Colossal magnetoresistance in manganese oxide perovskites

Journal of Magnetism and Magnetic Materials 177-181 (1998) 846-849

Journal of magnetism and magnetic materials

ELSEVIER

Invited paper

Colossal magnetoresistance in manganese oxide perovskites M.R. Ibarra*, J.M. De Teresa Departame~to de Fisica de la Materia Co~Tdensada& ICMA, Universidad de Zaragoza-CSIC, 50009-Zaragoza, Spain

Abstract The large magnetoresistance observed in the manganese-oxide-based perovskites is explained on the basis of a strong electron-phonon and ferromagnetic interactions which provide the mechanism for the existence of dynamic electronic phase segregation in the form of magnetic polarons. These entities are responsible for the magnetic and lattice effects observed in these compounds. In this experimental work we give evidence of their observation by using several techniques. ~ 1998 Elsevier Science B.V. All rights reserved. Keywords: Magnetoresistance-colossaI; Perovskites; Eiectron-phonon interaction

The recent interest in the spin-polarization-based magnetoresistance effects observed in artificial multilayers has given relevance to the so-called giant magnetoresistance (GMR) [1, 2]. Devices based on this effect, like 'spin valves', are of current interest for application and a new branch (called 'magnetoetectronics' 1-3"]) has emerged. Bulk materials with layered structures such as SmMn2G% (GMR of 4-8 %) 1-4] and FeRh (GMR of up to 50%) [5-1 have also interesting performances for application as G M R systems. Von Helmolt et al. 1.6] reported a large increase of the resisitivity in a thin film of the mixed-valence manganite La0.6vBa0.3~MnO3 near the para-ferromagnetic transition. This work triggered further investigations on these compounds. Jin et al. I-7"] discovered a value of magnetoresistance as high as 10~% in a bulk polycrystal of Lao.6oY0.oTCao.3sMnO> Then they coined the term colossal magnetoresistance (CMR). It is well known at present that this huge value for the magnetoresistance can even be increased by seven orders of magnitude because the nature of the CMR is related to the metal-insulator transition which takes place in these compounds, tn manganites of formula Rz -~AxMnO3 the substitution of a R 3~ ion by an alkaline earth A 2 + gives rise to the appearence of 3d holes (Mn 4+, 3d 3) in a narrow band with % symmetry. This is due to the octahedral

*Corresponding author. Tel.: + 34 976 761215; fax: + 34 976 761229; e-mail: [email protected].

0 2- environment of the Mn, which splits the five-fold orbital degeneracy of the ground state in a triplet ground state tzg and an % doublet. It is well established that a strong intra-atomic Hund coupling giving a local magnetic moment with the maximum multiplicity (also called high-spin state) exists. The itinerant character of the eg electron (or hole) is provided by the covalent nature of the M n - O bond, which produces a net charge transfer between Mn ions. This mechanism gives rise to an indirect ferromagnetic interaction (Zener-type) called double exchange ]-8] and explains the existence of long-range ferromagnetic order in these mixed-valence manganese oxides. Other interactions should also be considered in order to explain not only the magnetic behaviour but also the magnetotransport properties, i.e. the electron-phonon interaction. This interaction is particularly strong and relevant in the series Lal-xCaxMnO3 as has been experimentally demonstrated by thermal expansion and magnetostriction measurements [9], the isotopic effect ]-10] and infra-red spectroscopy [11]. At high temperatures in the paramagnetic and insulating phase the strong electron-phonon coupling provides the mechanism for the carrier localization as small lattice polarons as proposed in Holstein's model [12-15]. A narrow potaronic band is formed at temperatures higher than OD/2 (OD is the Debye temperature). At low temperatures the broadening of this band enhances the itinerancy of the carrier. The etectron-phonon interaction provides also the mechanism for the vibrational contribution as a result of

0304-8853/98/$19.00 © 1998 Elsevier Science B.V, All rights reserved PIt S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 8 0 1-9

847

M.R. Ibarra, J.M. De Teresa / Journal of Magnetism and Magnetic Materials 177-181 (1998) 846-849 t ( t o l e r a n c e factor) 089 0895 09 {]~05 0,91 . . . . , ,

3~

0915

(La I.xTbx)2g3Cal/3Mn03 35

~

30 - x=0.25

1

i

J

~.

~=0..~3 20

~'~ 20 0= I

08

SGI 06 3a X (Tb content)

02

0

Fig. 1. Magnetic and magnetotransport phase diagram of the series (La~-~Tb.02 3Ca,~3MnO> The dashed line regions indicate the existence of the cluster-glass phase. The inset shows the Mn-O=Mn bond angle determined from X-ray refinement at room temperature [19] for the whole series. the dynamical Jahn-Teller effect as proposed by Goodenough [I6, I7]. This last mechanism does not imply charge transfer between Mn 3 + ions but a resonant state in which the local moments tend to be aligned. If the La ion is substituted by another ion, electronic and structural changes can take place. The accommodation of a smaller A ion in a cubic perovskite produces a tilting and buckling of the Oxygen octahedra which give rise to orthorhombic or rhombohedrai structures in which the M n - O - M n bond angle is smaller than 180:. This distortion of the structure has a strong influence on the ferromagnetic interaction. In order to illustrate this effect we display in Fig. 1 the magnetic phase diagram obtained for the series of compounds (La~_~Tb.~)2/aCa~/3MnO3 [-18]. Along this series the number of carriers is fixed to 33% of M n "*+ and the replacement of La by Tb brings about the distortion of the structure which is indicated by a decrease in the tolerance factor. The undoped compound x = 0 is characterized by a M - I transition taking place simultaneously to the para-ferromagnetic transition. At the other end, x = 1, the system is an insulator over the whole temperature range and a para-antiferromagnetic transition takes place. For these extreme compounds the main difference is the value of the M n - O - M n bond angle which ranges between 161 (for x = 0) to 150- (for x = i) [19]. This low value makes the ferromagnetic interaction very weak and the antiferromagnetic superexchange interaction is favoured. Along the series the M n - O - M n angle decreases linearly with x (see inset Fig. 1) giving rise to a weakening of the ferromagnetic interaction. As a consequence, in the intermediate concentration range (0.75 ~> x i> 0.33), due to the competition between these two magnetic interactions, the low-temperature ground state is a cluster-glass, the compounds being insulators over the whole temperature range. The average cluster-size (about 17 A at low temperature for x = 0.33) is given by the ferromagnetic correlation length (4) which was obtained from small-angle neutron-scattering experiments (SANS) [20] using a Lorentzian-type dependence for I ( q ) = Io/(q2"+ - 1¢2) where (so = 1/~) and q is the scattering vector. In Fig. 2

°0~

10 5

! r 50

1

2

'

i i00

3

~ i 150

200

250

T(K) Fig. 2. Thermal dependence of the magnetic correlation length, ~, in (La~_xTbj~/3Ca~/3MnO3 obtained from SANS experiments for x = 0.33 and 0.25. Inset: Field dependence of ~ for x = 0.33. (La~.~Tb,) 2 ~Caw3MnO3

25000

/o

x=0.33

~ , 10~

~z

x=025

20000 15000

T(K)

10000

127

5000

0T

p

50

r

i00 150 T(K)

200

Fig. 3. Thermal dependence of the electricaI resistivity at several applied magnetic fields for the compound x = 0.33. The inset shoves the results for the compound x = 0.25. we show the thermal dependence of ~ for the compounds x = 0.33 and 0.25. A significant difference is observed between both compounds. While ~ remains finite even at the lowest temperatures indicating a cluster-glass state for x = 0.33, ~ diverges for x = 0.25. In the inset of Fig. 2 we display the field dependence of ~ at low-temperatures (insulator phase, R > 1012 f2). The value of ~ is practically constant at low field and diverges above 3 T. These results explain the huge change in the resisistivity, which drops down to i04 f) cm at 5 T (see Fig. 3). Certainly in this compound the magnetoresistance is related to the existence of phase segregation in the form of small clusters. However, the x = 0.25 compound shows a spontaneous divergency in ~ at To, which corresponds to the establishement of long-range ferromagnetic order. As can be seen in the inset of Fig. 3, at T c a strong decrease in the resistivity is observed. The behaviour observed for the x = 0.33 compound can be considered as that of a set of disordered ferromagnetic clusters in a paramagnetic background, This picture is close to the magnetic polarons observed in the paramagnetic phase of the compound x = 0 [21]. We wili summarize the concept of magnetic polaron introduced in Ref. [2i] and describe

848

M.R. lbarra, J.M. De Teresa~Journal of Magnetism and Magnetic Materials 177-18] (1998) 846-849

i+, ,+-OT

I01 [ ~ )+2

10°

Lal36Yo 07C~ 33MnO3 + + + +

'

~oo.o

,

,

t

2

)

,

3

4

4OO,O

w

,~oo.o 10-1

4ooo.o

o 10-2 0

50

10O

150

i

'

200

250

H~

1,0

T(K)

,

,

o,~

Fig. 4. Thermal dependence of the resistivity under several applied magnetic fields in Lao.60Y0.07Cao.3MnO3. Inset: Thermal dependence of the magnetoresistance at 12 T.

200

t

2H(T) 3

+

+ I 1N)0

lattice

222.3 .LT 100

~

0,2

o

0

~ T=L~3K

o

/

/],~j~/

.

0.4

'me~

T=210K

o

Lao,6Yo.o7 C a o 3 3 M i t O 3

.............

$

300

5 0

I

2

3

4

5

H(T) 300

400

35

T-mo

I

T(K) Fig. 5. Thermal expansion under several applied magnetic fields. Inset: Extra anharmonicity without field and at 5 T.

0.6

15

as an example the behaviour of the compound Lao.6oYo.oTCao.33MnOa (the role of Y is just to decrease T~ in regard to the La-Ca compound, consequently both can indistinctly be discussed). This compound illustrates the behaviour of the undoped compound of the series x = 0. At high temperature the compound is paramagnetic and the resisitivity has semiconductor behaviour as can be seen in Fig. 4. Below T~ = 150 K a sharp drop in the resistivity is observed attributable to the establishment of long-range ferromagnetic order. Under an applied magnetic field the resistivity is drastically reduced and consequently a large magnetoresistance is observed. In the inset of Fig. 4 we show the thermal dependence of the magnetoresistance, which reaches at T~ a value of 104%. It is now well established that the magnetoresistance at low temperatures is related to the electronic scattering on the grain boundaries [22] (given the extrinsic character of this contribution, this effect will not be treated here). Our observation of the spontaneous and field-induced lattice effects in this compound indicated the plausible existence of magnetic polarons [9]. We observed in the paramagnetic phase an extra anharmonic contribution over the theoretical phonon contribution given by the Griineisen law. In Fig. 5 we show the experimental and the theoretical results. We interpreted these results as the contribution from the polaronic effect. The strong electron-phonon coupling gives rise to the formation of small lattice polarons which brings about a charge

0.2 ')'

iO

0

H(T)

Fig. 6. Isotherms at selected temperatures of relevant magnitudes in the compound Eao.6oYo.ovCao.3MnO3: (a) Volume parastriction, (b) Resistivity, (c) Intensity of a Bragg peak, (d) Ferromagnetic correlation length, ~, and normalised SANS intensity at q = 0.1 ~ - t . localization as the temperature decreases. The localization of the eg electrons is reponsible for the extra anharmonic contribution (see, inset of Fig. 5). At T~ the carriers are released and the extra contribution to the thermal expansion disappears. This simple picture has overcome all kind of tests up to now, i.e. the localized state corresponds to a high-volume state and the delocalized or itinerant state to a low-volume state [23]. We also observed a large-field effect (see Fig. 5) that produces a completely unusual and huge volume magnetostriction in the paramagnetic phase (parastriction). The results of selected volume parastriction isotherms are given in Fig, 6a. Considering these results, the simple picture of small lattice polarons is not supported given the large-field effect observed. We consider that the small lattice polaron should be dressed by the short-range ferromagnetic correlations forming an entity that moves as a whole, This assumption is easity visualized as follows. The carriers are eg holes, whose average Iifetime at specific lattice sites (as Mn ¢+) increases for decreasing

M.R. Ibarra, J.M. De Teresa~Journal of Magnetism and Magnetic Materials 177-181 (1998) 846-849

this field long-range magnetic order occurs, releasing the eg holes trapped in the magnetic polarons. Consequently, the resistivity and the magnetostriction decrease as well. In an attempt to draw a general conclusion we should underline the relevance of the electronic phase-segregation as reponsible for the huge magnetorresistance effect observed in the manganites.

La o 6Yo 07Ca 0 33Mr103

3000.

//".._ ~¢)

O~[ 15

TK)

•

~ I •+ H=5 T

0

50

1130

150

200

250

849

•

300

T(K) Fig. 7. Thermal dependence of the SANS intensity at a fixed q = 0.1 ,~- 1 with and without applied field. The inset shows the thermal dependence of ~ in both cases.

temperatures. This hole is shared by the next M n 3+ neighbours providing a short-range ferromagnetic order (Zener mechanism). In order to confirm this hypothesis we performed susceptibility measurements and also SANS experiments E21] which showed the existence of short-range ferromagnetic interactions up to 1.8T¢. In Fig. 7 we display the thermal dependence of the SANS intensity with and without field. These results give the magnetic contribution from small ferromagnetic clusters. We can observe that the contribution below Tc is negligible (the long-range ferromagnetic order gives contribution only to the Bragg peaks). However, above To there exists a significant contribution. The fit of the SANS results gives the values for # shown in the inset of Fig. 7. The magnetic cluster size associated with the lattice polaron (magnetic polaron) is almost constant above To within the explored temperature range ( ~ 12 ~,). At To, ~ diverges because of the establishment of long-range ferromagnetic order. The effect of the applied magnetic field is to reduce the SANS intensity (the n u m b e r of clusters) and also shift T~ to higher temperatures as is clearly seen in the inset of Fig. 7. It is also worth comparing the results obtained from SANS and the extra anharmonic contribution shown in the inset of Fig. 5. This form of phase segregation is the responsible for the lattice effect and also for the magnetoresistance. In Fig. 6b we show the magnetoresistance isotherms. They show the same behaviour as the magnetostriction, increasing the conductivity as the holes are delocalized, i.e. as the magnetic polarons disappear. In Fig. 6c we show the field dependence of the intensity of a Bragg peak. The magnetic contribution represents the degree of magnetization. This behaviour is not typical of a pardmagnet but it reflects a kind of metamagnetic behaviour. We can explain this behaviour by considering that the effect of the field is to increase the size of the magnetic clusters (as can be inferred from the field dependence of in Fig. 6d). When they reach a critical value (temperature dependent) they start to percolate giving rise to a decrease of the SANS intensity with field (see also Fig. 6d), i.e. of the n u m b e r of magnetic poIarons. Above

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