48 Ë 2 Superconducting materials
magnetic field needs to reach at least somewhat above the level of the residual
magnetic field.
The magnetization process may lead to a local temperature rise in superconduc-
tors. Oka et al. [154] reported this warming effect. The temperature rise of a SmBCO
single-domain bulk superconductor was precisely measured during the ZFC process.
The highest temperature rise was reported as 7.5 K when the sample was magnetized
at a sweeping rate of 5.06 mT/s at 50 K.
The pulsed field magnetization (PFM) is considered to be the most practical me-
thod because it is inexpensive, small-volume, and mobile experimental set-up to apply
a magnetic field. However, the field trapped by PFM was smaller than that attained
by FCM below 77 K, because of the large temperature rise caused by the magnetic
flux motion in the bulk. In order to improve the magnetization performance of HTS
PM using PFM method, much work has been done. Several approaches were tried
and succeeded in enhancing trapped flux around 77 K, including such as iteratively
magnetizing pulsed field operation with reducing amplitudes (IMRA) [155] and multi-
pulse technique with stepwise cooling (MPSC) [156]. A single-pulse magnetization
gives a poor result compared to the ZFC, IMRA and MPSC magnetization. The trapped
flux of the latter can be made to be almost as high as for the ideal FC case.
Fujishiro et al. [157] succeeded in establishing a record of high-field trapping,
Btrap=5.20 T on the휙45 mm GdBCO bulk, using a 6.7-T applied field at 29 K. A
maximumBtrapof 3.6 T was attained by a single-pulse application.
Trapped field profiles for a 26-mm-diameter GdBCO bulk magnetized at 77 K in
FCM and PFM were investigated [158]. FCM trapped a higher peak field (0.90 T) than
PFM (0.63 T), but using IMRA allows pulsed magnetization to give approximately 95%
of the FC trapped flux. The real effectiveness of PFM came when using both the IMRA
and MPSC together.
In general, the pulsed magnetic field is provided by the solenoid in PFM. The
pulsed field of the solenoid is not uniform and imposes inhomogeneous pulsed-field
distribution upon the magnetization of HTS bulk samples. Ida et al. [159] studied the
magnetization of GdBCO which was inserted between two disks as pulsed-current
vortex-type Cu coils. As a result, the pulse current was smaller and the magnetic field
distribution was more uniform than for a solenoid tape.
Deng [160] presented the experimental results for different penetration patterns of
magnetic flux when employing vortex-type coils in PFM, that is, the magnetic flux will
primarily penetrate inside the bulk from the upper and lower surfaces using vortex-
type coils. In general, the size and current of vortex-type coils is smaller than that of
a solenoid; thus, so a reduced heat generation and temperature rise can be obtained
by vortex coils, which is very important for PFM applications.
A new compact NMR magnet using HTS bulk magnets has been developed.
However, it is difficult to trap a uniform magnetic field above 4.7 T at liquid nitrogen
temperature. Kim et al. [161] presented a remagnetization method in order to improve
the field homogeneity of a HTS bulk NMR relaxometry device, and obtained the 1.58 T