Magnetocaloric effect in 4d itinerant ferromagnet SrRuO3

Journal of Alloys and Compounds 459 (2008) 51–54

Magnetocaloric effect in 4d itinerant ferromagnet SrRuO3 Xiao-Yu Zhang a , Yajie Chen b,∗ , Zhen-Ya Li a a

b

Department of Physics and Jiangsu Key Laboratory of Thin Films, Suzhou University, Suzhou 215006, China Center for Microwave Magnetic Materials and Integrated Circuits, and the Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115, United States Received 17 April 2007; received in revised form 6 May 2007; accepted 8 May 2007 Available online 22 May 2007

Abstract The magnetocaloric effect (MCE) in 4d itinerant ferromagnet SrRuO3 prepared by conventional ceramic processing has been investigated using superconducting quantum interference device (SQUID). SrRuO3 material exhibits magnetic entropy change SM = −2.5 J/kg K and adiabatic temperature change Tad = 3.1 K at an applied field of 6.5 T and near Curie temperature (∼160 K). Based on the analysis of Debye approximation, the lattice entropy change reaches SL = 0.34 J/kg K and is 12% of total entropy changes. The present work demonstrates the importance of the lattice entropy change in SrRuO3 material with second-order phase transition since the large change in lattice volume exhibits near Curie temperature. © 2007 Elsevier B.V. All rights reserved. JEL classification: 75.30.Sg; 77.80.Bh Keywords: Magnetocaloric effect; Phase transition; Oxide materials

1. Introduction SrRuO3 is an itinerant ferromagnet (TC ∼ 160 K) with a GdFeO3 -type pseudocubic perovskite structure [1]. In the past decade, SrRuO3 has received much attention due to technological applications for metallic substrates and others [2]. 4d ferromagnet is rare because it only occurs in a handful of compounds and certain low-dimensional systems. The material can exhibit strong magnetocrystalline anisotropy in both single crystal bulk and single domain thin film samples [3]. Additionally, magnetic ordering decreases the electrical resistivity in SrRuO3 rendering it attractive for magnetotransport studies [4]. Recently, magnetic refrigeration based on magnetocaloric effect (MCE) has drawn an immense increase in interest due to the more considerable advantages than conventional gas compression refrigeration, such as the high cooling efficiency, environmentally friendly cooling technology, small volume, etc. [5,6]. The MCE displays an intrinsic thermodynamic property of magnetic materials. The potential magnetic materials in future magnetic refrigeration technology are reviewed by Gschneidner et al. [7] and Phan and Yu [8] For example, rare earth element ∗

Corresponding author. Tel.: +1 617 3735160; fax: +1 617 3734853. E-mail address: [email protected] (Y. Chen).

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Gd [9], Gd5 (Ge1−x Six )4 series [10], La(Fe13−x Mx )-based compounds (M = Si, Al) [11], manganites [12], and half-metallic CrO2 [13]. In the view of the summarized materials, magnetic oxides have been attracted more attentions for magnetic refrigeration technology due to adjustable temperature corresponding to MCE peak over a wide temperature range of 100–375 K and the cheapest material among existing magnetic refrigerants. However, the MCE for 4d transition ferromagnet SrRuO3 has been unclear so far. Since SrRuO3 is an intriguing material as a rare case of 4d itinerant ferromagnetism, the electronic states and the coupling of p–d hybridization are far different from that in 3d ferromagnet, therefore, it is interesting and meaningful to study and reveal the mechanism of MCE in 4d itinerant ferromagnet. In this paper, we have a systemic investigation of magnetocaloric effect including magnetic entropy change SM and adiabatic temperature change Tad in 4d itinerant ferromagnetic SrRuO3 . Our efforts are of significance not only to seek for new alternative materials, but also to understand physical mechanism of MCE in 4d itinerant ferromagnet SrRuO3 . 2. Experimental The polycrystalline sample of SrRuO3 was prepared from high-purity SrCO3 and RuO2 powders. The powders with stoichiometric composition were mixed and first sintered at 700 ◦ C, and then the mixed materials had extra four sin-

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X.-Y. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 51–54

Fig. 1. X-ray diffraction pattern of polycrystalline SrRuO3 sample.

ters at 1100–1250 ◦ C for a soak of 24–48 h. Finally, the sintered materials were reground for 10 min, pressed into pellets, and reacted at 1280 ◦ C for 20 h. The crystallography was characterized by a Rigaku D/Max 2500C X-ray diffractometer (XRD). Magnetic measurements were performed by superconducting quantum interference devices (SQUID) over the temperature range from 10 to 300 K.

3. Results and discussion The XRD pattern of the sample shows no secondary phase, as shown in Fig. 1. The sample can be indexed to tetragonal structure SrRuO3 from these diffraction peaks. Fig. 2 displays the magnetization as a function of temperature from 10 to 300 K. The magnetization drops rapidly at the Curie temperature TC of 160 K, which is derived from the maximum of dM/dT curve. It is worth noting that an abrupt change in magnetization with respect to the temperature may reveal a large MCE in the close vicinity of the phase transition temperature. Therefore, Fig. 3 presents the isothermal magnetization dependence of applied field up to 6.5 T and over a temperature range of 10–200 K. Thus, one can calculate the magnetic entropy changes SM based on a series of isothermal magnetization curves by following

Fig. 2. Magnetization as a function of temperature from 4 to 300 K at a magnetic field of 10 kOe for SrRuO3 sample.

Fig. 3. Isothermal M–H curves at different temperatures ranging from 10 to 200 K.

Maxwell equation:  SM (T, H) = S(T, H) − S(T, 0) = 0

H



∂M ∂T

 dH.

(1)

H

Fig. 4 displays the magnetic entropy change SM plotted against temperature at different magnetic field of 6.5, 3.5 and 1.5 T, respectively. As expected, the maximum |SM | approaches to 2.5, 1.3, and 0.6 J/kg K at three applied fields near Curie temperature TC . The magnetic entropy change for 4d itinerant ferromagnetic SrRuO3 originates mainly from a rapid drop in magnetization in the vicinity of Curie temperature TC . Namely, spin alignments induced by magnetic field lead to negative entropy change. Furthermore, the maximum of magnetic entropy change SM exhibit a linear increase (SM = p + kH) with increasing magnetic field H, as shown in the inset of Fig. 4. Although a larger SM can be expected at a high field, this is unable to meet the requirements of engineering applications. In order to profoundly discuss the critical phenomena in SrRuO3 materials, the measured data of the M–H isotherms are

Fig. 4. Temperature dependence of magnetic entropy change (SM ) at a magnetic field of 6.5, 3.5, and 1.5 T. Dash lines denote the calculation by molecular field model. Inset shows the maximum |SM | as a function of magnetic field. The straight lines are guide for eyes.

X.-Y. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 51–54

Fig. 5. The H/M vs. M2 plots at different temperatures for SrRuO3 sample.

Fig. 6. Temperature dependence of adiabatic temperature change (Tad ) at different magnetic fields.

plotted in H/M versus M2 curves, depicted in Fig. 5. According to a widely acceptable Banerjee criterion [14], a positive slope in H/M–M2 plots reveals that the ferromagnetic–paramagnetic process is a second-order phase transition for the SrRuO3 material, which is consistent with the thermodynamic investigation of SrRuO3 [15]. Therefore, it is observable that a large SM for SrRuO3 can retain at a broad temperature range of 28 K, which is attributed to the second-order phase transition. Additionally, a relative cooling power (RCP) useful engineering parameter for evaluating magnetocaloric materials, can be obtained by the following equation: RCP = −SM δTFWHM ,

(2)

where ␦TFWHM , as indicated in Fig. 4, denotes the full width temperature at half maximum of SM . Unfortunately, the RCP value in present sample is small compared to some doped manganites, such as RCP = 670 J/kg (7 T) for La0.845 Sr0.155 MnO3 , RCP = 462 J/kg (5 T) La0.7 Ca0.25 Sr0.05 MnO3 , etc. [8,16]. On the other hand, an estimation of adiabatic temperature change Tad , which represents another important criterion to evaluate the MCEs, is performed as below: Tad = −SM

T . C(T, H)

(3)

The specific heat C(T, H) in Eq. (3) is considered to be the sum of the lattice and magnetic contribution, namely, C(T, H) = CL + CM . The Debye specific heat CL is calculated from  CL = 9kB N

T Θ

3  0

θD /T

ex x 4 (ex − 1)2

dx,

(4)

where kB is the Boltzmann constant, N is the number of atoms per unit mass, and Θ = 368 K is the Debye temperature for SrRuO3 [17]. The magnetic contribution to specific heat capacity CM is given by the derivative of the magnetization with respect to temperature: CM = −Hext

∂M ∂M 2 1 − Hint . ∂T 2 ∂T

53

(5)

Here Hint = 3kB Tc /[Ns g2 μ2B J(J + 1)] is the mean field constant, and Ns is the number of spins per unit mass. The g is the Lande factor and J is the total angular momentum. The estimated adiabatic temperature change Tad is illustrated in Fig. 6. Surprisingly, the sample presents a maximum Tad of 3.1 K at a magnetic field of 6.5 T. Since the increase of magnetic ordering under an external field makes the magnetic entropy smaller (corresponding to a larger |−SM |), the Tad is certainly positive. This means that the magnetic material will be heated. However, the quantitative correlation between SM and Tad has been discussed in Ref. [5]. Since the behavior of Tad (T) is assumed to be quite similar to |SM (T)|, i.e. both Tad and SM exhibit maximum at the Curie temperature TC . The assumption is also in good agreement with our observations in present experiment. To obtain more knowledge about the entropy change in SrRuO3 materials, a further study on lattice entropy and electronic entropy changes is necessary because the thermal expansion coefficient and lattice volume exhibit large changes near TC [17,18]. Here, the lattice entropy is considered based on the Debye approximation [19]:    −Θ SL (T ) = −3NkB ln 1 − exp T  3  Θ/T T x3 dx +12NkB , (6) Θ exp(x) − 1 0 where Θ = 368 K and N is the number of the atoms per mole SrRuO3 . The variation of the Debye temperature due to the phase transition can be approximated by Θ = Θ0 (1 − ηV/V), where η is Gr¨uneisen parameter and V/V is the volume change. Here, we can take η = 1 for solid materials [20], and volume change V/V = 8.3 × 10−4 [18]. Thus, the lattice entropy change SL = SL (Θ) − SL (Θ0 ) of SrRuO3 can be estimated of 0.34 J/kg K. Meanwhile, the change of electronic entropy will be Se = (2/3)(V/V)γTc = 0.015 J/kg K, where γ = 30 mJ/mol K2 for SrRuO3 [17]. Obviously, the electronic entropy change Se is so small that it can be neglected, whereas, the lattice entropy change SL , 12% of total entropy changes at

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X.-Y. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 51–54

6.5 T, is remarkable. Clearly, the results are reasonable for the materials because the magnetic entropy change under high magnetic fields arises mainly from a second-order phase transition. Unlike those materials with first-order phase transition, a second-order phase transition is weakly responsible for lattice entropy change. Usually, lattice entropy is able to contribute 20–40% to total entropy change in Gd5 Six Ge4−x with firstorder phase transition [21]. Therefore, SrRuO3 demonstrates notable lattice entropy which is related to the remarkable change in specific heat and lattice volume near Curie temperature. 4. Conclusions In summary, we have addressed a detailed investigation on magnetocaloric effect in polycrystalline SrRuO3 material. The magnetic entropy SM = −2.5 J/kg K and adiabatic temperature change Tad = 3.1 K are found at an applied field of 6.5 T near Curie temperature. Second-order transition in SrRuO3 material leads to a relative cooling power (RCP) of 70 J/kg. Since the large lattice volume change exists at TC , the present work reveals the importance of lattice entropy change in 4d itinerant ferromagnetic SrRuO3 . Acknowledgment This work was supported by National Natural Science Foundation of China under grant No. 10474069.

References [1] G. Cao, S. McCall, M. Shepard, J.E. Crow, R.P. Guertin, Phys. Rev. B 56 (1997) 321. [2] C.B. Eom, R.J. Cava, R.M. Fleming, et al., Science 258 (1992) 1766. [3] R.A. Rao, D.B. Kacedon, C.B. Eom, J. Appl. Phys. 83 (1998) 6995. [4] L. Klein, J.S. Dodge, C.H. Ahn, G.J. Snyder, T.H. Geballe, M.R. Beasley, A. Kapitulnik, Phys. Rev. Lett. 77 (1996) 2774. [5] V.K. Pecharsky, K.A. Gschneidner Jr., J. Magn. Magn. Mater. 200 (1999) 44. [6] K.A. Gschneidner Jr., V.K. Pecharsky, J. Appl. Phys. 85 (1999) 5365. [7] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Tsokol, Rep. Prog. Phys. 68 (2005) 1479. [8] M.H. Phan, S.C. Yu, J. Magn. Magn. Mater. 308 (2007) 325. [9] G.V. Brown, J. Appl. Phys. 47 (1976) 3673. [10] V. Provenzano, A.J. Shapiro, R.D. Shull, Nature (London) 429 (2004) 853. [11] A. Fujita, S. Fujieda, Y. Hasegawa, K. Fukamichi, Phys. Rev. B 67 (2003) 104416. [12] Z.B. Guo, Y.W. Du, J.S. Zhu, H. Huang, W.P. Ding, D. Feng, Phys. Rev. Lett. 78 (1997) 1142. [13] X. Zhang, Y. Chen, L. L¨u, Z. Li, J. Phys.: Condens. Matter 18 (2006) L559. [14] S.K. Banerjee, Phys. Lett. 12 (1964) 16. [15] J.J. Neumeier, A.L. Cornelius, K. Andres, Phys. Rev. B 64 (2001) 172406. [16] A. Szewczyk, M. Gutowska, B. Dabrowski, T. Plackowski, N.P. Danilova, Y.P. Gaidukov, Phys. Rev. B 71 (2005) 224432. [17] P.B. Allen, H. Berger, O. Chauvet, L. Forro, et al., Phys. Rev. B 53 (1996) 4393. [18] G.M. Leitus, S. Reich, F. Frolow, J. Magn. Magn. Mater. 206 (1999) 27. [19] P.J. von Ranke, S. Gama, A.A. Coelho, A. de Campos, A. Magnus, G. Carvalho, F.C.G. Gandra, N.A. de Oliveira, Phys. Rev. B 73 (2006) 014415. [20] C. Kittel, Introduction to Solid State Physics, 5th ed., Wiley, New York, 1976. [21] G.J. Liu, J.R. Sun, J. Lin, Y.W. Xie, T.Y. Zhao, H.W. Zhang, B.G. Shen, Appl. Phys. Lett. 88 (2006) 212505.