Matea Trevino and Dr. Karine Chesnel, Department of Physics and Astronomy

Introduction

Magnetite (Fe3O4) nanoparticles have the unique ability of being superparamagnetic. When no field is applied to the nanoparticles, the nanoparticles’ assembly has a net magnetization of zero, meaning that the nanoparticles’ magnetic moments align in random positions leading to a net magnetization of zero. When a field is applied, the particles’ magnetic moments align with the magnetic field. This is considered superparamagnetic because this is looked at on the nanometer (nm) scale and not the atomic scale. Being able to understand this phenomenon will open up many doors in technology and medical usage. Nanocrystals, such as Fe3O4 particles, are good candidates for potential developments as they carry a strong magnetization. The nanoparticles can be used for high density magnetic storage and also for the delivery of medicine to pin point areas in the body, for example hyperthermia, since magnetite is not harmful to our bodies. While the magnetic properties of such particles are a central aspect for technological applications, their chemical stability is a very important factor that can impact all the other properties. It is therefore necessary to fully control and understand the influence of the chemical composition, the crystallographic structure, and the size and shape of the nanoparticles and how this affects their electronic and magnetic properties. At a larger spatial scale, it is important to understand how the density, the morphology, and the spatial distribution of the nanoparticles in self-assemblies can influence the inter-particle magnetic couplings, the long-range order and the dynamics of magnetic fluctuations.

Methodology and results

In order to be able to study the behavior of the nanoparticles, we have collaborated with the group of Dr. Roger Harrison and the group of Dr. Juliana Boerio-Goates at BYU’s Chemistry department, in order to prepare three different fabrication methods to synthesize the nanoparticles. We used an inorganic salt method, an inorganic solution method, and an organic solution method. The inorganic salt method produces the largest particles in size, the inorganic solution method produces medium size particles, and the organic solution method makes the smallest particles. We tested these samples by using x-ray diffraction (XRD) and transmission electron microscopy (TEM) to get an understanding of the physical properties of the nanoparticles. The XRD gives us information about the crystallographic structure of the sample, meaning if we really made magnetite (Fe3O4) or its counterpart hematite (Fe2O3). We do this by looking at the score that the XRD gives us, by comparing it to the template that is magnetite and hematite. This lets us know what our sample was made out of and also gave us a general overview of how big the particles are. This is done by looking at the peaks on the XRD spectrum and by using the width at half max of the peaks and the Scherrer equation; we get an estimate of how big the particles are. We also imaged the particles with TEM and verified these results by estimating the size of the individual particles themselves. On the TEM images, we can also see how the nanoparticles assemble themselves on a substrate, and the shape of the nanoparticles.

We noticed that the particles like to stick together and that they usually go into a hexagonal pattern. This lets us get a grasp of the physical properties of the nanoparticles and, with this in mind; we look at the bulk magnetic properties of the nanoparticles.

In order to study the bulk magnetic properties of the nanoparticles, we use the vibrating sample magnetometer (VSM). The VSM works by vibrating the sample in the center of a superconducting magnet. With this vibration, it creates a flux and changes the flux over time. This change in flux is picked up by the detection coil, in the center of the magnet, and uses the Faraday’s law to provide the magnetic moment of the sample. With this information, we performed magnetization loops, field-cooling (FC), and zero-field-cooling (ZFC) measurements. The magnetization loops inform us about when the sample reaches saturation, or when all of the magnetic moments are aligned with the external magnetic field. We also used the loops to make sure that our samples had not moved during the measurements. We also looked at the magnetization loops to see if there was any hysteresis in the loop, meaning that the ascending and descending branches of the loop do not match. We have noticed that the smaller the particles, the smaller the hysteresis is, but overall the hysteresis is not very big with any of our samples, which is expected for nanoparticles. We also performed FC and ZFC measurements. When we did the FC measurements, we usually started with the sample at 400 K and cooled it down to around 20 K and heated it back up to 400 K, all while applying a magnetic field. We then complemented the FC measurements with ZFC measurements. A ZFC measurement was usually done by starting the sample at 400 K and cooling the sample down to around 20 K, under no magnetic field. Once at low temperature, a magnetic field, of same magnitude for the FC measurement, was applied while the sample is heated back up. When comparing FC to ZFC curves, we noticed a separation between the two curves at low temperature, with a bump on the ZFC curve, and this is what we used to estimate the blocking temperature. At higher temperatures, the FC and ZFC curves join. We have noticed that the lower the magnetic field, the clearer it is to establish the blocking temperature. We are also expecting to see that the larger the particles, the higher the blocking temperature is; at this point, more testing needs to be done in order to confirm this.

Discussion and Conclusion

We have learned that we can successfully fabricate magnetite nanoparticles using three different methods; inorganic salt, inorganic solution, and organic solution. We looked at their physical properties through XRD and TEM and determined if the sample was truly magnetite or hematite. We also determined the size of the nanoparticles and how they assemble. Through our measurements, we measured the saturation point. We determined that there was a slight hysteresis in the magnetization loops, in which the smaller the particle, the smaller the hysteresis. We started to measure the blocking temperature of the samples. We will see how the particle size influences the blocking temperature. We are improving our scientific method by using lower magnetic fields for the FC/ZFC measurements. In addition, we have looked at nanoparticles self-assemblies, in order to understand the magnetic interactions and ordering between the nanoparticles. We do this by using synchrotron radiation. In our case, we used the Stanford’s synchrotron facility. We performed X-ray magnetic scattering on the nanoparticles. The scattering patterns give information about the magnetic order in the assemblies. I helped with this experiment and used most of my ORCA grant to finance the trip to SSRL at Stanford.