Magnetic composites are engineered materials made of magnetic particles embedded in a non-magnetic, insulating matrix. Such materials are of importance for inductors and antennae used in cellphones and other wireless frequency applications. At these frequencies, eddy currents losses preclude the use of bulk or thin film devices. We are investigating composite materials made of plate-shaped particles oriented in the matrix using a rotating, planar magnetic field. The orientation results in anisotropy, extending the useful frequency range of the material. This work combines modeling and experiments to understand and control the orientation of the particles in the applied magnetic field as well as to predict the permeability of the composite as a function of particle size, concentration and degree of orientation.
Magnetic nanoparticles are of interest for a wide range of technologies including inductor and transformer cores, radio frequency absorbing and antenna loading materials as well as biomedical applications such as hyperthermia (cancer) treatment and diagnostic imaging. Understanding the nonlinear and hysteretic behavior of the nanoparticles in the operating frequency and magnetic field range is requisite to the design and development of these applications; yet, few instruments particularly, with high field and high frequency measurement capability, are available to characterize the nanoparticles. To fill this metrology need, we have developed a high field, high-frequency ac magnetometer capable of providing 130 mT at 50 kHz. In continuing effort, hysteresis measurements, made using the magnetometer, of a variety of magnetic nanoparticles are being used to verify Stoner-Wohlfarth and Néel relaxation theories commonly employed to model nanoparticle behavior.
P. Lenox, L. Plummer, P. Paul, J. Hutchison, A. Jander, and P. Dhagat, "High-Frequency and High-Field Hysteresis Loop Tracer for Magnetic Nanoparticle Characterization", IEEE Magnetics Letters, v. 9, 6500405 (2017); doi: 10.1109/LMAG.2017.2768521.
We are developing a novel imaging technique using magnetic nanoparticle tracers, to observe biological processes occurring in live cells and deep tissue. The technique relies on the well tailored, non-linear magnetization response of the nanoparticles to obtain the desired (sub 100 µm) spatial resolution. Unlike light, the standard means for microscopy in biology, magnetic fields are not scattered or absorbed by tissue and hence, provide a unique advantage, as is exploited in this work, to probe deep tissue. The technique, once validated, will be used to study the interaction of the nanoparticles with cancer cells in zebrafish.
Imagine if you could print your own electronic circuits, sensors, wi-fi antennas in the same way as you do your Word document, where you create your text or picture and, at the stroke of a key, print it to a printer near you. Well, this is the goal of our project on inkjet printed magnetic and magnetodielectric materials. Here, we formulate ink containing magnetic or dielectric nanoparticles. Taking advantage of the unique opportunity afforded by inkjet printing to control material properties, such as composition and anisotropy, on an inkdrop by inkdrop basis, we are printing devices with custom functionality and performance for sensors, actuators, microwave frequency communication systems, and printed electronics. To learn more about our groundbreaking work, read:
Exchange stiffness is an important fundamental property of magnetic materials that is notoriously difficult to measure. The exchange stiffness determines how strongly the atomic magnetic moments within a material are all linked together and plays a key role in how, and how easily, the state of a magnetic memory element can be switched - i.e., does the magnetization reverse all together or can different regions within the magnetic bit switch independently? Thus, designers of magnetic random access memory (MRAM) and hard disk drive media have keen interest in knowing the exchange stiffness in the materials they use. We have developed a unique method of determining the exchange stiffness of magnetic thin films using a Kerr microscope. As seen in this video, when a material is demagnetized, it develops a complex but characteristic domain pattern (domain refers to regions of different magnetization: in the video, black domains are magnetization in one direction and white domains in the opposite direction). This domain pattern develops from a delicate balance between demagnetization energy (fields from a magnet tend to oppose its own magnetization) and domain wall (boundaries between different magnetization) energy. Since the domain wall energy depends critically on exchange stiffness, we can analyze these magnetic images to determine this important material property.
Many magnetic materials are affected by deformation or stress through a process called magnetostriction. Magnetostrictive materials elongate or contract in a magnetic field (the hum of a power transformer is due to magnetostriction in its iron core) or, in the inverse effect, squeezing the materials changes the magnetization. These effects can be useful for making sensors and actuators or for selecting which bits to write in a magnetic memory. On the other hand, in many technological applications of magnetic materials, magnetostriction is undesirable. We have built specialized, ultra-sensitive instrumentation in our laboratory for characterizing the strength of the magnetostrictive effects in a wide variety of materials. We are interested in unique engineering applications of magnetostrictive materials, including their use to increase the storage density of hard disk drives. We have shown, for example, that application of an acoustic wave (stress) to a magnetic disk can reduce the current needed to record data. Ultimately, this process could be used to increase the amount of data that can be stored on a hard drive.
W. Li, B. Buford, A. Jander, and P. Dhagat , "Acoustically Assisted Magnetic Recording: A New Paradigm in Magnetic Data Storage", IEEE Transactions on Magnetics, v. 50, no. 3, 3100704 (2014); doi: 10.1109/TMAG.2013.2285018.
Spin waves (propagating disturbances of the magnetization in a material) have been proposed as a means to transmit information in spintronic circuits (circuits in which the spin orientation of electrons is used to represent the ‘1’ and ‘0’ of binary states). However, even in the best low-loss materials, spin waves travel only about a centimeter before dying out. Thus, we are interested in developing amplifiers for spin waves. Our approach to amplification is to use acoustic waves to interact with the spin waves in magnetostrictive materials such as yttrium iron garnet (YIG). The acoustic waves periodically compress the YIG and modify a “parameter”, the magnetic anisotropy, which governs the propagation of spin waves. When the acoustic waves are applied at twice the frequency of the spin waves, this process can lead to “parametric amplification” of the spin waves. These tricky experiments require the generation of sound waves at 2.5 GHz (100,000 times beyond what can be heard by the human ear) in order to amplify spin waves at 1.25 GHz.