- Email: [email protected]
Control of the magnetic anisotropy of GdFe thin films
Journal of Magnetism and Magnetic Materials 165 (1997) 161 - 164
Journalof
magnetism
~ H and magnetic J H materials
ELSEVIER
Control of the magnetic anisotropy of GdFe thin films S. Mangin a,*, C. Bellouard b, G. Marchal b, B. Barbara a a Laboratoire Louis N&L CNRS, BP 166, 38042 Grenoble C£dex 9, France b LMPSM U. Henri Poincar~, Nancy-l, BP 239, 54506 Vandoeuvre, France
Abstract GdFe amorphous layers have been prepared with well defined in-plane magnetic anisotropy. The best results were obtained when the sample was deposited on glass substrate kept at 90 K during the deposition process. When the substrate was kept at room temperature the anisotropy is less well defined and a perpendicular anisotropy component appears as the thickness of the layer increases. The effect of the position of the sample during the preparation is shown. Keywords: Anisotropy; Thin films; Rare earth-transition metal alloys
1. Introduction Thin films of amorphous rare earth-transition metal ( R E - T M ) alloys represent a very interesting class of materials for magneto-optical storage technology [1]. However the use of these materials requires a control of the magneto-optical anisotropy, in particular the anisotropy related to the elaboration mode. On the other hand, recent studies, concerning fundamental understanding of the processes of magnetisation reversal, need thin magnetic films with a well defined in-plane anisotropy. Among them is the problem of domain wall junction [2] which requires the occurrence of in-plane 180 ° domain walls and as a consequence of an in-plane anisotropy. In fact, because of the randomness of the bounding directions, layers of amorphous alloys are expected to be magnetically isotropic in the plane. However special conditions of deposition or appropriate ex situ treatments can give rise to an in-plane magnetic anisotropy, probably related to an anisotropic short range order. In some materials, for example CoDyZr, an uniaxial anisotropy can be induced by applying a magnetic field during the deposition process [3] or an external stress after the deposition [4]. Nevertheless, up to now, the number of systems exhibiting a well defined and controlled in-plane anisotropy is limited. We report here a study of the in-plane anisotropy in the GdFe amorphous system prepared by co-evaporation. This R E - T M system is particular because gadolinium does not induce strong magnetocrystalline anisotropy unlike other
* Corresponding author. Email: [email protected]; fax: + 33-3-8391-2083.
rare-earth as terbium or dysprosium. The special behaviour of gadolinium is due to the S character of this ion (L = 0). We present the role of different parameters such as the nature of the substrate, its temperature during deposition, the incidence angle of deposition and the thickness of the layer.
2. Experimental procedure The samples were obtained by co-evaporation from two separated sources in a high vacuum chamber where the pressure was kept below 10 -8 Torr during the deposition process. Iron was evaporated from an electron gun and gadolinium from a Joule effect crucible. The distance between the two sources was 130 mm and the substrate holder was at a 300 mm height from the sources. The geometry of the system, which will appear as an important parameter is sketched in Fig. 1 where O z is the vertical, O x the direction which joins the two sources. In the (Ox, O y ) plane, the substrate can be placed at any position along the O x axis between A~ and A 2, the points located at the vertical of the gadolinium and iron sources respectively, which allowed us to tune (in a narrow range). Whatever the position of the substrate, the angle of incidence of the evaporated atoms never exceeds 20 ° . Two substrates were used: Coming 7059 glass and 25 ~ m thick kapton fixed on a rigid holder. The size of the samples were 5 mm large and 20 mm long. The composition Gd6oFe40 was chosen to provide a Curie temperature close to room temperature. The compositions were checked by electron microprobe analysis. The amorphous state was confirmed by electron diffraction and by M~ssbauer spectroscopy. Before the deposition of the alloy, a 500 A thick
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-885 3(96)00495-7
162
S. Mangin et a l . / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164
z A1
I
65 mm A2
o
I
x
3 0 0 mm
Ii Fe
t
i~ Gd
Fig. 1. Plan of the vacuum chamber indicatingthe position of the samples, placed between A I and A 2 on the Ox axis, with respect to the position of the sources. Oz is the vertical direction.
In the usual Stoner-Wohlfarth model of uniaxial anisotropy, when the applied field is applied along the easy axis of a monodomain sample, the loop should be strictly square with a coercive field Hc equal to Ha, where /4, is the anisotropy field. When the field is perpendicular to the easy axis, the magnetisation should vary linearly from /4, to -H~ without hysteresis. The discrepancies of our resuits with respect to this model (round hysteresis loop measured in the easy direction and existence of an hysteresis in the hard direction) show that the observed anisotropy is far from being uniaxial. As shown in Fig. 2b, the magnetlsation loops measured with a 500 ,~ thick sample also exhibit the existence of an uniaxial anisotropy, with a more square loop in the Oy direction than the 100 ,~ layer, but the loop is still open along the Ox direction. On the other hand, the magnetisation loops collected with a 1000 .~ sample are significantly different. This can
1
amorphous silicon layer was deposited at 420 K on both substrates to get a clean and controlled surface. Finally the sample was covered with a new 100 ,~ amorphous silicon layer which protected it from oxidation. The substrate could be maintained at a temperature down to 90 K. The evaporation rates, the composition of the samples and their thicknesses were controlled by a quartz system calibrated from optical methods. In some experiments the sample was submitted to a 1000 Oe magnetic field applied in the plane of the substrate. The magnetisation measurements were performed in a SQUID magnetometer (Quantum Design). Hysteresis loops were performed along the Ox and Oy axes at different temperatures. The results shown here concern data collected at 10 K. The loops presented were obtained after saturation of the magnetisation at 1000 Oe, which is very large compared to all the observed coercive fields. The magnetic field was always applied in the plane of the sample.
.o,K
~ 00.5 "'"°"'" 0 x
t Ji!l
-o.5 -
6/
I
i
1
i
__l_ [J
r.-~
0.5
~
(a): IOOA
,,,,~ ?}
0 ~ t -0.5
- 1
__
,,
J
r
(b): 500A
i
3. Experimental results and discussion 3.1. Deposition on glass and kapton at room temperature
Fig. 2 shows hysteresis loops collected along Ox and Oy with samples obtained by deposition on glass substrates maintained at room temperature. As shown in Fig. 2a, a 100 ,~ film presents some sign of an easy axis along the Oy direction: the hysteresis loop along Oy is rather square with a remanent magnetisation close to the saturation magnetisation Ms. The magnetisation decreases from + M s to - M s over 10 Oe between - 2 0 Oe and - 3 0 Oe. This loop is very different from that collected along Ox where the magnetisation decreases from + M s to - M s between 40 Oe and - 4 0 Oe with a flatter loop.
05
~ 0
I
U
H~'~ I-Ih~
-0,5
,~,~.£~,,Er
- 1C -100
~-50
I 0 H(Oe)
(c): IO00A
I 50
10
Fig. 2. Hysteresis loops measured at 10 K, with a field applied in the Ox and Oy directions,for samples deposited at room temperature on a glass substrate with respective thicknesses: (a) 100 A, (b) 5OO~,, (c) 1000 L
163
S. Mangin et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164
be seen in Fig. 2c. In this sample, along Oy, the magnetisation leaves the saturation magnetisation at H~ = + 50 Oe and decreases almost linearly down to - 3 0 Oe where the magnetisation drops to - M s. With the field applied along Ox, a similar curve is obtained with a significantly higher saturation field H,h = + 75 Oe. Such a behaviour is typical of samples presenting a biaxial anisotropy with an axis perpendicular to the plane of the sample and an axis in the plane of the sample. This has been previously observed by Suran et al. [5] and analysed as an extension of the Murayama model [4]. Our results are in agreement with the work [5] where a critical thickness, below which the perpendicular anisotropy disappears, has been experimentally observed. When a kapton substrate is used, no significant in-plane anisotropy is observed with the 100 A thick sample as shown in Fiog. 3a. On the other hand, the hysteresis loops of the 500 A film, plotted in Fig. 3b, exhibit more closely the characteristic features of an in-plane anisotropy. In that case, the M - H loops corresponding to the two directions of the field are indeed much different: for H parallel to Oy, the loop is still square, whereas that, with H parallel to Ox, is weakly opened and presents a slope from 50 Oe to - 5 0 Oe. The substitution of the glass substrate by a kapton film substrate makes then the uniaxial anisotropy more well defined. The loops collected with a 1000 ~, thick film exhibit the same tendency as the sample deposited on glass with features related to biaxial anisotropy.
0.5
(a)----c.--Ox • Oy
~o,o~O],d
//
o
/I
Is O
-0.5
"""
a r o "
, ~
O°~
,c..C/°
(~/:looA
- 1
1
(b)
0.5 0 -0.5
o
(b): 500.ik .
1 1 ~ -100
-50
0 H(Oe)
5o
100
Fig. 3. Hysteresis loops measured at 10 K, with an applied field in the Ox and Oy directions, for samples deposited at room temperature on a kapton substrate with respective thicknesses: (a) 100 ,~, (b) 500 ,~.
oO°
~ - - Oy ....:'"- O x
0.5
.Ol
0 -0.5 -1
j
00
-50
;1
5 4 0 H(Oe)
50
100
Fig. 4. Hysteresis loops measured at 10 K, with an applied field in the Ox and Oy directions, for a 500 ,~ thick sample deposited at 90 K on a glass substrate.
Finally, the conclusion of room temperature deposition is that a tendency to in-plane uniaxial axis occurs spontaneously in the plane of the sample along the O y direction when the substrate is glass. Nevertheless, two limits are observed: (i) the films exhibit a biaxial behaviour for a thickness of 1000 .~, (ii) the squareness of the O y loop and the linearity of the O x loop remain to be improved. For that reason, we have checked the influence of several parameters, the more effective was the temperature of the substrate as reported below. 3.2. Deposition on glass at low temperature
Fig. 4 shows typical loops obtained for a 500 ~, thick sample deposited on a glass substrate kept at the liquid nitrogen temperature during the deposition. These loops show that this sample possess a better defined in-plane uniaxial anisotropy than the 500 ,~ thick sample deposited on glass at room temperature (Fig. 2b). When the field is applied along Ox, the loop exhibits a very linear variation of the magnetisation from + M s to - M S in the field range H a to - H a without a significant coercive field (in comparison to H a) as predicted by the Stoner-Wohlfarth model of uniform rotation of the magnetic moments. When the field is applied along Oy, the loop is very square with a coercive field H c. As in most of the systems, H c is different from H a which means that when the field is applied along the easy axis, the magnetisation does not rotate uniformly but that there is nucleation and propagation of domain walls. The second result of the cooling of the sample is the absence of any features indicating the presence of a °perpendicular anisotropy even in films as thick as 3000 A. Finally during the deposition, we applied a magnetic field of 1000 Oe on the substrate as well at room temperature as at 90 K. In both cases, this field appears to be totally inefficient for an improvement of the in-plane anisotropy. Moreover, a magnetic field applied in the O x direction does not modify the direction of the easy axis.
164
S. Mangin et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164 8O
(a)
6O o
40
.
.r.,~
.
2O
o
.
.
.
© .
.
.
•
1000 A
o
500A
i
i
.
o
.
i/hi 25 -- ~ •
20
o
-500A 1000A
l#
15
I 10
0
i
0
10
i
i
~
a
20 30 40 50 position (Ox) (mm)
i
60
Fig. 5. (a) Anisotropy field Ha deduced from the loop measured in the Ox direction (hard axis) and (b) width of the hysteresis loop 2H c measured in the Oy direction (easy axis) for 500 A. and 1000 A thick samples deposited at 90 K on glass.
3.3. Effect of the position of the substrate In order to understand the origin of such an in-plane anisotropy, with an easy axis in the direction O y perpendicular to the axis of the sources, we placed substrates at different positions on the Ox axis, from the vertical of the iron electron gun (A n in Fig. 1) to the vertical of the gadolinium source (A 2 in Fig. 1). The first qualitative result is that the samples elaborated at the vertical of the iron source with a glass substrate exhibit the better defined anisotropy axis in comparison with the Wolfarth model. We focus now on the results obtained with a 500 ,~ thick layer and a 1000 ,~ one elaborated at 90 K on a glass substrate. The coercive field H c, deduced from the O y loop, and the anisotropy field H a, deduced from the Ox loop are plotted in Fig. 5 as a function of the position of the substrate. As shown in Fig. 5b, the coercitive field is the largest at the vertical of the iron source (A l position) and decreases when the position is shifted towards the gadolin-
ium crucible.° H c is practically the same for the 1000 and the 500 A samples. At the opposite, the anisotropy field H a plotted in Fig. 5a is rather constant with the position for both thicknesses. It is nevertheless larger for the 1000 A thick sample than for the 500 .& thick one. In conclusion, although the direction of the anisotropy field H a is determined by the position of the sources with respect to the sample, we do not observe an evolution of /4, within the studied range of angles of incidence. The dependence of H~ with the samples thicknesses indicates that the anisotropy is not only due to a local ordering of pair of atoms as previously suggested in Ref. [5]. 4. Conclusion In conclusion, we prepared GdFe amorphous layers with a well defined uniaxial in-plane anisotropy in a direction perpendicular to the plane containing the two sources and the sample. The best samples were obtained by deposition on glass substrates maintained at low temperature and placed at the vertical of the iron source. The anisotropy is obtained spontaneously along the direction perpendicular to the axis of the sources and is probably due to the fact that the elements are evaporated from two separate crucibles and then arrive on the sample with two different angles of incidence. The origin has to be studied in more detail but it can be suggested that in the absence of long range order, pair ordering due to the deposition of two atoms of very different sizes with different angles of incidence, even small, play an important role [6]. This effect can be enhanced by strains due to the deposition of the sample at low temperature. References [1] H.A.M. van den Berg and S. Winkler, IEEE Trans. Magn. 26 (1990) 184. [2] S. Mangin, G. Marchal, W. Wernsdorfer, A. Sulpice, D. Mailly and B. Barbara, J. Magn. Magn. Mater. 165 (1997) 13 (these Proceedings). [3] G. Suran, J. Sztern, B. Barbara. Appl. Phys. Lett. 58 (1995) 1338. [4] Y. Murayama, J. Phys. Soc. Jpn. 21 (1966) 2253. [5] G. Suran, K. Ounadjela and F. Machizaud, J. Appl. Phys. 61 (1987) 3658. [6] K. Tohma, Y. Kawawake and R. Sugita, J. Magn. Magn. Mater. 134 (1994) 342.
Journalof
magnetism
~ H and magnetic J H materials
ELSEVIER
Control of the magnetic anisotropy of GdFe thin films S. Mangin a,*, C. Bellouard b, G. Marchal b, B. Barbara a a Laboratoire Louis N&L CNRS, BP 166, 38042 Grenoble C£dex 9, France b LMPSM U. Henri Poincar~, Nancy-l, BP 239, 54506 Vandoeuvre, France
Abstract GdFe amorphous layers have been prepared with well defined in-plane magnetic anisotropy. The best results were obtained when the sample was deposited on glass substrate kept at 90 K during the deposition process. When the substrate was kept at room temperature the anisotropy is less well defined and a perpendicular anisotropy component appears as the thickness of the layer increases. The effect of the position of the sample during the preparation is shown. Keywords: Anisotropy; Thin films; Rare earth-transition metal alloys
1. Introduction Thin films of amorphous rare earth-transition metal ( R E - T M ) alloys represent a very interesting class of materials for magneto-optical storage technology [1]. However the use of these materials requires a control of the magneto-optical anisotropy, in particular the anisotropy related to the elaboration mode. On the other hand, recent studies, concerning fundamental understanding of the processes of magnetisation reversal, need thin magnetic films with a well defined in-plane anisotropy. Among them is the problem of domain wall junction [2] which requires the occurrence of in-plane 180 ° domain walls and as a consequence of an in-plane anisotropy. In fact, because of the randomness of the bounding directions, layers of amorphous alloys are expected to be magnetically isotropic in the plane. However special conditions of deposition or appropriate ex situ treatments can give rise to an in-plane magnetic anisotropy, probably related to an anisotropic short range order. In some materials, for example CoDyZr, an uniaxial anisotropy can be induced by applying a magnetic field during the deposition process [3] or an external stress after the deposition [4]. Nevertheless, up to now, the number of systems exhibiting a well defined and controlled in-plane anisotropy is limited. We report here a study of the in-plane anisotropy in the GdFe amorphous system prepared by co-evaporation. This R E - T M system is particular because gadolinium does not induce strong magnetocrystalline anisotropy unlike other
* Corresponding author. Email: [email protected]; fax: + 33-3-8391-2083.
rare-earth as terbium or dysprosium. The special behaviour of gadolinium is due to the S character of this ion (L = 0). We present the role of different parameters such as the nature of the substrate, its temperature during deposition, the incidence angle of deposition and the thickness of the layer.
2. Experimental procedure The samples were obtained by co-evaporation from two separated sources in a high vacuum chamber where the pressure was kept below 10 -8 Torr during the deposition process. Iron was evaporated from an electron gun and gadolinium from a Joule effect crucible. The distance between the two sources was 130 mm and the substrate holder was at a 300 mm height from the sources. The geometry of the system, which will appear as an important parameter is sketched in Fig. 1 where O z is the vertical, O x the direction which joins the two sources. In the (Ox, O y ) plane, the substrate can be placed at any position along the O x axis between A~ and A 2, the points located at the vertical of the gadolinium and iron sources respectively, which allowed us to tune (in a narrow range). Whatever the position of the substrate, the angle of incidence of the evaporated atoms never exceeds 20 ° . Two substrates were used: Coming 7059 glass and 25 ~ m thick kapton fixed on a rigid holder. The size of the samples were 5 mm large and 20 mm long. The composition Gd6oFe40 was chosen to provide a Curie temperature close to room temperature. The compositions were checked by electron microprobe analysis. The amorphous state was confirmed by electron diffraction and by M~ssbauer spectroscopy. Before the deposition of the alloy, a 500 A thick
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-885 3(96)00495-7
162
S. Mangin et a l . / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164
z A1
I
65 mm A2
o
I
x
3 0 0 mm
Ii Fe
t
i~ Gd
Fig. 1. Plan of the vacuum chamber indicatingthe position of the samples, placed between A I and A 2 on the Ox axis, with respect to the position of the sources. Oz is the vertical direction.
In the usual Stoner-Wohlfarth model of uniaxial anisotropy, when the applied field is applied along the easy axis of a monodomain sample, the loop should be strictly square with a coercive field Hc equal to Ha, where /4, is the anisotropy field. When the field is perpendicular to the easy axis, the magnetisation should vary linearly from /4, to -H~ without hysteresis. The discrepancies of our resuits with respect to this model (round hysteresis loop measured in the easy direction and existence of an hysteresis in the hard direction) show that the observed anisotropy is far from being uniaxial. As shown in Fig. 2b, the magnetlsation loops measured with a 500 ,~ thick sample also exhibit the existence of an uniaxial anisotropy, with a more square loop in the Oy direction than the 100 ,~ layer, but the loop is still open along the Ox direction. On the other hand, the magnetisation loops collected with a 1000 .~ sample are significantly different. This can
1
amorphous silicon layer was deposited at 420 K on both substrates to get a clean and controlled surface. Finally the sample was covered with a new 100 ,~ amorphous silicon layer which protected it from oxidation. The substrate could be maintained at a temperature down to 90 K. The evaporation rates, the composition of the samples and their thicknesses were controlled by a quartz system calibrated from optical methods. In some experiments the sample was submitted to a 1000 Oe magnetic field applied in the plane of the substrate. The magnetisation measurements were performed in a SQUID magnetometer (Quantum Design). Hysteresis loops were performed along the Ox and Oy axes at different temperatures. The results shown here concern data collected at 10 K. The loops presented were obtained after saturation of the magnetisation at 1000 Oe, which is very large compared to all the observed coercive fields. The magnetic field was always applied in the plane of the sample.
.o,K
~ 00.5 "'"°"'" 0 x
t Ji!l
-o.5 -
6/
I
i
1
i
__l_ [J
r.-~
0.5
~
(a): IOOA
,,,,~ ?}
0 ~ t -0.5
- 1
__
,,
J
r
(b): 500A
i
3. Experimental results and discussion 3.1. Deposition on glass and kapton at room temperature
Fig. 2 shows hysteresis loops collected along Ox and Oy with samples obtained by deposition on glass substrates maintained at room temperature. As shown in Fig. 2a, a 100 ,~ film presents some sign of an easy axis along the Oy direction: the hysteresis loop along Oy is rather square with a remanent magnetisation close to the saturation magnetisation Ms. The magnetisation decreases from + M s to - M s over 10 Oe between - 2 0 Oe and - 3 0 Oe. This loop is very different from that collected along Ox where the magnetisation decreases from + M s to - M s between 40 Oe and - 4 0 Oe with a flatter loop.
05
~ 0
I
U
H~'~ I-Ih~
-0,5
,~,~.£~,,Er
- 1C -100
~-50
I 0 H(Oe)
(c): IO00A
I 50
10
Fig. 2. Hysteresis loops measured at 10 K, with a field applied in the Ox and Oy directions,for samples deposited at room temperature on a glass substrate with respective thicknesses: (a) 100 A, (b) 5OO~,, (c) 1000 L
163
S. Mangin et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164
be seen in Fig. 2c. In this sample, along Oy, the magnetisation leaves the saturation magnetisation at H~ = + 50 Oe and decreases almost linearly down to - 3 0 Oe where the magnetisation drops to - M s. With the field applied along Ox, a similar curve is obtained with a significantly higher saturation field H,h = + 75 Oe. Such a behaviour is typical of samples presenting a biaxial anisotropy with an axis perpendicular to the plane of the sample and an axis in the plane of the sample. This has been previously observed by Suran et al. [5] and analysed as an extension of the Murayama model [4]. Our results are in agreement with the work [5] where a critical thickness, below which the perpendicular anisotropy disappears, has been experimentally observed. When a kapton substrate is used, no significant in-plane anisotropy is observed with the 100 A thick sample as shown in Fiog. 3a. On the other hand, the hysteresis loops of the 500 A film, plotted in Fig. 3b, exhibit more closely the characteristic features of an in-plane anisotropy. In that case, the M - H loops corresponding to the two directions of the field are indeed much different: for H parallel to Oy, the loop is still square, whereas that, with H parallel to Ox, is weakly opened and presents a slope from 50 Oe to - 5 0 Oe. The substitution of the glass substrate by a kapton film substrate makes then the uniaxial anisotropy more well defined. The loops collected with a 1000 ~, thick film exhibit the same tendency as the sample deposited on glass with features related to biaxial anisotropy.
0.5
(a)----c.--Ox • Oy
~o,o~O],d
//
o
/I
Is O
-0.5
"""
a r o "
, ~
O°~
,c..C/°
(~/:looA
- 1
1
(b)
0.5 0 -0.5
o
(b): 500.ik .
1 1 ~ -100
-50
0 H(Oe)
5o
100
Fig. 3. Hysteresis loops measured at 10 K, with an applied field in the Ox and Oy directions, for samples deposited at room temperature on a kapton substrate with respective thicknesses: (a) 100 ,~, (b) 500 ,~.
oO°
~ - - Oy ....:'"- O x
0.5
.Ol
0 -0.5 -1
j
00
-50
;1
5 4 0 H(Oe)
50
100
Fig. 4. Hysteresis loops measured at 10 K, with an applied field in the Ox and Oy directions, for a 500 ,~ thick sample deposited at 90 K on a glass substrate.
Finally, the conclusion of room temperature deposition is that a tendency to in-plane uniaxial axis occurs spontaneously in the plane of the sample along the O y direction when the substrate is glass. Nevertheless, two limits are observed: (i) the films exhibit a biaxial behaviour for a thickness of 1000 .~, (ii) the squareness of the O y loop and the linearity of the O x loop remain to be improved. For that reason, we have checked the influence of several parameters, the more effective was the temperature of the substrate as reported below. 3.2. Deposition on glass at low temperature
Fig. 4 shows typical loops obtained for a 500 ~, thick sample deposited on a glass substrate kept at the liquid nitrogen temperature during the deposition. These loops show that this sample possess a better defined in-plane uniaxial anisotropy than the 500 ,~ thick sample deposited on glass at room temperature (Fig. 2b). When the field is applied along Ox, the loop exhibits a very linear variation of the magnetisation from + M s to - M S in the field range H a to - H a without a significant coercive field (in comparison to H a) as predicted by the Stoner-Wohlfarth model of uniform rotation of the magnetic moments. When the field is applied along Oy, the loop is very square with a coercive field H c. As in most of the systems, H c is different from H a which means that when the field is applied along the easy axis, the magnetisation does not rotate uniformly but that there is nucleation and propagation of domain walls. The second result of the cooling of the sample is the absence of any features indicating the presence of a °perpendicular anisotropy even in films as thick as 3000 A. Finally during the deposition, we applied a magnetic field of 1000 Oe on the substrate as well at room temperature as at 90 K. In both cases, this field appears to be totally inefficient for an improvement of the in-plane anisotropy. Moreover, a magnetic field applied in the O x direction does not modify the direction of the easy axis.
164
S. Mangin et al. / Journal of Magnetism and Magnetic Materials 165 (1997) 161-164 8O
(a)
6O o
40
.
.r.,~
.
2O
o
.
.
.
© .
.
.
•
1000 A
o
500A
i
i
.
o
.
i/hi 25 -- ~ •
20
o
-500A 1000A
l#
15
I 10
0
i
0
10
i
i
~
a
20 30 40 50 position (Ox) (mm)
i
60
Fig. 5. (a) Anisotropy field Ha deduced from the loop measured in the Ox direction (hard axis) and (b) width of the hysteresis loop 2H c measured in the Oy direction (easy axis) for 500 A. and 1000 A thick samples deposited at 90 K on glass.
3.3. Effect of the position of the substrate In order to understand the origin of such an in-plane anisotropy, with an easy axis in the direction O y perpendicular to the axis of the sources, we placed substrates at different positions on the Ox axis, from the vertical of the iron electron gun (A n in Fig. 1) to the vertical of the gadolinium source (A 2 in Fig. 1). The first qualitative result is that the samples elaborated at the vertical of the iron source with a glass substrate exhibit the better defined anisotropy axis in comparison with the Wolfarth model. We focus now on the results obtained with a 500 ,~ thick layer and a 1000 ,~ one elaborated at 90 K on a glass substrate. The coercive field H c, deduced from the O y loop, and the anisotropy field H a, deduced from the Ox loop are plotted in Fig. 5 as a function of the position of the substrate. As shown in Fig. 5b, the coercitive field is the largest at the vertical of the iron source (A l position) and decreases when the position is shifted towards the gadolin-
ium crucible.° H c is practically the same for the 1000 and the 500 A samples. At the opposite, the anisotropy field H a plotted in Fig. 5a is rather constant with the position for both thicknesses. It is nevertheless larger for the 1000 A thick sample than for the 500 .& thick one. In conclusion, although the direction of the anisotropy field H a is determined by the position of the sources with respect to the sample, we do not observe an evolution of /4, within the studied range of angles of incidence. The dependence of H~ with the samples thicknesses indicates that the anisotropy is not only due to a local ordering of pair of atoms as previously suggested in Ref. [5]. 4. Conclusion In conclusion, we prepared GdFe amorphous layers with a well defined uniaxial in-plane anisotropy in a direction perpendicular to the plane containing the two sources and the sample. The best samples were obtained by deposition on glass substrates maintained at low temperature and placed at the vertical of the iron source. The anisotropy is obtained spontaneously along the direction perpendicular to the axis of the sources and is probably due to the fact that the elements are evaporated from two separate crucibles and then arrive on the sample with two different angles of incidence. The origin has to be studied in more detail but it can be suggested that in the absence of long range order, pair ordering due to the deposition of two atoms of very different sizes with different angles of incidence, even small, play an important role [6]. This effect can be enhanced by strains due to the deposition of the sample at low temperature. References [1] H.A.M. van den Berg and S. Winkler, IEEE Trans. Magn. 26 (1990) 184. [2] S. Mangin, G. Marchal, W. Wernsdorfer, A. Sulpice, D. Mailly and B. Barbara, J. Magn. Magn. Mater. 165 (1997) 13 (these Proceedings). [3] G. Suran, J. Sztern, B. Barbara. Appl. Phys. Lett. 58 (1995) 1338. [4] Y. Murayama, J. Phys. Soc. Jpn. 21 (1966) 2253. [5] G. Suran, K. Ounadjela and F. Machizaud, J. Appl. Phys. 61 (1987) 3658. [6] K. Tohma, Y. Kawawake and R. Sugita, J. Magn. Magn. Mater. 134 (1994) 342.