Existence of Griffiths phase in La0.67Ca0.33Mn0.93Fe0.07O3

Journal of Alloys and Compounds 479 (2009) 879–882

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Existence of Griffiths phase in La0.67 Ca0.33 Mn0.93 Fe0.07 O3 L. Joshi a , V. Dayal a , N. Rama b , S. Keshri a,∗ a b

Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi, Jharkhand 835215, India Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India

a r t i c l e

i n f o

Article history: Received 27 August 2008 Received in revised form 21 January 2009 Accepted 22 January 2009 Available online 13 February 2009 PACS: 75.47.Lx 76.30.−v 75.40−s

a b s t r a c t Temperature variation of electrical resistivity, AC susceptibility and electron spin resonance measurements are reported for La0.67 Ca0.33 Mn0.93 Fe0.07 O3 . The inverse of AC susceptibility shows a downturn with decreasing temperature before Tc indicating the existence of Griffiths phase for the temperature range Tc ≤ T ≤ TG , where TG is the Griffiths temperature. The ESR results show the existence of FM clusters in the PM region above Tc which supports the presence of Griffiths phase. The ESR results also show the existence of inhomogeneous distribution of magnetic phases in the sample. A possible mechanism of the obtained results has been explained. © 2009 Published by Elsevier B.V.

Keywords: Fe-doped CMR sample ESR AC susceptibility Griffiths phase

1. Introduction The discovery of colossal magnetoresistance (CMR) in perovskite manganese oxides A(1−x) Ax MnO3 (where A is a trivalent rare-earth element, e.g. La, Pr, etc., and A is a divalent metal element such as Ca and Sr) has generated a considerable interest because of their various electronic, magnetic and structural properties and potential applications [1,2]. Compound AMnO3 is antiferromagnetic (AFM), and when doped with A ion can be driven into a metallic ferromagnetic state due to conversion of proportional number of Mn3+ to Mn4+ via O2− , the process being known as “double exchange (DE)” mechanism proposed by Zenner [3]. Later study pointed out that DE mechanism alone cannot explain all the physics of these types of compound. The other important factors governing the magnetic and transport behaviour in manganites are Jahn–Teller distortion and electron–phonon coupling [4]. Introducing impurity at Mn site hampers the DE mechanism forcing the change in magnetic properties of the material and induces inhomogeneous magnetic distribution [5]. Doping of magnetic ions such as Fe3+ , Cr3+ and Ga3+ at Mn site causes additional magnetic coupling [6]. Different experimental results [8–10] on La0.67 Ca0.33 MnO3 (LCMO) suggest that the metal–insulator (M–I) transition is a grad-

∗ Corresponding author. Fax: +91 651 2275401. E-mail addresses: s [email protected], [email protected] (S. Keshri). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.01.140

ual process, in which over an extended region around paramagnetic to ferromagnetic (PM–FM) transition temperature (Tc ) there is a competition between the DE mechanism and the electron–phonon coupling. The M–I transition can be thought to occur due to percolation of FM metallic clusters. A percolation like problem on random Ising ferromagnets was considered by Griffiths. According to his model [11], in a percolative transition there is always a finite probability of finding an arbitrary large FM cluster above Tc with randomly distributed spins. Bray and Moore have extended this argument to a system with a disordering distribution and termed the temperature range Tc ≤ T ≤ TG where TG is Griffiths temperature as the Griffiths phase [12,13]. The Griffiths phase is characterized by the formation of FM clusters below TG . These clusters are formed thermodynamically during cooling and nucleated by the intrinsic randomness, which cause inhomogeneous magnetic distribution in the sample. The existence of the Griffiths phase is therefore intrinsic to magnetic systems including perovskite manganites. Salamon and Chun [14] have used Griffiths idea for explaining a similar behaviour observed in A-site disordered CMR systems. Such a system exhibits a sharp downturn in the inverse of high temperature susceptibility before Tc . In this context, we have attempted to analyze the correlation of the transport properties to the existence of Griffiths phase for a CMR having Mn-site disorder. We have reported the transport properties of La0.67 Ca0.33 Mn0.93 Fe0.07 O3 (LCMFO) CMR sample within the context of Griffiths phase.

880

L. Joshi et al. / Journal of Alloys and Compounds 479 (2009) 879–882

The inhomogeneous magnetic distribution can be confirmed by the electron spin resonance (ESR) spectrum which is a powerful microscopic probe to reveal magnetic phase transition and magnetic ordering and has been extensively utilized to investigate the magnetic properties of single crystals and polycrystalline samples of various manganites [15–18]. For example, through transport and ESR studies Zhou et al. [16] have shown that for La0.875 Sr0.125 MnO3 bulk composition there is a coexistence of ferromagnetic metallic (FMM) and ferromagnetic insulating (FMI) phases below Tc , whereas no FMI signal like the bulk is observed in the same nanosized composition. Shames et al. [18] have demonstrated that for bulk LCMO sample the magnetic inhomogeneity enhances in the vicinity of its Curie temperature when compared with nanosized LCMO. It is also reported that the appearance of Griffiths phase regimes can be mapped out by ESR study. For example, Deisenhofer et al. [15] have studied the existence of Griffiths phase for some samples of La1−x Srx MnO3 (0.0 ≤ x ≤ 0.2) series. Fan et al. [7] have also observed the presence of Griffiths phase for Nd0.5 Sr0.45 Mn1−x Gax O3 (0.08 ≤ x ≤ 0.10) samples. Although a good amount of work on transport properties and ESR related investigations of Fe-doped manganites has already been done [for example, [19–22]], but not much investigations on magnetic inhomogeneities of these samples, especially within the framework of Griffiths theory using ESR, has been reported to the best of our knowledge.

Fig. 1. XRD pattern of LCMFO sample. Inset: SEM of the same sample.

2. Experimental Polycrystalline sample of LCMFO was prepared by the conventional solid-state route. The stoichiometric amount of La2 O3 , CaCO3 , (CH3 COO)2 Mn·4H2 O and Fe2 O3 were taken. The powdered samples were first sintered in air at 900 ◦ C for 24 h and then at 1100 ◦ C for 18 h with intermediate grindings. The powder thus obtained was pelletized and annealed at 1250 ◦ C for 72 h with intermediate grindings and repelletization, and finally furnace cooled to room temperature. The apparent porosity was measured by using Archimedes method. The characterization of the sample was done by employing X-ray diffractometer (Rigaku) with Cu K␣ radiation and scanning electron microscope (SEM, make JEOL). Temperature variation of electrical resistivity was measured by standard four-probe method using a closed cycle He-cryostat (Oxford Instruments) down to 10 K. The magnetic phase transition was studied by the temperature variation of in-phase components of AC susceptibility ( ); the measurement was carried out at the field of 1 Oe and frequency 131 Hz using home made susceptometer down to 80 K using liquid N2 bath cryostat. The ESR measurements were performed using a Varian E 112 X-band spectrometer using a gas flow cryostat from 300 to 150 K on powdered sample.

3. Results and discussion The powder X-ray diffraction pattern of polycrystalline LCMFO sample is shown in Fig. 1. All the diffractograms could be indexed to the orthorhombic perovskite structure with Pbnm space group and lattice parameters a = 0.5467 nm, b = 0.5450 nm and c = 0.7701 nm. The surface morphology of the sample was studied using SEM; the corresponding micrograph is shown in the inset of Fig. 1. The grain size of the sample lies in the range ∼3.2–7.3 ␮m. The apparent porosity and mass density of the sample has been found to be 0.1% and 5.31 g/cm3 respectively. The temperature variation of electrical resistivity as shown in Fig. 2(a) shows M–I transition at temperature TMI = 163.0 K. In any CMR sample, TMI arises because of the competition between DE interaction Mn3+ –O–Mn4+ , super exchange antiferromagnetic interactions Mn3+ –O–Mn3+ and Mn4+ –O–Mn4+ , and the electron–phonon coupling (formation of polaron in the paramagnetic state). In LCMFO compositions, Fe enters at Mn site as Fe3+ , giving rise to an antiferromagnetic coupling between Mn and Fe ions that favors the super exchange mechanism [20]. However, the effect of magnetic lattice on hopping conductivity shows weak temperature dependence in paramagnetic regime [24]. Above TMI in the adiabatic regime, the electronic motion is much faster than the ionic motion of lattice. Data in the semiconducting

Fig. 2. (a) Temperature variation of resistivity for LCMFO, solid line indicates the theoretical fit by polaronic conduction model; (b) the real part of AC susceptibility plotted as a function of temperature for the same sample. Inset: ( )−1 vs T on a semilogarithmic scale indicating TG and Tc .

L. Joshi et al. / Journal of Alloys and Compounds 479 (2009) 879–882

881

Fig. 4. Temperature variation line of peak to peak linewidth HPP (T). Inset: Quasilinear behaviour of linewidth plotted on a reduced temperature scale.

Fig. 3. Line shapes of ESR spectra of LCMFO from 150 to 300 K with multiplication factors; inset shows magnified view of ESR pattern at 200 K.

region has been analyzed using adiabatic small polaronic model [23], according to which  can be defined as follows:  = 0 T exp

E  p kB T

,

(1)

where 0 is a constant and Ep is the activation energy which corresponds to the barrier potential that a polaron must surmount in order to hop into next site. For our sample Ep is found to be 0.10 eV. Fig. 2(b) shows the in-phase ( ) component of AC susceptibility with the variation of temperature from room temperature up to 80 K. On decreasing the temperature below Tc susceptibility curve shows a small down turn indicating the presence of FM phase with a possible amount of AFM phase [21]. Inset of Fig. 2(b) shows the dependence of ( )−1 vs T in a semi-logarithmic scale. According to the Griffiths model, a long range order starts at the downturn in ( )−1 vs T plot which is an indicator of onset of Griffiths phase (TG ) and the ordering is fully achieved at Tc [25]. A sharp downturn implying deviation from Curie–Weiss form and a saturation in ( )−1 vs T plot are observed at TG = 200.2 K and Tc = 156.0 K respectively as shown in the same inset. For our sample the Griffiths phase is obtained for the temperature range 156.0 K ≤ T ≤ 200.2 K. It may be mentioned that the polycrystalline LCMO sample undergoes a PM–FM transition at Tc ∼ 279 K and the M–I transition at TMI ∼ 265 K [26]. A comparison between our result and this one implies that Fe doping suppresses Tc and TMI . The decrease in transition temperatures of LCMFO compared to those of parent LCMO may be explained on the basis of weakening of the DE process and an increase in magnetic disorder with the increase in Fe concentration [27]. Fig. 3 shows the line shapes of the ESR spectra with corresponding multiplication factors between 150 and 300 K. In high temperature region ESR spectrum consists of a single line with a Lorentzian derivative (g ∼ 2). However, in low temperature region around 200 K a new line develops at low field side and the spectrum deviates from Lorentzian shape which has been illustrated separately in the inset of Fig. 3. Such deviation is largely seen in manganites and attributed to existence of FM clusters in the PM matrix. The presence of some AFM clusters due to Fe–Mn exchange cannot also be ruled out. The low field line continues to increase in intensity as we move towards Tc . Thus above TG , the parent

line maintains its Lorentzian shape and the sample is in PM phase. The parent line exhibits weak temperature dependence while the low field line shows greater temperature dependence and its signal broadens indicating an increase in magnetic inhomogeneities. Both these two lines below Tc , merge to form a FM resonance line. Below Tc the line broadens out and no analysis could be done to those line shapes. It is interesting to note that the temperature dependence of peak-to-peak linewidth (Fig. 4) exhibits a minimum at Tmin = 1.26Tc . ˛ et al. The value of Tmin is in agreement to that obtained by Zajac [28]. Below Tmin (refer Fig. 3) a new line develops at low field side, indicating the coexistence of FM metallic clusters Mn3+ –Mn4+ coupled via DE interaction between Mn3+ and Mn4+ embedded into a PM matrix. In addition to that, the substitution of magnetic ion Fe3+ possibly causes AFM super exchange interaction between Fe3+ and Mn3+ or Fe3+ and Mn4+ . Similar prediction has been made also through Mössbauer spectroscopy done on Fe-doped LCMO samples [27]. Our results are in agreement with the ESR results of Gutiérrez et al. [29], who have also shown that substitution of Fe at Mnsite gives rise to competing FM and AFM interactions resulting in the suppression of the DE mechanism and lowering of Tc values in LaPb(Mn1−x Fex )O3 (0.1 ≤ x ≤ 0.3) composition. Fig. 4 represents a quasi-linear increase in the ESR linewidth HPP (T) with increase of temperature, which follows a universal behaviour in PM region (above 200 K for this case) as proposed by Causa et al. [30]: HPP (T ) = H∞

C  T

,

(2)

where C/T is the single ion (Curie) susceptibility,  is the measured paramagnetic susceptibility of magnetically coupled clusters and H∞ corresponds to spin only interaction. For our sample, H∞ is found to be 1800 G. Inset of Fig. 4 shows a universal temperature dependence of HPP (T/Tc )/HPP (∞) in a T scale normalized to Tc , where a comparison with the function C/T is shown by solid line. The universal temperature dependence of the ESR linewidth is attributed to the existence of magnetic clusters well above Tc . Fig. 5 displays the temperature dependence of ESR intensity spectra I(T), normalized with respect to room temperature value. As the temperature decreases, ESR intensity rapidly increases towards Tc . Inset of same figure shows calculations of ESR intensity in the one phonon model leads to an activated behaviour of intensity given by

882

L. Joshi et al. / Journal of Alloys and Compounds 479 (2009) 879–882

new line develops at low field side, indicating the coexistence of FM metallic clusters which could be originated because of FM interaction between Mn3+ –Mn4+ coupled via DE interaction between Mn3+ and Mn4+ embedded into a PM matrix. Doping of magnetic Fe3+ ion possibly causes additional magnetic coupling resulting AFM super exchange interaction between Fe3+ and Mn3+ or Fe3+ and Mn4+ . References

Fig. 5. Temperature variation of ESR intensity. Inset: The activated behaviour of ESR intensity in PM regime.

the following equation: I = Io exp

E

Int



kB T

,

(3)

where EInt corresponds to the activation energy to form or dissociate a cluster [31]. For our sample, the value of EInt was found to be 0.140 eV which is greater than Ep (=0.10 eV). The higher value of EInt compared to Ep implies that the polaronic strength (i.e. Jahn–Teller strength) is lower than the DE strength since activation energy calculated from resistivity corresponds to free polarons while the activation energy from ESR intensity corresponds to the formation/disassociation of FM clusters in the PM regime. 4. Conclusions We have studied the temperature dependence of electrical resistivity, AC susceptibility and ESR line parameters for La0.67 Ca0.33 Mn0.93 Fe0.07 O3 compound. The inverse of AC susceptibility shows a downturn at temperature 200.2 K indicating the existence of Griffiths phase for the temperature range 156.0–200.2 K. It is interesting to note that the existence of FM clusters in the PM region above Tc as expected by Griffiths model has been evidenced by ESR results also. The temperature dependence of peak-to-peak linewidth of ESR exhibits a minimum below which a

[1] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxide, World Scientific, Singapore, 1998. [2] Y. Tokura (Ed.), Colossal Magnetoresistance Oxides, Gordon and Breach, Singapore, 2000. [3] C. Zenner, Phys. Rev. 82 (1951) 403. [4] A.J. Millis, Nature (London) 392 (1998) 147. [5] Y. Tomioka, Y. Tokura, Phys. Rev. B 66 (2002) 104416. [6] A. Banerjee, S. Pal, S. Bhattacharya, B.K. Chaudhuri, H.D. Yang, Phys. Rev. B 64 (2001) 104428. [7] J. Fan, L. Pi, Y. He, L. Ling, J. Dai, Y. Zhang, J. Appl. Phys. 101 (2007) 123910. [8] M. Egilmez, K.H. Chow, J. Jung, Appl. Phys. Lett. 92 (2008) 162515. [9] J.Q. Li, S.L. Yuan, Solid State Commun. 129 (2004) 431 (and references therein). [10] P.G. Li, M. Lei, H. Yang, H.L. Tang, Y.F. Guo, W.H. Tang, J. Alloys Compd. 460 (2008) 60. [11] R.B. Griffiths, Phys. Rev. Lett. 23 (1969) 17. [12] A.J. Bray, M.A. Moore, J. Phys. C 15 (1982) L765. [13] A.J. Bray, Phys. Rev. Lett. 59 (1987) 586. [14] M.B. Salamon, S.H. Chun, Phys. Rev. B 68 (2003) 014411. [15] J. Deisenhofer, D. Braak, H.-A. Krug von Nidda, J. Hemberger, R.M. Eremina, V.A. Ivanshin, A.M. Balbashov, G. Jug, A. Loidl, T. Kimura, Y. Tokura, Phys. Rev. Lett. 95 (2005) 257202. [16] S. Zhou, L. Shi, J. Zhao, H. Yang, L. Chen, Solid State Commun. 142 (2007) 634. [17] N. Rama, V. Sankaranarayan, M.S. Ramachandra Rao, J. Alloys Compd. 466 (2008) 12. [18] A.I. Shames, E. Rozenberg, Ya.M. Mukovskii, E. Sominski, A. Gedanken, J. Magn. Magn. Mater. 320 (2008) e8. [19] T.S. Orlova, J.Y. Laval, P. Monod, J.G. Noudem, V.S. Zahvalinskii, V. Svikhnin, Yu P. Stepanov, J. Phys. Condens. Matter 18 (2006) 6729. [20] M. Nadeem, M.J. Akhtar, A.Y. Khan, Solid State Commun. 134 (2005) 431. [21] K. De, S. Majumdar, S. Giri, J. Phys. D 40 (2007) 5810. [22] M.S. Sahasrabudhe, S.I. Patil, S.K. Date, K.P. Adhi, S.D. Kulkarni, P.A. Joy, R.N. Bathe, Solid State Commun. 137 (2006) 595. [23] R. Ang, W.J. Lu, R.L. Zhang, B.C. Zhao, X.B. Zhu, W.H. Song, Y.P. Sun, Phys. Rev. B 72 (2005) 184417. [24] D.C. Worledge, L. Mieville, T.H. Geballe, Phys. Rev. B 57 (1998) 15267. [25] M.B. Salamon, P. Lin, S.H. Chun, Phys. Rev. Lett. 88 (2002) 197203. [26] A. Gaur, G.D. Varma, Solid State Commun. 144 (2007) 138. [27] S.B. Ogale, R. Shreekala, R. Bathe, S.K. Date, S.I. Patil, B. Hannoyer, F. Petit, G. Marest, Phys. Rev. B 57 (1998) 7841. ˛ L. Folcik, A. Kołodziejczyk, H. Drulis, K. Krop, G. Gritzner, J. Magn. Magn. [28] T. Zajac, Mater. 120 (2004) 272. ˇ L. Lezama, T. Rojo, Physica B [29] J. Gutiérrez, V. Siruguri, J.M. Barandiarán, A. Pena, 372 (2006) 173. ˜ [30] M.T. Causa, M. Tovar, A. Caneiro, F. Prado, G. Ibanez, C.A. Ramos, et al., Phys. Rev. B 58 (1998) 6. [31] S.B. Oseroff, M. Torikachvili, J. Singley, S. Ali, S.-W. Cheong, S. Schultz, Phys. Rev. B 53 (1996) 6521.