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Sodium Doped Lanthanum Manganites Thin Films: Synthesis, Substrate Effect and Thickness Dependence

Paolo Ghigna Dipartimento di Chimica Fisica

“M. Rolla”, Università di Pavia, V.le Taramelli 16, I-27100,

Pavia, Italy

                                                                                                                                                                                                                                            

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Introduction One of the most exciting feature of manganites thin films is the

close correlations between the electronic transport and the strain effect induced by the substrate used for thin films growth. In fact the physical properties of the films (TI-M, TC, MR) can be nicely tuned by controlling the film thickness and the substrate nature, i.e., the two most important variable in affecting the film strain and growth direction.

Recently we focused on the sodium doped lanthanum manganites in order to deeply investigate this system which looks suitable for achieving better electronic and magnetic properties. This is mainly due to the fact that the tolerance factor (t) is practically unchanged by the sodium replacement of lanthanum; moreover, compared to the Ca-doped manganites, it is possible to achieve an equal amount of hole doping with a lower cation substitution since for the same amount of aliovalent dopant the hole density is twice with respect to the calcium doping. This should reflect in a lower cation disorder induced by the doping. Finally, for an optimal doping, these materials present a rhombohedral structure.

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Experimental La0.88Na0.12MnO3+ thin films were

deposited on SrTiO3 (100), NdGaO3 (100) and NdGaO3 (110) single crystals. The depositions were performed by an off-axis Rf-magnetron sputtering system.

X-ray reflectivity (XRR), as well as -2 X-ray diffraction (XRD) data were collected by using a Bruker “D8 Advance” diffractometer.

Resistance measurements were carried out in a Quantum Design Magnetic Property Measurement System (MPMS) in a four points configuration.

NC-AFM images were obtained with an Autoprobe CP microscope (Park Instruments- VEECO), operating in non-contact mode.

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Crystal Structure La0.88Na0.12MnO3+

(LNMO) starting powders present a rhombohedral structure; cell parameters: a=5.494 Ǻ, c=13.302 Ǻ, V=57.96 Ǻ3.

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Film growth and microstructure

Tempo di deposizione (min)

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Spess

ore

(nm

)

0

10

20

30

40

50

60

70

80

90

100

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X-ray diffraction and reflectivity

/deg

0 1 2 3

Intensity/ar.un.

1e-7

1e-6

1e-5

1e-4

1e-3

1e-2

1e-1

1e+0

1e+1

simulation

experimental

--154.2-3.88482

--188- 3.87933

--221.1-3.87711NGO (100)

240911142563.89282

2717071.62853.89833

--155-3.88911NGO (110)

259; 178

45; 37129260, 182

3.87670

25278.295.7603.87825

24077.994.22483.85113 STO (100)

MR peak 5 T (K)

MR (%)

5 T

Ea

(meV)

TI-M

(K)

Out-of-plane

parameter (Å)

Thick-ness

(nm)

Substrate

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Substrate induced strain The strain is defined as

((asubstrate-atarget)/asubstrate)100 For the STO (100) substrate is about +0.50%

(tensile strain). For the NGO (110) substrate is about -0.54%

(compressive strain) Finally, for the films grown on the NGO (100),

the epitaxial growth should place the b-c lattice planes of the orthorhombic Pbnm structure in the plane of the substrate while the out-of-plane lattice constant will be one of the two short axes of the orthorhombic cell (a, in this case)

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Magnetoresistance: STO (100) deposited films

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Magnetoresistance: NGO (110) deposited films

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Magnetoresistance: NGO (100) deposited films

T/K

150 200 250 300

R/

0.0

5.0e+6

1.0e+7

1.5e+7

2.0e+7

2.5e+7

11 nm

30 nm

82 nm

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Conclusions The first synthesis by means of Rf-sputtering of

optimally doped La1-xNaxMnO3 thin films has been performed, With three different substrates, and for each of them, three different thicknesses.

The synthetic procedure here applied showed to be suitable to produce well oriented epitaxial thin films.

The structural data showed that film grown onto STO (100) are under tensile strain while the one growth on NGO (110) are under a compressive strain of the same magnitude.

Films grown onto the NGO (110) substrate do not show magnetoresistance.