Focused ion beam (FIB) irradiation is a versatile means to tune magnetic thin films on the nanoscale [1]. Ion irradiation can create nucleation sites for Skyrmions [2] and even establish Skyrmion-tracks [3]. In this work, we present statistical data of Skyrmion motion driven by 50 ns long current pulses in a sputtered W/CoFeB/MgO trilayer. In this layers a Skyrmion phase can be stabilized by a an oop-field in the low single digit mT regime. Furthermore, we elaborate on how Skyrmion motion can be tuned and directed by non-destructive Ga+ Ion irradiation with ultra-low doses (&lt;1e13 Ions/cm2) and high dose resolution of &lt; 1e12 Ions/cm2. First, we will give an overview of how the magnetic properties of the thin film are affected by ion irradiation. Here we analyze the average domain size, film coercivity (Hc) and relative decrease of saturation magnetization (Ms) and effective magnetization (Meff), over different irradiation steps and compare this data to stepwise annealing of the samples. Next, we track the motion of at least 50 Skyrmions for various current densities in magnetic wires, utilizing widefield magneto-optical measurements, as depicted in Figure 1. Hereby half of the wire is treated with Ga+ ions. We show that ion irradiation can be used to adjust the motion of Skyrmions towards lower velocities. While the Skyrmion velocity is very similar in the pristine parts of the wires, the velocity in the irradiated region is heavily dependent on the ion dose (see Figure 2). This lower velocity is accompanied by a significantly reduced Skyrmion Hall angle in the irradiated part, which leads to a nearly straight motion in the direction of the current flow. Beyond that, also stochastical motion due to thermal effects is largely supressed in the irradiated parts.

![](https://assets.underline.io/uploads/markdown_image/1/image/bb1f0be62208fad403370b509e76634d.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/82950f12d80d3940195e22e18676c0c9.jpg)

Influence of Ga+ Ion Irradiation on Skyrmion Motion

Influence of Ga+ Ion Irradiation on Skyrmion Motion

We present focusing of spin waves with spherical lenses in YIG by creating a magnonic refractive index change via Ga+ focused ion beam (FIB) irradiation. Direct FIB irradiation in YIG induces an ion-dose-dependent change in the effective magnetization Meff [1], and we use this to locally modify the refractive index. The lenses were designed using the lensmaker’s equation where the relative refraction index is estimated from experiments as nrel=λ0/λFIB= 4.2μm/2.5μm = 1.68. In the experiments, we use an in-house sputtered 100 nm thick YIG film on GGG and obtain the largest △Meff for a dose of 1×1013 ions/cm2. For larger doses, the change in △Meff is reversed and damping increases significantly. The measured spin-wave pattern of two plano-convex spherical lenses is presented in Fig. 1. We use longitudinal time-resolved MOKE microscopy to image the spin-wave patterns in forward volume configuration (isotropic propagation). Bending of the wavefront is visible inside the lenses, as well as the shorter wavelengths. Ion irradiation in thin films to manipulate their magnetic properties has been used in many magnetic materials/compositions but is particularly challenging in insulators. Researchers in our field, to our knowledge, have not FIB irradiated YIG to locally modify the spin-wave dispersion relation, or to pattern YIG without etching. Using spin waves for analog computing (such as optical signal processing) requires precise local modification of the magnetic properties, and we recently demonstrated the potential of FIB patterning by fabricating a concave grating for spin waves realized with higher ion doses that locally destroy magnetic properties of YIG [2]. Furthermore, gradually changing refractive-index maps can be realized by irradiating a dose-gradient, giving access to create gradient-index magnonic elements like GRIN lenses [3],[4],[5].

![](https://assets.underline.io/uploads/markdown_image/1/image/522825ca15aa1d16debf523a689aec24.jpg)

Time-resolved MOKE image of a 20 (left) and a 10 μm (right) thick spin-wave lens fabricated via FIB. The irradiation pattern d-shaped lens is indicated in semitransparent red, the groundline of a coplanar antenna for coherent excitation in gray. The initially applied wavelength is 4.3 μm, and shortened to about 2.5 μm inside the lenses due to the FIB induced Meff increase.

Spin-wave optics fabricated by FIB irradiation of YIG films

**Session chair:** Aisha Aqeel, University of Munich



GOG - Fundamental Interactions in Skyrmions and Novel Phases

technical paper

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

**Session chair:** Jaroslaw Klos, Uniwersytet im Adama Mickiewicza w Poznaniu


Martina Kiechle

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Influence of Ga+ Ion Irradiation on Skyrmion Motion

Spin-wave optics fabricated by FIB irradiation of YIG films

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