intro
Before going off to college to study physics, I promised my parents I'd come home knowing why magnets are magnetic. I realize now that I have two choices:
1. Never come home (because no one knows)
2. Come home and say just cuz.
Inspired by the best just cuz ever (delivered by Feynman), I've decided to choose the second option. However, I can do slightly better if i trade out answering why magnets work for answering how magnets work.
Working in Professor Eckert’s magnetism lab at Harvey Mudd College, I’m finally tackling the second question. This is the third of three articles on how magnets work. Make sure to check out the first two posts if you haven’t already. The three posts are:
1. Building Blocks
2. Domains and Magnetization
3. Application and Experiment
In this post, we investigate magnetic storage devices and explore the experimental physics techniques used to research magnetic materials.
pt. 3 : application and experiment
In this article, we will focus on one of the most impactful implementations of magnetic materials: data storage. From hard drives to floppy disks, magnetic memory has played an integral role in the digital revolution.
data storage
Magnetic devices store information in the orientation of magnetic moments. Different upward and downward oriented “domains” represent binary ones and zeros.
“Domains” is a loose term here; hard drive “domains” consist of ~20 magnetic domains.
Hard drives have a write head that magnetizes domains and a read head that detects domain magnetization. Challenges in magnetic storage include increasing read/write speed and minimizing energy consumption.
Some approaches to these problems involve adding new components. For example, research has explored using lasers to heat domains. Magnetic moments are more fluid at higher temperatures, so heat makes it easier to magnetize domains. The energy saved during magnetization outweighs the energy cost of the laser, making the system more efficient overall.
Another approach is to search for new materials with novel magnetic properties. These materials could either perform better, last longer, or be better for the environment. Research in this field relies on being able to create samples and accurately measure various properties.
The rest of this article will be dedicated to this second approach. We will explore the experimental techniques that are used to create and measure magnetic materials, especially thin films.
creating thin films
In Prof. Eckert’s lab at Harvey Mudd College, thin films are grown through sputtering.
Sputtering involves ionizing argon, then accelerating it in a magnetic field. The ions smash into targets, generating a slight dusting of sample material, which can be metal, ceramic, or even plastic. This ejected material settles to form thin films on the nanometer scale. At the end of this process, the sample is annealed at high temperatures to fuse the sputtered material into its final form.
measuring sample properties
While we care about many different properties of magnetic materials, we will focus on two here: magnetization and anisotropic magnetoresistance.
magnetization
In this series, we’ve analyzed how magnetization changes in response to applied fields, temperature, etc. For example, we investigated hysteresis and predicted that graphing magnetic moment against applied field should form an s-shaped loop. Experimental data supporting these theories can be measured with a vibrating sample magnetometer (VSM). For example, measuring the magnetic moment of MnRh thin films in response to applied fields yields the following hysteresis graphs
which are reminiscent of predicted curves.
As the name suggests, VSMs vibrate a sample within a magnetic field. If the sample is magnetized, Faraday’s law predicts that its movement will induce a voltage difference in the coils, inducing a current from which the magnetization of the sample can be determined.
anisotropic magnetoresistance
Another property that is often researched is the resistance of magnetic materials. There are many dynamics that affect the resistance value. For one, electrons moving at an angle to an applied magnetic field feel an electromotive force that affects their motion. This affects resistance, and the effect is roughly equal regardless of the orientation of the sample.
What’s more interesting is anisotropic magnetoresistance (AMR); magnetoresistance that depends on orientation. Various circumstances can cause anisotropy in magnetoresistance, including irregular grain sizes and imbalanced stress forces. However, the strongest effect is the influence of crystal structure.
In crystals, there are so-called easy and hard axes of magnetization. The hard axis requires more domains to rotate during magnetization when compared to the easy axis. This is an energy intensive process. Thus, at the same applied field, the same sample may have different magnetizations depending on whether it is aligned along its easy axis, its hard axis, or somewhere in between.
Just as magnetization is different based on orientation, resistance varies with angle. This AMR can be measured, but to do so, a device must be able to control many different factors. The device must be able to control pressure, keeping samples under a vacuum to eliminate sample degradation and vapor crystallization. Additionally, the device must control temperature and magnetic field to affect magnetic moments. Lastly, the device must be able to sweep the sample through various angles and read off resistance values.
A Physical Property Measurement System is a device capable of doing all of these things and are thus able to perform a litany of measurements, including AMR. PPMSs can also make VSM measurements with a special vibrating head. Here is a picture of the PPMSs in Professor Eckert’s lab at Harvey Mudd College.
summary
In this post, we took a glance at the applications of magnetic materials as well as the research techniques that help advance the field.
This concludes my three-part series on magnetism. As far as I know, the jury is still out on why magnetism exists. However, we have still built hard drives storing more bits than we could count in a thousand lifetimes. Not bad for a species waiting for the jury to come back.
cited
Calvano's undergraduate senior thesis Magnetic and Structural Properties of MnRh Thin Films