
The contribution of the Fe sublattice has the same
magnitude as in the Nd
2
Fe
14
B compounds and favors
the tetragonal c-axis as the magnetically easy direc-
tion. But the crystal field-induced R sublattice an-
isotropy differs in sign and magnitude leaving only
samarium as a suitable R element because for the
other R elements either the c-axis is not favored or an
antiparallel coupling to the Fe sublattice is observed.
SmFe
12–x
M
x
, magnetically hardened by melt-spin-
ning or mechanical alloying (see Magnets: Mechani-
cally Alloyed) shows high coercivity and Curie
temperature but the remanence is somewhat low com-
pared to Nd
2
Fe
14
B. Interstitial nitrogen, introduced
using analogous procedures as described above, leads
to a sign reversal of the R sublattice anisotropy and
thus, of the second-order crystal field parameter. The
consequence is that now the samarium-based com-
pounds show easy-plane and the neodymium-based
compounds easy-axis magnetocrystalline anisotropy.
Nitrogen charging is limited to one nitrogen atom per
formula unit and good results have been obtained for
Nd
10
Fe
75
V
15
N
x
(Wang et al. 1992). For more details
the reader is referred to reviews by Buschow (1991,
1997) and Fuji and Sun (1995).
The monoclinic structure of the R
3
(Fe,M)
29
-type
compounds can be described as alternating stack of
1:12 and 2:17 units which also present modifications
of the CaCu
5
structure (e.g., Yang et al. 1994). Again,
an enhancement of the Curie temperature, saturation
magnetization, and a change to easy-axis magneto-
crystalline anisotropy is observed upon nitrogen-
ation. In terms of practical use the Sm
3
(Fe,V)
29
N
4
material appears to be most promising, having an
anisotropy field and Curie temperature higher than
Nd
2
Fe
14
B and a slightly lower saturation at room
temperature (Hu et al. 1996).
Finally, the interesting group of materials with the
hexagonal TbCu
7
-type structure (derived from the
Th
2
Zn
17
structure but without long-range ordering) is
worth mentioning. These metastable compounds can
be prepared by nonequilibrium processing and it has
been reported that, for example, zirconium substitu-
tion for the rare-earth atom facilitates the formation
of this structure and the hard magnetic properties are
dramatically improved upon nitrogenation (e.g.,
(Sm
0.75
Zr
0.25
) (Fe
0.7
Co
0.3
)
10
N
x
, Sakurada et al. 1996).
In the twentieth century, the (BH)
max
doubled ap-
proximately every 12 years (see Hard Magnetic Ma-
terials, Basic Principles of). Nowadays about 90% of
the limit for the energy density (BH)
max
(based on the
Nd
2
Fe
14
B phase) can be achieved in commercially
produced sintered Nd–Fe–B grades (see Magnets:
Sintered). At the end of 2000, however, it appeared
that the search for novel hard magnetic compounds,
helped by a basic understanding of the crystal field
parameters, had somewhat stagnated and no further
breakthrough is in sight. On the other hand, only a
small number of ternary and quaternary systems has
been investigated so far.
See also: Ferrite Magnets: Improved Performance;
Magnetic Films: Hard
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1086
Rare Earth Magnets: Materials