Original Paper phys stat sol (a) 204, No 12, 4113 – 4116 (2007) / DOI 10.1002/pssa.200777297 Influence of Nb substituted for Fe on the microstructure and magnetic properties of Fe-based nanocomposite alloy N Q Hoa1, D T H Gam2, N D The3, N Chau2, D V Son1, and S C Yu1, * BK21 Physics Program and Department of Physics, Chungbuk National University, Cheongju 361-763, Korea Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK Received May 2007, revised 23 October 2007, accepted 28 October 2007 Published online 10 December 2007 PACS 75.50.Tt, 75.75.+a The influence of Nb substituted for Fe on the microstructure and magnetic properties including the magnetoimpedance effect of a Fe-based have been investigated The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was obtained by annealing the Fe-based amorphous alloy prepared by rapidly-quenched method The measurements of thermomagnetic curves indicated that the Curie temperature of the amorphous phase of the samples decreases with increasing Nb content The optimal heat treatment was performed at Ta = 480 °C for 30 and showed that the ultrasoft magnetic properties of nanocomposite materials were obtained The magnetoimpedance (MI) of these samples has been studied in range frequency from 1MHz to MHz and varying a dc magnetic field within 300 Oe The correlation between the MI effect and the soft magnetic properties is discussed The incremental permeability ratio (PR) showed the drastic changes of soft magnetic properties as a function of annealing temperatures © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Introduction A giant magnetoimpedance (GMI) effect, discovered in amorphous magnetic materials, has generated growing interests owing to its high potential for magnetic sensing and recording application [1–3] The GMI phenomenon can be understood as the conjunction of a skin effect and a strong field dependence of the transverse magnetic permeability associated with transverse domain wall motion [4] As an ac current I = Ioexp(-jωt) is applied to such materials, their impedance Z = R + jωL changes sensitively with changes in the biasing dc magnetic field At low frequencies, the GMI effect is demonstrated to originate from the contribution of the induced magneto-inductive voltage to magnetoimpedance As the frequency increases, the GMI effect can be explained in terms of an external field dependence of the impedance as a result of the transverse magnetization with respect to the current direction in the sample and the skin effect of an ac current Because an alternating current tends to be concentrated near the surface of a conductor, the impedance Z changes according to the current distribution and the shape of the conductor For magnetic materials, the transverse permeability circumferential permeability µφ affects the penetration depth (δm) through δm = c/(4π2fµφ)1/2 [5] It is worth that as the skin effect becomes dominant (a/δm »1, a is the thickness of the ribbon) the impedance Z is proportional to (fµ)1/2 Hence, δm decreases rapidly upon the dc applied magnetic field, causing a significant change in MI Recently, GMI effect has been * Corresponding author: e-mail: scyu@chungbuk.ac.kr, Phone: +82-43-261-2269, Fax: +82-43-275-6416 © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 4114 N Q Hoa et al.: Influence of Nb substituted for Fe in Fe-based nanocomposite alloy reported in Fe-based nanocrystalline materials obtained by annealing their precursor amorphous alloys [6–11] In this paper, the influence of Nb substituted for Fe on microstructure and magnetic properties of Fe-based nanocrystalline alloy will be presented and discussed Experiment Amorphous ribbons with composition of Fe76.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5 at %) were prepared by the melt-spinning technique The nanocrystalline materials consisting of ultrafine grains dispersed in an amorphous matrix were obtained by annealing their amorphous alloys at 480 oC for 30 in vacuum The mirostructure of as-quenched amorphous and annealed ribbons was examined by X-ray diffraction (XRD) with Cu-Kα radiation The crystallization process of as-quenched specimens was examined through a thermal analysis using a differential scanning calorimetry (DSC) Grain structure was examined by a T20 Tecnai energy filtered transmission electron microscopy (TEM) Magnetic measurements were carried out using a vibrating sample magnetometer (VSM) Magnetoimpedance (MI) measurements were carried out along the ribbon axis under an external magnetic DC-field ranging from -300 Oe to 300 Oe The frequency of AC current was varied from to MHz and its magnitude was fixed at 10 mA Results and discussion Fig X-ray diffraction patterns of as-quenched amorphous Fe76.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5 at %) alloys Fig DSC curves of as-quenched Fe76.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5 at %) alloys with heating rate 20 oC/min Figure shows the XRD patterns of as-quenched ribbons of Fe76.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5) We can see that these exhibit only one broad peak around 2θ = 45o, showing that the ribbons of all asquenched samples are amorphous We carried out DSC measurements in order to investigate crystallization process and find out the most appropriate annealing temperature for as-quenched samples The DSC curve of as-quenched ribbons (x = 1.5, 3.0, 4.5) were performed at a heating rate 20 oC/min As clearly shown in Fig two exothermal peaks are observed for all samples The first peak Tp1 (502 – 587 oC) depending on the Nb content corresponds to the crystallization of α-Fe(Si) soft magnetic phase and the second peak Tp2 (676 – 722 oC) is related to the formation of the boride phases (Fe3B or Fe2B) It is clear that crystallization temperature of the α-Fe(Si) phase and boride phase increases with increasing Nb content in the studied samples The DSC measurements on as-quenched amorphous alloys were performed with a heating rate from 10 to 50 oC/min (Fig 3) Based on the DSC curves in Fig.3 we can estimate the crystallization activation energy according to Kissinger relation: ⎛β ⎞ E =− + const , ⎟ k B Tp ⎝ Tp ⎠ ln ⎜ © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim (1) www.pss-a.com Original Paper phys stat sol (a) 204, No 12 (2007) 4115 where β is the heating rate, Tp is the temperature at exothermal peak, kB is the Boltzmann constant and E is the crystallization activation energy From the equation (1), the values of E were determined It is show that the crystallization activation energy of the α-Fe(Si) phase and boride phase increases linearly with increasing Nb content in the studied samples The crystallization kinetics of the ribbons can be observed by measurements of thermomagnetic curve, as shown in Fig When the temperature increases, the magnetization is abruptly reduced marking the Curie temperature, TC, of amorphous phase With further increasing temperature, the magnetization is small over a large temperature interval up to an increase of the magnetization The increase of the magnetization at the crystallization onset indicates the forming of crystalline magnetic phase On returning from high temperature, a large amount of α-Fe(Si) grains are crystallized in sample and this leads to a strong increase of the magnetization below the TC of α-Fe(Si) From the thermomagnetic curves in the figure we can see that the Curie temperature of asquenched samples was decreased with increasing Nb content in the studied samples Fig DSC curves of as-quenched Fe75Nb1.5Si15.5B7Au1 alloys with different heating rate Fig Thermal magnetic curves of studied samples in field of 50 Oe (1: heating cycle, 2: cooling cycle) Basing on the DSC results, the as-quenched samples were annealed at 480oC for 30 in vacuum to obtain the nanocrystalline samples with ultrafine α-Fe(Si) grains To confirm this feature, the structure of the annealed samples was examined by XRD and TEM (see Fig 5) It is shown that the α-Fe(Si) phase was detected in all studied samples This indicated that, upon a proper heat treatment, the as-quenched amorphous state was transformed into the bcc structure nanograins with excellent soft magnetic properties Furthermore, the particle size, d, of α-Fe(Si) was determined according to the Scherrer expression: 0.9λ d= (2) B cos θ B where λ is the X-ray wavelength (λ = 1.54056 Å), θ is the diffraction angle at the peak, and B is the full width at half maximum (FWHM) During the crystallization process, it is assumed that niobium is rejected into the amorphous boundary phase causing an inhomogeneous distribution, consequently, an accumulation of Nb at the boundary of the nanocrystalline grains occurred The GMI profile were measured as a function of the external dc magnetic field (HDC) at various frequencies up to f = MHz The obtained results for optimized nanocrystallized (i.e., the amorphous alloy annealed at 480 oC for 30 min) samples are given in Fig for x = 1.5 We can see that the maximum value of GMI was observed at near zero field (HDC ∼ 0) and GMI profiles had a single peak feature Among the studied specimens (x = 1.5, 3.0, 4.5), the sample with x = 1.5 exhibited the best GMI effect and the maximum value of GMI reached the highest value of 105 % at a measuring frequency of MHz The GMI profile decreased with increasing frequency At frequencies below MHz, the maximum value of GMI was relatively low due to the contribution of the magneto-inductive voltage to MI When 1MHz ≤ f ≤ 5MHz, the skin effect was dominant, a higher MIR was found In the frequency region (f ≥ MHz) the www.pss-a.com © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 4116 N Q Hoa et al.: Influence of Nb substituted for Fe in Fe-based nanocomposite alloy domain wall displacements were strongly damped owing to eddy currents thus contributing less to the transverse permeability, i.e., a small MIR The sharpness of PR curves (not shown here) after annealing implies the decrease of local anisotropy distribution by the nanocrystallization and indicates that the magnetization can be saturated under very low external field Fig TEM image of the Fe75Nb1.5Si15.5B7Au1 alloy annealed at 480 oC for 30 Fig The MI curves for the nanocrystalline Fe75Nb1.5Si15.5B7Au1 sample at various frequencies up to f = MHz Conclusions The influences structure and crystallization process on the soft magnetic properties and the GMI effect in Fe76.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5 % at.) alloys have been investigated By substituting Nb for Fe, the onset crystallization temperature, which was shown in DSC curves, gradually increases with Nb content due to high melting temperature of Nb element Besides, the feature of high-melting temperature of Nb also leads to decreasing of volume fraction of bcc-Fe(Si) phase, and therefore it changes the magnetic properties of nanocomposite alloys obtained after annealing The existence of Nb decreases the ferromagnetic-ferromagnetic interaction between Fe atoms, thus, reduces the Curie temperature of the specimens The largest GMI effect was observed in the 1.5 at % Nb doping sample among the studied samples, which is mainly related to the best magnetic softness in the sample Acknowledgements This research, carried out at Center for Materials Science, Vietnam National University, was supported by Vietnamese Fundamental Research Program for Natural Science (Project 406506) and it was also supported by the Research Center for Advanced Magnetic Materials at Chungnam National University The authors would like to thank Department of Physics and Astronomy, University of Glasgow for supporting TEM measurement References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] L.V Panina and K Mohri, Appl Phys Lett 65, 1189 (1994) L.V Panina and K Mohri, J Magn Soc Jpn 19, 265 (1995) H.B Lee, K.J Lee, Y.K Kim, T.K Kim, C.O Kim, 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magnetic properties and the GMI effect in Fe7 6.5-xNbxSi15.5B7Au1 (x = 1.5, 3.0, 4.5 % at. ) alloys have been investigated By substituting Nb for Fe, the. .. the influence of Nb substituted for Fe on microstructure and magnetic properties of Fe- based nanocrystalline alloy will be presented and discussed Experiment Amorphous ribbons with composition