Magnetics Business & Technology - Spring 2015 - (Page 8)

FEATURE ARTICLE First-Order-Reversal-Curve Analysis of Multi-Phase Ferrite Magnets By B. C. Dodrill, Senior Scientist * Lake Shore Cryotronics Magnetically hard ferrite powders are widely used, due to their low-cost production and good performance in many electronic devices such as electrical motors, speakers and recording media. Usually ferrites are single-phase magnets but when the stoichiometry is not precise or the fabrication process is not adequate, the ferritic phase may be accompanied by other phases that promote magnetic interactions, which results in a decrease of the magnetic performance of the magnet. Additionally, there is currently strong interest in exchange spring magnets, which are comprised of a hard high coercivity phase exchange coupled to a soft high saturation magnetization phase, as this leads to a magnet with increased energy density. This results in reduced costs because less hard phase material is required. Other examples of multiphase magnets include nanostructures, such as soft shell/hard core nanowires, hybrid magnets, etc. The magnetic characterization of such materials is usually made by measuring a hysteresis loop. However, it is very difficult to unravel the complex magnetic signatures of multi-phase magnets, or to obtain information of interactions or coercivity distributions from the hysteresis loop alone. First-order-reversal-curves (FORC) provide a means for determining the distribution of switching and interaction fields between magnetic particles, and for distinguishing between magnetic phases in composite materials that contain more than one magnetic phase. In this article, we will discuss the FORC measurement and analysis technique, and present results for various ferrite multi-phase composites. Magnetization Measurements & First-Order-Reversal-Curves The most common measurement that is performed to characterize a materials magnetic properties is measurement of the major hysteresis or M(H) loop. The parameters that are usually extracted from the M(H) loop are illustrated in Figure 1 and include: the saturation magnetization Msat (the magnetization at maximum applied field), the remanence Mrem (the magnetization at zero applied field after applying a saturating field), and the coercivity Hc (the field required to demagnetize the material). For permanent magnet materials, the maximum energy product BHmax, which is determined from the second quadrant demagnetization curve, is also commonly of Figure 1. Hysteresis M(H) Loop for a NdFeB Sample 8 Magnetics Business & Technology * Spring 2015 interest. Note that the measured coercivity Hc is the average coercivity (or average distribution of switching fields) of the entire ensemble of magnetic particles that constitute a magnetic material. Figure 2. Measured First-Order-Reversal-Curves for a Ferrite Permanent Magnet More complex magnetization curves covering states with field and magnetization values located inside the major hysteresis loop, such as first-order-reversal-curves (FORC)1, can give information that is not possible to obtain from the hysteresis loop alone. These curves include the distribution of switching and interaction fields, and differentiation of multiple phases in composite or hybrid materials containing more than one phase. A FORC is measured by saturating a sample in a field Hsat, decreasing the field to a reversal field Ha, then sweeping the field back to Hsat in a series of regular field steps Hb. This process is repeated for many values of Ha, yielding a series of FORCs. This is illustrated in Figure 2. The measured magnetization at each step as a function of Ha and Hb gives M(Ha, Hb), which Figure 3. A 2-D FORC diagram for a periodic array of Ni nanowires showing the distribution of switching (Hc) and interaction (Hu) fields2. www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2015

Editor's Choice
First-Order-Reversal-Curve Analysis of Multi-Phase Ferrite Magnets
Magnetics Design Tool for Power Applications
Magnet Inspection Tool with High Magnetic and Mechanic Accuracy
Research & Development
Electromagnetics
Software & Design
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: The Patent Challenge

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