Investigation of Magnetic Recordings Made Throughout History

Ben Ecker

beckerpas@gmail.com

University of Rochester

 Department of Physics and Astronomy

Introduction 

The goal of this project for me was to gain experience looking at magnetic materials with the use of the facilities and microscopes at the URnano Center, where the experience could potentially be useful for future graduate work.  This was to be done by trying to investigate the magnetic domains in numerous sample with different techniques, but where the main technique for this project was to be magnetic force microscopy (MFM).  This technique is significantly more difficult than ordinary atomic force microscopy (AFM), and it generally involves a two pass technique where the first pass determines the surface morphology and the second determines the magnetic morphology of the sample.  It is so much more difficult that even exchanging the normal tips to the magnetic tips becomes problematic, the magnetic tips must first be magnetized on a permanent magnet but removing them from the magnet is often extremely difficult due to the interaction of the tip and magnet.  Unfortunately this is exactly the problem that I ran into and both magnetic tips were broken when the normal tips were being replaced and the resulting MFM data was not collected as planned.  Necessarily, the direction of the project changed due to the breaking of the magnetic tips.  Instead, the project focused on looking at the surface morphologies of the samples to see if any potential magnetic domain information could be inferred without directly observing them with MFM.

Samples

The common theme of the samples were that they are all magnetic recordings made throughout history, which included a (potential) meteorite shard, a floppy disk, a zip drive disk, and a hard disk.  The Campo del Cielo meteorite shard originated from a region in Argentina and was expected to have made impact approximately four to five thousand years ago, it was acquired from geological sample vendor and was expected to be authentic.  It turns out, that meteorites are an incredibly useful tool for the paleomagnetism community as it allows them to look back to a fixed moment in time.  When the meteorite makes impact, the shards undergo tremendous heating and stress allowing for the magnetic domains to interact with the earth’s current magnetic field before cooling back down and becoming fixed.  The floppy disk and zip drive disk are both information storages devices that encoded information into magnetic domains, and they used similar technologies that allowed them to be relatively inexpensive information storage devices that could be transported between locations.  The devices main component is a thin circular Mylar section that has a magnetic coating where the information is stored, the main difference between the two types of disks are their section sizes and domain size where the smaller domains allow for high information storage.  Unfortunately the technology that allowed them to be so inexpensive came at the cost of capacity, and largely fell out of fashion by the mid-2000s.  A similar information storage device to the floppy and zip disk is the hard disk drive, and operates under similar principals to the floppy and zip disk.  The technology has been further refined to where a magnetic coating is placed on circular aluminum platters, and they have become the prevailing secondary storage devices for general purpose computers.  Lastly, a read-write head of hard disk drive was also examined.

Methods

  1. Light Microscopy

    Sample were often viewed under a light microscope initially to ensure proper sample preparations before placing into the AFM or SEM and wasting valuable instrument time.

  2. Sample Preparation - Polishing

    The meteorite shard was naturally rough and required extensive polishing before viewing.  Sequential polishes on a polishing wheel with silicon carbide grinding sheets were performed starting at grit sizes (average grit size in microns) 300 (30), 800 (12), and 1200 (2.5).

  3. Sample Preparation – Etching

    It turned out that meteorite was initially rather dull and the main surface features were just the polishing lines.  To see the material grain sizes, the shard was placed in a Nital solution for about a minute, a mixture of nitric acid and alcohol.  Extreme care was used due to the volatility of the solution.

  4. Sample Preparation – Conductive Coating

    The floppy disks were partly made of Mylar, which is insulating, and to prevent any potential charging effects when viewing the samples in the SEM, the samples were coated with a thin layer of gold using a Denton Sputtering system.  Proper groundings to the samples were also added with the use of conductive carbon paint.

  5. Atomic Force Microscopy

    Extensive AFM was performed before attempting MFM to determine sample surface morphology.  This would have greatly helped if the tips had not been broken in that it would have allowed me to see if the tip was seeing the surface correctly with the first pass.  There was the potential for the first pass to become severely distorted due to both the surface and magnetic interactions.  AFM offered a way to look at the surface roughness and verticality, which the SEM is often unable to provide easily.
     

  6. Scanning Electron Microscopy – SE2, Inlens, BSD

    The SEM with appropriate viewing parameters offers superior visualization of the surface morphology compared to AFM, and potentially useful secondary information.  Care was taken to ensure the samples were firmly secure to protect the instrument from any possible magnetic interactions before being placed into the Zeiss Auriga SEM.

  7. Energy Dispersive X-ray Spectroscopy – X-ray collection, and Mappings

    EDX allows for the determination of sample elemental composition, and when mapping, spatial variations in elemental compositions can be seen.

Results & Discussions

Due to changes in the project direction midway, I ended up collecting a tremendous of data and images. Some were good images and some were pretty awful images.  For brevity’s sake, I am only displaying a collection of images from each sample that I believe best represent the sample while still allowing for comparisons between the different sample surfaces.  Most of the samples have an SEM images taken at a lower magnification (1kX) to see large surface morphology and an SEM image taken at a higher magnification (10kX) to see the coatings/grain surface morphologies.  AFM measurements are usually presented for 30umx30um and 2umx2um scale with both 2D and 3D perspectives.  Lastly, I tried to incorporate one or two additional bits of information that I believe were noteworthy or interesting.

Pre-Etched Meteorite Shard

SEM:  In the pre-etched SEM images shown below, the main surface streaks were related to the polishing.  The large clumps are most likely dust and polishing debris not properly removed before imaging.  You can clearly make out polishing streaks going in two different direction,  which is due to lack of prior knowledge polishing on behalf.  During polishing, I rotated the shard to get a better grip on it with respect to the wheel changing the direction of the polishing pattern.
pre-etched meteorite, 1kX SEM picpre-etched meteorite, 10kX SEM pic
AFM:   The AFM is very clearly able to resolve the two different polishing streaks.  In the images there are vertical streaks present, which are an artifact of the tip striking the surface a bit to hard.  Roughness calculations were performed in regions lacking the large polishing and dust contaiminations.

pre-etched meteorite, 30um 2D AFMpre-etched meteorite, 2um 2D AFM
pre-etched meteorite, 30um 3D AFMpre-etched meteorite, 2um 3D AFM


Sample
Scan Size		 Root Mean Square
(um)
Pre-Etched
30umx30um		 0.0259
Pre-Etched
2umx2um		 0.00804

EDX Spectrum:  The only elements visible in the characteristic x-ray spectrum is iron and nickel, which is a strong suggestion that the elemental composition is similar to verified meteorites.
pre-etched meteorite, EDX spectrum
Light Microscope Image:  Shown below is one of the light microscope images taken of the pre-etched sample.  The polishing lines are very clear.
pre-etched meteorite, light microscope


Post-Etched Meteorite Shard

SEM:  After the Nital etching process, the meteorite shard's polishing streaks are sufficiently suppressed and the grains become easily visible.  The surface is now remarkably patterned in a rough pattern reminiscent of sea waves.
post-etched meteorite, 1kX SEM picpost-etched meteorite, 10kX SEM pic

AFM:  The AFM very easily is able to resolve the etched shards roughness and is able to pick up the rolling up and down features of the grains.  The sample roughness for both scans has gone up a reasonable amount compared to the pre-etched shard.

post-etched meteorite, 30um 2D AFMpost-etched meteorite, 2um 2D AFM

post-etched meteorite, 30um 3D AFMpost-etched meteorite, 2um 3D AFM

Sample
Scan Size		 Root Mean Square
(um)
Post-Etched
30umx30um		 0.0363
Post-Etched
2umx2um		 0.0248

EDX Mapping:  It was my hope that by etching the shard, the grain boundaries would display some variations of elemental composition.  Unfortunately, nital does not etch based off of elemental composition but instead by the grains phase orientation.  Thus the EDX mapping of the grains proved not useful.  Afterward, I tried taking an EDX mapping of a crack in the sample.  The first EDX map is of iron and the second is of nickel.  From the mappings, it is clear that the crack is deficient in iron, and the nickel is pretty uniformly spread out.

post-etched meteorite, crack SEMpost-etched meteorite, crack EDX Fe Mappost-etched meteorite, crack EDX Ni Map

Floppy Disk

SEM:  The SEM images of he floppy disk disply the spindly structures made by the magnetic coating on the mylar sheet.  There are regions of bright and dark spots in both SE2 and Inlens detector images, at both high and low accelerating voltages.  EDAX mappings also provided little in the way of determining the variations in images.  As best as I can reason, the surface is not as flat as it appears from the AFM images.  Instead of flat the surface is forming mountaints and valleys and some of the escaping electrons are being blocked from reaching the detectors leading to regions of dark and bright.  It should also be noted that the bright edge effect is clearly visible in the second image, indicating an exposed surface.  These features are stationary indicating that it is not a charging pattern.
floppy disk, 1kX SEM picpost-etched meteorite, 10kX SEM pic

AFM:  The AFM picture further confirm the mountain and valley theory, which are clearly visible in the 2umx2um image.  It again appears that the tip was striking the surface a bit to hard leading to the streaking image shown in the 2um image.  Lastly below the roughness measurements, are two 30umx30um images of the similar areas.  The left image was scanned top to bottom, while the right image was scanned left to right.  The bright spot orientation appear to change their shape depending on the scan direction, indicating that there was most likely something wrong with the tip, perhaps it picked something up on one side leading to distorted scans.

floppy disk, 30um 2D AFMfloppy disk, 2um 2D AFM

floppy disk, 30um 3D AFMfloppy disk, 2um 2D AFM

Sample
Scan Size		 Root Mean Square
(um)
Floppy
30umx30um		 0.0153
Floppy
2umx2um		 0.00454


floppy disk, 30um 2D AFM, top to bottom scanfloppy disk, 30um 2D AFM, left to right scan

EDX Spectrum:  The EDX spectrum shows that the floppy disk was coated with a predominately iron oxide, with perhaps a bit of cobalt and chlorine. 

floppy disk, EDX spectrum


Zip Disk

SEM:  The zip disk sem images again show a spindly pattern on the surface, and they also display the dark and bright regions.  This is most likely due to how the surface coatings are deposited since the processes are similar for both zip and floppy disks.   The edge effect is particular visible in the 10kX magnification, and this again is not due to charging..  I also included images of the samples after taking a full EDX mapping of the bright and dark spots.  The zip disk post EDX image is on the left and the floppy disk post EDX image is on the right.   It is very easy to see the rip in surface exposing the insulating mylar films underneath.  What was so remarkable about these photos, is that the damage was not occuring where the beam was striking the surface, but instead further away.  The floppy disk image in particular even shows the carbon deposit from the beam showing very clearly where the mapping took place, yet the damage occured some distance away.
zip disk, 1kX SEM piczip disk, 10kX SEM pic

zip disk, beam damage resulting from EDX mapping scansfloppy disk, beam damage resulting from EDX mapping scans

AFM:  The AFM picutre again suggest mountains and troughs in the surface morphology.

zip disk, 30um 2D AFMzip disk, 2um 2D AFM

zip disk, 30um 3D AFMzip disk, 2um 3D AFM

Sample
Scan Size		 Root Mean Square
(um)
Zip
30umx30um		 0.00982
Zip
2umx2um		 0.00701

EDX Spectrum:  The EDX spectrum for the zip disk is particulary puzzling.  It appears to have another magnetic iron oxide coating, but it also features paramagnetic titanium.  I was unable to find any work suggesting the presence of titanium, but I was also not able to verify the coating used on zip disks.  What I do know is that the zip disk's magnetic coating was able to increase the storage capacity by about a factor of 10.

zip disk, EDX spectrum



Hard Disk

SEM:  The hard disk was particulary difficult to collect SEM images of, due to the incredibly smooth surface, there was just very little topologically going on.  So secondary electron detectors were unable to pick up much of anything.  There is also a backscattered detector image featuring a bit of dirt for contrast shown below, but it was also fairly dull of an image.
hard disk, ~70kX SEM, Extremely little surface variationsHard Disk, BSD image

AFM:  The AFM was able to clearly pick up streaking patterns across the surface.  While the scratches are incredibly small ~0.5 nm, shown by the surface roughness calculation and sectional analysis, it is likely damage caused by the head which lead to the crashing of the hard disk.  Even this small variation likely lead to the disk failing, which is how I managed to obtain a piece of one.


hard disk, 25um 2D AFMhard disk, 0.5um 2D AFM

hard disk, 25um 3D AFMhard disk, 0.5um 3D AFM

Sample
Scan Size		 Root Mean Square
(um)
Hard Disk
25umx25um		 0.000258
Hard Disk
0.5umx0.5um		 0.000523


hard disk, section AFM scan

EDX Spectrum:  The EDX spectrum for the hard disk  clearly shows peaks for cobalt, nickel, and phosphorus.  The magnetic coating for hard disk are usually an outermost 5um layer of cobalt, with a 20 um layer of nickel phosphorous.  The platter is mainly aluminum with a bit of magnesium, but the core of the platter is covered by too much magnetic coating to see any characteristic xrays.  The phosphorus was being used to make the nickel nonmagnetic, and perhaps the titanium and chlorine was being used in the zip disk in a similar fashion.
hard disk, EDX spectrum

Read-Write Hard Disk Head

SEM:  Lastly, images of the read-write head of the solid disk were imaged in the SEM.  The image on the left shows the entire cross section of one such head,  where the contact is the visibly bright area on the left of the image.  The middle section of the image is another magnetic coating using to read the magnetic field of the disk.  The right images shows the magnetic coating.
read-write head, entire sem imageread-write head, magnetic coating sem image

EDX Mapping & SEM Position:  EDX mapping were taken of the contact shown in the above image.  The first two images show the contact position and the SEM image of the spot where the mapping was taken.  The mapping shown below correspond to oxygen, lead, aluminum, magnesium, iron, and silicon going from left to right.  The black contact clearly contains oxygen, magnesium, and iron, while  it is deficient in lead, aluminum, and silicon.

read-write head, contact sem imageread-write head, contact sem image, EDX mapping location

read-write head mapping Oread-write head mapping Pbread-write head mapping Alread-write head mapping Mnread-write head mapping Feread-write head mapping Si

All Atomic Force Microscopy Roughness Comparison

The atomic force microscope software program was able to calculate the surface roughness of the scanned regions.  Where dust or contamination was present, regions avoid the dust were chosen for roughness calculations.  Changing directions for scanning had very little effect to surface roughness, even when the tip appeared to be slightly defected.  From the table, it is very clear that etching increased the surface roughness by a factor of about ~2-3.  It is also clear that as the magnetic domain size decreased from floppy, to zip, to hard disk, the surface roughness of the corresponding magnetic coatings also decreased.  
Sample
Scan Size		 Root Mean Square
(um)
Pre-Etched
30umx30um		 0.0259
Pre-Etched
2umx2um		 0.00804
Post-Etched
30umx30um		 0.0363
Post-Etched
2umx2um		 0.0248
Floppy Disk
30umx30um		 0.0153
Floppy Disk
2umx2um		 0.00454
Zip Disk
30umx30um		 0.00982
Zip Disk
2umx2um		 0.00701
Hard Disk
25umx25um		 0.000258
Hard Disk
0.5umx0.5um		 0.000523


Acknowledgements

I want to express my deepest thanks to Brian McIntyre, the course instructor, for all of his help with the class work and graduate work related questions.  Without his help and guidance, it would have been very unlikely that I got any data or results at all.  I would also like to thank my teaching assisntant Hanyuan Zhang for all his help during the labs, and my labmate Congcong Wang who helped keep me on time to class and lab this semester.  Lastly, I want to thank the Dean's office for making this course available to graduate students.  The knowledge and experience gained in this course, will prove invaluable in the future.

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