DSpace at VNU: Spin reorientation in Er1-xYxCo10Mo2 and ErCo10-yNiyMo2 compounds

Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

Spin reorientation in Er1ÀxYxCo10Mo2 and
ErCo10ÀyNiyMo2 compounds
N.H. Luonga,*, N. Chaua, N.D. Dunga, M. Kurisub, G. Nakamotob
a

Center for Materials Science, Faculty of Physics, College of Science, Viet Nam National University,
Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
b
School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsumokuchi, Ishikawa 923-1292, Japan

Abstract
Er1ÀxYxCo10Mo2 (x ¼ 0; 0.2, 0.4, 0.6, and 0.8) and ErCo10ÀyNiyMo2 (y ¼ 0; 1, 2, and 3) compounds have been
studied in magnetization and susceptibility measurements. Interesting magnetization curves at selected temperatures
have been observed. The results show that both these systems undergo a spin reorientation of the axis-to-plane type
(referred to decreasing temperature). The spin-reorientation temperature, TSR ; is found to decrease not only with
increasing Y content in the Er1ÀxYxCo10Mo2 compounds but also with increasing Ni concentration in the
ErCo10ÀyNiyMo2 compounds. The causes of the observed concentration dependence of TSR are discussed.
r 2003 Elsevier Science B.V. All rights reserved.
PACS: 75.25.+z; 75.30.Gw
Keywords: Hard magnetic materials; Rare earth—transition metal compounds; Spin reorientation; Anisotropy

1. Introduction
The intermetallic compounds of the type
R(T,M)12 (R=rare earth, T=Fe, Co, Ni and
M=Ti, V, Mo, Cr) have been regarded as
potential starting materials for permanent magnet
applications. These compounds crystallize in the
tetragonal ThMn12 structure with space group I4/
mmm. It can be derived from the magnetic

measurements on the RCo10Mo2 compounds
performed by Xu and Shaheen [1] that in these
compounds the moments of the light rare earth
couple parallel to the Co moments and that the
moments of heavy rare earths couple antiparallel
to the Co moments, as is usually observed [2].
*Corresponding author. Fax: +84-4-85-89496.
E-mail address: (N.H. Luong).

Zeng et al. [3] and Tang Ning [4] have shown that
in these compounds the Co sublattice displays easy
c-axis magnetization. These authors also reported
that an axis-to-plane spin reorientation occurs at
the spin-reorientation temperature, TSR ; (referred
to decreasing temperature) in ErCo10Mo2.
The present investigation has been undertaken in
order to study the spin-reorientation phenomena in
the Er1ÀxYxCo10-Mo2 and ErCo10ÀyNiyMo2 compounds. Magnetization isotherms in several of these
compounds have also been studied.

2. Experimental
The Er1ÀxYxCo10Mo2 (x ¼ 0; 0.2, 0.4, 0.6,
and 0.8) and ErCo10ÀyNiyMo2 (y ¼ 0; 1, 2,
and 3) compounds were prepared by arc-melting

0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0304-8853(03)00081-7


N.H. Luong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

stoichiometric amounts of high purity elements
(99.9% for Er and Y, 99.99% for Co, Ni and Mo).
Subsequently, the samples were annealed in vacuum at 980 C for 3 days. X-ray diffraction and
thermomagnetic analysis were used to check the
quality of the samples. All the samples were of
single phase.
Magnetic measurements were carried out in
applied magnetic fields up to 50 kOe and in the
temperature range from 1.8 to 300 K by using a
SQUID. Magnetization (M) as a function of
temperature from 100 to 700 K in a magnetic field
of 1 kOe has also been measured in a vibrating
sample magnetometer. The AC susceptibility (wac )
of the samples has been measured as a function of
temperature from 20 to 200 K. The spin-reorientation temperature TSR is determined as the temperature where in the magnetically ordered state
the maximum in the wac ðTÞ curve occurs.

6.0
ErCo10Mo2
T= 150 K
Magnetization (µB/f.u.)

480

4.0

T= 50 K

2.0

T= 1.8 K

0.0

0

10

20

30

40

50

Magnetic field (kOe)

Fig. 1. Magnetization curves at selected temperatures for
ErCo10Mo2.

3. Results and discussion
Results of magnetization measurements in
applied magnetic fields up to 50 kOe at selected
temperatures for ErCo10Mo2 are shown in Fig. 1.
The observation that the magnetic isotherm at
1.8 K passes through the origin means that in this
compound the Er-sublattice magnetization MEr
and the Co-sublattice magnetization MCo are

equal to each other. The value for MEr and MCo
is 9 mB/f.u. This value for MEr and MCo has been
also reported by Zeng et al. [3] and Tang Ning [4].
As one can see from Fig. 1, magnetization of the
compound increases with increasing temperature.
As the temperature increases, the two sublattice
magnetizations bend towards each other, giving
rise to the total magnetization. We note that the
highest lying magnetization curve in Fig. 1 is
measured at 150 K, i.e. at a temperature higher
than TSR (see below).
Fig. 2 presents the temperature dependence of
the magnetization for the Er1ÀxYxCo10Mo2 compounds in a magnetic field of 1 kOe. The MðTÞ
curves exhibit clear peaks which are associated
with the spin reorientation. In order to determine
precisely the values of TSR for these compounds,
we have measured the AC susceptibility as a

Fig. 2. Temperature dependence of the magnetization for the
Er1ÀxYxCo10Mo2 compounds.

function of temperature from 20 to 200 K. The
results are shown in Fig. 3. Sharp peaks have been
observed in the wac ðTÞ curves for the Er1ÀxYxCo10Mo2 compounds investigated. Apart from these
peaks at TSR ; there is an anomaly in the wac ðTÞ
curves for ErCo10Mo2 and Er0.8Y0.2Co10Mo2. The
TSR values for the Er1ÀxYxCo10Mo2 compounds
are presented in Table 1. As one can see from this
table, the spin-reorientation temperature is found
to decrease with increasing Y content. Our values

N.H. Luong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

x = 0.8

Er1-xYxCo 10 M o 2

x = 0.6
x = 0.4
x = 0.2

χac (a.u.)

x = 0.0

20

60

100
140
Temperature (K)

180

Fig. 3. Temperature dependence of the AC susceptibility for
the Er1ÀxYxCo10Mo2 compounds.

Table 1

Spin-reorientation temperature TSR in the Er1ÀxYxCo10Mo2
compounds
x

TSR (K)

0.0
0.2
0.4
0.6
0.8

132
127
102
89
60

for TSR for the compounds with x ¼ 0 and 0.2 are
in good agreement with the corresponding values
of 125 and 115 K reported by Zeng et al. [3] and
Tang Ning [4].
The sharp cusp observed in the MðTÞ and wac ðTÞ
curves are taken as evidence of spin reorientation of the axis-to-plan type with decreasing
temperature. The spin reorientation in the
Er1ÀxYxCo10Mo2 is explained by the competition
between Co-sublattice anisotropy and Er-sublattice anisotropy. When erbium atoms are replaced
by non-magnetic yttrium, the planar anisotropy of
the erbium sublattice decreases, causing a reduction of the spin-reorientation temperature.
We note that the spin reorientation in the

Er1ÀxYxCo10Mo2 compounds persists up to high
Y concentrations (x ¼ 0:8). In the Nd1ÀxYxFe11Ti
compounds, the spin reorientation is not observed
when the Y content is as high as x ¼ 0:8 [5],
whereas in the Tb1ÀxYxFe11Ti compounds the

481

spin reorientation is observed only in the samples
with x ¼ 0; 0.2 and 0.4 [6]. However, the above
observation for the Er1ÀxYxCo10Mo2 compounds
is similar to that for the Dy1ÀxYxFe11Ti compounds in which spin reorientation exists in the
whole series of samples (x ¼ 0; 0.2, 0.4, 0.6 and
0.8) [7].
On the basis of this analysis we could expect
that the TSR would increase with the reduction
of the Co-sublattice anisotropy. Therefore, it
is interesting to investigate the system ErCo10ÀyNiyMo2 in which Co atoms are partly
replaced by Ni.
Very interesting magnetization isotherms were
obtained in the ErCo10ÀyNiyMo2 compounds.
Fig. 4 shows magnetization curves for these
compounds with x ¼ 1; 2 and 3 at selected
temperatures. As it is seen in this figure, metamagnetism is observed at 1.8 K for all substituted
samples. The magnetization behaviour strongly
depends on bending process under influence of the
external magnetic field and temperature.
The temperature dependence of the magnetization of the ErCo9NiMo2 sample in a magnetic field
of 1 kOe and in the temperature range from 100 to
700 K is presented in Fig. 5 as an example. Similar

MðTÞ curves were obtained for ErCo10ÀyNiyMo2
with x ¼ 0; 2 and 3. Values for the Curie
temperature, TC ; were determined from MðTÞ
curves by plotting M 2 versus T and extrapolating
the steep part to M 2 ¼ 0: These values are
presented in Table 2. As one can see in this table,
the Curie temperature strongly decreases with
increasing Ni content. It is well known that in
the 4f–3d compounds the Curie temperature is
mainly determined by 3d–3d interaction. Substitution of nickel for cobalt reduces mainly this
interaction, leading to the decrease of TC :
As we can see in Fig. 5, the MðTÞ curve exhibits
at the low-temperature part a clear peak, which is
associated with spin reorientation. As in the case
of the Er1ÀxYxCo10Mo2 compounds, in order to
determine precisely the values of the spin-reorientation temperature, we have measured the AC
susceptibility in the ErCo10ÀyNiyMo2 samples as a
function of temperature. The results are shown in
Fig. 6. Sharp peaks have been observed in the
wac ðTÞ curves for all the samples investigated.


N.H. Luong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

482

20

6.0

Er Co9 Ni Mo 2

Magnetization (emu/g)

Magnetization (µB/f.u.)

ErCo 9NiMo 2
T= 150 K
4.0

T= 50 K
2.0
T= 1.8 K

15

10

5

0
100
0.0
0

H = 1kOe

200

300

400 500

600

700

Temperature (K)
10

(a)

20
30
Magnetic field (kOe)

40

50
Fig. 5. Temperature dependence of the magnetization for
ErCo9NiMo2 in a magnetic field of 1 kOe.

6.0

Magnetization (µB/f.u.)

ErCo 8Ni 2Mo 2

Table 2
Curie temperature, TC ; and spin-reorientation temperature,

TSR ; for the ErCo10ÀyNiyMo2 compounds. The values for TSR
obtained in Ref. [8] for the ErCo11ÀyNiyTi compounds are also
shown for comparison

4.0
T= 150 K

y
T= 1.8 K
2.0
T= 50 K

0.0
0

10

(b)

20
30
Magnetic field (kOe)

40

0
1
2
3

Magnetization (µB/f.u.)

4.0
T= 1.8 K
T= 150 K
T= 50 K

(c)

10

20
30
40
Magnetic field (kOe)

TC (K)

TSR (K)

TSR (K) [8]

482
416
332
250

132
127
109

108

145
155
160
165

Fig. 7 presents the temperature dependence of
the susceptibility measured by SQUID for the
ErCo10ÀyNiyMo2 compounds. The curves in this
figure exhibit also sharp peaks at the temperatures
where peaks in the wac ðTÞ curves occur, i.e. at the
spin-reorientation temperature. The values for TSR
determined from the curves in Fig. 6 are collected
in Table 2. As can be seen in this table, spinreorientation temperatures decrease with increasing Ni content. In the wac ðTÞ curve for the sample
with x ¼ 2 there is a less pronounced second peak,
the origin of which is still unclear.

ErCo7Ni3Mo2

0.0
0

ErCo11ÀyNiyTi

50

6.0

2.0

ErCo10ÀyNiyMo2

50

Fig. 4. Magnetization curves at selected temperatures for the
ErCo10ÀyNiyMo2 compounds. (a) x ¼ 1; (b) x ¼ 2; (c) x ¼ 3:


N.H. Luong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

500

ErCo10-yNiyMo2

y=0

ErCo 10-y Ni yMo 2

y=1

400
Temperature (K)

χac (a.u.)

y=2
y=3

80

100

120

140

160

180

Temperature (K)
Fig. 6. Temperature dependence of the AC susceptibility for
the ErCo10ÀyNiyMo2 compounds.

ErCo10–y Niy Mo2
H = 50 Oe

Susceptibility (emu/g)

0.02

y=0
y = 1.0
y = 2.0

0.01

y = 3.0

0.00
0

483

100
200
Temperature (K)

300

Fig. 7. Temperature dependence of the susceptibility for the
ErCo10ÀyNiyMo2 compounds.

As mentioned above, the spin reorientation in
the ErCo10Mo2 compound is due to competition
between uniaxial Co-sublattice anisotropy and
planar Er-sublattice anisotropy. Substitution of
nickel for cobalt reduces the uniaxial Co-sublattice
anisotropy. In this case, if the behaviour of the
Er-sublattice anisotropy remains the same,
we would expect an increase of the spin-reorientation temperature as the planar anisotropy of the

300
TC

200
TSR

100

0
0

1

2
3
Ni concentration y

4

Fig. 8. Dependence of the spin-reorientation temperature TSR
and of the Curie temperature TC on the Ni concentration in the
ErCo10ÀyNiyMo2 system.

Er-sublattice dominates in a wider temperature
range. An increase of the spin-reorientation
temperature with increasing Ni content indeed
has been observed in the ErCo11ÀyNiyTi compounds [8], as can be seen in Table 2. However, as
mentioned above, the situation in the ErCo10ÀyNiyMo2 compounds is different. When the
Ni content increases, the value of the Curie
temperature decreases rapidly. In the compounds
with lower values of TC ; the temperature dependence of the Er-sublattice anisotropy probably
changes, causing the reduction of spin-reorientation temperature. Fig. 8 presents the dependence
of the spin-reorientation temperature and of the
Curie temperature on the Ni concentration in
the ErCo10ÀyNiyMo2 system.
Apparently the Er-sublattice anisotropy in both
Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 systems
favour easy-plane anisotropy (second-order anisotropy constant K1R o0). The anisotropy of the rareearth sublattice is crystal-field (CF) induced. In

lowest-order approximation, it can be given by
K1R ¼ Àð3=2ÞaJ /r2 S/O02 SA02 ;

ð1Þ

where aJ is the second-order Stevens factor, /r2 S
the mean value of the second power of the 4f
radius, /O02 S the thermal average of the secondorder Stevens operator O02 ; and A02 the secondorder CF coefficient. Since aJ for Er is positive, we
can conclude on the basis of Eq. (1) that A02 > 0 for


484

N.H. Luong et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 479–484

the Er1ÀxYxCo10Mo2 and ErCo10ÀyNiyMo2 compounds. Tang Ning [4] and Tang et al. [9] have also
found that A02 > 0 for the RCo10Mo2 and RCo11Ti
series. Only DyCo10Mo2 and DyCo11Ti deviate
from the expected behaviour, which is probably
due to the neglect of higher-order CF terms in this
simple approach.
The generally accepted view is that the crystal
field should be roughly the same in isostructural
compounds containing chemically similar elements, like Co and Fe. However, investigation
on the isostructural series RFe10Mo2 [10] and
RFe11Ti [5–7,11] compounds, exhibiting a similar
competition between the rare-earth sublattice
anisotropy and the 3d-sublattice anisotropy, have
shown that A02 is negative. Thus sign reversal of the
rare-earth anisotropy in Fe-rich versus Co-rich

intermetallic compounds with ThMn2 structure
has been found. This puzzling fact is confirmed by
density functional calculations [12,13]. According
to Kuz’min et al. [12], the slightly different charge
distributions around the Fe and Co atoms situated
on the 8j sites neighbouring the rare-earth are
chiefly responsible for the difference in the sign of
A02 : It is interesting to investigate the R(T,M)12
compounds in which Co is partly replaced by Fe.
We have performed an experimental study of the
effect of substituting Fe for Co on the spin
reorientation in ErCo10Mo2 [14].

Acknowledgements
The authors are grateful to Prof. Nguyen Xuan
Phuc of the Institute of Materials Science (IMS),
NCST for collaboration, Dr. Vu Van Hong,

Dr. Dao Nguyen Hoai Nam and Dr. Nguyen
Huy Dan of the IMS for assistance in preparation
of some samples and in measurements of the AC
susceptibility. This work was supported by the
National Natural Science Council of Vietnam and
National Basic Research Program KT-420101.

References
[1] X. Xu, S.A. Shaheen, J. Magn. Magn. Mater. 118
(1993) 16.
[2] J.J.M. Franse, R.J. Radwanski, in: K.H.J. Buschow (Ed.),
Magnetic Materials, Vol. 7, Elsevier Science, Amsterdam,

1993, p. 308.
[3] D.C. Zeng, N. Tang, T. Zhao, Z.G. Zhao, K.H.J.
Buschow, F.R. de Boer, J. Appl. Phys. 76 (1994) 6837.
[4] T. Ning, Ph.D. Thesis, University of Amsterdam, 1998.
[5] N.H. Luong, N.P. Thuy, J.J.M. Franse, J. Magn. Magn.
Mater. 104–107 (1992) 1301.
[6] P.H. Quang, N.H. Luong, N.P. Thuy, T.D. Hien,
J.J.M. Franse, IEEE Trans. Magn. 30 (1994) 893.
[7] P.H. Quan, N.H. Luong, N.P. Thuy, T.D. Hien,
J.J.M. Franse, J. Magn. Magn. Mater. 128 (1993) 67.
[8] P.H. Quan, N.H. Luong, N.P. Thuy, N.L. Quang,
Proceedings of the 2nd International Workshop on
Materials Science, IWOMS’99, Hanoi, October 1995,
p. 145.
[9] N. Tang, D.C. Zang, J.H.V. Brabers, F.R. de Boer,
K.H.J. Buschow, J. Magn. Magn. Mater. 150 (1995) 241.
[10] X.C. Kou, R. Grossinger, G. Wiesinger, J.P. Liu, F.R. de
Boer, I. Kleirschorth, H. Kronmuler, Phys. Rev. B 51
(1995) 8254.
[11] X.C. Kou, T.S. Zhao, R. Grossinger, H.R. Kirchmayer,
X. Li, F.R. de Boer, Phys. Rev. B 47 (1993) 3231.
[12] M.D. Kuz’min, M. Richter, K.H.J. Buschow, Solid State
Commun. 113 (2000) 47.
[13] M.D. Kuz’min, M. Richter, H. Eschrig, K.H.J. Buschow,
J. Magn. Magn. Mater. 226–230 (2001) 1118.
[14] N.H. Luong, N. Chau, N.D. Dung, B.N.Q. Trinh,
in press.