Home Search Collections Journals About Contact us My IOPscience [Co/Pd]-CoFeB exchange spring magnets with tunable gap of spin wave excitations This content has been downloaded from IOPscience Please scroll down to see the full text 2014 J Phys D: Appl Phys 47 495004 (http://iopscience.iop.org/0022-3727/47/49/495004) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 157.89.65.129 This content was downloaded on 23/11/2014 at 16:31 Please note that terms and conditions apply Journal of Physics D: Applied Physics J Phys D: Appl Phys 47 (2014) 495004 (6pp) doi:10.1088/0022-3727/47/49/495004 [Co/Pd]-CoFeB exchange spring magnets with tunable gap of spin wave excitations S Tacchi1, T N Anh Nguyen2,3, G Gubbiotti1, M Madami4, G Carlotti4,5, M G Pini6, A Rettori5,7, V Fallahi8, R K Dumas9 and Johan Åkerman2,9 Istituto Officina dei Materiali del CNR (CNR-IOM) - Unità di Perugia, c/o Dipartimento di Fisica e Geologia, Perugia, Italy Materials and Nano Physics, Royal Institute of Technology (KTH), 164 40 Stokholm-Kista, Sweden Spintronics Research Group, Laboratory for Nanotechnology, Vietnam National University, Ho Chi Minh City, Vietnam Dipartimento di Fisica e Geologia, Università di Perugia, Perugia, Italy Centro S3, c/o Istituto Nanoscienze del CNR (CNR-NANO), I-41125 Modena, Italy Istituto dei Sistemi Complessi del CNR (CNR-ISC), Unità di Firenze, I-50019 Sesto Fiorentino (FI), Italy Dipartimento di Fisica ed Astronomia, Università di Firenze, I-50019 Sesto Fiorentino (FI), Italy Faculty of Physics, Amirkabir University of Technology, Hafez Ave., 1591634311 Tehran, Iran Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden E-mail: tacchi@fisica.unipg.it Received 14 August 2014, revised October 2014 Accepted for publication 20 October 2014 Published 17 November 2014 Abstract Exchange spring magnets, consisting of a [Co(0.5 nm)/Pd(1 nm)]5 multilayer with perpendicular magnetic anisotropy and a Co20Fe60B20 film with easy plane anisotropy, of variable thickness tCFB, are investigated using Brillouin light scattering On reducing tCFB in the range 0.8–2.3 nm, the spin-wave frequency gap displays a remarkable increase from nearly 4–48 GHz, reflecting the corresponding rapid growth of the tilting angle of the magnetization with respect to the film normal These findings are interpreted using a one-dimensional model in which each atomic layer is assumed to be uniformly magnetized, subjected to an effective out-of-plane or easy-plane anisotropy depending on the layer position in the stack, and exchange coupled to its two nearest neighbour atomic layers With respect to previously investigated [Co/Pd]-NiFe hybrid magnets, a largest frequency tunability, restricted to a narrower range of the soft layer thickness, is observed Keywords: spin waves, brillouin light scattering, exchange springs, magnetic properties of interfaces (Some figures may appear in colour only in the online journal) 1. Introduction for STT-MRAM to increase the switching speed, reduce the switching current, and improve the thermal stability and future scalability [10] In addition, zero-field STO operation has been demonstrated using PMA fixed [11] and free [12–14] layers Going beyond a purely perpendicular magnetization, it has been shown that spin-valve structures with a tunable outof-plane tilt angle of magnetization can further improve the performance of STT-based devices [15, 16], since the variable tilt angle offers an additional degree of freedom, leading to a rich phase diagram of STT switching and precession [17] Exchange-spring systems consisting of strongly coupled hard The great potential of the spin-transfer torque (STT) effect [1, 2] for applications in magnetic storage technology and spintronics [3, 4] has recently stimulated a considerable interest in engineering magnetic structures which meet the criteria of scalability, thermal stability, and low operating currents required by commercial devices, such as spin-torque oscillators (STOs) [5–8] and STT-magnetoresistive random access memories (STT-MRAM) [9] Materials with perpendicular magnetic anisotropy (PMA) have been introduced 0022-3727/14/495004+6$33.00 © 2014 IOP Publishing Ltd Printed in the UK S Tacchi et al J Phys D: Appl Phys 47 (2014) 495004 [22, 31–33]: a mean field approximation is made, so that a uniform magnetization Mi can be assumed for each atomic layer of the stack The free energy density of the system reads: and soft ferromagnetic films were recently investigated both theoretically and experimentally [18, 19] In our previous studies, we considered exchange-spring structures, made of [Co/Pd] [20–22] or [Co/Ni] [23] multilayers characterized by a PMA, coupled to a NiFe film with in-plane anisotropy In such systems, we found that the magnetization orientation can be tuned simply by changing the NiFe thickness More recently, the static properties of exchange springs composed of a [Co/Pd]5/CoFeB stack were studied [24] by alternating gradient magnetometry and one-dimensional (1D) micromagnetic simulations Thanks to the small value of its damping constant [25], CoFeB is a very promising material with regard to reducing the critical current density required for spin switching, as well as obtaining a mutual synchronization of STO arrays, mediated by propagation of spin waves (SW) [26–30] It was also observed that in the case of CoFeB the range of the tilting angle of the soft layer magnetization is reduced in comparison with NiFe, due to the different magnetic parameters of the two materials [24] In the present work, we investigate the dynamic properties of the [Co/Pd]5/CoFeB system Experimental analysis of the magnetic excitations with respect to the ground state was performed by Brillouin light scattering (BLS) from thermally excited spin waves, while an interpretation of the data was conducted in the framework of a 1D model of the composite system where the discreteness of the atomic layers was explicitly taken into account [31–33] The opening of a noticeable frequency gap at zero field has been observed on reducing the CoFeB thickness Moreover, we found that CoFeB films thinner than 1 nm are characterized by a sizeable out-of-plane anisotropy which competes with the easy-plane dipole-dipole interaction This results in a very large tunability of the SW frequency of [Co/Pd]5/CoFeB when the CoFeB film thickness is reduced by just one or two nanometers N−1 Aiex ,i+1 E = − ∑ ⎡⎣ sin θi sin θi + cos (ϕi + − ϕi ) d i, i + i=1 N + cos θi cos θi + 1⎤⎦ − ∑ (L i cos2 θi + HMi sin θi cos ϕi ) i=1 (1) where θi denotes the canting angle of the i-th magnetization vector with respect to the film normal, z, and ϕi is the angle formed by the in-plane projection of Mi with the x axis Each atomic layer is exchange coupled to its two nearest neighbours along the z direction by the exchange constants Aiex , i ± 1, and is subjected to an effective anisotropy L i = Ki − 2πMi2, which results from the balance between the out-of-plane anisotropy (Ki > 0) and the easy-plane shape anisotropy(2πMi2 ) By di, i ± 1 one denotes the distance between neighbouring layers, and by N = Nh + Ns the total number of atomic layers, where Nh is the number of hard layers and Ns the number of soft ones Finally, H is the intensity of a magnetic field applied in plane along the x direction The ground state configuration was calculated via a nonlinear map method [31–33], while the SW energies were obtained via the Landau–Lifshitz equations of motion [35] In order to determine the equilibrium configurations of the system, it is necessary to impose the vanishing of the first derivatives of the free energy density E in equation (1) with respect to the variables (ϕi, θi) (i = 1, ···, N) While the equations ∂E/∂ϕi = 0 are satisfied by ϕi = 0 ∀ i, the remaining N equations ∂E/∂θi = 0 can be reformulated, following a procedure described in previous works [22, 31–33], in terms of the two-dimensional nonlinear map: Aiex ,i+1 2. Experimental and computational methods di, i + 1Mi si + = Aiex − 1, i di2− 1, iMi si + 2L i sin θi cos θi − H x cos θi Mi θi + = θi + sin−1 (si + 1) 2.1. Experimental details (2) In the map, the auxiliary variables si = sin (θi − θi − 1) (i = 1, ···, N + 1) are introduced [22, 31–33] in order to allow the numerical determination of the equilibrium configurations as special trajectories in (θ, s) phase space, which evolve exactly in N steps from s1 = sin (θ1 − θ0) = 0 to sN + 1 = sin (θN + 1 − θN) = 0 The two latter equations represent the boundary conditions of the map Note that the angles θ0 and θN + 1 are fictitious: they simply express the lack of a nearest neighbour layer, and of the corresponding interlayer exchange coupling, for each of the terminal layers (i = 1 and i = N) in the stack In contrast, each of the inner layers (1