Development of Low Noise Fluxgate Sensors

Transcription

1 Development of Low Noise Fluxgate Sensors Luiz C.C. Benyosef* Resumen Este trabajo resume los resultados de cintas amorfas de dos aleaciones de CoFeSiB y otras dos de CoFeSiBCr, como sensores del dispositivo de paro de flujo magnético de ruido bajo. Este trabajo también muestra, sus propiedades suprimidas, después del recocido y finalmente su nivel de ruido como núcleo del anillo de los sensores de dispositivos de paro de flujo magnético. Abstract This paper compares the results of amorphous ribbons two alloys CoFeSiB and another two CoFeSiBCr as low noise fluxgate sensors. This paper shows also, their properties as-quenched, after stress annealing and finally their noise level as ringcore fluxgate sensors. Introduction The fluxgate sensor mechanism is based upon the non linear properties of soft magnetic materials, used as sensor core, that changes it's relative permeability in consonance with the core magnetization. Amorphous ribbons of Co base, having nearly zero magnetostriction at room temperatures, have been found to show excellent magnetic properties to be applied as fluxgate sensors cores (Shirae 1984 and Nielsen et al. 1991). These properties depend sensitively upon the composition and domain structure present in these materials which is related to their microstructure. Amorphous materials of Co base with Cr or Mo reduce values or the Curie CNPq/Observatório Nacional, Rua General José Cristino, 77, Rio de Janeiro R.J., Brazil.

2 112 Luiz C.C. Benyosef Revista Geofísica 48 temperature (Tc) and saturation magnetization also displacing the magnetostriction coefficients (Às) for positive and near zero values. Intensities Às:::: O correspond to the highest permeability intensities with lowest He values. For reducing Tc it is important to decrease the noise since it decreases with increasing temperature and goes to null value at the Curie temperature. A convenient annealing process (Primdahl et al. 1989) needs to be done to improve the ribbon properties inducing a hard axis anisotropy and promoting a domain rotation rather than a domain wall movement for the alternating excitation current that will saturate the sensor core reducing the Barkhausen noise. Materials and Methods Two nominal compositions from CoFeSiB base and another two nominal compositions from CoFeSiBCr base alloys melted by several processes were used: Table 1 Physical characteristics of the melted ribbons used Alloy I Alloy2 Alloy3 Alloy4 Composition CO!J75Fe_;sSiwB 11.6 CoMFe.iSÎl5B1s Co6sFesSii1BwCrs (C067FeSi1sB1s)y3Cn Melt spinner Fe roller-vacuum Cu-roller-Ambient Cu-roller-Ambient Cu-roller-Vacuum atmosphere atmosphere Density 7.6g/cm g/cm' 7.9 g/cm' 7.8 g/cm' Width 1.2mm 1.3mm I.Imm 1.3mm Thickness 25µm 27µm 30µm 26µm Curie temp. 226 ºC 190ºC 172 ºC 50ºC To measure the magnetic properties toroid and straight samples were used. The toroidal assemblies were built on a ring shaped support made of inconel with the following dimensions: outer/inner diameter: 26.2/25.0 mm; length: 2.5 mm and groove depth: 1 mm. Upon the non-magnetic support were used: (9-11) wraps for the amorphous ribbon. The excitation coils were built using ( 17-36) windings of cooper wire and the sensing coil with 60 windings. The straight ribbons were ( 10-12) cm long and their sense coil had ( ) windings. All the samples were magnetically saturated for fields (7-1 O) Oe. The annealing procedure took place in air atmosphere, in resistance's furnaces. A Cromel-alumel thermocouple was used for measuring the temperature. The rates of heating and cooling were approximately 5.0 K/min. Alloy number 4 was not considered in this experiment due its low Curie Temperature that's approximately like the ambient values. The constant of induced anisotropy Ku was calculated as the square of magnetization curve at minimal stresses applied during measurements. The saturation

3 enero-junio 1998 Development of Low Noise Fluxgate Sensors 113 magnetostriction were measured by the method of small-angle magnetization rotation (SAMR).The determination of Curie's Temperatures were done by inductance method. The saturation magnetization was obtained by a fluxmeter with an accuracy of ±1.5%. Experiments to determine magnetization values were carried out in fields of 400A/m and to determine coercive forces fields of 40A/m and frequencies of 1 KHz were used. Amorphous nature of the ribbons was checked by X-ray diffraction using CuKa radiation. To improve the magnetic properties and reduce the noise levels, the annealing procedure was divided into two steps. First the ribbon is submitted to a stress-annealing under temperatures shown in table 2 for an hour and a load. To decrease the internal tensions in the ribbon, it is convenient (after the stress-annealing process) to submit the ribbon to a stress-relief (Nielsen et al. 1991) that consists in a heating without load after the main stress-annealing. Table 2 presents values used in the stress-annealed and in the stress-relief. Table 2 Annealing process used (Stress-annealing + stress-relief) to reduce the noise levels Annealing Temp. (ºC) Time (h) Applied Stress (MPa) Relieftemp(ºC) Relief time (h) The noise. measurements were done with the sensors inside a magnetic shield through a short-circuited fluxgate described by Primdahl (Primdahl et al. 1989). Each sensor was built on a ring shaped support made of acrylic with the outer/inner diameter: 17.5/16.0 mm containing 12 wraps of ribbon. The ring-cores were surrounded by ( ) turns of AWG-29 copper wire and the sense winding used 450 turns of AWG-41 copper wire. Results Table number 3 shows the magnetic properties of the materials as-quenched and stress-annealed. It's possible to observe the strong stress dependence of the magnetostriction values, Às with stress annealing. Table 3 Magnetic properties of the material before and after stress annealing Alwy µoj(tj Tc(ºC) /)Âsx 10 7 l/)âsx 10 7 Ku(J/m 3 ) l)hc(a/m) li) Hc(Alm) I X X

4 114 Luiz CC. Benyosef Revista Geofísica 48 The alloy number I presented the greatest Ku and the most uniform transverse magnetic anisotropy because the magnetization curve was completely straight for all the applied stresses. Curves I to 7, Figure I, was measured with materials asquenched and curves 8 and 9 after stress-annealing. The curves after stress-annealing for alloy I are almost insensitive to tensile loading (figure I, curves 8 and 9) as a result of very low value of magnetostriction coefficient. The values of applied stress for alloy number I are showed in the Table 4: Table 4 Stress values applied to the sample number 1 Curve Stress (MPa) Figure 2 shows the magnetization curves for sample number 2. Curves I to 8 was measured after stress-annealing and curves 8 and 9 were measured in as-quenched state. Table 5 provides the applied stress in each curve from Figure 2: Table 5 Applied stress to the sample number 2 Curve Stress (MPa) Figure 3 shows the temperature dependence of magnetization of the alloy 3. This alloy differs from the alloys I and 2 in respect of special features which indicates the displacement of the domain boundaries during magnetization. The highest value of magnetostriction and the contribution of the displacement of the domain boundaries in magnetization are probably the reasons for the higher level of magnetic noise in alloy 3 (Table 7). Table 6 Applied stress to the sample number 3 Curve JO 11 Stress (MPa) Figure 4 shows the variation of coercive force H with tensile stresses for three alloys after stress-annealing. The increase of the coercive force for alloy 3 under the effect of tensile loading indicate (Benyosef et al. 1996) that winding the ribbon

5 enero-junio 1998 Development of Low Noise Fluxgate Sensors 115 Derechos Reservados

6 116 Luiz C.C. Benyosef Revista Geofísica 48 Derechos Reservados

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9 enero-junio 1998 Development of Low Noise Fluxgate Sensors 119 into the toroidal core of the magnetometer results in changes of the magnetic properties in comparison with the straight ribbon. It is possible that bending stresses are the reason for the higher level of magnetic noise in alloy 2 and in special in the alloy 3. Table 7 Noise in ring core sensors (0.04 mhz-12 Hz) Alloy Annealing ( stress-relief) Noise (pt RMS) I I 47.4 I I I Conclusion The magnetization and the Curie temperature is correlated with the alloy composition. So It is possible to observe a correlation between Às and Ku. The sample number I has the greatest Ku and the most uniform transverse magnetic anisotropy because the magnetization curves are completely straight for all the applied stresses (curves 8 and 9 at Figure 2). This kind of curve is due to relation magnetization mechanism. So we have the lowest values of He, and we need to observe that lower the He values better will be the sensitivity of the fluxgate sensor. The sample number 3 has the lowest Ku and the strongest curvature of magnetization. Its magnetization consists of some part of rotation and some part of domain wall movement. So it has the biggest values of He. By observation of colloid patterns, we confirm the suggestion of transverse type of magnetic mechanism. So it is possible to conclude that in all samples only perpendicular spontaneous magnetization orientation exists normal or transverse in the ribbon plane. From these results it is suggested that the biggest induced anisotropy results in the best uniformity for perpendicular spontaneous magnetization orientation. If the closure domains exist in great number like in the 2 and 3 samples, the displacement of their walls (90º type) leads to increasing of the coercive force. Such increase maybe by nonuniformity of perpendicular magnetic anisotropy too which is more probable for samples 2 and 3 with low Ku. The more probable source of such nonuniformity is the high Às magnitudes and the surface roughness. In these conditions intrinsic stresses may turn the spontaneous magnetization in local volumes out of perpendicular to ribbon axis direction that forms due to the stress annealing. Then to get the lowest coercive field, one needs to have some certain value of transverse magnetic anisotropy and as low a value of magnetostriction as possible. The best noise value, 18 pt, was obtained from alloy number I after a convenient stress-annealing followed for a stress-relief (annealing 2) indicated in Table 2.

10 120 Luiz C.C. Benyosef Revista Geofísica 48 References Benyosef, L.C.C.; Teodosio, J.R.; Taranichev, V.E.; Zalnin, B.V. and Nemova, O. Yu., "Effect of stresses on the magnetic properties of amorphous ribbons for fluxgate magnetometers," Journal of Advanced Materials, 3 (2) , Nielsen, O.V.; Petersen, J.R.; Fernandez, A.; Hernando, B.; Spisak, P.; Primdahl, F. and Mozer, N., "Analysis of a fluxgate magnetometer based on metallic glass sensors," Meas. Sci. Technol. 2, , Primdahl, F.; Petersen, J.R.; Olin, O. and Harbo Andersen, K., "The short-circuited fluxgate output current," J. Phys. E., Sci. Instruments 22, , Shirae, K., "Noise in amorphous magnetic materials," IEEE Trans on Magnetics, vol. MAG-20 (5) , 1984.