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Magnetic characteristics of bulk BPSCCO magnetic sensors fabricated by the shock compaction method: relationship between the magnetic sensitivity and thickness of the magnetic sensor

As one of the basic areas of research for the development of a highly sensitive magnetic sensor, the present authors have fabricated bulk Bi-Pb-Sr-Ca-Cu-O magnetic sensors by use of the shock compaction method, as a new fabrication technique for high-critical temperature superconductors. Values of the thickness t of the sensors, under a shock compaction pressure P of 4 GPa, are systematically changed between 0.3 and 1.1 mm to investigate the magnetic sensitivity S of the sensors. It was found that the value of S increases as the value of t decreases. The value of S for a magnetic sensor (1.1 mm in width, 8.7 mm in length) was about 62 %/(104 T) over the range of measurement of the magnetic field Bmeas which varied from 0 to ±5×10-4 T, under a constant value of the bias magnetic field Bbias of 20×10-4 T. Namely, it was approximately 62 times greater than that of giant magnetoresistance sensor. In addition, the present paper examined the dependence of resistivity ρ(T) on temperature, the dependence of ρ(T=77.4 K) of the sensor on current density, the dependence of Son t, and the dependence of measured resistance of the magnetic field Rmeas on Bmeas.


Introduction
The application of magnetic sensors can be divided into those related to the direct measurement of magnetic fields, and those related to the measurement of other quantities by means of variations in the magnetic field. With this in mind, it may be necessary to develop a highly sensitive magnetic sensor. Examples of such needs are geophysical applications such as in magnetic exploration, nondestructive tests for circuit boards and IC mapping, and in medical applications for measuring biomagnetic fields such as in a magnetocardiogram (MCG), among many others [1,2]. Fluxgate and superconducting quantum interference device (SQUID) magnetometers, using structure type sensors, measure the magnetic magnitude and direction of the dc or low frequency magnetic field, and are generally suitable for measurements over the magnetic ranges of approximately 10 -10 to 10 -4 T and 10 -14 to 10 -10 T, respectively. It is necessary, however, to fabricate a solid-state type magnetic sensor in order to extend the measurement range from the fluxgate magnetometer to the SQUID magnetometer.
As one of the basic areas of research for the development of a highly sensitive magnetic sensor, the . Little is unknown, however, of the characteristics and sensitivity when the sensor thickness t is changed under conditions of a constant bias magnetic field B bias . Therefore, the sensors thicknesses, under a shock compaction pressure P of 4 GPa, were systematically changed between 0.3 and 1.1 mm to investigate the magnetic sensitivity S of the sensor. As a result, it was found that the value of S increases as the thickness t decreases. As typical example, the sensitivity S for a sensor with a thickness t of 0.3 mm was determined as 62 %/(10 -4 T), which is approximately 62 times greater than that of giant magnetoresistance (GMR) sensor. Experimental results revealed the dependence of the resistivity ρ(T), without an application of a bias magnetic field B bias on temperature T, the dependence of resistivity ρ(Τ=77.4 Κ) of the specimen on current density J, the dependence of S on sensor thickness t, and the dependence of the measured resistance in the magnetic field R meas on the magnetic field B meas .

Experimental procedure
The particle size distribution of BPSCCO powder (Dowa Mining Co., Ltd., DSC-045001) used in the present experiment was approximately 3 µm, such as was reported in Ref. [3]. For the fabrication of the bulk BPSCCO specimens to be used as magnetic sensors, the BPSCCO powder was formed into pellets by a compressor under pressures ranging from 200 to 300 MPa. The pellets were then subjected to a shock compaction pressure of 4 GPa for approximately 1 µsec, using the shock compaction system as was reported in Refs. [4][5][6]. The pellet specimens were annealed under a temperature of 845 °C for 48 hours, by heating in dry air at the rate of 14 °C/min, cooled in dry air to a temperature of 300 °C at the rate of 5 °C/min, and then reduced to room temperature by natural cooling. The pellets, measuring 5.0 mm in thickness and 20 mm in diameter, were cut into rectangular-shaped test specimens to be used as magnetic sensors, measuring 8.7 mm in length and 1.1 mm in width. The cutting was performed with a diamond saw at a low cutting rate to limit the effects of heat. The final step involved reducing the various thicknesses of the sensors by sandpaper (No. 1500). The above procedures resulted in five sensors ranging in thickness from 0.3 to 1.1 mm.
The geometry of a bulk BPSCCO magnetic sensor is illustrated in Fig. 1 (a), where the letters I and V represent the current terminal and voltage terminals, respectively. The four copper leads were attached to the sensor by a conductive silver paste. The magnetic sensor and calibrated platinum thermometer were fixed with grease (Apiezon N) to a holder composed of oxygen free, highly conductive copper (OFHC) with a copper spiral (diameter of about 2.0 mm) used to simplify the temperature control, such as shown in Fig. 1 The changes in resistance of the sensor having a thickness of 0.3 mm, for example, due to the external magnetic flux density B ex , does not exhibit a normal state for values of B ex less than 10×10 -4 T and values of current density J less than 40 A/cm 2 . Therefore, the magnetic measuring system is constructed in such a manner to realize a normal state of the sensor, and is biased by a homogeneous  DC magnetic flux density B bias of 20×10 -4 T. The B bias is applied perpendicular to the surface of the sensor by use of a Helmholtz coil, such as was reported in Refs. [6,7]. The measuring system for obtaining the magnetic characteristics, in turn, is shielded by two layers consisting of cylinders of a Mu-metal around a metal dewar vessel, as was illustrated in Refs. [7]. The two cylinders were degaussed by an AC magnetic field (60 Hz) at room temperature, prior to performing the experiment.

Results and discussion
The dependence of the resistivity ρ(T=77.4 K), without an application of the bias magnetic field B bias , on temperature T for the magnetic sensors in the absence of an external magnetic flux density B ex is measured under a constant current density J of 1 A/cm 2 (not shown). From these results, the values of the temperature of zero resistance T zero of sensors were determined. The plotted points in      Figure 4 shows the dependence of B c on sensor thickness t (ranging from 0.3 mm to 1.1 mm), at 77.4 K. It can be seen in this figure that the values of B c increase as the values of t increase.
The plotted points in Fig. 5 denote the dependence of the resitivity ρ(T=77.4 K) for two sensors,    Figure 6 shows the dependence of the critical current density J c on sensor thickness t at 77.4 K, under a constant B bias of 20×10 -4 T. Measurements of the values J c were determined for specimens constructed with values of t from 0.3 to 1.1 mm. It can be seen in this figure that the values of J c increase as the values of t increase, and exhibit a similar tendency as was found for B c in Fig. 4. Figure 7 displays the magnetic sensitivity S as a function of the sensor thickness t, under constant conditions of J of 40 A/cm 2 and B bias of 20×10 -4 T. Here, S is defined by where R meas (B meas ) and R meas (B meas =0 T) are the resistances in the measurement of the magnetic field B meas . It is found that the values of S decrease as the values of t increase. Namely, a highly sensitive sensor can be obtained by making use of low values of t, contrary to the results found for measurements of critical current density, such as were shown in Fig. 6. Stated in different terms, the value of magnetic sensitivity S for thin sensors is greater than those of thick sensors. Figure 8 shows the typical dependence of resistance R meas (B meas ) for sensors with thicknesses t of  . This is about 62 times greater than that of a GMR sensor. Furthermore, it was found that the magnetic sensors exhibit directional and linear characteristics in the values of B meas . In addition, no evidence of hysteresis was found in these characteristics. After testing sensor specimens through more than 20 thermal cycles between temperatures of the boiling point of liquid nitrogen (77.4 K) and room temperature (300 K), the characteristics such as shown in Figs. 2-8 were found to exhibit no significant changes.

Conclusions
In the present research, bulk BPSCCO magnetic sensors were constructed by incorporating the shock compaction method as a new fabrication technique for bulk HTSs. The sensor thicknesses t were systematically changed, from 0.3 mm to 1.1 mm in order to investigate the in relationships with the values of magnetic sensitivity S, temperature of zero resistance T zero , critical current density J c , and critical magnetic flux density B c .
The values of magnetic sensitivity S for the sensors decrease as the values of t increase, such as was shown in Fig. 7. That is, a highly sensitive sensor can be obtained by making use of low values of t, contrary to the results found for measurements of critical current density J c , such as was shown in Fig. 6. Stated in different terms, the value of S for thin sensors is greater than those of thick sensors.
The sensitivity of the sensor with a t of 0.3 mm was found to be about 62 %/(10 -4 T) over the range of measurement of the magnetic field B meas between 0 T to ±5×10 -4 T, under constant conditions of B bias of 20×10 -4 T and J of 40 A/cm 2 . Namely, it was about 62 times greater than that of a GMR sensor. Furthermore, it was found that the measured resistances in the magnetic sensors exhibited directional and linear characteristics for the values of B meas . In addition, no evidence of hysteresis was found in these characteristics.
It was determined from the present results that the characteristics of the sensors were significantly influenced by the value of the sensor thickness t. These results were found to be important criteria for fabricating more highly sensitive magnetic sensors.