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14
H = 5 T
100
12
80
M (emu/g)
10
8
6
4
2
c
a
b
0 50 100 150 200 250 300
T (K)
Figure 3. Temperature dependency of the magnetization of
Co 2 SiO 4 along different crystallographic axes.
ΔI/ΔI max
60
40
20
0
(001)
(003)
(502)
0 10 20 30 40 50 60
T (K)
Figure 4. Temperature dependency of several
magnetic reflections of Co 2 SiO 4
According to the neutron diffraction data the magnetic contributions into intensities of Bragg reflections almost disappear
above sinΘ/λ ≈ 0.75 Å -1 . On the other hand side, the nuclear contributions are still strong above 0.75 Å -1 . Thus, we have
precisely refined the crystal structure separately first in order to refine the magnetic structure at the ground state very
accurately afterwards.
Neutron diffraction data at 2 K suggest the magnetic cell is equal to the crystallographic cell (k=0); magnetic structure
corresponds to the Shubnikov magnetic space group Pnma. Cobalt magnetic moments on M II are parallel to the b-axis
whereas on M I they are canted below T N (see figure 5). The magnetic moments equal to about 3.9 and 3.3µ B /Co ion for Co 2+
in M I and M II , respectively (table 1). These values significantly exceed the spin-only moment of 3µ B /Co 2+ ion in high-spin
state (t 2g 5 e g 2 , S = 3/2). Most probably the orbital contribution is significant and different for the two Co species. However, an
additional X-ray magnetic circular dichroism experiments are necessary to separate the spin and orbital contributions to the
total magnetic moment.
It is also worth to note that our calculated values of the magnetic moment of Co I are in a good agreement with ref. [5], but
the M(Co II ) value is smaller. Furthermore, we have found that the thermal behaviors of several magnetic reflections differ
(figure 4). This might be a hint to a different thermal evolution of the different Co-sublattices and should be studied more
precisely. The preliminary results of our refinement of the crystal and magnetic structures of Co 2 SiO 4 at 2 K are reported in
the table 1.
Table 1. Results of refinement of crystal and magnetic
structures for Co 2 SiO 4 in space group Pnma at 2 K.
Unit cell parameters: a = 10.2897(50) Å, b =
5.9886(30) Å, c = 4.7793(24) Å.
Figure 5. A schematic representation of Co 2 SiO 4
magnetic structure
atom x y z U iso
Co(1) 0 0 0 0.0012(2)
Co(2) 0.27604(6) 0.25 0.99131(15) 0.0010(2)
Si 0.09488(4) 0.25 0.42822(8) 0.0011(1)
O1 0.09190(3) 0.25 0.76733(6) 0.0022(1)
O2 0.44849(3) 0.25 0.21571(7) 0.0022(1)
O3 0.16427(2) 0.03325(5) 0.28123(4) 0.0024(1)
M X (µ B ) M Y (µ B ) M Z (µ B ) M (µ B )
Co(1) 1.258(31) 3.587(22) 0.706(87) 3.866(30)
Co(2) - 3.262(23) - 3.262(23)
[1] Hahn T., International Tables for Crystallography. London: Kluwer (1995).
[2] http://www.frm2.tum.de/heidi
[3] Rodriguez-Carvajal J. L., Physica B 55 (1992) 192.
[4] Ballet O., Fuess H. et al, J. Phys: Condens. Matter 1 (1989) 4955.
[5] Lottermoser W. and Fuess H., Phys. Stat. Sol. A 109 (1988) 589.
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