Pallavi Dhagat

Domain Structure in CoFeB Thin Films With Perpendicular Magnetic Anisotropy

Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain-wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with superconducting quantum interference device measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain-wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.

Multifunctional Nanomedicine Platform for Concurrent Delivery of Chemotherapeutic Drugs and Mild Hyperthermia to Ovarian Cancer Cells

A multifunctional tumor-targeting delivery system was developed and evaluated for an efficient treatment of drug-resistant ovarian cancer by combinatorial therapeutic modality based on chemotherapy and mild hyperthermia. The engineered iron oxide nanoparticle (IONPs)-based nanocarrier served as an efficient delivery vehicle for doxorubicin and provided the ability to heat cancer cells remotely upon exposure to an alternating magnetic field (AMF). The nanocarrier was additionally modified with polyethylene glycol and LHRH peptide to improve its biocompatibility and ability to target tumor cells. The synthesized delivery system has an average size of 97.1 nm and a zeta potential close to zero, both parameters favorable for increased stability in biological media and decreased elimination by the immune system. The nanocarrier demonstrated faster drug release in acidic conditions that mimic the tumor environment. It was also observed that the LHRH targeted delivery system could effectively enter drug resistant ovarian cancer cells, and the fate of doxorubicin was tracked with fluorescence microscope. Mild hyperthermia (40°C) generated by IONPs under exposure to AMF synergistically increased the cytotoxicity of doxorubicin delivered by the developed nanocarrier to cancer cells. Thus, the developed IONPs-based delivery system has high potential in the effective treatment of ovarian cancer by combinatorial approach.

Inkjet printing of magnetic materials with aligned anisotropy

3-D printing processes, which use drop-on-demand inkjet printheads, have great potential in designing and prototyping magnetic materials. Unlike conventional deposition and lithography, magnetic particles in the printing ink can be aligned by an external magnetic field to achieve both high permeability and low hysteresis losses, enabling prototyping and development of novel magnetic composite materials and components, e.g., for inductor and antennae applications. In this work, we report an inkjet printing technique with magnetic alignment capability. Magnetic films with and without particle alignment are printed, and their magnetic properties are compared. In the alignment-induced hard axis direction, an increase in high frequency permeability and a decrease in hysteresis losses are observed. Our results suggest that unique magnetic structures with arbitrary controllable anisotropy, not feasible otherwise, may be fabricated via inkjet printing.

Surface Acoustic Wave Magnetic Sensor using Galfenol Thin Film

The performance of a surface acoustic wave (SAW) magnetic field sensor using galfenol (FeGa) thin film is investigated in this effort. We measure the change in the SAW velocity as a function of an externally applied magnetic field for galfenol films of varying thicknesses. A maximum change of 0.64%, greater than results reported for similar sensors, is obtained with a 500 nm thick film.

Radiation Tolerance of Magnetic Tunnel Junctions With MgO Tunnel Barriers

The gamma and neutron radiation tolerance of magnetic tunnel junctions with MgO barriers were investigated. The electrical transport and magnetic properties were measured in this effort. The characterization results show no statistically significant change in either tunneling magnetoresistance or coercivity from the radiation.

Acoustically Assisted Magnetic Recording: A New Paradigm in Magnetic Data Storage

A new paradigm in energy-assisted magnetic recording, acoustically assisted magnetic recording (AAMR), is investigated. In this approach, a surface acoustic wave (SAW) is applied to a magnetostrictive recording medium to temporarily lower its coercivity below the write field. Akin to other energy-assisted recording technologies, AAMR provides a strategy to enable writing on high-coercivity media required for thermal stability in high-density disk drives. Using a contact recording tester, it is demonstrated that the write current needed to record data on a galfenol film is reduced in the presence of SAWs. Further, it is shown that standing and focused acoustic waves can, respectively, be used to lower the coercivity in selected regions on the medium.

Ferromagnetic Resonance Detection for Magnetic Microbead Sensors

This paper presents a novel detection scheme for magnetic beads used to label target molecules in immunoassay based sensors. The beads are detected inductively using a microwave circuit consisting of a slotline and coplanar waveguide (CPW) fabricated in a single metal layer. The waveguides are terminated at a short-circuited junction that serves as the active sensor area. When the slotline is excited by an input radio frequency (RF) signal, ac magnetic fields are generated at the junction. These fields are orthogonal to the propagation mode allowed in CPWs. As a result, the signal from the slotline does not nominally couple into the CPW. In the presence of a bead immobilized at the junction, fields from the slotline are distorted and the signal is coupled to the output at the CPW. The output signal is further enhanced by exciting ferromagnetic resonance in the bead. Simulation results indicate a single bead to be detectable with a sensitivity of 1-10 muV/V depending on its location in the active sensor area and the waveguide geometry. The distinctive advantages of this detection technique are ease of implementation, requiring simple and inexpensive fabrication processes; and suitability for integration in lab-on-a-chip systems.

Writing magnetic patterns with surface acoustic waves

A novel patterning technique that creates magnetization patterns in a continuous magnetostrictive film with surface acoustic waves is demonstrated. Patterns of 10 μm wide stripes of alternating magnetization and a 3 μm dot of reversed magnetization are written using standing and focusing acoustic waves, respectively. The magnetization pattern is size-tunable, erasable, and rewritable by changing the magnetic field and acoustic power. This versatility, along with its solid-state implementation (no moving parts) and electronic control, renders it as a promising technique for application in magnetic recording, magnonic signal processing, magnetic particle manipulation, and spatial magneto-optical modulation.

Coexistence of Low Damping and Strong Magnetoelastic Coupling in Epitaxial Spinel Ferrite Thin Films

Low-loss magnetization dynamics and strong magnetoelastic coupling are generally mutually exclusive properties due to opposing dependencies on spin-orbit interactions. So far, the lack of low-damping, magnetostrictive ferrite films has hindered the development of power-efficient magnetoelectric and acoustic spintronic devices. Here, magnetically soft epitaxial spinel NiZnAl-ferrite thin films with an unusually low Gilbert damping parameter (<3 × 10-3 ), as well as strong magnetoelastic coupling evidenced by a giant strain-induced anisotropy field (≈1 T) and a sizable magnetostriction coefficient (≈10 ppm), are reported. This exceptional combination of low intrinsic damping and substantial magnetostriction arises from the cation chemistry of NiZnAl-ferrite. At the same time, the coherently strained film structure suppresses extrinsic damping, enables soft magnetic behavior, and generates large easy-plane magnetoelastic anisotropy. These findings provide a foundation for a new class of low-loss, magnetoelastic thin film materials that are promising for spin-mechanical devices.

Planar Alignment of Magnetic Microdisks in Composites Using Rotating Fields

Soft magnetic composites with aligned anisotropic magnetic particles can have properties not achievable in traditional single-phase materials. These materials are promising for high-frequency inductor and antenna applications. Composites with uniaxial anisotropy, achieved by aligning rod-shaped particles in a constant magnetic field, have previously been reported. In this paper, we discuss the advantages and conditions for realizing composites with planar anisotropy, by aligning disk-shaped particles in a rotating magnetic field. We use Ni-Fe microdisks in a UV-curable matrix as the study system, and present an experimental and theoretical investigation of their alignment under a rotating field in a viscous fluid. The theoretical model is based on Stokes flow of an isolated magnetic oblate ellipsoidal particle in a rotating magnetic field and enables an understanding of the hydrodynamics of the microdisk alignment process. Alignment rate under varying aligning field strength, field rotation frequency, and fluid viscosity is observed using optical microscopy, and characterized by the magnetic properties resultant in the composite. Good agreement is found between measured results and the model. Our work demonstrates for the first time that planar anisotropy in a magnetic composite can be tuned by precise control of aligning field strength and duration, and provides the insight necessary to engineer the alignment dynamics for achieving high-frequency operation.