measurement of the meissner effect by a magneto-optic ac method using ferrimagnetic garnet films

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58 Journal of Magnetism and Magnetic Materials 95 (1991) 58-60 North-Holland Measurement of the Meissner effect by a magneto-optic ac method using ferrimagnetic garnet films Y. Yuan, J. Theile and J. Engemann Department of Electrical Engineering University of Wuppertal, W-5600 Wuppertal, German_v A new method for measuring the Meissner effect of high-T, superconductors based on the magneto-optic Faraday rotation in ferrimagnetic garnet films is described. In this method the ferrimagnetic garnet film is placed on a superconducting sample. Subsequently both the garnet film and superconductor are subjected to a perpendicularly oriented ac magnetic field. In the presence of the superconducting transition, the field expulsion due to the Meissner effect changes the field induced vibration amplitude of the domain walls in the garnet film. By observing the vibration amplitude as a function of temperature the Meissner effect and critical temperature of the superconducting sample can be measured accurately. 1. Introduction The Meissner effect is one of the most promi- nent properties of superconductors. It can be mea- sured by several methods [l-5]. We have devel- oped a new method for measuring the Meissner effect of high-T, superconductors. In this method, a superconducting sample and a thin ferri- magnetic garnet film with uniaxial anisotropy are placed together, and an ac external magnetic field is applied to them perpendicularly. In the ac field the domain walls of the garnet film vibrate accord- ing to the frequency and amplitude of the field. In the event of a superconducting transition the vibration amplitude of the domain walls is re- duced due to the field expulsion caused by the Meissner effect. By observing the wall vibration amplitude changes magneto-optically the Meissner effect can be observed and the critical temperature measured accurately. 2. Experiment The experimental setup is shown schematically in fig. 1. The sample temperature is monitored by a silicon diode located inside the cold finger. The accuracy of the temperature measurement is estimated to be +O.l K. The superconducting sample is pasted to the cold finger with thermally conductive paste. The garnet film is Bily 64 with the chemical composition and material constants shown in table 1. It is coated with aluminium on the film side for reflected light mode. Magnetic domains of the garnet film are observed in re- flected light by a polarizing microscope. A small coil on the garnet film produces an ac magnetic field (Ha,) with a frequency of 2 kHz and ampli- tudes ranging from 0.1 to 4 Oe. Because of the ac field the domain walls in the garnet film will be displaced about their equilibrium position. The displacement waC is proportional to Ha, - H,, H, being the coercive force. A photodiode detects the light intensity Z of the bright and dark domains, Z = Z,, + Z& Z,= and Idc being the ac and dc part of I, respectively. The lock-in amplifier only mea- sures the ac part of 1. The measured intensity Za, is proportional to waC. With a relative polarizer setting of 45” the intensity Z,, is given by Z,, = Z~,~sin(2O,h), Zdc = $exp( -ah), (1) (2) where I, is the intensity of the incident light. Fig. 2 demonstrates the linear dependence of Z,, on 0304~%X53/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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Page 1: Measurement of the Meissner effect by a magneto-optic ac method using ferrimagnetic garnet films

58 Journal of Magnetism and Magnetic Materials 95 (1991) 58-60

North-Holland

Measurement of the Meissner effect by a magneto-optic ac method using ferrimagnetic garnet films

Y. Yuan, J. Theile and J. Engemann

Department of Electrical Engineering University of Wuppertal, W-5600 Wuppertal, German_v

A new method for measuring the Meissner effect of high-T, superconductors based on the magneto-optic Faraday rotation

in ferrimagnetic garnet films is described. In this method the ferrimagnetic garnet film is placed on a superconducting sample.

Subsequently both the garnet film and superconductor are subjected to a perpendicularly oriented ac magnetic field. In the

presence of the superconducting transition, the field expulsion due to the Meissner effect changes the field induced vibration

amplitude of the domain walls in the garnet film. By observing the vibration amplitude as a function of temperature the

Meissner effect and critical temperature of the superconducting sample can be measured accurately.

1. Introduction

The Meissner effect is one of the most promi- nent properties of superconductors. It can be mea-

sured by several methods [l-5]. We have devel-

oped a new method for measuring the Meissner

effect of high-T, superconductors. In this method, a superconducting sample and a thin ferri- magnetic garnet film with uniaxial anisotropy are

placed together, and an ac external magnetic field

is applied to them perpendicularly. In the ac field the domain walls of the garnet film vibrate accord-

ing to the frequency and amplitude of the field. In the event of a superconducting transition the

vibration amplitude of the domain walls is re- duced due to the field expulsion caused by the Meissner effect. By observing the wall vibration amplitude changes magneto-optically the Meissner effect can be observed and the critical temperature measured accurately.

2. Experiment

The experimental setup is shown schematically in fig. 1. The sample temperature is monitored by a silicon diode located inside the cold finger. The accuracy of the temperature measurement is estimated to be +O.l K. The superconducting

sample is pasted to the cold finger with thermally conductive paste. The garnet film is Bily 64 with

the chemical composition and material constants shown in table 1. It is coated with aluminium on the film side for reflected light mode. Magnetic

domains of the garnet film are observed in re-

flected light by a polarizing microscope. A small

coil on the garnet film produces an ac magnetic

field (Ha,) with a frequency of 2 kHz and ampli- tudes ranging from 0.1 to 4 Oe. Because of the ac field the domain walls in the garnet film will be

displaced about their equilibrium position. The displacement waC is proportional to Ha, - H,, H,

being the coercive force. A photodiode detects the light intensity Z of the bright and dark domains,

Z = Z,, + Z& Z,= and Idc being the ac and dc part of I, respectively. The lock-in amplifier only mea- sures the ac part of 1. The measured intensity Za, is proportional to waC. With a relative polarizer setting of 45” the intensity Z,, is given by

Z,, = Z~,~sin(2O,h),

Zdc = $exp( -ah),

(1)

(2)

where I, is the intensity of the incident light. Fig. 2 demonstrates the linear dependence of Z,, on

0304~%X53/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

Page 2: Measurement of the Meissner effect by a magneto-optic ac method using ferrimagnetic garnet films

Y. Yuan et al. / Measurement of the Meissner effect 59

Lamp i current I AC supply

X-Y recorder

T t,

Superconductor

Fig. 1. Schematic diagram of the experimental setup.

Table 1

Chemical composition and material constants of the garnet

film Bily 64

Composition (YSmLu),(F~a)s% Film thickness h 5.0 Frn

Faraday rotation & 4.2”

Domain width w,, 6pm Coercive force H, 0.2 Oe

Saturation magnetization 4nM, 185 Oe (T= 300 K)

Absorption constant a 1200 cm-’

H,, (at T = 100 K). The linearity is to be expected from eqs. (1) and (2), since wac is proportional to H,, as explained previously.

,0X8

Hat (Oe)

Fig. 2. Light intensity signal Z, as a function of the ac magnetic field amplitude H, (T = 100 K).

By cooling the superconducting sample below a certain temperature it is observed, that the vibra- tion amplitude of the domain walls is reduced, and the light intensity signal I,, is decreased down to zero. Without a superconductor no such reductions are observed. The explanation of the observed phenomenon is clear when we take into account the Meissner effect. This temperature cor- responds to the critical temperature of the super- conducting sample. Below T, the Meissner effect expells the magnetic field from the superconduct- ing sample which subsequently reduces the field in the garnet film. By observing I,, as a function of

0.0. 80 90 100 110

T (K)

Fig. 3. Light intensity signal Z, vs. temperature T for the thin film sample at H, = 0.5 Oe.

Page 3: Measurement of the Meissner effect by a magneto-optic ac method using ferrimagnetic garnet films

60 Y. Yuan et al. / Measurement of the Meissner effect

0 11 I I I

80 90 100 110 T (K)

Fig. 4. Light intensity signal I,, vs. temperature T for the bulk

sample at H,, =1 Oe.

temperature the Meissner effect can be measured

and the critical temperature can be obtained.

This method is found to be very sensitive with respect to the exciting field amplitude [ H,,(min) = 0.1 Oe]. And it works contactlessly. Further-

more due to the magneto-optical detection tech-

nique the method allows for locally resolved mea-

surements down to the 1 pm range of supercon-

ducting properties. This feature is expected to be useful for the development of future devices based

on high-T, superconductors.

Acknowledgements

The authors would providing the Y Ba ,Cu

like to thank H. Pie1 for 10,_6 samples. This work

was supported by the Bundesministerium fir For- schung und Technologie.

3. Results

A YBa,Cu,O,_, thin film sample and a

YBa,Cu,O,_, bulk sample were measured. The

film is 0.5 pm thick, deposited on a MgO sub-

strate. The bulk sample is a usually sintered pellet.

The results for the film and bulk sample are shown in figs. 3 and 4, respectively. At Ha, = 0.5

Oe the film sample displays the complete Meissner effect below 86 K. Its critical temperature defined at the midpoint of the transition is 87 K. The

corresponding transition width is 1 K (90-10%).

At Ha, = 1 Oe the bulk sample shows a gradual transition, displaying a critical temperature of 84 K. Its transition width is approximately 5 K.

References

[l] M. Tokumoto, M. Hirabayashi, K. Murata, N. Terada, K.

Senzaki and Y. Kimura, Jpn. J. Appl. Phys. 26 (1987) L517.

[2] S. Hatta, H. Hiyashino and K. Wasa. Jpn. J. Appl. Phys. 26

(1987) L724.

[3] T. Hioki, M. Ohkubo, A. Itoh, H. Doi, J. Kawamoto and

0. Kamigaito, Jpn. J. Appl. Phys. 26 (1987) L636.

[4] Y. Oda, I. Nakada, T. Kohara, H. Fujita, T. Kaneko, H.

Toyoda, E. Sakagami and K. Asayama, Jpn. J. Appl. Phys.

26 (1987) L481.

[5] M. Hagen, M. Hein, N. Klein, A. Michalke, G. Mtiller, H.

Piel, R.W. Roth, F.M. Mueller, H. Sheinberg and J.-L.

Smith, J. Magn. Magn. Mat. 68 (1987) Ll.