DSpace at VNU: Novel exchange-spring configuration for excellent magnetic and magneto strictive softness

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 290–291 (2005) 800–803 www.elsevier.com/locate/jmmm Novel exchange-spring configuration for excellent magnetic and magnetostrictive softness N.H Duca,Ã, D.T Huong Giangb, N Chaub a Academic Affair Department, College of Applied Sciences and Technologies, Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau giay, Hanoi, Vietnam b Faculty of Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Available online 14 December 2004 Abstract Magnetization and magnetostriction data are reported for discontinuous type exchange-spring Tb(Fe0.55Co0.45)1.5/ (Y0.1Fe0.9) multilayers, in which nanograins coexist with amorphous phase in soft YFe layers This novel exchangespring configuration exhibits an excellent magnetic and magnetostrictive softness: low magnetic coercivity m0 H C ẳ mTị; large magnetostriction lg;2 ẳ 720 106 ị and large parallel magnetostrictive susceptibility ðwljj ¼ dljj =dm0 H ¼ 29:7  10À2 TÀ1 Þ: In addition, the observed phenomena of the negative contribution to magnetostriction, the formation of the extended domain wall at the interfaces and the exchange-bias are discussed r 2004 Elsevier B.V All rights reserved PACS: 75.60.Jk; 75.70.Ak; 75.80.+q Keywords: Exchange-spring multilayers; Exchange-bias; Magnetization; Domain wall The conventional exchange-spring concept has successfully been applied to the low-field giant magnetostriction development in the so-called magnetostrictive spring-exchange multilayer, in which high magnetostrictive (e.g TbFeCo) and soft magnetic layers (e.g FeCo) alternate [1] In these multilayers, magnetization reversal is nucleated within the soft layers in low applied fields and propagates from the soft layers into the magnetostrictive layers When the soft FeCo-layer is structurally homogeneous in either crystalline or amorphous state— named as continuous structure, the nucleation of reversal occurs at some defect points on the sample surface and interfaces The reversal is expected to be easily nucleated in discontinuous soft phase, in which nanograins coexist with amorphous phase in soft layers ÃCorresponding author Tel.: 84 7680461; fax: 84 7680460 E-mail address: ducnh@vnu.edu.vn (N.H Duc) Such a novel exchange-spring configuration is realized by controlling the Y-concentration in soft magnetic layers of sputtered TbFeCo/YFe and TbFeCo/YFeCo multilayers [2,3] This approach opens an alternative route towards new magnetostrictive materials and new generation of soft magnetic materials Furthermore, it provides a configuration for studying fundamental reversal mechanism Here, these aspects are presented in more details for the discontinuous type exchangespring multilayer of {Tb(Fe0.55Co0.45)1.5/(Y0.1Fe0.9)}, denoted as {Terfecohan/(Y0.1Fe0.9)} with the individual layer thicknesses tTbFeCo ¼ 12 nm and tYFe ¼ 10 nm: The samples were fabricated by RF-magnetron sputtering The sample nanostructure was investigated using high-resolution transmission electron microscopy (HRTEM) The magnetization was measured with a VSM in magnetic fields up to T and in the temperature range from 4.2 to 300 K The magnetostriction was 0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2004.11.368 ARTICLE IN PRESS N.H Duc et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 800–803 measured using an optical deflectometer (the resolution of  10À6 rad), in which the bending of the substrate due to the magnetostriction in the film was measured For the as-deposited sample, a periodic stripe structure of smooth and unsmooth layers viewed in HRTEM-cross-sectional micrograph in Fig 1a is a good evidence for the multilayered structure of continuous (amorphous) Terfecohan layers and discontinuous (nanocrystalline) Y0.1Fe0.9 layers, respectively Dark spots observed in unsmooth stripes are noticeable with an average size of the stripe thickness They are attributed to BCC-Fe nanograins with an average diameter of about 10 nm embedded within an amorphous matrix The electron diffraction patterns performed in this sample is further evidenced for observed microstructure behavior (Fig 1b) After annealing at T a ¼ 350 1C; the microstructure almost remains unchanged, whereas the room temperature magnetization is strongly enhanced (see Fig below) This reflects the fact that the role of annealing is not to lead to the evolution in the grain size, but to enrich the Fe concentration in the grains Shown in Fig is magnetostriction data Clearly, the magnetostriction develops rapidly at the magnetic fields of a few militestla (Fig 2a) Optimization of the large magnetostriction lg;2 ẳ 720 106 ị as well as large parallel magnetostrictive susceptibility wljj ẳ 0:3 T1 ị was obtained for the 350 1C-annealed film The obtained wljj value is almost 30 times higher than that obtained in the well-known Terfenol-D alloy and comparable with that of the Metglas 2605SC In higher fields, however, the magnetostriction exhibits a negative slope (Fig 2a) In multilayered systems, properties such as magnetization or anisotropy differ from one layer to the next, so that the magnetization reversal occurs at different coercive fields for each layer When the reversal takes place in a given layer but not in the adjacent one, a socalled extended domain wall (EDW) will be formed at the interfaces and results in a negative contribution to 801 the parallel magnetostriction [4] As will be indicated below, this is not the case for the films under investigation Magnetization data are presented in Fig 3a,b for the as-deposited and annealed films, respectively Except the difference in the value of the magnetization and coercivity, the magnetic hysteresis loops of both samples exhibits the following common behaviours: (i) a field-induced magnetic transition at m0 H t and (ii) a Fig High-field (a) and low-field (b) magnetostriction data of as-deposited (open circles) and 350 1C-annealed (closed circles) {Terfecohan/(Y0.1Fe0.9)} multilayers Fig Bright field TEM image (a) and electron diffraction pattern (b) of as-deposited {Terfecohan/Y0.1Fe0.9} multilayers ARTICLE IN PRESS 802 N.H Duc et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 800–803 Fig Magnetic hysteresis loops measured in the magnetic fields applied in the film-plane for as-deposited (a) and 350 1C-annealed (b) {Terfecohan/(Y0.1Fe0.9)} multilayers phenomenon of exchange-biasing at low temperatures The observed phenomena become more pronounced as the temperature decreases For these films, it is reasonable to assume that the magnetization in the Tefecohan layers is dominated by Tb [5] Thus, the corresponding magnetization configurations of the magnetization process are illustrated in the inset of Fig 3a The EDW formation is well established above H t ; which is much higher than fields where the negative contribution of magnetostriction starts to occur Moreover, the room-temperature field-induced transition is similar for both samples, but their high-field magnetostrictive susceptibility is quite different In this context, the observed negative magnetostriction component could be connected to the nature of the YFe layers, which is strongly modified by annealing The question, however, still opens for studies in more details For the as-deposited Terfecohan/Y0.1Fe0.9 multilayer, the observed room-temperature coercivity value of mT is still high However, it was about a half of that obtained in the corresponding conventional Terfecohan/ Fe(Co) systems [1–3] This may be attributed to the specific discontinuous structure, in which each Fe nanocrystal is largely decoupled from the other ones The coercivity as small as mT is reached in the film after annealed at T a ¼ 350 1C: The coercivity reduction is usually related to the releasing stress introduced during the deposition With regards to the proposed advantage of the discontinuous type exchange-spring configuration for the magnetic softness, this result can also be attributed to the enrichment of Fe in the nanograins, which enhance the decoupling of the Fe nanocrystallites with each other via the real nonmagnetic matrix The exchange-biasing phenomenon is a property of antiferromagnetic (AF)/ferromagnetic (F) bilayer systems Similar behavior is found in exchange-spring magnets, where the hard layer replaces the AF layer as biasing layer [6] At present, the observed phenomenon may relate to the enhancement of the hysteresis of the field-induced transitions below 100 K In this case, the investigated magnetization curves can be considered as minor loops only The recoil curves show the exchangespring behavior, which resembles the exchange-bias loops of other systems (see Fig 4) At T ¼ 10 K; the exchange field ðm0 H ex Þ equals to 0.17 and 0.09 T for the ARTICLE IN PRESS N.H Duc et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 800–803 803 This work was supported by the State Program for Nanoscience and Nanotechnology of Vietnam under the Project 811.204 and by the Italian–Vietnamese Program of Cooperation in S&T—Project 8BS3 References Fig Exchange-biasing observed in as-deposited {Terfecohan/(Y0.1Fe0.9)} multilayer as-deposited and annealed films, respectively Scaling the magnetization of the soft layer M s to the formula of m0 H ex ¼ g=M s ts ; it turns out that the energy of a domain wall g is the same order of magnitude in the two samples [1] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J Appl Phys 81 (1997) 5420 [2] D.T Huong Giang, N.H Duc, V.N Thuc, L.V Vu, N Chau, Appl Phys Lett 85 (2004) 1565 [3] N.H Duc, D.T Huong Giang, N Chau, J Magn Magn Mater 282 (2004) 44 [4] D Givord, J Betz, K Mackay, J.C Toussaint, J Voiron, S.D Wuăchner, J Magn Magn Mater 159 (2004) 71 [5] N.H Duc, D.T Huong Giang, V.N Thuc, I Davoli, F Richomme, J Magn Magn Mater 272–276 (2004) E1597 [6] E.E Fullerton, J.S Jiang, S.D Bader, J Magn Magn Mater 200 (1999) 392  ... will be formed at the interfaces and results in a negative contribution to 801 the parallel magnetostriction [4] As will be indicated below, this is not the case for the films under investigation... grains Shown in Fig is magnetostriction data Clearly, the magnetostriction develops rapidly at the magnetic fields of a few militestla (Fig 2a) Optimization of the large magnetostriction ðlg;2... investigation Magnetization data are presented in Fig 3a,b for the as-deposited and annealed films, respectively Except the difference in the value of the magnetization and coercivity, the magnetic hysteresis