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Deivanayagam Vijayan, Ramesh Ade, Joji Kurian, and Rajender Singh. School of Physics, University of Hyderabad, Hyderabad 500046, India. In this paper, the ...
IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015

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Electron Spin Resonance and Magnetization Studies of Bi-Rich La-Manganites Deivanayagam Vijayan, Ramesh Ade, Joji Kurian, and Rajender Singh School of Physics, University of Hyderabad, Hyderabad 500046, India In this paper, the temperature-dependent electron spin resonance and magnetization (M) studies of La0.7−x Bi x Ca0.3 MnO3 (x = 0, 0.35, 0.42, 0.56, and 0.63) manganites synthesized by the sol–gel method are reported. In all the samples, the coexistence of paramagnetic and ferromagnetic phases over a wide range of temperature is observed. The estimated Curie temperature (TC ), the Curie–Weiss temperature (θC ), and the temperature at which linewidth (H) becomes minimum (Tmin ) decrease with the increase in the Bi content. The decreasing trend of these parameters is discussed in view of the difference in the ionic radii of La3+ and Bi3+ and the variations in 6s2 lone pair character of Bi3+ ions with the increase in the Bi content. The evolution of various types of magnetic interactions as a function of temperature and Bi content is discussed. Index Terms— Colossal magnetoresistance, magnetization, manganese oxides, rare-earth metals.

I. I NTRODUCTION

I

N ABO3 (A is the trivalent rare-earth metal ions and/or alkaline earth ions and B is the Mn ion)-type perovskite compounds, doping-induced antiferromagnetic (AFM)–ferromagnetic (FM) and insulator–metal transitions as a function of temperature (T ) have been investigated extensively for many decades [1]–[5]. The interplay between magnetic structure and electronic transport in these materials gives rise to several novel phenomena, such as double exchange (DE) and superexchange interactions, which occur due to the hopping of charge carriers between the Mn ions (Mn3+ /Mn4+ ) via oxygen anions [6], [7]. The properties of manganese oxides are mainly influenced by a Mn3+ /Mn4+ ratio, a Mn–O–Mn bond length, and a Mn–O–Mn bond angle, which give the emergence of various physical properties [7]. The phenomenon of phase separation (PS) and the coexistence of magnetic phases at different length scales of FM metallic and AFM charge or orbital-ordered insulating domains in manganites have been a subject of study in recent years [8]. The substitution of La by Pr in the La5/8−y Pr y Ca3/8 MnO3 system destabilizes the FM state in favor of the charge-ordered AFM (CO-AFM) state [9]. Recent studies on doped manganites reveal that the nanoscale PS due to the competition between the FM phase and the CO-AFM phase yield the colossal magnetoresistance phenomenon [10], [11]. There are a few reports on Bi-doped La-manganites. Though trivalent bismuth (Bi3+ ∼ 1.24 Å) and trivalent lanthanum (La3+ ∼ 1.27 Å) ions have approximately similar ionic radii, the magnetic and electrical transport behaviors of Bi-doped La-manganite differ from that of undoped one [12], [13]. These discrepancies may arise from the highly polarizable 6s2 lone pair of Bi3+ ions [14]. Some interesting results have been reported in the literature comparing the two systems La1−x Cax MnO3 and Bi1−x Cax MnO3 . For x = 0, the end member of the solid solution, LaMnO3 is of Manuscript received March 20, 2015; revised May 11, 2015; accepted June 8, 2015. Date of publication June 10, 2015; date of current version October 22, 2015. Corresponding author: R. Singh (e-mail: [email protected]). 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.2015.2443911

orthorhombic structure and shows an A-type AFM behavior, while BiMnO3 is a triclinically distorted perovskite structure exhibiting ferromagnetism. The Bi0.1 La0.5 Ca0.4 MnO3 system lies in a highly blocked metastable state at low temperatures due to the competition between the FM and the CO-AFM phases [15]. In Bi-doped La-manganites, most of the results have been reported on the low levels of Bi-doping [12]–[16]. The studies on La-manganites containing high Bi-doping levels are few in the literature. In this paper, we report the systematic study of the evolution of different magnetic phases as a function of high Bi-doping levels and temperature in La0.7−x Bix Ca0.3 MnO3 (x ≥ 0.35) manganites, using electron spin resonance (ESR) and magnetization measurements. II. E XPERIMENTAL D ETAILS La0.7−x Bix Ca0.3 MnO3 (x = 0, 0.35, 0.42, 0.56, and 0.63) samples were prepared by the sol–gel method [17]. A fluffy material was obtained after the decomposition of gel, which was then sintered at 900 °C for few hours, with intermediate grinding. The homogenized powder and pellets of these samples were again sintered at 900 °C for 3–4 h. The X-ray diffraction (XRD) plots for the samples were obtained using the Philips Diffractometer with a Cu-K α radiation to check the phase formation. The ESR spectra were recorded on a Jeol X-band ESR spectrometer in the temperature range of 110–470 K. The temperature-dependent magnetization (M) studies on the samples were done using a quantum design physical properties measurement system in the temperature range of 5–320 K in zero-field cooled (ZFC) and field cooled (FC) conditions with an applied field of 0.05 T. III. R ESULTS AND D ISCUSSION A. XRD Studies The room temperature XRD data for various samples shown in Fig. 1 indicate the single-phase nature with a cubic perovskite structure. The doping does not affect the crystal structure of the sample. The estimated value of lattice parameter is ∼3.85 Å for all the samples, which is comparable with the values reported in [15]. The splitting of some reflections in the XRD spectra for the sample with x = 0.63 is observed due to monoclinic distortion.

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Fig. 1.

IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 11, NOVEMBER 2015

XRD plots of La0.7−x Bix Ca0.3 MnO3 samples.

Fig. 3. H versus T plots of La0.7−x Bix Ca0.3 MnO3 . Solid lines: theoretical model.

Fig. 2.

ESR spectra of La0.7−x Bix Ca0.3 MnO3 at various temperatures.

B. ESR and Magnetization Studies Fig. 2 shows the ESR spectra for the x = 0 and 0.63 samples at some selected temperatures. The observed ESR signal is the result of some complex magnetic entities made up of a collection of Mn3+ and Mn4+ ions. For all the samples, the ESR spectra above a particular temperature (∼260–270 K) show a single resonance line at the resonance field of 325 mT, which is a characteristic of the existence of the paramagnetic (PM) phase. Below this temperature, FM correlations evolve in the PM matrix as indicated by the appearance of hump (shown with ↓1) in the low-field region of the ESR spectra. This hump can be assigned to a low-moment state. With further decrease in temperature, FM correlations grow at the expense of PM interactions, as a result of which a complete PM–FM transition occurs below a particular temperature (∼180 K). This is indicated by the shift of the hump toward a further lower field region (shown with ↓2). This effect can be assigned to the development of a high-moment state as temperature is lowered. Fig. 3 shows the variation of linewidth (H ) as a function of temperature for various samples. H decreases with the decrease in temperature and reaches its minimum value

at Tmin , and then increases with the decrease in temperature. Below 250 K, the second peak is well pronounced, making H measurement uncertain. Hence, H data below 250 K are not included in Fig. 3. The data are analyzed using Huber et al.’s [18] and Auslender et al.’s [19] approaches. According to their approaches, H can be expressed as   H (∞) +B (1) H = (T − θC ) T where θC is the Curie–Weiss temperature, H (∞) is the high temperature asymptote value of linewidth, and B describes the interaction between the eg electrons and impurities with spin reversal. In (1), the first term corresponds to pure ion–ion spin relaxation and the second term corresponds to itinerant eg electrons. The H versus T fits are drawn using (1) in the temperature range T > Tmin and are shown with solid lines (Fig. 3). The estimated B values are 3.94, 3.32, 2, 1.92, and 2 GK−1 for x = 0, 0.35, 0.42, 0.56, and 0.63, respectively. Similarly, the values of H (∞) are found to be 332, 425, 1073, 2199, and 1747 G for x = 0, 0.35, 0.42, 0.56, and 0.63 respectively. The decrease in the values of B (except for x = 0.56) with doping is the indication of the weakening of the strength of electron spin–lattice interactions, whereas the increase in the values of H (∞) with doping is the indication of the strengthening of spin–spin relaxation [18], [19]. Fig. 4 shows M versus T plots both in the ZFC and FC modes, for all the samples. M decreases with the increase in the Bi content. d M/d T versus T [Fig. 4 (inset) for x = 0] plots show two cusps, indicating two magnetic transitions for all the samples except for the sample with x = 0.63. The first one indicates a PM to a weak FM state transition, and this

VIJAYAN et al.: ESR AND MAGNETIZATION STUDIES OF Bi-RICH La-MANGANITES

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Fig. 4. M versus T plots of La0.7−x Bix Ca0.3 MnO3 samples. Inset: d M/d T versus T plot of x = 0 sample.

temperature can be assigned to the Curie temperature (TC ) with a low-moment state [TC (L)]. The second one is a PM to a robust FM state transition, and this temperature is assigned to TC with a high-moment state [TC (H )] [13]. This magnetic transition is attributed to the successive spin transitions in a single phase. The temperature at which the evolution of humps in the low-field region of ESR spectra starts is shown in Fig. 2 with ↓1 and ↓2 for the x = 0 and 0.63 samples. These temperatures are close to the values of TC (L) and TC (H ), respectively. For the undoped sample, only one transition at TC ∼ 270 K has been reported in the literature. The observed value of TC (L) is consistent with this reported value of TC [20]. The observed value of TC (H ) ∼ 180 K is not reported so far. The observed features in the present ESR spectra, however, are similar to [20] and [21]. But, the earlier reports have not assigned these features to different spin states. The role of inhomogeneities on the observed anomalous features in the ESR and magnetization data cannot be ruled out. However, the systematic changes in the ESR and M data as a function of composition rule out the dominant role of inhomogeneities. For the samples with x ≥ 0.35, a strong bifurcation between the ZFC and the FC curves is observed below ∼40 K. It is also possible that the ZFC-magnetization curves correspond to an unsaturated FM state, and the FC-magnetization curves correspond to a saturated FM state. The temperature of bifurcation is assigned as spin freezing temperature (T f ), leading to a spin glass (SG) state in the case of Bi1−x Cax MnO3 (x ≤ 0.6) system [3]. The SG state evolves as the FM cluster size decreases due to the addition of Bi in the La-manganite matrix. To attribute this difference to an SG state, the confirmation that the M–H curve is a single-valued function without hysteresis down to low temperatures is needed. Since all the Mn ions contribute to the ESR signal, the double-integrated intensity (DI) of the ESR signal should be proportional to the number of ESR centers, which is a measure of ESR susceptibility (χESR ) [5]. Considering this, 1/DI is plotted as a function of temperature. Fig. 5(a) and (b) depicts 1/DI versus T and 1/χ versus T plots of various samples, respectively. The straight line fits are as per the Curie–Weiss law. In all the samples, with the decrease in temperature,

Fig. 5. (a) 1/DI versus T La0.7−x Bix Ca0.3 MnO3 samples.

and (b) 1/χ

versus T

plots of

1/DI deviates from a straight line when it reaches a minimum at a temperature, which is close to TC (L). However, for the samples with x = 0 and 0.35, a nonlinear behavior is observed in the 1/DI versus T plots in the entire temperature range. The deviation from the Curie–Weiss law near the Tmin or TC (L) can be assumed due to the formation of spin clusters as temperature approaches Tmin or TC (L) [22]. From the 1/χ versus T plots, the values of Curie–Weiss temperature, θC (L) and θC (H ), are estimated in the temperature range of ∼270 K < T < 320 K and ∼250 K < T < 270 K, respectively. The observed deviation of 1/DI versus T data from the Curie–Weiss law might be due to the evolving spin–lattice interactions in the vicinity of the Curie point. This is in agreement with the prediction of the adiabatic polaron hopping model [23]. The exponential decay of EPR line intensity with respect to temperature reflects the formation of polarons near the Curie point. These polarons can be centered by the Mn3+ and Mn4+ ions, and can also be mediated by the double-activated hopping of the electrons (via the O2− ion). Fig. 6 depicts various transition temperatures, TC (L), TC (H ), θC (L), θC (H ), θC , and Tmin as a function of Bi content. It is observed that doping does not affect significantly the values of TC (H ) and θC (H ). Except for the sample with x = 0.56, a significant decrease in the values of TC (L), θC , and Tmin is observed with the increase in the Bi content. An abrupt decrease in the values of θC (L) is observed for the samples with x > 0.35. The drop in θC (L) is an indication of decreasing FM correlations, and is in accordance with the drop in M value for the samples with x > 0.35. The M and θC (L) values are thus correlated. The microscopic origin for the decrease in the values of these parameters with the increase in the Bi content is probably the lone pair effect of

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Fig. 6. Various transition temperatures as a function of x (Bi content) of La0.7−x Bix Ca0.3 MnO3 samples.

Bi3+ ions [14], [24]. Due to the covalent nature of Bi3+ ions, the length of the Bi–O bond will be less than that of the length of La–O bond, and though it is mentioned that the ionic radii of Bi3+ and La3+ are similar, Bi3+ ions have slightly lower ionic radius, which results in the decrease in the value of effective ionic radii of A-site ions. As a result, the Mn–O–Mn bond angle decreases with the increase in the Bi content, which results in local distortion, and this local distortion increases with the increase in the Bi content. This, in turn, decreases the bandwidth, which hinders the movement of eg electrons between the Mn ions, causing the localization of electrons around the Bi3+ ions and weakening the FM–DE interactions. This results in the decrease of M and various temperature parameters, describing the evolution of various kinds of magnetic interactions with the increase in the Bi content. IV. C ONCLUSION In this paper, the magnetic properties of perovskite La0.7−x Bix Ca0.3 MnO3 (x = 0, 0.35, 0.42, 0.56, and 0.63) manganites synthesized by the sol–gel method have been systematically investigated. The ESR and magnetization (M) studies on these samples show that the higher level of Bi-doping decreases the M as well as various temperature parameters, describing the evolution of various kinds of magnetic interactions. These changes are ascribed to the difference in ionic radii of La3+ and Bi3+ and the variations in 6s2 lone pair character of Bi3+ ions with Bi-doping. ACKNOWLEDGMENT R. Ade would like to thank the award of University Grants Commission, India - Senior Research Fellowship from the Council of Scientific and Industrial Research, India. R EFERENCES [1] G. H. Jonker and J. H. Van Santen, “Ferromagnetic compounds of manganese with perovskite structure,” Physica, vol. 16, no. 3, pp. 337–349, 1950.

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