Appl. Magn. Reson. 24, 351-360 (2003) AppUed Magnetic Resonance �9 Springer-Verlag 2003 Printed in Austria

Spin Dynamics Investigated by EPR in the Paramagnetic Regime of La2/3Cal/3Mnl_xMexO 3 (Me = Al, In) Manganites

L. V. Giurgiu ~, M . N . Grer 2, X. Filip ~, O. Raita ~, Al. Darabont 3, D. Gavre ~ and J. Blasco 4

~National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania

2National Institute for Material Physics, Bucharest, Romania 3 Faculty of Physics, Babes-Bolyai University, Cluj-Napoca, Romania

4 Departamento de FŸ de la Materia Condensada, Universidad de Zaragoza, Zaragoza, Spain

Received September 30, 2002

Abstract. We repon an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of the colossal magnetoresistive manganites Lav3CamMn~_xMexO 3 (Me = Al, In; x < 0.05). The temperature dependences of the EPR linewidth and integral intensity have been ana- lyzed in temas of the bottleneck spin relaxation and small-polaron hopping models. The exchange cou- pling integral between Mn 3§ and Mn 4§ ions and the polaron activation energy decrease with increasing doping level. A discussion is given concerning the factors which could explain the observed changes.

1 Introduct ion

The perovski te manganites (Rl_yAyMnO 3 where R and A are t r ivalent rare-earth and divalent cations, respect ively) are wel l -known compounds for their colossal magnetoresis t ive propert ies (CMR). They are mixed-valent compounds contain- ing Mn 3+ and Mn 4+ ions. The coexistence o f ferromagnet ism and metal l ic con- duction in these materials has been explained in terms o f double exchange mecha- nism (DE) [1]. However , DE interact ion alone is not sufficient to expla in the observed CMR and an addit ional mechanism on the basis o f the polaronic ef- fects was included [2].

Since Mn ions p lay a key role in the electrical and magnet ic propert ies o f CMR materials , a better understanding o f the under lying phys ica l mechanisms could be achieved by destabil izing the Mn sublattice. Despite the exhaustive smdy o f the effects o f the rare-earth replacement in these mangani tes , the influence o f the subst imtion at Mn sites with closed-shell ions is much less invest igated [3 - 7]. It has been recognized so far that this substitution induces two types o f ran- domness: (i) random potential f luctuation because the electronic configurat ion o f

352 L.V. Giurgiu et al.

Me ions is different from that of Mn 3+ and Mn 4+ (electrical randomness) and (ii) local cutoff of the magnetic interaction between the t2g spins (magnetic ran- domness) [6].

The study of the spin dynamics in CMR manganites is very important be- cause the DE mechanism is basically a dynamical process. Electron paramag- netic resonance (EPR) spectroscopy is ideally suited in probing the dynamics of the spins over a wide temperature range [8-13]. In the present work we have investigated by EPR the effects of the substitution of Mn with diamagnetic ions (Al, In) in the paramagnetic regime of the La2/3Cal/3MnO 3 manganite. Particular attention is devoted to the discussion conceming the influence of the doping on the exchange coupling integral between Mn spins and the polaron activation energy. Some preliminary results have been recently reported [14-16].

2 Experimental

We have chosen Al and In to make the substitution because these ions have (i) a closed-shell configuration, with no d electrons and no magnetic moments and (ii) the same valence as the host manganese (+3). The only difference arises from their atomic radii, i.e., (r>Al < (r>M, < (r)i, .

Polycrystalline Laz/3Cal/3Mnl_xMexO 3 (Me = Al, In; x _< 0.05) were prepared with the conventional solid-state reaction method [3, 4]. The X-ray powder dif- fraction analysis confirmed that the samples are composed of a single phase. Table 1 shows the Mn 4+ contentas determined by redox titration, the transition temperature T c a n d the Curie-Weiss temperature 0 from the altemating-current magnetic susceptibility curves [3, 4, 16]. The samples with x < 0.05 and x < 0.03 for Al and In doping, respectively, are nominally stoichiometric; and in order to preserve the charge equilibrium, the substitution of Me for Mn leads to an oxi- dation from Mn 3+ to Mn 4+ (Table I). The T c and 0 values are reduced as x is increased showing a general weakening of the DE interaction upon doping.

In the paramagnetic regime the EPR spectrum for all investigated samples consists of a single line with g ~ 1.99 [14]. The lineshape of the powdered samples was found to be Lorentzian over the investigated temperature range and

Table 1. Percentage of Mn 4+, critical temperature T o Curie-Weiss temperature 0 and the ratio Tc/O for Laz3Ca~q 3.

Dopant x Mn 4+ (%) T c (K) 0 (K) Tc/O

Al

In

0.00 30 263 339 0.78 0.01 34 217 318 0.68 0.03 34 203 260 0.78 0.05 35 189 249 0.76 0.01 31 235 294 0.80 0.03 32 190 220 0.86

EPR of La2/3Cat/3Mnl_xIVIexO 3 (Me = Al, In) Manganites 353

for all x. It confirms the presence of the exchange-narrowed Mn dipolar fields [15, 16]. Since the sample size effects due to magnetic losses affect the spec- trum [11], we have used a small amount of powder material, m = 0.5 mg, for each x. In order to evaluate the EPR parameters, the de¡ spectra were fitted with a Lorentzian lineshape with fit parameters being the half-width at half- hight, 5H~/2, of the corresponding absorbtion line and the resonance field, H 0.

3 Spin Dynamics in the Paramagnetic Regime

In the mixed-valence manganites, Mn 3+ (3d 4 with S = 2) and Mn 4§ (3d 3 with S = 3/2) ions are in octahedral symmetry, their electronic configurations being t2geg3 ~ and t2geg3 o for Mn 3+ and Mn 4+, respectively. The basic feature of the DE mechanism is the hopping of the eg electron between neighboring Mn 3+ and Mn 4+ ions through the Mn-O-Mn path [1]. It could also be looked upon a s a transfer of a d-hole from Mn 4+ to Mn 3+ via the oxygen. The electron transfer is favored when the spins of the t2g electrons of Mn 3+ are aligned with the t2g spins of the adjacent Mn 4+ ion since in this case the carrier avoids the strong-site Hund ex- change energy and hops easily. Therefore, eg electrons are considered as mobile carriers being responsible for the electronic conduction and the ferromagnetism occurs simultaneously with metallic conduction.

The electronic ground state of the Mn 3+ ions is degenerate (orbital doublet) and the spin-tattice relaxation is very strong [8]. This degeneracy is removed by the dynamic Jahn-Teller effect (JT) and one expects a strong electron-phonon coupling. The mobile eg electron is also expected to carry the lattice distortion with it and thus the JT distortion of Mn 3+ may assist the polaron formation. Therefore, the polaronic effects have to be included in the DE mechanism re- sulting in a polaronic contribution to the conduction in the paramagnetic regime of CMR materials [2]. EPR could provide experimental evidence of JT polarons in Lal_yCayMnO 3 [8, 9, 17].

According to the scenario developed in refs. 8 and 9, the characteristic fea- tures of the EPR integral intensity and the linewidth in CMR manganites could be caused by spin-lattice relaxation of the entire exchange-coupled Mn3+-Mn 4§ spin system under the condition of a strong relaxation bottleneck. Here, the spin- relaxation rates for the exchange-coupled Mn 3§ and Mn 4§ ions are much larger than the spin-lattice relaxation rates. On the basis of the observed similarity be- tween the temperature dependence of the EPR linewidth and that of the polaron hopping conductivity in the paramagnetic regime [9], the energy transfer in the relaxation mechanism could be provided by the thermally activated hopping of small eg polarons.

In the bottleneck spin-relaxation scenario, the EPR integral intensity, IePR, is proportional to the total susceptibility, Xtotal of Mn 3+ and Mn 4+ spins [9]

]EPR OC ~total = Xs-k-X~, (1 )

354 L.V. Giurgiu et al.

where Z~ and Z~, are the renormalized static susceptibilities of the Mn 4+ and Mn 3+ spin systems, respectively,

1 + 2Z ~ 1 + 2Z ~ Z s = Z ~ 1 2 . 0 . 0 ' Z" = Z~ l _ & Z=Z~ - 2 ,�91 2 .0 .0"

Here Zs and Z,, are the bare (without exchange) susceptibilities of the Mn 4+ and Mn 3+ ions. The parameter 2 is given by

)7. = zJk8

Ng~go/-4/~o '

where z = 6 is the number of nearest neighbors, N is the total number of Mn spins per cubic centimeter, gs (g~) is the g factor of Mn 4+ (Mn 3+) ions and J is the exchange coupling integral for the exchange interaetion between Mn 4+ and Mn 3+ spins.

We assumed that the bare susceptibility of Mn 4+ ions follows a Curie law Zs ~ = CJT, while that of Mn 3+ is given by the Curie-Weiss law, Z ~ = C ~ / ( T - 0 ) , where 0 = - 2 2 0 K is taken to be the same as for undoped LaMnO 3 parent compound [18].

It is well-established that in the paramagnetic regime of manganites, the conductivity o- is dominated by the adiabatic hopping motion of small polarons with the temperature dependence of the form [19]

cr(T) = AT- ' exp( -EJkBŸ ).

Considering the proportionality between • and 8H1/2, the temperature-depen- dent part of the linewidth should arise from a thermally activated process in- volving the adiabatic hopping of small polarons. In this case, 8H~/: can be ex- pressed by [9]

8H~/2(T ) = 6H o + BT-'exp(-Ea/kBŸ (2)

where E a and 8H o are the polaron activation energy and the residual linewidth, respectively.

4 Resu l t s a n d D i s c u s s i o n

In what follows, the effects of the substitution of Mn with Me ions on the spin dynamics in paramagnetic regime are presented and discussed in terms of the bottleneck spin-relaxation scenario.

We can get an estimate of J by fitting Eq. (1) to the IEPR" T data (Figs. 1 and 2). Here, we used the multiplication of IEpR by T in order to eliminate the intrin- sic temperature dependence of the EPR signal caused by the Boltzmann popula-

EPR of La2/3Cal/3Mnl_~MexO 3 (Me = Al, In) Manganites

16

14-

12-

~-10- 8-

4-

2-

\ [] x=0

260' 250 ' 300 ' 350 ' 400 ' 450 ' 5 6 0 ' 550' 600 T(K)

Fig. 1. Temperature dependence of IEea" T for La2:3Cat/3Mnl_~AlxO 3 (x = 0.00, 0.03).

355

tion o f the Zeeman levels involved [8]. In both Figs. 1 and 2, the solid lines rep- resent the best fit to Eq. (1) in the temperature range o f 1.5T c _< T < 500 K.

The results are given in Table 2. For the x = 0.00 sample, the determined J value is o f the same order o f magnitude as the values obtained for Lao.sCao.2Mnl603+ ~ (3" = 78 K) [9], La0.sCa0.2MnlSO3+ 8 (J = 71 K) [9] and Pr0.6Ca0.4MnO 3 ( J = 154 K) [20] from EPR measurements by using the bottleneck scenario.

As one can see, the effect o f Me doping is to perturb the DE coupling be- tween Mn 4+ and Mn 3§ ions, causing a continuous reduct ion in J wi th doping level (Table 2). The dilution o f J in In-substi tuted samples is h igher that the cor- responding one for Al substitution.

14

12-

10~

2- o

\ \ ,x x=0.01 In

;0 3bo ' 3~0 ' 4bo ' 4~0 ' 5bo ' 5~0 ' T (K)

)0

Fig. 2. Comparison of the temperature dependences of IEpa' T for La2/3Calq lnxO 3 (x --- 0.01, 0.03).

356 L.V. Giurgiu et al.

Table 2. Exchange coupling integral J between Mn spins, polaron activation energy E, and the re- sidual linewidth 5H 0 in the paramagnetic regime of Laz~3Cal/3Mnt_xMexO 3.

Compound x J (K) E a (meV) 6H 0 (G)

La2/3Cal/3MnO3 La~/3Ca,3Mnl _xAlxO3

La2/3Cat/3Mnl _xlnxO3

116 120 24 0.01 93 109 111 0.03 84 103 126 0.O5 73 98 133 0.01 85 106 77 0.03 71 97 138

it is well established that the DE interaction depends on both the Mn-O dis- tance and Mn-O-Mn bond angle [21]. The bending of the Mn-O bonds in the ab plane suppresses the DE interaction and promotes the antiferromagnetic con- tributions. In the substituted samples the average Mn-O distance varies, depend- ing on the Me-atomic radius compared to that of Mn, with the increase of dop- ing level [3, 4]. On the other hand, the average Mn-O-Mn angle is rather con- stant in Al-substituted samples [3], while it decreases in In-substituted ones [4]. Ah enhanced bending of the Mn-O-Mn bond due to the large ionic size of In 3+ is expected to slow down the charge transfer and weaken the ferromagnetic cou- pling between Mn 3§ and Mn 4§ ions. It could be the reason for the observed larger reduction in J with In doping in comparison with the corresponding one for Al substitution (Table 2).

Therefore, the DE mechanism which is a process consisting of a Mn-Mn indirect exchange through the oxygen orbitals is very sensitive to the presence of Me impurities. The structural changes upon doping (x < 0.05) could qualita- tively explain the decrease of J in La2q161 samples because they weaken the DE interaction. A similar effect in Cr-doped La2/3Ca~q 3 was observed by EPR [22]. Here, a reduction of 20% in J for x = 0.025 was evalu- ated by means of the spin-only interaction scenario [11].

In the following we discuss the temperature dependences of the EPR linewidth 5Hi/2 which are shown in Figs. 3 and 4 for samples with various Al and In concentrations x, respectively.

The linewidth goes through a minimum at /'mi n ~ 1.1 T c and the temperature Tmi n shows a decrease with increasing x in accordance with the T c variation, a behavior similar to that observed for undoped compounds [14]. Its increase be- low Tmi n is attributed to the usual critical slowing down in ferromagnets.

In the paramagnetic regime, T > Tm~ n, the temperature dependence of 5H~/2 for the La08Ca0.2MnO 3 compound was found to be similar to that of the electri- cal conductivity and could well be described by the small polaron hopping model [9]. We have observed an alike behavior for La2/3Cal/3Mn~_xMexO 3 manganites.

Figure 5 shows a comparison between the tinewidth and the conductivity plot- t edas ln(SH~/2 �9 T) and ln(cr. T), respectively, vs. 1000/T for the x = 0.00 sample. The o- data are taken from ref. 19 and correspond to a La2/3CamMnO3+ 6 thin

EPR of Laz/3Cau3Mnl_xMexO 3 (Me = Al, In) Manganites 357

800

700-

600~

500-

400~

300~

200-

100 150 '

"~~ �9 x = 0.03Al

2b0 ' 2~0' 3b0' 3~0' 400' 4~0" 5()0 ' 5~0 r (K)

Fig. 3. Temperature dependence of the linewidth 8Hin for La2/3CamMnl_~Al~O3. The solid lines are the best fit to the small polaron model.

film having the highest annealing temperature. The solid lines are linear fits dem- onstrating that both 8H m and cr follow the temperamre dependence characteris- tic for the adiabatic small polaron hopping model with a similar value of the activation energy.

The small polaron hopping model could also describe the temperature depen- dence of 8HI/2 for the substituted manganites. In the temperature range T > 1.5T c we have found that 8HI/2(T) for La2/3Cal/3Mnl_xMe~O 3 samples (x < 0.05) increases and can wetl be fitted by Eq. (2). The solid lines in Figs. 3 and 4 represent the best fit to the data corresponding to some typical doping levels, The fitting parameters, the polaron activation energy E~ and the residual line-

800

700~

600~

500~

400~

300~

200~

100" 150'

D

2 ~ 0 2 5 0 ' 3b0 350' 4 0 0 ' 4 5 0 ' 500"550 r (i<)

Fig. 4. ~HI/2 v s . T for La2/3CamMnl_~In~O 3 manganites. The solid line are the best fit to Eq. (2).

358 L.V. Giurgiu et al.

12 ~...~_ [] Linewidth 11 ~ ~ �9 Conductivity

l

�91

7 116 ' 210 2.4 2.8 3.2 6

1000 / T ( K -l)

Fig. 5. The temperature dependence of 6H~/2 and tr in the paramagnetic regime of La~~3Ca~q 3.

width 8HI/2, for all investigated samples are summarized in Table 2. A fit to Eq. (2) was also used to evaluate the activation energy for polycrystall ine La0.sCa02MnO 3 and Pr0.65Ca0.2sSr0.07MnO 3 and results in E a = 106 meV [9] and 110 meV [23], respectively.

An analysis of Lal_yCayMnO 3 and LaMnO3+ a series has shown a strong in- crease in E a as the Mn 3+ content increases [24]. It was argued that the polaron binding energy was proportional to the Mn-O distortion and there was a linear relationship between the Mn-O distortion and the Mn 3+ content. Our results on La:q 3 manganites are in agreement to that conclusion. For small doping levels (x < 0.05), the oxidation process from Mn 3+ to Mn 4+ leads to an increase of Mn 4+ content with increasing x (Table 1). In addition, the substitu- tion of Mn with Me ions produces changes in Mn-O distance. From Table 2 one can see that E a decreases with increasing doping level x, i.e., the decreasing Mn 3§ content or, similarly, increasing Mn 4§ content. A more pronounced decrease in E a with In-doping level is also evidenced.

In contrast to our findings, the nonmagnetic Ga (identical ionic radius with Mn) substitution at the Mn sites in La2/3Cal/3MnO 3 implies an increase of E a with increasing doping level [25]. It was suggested in this case that the main influ- ente of Ga doping is not on the lattice distortion but on the local magnetic struc- ture. Obviously, more experimental work has to be done in order to clarify the influence of the Mn site substitution on the transport and magnetic properties of CMR materials.

5 Conclusions

The effects of the partial substitution at Mn sites with Me ions (Al, In) in Laz/3Ca~q 3 manganite have been investigated by EPR. The temperature de-

EPR of La2:3Ca)/3MnijVle~O3 (Me = Al, In) Manganites 359

pendences of the EPR integral intensity in the paramagnetic regime of the sub- stituted samples can be well described by the bottleneck spin relaxation sce- nario. The degradation of the DE interaction by Me substitution results in a decrease of the exchange coupling integral J between Mn spins with increasing doping level. The observed dilution in J might be explained by the structural changes upon substitution, i.e., the bending of Mn-O-Mn bond and the Mn-O distortion, which are expected to slow down the charge transfer and weaken the DE coupling. Considering that the relaxation path is provided by thermally ac- tivated hopping of small eg polarons, the activation energy Ea was evaluated from the temperature dependence analysis of the EPR linewidth. The observed de- crease of E a with increasing Me-doping level could result from both the decrease of Mn 3+ content and the lattice distortion.

Acknowledgements

L.V. Giurgiu acknowledges fruitful discussions with Prof. M. Mehring. This work was supported by MEC under CERES programme, project nr. 52.

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Authors' address: Liviu V. Giurgiu, National Institute for Research and Development of Isotopic and Molecular Technologies, P.O. Box 700, 3400 Cluj-Napoca, Romania