Stabilization of magnetic helix in exchange-coupled thin films

www.nature.com/scientificreports OPEN Stabilization of magnetic helix in exchange-coupled thin films L.V. Dzemiantsova1,2, G. Meier1,3,4 & R. Röhlsberger1,2 received: 22 June 2015 accepted: 29 September 2015 Published: 05 November 2015 Based on micromagnetic simulations, we report on a novel magnetic helix in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers with different anisotropies. We show that such a confined helix stays stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal magnetic field created by the exchange interaction. We show that this internal field stores a magnetic energy density of a few kJ/m3. We also find that it dramatically modifies ferromagnetic resonances, such that the helix can be used as a ferromagnetic resonance filter and a fast acting attenuator. Robust and energetically efficient magnetic structures that employ the spin degree of freedom to store and process information are at the heart of modern spin-based technology1–3. Many experiments have been performed to investigate the interaction of spins with electrical charges or external magnetic fields, using different device geometries like mechanically or lithographically fabricated point contacts, nanopillars or tunnel junctions4. It has recently been shown that the transmission and processing of information without electric currents or external fields can be achieved via the spin degree of freedom subjected to internal interactions such as exchange, Ruderman-Kittel-Kasuya-Yosida (RKKY) or long-range dipolar interactions5,6. In combination with boundary conditions including magnetic anisotropy, these interactions can topologically stabilize spin configurations like spin helices5,7 without presence of chiral Dzyaloshinskii-Moriya interaction8–10. In the work of E. Y. Vedmedenko et al.5, it has been pointed out theoretically that nano-sized stable helices can be used for magnetic energy storage. Although a large variety of magnetic devices with desirable parameters can be fabricated down to the sub-nanoscale, the creation of helices with stable magnetic properties can, however, be an experimental challenge. Here, we propose an approach for creating novel magnetic helices in exchange-coupled thin films. We show that such helices, initially twisted in an external magnetic field, stay stable even without the presence of the field. In contrast to the helimagnetism governed by the RKKY interaction in rare-earth materials11,12, or by the dipolar interaction in a [Co/Si] ×  3 multilayer13, the functionality of magnetic helices in this study relies critically on the exchange-coupling mechanism of thin layers consisting of a hard and a soft-magnetic material. As a characteristic property of exchange-coupled layers or exchange spring magnets, the magnetization of the soft-magnetic film is pinned to the hard-magnetic film at the interface as a result of the exchange interaction14. With increasing distance from the interface, the exchange coupling becomes weaker and the magnetic moments in the soft layer form a spiral structure under the action of an external field. To stabilize this spin spiral structure, we add on top a magnetic film with an anisotropy value that lies in between those of the hard and the soft material. When the external field is removed, such a trilayer can relax into a new stable configuration where a magnetic helix exists. The magnetic stability is the result of an internal field that is created by exchange interaction and stores magnetic energy. We find that this field dramatically modifies ferromagnetic resonance (FMR) frequencies in the GHz range, compared to the untwisted ferromagnetic state at zero applied magnetic fields. 1 The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany. 2Deutsches Elektronen-Synchrotron, Notkestraβe 85, 22607 Hamburg, Germany. 3Max-Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany. 4Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany. Correspondence and requests for materials should be addressed to L.V.D. (email: ) Scientific Reports | 5:16153 | DOI: 10.1038/srep16153 1 www.nature.com/scientificreports/ z ferromagnet x y b exchange spring, clockwise Bext c helix, clockwise d exchange spring, counterclockwise Bext e helix, counterclockwise +2 a Mx × 10 (A/m) hard Fe 20 nm 6 soft Fe 90 nm f x -2 hard FePt 10 nm z y φg φm φm Figure 1. 3D representation of micromagnetic simulation data for a hard FePt/soft Fe/hard Fe trilayer with localized magnetic moments (arrows inside slabs) arranged into (a) ferromagnet, (b,d) exchange spring, (c,e) helix. As the initial state, the trilayer shows a ferromagnetic alignment of moments (Bext = 0). Under the influence of an external magnetic field, Bext > 0, applied to the whole volume of the trilayer, the exchange spring can be created. When the external field is removed, the trilayer can relax into the metastable state, where the helix exists. The counterclockwise/clockwise rotation direction of Bext defines the clockwise/ counterclockwise chirality of the exchange spring, hence the helix. (f) Top view of the trilayer. ϕg (ϕm) is the relative angle between the magnetization directions of the bottom and top hard magnetic layers, when the trilayer is in the ground state (metastable state). The colorscale highlights the x-component of the magnetic moments, Mx, with respect to the minimum (blue) and the maximum (red) value. In experiments, magnetic materials such as FePt in the hard magnetic tetragonal L10 phase and Fe can be used as the bottom hard and mid soft magnetic layers, respectively15. The top layer with a coercive field up to 100 mT can be obtained by sputtering Fe via a technique known as oblique incidence deposition (OID) at room temperature16,17. The magnetic helix can be studied with Mössbauer spectroscopy18, nuclear resonant scattering19, resonant magnetic x-ray reflectometry20 or polarized-neutron reflectometry21, which all are capable to characterize vertical spin profiles in multilayers. Using broadband FMR with a vector network analyzer22, one can distinguish the helix from the ferromagnetic alignment. Nanocomposite materials with a stable helical order open broad perspectives for future scientific and technological applications in the field of spin engineering on smallest length scales. Since these structures store magnetic energy, they can serve as energy-storing elements in spin-based nanodevices. The stored energy can be released by switching the magnetization of the top layer with a laser pulse23,24, a radio-frequency field pulse25 or a current pulse26,27, and transformed into its mechanical, electric or magnetic counterparts. The generation of high-frequency signals without the presence of external fiel (...truncated)