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Anomalous effects of hydrogen absorption in Nb films
Journal of Magnetism and Magnetic Materials 240 (2002) 475–477
Anomalous effects of hydrogen absorption in Nb films H. Malettaa,*, Ch. Rehma,b, F. Kloseb, M. Fieber-Erdmanna, E. Holub-Krappea a
Department SF2, Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109, Berlin, Germany b Argonne National Laboratory/SNS Project, Argonne, IL 60439, USA
Abstract Hydrogen absorption in Nb films of epitaxial W/Nb and polycrystalline Fe/Nb multilayers was explored in-situ by small-angle neutron/X-ray reflectometry, high-angle diffraction and extended X-ray absorption fine structure spectroscopy. Besides the hydrogen-induced Nb lattice expansion, the experiments reveal a second, unexpected and macroscopic effect: The considerably larger relative expansion of the Nb film thickness, which is interpreted as a rearrangement of Nb lattice planes. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Hydrogen absorption; Reflectometry; EXAFS
Over the past several decades, hydrogen in metals has attracted considerable research interest. Improvements in experimental techniques have now allowed a focus on the behaviour of hydrogen in thin films. The interaction of hydrogen with the film can lead to significant modifications of the electronic, magnetic and structural properties [1]. Exciting new results have been achieved recently. Among those are (i) reversible switching of optical properties upon hydrogen absorption in yttrium and lanthanum thin films [2], and (ii) reversible switching of magnetic coupling and magnetoresistivity during hydrogen charging and decharging in Fe/Nb [3] or Fe/V [4] multilayers. In the present paper, we focus on structural changes that occur during the initial loading of thin Nb layers with hydrogen. We present results obtained from in-situ hydrogen charging experiments on epitaxial W/Nb(0 0 1) [5] and polycrystalline Fe/Nb(1 1 0) multilayers [6]. In both systems, the Nb layers can easily be charged with hydrogen from the gas phase. Due to thermodynamic reasons, and facilitated by the large negative enthalpy of mixing, only the Nb films of the multilayer absorb hydrogen. We never found evidence of a noticeable amount of hydrogen in the Fe or W layers. *Corresponding author. Tel.: +49-30-8062-2058; fax: +4930-8062-2523. E-mail addresses: [email protected] (H. Maletta)
Due to the strong interaction of neutrons with hydrogen atoms, neutron reflectivity is a direct and precise method of determining the average hydrogen concentration in thin films (see Ref. [6] for an extensive discussion). We performed in-situ measurements on W/ Nb and Fe/Nb multilayers at the V6 (Hahn-MeitnerInstitut Berlin) and the POSY2 (IPNS, Argonne National Laboratory) neutron reflectometers, and derived hydrogen solubility curves as displayed in Fig. 1 in the H2 pressure range up to 900 mbar. Note that during these and all the following experiments, the temperature was kept constant at T ¼ 1851C. This temperature allows for fast hydrogen uptake dynamics (see Ref. [6]) and is high enough to avoid a possible phase separation, which is well known for the bulk hydrogen/niobium system. Very similar results in Fig. 1 were obtained on W/Nb multilayers of comparable layer thicknesses [5]. Obviously, the hydrogen solubility in the Nb films strongly differs from that in bulk Nb and is dependent on the film thickness. The plateau-like pressure region, at which most of the hydrogen is absorbed, is found orders of magnitude higher than the bulk values. In ( particular the surprisingly large deviation of the 1000 A Nb sample from bulk behaviour emphasizes the importance of long-range mechanical interactions between the hydrogen absorbing Nb layers and the adjacent non-absorbing layers (Fe, W, or the substrate) for hydrogen solubility.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 0 5 - 2
H. Maletta et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 475–477
476
[26 Å Fe / X Å Nb]*n
Increase of Nb(110) interplanar spacing (%)
3
10
10
2
(a) 20 Å Nb
1
1
10
-2
0
10
-5
10
[26 Å Fe / X Å Nb]*n
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20 Å Nb 50 Å Nb 100 Å Nb 1000 Å Nb Bulk Nb
-2
10
-3
10
-4
10
0
20
40
60
80
100
Hydrogen concentration [H]/[Nb] (at.%)
Fig. 1. Solubility curves of hydrogen in Nb films measured in Fe/Nb multilayers with various Nb layer thicknesses at T ¼ 1851C. The dashed line for bulk Nb is extrapolated from literature.
Up to now it has always been assumed that hydrogen absorption in thin films is comparable to the absorption process in bulk metals, where by far the most dominating mechanism is the hydrogen occupation of interstitial lattice sites in the host metal. The main difference in structural changes caused by hydrogen absorption was thought to be the predominant one-dimensional lattice expansion in the out-of-plane direction of the films, because of the clamping of the film to the substrate, in contrast to the three-dimensional lattice expansion of bulk materials [7]. Our study of the Nb films in W/Nb and Fe/Nb multilayers reveals a second, unexpected effect of equal importance besides the lattice expansion due to hydrogen absorption at interstitial sites. Analyzing both the high-angle X-ray diffraction and the small-angle X-ray or neutron reflectivity measurements on the in-situ hydrogen loaded multilayers, we found a remarkable effect as displayed in Fig. 2. There is a huge difference between the out-of-plane lattice expansion (determined by diffraction in the high-angle regime) and the expansion of the film thickness (determined by reflection in the small-angle regime) during the initial hydrogen loading of the Nb films. For all investigated samples, the relative increase at the length scale of the Nb layer thickness is much larger than the relative increase at the level of the (atomic scale) interplanar spacing [8]. The details, however, depend on the actual thickness of the ( Nb behave Nb layers. The films with 100 and 1000 A very similarly. In both cases approximately 11% layer expansion vs. 4.5% lattice plane expansion are found. ( For smaller Nb thicknesses (dNb p50 A), however, interface effects become important, which at first restrict
Hydrogen pressure (mbar)
Hydrogen pressure (mbar)
10
10 10
-2
10
-5
10
(c) 100 Å Nb
1
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(b) 50 Å Nb
1
(d) 1000 Å Nb
1
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8
10
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Increase of Nb layer thickness (%) Fig. 2. Comparison between the hydrogen-induced relative increase of the out-of-plane Nb(1 1 0) interplanar spacing (filled symbols) and the increase of the Nb layer thickness (open symbols) in Fe/Nb multilayers for various hydrogen pressures.
( Nb sample is the layer expansion to about 7%. The 20 A a special case because the interface regions of the Nb ( at each side) do not significantly layers (about 6 A participate in the hydrogen uptake [9]. While reflectivity measurements clearly reveal a large layer expansion, the high-angle diffraction signal arises mostly from this interface region and therefore shows only minor lattice expansion. Based on the observation of increasing line width and background that occurs during charging, we conclude that the inner Nb part which actually absorbs most of the hydrogen must be largely disordered. This interface effect also explains why the measured hydrogen concentrations of thin Nb layers are significantly reduced compared to the case of thick Nb layers with ( (Fig. 1). dNb X100 A Recently, we have performed extended X-ray absorption fine structure (EXAFS) measurements on these insitu hydrogen loaded multilayers [5] which enable us to suggest an interpretation of the unexpected results displayed in Fig. 2. The experiments were carried out . at the ROMO II station at HASYLAB at photon energies near the K-absorption edge of Nb (18.985 keV) using the fluorescence detection mode. A typical example of the measured oscillating data in k-space and its Fourier transform is shown in Fig. 3. From such data one obtains selective information about structural
H. Maletta et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 475–477 0.15 Exp. data
χ (k)
0.10
Fit
0.05 0.00 -0.05 -0.10 -0.15 2
4
6
k (Å) R1
0.25 -1
FT kχ (Å )
8
10
-1
Exp. data Fit
0.20
R2
0.15 0.10 0.05 0.00 0
2
4
6
8
R ( Å) Fig. 3. Representative data from EXAFS measurements on the multilayers near the Nb K-edge, showing the oscillating data (on top) and its Fourier transform (at the bottom). R1 and R2 mark the distance to the first and second nearest Nb neighbours, respectively.
changes occurring perpendicular to the layer and within the layer, when measuring at grazing and normal incidence geometry, respectively. For instance, the position of the R2 peak of data measured in normal incidence in the lower part of Fig. 3 gives the in-plane distance between the next nearest Nb neighbours. The EXAFS experiments clearly reveal an in-plane expansion of the Nb lattice under hydrogen absorption. ( W/100 A ( Nb) multilayer at the hydrogen For the (26 A pressure of 900 mbar as an example, the in-plane interplanar spacing of the Nb lattice increases by 2.5%. Taking the corresponding out-of-plane lattice expansion of 6.2% derived both from our high-angle diffraction and EXAFS experiments on the same sample, one would expect a total increase in volume of 11.5%. This amount coincides within error with the relative increase of the Nb layer thickness expansion of 11.1% as determined from the reflection experiment. Hence, the unexpected results shown in Fig. 2 can be explained quantitatively by assuming a fixed in-plane film area and consequent one-dimensional out-of-plane layer expansion.
477
We have evidence for a new structural effect appearing during hydrogen absorption in the films, namely a three-dimensional rearrangement of Nb atoms. It is caused by massive lateral strain, which builds up during the charging process. As a result, individual Nb atoms are squeezed out of existing lattice planes and create new Nb lattice planes, which finally causes the unexpected differences in the various expansion values shown in Fig. 2. The newly created partial lattice planes imply a large amount of additional dislocations. These are evidenced by significant broadening of the out-of-plane diffraction peaks upon initial hydrogen loading. The proposed model is further supported by our studies on the reversibility of the hydrogen loading cycles (see Ref. [6], and results to be published). The rearrangement itself is most likely a one-time process, since in the second and subsequent loading cycles, the relative layer and lattice expansions are found to be almost equal. In summary, both the hydrogen solubility curves (Fig. 1) and the anomalously large macroscopic expansion of the Nb layers during the initial loading of the films (Fig. 2) emphasize the importance of geometrical constraints and give new important insights into the hydrogen absorption process in thin films.
References [1] H. Zabel, in Encyclopedia of Materials Science and Technology, Pergamon, Elsevier Science, 2001. [2] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G. de Groot, N.J. Koeman, Nature 380 (1996) 231. [3] F. Klose, Ch. Rehm, D. Nagengast, H. Maletta, A. Weidinger, Phys. Rev. Lett. 78 (1997) 1150. . [4] B. Hjorvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. Udovic, G. Andersson, C.F. Majkrzak, Phys. Rev. Lett. 79 (1997) 901. [5] F. Klose, Ch. Rehm, M. Fieber-Erdmann, E. Holub. Krappe, H.J. Bleif, H. Sowers, R. Goyette, L. Troger, H. Maletta, Physica B 283 (2000) 184. [6] Ch. Rehm, H. Fritzsche, H. Maletta, F. Klose, Phys. Rev. B 59 (1999) 3142. [7] G. Song, M. Geitz, A. Abromeit, H. Zabel, Phys. Rev. B 54 (1996) 14093. [8] Ch. Rehm, H. Fritzsche, H. Maletta, F. Klose, Physica B 276–278 (2000) 549. [9] D. Nagengast, Ch. Rehm, F. Klose, A. Weidinger, J. Alloys Compounds 231 (1995) 307.
Anomalous effects of hydrogen absorption in Nb films H. Malettaa,*, Ch. Rehma,b, F. Kloseb, M. Fieber-Erdmanna, E. Holub-Krappea a
Department SF2, Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109, Berlin, Germany b Argonne National Laboratory/SNS Project, Argonne, IL 60439, USA
Abstract Hydrogen absorption in Nb films of epitaxial W/Nb and polycrystalline Fe/Nb multilayers was explored in-situ by small-angle neutron/X-ray reflectometry, high-angle diffraction and extended X-ray absorption fine structure spectroscopy. Besides the hydrogen-induced Nb lattice expansion, the experiments reveal a second, unexpected and macroscopic effect: The considerably larger relative expansion of the Nb film thickness, which is interpreted as a rearrangement of Nb lattice planes. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Hydrogen absorption; Reflectometry; EXAFS
Over the past several decades, hydrogen in metals has attracted considerable research interest. Improvements in experimental techniques have now allowed a focus on the behaviour of hydrogen in thin films. The interaction of hydrogen with the film can lead to significant modifications of the electronic, magnetic and structural properties [1]. Exciting new results have been achieved recently. Among those are (i) reversible switching of optical properties upon hydrogen absorption in yttrium and lanthanum thin films [2], and (ii) reversible switching of magnetic coupling and magnetoresistivity during hydrogen charging and decharging in Fe/Nb [3] or Fe/V [4] multilayers. In the present paper, we focus on structural changes that occur during the initial loading of thin Nb layers with hydrogen. We present results obtained from in-situ hydrogen charging experiments on epitaxial W/Nb(0 0 1) [5] and polycrystalline Fe/Nb(1 1 0) multilayers [6]. In both systems, the Nb layers can easily be charged with hydrogen from the gas phase. Due to thermodynamic reasons, and facilitated by the large negative enthalpy of mixing, only the Nb films of the multilayer absorb hydrogen. We never found evidence of a noticeable amount of hydrogen in the Fe or W layers. *Corresponding author. Tel.: +49-30-8062-2058; fax: +4930-8062-2523. E-mail addresses: [email protected] (H. Maletta)
Due to the strong interaction of neutrons with hydrogen atoms, neutron reflectivity is a direct and precise method of determining the average hydrogen concentration in thin films (see Ref. [6] for an extensive discussion). We performed in-situ measurements on W/ Nb and Fe/Nb multilayers at the V6 (Hahn-MeitnerInstitut Berlin) and the POSY2 (IPNS, Argonne National Laboratory) neutron reflectometers, and derived hydrogen solubility curves as displayed in Fig. 1 in the H2 pressure range up to 900 mbar. Note that during these and all the following experiments, the temperature was kept constant at T ¼ 1851C. This temperature allows for fast hydrogen uptake dynamics (see Ref. [6]) and is high enough to avoid a possible phase separation, which is well known for the bulk hydrogen/niobium system. Very similar results in Fig. 1 were obtained on W/Nb multilayers of comparable layer thicknesses [5]. Obviously, the hydrogen solubility in the Nb films strongly differs from that in bulk Nb and is dependent on the film thickness. The plateau-like pressure region, at which most of the hydrogen is absorbed, is found orders of magnitude higher than the bulk values. In ( particular the surprisingly large deviation of the 1000 A Nb sample from bulk behaviour emphasizes the importance of long-range mechanical interactions between the hydrogen absorbing Nb layers and the adjacent non-absorbing layers (Fe, W, or the substrate) for hydrogen solubility.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 0 5 - 2
H. Maletta et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 475–477
476
[26 Å Fe / X Å Nb]*n
Increase of Nb(110) interplanar spacing (%)
3
10
10
2
(a) 20 Å Nb
1
1
10
-2
0
10
-5
10
[26 Å Fe / X Å Nb]*n
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20 Å Nb 50 Å Nb 100 Å Nb 1000 Å Nb Bulk Nb
-2
10
-3
10
-4
10
0
20
40
60
80
100
Hydrogen concentration [H]/[Nb] (at.%)
Fig. 1. Solubility curves of hydrogen in Nb films measured in Fe/Nb multilayers with various Nb layer thicknesses at T ¼ 1851C. The dashed line for bulk Nb is extrapolated from literature.
Up to now it has always been assumed that hydrogen absorption in thin films is comparable to the absorption process in bulk metals, where by far the most dominating mechanism is the hydrogen occupation of interstitial lattice sites in the host metal. The main difference in structural changes caused by hydrogen absorption was thought to be the predominant one-dimensional lattice expansion in the out-of-plane direction of the films, because of the clamping of the film to the substrate, in contrast to the three-dimensional lattice expansion of bulk materials [7]. Our study of the Nb films in W/Nb and Fe/Nb multilayers reveals a second, unexpected effect of equal importance besides the lattice expansion due to hydrogen absorption at interstitial sites. Analyzing both the high-angle X-ray diffraction and the small-angle X-ray or neutron reflectivity measurements on the in-situ hydrogen loaded multilayers, we found a remarkable effect as displayed in Fig. 2. There is a huge difference between the out-of-plane lattice expansion (determined by diffraction in the high-angle regime) and the expansion of the film thickness (determined by reflection in the small-angle regime) during the initial hydrogen loading of the Nb films. For all investigated samples, the relative increase at the length scale of the Nb layer thickness is much larger than the relative increase at the level of the (atomic scale) interplanar spacing [8]. The details, however, depend on the actual thickness of the ( Nb behave Nb layers. The films with 100 and 1000 A very similarly. In both cases approximately 11% layer expansion vs. 4.5% lattice plane expansion are found. ( For smaller Nb thicknesses (dNb p50 A), however, interface effects become important, which at first restrict
Hydrogen pressure (mbar)
Hydrogen pressure (mbar)
10
10 10
-2
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(c) 100 Å Nb
1
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4
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8
10
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Increase of Nb layer thickness (%) Fig. 2. Comparison between the hydrogen-induced relative increase of the out-of-plane Nb(1 1 0) interplanar spacing (filled symbols) and the increase of the Nb layer thickness (open symbols) in Fe/Nb multilayers for various hydrogen pressures.
( Nb sample is the layer expansion to about 7%. The 20 A a special case because the interface regions of the Nb ( at each side) do not significantly layers (about 6 A participate in the hydrogen uptake [9]. While reflectivity measurements clearly reveal a large layer expansion, the high-angle diffraction signal arises mostly from this interface region and therefore shows only minor lattice expansion. Based on the observation of increasing line width and background that occurs during charging, we conclude that the inner Nb part which actually absorbs most of the hydrogen must be largely disordered. This interface effect also explains why the measured hydrogen concentrations of thin Nb layers are significantly reduced compared to the case of thick Nb layers with ( (Fig. 1). dNb X100 A Recently, we have performed extended X-ray absorption fine structure (EXAFS) measurements on these insitu hydrogen loaded multilayers [5] which enable us to suggest an interpretation of the unexpected results displayed in Fig. 2. The experiments were carried out . at the ROMO II station at HASYLAB at photon energies near the K-absorption edge of Nb (18.985 keV) using the fluorescence detection mode. A typical example of the measured oscillating data in k-space and its Fourier transform is shown in Fig. 3. From such data one obtains selective information about structural
H. Maletta et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 475–477 0.15 Exp. data
χ (k)
0.10
Fit
0.05 0.00 -0.05 -0.10 -0.15 2
4
6
k (Å) R1
0.25 -1
FT kχ (Å )
8
10
-1
Exp. data Fit
0.20
R2
0.15 0.10 0.05 0.00 0
2
4
6
8
R ( Å) Fig. 3. Representative data from EXAFS measurements on the multilayers near the Nb K-edge, showing the oscillating data (on top) and its Fourier transform (at the bottom). R1 and R2 mark the distance to the first and second nearest Nb neighbours, respectively.
changes occurring perpendicular to the layer and within the layer, when measuring at grazing and normal incidence geometry, respectively. For instance, the position of the R2 peak of data measured in normal incidence in the lower part of Fig. 3 gives the in-plane distance between the next nearest Nb neighbours. The EXAFS experiments clearly reveal an in-plane expansion of the Nb lattice under hydrogen absorption. ( W/100 A ( Nb) multilayer at the hydrogen For the (26 A pressure of 900 mbar as an example, the in-plane interplanar spacing of the Nb lattice increases by 2.5%. Taking the corresponding out-of-plane lattice expansion of 6.2% derived both from our high-angle diffraction and EXAFS experiments on the same sample, one would expect a total increase in volume of 11.5%. This amount coincides within error with the relative increase of the Nb layer thickness expansion of 11.1% as determined from the reflection experiment. Hence, the unexpected results shown in Fig. 2 can be explained quantitatively by assuming a fixed in-plane film area and consequent one-dimensional out-of-plane layer expansion.
477
We have evidence for a new structural effect appearing during hydrogen absorption in the films, namely a three-dimensional rearrangement of Nb atoms. It is caused by massive lateral strain, which builds up during the charging process. As a result, individual Nb atoms are squeezed out of existing lattice planes and create new Nb lattice planes, which finally causes the unexpected differences in the various expansion values shown in Fig. 2. The newly created partial lattice planes imply a large amount of additional dislocations. These are evidenced by significant broadening of the out-of-plane diffraction peaks upon initial hydrogen loading. The proposed model is further supported by our studies on the reversibility of the hydrogen loading cycles (see Ref. [6], and results to be published). The rearrangement itself is most likely a one-time process, since in the second and subsequent loading cycles, the relative layer and lattice expansions are found to be almost equal. In summary, both the hydrogen solubility curves (Fig. 1) and the anomalously large macroscopic expansion of the Nb layers during the initial loading of the films (Fig. 2) emphasize the importance of geometrical constraints and give new important insights into the hydrogen absorption process in thin films.
References [1] H. Zabel, in Encyclopedia of Materials Science and Technology, Pergamon, Elsevier Science, 2001. [2] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G. de Groot, N.J. Koeman, Nature 380 (1996) 231. [3] F. Klose, Ch. Rehm, D. Nagengast, H. Maletta, A. Weidinger, Phys. Rev. Lett. 78 (1997) 1150. . [4] B. Hjorvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. Udovic, G. Andersson, C.F. Majkrzak, Phys. Rev. Lett. 79 (1997) 901. [5] F. Klose, Ch. Rehm, M. Fieber-Erdmann, E. Holub. Krappe, H.J. Bleif, H. Sowers, R. Goyette, L. Troger, H. Maletta, Physica B 283 (2000) 184. [6] Ch. Rehm, H. Fritzsche, H. Maletta, F. Klose, Phys. Rev. B 59 (1999) 3142. [7] G. Song, M. Geitz, A. Abromeit, H. Zabel, Phys. Rev. B 54 (1996) 14093. [8] Ch. Rehm, H. Fritzsche, H. Maletta, F. Klose, Physica B 276–278 (2000) 549. [9] D. Nagengast, Ch. Rehm, F. Klose, A. Weidinger, J. Alloys Compounds 231 (1995) 307.