Composites: Part A 37 (2006) 191–196 www.elsevier.com/locate/compositesa Effect of annealing on the microstructure and magnetic properties of Fe-based nanocomposite materials Manh-Huong Phana,*, Hua-Xin Penga, Michael R Wisnoma, Seong-Cho Yub, Nguyen Chauc a Department of Aerospace Engineering, Bristol University, Queen’s Building, University Walk, Bristol BS8 1TR, UK b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea c Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam Received October 2004; revised January 2005; accepted 11 January 2005 Abstract The influence of annealing on microstructure, magnetic properties including the giant magnetoimpedance (GMI) effect of a Fe-based nanocomposite has been investigated The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was attained by annealing the Fe-based amorphous alloy prepared by rapid quenching method The GMI profiles were measured for samples annealed at different temperatures ranging from 350 to 650 8C in vacuum and for 30 It is found that the mean grain size of the a-Fe(Si) crystallites in the order of 12 nm remains almost unchanged until the annealing temperature reached 540 8C A decrease of anisotropy field and an increase of GMI with increasing annealing temperature up to 540 8C were observed and ascribed to the increase of the magnetic permeability and the decrease of the coercivity, whereas the opposite tendency was found for the sample annealed above 600 8C which is likely due to the microstructural change caused by high-temperature annealing This indicates that variation in the magnetic characteristic of the amorphous phase upon annealing changed the intergrain exchange coupling This altered both the magnetic softness and the effective anisotropy and consequently modified the GMI features The study of the temperature dependence of the GMI effect provides further understanding of the magnetic exchange between these crystallized grains through the amorphous boundaries in Fe-based nanocrystalline materials q 2005 Elsevier Ltd All rights reserved Keywords: B Anisotropy; Magnetoimpedance Introduction Recent advances in magnetic sensing applications, especially in the high-density magnetic recording technology, has benefited from the discovery of new magnetic materials with amorphous structure [1–3] In contrast to crystalline magnetic materials where the periodicity of constituent atoms plays an essential part, in an amorphous substance, atoms are distributed randomly, taking a topologically disordered structure The absence of crystal structure (i.e the presence of a short range order and the absence of a long range order) leads to superior properties * Corresponding author Tel.: C44 783 823 2277; fax: C44 117 927 2771 E-mail address: m.h.phan@bristol.ac.uk (M.-H Phan) 1359-835X/$ - see front matter q 2005 Elsevier Ltd All rights reserved doi:10.1016/j.compositesa.2005.01.033 (e.g mechanical, chemical, electrical, and magnetic properties) observed in these materials It is known that the absence of magnetocrystalline anisotropy and grain boundaries in an amorphous magnetic material results in excellent soft magnetic properties (e.g high magnetic permeability and saturation induction), high electrical resistivity leading to small eddy current losses, high hardness and stiffness etc Importantly, a variety of properties can be achieved by the applications of external parameters (e.g magnetic field, pressure, temperature, etc.) and/or by controlling the fabrication processes [1] The combined magnetic, electrical, mechanical and chemical properties are making an amorphous magnetic material the most promising candidate material for many engineering applications [3] In view of the existing materials, the discovery of Finemet-type nanocomposite magnetic materials with a composition of Fe 73.5Si13.5B9Cu1Nb3 provided some insights into the science and technology of soft magnetic 192 M.-H Phan et al / Composites: Part A 37 (2006) 191–196 materials [4–8] This kind of materials, routinely obtained by an appropriate heat treatment of an amorphous precursor, exhibits excellent magnetic properties due to its unique microstructure, namely, ultrafine nanocrystalline a-Fe(Si) grains embedded in an amorphous matrix [6] This is directly depending on the magnetic exchange coupling between the grains through the amorphous boundaries [9,10] However, the underlying physical mechanism of the magnetic exchange coupling in a nanocrystalline magnetic material is not well understood Fortunately, a number of recent studies on the giant magnetoimpedance (GMI) effect in Fe-based amorphous soft magnetic alloys subjected to heat treatment showed some insights into the nature of the magnetic exchange coupling between these grains through the amorphous boundaries in Fe-based nanocrystalline materials [7,8,11– 13] Because of the fact that the two-phase Fe-based nanocomposite material has two distinct Curie temperatures, one for the nanocrystalline grains and the other belonging to the amorphous phase, the roles of the two magnetic phases in the intergrain magnetic coupling can be taken apart in a sufficiently high temperature region In this context, the study of the temperature dependence of the magnetic properties and the GMI effect in such a Febased nanocomposite material composed of a nanocrystalline phase in an amorphous matrix can be of significant importance in gaining more rudimentary insights into the nature of the magnetic coupling in the material This paper reports the effect of annealing on the structural and magnetic properties and the GMI effect in a Fe73.5Si15.5Nb3Cu1B7 amorphous alloy Experiment Fe73.5Si15.5Nb3Cu1B7 ribbons with a width of mm and a thickness of 20 mm were prepared by rapid quenching method The nanocomposite materials composed of a nanocrystalline phase in an amorphous matrix were obtained by annealing these as-quenched amorphous ribbons at different temperatures ranging between 350 and 650 8C for 30 in vacuum The structures of the asquenched amorphous ribbons and the annealed ones were examined by X-ray diffraction (XRD) Differential scanning calorimeter (DSC) measurements on as-cast and annealed ribbons were conducted with increasing temperature at a rate of 20 8C/min in Ar atmosphere Accordingly, the crystallization processes can be monitored by DSC Transmission electron microscopy (TEM) images of the nanocrystallized ribbons have been obtained for samples thinned by using a Philips C30 ion etching device The M–H hysteresis loops were measured using a vibrating sample magnetometer (VSM) Magneto-impedance (MI) measurements were carried out along the ribbon axis with the longitudinal applied magnetic field The samples with a length of about 15 mm were used for all MI measurements Details on a MI measurements system can be found elsewhere [14] Results and discussion 3.1 Microstructural analyses Fig shows the XRD pattern of the Fe-based asquenched amorphous ribbon It is clear that the pattern exhibited only one broad peak around 2qZ458, which is often known as a diffuse halo, indicating that the sample prepared is amorphous No indication of presence of crystallites was observed by TEM This reflects the absence of crystal structure, i.e the absence of a long range order To find out a proper annealing regime for as-quenched amorphous ribbon samples, we carried out DSC measurements Typical DSC curves for the as-cast and 540 8Cannealed ribbons are shown in Fig It is easy to see clearly from Fig that for the as-cast sample the curve has a typical behavior with the two mainly exothermic peaks; the first exothermic peak (first peak at w550 8C) is attributed to the primary crystallization of the nanocrystalline phase (e.g the a-Fe(Si) soft magnetic phase) while the second one is attributed either to the further crystallization of the remaining amorphous phase, or to phase transformation of existing metastable phases, such as Fe3B, following the primary crystallization In the case of the annealed sample, the crystallization peaks shifted to a higher temperature due to a significant contribution of nucleation It was also found that the first peak disappeared for the sample annealed at 650 8C for 30 min, indicating a full crystallization state Based on the DSC results, as-cast amorphous alloys were annealed at different temperatures ranging between 350 and 650 8C for 30 in vacuum to achieve the nanocrystalline materials with a-Fe(Si) phase Furthermore, it is known that the crystallization fraction determines magnetostriction of the ribbon while the grain Fig The X-ray diffraction pattern of the Fe73.5Si15.5Nb3Cu1B7 as-cast amorphous alloy M.-H Phan et al / Composites: Part A 37 (2006) 191–196 193 Fig DSC curves for Fe73.5Si15.5Nb3Cu1B7 ribbons (as-cast and annealed at 540 8C for 30 min) size determines the magnitude of the effective magnetic anisotropy Both magnetostriction and effective magnetic anisotropy play a decisive role in the soft magnetic properties of nanocrystalline magnetic materials It is therefore necessary to evaluate the crystallization fraction of the sample after annealing Recently, Leu and Chin [15] have first proposed the method that allows one to evaluate the crystallization fraction (cf) from the DSC diagram, which is expressed by: cf Z DHa KDHt ; DHa (1) where DHa and DHt are the crystallization enthalpy of the as-cast amorphous ribbon and the ribbon annealed for a time t, respectively An example is also shown in Fig 2, where the crystallization fraction of the sample annealed at 540 8C for 30 min, corresponding to the a-Fe(Si) phase at the first peak, reaches a value of 82% We have found that the amorphous sample became fully crystallized (cfZ100%) when annealed above 650 8C It should, however, be noted that the soft magnetic property may be degraded by excessive crystallization Because, for annealing over 650 8C, the BCC crystallites will grow, and large crystallites lead to the decoupling of magnetic exchange, and consequently the good soft magnetic properties are lost To further scrutinize this feature, the structure of the amorphous samples after annealing was examined by XRD and TEM [see Fig 3, for example] After the thermal treatments the XRD peaks of a-Fe(Si) are seen to emerge from the amorphous halos The relative intensity of the various peaks indicates that there is no preferred orientation in the crystallized phase Furthermore, the mean grain size (t) of a-Fe(Si) was determined according to the Scherrer expression [16]: 0:96l tZ ; B cos q (2) Fig X-ray diffraction pattern (in the upper panel) and TEM image (in the lower panel) for the Fe73.5Si15.5Nb3Cu1B7 amorphous alloy annealed at 540 8C for 30 ˚ ), q is the where l is the X-ray wavelength (lZ1.54056 A diffraction angle, and B is the full width at half maximum (FWHM) In Fig we display the annealing-temperature dependence of the mean grain size of the bcc phase estimated from broadening its relation to the peak in the X-ray diffraction patterns using Eq (2) It should be noted that the growth of a-Fe(Si) is controlled by the slow diffusion of Nb and Cu which leads to a nanocrystalline structure As can be seen Fig The annealing temperature dependence of the mean grain size of the bcc phase estimated from the broadening of the relations in the X-ray diffraction patterns using Eq (2) 194 M.-H Phan et al / Composites: Part A 37 (2006) 191–196 clearly from Fig 4, the mean grain size is about 12 nm and it remains almost constant until the annealing temperature reaches 540 8C This indicates that the primary crystallization (the first peak of the DSC curve in Fig 2) is actually the formation of nanocrystalline structure, where a-Fe(Si) grains are embedded in an amorphous matrix [see the TEM image in Fig 3(b)] Annealing at higher temperatures not only leads to grain growth of a-Fe(Si), but also additional phases are formed from the amorphous matrix phase at Ta Z 650 8C This second stage of crystallization corresponds to the second peak in the DSC curve (Fig 2) and boride phases (e.g Fe3B and Fe2B) are found 3.2 Magnetic characteristics The crystallization kinetics of the ribbons can be observed by measurements of thermomagnetic curve, as shown in Fig It is clear to see from this figure that the ribbon is amorphous at room temperature As the temperature increases, the magnetization is abruptly reduced marking the Curie temperature (TC) of the amorphous phase With further increasing temperature, the magnetization is small and constant over a large temperature interval up to a region where crystallization of a-Fe(Si) leads to an increase of the magnetization The increase of the magnetization at the crystallization onset (w500 8C seen in the DSC curve of Fig 2) indicates the formation of some crystalline magnetic phase(s) On returning from high temperature, a large amount of a-Fe(Si) grains are crystallized in the sample and this leads to a strong increase of the magnetization below the TC of a-Fe(Si) [see the curve in Fig 5] This reflects that any variation in the magnetic nature of the amorphous phase could change the intergrain exchange coupling and consequently the magnetic softness of the nanocomposite material In order to further evaluate influences of annealing on the magnetic properties, we measured hysteresis loops and the annealing-temperature dependence of the coercivity (Hc) is displayed in Fig It is clear that the coercivity decreased Fig The coercive force (Hc) as a function of annealing temperature for Fe73.5Si15.5Nb3Cu1B7 alloys annealed for 30 with increasing annealing temperature (Ta) up to 540 8C and then increased at higher temperatures This can be interpreted as following: the gradual decrease of Hc at Ta well below the onset crystallization temperature (i.e w500 8C, see Fig 2) is a result of structural relaxation, while the drop of Hc in the temperature range of the first crystallization stage (w540 8C) is likely due to the appearance of nanosized a-Fe(Si) grains where magnetocrystalline anisotropies are averaged out Annealing over 540 8C caused a rapid increase of Hc, indicating a large degradation of the soft magnetic properties This coincides well with microstructural change (i.e the abrupt increase of the mean grain size for annealing above 540 8C as seen in Fig 4) In this case, the increase of nanoparticles size can considerably reduce the magnetic exchange coupling in the nanocrystalline material [9] Furthermore, it is found that the change of Hc with annealing temperature is correlated well to the temperature dependence of the permeability, where the permeability resulting from the rotational magnetization increased with increasing annealing temperature up to 540 8C and then decreased at higher temperatures [13] It is known that the permeability is inversely proportional to the coercivity in the temperature range investigated 3.3 Magnetoimpedance analyses Fig Thermomagnetic curves of the Fe73.5Si15.5Nb3Cu1B7 amorphous alloy: (1) heating cycle and (2) cooling cycle The magnetoimpedance ratio DZ/Z can be defined as DZ=Zð%ÞZ ZðHÞ=ZðHmax ÞK1 where Hmax is the external magnetic field sufficient to saturate the impedance and equals to 150 Oe in the present study The GMI profiles of the amorphous samples annealed at different temperatures ranging between 350 and 650 8C were measured and used to assess the anisotropy field As reported in Ref [17], the contribution of the transverse permeability to GMI from magnetization rotation becomes dominant in the high frequency range (w10 MHz) and a simple single-domain model was proposed According to this model, the width of measured GMI peak could reflect M.-H Phan et al / Composites: Part A 37 (2006) 191–196 Fig GMI profile at fZ10 MHz in Fe73.5Si15.5Nb3Cu1B7 as-cast and annealed alloys the distribution of anisotropy field As shown in Fig 7, at a fixed frequency of 10 MHz, the anisotropy field and GMI change sensitively with annealing temperature This also implies that the permeability in the transverse direction changes sensitively with annealing temperature [13] The changes in anisotropy field (Hk, as depicted in Fig 7) and the magnitude of GMI [DZ/Z(%)] are plotted as a function of the annealing temperature in Fig It is clear that, with increasing temperature up to 540 8C, a decrease of the anisotropy field and an increase of GMI were observed, but an opposite tendency was found when the annealing temperature exceeded 600 8C This is respectively related to the increase of magnetic softness and the microstructural change of the sample as the annealing temperature is increased, as discussed in Sections 3.1 and 3.2 These results also coincided with the annealing-temperature dependence of the magnetostriction saturation and the effective anisotropy constant evaluated by separate magnetization measurements [4–8] Now let us discuss the interaction between the magnetic properties and the GMI effect in the Fe-based amorphous alloy upon annealing by considering a two-phase random anisotropy model [9] Within the framework of this model, the nanocrystalline grains in nanocrystalline alloys are Fig Variation of the anisotropy field and magnitude of GMI with annealing temperature 195 strongly coupled through magnetic exchange interactions, and the local magnetocrystalline anisotropies of grains are averaged out Meanwhile, the intergranular amorphous phase plays an indispensable role, because, only through it, can the exchange coupling be conveyed Thus, any variation in the magnetic nature of the amorphous phase will consequently change the intergrain exchange coupling, then alter the magnetic softness and the effective anisotropy, and finally modify the GMI features Here, we assume that, for an amorphous alloy (i.e as-cast state), the amorphous phase is ferromagnetic and maintaining the exchange coupling As the amorphous sample was annealed at a temperature close to the crystallization temperature of the soft magnetic phase of a-Fe(Si), a combination of stress release and the magnetocrystalline anisotropy decrease in the amorphous phase further softens the ribbon magnetically, thus enhance the GMI effect In the present work, the optimal GMI profile was observed for the alloy annealed at 540 8C, as a result of the largest increase in magnetic softness (i.e the largest permeability and the smallest coercivty as seen in Fig 6) The annealing of amorphous ribbons drastically reduced the coercive force and increased the effective magnetic permeability, thus resulted in an increase in GMI effect When annealing temperatures were relatively high, e.g over 600 8C, and close to the crystallization temperature of the hard magnetic Fe-B phase (725 8C, see Fig 2), annealing may damage the soft magnetic phase of a-Fe(Si) and cause a ferromagnetic to paramagnetic transition, the material becomes incapacitated in conveying the intergrain magnetic exchange coupling, an overwhelming decrease in GMI was observed Conclusions A thorough study of the effect of annealing on structural and magnetic properties and the giant magnetoimpedance effect in the Fe73.5Si15.5Nb3Cu1B7 amorphous alloy has been made It is found that the mean grain size of the aFe(Si) crystallites in the order of 12 nm remains almost unchanged until the annealing temperature reached 540 8C The decrease of anisotropy field and the increase of GMI with increasing annealing temperature up to 540 8C were observed and ascribed to the increase of the magnetic permeability and the decrease of the coercivity, whereas the opposite tendency was found for the sample annealed above 600 8C which is likely due to the microstructural change caused by high-temperature annealing This indicates that variation in the magnetic characteristic of the amorphous phase upon annealing changed the intergrain exchange coupling This altered both the magnetic softness and the effective anisotropy and consequently modified the GMI features It is proposed that the temperature-dependent GMI profile is useful to further understand the magnetic exchange coupling between these grains through the amorphous boundaries in Fe-based nanocrystalline materials 196 M.-H Phan et al / Composites: Part A 37 (2006) 191–196 Acknowledgements The authors wish to acknowledge the Scientific cooperation between UK, Korea and Vietnam Research at Chungbuk National University supported by the Korean Science and Engineering Foundation through the Research Center for Advance Magnetic Materials at Chungnam National University Research at Center for Materials Science was supported by the Vietnam National Program for Fundamental Research Grant No 420110 References [1] McHenry ME, Laughlin DE Nano-scale materials development for future magnetic applications Acta Mater 2000;48:223–38 [2] Lachowicz HK Magnetic materials-progress and challenges J Tech Phys 2001;42(2):127–48 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Sections 3.1 and 3.2 These results also coincided with the annealing- temperature dependence of the magnetostriction saturation and the effective anisotropy constant evaluated by separate magnetization... temperature, the magnetization is small and constant over a large temperature interval up to a region where crystallization of a-Fe(Si) leads to an increase of the magnetization The increase of the. .. magnetization at the crystallization onset (w500 8C seen in the DSC curve of Fig 2) indicates the formation of some crystalline magnetic phase(s) On returning from high temperature, a large amount of