- Email: [email protected]
Surface magnetic characterization of FeB amorphous ribbons
c:>N•
Journal of Magnetism and Magnetic Materials 157/158 (1996) 171-172
~
journalof magnetism and
~[~
ELSEVIER
magnetic materials
Surface magnetic characterization of FeB amorphous ribbons P. Vavassori b,,, L. Callegaro a, E. Puppin a, F. Malizia b, F. Ronconi b a INFM, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, ][-20133 Milano, Italy b INFM, Dipartimento di Fisica, Universitd di Ferrara, Via Paradiso 12, 1-44100 Ferrara, Italy Abstract
We report longitudinal Kerr hysteresis loops of the two surfaces of a FesoB2o amorphous melt-spun ribbon. We observe different magnetic properties on the two sides. The results could be explained as being due to the presence of surface inhomogeneities having particular shapes, which induces high anisotropy, and to the bulk underlying magnetic domain structure which strongly interacts with the surface. Keywords: Amorphous systems; Hysteresis loops; Kerr magneto-optic
Amorphous alloys ribbons have attracted a great deal of interest due to the outstanding magnetic properties of disordered systems when subjected to suitable temperature treatments, giving rise to more stable nanocrystalline materials [i]. The melt-spin quenching technique is widely used to produce these ribbons. Since the quenching rate is different along the ribbon thickness, the ribbon could show different magnetic properties, particularly at its surfaces with respect to the bulk. Regarding the characterization of such alloys, standard magnetic measurements can provide only the bulk properties. In the present paper we focus on the surface magnetic properties of a binary amorphous alloy, Fe80B20 . Magneto-optical Kerr measurements, with their remarkable surface sensitivity ( ~ 10 nm of thickness depth) and spatial resolution (down to 1 /zm) are particularly suitable for nondestructive surface magnetic characterization. We compared the hysteresis loops observed at the two surfaces by means of the magneto-optical longitudinal Kerr effect. Also, we varied the sampled area in order to investigate the possible presence of surface inhomogeneities. The FesoB20 amorphous ribbons were prepared by the melt-spinning technique. The samples were in strip form, 5 m m wide and 20 /xm thick. The amorphous nature of the samples was checked by differential scanning calorimetry and M~Sssbauer spectroscopy [2], X-ray and selected area electron diffraction. The topographic images of the surfaces were obtained at room temperature using atomic force microscopy. The profiles across the surfaces, shown in Fig. 1, reveal two different kinds of topography, with peak-to-valley surface roughnesses < 3 nm for the free
* Corresponding author. Fax: + 39-2-2399-6126; email: [email protected].
side with conical shape, and < 200 nm for the wheel side characterized by parallel thin sheets, respectively. The longitudinal Kerr hysteresis loops were taken with a magneto-optical ellipsometer [3]. In order to investigate spatial inhomogeneities, the probe beam was focused with a zoom lens. This configuration also allowed us to vary the diameter d of the sampled area. Typical loops for d = 3 m m and H parallel to the ribbon length for both surfaces, on the side in contact with the spinning wheel (wheel side) and the other side (free side), are shown in Fig. 2A. Similar shapes (although with different H c and slope) were obtained by rotating the sample so that H was perpendicular to the ribbon length. The coercive fields for all the configurations are reported in Table 1. We observed different Ho values by changing both the surface and the orientations (mutually perpendicular) of the sample in the field H. In general, H c for the free side is lower than the corresponding Ho for the wheel side, for both H orientations; this effect is probably due to structural or magnetic differences at the surfaces. This hypothesis is consistent with the different topographies of the two surfaces, as shown in Fig. 1. The sheet structures of the wheel side imply local shape anisotropies which tend to increase H c at variance with the smoothness of the free side. This shape anisotropy can also explain the greater anisotropy in Hc (see A H in Table 1) of the wheel side than that of the free side. Fig. 2B shows three hysteresis loops of the flee surface measured in the same configuration as in Fig. 2A, but with the beam properly focused in order to reduce the spot size at the surface to d ~- 0.5 mm, taken at different points on the surface. The variation in the hysteresis loop shape is a clear indication of inhomogeneities in the surface magnetic properties on the mm scale. The loops of Fig. 2B are characterized by the presence of horizontal steps (along the
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 0 9 8 - X
172
P. Vavassori et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 171-172
Table 1 Coercive force values of the surfaces for external fields parallel (HII) arid perpendicular ( H ± ) to the ribbon length; AH is the difference between Hll and H I
Fr
H c (Oe)
2 nm Free side Wheel side
..t,'/
1 gm
.~
1 gm
Ribt
50 nm
Wl ,~,
~luu
Fig. 1. Atomic force micrographs of the free side (top) and the wheel side (bottom).
A Free side
Oe
j e-
oe
J ~"
I -100
]
r--
= -
W h e e l side I I I 0 100
H (Oe)
g
,/
HII
H±
AH
3.0 5.1
3.9 7.3
0.9 2.2
external H axis) in their central part. In some cases these steps give rise to a crossings (Fig. 2B, right). This complex behavior, which is currently under investigation, explains why the coercive field of the average loop is lower than those of some local loops (Fig. 2B, left and center). Similar behavior is observed on the wheel side. The horizontal steps observed in the local loops could not be simply interpreted as resulting from the presence of a superimposed square loop. In such a case a vertical step (along the M axis) should be observed. We tentatively explain the particular shape of these loops by assuming that the local field sensed by the surface does not follow smoothly the externally applied field, but undergoes steps that could be caused by the interaction of the surface structures with the underlying bulk domains (not directly observable with our probe), which are known to have an average dimension of the same order of magnitude as the sampled area [4]; when a domain wall of the bulk crosses the sampled area, the local field sensed by the surface changes abruptly, resulting in the steps shown in Fig. 2B. In conclusion, we have used the longitudinal magnetooptical Kerr technique to measure the hysteresis loops of the surfaces of FesoB2o amorphous melt-spun ribbon. We have observed a greater coercive field and anisotropy on the side in contact with the quenching surface compared with the free side. W e relate this behavior to the different morphologies of the two surfaces. The shapes of the local loops (measured by focusing the laser beam) are related to the interaction between the surfaces and the underlying bulk magnetic domain structure.
Free side
e/_2 4.40e
References
e
Fig. 2. Longitudinal Kerr hysteresis loops with the external field parallel to the ribbon length of (A) the two sides of the ribbon with spot size d = 3 mm, and (B) the free side with d ~ 0.5 ram, varying the sampled area.
[1] U. K~Ssterand U. Herald, Glassy Metals I, ed. H.J. Giintherod and H. Beck (Springer, Berlin, 1981) p. 255. [2] F. Malizia and F. Ronconi, Mater. Res. Soc. Symp. Proc, 321 (1994) 355. [3] L. Callegaro, E. Puppin and A. Vannucchi, Rev. Sci. Instr. 67 (1995) 1065. [4] G. Schroeder, R. Schiifer and H. Kronmiiller, Phys. Status Solidi (a) 50 (1978) 475.
Journal of Magnetism and Magnetic Materials 157/158 (1996) 171-172
~
journalof magnetism and
~[~
ELSEVIER
magnetic materials
Surface magnetic characterization of FeB amorphous ribbons P. Vavassori b,,, L. Callegaro a, E. Puppin a, F. Malizia b, F. Ronconi b a INFM, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, ][-20133 Milano, Italy b INFM, Dipartimento di Fisica, Universitd di Ferrara, Via Paradiso 12, 1-44100 Ferrara, Italy Abstract
We report longitudinal Kerr hysteresis loops of the two surfaces of a FesoB2o amorphous melt-spun ribbon. We observe different magnetic properties on the two sides. The results could be explained as being due to the presence of surface inhomogeneities having particular shapes, which induces high anisotropy, and to the bulk underlying magnetic domain structure which strongly interacts with the surface. Keywords: Amorphous systems; Hysteresis loops; Kerr magneto-optic
Amorphous alloys ribbons have attracted a great deal of interest due to the outstanding magnetic properties of disordered systems when subjected to suitable temperature treatments, giving rise to more stable nanocrystalline materials [i]. The melt-spin quenching technique is widely used to produce these ribbons. Since the quenching rate is different along the ribbon thickness, the ribbon could show different magnetic properties, particularly at its surfaces with respect to the bulk. Regarding the characterization of such alloys, standard magnetic measurements can provide only the bulk properties. In the present paper we focus on the surface magnetic properties of a binary amorphous alloy, Fe80B20 . Magneto-optical Kerr measurements, with their remarkable surface sensitivity ( ~ 10 nm of thickness depth) and spatial resolution (down to 1 /zm) are particularly suitable for nondestructive surface magnetic characterization. We compared the hysteresis loops observed at the two surfaces by means of the magneto-optical longitudinal Kerr effect. Also, we varied the sampled area in order to investigate the possible presence of surface inhomogeneities. The FesoB20 amorphous ribbons were prepared by the melt-spinning technique. The samples were in strip form, 5 m m wide and 20 /xm thick. The amorphous nature of the samples was checked by differential scanning calorimetry and M~Sssbauer spectroscopy [2], X-ray and selected area electron diffraction. The topographic images of the surfaces were obtained at room temperature using atomic force microscopy. The profiles across the surfaces, shown in Fig. 1, reveal two different kinds of topography, with peak-to-valley surface roughnesses < 3 nm for the free
* Corresponding author. Fax: + 39-2-2399-6126; email: [email protected].
side with conical shape, and < 200 nm for the wheel side characterized by parallel thin sheets, respectively. The longitudinal Kerr hysteresis loops were taken with a magneto-optical ellipsometer [3]. In order to investigate spatial inhomogeneities, the probe beam was focused with a zoom lens. This configuration also allowed us to vary the diameter d of the sampled area. Typical loops for d = 3 m m and H parallel to the ribbon length for both surfaces, on the side in contact with the spinning wheel (wheel side) and the other side (free side), are shown in Fig. 2A. Similar shapes (although with different H c and slope) were obtained by rotating the sample so that H was perpendicular to the ribbon length. The coercive fields for all the configurations are reported in Table 1. We observed different Ho values by changing both the surface and the orientations (mutually perpendicular) of the sample in the field H. In general, H c for the free side is lower than the corresponding Ho for the wheel side, for both H orientations; this effect is probably due to structural or magnetic differences at the surfaces. This hypothesis is consistent with the different topographies of the two surfaces, as shown in Fig. 1. The sheet structures of the wheel side imply local shape anisotropies which tend to increase H c at variance with the smoothness of the free side. This shape anisotropy can also explain the greater anisotropy in Hc (see A H in Table 1) of the wheel side than that of the free side. Fig. 2B shows three hysteresis loops of the flee surface measured in the same configuration as in Fig. 2A, but with the beam properly focused in order to reduce the spot size at the surface to d ~- 0.5 mm, taken at different points on the surface. The variation in the hysteresis loop shape is a clear indication of inhomogeneities in the surface magnetic properties on the mm scale. The loops of Fig. 2B are characterized by the presence of horizontal steps (along the
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 0 9 8 - X
172
P. Vavassori et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 171-172
Table 1 Coercive force values of the surfaces for external fields parallel (HII) arid perpendicular ( H ± ) to the ribbon length; AH is the difference between Hll and H I
Fr
H c (Oe)
2 nm Free side Wheel side
..t,'/
1 gm
.~
1 gm
Ribt
50 nm
Wl ,~,
~luu
Fig. 1. Atomic force micrographs of the free side (top) and the wheel side (bottom).
A Free side
Oe
j e-
oe
J ~"
I -100
]
r--
= -
W h e e l side I I I 0 100
H (Oe)
g
,/
HII
H±
AH
3.0 5.1
3.9 7.3
0.9 2.2
external H axis) in their central part. In some cases these steps give rise to a crossings (Fig. 2B, right). This complex behavior, which is currently under investigation, explains why the coercive field of the average loop is lower than those of some local loops (Fig. 2B, left and center). Similar behavior is observed on the wheel side. The horizontal steps observed in the local loops could not be simply interpreted as resulting from the presence of a superimposed square loop. In such a case a vertical step (along the M axis) should be observed. We tentatively explain the particular shape of these loops by assuming that the local field sensed by the surface does not follow smoothly the externally applied field, but undergoes steps that could be caused by the interaction of the surface structures with the underlying bulk domains (not directly observable with our probe), which are known to have an average dimension of the same order of magnitude as the sampled area [4]; when a domain wall of the bulk crosses the sampled area, the local field sensed by the surface changes abruptly, resulting in the steps shown in Fig. 2B. In conclusion, we have used the longitudinal magnetooptical Kerr technique to measure the hysteresis loops of the surfaces of FesoB2o amorphous melt-spun ribbon. We have observed a greater coercive field and anisotropy on the side in contact with the quenching surface compared with the free side. W e relate this behavior to the different morphologies of the two surfaces. The shapes of the local loops (measured by focusing the laser beam) are related to the interaction between the surfaces and the underlying bulk magnetic domain structure.
Free side
e/_2 4.40e
References
e
Fig. 2. Longitudinal Kerr hysteresis loops with the external field parallel to the ribbon length of (A) the two sides of the ribbon with spot size d = 3 mm, and (B) the free side with d ~ 0.5 ram, varying the sampled area.
[1] U. K~Ssterand U. Herald, Glassy Metals I, ed. H.J. Giintherod and H. Beck (Springer, Berlin, 1981) p. 255. [2] F. Malizia and F. Ronconi, Mater. Res. Soc. Symp. Proc, 321 (1994) 355. [3] L. Callegaro, E. Puppin and A. Vannucchi, Rev. Sci. Instr. 67 (1995) 1065. [4] G. Schroeder, R. Schiifer and H. Kronmiiller, Phys. Status Solidi (a) 50 (1978) 475.