Solid State Communications 149 (2009) 1274–1277
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Reentrant spin glass behavior in La0.8 Sr0.2 Mn1−x Tix O3 manganites B. Aslibeiki, P. Kameli ∗ , H. Salamati Department of Physics, Isfahan University of Technology, Isfahan, 84156-83111, Iran
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Article history: Received 15 November 2008 Received in revised form 11 April 2009 Accepted 9 May 2009 by D.D. Sarma Available online 15 May 2009 PACS: 71.30.th 75.47Lx
abstract In this study, the effect of Ti substitution on structural, electrical and magnetic properties of La0.8 Sr0.2 Mn1−x Tix O3 (0 ≤ x ≤ 0.075) are investigated by XRD, electrical resistivity and ac susceptibility measurements. The XRD refinement result indicates that the lattice parameters and volumes increase by an increase of the Ti doping level. The resistivity measurement results show that by increasing the Ti doping level, the metal insulator transition temperatures decrease and system becomes an insulator. The paramagnet–ferromagnet transition temperature, Tc also decreases as the Ti content increases. The reentrant spin glass (RSG) state exists in the x = 0.05 and 0.075 samples. The RSG behavior is mainly ascribed to an increase of disorder in the FM matrix induced by the random Ti impurity substitution. © 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Manganite B. Doping D. Reentrant spin glass E. Ac susceptibility
1. Introduction Perovskite manganite with the formula La1−x Ax MnO3 (A = Sr, Ca, Ba or vacancies) has attracted considerable attention due to the discovery of the phenomenon of colossal magnetoresistance (CMR) and its potential applications [1–3]. By varying the composition x, the compound La1−x Ax MnO3 reveals various electronic, magnetic and structural phase transitions at different temperatures. These phase transitions have been attributed to strong coupling among spin, charge, orbital degree of freedom and lattice vibrations. The fundamental feature of the conductive mechanism and magnetic properties can be explained qualitatively, by the double exchange (DE) mechanism between Mn3+ and Mn4+ neighboring ions together with Jahn–Teller distortion and electron–phonon interactions. The basic properties of mixed valence manganites depend mainly on the relative amount of Mn3+ and Mn4+ ions [4–6]. The A-site substitution influences the magnetic properties and the CMR by tuning the Mn3+ /Mn4+ ratio, changing the Mn–Mn distance, and the Mn–O–Mn bond angle [7]. Similar effects are also obtained by substituting Mn with other transition metal cations and metal ions [8–11]. The effect of Ti substitution on the magnetic and transport behavior of manganites has recently been studied by several authors [12–17]. Some anomalous magnetic behavior
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at low temperatures is observed in these Ti doped samples. This anomalous magnetic behavior is observed in the form of a decrease in the zero field cooled dc magnetization and explained as being due to the glassy-spin configuration or spin frustration [14–17]. The low temperature decrease in magnetization of La0.9 Ca0.1 MnO3 sample is interpreted in terms of a domain wall pinning effect by Joy et al. [18]. Also, recently Mukherjee et al. showed that in the La0.9 Sr0.1 MnO3 manganite, this kind of fall in magnetization or susceptibility is due to the formation of orbital domains [19]. Therefore there are many possibilities for the low temperature phase of Ti doped samples. This paper is intended to obtain a comprehensive experimental characterization of the La0.8 Sr0.2 Mn1−x Tix O3 samples with 0 ≤ x ≤ 0.075, including the novel results of ac susceptibility measurements. 2. Experimental The La0.8 Sr0.2 Mn1−x Tix O3 samples (x = 0, 0.01, 0.025, 0.05 and 0.075) were prepared by a conventional Solid-State Reaction method. High purity powders of La2 O3 , SrCO3 , TiO2 and MnO2 were mixed in stoichiometric proportions, calcined at 1100 ◦ C and 1200 ◦ C for 24 h. The LSMO calcined powders were pelleted at a pressure of 105 N/cm2 and sintered at 1400 ◦ C for 24 h. The resistivity measurements were carried out by the four probe method, using a Leybold Closed Cycle Refrigerator. The ac Susceptibility measurements were performed using a Lake Shore ac Susceptometer Model 7000. X-ray diffraction (XRD) patterns of samples were taken on Philips XPERT X-ray Diffractometer.
B. Aslibeiki et al. / Solid State Communications 149 (2009) 1274–1277
Fig. 1. The XRD patterns for La0.8 Sr0.2 Mn1−x Tix O3 (x = 0, 0.01, 0.025, 0.05 and 0.075) samples. Table 1 Lattice parameters and volume of La0.8 Sr0.2 Mn1−x Tix O3 (x = 0, 0.01, 0.025, 0.05 and 0.075) samples. x
a (Å)
c (Å)
v (Å3 )
0.00 0.01 0.025 0.05 0.075
5.520 5.520 5.530 5.533 5.538
13.300 13.300 13.350 13.350 13.350
351.100 351.100 353.782 354.200 354.840
1275
Fig. 2. Temperature dependence of resistivity for La0.8 Sr0.2 Mn1−x Tix O3 (x = 0, 0.01, 0.025, 0.05 and 0.075) samples. -3
-3
-3
-3
-4
3. Results and discussion Fig. 1 shows the XRD patterns for all samples at room temperature. The results of XRD show that all samples have a rhombohedral crystal structure with no detectable secondary phases. The XRD refinement result indicates that the lattice parameters and volumes increase by the increase of the Ti doping level (see Table 1). There is general agreement that the Ti4+ substitutes Mn4+ [14], since the ionic radii of Ti4+ > Mn4+ (0.605 > 0.53) Å [20], so, the substitution of Ti4+ will increase the bond length, lattice parameters and the volumes. Fig. 2 shows the temperature dependence of resistivity for all samples. The low level doped samples (x = 0, 0.01, 0.025, and 0.05) show a Metal–Insulator (MI) transition. The MI transition temperature Tp , decreases by the increase in Ti concentration. However, for the sample (x = 0.075), Fig. 2 shows that it behaves like an insulator, in the whole measuring temperature range in spite of the appearance of an FM component (see Fig. 3). The resistance increases dramatically, in the high doped samples. This kind of behavior can be explained by the percolation model [21]. It is well known that there is a ferromagnetic DE interaction between Mn3+ –O–Mn4+ ions in the La1−x Ax MnO3 compounds. The doping of Ti4+ (non magnetic) ion in Mn4+ ion site, leads to the suppression of the DE interaction and ferromagnetism. An increase in Ti content leads to the formation of clusters because there is no DE interaction between Ti4+ and Mn4+ ions. So the long-range ordering gradually reduces to short-range ordering in these clusters. This means that the phase separation around the MI transition temperature in La0.8 Sr0.2 Mn1−x Tix O3 samples is in the form of many small isolated FM metallic clusters coexisting with PM insulating phase. In low doped samples, when the temperature is decreased the FM component increases and a percolation channel is formed and the resistivity decreases. For the heavily doped samples, due to the high degree of disorder, the FM clusters
-4
-4
-4
-4
-5
Fig. 3. Temperature dependence of ac susceptibility for La0.8 Sr0.2 Mn1−x Tix O3 (x = 0, 0.01, 0.025, 0.05 and 0.075) samples in an ac field of 5 Oe and frequency of 333 Hz.
are separated and the volume of the FM clusters does not reach the percolation threshold. Therefore the system exhibits a highresistance and an insulating behavior. Fig. 3 shows the real (χ 0 ) and imaginary (χ 00 ) parts of ac susceptibility for La0.8 Sr0.2 Mn1−x Tix O3 samples (x = 0, 0.01, 0.025, 0.05, and 0.075), which were measured in an ac field of 5 Oe and frequency of 333 Hz. It is evident that, all samples show the paramagnetic–ferromagnetic (PM–FM) transition and the transition temperature, Tc , decreases when the Ti content increases. By further
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Fig. 4. Temperature dependence of ac susceptibility for La0.8 Sr0.2 Mn1−x Tix O3 (x = 0.05) sample in an ac fields of 1, 2.5 and 10 Oe, and frequency of 333 Hz.
10-4
2.2x10-4
above Tc . We assume the existence of FM clusters in the PM matrix for an interpretation of resistivity data. In order to clarify the nature of this peak we have measured the χ 00 of sample x = 0.05 at different amplitudes of the ac field. As one can see in Fig. 4 the intensity of this peak increases with the increase of the ac field amplitude. It is well known that the imaginary component of ac susceptibility reflects the ac losses in magnetic systems. Therefore, the variation of this peak with ac field amplitude is related to the variation of cluster size. It seems that by the increase of ac field amplitude the FM cluster size increases and consequently the amount of ac loss increases. To further investigate the spin glass behavior, we have measured the ac susceptibility of x = 0.075 sample at different frequencies. Fig. 5 shows the temperature dependence of the x00 for the x = 0.075 sample at frequencies of 44, 111, 333, 666, and 1000 Hz. The measurements were performed at an ac field of 5 Oe. As one can see, the high temperature peaks are almost frequency independent. However, the low temperature peak shifts towards higher temperatures by the increase of frequency. In order to identify the nature of spin glass freezing, we have used three different interpretations. For isolated non-interacting superparamagnetic particles, the relaxation time has been given by the Neel–Arrhenius model [27],
2.0x10-4
10-4
τ = τ0 exp
1.8x10-4 1.6x10-4
10-4
1.4x10-4
90
10-4
95
100
105
110
115
T(K)
10-5
increasing the Ti doping level, a new peak is created above Tc for sample x = 0.05. Also, an anomalous decrease in the χ 0 of this sample, is evident at low temperatures. There is the same behavior in χ 0 of sample x = 0.075. The differences are that the intensity of new peak decreases, and an anomalous decrease in χ 0 , is developed at the higher temperature. At the same time, an imaginary part of ac susceptibility, χ 00 shows a peak that indicates a PM–FM transition for the low doped samples (x < 0.05). Very close to the first peak there is a second peak for samples x = 0.05 and 0.075 at high temperatures. Also, by further decreasing the temperature, a third peak is observed in χ 00 of the samples x = 0.05 and 0.075, in harmony with the sharp decline of χ 0 around the same temperature region. This peak is frequency dependent and shifts towards higher temperatures with increasing frequency (see Fig. 5). Therefore the possibility of the formation of orbital domains is ruled out for these samples. A similar behavior is also obtained by substituting Mn with Ga, Cr and other cations in manganites [22–24]. This is normally related to the existence of a reentrant spin-glass (RSG) phase in these samples. Recently Mukherjee et al. [25] and De Teresa et al. [26] have shown the presence of FM clusters in the PM phase of Sm0.55 Sr0.45 MnO3 and Ga doped La0.9 Sr0.1 MnO3 manganites respectively. It seems that, the occurrence of the peak above Tc in samples x = 0.05 and 0.075 suggests the formation of clusters
Ea
(1)
kB Tf
where τ0 for superparamagnetic systems is in the range of 10−9 –10−13 s, τ is related to measuring frequency as τ = 1/f and Tf is the temperature of peak position in the ac magnetic susceptibility. By fitting the experimental data from Fig. 5 with Eq. (1), we have found an unphysical small τ0 ∼ 1 × 10−26 s (the fit is not shown here). For interacting systems the relaxation time has been given by the Vogel–Fulcher law [27],
τ = τ0 exp Fig. 5. The temperature dependence of the χ 00 for the x = 0.075 sample at frequencies 40, 111, 333, 666, and 1000 Hz in an ac field of 5 Oe. The inset shows the evolution of the peak by the increase of the frequency.
Ea kB (Tf − T0 )
.
(2)
Here T0 is an effective temperature and Tf is the temperature of the peak position in the ac magnetic susceptibility. By fitting the experimental data from Fig. 5 with Eq. (2), For T0 = 78 K the values of τ0 = 5 × 10−7 s and Ea /k = 289 K have been obtained. The value of τ0 is larger than typical values (10−11 –10−13 s) for spin glasses (the fit is not shown here). We have checked the frequency dependent data by conventional critical slowing down model [26,27]: f = f0 (Tf − Tg /Tg )z ν .
(3)
Here Tg is the RSG transition temperature and Tf is the frequency dependent freezing temperature at which the maximum relaxation time of the system corresponds to the measured frequency. The Fig. 6 shows the fit for the case where we have set f0 = 1013 , typically taken in the spin-glass systems. The best fitting values are; z ν = 7.1 and Tg = 88 K. The estimated values are within the realm of three dimensional spin-glasses [28]. Reentrant behavior has been found in a variety of disordered magnetic materials in which there is a competition between spin-glass ordering and long-range FM ordering, i.e., in systems where there is a majority of FM couplings between the individual spins but a sufficiently large number of AFM couplings to create substantial frustration. When the temperature is lowered in such a system, it exhibits a transition from the PM to a FM phase. By lowering the temperature further a typical spin-glass, commonly called RSG, behavior appears. By substituting Ti for Mn, the density of holes (Mn4+ ) decreases and a portion of the Mn3+ –O–Mn4+ network is broken. So the double-exchange interaction is weakened due to the magnetic dilution and
B. Aslibeiki et al. / Solid State Communications 149 (2009) 1274–1277
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0
References
f
g
Fig. 6. Ln–Ln plot of the reduced temperature (Tf /Tg − 1) versus frequency for x = 0.075 sample.
increasing of disorder in high level doped samples (x = 0.05 and 0.075 samples).The increase of disorder in the FM matrix produces spin frustration and is responsible for the occurrence of RSG. 4. Conclusions We have studied the effect of Ti doping in La0.8 Sr0.2 Mn1−x Tix O3 manganite material. The XRD analysis, along with the resistivity and ac susceptibility measurement results support the fact that Ti4+ ions substitute for Mn4+ ions. By increasing Ti doping level, x, the MI transition temperatures decrease and system becomes an insulator. This can be explained by the percolation model. The PM–FM transition temperature, Tc also decreases when the Ti content increases. The RSG state accompanied by the FM transition exists in high doped samples. The RSG state is mainly ascribed to the increase of disorder in the FM matrix induced by the random Ti impurity substitution.
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