Flux Growth and Magnetic Properties of CaB6 Crystals

Full text of DOI:10.1143/JPSJ.71.1791

Journalof the PhysicalSociety of Japan
Vol. 71, No. 7, July, 2002, pp. 1791–1792
#2002 The PhysicalSociety of Japan
SHORT NOTES
Flux Growth and Magnetic Properties of
CaB6 Crystals
ꢀ
1
1;2
Shigeki OTANI and Takao MORI
1Advanced Materials Laboratory, National Institute for
Materials Science (NIMS),
Namiki 1-1, Tsukuba, Ibaraki 305-0044
2PRESTO, Japan Science and Technology Corporation (JST)
(Received March 29, 2002)
KEYWORDS: CaB6, Ca1Àx Lax B6, ferromagetism, iron impurity
DOI: 10.1143/JPSJ.71.1791
Lanthanum doped calcium hexaboride, Ca1ꢁxLaxB6,
attracts a lot of interest because of high temperature weak
ferromagnetism though the materialconsists of no magnetic
elements.1) However, there have been some doubts about
this material. For example, some of the samples did not have
ferromagnetism.2) Impurities in the samples were not
examined well in spite of weak ferromagnetism. In addition,
procedures of the sample preparation were not reported in
detail in the reports. Therefore, we prepared single crystals
of Ca1ꢁxLaxB6 while paying attention to a main impurity,
iron, and examined the relation between the impurity and the
ferromagnetism.
Undoped and lanthanum-doped CaB6 crystals were
prepared by the flux method. The CaB6 starting material
was synthesized by reaction of Ca(OH)2 and boron.3)
Aluminium metal, Al, was of nominal purity of 4N. The
CaB6 and aluminium, 50–52 g in a total weight, were put
into an AlN crucible. A molar ratio of CaB6 to Alwas
0.6%.4) Ca was also added as a form of Ca(OH)2 into the
crucible in order to suppress formation of AlB2 crystals. The
amount of Ca was 20 mol% of the CaB6 solute. Lanthanum
Fig. 1. As-grown CaB6 crystals. One division is 1 mm.
Table I. Contents of iron impurity.
Ca1ꢁxLaxB6 crystalFe (wt%)
x ¼ 0
(as-grown)
0.019
<0:001
0.010
(HCl-treated)
(as-grown)
x ¼ 0:0051
(HCl-treated)
0.001
Al(flux)
CaB6
0.003
(starting material)
0.002
dissolving the aluminium flux by the NaOH solution, the
aluminium matrix acts as a cathode. The crystals which are
held on the surface of the aluminium acts as an anode and
iron ions were reducted to iron on the crystalsurface. That
is, the crystals were electrochemically plated with iron.
Consequently, the crystals were covered with iron because
iron is stable in the NaOH solution. The iron was removed in
the HCltreatment because iron reacts with HClsoul tion.
The magnetization of lanthanum doped calcium hexabor-
ide Ca0:0049La0:0051B6 at 300 K is shown in Fig. 2. Similar to
what has been reported previously,1) a weak ferromagnetism
is observed with hysteresis. The lanthanum doping of
ꢃ0:51% of our sample is close to the stoichiometry
reported1) for the sample exhibiting the largest magnitude
of ferromagnetic component. The magnitude of ferromag-
netism (ꢃ2 emu/molf.u.) is comparabel to that reported
previously. However, we find that by washing these samples
was doped by adding commercialLaB powder.
6
The samples were kept at 1450ꢂC in a flow of argon for
24 h, slowly cooled to 800ꢂC by 15ꢂC every hour, and then
quenched. The aluminium flux was removed in NaOH
solution. Magnetization of undoped calcium hexaboride
CaB6 and lanthanum doped calcium hexaboride Ca1ꢁxLaxB6
(x ¼ 0:0051) single crystals were measured by using a
SQUID magnetometer up to fields of 55 kG.
The obtained crystals were less than several mm in size
and 0.2–1 mm thick. The shapes were rectangular plate-like
or cubic. The large crystals were shown in Fig. 1. The dope
of lanthanum did not influence the size and shape of the
CaB6 crystals.
More than half of both the undoped and La-doped crystals
were attracted or slightly moved by a magnet. Especially, the
thin crystals were strongly attracted. The cubic crystals were
hardly attracted. However, all the crystals were not attracted
at all after being kept in HCl solution for a few hours.
Table I shows contents of iron impurity, which came from
the aluminium flux. The undoped and La-doped CaB6
crystals used for magnetic measurement contained 190 ppm
and 100 ppm, respectively, before the HCl treatment. The
iron was removed by the HCltreatment, which means that
the iron existed on the surface of the CaB6 crystals. This
phenomena can be understood as follows. In the process of
ꢀE-mail: OTANI.shigeki@nims.go.jp
Fig. 2. Magnetization of Ca0:9949La0:0051B6 crystals.
1791

1792
J. Phys. Soc. Jpn., Vol. 71, No. 7, July, 2002
SHORT NOTES
S. OTANI and T. MORI
in HClthe ferromagnetism disappears and the magnetism
washing. For the measurements given here, more than 10
becomes diamagnetic (Fig. 2). As mentioned above, we crystals of CaB6 and Ca1ꢁxLaxB6 (x ¼ 0:0051) were used
have conducted chemicalanaylsis on our sampels and find with a typicaltotalweight of 100 mg. The CaB crystals
6
an iron concentration of 0.010 wt% for Ca0:9949La0:0051B6. used for measurements had a tendency to be thinner which
The iron concentration after washing is 0.001 wt% while the accounts for the larger relative magnetic moment.
lanthanum concentration does not change. Therefore, it is
It should also be noted that we have investigated the
strongly indicated that the ferromagnetism originates from magnetism of CaB2C2, which has similarities to CaB6 and
the iron impurities rather than being an effect of the has previously been reported to exhibit ferromagnetism at
lanthanum doping of CaB6.
We have found that all the CaB6 related samples treated concentration of less than 0.001 wt%, we find that the
room temperature.6) On preparing a pure sample with iron
with NaOH for the removalof flux (which is the method
magnetism is diamagnetic7) and it should be considered that
employed by previous researchers1,5)) with or without the ferromagnetism observed in that system is also due to
lanthanum doping exhibit a ferromagnetic moment when iron impurities since the results of analysis have not been
sufficient thin crystals were added to the sample batch presented in the previous studies.
measured. In other words, the magnetism was sample
The authors express thanks to Y. Yajima, NIMS, for
dependent, with thin single crystals having a larger ratio of chemicalanayl sis.
the ferromagnetism, which agrees with the idea that iron
impurities are electrochemically plated onto the crystal
1) D. P. Young, D. Hall, M. E. Torelli, Z. Fisk, J. L. Sarrao, J. D.
surface during the NaOH treatment.
For example, the magnetization of CaB6 is shown in
Fig. 3 and it has ferromagnetism which disappears after
Thompson, H.-R. Otto, S. B. Oseroff, R. G. Goodrich and R. Zysler:
Nature 397 (1999) 412.
2) T. Terashima, C. Terakura, Y. Umeda, N. Kimura, H. Aoki and S.
Kunii: J. Phys. Soc. Jpn. 69 (2000) 2423.
3) S. Otani: J. Cryst. Growth 192 (1998) 346.
4) S. Muranaka and S. Kawai: J. Cryst. Growth 26 (1974) 165.
5) For example, S. Kunii: J. Phys. Soc. Jpn. 68 (1999) 3189.
6) J. Akimitsu, K. Takenawa, K. Suzuki, H. Harima and Y. Kuramoto:
Science 293 (2001) 1125.
7) T. Mori and S. Otani: to be published in J. Phys. Soc. Jpn.
Fig. 3. Magnetization of CaB6 crystals.