DSpace at VNU: Discontinuous spring magnet-type magnetostrictive Terfecohan YFeCo multilayers: A novel nanostructured material principle for excellent magnetic softness

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 www.elsevier.com/locate/jmmm Discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers: A novel nanostructured material principle for excellent magnetic softness N.H DucÃ, D.T Huong Giang Laboratory for Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nano Technology, College of Technology, Vietnam National University, Hanoi, Building E3, 144 Xuan Thuy Road, Cau Giay, Hanoi, Vietnam Available online 27 November 2006 Abstract Novel physics and reversal mechanisms of the whole system switching (WS) and individual switching (IS) type are reported for hard/ soft TbFeCo/YFeCo exchange-spring multilayers The WS type usually occurs in multilayered systems, in which the magnetic anisotropy of hard TbFeCo layers is neglectable For such a system, the ferrimagnetically coupled hard/soft multilayered state is recovered after removing applied fields from the magnetized state At low negative fields, the magnetization switching occurs collectively for all magnetic moments in the whole system In this case, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact This configuration is realized for TbFeCo/YFeCo discontinuous exchange-spring multilayers, in which the magnetic (Fe,Co) nanograins coexist with non-magnetic amorphous phase in the soft layers In this state, a magnetic coercivity as small as 0.4 mT is achieved It is considered as an excellent magnetic softness of rare-earth-based systems Enhancing the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type at low temperatures Starting to decrease the applied magnetic field from the high-field state, one observes the first reversal of the magnetic moments in the soft high-magnetization YFeCo-layers in positive magnetic fields This is the reason for the observation of the negative coercivity as well as negative-biasing phenomena r 2006 Elsevier B.V All rights reserved PACS: 75.70.Ài; 75.30.Et; 75.60.Ej Keywords: Multilayer; Coercivity; Magnetic softness; Spring-exchange bias; Magnetization reversal Introduction: discontinuous spring magnet-type magnetostrictive Terfecohan/YFeCo multilayers Hard/soft exchange-coupled magnetic nanocomposites, which refer to the ‘‘exchange-spring’’ magnets, have provided a pathway to increased energy product (Fig 1a) [1] The fundamental understanding of the exchange-spring mechanism has also been improved in multilayered form (Fig 1b and c) Such materials have attracted much attention in the last two decades Regarding the highperformance permanent magnets, the experimental achievements are still far from the theoretical predictions, while the exchange-spring term has been successfully ÃCorresponding author Tel.: +84 7547203; fax: +84 7547460 E-mail address: ducnh@vnu.edu.vn (N.H Duc) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.11.011 applied to the so-called low-field giant magnetostrictive exchange-spring multilayers, in which high magnetostrictive (e.g TbFeCo) and soft magnetic (e.g FeCo) layers alternate [2–4] Magnetostrictive materials have particular interest in actuators as well as in sensors Crystalline and amorphous rare earth (R)–transition metal (T) alloys, e.g TbFeCo, usually exhibit giant magnetostriction The combination of RT alloys and T metals in the exchange-spring multilayers opens a new approach for combining both high magnetostriction (ls) and magnetostrictive susceptibility (wlJ) Indeed, the magnetostriction of the order of 10À3 and a parallel magnetostrictive susceptibility of about 10À1 TÀ1 were reported for TbFeCo/FeCo multilayers ([4] and references therein) The magnetostrictive softness obtained in these multilayers is attributed to the magnetization ARTICLE IN PRESS 2460 N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 In this paper, we will highlight our recent work on Terfecohan/YFe and Terfecohan/YFeCo DSMs that display several novel physics and reversal mechanisms of the exchange-spring magnets The paper is organized as follows After the introduction, Section presents a description of hard/soft interfacial point contact model for the discontinuous exchange-spring multilayer—a novel nanostructured material principle for excellent magnetic softness Section deals with magnetization reversal types Among them, the so-called negative coercivity is shown Section describes the negative exchange-biasing phenomenon Finally, conclusion remarks are given in Section Hard/soft interfacial point contact model—a novel nanostructured material principle for excellent magnetic softness Fig Illustration of the exchange-spring materials: nanocomposite (a), conventional multilayered (b) and novel discontinuous multilayered (c) types reversal, which is initially nucleated within the soft layers at low fields and propagates into the magnetostrictive layers [5,6] In these conventional spring magnet-type multilayers (CSMs) (Fig 1b), the individual layers are structurally homogenous either in the amorphous (TbFeCo) or crystalline (FeCo) state, thus the reversal nucleation occurs at some defect points on the sample surface and interfaces In this context, the magnetization reversal is expected to be nucleated more easily in the heterogeneous soft layers consisting of Fe(Co) nanograins embedded in an amorphous matrix (Fig 1c) This novel exchange-spring configuration is named as discontinuous spring magnettype multilayer (DSM) Such DSMs have been realized for sputtered {Tb0.4(Fe0.55Co0.45)0.6/(YxFe1Àx)}n and {Tb0.4(Fe0.55Co0.45)0.6/(Yx(Fe0.7Co0.3)1Àx)}n (denoted as {Terfecohan/(YxFe1Àx)}n and {Terfecohan/Yx(Fe,Co)1Àx}n, respectively [5,6]) multilayers with a variable Y content x ¼ 0, 0.1, 0.2 and the period number n ¼ 50 The thicknesses of the individual layers are tTbFeCo ¼ 12 nm and tYFeCo ¼ 10 nm As regards the R (and Y) concentration dependence of the microstructure, in these studies, a rather high Tb content of 40 at% is fixed in order to maintain the amorphous structure in the magnetostrictive Terfecohan layers The soft magnetic YFeCo layers, however, can be formed either in homogeneous crystalline (c), amorphous (a) or in heterogeneous nanostructure (n) state depending on the Y content and/or additional heat treatments The structural, magnetic and magnetostrictive investigations have been performed for various configurations of the exchange-spring multilayers [5–8] According to Kneller and Hawig [1], in exchange-spring magnets the magnetization reversal is originally attributed to domain wall motion Recently, Liu [9] proposed a new coercivity mechanism, in which the interfacial exchange coupling restricts the magnetization rotation Such magnetization rotation is incoherent, leading to a useful magnetic coercivity This new coercivity concept was supported by experimental results obtained in rare-earth nanocomposites YCo5/a-Fe, Sm2Co17/Co, Nd2Fe14B/a-Fe, etc Contrary to high-performance magnets, in several application aspects in microsystems, e.g magnetostriction, the rotation of the rare earth magnetization must be developed at low fields In this case, spring magnet-type magnetostrictive multilayers have been prepared as already mentioned above With regards to the magnetic (and/or magnetostrictive) softness, one can apply the traditional way to increase the thickness of the soft layer (roughly twice the width of a domain wall dh in the hard phase [10]) Presently, we propose another way by minimizing the above-mentioned incoherent rotation in spring magnettype multilayers This can be realized in a configuration where soft/hard interfaces become magnetically soft/hard interfacial point coupling The idea is illustrated in Fig In a two-dimensional description, the orientation of magnetic moments in hard and soft layers of the CSM in the zero demagnetizing field is shown in Fig 2a In this case, the soft/hard interfacial exchange coupling is homogenous and the incoherent rotation of the magnetic moments in the soft layers is reinforced The interfacial exchange coupling is a short-range interaction that is effective only for hard/soft atomic neighbors In order to lift the restriction of the rotation of the magnetic moments in the soft layers, the magnetic moments of atoms near the interface should be reduced or annulled as illustrated in Fig 2b and c, respectively This corresponds to the two typical configurations of soft layers in DSMs: the weakly magnetic amorphous matrix (wmm) (Fig 2b) and the nonmagnetic amorphous matrix (nmm) (Fig 2c) TEM electron diffraction patterns of the Terfecohan/ YFe multilayers are given on the left in Fig The ARTICLE IN PRESS N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 2461 Fig Magnetic configurations in CSM (a), DSM with wmm (b) and DSM with nmm (c) amorphous state existing in the Terfecohan layers of all three samples is characterized by the (typical) first bright spread ring from the inside diffraction spot, whereas the other rings, which are characteristics of the YxFe1Àx layers, exhibit drastically different behaviors with the variable Y concentration They are almost complete sharp rings for x ¼ (Fig 3a, left) and spotty rings for x ¼ 0.1 (Fig 3b, left) indicating the crystalline state of Fe layers and the coexistence of Fe nanocrystallites in an amorphous matrix of the Y0.1Fe0.9 layers, respectively Finally, for x ¼ 0.2, these rings become spread (Fig 3c, left) that evidence for amorphous state of Y0.2Fe0.8 layers Periodic stripe structures of layers are viewed in HR-TEM cross-sectional micrographs on the right of Fig for the as-deposited samples These images are characterized by typical (dark) smooth trips of amorphous Terfecohan layers and different microstructure of YxFe1Àx layers A percolation of BCCFe grains is observed in the x ¼ sample The Terfecohan/ Fe multilayer is considered as the CSM with an almost homogenous interfacial hard (amorphous)/soft (crystalline) exchange coupling Well-separated dark spots observed in unsmooth Y0.1Fe0.9 stripes are noticeable with an average Fig TEM electron diffraction patterns (left) and HR-TEM micrographs (right) of TbFeCo/YxFe1Àx multilayers: (a) x ¼ 0; (b) x ¼ 0.1 and (c) x ¼ 0.2 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 This is a typical observation of the DSM, in which hard/soft interfacial point contacts are formed As usual, the stripes are (light) smooth for the amorphous Y0.2Fe0.8 layers The Terfecohan/Y0.2Fe0.8 multilayer, thus, is the CSM with the almost homogenous hard (amorphous)/soft (amorphous) interfacial exchange coupling The observed transformation to the nanostructure was associated with the reduction of the thermodynamic driving force for the crystallization caused by the Y substitution [11] This is the direct approach to the nanostructure based on a critical concentration of the yttrium element Similar behaviors are found for Terfecohan/Yx(Fe,Co)1Àx system In addition, the DSM can also be obtained from the CSM by the conventional bottom-up ARTICLE IN PRESS N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 2462 Table The values of coercivity moHC (mT) of TbFeCo/YxFe1Àx and TbFeCo/Yx(Fe,Co)1Àx CSMs and DSMs with non-magnetic amorphous matrix (nmm) and weakly magnetic amorphous matrix (wmm) in soft layers Samples TbFeCo/Fe TbFeCo/Y0.1Fe0.9 TbFeCo/Y0.2Fe0.8 TbFeCo/FeCo TbFeCo/Y0.1(FeCo)0.9 TbFeCo/Y0.2(FeCo)0.8 TA ¼ 450 1C TA ¼ 350 1C As-deposited Type moHC Type CSM DSM CSM CSM DSM CSM 5.1 3.0 6.5 5.0 3.0 5.0 CSM DSM DSM CSM DSM DSM with nmm with nmm with wmm with wmm moHC Type 1.9 1.1 1.2 1.7 1.6 1.6 CSM DSM DSM CSM DSM DSM moHC with nmm with nmm with wmm with nmm 1.5 0.4 0.4 1.6 1.5 0.6 Fig Magnetization data measured at K (top) and 100 K (bottom) for Terfecohan/Y0.1(Fe,Co)0.9 (a) and Terfecohan/Y0.2(Fe,Co)0.8 (b) approach This is the case of the 350 and 450 1C annealed Terfecohan/Y0.2Fe0.8 multilayers Magnetic coercivity of investigated samples is determined from the magnetic and magnetostrictive hysteresis loops The results are collected in Table The hard/soft interfacial point contact model seems to be valid for the asdeposited samples: the observed magnetic coercivity is higher in CSM (moHC45 mT) with respect to that in DSM (moHC ¼ mT) The low-coercive force of the as-deposited Terfecohan/Y0.1Fe0.9 multilayer may be attributed to the specific nanostructure, in which each Fe nanograin is largely decoupled from the other ones via the non-magnetic matrix forming hard/soft interfacial point contacts (denoted as DSM with nmm) (Fig 2c) After releasing the stress induced during the deposition, the coercivity of $1 mT is reached in the 350 1C-annealed Terfecohan/ Y0.1Fe0.9 and Terfecohan/Y0.2Fe0.8 DSMs with nmm (see in Table 1) The coercivity reduction, in particular, is much effective after annealing at 450 1C: the difference in moHC between CSM and DSM with nmm is four times However, almost no difference in the coercivity is found between 350 1C-annealed Terfecohan/FeCo CSMs and Terfecohan/ Y(Fe,Co) DSMs At present, it is possible to assume that although the nanograins were formed in the YFeCo layers, their matrix is still magnetic (wmm) This reflects the fact that the effect on the coercivity between the CSM and DSM with wmm described in Fig 2a and b, respectively, is not much different Further increasing the annealing temperature, the evolution of nanograins and the phase segregation lower the FeCo concentration in the matrix As a consequence, the DSM with nmm is obtained for Terfecohan/Y0.2(Fe,Co)0.8 In this case, a magnetic coercivity as small as 0.4 mT is achieved This could be an excellent magnetic softness being able to obtain for ARTICLE IN PRESS N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 magnetic rare-earth-based materials For the 450 1C annealed Terfecohan/Y0.1(Fe,Co)0.9, the ferromagnetic behavior seems to be maintained in the YFeCo matrix, so that its coercivity is comparable with that in CSMs As a remark, the presented experimental results seem to phenomenologically support the hard/soft interfacial point contact model for the low-coercivity mechanism This model is, however, rather ideal Practically, the HR-TEM micrographs give a good impression that the Terfecohan layers block the grain growth, so there must be a rather large contact area One could perhaps surmise that some nucleation centers for re-crystallization are quite near the border, and that such a grain could be a handle to rotate the moments in the Terfecohan layer Magnetization reversal types and the observation of the so-called negative coercivity In artificial ferrimagnetic multilayered systems, magnetization and magnetic anisotropy differ from one layer to the next, so the magnetization reversal occurs at different coercive fields for each layer For TbFeCo/YFeCo DSMs under consideration, in the low-field saturation state (LFSstate), magnetization is dominated by the soft YFeCo layers and the ferrimagnetic multilayered state is characterized by the parallel orientation of 3d magnetic moments in the whole sample In external magnetic fields, the ferromagnetic multilayered state is established by the rotation of the hard TbFeCo-layer magnetization along the applied field direction In this case, the 3d magnetic moments in soft and hard layers are antiparallelly oriented, leading to the formation of a so-called extended domain wall (EDW) at the interfaces Thus, the high-field saturation state (HFS-state) always accompanies with the existence of the interfacial EDW As the external field is decreased, this EDW is destroyed in a middle field and finally, the ferrimagnetic multilayered state (without EDW) returns at low fields The details of these (reversal) phenomena, however, depend on not only the net magnetic moment of the system, but also the magnetic anisotropy of the hard layers In special conditions, one also observes the so-called negative coercivity, at which the reversal causes a negative magnetization when the applied field is still positive [12] The magnetization loops measured using a SQUID in the fields up to T are shown in Figs and Let us consider the 100 K magnetization loops shown at the bottom of Fig At this temperature, the magnetic anisotropy of the TbFeCo layers is still negligible and the Zeeman energy is dominated As the magnetic field is decreased from the HFS-state, the magnetization reversal takes place firstly in the smaller magnetization TbFeCo layers as expected (individual switching (IS) type) In this case, the LFS-state corresponds to the ferrimagnetic multilayered one and in low negative fields the reversal occurs for whole systems (whole system switching (WS) type) Similar phenomenon can be described for reversal 2463 processes at room temperature At K, all the samples are still the Fe magnetically dominated systems Thus, the Fe magnetization is, in principle, in priority to favor the applied field direction At present, however, on decreasing the applied magnetic field from the HFS-state, the first reversal of IS type occurs with 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 For Terfecohan/Y0.1(Fe,Co)0.9, the rotation of Fe magnetization starts in positive fields, almost compensates with TbFeCo magnetization in zero fields and the Fe magnetization reversal process is completed in negative fields (Fig 4a, top) For Terfecohan/Y0.2(Fe,Co)0.8, the IS-type reversal of the Fe magnetization is completed and causes a negative magnetization when the applied field is still positive (Fig 4b, top) This is the observation of the so-called negative coercivity The nature of the Fe magnetization reversal in the positive Fig Magnetization data measured at K for Terfecohan (tTFC)/Fe (10 nm): tTFC ¼ 20 nm (a), 16 nm (b) and 12 nm (c) ARTICLE IN PRESS 2464 N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 fields is confirmed in the magnetization loops of the Terfecohan (tTFC)/Fe (10 nm) multilayers with a fixed Fe layer thickness of 10 nm and the TbFeCo layer thickness varying as tTFC ¼ 12, 16 and 20 nm (Fig 5): with decreasing TbFeCo layer thickness, the Fe magnetization almost compensates with TbFeCo magnetization for tTFC ¼ 20 nm (Fig 5a) and causes negative net magnetization for tTFC ¼ 12 and 16 nm (Fig 5b and c) Note that these magnetization reversal types can be described by Monte Carlo simulations [10–13] Negative exchange biasing The phenomenon of exchange biasing is a property of many antiferromagnetic (AF)/ferromagnetic (F) bilayer systems It is observed by a displacement of hysteresis loop of the ferromagnetic layer toward negative fields when the sample is cooled through the Ne´el temperature of the AF layer This is defined as the positive exchange biasing Recently, similar physical and reversal mechanisms were found in exchange-spring magnets, where the hard layer replaces the AF layer as biasing layer [10] In this case, a complement understanding of the exchange-bias problem is involved with the development of domain walls, i.e with a twisted magnetic structure at the AF/F interfaces For Terfecohan/Y0.1Fe0.9 multilayer, besides the fieldinduced magnetic transition, at which the EDW is destroyed, the observation of the phenomenon of positive exchange biasing at low temperatures was reported [8] At present, as showed in Fig 6, a so-called negative exchange biasing, i.e a displacement of the hysteresis loop of the ferromagnetic layers toward positive fields, is observed Indeed, the minor hysteresis loops have been recorded in the temperature range from to 100 K for TbFeCo/ Y0.2(Fe,Co)0.8 multilayer The measurement was obtained by sweeping the field from to À0.4 T In the hysteresis loops that correspond only to the reversal of the soft layers (IS type), the value of the exchange-bias fields is found to decrease from 165 to 115 mT and 36 mT when the temperature increases from to 25 K and 50 K, respectively Finally, the positive exchange biasing disappears at 100 K, at which the WS-type reversal is governed (Fig 6d) This observation exhibits the important role of the hard layer anisotropy Concluding remarks This paper deals with two types of magnetization switching, denoted as WS type and IS type For the WS type, the low-coercivity mechanism is discussed on the basis of a hard/soft interfacial point contact This is the configuration realized for TbFeCo/YFeCo DSMs, in which the magnetic (Fe,Co) nanograins coexist with non-magnetic amorphous phase in soft layers In this state, a magnetic coercivity as small as 0.4 mT is achieved This is considered as the novel nanostructured material principle for excellent magnetic softness Due to the enhancement of the magnetic anisotropy in the hard TbFeCo layers, the magnetization switching follows the IS type firstly in soft layers at low temperatures This is the reason for the observation of negative coercivity as well as negativebiasing phenomena Fig Minor loops measured by sweeping the field from to À0.4 T (closed circles) and magnetization data (opened circles) at K (a), 25 K (b), 50 K (c) and 100 K (d) for TbFeCo/Y0.2(Fe,Co)0.8 films ARTICLE IN PRESS N.H Duc, D.T Huong Giang / Journal of Magnetism and Magnetic Materials 310 (2007) 2459–2465 Acknowledgments This work is supported by the State Program for Fundamental Research in Natural Sciences under Project 410.406 and by the College of Technology, Vietnam 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Magnetism and Magnetic Materials 310 (2007) 2459–2465 magnetic rare-earth-based materials For the 450 1C annealed Terfecohan/ Y0.1(Fe,Co)0.9, the ferromagnetic behavior seems to be maintained in the YFeCo. .. must be a rather large contact area One could perhaps surmise that some nucleation centers for re-crystallization are quite near the border, and that such a grain could be a handle to rotate the