The coercivity of this sample was decreased by half PACS: 75.50.Ww; 81.05.Ys; 81.40.Ef Keywords: FePt magnets; Cold rolling; Nanostructured magnetic materials; Bulk multilayers; Micro-sy
Trang 1Journal of Magnetism and Magnetic Materials 257 (2003) L139–L145
Letter to the Editor
An original route for the preparation of hard FePt
N.H Haia,b, N.M Dempseya, M Veronc, M Verdierc, D Givorda,*
a Laboratoire Louis N !eel, 25 Avenue des Martyrs, BP 166, 38042 Grenoble cedex 9, France
b Cryolab, Faculty of Physics, Vietnam National University, Hanoi, 334, Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
c LTPCM-INPG, Domaine Universitaire, BP 75, 38402 St Martin d’H "eres, France
Received 16 September 2002
Abstract
The preparation of FePt hard magnetic foils by an original procedure is described in this paper The process
results from controlled diffusion and an ordering phase transformation Coercivities as high as 0.9 T were measured in a VSM at room temperature following annealing at 4501C for 48 h The coercivity of this sample was decreased by half
PACS: 75.50.Ww; 81.05.Ys; 81.40.Ef
Keywords: FePt magnets; Cold rolling; Nanostructured magnetic materials; Bulk multilayers; Micro-system magnets
1 Introduction
Equiatomic FePt alloys may exist in a
disor-dered face-centered-cubic phase (high-temperature
phase) or an ordered face-centered-tetragonal
(FCT) phase (low-temperature phase) [1]
Although the ordered phase is stable below about
13001C, the disordered phase may be stabilized by
quenching bulk samples from high temperature or
depositing thin film samples at room temperature
The ordered FCT phase, commonly known as the
L10 phase, is of interest for permanent magnet
applications due to its excellent intrinsic magnetic properties (m0MS¼ 1:43 T and K1¼ 6:6 MJ/m3at
300 K; TC¼ 750 K) and its corrosion resistance
[2] It can be formed by the controlled annealing of the disordered phase through a first-order phase transition [3–6] Alternatively, the ordered phase can be directly prepared in thin-film form by deposition on substrates heated to the appropriate temperature[7] Ordered FePt thin films have also been prepared by optimized heat treatments of Fe/
Pt multilayers with individual layer thickness of the order of 1–3 nm [8,9] Bulk samples are magnetically isotropic owing to the three variants for the tetragonal distortion of the cubic phase Under certain conditions, crystallographic texture may be obtained in thin films owing to the preferential crystallization along the materials
*Corresponding author Laboratoire Louis N!eel, 25 Avenue
des Martyrs, BP 166, 38042 Grenoble cedex 9, France Fax:
+4-76-88-11-91.
E-mail address: givord@grenbole.cnrs.fr (D Givord).
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 2 8 4 - 2
Trang 2most dense crystallographic directions on
poly-crystalline substrates (fiber texture) or epitaxial
growth on single crystalline substrates [5–7]
Room-temperature coercivity values of typically
0.5 T have been developed in bulk samples [3]
while higher values are reported for thin-film
samples (typically 1–2 T)[4–9]
In this paper we report on the novel use of a
cyclic cold-rolling process to produce
nanocom-posite Fe/Pt multilayers which are then heat
treated to produce hard magnetic FePt foils This
study forms part of a body of work on the use of
classical mechanical deformation techniques (e.g
drawing, rolling and extrusion) to prepare
nano-composite materials by cyclic deformation
invol-ving sample re-assembly of macrocomposite
materials[10–15]
2 Experimental
Astack of iron and platinum foils {Fe(75 mm)/
Pt(100 mm)}10 (49 at% Fe—calculated from the
measured masses; stack dimensions: 1.8 4.5
15 mm3) was inserted into a sheath (stainless-steel
tube) and the ensemble was compressed in a
hydraulic press under ambient conditions
(Pmax¼ 5 ton/cm2
) so as to compact the sample vertically and produce parallel upper and lower
surfaces The thickness of the ensemble was then
progressively reduced by multiple-pass cold
roll-ing, the inter-cylinder spacing being slightly
reduced for each new pass In a given rolling
cycle, involving about 100 passes, the total
thickness was reduced by a factor 10 (i.e
tinit=tfinalE10) The low deformation rate per pass
typical of cold rolling allowed progressive
defor-mation without stress-relief heat treatment, a very
important factor for multilayers consisting of
metals which are miscible at the temperatures
required for stress-relief heat treatment The
sample was then removed from the stainless-steel
sheath by cutting off the edges of the sheath and
simply lifting off the upper and lower steel layers
Following this, the multilayer sample was cut into
short lengths, piled up to form a stack and inserted
into a new sheath The sample was submitted to
four such rolling cycles (cumulative reduction
factor E104
) and the final multilayered foil had
a total thickness of about 100 mm The nanos-tructured multilayer was then sealed under va-cuum (105mbar) in a quartz tube, annealed in a muffle furnace at temperatures in the range 450– 5501C (t=30 s to 48 h) and then water quenched SEM images were taken with a LEO 1530 electron microscope equipped with a field emission gun and operated at 20 kV, the TEM images were taken with a 3010 FX Jeol electron microscope operated
at 300 kV XRD diffraction patterns were made with Cu Karadiation and magnetization measure-ments were performed on either a VSM or an extraction magnetometer
3 Structural analysis SEM images of the Fe/Pt multilayers after two and four rolling cycles are shown in Fig 1 The thickness of the individual layers after two cycles is typically less than 1 mm and after four cycles it is of the order of some tens of nanometers, in agree-ment with the bulk reduction factor
X-ray diffraction analysis reveals that, after four deformation cycles, the Pt layers are (2 2 0) in-plane textured, as expected for rolled FCC metals
the rocking curve of the (2 2 0) peak is about 161 (inset ofFig 2) The intensities of the Fe peaks are much lower than those of the Pt peaks owing to the low atomic number of Fe relative to Pt; nevertheless, the (2 0 0) texture expected for BCC metals is discernible The diffraction spectra of the samples annealed for very short times (e.g 30 s at
Fig 1 SEM images of Fe/Pt multilayers after (a) two and (b) four rolling cycles.
Trang 35001C) are comparable to those of non-annealed
samples However, peaks of both the fundamental
and superstructure reflections of ordered FCT
FePt are observed for annealing times as short as
2 min at 5001C, indicating that both diffusion and
ordering occur under these conditions Only trace
quantities of the FCC phase are evident for short
annealing times (e.g 5 and 15 mins at 4501C,
extended annealing times (e.g 48 h at 4501C,
is formed almost immediately upon heating
nanostructured Fe/Pt multilayers in this
tempera-ture range Alattice parameter aE3.86 (Aand a
tetragonal distortion c=aE0.96 are estimated for
all annealing conditions studied The diffraction
peak widths become narrower as the annealing
time increases, which may be attributed to grain
growth The XRD spectrum of the sample
annealed at 4501C/48 h is shown inFig 4 Partial
crystallographic texture is identified as the
inten-sities of the (0 0 1) and (0 0 2) peaks are twice as
high as those expected for isotropic material
(intensity values taken from the PDF file
43-1359) The degree of long range order in the L10
phase can be quantified by the chemical ordering
2θ
θ (°)
fwhm = 16°
Pt (220)
Fig 2 XRD patterns (Cu K a radiation) of Fe/Pt multilayer
after four deformation cycles (powder diffraction intensities for
Pt and Fe are represented by and , respectively) Inset:
rocking curve of Pt (2 2 0) peak.
2θ
5 min.
15 min.
48 hrs
(311) fcc (311) fct (113) fct (222) fcc (222) fct
Fig 3 High-angle section of the XRD spectra (Cu K a radia-tion) of FePt foils produced by annealing Fe/Pt multilayers at 4501C for times ranging from 5 min to 48 h The vertical dashed lines represent the 2y positions of peaks belonging to the disordered FCC structure, taken from PDF file 29-0717, and those of the ordered FCT FePt structure calculated using the lattice parameters estimated with lower angle peaks (taking
l ¼ 1:54056 ( A).
2θ
Fig 4 XRD patterns (Cu K a radiation) of FePt foil produced
by annealing Fe/Pt multilayer at 4501C/48 h, the superstructure reflections of the L1 0 phase are denoted by the letter ‘‘s’’ (experimental data are represented by solid lines; powder diffraction intensities for FCT FePt (L1 ) are represented by ).
Trang 4parameter S; defined as
S ¼ ðrFe xFeÞ=yPt¼ ðrPt xPtÞ=yFe;
where xFeðPtÞis the atomic fraction of Fe(Pt) in the
sample, yFeðPtÞ the fraction of Fe(Pt) sites, and
rFeðPtÞ the fraction of Fe(Pt) sites occupied by the
right atomic species[17,7] It can be estimated by
comparing the integrated intensities of the (0 0 1)
superstructure peak and the (0 0 2) fundamental
peak [7] SE0:8 was estimated for the sample
annealed at 4501C/48 h (Fig 4), which is
compar-able to the value, 0.970.1, reported for epitaxial
films[7] The fact that this value is less than 1 may
be attributed to incomplete order in the FCT
phase (possibly due to the relatively low annealing
temperature) and/or a variation in composition
away from the 50:50 stoichiometry of the perfectly
ordered L10phase Plane-view TEM analysis of a
heat-treated sample (4501C/48 h), reveals many
interesting microstructural features (Figs 5 and 6)
The in-plane dimensions of the FePt grains vary
from tens of nanometers to some hundreds of
nanometers; an area with relatively large grains is
shown in Fig 5 Diffraction patterns taken on
individual grains reveal superstructure reflections,
confirming that the FePt is ordered Aband-like structure, with bandwidths of the order of 10 nm,
is observed in many grains Diffraction analysis of
an area of a large grain containing these bands
structure, i.e these bands are not twins, and that the bands are roughly perpendicular to the /
2 2 0S direction Twinning is observed in the diffraction pattern of another area of this grain,
in which the orientation of the bands changes along linear fronts (Fig 6) This band structure is not well understood and may simply be due to heavy faulting However, the fact that the bands are perpendicular to the /2 2 0S direction, a direction along which Fe and Pt atoms form alternate layers in the ordered FCT FePt structure, may suggest that the bands are related to an anti-phase-domain structure Moir!e patterning and boundary traces due to the superposition of grains are also clearly visible Cross-sectional observa-tions are under way to determine the thickness of the grains Adetailed investigation of the micro-structure of as-rolled and annealed Fe/Pt struc-tures will be reported elsewhere
4 Magnetic properties Room-temperature VSM magnetization mea-surements reveal that coercivity is developed even after very short annealing times, demonstrating the early onset of ordering into the high-aniso-tropy L10 phase (Fig 7) Coercivity continues to increase with annealing time, which can be attributed to progressive changes in the complex microstructure of this system The optimum coercivity value obtained in these foils (m0H ¼ 0:9 T) is significantly higher than that usually obtained in bulk FePt, which can be related to the foil’s specific nanostructure A detailed study of the relationship between coerciv-ity and nanostructure is under way Hysteresis loops of the sample with the highest observed value of coercivity (annealed at 4501C/48 h), measured in three orthogonal directions in an extraction magnetometer, are shown in Fig 8(for
a given sample the value of coercivity measured in the VSM was higher than that measured in the
Fig 5 TEM image and diffraction pattern of a section of an
FePt foil, produced by annealing an Fe/Pt multilayer (4501C/
48 h), containing a number of grains with in-plane dimensions
in the range 100–300 nm.
Trang 5extraction magnetometer owing to the higher field sweep rate in the former; kinetic effects on magnetization reversal in this system were re-ported by Luo et al [18]) The higher value of remanent magnetization in the out-of-plane loop is
in agreement with the observation of partial (0 0 1) texture in the XRD pattern of this sample (Fig 4) The temperature dependence of coercivity and remanent magnetization in the sample annealed at 4501C/48 h, measured in-plane in the range 10–700 K in an extraction magnetometer, are shown in the inset of Fig 7 Asignificant high-temperature coercivity of 0.37 T was measured at
600 K The energy product of this foil was estimated to be 100 kJ/m3 at 300 K and about
25 kJ/m3at 600 K No decrease in room-tempera-ture coercivity was detected following repeated high-temperature measurements up to 700 K This indicates that the microstructure is stable up to this temperature, which is very important if
Fig 6 TEM image of one large grain of an FePt foil, produced by annealing an Fe/Pt multilayer (4501C/48 h), displaying a band structure (see text) The diffraction pattern of the left-hand side of the grain is characteristic of a single crystalline structure (top left inset) while that of the right-hand side is characteristic of a twinned structure (top right inset).
0
0.2
0.4
0.6
0.8
1
0.001 0.01 0.1 1 10 100 1000 10 4
450°C
500°C
550°C
µ0
H C
annealing time (minutes)
0 0.2 0.4 0.6 0.8 1
0 100 200 300 400 500 600 700
µ0
µ0
T (K)
Fig 7 Variation of room-temperature coercivity of FePt foils
prepared by annealing Fe/Pt multilayers plotted as a function
of annealing conditions (VSM measurements); inset:
tempera-ture dependence of coercivity and remanent magnetization of
FePt foil produced by annealing Fe/Pt multilayer at 4501C/48 h
(extraction magnetometer measurements).
Trang 6these foils are to be used in high-temperature
applications
5 Conclusions
Cyclic cold rolling has been used to prepare
textured Fe/Pt multilayer foils of total thickness
100 mm and individual layer thicknesses of the
order of 10 nm Ordered FCT FePt (L10) is rapidly
formed upon annealing in the temperature range
450–5501C Coercivities as high as 0.9 T at 300 K
have been measured in annealed foils This
coercivity value is intermediate between the
high-est values reported for bulk and thin film samples
Coercivity decreases relatively slowly with
increas-ing measurincreas-ing temperature, attainincreas-ing a value of
0.37 T at 600 K Efforts are now under way to
increase the texture of the hard foils obtained and
thus further increase remanence/energy product
values Finally, the form and size of the samples
produced (foils of thickness 100 mm) make them
suitable candidates for magnetic micro-system applications [19]
Acknowledgements This work was carried out within the framework
of the European project for the development of high-temperature magnets ‘‘HITEMAG’’ (G5RD-2000-00213) which is supported by the Commis-sion of the European Union (D.G XII) The authors would like to thank Dr M Venkatesan for fruitful discussions Scanning electron micro-scope imaging was performed with Nanofab (CNRS Grenoble) facilities N.H Hai gratefully acknowledges support of the CNRS-PICS pro-gram (Nanomat!eriaux) and the French Embassy in Vietnam
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-0.4
0
0.4
0.8
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
x y z
µ0
µ0 H (T)
-1.2 -0.4 0 0.4 0.8
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