Creating Detecting and Analyzing Magnetic Skyrmions

Grace Dirks Author

07/28/2021 Added

63 Plays

Description

The goal of this research was to create large and optically visible magnetic skyrmions

Searchable Transcript

Search:

[00:00:01.080]Hello everyone. I am Grace Dirks, and this summer I worked with my advisor,
[00:00:04.200]Dr. Andrew Baruth in the Adenwalla lab. We worked on creating,
[00:00:07.650]detecting and analyzing magnetic skyrmions.
[00:00:10.080]And we were funded by the MRSEC professor/student pair REU
[00:00:13.620]So what are we doing?
[00:00:14.490]We are trying to create optically visible magnetic skyrmions.
[00:00:17.520]A skyrmion is a magnetic structure where the domain walls become
[00:00:20.130]self-enclosed, creating a spiral. Skyrmions have a fixed chirality.
[00:00:24.480]This makes them topologically protected and stable. For this project
[00:00:27.830]we focus on B, which is a Neel skyrmion. So why do we care about skyrmions?
[00:00:32.420]They have potential applications in spintronic devices
[00:00:35.150]like information storage.
[00:00:36.770]a possibility for improving the efficiency and durability of these devices is
[00:00:40.160]something called racetrack memory. Um,
[00:00:42.890]that involves a train of domain walls, and skyrmions have the potential to
[00:00:46.220]make these faster,
[00:00:47.050]more efficient and stable. DMI and PMA are essential parts of
[00:00:51.740]skyrmion research because they work together to form a chiral domain wall or a
[00:00:55.610]skyrmion. DMI determines the chirality. For data storage,
[00:00:59.330]we want to control the chirality or handedness. Um, for example, in this image,
[00:01:03.740]if you point your thumb in the direction of the black arrow,
[00:01:06.170]you have to use your right hand to curl in the direction of the colored arrows.
[00:01:09.590]Um, however, if we mirror that image, it would be left-handed.
[00:01:12.800]So DMI is determined by materials used.
[00:01:16.280]And then PMA is a magnetization that points perpendicular to the plane of the
[00:01:20.390]sample. Um, a platinum underlayer promotes PMA in cobalt,
[00:01:25.280]and the PMA of cobalt is thickness dependent. So we can tune this.
[00:01:28.820]And our goal is to test the effect of thickness on magnetization and get a trend
[00:01:33.110]similar to this graph. Both PMA and DMI are caused by spin orbit
[00:01:37.580]coupling of the cobalt with the adjacent heavy metal.
[00:01:41.600]So overall we need both DMI and PMA to be present.
[00:01:44.420]DMI creates the chirality and PMA holds the skyrmion together,
[00:01:47.870]but we want a weak DMI and a weak PMA so that the skyrmions are large and
[00:01:51.350]optically visible. So how do we measure this?
[00:01:53.930]The magnetic fields influence the polarization state of optical radiation.
[00:01:57.860]And so MOKE experiments are meant to detect a change in magnetization.
[00:02:01.700]So we use two types of MOKE, polar MOKE,
[00:02:03.440]where the magnetic field is perpendicular to the sample.
[00:02:05.960]So the magnetization is out-of-plane. And then longitudinal MOKE is where the
[00:02:10.280]magnetic field is in-plane with the sample.
[00:02:11.930]And so the magnetization is in-plane.
[00:02:14.240]So we are focused on PMOKE for this research. So we have the laser here.
[00:02:18.890]It goes through the chopper to kind of hone in on that signal.
[00:02:21.350]And then through a polarizer, uh,
[00:02:23.080]through this magnet, reflects off the sample back and reflects off this glass,
[00:02:27.290]into the polarizer and the signal detector.
[00:02:30.230]And then MOKE microscopy is the same as polar MOKE,
[00:02:33.170]but there's a microscope and a camera attached that allows us to actually look at the
[00:02:36.560]magnetic changes in the sample pixel by pixel.
[00:02:39.800]So these MOKE experiments I just described give us hysteresis
[00:02:42.530]loops a hysteresis loop shows the variation of magnetic flux when the field
[00:02:47.000]goes through a cycle of positive field to negative field,
[00:02:49.730]and then back to positive. As labeled on this diagram,
[00:02:52.580]the saturation point is the starting point.
[00:02:55.400]and this is where all magnetism is uniform.
[00:02:58.490]And then as field changes and goes to zero, um,
[00:03:03.010]the loop shows the magnetic response.
[00:03:05.320]So this remanence point shows the saturation that remains at the zero field.
[00:03:10.090]Um, and then as the field becomes negative,
[00:03:13.210]the coercivity point shows how much negative field is needed for half the
[00:03:17.230]magnetization to switch.
[00:03:18.970]And then finally the saturation point is where all the magnetization is in the
[00:03:22.330]same direction, but this time it's opposite.
[00:03:24.640]And this process repeats itself back to the positive field,
[00:03:28.300]but on a different path. So we often analyze the shape of these loops.
[00:03:32.230]For example, in a more S shaped loop (so the blue) would correspond to a lower remanence,
[00:03:36.150]lower coercivity and weak PMA, whereas the loops become more square
[00:03:39.760](so the orange) the remanence and the coercivity are higher and the PMA is strong.
[00:03:43.900]So we can actually watch this happen, pixel by pixel in a MOKE movie.
[00:03:47.530]So for a nice square loop,
[00:03:48.820]you start at the remanence point where we see most of the signal is in
[00:03:53.530]one direction, so that light color, um,
[00:03:55.900]and then we send pulses down where the magnetization changes and we watch what
[00:04:00.550]happens in the MOKE movie. So we can see that when about half the screen is black,
[00:04:05.170]that's the coercivity point. And then towards the end,
[00:04:07.690]we see most of the screen is black and most of it has switched and that is the
[00:04:10.780]saturation point.
[00:04:13.500]So what we realized is that optically detecting a full skyrmion is tough,
[00:04:17.130]but we were encouraged by a recent paper from Science Advances where they
[00:04:20.040]consider they are making half skyrmions.
[00:04:22.230]So a half skyrmion is exactly what it sounds like, half of a skyrmion.
[00:04:25.380]It has a topological winding number of one half. Um,
[00:04:28.200]it goes from up to down or down to up instead of going all the way around,
[00:04:31.530]like a full skyrmion.
[00:04:33.240]So the reason we're interested in half skyrmions is because they're
[00:04:35.670]potentially larger and optically detectable. Um,
[00:04:38.670]the images show that when a current is applied to a sample,
[00:04:42.970]the domains grow and at the end of that domain,
[00:04:45.150]these curves are what we believe to be half skyrmions.
[00:04:47.760]And this is what we want to replicate. So we have the, um,
[00:04:51.090]arrow pointing one direction, rotate 180 degrees pointing the other direction.
[00:04:54.180]This is what we think is a half skyrmion, with a topological winding number of one
[00:04:58.200]half. So to make samples, we used, um,
[00:05:01.980]magnetron sputtering with platinum cobalt and tantalum.
[00:05:05.910]The image shows, um, the optimized recipe. Um,
[00:05:10.050]and then the sputtering machine has four guns positioned around a sample holder
[00:05:13.590]at an angle, and the sample can be rotated.
[00:05:16.290]The rotation allows for an even distribution of the material. In order to make,
[00:05:19.740]um, the cobalt, however, we stop the rotation.
[00:05:23.330]And the cobalt deposition time was set for about 1.5 nanometers meaning the middle
[00:05:27.780]of the wedge is 1.5 nanometers.
[00:05:29.310]And then the actual wedge ranges from one to two nanometers.
[00:05:35.400]So after we created the sample that I showed in that last slide,
[00:05:38.100]we performed polar MOKE measurements across the sample to get
[00:05:41.040]a good idea of how the thickness affects the PMA.
[00:05:43.860]We know we should see a transition like this graph, um,
[00:05:46.830]where thin cobalt corresponds to strong PMA
[00:05:48.930](so a square loop) and then thick cobalt corresponds to a weak PMA
[00:05:52.360](so S shaped). Um, and so the graph, uh,
[00:05:56.640]shows data from the wedge sample.
[00:05:58.610]We can see the trend from S shaped to square (red to blue) or red
[00:06:03.080]to blue in the sample over here as the thickness decreases. Uh,
[00:06:06.740]so we presume that this orange loop is important because this is the transition
[00:06:10.250]from um, from square to S shape.
[00:06:13.370]And this is about halfway across the sample where we think is 1.4 nanometers.
[00:06:19.730]So from the PMOKE data in the last slide, uh,
[00:06:22.010]these graphs show how remanence and coercivity vary over position.
[00:06:25.640]The overall trend is that they both increase as the thickness decreases
[00:06:30.620]of the cobalt, the cobalt thickness decreases.
[00:06:33.710]And so these characteristics align with the fact that the PMA increases as the
[00:06:37.760]thickness decreases. So we can't actually make devices out of wedges.
[00:06:42.380]So we made the same samples as before, but with an even cobalt layer,
[00:06:46.820]um, and this data shows that the 1.3 nanometer thickness, uh,
[00:06:51.680]shows a strong PMA, but as we switched to the 1.4 nanometer thickness,
[00:06:55.310]we get a pinched loop. And this loop could be a sign of half skyrmions. Uh,
[00:06:59.540]so we're working on the thickness calibration in our sputtering chamber,
[00:07:02.240]but based on our hysteresis loops,
[00:07:03.920]we expect to see MOKE images similar to the two from the science paper.
[00:07:07.580]So these will correlate, and these will correlate.
[00:07:09.620]And so we are focused on this orange loop because we want to see these half
[00:07:13.130]skyrmions instead of just this blob.
[00:07:16.220]So at this cobalt thickness, uh,
[00:07:18.500]hysteresis loops match the science paper.
[00:07:21.440]So a 1.3 here corresponds to this blob that we saw.
[00:07:26.720]Um, however, as we try to improve MOKE microscopy and improve the resolution,
[00:07:31.520]we are hoping that we can see, um,
[00:07:34.280]this magnetization change at the thicker cobalt,
[00:07:37.820]the 1.4 nanometers, and possibly even see half skyrmions. Thank you.

The screen size you are trying to search captions on is too small!

You can always jump over to MediaHub and check it out there.

Comments

0 Comments

Creating Detecting and Analyzing Magnetic Skyrmions

Description

Searchable Transcript

Comments icon comment

Related Channels

Comments