The instrumental set up used for ac susceptibility measurements consists a cryostat assembly with coil system. Here the change in the mutual inductance of a primary and two identical but oppositely wound secondary coils is measured when a sample is placed in it. This change is a measure of the magnetic susceptibility of the sample.
The length of the glass cryostat is around 300mm. The lower tail of the glass cryostat is a double walled cylinder of inner diameter 7mm and length 200mm. The glass cryostat is attached to a tube through a glass-to-metal seal. This tube has a Wilson seal at the top to introduce the sample rod to the cryostat. There is a glass valve in the tube to facilitate the change of the sample without disturbing the vacuum. The evacuated glass cryostat along with the coil system is dipped into a glass dewar filled with liquid N2. Primary and a set of secondary coils are wound on separate hylum formers. Hylum was chosen for its thermal and mechanical properties. The secondary coils are generally kept inside the primary coil for a maximum filling factor of the secondary and the sample. The offset voltage, which is undesirable, is minimized by unwinding a few turns of the secondary while construction of the coil assembly. Even then a small offset voltage will remain which is compensated by an electronic offset compensation circuit. Here a voltage tapped with the primary coil is phase resolved using Butterworth filters, and is attenuated to generate voltages that are equal in magnitude and opposite in sign to the offset voltage. The sample holder is a sapphire plate of length 70mm and width 4mm. It is attached to a hollow ss rod through a nonmetallic hylum extension of 60mm length. The sample holder moves up and down through the Wilson seal inside the glass cryostat.
Initial excitation of primary coil and extraction of output have been done by using a Standard Research Systems lock-in amplifier, model SR 850. The variable sine out of the lock-in amplifier can directly provide ac fields up to 15 Oe. A current booster is used to generate fields up to 300 Oe. Temperature is sensed using a platinum resistance thermometer attached to sapphire rod near the sample and heater and a Lakeshore temperature controller is used to achieve the fine temperature control. Necessary care was taken to minimize the noise.
Automation software was developed in TURBO PASCAL version 6.0, and complete automation of the system was achieved for the simultaneous measurement of ac magnetic susceptibility. A provision to interact with the program allows changing the parameters like temperature, ac field and frequency. The overall sensitivity of the set up is better than 10-7 emu, and reproducibility better than 0.1% in low fields.
The length of the glass cryostat is around 300mm. The lower tail of the glass cryostat is a double walled cylinder of inner diameter 7mm and length 200mm. The glass cryostat is attached to a tube through a glass-to-metal seal. This tube has a Wilson seal at the top to introduce the sample rod to the cryostat. There is a glass valve in the tube to facilitate the change of the sample without disturbing the vacuum. The evacuated glass cryostat along with the coil system is dipped into a glass dewar filled with liquid N2. Primary and a set of secondary coils are wound on separate hylum formers. Hylum was chosen for its thermal and mechanical properties. The secondary coils are generally kept inside the primary coil for a maximum filling factor of the secondary and the sample. The offset voltage, which is undesirable, is minimized by unwinding a few turns of the secondary while construction of the coil assembly. Even then a small offset voltage will remain which is compensated by an electronic offset compensation circuit. Here a voltage tapped with the primary coil is phase resolved using Butterworth filters, and is attenuated to generate voltages that are equal in magnitude and opposite in sign to the offset voltage. The sample holder is a sapphire plate of length 70mm and width 4mm. It is attached to a hollow ss rod through a nonmetallic hylum extension of 60mm length. The sample holder moves up and down through the Wilson seal inside the glass cryostat.
Initial excitation of primary coil and extraction of output have been done by using a Standard Research Systems lock-in amplifier, model SR 850. The variable sine out of the lock-in amplifier can directly provide ac fields up to 15 Oe. A current booster is used to generate fields up to 300 Oe. Temperature is sensed using a platinum resistance thermometer attached to sapphire rod near the sample and heater and a Lakeshore temperature controller is used to achieve the fine temperature control. Necessary care was taken to minimize the noise.
Automation software was developed in TURBO PASCAL version 6.0, and complete automation of the system was achieved for the simultaneous measurement of ac magnetic susceptibility. A provision to interact with the program allows changing the parameters like temperature, ac field and frequency. The overall sensitivity of the set up is better than 10-7 emu, and reproducibility better than 0.1% in low fields.
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