Bull. Mater. Sci., Vol. 27, No. 2, April 2004, pp. 169–173. © Indian Academy of Sciences.
169
Magnetic properties of ball-milled TbFe
2
and TbFe
2
B
J AROUT CHELVANE, S KASIVISWANATHAN, M V RAO and G MARKANDEYULU*
Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India
MS received 25 August 2003; revised 13 February 2004
Abstract. The magnetic properties of ball-milled TbFe
2
and TbFe
2
B were studied by magnetization mea-
surements. X-ray diffraction studies on TbFe
2
B showed that boron occupied interstitial position in the crystal
structure, just as hydrogen did. The value of the saturation magnetization of TbFe
2
B was found to be smaller
than that of TbFe
2
. This is explained on the basis of a charge transfer between the boron atoms and the 3d
band of Fe. The anisotropy of TbFe
2
B was found to be large compared to that of TbFe
2
. X-ray diffractograms
for the ball milled samples showed that after 80 h of milling, a predominantly amorphous phase was obtained.
TbFe
2
B was found to undergo easy amorphization compared to TbFe
2
. Magnetization of TbFe
2
was found to
decrease rapidly with initial milling hours and was found to be constant with further hours of milling. TbFe
2
B
exhibited an anomalous behaviour with an increase in moment with milling hours and this may be due to the
segregation of αα-Fe.
Keywords. Amorphous materials; intermetallic compounds; magnetic materials; magnetic properties.
1. Introduction
The RFe
2
(R = rare earth) Laves phase compounds are
known to possess large cubic anisotropy (Clark et al 1972)
and highest Curie temperature (T
C
) of all RT
2
compounds
(T = transition metal). RFe
2
compounds crystallize in
cubic MgCu
2
(C15) type structure. These compounds are
known to exhibit diverse magnetic properties due to the
competing effects of exchange interaction and crystalline
electric field effects. Koon et al (1991) reviewed the ani-
sotropy and other magnetic properties in these materials.
Dhilsha and Rama Rao (1990) reported single particle
excitations in Co substituted Ho
0⋅85
Tb
0⋅15
Fe
2
, through ele-
ctrical resistivity measurements. Low temperature mag-
netization studies on Ni substituted Dy
0⋅73
Tb
0.27
Fe
2
and
Ho
0⋅85
Tb
0⋅15
Fe
2
have revealed domain wall pinning (Senthil
Kumar et al 1995). Annapoorni et al (1989, 1990) inves-
tigated the effect of hydrogen on the structural, magnetic
and electrical properties of Dy
0⋅73
Tb
0⋅27
Fe
2
and Ho
0⋅85
Tb
0⋅15
Fe
2
. They observed the solubility of hydrogen
through isotherms, compensation temperatures near T
C
that decreased with increasing hydrogen concentration
(Annapoorni et al 1989) and reported a metal–semicon-
ductor transition induced by hydrogen (Annapoorni et al
1990). The modifications of the magnetic properties due
to hydrogenation have been understood as due to the
transfer of charges from the 3d band of Fe to hydrogen
(Annapoorni and Rama Rao 1990). Kishore et al (1998)
observed spin flip meta-magnetism and domain wall pin-
ning resulting from the modification of magnetoelastic
properties in hydrogenated Dy
0⋅73
Tb
0⋅27
Fe
2–x
Co
x
compounds.
Lee et al (2000) measured magneto-optical equatorial
spectra of polycrystalline RFe
2
and derived half-diagonal
conductivities.
In recent years, structural and magnetic properties of
ball-milled RFe
2
compounds have been investigated.
57
Fe
Mossbauer, magnetization and a.c. susceptibility studies
on ball milled YFe
2
showed significant changes in mag-
netic properties and these have been attributed to the sys-
tematic disordering of the crystal structure and subsequent
reduction in ordering temperature with milling time
(Larica et al 1998). In addition, the formation of disor-
dered distribution of magnetic moments (spin glass/aspero-
magnetic behaviour etc) has been reported.
155,157
Gd
NMR studies on the ball-milled GdFe
2
and GdCo
2
have
shown a broadening of the spectrum due to the trans-
ferred hyperfine interaction from the segregation of ele-
mental particles upon milling (Tribuzy et al 1999).
Modder et al (1999) studied the structural and magnetic
properties of ball-milled GdX
2
(X = Al, Mg, Pt, Ir, Rh)
Laves phase compounds. Geshev et al (1997) reported a
significant increase in coercivity in TbFe
2
upon ball mill-
ing. In this paper, we report the effect of the addition of
boron on the magnetic properties of TbFe
2
and also the
magnetic properties of the above compounds as a func-
tion of reduction in particle size.
2. Experimental
TbFe
2
and TbFe
2
B were prepared by arc melting the high
pure elements (Tb and B, 99⋅9% purity; Fe, 99⋅95% purity).
The ingots were melted several times to obtain a homo-
*Author for correspondence
J Arout Chelvane et al
170
geneous mixture. In all the compounds, the weight loss
after melting was less than 0⋅5%. The ingots were anne-
aled in vacuum at 950°C for 7 days and were then furnace
cooled to room temperature. Powder X-ray diffraction
studies were carried out on the compounds employing
FeK
α
radiation for structural characterization.
Ball milling was carried out using a planetary ball mill
(FRITSCH) at a speed of 300 rpm and a ball-to-powder
weight ratio of 15 : 1. The milling was carried out in toluene
for 80 h employing tungsten carbide balls of 10/20 mm
diameter. The magnetization measurements were carried
out using a PAR vibrating sample magnetometer up to a
field of 10 kOe.
3. Results and discussion
Figures 1 (a) and (b) show the XRD patterns of TbFe
2
and TbFe
2
B, respectively. Both the compounds formed in
cubic Laves phase structure. The lattice parameter of
TbFe
2
is found to be 7⋅340 Å, which is in agreement with
that reported in literature.
The XRD pattern of TbFe
2
B indicates two sets of
peaks corresponding to two lattices with MgCu
2
struc-
ture. These have been indexed, one corresponding to a
lattice (lattice I) with a lattice parameter of 8⋅017 Å and
the other set corresponding to a further expanded lattice
(lattice II) with a lattice parameter of 8⋅268 Å. Ren et al
(2002) reported that boron in Dy
0⋅7
Pr
0⋅3
(Fe
1–x
B
x
)
2
(x =
0⋅1–0⋅3) occupies Fe sites, with a decrease in lattice
parameter with increasing boron concentration. The
effect of boron in Tb
0⋅7
Pr
0⋅3
(Fe
1–x
B
x
)
2
(x = 0⋅05–0⋅35) has
been studied by Ren et al (2001). They have shown that
the lattice expands when boron occupies the interstitial
sites for concentration up to x = 0⋅1 and for further in-
crease in boron concentration the lattice contracts due to
the occupancy of boron in lattice at the Fe sites. How-
ever, in the present case of TbFe
2
B there is only a lattice
expansion and therefore, the probability that boron occu-
pies the interstitial sites could be more than that for the
occupation at the substitutional site. This situation may
be likened to the case of hydrogen occupying the intersti-
tial sites. However, while hydrogen preferentially occu-
pies the tetrahedral site formed by two Tb atoms and two
iron atoms, boron may prefer to occupy the site formed
by three iron atoms and a Tb atom. This could expand the
lattice more than in the case of hydrogenated compounds.
This combined with the fact that boron atom is bigger
than hydrogen, might be responsible for the lattice I itself
to be an expanded lattice.
Figures 2 (a) and (b) show the magnetization curves of
TbFe
2
and TbFe
2
B, respectively at 12 K. Both the mag-
netization values did not saturate up to a field of 10 kOe.
It is seen that the magnetization decreases with the intro-
duction of B in TbFe
2
. In TbFe
2
, the Tb and Fe moments
Figure 1. (a) X-ray diffraction patterns of unmilled and ball milled TbFe
2
and (b) X-
ray diffraction
patterns of unmilled and ball milled TbFe
2
B, peaks indexed as ‘α
’ correspond to lattice I and those peaks
indexed as ‘β’ correspond to lattice II.
Magnetic properties of ball-milled TbFe
2
and TbFe
2
B
171
are antiparallel and the net magnetic moment is domi-
nated by the Tb moment. The magnetic moment per iron
atom has been reported to be 1⋅64 µ
B
. As in the case of
hydrogenated compounds, the addition of boron may
cause the depletion of the 3d band of Fe thereby increas-
ing the Fe moment and hence a decrease in the net mag-
netic moment of the alloy. However, while hydrogen
causes a fanning of the Tb moments leading to a decrease
in the anisotropy, with the addition of boron, the aniso-
tropy is seen to increase. This again, may be due to the
affinity of boron for iron, causing the anisotropy of the
iron sublattice to increase, adding to the anisotropy of the
Tb sublattice.
The ball-milled samples of TbFe
2
and TbFe
2
B were
analysed using XRD, after various milling times (figures
1 (a), (b)). The intensities of the peaks for both the com-
pounds are seen to decrease with the increase of milling
time. The XRD pattern for TbFe
2
taken after 80 h of mill-
ing shows a predominantly amorphous phase along with
the MgCu
2
type crystalline phase. In fact, the develop-
ment of amorphous phase is seen after 50 h of milling. In
addition to the MgCu
2
and amorphous phases, Tb
2
O
3
phase
is seen to develop. This could have been picked up from
the air molecules present in the vial used for ball milling,
even though the milling was carried out in toluene me-
dium, due to the small size of the particles leading to a
large effective surface area. Tang et al (1997), in fact,
reported the formation of oxides of Dy and Tb when
Dy
0⋅7
Tb
0⋅3
Fe
1⋅97
was ball-milled in ethanol or argon
medium. The XRD pattern of TbFe
2
B after 80 h of mill-
ing shows reflections corresponding to terbium oxide, an
amorphous phase and α-Fe, in addition to the MgCu
2
phase. The amorphous phase in this material is seen to
form after < 30 h of milling, in contrast to TbFe
2
.
Figure 3 shows the magnetization curves at 12 K, of
ball milled TbFe
2
at various stages of milling. The mag-
netization does not saturate up to a field of 1 Tesla, irre-
spective of the particle size. However, the magnetization
at 10 kOe decreases rapidly for the material ball milled
for 30 h, compared to the starting material. The magneti-
zation is found not to change much subsequently. Inset
shows the variation of magnetization at 10 kOe as a func-
tion of milling time. The decrease in the magnetic mo-
ment can be understood as follows: the Fe moments that
are suppressed in the alloy, from the elemental value
might have been recovered due to the localization upon
reduction in the particle size. Since Fe and Tb moments
are anti-parallel there is a net decrease in the magnetic
moment of ball-milled TbFe
2
. Of course, it is assumed
that the Tb moments are not affected as they are loca-
lized. In addition, the reduction in particle size causes
disordering at the surfaces that may also cause a reduc-
tion in magnetization. Tang et al (1997) reported similar
observations in Dy–Tb–Fe alloy.
The magnetization curves of TbFe
2
B at 12 K (figure 4)
are interesting in the sense that the magnetization de-
creases with milling up to 30 h and subsequently in-
creases (see inset). The decrease can be attributed to the
localization of Fe moments, causing a net decrease in the
magnetization due to anti-ferromagnetic ordering of Fe
and Tb moments. Rhombohedral distortion in the cubic
Figure 2. Magnetization curves of (a) TbFe
2
and (b) TbFe
2
B,
at 12 K.
Figure 3. Magnetization curves of ball milled TbFe
2
at 12 K.
J Arout Chelvane et al
172
Laves phase compounds due to magnetoelastic interac-
tions has been reviewed by Clark (1980). The milling
could enhance the already enhanced rhombohedral distor-
tion due to the addition of boron. The increased distortion
could in turn lead to the separation of α-Fe. Therefore,
the increase in the magnetic moment with increasing
milling time beyond 30 h could be due to the separation
of α-Fe upon ball milling.
Figure 5 shows the magnetization curves up to a field
of 2 kOe and at 12 K, of ball milled TbFe
2
B at various
stages of milling and discontinuous increase of magneti-
zation is seen in all the compounds. Such a phenomenon,
termed as spin flip metamagnetism, is encountered in
several rare earth iron intermetallic systems with pro-
nounced localized and uniaxial anisotropy (Gignoux and
Schmitt 1995). Kishore et al (1998) observed this phe-
nomenon for the first time in hydrogenated RFe
2–x
Co
x
compounds. They attributed the metamagnetism as due to
Figure 4. Magnetization curves of ball milled TbFe
2
B at
12 K.
Figure 5. Magnetization curves of ball milled TbFe
2
B at 12 K shown up to a field of 2 kOe.
Magnetic properties of ball-milled TbFe
2
and TbFe
2
B
173
the localization of anisotropy as a result of the ordering
of hydrogen with periodicity incommensurate to the peri-
odicity of the main lattice, in the already rhombohedrally
distorted lattice. A similar effect, though to a smaller
magnitude, could be responsible for the observed behav-
iour of magnetization in TbFe
2
B. In the present case, the
discontinuous changes in magnetization is more pro-
nounced and extends up to 2 kOe in the unmilled com-
pound and is less pronounced and shifted towards lower
fields, in the milled compounds. This is of course, to be
expected due to the amorphous nature of the compound,
after milling.
4. Conclusions
The decrease in magnetization of TbFe
2
with the intro-
duction of boron has been explained on the basis of the
depletion of 3d band. The severe milling of TbFe
2
and
TbFe
2
B show the development of predominant amorphous
phases in both the compounds, after 80 h of milling. The
magnetization of ball milled TbFe
2
decreases rapidly up
to 30 h of milling and remains constant with further mill-
ing as has been attributed to the localization of Fe mo-
ments with decrease in particle size. Magnetization of
ball-milled TbFe
2
B decreases up to 30 h of milling and
increases with further milling. The initial decrease may
be attributed to the localization of the Fe moments and
the subsequent increase is due to the separation of α-Fe
due to the inherent rhombohedral distortion which is fur-
ther enhanced by boron during milling. Spin-flip meta-
magnetism is observed in 12 K magnetization curves of
ball-milled TbFe
2
B up to a field of 2 kOe.
Acknowledgements
The authors thank Prof. K V S Rama Rao, Department of
Physics, Indian Institute of Technology Madras, Chennai,
for fruitful discussions. One of the authors (JAC) thanks
the Indian Institute of Technology Madras, Chennai, for
financial support.
References
Annapoorni S and Rama Rao K V S 1990 J. Appl. Phys. 67 424
Annapoorni S, Markandeyulu G and Rama Rao K V S 1989 J.
Appl. Phys. 65 495
Annapoorni S, Markandeyulu G and Rama Rao K V S 1990 J.
Phys. Soc. Jap. 59 3014
Clark A E 1980 Ferromagnetic materials (ed.) E P Wholfrath
(Amsterdam: North Holland) Vol. 1, p. 531
Clark A E, Belson H S and Tamagawa N 1972 Phys. Lett. A42
160
Dhilsha K R and Rama Rao K V S 1990 J. Appl. Phys. 68 259
Geshev J, Bozukov L, Coey J M D and Mikhov M 1997 J.
Magn. Magn. Mater. 170 219
Gignoux D and Schmitt D 1995 J. Alloys and Compounds 225
423
Kishore S, Markandeyulu G and Rama Rao K V S 1998 Solid
State Commun. 108 313
Koon N C, Williams C M and Das B N 1991 J. Magn. Magn.
Mater. 100 173
Larica C, Passamani E C, Nunes E, Orlando M T D, Alves K M B
and Baggio-Saitovitch E 1998 J. Alloys and Compounds 274
23
Lee S J, Lange R J, Canfield P C, Harmon B N and Lynch D W
2000 Phys. Rev. B61 9669
Modder I W, Bakker H and Zhou G F 1999 Physica B262 141
Ren W J, Zhang Z D, Markosyan A S, Zhao X G, Jin X M and
Song X P 2001 J. Phys. Condens. Matter 34 3024
Ren W J, Zhang Z D, Liu J P, Zhao X G, Liu W, Geng D Y and
Jin X M 2002 J. Appl. Phys. 91 8207
Senthil Kumar M, Reddy K V and Rama Rao K V S 1995
IEEE Trans. Magn. 31 4160
Tang Jinke, Zhao Wei, Charles J, Cannor O and Li Sichu 1997
J. Alloys and Compounds 250 482
Tribuzy C V B, Biondo A, Larica C, Alves K M B and Gui-
maraes A P 1999 J. Magn. Magn. Mater. 195 49