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Spin dynamics, electrical and magnetic properties of( La,Pr)0.7Pb0.3Mn1-xCuxO3 perovskites

?Corresponding author.Department of Physics,University of Bristol, Bristol BS81TL,UK.Tel.:+44(0)1179288750;fax:+82432747811.

E-mail address:ptlong2512@https://www.wendangku.net/doc/ef2864371.html,(T.L.Phan).

applications such as magnetic sensors,magnetoresistive read heads,and magnetic refrigerators[13,16].

In accessing the physical mechanism of the CMR effect in the doped manganites,a double-exchange(DE)interac-tion model was proposed by Zener[17].This model considers electronic-exchange processes between Mn3+ and Mn4+ions via the Mn–O bond and the Mn–O–Mn bond angle[17,18].However,Millis et al.[19,20]recently indicated that the only DE model is not suf?cient enough to elucidate the entire physical picture of the CMR effect in the manganites and that addition into the DE model the Jahn–Teller effect,which arises from a strong electron-phonon coupling,is extremely important.This has been experimentally veri?ed by several studies of Raman scattering,neutron powder diffraction,and electron spin resonance(ESR)[5,18].Despite a number of previous studies[18–20],the understanding of several physical phenomena such as phase-separation and magneto-trans-port mechanism in the manganites still remains controver-sial in part due to the complex nature of the problem[21]. With the hope of gaining some more insight into the nature of the electrical transport and magnetic properties as well as internal dynamics of such a doped manganite,in the present work,a thorough study of the electrical and magnetic properties of polycrystalline(La0.5Pr0.5)0.7Pb0.3 Mn1àx Cu x O3(x?0,0.02)perovskites were carried out by means of magnetization,resistivity,and electron spin resonance(ESR)measurements.The experimental results reveal that the partial substitution of Mn by Cu resulted in the weakening of double-exchange ferromagnetic interactions in the Cu-doped sample.No DE process took place between Mn3+and Cu2+ions.The internal dynamic properties of the samples were exposed by ESR spectra.

2.Experimental details

(La0.5Pr0.5)0.7Pb0.3Mn1àx Cu x O3(x?0,0.02)polycrys-talline samples were prepared by conventional solid-state reaction.Powder precursors of La2O3,Pr2O3,PbO,CuO and MnCO3with a high purity were well-mixed stoichio-metrically and then pressed into two pellets.The pellets were pre-sintered in turn at850and9001C for15h,after several intermediate grinding and pressing.Finally,they were annealed at10001C for15h,and then slowly cooled down to room temperature.The whole processes were carried out in the normal air condition.The quality of the ?nal samples was checked by powder X-ray diffraction patterns(D5005-Brucker).The temperature dependence of the DC resistivity was measured by the four-probe technique using a closed cycle-helium refrigerator.Zero-?eld-cooled(ZFC)and?eld-cooled(FC)magnetiza-tions were performed on a vibrating sample magneto-meter(VSM),DMS-880,with a maximum?eld value of 1.5T.ESR measurements were carried out with a JEOL-JES-TE300ESR spectrometer operating at9.2GHz (X-band).3.Results and discussion

Fig.1shows the X-ray diffraction patterns of (La0.5Pr0.5)0.7Pb0.3Mn1àx Cu x O3(x?0,0.02)samples. The samples are single-phase with an orthorhombic structure;the lattice parameters(a,b,and c)are summarized in Table1.As one can see clearly from the table,with addition of Cu,the lattice constants of the Cu-doped sample(x?0:02)slightly decreased comparing to the Cu-free sample(x?0).This is understandable because of the fact that the radius of the Cu2+ion is smaller than that of the Mn3+ion.Similar trend was reported by other authors[21].

M ZFC(T)and M FC(T)curves measured at20Oe are displayed in Fig.2.The ferromagnetic(FM)-to-paramag-netic(PM)phase transition temperature,T C,determined from these magnetization curves,is about326and300K for x?0and0.02compositions,respectively.This indicates a considerable reduction of T C in samples with Cu substitution for Mn.However,the T C of the x?0:02 sample is still of300K,which may be of interest in the development of magnetic refrigerants for room-tempera-ture magnetic refrigeration applications[16,22].A reduc-tion in magnetization of the x?0:02sample compared to the x?0sample could be ascribed to the decrease in FM interactions due to the Cu doping effect[21].It is worth noting that,at temperatures below T C,a separation of 30

2030

(

)

40506070

2Θ (°)

x = 0.02

x = 0

Fig.1.The X-ray diffraction patterns of(La0.5Pr0.5)0.7Pb0.3Mn1àx Cu x O3 (x?0,0.02)samples.

Table1

The lattice parameters of(La0.5Pr0.5)0.7Pb0.3Mn1àx Cu x O3(x?0,0.02) samples

x a(A)b(A)c(A)V(A3)

0 5.499 5.4647.736233.44

0.02 5.478 5.4827.705231.38

T.L.Phan et al./Physica B371(2006)317–322 318

M FC (T )from M ZFC (T )for both compositions was observed.This is typically symptomatic of ferromagnets with a strong anisotropy ?eld arising from FM clusters that are usually observed in unconventional ferromagnets.With the presence of the FM clusters,magnetic moments of spins inside the cluster could be frozen in directions energetically favored by their local anisotropy or an external magnetic ?eld as the system is cooled down from a high temperature in zero or non-zero magnetic ?eld,respectively [23].This feature seems to be very widespread in manganese perovskites as FC magnetization is per-formed at low applied ?elds.When the applied magnetic ?eld,H ex ,is high enough,such feature will disappear and,M ZFC (T )and M FC (T )curves become coincide with each other because of the fact that the suf?ciently high ?eld could suppress entirely anisotropy ?elds in the system (see the inset of Fig.2,magnetization of the two samples measured at 1kOe).

Fig.3shows the temperature dependence of the DC resistivity of the x ?0and 0.02samples.As one can see from Fig.3,with respect to the increase of temperature the x ?0sample exhibited a metallic-to-insulator transition at $130K while only insulating behavior was observed in the x ?0:02sample in the whole temperature range investi-gated.In the present case,we did not measure resistivity values at lower temperatures,maybe,there was the metallic behavior [24].As shown earlier in Refs.[7,24]on a (La 0.5Pr 0.5)0.7Ca 0.3MnO 3expitaxy ?lm and Pr 0.65Ca 0.35àx Sr x MnO 3compounds,a metallic-to-insulator transition followed by a broadening band around the phase transition temperature was observed.This coincides with what we observed here on (La 0.5Pr 0.5)0.7Pb 0.3MnO 3(x ?0),and can be explained due to the presence of charge-ordering states [24].It should be noted that the magnitude of the resistivity of the x ?0:02sample at a given temperature is larger than that of the x ?0sample.In connection with the magnetization data,it is reasonable to conclude that the Cu addition leads to a decrease of the ferromagnetic interaction and conductivity of the parent compound (x ?0).This might be related to changes in the exchange mechanism occurring among e g electrons of Mn and Cu ions [21].Here,an emerging question is why the resistivity of the (La 0.5Pr 0.5)0.7Pb 0.3MnO 3(x ?0)sample becomes higher with Cu addition?

In order to address this question,the model of Mott’s variable-range-hopping (VHR)insulating behavior r (T )?r 0exp[(T 0/T )1/4](where r 0is a pre-exponential factor and T 0is a constant)has been used to analyze the resistivity data of the presently investigated samples.Determined values of T 0are 4.55?107and 16.12?107(K)for x ?0and 0.02compositions,respectively.In the present work,we have taken into account the density of states of the system at the Fermi level N (E F )-which is obtained using the equation of T 0?18a 3/k B N (E F ),where k B is the Boltzmann factor and a is the electron wave-function decay https://www.wendangku.net/doc/ef2864371.html,ing a ?2:22nm à1[24,25],N (E F )is deduced to be 1.04?1025and 0.63?1020eV à1cm à1for x ?0and 0.02compositions,respectively.This enables us to state that the decrease of the density of states on the Fermi level in the Cu-doped sample is the origin leading to the decrease of the electrical conductivity of the sample,as compared to the Cu-free sample.As can be seen in Fig.3(b),the VHR model can describe the r (T )data for the x ?0:02sample in a large temperature range,rather than for the x ?0sample.This is in good agreement with what was reported in Ref.[21]with respect to an increase of Cu content.To understand internal spin dynamics of the presently investigated samples,we recorded ESR spectra at different temperatures above from T C ,as shown in Fig.4.It can be seen that asymmetrical ESR signals at low temperatures

100

160

120180

240

3000.24

0.270.300.33

T (K)

1/4

-1/4

)

ρ (?.c m )

L n (ρ) (?.c m )

x = 0x = 0.02

(a)

(b)

Fig. 3.(a)The temperature dependence of resistivity,r (T ),for (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx Cu x O 3(x ?0,0.02)samples;(b)the logarithm of resistivity Ln(r )versus T à1/4,solid lines are ?tting curves according to Mott’s variable-range-hopping model.

2.0

1.51.0

0.5

0.0

100150200250

300350400450500

T (K)

M (e m u /g )

ZFC

H ex = 20 Oe

H ex = 1 kOe

x = 0x = 0.02

6045301580

160240320400

T (K)

M (T )

x = 0x = 0.02

Fig. 2.Temperature dependence of ?eld-cooled (FC)and zero-?eld-cooled (ZFC)magnetizations taken at 20Oe for (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx-Cu x O 3(x ?0,0.02)samples.The inset shows the temperature dependence of FC and ZFC magnetization for (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx Cu x O 3(x ?0)measured at 1000Oe.

T.L.Phan et al./Physica B 371(2006)317–322

319

became symmetrical-Lorentzian at temperatures T 4T min (T min is,in Table 2,the temperature corresponding to the narrowest ESR linewidth).With decreasing temperature in the region T C o T o T min ,the ESR signals were splitted into two lines,in which the resonant line at a lower ?eld changed in its position with temperature [21,26],see Fig.5.In particular,these two lines strongly competed in amplitude at temperatures around T min .This may be considered a signature of coexistence of two competing phases,namely,FM and PM phases.At temperatures T p T C ,the FM phase is dominant and it strongly suppresses ESR signals of the PM phase.As a matter of fact,it was found in the ferromagnetic-low-temperature region appearing several resonance lines [27–29]due to the presence of FM correlations and spins in FM micro-regions (or FM clusters).These spins were strongly in?uenced by an anisotropy ?eld (arising from itself FM clusters)added to the external magnetic ?eld.On the other hand,recent reports also pointed out experimental evidences on the presence of phase separation,this led to the additional appearance of resonance lines of mixed phases [27,28],introducing to the broadening of the ESR linewidth at low temperatures.However,at temperatures above T min ,single ESR signals in the Lorentzian shape for the samples were observed;where the samples are completely in the PM state.This differs from the results of Ref.[21],where for the Cu-doped samples the two lines arising from the coexistence of double-exchange FM and

super-exchange antiferromagnetic (AFM)interactions were observed even above T min .This differential can be understood,because of the fact that the Cu-doping level in our system is so small that formation of AFM clusters due to Cu 2+–Mn 3+and/or Cu 2+–Mn 4+interactions is trivial only.It is furthermore suggested that the origin of the observed ESR signals comes from the participation of electron spins of Mn 3+,Mn 4+,and Cu 2+ions in correlation to crystal ?elds caused by the neighboring ions [4–6,9,26].

Fig.6shows the temperature dependence of the ESR linewidth,D H eT T,for the x ?0and 0.02samples.D H eT Treached a minimum value D H min at T min (Table 2),and its

x = 0

x = 0.02

328 K 308 K

363

353

423

428x3

E S R s i g n a l (a . u )

0200400

600800200

400600800

DC field (mT)

Fig.4.X-band ESR spectra for (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx Cu x O 3(x ?0,0.02)samples at selected temperatures around T min .

Table 2

Experimental parameters obtained for (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx Cu x O 3(x ?0,0.02)samples x

T C (K)T 0?107(K)T min (K)Y (K)B (Oe/K)D H min

(Oe)E a (eV)0326 4.553633417.025590.160.02

300

16.12

351

327

6.64

716

0.14

340320

300280260340320

300280260300

320

340

360

380400420

440

460

480

T (K)

x = 0.02

x = 0

H r (m T )

Fig.5.The plot of the resonance position of high and low ?eld lines for x ?0and 0.02compositions with respect to temperature.Solid lines are guides to the eye.Two lines at T o T min due to FM and PM correlations become a sole line at T 4T min .

x = 0x = 0.02

1.4

1.21.0

0.8

0.6

340

360

380

400420440

460

480

T (K)

?H (k O e )

Fig.6.The temperature dependence of the ESR linewidth,D H eT Tfor (La 0.5Pr 0.5)0.7Pb 0.3Mn 1àx Cu x O 3(x ?0,0.02)samples;the solid lines ?t to a function of D H eT T?AT tB .

T.L.Phan et al./Physica B 371(2006)317–322

320

appearance is probably related to the exchange narrowing and decay values of the correlation function around the phase transition[9].In the temperature range studied, D HeTTof the x?0:02sample was higher than that of the x?0sample;this could be related to difference in the concentration of Mn3+and Mn4+ions in the samples. In general,the ESR linewidth for manganites con-taining either Mn3+or Mn4+ions individually is usually larger than that for manganites containing both these ions[4,9].Accordingly,in our system the Mn substitution by Cu resulted in an increase in the concentration of Mn4+ ions.

Paying attention to the variation of D HeTTat tempera-tures(lower T min),it increases because of the development of FM correlations and forming FM clusters[27–31]. However,the increase of D HeTTin the region T4T min is different,it relates to correlation of FM clusters existing on a large range of temperature[14,26];this is concerned via the activation energy values as being presented later.Based upon the relation between the resistivity reTTand D HeTTdata at temperatures above T min,on the other hand,it is stated that the hopping rate of charge carriers could limit the lifetime of the spin state,thereby resulting in a broadening of EPR spectra with increasing temperature [31,32].

Regarding the interaction mechanism in the PM region, one see that the Lande factor g of the samples is close to 2.00,and it is temperature independent.This means that the spin–spin interaction is dominant in this temperature range.And,the obtained value of g is appropriate to the one-phonon process[12,33],i.e.the variation of the ESR linewidth above T min obeys to a linear function.As can be seen in Fig.6,a function D HeTT?AtBT?ts well to the ESR linewidth data above T min,where A is a constant and B is a parameter being related to exchange,dipolar,lattice distortions,and/or Jahn–Teller?uctuations[33].The B value is determined to be7.02and6.64Oe/K for the x?0 and0.02samples,respectively.According to earlier studies

[12,32–36],we realize that La1àx A0

x MnO3manganites were

often found to have the low B value($3Oe/K)[34–36]

compared to praseodymium manganites Pr1àx A0

x MnO3

($5–7Oe/K)[33].This is perhaps due to in?uences of crystal?elds,which are caused by La3+and Pr3+ions,on Mn ions.In our case,the estimated value of B is$7.0Oe/K and this is acceptable because of the presence of both La3+ and Pr3+ions in the samples.

The temperature dependence of the ESR intensity I(T)at T4T min,determined by taking double integration of the experimental curve,for the samples is shown in Fig.7. With increasing temperature,I(T)exponentially decreased. According to the one-phonon process,I(T)p w DC(T), where w DC is the DC susceptibility,[33,36,37]and the relation between D HeTTand r(T)[6,32],I(T)can be expressed by the function IeTT?I0expeàE a=k B TT,where E a is the thermal activation energy for the dissociation of the FM spin clusters[26,32,35].This expression describes well the experimental I(T)data as shown in Fig.6.The E a value is determined to be$0.16and0.14eV for the x?0

and0.02samples,respectively.The1=IeTTvs.T plot and the extrapolation of the linear-high-temperature part of this curve to zero value,according to the Curie-Weiss law, allowed us to determine the Curie–Weiss temperatures of the samples,Y,which are summarized in Table2[4,34–36].

A reasonable agreement of this law was found at high temperatures(see the inset of Fig.7)implying an existence of FM clusters as exposed earlier by the magnetization data and the activation energy E a.

Finally,to interpret the magneto-transport mechanism of the samples,the electronic con?guration of ions present in the compounds has been taken into account.In the case of the x?0sample,both Mn3+and Mn4+ions(Mn3+is 3d4,t3

,with S?2,whereas Mn4+is3d3,t3

,with S?3=2)coexist and follow the strong Hund’s rule coupling.The spins of these ions orient parallel to each other thus leading to the fact that the interaction governing them is FM[17].Meanwhile,the exchange couplings of Mn4+–Mn4+and/or Mn3+–Mn3+ions are AFM super-exchange interactions[38,39].When substituting a small amount of Cu for Mn,the Mn4+concentration in the sample increases,and the AFM interaction is intensi?ed due to the additional appearance of the super-exchange couplings of Cu2+–O–Mn3+,4+and Cu2+–O–Cu2+.As a result,the FM interaction is declined.That is why both the T C and the activation energy value E a decreased in the Cu-doped sample.It should be furthermore noted that the x?0:02sample contains Cu2+ions with3d9con?guration,

and S?1=2,one free-electron spin on the e g level which couples weakly with the t2g-core[38–40].Therefore, the interaction between Cu2+and Mn3+/4+ions is actually AFM.

330360390420450480

T (K)

(

)

(

)

x = 0

x = 0.02

Fitting curve

360380400420440460480

Fig.7.The temperature dependence of the ESR intensity for (La0.5Pr0.5)0.7Pb0.3Mn1àx Cu x O3(x?0,0.02)samples;the symbols shows the experimental data and the solid lines are?tting curves with the function IeTT?I0expeàE a=k B TT.The inset shows the1=IeTTvs.T curve,according to the Curie–Weiss law.

T.L.Phan et al./Physica B371(2006)317–322321

4.Conclusions

The electrical and magnetic properties of(La0.5Pr0.5)0.7 Pb0.3Mn1àx Cu x O3(x?0,0.02)perovskites have been thoroughly studied.It was found that with Cu addition the Curie temperature decreased from326K for x?0 composition to300K for x?0:02composition.The resistivity of the Cu-doped sample was higher than that of the Cu-free sample.It is interesting to note that the x?0sample underwent a metallic–insulator transition at $130K,whereas the x?0:02sample exhibited an insulat-ing behavior in the entire temperature investigated.The results obtained from ESR measurements show that asymmetrical ESR signals at low temperatures became Lorentzian at high temperatures above T min.Temperature dependence of the linewidth,D HeTT,at T4T min?tted well to the one-phonon model,D HeTT?ATtB.The activa-tion energy E a are estimated to be0.16and0.14eV for x?0and0.02compositions,respectively.In terms of our experimental results,it is reasonable to conclude that the Cu addition led to a suppression of the ferromagnetism and conductivity of the parent compound(x?0). Acknowledgements

This work in Vietnam was supported by the National Fundamental Research Program(Project421004),and in Korea was supported by the Korea Research Foundation Grant(KRF-2003-005-C00018).

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