Large magnetocaloric effect in hexagonal Yb1

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fuel industry.1,2 Consequently, magnetic refrigeration tech- nology is building ... change in temperature (DTad) and relative cooling power. (RCP) in Mg ..... (2015). 7B. Sattibabu, A. K. Bhatnagar, D. Singh, D. Das, and V. Ganesan, Mater. Lett.
Large magnetocaloric effect in hexagonal Yb1−xHoxMnO3 Bhumireddi Sattibabu, A. K. Bhatnagar, K. Vinod, Awadhesh Mani, and D. Das Citation: Applied Physics Letters 107, 262904 (2015); doi: 10.1063/1.4938750 View online: http://dx.doi.org/10.1063/1.4938750 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/107/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evidence of large magneto-dielectric effect coupled to a metamagnetic transition in Yb2CoMnO6 Appl. Phys. Lett. 107, 012902 (2015); 10.1063/1.4926403 Phase diagram and magnetocaloric effects in Ni1-xCrxMnGe1.05 J. Appl. Phys. 117, 17A711 (2015); 10.1063/1.4907765 Large magnetocaloric effect induced by intrinsic structural transition in Dy1− x Ho x MnO3 Appl. Phys. Lett. 100, 222404 (2012); 10.1063/1.4722930 Large reversible magnetocaloric effect in HoTiO3 single crystal J. Appl. Phys. 110, 083912 (2011); 10.1063/1.3653838 Correlation of structural distortion with magnetic properties in electron-doped Ca 0.9 R 0.1 MnO 3 perovskites ( R = rare -earth) J. Appl. Phys. 108, 063928 (2010); 10.1063/1.3481419

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APPLIED PHYSICS LETTERS 107, 262904 (2015)

Large magnetocaloric effect in hexagonal Yb12xHoxMnO3 Bhumireddi Sattibabu,1,a) A. K. Bhatnagar,1,2,a) K. Vinod,3 Awadhesh Mani,3 and D. Das1 1

School of Engineering Sciences and Technology, University of Hyderabad, Hyderabad 500046, India School of Physics, University of Hyderabad, Hyderabad 500046, India 3 Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2

(Received 6 October 2015; accepted 13 December 2015; published online 29 December 2015) Magnetocaloric properties of polycrystalline hexagonal Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) compounds are studied through magnetization measurements. Temperature dependence of Zero Field Cooled magnetic moment measurements shows Neel temperature (TN1) of 83 K, corresponding to the Mn3þ antiferromagnetic ordering. At low temperatures (TN2  5 K), all compounds show ferromagnetic ordering due to alignment of the Yb moments and the field induced magnetic transition is observed in the isothermal magnetization measurements. The maximum entropy change jDSMmaxj and the relative cooling power (RCP) of Yb1xHoxMnO3 are 3.75 6 0.78 J/(mol K) and 90.0 6 27 J/mol for x ¼ 0.3 at DH ¼ 100 kOe. Values of both jDSMmaxj and RCP found to increase C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4938750] with increasing Ho content. V

For practical refrigeration applications, magnetocaloric materials are considered to be energetically efficient and environmentally friendly than the conventional refrigeration route based on the compression of a gas. Recent research in this area has been focused on identifying materials with large magnetocaloric effect (MCE) in two temperature regimes; either above 250 K for domestic and several technological applications, such as in air conditioning and commercial refrigeration, or below 80 K for specific technological applications, such as space science and liquefaction of hydrogen in fuel industry.1,2 Consequently, magnetic refrigeration technology is building up with time, and search for materials with large MCE is amplified in recent times. Multiferroic rare earth hexagonal manganites RMnO3(R ¼ Ho, Er, Tm, Yb, and Lu) have been reasonably well studied for its crystal structure, dielectric and magnetic properties.3–5 YbMnO3 exhibits interesting physical properties. Most of the studies on YbMnO3 have been focused on its magnetic and ferroelectric behavior and less work has been undertaken on exploring the MCE properties of YbMnO3. Recently, significant values of magnetic entropy change (DSM), the adiabatic change in temperature (DTad) and relative cooling power (RCP) in Mg and Er doped YbMnO3 have been reported.6,7 The magnetic phase diagram of YbMnO3 is rather complex. Results of magnetic, dielectric, and magnetoelectric measurements on single crystals of YbMnO3 by Sugie et al.8 show that below TN2  3.5 K, long range ferromagnetic (FM) ordering of Yb3þ moments develops and a magnetic field applied along the c-axis induces a first order magnetic transition at a critical field of Hjjc  35 kOe, where the magnetic ordering along the c-axis becomes ferrimagnetic. They have identified the low temperature magnetic phase (H-T) diagram of YbMnO3 with three different regions. Based on the neutron diffraction and M€ossbauer spectroscopy studies on polycrystalline samples, Fabreges et al.9 have reported that the Mn3þ moments get aligned in the ab-plane order antiferromagnetically, at the Neel temperature (TN1 ¼ 85 K) a)

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]

0003-6951/2015/107(26)/262904/5/$30.00

and starts the Yb moments at the 4b crystallographic site to order due to the Mn molecular field. The Yb(4b) moments aligned along the c-axis and are antiferromagnetically (ferromagnetically) coupled along the ab-plane (c-axis). Below TN2  3.5 K, Yb3þ moments at the 2a crystallographic site aligned along the ab-plane start long range ferromagnetic ordering through Yb-Yb interactions. They suggest a spin flip/reorientation of the Yb(2a) moments from ab-plane to c-axis as the source/reason for the field induced magnetic transition.9 Based on the heat capacity and magnetization measurements on single crystals, Abramov et al.10 also identified a three phase H-T phase diagram for this compound at low temperatures. However, they have attributed the Yb(4b) moments spin flip as the reason for the field induced magnetic transition, rather than the spin flipping of Yb(2a) moments as suggested by Fabreges et al.9 The above reported behavior of YbMnO3 is very interesting, and therefore, it was considered to be useful to investigate changes in the above behavior due to other rare-earth substitution for Yb. This paper reports the effect of Ho substitution on the magnetic and magnetocaloric properties of Yb1xHoxMnO3 system with x ¼ 0.1, 0.2, and 0.3. Considering that the Ho3þ ion size is a little larger than that of Yb3þ and also has higher magnetic moment than that of Yb3þ, it was considered interesting to study doping of Ho for Yb in YbMnO3 on its magnetic behavior and magnetocaloric properties. The large field induced magnetization observed in Yb1xHoxMnO3 has motivated us to investigate the magnetocaloric behavior in this system. Here, we present a comprehensive study of the magnetocaloric effect of Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) measured on polycrystalline compound near the Yb spin ordering transition. Polycrystalline samples of Yb1xHoxMnO3 system with x ¼ 0.1, 0.2, and 0.3 were synthesized by a conventional solid-state reaction method from stoichiometric mixtures of Yb2O3, Ho2O3, and MnCO3 powders. XRD measurements confirmed that the synthesized Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) samples are in single phase and have the hexagonal symmetry with P63cm space group, in agreement with previous reports.6,11

107, 262904-1

C 2015 AIP Publishing LLC V

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Temperature dependence of susceptibility v(T) of the compounds measured under an applied magnetic field of 500 Oe in zero field cooled (ZFC) mode is shown in Fig. 1(a). Temperature variation of the inverse magnetic susceptibility is plotted as right inset in Fig. 1(a) which shows that the Curie–Weiss (CW) law is well obeyed in temperature interval between 200 and 300 K. The Curie-Weiss temperatures are found to decrease with Ho doping (hCW ¼ 92 K, 65 K, and 52 K for x ¼ 0.1, 0.2, and 0.3), while the effective moment (leff) increases with the Ho content (leff ¼ 6.81, 7.36, and 7.9 for x ¼ 0.1, 0.2, and 0.3). The Mn3þ antiferromagnetic (AFM) ordering temperature is clearly visible in the derivative of 1/v vs temperature ðdð1=vÞ dT Þ plot shown in the left inset of Figure 1(a). The ferromagnetic ordering of Yb3þ(2a) moments8,9 is observed below T  5 K, as a sudden rise in v(T) of all the samples. The isothermal magnetization (M-H) curves measured at 2.5 K (shown in Fig. 1(b)) also show the weak FM behavior of the samples, reported due to ordering Yb3þ moments at the 2a crystallographic sites through Yb-Yb interactions.8,9 As for Yb0.9Ho0.1MnO3 and Yb0.7Ho0.3MnO3, the magnetization at 100 kOe is 2.2 lB/f.u. and 3.8 lB/f.u., respectively. To calculate the magnetic entropy change DSM associated with the magnetocaloric effect, we have measured H

FIG. 1. (a) Temperature dependences of the zero-field-cooled (ZFC) magnetization for Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) samples under applied magnetic field 500 Oe. Left inset shows first derivative of temperature dependent inverses susceptibility, 1/v(T), and the right inset shows temperature dependent inverses susceptibility, 1/v(T) curves with Curie-Weiss fit. (b) The hysteresis loops of Yb1xHoxMnO3 (x ¼ 0.1 and 0.3) measured at 2.5 K. Inset shows the fields dependence of the magnetization plots in magnetic fields strengths of up to 100 kOe.

Appl. Phys. Lett. 107, 262904 (2015)

dependence of M of Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) in the vicinity of Yb ordering (TN2  5 K). For each isotherm, the magnetic field has been varied from 0 to 100 kOe. Representative plots of isothermal M vs H plots in the temperature range 2–40 K are shown in Figs. 2(a)–2(c). The field-induced magnetic transition is observed as a sudden slope change of M vs H at a critical field of H  30 kOe. Figure 2(d) shows the Arrot plots for the samples at T ¼ 2.5 K (below the TN2). According to Banerjee,12 the Arrott plot will have negative (positive) slope for a first (second) order transition. In Fig. 2(d), Arrott plots show that the samples have positive slopes in the complete M2 ranges, indicating that the system exhibits a second order transition. The magnetic entropy change DSM associated with the magnetocaloric effect can be calculated from the magnetization data using the Maxwell relation ðH   @M dH: (1) DSM ðT; H Þ ¼ @T H 0 The value of DSM is determined from the isothermal magnetization curves measured at different temperatures with an appropriate interval of temperature DT, using Equation (2.3b), and the error/uncertainty in DSM is calculated using Equation (2.4b) of Ref. 13. For the calculation of relative error in the entropy change following,13 the accuracy of magnetization measurements is taken as 0.5% and the accuracy of the magnetic field as 0.1%. The manufacturer quoted temperature stability of 0.25% is used as the error for temperatures. The values of DSM for different DH as a function of temperature are presented in Figs. 3(a)–3(c). The inset shows the relative errors in DSM, calculated using (2.4b) of Ref. 13. Except for very low temperatures and high fields, the relative error is 10%–20%. DSM is positive in the entire temperature range for all the samples. The curves present a characteristic shape with a broad maximum in the vicinity of the FM transition of the Yb moment. The magnitude of the peak increases with increasing of DH for each composition, and the position of the maximum shifts from 5 to 10 K when the magnetic field change increases from 10 to 100 kOe. The values of the peak, DSMmax of Yb1xHoxMnO3, are 2.85 6 0.48 J/(mol K) for x ¼ 0.1, 3.83 6 0.65 J/(mol K) for x ¼ 0.2, and 3.75 6 0.78 J/(mol K) for x ¼ 0.3 with DH ¼ 100 kOe. Magnitude of DSMmax increases with increasing the magnetic field. These values (of jDSMmaxj) are large compare to other reported rare earth manganites.14–17 The magnetic materials with a second order 2 3 transition generally obey the relation, DSmax M ¼ kMS ð0Þh Sð0; 0Þ, where h is the reduced field just around TC, h ¼ l0lBH/kBTc, k is a constant, MS(0) is the saturation magnetization at low temperatures, and S(0, 0) is the reference parameter, which may not be equal to zero.18 Fig. 3(d) shows the linear dependence of DSMmax versus h2//3 which further confirms the second order transition observed in Arrot plots, for Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3). The phenomenological universal curve for second order transitions, as suggested by Franco and Conde,19 converges DSM versus temperature curves to a single universal curve if (DSM/DSMmax) versus a reduced number (h) is plotted with rescaling the temperature axis below and above TC, as defined in the following equation:

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FIG. 2. Field dependence of isothermal magnetization at various temperatures for Yb1xHoxMnO3 (x ¼ 0.1 (a), 0.2 (b) and 0.3 (c)) samples and (d) the Arrott plot (H/M vs M2) at T ¼ 2.5 K for the samples.

h¼ h¼

T  TC ; Tr1  TC

T  TC ; Tr2  TC

T  TC; T > TC ;

(2a) (2b)

where Tr1 and Tr2 are the temperatures of the two reference points that, for the present study, have been selected as those corresponding to DSM (Tr1,2) ¼ 1/2 DSMmax.20 Figure 3(e) shows the dependence of DS* ¼ (DSM/DSMmax) for typical

field changes for Yb0.7Ho0.3MnO3. It can be clearly seen that the experimental points of the samples distribute on one universal curve of the magnetic entropy change (ranging from 50 to 100 kOe), demonstrating the predictions of universal curve behavior for different magnetic fields of the same sample. The universal curve can be well fitted by a Lorentz function20 DS ¼

a b þ ðh  c Þ

2

;

(3)

FIG. 3. (a)–(c) Temperature variation of magnetic entropy change for different field change for Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) samples. Inset shows the relative errors calculated using (2.4b) of Ref. 13. (d) Temperature dependence of magnetic entropy change jDSMmaxj vs h2/3. (e) The universal curve behavior of the normalized entropy change curves as a function of the rescaled temperature for different magnetic field for Yb0.7Ho0.3MnO3. (f) Relative cooling power (RCP) as a function of field for Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3) samples.

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Appl. Phys. Lett. 107, 262904 (2015)

where a, b, and c are the free parameters a, ¼ 1.28, b ¼ 1.25, and c ¼ 0.13. According to Eq. (3), only the position and magnitude of the peak, namely, TC, DSMmax, and two reference temperatures Tr1 and Tr2, are needed to characterize the entropy change, where Tr1 < TC and Tr2 > TC. That is to say, to translate S into the “real” DSM(T), one need only these values that are determined by the properties of the materials. Thus, asymmetric DSM(T) curves, which are experimentally determined from a small temperature span in the vicinity of TC for the isothermal magnetization measurements, can be easily transformed into almost symmetrical universal curves, which is a helpful tool for the evaluation of material properties. For practical applications, both high value of DSM and the temperature range over which it remains large are important. The characteristic parameter that determines the magnetic cooling efficiency of a magnetocaloric material is the RCP and is defined as RCP ¼ jDSmax mag j  jdTFWHM j:

(4)

It is a measure of the quantity of heat transferred by the magnetic refrigerant between hot and cold sinks. Calculated values of RCP of Yb1xHoxMnO3 are 53.43 6 14 J/mol, 67.52 6 21 J/mol, and 90.0 6 27 J/mol with DH ¼ 100 kOe for x ¼ 0.1, 0.2, and 0.3, respectively. Furthermore, the values of the RCP increase with increasing of the Ho content. These values are higher than single crystal YbMnO3 (RCP ¼ 26 J/mol with DH ¼ 80 kOe)15 and other rare earth manganites except for Ho/DyMnO3.16 The RCP values exhibit a linear increase with increasing field for all compounds as shown in Fig. 3(f). The high RCP values for the present compounds indicate that they can be considered as potential candidates at low temperature applications for magnetic refrigeration. The introduction of Ho3þ (10.6 lB) at the Yb3þ (4.53lB) resulted increased magnetization (Figures 2(a)–2(c)), increased magnetic entropy change, and consequently enhanced the RCP values. Thus the higher moment of the doped Ho ion is responsible for the enhancement of the DSM and RCP in the samples. This argument is supported by the fact that with non-magnetic Sc substitution at the Yb site, the jDSMj and RCP values decreased.11 The DSM, RCP values at 100 kOe versus effective moment (theoretically calculated: leff ¼ ((1  x)l2Yb þ l2Mn)1/2 for Sc doping and leff ¼ ((1  x)l2Yb3þ þ xl2Ho3þ þ l2Mn3þ)1/2 for Ho doping) and DSM, RCP values versus the maximum magnetization M (at 2.5 K, 100 kOe) are shown in Figures 4(c)–4(f). The data points are shown in symbols with error bars, and the solid line in the figures is linear fit to the data. There is a clear trend between the DSM, RCP and effective moment of the samples. We have reported detailed investigations of magnetic and magnetocaloric properties of Yb1xHoxMnO3 (x ¼ 0.1, 0.2, and 0.3). We found that around 10 K, RCP of Yb1xHoxMnO3 are 53.43 6 14 J/mol, 67.52 6 21 J/mol, and 90.0 6 27 J/mol with DH ¼ 100 kOe for x ¼ 0.1, 0.2, and 0.3, respectively. Thus, Yb1xHoxMnO3 seems to be potential materials for magnetic refrigeration in the low temperature

FIG. 4. (a) and (b), jDSMmaxj and RCP at 100 kOe vs magnetization at 2.5 K, 100 kOe. (c) and (d), jDSMmaxj and RCP at 100 kOe vs effective moment for the Yb1xHoxMnO3 and Yb1xScxMnO3.11

region. The normalized entropy change vs the rescaled temperature for Yb0.7Ho0.3MnO3 follows a universal curve behavior with a Lorentz function, and the universal curve is a helpful tool for the evaluation of properties such as the refrigerant capacity RCP. This work has been supported by UGC-DAE Consortium for Scientific Research, Mumbai Centre, India in the form of a collaborative research scheme (CRS) through Project No. CRS-M-199. BSB acknowledges UGC-DAE CSR, Mumbai Centre for project fellowship. A.K.B. is thankful to the National Academy of Sciences, India for their support to this work through Senior Scientist Platinum Jubilee Fellowship Scheme. We also thank to A. Banerjee of UGC-DAE-CSR, Indore, India, for providing facilities for magnetic (M-T) measurements. 1

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