4348 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

Electron Magnetic Resonance Study of Magnetic Order,Paramagnetic Magnetic Correlations, and Spin Dynamics

in La- and Mn-Deficient LaMnO� ManganitesM. Auslender�, A. I. Shames�, E. Rozenberg�, G. Gorodetsky�, and Ya. M. Mukovskii�

Department of Electrical Engineering and Computers, BGU of the Negev, Beer-Sheva 84105, IsraelDepartment of Physics, BGU of the Negev, Beer-Sheva 84105, Israel

The New Materials Synthesis Laboratory, Moscow Steel and Alloys Institute, Moscow 119049, Russia

To probe magnetic ordering in single crystals of La� MnO� � � � �� � �� � ��� and in ceramics of LaMn� �O� � �� � �� � ��, the X-band electron magnetic resonance measurements were carried out in the temperature range � � �� K.The temperature dependences of doubly integrated intensity of electron paramagnetic resonance signal and its linewidth were fitted withknown theoretical models modified for taking into account the different mechanisms of spin relaxation. The experimental data and theirfits evidence that Mn- and La-site vacancies dope the holes in parent LaMnO�, which induces ferromagnetic double exchange interaction.However, strong structural and chemical disorder of La� MnO� crystals makes the ferromagnetic ground state eventually impossibleeven at � � ��. In a contrast, better structural/chemical homogeneity together with a stronger impact of Mn-site vacancies on mixedmanganese valence and double exchange are characteristic for LaMn� O� ceramics. As a result, the LaMn� ��O� compound appearsto be ferromagnetic-like ordered and demonstrates band-like propagation of doped carriers.

Index Terms—Magnetic resonance, magnetization processes, manganese compounds.

I T is the matter of consensus now that ferromagnetism (FM)and metallic conductivity in doped rare-earth manganites,

characterized by mixed valence of manganese Mn -Mn , aregoverned by double-exchange (DE) phenomenon [1]. So called“self-doping” effect, i.e., an appearance of some surplus Mnions in, e.g., La- and Mn-sites deficient LaMnO (LMO) com-pound [2], [3], also results in the FM correlations or even in theFM-like ordering of this former antiferromagnetic (AFM) insu-lator [4]. Such change of magnetic interactions and, even, in theground state of self-doped LMO seems to be a complex phe-nomenon, in which the coupled spin, charge and lattice degreesof freedom are involved [1]. Thus, usual magnetic and transportmeasurements [2]–[6] may be insufficient for detail characteri-zation of this complex system.

Recently, we have suggested a new approach for probing theeffective paramagnetic (PM) spin-spin interactions and spin dy-namics in doped manganites [7], [8], [20]. These effects arewell-defined precursors of the magnetic ordering in a wide classof such compounds. The main idea addressed in [7], [8], and[20] was using of high-sensitive electron magnetic resonance(EMR) technique, incorporating electron PM resonance (EPR)and ferromagnetic resonance, and the model fitting of the EPRparameters. This allowed us to reveal a contribution of dopedelectron spins to both the PM spin-spin interactions and spin dy-namics in Ca-doped LMO, increasing regularly versus Ca con-tent [7]. Also, these effects proved high sensitivity to inhomoge-neous distribution of La-site dopant in low-doped LMO singlecrystals [8], [20].

The aim of this work is to explore the self-doping effect insingle crystals of La MnO ( and ) with

Manuscript received March 01, 2009. Current version published September18, 2009. Corresponding author: E. Rozenberg (e-mail: evgenyr@bgu.ac.il).

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

Digital Object Identifier 10.1109/TMAG.2009.2024630

La-site vacancies and in LaMn O ( and )ceramics with Mn-site vacancies using the above-describedapproach. To this effect, the temperature dependences ofthe doubly integrated intensity of EPR signal (DIN) and itslinewidth were fitted with known theoretical models[9], [10], modified to allow for the different mechanisms of spinrelaxation. It is shown that Mn- and La-site vacancies dope theholes in parent LMO, which induces FM DE Mn -O-Mninteraction, enhancing versus both and . However, strongstructural and chemical disorder of La MnO crystals makesthe FM ground state eventually impossible even at .In a marked contrast, better structural/chemical homogeneitytogether with a stronger impact of Mn-site vacancies on mixedmanganese valence and DE are characteristic for LaMn Oceramics. This results in a mixed-phase AFM-FM groundstate and finite- FM state in the ceramic, whilethe one is likely FM compound. It appears that the“bottleneck” regime [10] is not achieved for La MnO ,whereas these conditions are fulfilled for Mn-deficient samples,which influences strongly their EMR spectra.

The La MnO crystals were grown by floating zonemethod [11], and LaMn O samples were synthesized usinga standard ceramic route [3]. The structure, chemical composi-tion, and oxygen stoichiometry of the samples were controlledusing X-ray powder diffraction, energy dispersive, and induc-tively coupled plasma atomic emission spectroscopy, as wellas iodometric titration techniques [3], [12]. The single crystalshas an orthorhombic structure close to that of parent LMO [12]and show weak FM moment below K due to the presenceof canted AFM and FM DE-coupled Mn -Mn ions clustersubsystems. The orthorhombic structure is also characteristicfor LaMn O ceramics, but their unit cell volume notablyreduces versus [3]. The relatively small low- magneticmoments were observed for and samples, whileit increases drastically for one [13]. EMR was mea-sured in the -range 5–600 K with Bruker EMX-220 -band

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AUSLENDER et al.: ELECTRON MAGNETIC RESONANCE STUDY 4349

Fig. 1. Temperature dependences of the DIN normalized to its value at 300 Kfor: (a) La MnO crystals and (b) LaMn O ceramics. The inset in(b) shows enlarged plots for � � � and ���� samples.

( GHz) spectrometer using few mg of loose packed finepowdered samples. The model fittings were carried out for theEPR signal DIN and characterizing PM spin correlationsand spin dynamics, respectively.

Temperature dependences of DIN normalized to its value at300 K are presented in Fig. 1. One can note that intensities ofthe EMR spectra increase continuously upon cooling from hightemperatures, reaching a maximum and then dropping on fur-ther decrease of for both La MnO and LaMn O se-ries. The maxima of curves are observed at about thesame – K for single crystals in Fig. 1(a), whilefor ceramics corresponding maxima shift to lower temperatureswith the increase of Mn-site vacancies content, i.e., de-creases from K down to K inFig. 1(b). The notable increase of the peak DIN values versusboth and is observed, but the LaMn O ceramics demon-strate that peak DIN for sample rises on about of twoorders of magnitude as compared to the values for and

ceramics; see Fig. 1(b). At the same time, the peak DINshows only five-fold increase upon a rise of La-site vacanciescontent from up to —Fig. 1(a). The propor-tionality of DIN to the EMR magnetic susceptibility[7], [8] allows to conclude that all these features are connectedto the magnetic ordering of the samples studied.

Let us note that the complex EMR spectra, consisting of thenarrow and broad lines, are observed for La MnO crystalsin wide -intervals above and near [12]. Below of about140 K (close to the Néel temperature for LMO), the broad linesdisappear and only weak asymmetric signals continue to exist.Their intensity decreases upon cooling to lowest , which isresponsible for the low- “tails” of curves—only onesuch low- part of curve for sample is plottedin Fig. 1(a) for clarity. In a contrast, a single EMR line is ob-served for LaMn O series at ; an additional narrowsignal appears only in some -range around K in

Fig. 2. Temperature dependences of the inversed DIN normalized to its valueat 300 K for: (a) La MnO crystals and (b), (c) LaMn O ceramics. Thepoints are experimental data and the lines are Curie–Weiss and Néel fits as de-scribed in the text.

ceramic [14]. Note that the intensity of EMR signals forand 0.02 samples drop sharply below respective of about170 K and 130 K, and at lowest temperatures, only weak dis-torted lines are observed, similarly to the case of La MnOcrystals—compare Fig. 1(a) and inset in Fig. 1(b). Forceramic, the EMR spectra change upon cooling in a stronglydifferent manner. Namely, the DIN starts to increase sharply al-ready below 260 K; beginning from about 130 K, the EMR linedistorts and shifts to lower resonance field. This shift becomesstronger below of K, which results in the well-pronouncedmaximum on corresponding curve in Fig. 1(b). Suchbehavior is characteristic for typical FM ordered doped man-ganites; see, e.g., [15]. Some weak EMR lines, which are ob-served at low- region together with the above-described mainsignal, may point out the presence of several FM like phases in

sample [15].For getting a deeper insight into the nature of magnetic corre-

lations and magnetic order in La- and Mn-sites deficient LMO,we have used the model fittings of EPR parameters as it wasproposed previously [7]. This is, we assume further that DIN isproportional to transversal susceptibility , i.e., they have thesame dependence. The inversed DIN (normalized to its valueat 300 K) versus dependences are plotted in Fig. 2. One cannote linearity of plots in some temperature intervalsfor and , as well as for and sam-ples in Fig. 2(a) and (c). This allows us to fit these data with thestandard Curie–Weiss (CW) law: ( is the CWtemperature).

4350 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009

At the same time, the curves are sub-linear witha straight-line high- asymptote for and sam-ples—Figs. 2(a) and (b). They were fitted in the respective -in-tervals of 220–440 K and 200–370 K using the Néel PM-suscep-tibility formula:

(1)

derived initially [9] for ferrites and applied further to dopedmanganites [7], [16]. The applicability of (1) evidences on apresence of two distinct exchange coupled AFM and FM corre-lated spin subsystems. Here is the total Curie constant,and are known [9] combinations of the subsystems’ CW tem-peratures and Curie constants.

The above-noted complexity of EPR spectra, characteristicfor La MnO crystals, makes problematic an accurate extrac-tion of their PM in wide enough for model fitting -inter-vals. Thus, we have analyzed for LaMn O seriesonly; see Fig. 3. Our analysis starts from Mori formula used byHuber et al. [10]:

(2)

where and are the Curie and actual susceptibility,respectively, of the resonating spins, and is the Laplace-transformed normalized time correlation function of quantumtorques, causing the total system spin to relax. The original ap-proximation of ( beinglarger than the CW temperature) [10] works well forsample and the model fit of its PM in Fig. 3 havebeen performed using (2). However, the excess charge carriersmay drastically modify as compared to this Huber’s para-digm [7]. It seems to be exactly the case for the analyzed PM linewidth versus dependence of the ceramic in Fig. 3.To model the carriers-assisted mechanism of spin relaxation inthis sample, we assumed that

(3)

where the first term is due to pure ion-ion spin relaxation [10]and the second one is due to mobile electrons, being a pa-rameter of an interaction between electrons and impuritieswith spin-reversal; see [17]. One can note that (2) and (3) excel-lently fit the experimental data in Fig. 3. The results of the CWfits and those with (1)–(3) are discussed further on.

The curves in Fig. 1(a) show that the FM cor-relations strengthen and then fade upon cooling. This defi-nitely evidences on non-FM ground state in the La MnOsingle crystals. However, the presence of FM DE-coupledMn -Mn ions clusters [12] means that the La-site vacan-cies dope the holes to the parent LMO. But nonzero DIN valuesat low testify for very random distribution of these clusterswithin the crystals, which is enhanced by chemical/structuralinhomogeneity, inherent to low-doped LMO crystals [8], [20].This claim is strongly supported by experimentally observedfreezing of FM spin moments in the strong field of AFManisotropy and resulting vanishing of -band EMR signal atlow- in homogeneous doped LMO ceramics [18]. Thus, it

Fig. 3. Temperature dependences of the linewidths for the representativeLaMn O ceramics (points) and their fits at PM temperatures (lines) asdescribed in the text.

is not surprising that the above-noted strong magnetic inho-mogeneity in La MnO crystals leads to multiline EMRspectra reported in [12]. This fact shows that the “bottleneck”regime (in-phase precession of the total spins of all magneticsubsystems in sweeping magnetic field), assumed in [10], isnot valid in the considered case. The CW fittings ofdependences in Fig. 2(a) evidence that AFM exchange prevailsin the crystal ( K), whileK is observed for , so the FM exchange dominatesDIN in the PM state. Finally, the Néel fit (1) reveals for the

sample the presence of two spin subsystems showingK, i.e., an enhancement and domi-

nance of the FM correlations in the PM state. The downwardconvex of curves at above the linear fit intervalsfor and crystals in Fig. 2(a) may be plausiblyexplained by spatial changes of values due to doped holesdensity fluctuations (i.e., strong spatial fluctuations of localDE due to enhanced chemical/structural disorder). The -in-tervals of model fits in Fig. 2(a) increase versus , which maytestify for decrease of such fluctuations, i.e., improvement ofLa MnO homogeneity. While, low- DIN data evidenceon non-FM ground state in the La MnO crystal contrary toits ceramic counterparts [2], [6].

The weak extra EPR line observed in sample [14]appears due to technologically unavoidable presence of someMn impurity in nominally stoichiometric LaMnO [19]. Fur-ther introduction of Mn-site vacancies dopes the holes in LMOmuch more effectively than in the case of La-site vacancies[3]. Thus, DE-coupled Mn -Mn clusters are introduced inLMO matrix, and an effect of its spatial disorder, similar tothose in La MnO crystals, is observed for andLaMn O ceramics; compare inset in Fig. 1(b) and Fig. 1(a).The best fit of (1) to data for and correspondingCW fit for samples in Fig. 2(b) and (c) revealed

K and K, respectively. The positiveCW temperature for basically AFM LMO appears due to thecanting of its AFM structure [7], [13]. The CW fit todata for ceramic in Fig. 2(c) confirms the prevailingof FM short range ordering in its PM state, which results in

K, being in agreement with the Curie tempera-ture of this compound K [14].

AUSLENDER et al.: ELECTRON MAGNETIC RESONANCE STUDY 4351

The best fits of (2) and (3) to PM in Fig. 3 result inthe parameters: K and K;G and G for and ceramics, respectively. Afine agreement between the values of CW temperatures obtainedfrom model fits of both PM and depen-dences evidences on self-consistency of our fittings. The notabledecrease of versus testifies for weakening of ion-ionspin relaxation. This suggestion is also confirmed by revealingthe nonactivated and linear versus contribution to for

ceramic; see (3) and Fig. 3. Such a contribution ischaracterized by parameter GK . This valueis close to those obtained for nano-grained La Ca MnO[17]. Upon this similarity and analysis of [17], we suggest that aspin relaxation of band-like electrons via impurity centers is alsocharacteristic for LaMn O . This fact suits well with the FMlike ground state of this sample; see Fig. 1(b) and [13]. Thus, for

ceramic, at least on spatial scales probed by the PMspin dynamics, the FM correlations develop in a homogeneousmatrix. However, some magnetic inhomogeneities may exist onlarger scale even in this ceramic, which causes appearance ofthe above-noted multiphase FM ground state.

In summary, the results obtained allow us to propose thatthe inhomogeneous La MnO single crystals demonstrateenhanced spatial disorder of La-site vacancies, leading tostrong fluctuations of doped holes density and local DE in-teractions through the volume of crystals. This may be amechanism, responsible for suppression of long range ferro-magnetism in La MnO crystals and distinguishing it fromboth La MnO and LaMn O ceramics. So, the seriesof LaMn O ceramics, synthesized using relatively low-solid state reaction, appear to be much more structurally/chem-ically homogeneous as compared to La MnO crystals. Thisresults in effective suppression of spatial disorder of Mn-sitevacancies distribution versus , and the LaMn O ceramicdemonstrates FM-like ground state together with band-likepropagation of doped carriers. These data emphasize a crucialeffect of strong chemical/structural disorder on magnetic andtransport properties of self-doped LMO single crystals in agree-ment with the results, derived previously for their low-dopedcounterparts [8], [17], [20].

ACKNOWLEDGMENT

This work was supported in part by the ISF under Grant845/05.

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