~ Jein'nal ef

milsdil$ Journal of Magnetism and Magnetic Materials 168 (1997) 1-8 ELSEVIER

Anomalies in the structural and magneto-transport properties of thin films of Bi substituted La-Ca-Mn-O

G. Srinivasan a'*, R.M. Savage a, V. Suresh Babu b, M.S. Seehra b

a Physics Department, Oakland University, Rochester, MI 48309, USA b Physics Department, West Virginia University, Morgantown, WV 26506, USA

Received 19 September 1996

Abstract

The effects of Bi-substitution on structural, magnetic, and transport properties of polycrystalline and RF sputtered thin films of L a - C a - M n - O are reported. Films of La0.62Bio.05Ca0,aaMnO3 sputtered on (1 1 1)Si are polycrystalline whereas textured films with (1 1 0) orientation are obtained for (1 0 0)Si. In bulk polycrystalline samples, Bi substitution leads to (i) a very small decrease in the lattice constant and (ii) a large reduction in the Curie temperature Tc (230 K vs. 240 K for Lao.67Cao.3aMnO3). The films, however, show a 0.4% decrease in the lattice constant and a relatively high Tc ( = 262-266 K) due to reduced M n - O - M n bond lengths. Although the magnetic parameters are identical for polycrystalline and oriented films, electrical resistivity P and MR data for the (1 1 0) textured films show the following anomalous features: (i) a semiconductor like conduction and the absence of the metal-semiconductor transition observed near To in polycrystalline bulk samples and thin films, (ii) a two-order of magnitude increase in low temperature p relative to the polycrystalline films and (iii) MR-values much higher than for polycrystalline films. These data are indicative of a giant anisotropy in the magneto-transport properties of thin films of the oxides.

PACS: 73.50.Jt; 75.30.-m; 75.70.-i; 76.50. + g

Keywords: Magnetoresistance; Thin films; Ferromagnetic order; Perovskite oxides; Transport properties

1. Introduction

The p h e n o m e n o n of spin-polar ized t ranspor t in ordered materials is of fundamenta l and technolo- gical impor tance [1]. In part icular , the observa t ion

*Corresponding author. Fax: + 1 810 370-3408; e-mail: srinivas@oakland.edu.

of giant magnetores is tance (GMR) in La-based mangana te s has genera ted renewed interest in these fe r romagnets first synthesized in the 1950s [2]. The oxides are fer romagnet ic for specific com- posi t ions due to the double exchange (DE) bet- ween Mn 3+ and M n 4+ ions [3]. Magne to - esistance on the order of 105% in inflat ionary units has been repor ted in m a n y c o m p o u n d s in- cluding L a - C a - M n - O , L a - T b - - C a - M n - O , and

0304-8853/97/$17.00 ~ 1997 Elsevier Science B.V. All rights reserved Pl l S0304- 88 53(96)0068 3-X

2 G. Srinivasan et al. /Journal of Magnetism and Magnetic Materials' 168 (1997) 1-8

N d - S r - M n - O [4-6]. The very large MR, however, occurs at very high applied magnetic fields and is suggested to arise from a metal-to-semiconductor type transition, DE assisted processes, and mag- netic polaron scattering [4, 7, 8]. In addition to GMR, a rich variety of phenomena including mag- netic field induced crystallographic transitions and giant magnetostriction are observed in these perov- skite oxides [9].

There are several reports on the effects of substi- tutions for La in DE ferromagnets. Of particular interests are a factor of ten enhancement in MR and a large magneto-volume effect in polycrystal- line L a - C a - M n - O substituted with yttrium which are attributed to a decrease in lattice dimensions and the resulting changes in DE interactions [10, 11]. The present study on Bi substituted L a - C a - M n - O was motivated by reports of fer- romagnetism at temperatures as high as 510 K and semiconductor-to-metal transitions at even higher temperatures in Bi(Ca,Sr)Mn206 [12].

X-ray diffraction, initial susceptibility, magnetiz- ation and magneto-resistance (MR) studies were performed on polycrystalline samples and an- nealed radio frequency sputtered thin films of La0.62Bi0.osCao.33MnO3 on (1 1 1) and (1 0 0)Si. Films sputtered on (l 1 1)Si are polycrystalline and textured films with (1 1 0) orientation are deposited on (1 00)Si. Both films show 0.4% decrease in the lattice constant compared to polycrystalline bulk samples and a correspond- ing increase in the Curie temperature To. Magnetic characterization of polycrystalline and oriented films reveal identical features and equal magnetic parameters, but we observe a strong dependence of electrical transport and MR on crystallographic orientation. In bulk and thin film polycrystalline samples, a transition from metal to semiconductor like conduction is observed and a maximum MR = Ap/po of 50% is measured for 40kOe fields at temperatures below To. The (1 1 0) textured film on (1 0 0)Si, however, show the fol- lowing important characteristics: (i) a semiconduc- tor-like thermally activated conduction for 5 K < T < 300 K, (ii) the absence of any tran- sitions in the resistivity data, and (iii) a relatively large MR of 80%. Details on these studies are provided in the subsequent sections.

2. Experiment

Samples of bulk polycrystalline Lao.62Bio.osCao.33MnO 3 were prepared by the standard ceramic technique involving wet ball mill- ing the mixture of La203, Bi203, CaCO3 and MnCO3, a series of presintering at 1100°C for 72 h followed by wet ball milling and final sintering at 1300°C for 10 h.

Thin films of the oxide were prepared by radio frequency sputtering techniques [13]. A 2 in dia- meter polycrystalline sintered target with the com- position Lao.62Bio.o6Cao.4sMnO 3 was used in order to obtain stoichiometric films with the re- quired composition. The target was mounted in a Torus 2C (Kurt Lesker) magnetron sputter gun. The sputtering system was evacuated to a base pressure of 0.01 mTorr and was filled with a mix- ture of 20% oxygen and 80% argon to a pressure of 12 mTorr, An RF power of 50 W at 13.6 MHz was supplied to the target. The substrates were located on a heated platform at 2 in below the target. Films were deposited for 20 h on (1 1 1) and (1 0 0)Si. Films with thickness on the order of 1 ~tm were prepared. The chemical composition of the films was determined by energy dispersive X-ray spectro- scopy. As mentioned earlier, it was necessary to tailor the target composition to obtain stoichiomet- ric films.

Studies on the crystal structure of bulk polycrys- talline samples and thin films were performed with a Philips-Norelco X-ray diffraction (XRD) system and the Cu Ks radiation. As deposited films were found to be amorphous and the films were crystal- lized by annealing in an oxygen atmosphere at 925°C for 60rain. Fig. 1 shows XRD data for a bulk polycrystalline sample and annealed films on (1 1 1) and (1 0 0)Si substrates. The data for the polycrystalline sample in Fig. l(a) indicate a cubic perovskite structure with a lattice constant of 3.861 A. Thus Bi substitution leads to a very small decrease in the lattice constant, from a value of a = 3.863 .A reported for Lao.67Cao.33MnO 3 [14], since the ionic radius for Bi (1.03 .A) is nearly equal to that of to La (1.032 ,A). In the data in Fig. l(b) for the film on (1 1 1)Si, the peak centered at 95 ° is due the substrate. It is evident from the data that the film is polycrystalline, and the estimated

G. Srinivasan et al. /Journal of Magnetism and Magnetic Materials 168 (1997) 1-8 3

a = 3.847 A amounts to a reduction of 0.4% com- pared to the bulk sample. The XRD data also confirm the (1 1 0) textured nature of films depo- sited on (1 0 0)Si. The data in Fig. l(c) do indicate the presence of small but measurable volume frac- tion of(1 00), (1 1 1) and (1 1 2) phases. The lattice constant of 3.845 ~, is in excellent agreement with the value for the polycrystalline film on (1 1 1)Si. The film-substrate lattice mismatch for the textured film is on the order of 0.4%.

1101

(112 (200)

i

a = 3.861 ± 0.002 ~,

BULK POLYCRYSTAL

a,

(220)

1103) (12 i)

a = 3,847 :t: 0.002 A

FILM ON {111)Si

,- 1 ..... k

b,

J a = 3.845 + 0.OOIA

FILM ON (1OO)Si

30 40 50 60

Si

C.

70 80 90 100

2 e (Degrees)

Fig. 1. X-ray diffraction intensity vs. angle profiles obtained using Cu K~ radiation for (a) bulk polycrystalline and (b), (c) thin films of Lao.62Bio.osCao.aaMnO3 . The thin film data are for polycrystalline film on (1 I 1)Si (b), and films with (1 1 0) orienta- tion on (1 0 0)Si (c). The films were annealed in oxygen at 925°C for 60 min.

The most important observations from X RD data are (i) the deposition of textured films with (1 1 O) orientation on (1 00)Si and (ii) the substan- tial reduction in the lattice constant for the films compared to bulk polycrystalline samples. A some- what similar effect has been reported for epitaxial thin films of Lao.67Cao.aaMnOs with a -- 3.89 ~, grown on LaA103 (a = 3.79 A) [4], and the larger a-value for the film compared to bulk polycrystals is suggested to arise due to the strain imposed on the film by the substrate. In the present case, how- ever, we observe changes in the lattice dimension even for polycrystalline films on Si. The possible presence of excess Mn 4+ in these oxygen annealed films could partially account for the smaller than expected a-value.

3. Results

3.1. Magnetic characterization

Measurements on 1 pm thick annealed films were made with an AC susceptometer for low fre- quency susceptibility, a Faraday balance for low field magnetization (H < 5 kOe), and a SQUID magnetometer for high field magnetization studies.

3.1.1. Low frequency susceptibility In order to study the influence of Bi substitution

on the magnetic ordering temperature, AC suscep- tibility measurements were carried out on bulk polycrystalline samples and thin films. Initial sus- ceptibility Z vs. temperature data in Fig. 2 were obtained for a frequency of 1 kHz and a magnetic field of 0.6 Oe. Consider first the data for bulk polycrystals. The susceptibility shows a broad max- imum at low temperatures and a rapid decrease with increasing temperature for T >200 K. The Tc corresponds to the maximum in d;ddT which occurs due to the onset of a long range magnetic order. Data in Fig. 2a yields Tc = 230 K and is smaller than the value of 240 K determined from initial susceptibility data for polycrystalline Lao.67Cao.33MnO3 [15]. This observed decrease in Tc for Bi-substituted sample is contrary to the anticipated increase in Tc due to a small decrease in a-value measured for La -B i -Ca -Mn -O . Thus

4 G. Srinivasan et al. /Journal of Magnetism and Magnetic Materials 168 (1997) 1-8

Bi-substitution appears to weaken the magnetic interactions in the oxide. A similar effect in which T¢ decreases with decreasing unit cell volume is reported for Y-substitution for La in polycrystal- line L a - C a - M n - O 1-15].

Fig. 2 also shows Z vs. T data for films annealed in oxygen at 925°C. Films sputtered on (1 1 1) and (1 0 0)Si have equal Z, their temperature depend- ence is identical, but z-values are slightly smaller than for the bulk sample. The most important ob- servations are (i) a relatively sharp ferromagnetic transition which is indicative of structural and magnetic homogeneity of the films and (ii) a Tc of 266 K for the polycrystalline film on (1 1 1)Si and 262 K for (1 1 0) textured film on (1 0 0)Si. The thin film T~ is 14% larger than for the bulk material and could result from (i) the observed 0.4% decrease in the lattice constant for the films and (ii) a compres- sive stress on the film imposed by the substrate.

The volume shrinkage observed in thin films and any compressive stress on the film, either growth induced or due to film-substrate thermal expansion mismatch, are equivalent to the application of an external pressure on the system. High pressure Z measurements on Lal _ xCaxMnO3 systems reveal

=

%

100

( a ) ~ (b)

(c)

, I ~ I L , 1 , ,

150 200 250 300

!

Temperature (K)

Fig. 2. Temperature dependence of the real part of the AC susceptibility measured at 1 kHz with H = 0.6 Oe for (a) bulk polycrystaUine, (b) polycrystalline film on (1 I l)Si, and (c) (1 1 0) textured film on (1 0 0)Si. The Curie temperatures determined from the peak position in dz/dT vs. T plots are 230, 266 and 262 K for the bulk polycrystalline, film on (1 1 1)Si, and film on (1 0 0)Si, respectively.

a very strong increase in Tc with the external pres- sure P, with dTc/dP as large as 37 K/GPa for x = 0.21 and 16 K /G P a for x = 0.33 1-16]. The pressure effect could be understood on the basis of expected changes in the Mn -O bond angles and bond lengths. In addition, pressure also influences the carrier concentration and mobility which would in turn influence the ferromagnetic double exchange and T¢. Thus the substitution of Bi in L a - C a - M n - O leads to remarkable changes in the ordering temperature for both bulk samples and thin films. The thin film T¢ is relatively large, possibly due to effects related to the choice of substrates.

3.1.2. Magnetization Magnetization (M) measurements were carried

out with a Faraday balance and a SQUID mag- netometer. The field dependence of M were mea- sured at low temperatures for information on the nature of magnetic order in the bulk samples and thin films. Representative data on M vs. H in Fig. 3 are for the film on (1 1 1)Si. The studies were done at 5 K for H applied parallel to the sample plane. The data show ferromagnetic behavior with hyster- esis and remanence and a coercive force of 250 Oe. Studies on the temperature dependence of M were carried out on bulk samples and thin films. Fig. 4

-4O

80

40

-80 -1000 -500 0 500 1000

H IOe)

Fig. 3. Magnetization M vs. applied magnetic field H data at 5 K for the annealed film on (1 1 l)Si. The field was applied parallel to the film plane.

G. Srinivasan et al. / Journal of Magnetism and Magnetic Materials 168 (1997) 1-8 5

14

12

= 8 o

m 6

2

0

100

80

60 O

N • ,~ 40

20

0 100

i a i I i L i 1 i i J I i i i ~ l J

E J i i = , J i I i , , i , , , , " ~ 0 , ,

140 180 220 260 300

Temperature (K)

Fig. 4. M a g n e t i z a t i o n as a funct ion t empera tu re for the bu lk

po lycrys ta l l ine sample for H = 85 and 5 kOe.

shows such data for bulk polycrystals for H = 85 and 5 kOe. The low field measurements were made for both zero field cooled (ZFC) and field cooled (FC) conditions in order to obtain information on the possible presence of impurity phases with spin freezing temperatures different from To. M-values remained identical for both ZFC and FC condi- tions. The low field M data in Fig. 4 are for ZFC conditions. A sharp ferromagnetic to paramagnetic transition at 230 K is evident from the data. The temperature dependence of the saturation value of M was measured with a field of 5 kOe. The high field M-data in Fig. 4 shows a long tail in the high temperature region. The saturation M of 95 emu/g at low temperatures is in excellent agreement with the reported value of 97 emu/g and the theoretical value of 100emu/g for Lao.67Cao.33MnO3 [2-1. Thus Bi substitution for La does not appear to influence Mn 3+ and Mn 4+ concentrations as expected. It is clear from the sharpness of the

A

i

lOO

8 0

6 0

4O

20

0 0

*'eaa a

I I I I

50 100 150 200

• Film on (100) Si 0 Film on (111) Si

250 300 350

T e m p e r a t u r e (K)

400

Fig. 5. D a t a on M vs. T for an inp lane H = 5 kOe for the

polycrys ta l l ine film on (1 1 1)Si and textured film on (1 0 0)Si.

magnetic transition and M data in Fig. 4 that the sample is free of any structural and chemical imper- fections.

Data on saturation magnetization as a function of temperature for the films are shown in Fig. 5. A magnetic field of 5 kOe was applied parallel to the film plane for these measurements. Both films, the polycrystalline film on (1 1 1)Si and (1 1 0) tex- tured film on (1 0 0)Si, have identical M-values. The low temperature M is 15% smaller than the theor- etical value and could be due to the possible pre- sence of excess Mn 4+. Such an excess Mn 4+ concentration would account for the relatively small unit cell volume and the low magnetization for the films compared to bulk polycrystalline samples.

3.2. Magneto-transport properties

Magneto-resistance (MR) measurements were done by the standard four probe technique and using a superconducting magnet for fields up to a maximum of 50 kOe. The resistivity p was mea- sured for in-plane H. First we consider results of our studies on bulk polycrystalline samples. The H-dependence of p at 5 K is shown in Fig. 6. The zero field p is on the order of 10 .4 Q cm, in agree- ment with values reported for similar compounds. The overall p vs. H behavior is linear with two dis- tinct regions with different slopes. For H < 10 kOe,

6 G. Srinivasan et al./Journal of Magnetism and Magnetic Materials 168 (1997) 1-8

0.16

'~ 0.16

,,p

0

0.14

m 0.12

b &

A

A

& &

&

AA ~

• *

, , " %

A A

T = S K

0.10 n -60 -30 0 30

H (kOe]

6O

50

A 4O

|3o f r

o a o

,= 10

J I , - -

100 200 300

Temperature (K)

Fig. 6. Electrical resistivity p as a function of H at 5 K for the bulk polycrystal. Arrows indicate data obtained for increasing and decreasing fields.

Fig. 7. Variation of absolute magnitude of the magnetoresis- tance MR = (P. - Po)/Po with temperature at H = 40 kOe for the bulk polycrystal.

p decreases much more rapidly than for H = 10-50 kOe. The sample resistivity is not saturated even at the highest H. The data also show hyster- esis; an 8% decrease in Po is measured when H is decreased to 0 from 50 kOe. With increasing H one notices a decrease in p, indicative of negative values for MR = (P0 - pH)/Po. An MR-value of 29% is obtained for a field of 50 kOe at 5 K.

Resistivity as a function of temperature were then measured for H = 0 and H = 40 kOe. The zero-field data showed the typical behavior ob- served in several G M R oxides, i.e., a transition from metallic type conduction for T < T~ to a semicon- ductor-like conduction for T > T~. Fig. 7 shows the variation in MR with temperature. As T is increased from 5K, the MR-value increases, reaches a maximum of 48% close to T~ and de- creases at higher temperatures. A reasonably large MR of 3-20% is measured in the paramagnetic state.

Thin films of the oxides show p-values several orders of magnitude larger than for bulk polycrys- tals. Data on T dependence of p for H = 0 and 40 kOe for polycrystalline films on (1 1 1)Si are shown in Fig. 8. The zero field p is quite large, on the order of 104~ ") c m . Such high resistivities for thin films were reported for other G M R oxides including L a - C a - M n - O and are suggested to arise due to small grains in films [4]. For H -- 0, p shows

a broad maximum centered at 150 K and a sharp peak at 245 K, about 21 K below To. The peaks are suppressed in the presence of an applied field as seen for the data in Fig. 8 for H = 40 kOe. The estimated MR-values as a function of temperature are also shown in the figure. An MR-value of 38% is measured at 5 K. The data show (i) a series of peaks in MR, (ii) a maximum MR of 58% at 240 K, about 26 K below To, and (iii) MR of 20-40% in the paramagnetic state. These observations are somewhat qualitatively similar to the data in Fig. 7 for the bulk polycrystal.

Remarkable changes in the MR characteristics are observed for (1 1 0) textured films deposited on (1 0 0)Si. Resistivities and their field and temper- ature dependence for the film are shown in Fig. 9. The most important observations are: (i) the low temperature p-values are two-orders of magnitude higher than for the polycrystalline film (Fig. 8), (ii) the zero field p shows a rapid fall-off with temper- ature and the metal-semiconductor type transition in polycrystalline samples is not seen for the tex- tured film and (iii) the film shows a maximum MR of 80% at 150 K, a much higher value than for the film on (1 1 1)Si. For 200 K < T < 300 K, MR data show some scatter with an average value of 35%.

To our knowledge, there has been no previous reports of such large anisotropies in magneto- transport in thin films or single crystals of

G. Srinivasan et al. / Journal o f Magnetism and Magnetic Materials 168 (1997) l - 8 7

E 0

% v

Q.

100

80

60

40

20

0

60

40

:[ 20

H=O

" '4

0 1 O0 200 300

Temperature (K)

Fig. 8. Data on (a) resistivity for H = 0 and 40 kOe and (b) M R at 40 kOe as a function of tempera ture for the annealed poly- crystalline film on (1 1 l)Si.

10 7

E lOe 0

Q.. 10 s

104

_ H = O

ee I [

75

50

25

0

r, T i '~

, I . . . . I ~ l

0 100 200 300

Temperature (K)

Fig. 9. Da ta as in Fig. 8 for the (l 1 0) textured film on (1 0 0)Si.

ter understanding of this important effect related to anisotropic p and MR for the oxide.

GMR oxides. The data in Figs. 8 and 9 clearly demonstrate the anisotropic nature of p and MR in Bi substituted films of La-Ca-Mn-O. According to a recent phenomenological model, electrical con- ductivity in CMR oxides is given by the sum of two terms: a magnetic term proportional to M 2 domi- nant at T < Tc and a thermally activated conduc- tion that dominates for T > Tc [17]. Here, M is the microscopic magnetization within a magnetic do- main. The observed anisotropies in p are not likely to arise from the magnetic term since the magnetic parameters, i.e., the initial susceptibility, magnetiz- ation and To, are virtually the same for both the films on (1 1 1) and (1 0 0)Si. The data do indicate a very large contribution to p from the thermally activated conduction term for the (1 1 0) textured film and the origin of such an effect is not clear at this time. Further studies, in particular on epitaxial thin films and single crystals, are essential for a bet-

4. Conclusions

A large anisotropy in the electrical resistivity and magnetoresistance of RF sputtered films of Lao.62Bio.osCao.33MnO3 on Si is observed. Films sputtered on (1 1 1)Si are polycrystalline whereas textured films with (1 1 0) orientation are obtained for (1 00)Si. Both films have identical magnetic parameters, but the textured films show a two order of magnitude enhancement in the low temperature p-value compared to polycrystalline films. In addi- tion, the metal-to-semiconductor type transition is absent in the textured film. The film shows a rela- tively large MR, as high as 80% at 40 kOe. These observations provide clear evidence for anisotropic magneto-transport in Bi substituted L a - C a - M n - O films.

8 G. Srinivasan et al./Journal of Magnetism and Magnetic Materials 168 (1997) 1-8

Acknowledgements

T h e w o r k at O a k l a n d U n i v e r s i t y was s u p p o r t e d

by a g r a n t f r o m the P e t r o l e u m Resea rch F u n d ,

a d m i n i s t e r e d by the A m e r i c a n C h e m i c a l Society.

References

[1] G.A. Prinz, Physics Today 48 (1995) 58. [2] G.H. Jonker, Physica 22 (1956) 707 and references therein. [3] C. Zener, Phys. Rev. 82 (1951) 403. [4] S. Jin, M. McCormack, T.H. Tiefel and R. Ramesh, J. Appl.

Phys. 76 (1994) 6929 and references therein. [5] A. Maignan, Ch. Simon, V. Caignaert and B. Raveau, Solid

State Commun. 96 (1995) 623. [6] G.C. Xiong, Q. Li, S.N. Mao, L. Senapathi, X.X. Xi, R.L.

Greene and T. Venkatesan, Appl. Phys. Lett. 66 (1995) 1427.

[7] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz and K. Samwer, Phys. Rev. Lett. 71 (1993) 2331.

[8] K. Chahora, T. Ohno, M, Kasai and Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990.

[9] A. Asamitsu, Y. Moritomo, Y. Tomioka, T. Arima and Y. Tokura, Nature 373 0995) 407.

[10] S. Jin, H.M. O'Bryan, T.H. Tiefel, M. McCormack and W.W. Rhodes, Appl. Phys, Lett. 66 (1995) 382.

[1 l] M.R. Ibarra, P.A. Algarabel, C. Marquina, J. Blasco and J. Garcia, Phys. Rev. Lett. 75 (1995) 3541.

[12] Z. Zuotao and R. Yufang, J. Solid State Chem. 121 (1996) 138.

[13] G. Srinivasan, V.S. Babu and M.S. Seehra, Appl. Phys. Lett. 67 (1995) 2090.

[14] R. Mahesh, R. Mahendiran, A.K. Raychaudhri and C.N.R. Rao, Appl. Phys. Lett. 68 (1996) 2291.

[15] Z. Arnold, K. Kamenev, M.R. Ibarra, P.A. Algarabel, C. Marquina, J. Blasco and J. Garcia, J. Appl. Phys. Lett. 67 (1995) 2875.

[16] J.J. Neumeier, M.F. Hundley, J.D. Thompson and R.H. Heffner, Phys. Rev. B 52 (1995) 7006.

[17] J.E. Nunez-Reguerio and A.M. Kadin, Appl. Phys. Lett. 68 (1996) 2747.