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Anomalous hysteresis loops in single and double layer sputtered TbFe films
Journal of Magnetism and Magnetic Materials 35 (1983) 269-271 North-Holland Publishing Company
269
A N O M A L O U S H Y S T E R E S I S L O O P S IN S I N G L E AND D O U B L E LAYER S P U T I ' E R E D TbFe FILMS T u C H E N a n d R. M A L M H t i , LL Xerox Palo Alto Research Centers Palo Alto, CA 94304, USA
Hall and Kerr effect observations indicate that the anomalous loop of the single layer film is associated with the propagation of a compensation wall through the thickness which can occur only in conjunction with a gradual composition variation, whereas the loop in the double layer film is a simple addition of the loops of each layer.
Introduction
HALLVOLTAGE
a
I
Anomalous Kerr and Hall effect loops were observed in a single layer thin amorphous TbFe film. In order to investigate this phenomenon, a comparison with the hysteresis properties of a double layer TbFe film containing a compensation plane between the two layers was also made.
293 K
326 K
Experimental Single layer TbFe films (30 nm thick), having a nominal Tb content higher than that at the room temperature compensation composition, were prepared by rf sputtering on a glass substrate from a small diameter (76 ram) composite target [1]. Due to the geometry of the sputtering system the deposited films will exhibit a composition gradient along the film thickness. The double layer film, on the other hand, was deposited in a sputtering system with a larger target (203 mm) to ensure film homogeneity [2,3]. This film consisted of two TbFe film layers, 12 and 17 nm thick, with their compensation temperatures being above and well below room temperature, respectively. The composition difference between the two film layers was achieved by using two different argon pressures [2,3]. The hysteresis loop measurements as a function of temperature were made by Kerr and Hall effect magnetometers.
'/
332K
I 0 I
b
5 I kOe
10 [
KERRSIGNAL
-
I
. ~
'ff"~ q25K
Results and discussion
Figs. la and lb show, respectively, the Hall and Kerr loops of the single layer film taken at various temperatures. It is found that the field dependence of the Hall and Kerr effects behaves identically at a given temperature, as well as when the anomalous loops appear above 320 K. This result is quite different from the previously reported anomalies in GdCo [4,5] and GdFe [61 films. In comparison, the Hall loops of the double layer film at the temperature range of 200 to 300 K behave like a sum of loops with opposite polarity from two
0 I
5 I kOe
10 I
Fig. 1. Anomalous hysteresis loops in a single layer TbFe film measured by (a) Hall effect and (b) Kerr magneto-optic effect.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1982 N o r t h - H o l l a n d
Tu Chen and R. Malmhi~ll / Anomalous hysteresis loops in sputtered'TbFe films
270
1 H A L L VOLTAGE
225 K
"e,/ % \
tO
.--To ~ c 1 l~"
Hc2
295 K
~c1
Hc/
t~
I
I
I
I
I
I
-18
-15
-12
-9
-6
-3
0
I
I
I
3
6
9
Hc,
I
i
I
12
15
18
kOe Fig. 2. Anomalous hysteresis loops in a double layer TbFe film measured by Hall effect at 225 and 295 K. The open arrows indicate the direction and magnitude of the net magnetization of each layer, whereas the solid arrow shows the direction of the Tb sublattice moment.
function of field can be consistently explained without introducing the spin alignment due to the exchange interactions between the layers as is the case of GdCo [4,5] and G d F e [6] films. Hence this double layer film does not exhibit an anomalous behavior. A study of the loop in one film biased by the other show that the loop
independents films placed on top of each other, even though there is a compensation plane existing between the two layers. Typical loops of the double layer film along with a schematic representation of sublattice spins and magnetic moments of each layer are shown in fig. 2. As seen in the figure, the change in the Hall voltage as a
t~t9
•
t9 / ~
O
\1~
326 K
lo
. . . . . . "~]== ~.o
......
-o-
21=o--,
t \
/
!~
/~
t m_
o
iO,
.
t
to
,~
\__EL~
/
--f-o--
\1~ I o .... ~1~ - - ~ - g -
-'
/
~
;_,
332 K
~ g
t ~?
-
-ft I
I
I
I
I
I
I
I
I
-18
-15
-12
-9
-6
-3
3
6
9
I2
1
/ I
I
15
18
kOe Fig. 3. Schematics of the net magnetization (open arrows) and Tb sublattice moment (solid arrows) for different state of magnetization in a single layer TbFe film at 326 and 332 K.
Tu Chen and R. Malmhall / Anomalous hysteresis loops in sputtered TbFe films
is asymmetrically shifted along the field axis. The analysis of this phenomenon indicates that the loop shift is caused by a combination of a magneto-static and exchange coupling between the two layers. These results show that, although the exchange coupling exists, no exchange driven spin realignment similar to that of the GdCo and G d F e can take place in this film. This might be related to the fact that each layer in the double layer film has a homogenous composition and that there is a discontinous change in composition at the interface. In the case of the single layer film having a composition gradient along the film thickness and a compensation plane within the film, the occurrence of loop anomalies as shown in figs. 1a and 1b can be explained in terms of the formation of a mobile compensation wall within the film interior. The sublattice spin state and net magnetization state of each sub-layer of two typical anomalous loops are illustrated in fig. 3. The magneto-static energy gradient, which is affecting the wall movement, is associated with the magnitude and direction of the net magnetization in each sub-layer, as represented by the open arrows in the figure. Here the length of the open arrows is proportional to the intrinsic magnetization which is varying within the film due to the composition gradient. Under the combined effects
271
of the exchange interactions of the sublattice spin in each sub-layer and the field-induced magneto-static energy gradient, the wall will move in the direction along the film thickness under various applied fields. As shown in the figure, the change in the Hall and Kerr loops can be explained consistently by the corresponding change in the sublattice spins. In conclusion, it appears that the anomalous hysteres i s loop can occur in a TbFe film only when a composit i o n gradient normal to the film plane is present. This composition variation provides the magneto-static energy gradient to drive the compensation wall through the film to cause the anomalous behavior. References [1] Tu Chen, D. Cheng and G.B. Charlan, IEEE Trans. Magn. MAG-16 (1980) 1194. [2] Tu Chen, R. Malmhilll and G.B. Charlan, J. Appl. Phys. 53 (1982) 2356. [3] R. Maiml'~ll and Tu Chen, J. Appl. Phys. 53 (1982) 7843. [41 S. Esho, Jpn. J. Appl. Phys. 15 (1976) 93. [5] O.S. Lutes, J.O. Holmen, R.L. Kooycr and O.S. Andland, IEEE Trans. Magn, MAG-13 (1977) 1615. [6] M. Amatsu, S. Honda and q'. Kusuda, IEEE Trans. Magn. MAG-13 (1977) 1612.
269
A N O M A L O U S H Y S T E R E S I S L O O P S IN S I N G L E AND D O U B L E LAYER S P U T I ' E R E D TbFe FILMS T u C H E N a n d R. M A L M H t i , LL Xerox Palo Alto Research Centers Palo Alto, CA 94304, USA
Hall and Kerr effect observations indicate that the anomalous loop of the single layer film is associated with the propagation of a compensation wall through the thickness which can occur only in conjunction with a gradual composition variation, whereas the loop in the double layer film is a simple addition of the loops of each layer.
Introduction
HALLVOLTAGE
a
I
Anomalous Kerr and Hall effect loops were observed in a single layer thin amorphous TbFe film. In order to investigate this phenomenon, a comparison with the hysteresis properties of a double layer TbFe film containing a compensation plane between the two layers was also made.
293 K
326 K
Experimental Single layer TbFe films (30 nm thick), having a nominal Tb content higher than that at the room temperature compensation composition, were prepared by rf sputtering on a glass substrate from a small diameter (76 ram) composite target [1]. Due to the geometry of the sputtering system the deposited films will exhibit a composition gradient along the film thickness. The double layer film, on the other hand, was deposited in a sputtering system with a larger target (203 mm) to ensure film homogeneity [2,3]. This film consisted of two TbFe film layers, 12 and 17 nm thick, with their compensation temperatures being above and well below room temperature, respectively. The composition difference between the two film layers was achieved by using two different argon pressures [2,3]. The hysteresis loop measurements as a function of temperature were made by Kerr and Hall effect magnetometers.
'/
332K
I 0 I
b
5 I kOe
10 [
KERRSIGNAL
-
I
. ~
'ff"~ q25K
Results and discussion
Figs. la and lb show, respectively, the Hall and Kerr loops of the single layer film taken at various temperatures. It is found that the field dependence of the Hall and Kerr effects behaves identically at a given temperature, as well as when the anomalous loops appear above 320 K. This result is quite different from the previously reported anomalies in GdCo [4,5] and GdFe [61 films. In comparison, the Hall loops of the double layer film at the temperature range of 200 to 300 K behave like a sum of loops with opposite polarity from two
0 I
5 I kOe
10 I
Fig. 1. Anomalous hysteresis loops in a single layer TbFe film measured by (a) Hall effect and (b) Kerr magneto-optic effect.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1982 N o r t h - H o l l a n d
Tu Chen and R. Malmhi~ll / Anomalous hysteresis loops in sputtered'TbFe films
270
1 H A L L VOLTAGE
225 K
"e,/ % \
tO
.--To ~ c 1 l~"
Hc2
295 K
~c1
Hc/
t~
I
I
I
I
I
I
-18
-15
-12
-9
-6
-3
0
I
I
I
3
6
9
Hc,
I
i
I
12
15
18
kOe Fig. 2. Anomalous hysteresis loops in a double layer TbFe film measured by Hall effect at 225 and 295 K. The open arrows indicate the direction and magnitude of the net magnetization of each layer, whereas the solid arrow shows the direction of the Tb sublattice moment.
function of field can be consistently explained without introducing the spin alignment due to the exchange interactions between the layers as is the case of GdCo [4,5] and G d F e [6] films. Hence this double layer film does not exhibit an anomalous behavior. A study of the loop in one film biased by the other show that the loop
independents films placed on top of each other, even though there is a compensation plane existing between the two layers. Typical loops of the double layer film along with a schematic representation of sublattice spins and magnetic moments of each layer are shown in fig. 2. As seen in the figure, the change in the Hall voltage as a
t~t9
•
t9 / ~
O
\1~
326 K
lo
. . . . . . "~]== ~.o
......
-o-
21=o--,
t \
/
!~
/~
t m_
o
iO,
.
t
to
,~
\__EL~
/
--f-o--
\1~ I o .... ~1~ - - ~ - g -
-'
/
~
;_,
332 K
~ g
t ~?
-
-ft I
I
I
I
I
I
I
I
I
-18
-15
-12
-9
-6
-3
3
6
9
I2
1
/ I
I
15
18
kOe Fig. 3. Schematics of the net magnetization (open arrows) and Tb sublattice moment (solid arrows) for different state of magnetization in a single layer TbFe film at 326 and 332 K.
Tu Chen and R. Malmhall / Anomalous hysteresis loops in sputtered TbFe films
is asymmetrically shifted along the field axis. The analysis of this phenomenon indicates that the loop shift is caused by a combination of a magneto-static and exchange coupling between the two layers. These results show that, although the exchange coupling exists, no exchange driven spin realignment similar to that of the GdCo and G d F e can take place in this film. This might be related to the fact that each layer in the double layer film has a homogenous composition and that there is a discontinous change in composition at the interface. In the case of the single layer film having a composition gradient along the film thickness and a compensation plane within the film, the occurrence of loop anomalies as shown in figs. 1a and 1b can be explained in terms of the formation of a mobile compensation wall within the film interior. The sublattice spin state and net magnetization state of each sub-layer of two typical anomalous loops are illustrated in fig. 3. The magneto-static energy gradient, which is affecting the wall movement, is associated with the magnitude and direction of the net magnetization in each sub-layer, as represented by the open arrows in the figure. Here the length of the open arrows is proportional to the intrinsic magnetization which is varying within the film due to the composition gradient. Under the combined effects
271
of the exchange interactions of the sublattice spin in each sub-layer and the field-induced magneto-static energy gradient, the wall will move in the direction along the film thickness under various applied fields. As shown in the figure, the change in the Hall and Kerr loops can be explained consistently by the corresponding change in the sublattice spins. In conclusion, it appears that the anomalous hysteres i s loop can occur in a TbFe film only when a composit i o n gradient normal to the film plane is present. This composition variation provides the magneto-static energy gradient to drive the compensation wall through the film to cause the anomalous behavior. References [1] Tu Chen, D. Cheng and G.B. Charlan, IEEE Trans. Magn. MAG-16 (1980) 1194. [2] Tu Chen, R. Malmhilll and G.B. Charlan, J. Appl. Phys. 53 (1982) 2356. [3] R. Maiml'~ll and Tu Chen, J. Appl. Phys. 53 (1982) 7843. [41 S. Esho, Jpn. J. Appl. Phys. 15 (1976) 93. [5] O.S. Lutes, J.O. Holmen, R.L. Kooycr and O.S. Andland, IEEE Trans. Magn, MAG-13 (1977) 1615. [6] M. Amatsu, S. Honda and q'. Kusuda, IEEE Trans. Magn. MAG-13 (1977) 1612.