DSpace at VNU: Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricated by PLD Technique

Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricatedby PLD Technique Quoc Thanh Phung1;2, Seong-Cho Yu1, Duc Tho Nguyen2, and Nam Nhat Hoang2 BK21 Physics Program and Department

Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricated

by PLD Technique

Quoc Thanh Phung1;2, Seong-Cho Yu1, Duc Tho Nguyen2, and Nam Nhat Hoang2 BK21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763, Korea

Department of Technical Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University

Hanoi, Hanoi, Vietnam

The Ca0 85Pr0 15Mn1 yRuyO3( y = 0, 0.04, 0.08, 0.12, 0.16, and 0.20) manganate thin films were prepared by pulsed laser deposi-tion (PLD) technique The X-ray diffracdeposi-tion (XRD) analysis revealed that the samples were single-phased with orthorhombic structure The scanning electron microscopy (SEM) images indicated that the samples were composed of homogeneous grains The Hall-effect mea-surements showed that the carrier density and Hall mobility of films increased with increasing Ru-doped content The magnetic field de-pendence of magnetization at various temperatures were measured by using the superconducting quantum interference device (SQUID) and showed that the increase of Ru-doping content induced the large 1 change in broad range of temperature This demonstrates

a possible application of these materials in cooling devices as the relative cooling power (RCP) is proportial to (1 )max1TFWHM

(where1TFWHMis the full width at half-maximum of 1 ).

Index Terms—Magnetocaloric effect, manganate, pulsed laser deposition (PLD), thin film.

I INTRODUCTION

continuously attract the interests of scientists worldwide

be-cause of their outstanding electrical and magnetic properties,

such as colossal magnetoresistive effect (CMR) [1], and large

magnetocaloric (LMCE) effects [2] Recently, among the

materials of great focus, the Ru-doped Ca-Pr manganates of

as they showed a giant thermoelectric effect (GTE) at high

magnetic properties, electrical conduction mechanism, and

car-rier transport through the grain boundaries of these perovskites

were investigated [4], [5] However, a complete understanding

of all physical aspects still remains under discussion, which

arises in part due to poor control over the quality of samples

prepared by conventional ceramic methods [3]–[5]

The overall aim of this paper is the fabrication of high-quality

(where 0, 0.04, 0.08, 0.12, 0.16, and 0.20) by pulsed

laser deposition (PLD) technique and as a first step to

charac-terize their structure and magneto-electrical properties The

re-sults obtained revealed that the samples prepared by PLD

tech-nique possess better quality, with more uniformity in size and

boundary of grains The structure of all films showed the

or-thorhombic symmetry with space group Pnma The transport

characteristics of samples were strongly Ru-doping dependent

The investigations were also taken on the address of LMCE, and

in contrary to the samples prepared by ceramic route, the PLD

gave LMCE that might attract attention for potential application

in cooling devices

Manuscript received February 20, 2011; accepted May 03, 2011 Date of

cur-rent version September 23, 2011 Corresponding author: Q T Phung (e-mail:

phung.qth@gmail.com).

Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2011.2154356

II EXPERIMENTAL The thin films CPMRO with Ru-doping content from 0 up

to 0.20 were prepared by PLD using the CPMRO targets of the same compositions The CPMRO targets were fabricated from the pure oxides using a solid-state reaction technique with re-peated grinding and sintering at 1180 C for 24 h in open air The CPMRO thin films were then deposited onto a SiO (001) sub-strate During the deposition, all substrates were heated and kept

at 700 C in an oxygen atmosphere of 450 mTorr The process condition was controlled so that the thickness of the samples was kept at the desired value of 30 nm The as-deposited sam-ples were then annealed at 600 C for 2 h in open air

The structure and microstructure of the films were char-acterized by X-ray diffraction (XRD) technique using the Brucker D5005 diffractometer and by the scanning electron microscopy (SEM) using Jeol’s JSM-5400LV microscope The electrical properties of the samples were investigated using the Hall-effect method (Lake Shore 7500/9500) To investigate the LMCE, we measured the field dependence of magnetization

at various temperatures by using a superconducting quantum interference device (SQUID), where temperature incremental step and applied field step were 5.0 K and 10 kOe, respectively

III RESULTS ANDDISCUSSION

In Fig 1, we showed the XRD patterns of the as-deposited thin films We can clearly observe that all samples are of single phase and possess the same symmetry The calculation showed that the appropriate space group was Pnma As the maxima are moved left in comparison with that of the standard bulk samples, the lattice structures of as-deposited films were strained [6], [7] This strained state is analogous to the strain-glass state in bulk samples, but is obviously strong by influence of the effect of quenched disorder This strained state disappeared when the films were annealed in air (or in oxygen) In this case, their XDR patterns were quite similar to those of the bulks

Table I shows the lattice constants of annealed samples obtained by using the Rietveld refinement method [8] The increase in lattice parameters and volume was observed with 0018-9464/$26.00 © 2011 IEEE

Fig 1 XRD patterns of the CPMRO as-deposited films.

TABLE I

L ATTICE P ARAMETERS AND S OME I MPORTANT M EAN B OND L ENGTHS

AND B OND A NGLES FOR CPMRO A NNEALED T HIN F ILMS

increasing Ru-doped content Since the radii of Ru (0.565 )

and Mn (0.531 ) cations are similar, the induced changes

in lattice parameters were relatively weak, which indicates

that the Ru substitution was taken possibly in Mn sites in base

matrix The calculation also showed the increase in bond angle

(Mn-O-Mn) and bond distances (Ca-O, Mn-O) This led to the

reduction in double exchange interaction (DE) since the total

overlap integral (which is proportional to the square of cosine

of angle per bond distance) reduced

The influences of Ru-doping content and of applied magnetic

field on Hall mobility and carrier density of the thin films were

also investigated The results are shown in Fig 2(a) We

ob-served that the Hall mobility and carrier density of samples

re-mained almost constant when the applied magnetic fields varied

The independence of the both on magnetic field was observed

even at high fields such as from 1.2 to 1.3 T [Fig 2(a)], therefore

the intrinsic elastic energy of strain state of the thin films was

uneliminated by external magnetic field [6] This independence

of carrier mobility and density on magnetic field argues for

the fact that the conduction electrons are not probably involved

in the magnetic exchange interactions This observation agrees

in principle with the statement about the percolative

conduc-tion mechanism in ruthenates and Ru-doped manganates [5] In

Fig 2(b), we demonstrate the dependence of Hall mobility and

carrier density of thin films on Ru-doped content As seen, these

quantities developed almost linearly with Ru-doped content It

is worthwhile noting that the values of carrier density developed

about 10 times larger (from 10 /cm to 10 /cm ), where the

Fig 2 (a) Hall mobility and the carrier density versus magnetic field and (b) Ru concentration for the films The inset in (b) depicts the development of resistivity according to the content of substitution.

ruthenium content increased from 0.00 to 0.20 In other words, the Ru substitution in B-position heavily influenced the trans-port properties of the undoped sample Ca Pr MnO In a similar fashion, the Hall mobility of samples also increased ap-proximately ten times when the Ru content increased

Since the mobility and carrier density of the samples in-creased with Ru-doped content, their electrical resistivities decreased [inset, Fig 2(b)] The substitution of Ru cations

in the lattice weakly distorted the structure of material as the

Ru and Mn ionic radii are quite similar, but the sub-stitution strongly affected the Mn /Mn ratio because of higher valence of Ru in comparison to of Mn Furthermore, when Ru was substituted for Mn, one should consider the FM interactions and consequently the metallicity of samples From the two effects, the double exchange (DE) between Mn and

Fig 3 Temperature dependences of maximum magnetic-entropy changes for

Ru-doped samples under magnetic field of 1 T The inset shows the FC-ZFC

curve for two samples with y = 0.08 and 0.16

Mn and the super-exchange (SE) between Mn and Ru

have to be considered [9], [10] From the inset of Fig 2(b), we

can observe that the electrical resistivity of samples was visibly

reduced when the Ru content increased

The existence of magnetic ordering in general and the

com-petitive coexistence of AFM and FM states in particular

sug-gest that the magnetocaloric behavior may exist in these

sam-ples; some enhancements may even occur Indeed, the Ru-doped

manganates prepared by the ceramic route showed some

mag-netocaloric effects, but the relative cooling efficiency was much

smaller in comparison with that of the other doped perovskites

such as La Pb MnO [12] To investigate the measure of

LMCE in the samples prepared by PLD, we recorded the

de-velopment of magnetization according to temperature and

ap-plied field and evaluated the magnetic entropy change

from obtained isotherms according to [13]

(1)

ab-sence and in the presence of applied field

was evaluated as the function of temperature

Fig 3 shows the obtained results for Ru contents 0.08,

0.12, and 0.16 As seen, the maxima occurred at around

0.47 J/Kg K under the field of 1 T One set of maxima (left

part) showed the reduction to 0.4 and 0.3 J/kg K, but the other

set (right part) remained unchanged Obviously, the increase

in Ru-doped content induced the large variation of in a

broad range of temperature ( 150 K) This means the

enhance-ment in cooling efficiency and demonstrates that these thin

films may be used for cooling devices As defined, the relative

TABLE II

R ELATIVE C OOLING E FFICIENCY (RCP) FOR S EVERAL P EROVSKITE S YSTEMS

therefore the spreading of when Ru content increased sufficiently enhanced the cooling power Although the maxima

of here are a bit smaller than that of La Pb MnO [12]

comparable to that of those compounds (even exceeded for the case of ) The summary of several RCPs is given in Table II The surprising expression of LMCE obtained in the films prepared by PLD is the primary result of this work Such LMCE was not observed in the films fabricated by traditional ceramic route The enhancement of LMCE in Ru-doped man-ganates may originate in the competitive coexistence of AFM and FM ordering states induced by doping (observe the FC and ZFC curves shown in the inset of Fig 3) The improvement

research

IV CONCLUSION

We have successfully fabricated the high-quality thin films of

0.12, 0.16, 0.20) using the PLD technique The structural and electrical properties of these films have been analyzed and dis-cussed The as-deposited thin films were strained, therefore their structures were deformed but remained orthorhombic, similar

to that of the bulk samples after annealing The conductivity of samples showed the increase with increasing Ru-doping con-tent The obtained results indicated that the substitution of Mn

by Ru belongs to the electron-doped type The magnetic-entropy changes span over a large range of temperature, and the prepared films can be considered as the potential LMCE candidates for magnetic refrigerators

ACKNOWLEDGMENT This work was supported by the Brain Korea 21 project and the Korea Foundation for Advanced Studies’ Interna-tional Scholar Exchange Fellowship for the academic year of 2008–2009 The authors would like also to thank the finan-cial support from the National Foundation for Scientific and Technology Development of Vietnam (NAFOSTED), research project “Magnetism in novel perovskite composites” (2010)

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