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Inverse wiedemann effect on magnetic ribbons for very low torsions
278
Journal
INVERSE WIEDEMANN M. TEJEDOR, Depnrtamento Received
J. GARCIA
EFFECT ON MAGNETIC *, B. HERNANDO
of Magnetism
and Magnetic
Materials 61(1986) 278-282 North-Holland. Amsterdam
RIBBONS FOR VERY LOW TORSIONS
and A. FERNANDEZ
de Fmica, Fact&ad de Ciencias, Uniuersidad de Oviedo, Spain
15 April 1986
Measurements of inverse Wiedemann effect for very low torsions ( cl.75 rad m-‘) have been performed in high magnetostrictive amorphous and polycrystafline magnetic ribbons, (Metglas 2826: Fe+,NiwPr4$ and Deltamax: Fe,,Ni,,), in order to study their behaviour as sensitive elements in magnetoelastic torque magnetometers.
1. Introduction
2. Experimental
A first study about the magnetic properties of magnetic ribbons under torsion was reported by Becker [l]. A more detailed work about the influence of torsion on the magnetic properties of an amorphous ribbon (Metglas 2826) is achieved by Barandiaran et al. [2]. They study the magnetization and the Matteucci and inverse Wiedemann effects for various degrees of torsion. All these studies are very interesting because the torsioned ribbon has been revealed as a new magnetic element with many applications, as has been pointed out by Dalpadado [3], who named it the “striptor” (torqued magnetic strip). The applications include sensitive torque magnetometers that have been developed by us [4,5]. For torque measurements it is necessary to make a careful study of the inverse Wiedemann effect (IWE) for very low torsions, question to which the above mentioned papers have not devoted special attention. This is the object of our work in which we study two different magnetoestrictive ribbons, one amorphous, the Metglas 2826 Fe,,Ni,P,,$ (Allied Chemical Corp.), and another polycrystalline, the oriented grain alloy Fe,,Ni,, named Deltamax (The Arnold Engineering Comp.).
For the measurements, the elements of a magnetoelastic torque magnetometer [5] have been used. As described in the previous paper the magnetic ribbon of about 6 cm is attached without tension to the balancing square of the magnetometer. To get the low torsions a small coil supplied with a variable small direct current is mounted in the sample holder, located between the polar pieces of an electromagnet. The interaction between the coil and the magnetic field of the electromagnet supplies the torques that are transmitted to the ribbon through a shaft attached to the magnetometer balancing square. A special design allows to inject the direct current to the coil without introducing spureous torques in the measuring system. The rotation angle of the ribbon is measured in the conventional way by means of a light beam reflected on a small mirror glued to the balancing square of the magnetometer. The longitudinal magnetization M, that appears in the torsioned ribbon when an alternating current is flowing throught it (IWE), is measured integrating the EMF that appears in a coil wound around the ribbon. Figs. 1, 2 and 3 show, in arbitrary units, the variation of M, for different degrees of low torsions in a ribbon of Metglas 2826 of about 1.25 mm by 47 pm cross section. Each curve of the
* Escuela
Superior
de la Marina
Civil, Gijbn,
Spain
0304-8853/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
procedure and results
M. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
279
Fig. 1. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 100 mA; 0 150 mA; a 200 mA; 0 250 mA, in a Metglas 2826 ribbon free of initial torsion.
Fig. 2. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0100 mA; 0 150 mA; A 200 mA; 0 250 mA, in a Metglas 2826 ribbon. The applied initial torsion was 1.75 rad m-t.
figures corresponds to a different values of the IWE exciting current. Each figure corresponds to a different degree of initial ribbon torsion. So that what we really measure in figs. 2 and 3 is an “incremental” longitudinal magnetization AM, obtained for small amounts of torsion when the ribbon is already previously torsioned. Figs. 4, 5 and 6 show the obtained results for analogous measurements of a Deltamax ribbon of about 3.15 mm by 50 pm cross section.
of about 1 rad m- *. The best linearity is obtained for the Deltamax ribbon, specially for an exciting intensity of 1.0 A as can be seen in figs. 5 and 6. The amorphous ribbon presents a more irregular behaviour. The linearity is not so perfect and in some cases the values of M, present sudden changes that clearly break the linearity, as can be observed in figs. 1 and 2, mainly in fig. 1 where one line of results (I = 150 mA) crosses over another. The sudden changes in magnetization is a known feature of these amorphous quenched materials. The magnetization curves of Metglas 2826 ribbons are not smooth and show a kind of giant Barkhausen jumps [6]. This behaviour is probably due to the irregular distribution of the anisotropy in these materials as the domain pat-
3. Discussion
and conclusions
As a general feature of the obtained results we can conclude that the magnetization M, varies almost linearly in a range of incremental torsion
M. Tejedor et al. / Inverse Wiedemann
effect on magnetic ribbons
AM,lau
I
I 1.50 5 lrad
+
m-l)
-120
t Fig. 3. Variation of M. with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 100 mA; 0 150 mA; A 200 mA; 0 250 mA, in a Metglas 2826 ribbon. The applied initial torsion was 3.50 rad m- ‘.
terns reveal [7]. These patterns indicate the presence of areas with strong perpendicular anisotropy, in which the magnetization is perpendicular to the ribbon surface. The magnetization jumps are probably caused by the change of magnetization from the perpendicular to the in plane direction of the ribbon, for certain critical values of the magnetic applied field. These sudden variations can also be reflected in a minor scale in the magnetization M, obtained by IWE for various degrees of torsion as some of the curves obtained show. The linearity of the effect, necessary to employ a ribbon as sensitive element in a magnetoelastic torque magnetometer, must be carefully checked in the case of quenched amorphous ribbons. and we must seek the best conditions of exciting cur-
t t Fig 4. Variation of Mz with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon free of initial torsion.
rent and initial torsion that assure in every case a linear behaviour. Another feature that can be deduced from the obtained results is that the slope of the curves always increases with the exciting current. Measurements of the IWE up to the maximum values of the current allowed by the sample have been
M. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
281
I
1.50
I
,
I
0.75
5 (rad m-1)
I
1.50 5 (rod
m-1)
-60
- -90
t Fig. 5. Variation of Mz with applied low torsions, for different values of the IWE exciting cur&t at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon. The applied initial torsion was 1.5 rad m-l.
made. The maximum value of these currents is limited by the heating of the ribbon by Joule effect. The heat generated can greatly alter the magnetic properties of the sample mainly in amorphous ribbons by recrystallization. This increase is easily understood because the longitudi-
Fig. 6. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon. The applied initial torsion was 3.0 rad m- ‘.
nal magnetization M, of the IWE is a projection of the helical magnetization obtained in a ribbon or wire twisted and circularly excited by a current flowing through it. The helical magnetization will be greater with increasing current and consequently its projection M,. This will occur until the
282
IU. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
saturation of the sample is reached, in which case no more changes can be observed. In our case it is clear that we have not reached the saturation, because we always observe an increase of A4,. So, we can conclude that it is impossible to saturate “circularly” this kind of samples by means of a current that flows through them, if we use low enough currents so that damages in the samples are not produced by heating. The samples are very soft magnetically, therefore the impossibility of total saturation is probably due to the shape of them that have rectangular section instead of circular, ideal for this type of magnetization. The rectangular geometry generates strong demagnetizing factors for circular magnetization, mainly at the edges of the strip that impede easy saturation. Comparing the graphics obtained for different initial ribbon torsions we can see that in the case of Deltamax the slope of the curves first increases with initial torsion angle, and later decreases, with a maximum for an initial torsion angle of 1.5 rad m -‘. For Metglas 2826 the slope always shows a slight decrease with initial torsion. It is difficult to explain this different behaviour, but we think that is related to the magnetization process in these materials. Deltamax is a soft policrystalline material of which magnetization process is mainly due to the wall displacement. In the case of amorphous Metglas 2826 the rotation plays an important role, mainly in the areas with perpendicular anisotropy [S]. This difference in the magnetization process can produce the different variation of the IWE with initial torsion for the two materials, although we do not know exactly how. Finally, we can comment that there are other factors that can affect the IWE in the ribbons employed for torque measurements. These could be the frequency of the exciting current and the applied tension to the ribbon. It is known that increasing frequency decreases the IWE for low torsions [2]. The decrease is not very large and at frequencies of the order of 50 Hz it is practically null. So it is advisable to use this easily available frequency.
Fig. 7. Influence of tension on the IWE at 50 Hz for a exciting current of 100 mA, in a Metglas 2826 ribbon.
The tension also diminishes the IWE as it is seen in fig. 7, which shows the obtained results for a Metglas 2826 ribbon, Deltamax exhibits a similar behaviour. The reason of this behaviour seems to be clear. Due to the positive magnetoestriction of the materials the tension produces an easy axis along the ribbon that opposes to the circular magnetization which is the origin of the IWE, because this magnetization is perpendicular to the axis. Magnetoelastic torque magnetometers are designed taking into account this characteristic, avoiding tension on the sensitive ribbon [5].
References Ill J.J. Becker, IEEE Trans. Magn. MAG-11 (1975) 1326. I21 J.M. BarandiarBn, A. Hernando and E. Ascasibar, J. Phys. D 12 (1979) 1943. 131 R.N.G. Dalpadado, IEEE Trans. Magn. MAG 19 (1983) 2027. 141 M. Tejedor, J. Garcia and B. Hernando, J. Phys. E 17 (1984) 869. I51 M. Tejedor, J. Garcia and B. Hernando, J. Phys. E 18 (1985) 265. I61 T. Egami, P. Flanders and C.D. Graham, AIP Conf. Proc. 24 (1975) 697. 171 M. Tejedor and B. Hernando, J. Phys. D 13 (1980) 1709. 181 M. Tejedor and B. Hemando, J. Phys. D 16 (1983) L85.
Journal
INVERSE WIEDEMANN M. TEJEDOR, Depnrtamento Received
J. GARCIA
EFFECT ON MAGNETIC *, B. HERNANDO
of Magnetism
and Magnetic
Materials 61(1986) 278-282 North-Holland. Amsterdam
RIBBONS FOR VERY LOW TORSIONS
and A. FERNANDEZ
de Fmica, Fact&ad de Ciencias, Uniuersidad de Oviedo, Spain
15 April 1986
Measurements of inverse Wiedemann effect for very low torsions ( cl.75 rad m-‘) have been performed in high magnetostrictive amorphous and polycrystafline magnetic ribbons, (Metglas 2826: Fe+,NiwPr4$ and Deltamax: Fe,,Ni,,), in order to study their behaviour as sensitive elements in magnetoelastic torque magnetometers.
1. Introduction
2. Experimental
A first study about the magnetic properties of magnetic ribbons under torsion was reported by Becker [l]. A more detailed work about the influence of torsion on the magnetic properties of an amorphous ribbon (Metglas 2826) is achieved by Barandiaran et al. [2]. They study the magnetization and the Matteucci and inverse Wiedemann effects for various degrees of torsion. All these studies are very interesting because the torsioned ribbon has been revealed as a new magnetic element with many applications, as has been pointed out by Dalpadado [3], who named it the “striptor” (torqued magnetic strip). The applications include sensitive torque magnetometers that have been developed by us [4,5]. For torque measurements it is necessary to make a careful study of the inverse Wiedemann effect (IWE) for very low torsions, question to which the above mentioned papers have not devoted special attention. This is the object of our work in which we study two different magnetoestrictive ribbons, one amorphous, the Metglas 2826 Fe,,Ni,P,,$ (Allied Chemical Corp.), and another polycrystalline, the oriented grain alloy Fe,,Ni,, named Deltamax (The Arnold Engineering Comp.).
For the measurements, the elements of a magnetoelastic torque magnetometer [5] have been used. As described in the previous paper the magnetic ribbon of about 6 cm is attached without tension to the balancing square of the magnetometer. To get the low torsions a small coil supplied with a variable small direct current is mounted in the sample holder, located between the polar pieces of an electromagnet. The interaction between the coil and the magnetic field of the electromagnet supplies the torques that are transmitted to the ribbon through a shaft attached to the magnetometer balancing square. A special design allows to inject the direct current to the coil without introducing spureous torques in the measuring system. The rotation angle of the ribbon is measured in the conventional way by means of a light beam reflected on a small mirror glued to the balancing square of the magnetometer. The longitudinal magnetization M, that appears in the torsioned ribbon when an alternating current is flowing throught it (IWE), is measured integrating the EMF that appears in a coil wound around the ribbon. Figs. 1, 2 and 3 show, in arbitrary units, the variation of M, for different degrees of low torsions in a ribbon of Metglas 2826 of about 1.25 mm by 47 pm cross section. Each curve of the
* Escuela
Superior
de la Marina
Civil, Gijbn,
Spain
0304-8853/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
procedure and results
M. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
279
Fig. 1. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 100 mA; 0 150 mA; a 200 mA; 0 250 mA, in a Metglas 2826 ribbon free of initial torsion.
Fig. 2. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0100 mA; 0 150 mA; A 200 mA; 0 250 mA, in a Metglas 2826 ribbon. The applied initial torsion was 1.75 rad m-t.
figures corresponds to a different values of the IWE exciting current. Each figure corresponds to a different degree of initial ribbon torsion. So that what we really measure in figs. 2 and 3 is an “incremental” longitudinal magnetization AM, obtained for small amounts of torsion when the ribbon is already previously torsioned. Figs. 4, 5 and 6 show the obtained results for analogous measurements of a Deltamax ribbon of about 3.15 mm by 50 pm cross section.
of about 1 rad m- *. The best linearity is obtained for the Deltamax ribbon, specially for an exciting intensity of 1.0 A as can be seen in figs. 5 and 6. The amorphous ribbon presents a more irregular behaviour. The linearity is not so perfect and in some cases the values of M, present sudden changes that clearly break the linearity, as can be observed in figs. 1 and 2, mainly in fig. 1 where one line of results (I = 150 mA) crosses over another. The sudden changes in magnetization is a known feature of these amorphous quenched materials. The magnetization curves of Metglas 2826 ribbons are not smooth and show a kind of giant Barkhausen jumps [6]. This behaviour is probably due to the irregular distribution of the anisotropy in these materials as the domain pat-
3. Discussion
and conclusions
As a general feature of the obtained results we can conclude that the magnetization M, varies almost linearly in a range of incremental torsion
M. Tejedor et al. / Inverse Wiedemann
effect on magnetic ribbons
AM,lau
I
I 1.50 5 lrad
+
m-l)
-120
t Fig. 3. Variation of M. with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 100 mA; 0 150 mA; A 200 mA; 0 250 mA, in a Metglas 2826 ribbon. The applied initial torsion was 3.50 rad m- ‘.
terns reveal [7]. These patterns indicate the presence of areas with strong perpendicular anisotropy, in which the magnetization is perpendicular to the ribbon surface. The magnetization jumps are probably caused by the change of magnetization from the perpendicular to the in plane direction of the ribbon, for certain critical values of the magnetic applied field. These sudden variations can also be reflected in a minor scale in the magnetization M, obtained by IWE for various degrees of torsion as some of the curves obtained show. The linearity of the effect, necessary to employ a ribbon as sensitive element in a magnetoelastic torque magnetometer, must be carefully checked in the case of quenched amorphous ribbons. and we must seek the best conditions of exciting cur-
t t Fig 4. Variation of Mz with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon free of initial torsion.
rent and initial torsion that assure in every case a linear behaviour. Another feature that can be deduced from the obtained results is that the slope of the curves always increases with the exciting current. Measurements of the IWE up to the maximum values of the current allowed by the sample have been
M. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
281
I
1.50
I
,
I
0.75
5 (rad m-1)
I
1.50 5 (rod
m-1)
-60
- -90
t Fig. 5. Variation of Mz with applied low torsions, for different values of the IWE exciting cur&t at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon. The applied initial torsion was 1.5 rad m-l.
made. The maximum value of these currents is limited by the heating of the ribbon by Joule effect. The heat generated can greatly alter the magnetic properties of the sample mainly in amorphous ribbons by recrystallization. This increase is easily understood because the longitudi-
Fig. 6. Variation of M, with applied low torsions, for different values of the IWE exciting current at 50 Hz: 0 0.5 A; 0 1.0 A; A 1.5 A; 0 2.0 A, in a Deltamax ribbon. The applied initial torsion was 3.0 rad m- ‘.
nal magnetization M, of the IWE is a projection of the helical magnetization obtained in a ribbon or wire twisted and circularly excited by a current flowing through it. The helical magnetization will be greater with increasing current and consequently its projection M,. This will occur until the
282
IU. Tejedor et al. / Inverse Wiedemann effect on magnetic ribbons
saturation of the sample is reached, in which case no more changes can be observed. In our case it is clear that we have not reached the saturation, because we always observe an increase of A4,. So, we can conclude that it is impossible to saturate “circularly” this kind of samples by means of a current that flows through them, if we use low enough currents so that damages in the samples are not produced by heating. The samples are very soft magnetically, therefore the impossibility of total saturation is probably due to the shape of them that have rectangular section instead of circular, ideal for this type of magnetization. The rectangular geometry generates strong demagnetizing factors for circular magnetization, mainly at the edges of the strip that impede easy saturation. Comparing the graphics obtained for different initial ribbon torsions we can see that in the case of Deltamax the slope of the curves first increases with initial torsion angle, and later decreases, with a maximum for an initial torsion angle of 1.5 rad m -‘. For Metglas 2826 the slope always shows a slight decrease with initial torsion. It is difficult to explain this different behaviour, but we think that is related to the magnetization process in these materials. Deltamax is a soft policrystalline material of which magnetization process is mainly due to the wall displacement. In the case of amorphous Metglas 2826 the rotation plays an important role, mainly in the areas with perpendicular anisotropy [S]. This difference in the magnetization process can produce the different variation of the IWE with initial torsion for the two materials, although we do not know exactly how. Finally, we can comment that there are other factors that can affect the IWE in the ribbons employed for torque measurements. These could be the frequency of the exciting current and the applied tension to the ribbon. It is known that increasing frequency decreases the IWE for low torsions [2]. The decrease is not very large and at frequencies of the order of 50 Hz it is practically null. So it is advisable to use this easily available frequency.
Fig. 7. Influence of tension on the IWE at 50 Hz for a exciting current of 100 mA, in a Metglas 2826 ribbon.
The tension also diminishes the IWE as it is seen in fig. 7, which shows the obtained results for a Metglas 2826 ribbon, Deltamax exhibits a similar behaviour. The reason of this behaviour seems to be clear. Due to the positive magnetoestriction of the materials the tension produces an easy axis along the ribbon that opposes to the circular magnetization which is the origin of the IWE, because this magnetization is perpendicular to the axis. Magnetoelastic torque magnetometers are designed taking into account this characteristic, avoiding tension on the sensitive ribbon [5].
References Ill J.J. Becker, IEEE Trans. Magn. MAG-11 (1975) 1326. I21 J.M. BarandiarBn, A. Hernando and E. Ascasibar, J. Phys. D 12 (1979) 1943. 131 R.N.G. Dalpadado, IEEE Trans. Magn. MAG 19 (1983) 2027. 141 M. Tejedor, J. Garcia and B. Hernando, J. Phys. E 17 (1984) 869. I51 M. Tejedor, J. Garcia and B. Hernando, J. Phys. E 18 (1985) 265. I61 T. Egami, P. Flanders and C.D. Graham, AIP Conf. Proc. 24 (1975) 697. 171 M. Tejedor and B. Hernando, J. Phys. D 13 (1980) 1709. 181 M. Tejedor and B. Hemando, J. Phys. D 16 (1983) L85.