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Effects of application of axial fields on asymmetric magnetization reversal
Journal of Magnetism and Magnetic Materials 177-181 (1998) 225-226
ELSEVIER
~
Journal of mnalneusm magnetic , i ~ materials
Effects of application of axial fields on asymmetric magnetization reversal K.H. Shin a'*, C.D. Graham Jr. b, P.Y.
Zhou c
a Thin Film Technology Research Center, K1ST, P.O. Box 131, Cheongryang, Seoul, South Korea bDepartment of Material Sciences and Engineering, University o f Pennsylvania, Philadelphia, PA 19104, USA cSentry Technology Corp., Hauppauge NY 11788, USA
Abstract We observed that asymmetric magnetization reversal (AMR) can be destroyed partially or totally by magnetizing the sample in a sufficiently high magnetic field. The single AMR is not altered for applied fields below + 3 Oe. For applied fields between + 3 and + 250 Oe, the single AMR is increasingly altered by higher fields, but can be restored close to the original state. For fields greater than _ 300 Oe, the single AMR is destroyed and cannot be restored to its original state by applying a field at room temperature. The experimental results can be explained by means of a metastable domain structure characterized by the induced anisotropy and the exchange anisotropy. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous systems
soft magnetic; Anisotropy induced; Domain wall pinning; Magnetization - asymmetrical
I. Introduction Asymmetric magnetization reversal (AMR) develops when amorphous ferromagnetic ribbons are annealed in a carefully controlled magnetic field [1]. A material having this phenomenon is much superior to any other soft magnetic materials as a security tag because it produces a magnetization reversal signal that is unique. Searching for a way to deactivate or erase AMR, we observed that AMR can be destroyed partially or totally by magnetizing the sample in a sufficiently high magnetic field.
a sample would inevitably be at least partially destroyed after the test. About 20 samples were prepared by annealing Covo.sFea.sSi10B15 amorphous ribbons for 8 h at 380°C in a magnetic field of 100 mOe. A clean single AMR was produced in most of the samples, with jump fields from 250 to 320 mOe. The jump field is the negative field at which a magnetization jump occurs after annealing in a positive field. A pair of Helmholtz coils was used for providing a field up to 7 Oe; a solenoid coil for fields from 10 to 1000 Oe; and water-cooled electromagnet for fields from 1000 to 5000 Oe. The magnitude of the jump field Hj was taken as a measure of the AMR effect.
2. Experimental procedures
3. Results and discussion
For the purpose of investigating the effects of application of axial magnetic fields on AMR, several samples having similar properties must be prepared because
For applied fields below _+ 3 0 e , little change in the jump field Hj was observed. However, Hj changed by applying a static magnetic field as low as 5 0 e . There seem to be two different types of samples which behaved in different ways when exposed to a field up to 7 0 e . The value of Hj in a sample designated type A increased by 10% to 15% after the application of a field of 7 0 e in the negative direction (opposite to the direction during the
*Corresponding author. Tel.: + 822958 5418, fax: + 822 958 5409, e-mail: [email protected].
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved P l l S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 9 1 5-3
226
K.H. Shin et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 225-226
anneal). The original (as-annealed) value was regained by applying a field of 7 0 e in the positive direction (parallel to the direction during the anneal). On the other hand, Hj of a sample designated type B decreased about 15% by applying a field of 7 0 e in the negative direction and returned to the original value or a bit higher by subsequent application of a field of 7 0 e in the positive direction. The jump field Hj measured from a maximum field of 1000 mOe decreased dramatically when a field up to 100 Oe in the negative direction applied, while only a minor change in Hj can be observed when the applied field reaches 200 Oe or higher in the positive direction (Fig. 1). A Barkhausen jump a t Hi, negative during the magnetic field reversal from - to ÷ showed up at an applied field of 100 Oe or higher in the negative direction. Note that - 100 Oe was the field up to which Hj decreased drastically. A hysteresis loop partially destroyed due to the application of a field up to 200 Oe in the negative direction could be returned to nearly its original (as-annealed) AMR shape by applying a positive field of a similar magnitude to the negative field that caused the destruction. However, the single A M R faded out permanently by applying a field of 400 Oe or higher in either direction. By applying a positive field ranging from 225-300 Oe, a clean single AMR effect could be regained, but was unstable. Hj kept changing with the maximum measuring field. Fig. 2 displays this instability with the maximum measuring field when a single AMR destroyed by the application of a negative field was restored by the subsequent application of a positive field of 225 to 300 Oe. As shown in Fig. 2, Hj changed more readily when a higher field applied, and Hj was reduced suddenly or rapidly when the maximum positive field during measuring exceeded a certain value. Fig. 2d demonstrates that, after applying a positive field of 400 Oe, Hj was small and did not change with the maximum measuring field. Extended experiments with the sample of Fig. 2d resulted in a range of values of Hj in a maximum measuring field of 1000 mOe. Hj was about 50 mOe in most measurements, but had quantized values of 80, 165, 250 and 310 mOe, apparently at random. The experimental results strongly support our theory on AMR [1] as follows: The annealing treatments create a metastable domain structure, which can be destroyed by the application of a sufficiently high field at room temperature. A metastable domain structure is characterized by the anisotropy induced during annealing (the
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-200 -400
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Hmax,external (Oe)
Fig. 1. Variation of Hj after application of external magnetic field up to ___400 Oe.
=:iii°Io " ,oo
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.
.
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.
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Fig. 2. Variation of Hj with the maximum measuring field after the application of a positive field of (a) 225, (b) 250, (c) 300 and (d) 400 Oe following the application of a negative field of (a) 225, (b) 250, (c) 300 and (d) 400 Oe.
induced anisotropy) and the anisotropy formed during cooling in a magnetic field (the exchange anisotropy). The Barkhausen jump at Hj in a sample showing an AMR results from the nucleation and propagation of the reverse domain whose activation energy for overcoming the induced anisotropy combined with the exchange anisotropy is smallest. The origin of the alteration of AMR lies in an alteration of the direction and magnitude of the anisotropies created during annealing and/or cooling due to the application of the external field.
References
[1] K.H. Shin, C.D. Graham Jr., P.Y. Zhou, IEEE Trans. Magn. 28 (1992) 2772.
ELSEVIER
~
Journal of mnalneusm magnetic , i ~ materials
Effects of application of axial fields on asymmetric magnetization reversal K.H. Shin a'*, C.D. Graham Jr. b, P.Y.
Zhou c
a Thin Film Technology Research Center, K1ST, P.O. Box 131, Cheongryang, Seoul, South Korea bDepartment of Material Sciences and Engineering, University o f Pennsylvania, Philadelphia, PA 19104, USA cSentry Technology Corp., Hauppauge NY 11788, USA
Abstract We observed that asymmetric magnetization reversal (AMR) can be destroyed partially or totally by magnetizing the sample in a sufficiently high magnetic field. The single AMR is not altered for applied fields below + 3 Oe. For applied fields between + 3 and + 250 Oe, the single AMR is increasingly altered by higher fields, but can be restored close to the original state. For fields greater than _ 300 Oe, the single AMR is destroyed and cannot be restored to its original state by applying a field at room temperature. The experimental results can be explained by means of a metastable domain structure characterized by the induced anisotropy and the exchange anisotropy. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous systems
soft magnetic; Anisotropy induced; Domain wall pinning; Magnetization - asymmetrical
I. Introduction Asymmetric magnetization reversal (AMR) develops when amorphous ferromagnetic ribbons are annealed in a carefully controlled magnetic field [1]. A material having this phenomenon is much superior to any other soft magnetic materials as a security tag because it produces a magnetization reversal signal that is unique. Searching for a way to deactivate or erase AMR, we observed that AMR can be destroyed partially or totally by magnetizing the sample in a sufficiently high magnetic field.
a sample would inevitably be at least partially destroyed after the test. About 20 samples were prepared by annealing Covo.sFea.sSi10B15 amorphous ribbons for 8 h at 380°C in a magnetic field of 100 mOe. A clean single AMR was produced in most of the samples, with jump fields from 250 to 320 mOe. The jump field is the negative field at which a magnetization jump occurs after annealing in a positive field. A pair of Helmholtz coils was used for providing a field up to 7 Oe; a solenoid coil for fields from 10 to 1000 Oe; and water-cooled electromagnet for fields from 1000 to 5000 Oe. The magnitude of the jump field Hj was taken as a measure of the AMR effect.
2. Experimental procedures
3. Results and discussion
For the purpose of investigating the effects of application of axial magnetic fields on AMR, several samples having similar properties must be prepared because
For applied fields below _+ 3 0 e , little change in the jump field Hj was observed. However, Hj changed by applying a static magnetic field as low as 5 0 e . There seem to be two different types of samples which behaved in different ways when exposed to a field up to 7 0 e . The value of Hj in a sample designated type A increased by 10% to 15% after the application of a field of 7 0 e in the negative direction (opposite to the direction during the
*Corresponding author. Tel.: + 822958 5418, fax: + 822 958 5409, e-mail: [email protected].
0304-8853/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved P l l S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 9 1 5-3
226
K.H. Shin et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 225-226
anneal). The original (as-annealed) value was regained by applying a field of 7 0 e in the positive direction (parallel to the direction during the anneal). On the other hand, Hj of a sample designated type B decreased about 15% by applying a field of 7 0 e in the negative direction and returned to the original value or a bit higher by subsequent application of a field of 7 0 e in the positive direction. The jump field Hj measured from a maximum field of 1000 mOe decreased dramatically when a field up to 100 Oe in the negative direction applied, while only a minor change in Hj can be observed when the applied field reaches 200 Oe or higher in the positive direction (Fig. 1). A Barkhausen jump a t Hi, negative during the magnetic field reversal from - to ÷ showed up at an applied field of 100 Oe or higher in the negative direction. Note that - 100 Oe was the field up to which Hj decreased drastically. A hysteresis loop partially destroyed due to the application of a field up to 200 Oe in the negative direction could be returned to nearly its original (as-annealed) AMR shape by applying a positive field of a similar magnitude to the negative field that caused the destruction. However, the single A M R faded out permanently by applying a field of 400 Oe or higher in either direction. By applying a positive field ranging from 225-300 Oe, a clean single AMR effect could be regained, but was unstable. Hj kept changing with the maximum measuring field. Fig. 2 displays this instability with the maximum measuring field when a single AMR destroyed by the application of a negative field was restored by the subsequent application of a positive field of 225 to 300 Oe. As shown in Fig. 2, Hj changed more readily when a higher field applied, and Hj was reduced suddenly or rapidly when the maximum positive field during measuring exceeded a certain value. Fig. 2d demonstrates that, after applying a positive field of 400 Oe, Hj was small and did not change with the maximum measuring field. Extended experiments with the sample of Fig. 2d resulted in a range of values of Hj in a maximum measuring field of 1000 mOe. Hj was about 50 mOe in most measurements, but had quantized values of 80, 165, 250 and 310 mOe, apparently at random. The experimental results strongly support our theory on AMR [1] as follows: The annealing treatments create a metastable domain structure, which can be destroyed by the application of a sufficiently high field at room temperature. A metastable domain structure is characterized by the anisotropy induced during annealing (the
4oo
300
~
2OO
"
.. mMH.I
~ ~o
it
-lOO
-200 -400
.H
,.1..1"
' 300
200
100
I
0
100
200
SO0
400
Hmax,external (Oe)
Fig. 1. Variation of Hj after application of external magnetic field up to ___400 Oe.
=:iii°Io " ,oo
!iii,';f'i' It'";....
.........
°°
; ......
o
,°, o
,,, 5o0
(a)
,
, , looo
. . . . . . . 15oo
'
°°[ o-,*, o
2o0°
Hmax,measuring (mO~)
.
.
(b)
~oo
. . 500
.
.
. . 1ooo
.
.
. . ,5oo
°
" 2o0o
Hmax,measurin8 (mOe)
o ,so
= 1oo
0 (c)
500 1000 1500 Hm~,measuring (mOe)
o
2000
,,
0 (d)
,
. . . .
,,,
. . . . . . .
500 1000 1500 Hm~.measuring (mOe)
200
Fig. 2. Variation of Hj with the maximum measuring field after the application of a positive field of (a) 225, (b) 250, (c) 300 and (d) 400 Oe following the application of a negative field of (a) 225, (b) 250, (c) 300 and (d) 400 Oe.
induced anisotropy) and the anisotropy formed during cooling in a magnetic field (the exchange anisotropy). The Barkhausen jump at Hj in a sample showing an AMR results from the nucleation and propagation of the reverse domain whose activation energy for overcoming the induced anisotropy combined with the exchange anisotropy is smallest. The origin of the alteration of AMR lies in an alteration of the direction and magnitude of the anisotropies created during annealing and/or cooling due to the application of the external field.
References
[1] K.H. Shin, C.D. Graham Jr., P.Y. Zhou, IEEE Trans. Magn. 28 (1992) 2772.