Rare earth effect on the critical behavior of La0.75Ln0.05Sr0.2MnO3 manganites
Introduction
The discovery of colossal magnetoresistance (CMR) in the manganese oxides RE1-xAExMnO3 (RE=rare earth and AE=alkaline earth) close to their magnetic transition temperature has attracted renewed interest in the physical behavior of these systems [1], [2], [3], [4], [5], [6]. The substitution of RE by another trivalent cation leads to a dramatic change in the physical properties of these oxides i.e. structural, electrical and magnetic properties [7], [8]. CMR effects in these systems can be ascribed to the percolative nature of the paramagnetic (PM)-ferromagnetic (FM) transition [9] and phase separation phenomenon [3], [5], [10]. In order to understand both CMR effect and the metal-insulator transition in manganites, it is important to explain the order as well as the nature of the PM-FM phase transition. To do so, it is interesting to study the critical exponents associated with such transition.
In manganites, the magnetic interactions can be generally described by four models: mean field, 3D-Heisenberg, 3D-Ising and tricritical mean field. Mean field model describes long-range exchange interactions with negligible critical fluctuations near the Curie temperature TC. The 3D-Heisenberg model is a statistical model used to study the critical points and the magnetic phase transitions. In this model, short-range exchange interactions exist between spins with the presence of significant spin fluctuations in the vicinity of TC. In the 3D-Ising model, magnetic dipolar moments of atomic spins can be in one of the two states ±1. The spins are usually arranged in a lattice, allowing each spin to interact with its neighbors. The tricritical mean field model suggests that the sample's composition should be close to a tricritical point in the phase diagram of the used material. The theoretical investigations on critical exponents of different manganites have yielded several values of the critical exponents β, γ and δ, which correspond to the mean-field (β = 0.5, γ = 1 and δ = 3), the 3D-Heisenberg (β = 0.365, γ = 1.336 and δ = 4.80), the 3D-Ising (β = 0.325, γ = 1.241 and δ = 4.82), and the tricritical mean-field (β = 0.25, γ = 1 and δ = 5) models [11], [12].
Previous studies were carried out on the rare earth effect on the structural, magnetic and magnetocaloric properties of the samples La0.75Ln0.05Sr0.2MnO3 (Ln˭La, Sm, Eu, Gd, Dy and Ho) [13]. Structural study has shown a transition from rhombohedral to orthorhombic symmetry for highly disordered samples. The magnetic study revealed that all our samples exhibit a FM-PM transition with increasing temperature and this temperature decreases by substitution. The TC values for all our samples were located between 330 and 225 K. The substitution also enhances the magnetocaloric effect through the enhancement of relative cooling power values, which suggests the possibility of using these compounds for magnetic refrigeration. However, we have found that the cationic mismatch at the A-site is not the only parameter that governs the evolution of the physical properties of the studied samples. In fact, both Dy and Ho substituted compound have shown some peculiarities. For Ho based sample, we have suggested that the large <Mn-O-Mn> bond angle is responsible for the enhancement of ferromagnetism. Although we have found a possible explanation for the behavior of Ho based sample, the peculiarities observed in Dy based one still need further investigations. This compound possesses the smaller TC value and a high unit cell volume per formula unit compared to other substituted specimens. Previous studies on manganites has shown that Dy [5] and Ho [14] ions may generate unexpected physical behavior.
The critical behavior of La1-xSrxMnO3 system (0.2 ≤x ≤ 0.3) was previously investigated and several models were obtained [15], [16], [17], [18], [19], [20], [21]. In this work, we have tried to study the critical behavior near the FM-PM phase transition temperature in La0.75Ln0.05Sr0.2MnO3 (Ln˭La, Sm, Eu, Gd, Dy and Ho) samples in order to study the rare earth element substitution effect on the critical behavior of the parent compound La0.8Sr0.2MnO3. The obtained results may throw some light on the peculiar behavior of Dy based sample.
Section snippets
Experimental techniques
La0.75Ln0.05Sr0.2MnO3 (Ln˭La, Sm, Eu, Gd, Dy and Ho) polycrystalline samples were synthesized using the sol-gel technique method described in our previous study [13]. Magnetic measurements were performed by vibrating sample magnetometers BS1 and BS2. The isothermal magnetization measurements versus applied magnetic field were carried in the temperature range near the Curie point TC with 2 K step.
Scaling analysis
The critical behavior near TC and for a second-ordered PM-FM phase transition, is characterized by a set of critical exponents, namely β, γ, and δ [22], which are related to the spontaneous magnetization (MS), the inverse of initial PM susceptibility (χ−1), and the critical magnetization isotherm M(H, T = TC), respectively. The scaling hypothesis introduces the relations in the vicinity of the critical region as follows:where ε = (T-TC)/TC is the
Results and discussion
The temperature dependent magnetization measurements at 0.05 T are shown in Fig. 1. All our samples show a magnetic transition from PM to FM state as the temperature decreases. The TC values can be obtained from the minimum of dM/dT. The obtained results are gathered in Table 1 [13]. It is clear that Dy based sample shows the lowest TC value. Although both Dy3+ and Ho3+ ions have similar radii and magnetic moment values (~10.6 µB), a great difference between TC values can be observed for these
Conclusion
We have studied the critical behavior of La0.75Ln0.05Sr0.2MnO3 (Ln˭La, Sm, Eu, Gd, Dy and Ho) manganites around their TC values. The critical exponents values for La0.8Sr0.2MnO3 are similar to those corresponding to the 3D-Ising model, suggesting the presence of short-range FM interactions in this sample. Dy substituted compound changes the universality class from the 3D-Ising model to the tricritical mean field model but the other rare earth substituted samples preserve the same universality
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