ARTICLE IN PRESS
T. Tajiri et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e566–e568
e567
1
wavelength of 0.6876 A calibrated with CeO powder
diffraction pattern.
B of the Photon Factory (in KEK, Japan) operated at a
˚
1200
000
P = 0 kbar
2
0
1
1
1
.63
.21
.54
.74
1
T = 5 K
8
6
4
2
00
00
00
00
0
3
. Results and discussion
2.76
after depressurization
Fig. 1 shows the pressure dependence of susceptibility of
the CFT single crystal for the field H ¼ 0:1 kOe applied
along the b axis, which is smaller than critical field of the
field-induced phase transition. The susceptibility at ambi-
ent pressure shows the peak corresponding to the
antiferromagnetic ordering at TN ¼ 17 K. Below 1.4 kbar,
the susceptibility is similar to that at ambient pressure,
while the paramagnetic behavior suddenly appears above
0
2000
4000
6000
8000
10000
H [Oe]
1
.4 kbar. The paramagnetic component increases continu-
Fig. 2. The pressure dependence of magnetization curve of the CFT single
crystal at 5 K. The solid lines are the calculated value (see text).
ously with increasing pressure, and the antiferromagnetic
order vanishes above 2.8 kbar. The magnetic ordering
temperature does not depend on pressure below 2.8 kbar.
Fig. 2 shows the pressure dependence of magnetization
curve of CFT of single crystal at 5 K for the filed applied
along the b-axis. The magnetization curve at ambient
paramagnetic one with the Curie–Weiss law and Brillouin
function BS:
C
wðPÞ ¼ awðP ¼ 0 kbarÞ þ ð1 ꢁ aÞ
,
T ꢁ Y
MðPÞ ¼ bMðP ¼ 0 kbarÞ þ ð1 ꢁ bÞNgm SB
ꢀ
ꢁ
pressure has a jump at H ¼ 4 kOe due to the field-induced
c
gm H
k T
B
B
phase transition. The magnetization curve below 1.4 kbar is
similar to that at ambient pressure; however, the para-
magnetic component is induced above 1.4 kbar. The jump
of magnetization curve disappear above 2.8 kbar. The
critical field does not depend on pressure below 2.8 kbar.
After depressurization, the susceptibility and the magneti-
zation curve does not return to those before pressurization,
however, the antiferromagnetic order reappeared at same
temperature.
,
B
S
where a and b are the amount of spin of the antiferro-
magnetic components of CFT and the quantity (1ꢁa) and
(1ꢁb) correspond to the amount of the induced S ¼ 1=2
paramagnetic one. As shown in Figs. 1 and 2, the
experimental results under pressure are well reproduced
using w(P) and M(P) with the values of a and b shown in
the inset of Fig. 1. The two values begin to decrease at
1.4 kbar and are reduced to zero above 2.8 kbar. In the case
of powder samples, we obtained similar results with those
of the single crystal.
We analyzed the results for susceptibility w(P) and
magnetization M(P) under pressure as the summation
of the antiferromagnetic component of CFT and the
We performed X-ray diffraction measurements under
pressure to investigate the origin of the pressure-induced
paramagnetic behavior. The diffraction pattern at 6.6 kbar
changes from that of ambient pressure and is the similar
profile to copper formate dihydrate Cu(HCOO) ꢀ 2H O
P
= 0 kbar
1.54
0.63
1.74
1.21
2.76
after depressurization
1
.0
2
2
0
0
0
0
.06
.04
.02
.00
0.8
(abbreviated as CFD). The results indicate that the crystal
structure changes from CFT to CFD by pressurization.
The diffraction pattern after releasing the pressure
contained both CFD profile and CFT one, which indicate
that depressurization induced recrystallization of CFT
partially.
0
0
0
0
.6
.4
.2
.0
after
depressurization
0
1
2
3
P [kbar]
CFD is a ferromagnetic compound with the Curie
temperature T ¼ 0.75 K [5] and exhibits paramagnetic
C
behavior in the present experimental region. Therefore, the
experimental results suggest that the pressure-induced
paramagnetic component is attributed to the paramagnetic
behavior of CFD and CFD coexists with CFT between 1.4
and 2.8 kbar. The pressure dependence of a and b shows
that the fraction of CFD increases continuously between
5
10
15
20
T [K]
25
30
35
40
Fig. 1. The pressure dependence of susceptibility of the CFT single
crystal. The solid lines are the calculated value (see text). The inset shows
1.4 and 2.8 kbar, and above 2.8 kbar the system changes to
the pressure dependence of a (
M(P), respectively.
J) and b (&) determined using w(P) and
that of CFD in both the magnetic properties and the
crystal structure.