Creating Detecting and Analyzing Magnetic Skyrmions

Grace Dirks Author

07/28/2021 Added

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

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[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.

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Creating Detecting and Analyzing Magnetic Skyrmions

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