At increasing the Ti or V content, the crystalline fraction in the samples increases and for x ¼ 10; the alloys are fully crystalline.. All samples, some of them in a partly crystalline
Trang 1Influence of Ti and V substitution for Al on the properties of
N Chau*, N.H Luong, C.X Huu, N.T Thep, H.D Anh
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
Abstract
Systems with compositions Nd60Fe30Al10xTixand Nd60Fe30Al10xVx(x ¼ 2; 4, 6, 8, and 10) have been prepared by arc melting and copper mold suction-casting The cast surface of the samples is partially crystalline At increasing the Ti
or V content, the crystalline fraction in the samples increases and for x ¼ 10; the alloys are fully crystalline Melting temperatures also increase with increasing Ti or V content in the samples All samples, some of them in a partly crystalline state, exhibit good hard magnetic properties at room temperature With increasing Ti or V content, the magnetization continuously decreases, whereas both the coercivity and the Curie temperature increase
Thermomagnetic curves measured for all samples at low field exhibit an interesting shape The temperature dependence of the coercivity of the samples has been determined
r2003 Published by Elsevier Science B.V
PACS: 75.50.Bb; 75.50 Lk; 75.50.Vv
Keywords: Magnetization; Hard magnetic materials; Crystallization–amorphous systems; Rare-earth alloys
Recently, Inoue et al [1–3] prepared bulk
amorphous alloys of the type Ln–Fe–Al (Ln=Nd,
Pr) by a copper mold casting technique These
alloys have generated considerable interest in both
fundamental research and applications because
they exhibit a large coercivity at room
tempera-ture Unlike the magnetic ribbons, in which the
amorphous state can be only obtained by cooling
from the melt with very high cooling rates of about
106K/s, the amorphous state in the bulk alloys
occurs at cooling rates as low as 1–102K/s
However, the authors of Refs [4–8] pointed out
that it is difficult to prepare samples with a fully
amorphous state from these alloys According to
Inoue et al [2,3], the hard magnetic properties occur in the alloys Ln–Fe–Al only when they are in the bulk amorphous state The same authors of Refs [4–8] have shown that the hard magnetic properties also persist in partially crystalline bulk samples The effects of B substitution for Al [7]
and Co substitution for Al[8]have been studied This work presents the influence of Ti and V substitutions for Al in the alloy Nd60Fe30Al10, on its structure and magnetic properties
Two alloy systems with compositions
Nd60Fe30Al10xTix (x ¼ 2; 4, 6, 8 and 10) and
Nd60Fe30Al10xVx (x ¼ 2; 4, 6, 8 and 10) have been prepared by arc-melting a mixture of high-purity elements in an argon atmosphere After melting all elements together, the samples were prepared by suction-casting of the molten alloy
*Corresponding author Tel./fax: +84-4-858-9496.
E-mail address: chau@cms.edu.vn (N Chau).
0304-8853/03/$ - see front matter r 2003 Published by Elsevier Science B.V.
doi:10.1016/S0304-8853(03)00075-1
Trang 2into a water-cooled copper mold The structure of
the as-cast samples was examined with X-ray
diffraction using a Bruker D5005 apparatus The
composition was checked by EDS included in the
JEOL 5410 LV SEM The thermal stability of the
alloys has been determined by DSC using a TA
Instruments SDT 2960 Magnetization and
hyster-esis loops were measured using a Digital
Measure-ment Systems VSM model DMS 880
X-ray diffraction patterns of samples of both
alloy systems show that the cast surfaces of all
samples are crystalline, probably due to
crystal-lization originating from the copper mold wall
To study the variation of crystallinity with
respect to the position in the sample, we have
taken X-ray diffraction patterns at depths of
d ¼ 0:2 and 0.4 mm beneath the surface of the
samples The results show that with increasing
depth the samples essentially remain crystalline,
although the crystallinity fraction somewhat
decreases Quantitative analysis by EDS
reveals that the alloys have the intended nominal
compositions
The DSC curves of all samples have been
determined The samples with lower Ti content
show an exothermic peak associated with
crystal-lization With increasing Ti concentration the
crystallization transition becomes less clear and
the sample with x ¼ 10 does not show any
crystallization transition It indicates that the
sample Nd60Fe30Ti10 is fully crystalline In the
samples with lower V content a supercooled,
partially amorphous state exists Nd60Fe30V10 is
also fully crystalline We find that the melting
temperature of the studied samples clearly
in-creases with increasing Ti or V content The
authors of Ref [9]pointed out that the addition
of Co significantly enhances the glass-forming
ability of Nd70xFe20Al10Coxalloys Our
observa-tion here is in agreement with the results obtained
in Ref [7] for Nd60Fe30Al10xBx alloys in which
the glass-forming ability decreases with increasing
B content
Nd60Fe30Al10xTix (Fig 1a) and Nd60Fe30
Al10xVx (Fig 1b) samples at room temperature
We see that with increasing Ti or V content, both
the magnetization measured at maximum field and
the remanence decrease whereas the coercivity increases
Thermomagnetic curves of the samples were measured at temperatures up to 620 K At low fields, the Hopkinson effect is clearly seen Fig 2
-20 -10 0 10
20
x = 2
x = 4
x = 6
x = 8
x = 10
H (kOe)
14.0 10.5 7.0 3.5 0 -3.5 -7.0 -10.5 -14.0
-20 -10 0 10 20
x = 2
x = 4
x = 6
x = 8
x = 10
H (kOe)
(a)
(b) Fig 1 Hysteresis loops of the Nd 60 Fe 30 Al 10x Ti x (a) and
Nd 60 Fe 30 Al 10x V x (b) samples at room temperature for different x-values.
0 2 4 6 8 10 12 14 16
300 350 400 450 500 550 600
10000 Oe
5000 Oe
2500 Oe
1000 Oe
500 Oe
100 Oe
T (K) Fig 2 Thermomagnetic curves of Nd 60 Fe 30 Al 6 V 4 measured at different applied fields.
Trang 3presents a representative series of thermomagnetic
curves for Nd60Fe30Al6V4 Note that crystalline
fraction in this V substituted sample is rather high
The occurrence of a Hopkinson effect here
indicates that the magnetic anisotropy decreases
rapidly with increasing temperature and the
magnetic saturation can be easily reached at low
magnetic fields due to a rotation of domain
moments
measurements at low field we derived the
composi-tion dependence of the magnetic characteristics of
the samples as presented in Table 1 One can see
fromTable 1that in both systems of samples, the
Curie temperature increases with increasing Ti or
V content substituted for Al
Hysteresis loops are measured from low to high
temperatures The available maximum field is not
enough to saturate the samples, i.e the hysteresis
loops are minor loops However, the coercivity
increases rapidly with decreasing temperature For
example, HC of Nd60Fe30Al8Ti2 is 8.5 kOe at
201 K and 0.5 kOe at 390 K whereas HC of
Nd60Fe30Al8V2is 8.35 kOe at 240 K and 0.46 kOe
at 390 K
Field-cooled (FC) and zero-field-cooled (ZFC)
curves have been assessed for all samples Fig 3
shows representative data, for Nd60Fe30Al6Ti4
(Fig 3a) and Nd60Fe30Al6V4(Fig 3b) It is
inter-esting to note that these curves are clearly
separated from each other at low temperatures
This remarkable feature of magnetic properties in
two series of samples indicates the appearance of
short-range ferromagnetic interactions This is
confirmed by observing spin-glass-like behavior
at low temperature and in low field ranges In low
applied fields, both the ZFC and FC
magnetiza-tion curves split at a temperature below the
so-called irreversibility temperature, Tr (oTC) Both the ZFC and FC MðTÞ curves resemble each other substantially in the range of TXTr; implying the irreversibility of magnetization In addition, the low field ZFC MðTÞ curves clearly show a cusp at
a so-called spin freezing (or spin–glass transition) temperature, Tg; where a maximum magnetization
is attained, as demonstrated inFig 3 This research is performed within the project
420101 of the Vietnam National Program for Fundamental Sciences and the Vietnamese—US
Table 1
Composition dependence of magnetic characteristics
X Nd 60 Fe 30 Al 10x Ti x Nd 60 Fe 30 Al 10x V x
Z F C
0.0 1.2 2.4 3.6
T (K)
(a)
(b)
0 1 2 3 4 5
F C
T (K)
Z F C
F C
Fig 3 FC and ZFC curves measured in a field of 20 Oe for
Nd 60 Fe 30 Al 6 Ti 4 (a) and Nd 60 Fe 30 Al 6 V 4 (b).
Trang 4Cooperation project supported in part by the US
National Science Foundation
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