Nanoscale Confinement of All-Optical Magnetic ... - ACS Publications

Subscriber access provided by TEXAS A&M INTL UNIV

Communication

Nanoscale confinement of all-optical magnetic switching in TbFeCo – competition with nanoscale heterogeneity Tian-Min Liu, Tianhan Wang, Alex Hume Reid, Matteo Savoini, Xiaofei Wu, Benny Koene, Patrick Granitzka, Catherine E Graves, Daniel J Higley, Zhao Chen, Gary Razinskas, Markus Hantschmann, Andreas Scherz, Joachim Stöhr, Arata Tsukamoto, Bert Hecht, A. V. Kimel, Andrei Kirilyuk, Theo Rasing, and Hermann A Dürr Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02743 • Publication Date (Web): 27 Aug 2015 Downloaded from http://pubs.acs.org on September 1, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

Nanoscale confinement of all-optical magnetic switching in TbFeCo – competition with nanoscale heterogeneity Tian-Min Liu, †,▲ Tianhan Wang, †,○ Alexander H. Reid, *,† Matteo Savoini, §,▼ Xiaofei Wu, ǁ,∆ Benny Koene, § Patrick Granitzka, †,◊ Catherine E. Graves,†, ¶ Daniel J. Higley, †,¶ Zhao Chen, †,▲ Gary Razinskas, ǁ Markus Hantschmann, ‡ Andreas Scherz, † Joachim Stöhr, † Arata Tsukamoto, # Bert Hecht, ǁ Alexey V. Kimel, § Andrei Kirilyuk, § Theo Rasing,§ & Hermann A. Dürr*,† AUTHOR ADDRESS (Word Style “BC_Author_Address”). †

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,

2575 Sand Hill Road, Menlo Park, CA 94025, USA. §

Radboud University Nijmegen, Institute for Molecules and Materials, Heyendaalseweg 135,

6525 AJ Nijmegen, The Netherlands. ǁ

Nano-Optics and Biophotonics Group, Experimentelle Physik 5, Physikalisches Institut,

Wilhelm-Conrad-Röntgen-Center for Complex Material Systems, Universität Würzburg, Am Hubland, Würzburg D-97074, Germany.

ACS Paragon Plus Environment

1

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

‡

Institute Methods and Instrumentation for Synchrotron Radiation Research, G-ISRR,

Helmholtz-Zentrum Berlin, Albert-Einstein-Str 15, 12489 Berlin, Germany. #

College of Science and Technology, Nihon University, 7-24-1 Funabashi, Chiba 274-8501,

Japan. ¶

Department of Applied Physics, Stanford University, Stanford, California 94305, USA.

∆

Experimentalphysik III, Universität Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany.

○

Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, USA. ▲

Department of Physics, Stanford University, Stanford, California 94305, USA.

◊

Van der Waals-Zeeman Institute, University of Amsterdam, 1018XE, Amsterdam, The

Netherlands.

ABSTRACT Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant x-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance, but also demonstrate the highly inhomogeneous nature


2

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

of the switching process, which is attributed to the process depending on the material’s heterogeneity.

KEYWORDS (Word Style “BG_Keywords”). All-Optical Switching; Plasmonics; Magnetic Inhomogeneity; Magnetic Recording; X-ray Holography; XMCD.

TEXT All-optical switching (AOS) of magnetic domains by femtosecond laser pulses was first observed in the rare-earth–transition metal alloy GdFeCo.1-5 AOS demonstrates the potential for optical control of magnetism and for the development of faster future magnetic recording technologies. The technological potential of AOS has recently increased due to the discovery of the same effect in other materials, including rare-earth free magnetic multilayers.6,7 However, to be technologically meaningful, AOS must be able to compete with the bit densities of conventional storage devices, which means it must be able to restrict the optically-switched magnetic areas to sizes well below the diffraction limit.

Understanding and utilizing optical manipulation of magnetic order by ultrafast laser pulses has been a longstanding goal in magnetic research.8 The first demonstration of all-optical switching of magnetization—performed by Stanciu et al.1 in the rare-earth–transition metal (RE-TM) alloy GdxFeyCo100-x-y—showed switched areas with a diameter of 10 µm, defined by the laser spot size used. In addition to the originally observed switching with circular optical polarization, linear polarization was found to display AOS above a threshold fluence.2-5,9 Element specific x-ray magnetic circular dichroism experiments revealed the importance of the two-sublattice character of the RE-TM alloys and their antiferromagnetic coupling to the AOS process,2 whereas recent ultrafast x-ray scattering experiments revealed the presence of nanoscale chemical inhomogeneous, local magnetization switching in these nanoscale patches, and spin transport


3

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

between them.10 Because of its magnetic softness, the minimum stable domain size in a continuous GdFeCo film is about 1 µm. All-optical switching was further localized in GdFeCo down to 200 nm using sample patterning,11–13 while in the magnetically harder TbFeCo alloptically switched domain sizes down to 300 nm were achieved by focusing the light with a microscope objective14 and exploiting the AOS-threshold character.9 However, shrinking the switched domain sizes further, towards the Tbit/inch2 density of projected magnetic storage devices, requires a different approach.

In order to achieve unprecedented nanoscale control of AOS, we employed two-wire plasmonic gold nano-antennas to obtain localized enhancement of the optical field around the structure.15–17 The antenna structure consists of two end-to-end aligned wires with a narrow gap between them. By placing such plasmonic resonators in close proximity to the active magnetic layer, we exploit the near-field intensity enhancement in order to confine the area to be magnetically switched to dimensions that are defined by the antenna geometry. The two-wire antennas resonate with linearly polarized light with an electric field vector parallel to the antenna axis. We note that using linearly polarized light does not limit the investigation of AOS, as switching with linearly polarized light is fundamentally similar to switching with circularly polarized light—the AOS polarization dependence is due to circular dichroism which leads to a difference in the AOS thresholds for opposite light helicities.9 In order to resolve the all-optically switched magnetic domain structure, we use an x-ray holographic imaging technique, which has a spatial resolution down to 16 nm.18 The x-ray magnetic contrast is obtained via x-ray magnetic circular dichroism.2,10,19


4

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

X-ray holography requires the nanoscale integration of the magnetic sample layer and a holography mask. The sample stack, as shown in Fig. 1a, consists of a 20 nm thick TbFeCo magnetic layer sandwiched between a 100 nm thick Si3N4 membrane, and a 10 nm Si3N4 capping layer to prevent oxidation. A gold x-ray holography mask is deposited and fabricated on the other side of the membrane (see methods). The optimal antenna geometry is determined by performing finite difference time domain (FDTD) simulations (see methods), which considers the sample stack, laser wavelength, polarization and other experimental parameters (see methods). Three antenna sets were prepared with total lengths of 230, 270 and 310 nm and with gaps between 20 to 30 nm (see Fig. 1b & 1d). The calculated near-field enhancement within the magnetic TbFeCo layer is shown in Fig. 1c. The single-crystalline gold antennas were first fabricated by focused ion beam milling on a glass substrate coated with gold and SiO2 layers before being transferred to the actual samples (see methods).20 This fabrication procedure prevents damage to the magnetic material underneath the antennas by the milling process.21 Magnetic switching is achieved by directing single femtosecond optical pulses onto the sample surface that contains the antennas. The optical beam focus is 75 µm full-width-half-maximum and we can therefore assume that the antennas within the imaging window to be excited by a plane wave. After the optical exposure, the magnetic state is probed using a circularly polarized x-ray beam. The diffraction pattern in the transmission geometry, as shown in Fig. 2a, is formed by interference between the transmitted radiation through the imaging window and the reference slit, and is recorded on a CCD detector. To reconstruct the real space transmitted x-ray field through the sample stack, we use holography with an extended reference by autocorrelation linear differential operation (HERALDO)17,22 (see methods).


5

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

The x-ray holography experiment is performed at SSRL beamline 13-3, with the incident xrays tuned to Fe L3 edge at 706.8 eV. The x-ray transmission profile obtained from the HERALDO reconstruction resolves both magnetic and non-magnetic absorption contrasts (Fig. 2b & 2c). To isolate the magnetic information, we take the difference of the reconstructed images obtained with right- and left-circularly polarized x-rays. This difference reveals magnetic switching both near and in-between the plasmonic nano-antennas (schematically indicated in yellow). The sources of non-magnetic contrast, such as the gold antennas observed in the individual images taken with left- (LCP) and right-circularly polarized (RCP) x-rays, disappear from the difference image (see Fig. 2c). The image in Fig. 2c was taken after illuminating the sample with a laser pulse of 10.3 mJ/cm2 fluence. The fluence is clearly high enough to switch areas of the TbFeCo film even without plasmonic antennas. From the switched regions between antennas we obtain domains with sizes between 50 and 150 nm diameter. It is noted that homogenous switching does not occur in this amorphous TbFeCo alloy, unlike in amorphous materials that only support micrometer sized domains such as in GdFeCo.1-4. The switched regions are randomly distributed between the antennas. We assign the distribution

to the

inhomogeneous chemical nanostructure of the sample which will be described below. The magnetic TbFeCo layer possesses a nanoscale chemical inhomogeneity, which is revealed by real-space scanning transmission electron microscopy images taken with energy dispersive xray spectroscopy (see methods), as shown in Figure 3a. Tb-rich and Fe-rich regions are visible with concentrations that are anti-correlated with each other. A spatial variation of the Co composition is also observed, but is not directly correlated with the Fe and Tb concentration maps. The measured average concentrations of Tb16.9Fe71.8Co11.3 agree reasonably well with the nominal values(see methods). It is important to note that the electron microscopy images in Fig.


6

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

3a were taken on TbFeCo samples without Si3N4 support membranes due to the lower penetration depth of keV electrons compared to the penetration depth of soft x-rays. The correlation length scales obtained from the resonant scattering patterns as shown in Fig. 3b are 6 nm and 12 nm for Tb and Fe respectively in the samples used for AOS. The scattering patterns are in agreement with the chemical correlation maps as shown in Fig. 3a. The scattering data also show evidence for larger-scale (i.e. smaller wavevector) chemical variations that lead to changes in the average stoichiometry on length scales larger than the ones displayed in the chemical maps shown in Fig. 3a. The variations in the elemental composition of the alloy will directly influence the local compensation and Curie temperatures of the sample, and change its magnetic properties. Correspondingly, the local AOS threshold will also be affected, as it depends on these parameters, in particular the compensation temperature (see supporting information). We will now describe the reversible writing of magnetic information by using plasmonic nanoantennas with AOS. This is done for a laser fluence of 3.7 mJ/cm2 that is below the onset of AOS in-between antennas as shown in Fig. 4f. Figure 4 a-e show our demonstration of AOS at a length scale below the average TbFeCo domain size (Fig. 2b). A reversed magnetic domain is written in a uniformly magnetized region below one end of the 230 nm antenna arms with a single optical pulse of 3.7 mJ/cm2 (Fig. 4a, b). The diameter of the switched area is 53 nm, almost six times smaller than previous observations.14 Areas with switched magnetic orientation are observed below the antenna. The domain size compares favorably to the track width of 55 nm used in a recent demonstration of 1+ Tb/in2 heat-assisted magnetic recording.23 The magnetic state can be toggled back and forth deterministically, as previously observed in GdFeCo on the macroscopic scale3 demonstrating that the optically induced switching is controllable on the nanoscale (see Fig. 4). Following the writing of the initial switched domain in


7

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Fig. 4b, the sample is then magnetically reset ex-situ by applying a magnetic field to the sample (Fig. 4c), to return the magnetization to the initial state. Subsequently, it receives another laser pulse at a slightly higher fluence of 4 mJ/cm2—the fluence change is due to the necessary x-ray– optical realignment following the magnetic reset. Magnetic reversal is again observed in the same region (Fig. 4d); this demonstrates that the switching occurs reproducibly. However, the area and shape of the switched region is not identical. Illuminating the sample again with another laser pulse identical to the latter one of 4 mJ/cm2, the magnetic state is returned back to its original uniformly magnetized state demonstrating the reversibility (see Fig. 4e). Although the switching is reproducible, we do not yet have full control over the magnetic switching location due to the inhomogeneity in the Tb, Fe and Co composition in the alloy. Such inhomogeneous concentration can lead to strong local variations in the compensation temperature and thus to local variations in the switching fluence threshold. Switching in GdFeCo has been theoretically shown to be strongly concentration dependent,24 and a similar behavior is expected for TbFeCo. Fabrication imperfections, surface roughness and a non-perfect positioning of the antennas may also affect the optical illumination profile, but SEM measurements reveal little evidence of these imperfections. The effect of the nano-antennas on the switching fluence threshold is investigated by examining the fluence dependence of different sample regions as shown in Fig. 4f. The plot shows switched area for regions in the near-field of the antennas and regions away from the antennas as a function of fluence. Regions within the antennas’ near-field start to magnetically reverse at fluences of 3.7 mJ/cm2. At fluences of 5.8 mJ/cm2, switching also occurs in areas where no near-field enhancement is expected. These results demonstrate inhomogeneity in the switching threshold across the sample. The switched area for both regions—within the antennas’


8

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

near fields and those away from the near fields—increase linearly with fluence above a threshold. The slope of this increase is a measure of the switching susceptibility. The slope is 1.98 %/mJcm-2 for the regions within the antennas’ near-field enhancement and 0.33 %/mJcm-2 for regions away from the near field. The ratio of these slopes is 6.0, which is above the modelled average near-field intensity enhancement by the antennas of approximately 4. It is also noted that the switching susceptibility within the antenna near-field regions is higher than the maximum extrapolated switching area away from the antennas. We attribute this difference to the importance of lateral heat transport within the material to the switching process (for a more detailed discussion of heat generation and transport, please see the supporting information). In order to study the reliability of the simulated field intensities and to provide a thorough study of maximum achievable localization of the all-optical switching, a material with a more uniform switching threshold, such as multilayered or crystalline magnetic materials,6 would be required. In conclusion, by exploiting the field-confining properties of plasmonic nano-antennas, we successfully demonstrated all-optical switching with a lateral size of 53 nm in a ferrimagnetic TbFeCo thin film, a size that is comparable to what is achieved in heat assisted-magnetic recording. To visualize the switching we used resonant x-ray holography. The switching is shown to be reproducible and back-and-forth switching is also demonstrated. Moreover, our results show the importance of a chemically homogenous sample structure for AOS-based recording technologies. It is suggested that better control of spatial switching could be achieved in magnetic multi-layer systems or crystalline magnetic materials such as L10 FePt. In addition, further development of the plasmonic antenna is required. A non-contact geometry also is required to make this a viable alternative technology; it could be achieved using the present twowire antenna structure atop a flattened atomic force microscopy tip.25


9

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Methods: Samples: 20 nm films of Tb22Fe69Co9 are fabricated by magnetron sputtering onto 100 nm Si3N4 membranes. The sample is sandwiched between 2 extra protective layers of Si3N4 respectively of 5 nm (bottom) and 10 nm (top). The samples exhibit out-of-plane magnetization with a coercive field of 0.65 T at room temperature. The Curie temperature is at 550K, while the magnetization compensation temperature—the temperature at which the ferrimagnetic alloy behaves as a pure antiferromagnet—is below room temperature. To fabricate the nano-antennas, first single crystalline gold flakes grown on a glass coverslip were transferred to another glass coverslip coated with 50 nm gold and 40 nm SiO2 layers sequentially.20 Then antenna arrays were milled out of the gold flakes with focused ion beam (FIB). Afterwards a Poly(methyl methacrylate) (PMMA) layer (about 300 nm) was spin-coated on the sample and then baked in an oven at 170 °C for 2 hours. Due to the poor adhesion between the gold layer and coverslip and the hydrophobicity of PMMA, the gold/SiO2/PMMA layers and the sandwiched antennas were peeled off from the coverslip by dipping the sample into water obliquely because of the surface tension of water. After etching away the gold and SiO2 layers with KI/I2 aqueous solution and buffered oxide etch respectively, the antennas embedded in the PMMA layer were placed on top of the membranes with help of micromanipulators under an optical microscope. As the last step, the PMMA layer was dissolved in acetone vapor. The holography mask consisting of an 800-nm-thick gold layer is sputtered on the backside of the sample. Imaging aperture and reference slots are fabricated with a FIB of 20 nm focus. A FEI Helios NanoLab 600i DualBeam FIB/SEM was used for imaging structure fabrication. The


10

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

distance between this slit and the imaging window is within the x-ray spatial coherence length in order to fulfill the necessary conditions for holographic imaging17. The chemical inhomogeneity of the samples was characterization with scanning transmission electron microscopy with x-ray fluourescence analysis (STEM-EDX). Samples for STEM-EDX were deposited simultaneously with those for AOS ensuring the same stoichiometry. However, the STEM Si3N4 substrate was thinner at 20 nm compared to 100 nm for those used in the switching experiment. A FEI Tecnai G2 F20 X-TWIN TEM with an EDAX SUTW (super ultrathin window) and analyzer was used for elemental mapping. Experiment: The x-ray scattering experiment is performed at Stanford Synchrotron Radiation Lightsource (SSRL) beamline 13-3. A 1030 nm Calmar Cazadero Er-doped fiber laser is operated in pulse mode at 320 kHz with 500 fs long pulses of 8 µJ/pulse. Single pulses are selected from the train by use of an acousto-optic modulator and manual shutter. The laser pulse is S-polarized with the electric field vector along the long axis of the dipole antennas. The incident circularly polarized x-ray has a spot size of 220x70 µm2 and is tuned to the Fe L3 edge to probe the Fe atoms selectively. To improve the x-ray beam coherence a 100 µm aperture is placed 600 mm upstream of the sample position. The x-ray diffraction pattern is then collected using an in-vacuum CCD camera at a distance of 200 mm away from the sample plane. The camera comprised of 2048 by 2048 pixels of 13.5 µm in size. A circular beamstop of approximately 0.1 mm in radius is used to block the undiffracted beam.


11

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Holographic reconstructions are performed by the method of holography with extended reference by autocorrelation linear differential operation (HERALDO). The reference is an L-shaped slit milled approximately 5 µm away from the imaging aperture. To reconstruct the absorption contrast x-ray image, a linear operator, whose inverse Fourier transform (IFT) is a differential operator, is applied to the diffraction image. Simulations: The simulations are Finite Difference Time Domain (FDTD) simulations performed with the commercial software Lumerical FDTD.26 The simulation consists of an area of 1x1x1.1µm3 with a non-uniform meshing with the smallest mesh cell being 1 nm3 around the antenna structure. The boundary conditions consist of perfectly matched layers and we make use of the symmetry of the structure to reduce the calculation time. The dielectric constants used for the different materials are ε=-45.1+3.25i (ref 27) for gold, ε =4 (ref 28) for Si3N4 and measured to be ε =23.6+41.3i for TbFeCo. The antenna is modelled to match the fabricated structure as realistically as possible by rounding its corners. The height and the width at the base of the antenna are both 55 nm. The gap at the base is 20 nm. A plane wave source at a wavelength of 1030 nm is used for excitation.


12

Nano Letters

Page 14 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

13

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figures

Figure 1. Layout of TbFeCo sample, plasmonic antennas and holography mask: (a) schematics of the sample structure showing the depth profile (not to scale). Gold nanoantennas are positioned onto the topmost surface, a 10 nm Si3N4 capping layer which protects the magnetic TbFeCo layer from oxidation. The reverse side is covered by an 800 nm gold layer, in which a 2x2 µm2 imaging window is milled. An L-shaped holographic reference slot is cut through all layers. (b) a scanning electron microscope image of the gold antenna structures on the top surface of the sample. Three different antenna lengths of 230, 270, and 310 nm are present. (c) simulated near-field intensity enhancements for the three antenna types using a plane wave with a wavelength of 1030 nm for excitation. (d) enlarged scanning electron microscope images of the three antenna types.


14

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Figure 2. Optical switching and resonant x-ray holography schematic: (a) Experiment schematic—a 500 fs incident laser pulse (center wavelength 1030 nm and linearly polarized along the antennas’ long axes) induces the optical switching. This pulse is incident onto the sample at 30 degrees angle from normal. X-ray diffraction patterns are collected using a normally incident beam of right- and left-circularly polarized x-rays from the SSRL tuned to the Fe L3 resonance at 706.8 eV. The reconstructed image of the sample is obtained from the inverse Fourier transform of the diffraction image after filtering18,22. An SEM image of the gold holographic mask on the backside of the sample is shown on the top left, with the center imaging window, L-shaped reference slit and a point reference. (b) Holographic reconstructions for diffraction images taken with left- (LCP) and right-circularly polarized (RCP) x-rays. (c) Magnetic contrast image, obtained as the difference of the LCP and RCP images.


15

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. Chemical nanostructure of TbFeCo: (a) 20nm x 50nm chemical maps of the Tb, Fe and Co chemical distribution taken with scanning electron microscopy with x-ray fluorescence analysis (EDX). The color levels show the deviation from the nominal Tb22Fe69Co9 stoichiometry. (b) Resonant x-ray diffraction analysis of the samples used in Fig. 4 except without nanoantennas and holography mask. The graphs are the scattering yields obtained at the Tb (1240 eV photon energy), Fe (707 eV) and Co (778 eV) resonances respectively, divided by off-resonance scattering data. The curves shown correspond to the scattering factors due to the chemical variation of the respective elements. The observed peaks in the Tb and Fe scattering factors in (b) are due to the correlation lengths of 6 nm and 13 nm of the respective Tb and Fe chemical distribution shown in (a).


16

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

Figure 4. | Antenna-mediated switching is reproducible and reversible. (a) initial magnetic contrast around the selected antenna after being magnetic saturated in a field of 1.6 T. The gold scale bar is 100 nm in length. (b) magnetic contrast after 1st laser pulse of 3.7 mJ/cm2. A small domain with a FWHM of 53.4 nm is switched. (c) the magnetization is reset again using an external magnetic field. (d) magnetic contrast after the 1st laser pulse of 4.0 mJ/cm2 on the newly saturated sample. A domain of comparable size as shown in (b) is switched in the same region. (e) magnetic contrast after a 2nd laser pulse of 4.0 mJ/cm2. The magnetization of the region switched in (d) is toggled back to its original state. (f) blue data show the laser fluence dependent switched sample area within 100 nm x 250 nm, 100 nm x 290 nm, and 100 nm x 330 nm regions around the 230 nm, 290nm, and 310 nm Au antennas respectively. Red data show the switched area in regions without any near-field enhancement. Above 3.7 mJ/cm2 incident fluence, all optical switching in the vicinity of the antennas is observed. Above 5.8 mJ/cm2 incident fluence, switching is observed in multiple locations away from the antennas. In both regions, a linear increase in the switched area is observed with increasing laser fluence above an onset threshold. (g) The calculated near-field enhancement around the three antenna lengths overlaid with white boxes to illustrate the “around antenna” regions defined above.


17

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ASSOCIATED CONTENT (Word Style “TE_Supporting_Information”). Supporting Information. Supporting Information includes: 1) FDTD simulations of the near-field antenna enhancement; 2) a discussion of the effects of chemical inhomogeneity to the optical switching effect, and 3) Resonant x-ray scattering data characterizing the chemical inhomogeneity. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], *E-mail: [email protected] Present Addresses ▼

Institute for Quantum Electronics, Eidgenössische Technische Hochschule (ETH) Zürich,

Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) A.H.R. & M.S. conceived the experiment. B.K., M.S., X.W., T.W. & B.H. designed the antennas. A.T. grew the TbFeCo films. X.W. milled and transferred the antennas. T.W. & T.L. milled the references. T.W., A.H.R., T.L., P.G. & M.S. constructed the experiment. T.W., A.R., M.S., B.K., P.G., C.G., D.H., Z.C., & M.H. performed the


18

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

measurements and online analysis. T.L., T.W., H.A.D. & A.R. performed the offline analysis. B.K., G.R. & M.S. performed the FDTD simulations. A.R., T.L., M.S., B.K., A.K., T.R. & H.A.D. co-wrote the manuscript with input from all authors Funding Sources Research is supported by US DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC02-76SF00515. Stichting voor Fundamenteel Onderzoek der Materie (FOM), De Nederlandse Organisatie voor Wetenschappelijk Onderzoek(NWO), the European Union (EU) Nano Sci-European Research Associates (ERA) project FENOMENA, ERC Grant agreements No. 257280 (Femtomagnetism) and No.339813 (EXCHANGE) and EC FP7 No. 281043 (FEMTOSPIN). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Research at Stanford is supported by US DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC02-76SF00515. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the US Department of Energy Office of Science by Stanford University. Furthermore this research has received funding from Stichting voor Fundamenteel Onderzoek der Materie (FOM), De Nederlandse Organisatie voor Wetenschappelijk Onderzoek(NWO), the European Union (EU) Nano Sci-European Research Associates (ERA) project FENOMENA, ERC Grant agreements


19

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

No. 257280 (Femtomagnetism) and No.339813 (EXCHANGE) and EC FP7 No. 281043 (FEMTOSPIN). We thank Diling Zhu for useful discussions. ABBREVIATIONS AOS, all-optical switching; TM, transition metal; RE, rare-earth metal; HERALDO, holography with an extended reference by autocorrelation linear differential operation; REFERENCES 1.

Stanciu, C. D.; Hansteen, F.; Kimel, A. V.; Kirilyuk, A.; Tsukamoto, A.; Itoh, A.; Rasing,

Th.; Phys. Rev. Lett. 2007, 99, 047601. 2.

Radu, I.; Vahaplar, K.; Stamm, C.; Kachel T.; Pontius, N.; Dürr, H. A.; Ostler, T. A.;

Barker, J.; Evans, R. F. L.; Chantrell, R. W.; Tsukamoto, A.; Itoh, A.; Kirilyuk, A.; Rasing, Th.; Kimel, A. V.; Nature 2011, 472, 205-208. 3.

Ostler, T. A.; Barker, J.; Evans, R. F. L.; Chantrell, R. W.; Atxitia, U.; Chubykalo-

Fesenko, O.; El Moussaoui, S.; Le Guyader, L.; Mengotti, E.; Heyderman, L. J.; Nolting, F.; Tsukamoto, A.; Itoh, A.; Afanasiev, D.; Ivanov, B. A.; Kalashnikova, A. M.; Vahaplar, K.; Mentink, J.; Rasing, Th.; Kimel, A. V.; Nature Commun. 2012, 3, 666 1–6. 4.

Vahaplar, K.; Kalashnikova, A. M.; Kimel, A. V.; Hinzke, D.; Nowak, U.; Chantrell, R.;

Tsukamoto, A.; Itoh, A.; Kirilyuk, A.; Rasing, Th.; Phys. Rev. Lett. 2009, 103, 117201. 5.

Steil, D.; Alebrand, S.; Hassdenteufel, A.; Cinchetti, M.; Aeschlimann, M.; Phys. Rev. B,

2011, 84, 224408.


20

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6.

Page 22 of 28

Mangin, S.; Gottwald, M.; Lambert, C-H.; Steil, D.; Uhlíř, V.; Pang, L.; Hehn, M.;

Alebrand, S.; Cinchetti, M.; Malinowski, G.; Fainman, Y.; Aeschlimann, M.; Fullerton, E. E.; Nature Mater. 2014, 13, 286–292. 7.

Lambert, C-H.; Mangin, S.; Varaprasad, B. S. D. Ch. S.; Takahashi, Y. K.; Hehn, M.;

Cinchetti, M.; Malinowski, G.; Hono, K.; Fainman, Y.; Aeschlimann, M.; Fullerton, E. E.; Science 2014, 345, 1337–1340. 8.

Kirilyuk, A.; Kimel, A. V.; Rasing, T.; Rev. Mod. Phys. 2010, 82, 2731–2784.

9.

Khorsand, A. R.; Savoini, M.; Kirilyuk, A.; Kimel, A. V.; Tsukamoto, A.; Itoh, A.;

Rasing, Th.; Phys. Rev. Lett., 2012, 108, 127205. 10. Graves, C. E.; Reid, A. H.; Wang, T.; Wu, B.; de Jong, S.; Vahaplar, K.; Radu, I.; Bernstein, D. P.; Messerschmidt, M.; Müller, L.; Coffee, R.; Bionta, M.; Epp, S. W.; Hartmann, R.; Kimmel, N.; Hauser, G.; Hartmann, A.; Holl, P.; Gorke, H.; Mentink, J. H.; Tsukamoto, A.; Fognini, A.; Turner, J. J.; Schlotter, W. F.; Rolles, D.; Soltau, H.; Strüder, L.; Acremann, Y.; Kimel, A. V.; Kirilyuk, A.; Rasing, Th.; Stöhr, J.; Scherz, A. O.; Dürr, H. A.; Nature Mater. 2013, 12, 293–298. 11. Le Guyader, L.; El Moussaoui, S.; Buzzi, M.; Chopdekar, R. V.; Heyderman, L. J.; Tsukamoto, A.; Itoh, A.; Kirilyuk, A.; Rasing, Th.; Kimel, A.V.; Nolting, F.; Appl. Phys. Lett. 2012, 101, 022410. 12. Savoini, M.; Medapalli, R.; Koene, B.; Khorsand, A. R.; Le Guyader, L.; Duò, L.; Finazzi, M.; Tsukamoto, A.; Itoh, A.; Nolting, F.; Kirilyuk, A.; Kimel, A. V.; Rasing, Th.; Phys. Rev. B, 2012, 86, 140404(R).


21

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

13. Le Guyader, L.; Savoini, M.; El Moussaoui, S.; Buzzi M.; Tsukamoto, A.; Itoh, A.; Kirilyuk, A.; Rasing, T.; Kimel, A. V.; Nolting, F.; Nature Commun. 2014, 6, 5839, doi:10.1038/ncomms6839. 14. Finazzi, M.; Savoini, M.; Khorsand, A. R.; Tsukamoto, A.; Itoh, A.; Duò, L.; Kirilyuk, A.; Rasing, Th.; Ezawa, M.; Phys. Rev. Lett., 2013, 110, 177205 15. Mühlschlegel, P.; Eisler, H.-J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W.; Science, 2005, 308, 1607–1609. 16. Koene, B.; Savoini, M.; Kimel, A. V.; Kirilyuk, A.; Rasing, Th.; App. Phys. Lett. 2012, 101, 013115. 17. Challener, W. A.; Peng, C.; Itagi, A. V.; Karns, D.; Peng, W.; Peng, Y.; Yang, X.-M.; Zhu, X.; Gokemeijer, N. J.; Hsia, Y.-T.; Ju, G.; Rottmayer, R. E.; Seigler, M. A.; Gage, E. C.; Nature Photonics 2009, 3, 220–224. 18. Zhu, D.; Guizar-Sicairos, M.; Wu, B.; Scherz, A.; Acremann, Y.; Tyliszczak, T.; Fischer, P.; Friedenberger, N.; Ollefs, K.; Farle, M.; Fienup, J. R.; Stöhr, J.; Phys. Rev. Lett. 2010, 105, 043901. 19. Von Korff Schmising, C.; Pfau, B.; Schneider, M.; Grünther, C. M.; Giovannella, M.; Perron, J.; Vodungbo, B.; Müller, L.; Capotondi, F.; Pedersoli, E.; Mahne, N.; Lüning J.; Eisebitt, S.; Phys. Rev. Lett., 2014, 112, 217203. 20. Wu, X.; Kullock, R.; Krauss, E.; Hecht, B; Cryst. Res. Technol., 2015 50, 595–602.


22

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

21. Savoini, M.; Reid, A. H.; Wang, T.; Graves, C. E.; Hoffmann, M. C.; Liu, T.-M.; Tsukamoto, A.; Stöhr, J.; Dürr, H. A.; Kirilyuk, A.; Kimel, A. V.; Rasing, T.; Proc. Of SPIE, 2014, 9167-9184. 22. Guizar-Sicairos, M.; Fienup, J. R.; Opts. Express, 2007, 15, 17592–17612. 23. Wu, A. Q.; Kubota, Y.; Klemmer, T.; Rausch, T.; Peng, C.; Peng, Y.; Karns, D.; Zhu, X.; Ding, Y.; Chang, E. K. C.; Zhao, Y.; Zhou, H.; Gao, K.; Thiele, J.-U.; Seigler, M.; Ju, G.; Gage, E.; IEEE Trans. Magn., 2013, 49, 779–782. 24. Barker, J.; Atxitia, U.; Ostler, T. A.; Hovorka, O.; Chubykalo-Fesenko, O.; Chantrell, R. W.; Scientific Reports 2013, 3, 3262, DOI: 10.1038/srep03262. 25. Farahani, J. N.; Eisler, H.-J.; Pohl, D. W.; Pavius, M.; Flückiger, P.; Gasser, P.; Hecht, B.; Nanotechnology 2007, 18, 125506. 26. FDTD Solutions v8.5.3, 2012, Lumerical Solutions Inc., Vancouver, Canada. 27. Johnson, P. B.; Christy, R. W.; Phys. Rev. B, 1972, 6, 4370–4379. 28. Palik, E. D. Handbook of Optical Constants of Solids I-III; Academic Press: San Diego, London, Boston New York, Sydney, Tokyo, Toronto, 1997.


23

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Insert Table of Contents Graphic and Synopsis Here


24

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30


Page 26 of 28

a Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 b 23 24 25 26 27 28 29 30 31 32 33 34 35

Nano Letters

2 um Laser Pulse

30°

c

M+

ACS Paragon Plus Environment400 nm

LCP

RCP

Dif erence

M-

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30


Page 28 of 28

a 29 of 28 Page

b

Switching Area Percentage (%)

1 2 3 4 5100 nm 6 initial single shot 7 8f 9 20 10 Away from antenna 11 Around antenna 12 15 13 14 15 10 16 17 5 18 19 20 0 21 0 2 4 6 8 22 Laser Fluence (mJ/cm2) 23

c

Nano Letters

reset g

d

single shot


10

12

M+

e

next single shot

M-

Nanoscale Confinement of All-Optical Magnetic ... - ACS Publications

Recommend Documents