Magnetic Materials

Evolution of permanent magnet technologies.
Complex lattice structure of modern permanent magnets based on rare-earth elements, compared with much simpler magnetite.
A neodymium magnet holding up 1300 times its own weight.
Magnetic impurities placed into a host semiconductor yield ferromagnetism when effective interaction is mediated by carriers.

The study and applications of magnetic materials date back to ancient China, Greece, and the dawn of modern science when in 1600 William Gilbert published his great study of magnetism De Magnete which  gave the first rational explanation to the mysterious ability of the compass needle to point north-south. Around the same time, Decartes formulated mechanical model to explain magentism. The XX century models of magnetic materials  have influenced greatly the development of quantum mechanics, statistical mechancis, metalurgy, and biology. The ferromagnets are typically classified as: 

  • hard (or permanent), with coercivity (applied magnetic field to bring their magnetization to zero) usually above 0.25 T which makes them useful in electromotors, car starters, alternators for wind power generation, computer hard drives, loudspeakers, locks, and microphones.
  • soft with coercivities below 1 mT which finds applications in transformer cores, high frequency and microwave applications, and recording heads.

Depending on their electronis structure, itinerant magents involving delocalized electrons are classified as: iron-series transition metals and their allows; rare-earth magnets; alloys containing heavy transition metals; metallic oxides, and recently pursued diluted magnetic semiconductors.

The Materials Physics projects in Delaware involve synthesis, characterization, and applications of rare-earh magnets, nanostructures hard-soft composites, and diluted magnetic semiconductors (DMS) . Apart from the synthesis, we characterize the materials  using X-ray diffraction (XRD), scanning electron microscopy (SEM), tunneling electron microscopy (TEM), atomic force microscopy (AFM),  and X-ray Photoelectron Spectroscopy (XPS). Physical properties measurements system (PPMS) and SQUID are used to measure materials magnetic properties. 

1. Rare-earth permanent magnets (Hadjipanayis):

Todays high-performance magnets  are made from rare-earth transition-metal intermetallics, especially Nd-Fe-B and Sm-Co. They exhibit a large and persistent magnetization after a magnetizing field is removed. The Nd-F-B are so strong that 10 g can replace 1 kg of carbon steel! This is largely due to the presence of rare-earth ions in their structure (Nd-Neodynium), which add magnetic stability by way of the interaction between their anisotropic atomic orbitals and surrounding charges on neighboring atoms. This in turn determines a preferred orientation of the magnetic moments relative to the material's crystalline axes, effectively pinning the magnetic moments.

Rare-earth ions come from metallic elements that share similar chemical properties; they are not, in fact, especially rare, but they are used sparingly because of the high cost of preparing the material. Rare earths are the 14 elements in the periodic table that start with the element Lanthanum and end with the element Luthecium. The rare earth elements are magnetic because their 4f shells are not full. Rare earth elements have f shell electrons which can accommodate 14 electrons. The f shells is filled initially with all spins pointing in one direction, in accord with the Hunds' rule. Having these electrons align give rise to stronger magnetic fields. Rare earth magnets are made from rare-earth elements. Typically these are compounds or composites with Fe.

2. Hard-soft composites (Hadjipanayis):

Adding a soft-magnetic phase to a hard-magnetic phase can improve the performance of the hard phase.They can be fabricated as multilayers or granular composites with hard and soft phase having volume fractions f and 1-f, respectively. In both cases, the hard regions act as skeleton to stiffen the magnetization direction of the soft phase. The improvement of magnetic properties due to nanostructuring is limited to extrinsic properties, such as energy product (BH)max. The intrinsic properties, such as the Curie temperature, are realized on atomic scales of about 1 nm and cannot be improved by nanostructuring.

3. Diluted magnetic semiconductors (Shah, Xiao):

Magnetoelectronics—electronics that make use of the magnetic (spin) property of the charge carriers—has revolutionized the information storage industry over the past two decades. However, in doing so it has also created a new bottleneck in the process of handling information. Storage is done in metal-based magnetic devices, whereas information processing is optimally realized using semiconductor devices. The exchange of information between these two segregated systems is both energy and time consuming. Significant gain would be achieved if storage and processing could be brought together on a single chip. Semiconductor spintronics as the extension of magnetoelectronics has magnetic semiconductors as one of its most active subfields. It combines all properties needed for both storage and processing of information in one material, and therefore is a promising solution to the component segregation issue.

The primary practical challenge to the commercialization of DMS spintronic devices is their operation temperature. The onset of ferromagnetism in a material occurs at its Curie temperature, but so far, a magnetic semiconductor with a Curie temperature above 200 K has not been found. The DMS fabricated in Delaware include:

  • Co doped ZnO  in which defects play a vital role in promoting carrier-mediated ferromagnetism (Xiao)
  • CoxTi1-xO2 anatase and rutile has been observed to have high Curie temperature (~400 K), while being   transparent in the visible and near infrared regions with a sizable band gap and appreciable conductivity (Shah).

4. Micromagnetics simulations of nanostructures of hard/soft two phase systems (Chui, Hadjipanayis).

Theory & Computation: