giant low field magnetocaloric effect in single crystalline euti0 85nb0 15o3

Giant low-field magnetocaloric effect in single-crystalline EuTi0.85Nb0.15O3 S Roy, N Khan, and P Mandal Citation: APL Mater 4, 026102 (2016); doi: 10.1063/1.4940960 View online: http://dx.doi.org/10.1063/1.4940960 View Table of Contents: http://aip.scitation.org/toc/apm/4/2 Published by the American Institute of Physics APL MATERIALS 4, 026102 (2016) Giant low-field magnetocaloric effect in single-crystalline EuTi0.85Nb0.15O3 S Roy, N Khan, and P Mandala Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700 064, India (Received 29 October 2015; accepted 18 January 2016; published online February 2016) The magnetocaloric effect in ferromagnetic single crystal EuTi0.85Nb0.15O3 has been investigated using magnetization and heat capacity measurements EuTi0.85Nb0.15O3 undergoes a continuous ferromagnetic phase transition at TC = 9.5 K due to the long range ordering of magnetic moments of Eu2+ (4 f 7) With the application of magnetic field, the spin entropy is strongly suppressed and a giant magnetic entropy change is observed near TC The values of entropy change ∆Sm and adiabatic temperature change ∆Tad are as high as 51.3 J kg−1 K−1 and 22 K, respectively, for a field change of 0–9 T The corresponding magnetic heating/cooling capacity is 700 J kg−1 This compound also shows large magnetocaloric effect even at low magnetic fields In particular, the values of ∆Sm reach 14.7 and 23.8 J kg−1 K−1 for field changes of 0–1 T and 0–2 T, respectively The low-field giant magnetocaloric effect, together with the absence of thermal and field hysteresis makes EuTi0.85Nb0.15O3 a very promising candidate for low temperature magnetic refrigeration C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4940960] In the last four decades, scientists and engineers are engaged in exploring environmental friendly, highly efficient, and low-cost technologies Refrigeration based on the magnetocaloric effect (MCE) of solid materials is one of the remarkable discoveries in the field of refrigeration technology because it does not use the harmful chlorofluorocarbon gas.1–4 Although this technique was discovered about a century ago, its use is limited mainly to achieve ultralow temperature which in turn allows to study low temperature physical phenomena Extensive research is going on to extend the temperature range of refrigeration from room temperature down to very low temperatures The adiabatic temperature change (∆Tad) and isothermal magnetic entropy change (∆Sm) under varying magnetic field are the important parameters which determine the efficiency of magnetic refrigeration The main challenge in this field is to search for new materials, which exhibit large ∆Tad and ∆Sm close to room temperature for domestic and industrial purposes However, the low temperature refrigeration is important for basic research and some specific technological applications such as space science and liquefaction of hydrogen in fuel industry.1,5 Whether in the vicinity of room temperature or at low temperature, the large MCE of magnetic material is the key to cooling capacity Further, it is desirable to search for magnetic materials that exhibit large ∆Sm and ∆Tad under moderate magnetic fields (∆H ≤ T) without field and thermal hysteresis The perovskite titanate RTiO3 (R is the rare-earth element or Y) has attracted interest because of the rich orbital physics.6–9 In RTiO3, R is usually trivalent and Ti is also trivalent with 3d electron configuration These compounds exhibit orthorhombic distortion due to the relatively large Ti3+ ion and the small R3+ ion Among the titanates, EuTiO3 is somehow exceptional In EuTiO3, Ti is tetravalent (3d 0) and Eu is divalent with large spin moment (S = 7/2) due to the stable f electronic configuration EuTiO3 has attracted much attention in recent years due to the observations of large magnetodielectric effects in single crystals10 below the antiferromagnetic (AFM) transition a Electronic mail: prabhat.mandal@saha.ac.in 2166-532X/2016/4(2)/026102/7 4, 026102-1 © Author(s) 2016 026102-2 Roy, Khan, and Mandal APL Mater 4, 026102 (2016) TN = 5.5 K and strain induced ferromagnetism and ferroelectricity in thin films.11 Efforts have also been made to transform this compound from AFM to ferromagnetism (FM) using substitution It has been shown that a small amount of Nb substitution at the Ti site drives the system into a FM metallic state.12–14 However, the magnetocaloric effect and the detailed magnetic properties of Nb-doped EuTiO3 have not been studied so far In this regard, we have studied the field and temperature dependence of magnetization and heat capacity of an EuTi0.85Nb0.15O3 single crystal and calculated several magnetocaloric parameters in order to test whether this material is suitable for low temperature magnetic refrigeration Polycrystalline EuTi1−x Nb x O3 samples were prepared by the solid state reaction method Stoichiometric mixtures of Eu2O3 (pre-heated), Nb2O5, and TiO2 were heated in reduced atmosphere (5% H2 and 95% Argon) at 1000-1100 ◦C with intermediate grindings The obtained powder was then pressed into feed and seed rods and sintered at 1100 ◦C in the same environment The single crystals were grown in reduced atmosphere by travelling solvent floating zone technique using a four-mirror image furnace (Crystal Systems) The typical growth rate was mm/h The x-ray diffraction pattern of powdered single crystals reveals that these materials are of single phase The temperature and field dependence of the dc magnetization and heat capacity measurements were carried out in a physical property measurement system (Quantum Design) The heat capacity was measured using thermal relaxation technique The thermal variation of the zero-field-cooled dc susceptibility χ (=M/H) and the inverse susceptibility χ−1 for the EuTi0.85Nb0.15O3 single crystal are shown in Fig With decreasing temperature, χ increases down to the FM transition point TC = 9.5 K Below TC, χ exhibits a saturation-like behavior similar to that reported earlier.13,14 The FM transition temperature was determined from the position of the minimum of the dM/dT versus T curve In EuTiO3, the Eu spin orders antiferromagnetically around 5.5 K However, the Curie-Weiss fitting of the susceptibility, χ = C/(T − θ CW), reveals a positive value for the Curie-Weiss temperature θ CW.10 This indicates that the strength of AFM and FM interactions are comparable, i.e., a subtle balance between two competitive interactions Using the mean field approximation of the Heisenberg model, the positive value of θ CW (=3.17 K) and the antiferromagnetic transition of EuTiO3 can be reproduced assuming that the absolute value of nearest neighbour antiferromagnetic interaction is smaller than the next-nearest neighbour ferromagnetic interaction.10 When a small amount of Nb is doped at the Ti site, the AFM ordering collapses and the system becomes an itinerant FM.12–14 The FM ordering of the localized Eu2+ spin is mediated by the itinerant electrons of the dopant Nb4+ (4d 1) It is evident from the figure that χ(T) obeys the Curie-Weiss law over a wide temperature range above TC From the high temperature linear fit, we have calculated θ CW = K and the effective moment, Peff = 7.9 µ B Though θ CW is positive for both EuTi0.85Nb0.15O3 and EuTiO3, θ CW is more than two FIG Temperature dependence of the zero-field-cooled susceptibility and the inverse susceptibility at kOe for EuTi0.85Nb0.15O3 The solid line is the Curie-Weiss fit 026102-3 Roy, Khan, and Mandal APL Mater 4, 026102 (2016) FIG Isothermal magnetization curves for EuTi0.85Nb0.15O3 Inset shows the five-segment magnetization curve at K times larger for the former than the later one The experimental value of Peff is close to the expected moment corresponding to f spin configuration of Eu2+ We have measured the field dependence of magnetization for EuTi1−x Nb x O3 at different temperatures from to 60 K Some representative plots of M(H) are shown in Fig up to T magnetic field Below TC, the magnetization increases sharply in the low-field region and the M(H) curve changes its slope above a threshold value of field and tends to saturate at high fields At K and T, the value of M is found to be 7.3 µ B per formula unit This value of the saturation moment is slightly larger than the expected spin only moment of Eu2+, indicating an additional contribution from the itinerant 4d electron of Nb4+ We would like to mention that Li et al also observed similar value of the saturation moment.13,14 The inset of the figure shows a five segment magnetization curve at K without any visible hysteresis loop To explore the nature of the magnetic transition, we have transformed the M(H) curves into the well known Belov-Arrott plots and observed a positive slope for these M versus H/M (not shown) curves which indicates that the FM transition is continuous in nature The strong H and T dependences of M suggest that EuTi1−x Nb x O3 may exhibit large entropy change with the application of magnetic field The entropy change can be calculated from the H Gibb’s free energy and is given by ∆S(H,T) = ∂M ∂T H dH However, to implement a numerical calculation this integral equation for the entropy change can be written in a summation form ∆Sm =  Mi+1 − Mi ∆Hi , Ti+1 − Ti i (1) where Mi and Mi+1 are the experimentally measured magnetizations at temperatures Ti and Ti+1, respectively, for a small magnetic field change ∆Hi The temperature dependence of the magnetic entropy change for some selected magnetic fields between and T is shown in Fig as representative The nature of ∆Sm(T) curve is typical of a FM system For a given field change, ∆Sm decreases on the both sides of TC ∆Sm is negative over the whole range of T and its value increases monotonically with increasing field The maximum of ∆Sm is 51.3 J kg−1 K−1 for a field change of T However, the peak position is almost the same for all the curves for different magnetic fields The magnetic heating/cooling capacity (MC) which determines the amount of heat exchange between the cold and hot reservoirs in an ideal heating/cooling cycle depends on the height and T width of the peak of the ∆Sm(T) curve and is defined as MC = T12 ∆Sm dT, where T1 and T2 are the 026102-4 Roy, Khan, and Mandal APL Mater 4, 026102 (2016) FIG Temperature dependence of the magnetic entropy change (−∆Sm) for EuTi0.85Nb0.15O3 Inset shows the field variation of magnetic heating/cooling capacity (MC) temperatures corresponding to both sides of the half-maximum of ∆Sm(T) peak The inset shows the magnetic field variation of the magnetic heating/cooling capacity MC increases almost linearly with H and reaches 700 J kg−1 for a field change of T The observed value of ∆Sm for the present system is comparable with that for the AFM EuTiO3 and Eu1−xBaxTiO3 systems.15,16 In these compounds also the position of maximum in ∆Sm(T) curve is field independent and it occurs at TN However, the value of ∆Sm for EuTi0.85Nb0.15O3 is significantly larger than that reported for several other rare-earth titanates17 and Eu-based compounds including EuO and Eu3O4.18–21 We would like to mention that the reported values of ∆Sm for several multiferroic compounds are also smaller as compared to that for the present compound.22–26 The effect of the magnetic field on the heat capacity has also been studied The temperature dependence of the specific heat capacity (Cp ) at different magnetic fields is shown in Fig 4(a) Cp at zero field decreases with decreasing temperature down to 12 K and then increases sharply and displays a λ-like peak at TC The λ-like peak at TC indicates that the phase transition is continuous in nature It can be seen from the plots that the nature of the specific heat curve is strongly influenced by the magnetic field The peak broadens with increasing field strength For a better understanding of the nature of the magnetic ground state, the magnetic contribution to the specific heat (Cm) in the vicinity of FM transition and beyond has been estimated The zero-field Cp(T) curve was fitted using a combined Debye and Einstein model in the temperature range of 30–220 K in order to calculate the lattice heat capacity The obtained lattice part was then subtracted from the total heat capacity to determine Cm and hence the entropy (Sm) associated with the FM ordering at 9.5 K Sm is obtained by integrating (Cm/T)dT At zero field, the estimated saturation value of Sm is 17.3 J mol−1 K−1 which is very close to that expected for the Eu2+ (S = 7/2) It may be further noted that more than 95% of the magnetic entropy is released just below the FM transition This suggests that a major fraction of f spins is taking part in the magnetic ordering However, the magnetic entropy shifts rapidly towards the higher temperature side with the application of magnetic field To check the consistency in our results on magnetic entropy change as estimated from the M(H) data with that estimated from the heat capacity data, the magnetic entropy change has also been calculated from the field dependence of the heat capacity using the relation  T [Cp (H2, T) − Cp (H1, T)] dT, (2) ∆Sm = T where Cp (H, T) is the specific heat at a field H ∆Sm as calculated from Equation (2) for different applied fields is shown in Fig 4(b) as a function of temperature It is clear from the plot that the values of ∆Sm calculated from magnetization and heat capacity data are close to each other The inset of Fig 4(b) shows the temperature dependence of thermal conductivity at and T for 026102-5 Roy, Khan, and Mandal APL Mater 4, 026102 (2016) FIG (a) Temperature dependence of the total specific heat (C p) for EuTi0.85Nb0.15O3 at different magnetic fields and the combined Debye plus Einstein fit to the zero-field specific heat data (solid line) Inset: thermal variation of zero-field magnetic entropy (Sm) (b) Temperature dependence of magnetic entropy change (−∆Sm) from heat capacity measurement and the inset shows the temperature dependence of thermal conductivity at and T magnetic fields (c) Adiabatic temperature change (∆Tad) calculated from the zero field heat capacity and magnetization data (d) Temperature dependence of total entropy, S(H, T ), at different magnetic fields obtained from heat capacity data ic → fc and ih → fh represent cooling and heating effects in adiabatic demagnetization and adiabatic magnetization at the transition temperature Tc , respectively the present compound In the magnetic refrigeration, besides large isothermal entropy change the cycle frequency is also important.27 The latter involves a fast heat exchange, i.e., the material should possess a high thermal diffusivity, which is the ratio of thermal conductivity to the thermal capacity per unit volume From the zero-field specific heat and thermal conductivity data for the present sample and using the density14 of 6.758 × 103 kg m−3, we obtain thermal diffusivity in the range of × 10−5 to × 10−6 m2 s−1 below the transition temperature which is significantly larger than that observed for well known perovskite La0.67Ca0.33MnO2.95.28 026102-6 Roy, Khan, and Mandal APL Mater 4, 026102 (2016) TABLE I Comparison of magnetocaloric parameters of EuTi0.85Nb0.15O3 estimated from magnetization and heat capacity measurements Magnetic field change kg−1 K−1) −∆S m (M ) (J −∆S m (C P ) (J kg−1 K−1) ∆Tad (M ) (K) ∆Tad (C P ) (K) 0–2 T 0–5 T 0–7 T 23.8 21.0 9.8 8.6 39.6 36.3 16.5 15.5 46.2 43.7 19.5 19.2 Another very important parameter related to the magnetocaloric effect is the adiabatic temperature change (∆Tad), which can be calculated from the field-dependent magnetization and zero-field heat capacity data The total entropy S(0,T) in absence of magnetic field is given by S(0,T) T = Cp (0,T)/T dT and then S(H,T) may be evaluated by subtracting the corresponding ∆Sm (H,T) determined using Equation (1) from this calculated value of S(0,T) The isentropic temperature change between the entropy curves S(0,T) and S(H,T) gives the value of ∆Tad The thermal variation of ∆Tad for different magnetic fields is shown in Fig 4(c) The nature of the ∆Tad(T) curves is quite similar to that of ∆Sm(T) ∆Tad is also quite large, increases monotonically with increasing field and attains a maximum value of 22 K at 9.5 K for a field change of T However, there is a large asymmetry in the heating effect (∆Tad heating) when applying a field adiabatically and the cooling effect when removing the field adiabatically (∆Tad cooling) at this low temperature, where the entropy increases very rapidly in zero-field and much slower in an applied magnetic field So to interpret the results of adiabatic temperature change from a realistic physics point of view, we have shown in Fig 4(d) the actual cooling (ic → fc ) and heating (ih → fh ) effects due to adiabatic demagnetization and adiabatic magnetization, respectively, at 9.5 K in a typical magnetocaloric process These two processes show the difference between cooling and heating effects The very different temperature changes that are achieved in materials with magnetic transitions at very low temperatures are evident from Figure 4(d) Similar to ∆Sm, ∆Tad can also be calculated solely from the heat capacity data We have tabulated the maximum values of both ∆Sm and ∆Tad determined from the field dependence of M and Cp at three different fields Table I shows that these parameters as determined from Equations (1) and (2) using two different techniques are close to each other It is evident from the figures that the magnetocaloric parameters are also quite large for a small magnetic field change Although the magnetocaloric entropy change has been estimated for pure and Ba-doped EuTiO3 using magnetization data, the adiabatic temperature change has not been reported so far for these materials We would also like to mention that 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(2016) Giant low- field magnetocaloric effect in single- crystalline EuTi0. 85Nb0. 15O3 S Roy, N Khan, and P Mandala Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700 064, India (Received... published online February 2016) The magnetocaloric effect in ferromagnetic single crystal EuTi0. 85Nb0. 15O3 has been investigated using magnetization and heat capacity measurements EuTi0. 85Nb0. 15O3 undergoes... for field changes of 0–1 T and 0–2 T, respectively The low- field giant magnetocaloric effect, together with the absence of thermal and field hysteresis makes EuTi0. 85Nb0. 15O3 a very promising