IEEE TRANSACTIONS ON MAGNETICS. VOL 31. NO 2, MARCH 1995 1261 INFLUENCE OF MAGNETOSTRICTION ON MAGNETO-IMPEDANCE IN AMORPHOUS SOFT FERROMAGNETIC WIRES J. L. Costa- Kramer and K. V. Rao Dept. of Condensed Matter Physics, Royal Institute of Technology, Stockholm, Sweden Abstract-Very large changes are observed in the impedance of a (CogqFeg)72,5Si12.5B15 soft ferromagnetic wire in the presence of axial fields of the order of 1 Oe. Changes of the impedance of 200% or more have been reported at axial field values less than 1 Oe in soft nearly zero magnetostrictive Co-based amorphous wires with induced circumferential anisotropies. In this work the effect of different small saturation magnetostriction constants and induced circumferential anisotropies on the magneto- impedance for various annealed wires is studied. During current annealing a circumferential anisotropy is induced. Also, the saturation magnetostriction constant changes continuously from a small negative (-7x10-8) to a small positive value (+6x10-8) depending on the current amplitude. The impedance as a function of current, frequency and dc axial magnetic field is measured for these wires. All of the wires exhibit a sharp threshold of the impedance when subjected to axial fields less than 1 Oe. Changes in the impedance, AVN=(V(H)-V(O))N(O), of the order of 200% with maximum slopes reaching up to 1700 %/Oe are measured in this series. The role of the saturation magnetostriction constant and the induced circumferential anisotropy in the observed magneto-impedance is discussed. IJNTRODUCTION Amorphous ferromagnetic wires made by the in-rotating water method [ 11 were a late-comer among amorphous magnetic metals, being now available for about a decade [2]. The special mechanical properties of the amorphous wires [3], better than those of ribbons, were expected; however, quite unique magnetic characteristics [4]- [6], like for example: re-entrant reversal with a perfect square loop at very low exciting fields, soft magnetic properties, possibilities to tailor the overall anisotropy etc., were soon discovered. The special mechanical and magnetic properties of these wires can be understood in terms of the symmetry of the quenching process, which creates an amorphous metallic material with a circular cross section and an unusual residual stress distribution [7]. In the as-quenched state the wire diameter is about 125 pm and can be cold-drawn [8] without any intermediate annealing down to lOpm -a feature which is very useful in developing new types of smart sensors. From the magnetic point of view it is amazing to note how just a change of geometry can initiate a whole new area for applications of amorphous materials. Amorphous wires provide us with long lengths of good quality soft ferromagnetic metals, something not easily available before [9]. One of the early recognized advantages of the amorphous wires was the ability to excite longitudinal magnetization processes with longitudinal currents [ 101 flowing through the wire, which created a well defined circumferential magnetic field. It was later realized [ 111 that the ac current driven changes in circumferential flux produced an inductive component of the voltage between the ends of the wire which was very sensitive to the axial fields, a phenomenon termed magneto-inductance/magneto- impedance or the so-called MI effect. This inductive component of the voltage between the wire ends is the one that integrated provides us with the circumferential magnetic flux [ 12,111. For the as-quenched wires, the ratio of the magneto- resistive to the magneto-inductive changes was found to be very sensitive to the value and sign of the magnetostriction and to the current frequency [13]. The resistive part is frequency independent while the inductive part increased approximately linearly with the frequency of the excitation [14]. Recently, we have reported [15] the effect of 0018-9464195$4.00 G 1995 IEEE
Transcript
IEEE TRANSACTIONS ON MAGNETICS. VOL 31. NO 2 , MARCH 1995 1261
INFLUENCE OF MAGNETOSTRICTION ON MAGNETO-IMPEDANCE IN AMORPHOUS SOFT FERROMAGNETIC WIRES
J. L. Costa- Kramer and K. V. Rao Dept. of Condensed Matter Physics, Royal Institute of Technology, Stockholm, Sweden
Abstract-Very large changes are observed in the impedance of a (CogqFeg)72,5Si12.5B15 soft ferromagnetic wire in the presence of axial fields of the order of 1 Oe. Changes of the impedance of 200% or more have been reported at axial field values less than 1 Oe in soft nearly zero magnetostrictive Co-based amorphous wires with induced circumferential anisotropies. In this work the effect of different small saturation magnetostriction constants and induced circumferential anisotropies on the magneto- impedance for various annealed wires is studied. During current annealing a circumferential anisotropy is induced. Also, the saturation magnetostriction constant changes continuously from a small negative (-7x10-8) to a small positive value (+6x10-8) depending on the current amplitude. The impedance as a function of current, frequency and dc axial magnetic field is measured for these wires. All of the wires exhibit a sharp threshold of the impedance when subjected to axial fields less than 1 Oe. Changes in the impedance, AVN=(V(H)-V(O))N(O), of the order of 200% with maximum slopes reaching up to 1700 %/Oe are measured in this series. The role of the saturation magnetostriction constant and the induced circumferential anisotropy in the observed magneto-impedance is discussed.
IJNTRODUCTION
Amorphous ferromagnetic wires made by the in-rotating water method [ 11 were a late-comer among amorphous magnetic metals, being now available for about a decade [2]. The special mechanical properties of the amorphous wires [3], better than those of ribbons, were expected; however, quite unique magnetic characteristics [4]- [6], like for example: re-entrant reversal with a
perfect square loop at very low exciting fields, soft magnetic properties, possibilities to tailor the overall anisotropy etc., were soon discovered. The special mechanical and magnetic properties of these wires can be understood in terms of the symmetry of the quenching process, which creates an amorphous metallic material with a circular cross section and an unusual residual stress distribution [7]. In the as-quenched state the wire diameter is about 125 pm and can be cold-drawn [8] without any intermediate annealing down to lOpm -a feature which is very useful in developing new types of smart sensors. From the magnetic point of view it is amazing to note how just a change of geometry can initiate a whole new area for applications of amorphous materials. Amorphous wires provide us with long lengths of good quality soft ferromagnetic metals, something not easily available before [9]. One of the early recognized advantages of the amorphous wires was the ability to excite longitudinal magnetization processes with longitudinal currents [ 101 flowing through the wire, which created a well defined circumferential magnetic field. It was later realized [ 111 that the ac current driven changes in circumferential flux produced an inductive component of the voltage between the ends of the wire which was very sensitive to the axial fields, a phenomenon termed magneto-inductance/magneto- impedance or the so-called MI effect. This inductive component of the voltage between the wire ends is the one that integrated provides us with the circumferential magnetic flux [ 12,111. For the as-quenched wires, the ratio of the magneto- resistive to the magneto-inductive changes was found to be very sensitive to the value and sign of the magnetostriction and to the current frequency [13]. The resistive part is frequency independent while the inductive part increased approximately linearly with the frequency of the excitation [14]. Recently, we have reported [15] the effect of
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different induced anisotropies (axial, circumferential and helical) in the magneto- impedance of nearly zero magnetostrictive wires, concluding that a maximum MI efSect is observed in wires with circumferential anisotropy. In this work we study the effect of current amplitude, frequency and induced circumferential anisotropy and saturation magnetostriction constant on the
per mA at the surface, decreasing linearly in amplitude to zero at the center of the wire. An FFT spectrum analyzer, model SR760 from Stanford Research Systems, was used to measure the different spectral components of the magneto- impedance. In this work we present the amplitude of the first harmonic of the voltage as a function of the axial field for the above mentioned wires.
magneto-impedance of soft ferromagnetic amorphous wires. III.RESULTS AND DISCUSSION
1I.EXPERIMENTAL
Amorphous wires with a nominal composition (Co,,Fe,),,,,Si,,,B,,, 125 pm diameter, and 45cm long, have been current annealed using different current amplitudes (290-350 mA rms for 30 min.). During current annealing a circumferential magnetic anisotropy develops [ 161. The axial hysteresis loops of the wires are meas- ured with a conventional home-made induction technique. The experimental system uses a primary coil 27 cm long, 3 cm diameter, with a field to current constant of 22.9 Oe/A. A 31 Hz sinusoidal current is fed to the primary, the voltage across a resistor connected in series with the primary coil is fed to the X-channel of an oscilloscope. The voltage induced in a lo00 turn 0.8 cm long secondary, properly compensated from the applied field induced voltage, is integrated and displayed in the Y-channel of the oscilloscope where the longitudinal hysteresis loops are monitored. The saturation magnetostriction constant has been measured using the SAMR method [ 171 in the same coil, using a lo00 Hz current passing through the wire and a Lock-in amplifier model PAR 5209. The displayed value, &(o=O), is obtained by fitting the HaXial VS. 0lV2,=constant dependence to a second degree polynomial, extracting the first order coefficient. We estimated an error less than 10-8. The I-V characteristics of the wires were studied passing an ac current through the wire and a resistor connected in series, using an oscilloscope to visualize the voltage between the amorphous wire ends against the voltage across the resistor. The current through the wire creates a
In Fig. 1 the saturation magnetostriction constant, measured with the SAMR method, of the wires annealed at different current amplitudes for 30 min. is shown. The value of the saturation magnetostriction constant of the as-quenched wire is about -7x10-'. As observed, the saturation magnetostriction constant changes continuously from a small negative value to a small positive value, crossing the "zero" magnetostriction line at a current annealing amplitude of about 320 mA rms. This wire, anpealed at 320 mA, displays a value of h, <= 10-9, and stress independent magnetic properties for stress values less than 100 MPa. The wires with positive saturation magnetostriction constant, AS, display a change of sign of hs as a function of the longitudinal stress [18,19], a well known phenomenon only observable for extremely small magnetostriction. This is in agreement with
Fig. 1 Dependence of the saturation magnetostriction constant (SAMR method) of (Cog@e&2.5Si 12.5Bl5 amorphous wires on the current amplitude during a 30 min
circumferential magnetic field of about 0.03 1 Oe current annealing.
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our previous studies in positive and small magnetostriction amorphous ribbons and wires.
The axial hysteresis loops of the current annealed wires exhibit a shape typical of a magnetic field applied in a hard axis direction (Max vs. Ha, linear with M/M,=O) with an axial anisotropy field ranging from 0.7 to 1.3 Oe depending on the current amplitude.
The I-V characteristics of all the wires, at zero applied field, and different current frequencies through the wire, are qualitatively similar, displaying an inductive peak [ 12,111 at the current value that creates a circumferential field that sets the magnetization oscillation in the circumferential direction, Fig.2. Below that current value, the I-V characteristic is linear, demonstrating a purely resistive behaviour. This points to the existence of a critical circumferential switching field below which, there is no significant circumferential flux change and accordingly, no axial inductive voltage. The position of the inductive peak displaces to higher current values as the frequency of the current is increased, an instance of the increase in circumferential coercive field as the frequency of the excitation increases. The position of this inductive peak is different for different current annealed wires, demonstrating a different induced circumferential anisotropy. In Fig.2 we mark the position of the main features of the inductive peak, at lkHz current frequency, i.e., the onset of the circumferential flux change l&,onset, the maximum of the peak, related to the circumferential coercive field &,c, and the completion of the circumferential flux change related to the circumferential anisotropy field, Ha,K. The inductive component of the voltage eventually disappears with an applied axial field, leaving a purely resistive behaviour which is frequency independent. The circumferential anisotropy field at the surface of the wire, Ha,K, as a function of the annealing current, is displayed in Fig.3. A continuous increase of Ha,K is observed, illustrating an increase of the induced circumferential anisotropy for higher
Fig.2 I-V characteristics at three different frequencies for a current annealed (Co,,Fe$,,~i,~,,$,, wire. Note the position of Ha,onset , Ha,, and H0,K
currents during current annealing. This is physically expected since, higher current amplitudes produce both higher Joule heating and a higher circumferential field during the annealing.
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I I*
/ /
/ /
/ / * /
I /
*I
,
071 280 { 290 MO 310 320 330 340 350 360
mA rms
Fig3 Circumferential anisotropy field as a function of the current amplitude during annealing for (Cog,FeJ,,$i12,,B15 amorphous wires.
The magneto-impedance behaviour for all the wires subject of this study at 90 kHz current frequency, 10 mA peak amplitude, is shown in Fig.4 together with a schematic of the physical situation. All the wires exhibit a sharp increase of the first harmonic voltage amplitude, reaching a maximum total change between 100 and 200% at axial fields below 1 Oe. The field at which this threshold occurs is a continuously increasing function of the current amplitude during the annealing. This demonstrates again a smaller circumferential anisotropy for the wires annealed at low current values and consequently a smaller dc axial field required to overcome the induced anisotropy to a point in which the ac circumferential field manages to start the switching in the circumferential direction. The maximum slope at the threshold changes from 570 %/Oe at about 0.7 Oe for the wire annealed with 350 mA rms, to 1740%/0e at about 0.3 Oe for the wire annealed at 290 mA rms. The maximum total change, about 220%, is observed for the wire annealed with 300 mA which exhibits a small negative magnetostriction. This may be caused by a magneto elastic anisotropy that compensates the effect of the demagnetizing field, producing a maximum differential susceptibility. It is yet unclear if a totally stress free material is produced by the current annealing procedure.
Amorphous soft ferromagnetic wire i - 2
r
0.00 0.1
I
H (oe) 1
Fig.4 Dependence of the impedance on the axial de magnetic field for (Cog~eJ72,$i12,,B15 wires current annealed with different current amplitudes.
1V.CONCLUSIONS
Current annealing proves to be a powerful method to tailor the magnetic response, particularly the magneto-impedance, of amorphous soft ferromagnetic wires. The magneto-impedance effect has been studied in low magnetostrictive amorphous wires with different saturation magnetostriction constants and induced circumferential anisotropies. Changes in the magneto-impedance up to +220% with maximum slopes up to +1740%/0e are observed. The maximum magneto-impedance at a constant current amplitude (about +220%) is found for a slightly negative magnetostricl ive material and the maximum slope is observed for the wire with the smallest induced circumferential anisotropy
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(+ 1740%/0e). From the point of view of device applications the small saturation magnetostriction constant of the wires studied in this work seems to have a modest influence in the observed magneto-impedance characteristics of the wires. This points to a differential circumferential susceptibility controlled mainly by the induced circumferential anisotropy in these low magnetostrictive materials.
ACKNOWLEDGMENTS
The authors wish to express their gratitude to Prof. Floyd B. Humphrey and John C. Mallinson for helpful discussions and to Jan-Erik Schuch and Fredrik Wastlund for technical help.
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Costa-Kramer J. L. , Ph.D. Thesis. Dept. of Condensed Matter Physics, Royal Institute of Technology, Stockholm, Sweden, February 1994. J. L. Costa, F. Wastlund and K. V. Rao, "Very large magneto impedance in (Co94Fe6)72.5Si12.5B15 as-quenched amorphous wire," To be published. K. V. Rao, F. B. Humphrey and J. L. Costa- Kramer, "Very Large Magneto-Impedance in Amorphous Soft Ferromagnetic Wires". The 6th Joint MMM-INTERMAG conference, Albuquerque, New Mexico, June 20-23. 1994. For references in field induced anisotropy see for example Amorphous Metallic Alloys, ed. F. E. Luborsky, Butterworths 1983, pp.103 and
K. Narita, J. Yamasaki and H. Fukunaga, "Measurement of Saturation Magnetostriction of a Thin Amorphous Ribbon by Means of Small-Angle Magnetization Rotation". IEEE Trans. Magn. vol. 16, No.2, pp. 435-439, March 1980. H. K. Lachowicz and H. Symczak, "Stress dependence of the saturation magnetostriction constant in metallic glasses", Magn. Prop. of Amorphous Metals, ed. A.Hemando et al., Elsevier 1987, pp.232-237. G6mez-Polo C., Ph.D. Thesis, Universidad Complutense de Madrid, Facultad de Ciencias Fisicas, Madrid 1992.
