This week I will introduce the second way we analyze a sample’s magnetic properties. There are two key things this measurement relies on: Eddy currents and the exchange bias.

Eddy currents are small loops of electric current that are induced inside a material when it is passed through a magnetic field. The reason these currents form is similar to Faraday’s law of induction. When we move a material through a magnetic material, the amount of magnetic flux penetrating through the material  changes. In order to try and keep a constant amount of flux, if the material is conductive current loops will form that create a magnetic field to either aid or oppose the applied field depending on direction of the movement 

The exchange bias is a phenomenon which happens at the interface between an antiferromagnet and a ferromagnet. In an antiferromagnet, every atomic layer has the opposite magnetic moment of the adjacent ones. However, the end layers will act as small ferromagnets that can either help or resist the ferromagnetic layer from switching its polarization. For example if a ferromagnet usually switches its polarization direction at -2 Oe and +2 Oe, the antiferromagnet may cause the layer to switch at -4 Oe and 0 Oe instead. In this example, +2 Oe shift is the effective field created by exchange bias. Having a good exchange bias is crucial for us because it allows us to confirm that our antiferromagnet actually has good antiferromagnetic properties. 

Now onto the actual measurement, vibrating-sample magnetometry (VSM). A sample is vibrated up and down at a very fast speed. Because of how fast it is, even though the current produced is very low, the magnetic field created by the eddy currents is measurable. As the sample vibrates, we apply a changing magnetic field that goes from say -300Oe to 300Oe. Then we graph the measured emu (electromagnetic unit) v.s. the applied field. A large dip/rise in the graph represents a ferromagnetic layer switching its polarization. In order to see exchange bias, we first apply a large magnetic field (2 Tesla) to set the direction of the ferromagnets and corresponding atomic antiferromagnetic layer touching the ferromagnet and cool it down to a very low temperature (-100 C). This lets us be sure we are looking for the change in magnetic field in the right direction (e.g. when we apply a positive field the effective field will be in the positive direction and the switching field values shift towards the negative side). After measuring this sample, we must measure again after cooling at -2 Tesla to ensure that any shift we see is actually due to exchange bias. The reason we do the measurement at low temperature (also around -100 C) is because exchange bias usually has a larger effect at lower temperatures when the antiferromagnet has a better structure. We do VSM measurements in this project to ensure that the NiO in the MTJs is actually acting as an antiferromagnet and it has the properties needed for the magnon theory to work. We have done some VSM measurements, but unfortunately so far there has been negligible exchange bias. This is likely because the CoFeB layer on top of the NiO was oxidized, and the oxide forms a non-ferromagnetic and non-antiferromagnetic material. This means we cannot see the effect of NiO on CoFeB to determine if it is a good antiferromagnet as the surface between NiO and CoFeB is not flat or continuous.