The DSC measurements indicated that both first exothermal peak Tp1of a-FeSi phase and second peak Tp2of boride phase as well as crystallization activation energy increase with increasing
Trang 1Journal of Magnetism and Magnetic Materials 310 (2007) 2483–2485
The crystallization, magnetic and magnetocaloric properties in
Fe 76.5x Nb x Si 15.5 B 7 Au 1 ribbons
N.Q Hoaa,b, D.T.H Gama, N Chaua, N.D Thea,b, S.-C Yub,
a Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Road, Hanoi, Vietnam
b Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
Available online 27 November 2006
Abstract
Fe76.5xNbxSi15.5B7Au1ribbons (x ¼ 0.0, 1.5, 3.0, 4.5) have been fabricated by rapid quenching technique The DSC measurements indicated that both first exothermal peak Tp1(of a-Fe(Si) phase) and second peak Tp2(of boride phase) as well as crystallization activation energy increase with increasing Nb content substituted, whereas saturation magnetization of samples decreases with x, due to ferromagnetic dilution Besides, Curie temperature of amorphous phase decreases with x, i.e Nb stabilizes amorphous structure of ribbons The investigation of magnetic entropy change of studied samples showed that it may lead to magnetocaloric effect around respective Curie temperature of amorphous phase
r2006 Elsevier B.V All rights reserved
PACS: 75.50.Tt; 75.30.Sg; 75.50.Kj; 75.50.Bb
Keywords: Soft magnetic amorphous system; Melt-spun; Magnetocaloric effect; Nanocrystalline matirials
It was shown that the crystallization of Fe–Si–B
amorphous alloys containing Nb and Cu causes the
formation of nanoscale BCC structure [1] Cu and Nb
atoms play a very important role in producing the
nanocrystalline structure A small amount of Cu (with
melting temperature Tm¼1083 1C) to form a-Fe(Si) phase
as crystallization nucleation, but Nb with high melting
temperature (Tm¼2468 1C) ascribed to hinder the grain
growth, which are decisive factors to achieve the ultra-soft
magnetic properties In the previous papers, we have
studied the influence of Ag, Zn and Au in Finemet[2–4]on
the properties of ribbons This article presents study on
influence of Nb content substituted for Fe in Fe76.5x
NbxSi15.5B7Au1 alloys on their structure, crystallization
and properties
Fe76.5xNbxSi15.5B7Au1 ribbons (x ¼ 0.0, 1.5, 3.0, 4.5)
have been fabricated by rapid quenching with 25 mm
thickness and 6 mm wide Structure of as-cast samples
was checked by X-ray diffractometer D5005 and the results
(Fig 1) show that beside sample x ¼ 0 which is partly crystallized, the containing Nb alloys are fully amorphous The DSC measurements indicated that first exothermal peak corresponding to crystallization of a-Fe(Si) phase increased from 502 to 555 and 576 1C for ribbons x ¼ 1.5, 3.0, 4.5, respectively A similar tendency is found for second crystallization peak of boride phase (676, 716 and
722 1C, respectively)
The DSC curves were measured in different heating rates from 10 to 50 1C/min and from the Kissinger plots, the crystallization activation energy for a-Fe(Si) phase, Ea1, as well as for boride phase, Ea2, has been determined and shown to be increased, both, with increasing Nb content existed in the samples
The magnetic measurements showed that saturation magnetization of as-cast ribbons consequently reduced with higher value of x, namely, Ms¼147 (x ¼ 1.5), 134 (x ¼ 3.0) and 112 emu/g (x ¼ 4.5) due to ferromagnetic dilution
The M(T) curves of samples measured in low field of
5 mT with heating and cooling cycle are presented inFig 2
We define DTcr¼TonsetTCas the stabilizing temperature
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0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.jmmm.2006.11.088
Corresponding author Tel.: +82 43 2612269; fax: +82 43 2756415.
E-mail address: scyu@chungbuk.ac.kr (S.-C Yu).
Trang 2region of amorphous phase, in which no phase transition is
detected Table 1 collects parameters of alloys including
DTcr and obviously DTcr increases with increasing Nb
content, namely DTcr¼80 (x ¼ 1.5), 220 (x ¼ 3.0) and
320 1C (x ¼ 4.5), i.e Nb plays the role of stabilizing
amorphous structure
The samples are annealed at appropriate temperatures and keeping time with crystallization volume fraction of around 82% After annealing, materials become nanocom-posites with nanocrystallites a-Fe(Si) embedded in remain-ing amorphous matrix Once again from Table 1,we can see that Nb plays the role of limitation of grain growth, namely, particle size D ¼ 21.5 (x ¼ 1.5), 10.8 (x ¼ 3.0) and 8.6 nm (x ¼ 4.5) From Table 1, we also recognize that lattice parameter of annealed samples increases with increasing Nb content Yavari et al pointed out that possibility of some atomic percentages of niobium, which are solved in the nanocrystallites, might be taken into account [5] 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 Hence, it seems that the higher value of the lattice constant was originated from the diffusion of Nb into the Fe–Si crystallites (seeTable 1) Because of nanosize and appropriate volume fraction of a-Fe(Si) phase, the soft magnetic properties of annealed sample have been remarkably improved comparing with those of as-cast sample (Fig 3) Magnetic entropy change,
|DSm|, as a function of temperature was evaluated by measuring a series of isothermal magnetization curves around Curie temperature of amorphous phase The results indicate that |DSm| established as high as around respective Curie temperature of amorphous phase (seeTable 1)
In conclusion, the structure, crystallization and magnetic properties of Fe76.5xNbxSi15.5B7Au1 alloys have been examined The crystallization temperature, crystallization activation energy, stabilizing temperature region of amor-phous phase as well as lattice parameter increase with increasing Nb content in the samples, whereas saturation magnetization, Curie temperature of amorphous phase as well as particle size decrease The role of Nb is discussed in detail
Fig 2 Thermomagnetic curves of studied samples Fe 76.5x Nb x Si 15.5
B 7 Au 1 in field of 5 mT, (1) heating cycle, and (2) cooling cycle.
Table 1
Several characteristics of Fe 76.5x Nb x Si 15.5 B 7 Au 1 ribbons
M s (annealed) (emu/g) 160 142 139
Fig 1 X-ray diffraction patterns of as-cast ribbons Fe 76.5x Nb x Si 15.5
B 7 Au 1
Fig 3 Hysteresis loops of samples Fe 76.5x Nb x Si 15.5 B 7 Au 1 (x ¼ 1.5).
Trang 3We acknowledge Korean Science and Engineering
Foundation through the Research Center for Advanced
Magnetic Materials at Chungnam National University and
Vietnam National Fundamental Research Program for
Natural Sciences for financial support of this work
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