Metal-semiconductor transition in the double exchange system La0.8Sr0.2Mn1−xCuxO3

Physics Letters A 165 (1992) 473—479 North-Holland

PHYSICS LETTERS A

Metal—semiconductor transition in the double exchange system La0 8Sr0 2Mn1 _~Cu~O3 L. Haupt, R. von Helmolt, U. Sondermann’, K.-Bärner 4. Physikalisches Instilut der Universität Gottingen, W-3400 Göttingen, Germany

Y. Tang, E.R. Giessinger

2,

E. Ladizinsky and R. Braunstein

Physics Department, University of California at Los Angeles, CA 90024, USA Received 17 February 1992; accepted for publication 2 April 1992 Communicated by J. Flouquet

Magnetization and resistivity ofLa03Sr02Mn1 _~Cu~O3 compounds are measured for 10< T< 500 K and 0
1. Introduction

dimensional structure seems to be favoured: La 0 5Sr15MnO4, which has the quasi-two-dimen-

Metallic conducting ferromagnetic compounds on an oxide basis and with a perovskite structure such as La1 _2,Sr~MnO3have been known for quite a while [1,21. Recently, high temperature superconductivity has been observed in closely related classes of cornpounds such as La2 ~Ba~CuO4[3,4]. This invites questions as to what are the typical differences in the electronic structure and whether there is something like a coexistence or exclusion principle between ferromagnetism and superconductivity in oxide based metallic compounds. In the Cu-based compounds, for the occurrence of high T~superconductivity, apparently CuO layers play an important role [5]. Superconductivity in metallic oxides of this kind itself had been predicted earlier, but quite low transition temperatures were expected [6]. In contrast, for the ferromagnetic metallic cornpounds, for example La0 5Sr05MnO3 [7], a three-

2

Institut für Mineralogie, Kristallographie und Petrologic der UniversitSt Marburg, Lahnberge, W-3550 Marburg, ~ Virginia Commonwealth University, Richmond, VA, USA.

sional K2NiF4 structure, is only weakly ferromagnetic, presumably because of weak interlayer couplings. Another characteristic difference seems to be the formal valences of the transition metal: Cu in high T~superconductors is supposed to appear mostly as 2 + and 3 + while the Mn ions in double exchange ferromagnets are thought to have 3 + and 4+. However, in an attempt to see how double exchange ferromagnetism is eventually converted to superconductivity, we have prepared a mixed crystal series La0~8Sr02Mn1~Cu~O3~. We find that a substitution of 10% Cu already destroys the ferromagnetic state but instead ofsuperconductivity a spin-glass like insulating state occurs.

2. Experimental Starting from stoichjometric oxides/carbonates, the samples were prepared as described elsewhere [8]. All samples were finally obtained in the form of black, compact sintered pellets. Magnetization data

0375-9601/92/S 05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

473

Volume 165, number 5,6

PHYSICS LETTERS A

1 June 1992

Table 1 Lattice parameters, lattice volume and deviation from cubicsymmetry for the five compounds investigated. 3)

x

a(nm)

b(nm)

c(nm)

fi

V(nm

0.0 0.1 0.2 0.3 0.4

0.5477(3) 0.5471(1) 0.5463(2) 0.5457(4) 0.5458(1)

0.5524(5) 0.5519(4) 0.5513(2) 0.5507(6) 0.5510(3)

0.7779(4) 0.7771(5) 0.7761(5) 0.7753(2) 0.7755(9)

90.7(l) 90.7(2) 90.7(5) 90.7(5) 90.7(8)

0.2353(8) 0.2346(6) 0.2337(5) 0.2330(2) 0.2332(4)

are obtained as described elsewhere [9]. For the powder X-ray determination we used a Siemens D 500 diffractometer. Whenever possible, small amounts of secondary phases were identified using JCPDS data. Table 1 contains the lattice parameters ofa monoclinic structurewhich could be fitted to the

latticefor data parameter). x~0.4. This The “(1—1)” differencewith structure the empirical is a one distorted perovskite structure; without distortion would have a=b=.J~a 0and c=2a0 (a0 is the cubic values ofa, b, c are less than 0.5%. For samples with x>~50% two or more phases in comparable amounts were observed, most of them could be identified in respect of their lattice symmetry [101. For the measurement ofthe resistivity we used the four-point method together with conducting glue contacts on rectangular blocks cut from the pellets. For the low resistivity samples an ac current was used in connection with a lock-in detector (Ithaco 393M102) while for the high resistivity samples we used a dc current together with a high impedance DVM (Keithley 7116); here, the polarity was switched manually in order to subtract possible thermo e.m.f.’s.

4 M FIIB/FEJ

~ 100

200 i~

-~

300

400

Fig. 1. Magnetic moment versus temperature M( T) of the two ferromagnetic metallic compounds (x= 0.1, 0).

0.28 0.24 0.20

-

0.16 0.12

0.08 0 04 0.00

40

80

120

160

200

240 280

T [K]

3. Results

Fig. 2. M(T) curves of the semiconducting compounds (x=0.2,

0.3,0.4). Upper curve: B=0.6 T, lower curve: B=0.

3.1. Magnetization measurements M(B, T) Compounds with 0 ~ x~0.1 are ferromagnetic; this is shown in fig. 1 by way of the M( T) curves. The Curie temperature decreases with increasing x. For 0.2 ~ 0.4 we observe a spin-glass like behaviour; this is shown by the differences in the M( T) curves which are obtained by cooling the samples with and without an applied magnetic field (fig. 2 and ref. [11]): for T> T~the curves run together, but they 474

branch for T< T,g. Competing magnetic couplings (for example: superexchange, double exchange from free carriers) and their distribution are plausible because ofthe doping and the Cu~Mn1—x mixture. Possibly, also oxygen vacanciesexist and add to the randomness in the couplings. Clustering of magnetic moments, i.e. a non-ideal spin-glass behaviour is suggested by the curved M(B) dependences (fig. 3a) and the non-linearity of

Volume 165, number 5,6

PHYSICS LETTERS A

0.24

~

a

-

10K

x

—

=

0.2

/

1.2

-

M(~BlfU)

0.12

1 June 1992

y~-l ~

_—~

/•

80K

.-~

-.

150K

0

0.:

0.3

0

B(T)

0.6

-.

0

I

100

200 T(K) 300

Fig. 3. (a) Magnetic moment versus field curves M(B) for x=0.2 and different temperatures as indicated. fu: per formula unit. (b) Reciprocal mass susceptibility versus temperature for x=0.2. Solid line: high temperature asymptotic behaviour: p~=4.2jz 1~, ø~=190 K. Table 2 Calculated (spin-only values) saturation moment M1~,and effective paramagnetic moment p~as compared to the experiment; also paramagnetic and ferromagnetic Curie temperatures. x

S (calc.)

M0 (~z8/fu)

peff(calc.)

0.0 0.1 0.2 0.3 0.4

1.9 1.7 1.5 1.3 1.1

3.8 3.4 3.0 2.6 2.2

4.69 4.28 3.87 3.46 3.04

x

‘(T) (fig. 3b). Because ofpossible compositional variations in our samples we cannot decide whether these results are due to local magnetic inhomogeneities or whether we have larger spin clusters intrinsically. The high temperature asymptotic behaviour of the susceptibility versus temperature curve, however, allows the determination of the effective paramagnetic moments which are compiled in table 2 together with the paramagnetic Curie temperatures. —

Peir(exp.) T0.7

M0 (i~B/fu) T0.l

T, (K) T5

3.6 3.2

320 280

4.2 3.9 3.7

190 160 120

p1Qi~) 0.28

I

0.24 0.20

Tc

.12

~

0.16 0 12

.08 Ic X = 0.0

.

0.08 0.04

.04 100

3.2. Resistivity measurements

O~,(K) ~25

150

200

250

300

350

I EKJ

Fig. 4. Resistivity versus temperature p( T) of the two metallic

Fig. 4 shows the resistivity of the metallic ferromagnetic compounds (x= 0, x= 0.1). A double peak structureis found and is discussed in detail later. For the semiconducting compounds x=0.2, 0.3, 0.4, Arrhenius plots are displayed (fig. 5) which show strong deviations from linearity at medium temperatures. From the high temperature asymptotic be-

ferromagnetic compounds (x=0, 0.1). T~is the Curie temperature from fig. 1.

haviour, however, activation energies can be extracted which are compiled in table 3 together with the room temperaturevalues of the resistivity and its temperature derivative. 475

Volume 165, number 5,6

4

:

PHYSICS LETTERS A

(,

0.5

1OlXJ/T (K’] (*) 8

1 June 1992

—

10

R

~//xO2 7

7

tO

to~

*

X0.3

to~

__________

~~2O%Cu

0

10%Cu

I

-

0%Cu

102

1

2

3

4

l0

l0~

K (1000 cm-i)

-~

~

semiconducting (x= 0.2, 0.3) compounds.

4

______

~/~X=0.

I

Fig. 6. Infrared reflectivity of two metallic (x=0, 0.1) and two 0.5

10

2.4

2.6

2.8 ~

3.0

3.2

4]

a (a.u.)

(K~’

Fig. 5. Arrhenius and Mott plots ofp( T) for the semiconducting compounds (x=0.2, 0.3, 0.4). 0%Cu Table 3 Thermal energygap values as derived from the Arrhenius plots; also room temperature resistivity Po and temperature coefficient a. _____________________________________________ x p (~7cm) a=p~ c9p/,9T a E~E(eV) 300K lOOK ~0.0l ________________________________________________________ 0.0 0.1 0.2 0.3 0.4

0.10 0.27 2.65 2.96 9.45

—0.004 —0.004 —0.018 —0.014 —0.013

1

2

3

4

K(1000 cm-i) Fig. 7. Optical conductivity of two metallic (x= 0, 0.1) and two semiconducting (x=0.2, 0.3) compounds as obtained from a Kramers—Kronig analysis.

0.008

0.009 0.054 —0.080 —0.024 —

0.24 0.25 0.20

3.3. Infrared measurements

Fig. 6 shows room temperature reflectivity spectra of the La 08Sr02Mn1 _~Cu~O3 compounds (0 ~ x < 0.4) between 150 and 4000 cm* The surfaces of the samples were coated with aluminum and reflectivity measurements were performed on the coated 476

0

surfaces in order to estimate the absolute value of the reflectance. The frequency dependent optical conductivity, which is a direct measure of microscopic energy absorbing processes, was obtained from a Kramers—Kronig analysis and is presented in fig. 7. It shows a phonon doublet around 400 and 600 cm-’ and a broad mid-infrared band at about 1800 cm—’.

Volume 165, number 5,6

PHYSICS LETTERS A

4. Discussion 4.1. Magnetic properties ofcompounds with x~0.4 For classical perovskites ABX3 the magnetic ions 2~,Mn3~)are arranged in a simple cubic B (hereand Cuare separated by the diamagnetic ions X lattice (here 02). IfA ions of a different valence are mixed (here La3~,Sr2~),also for the B ions mixed valences are introduced (Mn3~,Mn4~).If the excess electron of such a pair is mobile it becomes spinpolarized and simultaneously a ferromagnetic coupling occurs between the residual localized spin of the Mn ions (“double exchange” model). In addition, competitive negative superexchange couplings via the X ions can occur. If n.=N,/N are the relative number densities with

I June 1992

metallic conduction should vanish. In fact, at x,, there seems to appear a two-phase region with two insulating phases. That does not necessarily mean that ferromagnetism_xCaxMnOs vanishes compounds at x,, = 0.4: only for at mixed valence La, x= 15 at.% Ca ferromagnetism appears [1,21; apparently a percolation of the Mn3+ /Mn4~pairs is necessary to simultaneously install metallic conduction and double exchange ferromagnetic coupling. If, because of the similarity of structures, we subtract 15 at.% from x~,we should have the metal—insulator transition at x, = 0.25; empirically it is found between 0.1 and 0.2. This might be connected with a distribution of the competing super exchange couplings (see spin-glas like behaviour): in that case, in order to observe ferromagnetism, another condition

N >N is

1 the average spin moment S per formula unit

—

S ~ n1Sj, (1) where S, are the atomic spin moments (possible orbital contributions are neglected). The relative number densities n, can be related to the formal valences as follows: for La, .~,Sr,Mn1 _~Cu~O3 there exist three 3~(S=~),Mn4~(S=~),Cu2~ magnetic ions:Assuming Mn (S= ~) [121. that La and Sr observe fixed valences (3+, 2+) and that 02 is stoichiometric, according to charge neutrality the number densities are nMn3+ =

1 —y--2x,

flMn4+

=x+y,

Having thus defined an equivalent homogeneous system, the saturation magnetization M~and the effective paramagnetic moment p~can be calculated in a molecular field approach, Mo~=gSuB,

Peff =g[~(S+

) ] “a.

4.2. Resistivity According to the absolute values of the resistivity, the ferromagnetic samples (x~0.1) can be considered metals while those with spin-glass like behavjour are (doped) semiconductors (table 3). 4.2.1. Metallic compounds

(2)

flCu2+ =~.

must be met: the average J must be larger than the halfwidth of the distribution. Indeed, statistical calculations for a ferromagnetic spin lattice under such conditions yield stability lines T* (H), and T must be lower than the Curie temperature T~for ferromagnetism to be observed [13].

(3)

The calculated values are compiled in table 2 and are compared with the experiment. The discrepancies suggest the existence of frustrated spins inside the average ferromagnetic order in particular if Cu is substituted. For y= 0.2 and x,, = 0.4, flMn3+ = 0. Thus, for x> 0.4 there are no Mn3~/Mn4~pairs anymore, the ferromagnetic double exchange coupling and the

Aside from the absolute values, for x ~ 0.1 at low temperatures, the temperature coefficient of the resistivity, a=p ‘ap/oT, is positive; this is generally expected for metals: with phonon and impurity scattering dominant, a remains positive up to the melting point. Here, in particular, a—~0for T= T~,this indicates that magnon scattering is dominant. However, as for T> T~the spin lattice is “molten” a 0 is expected for T> T~[141. In our case, a <0 for T> T,~,suggesting a transition from a metal to a narrow bandgap semiconductor at T~.For double exchange ferromagnets, with the introduction of an internal polarization, significant changes of the electronic states are to be expected [15]. In particular, given a doped (double exchange coupled) semiconductor, -

477

Volume 165, number 5,6

PHYSICS LETTERS A

it has been predicted that the gap Eg generally is reduced for T< T~[15]. Thus, in the case of a narrow bandgap semiconductor, an overlapping of bands at T~appears to be feasible, too. An Arrhenius plot of a( T) yields 20—25 meV as activation energy. A closer inspection of the temperature region around the Curie temperature reveals a plateau or a second maximum close to, but definitely below, T~. In other systems, this has been attributed to scattering on critical fluctuations of the spin systems which in the case of highly correlated electron systems might extend rather far below the critical temperature [161. Alternatively, a mutual polarizing influence of the conduction electrons (“spin-polarons”) and the spin lattice has been suggested, which leads to a “frozen in” magnetic disorder below T~where the carriers are almost localized, leading to a hopping type of conduction in a certain temperature regime [17]. Lowering the temperature or increasing the magnetic field would increase the magnetic order and thus reenhance the carrier mobility such as to approach the band conduction situation again. Phenomena of this kind (secondary maximum below T,~,negative magnetoresistance) have been found even more pronounced in the similar metallic ferromagnet La 0 67Ba0 33Cu0 ,Mn0903 [18]. Here also, for the metallic compound with x=0.1, the secondary maximum gets larger. Thus, for the proposed critical spin fluctuations or the carrier localization sites, apparently frustrated spinsplay which come in because of the Cu~Mn, —x mixture an important role. If one reduces the carrier concentration (metal to doped semiconductor transition), the localization effects are expected to become even more prominent. 4.2.2. Semiconducting compounds For 0.2 ~x~0.4 the absolute values and the temperature coefficient a suggests semiconducting behaviour. However, Arrhenius plots show strong deviations from a linear slope except for high temperatures, rejecting normal band conduction for intermediate temperatures. Indeed, variable range hopping lnp—~T”4 (Mott plot) gives a better fit here. Variable range hopping occurs between (distributed) localized states which occur in chemical or structural random systems [17]. Since for the ferromagnetic metallic compound, x= 0, we find spin 478

I June 1992

scattering to be dominant, when our systems get spin disordered, again we expect the spin scattering to be important, i.e. the spin disorder should introduce the localized states and with it the successful application of the variable range hopping model. This is supported by the susceptibility versus temperature curves (fig. 3b) ofthe spin-glass like compounds which suggest a strongly short-range ordered temperature regime to follow the “frozen in” spin state and only at high temperatures a Curie—Weiss law is approached. Here, one observes the transition to the Arrhenius slope. Indeed, with total dynamical spin disorder one would have difficulties to define local (spin) sites which could provide a localized (spin) hopping state. For ~ 0.1 such sites, which are possibly connected with local spin defects, are rare and probably embedded in conduction states rather than inside a semiconducting gap; that is why we do not expect a T—”4 regime here. 4.3. Optical conductivity We see two main contributions to the optical conductivity of the oxide based compounds: a Drude part superimposed to a phonon doublet in the far infrared and a broad mid-infrared band. This suggests the use of a model dielectric function of the form 2 ~

=

2 + iw/t

—

2

+ ~,~

SjW~ & ~(02 —

W

+&

—

~

+ too,

(4)

where the first term is a Drude contribution, the second an ocillator type contribution which should take care of the phonon doublet, the third term is supposed to account for the broad mid-infrared absorption and the core dielectric constant e~,,for higher frequency contributions. Then, eq. (4) can be used to calculate the real part of the optical conductivity a, (w) = a Im [e (w) ] /4it. (5) From the Drude part of preliminary curve fits relaxation times between 5x 10 ‘~and 8x l0 ‘~s are obtained and together with the dc conductivity data carrier concentrations between 7 x 1 0’~(metals) and 5 x l016 (semiconductors). A more detailed analysis has to await data below 150 cm-’. We can, however,

Volume 165, number 5,6

PHYSICS LETTERS A

comment qualitatively on the phonon and midinfrared bands: Fig. 6 shows that the band at 400 cm’ decreases relative to the band at 600 cm—’ as the Mn concentration increases, suggesting that the phonon doublet is directly coupled to the electron system. Also, as the conduction type changes from semiconducting to metallic (at 15 at.% Cu), the overall amplitude of the phonon peaks is significantly reduced for x< 0.15, suggesting an increased screening by the metal conduction electrons. In addition, a damping contribution due to alloy scattering seems to be superimposed as the phonon peaks are reduced for 10% Cu relative to 0% Cu. The broad mid-infrared absorption is similar to that found in La2~Sr~CuO4 compounds and in that case has been connected with absorption due to a nonadiabatic polaron hopping regime [191; if one could extend this approach to spin—polaron hopping, it would nicely tie in with the interpretation of our dc conductivity. However, other mechanisms, such as normal polaron hopping, direct charge transfer or excitonic interband interactions which often occur in strongly correlated electron systems are also possible.

1 June 1992

US Air Force Office of Scientific Research No. AFOSR-84-0 1698, US Army Research Office Durham No. DAAG-29- 12-1064 and the State of California MICRO Program. References [I] G.H. Jonker, Physica 16 (1950) 337.

[2] G.H. Jonker, Physica 22 (1956) 707. [3] J.G. Bednorz and K.A. Muller, Z. Phys. B 64 (1989) 189. [4] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58(1987)908. [5] K. Yvon and M.Z. Francois, Z. Phys. B 76 (1989) 413. [6] Landolt-Börnstein, New series, Vol. III, 4a (Springer, Berlin, 1970) pp. 255 if. [7] R.A. Mohan Ram, J. Solid State Chem. 70 (1987) 82. [8] L. Haupt, Diplomarbeit, Gdttingen (1990). [9] K. Heinemann, Dissertation, Gottingen (1987). [10] E.R. Giessinger, R. Braunstein, E. Ladizinsky, L. Haupt, J.W. Schünemann, K. Bärner, U. Sondermann and A.F.

Andresen, Solid State Commun. 78 (1991) 503. [II] K. Kopitzki, Einfiihrung in die Festkdrperphysik (Teubner, Leipzig, 1989) pp. 233 if. [12] A.W. Webb, K.H. Kim and C. Bouldin, Solid State Commun. 79(1991)507. [131 J.R.L. Almeida and D.J. Thouless, J. Phys. A Il (1978)

983. [14]T. Kasuya, Prog. Theor. Phys. 16 (1956) 58. [15] J. Mazzaferro, J. Phys. Chem. Solids 46 (1985)1339. [16] T. Moriya and K. Usami, Solid State Commun. 23 (1977)

Acknowledgement

935. [l7]N.F. Mott and E.A. Davies, Electronic processes in noncrystalline materials (Clarendon, Oxford).

This work was supported by the Deutsche Forschungsgemeinschaft (Unusual Valency States in Solids), VCU Grants-In-Aid grant No. 2-9335 1, the

[18] R. von Helmolt, Diplomarbeit, Gottingen (1991). [19] D. Mihailovic, C.M. Foster, K. Voss and A.J. Heeger, Phys. Rev. B42 (1990) 7989.

479