Journal of Magnetism and Magnetic Materials 316 2007 e269–e272Longitudinal Hall effect in Terfecohan thin films with perpendicular magnetic anisotropy N.H.. Etienne du Rouvray, France Ava
Trang 1Journal of Magnetism and Magnetic Materials 316 (2007) e269–e272
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 am¼201, 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
r2007 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
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 ‘‘cone-angle’’ 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 sin2b 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
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0304-8853/$ - see front matter r 2007 Elsevier B.V All rights reserved.
doi: 10.1016/j.jmmm.2007.02.116
Corresponding author Tel.: +84 4 7547203; fax: +84 4 7547460.
E-mail address: ducnh@vnu.edu.vn (N.H Duc).
Trang 2spectra: /BhfS ¼ 23.57(0.5) T and /bS ¼ 18 (75)1 The
/bS angle reflects the perpendicular magnetic
aniso-tropy 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 TbFe2alloy
Similar behaviors were previously reported[4]
Domain structure was studied using magnetic force
micro-scopy (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
perpendicu-lar 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 (seeFig 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 inFig 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 102V/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 102V/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 5shows the high-field Hall voltage susceptibility (wHFHV) variation with a At a ¼ 0, wHFHV has a positive value (as already observed inFig 4) With increasing a, firstly wHFHV
decreases, cancels around am¼201 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 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.
Fig 3 The M/M(0.6 T) and V H /V(0.6 T) curves of Terfecohan film at angle a ¼ 01.
Trang 3which 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
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 4 Hall voltage measured at various angles a (see in the text).
Fig 5 Angular dependence of high-field Hall voltage susceptibility
(w HFHV ).
Fig 6 Illustration of magnetization processes and angular dependence of
Hall voltage for different orientations of applied fields.
Fig 7 Angular dependence of Hall voltage measured in 0.6 T.
Fig 8 Hall voltage measured at various angles a for 350 1C-annealed Terfecohan film.
Trang 4TaX350 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).