IEEE Transactions on Magnetics 48(4) 1293(2012)

IEEE TRANSACTIONS ON MAGNETICS, VOL 48, NO 4, APRIL 2012 1293 Effects of the Cu Doping on Critical Behavior of La0 Ca0 MnO3 T L Phan1 , P Q Thanh2 , N H Sinh2 , Y D Zhang1 , and S C Yu1 BK-21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea Department of Physics, Hanoi University of Natural Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam We have studied the influence of the Cu-doping on the critical properties of a polycrystalline sample of La0 Ca0 Mn0 95 Cu0 05 O3 prepared by solid-state reaction Analyses of static magnetization data in the vicinity of the ferromagnetic-paramagnetic phase transi197 K, = 49 03 and = 04 04 tion based on the modified Arrott plot reveal critical parameters of C + ), where = ( These parameters are in good agreement with a magnetic equation of state ( )= ( ) (the reduced temperature), + for for C and C Comparing with theoretical models, the critical exponents in our case are close to those expected for the mean-field theory This reflects that the Cu doping leads to the second-order phase transition in La0 Ca0 Mn0 95 Cu0 05 O3 while the parent compound of La0 Ca0 MnO exhibits the first-order phase transition Such the phenomenon is assigned to changes in structural parameters, Mn3+ Mn4+ ratio, and magnetic interaction mechanisms caused by Cu dopants Index Terms—Critical behavior, magnetic phase transition, perovskite manganites I INTRODUCTION HE discovery of colossal magnetoresistance (CMR) and other interesting magneto-transport properties in ( , hole-doped perovskite manganites of Pr; , Ba, and Sr) around their ferromagnetic-paramagnetic phase transition temperature ( , the Curie temperature) has attracted much interest [1]–[5] Among these, -based materials have fascinated intensive interest [2], [4]–[8], particularly for the compositions with – , because of showing both CMR and giant magnetocaloric effect near room temperature [9] These features are applicable for magnetic sensitive sensors and refrigeration technology The explanation of such the physical phenomena is usually based on double-exchange (DE) interaction model proposed by Zener [10], in which electron-exchange interand ions play an important actions between role The ferromagnetic interaction becomes strongest when is about 7/3 However, theoretical the ratio of calculations carried out by Millis et al [1] indicated that the only DE model was not enough to elucidate the entire physical picture in perovskite manganites An addition of the polaron effect, arising from a strong electron-phonon coupling, known as the Jahn-Teller effect, is necessary The magnetic and ions is thus influenced interaction between by structural parameters of the bond length and the bond angle With this assumption, one can explain qualitatively the CMR and giant magnetocaloric materials phenomena taking place in In an attempt to explain quantitatively, it has been paid attention to the critical exponents in the vicinity of [2], [6]–[8], [11] Various manganite compositions usually have different sets of the critical exponents, which are characteristic of magnetic phase transition types Their values thus depend on struc- T Manuscript received June 27, 2011; revised October 08, 2011 and October 14, 2011; accepted October 27, 2011 Date of current version March 23, 2012 Corresponding author: S C Yu (e-mail: scyu@chungbuk.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2011.2174202 tural parameters and magnetic interactions (i.e., ferromagnetic and antiferromagnetic interactions) present in the investigated system exhibits the firstConcerning our work, order magnetic transition [2], [6], with the critical exponents found to be far from those expected from theoretical models [8] This means that magnetic phase transition in cannot be described by the critical equations, but can be explained in term of a tricritical point However, the Ca substitution by Sr changes the magnetic phase transition from the first-order to the second-order types [2], [4] Having reviewed previous studies, we find a very few works on the critical behavior of perovskite manganites in which impurities are doped [5] To get more insight into the Mn site of into this problem, we plan to investigate the critical behavior of With low doping concentrations, it has been suggested that Cu ions are dominant in an oxidation state of (with an ionic radius of 0.54 ), where they substi(0.53 ) instead of (0.65 ) [13]–[15] tuted for By analyzing isothermal magnetization data in the vicinity of its , we point out that while exhibits the firstorder magnetic transition, the Cu doping results in the secondorder phase transition, where its critical exponents determined are close to those derived from the mean-field theory II EXPERIMENT was prepared by conventional solid-state reaction, used commercial powders of , , and CuO (99.9%) as precursors The powders combined with stoichiometric quantities were well-mixed, for 12 pressed into a pellet, and then pre-sintered at 900 hrs After intermediate grinding processes, the mixture were re-pressed into a pellet and annealed at 1050 for 24 hrs The single phase of the final product in an orthorhombic structure was confirmed by an x-ray diffractometer (Brucker-D5005) Basing on x-ray diffraction data, the lattice constants , , and for obtained are 5.467, 5.527, and 7.762 , respectively Comparing with ( , , and ), the lattice constants of the Cu-doped sample slightly decrease These results are in good agreement with those reported by Wang et al [13] 0018-9464/$31.00 © 2012 IEEE 1294 IEEE TRANSACTIONS ON MAGNETICS, VOL 48, NO 4, APRIL 2012 Fig Temperature dependence of magnetization around T under an applied field of 50 Oe The corresponding dM=dT curve shows a minimum value at Cu O about 200 K marked the phase transition of La Ca Mn Magnetic measurements were performed on a superconducting quantum interference device (SQUID) III RESULTS AND DISCUSSION In Fig 1, it shows the temperature dependence of magne, around for tization, under an applied-field of 50 Oe One can see that magnetization decreases with increasing temperature, revealing the collapse of ferromagnetic order caused by thermal activation , its variation versus temperature energy If taking of exhibits a minimum value at about 200 K, close to the sample, see Fig Comparing with , our sample has a value This is assigned to an additional appearance smaller -related antiferromagnetic interactions besides antiferof romagnetic pairs of and , such as and/or They compete with the Consequently, ferromagnetic DE interaction of value are decreased Such both the ferromagnetic order and the circumstance also happens for Cu-doped and materials [15], [16] Notably, that Wang et al [13] reported on slightly decreased as – , quite different from our result Differences in sample-fabrication conditions could lead to this phenomenon value, and the critical exponents of To obtain exactly the , , and , we have measured isothermal magnetization curves (i.e., curves) in the temperature range of 160–220 K, with an increment of 1.0–2.0 K Typical curves at temperatures ranging from 182 to 210 K, in the vicinity of , are shown in Fig It appears that magnetization increases gradually with increasing the magnetic field, and does not meet a saturation value, although the applied field goes up to 45 kOe The similar behavior was also found in some other manganite materials [6], [17], [18], which is characteristic of magnetic materials without true long-range-ferromagnetic order, as can be seen from values of critical exponents afterwards If performing data, versus , the curves the Arrott plot for the are divided into two characteristic regions with temperatures and , see Fig Around , the curves Cu Fig Isothermal magnetization curves for La Ca Mn temperatures in the range of 182–210 K, with an increment of K O at are nearly linear If paying attention to the curves of versus (not shown) at high fields, we find their slope to be positive This demonstrates the magnetic phase transition in belongs to the second-order type, according to the Banerjee criterion [19] The critical exponents are thus expected to be close to those of the mean-field theory To determine these exponents, the modified Arrott plot has been used [20] It is known that in the critical region the second-order phase transition is described in terms of asymptotic relations: (1) (2) (3) where , , and are critical amplitudes, and is the reduced temperature Basically, the and were determined by plotting values versus Trial critical exponents could be selected and (close to the critical exponents as of the mean-field theory) The linear extrapolation from high and axes fields to the intercepts with the and , respectively Variations gives the values of and values are then fitted to (1) and (2) to of determine new exponents These exponents are re-introduced to the scaling of the modified Arrott plot After repeating several times of the above processes, the exponents converge to their critical values and In Fig 4, it shows the optimal data of for The fitting of the data and while to (1) gives data to (2) gives the fitting of the and The modified Arrott plot of versus corresponding to these and values is shown in values obtained by the two approaches Fig Here, the related to ferromagnetic and paramagnetic phases are in good agreement with each other For later calculations and discuswill be used According to sion, the average of [21], the value the Widom scaling relation of was obtained to be 3.12 0.02 Alternately, if plotting the critical isotherm - at a temperature of 197 K close to , and PHAN et al.: EFFECTS OF THE Cu DOPING ON CRITICAL BEHAVIOR OF Fig The Arrott plot of Fig tion La M, 1295 M H data recorded at temperatures 182–210 K : Fig Modified Arrott plots of = 04 M vs (H=M )  Temperature dependences of the spontaneous magnetizaand the inverse initial magnetic susceptibility for Ca Mn Cu O are fitted to Eqs (1) and (2), respectively fitting it to (3), we would also obtain , which is not far from the value calculated from the Widom scaling relation It should be noticed that the critical exponents in our case are close to those expected for the mean-field theory [21] ( , , and ) This also happened for several manganite materials, such as [22], [23], and [24] A small deviation in value of the exponents between our sample and the theoretical model can be due to an existence of ferromagnetic clusters [16] caused by random distribution of ions in the manganite host lattice With the above critical parameters, our isothermal magnetization data in the vicinity of the phase transition can be described by the magnetic equation of state [21] (4) where for and for are scaling functions versus is shown in Fig It appears The plot of data fall on two branches, one clearly that experimental for and the other for This proves that the Fig Normalized isotherms above and below from modified Arrott plots T , with using = 0:49 and and determined critical parameters ( , , and ) obtained in our work are reliable and in accordance with the scaling hypothesis Having compared with the parent compound of , the Cu doping changes the phase transition from the first-order to the second-order types, similar to the reported by Rưßler et case of al [5] This is associated with the substitution Cu ions for ions in that causes the phenomena: (i) the weakening of the ferromagnetic interaction and ions due to the additional presence between -related anti-ferromagnetic interactions and to a of decrease in the ratio; and (ii) the variation in bond length structural parameters, particularly for the bond angle [13], which influence directly and the ferromagnetic interaction It seems that the the concentration ratio of only affects significantly compositions in an the phase transition of orthorhombic structure [6]–[8] They can exhibit the first-order phase transition (with no critical exponents) [4], [6], [7] or the continuous/pseudo-continuous transition (with exponents 1296 IEEE TRANSACTIONS ON MAGNETICS, VOL 48, NO 4, APRIL 2012 close to those of the Heisenberg model [6] or of tricritical point [7], [8]) However, for the case of compounds, the Sr dopant substitutes for Ca, the variation to the from the first-order transition in is due to the change in crystal second-order one as structure from the orthorhombic to the rhombohedral [2], ratio keeps unchanged Such [4], while the the features are different from some material systems of ( 0.125–0.3), ( 0.15–0.4) [4], ( 0.1–0.3) [22], and ( 0.05–0.2) [25] where the second-order transition is independent of concentration of dopants and the ratio Also, their critical exponents close to and those expected for the Heisenberg model ( ) for a short-range ordered ferromagnet [21] With such the results, one can say that the phase transition in doped materials (i.e., dopants substitute for either Ca or Mn) is very sensitive to changes related to crystal ratio structure, the structural parameters, and the Depending on the nature of dopants and their concentration, the first- or second-order phase transition is introduced This is a typical feature of the system that it can be employed to control its magnetic, electrical, and transport properties for various application directions IV CONCLUSION We have studied the critical behavior of synthesized by conventional solid-state reaction The analyses of isothermal - data by using the modified Arrott plot introduced the critical exponents of and close to those expected for the mean-field universality class It means that the first-order phase transition becomes continuous by Cu substituin tion Here, the substitution ions for ions caused the changes in the structural parameters, and concentraand ions Also, there are additional tion of -related antiferromagnetic interaccontributions of tions All of these factors influence directly ferromagnetic interactions, and thus the magnetic phase transi tion of ACKNOWLEDGMENT This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2011K000777), Korea; and partly supported by NAFOSTED of Vietnam (103.02-2010.38) REFERENCES [1] A J Millis, B I Shraiman, and R Mueller, Phys Rev Lett., vol 77, pp 175–178, 1996 [2] J Mira, J Rivsa, F Rivadulla, C V Vazquez, and M A L Quintela, “Change from first- to second-order magnetic phase transition in La (Ca; Sr) MnO perovskites,” Phys Rev B, vol 60, pp 2998–3001, Aug 1999 [3] P G Radaelli, D E Cox, M Marezio, S.-W Cheong, P E Schiffer, and A P Ramirez, “Simultaneous structural, magnetic, and electronic transitions in La Ca MnO with x = 0:25 and 0:50,” Phys Rev Lett., vol 75, pp 4488–4491, Dec 1995 [4] M H Phan, V Franco, N S Bingham, H Srikanth, N H Hur, and S C Yu, “Tricritical point and critical exponents of Sr 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ions caused the changes in the structural parameters, and concentraand ions Also, there are additional tion of -related antiferromagnetic interaccontributions of tions All of these...1294 IEEE TRANSACTIONS ON MAGNETICS, VOL 48, NO 4, APRIL 2012 Fig Temperature dependence of magnetization around T under an applied field of 50 Oe The corresponding dM=dT curve