- Email: [email protected]
Young's modulus of MnAl and MnBi films
Materials Science and Engineering, B25 (1994) L7-L8
L7
Letter
Young's modulus of MnA1 and MnBi films M. Angadi and V. Thanigaimani Thin Film Labomto~, Department of Physics, The University of the West Indies, St. Augustine (Trinidad and Tobago) (Received November 19.1993)
Abstract The dependence on thickness of Young's modulus of vacuum evaporated MnAI and MnBi films is reported for thicknesses in the range 30-120 nm. A vibrating reed method is uded to determine Young's modulus of films.
Thin film materials possessing large magneto-optic effects and suitable magnetic properties are of great importance for many applications in thin film devices. Manganese forms very interesting compounds with many semi-metals such as bismuth, antimony and arsenic, of which the bulk crystals exhibit ferromagnetic properties [ 1]. Of these, MnBi has been investigated extensively, since it is a hard magnetic material with the easy direction of magnetization usually perpendicular to the film plane. MnBi films possess large magnetic Faraday rotation in the visible region [2, 3]. Of late, ferromagnetic MnBi thin films have been used in high density recording [4] and erasable magnetic holographic applications [5]. The MnA1 system, known as one of the Heusler alloys, possesses excellent properties for use as a permanent magnet [6]. MnA1 films containing about 2 wt.% Cu are used for high density, longitudinal recording [7]. Owing to the presence of paramagnetic Mn-' ÷ ions it is expected that ferromagnetic thin films of MnAI can be used for magneto-optic and integrated memory devices in the near future [8]. Both MnA1 and MnBi undergo a phase transition from the ferromagnetic to paramagnetic state near their Curie temperatures which are reported [1, 9] to be 653 K and 630 K respectively. Although there are several reports on the magnetic, structural and transport properties of MnBi [10-12] and MnAI [13, 14] films, to the best of our knowledge the mechanical properties of these films have not yet been reported. In this letter, we report on the dependence on thickness of 0921-5107/94/$7.00 SSDI 0921-51 (17(94)t)1053-K
Young's modulus of MnBi and MnA1 films in the thickness range 30-120 nm. Techniques which have been used to study the mechanical properties of thin films, such as the bulge test and ultrasonic wave measurements, require the film to be removed from the substrate and thereby give rise to large uncertainties in the measured values. A technique which overcomes these difficulties is the vibrating reed test [15]. Flexural resonant vibrations are induced in the substrate, and the shift in resonant frequency which occurs when a film is deposited onto the substrate is used to calculate the Young modulus of the film. Other details of this method are reported elsewhere [15]. MnBi and MnAI films were prepared by thermal evaporation on copper substrates ( 18 x 5 × 0.15 mm 3), held at different substrate temperatures, and in a vacuum of 10-4 Pa. Other experimental details of alloy formation are given in several of our earlier publications [12, 14]. The relative Young's modulus Er (the ratio of the film to the substrate modulus)is [16], = 1 2Af+ r
where A t,=f-fr is the resonant frequency of the coated sample, f~ is the resonant frequency of the substrate, t~ is the relative thickness and Pr is relative density. Figure 1 shows the thickness dependence of Young's modulus E r for (a) MnAI (59.5 at.% Mn) films deposited at a substrate temperature Ts--473K and annealed at 653 K for 4 h and (b) MnBi (59.1 at.% Mn) films deposited at T~= 423 K and annealed at 573 K for 2 h. We observe a monotonic decrease in E~ with increasing thickness. For very thin films, E r is markedly high and approaches a constant value at greater thicknesses. The size effect is frequently observed in the transport properties of most metallic films [17] but is not reported in the case of mechanical properties. The decrease in Young's modulus of MnAI and MnBi films with increasing thickness may arise from structural changes such as an increase in grain size and decrease in interatomic spacing, although elastic moduli are not supposed to be structure sensitive. We have reported a similar size effect for the thickness dependence of Young's modulus of MnTe and MnSe films [18]. © 1994 - Elsevier Science S.A. All rights reserved
L8
Letter
Acknowledgments ,2 k
c~ E
T h e authors are grateful to the Campus Research Fund Committee of the University of the West Indies, St. Augustine for financial assistance. One of the authors (VI) is grateful to the Department of Physics, UWI for financial assistance.
i
8 L
References
oE 20
40
60
80
I00
120
140
t (rim)
Fig. 1. Dependence on thickness of Young's modulus for (a) MnAl film (59.5 at.% Mn), T~= 473 K, and (b) MnBi (59.1 at.% Mn), T, =423 K.
T h e r e are very little experimental data reported on the thickness dependence of Young's modulus of thin films, probably because of the difficulty of measurement at small thicknesses using conventional methods which often require the film be removed from the substrate before measurement. T h e only previous reports of Young's modulus at low thicknesses are for carbon film on glass substrates. Kinbara et al. [ 19] using a vibrating reed method, found the modulus of carbon films to increase almost three-fold below 1000 nm, and Tsukamoto et al. [20] using a static cantilever method showed that in addition to this increase, the Young modulus subsequently decreased below 100 nm. These authors have not provided a satisfactory explanation for the anomalous behaviour of these films at low thicknesses. Recently, Whiting et al. [21] reported an enhanced Young's modulus at small thicknesses for copper films deposited onto glass, copper and nickel substrates. According to these authors, the increase in Young's modulus is not a size effect, but arises from the surface roughness of the substrate. T h e increase in Young's modulus is found to be quite small for films deposited onto glass rather than onto metallic substrates. We do not have a satisfactory explanation at the m o m e n t for the effect of size on the Young modulus of MnAI and MnB films. It may be necessary to carry out these measurements in situ to minimize further the effect of external factors on the experimental results. These efforts are currently underway and the findings will be reported in a future communication.
1 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 5th edn., 1976, p. 456. 2 D. Chan, J. F. Ready and E. Bernal, J. Appl. Phys., 39 (1968) 3916. 3 W.K. Unger and M. Stolz, J. Appl. Phys., 42 ( 1971 ) 1I)85. 4 Y. Takeno and Y. lwama, Jpn. J. Appl. Phys., 18 (1979) 269. 5 T. Takagi, K. Matsubara and N. Konda, Jpn. ,L AppL Phys., 19 (1979)507. 6 A. J. J. Koch, P. H. Hokkeling, M. G. V. D. Steeg and K. J. de Vos, J. Appl. Phys., 3111960) 75S. 7 A. Morisako and M. Matsumoto, Proc. Int. Con[ on Ferrites, 1980, p. 661. 8 Y. Kido, N. Suzuki and J. Kawamoto, Jpn. J. Appl. Phys., 26 (1987) L1900. 9 H. Miyajima, K. Sato and T. Matsuda, Proc. 7th Annu. (ot~[i on Magnetism, Japan, 1975, p. B- 13 (in Japanese). 10 Y. Iwama and Y. Takeno, Phys. Stares Solidi A, 76 ( 1983 ) 75. 11 T.S. Liu, J. Mater. Sci., 7 (1972) 559. 12 M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 10 11991)285. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 7 (1988) 717. M. Angadi and V. Thanigaimani, AppL Phys. A. 44 (1987) 261. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 6 (1987) 1004. 13 A. Morisako, M. Matsumoto and N. Naoe, J. AppL Phys., 61 (1987) 4281. 14 M. Angadi and V. Thanigaimani, Thin Solid Films, 195 11991)7. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 11 11992) 1213. 15 R. Whiting and M. Angadi, Meas. Sci. TechnoL, 1 11990) 662. 16 A. Kinbara, S. Baba and N. Matuda, Thin Solid Films, 141 (1986) 229. 17 M. Angadi, J. Mater. Sci., 20 (1985) 761. 18 M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., in press. 19 A. Kinbara, S. Baba and N. Matuda, Thin Solid Films, 84 11981)205. 211 Y. Tsukamoto, H. Yamaguchi and M. Yaragisawa, 1bin Solid Films, 154 11987) 171. 21 R. Whiting, M. A. Angadi and S. Tripathi, Thin Solid Films, submitted for publication.
L7
Letter
Young's modulus of MnA1 and MnBi films M. Angadi and V. Thanigaimani Thin Film Labomto~, Department of Physics, The University of the West Indies, St. Augustine (Trinidad and Tobago) (Received November 19.1993)
Abstract The dependence on thickness of Young's modulus of vacuum evaporated MnAI and MnBi films is reported for thicknesses in the range 30-120 nm. A vibrating reed method is uded to determine Young's modulus of films.
Thin film materials possessing large magneto-optic effects and suitable magnetic properties are of great importance for many applications in thin film devices. Manganese forms very interesting compounds with many semi-metals such as bismuth, antimony and arsenic, of which the bulk crystals exhibit ferromagnetic properties [ 1]. Of these, MnBi has been investigated extensively, since it is a hard magnetic material with the easy direction of magnetization usually perpendicular to the film plane. MnBi films possess large magnetic Faraday rotation in the visible region [2, 3]. Of late, ferromagnetic MnBi thin films have been used in high density recording [4] and erasable magnetic holographic applications [5]. The MnA1 system, known as one of the Heusler alloys, possesses excellent properties for use as a permanent magnet [6]. MnA1 films containing about 2 wt.% Cu are used for high density, longitudinal recording [7]. Owing to the presence of paramagnetic Mn-' ÷ ions it is expected that ferromagnetic thin films of MnAI can be used for magneto-optic and integrated memory devices in the near future [8]. Both MnA1 and MnBi undergo a phase transition from the ferromagnetic to paramagnetic state near their Curie temperatures which are reported [1, 9] to be 653 K and 630 K respectively. Although there are several reports on the magnetic, structural and transport properties of MnBi [10-12] and MnAI [13, 14] films, to the best of our knowledge the mechanical properties of these films have not yet been reported. In this letter, we report on the dependence on thickness of 0921-5107/94/$7.00 SSDI 0921-51 (17(94)t)1053-K
Young's modulus of MnBi and MnA1 films in the thickness range 30-120 nm. Techniques which have been used to study the mechanical properties of thin films, such as the bulge test and ultrasonic wave measurements, require the film to be removed from the substrate and thereby give rise to large uncertainties in the measured values. A technique which overcomes these difficulties is the vibrating reed test [15]. Flexural resonant vibrations are induced in the substrate, and the shift in resonant frequency which occurs when a film is deposited onto the substrate is used to calculate the Young modulus of the film. Other details of this method are reported elsewhere [15]. MnBi and MnAI films were prepared by thermal evaporation on copper substrates ( 18 x 5 × 0.15 mm 3), held at different substrate temperatures, and in a vacuum of 10-4 Pa. Other experimental details of alloy formation are given in several of our earlier publications [12, 14]. The relative Young's modulus Er (the ratio of the film to the substrate modulus)is [16], = 1 2Af+ r
where A t,=f-fr is the resonant frequency of the coated sample, f~ is the resonant frequency of the substrate, t~ is the relative thickness and Pr is relative density. Figure 1 shows the thickness dependence of Young's modulus E r for (a) MnAI (59.5 at.% Mn) films deposited at a substrate temperature Ts--473K and annealed at 653 K for 4 h and (b) MnBi (59.1 at.% Mn) films deposited at T~= 423 K and annealed at 573 K for 2 h. We observe a monotonic decrease in E~ with increasing thickness. For very thin films, E r is markedly high and approaches a constant value at greater thicknesses. The size effect is frequently observed in the transport properties of most metallic films [17] but is not reported in the case of mechanical properties. The decrease in Young's modulus of MnAI and MnBi films with increasing thickness may arise from structural changes such as an increase in grain size and decrease in interatomic spacing, although elastic moduli are not supposed to be structure sensitive. We have reported a similar size effect for the thickness dependence of Young's modulus of MnTe and MnSe films [18]. © 1994 - Elsevier Science S.A. All rights reserved
L8
Letter
Acknowledgments ,2 k
c~ E
T h e authors are grateful to the Campus Research Fund Committee of the University of the West Indies, St. Augustine for financial assistance. One of the authors (VI) is grateful to the Department of Physics, UWI for financial assistance.
i
8 L
References
oE 20
40
60
80
I00
120
140
t (rim)
Fig. 1. Dependence on thickness of Young's modulus for (a) MnAl film (59.5 at.% Mn), T~= 473 K, and (b) MnBi (59.1 at.% Mn), T, =423 K.
T h e r e are very little experimental data reported on the thickness dependence of Young's modulus of thin films, probably because of the difficulty of measurement at small thicknesses using conventional methods which often require the film be removed from the substrate before measurement. T h e only previous reports of Young's modulus at low thicknesses are for carbon film on glass substrates. Kinbara et al. [ 19] using a vibrating reed method, found the modulus of carbon films to increase almost three-fold below 1000 nm, and Tsukamoto et al. [20] using a static cantilever method showed that in addition to this increase, the Young modulus subsequently decreased below 100 nm. These authors have not provided a satisfactory explanation for the anomalous behaviour of these films at low thicknesses. Recently, Whiting et al. [21] reported an enhanced Young's modulus at small thicknesses for copper films deposited onto glass, copper and nickel substrates. According to these authors, the increase in Young's modulus is not a size effect, but arises from the surface roughness of the substrate. T h e increase in Young's modulus is found to be quite small for films deposited onto glass rather than onto metallic substrates. We do not have a satisfactory explanation at the m o m e n t for the effect of size on the Young modulus of MnAI and MnB films. It may be necessary to carry out these measurements in situ to minimize further the effect of external factors on the experimental results. These efforts are currently underway and the findings will be reported in a future communication.
1 C. Kittel, Introduction to Solid State Physics, Wiley, New York, 5th edn., 1976, p. 456. 2 D. Chan, J. F. Ready and E. Bernal, J. Appl. Phys., 39 (1968) 3916. 3 W.K. Unger and M. Stolz, J. Appl. Phys., 42 ( 1971 ) 1I)85. 4 Y. Takeno and Y. lwama, Jpn. J. Appl. Phys., 18 (1979) 269. 5 T. Takagi, K. Matsubara and N. Konda, Jpn. ,L AppL Phys., 19 (1979)507. 6 A. J. J. Koch, P. H. Hokkeling, M. G. V. D. Steeg and K. J. de Vos, J. Appl. Phys., 3111960) 75S. 7 A. Morisako and M. Matsumoto, Proc. Int. Con[ on Ferrites, 1980, p. 661. 8 Y. Kido, N. Suzuki and J. Kawamoto, Jpn. J. Appl. Phys., 26 (1987) L1900. 9 H. Miyajima, K. Sato and T. Matsuda, Proc. 7th Annu. (ot~[i on Magnetism, Japan, 1975, p. B- 13 (in Japanese). 10 Y. Iwama and Y. Takeno, Phys. Stares Solidi A, 76 ( 1983 ) 75. 11 T.S. Liu, J. Mater. Sci., 7 (1972) 559. 12 M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 10 11991)285. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 7 (1988) 717. M. Angadi and V. Thanigaimani, AppL Phys. A. 44 (1987) 261. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 6 (1987) 1004. 13 A. Morisako, M. Matsumoto and N. Naoe, J. AppL Phys., 61 (1987) 4281. 14 M. Angadi and V. Thanigaimani, Thin Solid Films, 195 11991)7. M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., 11 11992) 1213. 15 R. Whiting and M. Angadi, Meas. Sci. TechnoL, 1 11990) 662. 16 A. Kinbara, S. Baba and N. Matuda, Thin Solid Films, 141 (1986) 229. 17 M. Angadi, J. Mater. Sci., 20 (1985) 761. 18 M. Angadi and V. Thanigaimani, J. Mater. Sci. Lett., in press. 19 A. Kinbara, S. Baba and N. Matuda, Thin Solid Films, 84 11981)205. 211 Y. Tsukamoto, H. Yamaguchi and M. Yaragisawa, 1bin Solid Films, 154 11987) 171. 21 R. Whiting, M. A. Angadi and S. Tripathi, Thin Solid Films, submitted for publication.