IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005 2745

EPR and Resistivity Study ofPr0:7Ba0:3MnO3 Manganite

Alexander N. Ulyanov1;2, Hoang Duc Quang1, Nataliya E. Pismenova2, and Seong-Cho Yu1

Department of Physics, Chungbuk National University, Cheongju 361-763, KoreaDonetsk Physico-Technical Institute, National Academy of Sciences, 83114 Donetsk, Ukraine

The properties of Pr0 7Ba0 3MnO3 composition have been studied by means of magnetic and resistance measurements. According toX-ray CuK diffraction analysis, the sample was single-orthorhombic phase. The temperature dependence of resistance in the para-magnetic region followed the nearest neighbor hopping of the small polaron model. The value of activation energy obtained with theresistivity measurement was higher than that deduced from the temperature dependence of electron paramagnetic resonance line inten-sity.

Index Terms—Magnetic resonance, magnetoresistance (MR), resistance.

I. INTRODUCTION

PEROVSKITE-LIKE lanthanum manganites have attracteda renewed interest due to colossal magnetoresistance

(CMR) observed near the ferromagnetic ordering temperatureand their potential application in MR devices [1]–[3].

However, despite a large number of publications on the lan-thanum manganites, the CMR nature is not clear enough. Thedifficulties consist of a close connection between structuresand electronic and magnetic properties of perovskite-typemanganites. There are also some questions in interpretation ofspin (-lattice) dynamics near the . To clarify the situation wecarried out the electron paramagnetic resonance (EPR) and re-sistivity measurements of Pr Ba MnO (PBMO) manganiteto study the nature of the interaction near the Curie point.

II. MATERIALS AND METHOD

The PBMO, prepared by conventional ceramic technology,was single-orthorhombic phase according to X-ray diffraction(XRD) analysis. Resistance measurements were made usingthe four-probe technique. Magnetization was obtained with avibrating sample magnetometer (VSM). The EPR study wasperformed at 9.2 GHz (X band) with a Jeol JES-TE300 ESRSpectrometer.

III. RESULTS AND DISCUSSION

Fig. 1 shows temperature dependencies of resistivity andmagnetization . The dependence showed a metal-iso-lator transition at temperature ( K) that was lower thanthe Curie temperature ( K), deduced with the mag-netization measurement. The curve was obtained at afield-cooled (FC) warming rate in an applied magnetic field of50 Oe and revealed unusually for the FC dependence a bellshape. The unusual decrease in the magnetization with the de-crease in temperature was also observed before [4], [5] and canbe explained by the competition between the magnetization and

Digital Object Identifier 10.1109/TMAG.2005.854828

Fig. 1. Resistivity and magnetization versus temperature for PBMO sample.

magnetic domain orientations processes. The above anomalydisappeared in the field of 1.0 T due to the orientation of do-main magnetic moments along the field. It is interesting to notethat the dependence revealed a “kink” at the temperaturewhere the magnetization showed the highest slope. A high-tem-perature part of the dependence (see Fig. 2) was well fittedusing the nearest neighbor hopping model of a small polaron [6]

(1)

with the activation energy eV ( is Boltzmannconstant).

In the paramagnetic region the EPR spectra were singleLorentzian lines with the gyromagnetic ratio , which wastemperature independent. The temperature dependence of thepeak-to-peak EPR line width is shown in Fig. 3. Theline width showed the minimum at temperatureand then increased with temperature, showing a small tendencyfor saturation. The tendency for saturation was also found [7],[8] when studying the La Ca MnO compositions, butfull saturation was not achieved up to 1000 K. We have to notethat the nature of temperature dependence of EPR line width isstill under strong discussion. Some authors [7], [9] attributedthe temperature dependence of the line width to a spin-phononinteraction and explained by spin-lattice relaxation of an

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2746 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 2. Resistivity versus temperature and fitting curve according to (1).

Fig. 3. Temperature dependence of the peak-to-peak EPR line width �Hfor Pr Ba MnO . Inset shows derivative (dP=dH) power absorption curveof EPR spectrum, taken at 253 K, and definition for the �H .

exchange-coupled Mn –Mn spin system [10], [11]. Theconclusion of Shengelaya with coworkers [11] was based on theobserved proportionality of the EPR line width to conductivity(taken from [12]), which followed the adiabatic small polaronhopping model [6]. In our paper, the EPR linewidth also fol-lowed the adiabatic law withactivation energy eV. According to other authors[8], [13], the temperature dependence of EPR line width iscaused by the spin–spin (exchange) interaction, and determinedby the expression is a static magneticsusceptibility. Both noted mechanisms, describing the tempera-ture dependence of the EPR line width, were carefully analyzedon the basis of the measurement of longitudinal electron-spinrelaxation time and transverse relaxation time and didnot make clear this question [14].

The EPR line intensity was determined by double in-tegration of the experimental derivative absorption curve. The

dependence (see Fig. 4) was well fitted by the

(2)

expression deduced in [15] on the basis of the adiabatic polaronhopping model. It permits us to obtain activation energy

Fig. 4. Temperature dependence of the EPR line intensity (filled square) andfitting curve, I(T ) = I exp(E =k T ) (solid line); inverse line intensity1=I(T) (open cycle) and Curie–Weiss law (dashed line) for Pr Ba MnO .

( eV). The exponential decreasing in EPR line intensitywith temperature was also observed, e.g., in [16] and [17]. ACurie–Weiss temperature ( K) was obtained by extrapo-lating a linear (high temperature) part of an inverse intensity de-pendence to zero value (see Fig. 4). As one can see, theCurie–Weiss law, does not describe properlythe temperature dependence of the EPR line intensity in a widetemperature range around the . The deviation of depen-dence from the Curie–Weiss law near was explained [8] bythe formation of spin clusters at the frame of the constant cou-pling approximation model. At the same time, the polaron for-mation across the ferromagnetic-paramagnetic phase transitionwas observed in La A MnO ( Ca, Pb) perovskites bythe extended X-ray absorption fine structure spectroscopy [18].Recently, the correlated polarons were observed up to Kwhen studying the La Ca MnO and Pr Ca MnO com-positions by X-ray scattering [19]. It seems that deviation ofexperimental data from Curie–Weiss law can be causedby the spin-lattice interaction in the vicinity of . From thispoint of view, the exponential dependence of EPR line inten-sity on temperature reflects the formation of polarons near theCurie point. The polarons are centered by the Mn and Mnions and mediated by the double (through the O ion) activatedhopping of the itinerant electrons. The exponential decayand Curie–Weiss law coincide at high temperatures, indicatinga decrease of spin-lattice interaction and the dominance of aspin–spin one. The lesser value of the energy obtained usingthe (and line width) dependence than that deduced withthe curve is in question and cannot be explained by thedecrease in the energy in magnetic field, where the EPR res-onance was observed. Namely, we analyzed the resistivity de-pendences, obtained in the , and T magnetic fieldwhen studying the Pr Ba MnO manganite [20]. The anal-ysis showed that a decrease in activation energy ( 5% in1.0 T) with field is very weak and essentially less than the differ-ence between the and deduced by the resistivity and EPRmeasurements, respectively. We suppose that the difference canbe caused by the extremely slow spin(-lattice) dynamics in man-ganites near , which affect the EPR spectra.

ULYANOV et al.: EPR AND RESISTIVITY STUDY OF Pr Ba MnO MANGANITE 2747

IV. CONCLUSION

In this paper, we present a systematic experimental study ofpraseodymium manganite of Pr Ba MnO . The temperaturedependencies of resistivity and EPR line intensity and line widthare first measured for the same sample. The dependences canbe explained by the polaron hopping model. At the same time,the obtained difference in the activation energy, deduced withthe resistivity and resonance methods, is very discussable andshould be studied further to get an answer on the spin(-lattice)dynamics near the Curie point.

ACKNOWLEDGMENT

This work was supported in by the Korean Research Foun-dation under Grant KRF—2003-005-C00018. The work ofA. N. Ulyanov was supported by the Brain Pool Program,Korean Ministry of Education.

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Manuscript received January 25, 2005.