Magnetics Business & Technology - Fall 2012 - (Page 12)

FEATURE ARTICLE By Brad c. dodrill, Vice President of sales & senior scientist • laKe sHore cryotronics Characterizing Permanent Magnet Materials With a Vibrating Sample Magnetometer Rare-earth permanent magnet materials are indispensable elements in many electronic devices such as electrical motors, hybrid vehicles, portable communications devices, Aeolian generators used in wind turbines, etc. The magnets have major influence on the size, efficiency, stability and cost of these devices and systems. The development of rare-earth magnets in the 1970’s profoundly influenced the application of permanent magnetic materials owing to their large energy product (i.e., BHmax) and increased volume efficiency. The rare-earth magnets that have been most extensively studied and developed are SmCo and NdFeB. China’s rare-earth export restrictions have led to a resurgence of interest in identifying and exploiting sources of rare-earths outside of China. It also led to the re-opening of Molycorp’s mine in Mountain Pass California, which up until the 1980s was the world’s flagship rare-earth mine. Additionally, there’s renewed interest in developing strong permanent magnet materials that do not rely as heavily on rare-earth constituents, for example nanostructured magnets that would use smaller amounts of rare-earth metals than standard magnets. Vibrating sample magnetometers (VSM) are the most commonly employed tool for measuring the magnetic properties of permanent magnet materials, because measurements can be performed on solids, powders, single crystals, thin films, nanostructures and liquids. VSMs are also regularly employed because measurements can be performed over a broad range of temperatures (4 K to 1,273 K) that mimic the environmental conditions to which the magnets may actually be exposed. Since measurement of the magnetic properties is exceedingly important in connection with research and development of new magnetic materials, VSMs will prove indispensable in researcher’s efforts to synthesize and develop the next generation of “non-rare-earth” or “rare-earthlight” permanent magnet materials. The VSM Technique monly used to measure a material’s hysteresis or M(H) loop. In some cases, it is preferred to present the magnetization data in terms of the magnetic induction B, which in cgs units has the unit Gauss (G). The relation between M and B is: B(G) = H + 4πM where M is the volume magnetization (emu/cc). Figure 1. Initial magnetization and major hysteresis M(H) loop for a 79 mg NdFeB sample. The data is presented in terms of mass magnetization (emu/g) versus applied magnetic field (Oe). VSM Measurements of Permanent Magnet Materials If a material is placed within a uniform magnetic field H, a magnetic moment M will be induced in the sample. In a VSM, a sample is placed within suitably placed sensing coils, and is made to undergo sinusoidal motion, i.e., mechanically vibrated. The resulting magnetic flux changes induce a voltage in the sensing coils that is proportional to the magnetic moment of the sample. The magnetic field may be generated by an electromagnet, or a superconducting magnet. Variable temperatures from cryogenic to high temperatures may be achieved using either cryostats or furnace assemblies, respectively. While a VSM measures magnetic moment m, the quantity of interest is the material’s magnetization M. The magnetization M (in cgs units) can be expressed in terms of the mass (emu/g) or volume magnetization (emu/cc), and is the moment m divided by the sample mass or volume, respectively. A VSM is most com- Figure 1 shows the initial magnetization curve, and major M(H) loop for a 79 mg NdFeB powder sample. The most common parameters extracted from the hysteresis loop that are used to characterize the magnetic properties of permanent magnet materials include: the saturation magnetization Ms(the magnetization at maximum applied field), the remanence Mr (the magnetization at zero applied field after applying a saturating field), the coercivity Hc (the field required to demagnetize the sample), and the energy product BHmax which is the magnetic field strength at the point of maximum energy product of a magnetic material. BHmax is obtained from the 2nd quadrant or demagnetization curve of the material (i.e., upper left quadrant in Figure 1) where the curve is recorded starting at remanence Mr and ending at the coercivity Hc. Room temperature 2nd quadrant demagnetization curves for Nd and Sm-based permanent magnet materials are shown in Figure 2. From this measurement, you would conclude that the Nd-based magnet Figure 2. Second quadrant demagnetization maintains its prop- curves for Nd and Sm-based permanent magnet erties better when materials at room temperature. exposed to a higher demagnetizing field. However, this measurement was performed at room temperature. Quite often in real-life applications, the 12 Magnetics Business & Technology • Fall 2012 www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Fall 2012

Magnetics Business & Technology - Fall 2012
Contents
Editor's Choice
Permanent Magnet Motors in Application
Three-Axis Magnetic Field Measurement: From Nanoteslas to 14 Tesla
Characterizing Permanent Magnet Materials with a Vibrating Sample Magnetometer
Research & Development
Magnets • Materials • Measurement
Application • Component Developments
Industry News
Marketplace
Advertising Index
Spontaneous Thoughts: And the Wisdom to Know the Difference

Magnetics Business & Technology - Fall 2012