Magnetics Business & Technology - Summer 2017 - 7

FEATURE ARTICLE
can distinguish between different magnetic mineral species[2]. It has
also been used to characterize interactions and coercivity distributions in magnetic recording media[3,4], nanowire arrays[5], exchange
coupled permanent magnets[6] and exchanged biased magnetic
multilayers[7]. Finally, while it is very difficult to unravel the complex
magnetic signatures of multiphase magnetic materials from a hysteresis loop measurement alone, FORC can differentiate between
phases in such materials[8, 9, 10].

Magnetic Measurement Techniques

Vibrating Sample Magnetometry
In vibrating sample magnetometry, originally developed by Simon Foner[11] of MIT's Lincoln Laboratory, a magnetic material is
vibrated within a uniform magnetic field H, inducing an electric current in suitably placed sensing coils. The resulting voltage induced
in the sensing coils is proportional to the magnetic moment of the
sample. The magnetic field may be generated by an electromagnet or a superconducting magnet. VSM measurements can be performed from <2 K to 1,273 K using integrated cryostats or furnaces.
Commercial VSM systems provide measurements to field
strengths of ~3.4 T (34,000 Oe) using conventional electromagnets
[12,13], as well as systems employing superconducting magnets to
produce fields to 16 T [14,15]. When used with electromagnets,
very small step changes in field can be made (i.e., ~1 mOe) and the
measurement is very fast. A typical hysteresis loop measurement
can take as little as a few seconds to a few minutes.
When used with superconducting magnets, higher field strengths
are possible; however, this limits the field setting resolution, and the
measurement speed is inherently slower due to the speed at which
the magnetic field can be varied using superconducting magnets.

A typical hysteresis M(H) loop measurement can take one hour
or more. Additionally, magnetometers employing superconducting
magnets are more costly to operate since they require liquid helium. Cryogen-free systems employing closed cycle refrigerators, and
also liquefiers that recover helium in liquid helium based systems
are available, but these represent an expensive capital equipment
investment. An advantage of superconducting magnet systems is
that they reach higher magnetic fields than air- or water-cooled
electromagnets, which is necessary to saturate some magnetic materials, such as rare earth permanent magnet materials. The noise
floor of commercially available VSMs is 10-7 to 10-8 emu.
Superconducting Quantum Interference Device Magnetometry
Quantum mechanical effects in conjunction with superconducting detection coil circuitry are used in superconducting quantum
interference device (SQUID) based magnetometers to measure the
magnetic properties of materials. Theoretically, SQUIDs are capable
of achieving sensitivities of 10-12 emu, but practically, they are limited to sensitivities of 10-8 emu because the SQUID also picks up
environmental noise. As in a VSM, SQUIDs may be used to perform
measurements from low to high temperatures (from <2 K to 1,000
K). Superconducting magnets with field strengths up to 7 T are
employed in SQUIDs[14,15]; therefore, the measurement is inherently
slow due to the speed at which the magnetic field can be varied, as
is the case for superconducting magnet-based VSM systems. A typical hysteresis M(H) loop measurement can take one hour or more.
Alternating Gradient Magnetometry (AGM)
Force methods involve determination of the apparent change in
weight for a material when placed in an inhomogeneous magnetic

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