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Epitaxial growth of (110) DyFe2, TbFe2 and Dy0.7Tb0.3Fe2 thin films by molecular beam epitaxy
,. . . . . . . .
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 165 (1996) 175-178
L e t t e r to t h e E d i t o r s
Epitaxial growth of(110) DyFe 2, TbFe 2 and Dy0.vTbo.3Fe2 thin films by molecular beam epitaxy V. O d e r n o
*, C. D u f o u r , K. D u m e s n i l , P h . M a n g i n , G. M a r c h a l
Laboratoire M~tallurgie Physique et Science des Mat~riaux, URA CNRS 155, Unil'ersit~ H. Poincar~-Nancy 1, BP 239, F-54506 Vandoeurre Cedex, France
Received 28 November 1995; accepted 9 January 1996
Abstract We present the first epitaxial growth of some ( l l 0 ) rare earth-Fe 2 (DyF%, TbFe 2 and Dyo.TTbo.3Fe 2 known as terfenol-D) thin films on (110) Nb/(1170) sapphire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. The structures are investigated by RHEED and X-ray scattering. Depending on the thermal treatment of the iron thin layer, the films of R E - F e 2, epitaxially grown on it, are either single crystals or present twins related by a 110 ° rotation about the surface normal. The growth of epitaxial thin films of these compounds is of interest because of the magnetic and magnetostrictive properties these materials may exhibit.
The study of the magnetic properties of thin films is currently a very active field of research. The goal is to prepare small devices in which magnets, transformers or sensors able to respond to magnetic environment are incorporated. In particular, it is of obvious interest to elaborate epitaxial thin films of the phases that exhibit interesting magnetic bulk properties. The first step has been to epitaxy pure magnetic metals (transition metals (TM) or rare earth metals (RE)). Their growth is now well controlled. A lot of disordered alloys (amorphous or polycrystalline) have been previously obtained in the whole or in a restricted composition range. The challenge is now to prepare single crystalline films of these compounds.
* Corresponding author.
The R E / T M compounds combine the large magnetoelasticity of the rare earth and the strong exchange of the transition metal, which enhances the ordering temperature. The R E - F e 2 alloys (cubic Laves phases, MgCu 2 type) show interesting magnetostrictive properties. The compound Tb0.3Dy0.vF% (known as terfenol-D), which simultaneously presents a large magnetostriction and a weak anisotropy, is the most used magnetostrictive material [1,2]. Up to now, a small number of thin films of epitaxial R E - T M 2 compounds have been obtained. (111) TbFe 2 has been prepared by sputtering from two separate targets [3]. (111) YCo 2 has been grown by laser ablation from a stoichiometric target [4]. These compounds were obtained on buffers of bcc refractory metals: TbFe 2 was grown on a (1 10) Mo buffer and YC_o 2 on a (110) W buffer, both deposited on (1120) sapphire. In both cases, the compounds present twins.
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00187-X
176
v. Ode~vlo et al. /Journal ql'Co'sml Growth 165 (1996) 175-178
In this Letter, we report the first epitaxial growth of (110) R E - F e 2 Laves phases: DyFe 2, TbFe 2 and Dy0.vTb0.3Fe 2 (terfenol-D). These compounds have been grown on a (I 10) niobium buffer deposited on (1120) sapphire. The epitaxy has been initiated by a 30 A iron layer deposited on the niobium. It is shown that the in-plane epitaxial orientations depend on the thermal treatment of this thin layer of iron. We have noticed no significant differences between the growth of the three systems. The samples have been prepared by molecular beam epitaxy (MBE) in a chamber whose base pressure was typically 4 × 10-l~ Torr. They have been studied by RHEED, with a grazing incidence angle of the beam of about 1° and a beam energy of 30 keV. Niobium and terbium were deposited from two electron beam evaporation sources and iron and dysprosium from two effusion cells. The deposition rates were between 5 and I0 ,~/min. Prior to the
deposition, the (1120) sapphire substrate was outgassed at 800°C for several hours. Then the 500 niobium buffer was deposited at 700°C. As shown by X-ray scattering and RHEED, and as first observed by Kwo et al. [5], niobium grows as a single crystal with (110) planes parallel to the plane of the substrate. The in-plane epitaxial relationships are: [ 111 ] Nb]][0001] sapphire and []12] Nb]l[1]00] sapphire. RHEED performed during the deposition process exhibits extra streaks showing a surface reconstruction [4]. A thin iron layer was then deposited on niobium. After many tentative tries, two temperatures of substrate were selected for iron deposition: 200 and 500°C (but R E - F e 2 is always grown at 500°C). As shown below, the epitaxial growth of R E - F e 2 is possible on both iron layers (deposited at 200°C and deposited at 500°C) but with different in-plane orientations. The compositions of the R E - F e 2 compounds
Fig. 1. RHEED patterns along lhe azimuthal direction of [001] of DyFe2 after a deposition of (a) 50 ,~, (b) 250 A, and (c) 1000 ,~.
V. Oderno et al. / Journal of Crystal Growth 165 (1996) 175 178
177
o
were checked a posteriori by microprobe analysis. It was found to be within 5% of the expected stoichiometric compositions. During the first set of experiments, the thin iron layer was deposited at 200°C on (110) niobium. RHEED patterns showed that a bcc (110) surface with the iron parameters takes place after a 30 deposition, but no RHEED oscillations could be observed. It can therefore be concluded that, at 200°C, (110) bcc iron is epitaxied on (110) bcc niobium despite the large mismatch between the parameters (15%). The in-plane epitaxial relationship is: [001] Fell[001 ] Nb. The different elements (one or two rare earths and iron) were then codeposited on this iron buffer at 500°C. At first the RHEED iron streaks become less and less clear and the patterns become diffuse. However, when the deposited thickness reaches approxi-
mately 10 A, 3D diffraction patterns appear. The new streaks are very spotty, which evidences a large roughness. With increasing thickness, the RHEED patterns gradually transform into streaks (however, with some residual spots), which is the signature of a decrease of the roughness. From the spacing between the streaks, the surface unit mesh is rectangular and corresponds to a ( l l 0 ) fcc plane with, because of two equivalent orientations between the substrate and the film, twins related by a 110 ° rotation about the [110] surface normal (both domains have however the same growth direction). In-plane relationships are: [001] Nbll[ll2] R E - F e : , [110] Nbll[lll] R E - F e 2 for the first domain, and [001] Nbl][ll2] R E - F e 2, [1~0] Nb]l[lll] R E - F e 2 for the second domain. The a parameter of the DyFe 2 film (about 7.3 A, according to RHEED observations) is close to that of o
Fig. 2. RHEED patterns after a deposition of 1000 A of DyFe 2 along the azimuthal directions of (a) [001], (b) [1~0], (c) [l~l] or [111], and (d) [112] or [112].
178
V. Oderno et al. / Journal of Cr3'stal Growth 165 (1996) 175 178
bulk DyFe 2 (7.324 A). No modification of this parameter is observed during the growth of a 1000 A DyFe 2 layer. The results are the same for TbFe 2 and terfenol-D. During a second set of experiments, the thin layer of iron has been deposited on (110) niobium maintained at 500°C. After a 30 ,~ deposition of iron, RHEED patterns show unambiguously a rectangular surface unit mesh with main directions parallel to the main directions of niobium. The parameters of this surface, which is quite stable and reproducible, are approximately 6.9 and 4.8 ,~. One should note that this structure is close to the structure of a cubic Laves phase (110) plane and will therefore enable its epitaxy. When iron is deposited at high temperature, an interdiffusion with niobium probably occurs and the surface is consequently made of both types of atoms (one should note that the rectangular unit mesh is different from the Fe or Nb unit meshes). The different elements were then codeposited at 500°C. The evolution of the first stages of the growth is the same as previously: the patterns are at first diffuse, then spotty patterns appear (Fig. l a for DyFe z) and transform gradually into streaks with increasing thickness (Figs. lb and lc for DyFez). However, contrary to the previous case, the RHEED patterns transform into well defined and high quality streaks. After a 1000 ,~ deposition, the roughness and the background on RHEED patterns are weak. The streaks are numerous and clear (Fig. 2 for DyFez). Kikuchi lines and two Laue zones can be observed. Under these experimental conditions, the in-plane epitaxial relationships are: [001] Nbll[001] R E - F e 2, [110] Nb[][ll0] R E - F e 2. This orientation is different from the previous one and the R E - F e 2 sample is now a unique single crystal (there are no equivalent orientations in the plane). X-ray scattering (Fig. 3 for DyFe z) confirmed that the growth direction is always [110] and showed that the R E - F e z parameters along this direction are
4
(1120) AI20
(11o) Nb
>. (220) DyFe2
g
~E
I
1
20
21
22 theta (°)
23
Fig. 3. XRD 0-20 pattern of a 1000 ,~ thick (110) DEFe 2 fihn grown on (1120) sapphire and (110) niobium.
7.29 ~, for DyFe 2, 7.32 A for TbFe 2 and 7.30 A for terfenol-D. The dispersion of the [220] orientation corresponds to a mosaic spread of about 1.5 ° for DyFe z and TbFe 2 and 2.5 ° for terfenol-D. Bragg peak broadenings correspond to coherence lengths of about 400 A along the growth directions of the films. In conclusion, we have grown for the first time (110) DyFe2, TbFe z and terfenol-D single-crystalline thin films. Magnetic measurements and M~Sssbauer spectroscopy, which have been very efficient tools for the determination of the magnetic properties of R E - F e 2 alloys, are in progress for the three systems.
References [1] A.E. Clark, Magnetostrictive RFe 2 intermetallic compounds, in: Handbook on the Physics and Chemistry of Rare Earths, Eds. K.A. Gschneidner, Jr. and L. Eyring (North-Holland, Amsterdam, 1978). [2] E. du Tremolet de Lacheisserie, Les applications industrielles de la magn&ostriction, in: Silicates Industrielles, 1993/9-10. [3] C.T. Wang, R.M. Osgood, R.L. White and B.M. Clemens, MRS Spring Meeting Proc., San Francisco, CA, 1995. [4] F. Robaut, Thesis, Universit6 J. Fourier (Grenoble) 1995. [5] J. Kwo, M. Hong and S. Nakahara, Appl. Phys. Lett. 49 (1986) 319.
C R Y S T A L G R O W T H
ELSEVIER
Journal of Crystal Growth 165 (1996) 175-178
L e t t e r to t h e E d i t o r s
Epitaxial growth of(110) DyFe 2, TbFe 2 and Dy0.vTbo.3Fe2 thin films by molecular beam epitaxy V. O d e r n o
*, C. D u f o u r , K. D u m e s n i l , P h . M a n g i n , G. M a r c h a l
Laboratoire M~tallurgie Physique et Science des Mat~riaux, URA CNRS 155, Unil'ersit~ H. Poincar~-Nancy 1, BP 239, F-54506 Vandoeurre Cedex, France
Received 28 November 1995; accepted 9 January 1996
Abstract We present the first epitaxial growth of some ( l l 0 ) rare earth-Fe 2 (DyF%, TbFe 2 and Dyo.TTbo.3Fe 2 known as terfenol-D) thin films on (110) Nb/(1170) sapphire by molecular beam epitaxy. The epitaxy is initiated by the deposition of a thin layer of iron on niobium. The structures are investigated by RHEED and X-ray scattering. Depending on the thermal treatment of the iron thin layer, the films of R E - F e 2, epitaxially grown on it, are either single crystals or present twins related by a 110 ° rotation about the surface normal. The growth of epitaxial thin films of these compounds is of interest because of the magnetic and magnetostrictive properties these materials may exhibit.
The study of the magnetic properties of thin films is currently a very active field of research. The goal is to prepare small devices in which magnets, transformers or sensors able to respond to magnetic environment are incorporated. In particular, it is of obvious interest to elaborate epitaxial thin films of the phases that exhibit interesting magnetic bulk properties. The first step has been to epitaxy pure magnetic metals (transition metals (TM) or rare earth metals (RE)). Their growth is now well controlled. A lot of disordered alloys (amorphous or polycrystalline) have been previously obtained in the whole or in a restricted composition range. The challenge is now to prepare single crystalline films of these compounds.
* Corresponding author.
The R E / T M compounds combine the large magnetoelasticity of the rare earth and the strong exchange of the transition metal, which enhances the ordering temperature. The R E - F e 2 alloys (cubic Laves phases, MgCu 2 type) show interesting magnetostrictive properties. The compound Tb0.3Dy0.vF% (known as terfenol-D), which simultaneously presents a large magnetostriction and a weak anisotropy, is the most used magnetostrictive material [1,2]. Up to now, a small number of thin films of epitaxial R E - T M 2 compounds have been obtained. (111) TbFe 2 has been prepared by sputtering from two separate targets [3]. (111) YCo 2 has been grown by laser ablation from a stoichiometric target [4]. These compounds were obtained on buffers of bcc refractory metals: TbFe 2 was grown on a (1 10) Mo buffer and YC_o 2 on a (110) W buffer, both deposited on (1120) sapphire. In both cases, the compounds present twins.
0022-0248/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00187-X
176
v. Ode~vlo et al. /Journal ql'Co'sml Growth 165 (1996) 175-178
In this Letter, we report the first epitaxial growth of (110) R E - F e 2 Laves phases: DyFe 2, TbFe 2 and Dy0.vTb0.3Fe 2 (terfenol-D). These compounds have been grown on a (I 10) niobium buffer deposited on (1120) sapphire. The epitaxy has been initiated by a 30 A iron layer deposited on the niobium. It is shown that the in-plane epitaxial orientations depend on the thermal treatment of this thin layer of iron. We have noticed no significant differences between the growth of the three systems. The samples have been prepared by molecular beam epitaxy (MBE) in a chamber whose base pressure was typically 4 × 10-l~ Torr. They have been studied by RHEED, with a grazing incidence angle of the beam of about 1° and a beam energy of 30 keV. Niobium and terbium were deposited from two electron beam evaporation sources and iron and dysprosium from two effusion cells. The deposition rates were between 5 and I0 ,~/min. Prior to the
deposition, the (1120) sapphire substrate was outgassed at 800°C for several hours. Then the 500 niobium buffer was deposited at 700°C. As shown by X-ray scattering and RHEED, and as first observed by Kwo et al. [5], niobium grows as a single crystal with (110) planes parallel to the plane of the substrate. The in-plane epitaxial relationships are: [ 111 ] Nb]][0001] sapphire and []12] Nb]l[1]00] sapphire. RHEED performed during the deposition process exhibits extra streaks showing a surface reconstruction [4]. A thin iron layer was then deposited on niobium. After many tentative tries, two temperatures of substrate were selected for iron deposition: 200 and 500°C (but R E - F e 2 is always grown at 500°C). As shown below, the epitaxial growth of R E - F e 2 is possible on both iron layers (deposited at 200°C and deposited at 500°C) but with different in-plane orientations. The compositions of the R E - F e 2 compounds
Fig. 1. RHEED patterns along lhe azimuthal direction of [001] of DyFe2 after a deposition of (a) 50 ,~, (b) 250 A, and (c) 1000 ,~.
V. Oderno et al. / Journal of Crystal Growth 165 (1996) 175 178
177
o
were checked a posteriori by microprobe analysis. It was found to be within 5% of the expected stoichiometric compositions. During the first set of experiments, the thin iron layer was deposited at 200°C on (110) niobium. RHEED patterns showed that a bcc (110) surface with the iron parameters takes place after a 30 deposition, but no RHEED oscillations could be observed. It can therefore be concluded that, at 200°C, (110) bcc iron is epitaxied on (110) bcc niobium despite the large mismatch between the parameters (15%). The in-plane epitaxial relationship is: [001] Fell[001 ] Nb. The different elements (one or two rare earths and iron) were then codeposited on this iron buffer at 500°C. At first the RHEED iron streaks become less and less clear and the patterns become diffuse. However, when the deposited thickness reaches approxi-
mately 10 A, 3D diffraction patterns appear. The new streaks are very spotty, which evidences a large roughness. With increasing thickness, the RHEED patterns gradually transform into streaks (however, with some residual spots), which is the signature of a decrease of the roughness. From the spacing between the streaks, the surface unit mesh is rectangular and corresponds to a ( l l 0 ) fcc plane with, because of two equivalent orientations between the substrate and the film, twins related by a 110 ° rotation about the [110] surface normal (both domains have however the same growth direction). In-plane relationships are: [001] Nbll[ll2] R E - F e : , [110] Nbll[lll] R E - F e 2 for the first domain, and [001] Nbl][ll2] R E - F e 2, [1~0] Nb]l[lll] R E - F e 2 for the second domain. The a parameter of the DyFe 2 film (about 7.3 A, according to RHEED observations) is close to that of o
Fig. 2. RHEED patterns after a deposition of 1000 A of DyFe 2 along the azimuthal directions of (a) [001], (b) [1~0], (c) [l~l] or [111], and (d) [112] or [112].
178
V. Oderno et al. / Journal of Cr3'stal Growth 165 (1996) 175 178
bulk DyFe 2 (7.324 A). No modification of this parameter is observed during the growth of a 1000 A DyFe 2 layer. The results are the same for TbFe 2 and terfenol-D. During a second set of experiments, the thin layer of iron has been deposited on (110) niobium maintained at 500°C. After a 30 ,~ deposition of iron, RHEED patterns show unambiguously a rectangular surface unit mesh with main directions parallel to the main directions of niobium. The parameters of this surface, which is quite stable and reproducible, are approximately 6.9 and 4.8 ,~. One should note that this structure is close to the structure of a cubic Laves phase (110) plane and will therefore enable its epitaxy. When iron is deposited at high temperature, an interdiffusion with niobium probably occurs and the surface is consequently made of both types of atoms (one should note that the rectangular unit mesh is different from the Fe or Nb unit meshes). The different elements were then codeposited at 500°C. The evolution of the first stages of the growth is the same as previously: the patterns are at first diffuse, then spotty patterns appear (Fig. l a for DyFe z) and transform gradually into streaks with increasing thickness (Figs. lb and lc for DyFez). However, contrary to the previous case, the RHEED patterns transform into well defined and high quality streaks. After a 1000 ,~ deposition, the roughness and the background on RHEED patterns are weak. The streaks are numerous and clear (Fig. 2 for DyFez). Kikuchi lines and two Laue zones can be observed. Under these experimental conditions, the in-plane epitaxial relationships are: [001] Nbll[001] R E - F e 2, [110] Nb[][ll0] R E - F e 2. This orientation is different from the previous one and the R E - F e 2 sample is now a unique single crystal (there are no equivalent orientations in the plane). X-ray scattering (Fig. 3 for DyFe z) confirmed that the growth direction is always [110] and showed that the R E - F e z parameters along this direction are
4
(1120) AI20
(11o) Nb
>. (220) DyFe2
g
~E
I
1
20
21
22 theta (°)
23
Fig. 3. XRD 0-20 pattern of a 1000 ,~ thick (110) DEFe 2 fihn grown on (1120) sapphire and (110) niobium.
7.29 ~, for DyFe 2, 7.32 A for TbFe 2 and 7.30 A for terfenol-D. The dispersion of the [220] orientation corresponds to a mosaic spread of about 1.5 ° for DyFe z and TbFe 2 and 2.5 ° for terfenol-D. Bragg peak broadenings correspond to coherence lengths of about 400 A along the growth directions of the films. In conclusion, we have grown for the first time (110) DyFe2, TbFe z and terfenol-D single-crystalline thin films. Magnetic measurements and M~Sssbauer spectroscopy, which have been very efficient tools for the determination of the magnetic properties of R E - F e 2 alloys, are in progress for the three systems.
References [1] A.E. Clark, Magnetostrictive RFe 2 intermetallic compounds, in: Handbook on the Physics and Chemistry of Rare Earths, Eds. K.A. Gschneidner, Jr. and L. Eyring (North-Holland, Amsterdam, 1978). [2] E. du Tremolet de Lacheisserie, Les applications industrielles de la magn&ostriction, in: Silicates Industrielles, 1993/9-10. [3] C.T. Wang, R.M. Osgood, R.L. White and B.M. Clemens, MRS Spring Meeting Proc., San Francisco, CA, 1995. [4] F. Robaut, Thesis, Universit6 J. Fourier (Grenoble) 1995. [5] J. Kwo, M. Hong and S. Nakahara, Appl. Phys. Lett. 49 (1986) 319.