

T = 3 K Heisenberg

331 K

391 K M(1/) )g/ume((1/)

(b)

 = 1.336

 = 0.365

0.1 0.2 0.3 0.4

0 800 1600

 = 1

(H/M)^1/(T.g/emu)^1/

T = 3 K 331 K

361 K Mean field

M(1/) )g/ume((1/)

(c)

 = 0.5

0.1 0.2 0.3 0.4

0 50000 100000

Ising

M(1/) )g/ume((1/)

(H/M)^1/(T.g/emu)^1/ 331 K

391 K

T = 3 K

(a)

 = 1.241

 = 0.325

RS values close to 1, showing that this model describes better the critical behavior of samples.

To determine the critical exponents, we plot in Fig. 10 the spontaneous magnetization M

(T ) and inverse of magnetic susceptibility χ

(T ) obtained from the linear extrapolation of the modified Arrott plots in the high-field region to the intercept with the M

and the (H /M)

axes. The criti- cal exponents can be calculated using the fit of these curves with the following equations:

M

(T ) = M

( − t)

, t < 0, T < T

, (5)

340 350 360 370 380

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

Ising Heisenberg Mean field

RS

T (K)

Fig. 9 Temperature dependence of the relative slope (RS) for the different models of La0.67Sr0.33MnO3samples

χ

(T )

= (h

/M

)t

, t > 0, T > T

,

where t = (T − T

)/T

is the reduced temperature and h

/M

is the critical amplitude. β , γ , and δ = 1 +

are the universal critical exponents.

The obtained values are calculated to be β = 0.506, γ = 1.019, and δ = 3.014, which are different than those reported for LSMO powders. Several authors have reported critical exponents values around β = 0.37, γ = 1.22, and δ = 4.25 for samples with a similar composi- tion [19, 20]. These values correspond to the mean field model, highlighting that the universality class of the crit-

340 360 380

8 12 16 20 24

Ms)g/ume(

T (K)

= 0,506

T_C=363.51 ^{= 1,019}

T_C= 364.737

0.00 0.05 0.10 0.15 0.20

 -1 (T.g/emu)

Fig. 10 Plot of the spontaneous magnetizationMs(T )and inverse of magnetic susceptibilityχ₀⁻¹(T )as a function of temperature

ical behavior of the present samples is different from the one previously reported. Generally, the LSMO with a large grain size is described as a 3D-Heisenberg model with short- range interactions. In our case, the mean field model is most appropriate, which can be understood by the smaller grain size of the present samples. Similar conclusions have been drawn by Khiem and Bau [21], who have studied LSMO in the thin-film form. These authors have shown that for LSMO thin films (small grain size), the universality class changes from the 3D-Heisenberg to the mean field model.

The decrease of the grain size increases the surface contri- bution, where uncompensated spins at the surface and strain anisotropies can drastically affect the universality class of the material.

4 Summary

In summary, we have investigated the crystalline, mag- netic, magnetocaloric, and critical exponent properties of La

Sr

MnO

samples synthesized via the solid-state reaction process. The structural study showed the formation of the rhombohedral (R-3c) LSMO structure. The samples undergo a second-order magnetic transition from ferromag- netic to paramagnetic states with a Curie temperature of 361 K. For a magnetic field change of H = 2 T, the max- imum magnetic entropy and adiabatic temperature changes are found to be 1.29 J/kg K and 0.75 K, respectively.

These parameters are smaller than those reported for La

Sr

MnO

samples with larger grain sizes. By com- paring our results with those available in the literature, we showed that a large grain size is a crucial parameter in order to have high magnetic refrigeration efficiency for LSMO compounds.

Acknowledgments Financial support from the “PHC Maghreb”

research project is gratefully acknowledged.

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