IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 3049

Magnetic Correlations and Spin Dynamics in CrystallineLa1 Ca MnO3 ( = 0 0 1 0 2 0 3): Analysis of Basic EPR Parameters

M. Auslender1, A. I. Shames2, E. Rozenberg2, G. Gorodetsky2, and Ya. M. Mukovskii3

Department of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105 IsraelDepartment of Physics, Ben-Gurion University of the Negev, Beer-Sheva 653, 84105 IsraelMoscow Steel and Alloys Institute, Leninskii Prospect Moscow 119049 Russian Federation

Electron paramagnetic resonance data of La1 Ca MnO3 compounds with = 0 0 1 0 2 0 3 were analyzed in order to explorethe changes in effective spin-spin interactions and spin dynamics parameters. The presence of two magnetic subsystems — host of su-perexchange coupled Mn3+ ions and Mn4+ associated clusters is characteristic for = 0 and 0 1 samples, while ferromagneticallycorrelated host is observed in = 0 2 and 0 3 ones. The contributions of both ionic host and doped carriers to spin relaxation param-eters were determined using general spin-dynamics theory.

Index Terms—Magnetic resonance, magnetic susceptibility, manganese compounds.

THE MAGNETIC phase diagram of La Ca MnO(LCMO) manganite system is well established nowadays

using diverse experimental techniques, see e.g., [1], [2]. Theend LaMnO compound is antiferromagnetic (AFM) insulatorwith A-type AFM structure and Neel temperatureK [1]. The Ca substitution in La-site results in appearance ofmixed- valence Mn ions, and in a competition between AFMsuper exchange and ferromagnetic (FM) double exchange(DE) [1], [2]. Neutron scattering experiments at low dopinglevels have shown an inhomogeneous magneticground state consisting of hole-rich FM clusters embedded ina hole-poor canted AFM matrix [3]. At higher doping levels

non-typical for common DE, in whichcarriers are delocalized, FM insulating state emerges [1], [2].At critical the transition to a FM metallic stateoccurs [4].

The aim of this work is the study of changes in magneticcorrelations and spin-dynamics characteristics of LCMO withthe increase of Ca-content by means of electron magnetic reso-nance (EMR) technique. To this end the temperature depen-dences of electron paramagnetic resonance (EPR) peak-to-peakline width and double integrated intensity (DIN) wereanalyzed. Four Ca-contents, and , were se-lected for study the transition from AFM insulator to FM metalin LCMO. Previous analysis of the EPR data [5] showed that forLCMO ceramics has high- asymptoteand DE affects much weaker than magnetic orderingtemperatures. In a contrast, our work proves that for the

, 0.2 and 0.3 LCMO single crystals includes anon-saturating in contribution of doped carriers in additionto the purely ionic term of [5], but the high- asymptote of thelatter decreases fast with increasing . The EPR line broadeningon approaching the magnetic transition temperatures was inter-preted using the critical spin-dynamics theory [6].

Digital Object Identifier 10.1109/TMAG.2007.892174

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

TABLE IPARAMETERS OF THE FITS DESCRIBED IN TEXT

Polycrystalline LaMnO was sintered using standardsolid state reaction [7]. Three LCMO single crystals

were grown by a radiative heatingfloating-zone method [8]. These samples are labeled fur-ther as Ca0, Ca0.1, Ca0.2 and Ca0.3. The cationic compositionand crystallinity were controlled using the x-ray powderdiffraction and the energy dispersive spectroscopy. Magneticordering temperatures for the samples studied [9]–[11] areshown in Table I. The EMR spectra were recorded using aBruker EMX-220 X-band ( GHz) spectrometer at

K on the loose-packed crushed samplesconsisting of micron-sized (5–20 m) grains [11], [12].

The main EPR line with g-factor and aweak narrow one with were resolved in Ca0,while Ca0.1-0.3 samples have single line EPR spectra with

. It means that Mn ions are responsible for EPR inCa0.1-0.3, while mostly Mn ones—in Ca0. The weak extraEPR line observed in Ca0 supports the conclusion [13] on anunavoidable presence of some Mn impurity in nominally sto-ichiometric LaMnO .

We assume further that DIN is proportional to zero-fieldsusceptibility , i.e., they have the same dependence.For FM, this is valid when EPR line has Lorentzian shapeand is in paramagnetic (PM) range apart a vicinity of ,where sweeping field may have a strong effect, which issatisfied in our analysis. dependencesare shown in Fig. 1. For Ca0.2-0.3, these dependences fitwell with Curie-Weiss (CW) law, see Figs. 1(b) and (c). For

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3050 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007

Fig. 1. Temperature dependences of the inverse DIN. The points and lines on(a)–(c) are experimental data and its theoretical fits, respectively. The dashedline — fit to Neel formula, while solid ones — fits to CW law (see text).

Ca0-0.1, the dependences are generally sub-linear, but theydiffer: is linear at low- and sub-linear at high forCa0.1 and vice versa for Ca0 — see Fig. 1(a). We displayedthe fitted CW temperatures in Table I. Large differencebetween and is typical of DE, for which both depend ondensity of mobile carriers. The behavior of (DIN) in Ca0 isvery similar to that in ferrites [14], which is not accidental dueto the presence of the impure Mn ions in Ca0. For this reasonwe applied to of Ca0 Neel formula [14] in the form

(1)

where is the total Curie constant, is CW temperature ofMn host, while that of the impure spins is put zero; andare known functions [14] of and , the ratio of the Curie con-stants of the impure and host spins. The best fit of (1) todata for Ca0 results with (see Table I) obtained from inde-pendent fit for , see below. This yields –being about ten times larger than estimated Mn content [13].This means that each Mn and surrounding it Mn ions formcluster with the mean-square spin much larger than that in thehost.

The dependences are shown in Fig. 2. They changedrastically with . Our analysis of starts from Moriformula used by Huber et al. [5]

(2)

where and are the Curie and actual susceptibility,respectively, of resonating spin and is an integral over timein of a time-correlation function of torques acting on thespin and causing its relaxation. What spin enters (2) depends on

relaxation regime. There exist two gross spins in our samples:those of the Mn host and Mn associated clusters in Ca0 andthe Mn host and electrons in Ca0.1-0.3. For Ca0, anisotropicinteraction between the Mn spins is stronger than their ex-change interaction with the clusters. In this case (2) refers tothe broad line, it is the Mn host spin that enters (2), hence

is not the total susceptibility we measure by DIN but thehost CW susceptibility. In Ca0.1-0.3, due to strong Hund ex-change, the spin system is in ‘bottleneck’ regime when the spinthat enters (2) is total spin, so is the measured suscepti-bility. In this case the curve may be (up to some factor)extracted from the experimental data. For Ca0, the fit of (2) withconstant resulting in [5]works well, see Fig. 2(a), and the fit values and , seeTable I, agree with the results of [13] and [5], respectively. ForCa0.1-0.3, constancy of conflicts with the data.General expression of reads

(3)

where is Mn spin-spin, is Mn spin-car-rier-orbit and is carrier spin relaxation contribution.We modeled these contributions in PM range by

(4)

where a term in the high-temperature expansion, additional tothe term due to Huber et al. [5], was retained and

(5)

where and are the parameters of ion spin-carrier-orbit andcarrier spin-lattice interaction, respectively, is carriers acti-vation energy. Equation (5) follows from the theory of electronspin relaxation [15]. We fitted (3)-(5) to the data, seeFigs. 2(b) and (c); all fits yield . The dependentterm in (4) is not crucial for all samples but for Ca0.1, which fityields K, and meV. For Ca0.1, we used

extracted as noted above, since CW law doesn’t hold inall the fit interval. The wide minimum of around 420K, see Fig. 2(b), associated with Jahn-Teller transition [16] isnot observed in . For Ca0.2-0.3, we employed obtainedfrom the fits of CW to the DIN data. The fit here resultsin . The fitting parameters and are shown inTable I. Note that drops with increasing of and dueto DE narrowing of EPR line so that at the term givenby (4) is negligibly small, and the fit results in . Thechanges of parameters and , at increasing correspond totrends anticipated from their nature.

As approaches the magnetic critical points the EPR linebroadens, so transverse spin relaxation time decreases. Ourresults for are similar to those of [17]. We analyzethis phenomenon in terms of . Residing in far critical PMrange, we fit the scaling law [6]

(6)

to our data. The normalized (300 K) and fitted curvesare shown in Fig. 3; the parameters and are given

AUSLENDER et al.: MAGNETIC CORRELATIONS AND SPIN DYNAMICS 3051

Fig. 2. Temperature dependences of �H . The points and solid lines on(a)–(c) are experimental data and theoretical fits, respectively.

Fig. 3. Temperature dependences of coefficient L in far critical range. Thepoints and solid lines on (a)–(c) are experimental data and fits, respectively.

in Table I. The values of agree with the correspondingmagnetic transition temperatures. Comparing Figs. 3(a), (b),and (c) shows the strengthening of FM correlations withincrease. The indexes in Table I differ from those predicted

by Heisenberg model, which is well known and was discussedin the literature [6].

In summary, the presence of two magnetic subsystems — theAFM host of super-exchange coupled Mn ions and FM clus-ters, existing due small impurity of Mn ions [13] is observedin Ca0. The small amount of charge carriers do not influenceon spin dynamics in Ca0, which is well described by Huber etal.. [5] ion-ion spin relaxation mechanism. Ca-doping results inappearance of strong FM subsystem together with canted AFMmatrix in Ca0.1, which manifests itself in both the EPR suscep-tibility and the line width temperature dependences. The car-rier-assisted spin relaxation contribution is definitely observedin Ca0.1. The FM correlations become prevailing in Ca0.2-0.3compounds. This results in strong suppression of ion-ion spinrelaxation contribution, which is unobservable in Ca0.3. In acontrast, the increase of the FM correlations with strengthensthe EPR line broadening effect at approaching the magneticcritical points.

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Manuscript received October 29, 2006 (e-mail: marka@ee.bgu.ac.il).