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Resistance and magnetic behavior of Ce1− xA1x thin films
Journal of Magnetism and Magnetic Materials 177-181 (1998) 377-378
, ~ l Jewna[el
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ELSEVIER
magnetism and magnetic
materlais
Resistance and magnetic behavior of Cel-xAlx thin films G. Dumpich a, U. Eberhardt a'*, E.F. Wassermann a, Y. Miyako b, R. Hamada b, T. Taniguchi b Experimentelle Tiefiemperaturphysik, Gerhard-Mercator-Universitfit Duisburg, Lotharstrafle 65, D-47048 Duisburg, Germam, b Department of Earth and Space Science, Osaka University, Toyonaka 560, Osaka, Japan
Abstract Cea _xAl~ thin films (thickness < 50 nm) of various compositions 0.4 < x < 0.96 are prepared under high-vacuum conditions (p ~ 10- ~ mbar) on amorphous carbon films at room temperature. Structural properties of these films are obtained using transmission electron microscopy. The resistance of the Ce~ xAl~ films is measured for temperatures between 1.5 and 300 K and applying magnetic fields (perpendicular to the film plane) up to B = 5 T. For some of the films magnetic measurements are carried out using a SQUID magnetometer. The results show that Cex-xAl~ thin films with 0.6 < x < 0.8 exhibit heavy-fermion (HF) properties similar to bulk alloys, although the films in this concentration range are nanocrystalline. We discuss the results by comparing the properties of the films to bulk properties and in the framework of 2d-resistance behaviour. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth compounds; Thin films; Heavy fermions; Nanocrystalline materials
The study of the relation between magnetism and structure is important from the point of view not only of the development of magnetic materials but also of fundamental research of magnetism. Magnetic properties are strongly related to crystal structure as well as crystal size. The evolution of ferromagnetism with increasing crystal size has been studied for small transition-metal particles [1]. The study of thin films has also brought us a new field in magnetism although it contains complex compositional problems. Suzuki et al. studied the magnetic properties of Ce amorphous alloys suggesting that structural disorder tends to break the formation of coherent Kondo effects [2]. Here we studied the transport and magnetic properties of Cel-xAl~ thin films which are nanocrystalline for compositions 0.6 < x < 0.8. The Cel-xAlx films of various compositions x are prepared on carbon-coated substrates (quartz, NaC1) at room temperature by simultaneously evaporating Ce and A1 from tungsten filaments. To avoid oxidation we used high evaporation rates of typical F ~ 2 nm/s. The structure and the composition x of Ce~_~Alx films are investigated by transmission electron microscopy (TEM) and by energy-dispersive X-ray analysis (EDX). Samples
*Corresponding author. Tel.: + 49 203 379 2024; fax: + 49 203 379 2098; e-mail: [email protected].
prepared on quartz crystals are used for resistance and magnetization measurements. The resistance R is measured between 1.5 K < T < 300 K using a conventional four-terminal DC technique in a 4He-cryostat where magnetic fields up to B = 5 T can be applied perpendicular to the film plane. We prepared various Cel -xAlx thin films with different compositions between 0.4 < x < 0.96. For films with high Al-content, i.e. for x > 0.9 we find the Ce1_xAlx films to be superconducting with a transition temperature of Tc = 1.5 K. On the other hand, we find for Cel xAl~films with lower Al-concentration (x < 0.6) the resistance to continuously increase with decreasing temperature indicating the onset of insulating behaviour. Most interesting resistance behaviour occurs for Cel -xAlx films with compositions between 0.6 < x < 0.8. Fig. 1 shows the resistance R[](T) for a Ce0.zsAlo.75 film with a thickness of t = 62 nm exhibiting a pronounced minimum at Tm~n= 50 K with R[] linearly increasing for T > Tmi. indicating metallic behaviour. Below Tm~nthe resistance increases with decreasing temperature. However, when further lowering the temperature the resistance sharply drops at T m a x = 1.8 K from R~ = 18 I)/D down to REz = 2.5 ~/ff] (see inset in Fig. 1). Fig. 2 shows for the same film the R~(T) data around T = 1.8 K when magnetic fields of various strength B with 0 T < B < 1 T are applied perpendicular to the film plane. As one can
0304-8853/98/$19.00 {£ 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 6 8 3 - 5
378
G. Dumpich et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 377-378 18.2
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i
i
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i
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Temperature
,
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300
(K)
Fig. 1. Resistance RE2 versus temperature T for a Ce0 2sAl0 7s film with a thickness of t = 62 nm. 18
1.0 ¢ r e s l a
"
_
_
_
_
_
.
.
.
Fig. 3. Electron micrograph of a Ce0.25Alo.Ts film. The inset shows the corresponding diffraction pattern.
.
14 ~.
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i :!:::: j 00Tesla
~8
•
2| 1.5
1.6
1.7 Temperature
1.8
i 1.9
2.0
(K)
Fig. 2. RD versus T for the same film as in Fig. I applying various magnetic fields B perpendicular to the film plane. see from Fig. 2 the resistance clearly 'saturates' (for B = 0) at lower temperatures exhibiting a residual resistance of R:2 = 3.5 f~/[2]. The sharp resistance drop around T = 1.8 K is suppressed by applying a magnetic field resulting in a temperature independent resistance for B = 1 T. The resistance behaviour of the Ce0.zsAlo.vs film as shown in Figs. 1 and 2 is very similar for samples with a composition between 0.6 < x < 0.8. For a C e l - x A l x film of nominal concentration x = 0.75, Fig. 3 shows a bright-field electron micrograph and the corresponding diffraction pattern. As one can see the film shows a granular structure which can be identified as being composed of nanocrystalline CeAI3 and small Al-grains (bright contrast). It is well known that CeA13-bulk samples display heavy fermion behaviour exhibiting, e.g. resistivity maxima around Tm~x -~ 35 K with typical values of p,,,x -~ 110 p.f~ cm and a large resistivity drop down to p = 1 0 1 a f l c m for T ~ 0 K [3]. Above T,,,~ the resistance decreases which is attributed to incoherent K o n d o scattering processes, whereas for the low-temperature resistance behaviour coherent scattering processes are involved. The resistance behaviour of the present Ce~_~AI~ films appears to be similar to that of
CeA13 bulk samples, although some features are rather different. The temperature Tmax(film) = 1.8 K is much lower than the bulk value T,,,x(bulk)-~ 35 K. This is possibly due to the nanocrystalline structure of these films. On the other hand, the resistivity pm,x(film)~110 lafl cm is rather close to the bulk value. Above Tmax = 1.8 K the decrease of resistance could arise from (weak) localization and enhanced electron-electron interactions occurring in thin films [4]. However, the resistance variation beween Tmax < T < Train - as given by I~RE2/R2 is for the present film about two orders of magnitude larger than e.g. granular thin films [5]. We thus conclude that for T > Tma x the resistance behaviour of the present Ceo.25Alo.75 film is more likely the result of incoherent K o n d o scattering processes rather than q u a n t u m transport phenomena. Below Tmax coherent K o n d o behaviour may be masked by the superconductivity of Al-rich grains, since the resistance drop is rather steep and exhibits an almost temperature-independent behaviour applying a magnetic field of about B = 1 T. O n the other hand, the decrease of the resistance is of the same order of magnitude as for bulk CeAI3. Further resistance measurements could reveal whether or not these states exist in the present Ceo.zsAlo.75 films at lower temperatures.
References [1] J.L. Moran-Lopez, J.M. Sanchez (Eds.), New Trends in
[2] [3] [4] [5]
Magnetism, Magnetic Materials, and their Application, Plenum Press, New York, 1994. K. Suzuki, K. Sumiyama, H. Amans, H. Yamauchi, T. Suzuki, J. Magn. Magn. Mater. 108 (1992) 161. A. Percheron, J.C. Achard, O. Gorochov, B. Cornut, D. J6rome, B. Coqbtin, Solid State Communi. 12 (1973) 1289. B.L. Al'tshuler, A.G. Aronov, M.E. Gershenzon, Yu.V. Sharvin, Soy. Sci. Rev. A. Phys. 9 (1987) 223. A. Carl, G. Dumpich, D. Hallfarth, Phys. Rev. B 39 (1989) 3015.
, ~ l Jewna[el
~
ELSEVIER
magnetism and magnetic
materlais
Resistance and magnetic behavior of Cel-xAlx thin films G. Dumpich a, U. Eberhardt a'*, E.F. Wassermann a, Y. Miyako b, R. Hamada b, T. Taniguchi b Experimentelle Tiefiemperaturphysik, Gerhard-Mercator-Universitfit Duisburg, Lotharstrafle 65, D-47048 Duisburg, Germam, b Department of Earth and Space Science, Osaka University, Toyonaka 560, Osaka, Japan
Abstract Cea _xAl~ thin films (thickness < 50 nm) of various compositions 0.4 < x < 0.96 are prepared under high-vacuum conditions (p ~ 10- ~ mbar) on amorphous carbon films at room temperature. Structural properties of these films are obtained using transmission electron microscopy. The resistance of the Ce~ xAl~ films is measured for temperatures between 1.5 and 300 K and applying magnetic fields (perpendicular to the film plane) up to B = 5 T. For some of the films magnetic measurements are carried out using a SQUID magnetometer. The results show that Cex-xAl~ thin films with 0.6 < x < 0.8 exhibit heavy-fermion (HF) properties similar to bulk alloys, although the films in this concentration range are nanocrystalline. We discuss the results by comparing the properties of the films to bulk properties and in the framework of 2d-resistance behaviour. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth compounds; Thin films; Heavy fermions; Nanocrystalline materials
The study of the relation between magnetism and structure is important from the point of view not only of the development of magnetic materials but also of fundamental research of magnetism. Magnetic properties are strongly related to crystal structure as well as crystal size. The evolution of ferromagnetism with increasing crystal size has been studied for small transition-metal particles [1]. The study of thin films has also brought us a new field in magnetism although it contains complex compositional problems. Suzuki et al. studied the magnetic properties of Ce amorphous alloys suggesting that structural disorder tends to break the formation of coherent Kondo effects [2]. Here we studied the transport and magnetic properties of Cel-xAl~ thin films which are nanocrystalline for compositions 0.6 < x < 0.8. The Cel-xAlx films of various compositions x are prepared on carbon-coated substrates (quartz, NaC1) at room temperature by simultaneously evaporating Ce and A1 from tungsten filaments. To avoid oxidation we used high evaporation rates of typical F ~ 2 nm/s. The structure and the composition x of Ce~_~Alx films are investigated by transmission electron microscopy (TEM) and by energy-dispersive X-ray analysis (EDX). Samples
*Corresponding author. Tel.: + 49 203 379 2024; fax: + 49 203 379 2098; e-mail: [email protected].
prepared on quartz crystals are used for resistance and magnetization measurements. The resistance R is measured between 1.5 K < T < 300 K using a conventional four-terminal DC technique in a 4He-cryostat where magnetic fields up to B = 5 T can be applied perpendicular to the film plane. We prepared various Cel -xAlx thin films with different compositions between 0.4 < x < 0.96. For films with high Al-content, i.e. for x > 0.9 we find the Ce1_xAlx films to be superconducting with a transition temperature of Tc = 1.5 K. On the other hand, we find for Cel xAl~films with lower Al-concentration (x < 0.6) the resistance to continuously increase with decreasing temperature indicating the onset of insulating behaviour. Most interesting resistance behaviour occurs for Cel -xAlx films with compositions between 0.6 < x < 0.8. Fig. 1 shows the resistance R[](T) for a Ce0.zsAlo.75 film with a thickness of t = 62 nm exhibiting a pronounced minimum at Tm~n= 50 K with R[] linearly increasing for T > Tmi. indicating metallic behaviour. Below Tm~nthe resistance increases with decreasing temperature. However, when further lowering the temperature the resistance sharply drops at T m a x = 1.8 K from R~ = 18 I)/D down to REz = 2.5 ~/ff] (see inset in Fig. 1). Fig. 2 shows for the same film the R~(T) data around T = 1.8 K when magnetic fields of various strength B with 0 T < B < 1 T are applied perpendicular to the film plane. As one can
0304-8853/98/$19.00 {£ 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 6 8 3 - 5
378
G. Dumpich et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 377-378 18.2
,
.
18.1
,
.
,
.
,
,
,
14
18.0
~
...""
lo
17.9
,..-'"
4
:
17.8
17.7
i7.6
2
i
i
i
,
i
,
1
i00
50
i
i
150
i
i
200
Temperature
,
250
300
(K)
Fig. 1. Resistance RE2 versus temperature T for a Ce0 2sAl0 7s film with a thickness of t = 62 nm. 18
1.0 ¢ r e s l a
"
_
_
_
_
_
.
.
.
Fig. 3. Electron micrograph of a Ce0.25Alo.Ts film. The inset shows the corresponding diffraction pattern.
.
14 ~.
12
0.5 Tesl
i :!:::: j 00Tesla
~8
•
2| 1.5
1.6
1.7 Temperature
1.8
i 1.9
2.0
(K)
Fig. 2. RD versus T for the same film as in Fig. I applying various magnetic fields B perpendicular to the film plane. see from Fig. 2 the resistance clearly 'saturates' (for B = 0) at lower temperatures exhibiting a residual resistance of R:2 = 3.5 f~/[2]. The sharp resistance drop around T = 1.8 K is suppressed by applying a magnetic field resulting in a temperature independent resistance for B = 1 T. The resistance behaviour of the Ce0.zsAlo.vs film as shown in Figs. 1 and 2 is very similar for samples with a composition between 0.6 < x < 0.8. For a C e l - x A l x film of nominal concentration x = 0.75, Fig. 3 shows a bright-field electron micrograph and the corresponding diffraction pattern. As one can see the film shows a granular structure which can be identified as being composed of nanocrystalline CeAI3 and small Al-grains (bright contrast). It is well known that CeA13-bulk samples display heavy fermion behaviour exhibiting, e.g. resistivity maxima around Tm~x -~ 35 K with typical values of p,,,x -~ 110 p.f~ cm and a large resistivity drop down to p = 1 0 1 a f l c m for T ~ 0 K [3]. Above T,,,~ the resistance decreases which is attributed to incoherent K o n d o scattering processes, whereas for the low-temperature resistance behaviour coherent scattering processes are involved. The resistance behaviour of the present Ce~_~AI~ films appears to be similar to that of
CeA13 bulk samples, although some features are rather different. The temperature Tmax(film) = 1.8 K is much lower than the bulk value T,,,x(bulk)-~ 35 K. This is possibly due to the nanocrystalline structure of these films. On the other hand, the resistivity pm,x(film)~110 lafl cm is rather close to the bulk value. Above Tmax = 1.8 K the decrease of resistance could arise from (weak) localization and enhanced electron-electron interactions occurring in thin films [4]. However, the resistance variation beween Tmax < T < Train - as given by I~RE2/R2 is for the present film about two orders of magnitude larger than e.g. granular thin films [5]. We thus conclude that for T > Tma x the resistance behaviour of the present Ceo.25Alo.75 film is more likely the result of incoherent K o n d o scattering processes rather than q u a n t u m transport phenomena. Below Tmax coherent K o n d o behaviour may be masked by the superconductivity of Al-rich grains, since the resistance drop is rather steep and exhibits an almost temperature-independent behaviour applying a magnetic field of about B = 1 T. O n the other hand, the decrease of the resistance is of the same order of magnitude as for bulk CeAI3. Further resistance measurements could reveal whether or not these states exist in the present Ceo.zsAlo.75 films at lower temperatures.
References [1] J.L. Moran-Lopez, J.M. Sanchez (Eds.), New Trends in
[2] [3] [4] [5]
Magnetism, Magnetic Materials, and their Application, Plenum Press, New York, 1994. K. Suzuki, K. Sumiyama, H. Amans, H. Yamauchi, T. Suzuki, J. Magn. Magn. Mater. 108 (1992) 161. A. Percheron, J.C. Achard, O. Gorochov, B. Cornut, D. J6rome, B. Coqbtin, Solid State Communi. 12 (1973) 1289. B.L. Al'tshuler, A.G. Aronov, M.E. Gershenzon, Yu.V. Sharvin, Soy. Sci. Rev. A. Phys. 9 (1987) 223. A. Carl, G. Dumpich, D. Hallfarth, Phys. Rev. B 39 (1989) 3015.