Research Topics and Plans
1. Exchange
Bias (EB)
Exchange bias refers to the shift of magnetic hysteresis loop of a ferromagnet (FM) after being coupled to an antiferromagnet (AFM), due to the unidirectional anisotropy developed in the field cooling process. This was first discovered in 1956 by Meiklejohn and Bean at Bell Lab., as shown in the following figure. Along with the shift of the hysteresis loop is the enhancement of the coercivity due to the interfacial exchange coupling.
The origin of the exchange bias effect is still hotly debated after almost fifty years of research, due to the difficulty in determining the magnetic interfacial structure as mentioned above. Nevertheless, there is little doubt about the crucial importance of the interfacial spin structure. More specifically, the existence of uncompensated interfacial spins give rise to the exchange bias phenomenon.
2. Magnetic
Tunneling Junction (MTJ)
MTJ was introduced in 1995 after the discovery of GMR in 1980’s. It occurs in a trilayer structure composed of two ferromagnetic layers separated by a thin insulating barrier, such as AlOx, around 1~2 nm. Electron tunneling through an MTJ from one magnetic layer to the other results in the current. Resistance of an MTJ is directly related to the tunneling probability, which depends on the relative orientation of the magnetization vectors (M) of the magnetic layers (see figure below). Since the orientation of M depends on the applied field, the result is what’s called magnetoresistance.
The dependence of tunneling on M is duet to the asymmetry
in the electron density of states of the majority/minority energy bands in a
ferromagnet. When the magnetizations in these ferromagnets are parallel, there
is a maximal match between the numbers of occupied states in one electrode and
available states in the other. Thus, the electron tunneling probability is
maximum and the tunneling resistance (R) is minimum. In the antiparallel
configuration, electron tunneling is between majority states in one electrode
and minority states in the other. This mismatch reduces tunneling, causing
resistance to increase. The maximum MR of an MTJ depends on the spin
polarization parameters of the two magnetic electrodes.
3. Magnetic
Nanostructures and Electron Beam Lithography
Low dimensional magnetic systems, such as thin films, multilayers, and surfaces, as well as reduced dimensional systems, such as granular materials and microscopic lithographic devices exhibit many scientifically interesting and technologically useful properties. The physical and magnetic nanostructure of these materials is crucially important in determining their macroscopic magnetic properties.
SEM lithography can be used for the fabrication of a wide variety of devices. Research areas include: quantum structures, such as single electron transistors; optical structures, such as binary holograms and linear/circular gratings; electro-mechanical structures, such as Surface Acoustic Wave (SAW) and MEMS devices; as well as the testing of novel resists and ultra-small sensor fabrication. The above picture shows a two-dimensional array of holes with 300 nm separation. Virtually, SEM lithography can get any patterns done without having to design the expensive masks. Low dimensional magnetic structures can be fabricated and domain states in these systems can therefore be studied.
4. Plans
We are planning to set up a DC magnetron sputtering chamber in William M. Keck microanalysis laboratory, as well as to get the evaporator to work so that we will be able to grow these magnetic thin films. With the powder XRD, AFM, SEM and EDX, we will be able to analyze the structure, surface morphology, chemical components, etc. of these samples. SEM lithography will enable us to make these nano-dots, nano-wires and therefore to study the electrical, magnetic properties of these patterned samples. Students are encouraged to participate in to these projects if they are interested in high vacuum technology, characterizations of thin films, as well as those nano-patterned magnetic structures.