Field dependence of the magnetocaloric effect in core-shell nanoparticles

Field dependence of the magnetocaloric effect in core-shell nanoparticles V. Franco, A. Conde, Deepti Sidhaye, B. L. V. Prasad, P. Poddar, S. Srinath, M. H. Phan, and H. Srikanth Citation: Journal of Applied Physics 107, 09A902 (2010); doi: 10.1063/1.3335514 View online: http://dx.doi.org/10.1063/1.3335514 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low-temperature large reversible magnetocaloric effects of ErNi 1-x Cu x Al (x = 0.2, 0.5, 0.8) intermetallic compounds J. Appl. Phys. 114, 213907 (2013); 10.1063/1.4838040 Low-temperature reversible giant magnetocaloric effect in the HoCuAl compound J. Appl. Phys. 114, 163915 (2013); 10.1063/1.4826270 Low-temperature large magnetocaloric effect in the antiferromagnetic ErNi0.6Cu0.4Al compound J. Appl. 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Box 1065, 41080 Sevilla, Spain 2Department of Physics, Functional Materials Laboratory, University of South Florida, Tampa, Florida 33620, USA 3Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India 4School of Physics, University of Hyderabad, Andhra Pradesh, Hyderabad 500046, India 共Presented 19 January 2010; received 15 October 2009; accepted 4 November 2009; published online 15 April 2010兲 The field dependence of the magnetic entropy change peak at the low temperature surface spin freezing transition in chemically synthesized, monodispersed Co, Co–Ag, and Ni–Ag core-shell nanoparticles is studied, with the aim of gaining insight into the critical exponents of this transition. It is evidenced that although the magnitude of the peak entropy change and position of the peak can be tuned by changing the composition and nature 共metallic or organic兲of the shell and surfactant layers, the characteristics of the spin freezing transition are not altered. The field dependence of the refrigerant capacity also confirms this finding. © 2010 American Institute of Physics. 关doi:10.1063/1.3335514兴 Magnetocaloric effect 共MCE兲and its most straightforward application, magnetic refrigeration, are becoming fields of increasing research interest 共for recent reviews, the reader can consult Refs. 1–5兲. The reason for this is twofold. On the one hand, magnetic refrigerators are expected to be energetically more efficient and more environmentally friendly than those based on the compression-expansion of gases, which are extremely important considerations in applied scientific research in an era of increasing energy costs and environmental awareness. On the other hand, as magnetic refrigeration close to room temperature is only possible with materials which undergo a phase transition close to the working temperature of the refrigerator, the characterization of magnetocaloric materials generally tends to become associated with a detailed study of their phase transition, which is also interesting from a fundamental point of view. Apart from maximizing the magnetocaloric response of specific materials for a certain value of the applied magnetic field, the response of the materials should be optimized for the applied field range 共10–20 kOe兲which is expected for future refrigerator appliances. Therefore, the study of the field dependence of magnetic entropy change, ⌬SM,isan important subject of research. Recently, the limitation of a mean field approach has been overcome by using the equation of state for materials with a second order magnetic phase transition.6,7Expressing the field dependence as ⌬SM⬀Hn, this approach allows for establishing a relationship between the exponent nand the critical exponents of the material and to propose a phenomenological universal curve for the field dependence of ⌬SM, which was successfully tested for series of soft magnetic amorphous alloys6,8and lanthanide based crystalline materials.9,10 As in many other fields of research, most magnetocaloric materials investigated have primarily been bulk in nature. However, the emerging interest in novel magnetic behavior of nanostructured magnets can also open possibilities for practical applications which could not be envisaged before. There has been an intensive research on the magnetocaloric response of superparamagnetic nanoparticles.11,12 More recently, it has been shown that the adequate control of the magnetic anisotropy of a nanostructured system can originate behaviors that are qualitatively different from those of the bulk materials.13 There have been studies of the influence on the MCE of the control of the anisotropy of the material, especially through surface modification processes, either by ball-milling zinc ferrite particles,14 which lead to disorder on the surface of the particles causing some of the ions to change from spinel to the inverse spinel structure, or by systems of core-shell nanoparticles,15 where the total magnetization of each particle may be considered as consisting of two parts: the core magnetic moment 共which is ordered and behaves as a superspin兲and the shell magnetization 共which is mostly disordered and somewhat similar to spin-glass systems兲. The exchange coupling between the surface and core spins can lead to short-range order of the surface spins below a certain temperature with the surface anisotropy also competing in this process. This surface anisotropy that these particles exhibit at low temperatures causes the ⌬SMvalues obtained for these core-shell nanoparticles at their associated spin freezing transition to be the largest reported on any nanoparticle system so far.15 This transition, detected in the magnetization curves as an abrupt change in temperature dependence of magnetization, can be perceived as an orderdisorder transition. While the critical characteristics of this phase transition cannot be studied using standard procedures such as the Kouvel–Fisher method, the analysis of the field dependence of MCE proposed recently by us is believed to be a useful tool for this purpose.16 In this work, we will show that the study of MCE of Co, Co–Ag, and Ni–Ag core-shell nanoparticles can give us additional information about the characteristics of that phase a兲Electronic mail: [email protected]. JOURNAL OF APPLIED PHYSICS 107, 09A902 共2010兲 0021-8979/2010/107共9兲/09A902/3/$30.00 © 2010 American Institute of Physics107, 09A902-1 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.17 On: Wed, 20 Jan 2016 18:17:44 transition. These different samples involve two different synthetic strategies based on the idea of forming core-shell structures. In the first approach, the passivating organic surfactant layer acts as the shell and monodisperse cobalt particles constitute the core thereby increasing the active magnetocaloric content and also decreasing the effective mass of the nanocomposite. In the second approach, a conductive shell in the form of Ag is used, with Co or Ni as the magnetic core. The presence of Ag not only provides a desirable high thermal conducting surface 共beneficial for heat transfer to the lattice in practical refrigerator design兲but also prevents agglomeration and surface oxidation that tend to degrade the magnetic and MCE properties of the composite. Synthesis of monodisperse Co and Ni nanoparticles was performed by a simple, aqueous medium based wet chemical technique at ambient temperature with optimized concentration of a mixture of surfactants—sodium dodecyl sulfate and oleic acid.17 The CocoreAgshell and NicoreAgshell particles were synthesized by replacing the surface layer of Co and Ni nanoparticles with Ag using the transmetallation reaction.18 The particle morphologies were probed by transmission electron microscopy 共Fig. 1兲. Magnetic characterization was done using a commercial 7 T physical property measurement system from Quantum Design. For the temperature- and field-dependent magnetization measurements, dried samples in powder form were packed into a gel cap and care was taken to enclose them tightly so that no mechanical motion of particles was possible under external fields. The details of the magnetic characterization of these samples, including the collection of the family of isothermal magnetization curves for extracting the MCE, have been reported in Ref. 15. The MCE can be characterized by the magnetic entropy change due to the application of a magnetic field H, which can be evaluated from the processing of the temperature and field dependent magnetization curves using a numerical approximation to the equation, ⌬SM= 冕 0 Hmax 冉 ⳵ M ⳵ T 冊 H dH,共1兲 where the partial derivative is replaced by finite differences and the integration is performed numerically. The phenomenological universal curve is constructed by normalizing the magnetic entropy change curves with respect to their peak, ⌬SM pk, and rescaling the temperature axis6using a reference temperature,19 Tr,关the one corresponding to ⌬SM共Tr兲=0.5⌬SM pk兴, ␪ =共T−TC兲/共Tr−TC兲,共2兲 where TCis the transition temperature. In the present study, we are not dealing with a Curie temperature but rather with a surface spin freezing temperature, so the transition temperature has been identified as the temperature of the peak entropy change, Tpk. The upper panel of Fig. 2shows the temperature dependence of the magnetic entropy change for different values of the maximum applied magnetic field for the Co–Ag coreshell nanoparticles. When these data are processed to construct the universal curve, they collapse onto a single curve, as shown in the lower panel of Fig. 2. This collapse of the ⌬SMcurves has been found for the case of magnetocaloric materials with a second order phase transition6–10 and is therefore expected for the case of the core/shell nanoparticles as the surface spin freezing transition is typically an orderdisorder transition. More remarkable is the collapse of the ⌬SMcurves of the different Co, Co–Ag, and Ni–Ag samples. This implies that the critical exponents associated to this transition in the different studied samples should be equal, within the experimental error margin. The surface anisotropy in nanoparticles is largely affected by the chemical as well as physical environment surrounding the particles. The surface magnetic properties may change under the influence of interparticle magnetic interactions as well as intraparticle exchange interaction between the core and shell regions.20 Thus the overall change in the surface magnetic properties is most likely driven by the competition between intra- and interparticle interactions. At lower temperatures, the spins are frozen in the directions determined by these competing interactions as well as the effective anisotropy energy. For the Co core-shell nanoparticles, Co nanocrystals are separated by the organic surfactant layer. In the other two cases, it is the Ag layer that separates the magnetic particles. 500 nm 20 nm 100 nm A B C FIG. 1. Transmission electron micrographs of the nanoparticles 共a兲Co, 共b兲 CocoreAgshell, and 共c兲NicoreAgshell. FIG. 2. 共Color online兲Left panel: Temperature dependence of the magnetic entropy change of the Co–Ag nanoparticles measured up to different maximum applied fields. Right panel: Collapse of the magnetic entropy change curves of all the studied samples onto a single universal curve. 09A902-2 Franco et al. J. Appl. Phys. 107, 09A902 共2010兲 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.17 On: Wed, 20 Jan 2016 18:17:44 While the surface spin disorder in Co and Ni nanoparticles with just an organic surfactant coating is expected to be present, the ordering of the interfacial spins and the spin freezing characteristics are expected to be considerably altered with the presence of nonmagnetic metallic interfaces such as Ag. The configuration of interfacial spins is decided by the disorder 共which will be different for the organic surfactant coating and for the metal shell layer兲at the surface. This is generally influenced by several factors such as random anisotropy, broken bonds, and associated surface charge distributions and possible charge transfer across the interface. The change in composition of the particles and nature of the shell can be used to tailor the magnitude and position of the peak,15 but the present results indicate that it does not alter the characteristics of the phase transition, as evidenced by the collapse of the ⌬SMcurves. The low temperature at which the transition takes place, together with the fact that there is another magnetic transition 共the superparamagnetic/blocking transition of the core spins兲at higher temperatures, prevents an accurate determination of the critical exponents from the magnetization curves, which could be used to compare the similarities of the spin freezing transitions of the studied samples. Nevertheless, taking into account that the magnetic entropy change has a peak at the surface spin freezing temperature and that its width is smaller than the distance to the superparamagnetic transition 共i.e., the peaks corresponding to these two different magnetic transitions are not overlapped兲, the study of the field dependence of the magnetic entropy change peak can be used to qualitatively compare the transitions of the different samples. It has been recently shown experimentally19 and demonstrated theoretically7that there is a scaling law for the refrigerant capacity 共RC兲for materials with a second order phase transition, RC ⬀H1+1/ ␦ .共3兲 Therefore, if the critical exponents characterizing the spin freezing transition are the same, the field dependence of RC for all these alloys should be the same and characterized by a single exponent. To test this feature, a nonlinear fit of RC versus field for the Ni–Ag sample measured up to 70 kOe has been performed to extract the value of the exponent in Eq. 共3兲. That experimentally determined exponent value has been used to construct the abscissa axis of Fig. 3and the RC data for the other samples have been plotted versus this axis. It is shown that, as expected, the behavior for all the data is linear, which hints again at an independence of the characteristics of the spin freezing transition with the nature of the shell. In conclusion, the magnetocaloric peak of the spin freezing transition of Co, Co–Ag, and Ni–Ag core-shell nanoparticles has been studied under the point of view of the universal curve for the magnetic entropy change. It is shown that all the experimental data collapse onto the same universal curve, indicating that the nature of the phase transition is not altered by the characteristics of the shell 共either organic surfactant layer of metallic Ag兲. This is confirmed by the scaling law of the RC of these samples. This work has been supported by the Spanish MEC and EU FEDER 共Project MAT No. 2007-65227兲,bythePAIof the Regional Government of Andalucía 共Project No. P06- FQM-01823兲. H.S. acknowledges support from the U.S. Department of Army through Grant No. W911NF-08-1-0276. NCL authors acknowledge the financial support from the Department of Science and Technology, India, to set up a unit on nanoscience and the Grant No. SR/S5/NM-104/2006 under the “Nano Mission” program. 1A. M. Tishin, in Handbook of Magnetic Materials, edited by K. H. J. Buschow 共Elsevier, Amsterdam, 1999兲, Vol. 12, pp. 395–524. 2K. A. Gschneidner, Jr. and V. K. Pecharsky, Annu. Rev. Mater. Sci. 30, 387 共2000兲. 3A. M. Tishin and Y. I. Spichkin, The Magnetocaloric Effect and Its Applications 共Institute of Physics Publishing, Bristol, 2003兲. 4E. Brück, J. Phys. D: Appl. Phys. 38, R381 共2005兲. 5A. M. Tishin, J. Magn. Magn. Mater. 316, 351 共2007兲. 6V. Franco, J. S. Blázquez, and A. Conde, Appl. Phys. Lett. 89, 222512 共2006兲. 7V. Franco, A. Conde, J. M. Romero-Enrique, and J. S. Blázquez, J. Phys.: Condens. Matter 20, 285207 共2008兲. 8V. Franco, J. S. Blázquez, M. Millán, J. M. Borrego, C. F. Conde, and A. Conde, J. Appl. Phys. 101, 09C503 共2007兲. 9V. Franco, A. Conde, V. K. Pecharsky, and K. A. Gschneidner, Jr., Europhys. Lett. 79, 47009 共2007兲. 10Q. Y. Dong, H. W. Zhang, J. R. Sun, B. G. Shen, and V. Franco, J. Appl. Phys. 103, 116101 共2008兲. 11R. D. McMichael, R. D. Shull, L. J. Swartzendruber, L. H. Bennett, and R. E. Watson, J. Magn. Magn. Mater. 111,29共1992兲. 12P. Poddar, J. Gass, D. J. Rebar, S. Srinath, H. Srikanth, S. A. Morrison, and E. E. Carpenter, J. Magn. Magn. Mater. 307,227共2006兲. 13V. Franco, K. R. Pirota, V. M. Prida, A. M. J. C. Neto, A. Conde, M. Knobel, B. Hernando, and M. Vazquez, Phys. Rev. B 77, 104434 共2008兲. 14J. Gass, H. Srikanth, N. Kislov, S. S. Srinivasan, and Y. Emirov, J. Appl. Phys. 103, 07B309 共2008兲. 15P. Poddar, S. Srinath, J. Gass, B. L. V. Prasad, and H. Srikanth, J. Phys. Chem. C 111, 14060 共2007兲. 16V. Franco, A. Conde, V. Provenzano, and R. D. Shull, J. Magn. Magn. Mater. 322, 218 共2010兲. 17D. S. Sidhaye, T. Bala, S. Srinath, H. Srikanth, P. Poddar, M. Sastry, and B. L. V. Prasad, J. Phys. Chem. C 113,3426共2009兲. 18R. Jakubiak, T. J. Bunning, L. V. Natarajan, and V. P. Tondiglia, Adv. Mater. 共Weinheim, Ger.兲15,241共2003兲. 19V. Franco, J. S. Blázquez, and A. Conde, J. Appl. Phys. 103, 07B316 共2008兲. 20B. Martínez, X. Obradors, L. Balcells, A. Rouanet, and C. Monty, Phys. Rev. Lett. 80, 181 共1998兲. FIG. 3. 共Color online兲Field dependence of the RC 共calculated as the product of the peak entropy change times the full width at half maximum of the peak兲for the different studied samples. Lines are linear fits to the data. The exponent of the abscissa has been selected from the power law fit of the Ni–Ag 共70 kOe兲data. 09A902-3 Franco et al. J. Appl. Phys. 107, 09A902 共2010兲 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 150.214.182.17 On: Wed, 20 Jan 2016 18:17:44