Solid State Communications 167 (2013) 49–53 Contents lists available at SciVerse ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc Ferromagnetic short-range order and magnetocaloric effect in Fe-doped LaMnO3 The-Long Phan a, P.Q Thanh b, P.D.H Yen c, P Zhang a, T.D Thanh a,d, S.C Yu a,n a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea Faculty of Physics, Hanoi University of Science, Vietnam National University, Thanh Xuan, Hanoi, Vietnam c Faculty of Engineering Physics and Nanotechnology, VNU - University of Engineering and Technogoly, Xuan Thuy, Cau Giay, Hanoi, Vietnam d Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam b art ic l e i nf o a b s t r a c t Article history: Received 16 May 2013 Accepted 14 June 2013 by M Grynberg Available online 21 June 2013 We have studied the critical behavior and magnetocaloric effect of a perovskite-type manganite LaMn0.9Fe0.1O3 with a second-order phase transition Detailed critical analyses based on the modified Arrott plot method and the universal scaling law gave the critical parameters TC≈135.7 K, β¼ 0.35870.007, γ¼ 1.328 0.003, and δ ¼4.71 0.06 Comparing to standard models, the critical exponent values determined in our work are close to those expected for the 3D Heisenberg model (with β ¼0.365, γ¼ 1.336, and δ ¼4.80) This reflects an existence of ferromagnetic short-range order in LaMn0.9Fe0.1O3 Around TC, the magnetic entropy change reaches the maximum value (ΔSmax), which is about 3.8 J Á kg−1 Á K−1 for the applied field of 50 kOe Particularly, its magnetic-field dependence obeys the power law |ΔSmax|∝Hn, where n ¼0.63 is close to the value calculated from the relation n ¼1+(β−1)/(β+γ) & 2013 Elsevier Ltd All rights reserved Keywords: A Perovskite manganites D Critical behavior D Magnetocaloric effect Introduction It is known well that LaMnO3 (lanthanum manganite) is an insulating antiferromagnetic (AFM) material with orthorhombic perovskite structure, and has the Neél temperature TN≈140 K [1] Its magnetic properties are generated from super-exchange interactions of Mn3+ (3d4, t 32g e1g ) cations located in an octahedral crystal field formed by six oxygen anions (i.e., MnO6 octahedron) A strong coupling between the electron spins on eg and t2g orbitals causes the Jahn–Teller (JT) distortion of MnO6 octahedra around Mn3+ ions, as confirmed by Elemans et al [2] A small change related to oxygen excess (LaMnO3+s) creates more Mn4+ ions [3] This leads to ferromagnetic (FM) double-exchange interactions of Mn3+–Mn4+ pairs, and AFM Mn4+–Mn4+ ones [4,5] The creation of Mn4+ ions can also be carried out by replacing partly La ions by an alkaline-earth ion A ( ¼ Ca, Ba, and Sr) These A-doped compounds are known as hole-doped perovskite manganites with general formula La1−xAxMnO3 [5], where Mn4+ concentration is modified by varying A-doping content It has been discovered that the FM interaction becomes strongest when the Mn3+/Mn4+ ratio is about 7/3, corresponding to La1−xAxMnO3 compounds with x≈0.3 With such the optimal ratio, colossal magnetoresistance and magnetocaloric effects would be obtained around the n Corresponding author Tel.: +82-43-261-2269; fax: +82-43-275-6416 E-mail address: scyu@chungbuk.ac.kr (S.C Yu) 0038-1098/$ - see front matter & 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.ssc.2013.06.009 FM-paramagnetic (PM) phase transition temperature (TC) [6–10] Physically, these effects are explained by the double-exchange mechanism in addition to dynamic JT distortions generated from strong electron–phonon coupling [4,7,8] The exchange interaction strength of Mn ions thus depends on both the bond length 〈Mn–O〉, and angle 〈Mn–O–Mn〉 of perosvkite manganites [11] Together with the doping into the La site, the modification of the FM phase can also be carried out by substituting a transition metal into the Mn site, known as LaMn1−yMyO3 compounds (M ¼Ni, Fe, Ti, Co, Cr, and so forth) [12–17] This route usually decreases the strength of FM Mn3+–Mn4+ interactions (i.e., TC value decrease) with increasing M-doping concentration [12–14] because its presence changes the bond length and angle of the perosvkite structure, and results in additional contributions of AFM and/or FM interactions related to M ions Depending on the doping level and nature of M, there is the possibility of double exchange between Mn and M ions to enhance the magnetization [13,17] Typically, it was found double-exchange interaction of Mn3+–Fe3+ besides Mn3+–Mn4+ in LaMn1−yFeyO3 [13], which contributes to an enhancement of magnetization for x¼0.1 (i.e., LaMn0.9Fe0.1O3) [12] One can realize that though many previous works focused on LaMn1−yFeyO3 compounds, the FM order related to the magnetic mixed-valence state of Mn and Fe ions, and their magnetocaloric effect have not been studied yet To get more insight into this problem, we prepared LaMn0.9Fe0.1O3 (an optimal ferromagnet studied preliminarily in Ref [12]), and then investigated its critical behavior and magnetocaloric effect based on the T.-L Phan et al / Solid State Communications 167 (2013) 49–53 Experiment A perovskite manganite LaMn0.9Fe0.1O3 was prepared by conventional solid-state reaction, used high-purity powders La2O3, MnCO3, and Fe2O3 (99.9%) as precursors These powders combined with stoichiometric quantities were carefully ground and mixed, and then calcinated in air at 1100 1C for 24 h The obtained mixture was reground and pressed into a pellet under a pressure of about 5000 psi by a hydraulic press Finally, it was annealed at 1200 1C for 24 h The single-phase rhombohedral structure (space group R-3c) of the obtained product was confirmed by an X-ray diffractometer (Bruker AXS, D8 Discover) Magnetic measurements versus temperature (in the range of 4.2–350 K) and external magnetic fields ranging from to 50 kOe were carried out on a superconducting quantum interference device (SQUID) magnetometer Results and discussion Fig shows temperature dependences of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations (denoted as MZFC and MFC, respectively) for LaMn0.9Fe0.1O3 under an applied field (H) 100 Oe With increasing temperature above 120 K, one can see clearly a rapid decrease of magnetization associated with the FM–PM phase, where magnetic moments become disordered under the impact of thermal energy If performing the dMFC/ZFC/ dT versus T curve, see Fig 1, its minimum is the Curie temperature (TC) of the sample, which is about 137 K Particularly, below TC there is the bifurcation of the MZFC(T) and MFC(T) curves, with opposite variation tendencies as lowering temperature Their deviation is about 6.5 emu/g at K (for H¼100 Oe), and gradually decreases with increasing temperature Such the feature is popular in perovskite manganites [12,13,19], and assigned to the existence of an anisotropic field generated from FM clusters (due to magnetic inhomogeneity) Magnetic moments of Mn ions may be frozen in the directions favored energetically by their local anisotropy field or by an external field Depending on the magnetic homogeneity of a FM sample and on the applied-field magnitude, -0.2 -0.4 ZFC -0.6 40 80 120 160 160 K 45 30 15 0 10 20 30 40 50 60 H (kOe) 160 K dMFC/dT (emu/g.K) M (emu/g) 120 K 60 0.0 137 K FC 75 12 H = 100 Oe 12 the deviation between MFC(T) and MZFC(T) would be different In general, a large deviation is usually observed in FM materials exhibiting a coexistence of FM and anti-FM phases and exhibiting magnetic frustration [19–21] At temperatures above TC, performing the temperature dependence of χ−1( ¼H/M) reveals its variation according to the Curie–Weiss (CW) law of χ(T)∝1/(T−θ), with the CW temperature θ≈140 K, see the inset of Fig For doped manganites, the high-temperature PM region is usually dominated by FM fluctuation generating from dynamic double-exchange interactions [22,23] In other words, no real PM state exists above TC This is ascribed to the reason causing a small difference between TC and θ values To further understand the magnetic nature of LaMn0.9Fe0.1O3, we have studied its critical behavior around TC Fig 2(a) shows the data of isothermal magnetization versus the magnetic field (M–T–H), with T¼120–160 K One can see that M increases nonlinearly with increasing H At a given magnetic field, M gradually decreases with increasing T because of thermal energy Though the phase transition temperature TC is about 137 K, there is no linear feature observed for the M–H curves at T4TC This reflects an existence of FM shortrange order in LaMn0.9Fe0.1O3 More evidence of FM short-range order can be seen clearly from performing the H/M versus M2 curves (inverse Arrott plots [24]) Basically, if a magnetic system possesses FM long-range order (as described by the mean-field theory [25]), the H/M versus M2 curves in the vicinity of TC are parallel straight lines The straight line at the critical point TC goes through the original coordinate However, these criteria are not met in our system, as can be seen in Fig 2(b) Notably, the slopes of the H/M versus M2 curves are positive This proves the system LaMn0.9Fe0.1O3 undergoing a second order magnetic phase transition (SOMT) [26] M (emu/g) data of magnetic-field dependences of magnetization (M–H) Experimental results indicate an existence of FM short-range order in LaMn0.9Fe0.1O3 Around TC, the magnetic-entropy change reaches the maximum value (ΔSmax) of ∼3.8 J Á kg−1 Á K−1 for an applied field of 50 kOe Additionally, its magnetic-field dependence can be described by the power law ΔSmax∝Hn, which was proposed by Oesterreicher and Parker for a material with a second order magnetic phase transition (SOMT) [18] 200 T (K) Fig (Color online) Temperature dependences of ZFC and FC magnetizations for LaMn0.9Fe0.1O3 under an applied field of 100 Oe The inset shows χ−1(T) data fitted to the Curie–Weiss law H/M (x102, Oe.g/emu) 50 ΔT = 2K 120 K 0 1000 2000 M2 3000 4000 5000 (emu/g) Fig (Color online) (a) Magnetic-field dependences of magnetization (M–H), and (b) an inverse performance of Arrott plots (H/M versus M2) for LaMn0.9Fe0.1O3 recorded around TC, where a temperature increment is K T.-L Phan et al / Solid State Communications 167 (2013) 49–53 51 According to the mean-field theory approximation for a ferromagnet with the SOMT, the M–T–H relation obeys the scaling equation of state [25,27] H=Mị1= ẳ a þ bM 1=β ; ð1Þ where a and b are constants, and ε¼(T−TC)/TC is the reduced temperature The critical exponents β and γ are associated with the spontaneous magnetization (Ms) and inverse initial susceptibility (χ0–1), respectively As described above, for a magnetic system with true FM long-range order, the performance of (H/M)1/γ versus M1/β curves with β¼ 0.5 and γ¼1.0 [25] leads to their linear property However, the absence of such the feature demonstrates that β and γ values characteristic of our system LaMn0.9Fe0.1O3 are different from those expected for the mean-field theory (MFT) To determined their values and TC, one usually bases on modified Arrott plots [27], and the asymptotic relations [25] o0 ; Tị ẳ h0 =M ị ; M ẳ DH 1= ; 0; ẳ 0; 2ị 3ị 4ị where M0, h0, and D are critical amplitudes, and δ is associated with the critical isotherm With the correct values of β and γ, the M–H data around TC fall into a set of parallel straight lines in the performance of M1/β versus (H/M)1/γ The method content can be briefed as follows: starting from trial critical values (for example: β¼0.34 and γ¼1.29), Ms(T) and χ0(T) data are obtained from the linear extrapolation for the isotherms at high fields to the co-ordinate axes of M1/β and (1/χ0)1/γ ¼(H/M)1/γ, respectively These Ms(T) and χ0(T) data are then fitted to Eqs (2) and (3), respectively, to achieve better β, γ, and TC values The new values of β, γ, and TC obtained are continuously used for the next modified Arrott plots until they converge to stable values In Fig 3(a), it shows Ms(T) and χ0(T) data fitted to Eqs (2) and (3), respectively, and the critical parameters obtained from the final step of modified Arrott plots, where the critical exponents are β¼ 0.35870.007 and γ¼ 1.3287 0.003 The TC values obtained from extrapolating the FM and PM regions are 135.870.1 and 135.570.2 K, respectively Their average value is thus about 135.7 K In general, TC obtained from the M–H data is smaller than that determined from the M–T data This deviation will be small if the magnetic system is true FM longrange order With the obtained values of β and γ, M1/β versus (H/M)1/γ curves at high fields around TC are linear, as can be seen clearly in Fig 3(b) For δ, it can be obtained from fitting the critical isotherm to Eq (4) At the temperature T¼136 K (≈TC), δ is about 4.25, which is the value δ¼4.7170.06 calculated from the Widom scaling relation δ¼1+γ/β [25] Assessing the reliability of these critical values can be based on the static-scaling hypothesis, which predicts that the M–T–H behavior obey the universal scaling law [25] MðH; ị ẳ jj f H=jjỵ ị; 5ị where f+ and f− are regular functions for T4TC and ToTC, respectively The equation hints that plotting M/|ε|β versus H/|ε|β+γ makes all data points falling into two universal branches characteristic of temperatures ToTC and T4TC Clearly, such the conditions are fully met for the M–T–H data of our magnetic system LaMn0.9Fe0.1O3, see Fig This proves the reliability of the values β, γ, δ, and TC determined from modified Arrott plots If comparing these critical exponents to theoretical models [25], one can see that their values are close to those expected for the Heisenberg universality class relevant for conventional isotropic magnets (with β¼ 0.365, γ¼1.336, and δ¼ 4.80) This reflects an existence of FM short-range order in LaMn0.9Fe0.1O3, where FM interactions persist at temperatures above TC As proved by Tong and co-workers [13], there are magnetic Fig (Color online) (a) Ms(T) and χ0−1(T) data fitted to Eqs (2) and (3), respectively (b) Modified Arrott plots of M1/β versus (H/M)1/γ with TC ¼135.7 K, β¼ 0.358 and γ ¼ 1.328 300 250 M/| | (emu/g) M s T ị ẳ M ð−εÞβ ; 200 150 100 50 0.0 2.0x107 4.0x107 H/| | 6.0x107 (Oe) Fig (Color online) Scaling performance of M/|ε|β versus H/|ε|β+γ shows two universal curves for temperatures T oTC and T TC The inset shows the same scaling performance in the log–log scale inhomogeneous regions (FM clusters) in LaMn1−xFexO3 due to interaction series …Mn3+–O–Fe3+–O–Mn4+…, …Mn3+–O–Fe3+–O–Mn3+…, …Mn4+–O–Fe3+–O–Mn4+…, etc It means that the mixed valence of Mn and Fe ions promotes both FM double-exchange and AFM superexchange interactions The FM interaction is favored to exist for Fe3+– O–Mn3+ and Mn3+–O–Mn4+ [13], while the other interaction pairs are assigned to be AFM The coexistence of such the FM and AFM regions leads to FM short-range order in LaMn0.9Fe0.1O3 Recently, Yang et al [14] also observed FM short-range order in LaMn1−xTixO3 compounds, with 0.359≤β≤0.378, 1.24≤γ≤1.29 and 4.11≤δ≤4.21, depending on 52 T.-L Phan et al / Solid State Communications 167 (2013) 49–53 4 Smax H n (at T , with n = 0.63) C 50 kOe 40 kOe 30 kOe Smax (J.kg-1.K-1) Sm (J.kg-1.K-1) 20 kOe 10 kOe 120 135 150 10 165 20 30 40 50 H (kOe) T (K) Fig (Color online) (a) Temperature dependences of −ΔSm with magnetic-field intervals of 10–50 kOe (b) The magnetic-field dependence of −ΔSmax at T ¼ TC fitted to the power law, Eq (7), with n¼ 0.63 Ti-doping content For LaMnO3.14 [3], however, the FM phase is mainly due to Mn3+–O–Mn4+ It has been found the critical exponent values β¼ 0.415, γ¼ 1.470, and δ¼4.542 close to those expected for the MFT The above results prove that the critical property is sensitive to impurities and defects These important factors can be employed in rounding the magnetic phase transition of manganites [28,29] In the critical region, the magnetocaloric (MC) effect of LaMn0.9Fe0.1O3 can be assessed upon the magnetic entropy change (ΔSm) For a FM material undergoing the SOMT, the ΔSm in a magnetic-field interval of 0−H is determined from Maxwell's relation [10] Z H ∂M Sm T; H ị ẳ dH: 6ị T H Fig 5(a) shows temperature dependences of −ΔSm with various magnetic fields from 10 to 50 kOe At a given temperature, −ΔSm increases with increasing the applied field Particularly, the −ΔSm(T) curves exhibit the maxima (denoted as −ΔSmax) in the vicinity of TC Under the applied field H ¼50 kOe, the −ΔSm(T) curve has |ΔSmax| and the full-width-at-half maximum (δTFWHM) of about 40 K and 3.8 J Á kg−1 Á K−1, respectively If using this material for magnetic refrigeration application, its relative cooling power defined by RCP ¼|ΔSmax| Â δTFWHM is about 152 J/kg, and comparable to some perovskite manganites [10] In Fig 5(b), it shows the field dependence of −ΔSmax at T¼ TC For a material with the SOMT, this dependence obeys the power law jΔSmax j∝H n ; ð7Þ where n¼ 1+(β−1)/(β+γ) is assigned to a parameter characteristic of magnetic ordering [18,30] With β¼0.358 and γ ¼1.328, the calculated value of n is about 0.62, which is close to the value n¼0.63 obtained from fitting the |ΔSmax| data to Eq (7), see Fig 5(b), but different from that expected for the MFT (with n¼2/3 [18]) The deviation in the n value from the mean-field behavior is due to magnetic inhomogeneities This is in good agreement with the results deduced from analyzing the MZFC/FC(T) and M−T−H data, as mentioned above Conclusion The investigation into the critical behavior revealed LaMn0.9Fe0.1O3 exhibiting the SOMT in the vicinity of TC≈136 K Basing on the modified Arrott plots and universal scaling law, we determined the critical exponents β¼ 0.358 70.007, γ¼1.328 70.003, and δ¼4.71 70.06, which are close to those expected for the 3D Heisenberg model This reflects the existence of FM short-range order in LaMn0.9Fe0.1O3 The mixed valence of Mn and Fe ions promotes both the FM double-exchange and AFM super-exchange interactions, and thus leads to inhomogeneous regions in magnetism Due to magnetic inhomogeneities (or FM short-range order), the magnetic-field dependences of |ΔSmax| obey the power law |ΔSmax|∝Hn with n ¼0.63, instead of the law with n ¼2/3 for the mean-field case Under an applied field interval of 50 kOe, we obtained |ΔSmax| ¼3.8 J Á kg−1 Á K−1 and δTFWHM≈40 K, which correspond to the RCP of about 152 J/kg Acknowledgment This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001431) in Korea, and by the VNU Science and Technology Project QG-11-02 in Vietnam References [1] J.A Alonso, M.J Martínez-Lope, M.T Casáis, A Muñoz, Solid State Commun 102 (1997) [2] J.B.A.A Elemans, B Van Laar, K.R Van der Veen, B.O Loopstra, J Solid State Chem (1971) 238 T.-L Phan et al / Solid State Communications 167 (2013) 49–53 [3] R.S Freitas, C Haetinger, P Pureur, J.A Alonso, L Ghivelder, J Magn Magn Mater 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These powders combined with stoichiometric quantities were carefully ground and mixed, and then calcinated in air at 1100 1C for 24 h The obtained mixture was reground and pressed into a pellet under... the deviation between MFC(T) and MZFC(T) would be different In general, a large deviation is usually observed in FM materials exhibiting a coexistence of FM and anti-FM phases and exhibiting magnetic