Magnetocaloric Effect in La0.88Sr0.12MnO3 films

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ScienceDirect Materials Today: Proceedings 14 (2019) 104–108

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SLAFES XXIII

Magnetocaloric Effect in La0.88Sr0.12MnO3 films Passanante S.a,c,d, Goijman D.a, Linares Moreau M.a,c, Leyva A.G.a,b,d, Albornoz C.a,d , Rubi D.a,b,d , Ferreyra C.a,d, Vega D.a,b, Granja L.a,d, Quintero M.a,b,d,1 a

Departamento de Física de la M ateria Condensada,GIyA, GAIyANN,CAC,CNEA b Escuela de Ciencia y Tecnología, UNSAM c Instituto Sábato,UNSAM, CNEA d INN, CONICET-CNEA, Av. General Paz 1499, San Martín (1650), Provincia de Buenos Aires, Argentina.

Abstract The magnetocaloric effect (MCE) is the isothermal change of entropy and the adiabatic change of temperature that appears in some materials during the application of a magnetic field. Manganites are a family of compounds with an important MCE. Moreover, the strong coupling between their electronic, magnetic and structural degrees of freedom makes them very flexible to modify their magnetic properties with different stimulus. With the aim of exploiting the MCE of manganites in microdevices, we studied this effect in La0.88 Sr0.12 MnO3 thin films deposited by pulsed laser deposition (PLD) on silicon substrates. We present here the dependence of the magnetic properties and the MCE with the thickness and the thermal treatment of the films. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018. Keywords:magnetocaloric; thin films; magnetic refrigeration

* Corresponding author E-mail address:[email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the organizing committee of the XXIII Latin American Symposium on Solid State Physics (SLAFES XXIII), San Carlos de Bariloche, Argentina, 10–13 April 2018.

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1. Introduction The Magnetocaloric Effect (MCE) is the isothermal change of entropy observed in a magnetic material when an external magnetic field is applied [1]. The main motivation for the study of this effect is the possibility to build refrigeration devices based on this effect, increasing the energy efficiency and being less harmful for the environment compared with the traditional gas compression technology [2]. The discovery of large MCE at room temperature in Gadolinium based compounds [3, 4] was a breakthrough in the study of this topic. Due to the large costs of production of gadolinium, a lot of work was devoted to find materials that would replace it. In that sense, a large number of compounds were studied, such as As based compounds [5], hustler alloys [6], LaFe(Si,La)[7] and mixed-valence manganese oxides (manganites)[8], most of them as bulk systems. Only a limited number of works were focused on the study of the MCE in "reduced" dimensions, such as ribbons, microwires and thin films[9]. The use of these geometries improves the heat exchange between the active material and the surroundings, decreasing the duration of the cooling cycles. Unfortunately, the counterpart of this improvement is the complexity related with the preparation of the samples and their effects in the magnetic properties of the system. Unlike the widely studied La1-xSrxMnO3 composition with 0.17 < x < 0.5, which is ferromagnetic (FM) metallic at low temperature, the system becomes quite more complex for x < 0.17[10]. Particularly, the phase diagram for the narrow region of 0.1 < x < 0.17 presents a low temperature FM insulating state for single crystals samples[11,12]. Regarding previous reports, the paramagnetic (PM) to FM transition for 0.17 < x < 0.5 was characterized as a second order transition [13,14], but it would not seem to be the case for x < 0.17, where the magnetic transition would appears along with a structural one. [10] However it is known that the synthesis parameters[15], the thermal treatments [16,17] and the sample morphology [18,19] could strongly influence the structural, magnetic and electronic characteristics in manganites. In the present work we explore the magnetocaloric properties of polycrystalline thin films of La0.88Sr0.12MnO3 and their dependence with the thickness and the thermal treatment. Within this context, this compound proposes an almost unexplored and very interesting scenario for the study of the MCE in manganites thin films. 2. Experimental Thin films of La0.88Sr0.12MnO3 were deposited on Si/SiO2 substrates by pulsed laser deposition (PLD), using a 266 nm Nd: YAG solid state laser with a pulse frequency of 10 Hz. The target used was a La0.88Sr0.12MnO3 polycrystalline ceramic. The deposition temperature was 850 ºC under a pressure of 0.1 mbar of O2. In some cases, an additional annealing of 1 hour at 850 ºC under a pressure of 100 mbar of O2 was performed in-situ. X-Ray diffraction was measured using a Panalytical Empyrean diffractometer. Magnetization measurements were performed using a commercial vibrating sample magnetometer Versalab manufactured by Quantum Design. The thickness and rugosity of the films were determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, respectively. 3. Results Three films of 100 nm, 160 nm and 200 nm (named as LS100, LS160 and LS200 respectively) were grown by PLD. A fourth film of 160 nm (LS160x) was deposited with the same conditions but without the annealing process. All the films resulted polycrystalline with grains size ≤ 50 nm and surface rugosity of  1.5nm. The structural and magnetic properties of the target were also studied (sample LSB). X-ray Diffraction results confirmed the crystalline structure of x=0.12 composition, without any evidence of phase segregation for all the films and the target. The target has a rombohedric structure with a=b=5.51 Å and c = 13.35 Å. This result is in good agreement with detailed studies previously reported for this compound [15]. J. F. Mitchell et al. showed that the structure, and consequently the magnetic and electronic properties, is strongly determined by synthesis parameters, as it is the case of oxygen partial pressure. Otherwise the crystalline structure of the films is pseudocubic with a lattice parameter of 3.91 Å.

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In figure 1 we present the magnetization (M) for the films as a function of temperature (T), for the applied magnetic field (H) parallel to the film. All the samples are paramagnetic at room temperature, presenting a ferromagnetic transition (TC), defined as the temperature for which the M(T) curves have the largest slope . It is interesting to note that TC remains around the value typically reported for x = 0.12[10,11], but it is in all the cases much smaller than the one of the target (290 K) which agrees with ref. [15]. This fact would be directly associated with the structural differences found between the films and the target. Moreover, in the Inset of Figure 1 (d) can be observed that TC decreases with the increase of the thickness of the film. Another parameter to be considered is the saturation magnetization (MSAT), defined in this case as the magnetization value reached at 50 K. The obtained values are in good agreement with those reported by X. J. Chen and co-worker on thin films of La0.9Sr0.1MnO3 [18]. It can be observed in the Inset of Figure 1(a) an increment of this value with the thickness of the film. Otherwise, comparing the results for LS160 and LS160x, we do not observe a significant difference in the Tc, but lower values of MSAT were obtained in the absence of annealing. Thickness (nm) 150

200 440

360 320

LS100 LS160 LS160x LS200

LSB TC(K)

3

Magnetization (emu/cm )

400 500 400 300 200 100 0 500 400 300 200 100 0 500 400 300 200 100 0 110 100 90 80 70 60 50 40 30 20 10 0

MSAT(emu/cm3)

100

170 160 150 100

150

200

Thickness (nm) 60

80 100 120 140 160 180 200 220 240 260 280 300

Temperature (K)

Figure 1: Magnetization (M) vs. temperature (T) measured with H = 5 kOe applied parallel to the film, comparing M(T) for films with different thicknesses, 100 nm (a), 160 nm (b) and 200 nm (c), with M(T) for the target (H = 1 kOe) (d). Insets of Figure 1 (a) and (d) are the saturation magnetization MSAT and TC respectively as a function of the thickness of the films.

In order to study the MCE, we measured curves of M(H) between 0 and 30 kOe at different temperatures. Hence the entropy change (S) associated with the application of a magnetic field, can be obtained from M(H) curves using the following relation.

S (T , H ) 

1 T

H

  M (T  T , H ')  M (T , H ')dH ' 0

Figure 2 displays S(T) calculated for H = 30 kOe. Note that the magnetization is saturated at this H for all the samples presented here. In all the cases, the absolute value of S(T) reaches its maximum at TC where it is comparable to those observed in thin films of La0.87Sr0.13MnO3 [20] and Heusler alloys[21]. In contrast with the

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0.0

-0.6

-1.8

-1.8

-1.2

-3.6

-1.8

b) -5.4

-0.6

-1.8

LS200

-1.2

-3.6

c)

-5.4 0

S(J/KgK)

LSB

200

Thickness (nm) 100

d) 200

300

-24

3

150

-6

100 -6

RC (J/Kg)

160 -4

-12

S(10 J/mm K)

200 -2

3

-1.8 0

-6

0.0

S(10 J/mm K)

0.0

3

LS160

-6

S(J/KgK)

0.0

LS160x

-0.6

S(10 J/mm K)

0.0

S(J/KgK)

-3.6

a) -5.4

-1.8

3

LS100

-1.2

-6

0.0

S(10 J/mm K)

S(J/KgK)

abrupt PM-FM transition observed in Figure 1(d) for LSB, a broadening of the phase transition appears for the films (Figures 1(a)-(c)) which is clearly reflected in the width of S(T), whose magnitude depends slightly with the thickness as MSAT. These results suggest that the MCE could be controlled by modifying the growth parameters of the films. The cooling or refrigerant capacity (RC), which is defined as the area enclosed by S(T), is another magnitude to consider when a material is being studied for potential applications in magnetic refrigeration [22]. In the inset of Figure 2 we present RC for the different samples. An increase of the RC is observed when the thickness of the film increases. Moreover, the RC of the films is larger than the obtained for bulk (135.43 J/kg) following the behavior the MSAT.

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Temperature (K) Figure 2: Entropy change (ΔS) as a function of temperature (T) calculated for H = 30 kOe from the M(H) curves measured at different temperatures, comparing ΔS (T) for films with different thicknesses, 100 nm (a), 160 nm (b), 200 nm (c) and for the target (d). In the inset we present the refrigerant capacity (RC) as a function of the thickness of the film.

4. Conclusions We have obtained polycrystalline thin films of La0.88Sr0.12MnO3 by PLD, with homogeneous grain size and low roughness. These films were characterized structurally, morphologically and magnetically. The films present a pseudocubic structure that differs from the rombohedric one of the target used in the PLD. This structural difference is enough to affect the PM-FM transition and the MCE properties [14]. The magnetic results obtained as a function of thickness and the thermal treatment, show that the synthesis parameters in polycrystalline films are critical for the magnetic properties within this particular composition range (0.1 < x < 0.17). Therefore the tuning of the morphological and structural parameters could be the key to control the magnetocaloric effect. This work suggests an interesting scenario for the design and develop of new experiments to measure the effect in a more direct way, with the aim of a better understanding and potential applications of the MCE in manganite thin film devices.

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