- Email: [email protected]
The influence of the magnetic field on the electromagnetic wave absorption in Y–Ba–Cu–O
Superlattices and Microstructures, Vol. 21, No. 3, 1997
The influence of the magnetic field on the electromagnetic wave absorption in Y–Ba–Cu–O ´ cs, T. Porjesz† N. G. Khatiashvili, Gy. Kova Department for Low Temperature Physics, E¨otv¨os University, H-1088 Budapest, Hungary
(Received 8 July 1996) In this investigation resonance interaction between superconductors and radio frequency field has been studied by a modified electron spin resonance equipment. At transition temperature (Tc ) the amplitude of the response increased about 20–25 times and the resonance frequency of the circuit from 1 (at room temperature) to 3.5 MHz. The amplitude of the response increases only in the transition region. The frequency versus temperature curve is much alike the resistance versus temperature curve, i.e. the frequency and the resistance first slowly, then sharply decrease near Tc . At temperatures below Tc (80 K) the frequency approaches a saturation while the resonance interaction features a magnetic memory effect. c 1997 Academic Press Limited
Key words: electromagnetic wave absorption in HTSC, magnetic field effect in HTSC.
1. Introduction Much space has been devoted in the literature to the effect of the magnetic field on the superconducting parameters [1, 2]. The early experiments on high frequency properties of high Tc materials carried out by Electron Spin Resonance (ESR) equipment showed interesting and basically important effect of weak magnetic field on electromagnetic wave absorption. .
.
.
.
2. Experimental Radio spectrometer in the 13 ÷ 130 MHz frequency range has been applied for the investigation of the absorption property of Y–Ba–Cu–O in wide temperature interval 77 ÷ 300 K. To cover the whole diapason three coils have been used. The sample was made of sintered Y–Ba–Cu–O of 21 × 5 × 1 mm3 dimension. The sample could be rotated relative to the applied magnetic field, a platinum resistance thermometer was fixed to the sample surface to establish a good heat contact. This system was put into a glass tube and placed into the coil (inductance) of the radio spectrometer. The basic magnetic field oriented parallel to the field of Earth was excited by Helmholtz Coils. The positive direction of the field is the direction of the Earth field. The sample orientation was perpendicular (i.e. its 25 × 5 mm2 plane) to the field. In a temperature range near the transition temperature (Tc ) there is a sharp increase in the resonance interaction between the high frequency electromagnetic field and the superconductor. To evaluate the electron spin resonance absorption the signal on the resonance circuit was measured, and A1 (ω) and A2 (2ω) has been † E-mail: [email protected]
0749–6036/97/030377 + 04 $25.00/0
sm960404
c 1997 Academic Press Limited
378
Superlattices and Microstructures, Vol. 21, No. 3, 1997 40.5 freq
40
A2/A1
30
39.5
20
39.0
38.5 80
Fig. 1.
A2/A1
f [MHz]
40.0
10
85
90
95
100 T [K]
105
110
0 115
Temperature dependence of the resonance interaction and of the resonance frequency for zero field cooled sample. .
.
calculated by the aid of Fourier analysis. The ratio of A2 to A1 is more demonstrative than A2 itself. The superconductors display a resonance at normal to superconducting transition analog to the resonance of the paramagnetic salts. The main difference between the ESR and the resonance interaction of superconductors is that at ESR both the magnetic field and the frequency sweeping can be applied while in the case of superconductors only the magnetic field sweeping gives effect. In the transition range not only a peak of A2 /A1 occurs but the frequency also changes sharply. As it is widespread for the detection of electron spin resonance by radio spectroscopic method an a.c. modulating magnetic field (Bmod ) is applied. In our measurements the possible weakest modulating field Bmod = 0.02 mT has been selected for all the experiment.
3. Results The field cooled specimens display a steep change in the resonance frequency. At the same time the A2 /A1 ratio shows a peak (see Fig. 1) in the transition temperature range. It can be considered a resonance interaction but not the highest. The maximum can only be reached by applying a compensating field. The existence of a magnetic memory effect can be concluded by comparing the dependence of f and A2 /A1 on temperature before and after the sample was exposed to a magnetic field (Figs 1 and 2). After the field Bmax had been applied, during the measurement it was switched off. In our experiment the superconductors displayed the resonance peak only in the function of the magnetic field B but not of the frequency. The applied field Bmax1 causes not only a resonance but its intensity and orientation determine the value and direction of the field (Bpeak1 ) at which it occurs. Increasing the applied field from the resonance value, A2 /A1 and the f frequency decreases. Reaching a new, higher applied field Bmax2 , sweeping the field between 0 and Bmax2 , a new peak appears at a new Bpeak2 . The latter resonance amplitude has become lower but appears at a higher field: Bpeak1 < Bpeak2 . This is a magnetic memory effect demonstrated in Fig. 3.
Superlattices and Microstructures, Vol. 21, No. 3, 1997
42.5
379
freq
A2/A1
15
f [MHz]
10
A2/A1
42.0
41.5 5
41.0
80
85
90
95
100
105
110
0 115
T [K] Fig. 2. Temperature dependence of the resonance interaction and of the resonance frequency in B = 5 mT field. .
.
0.061 0.060
up 1. down 2. up 3. down 4.
8
6
0.058
A2/A1
∆f/f0
0.059
10 up 1. down 2. up 3. down 4.
0.057
4
0.056 2 0.055 0.054 –2.3
–1.9
–1.4
–0.9
–0.5
0.0 0.5 B [mT]
0.9
1.4
1.9
0 2.3
Fig. 3. The dependence of the resonance interaction and the normalized frequency variation on the value and direction of the applied maximum magnetic field. .
.
4. Discussion The normal to superconducting transition causes a resonance effect and the inductance of the resonance circuit changes sharply (Fig. 1). The inductance decreases as the excluded field reduces the effective volume of the coil. This effect appears in the transition range of about 5 deg. The resistivity measurement verifies the transition as well. The effect of the applied maximum d.c. magnetic field Bmax on the resonance field Bpeak
380
Superlattices and Microstructures, Vol. 21, No. 3, 1997
seems to show a behaviour like memory. In the first run, starting from B = 0 to a Bmax1 there is no resonance. Then, decreasing the field or sweeping between B = 0 and Bmax1 a resonance occurs at a value of Bpeak1 (Fig. 3). This peak exists only after the magnetic field (Bmax1 ) has been applied. This resonance can be measured as many times as the value of the applied field reaches the Bpeak1 value and the maximum field has not exceeded Bmax1 and the field has not changed direction. As a result of the appearance of flux lines an interaction with external d.c. field comes into existence. If the direction of the applied magnetic field changes the resonance field Bpeak also changes its sign (see Fig. 3). The existence of the trapped fluxes can easily be understood by comparing the dependence of the frequency on temperature for zero field cooled sample (Fig. 1) and the same temperature dependence of the frequency after the sample has been exposed to a Bmax = 5 mT magnetic field (Fig. 2).
5. Conclusion The resonance effect points towards the existence of fluxoids with an optimum number in the sample in question. It gives a possible explanation: when a certain maximum d.c. magnetic field was applied perpendicularly to the high frequency electromagnetic field, a number of flux lines have come into existence. Then, sweeping the external magnetic field between 0 and Bmax , the field intensity of the trapped fluxoids can also be reached. If the external field is equal to the field of fluxoids, the minimum energy is needed for putting into motion these flux lines by the Lorentz force of the external and high frequency electromagnetic fields. As long as the number of fluxoids does not change the value of the resonance field would not be changed either. Application of this effect is also possible: sensors for measuring small changes in the magnetic field and field controlled high frequency devices can also be constructed based on this principle.
References .
[1] }I. Hal´asz, I. Kirschner, T. Porjesz, Gy. Kov´acs, T. K´arm´an, G. Zsolt, Cs. S¨uk¨osd, N. S.-Rozlosnik, and J. K¨urti, Physica 153C, 379 (1988). } Kirschner, I. Hal´asz, Cs. S¨uk¨osd, T. Porjesz, J. K¨urti, Gy. Kov´acs, L. Korecz, T. K´arm´an, N. S.-Rozlosnik, [2] I. and G. Zsolt, Phys. Lett. 130A, 39 (1988). .
.
.
.
.
The influence of the magnetic field on the electromagnetic wave absorption in Y–Ba–Cu–O ´ cs, T. Porjesz† N. G. Khatiashvili, Gy. Kova Department for Low Temperature Physics, E¨otv¨os University, H-1088 Budapest, Hungary
(Received 8 July 1996) In this investigation resonance interaction between superconductors and radio frequency field has been studied by a modified electron spin resonance equipment. At transition temperature (Tc ) the amplitude of the response increased about 20–25 times and the resonance frequency of the circuit from 1 (at room temperature) to 3.5 MHz. The amplitude of the response increases only in the transition region. The frequency versus temperature curve is much alike the resistance versus temperature curve, i.e. the frequency and the resistance first slowly, then sharply decrease near Tc . At temperatures below Tc (80 K) the frequency approaches a saturation while the resonance interaction features a magnetic memory effect. c 1997 Academic Press Limited
Key words: electromagnetic wave absorption in HTSC, magnetic field effect in HTSC.
1. Introduction Much space has been devoted in the literature to the effect of the magnetic field on the superconducting parameters [1, 2]. The early experiments on high frequency properties of high Tc materials carried out by Electron Spin Resonance (ESR) equipment showed interesting and basically important effect of weak magnetic field on electromagnetic wave absorption. .
.
.
.
2. Experimental Radio spectrometer in the 13 ÷ 130 MHz frequency range has been applied for the investigation of the absorption property of Y–Ba–Cu–O in wide temperature interval 77 ÷ 300 K. To cover the whole diapason three coils have been used. The sample was made of sintered Y–Ba–Cu–O of 21 × 5 × 1 mm3 dimension. The sample could be rotated relative to the applied magnetic field, a platinum resistance thermometer was fixed to the sample surface to establish a good heat contact. This system was put into a glass tube and placed into the coil (inductance) of the radio spectrometer. The basic magnetic field oriented parallel to the field of Earth was excited by Helmholtz Coils. The positive direction of the field is the direction of the Earth field. The sample orientation was perpendicular (i.e. its 25 × 5 mm2 plane) to the field. In a temperature range near the transition temperature (Tc ) there is a sharp increase in the resonance interaction between the high frequency electromagnetic field and the superconductor. To evaluate the electron spin resonance absorption the signal on the resonance circuit was measured, and A1 (ω) and A2 (2ω) has been † E-mail: [email protected]
0749–6036/97/030377 + 04 $25.00/0
sm960404
c 1997 Academic Press Limited
378
Superlattices and Microstructures, Vol. 21, No. 3, 1997 40.5 freq
40
A2/A1
30
39.5
20
39.0
38.5 80
Fig. 1.
A2/A1
f [MHz]
40.0
10
85
90
95
100 T [K]
105
110
0 115
Temperature dependence of the resonance interaction and of the resonance frequency for zero field cooled sample. .
.
calculated by the aid of Fourier analysis. The ratio of A2 to A1 is more demonstrative than A2 itself. The superconductors display a resonance at normal to superconducting transition analog to the resonance of the paramagnetic salts. The main difference between the ESR and the resonance interaction of superconductors is that at ESR both the magnetic field and the frequency sweeping can be applied while in the case of superconductors only the magnetic field sweeping gives effect. In the transition range not only a peak of A2 /A1 occurs but the frequency also changes sharply. As it is widespread for the detection of electron spin resonance by radio spectroscopic method an a.c. modulating magnetic field (Bmod ) is applied. In our measurements the possible weakest modulating field Bmod = 0.02 mT has been selected for all the experiment.
3. Results The field cooled specimens display a steep change in the resonance frequency. At the same time the A2 /A1 ratio shows a peak (see Fig. 1) in the transition temperature range. It can be considered a resonance interaction but not the highest. The maximum can only be reached by applying a compensating field. The existence of a magnetic memory effect can be concluded by comparing the dependence of f and A2 /A1 on temperature before and after the sample was exposed to a magnetic field (Figs 1 and 2). After the field Bmax had been applied, during the measurement it was switched off. In our experiment the superconductors displayed the resonance peak only in the function of the magnetic field B but not of the frequency. The applied field Bmax1 causes not only a resonance but its intensity and orientation determine the value and direction of the field (Bpeak1 ) at which it occurs. Increasing the applied field from the resonance value, A2 /A1 and the f frequency decreases. Reaching a new, higher applied field Bmax2 , sweeping the field between 0 and Bmax2 , a new peak appears at a new Bpeak2 . The latter resonance amplitude has become lower but appears at a higher field: Bpeak1 < Bpeak2 . This is a magnetic memory effect demonstrated in Fig. 3.
Superlattices and Microstructures, Vol. 21, No. 3, 1997
42.5
379
freq
A2/A1
15
f [MHz]
10
A2/A1
42.0
41.5 5
41.0
80
85
90
95
100
105
110
0 115
T [K] Fig. 2. Temperature dependence of the resonance interaction and of the resonance frequency in B = 5 mT field. .
.
0.061 0.060
up 1. down 2. up 3. down 4.
8
6
0.058
A2/A1
∆f/f0
0.059
10 up 1. down 2. up 3. down 4.
0.057
4
0.056 2 0.055 0.054 –2.3
–1.9
–1.4
–0.9
–0.5
0.0 0.5 B [mT]
0.9
1.4
1.9
0 2.3
Fig. 3. The dependence of the resonance interaction and the normalized frequency variation on the value and direction of the applied maximum magnetic field. .
.
4. Discussion The normal to superconducting transition causes a resonance effect and the inductance of the resonance circuit changes sharply (Fig. 1). The inductance decreases as the excluded field reduces the effective volume of the coil. This effect appears in the transition range of about 5 deg. The resistivity measurement verifies the transition as well. The effect of the applied maximum d.c. magnetic field Bmax on the resonance field Bpeak
380
Superlattices and Microstructures, Vol. 21, No. 3, 1997
seems to show a behaviour like memory. In the first run, starting from B = 0 to a Bmax1 there is no resonance. Then, decreasing the field or sweeping between B = 0 and Bmax1 a resonance occurs at a value of Bpeak1 (Fig. 3). This peak exists only after the magnetic field (Bmax1 ) has been applied. This resonance can be measured as many times as the value of the applied field reaches the Bpeak1 value and the maximum field has not exceeded Bmax1 and the field has not changed direction. As a result of the appearance of flux lines an interaction with external d.c. field comes into existence. If the direction of the applied magnetic field changes the resonance field Bpeak also changes its sign (see Fig. 3). The existence of the trapped fluxes can easily be understood by comparing the dependence of the frequency on temperature for zero field cooled sample (Fig. 1) and the same temperature dependence of the frequency after the sample has been exposed to a Bmax = 5 mT magnetic field (Fig. 2).
5. Conclusion The resonance effect points towards the existence of fluxoids with an optimum number in the sample in question. It gives a possible explanation: when a certain maximum d.c. magnetic field was applied perpendicularly to the high frequency electromagnetic field, a number of flux lines have come into existence. Then, sweeping the external magnetic field between 0 and Bmax , the field intensity of the trapped fluxoids can also be reached. If the external field is equal to the field of fluxoids, the minimum energy is needed for putting into motion these flux lines by the Lorentz force of the external and high frequency electromagnetic fields. As long as the number of fluxoids does not change the value of the resonance field would not be changed either. Application of this effect is also possible: sensors for measuring small changes in the magnetic field and field controlled high frequency devices can also be constructed based on this principle.
References .
[1] }I. Hal´asz, I. Kirschner, T. Porjesz, Gy. Kov´acs, T. K´arm´an, G. Zsolt, Cs. S¨uk¨osd, N. S.-Rozlosnik, and J. K¨urti, Physica 153C, 379 (1988). } Kirschner, I. Hal´asz, Cs. S¨uk¨osd, T. Porjesz, J. K¨urti, Gy. Kov´acs, L. Korecz, T. K´arm´an, N. S.-Rozlosnik, [2] I. and G. Zsolt, Phys. Lett. 130A, 39 (1988). .
.
.
.
.