Effects of Ga substitution on structure and magnetocrystalline anisotropy of Tm2Fe17

A detailed investigation of the structure and magnetic properties of
Tm
2
Fe
17−x
Ga
x
Tm2Fe17−xGax
(x=0,
(x=0,
1, 2, 3, 4, 5, 6, 7, and 8) compounds has been performed by means of x-ray-diffraction, neutron-diffraction, magnetization, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal
Th
2
Ni
17
Th2Ni17
for
x⩽3
x⩽3
and the rhombohedral
Th
2
Zn
17
Th2Zn17
structure for
x⩾5.
x⩾5.
In
Tm
2
Fe
13
Ga
4
Tm2Fe13Ga4
the
Th
2
Zn
17
Th2Zn17
structure coexists with the
Th
2
Ni
17
Th2Ni17
structure. Substitution of Ga for Fe in
Tm
2
Fe
17
Tm2Fe17
leads to an increase of the unit-cell volume, which is linear with the Ga concentration. In
Tm
2
Fe
17−x
Ga
x
,
Tm2Fe17−xGax,
the saturation magnetization at 1.5 K decreases linearly with increasing Ga content with a rate of
2.3 μ
B
2.3 μB
per substituted Ga atom. The Curie temperature is found first to increase with increasing Ga content, going through a maximum value of 485 K at about
x=3,
x=3,
then to decrease. Between
x=6
x=6
and 7, a minimum value of
T
C
TC
is reached and for higher
x
x
values
T
C
TC
increases again. X-ray-diffraction measurements on magnetically aligned
Tm
2
Fe
17−x
Ga
x
Tm2Fe17−xGax
powders show that the compounds with
x⩽6
x⩽6
have an easy-plane type of magnetic anisotropy, whereas the compounds with
x⩾7
x⩾7
exhibit easy
c
c
-axis anisotropy at room temperature. All
Tm
2
Fe
17−x
Ga
x
Tm2Fe17−xGax
compounds exhibit a spin-reorientation transition at low temperature, except for the sample with
x=6,
x=6,
which shows an easy-magnetization direction perpendicular to the
c
c
axis in the temperature range from 5 to 300 K. For
x⩽5,
x⩽5,
the spin-reorientation temperature is found first to increase with
x
x
and then to decrease, having a maximum value of 211 K at about
x=3.
x=3.
In the samples with
x⩾7,
x⩾7,
an easy-plane anisotropy was observed at low temperature, but an easy-axis preference of the magnetization at room temperature was observed. The results obtained for
Tm2 Fe17−x Gax Tm2Fe17−xGax
indicate that the mutually competing Tm- and Fe-sublattice anisotropies both change their sign with increasing Ga concentration.