Appearance of Griffiths phase in oxygen deficient La0.4Ca0.6MnO3−δ oxides

Materials Letters 84 (2012) 48–51

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Appearance of Griffiths phase in oxygen deficient La0.4Ca0.6MnO3  d oxides M. Triki a,n, E. Dhahri a, E.K. Hlil b a b

´e, Faculte´ des Sciences de Sfax, Universite´ de Sfax, B. P. 1171, SFAX 3000, TUNISIE Laboratoire de Physique Applique ´el, CNRS-Universite´ J. Fourier, BP 166, 38042 Grenoble, France Institut Ne

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2012 Accepted 10 June 2012 Available online 18 June 2012

The crystal structure and the magnetic properties of polycrystalline La0.4Ca0.6MnO3  d oxides were studied depending on the concentration of the oxygen vacancies. X-ray diffraction reveals that all samples (0r d r 0.2) crystallize in the orthorhombic structure with Pbnm space group. Although vacancies have no effect on the structure of the parent sample, they induce a great variation in the magnetic properties. The parent sample La0.4Ca0.6MnO3 show a paramagnetic behavior at high temperature and a competition between an antiferromagnetic (AFM) and charge ordering (CO) state at low temperature. When we create a deficient of oxygen a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) ordering appears with the temperature decreasing. This fact is explained by the change of Mn3 þ and the Mn4 þ amounts in the samples. The magnetization measurements reveal features consistent with the appearance of Griffiths phase (GP) in the La0.4Ca0.6MnO2.8 sample. & 2012 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Sintering Oxygen deficit Griffiths phase

Introduction Perovskite manganites of the composition Ln1 xAxMnO3 (Ln¼rare earth metal, A¼alkaline earth metal) have attracted wide attention because of their fascinating properties such as colossal magnetoresistance (CMR). This effect was initially explained in terms of the double exchange (DE) mechanism which linked successfully metallicity with ferromagnetism in these materials [1,2], but failed to reproduce the resistivity exhibited in the disordered paramagnetic regime [3]. Recently it has been reported that the origin of CMR is due to the existence of quenched disorder which induces nanoscale spin clusters in the paramagnetic region [4,5]. The essential of these singularities would develop in a temperature region between T Rand o T o T G , where T Rand is the C C disorder dependent FM ordering temperature and TG is the pure transition temperature. In this region, called Griffiths phase there exist local regions of large susceptibility with short range spin order due to the dispersed ferromagnetic spin clusters in the PM matrix [6,7]. The phase competition between metallic FM and CO/AFM phases in the vicinity of phase transition also promotes the occurrence of Griffiths singularity [5]. An examination of the phase diagram of the La1 xCaxMnO3 system reveals electron–hole asymmetry in these materials. Manganese ions play a key role in electrical and magnetic properties of these manganites, by the mechanism of DE and superexchange interactions between Mn4 þ and Mn3 þ ions. These properties can

be enhanced or induced by varying the Mn3 þ /Mn4 þ ratio which can be changed by various methods such as controlling oxygen content [8]. In the present work, we report the structural and magnetic properties of La0.4Ca0.6MnO3  d with different oxygen contents. We show that a ferromagnetic–paramagnetic transition can be induced in the CO La0.4Ca0.6MnO3 oxide by creating an oxygen deficit. Here we report also the appearance of an entire GP in the La0.4Ca0.6MnO2.8 compound. Experimental Polycrystalline La0.4Ca0.6MnO3 sample was prepared by the standard ceramic technology. The detailed preparation procedure is the same as in ref [8]. The La0.4Ca0.6MnO3  d samples were obtained from parental oxide La0.4Ca0.6MnO3 by annealing the samples in evacuated quartz ampoules at 900 1C for 24 h with the use of iron as an oxygen absorber. X-ray powder diffraction (XRD) data were recorded at room temperature with Siemens D5000 diffractometer with CuKa radiation. The magnetization measurements were carried out using a Foner magnetometer in different magnetic fields at Neel institute.

Results and discussion X-ray diffraction analysis

n

Corresponding author. Tel.: þ216 21501866; fax: þ 216 74862432. E-mail address: [email protected] (M. Triki).

0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.036

Room temperature X-ray powder diffraction (XRD) measurements shows clean single phase patterns for all samples.

M. Triki et al. / Materials Letters 84 (2012) 48–51

The analysis of XRD patterns by the Rietveld refinement method has shown that the crystal structure of all synthesized samples is orthorhombic corresponding to the Pbnm space group [9]. We list in Table 1 the cell parameters and the unit cell volume obtained from the structural analysis of the XRD patterns for all samples. The unit cell volume increases slightly for d r 0:15 with the decrease of oxygen content in La0.4Ca0.6MnO3  d and than sharply for d ¼ 0:2 This evolution is explained by two effects: ˚ (i) the reduction of manganese ions from Mn4 þ (r Mn4 þ ¼ 0:530 A) ˚ [10] and to Mn3 þ state with larger ionic radius (r Mn3 þ ¼ 0:645 A) (ii) the distortion of MnO6 octahedron due to the increase of the Mn3 þ ions which presents a Jahn–Teller effect. Magnetic properties We report in Fig. 1 the temperature dependence of magnetization in a magnetic applied field of 0.05 T for La0.4Ca0.6MnO3  d, (0r d r0.2). The parent compound La0.4Ca0.6MnO3 (inset of Fig. 1) is showing a paramagnetic state at high temperature. When the temperature decreases a first transition to charge ordered state appears at TCO 250 K and a second transition to an antiferromagnetic state at TN  145 K which is confirmed by the phase diagram of La1  xCaxMnO3 samples (0rx r1) [11]. The deficit of oxygen in La0.4Ca0.6MnO3 tends to weaken the AFM/CO state and to induce ferromagnetism at low temperature. Thus as shown in Fig. 1 the samples with d ¼ 0.15 and d ¼ 0.2 exhibit a transition from paramagnetic to ferromagnetic state when the temperature decreases at 125 K and 165 K respectively. Therefore, when oxygen content is reduced, the amount of Mn4 þ ions is expected to decrease and shifts to the CMR side. That means the sample should approach the CMR region by Table 1 Lattice parameters and unit cell volume for La0.4Ca0.6MnO3  d (0r d r 0.2). 0

d ˚ a (A) ˚ b (A)

0.05

0.15

0.2

5.3857

5.3923

5.4072

5.4096

5.4294

5.3910

5.3956

5.4077

5.4097

5.4639

7.6480

˚ c (A) V (A˚ 3)

0.1

222.05

7.6428 222.36

7.6424 223.47

7.6579

7.6695

224.10

227.52

49

showing magnetic transition in the magnetization versus temperature curve according to the phase diagram. To verify the magnetic behavior for our samples magnetization versus magnetic applied field data at different temperatures were collected and are plotted in Fig. 2 for La0.4Ca0.6MnO3, La0.4Ca0.6MnO2.85 and La0.4Ca0.6MnO2.80. As confirmed by the phase diagram of La1  xCaxMnO3 (0 rxr1), the electron doped La0.4Ca0.6MnO3 sample is paramagnetic at high temperature and show a competition between a CO and an AFM state when the temperature decreases. We notice at low temperatures the coexistence of minor charge disordered FM and major AFM/CO phases. This is confirmed also by the broad shoulder seen around 150– 250 K and also at low temperature in the M(T) curve for this sample. For the oxygen deficit samples the magnetization curves measured at T4100 K show no appreciable hysteresis for both samples d ¼0.15 and 0.20. An apparent hysteresis loop was obtained at 5 K. The magnetization do not saturate even for a magnetic applied field of 6 T. The samples present a magnetization of 1 mB/fu, much lower than the magnetization of no less than  3.4 mB/fu supposed by the composition, if we consider a full alignment of manganese spins. This suggests that the ferromagnetic clusters involve only a small fraction of the Mn spins and correspondingly is rather short ranged [12]. On the other hand the DE between Mn3 þ and Mn4 þ ions around oxygen vacancies should been broken, further decrease of the oxygen content leads to increasing amount of Mn ions with a five-fold coordination and destroying long-range ferromagnetic order [13]. This could be explained also by the persistence of an AFM/CO component at low temperature confirming the stability of this state against a field of 6 T. We further investigate the temperature dependence of the inverse dc susceptibility w  1(T) for La0.4Ca0.6MnO3 d, (0r d r0.2) samples at an applied magnetic field of 500 Oe. The results are plotted in Fig. 3(a, b and c). All samples show for high temperatures typical PM behavior, where the susceptibility data can be well described by the Curie–Weiss law:

w¼

C ðTyw Þ

ð1Þ

where C ¼ N m2ef f =3kB is the molar Curie constant and yW is the PM Curie temperature. The behavior shown for the parent compound confirms the results mentioned below. For oxygen-deficient compounds the

0.25 5K 10K

0.008

200K 240K

300K

0.6 δ=0

0.006

0.5

0.004 0.002

0.15

0

100

200 T(K)

300

0.4

400

M(µB/fu)

M(µB/Formula)

M(µ /formua)

δ=0.2

0.2

100K 150K

0.1 δ=0.15

0.3 0.2

0.05

0.1 δ=0.1 0

δ=0

0 0

50

100

150 T(K)

200

250

300

Fig. 1. Temperature dependence of magnetization for La0.4Ca0.6MnO3 d (0r d r0.2) at 0.05 T. The inset shows a zoom for the parent compound (d ¼ 0).

0

2

4

6 µ0H(T)

8

10

12

Fig. 2. Field dependence of magnetization measured at different temperatures for La0.4Ca0.6MnO3, La0.4Ca0.6MnO2.85 (the left inset) and La0.4Ca0.6MnO2.8 (the right inset).

50

M. Triki et al. / Materials Letters 84 (2012) 48–51

1.5

1

1 1/χ(T.g/emu)

1/χ(T.g/emu)

δ=0.1

δ=0.05 δ=0

0.5

0

0

100

200 T(K)

300

1/χ(T.g/emu)

1/χ(T.g/emu)

δ=0.2

λ=0.61(9)

0.01

δ=0.15

0.5

0

100

200 T(K)

300

0.1 t

1

δ=0.15

600

300 50

400

1.5

100

150 T(K)

200

250

0.08

1

1/χ(T.g/emu)

1/χ(T.g/emu)

0.1

900

1

δ=0.2

0.5

0

=179K λ=0.00063

400

1.5

0

T

0

100

200 T(K)

300

400

0.04

0 100

δ=0.2

150 T(K)

200

Fig. 3. (a), (b) and (c) Temperature dependence of the inverse dc magnetic susceptibility obtained from magnetization measurements at 0.05 T for La0.4Ca0.6MnO3  d; (e) and (f) show the same measure for La0.4Ca0.6MnO2.85 at 2 T and for La0.4Ca0.6MnO2.8 at 0.1 T, respectively; (d) shows the replot of the data for La0.4Ca0.6MnO2.8 on a double logarithmic scale, testing the power law Eq. (1) with reduced temperature t ¼ ðTT Rand Þ=T Rand and yielding estimates for exponent l. C C

curves show a gradual change in shape and for high rates of oxygen vacancies (for the samples d ¼ 0.15 and 0.2) a significant downturn is observed above TC which could be explained by a Griffiths phase response, corresponding to the appearance of FM spin clusters within the PM region below a characteristic temperature TG and the ordering is fully achieved at TC [14]. To verify this behavior for these two samples we plot the inverse dc susceptibility w  1(T) at higher fields. For La0.4Ca0.6 MnO2.85 Fig. 3e shows the persistence of the downturn even with a magnetic field of 2 T, this result is unmatched with the critical behavior of the Griffiths ferromagnets and explains the lessening of the magnetic susceptibility by the antiferromagnetic interaction due to the charge ordering of Mn3 þ and Mn4 þ ions, which favors the antiparallel alignment of neighboring clusters [15]. At the other hand Fig. 3f for the La0.4Ca0.6MnO2.8 compound shows that the downturn is found to get suppressed at a field of only 0.1 T and the w  1(T) fully obeys the conventional Curie–Weiss law in the paramagnetic state. The magnetic field suppression of the anomaly in the inverse magnetic susceptibility is due to polarization of spins outside the clusters or the linear increase

of PM contribution with increase in magnetic field which masks the contribution from FM spin clusters [16]. Hence in the Griffiths phase, the magnetic is dominated by the largest cluster and correlated volume, leading to the following prediction for the inverse susceptibility w1 ðTÞpðTT Rand Þ1l , C ð0 r l r 1Þ (Eq. (1)) [17]. Fig. 3d shows double logarithmic plot of w  1 against reduced temperature t ¼ ðTT Rand Þ=T Rand . This C C graph confirm the power law prediction (Eq. (1)) and the fit is carried with T Rand ¼ 179 K, higher than TC ¼165 K and yielding C l  0:62 (180 K oTo190 K). The value of l deduced here is comparable to that found in a variety of some doped manganites [18]. In the PM region (T4 TG) the corresponding exponent l is very close to zero indicating that the GP does not extend to temperatures higher than TC ¼225 K.

Conclusion We studied the effect of oxygen content on the structural and magnetic properties of La0.4Ca0.6MnO3  d. The decrease of oxygen

M. Triki et al. / Materials Letters 84 (2012) 48–51

content leads to: (i) an increase of the lattice parameters and the unit cell volume, (ii) appearance of ferromagnetic order at low temperatures. This work shows that the ratio of Mn4 þ /Mn3 þ may play a role in determining the properties of La0.4Ca0.6MnO3  d with less oxygen content, so for d ¼0.2 we notice the existence of GP in the temperature range T Rand o T o T G . The correlated C disorder induced by oxygen deficiency and the competition between the AFM and FM interactions between Mn ions are responsible of the Griffiths singularity. References [1] [2] [3] [4]

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