# User:Andy Maloney/Notebook/Lab Notebook of Andy Maloney/2010/02/10/PBL1

## Problem statement

In class, we were given a problem and an associated grade sheet. My portion of the presentation is Part B stated in the problem sheet. Below, I have outlined what I must talk about.

• Iron nanoparticles
• Properties
• Coatings
• Nobel metals
• Silica
• Oxides
• Polymers
• Discuss the above coatings and how:
• They prevent degradation from oxygen and H2O.
• The coatings affect the iron nanoparticle properties.

This is a problem based learning exercise that necessitates group activity. The members of my group are dealing with Part A outlined in the problem sheet where they have doled out the various parts of making a quantum dot for a specific biological investigation. Their components include: picking a target (one person), synthesis of the quantum dot (one person), and characterization of the quantum dot (one person). It looks like I got the short end of the stick here...

## Introduction

Since I will be dealing with magnetic iron nanoparticles, I need to explain a little bit about magnets.

### Diamagnetism

Some materials will actually deflect external magnetic fields such that they can exclude the external magnetic field from going through them. An example is the picture below which show pyrolytic carbon, taken from Wikipedia's article on diamagnetism.

Insert picture of field lines going through a material

Another cool image is this movie which shows a levitating frog, double click the image to play.

Heuristically, diamagnetism comes from the material producing an opposing magnetic field that excludes the external magnetic field.

### Paramagnetism

Paramagnetism is a weak effect of materials and can only be detected when an external magnetic field is applied. Paramagnets follow Curie's law (if the magnetization of the material is weak) which can be stated as:

$\boldsymbol{M} = \chi \cdot\boldsymbol{H}=C\cdot \frac{\boldsymbol{H}}{T}$

M is the resulting magnetization
χ is the magnetic susceptibility
H is the auxiliary magnetic field, measured in amperes/meter
T is absolute temperature, measured in kelvins
C is a material-specific Curie constant

Or, more generally as:

The magnetization of a paramagnet depends linearly on the external magnetic field and inversely on the temperature.

### Ferrimagnetism

Ferrimagnetism is a property of materials in which the magnetic moments of the atom will align themselves with an external electric field but, sublatices will antialign themselves to the magnetic field with a smaller amplitude. The overall effect is that the material has a slightly less magnetization than a ferromagnet.

### Ferromagnetism

This the most familiar form of magnetism since refrigerator magnets are ferromagnets. The material retains its magnetization even after an external magnetic field has been removed.

The material creates domains where the magnetic moments of atoms are all aligned in that domain, thus forming a magnet. However, ferromagnets do not necessarily show a bulk magnetization. This is due to the fact that the material will make many domains that ultimately will cancel the bulk magnetization. Unless, the ferromagnet is placed in a strong magnetic field. Domains will align with the external magnetic field and will get stuck in that configuration due to imperfections in the bulk. Thus creating a permanent magnet.

### Superparamagnetism

Now we get to the important part of magnetism for nanoparticles. If an iron nanoparticle is below about 40 nm in diameter, then it can be approximated as being a single magnetic domain (see the section above).