Pulsed Field Magnetometers: Closing the loop on hard magnetic material characterization

The Pulsed-Field Magnetometer (PFM) is internationally recognised for the complete

characterisation of modern magnetic materials. It is a method for generating a hysteresis

graph similar to that of a permeameter.

Hirst recently launched its 8th generation of PFM, magnet characterisation magnetometers.

Uniquely these include the Hirst proprietary Self De-magnetisation Field Function SDFF™

which accurately generates an open to closed circuit mapping (O2C™). This finally gives

permeameter like measurements. With a maximum field of 10.5T (8356 kA/m / 105 kOe)

even the most coercive materials and highest grades of NdFeB magnets and SmCo can be

measured, while traditional permeameters cannot measure these high coercivity materials

due to pole piece saturation limitations. The International Electrotechnical Commission (IEC) has issued IEC TR62331, a draft to define the standard methodology for the characterisation of permanent magnetic materials using the PFM technique.

PFMs have gained in popularity over the years as a measurement technique as they are

easily scalable in sample size (from 1 mm to several tens of mm), with a significantly faster

measurement time.

PFMs have historically found application in production testing and quality control in magnet

factories across the world for rapid standard sample block tests, as well as materials

research and development. Small size testing down to 1x1x1 mm and thin slices have been

used over the years in grain boundary diffusion (GBD) magnet development, and more

recently developing selective diffusion of heavy rare earth in magnet development and

production testing.

PFMs can accurately characterise the high coercivity magnet samples used in today’s high

performance motor applications such as electric vehicles (EV). Larger sample sizes meet the

requirements of magnets used in EV rotors, allowing accurate quality control for batch

testing of production magnets for the key performance metrics including high temperature

coercivity. If a batch does not have the correct composition for whatever reason, its high

temperature performance can be compromised causing in extreme cases loss of motor

power and possible warranty claims. The presentation will review this application and

highlight the need for quality control for motor makers.