Science and Techniques Using Pulsed Magnetic Fields
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field research of ANML comprises semiconductors, magnetism in transition-metal compounds, heavy-fermion physics, superconductors, organic conductors, and magnetic separation. We present here a few selected topics.
Semicontinuous Magnets Since the late 1960s, the University of Amsterdam has operated a semicontinuous magnetic field installation that produces magnetic fields up to 40 T with typical time constants of about one second.1 The magnet coil is constructed from hard-drawn copper wire with a reinforcement cylinder of maraging steel positioned at roughly one third of the outer diameter. Before operation, the coil is cooled to 30 K by cold neon gas. The power for this installation is taken directly from a 10 kV connection to the public electricity grid. By means of a thyristor-based power control system, highly flexible field-time profiles can be realized: step-wise pulses can be generated with field levels constant within 10~4 during 100 ms; linearly increasing and decreasing fields as well as exponentially ripple-free decreasing fields are other examples of standard field-time profiles. Among the measuring techniques frequently used are magnetization, magnetotransport, quantum oscillations, relaxation phenomena, etc. Temperatures at which experiments can be performed range from 400 mK to room temperature. In the Netherlands, the Amsterdam High Field Facility has recently been combined with the High Magnetic Field Laboratory in Nijmegen, where static magnetic fields up to 30 T are produced in hybrid magnet systems, to form the Amsterdam-Nijmegen Magnet Laboratory (ANML). The high
Magnetic Anisotropy and Exchange Interactions in Rare-Earth, Transition Metal Intermetallics
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Since the discovery in 1983 of the excellent magnetic properties of the compound Nd2Fei4B, a large effort has been spent to elucidate its intriguing magnetic behavior.2 The magnetic properties of these materials are governed by the interplay between crystalline electric field and exchange interactions. Because of the relatively large values of the anisotropy and exchange fields, relevant magnetization measurements have to be carried out in high magnetic fields. In Figure la, we give an example of the magnetization data for Nd2FeMB at 4.2 K obtained at fields that were constant for 100 msec.3 Combining the data along the different crystallographic directions, we deduce a tilt angle of the easy-magnetization direction with respect to the hexagonal axis of 30.8° Furthermore, a first-order transition is seen in the [100] curve at a field of 16.3 T. This transition originates from a competition between lower- and high-order anisotropy constants. For comparison, we present in Figure lb the magnetization curves of the ferrimagnetic hexagonal compound Ho 2Co 17 along the a and b directions in the easy hexagonal plane where, at fields of 28 and 20 T, magnetic transitions occur as well.4 At these transition fields, the antiparallel configuration of the holmium and cobalt moments is lost. Values for the transition field provide accu-
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