- Email: [email protected]
Longitudinal Hall effect in Terfecohan thin films with perpendicular magnetic anisotropy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272 www.elsevier.com/locate/jmmm
Longitudinal Hall effect in Terfecohan thin films with perpendicular magnetic anisotropy N.H. Duca,, N.T.M. Honga, J. Teilletb a
Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, College of Technology, Vietnam National University, Hanoi, Buiding E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam b Groupe de Physique des Mate´riaux, Universite´ de Rouen, UMR CNRS 6634, 76801 St. Etienne du Rouvray, France Available online 28 February 2007
Abstract Longitudinal extraordinary Hall Effect (LEHE) of magnetic Tb(Fe0.55Co0.45)1.5 (known as Terfecohan) thin films with perpendicular magnetic anisotropy has been investigated as a function of both the intensity of applied magnetic fields and the angle a between the applied field and film normal directions. The Hall voltage loops exhibit a parallelogram shape, which is almost similar to those of the perpendicular magnetization. The high-field Hall voltage susceptibility is positive at a ¼ 0. Its value decreases with increasing a and changes in sign at am ¼ 201, which is considered as the easy magnetizable direction of the film. This finding is comparable with those obtained from the magnetization, magnetic force microscopy (MFM) and conversion electron Mo¨ssbauer spectra (CEMS) measurements. The obtained LEHE behaviors are rather promising for applications such as magnetic recording heads and magnetic field detectors, where a large output signal is required at low magnetic fields. r 2007 Elsevier B.V. All rights reserved. PACS: 72.20.My; 75.70.Kw; 75.70.j; 76.80.+y Keywords: Longitudinal extraordinary Hall effect; Magnetization; Magnetic force microscopy; Conversion electron Mo¨ssbauer spectrometry
In the magnetic films, the longitudinal extraordinary Hall effect (LEHE) is well known to be governed by the perpendicular magnetization component [1]. Thus, LEHE is recognized as an useful tool to study magnetic properties of magnetic films having perpendicular anisotropy. In this case, a large LEHE is usually obtained at low fields. The high-field magnetization state is, however, determined by the applied field direction and the high-field LEHE susceptibility can be positive or negative depending on the relative orientation of the intrinsic easy magnetization axis with respect to the applied magnetic field. The maximum of the LEHE voltage is obtained when the intrinsic easy magnetization axis is in coincidence with the applied magnetic field direction. As a consequence, the high-field LEHE voltage susceptibility can be used to determine the magnetization orientation in magnetic films with perpendicular anisotropy. This problem is tackled in Corresponding author. Tel.: +84 4 7547203; fax: +84 4 7547460.
E-mail address: [email protected] (N.H. Duc). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.116
this paper for the Tb(Fe0.55Co0.45)1.5 films (known as Terfecohan [2,3]) with a thickness of 570 nm deposited on glass substrates using RF-sputtering technique. The conversion electron Mo¨ssbauer spectrum (CEMS) at room temperature was recorded using a conventional spectrometer equipped with a homemade helium–methane proportional counter. The source was a 57Co in rhodium matrix. The film was set perpendicular to the incident g-beam. The CEMS for the as-deposited Terfecohan film is presented in Fig. 1. The spectrum is typical of a distribution of iron environments. It was fitted with a distribution of hyperfine fields only. The average ‘‘coneangle’’ b between the incident g-ray direction (being along the film-normal direction) and that of the hyperfine field Bhf (or the Fe-magnetic moment direction) is estimated from the line-intensity ratios 3:x:1:1:x:3 of the six Mo¨ssbauer lines, where x is related to b by sin2 b ¼ 2x/ (4+x). Despite a poor statistics, the information about the average hyperfine field /BhfS and the Fe-spin reorientation (/bS angle) can be extracted from this
ARTICLE IN PRESS e270
N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
Fig. 1. Mo¨ssbauer spectrum at 300 K of as-deposited Terfecohan film
Fig. 2. The MFM image of Terfecohan film in zero field. The light and the dark areas present domains with magnetization pointing out-of and into the film plane.
spectra: /BhfS ¼ 23.57(0.5) T and /bS ¼ 18 (75)1. The /bS angle reflects the perpendicular magnetic anisotropy. The /BhfS value obtained for this amorphous Tb(Fe0.55Co0.45)1.5 phase is, as expected, slightly higher than that of 21 T reported for the amorphous TbFe2 alloy. Similar behaviors were previously reported [4]. Domain structure was studied using magnetic force microscopy (MFM) with magnetic tip that was magnetized perpendicular to the sample plane. The result is presented in Fig. 2. It exhibits an interlacement of bright and dark color corresponding to stripe domains with alternating perpendicular magnetization component. In zero-field, the two stripes were found to have almost the same size and to possess equal areas. In an applied magnetic field, the domain structure is modified by the magnetization process (domain width, geometrical type) until the sample approached saturation state. Magnetization was determined using a vibrating sample magnetometer and LEHE measurements are carried out at room temperature by the standard DC four-probe method on square samples of 4 4 mm2. For the magnetic field
applied perpendicular to the film plane (a ¼ 01), the Hall voltage is found to be negative (see Fig. 4 below), which implies that the negative contribution of Tb is important. This is in good agreement with what was reported for heavy rare earth—transition intermetallics [5]. The normalized Hall voltage VH(H)/VH(0.6 T) and the normalized M(H)/M(0.6 T) hysteresis loops for the case a ¼ 01, where VH(0.6 T) and M(0.6 T) are the Hall voltage and magnetization measured in the magnetic field of 0.6 T, are reported in Fig. 3. These two loops are merged, what is a good evidence that the longitudinal Hall measurement can serve as a direct determination of the perpendicular magnetization component. In Fig. 3, a low-field Hall sensitivity as large as 2 102 V/T is achieved. This LEHE behavior is rather promising for applications such as magnetic recording heads and magnetic field detectors, where the large output signal is an important parameter at low magnetic fields. In practice, a Hall sensitivity of about 7 102 V/T can be realized for films showing a more perfect rectangular magnetic hysteresis loops. The Hall voltage VH measured as a function of angle a between the applied field and film normal directions is shown in Fig. 4 for a ¼ 01, 451 and 901. One observes that with increasing a, not only the Hall voltage decreases, but also the hysteresis parallelogram becomes more oblique. In addition, Fig. 5 shows the high-field Hall voltage susceptibility (wHFHV) variation with a. At a ¼ 0, wHFHV has a positive value (as already observed in Fig. 4). With increasing a, firstly wHFHV decreases, cancels around am ¼ 201 and then changes in sign. This am-value is close to the ‘‘cone angle’’ b value determined from the CEMS measurement for the orientation of the Fe magnetic moments. Consequently, the observed positive high-field susceptibility at a ¼ 0 in both Hall voltage and magnetization curves (Fig. 3) is, in accordance to the CEMS results, expected to relate to a non-perfect 901 -out-of-plane magnetic anisotropy. The variation of the high-field Hall voltage susceptibility with a can be explained from the magnetization processes,
Fig. 3. The M/M(0.6 T) and VH/V(0.6 T) curves of Terfecohan film at angle a ¼ 01.
ARTICLE IN PRESS N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
Fig. 4. Hall voltage measured at various angles a (see in the text).
e271
magnetization axis. In higher fields, the magnetization rotates progressively from easy axis to the field direction and the final magnetizable state is always along the applied field direction. When aoam, this process causes the perpendicular magnetization component (as well as Hall voltage) to be increased (Fig. 6a), corresponding to a positive Hall voltage susceptibility. On the other hand, when a4am, the rotation of magnetization towards the applied field causes the magnetization to decrease (Fig. 6c). As a consequence, a negative Hall voltage susceptibility is observed. For a ¼ am, however, the magnetization rotation process is absent and a zero high-field Hall susceptibility is observed (Fig. 6b). In addition, the Hall voltage data measured in 0.6 T are plotted as a function of a angle in Fig. 7. The experimental results are well fitted with a sinus function VH(a) ¼ VH(0) cos a. This confirms the contribution of the final magnetization state (magnetization rotation process described above) to the Hall voltage. Finally, it is interesting to note that the perpendicular anisotropy of the film is destroyed after annealing at
Fig. 5. Angular dependence of high-field Hall voltage susceptibility (wHFHV). Fig. 7. Angular dependence of Hall voltage measured in 0.6 T.
Fig. 6. Illustration of magnetization processes and angular dependence of Hall voltage for different orientations of applied fields.
which can be described by the model illustrated in Fig. 6 for different magnetic field directions. At low fields, magnetization is contributed mainly by the orientation of the magnetic moments along the easy
Fig. 8. Hall voltage measured at various angles a for 350 1C-annealed Terfecohan film.
ARTICLE IN PRESS e272
N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
TaX350 1C. In the case of samples with parallel magnetic anisotropy, the Hall voltage loop reflects the out-of-plane rotation of the magnetization (Fig. 8) and the relationship between the final magnetization state and Hall voltage is confirmed also. In a bias magnetic field, the field orientation dependence of Hall voltage exhibits either the perfect sinus or saw tooth angular symmetries depending on the nature of the magnetic anisotropy. In summary, the perpendicular magnetic anisotropy of the Terfecohan film is well evidenced by means of MFM, CEMS, VSM as well as LEHE measurements. In particular, this paper shows that LEHE investigations allow determining the magnetization orientation in films having perpendicular magnetic anisotropy. This LEHE behavior is rather promising for various applications at low magnetic fields.
This work is supported by the State Program for Fundamental Research in Natural Sciences under Project 410.406 and by the College of Technology, Vietnam National University. References [1] D.G. Stinson, A.C. Palumbo, B. Brandt, M. Berger, J. Appl. Phys. 61 (1987) 3816. [2] N.H. Duc, J. Magn. Magn. Mater. 242–245 (2002) 1411. [3] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 14, Elservier Science, North-Holland, Amsterdam, 2002, p. 89. [4] T.M. Danh, N.H. Duc, H.N. Thanh, J. Teillet, J. Appl. Phys. 87 (2000) 7208. [5] P. Hansen, Aritlce, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991 (Chapter 4).
Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272 www.elsevier.com/locate/jmmm
Longitudinal Hall effect in Terfecohan thin films with perpendicular magnetic anisotropy N.H. Duca,, N.T.M. Honga, J. Teilletb a
Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, College of Technology, Vietnam National University, Hanoi, Buiding E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam b Groupe de Physique des Mate´riaux, Universite´ de Rouen, UMR CNRS 6634, 76801 St. Etienne du Rouvray, France Available online 28 February 2007
Abstract Longitudinal extraordinary Hall Effect (LEHE) of magnetic Tb(Fe0.55Co0.45)1.5 (known as Terfecohan) thin films with perpendicular magnetic anisotropy has been investigated as a function of both the intensity of applied magnetic fields and the angle a between the applied field and film normal directions. The Hall voltage loops exhibit a parallelogram shape, which is almost similar to those of the perpendicular magnetization. The high-field Hall voltage susceptibility is positive at a ¼ 0. Its value decreases with increasing a and changes in sign at am ¼ 201, which is considered as the easy magnetizable direction of the film. This finding is comparable with those obtained from the magnetization, magnetic force microscopy (MFM) and conversion electron Mo¨ssbauer spectra (CEMS) measurements. The obtained LEHE behaviors are rather promising for applications such as magnetic recording heads and magnetic field detectors, where a large output signal is required at low magnetic fields. r 2007 Elsevier B.V. All rights reserved. PACS: 72.20.My; 75.70.Kw; 75.70.j; 76.80.+y Keywords: Longitudinal extraordinary Hall effect; Magnetization; Magnetic force microscopy; Conversion electron Mo¨ssbauer spectrometry
In the magnetic films, the longitudinal extraordinary Hall effect (LEHE) is well known to be governed by the perpendicular magnetization component [1]. Thus, LEHE is recognized as an useful tool to study magnetic properties of magnetic films having perpendicular anisotropy. In this case, a large LEHE is usually obtained at low fields. The high-field magnetization state is, however, determined by the applied field direction and the high-field LEHE susceptibility can be positive or negative depending on the relative orientation of the intrinsic easy magnetization axis with respect to the applied magnetic field. The maximum of the LEHE voltage is obtained when the intrinsic easy magnetization axis is in coincidence with the applied magnetic field direction. As a consequence, the high-field LEHE voltage susceptibility can be used to determine the magnetization orientation in magnetic films with perpendicular anisotropy. This problem is tackled in Corresponding author. Tel.: +84 4 7547203; fax: +84 4 7547460.
E-mail address: [email protected] (N.H. Duc). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.116
this paper for the Tb(Fe0.55Co0.45)1.5 films (known as Terfecohan [2,3]) with a thickness of 570 nm deposited on glass substrates using RF-sputtering technique. The conversion electron Mo¨ssbauer spectrum (CEMS) at room temperature was recorded using a conventional spectrometer equipped with a homemade helium–methane proportional counter. The source was a 57Co in rhodium matrix. The film was set perpendicular to the incident g-beam. The CEMS for the as-deposited Terfecohan film is presented in Fig. 1. The spectrum is typical of a distribution of iron environments. It was fitted with a distribution of hyperfine fields only. The average ‘‘coneangle’’ b between the incident g-ray direction (being along the film-normal direction) and that of the hyperfine field Bhf (or the Fe-magnetic moment direction) is estimated from the line-intensity ratios 3:x:1:1:x:3 of the six Mo¨ssbauer lines, where x is related to b by sin2 b ¼ 2x/ (4+x). Despite a poor statistics, the information about the average hyperfine field /BhfS and the Fe-spin reorientation (/bS angle) can be extracted from this
ARTICLE IN PRESS e270
N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
Fig. 1. Mo¨ssbauer spectrum at 300 K of as-deposited Terfecohan film
Fig. 2. The MFM image of Terfecohan film in zero field. The light and the dark areas present domains with magnetization pointing out-of and into the film plane.
spectra: /BhfS ¼ 23.57(0.5) T and /bS ¼ 18 (75)1. The /bS angle reflects the perpendicular magnetic anisotropy. The /BhfS value obtained for this amorphous Tb(Fe0.55Co0.45)1.5 phase is, as expected, slightly higher than that of 21 T reported for the amorphous TbFe2 alloy. Similar behaviors were previously reported [4]. Domain structure was studied using magnetic force microscopy (MFM) with magnetic tip that was magnetized perpendicular to the sample plane. The result is presented in Fig. 2. It exhibits an interlacement of bright and dark color corresponding to stripe domains with alternating perpendicular magnetization component. In zero-field, the two stripes were found to have almost the same size and to possess equal areas. In an applied magnetic field, the domain structure is modified by the magnetization process (domain width, geometrical type) until the sample approached saturation state. Magnetization was determined using a vibrating sample magnetometer and LEHE measurements are carried out at room temperature by the standard DC four-probe method on square samples of 4 4 mm2. For the magnetic field
applied perpendicular to the film plane (a ¼ 01), the Hall voltage is found to be negative (see Fig. 4 below), which implies that the negative contribution of Tb is important. This is in good agreement with what was reported for heavy rare earth—transition intermetallics [5]. The normalized Hall voltage VH(H)/VH(0.6 T) and the normalized M(H)/M(0.6 T) hysteresis loops for the case a ¼ 01, where VH(0.6 T) and M(0.6 T) are the Hall voltage and magnetization measured in the magnetic field of 0.6 T, are reported in Fig. 3. These two loops are merged, what is a good evidence that the longitudinal Hall measurement can serve as a direct determination of the perpendicular magnetization component. In Fig. 3, a low-field Hall sensitivity as large as 2 102 V/T is achieved. This LEHE behavior is rather promising for applications such as magnetic recording heads and magnetic field detectors, where the large output signal is an important parameter at low magnetic fields. In practice, a Hall sensitivity of about 7 102 V/T can be realized for films showing a more perfect rectangular magnetic hysteresis loops. The Hall voltage VH measured as a function of angle a between the applied field and film normal directions is shown in Fig. 4 for a ¼ 01, 451 and 901. One observes that with increasing a, not only the Hall voltage decreases, but also the hysteresis parallelogram becomes more oblique. In addition, Fig. 5 shows the high-field Hall voltage susceptibility (wHFHV) variation with a. At a ¼ 0, wHFHV has a positive value (as already observed in Fig. 4). With increasing a, firstly wHFHV decreases, cancels around am ¼ 201 and then changes in sign. This am-value is close to the ‘‘cone angle’’ b value determined from the CEMS measurement for the orientation of the Fe magnetic moments. Consequently, the observed positive high-field susceptibility at a ¼ 0 in both Hall voltage and magnetization curves (Fig. 3) is, in accordance to the CEMS results, expected to relate to a non-perfect 901 -out-of-plane magnetic anisotropy. The variation of the high-field Hall voltage susceptibility with a can be explained from the magnetization processes,
Fig. 3. The M/M(0.6 T) and VH/V(0.6 T) curves of Terfecohan film at angle a ¼ 01.
ARTICLE IN PRESS N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
Fig. 4. Hall voltage measured at various angles a (see in the text).
e271
magnetization axis. In higher fields, the magnetization rotates progressively from easy axis to the field direction and the final magnetizable state is always along the applied field direction. When aoam, this process causes the perpendicular magnetization component (as well as Hall voltage) to be increased (Fig. 6a), corresponding to a positive Hall voltage susceptibility. On the other hand, when a4am, the rotation of magnetization towards the applied field causes the magnetization to decrease (Fig. 6c). As a consequence, a negative Hall voltage susceptibility is observed. For a ¼ am, however, the magnetization rotation process is absent and a zero high-field Hall susceptibility is observed (Fig. 6b). In addition, the Hall voltage data measured in 0.6 T are plotted as a function of a angle in Fig. 7. The experimental results are well fitted with a sinus function VH(a) ¼ VH(0) cos a. This confirms the contribution of the final magnetization state (magnetization rotation process described above) to the Hall voltage. Finally, it is interesting to note that the perpendicular anisotropy of the film is destroyed after annealing at
Fig. 5. Angular dependence of high-field Hall voltage susceptibility (wHFHV). Fig. 7. Angular dependence of Hall voltage measured in 0.6 T.
Fig. 6. Illustration of magnetization processes and angular dependence of Hall voltage for different orientations of applied fields.
which can be described by the model illustrated in Fig. 6 for different magnetic field directions. At low fields, magnetization is contributed mainly by the orientation of the magnetic moments along the easy
Fig. 8. Hall voltage measured at various angles a for 350 1C-annealed Terfecohan film.
ARTICLE IN PRESS e272
N.H. Duc et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
TaX350 1C. In the case of samples with parallel magnetic anisotropy, the Hall voltage loop reflects the out-of-plane rotation of the magnetization (Fig. 8) and the relationship between the final magnetization state and Hall voltage is confirmed also. In a bias magnetic field, the field orientation dependence of Hall voltage exhibits either the perfect sinus or saw tooth angular symmetries depending on the nature of the magnetic anisotropy. In summary, the perpendicular magnetic anisotropy of the Terfecohan film is well evidenced by means of MFM, CEMS, VSM as well as LEHE measurements. In particular, this paper shows that LEHE investigations allow determining the magnetization orientation in films having perpendicular magnetic anisotropy. This LEHE behavior is rather promising for various applications at low magnetic fields.
This work is supported by the State Program for Fundamental Research in Natural Sciences under Project 410.406 and by the College of Technology, Vietnam National University. References [1] D.G. Stinson, A.C. Palumbo, B. Brandt, M. Berger, J. Appl. Phys. 61 (1987) 3816. [2] N.H. Duc, J. Magn. Magn. Mater. 242–245 (2002) 1411. [3] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 14, Elservier Science, North-Holland, Amsterdam, 2002, p. 89. [4] T.M. Danh, N.H. Duc, H.N. Thanh, J. Teillet, J. Appl. Phys. 87 (2000) 7208. [5] P. Hansen, Aritlce, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier, Amsterdam, 1991 (Chapter 4).