Metrology Needs for Magnetic Nanoparticles: How to Measure the Magnetic Properties of
Individual Magnetic Nanoparticles?
David E. Nikles, The University of Alabama
The science of magnetic nanoparticles, with sizes of 3 to 10 nm has been growing rapidly due to
potential magnetic recording and biomedical applications. The particles present two different
metrology problems structural characterization and magnetic characterization. The particle size
range pushes the limits of electron microscopy for structural characterization and we are
anticipating that local electrode atom probe will provide 3D atomic positional information with
near atomic resolution.
L10 FePt nanoparticles are a serious candidate for future magnetic recording media with data
storage densities beyond 1 terabit/in2. We pursue basic research to solve the materials science
problems that would allow these particles in be used in future disk drive with terabyte data
storage capacities. We have prepared FePt, CoPt, FePd, FeCoPt, FeCuPt, FePtAg, FePtAu and
FePdPt nanoparticles. As prepared the particles have a fcc structure, are superparamagnetic and
had a narrow distribution of particle sizes. Casting dispersions onto silicon wafers gives selfassembled films consisting of close-packed arrays of particles. The films must be heated to
temperatures above 550°C to transform the particles to the L10 phase, having high
magnetocrystalline anisotropy, giving ferromagnetic films. The heating also gives rise to particle
agglomeration and grain growth, resulting in a distribution of particle volumes. We have shown
that FePtAg or FePtAu nanoparticles can be transformed by heating at lower temperatures.
During heating the Ag or Au leave the particles, creating lattice vacancies thereby allowing the
Fe and Pt atoms to move to their L10 lattice positions.
High resolution EDX analysis on films containing as prepared FePt nanoparticles revealed a
distribution of particle compositions. Although the chemistry to prepare FePt nanoparticles gives
narrow particle size distributions, the composition varied from particle to particle. A distribution
of particle compositions was also observed for FePtAu nanoparticles. Measuring the composition
of individual particles having diameters of 3 to 4 nm is very difficult, beyond our capability, and
we collaborate with some of the best electron microscopy groups in the world (Hitachi Maxell
and J. Chapman at U. of Glascow). It would be most useful to find routine methods to determine
the composition of individual nanoparticles so that the chemists learn how to prepare particles
with narrow composition distributions.
The ultimate of high-density limit magnetic recording technology is expected to be recording in
monolayer films of FePt nanoparticles, where one bit is recorded in a single particle. There are
many challenging problems to be solved. The most fundamental problems are how to record and
how to read at this extreme spatial resolution. Furthermore, we know that we have a distribution
of particle compositions and the sintering the occurs during heat treatment gives rise to a
distribution of particle volumes. The magnetic characterization tools currently available to us
(VSM and AGM) measure the magnetic properties of a large collection of particles. Our ability
to extract information from these measurements is limited by the need to account for the
unknown distribution of magnetic properties. Accordingly the metrology challenge is how to
measure the magnetic properties of individual particles. A 3.5 nm diameter L10 FePt nanoparticle
has a volume of 2.2 x 10-20 cm3 and a magnetization of 2.5 x 10-17 emu.
Magnetic nanoparticles have potential biomedical applications including detection of
biomolecules, separation of biomolecules and cells, MRI contrast enhancement and hyperthermia
therapy. For example the Naval Research Laboratory has developed the Bead Array Counter
(BARC biosensor) for the rapid, multi-analyte detection of biowarfare agents. The counter
consists of an array of GMR sensors with each sensor element having a different type of singlestranded DNA bound to its surface. Single-stranded DNA is extracted from samples taken from
the field, amplified by PCR, biotinylated and then passed over the BARC biosensor. If the DNA
is from a biowarfare agent, it will hybridize only with the complimentary DNA on the surface of
the biosensor. Streptavidin labeled magnetic particles are then passed over the sensor and bind
only at those sensor elements having biotin, resulting from sample DNA binding. The field from
the bound magnetic particle will cause a change in the resistance of the GMR element. Thus the
BARC biosensor can identify and quantify multiple biowarfare agents simultaneously.
Beyond detection of biomolecules is the use of magnetic fields to manipulate biological
processes. One approach is to selectively bind a magnetic particle to a site and use an ac
magnetic field to heat the particle and the region around the particle, thereby influencing a
biological process (e.g. denaturing a protein). In order to achieve selective binding, proteins,
DNA or antibodies are covalently linked to the particle surface. To develop this science it would
be useful to manipulate individual nanoparticles so as to place them precisely in a desired
location perhaps by a magnetic field.