ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 316 (2007) 379–382 www.elsevier.com/locate/jmmm Magnetization and magnetostriction process in spring-magnet TbFeCo/Fe multilayers with variable TbFeCo thickness D.T Huong Gianga, N.H Duca, J Juraszekb, J Teilletb,Ã a Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, College of Technology, Vietnam National University, Building E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam b Groupe de Physique des Mate´riaux, UMR CNRS 6634, Universite´ de Rouen, 76801 St Etienne du Rouvray, France Available online March 2007 Abstract Different natures of the magnetization reversal are studied by means of magnetization and magnetostriction measurements for magnetostrictive ‘‘spring-magnet’’ multilayers of TbFeCo/Fe with a fixed Fe layer thickness of 10 nm and variable TbFeCo layer thickness of 12, 16 and 20 nm In such multilayered systems, magnetization reversal occurs at different coercive fields for each layer related to formation of the domain wall at interfaces The results show that at low temperatures, transition from the ferromagnetic saturation state (with domain walls) to the ferrimagnetic transient saturation state (without domain walls) takes place by the reversal of the magnetic moments in the high-magnetization Fe layers This process is governed by large magnetic anisotropy of the TbFeCo layer Increasing temperature, this anisotropy decreases and transition results from the reversal of magnetic moments in the small magnetization TbFeCo layers Formation of the domain wall at interfaces is clearly evidenced by the negative contribution to the parallel magnetostriction r 2007 Elsevier B.V All rights reserved PACS: 75.60.Ej; 75.60.Ch; 75.70.Ài; 75.80.+q Keywords: Spring-magnet multilayer; Magnetostriction; Magnetization process; Domain wall Introduction The combination of rare earth–transition metal (RE–TM) alloys and transition metals in spring-magnet RE–TM/TM multilayers opened a new approach for developing low-field giant magnetostriction [1,2] These conventional magnetostrictive spring-magnet-type multilayers (CMSMM) were recently developed in the novel magnetostrictive multilayers—named discontinuous magnetostrictive spring-magnet type multilayers (DMSMM), in which the soft FeCo-layer is structurally heterogeneous in nanostructured state Indeed, an excellent magnetic softness with magnetostriction (l) of the order of 10À3 and magnetostrictive susceptibility (wl) of about 10À1 TÀ1 was reported for Tb(Fe0.55Co0.45)1.5/(YFeCo) (named as Terfecohan/YFeCo) DMSMM [3,4] In such ferrimagnetic ÃCorresponding author Tel.: +33 35 14 63 11; fax: +33 35 14 69 09 E-mail address: jacques.teillet@univ-rouen.fr (J Teillet) 0304-8853/$ - see front matter r 2007 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2007.03.028 multilayered systems, magnetization and magnetic anisotropy differ from one layer to the next, so 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, the so-called extended domain wall (EDW) formes at the interfaces and results in a negative contribution to the parallel magnetostriction [5] In general, the high-field saturation state (HFS state) is usually related to the existence of the interfacial EDW This EDW is destroyed in middle field Finally, a temporary saturation state (TS state) without DW exists at low fields Details of these phenomena, however, depend not only on the intrinsic properties of the soft layers but also on the net magnetic moment of the system In a special condition, one observes also the so-called negative coercivity, at which the reversal causes a negative magnetization when the applied field is still positive [6,7] This paper deals with the influence of individual TbFeCo-layer thickness on magnetic and magnetostrictive ARTICLE IN PRESS D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 316 (2007) 379–382 380 properties in sputtered Tb(Fe0.55Co0.45)1.5/Fe multilayers with a fixed Fe layer thickness of 10 nm and TbFeCo layer thickness varying between tTFC ¼ 12, 16 and 20 nm (named as M12, M16 and M20, respectively) Results and discussion The magnetization loops were measured using a SQUID from the helium liquid temperature to room temperature in fields up to T Shown in Fig are the parallel magnetization curves measured at 5, 100 and 300 K for the as-deposited samples For single-layer films, investigations reveal that the Fe magnetization can take the almost temperature-independent value of (MFe) ¼ 1700 kA/m, whereas the Terfecohan magnetization (MTFC) decreases from 1100 to 250 kA/m when going from to 300 K [8] This can be confirmed by analyzing the magnetization data in the TS state (MTS): M TS ¼ jtFe M Fe tTFC M TFC j , tFe ỵ tTFC (1) where t indicates thickness For the HFS state, however, net magnetization (MHFS) depends also on dimension of the EDW through M HFS ẳ t0Fe M Fe ỵ t0TFC M TFC þ dDW M DW , tFe þ tTFC (2) where t0 is the remainder of thickness in the ferromagnetic Fe and Terfecohan core and d is thickness of the DW By analyzing HFS magnetization data, one can obtain information for d This problem, however, will be treated elsewhere Presently, we are interested in the nature of the magnetization reversal only Let us start by considering the K magnetization loops (Fig 1(a)) Note that M20 is an almost magnetically compensated system, but the M12 and M16 are the Fe magnetically dominating samples Thus, Fe magnetization is strongly sensitive to applied field direction In these experiments, however, starting to decrease the applied magnetic field from the HFS state, one observes first the reversal of the magnetic moment in the Fe layers This is due to the fact that although the Terfecohan layers have smaller net magnetization (with respect to that of Fe layers), their strong magnetic anisotropy continues to pin their magnetization against the magnetic field direction At K, for all investigated systems, the rotation of the Fe magnetization starts in positive fields, almost compensated with TbFeCo magnetization in zero fields and the Fe magnetization rotation process is completed in negative fields (in particular for M12 and M16 samples) In addition, this rotation process (i.e destroying the EDW) starts in the higher field for the sample with thinner TbFeCo thickness At 100 K, the reversal of the Fe magnetization in positive fields remains for the M16 and M20 samples only (Fig 1(b)) For the M12, due to reduction of TbFeCo magnetic anisotropy, TbFeCo magnetization cannot pin against the field direction anymore but it reverses in Fig Parallel magnetization loops of TbFeCo (tTFC)/Fe (10 nm) multilayers at different temperatures ARTICLE IN PRESS D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 316 (2007) 379–382 positive fields (Fig 1(b) at the top) Finally, at room temperature, magnetic anisotropy of amorphous TbFeCo phase can be neglected, the Zeeman energy is dominating and magnetization reversal takes place firstly in the smaller magnetization TbFeCo layers as expected (Fig 1(c)) The magnetization anomalies evidenced for this transition are, however, rather weak It will be discussed below Magnetostriction was measured using an optical deflection method [8] Fig presents room-temperature parallel and perpendicular magnetostriction for the investigated samples Note that, while the parallel magnetostriction curves show big anomalies, e.g negative contributions, the perpendicular magnetostriction exhibits almost ordinary behavior with a rather weak positive curvature in high fields This is a good evidence for the formation of the EDW, which is weakly indicated from the room-temperature magnetization curves Indeed, the formation of the Bloch-type DW results in a disorder of the magnetic moments at the interfaces along which is measured the parallel magnetostriction This causes a decrease of the parallel magnetostriction, but preserves the perpendicular 381 magnetostriction [5] The magnetostriction coefficient lg,2 ¼ lJÀl? is presented in Fig This presentation allows a good comparison not only for the magnitude of the domain wall magnetostriction, but also for its contribution with respect to the total magnetostriction Finally, considering the maximum in the parallel magnetostriction curves as the starting point of the formation of EDW, one can say that the critical field for this transition decreases as TbFeCo thickness increases It is in good agreement with the observation mentioned for the K magnetization data In summary, different natures of the magnetization reversal are clarified for the magnetostrictive ‘‘springmagnet’’ multilayers of TbFeCo/Fe with a variable TbFeCo layer thickness At low temperatures, transition from ferromagnetic HFS state (with the existence of the EDW) to the ferrimagnetic TS state (without the existence of EDW) takes place by the reversal of magnetic moments in high-magnetization Fe layers This is due to the large magnetic anisotropy of the TbFeCo With the increase of temperature, TbFeCo anisotropy decreases and transition Fig Room-temperature parallel (lJ) and perpendicular (l?) magnetostriction of TbFeCo (tTFC)/Fe (10 nm) multilayers Fig Room-temperature magnetostriction coefficient (lg,2) of TbFeCo (tTFC)/Fe (10 nm) multilayers ARTICLE IN PRESS 382 D.T Huong Giang et al / Journal of Magnetism and Magnetic Materials 316 (2007) 379–382 takes place by the reversal of magnetic moments in the TbFeCo layers with smaller magnetization The formation of the domain wall at the interfaces is evidenced by the negative contribution to the parallel magnetostriction Acknowledgments This work was supported by the College of Technology, Vietnam National University, under the project QG.06.22, the Programme International de Cooperation Scientifique (PICS) of France and the INTEREG IIIa European Research Program no 198 References [1] E Quandt, A Ludwig, J Betz, K Mackay, D Givord, J Appl Phys 81 (1997) 5420 [2] N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 [3] D.T Huong Giang, N.H Duc, V.N Thuc, L.V Vu, N Chau, Appl Phys Lett 85 (2004) 1565 [4] N.H Duc, D.T Huong Giang, N Chau, J Magn Magn Mater 290–291 (2005) 800 [5] E Quandt, A Ludwig, J Appl Phys 85 (1999) 6232 [6] G.J Bowden, J.M.L Beaujour, A.A Zhukov, B.D Rainford, P.A.J de Groot, R.C.C Ward, M.R Wells, J Appl Phys 93 (2003) 8639 [7] N.H Duc, D.T Huong Giang, N.T Lam, Invited paper presented at ICM’06, Kyoto, Japan, August 2006, to be published in J Magn Magn Mater [8] D.T Huong Giang, Thesis, University of Rouen, France, 2005 ... direction At K, for all investigated systems, the rotation of the Fe magnetization starts in positive fields, almost compensated with TbFeCo magnetization in zero fields and the Fe magnetization rotation... anisotropy, TbFeCo magnetization cannot pin against the field direction anymore but it reverses in Fig Parallel magnetization loops of TbFeCo (tTFC) /Fe (10 nm) multilayers at different temperatures... magnetostrictive ‘‘springmagnet’’ multilayers of TbFeCo/ Fe with a variable TbFeCo layer thickness At low temperatures, transition from ferromagnetic HFS state (with the existence of the EDW) to the ferrimagnetic