Magnetic materials and processes for their production

A process is provided for modifying the magnetic properties of an intermetallic compound comprising at least iron, or a combination of iron with at least one transition metal, and at least one rare earth element. The process comprises heating the intermetallic compound in a reaction gas containing at least one element of groups IIIA, IVA or VIA of the Periodic Table in the gaseous phase to interstitially incorporate the element or elements of these groups into the crystal lattice of the intermetallic compound. Novel magnetic materials showing easy uniaxial anisotropy, increased spontaneous magnetization and Curie temperatures are produced by the process.

BACKGROUND OF THE INVENTION 
The invention relates to a process for producing magnetic materials, to new 
and improved materials produced thereby and to the use of these materials 
to make permanent magnets. 
Magnets have many applications in engineering and science as components of 
apparatus such as electric motors, electric generators, focussing 
elements, lifting mechanisms, locks, levitation devices, anti-friction 
mounts and so on. In order for a magnetic material to be useful for making 
a permanent magnet three intrinsic properties are of critical importance. 
These are the Curie temperature (Tc) i.e. the temperature at which a 
permanent magnet loses its magnetism, the spontaneous magnetic moment per 
unit volume (M.sub.s) and the easy uniaxial anisotropy conventionally 
represented by an anisotropy field B.sub.a. The Curie temperature is of 
particular significance because it dictates the temperature below which 
apparatus containing the magnet must be operated. 
During this century much research has been directed to developing magnetic 
materials which combine high Curie temperatures and improved magnetic 
moments with strong uniaxial anisotropy. For many years magnetic materials 
of the AlNiCo type were used in permanent magnets for practical 
applications. In the late 1960's it was discovered that alloys of the rare 
earth elements, particularly samarium when alloyed with cobalt, had 
magnetic properties which made them superior as permanent magnets to the 
AlNiCo type. Compounds of samarium and cobalt provided magnets which were 
particularly successful in many demanding practical applications requiring 
a magnet with a high energy product. However the high cost of cobalt as a 
raw material led investigators in the early 1980's to consider the 
possibility of combining the cheaper and more abundant iron with the 
magnetically superior rare earth elements to produce permanent magnets 
with improved magnetic properties. A major breakthrough came in 1983 when 
the Sumitomo Special Metals Company. and General Motors of America 
independently developed a magnetic material which combined a rare earth 
element and iron and incorporated a third element, boron, into the crystal 
lattice to give an intermetallic compound, Nd.sub.2 Fe.sub.14 B which can 
be used to produce magnets with an excellent energy product, but a lower 
Curie temperature than the Sm-Co materials. These Nd-Fe-B magnetic 
materials can have a Curie temperature of up to 320.degree. C. and are 
particularly described in three European applications, EP-A-0101552, 
EP-A-0106948 and EP-A-0108474. Derivatives of these boride materials 
represent the state of the art to date in magnet technology. However they 
are somewhat unstable in air and change chemically, gradually losing their 
magnetic properties so that despite Curie temperatures in excess of 
300.degree. C. in practice they are not suitable for operating at 
temperatures greater than 150.degree. C. 
The fact that the incorporation of boron into the crystal lattice of 
intermetallic materials containing a rare earth element and iron serves to 
improve magnetic properties has encouraged investigators to search for new 
compounds of elements other than boron in combination with rare earth 
elements and iron. 
In EP-A-0320064 hard magnetic materials are described containing neodymium 
and iron but having carbon incorporated to give compounds of the formula 
Nd.sub.2 Fe.sub.14 C having a similar crystal structure to the known 
boride materials. In EP-A-0334445 variations of the above type of material 
having carbon incorporated are described in which neodymium is replaced 
with praseodymium, cerium or lanthanum and the iron is partly substituted 
with manganese. Finally EP-A-0397264 describes compounds of the formula 
RE.sub.2 Fe.sub.17 C where RE is a combination of rare earth elements of 
which at least 70% must be samarium. The preferred compound described in 
the last of the above three patent applications, which has carbon 
interstitially incorporated into a Sm.sub.2 Fe.sub.17 crystal lattice, 
demonstrates improved Curie temperatures and uniaxial magnetic anisotropy. 
However it is produced by arc melting of the constituent elements to 
obtain a casting which is then subjected to an annealing treatment at very 
high temperatures (900.degree.-1100.degree. C.) in an inert gas. Using 
such a process puts a limitation on the amount of additional elements 
which can be interstitially incorporated. 
A process for bringing about interstitial incorporation of an element of 
group VA of the Chemical Abstract Service (CAS) Periodic Table (all 
references made herein to the "Periodic Table" are being made to the CAS 
Periodic Table) into intermetallic compounds containing one or more rare 
earth elements and iron has already been developed by the present 
inventors and is described in the Applicants' co-pending European Patent 
Application No 91303442.7 which process comprises heating the 
intermetallic starting material in a gas containing the group VA element 
in the substantial absence of oxygen. 
SUMMARY OF THE INVENTION 
The present invention is a magnetic material consisting essentially of at 
least one rare earth element, iron or a combination of iron with one or 
more transition metals, a stabilizing element and one or more of the 
elements of groups IIIA, IVA, or VIA of the periodic table having the 
following general formula: 
EQU R(T.sub.n-x M.sub.x)Zy 
where N is approximately 12 and wherein 0.5&lt;x&lt;3.0 and 0.1&lt;y&lt; or &gt;1.0. 
The present invention further includes a process of modifying the magnetic 
properties of an intermetallic compound comprising at least iron, or a 
combination of iron with at least one transition metal and at least one 
rare earth element. The process of the present invention comprises heating 
said intermetallic compound and a reaction gas containing at least one 
element selected from the group consisting essentially of groups IIIA, IVA 
or VIA of the CAS periodic table in the gaseous phase. The element or 
elements of group IIIA, IVA or VIA are interstitially incorporated into 
the crystal lattice of said inner metallic compound forming a modified 
magnetic compound with improved properties. 
The novel magnetic materials of the present invention show easy uniaxial 
anisotropy, increased spontaneous magnetization and increased Curie 
temperatures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A process has now been developed which permits interstitial incorporation 
of elements of groups IIIA, IVA and VIA of the Periodic Table into the 
rare-earth/iron type compounds to produce novel materials having improved 
magnetic properties with regard to Curie temperatures (Tc), spontaneous 
magnetic moment per unit volume (Ms) and easy uniaxial anisotropy (Ba). 
Such materials are suitable for further processing to make permanent 
magnets with a large energy product exceeding 80 kJ/m.sup.3. 
A process for modifying the magnetic properties of an intermetallic 
compound comprising at least iron, or a combination of iron with at least 
one transition metal, and at least one rare earth element comprises 
heating said intermetallic compound in a reaction gas containing at least 
one element of groups IIIA, IVA or VIA of the Periodic Table in the 
gaseous phase to interstitially incorporate said element or elements of 
groups IIIA, IVA or VIA into the crystal lattice of said intermetallic 
compound. 
It is to be understood that herein the term rare earth element also 
includes the elements yttrium, thorium, hafnium and zirconium and that 
groups IIIA, IVA and VIA of the Periodic Table are those defined by the 
CAS version of that table, i.e. Group IIIA, B, Al, Ga, In, Tl, Group IVA, 
C, Si, Ge, Sn, Pb; Group VIA O, S, Se, Te, Po. 
The intermetallic compounds which may be modified by the process of the 
invention include those of the ThMn.sub.12 type with a tetragonal crystal 
structure and those of the Th.sub.2 Ni.sub.17 or ThZn.sub.17 type having 
hexagonal or rhombohedral crystal structures respectively. Those of the 
crystal structure type BaCd.sub.11 and CaCu.sub.5 may also be modified by 
the process. 
In one embodiment of the invention the intermetallic starting materials 
heated in a reaction gas in accordance with the process of the invention 
may be tetragonal compounds of the general formula: 
EQU R(T.sub.n-x M.sub.x) 
in which R is at least one rare earth element as herein defined, T is iron 
or a combination of iron with one or more transition metals, M is an 
element that serves to stabilise the structure-type, n is approximately 12 
and 0.5&lt;x&lt;3.0. 
Preferred components for R are yttrium, cerium, praseodymium, neodymium, 
samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium or 
lutetium or a mixture of two or more thereof. Particuarly preferred 
compounds are those where R is praseodymium or neodymium such as for 
example PrFe.sub.11 Ti or NdFe.sub.11 Ti or compounds where praseodymium 
or neodymium are combined with another rare earth element. For example in 
a compound such as NdFe.sub.11 Ti some of the neodymium can be substituted 
with cerium to reduce cost or substituted with a heavy rare earth such as 
terbium or dysprosium to improve uniaxial anisotropy. 
In compounds of the formula R(T.sub.n-x M.sub.x) described above the iron 
may be in combination with a transition metal such as cobalt, nickel or 
manganese. In particular the iron may be substituted with up to 45% 
cobalt. 
The stabilizing element M is preferably an early transition metal such as 
those of groups IVB, VB and VIB of the Periodic Table. Particularly 
preferred stabilizing elements are titanium, vanadium, molybdenum, 
tungsten or chromium. 
In another embodiment of the invention the intermetallic starting material 
which is heated in a reaction gas in accordance with the process of the 
invention may be a hexagonal or rhombohedral compound of the general 
formula: 
EQU R'.sub.2 (T'.sub.n-x' M'.sub.x') 
in which R' is at least one rare earth element, T' is iron, M' is one or 
more transition metals, n is approximately 17 and 0.ltoreq.x'&lt;6.0. 
Preferred components for R' for these hexagonal or rhombohedral starting 
materials are yttrium, cerium, praseodymium, neodymium, samarium, 
gadolinium, terbium, dysposium, holmium, erbium, thulium or lutetium or a 
mixture of two or more thereof and particularly preferred are those 
compounds where R is samarium such as for example SmFe.sub.17 or where R 
is samarium partially substituted with neodymium, praseodymium or cerium. 
Further, a transition metal M' may substitute for the iron such as cobalt, 
nickel or manganese. 
In yet another embodiment of the invention the intermetallic starting 
materials may be of the tetragonal crystal structure type BaCd.sub.11 for 
example RFe.sub.5 Co.sub.4 M" where M' is a stabilizing element such as 
silicon or of the crystal structure type CaCu.sub.5, for example RCo.sub.3 
FeM"' where M"' is a stabilizing element such as boron. 
The preferred group IIIA, IVA or VIA elements which may be interstitially 
incorporated into the crystal lattice of the intermetallic compounds of 
tetragonal, rhombohedral or hexagonal crystal structure described above 
are boron in Group IIIA, one or more of carbon, silicon and germanium in 
Group IVA or one or more of sulphur, selenium and tellurium in Group VIA. 
Optionally the interstitially incorporated element may be combined with 
hydrogen. 
The elements of Groups IIIA, IVA or VIA which are interstitially 
incorporated, whether or not in combination with hydrogen, will 
hereinafter be designated Z. 
Thus in accordance with another aspect of the invention there are provided 
novel magnetic materials of the general formula: 
EQU R(T.sub.n-x M.sub.x)Z.sub.y 
wherein R, T, x, M and Z are as herein defined and 0.1&lt;y.ltoreq.1.0. 
The invention also provides compounds of the general formula: 
EQU R'.sub.2 (T'.sub.n-x' M'.sub.x')Z.sub.y ' 
wherein R', T', M', Z and x' are as herein defined and 0.5&lt;y'&lt;3.0. 
Particularly preferred examples of these latter compounds are those where 
y'&gt;1.5. 
The invention further provides compounds of the formula RTCo.sub.n-x" 
M".sub.x" Z.sub.y" where R,T,Z and M" are as hereinbefore defined, n is 
11 1&lt;x"&lt;3 and 0&lt;y"&lt;1 and also compounds of the formula RCo.sub.3 FeM"'Z 
where R and Z are as hereinbefore defined and M"' is a stabilizing element 
such as boron. 
The precise formula of the novel materials will depend upon the starting 
materials, which of course may have all the variations already discussed 
herein, and the element or elements of Group IIIA, IVA or VIA of the 
Periodic Table which are present in the reaction gas. 
For example, if the element Z is to be carbon then the reaction gas may be 
a hydrocarbon such as methane, any C.sub.2 to C.sub.5 alkane, alkene or 
alkyne or an aromatic hydrocarbon such as benzene. If the element Z is to 
be boron the reaction gas may be a boron containing gas such as borane, 
diborane or decaborane vapour. If the element Z is silicon then the 
reaction gas may be a silane and if the element Z is sulphur the reaction 
gas may be hydrogen sulphide. The reaction gas may be mixed with an inert 
carrier gas such as helium or argon. 
Particularly preferred magnetic materials are those where the 
interstitially incorporated element is carbon such as, for example 
Sm.sub.2 Fe.sub.17 C.sub.y ' where y'&gt;2.0 and more preferably y=2.5 or 
NdFe.sub.11 TiC.sub.y and PrFe.sub.11 TiC.sub.y where 0.5&lt;y.ltoreq.1.0, 
preferably 0.6&lt;y&lt;0.9 and more preferably y=0.8. 
Other preferred magnetic materials are those where the interstitially 
incorporated element is boron such as Sm.sub.2 Fe.sub.17 B.sub.y ' where 
y'&gt;1.5. 
To carry out the process of the invention an ingot of the rare earth/iron 
intermetallic starting material is preferably crushed to a fine powder 
having a particle size of less than 50 microns diameter. Such a powder may 
be optionally prepared by mechanical alloying. The powder is then placed 
in a suitable reactor vessel which is evaporated and filled with the 
reaction gas at a pressure of from 0.01 to 1000 bar. Typically the 
pressure is from 0.1 to 10 bar. The powder is then heated in the vessel in 
the presence of the gas to a temperature in the range 300.degree. to 
800.degree. C., preferably in the range 400.degree. to 650.degree. C., and 
most preferably about 500.degree. C. for a period sufficient to permit 
diffusion of the element to be incorporated into the interstitial sites 
throughout each grain of powder. The heating time may be anything up to 
100 hours but a suitable period can be readily determined from the 
diffusion constants of the interstitial atoms in the intermetallic 
compound. A typical heating period is from 2 to 10 hours. 
Except in the case where the interstitial element to be incorporated is 
oxygen it is preferable if the starting materials are heated in the 
reaction gas in the substantial absence of oxygen. 
Following heating the reactor vessel is evacuated to remove excess reaction 
gas before cooling or alternatively it may be purged with an inert gas. 
The cooled product can then be processed to form permanent magnets. In the 
case of Sm.sub.2 Fe.sub.17 ingots, for example, it has been found 
advantageous to include in the cast ingot up to 5% by weight of an early 
transition metal additive. Suitable additives include niobium, zirconium 
or titanium. The additive suppresses the formation of alpha-Fe dendrites 
which occur because the phrase does not melt congruently. Without the 
additive the .alpha.-Fe phase, which tends to destroy coercivity in the 
interstitially modified material, may be removed by lengthy high 
temperature annealing at about 1000.degree. C. 
It is an advantage of the novel process of the invention that interstitial 
incorporation of an element such as carbon, for example into an 
intermetallic rare earth/iron compound can be brought about at a much 
lower temperature than the arc melting method used in EP-A-0397264. 
Further the gas phase process of the invention allows a higher level of 
interstitial incorporation to be achieved compared with the arc melting 
method. As a result the uniaxial anisotropy is much greater and the Curie 
temperatures significantly higher than materials produced by hitherto 
known methods. 
By way of example Table I compares the properties of compounds of the 
formula Sm.sub.2 Fe.sub.17 C.sub.y made by the process described in 
EP-A-0397264 with compounds of that formula made by the process of the 
present invention. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize the 
changes may be made in form and detail without departing from the spirit 
and scope of the invention. 
TABLE I 
______________________________________ 
Process of 
EP-A-0397264 
present invention 
______________________________________ 
Compound Sm.sub.2 Fe.sub.17 C.sub.y 
Sm.sub.2 Fe.sub.17 C.sub.y 
Range made 0.5 &lt; y' &lt; 1.5 
0.5 &lt; y' &lt; 2.8 
Tc (maximum) 
540 K 673 K 
Ba (maximum) 
4.0-5.3 T 16 T 
Process Arc melting Heating in 
of elements hydrocarbon gas 
______________________________________ 
From the above table the improvement in magnetic properties of the 
compounds produced by the process of the invention is readily apparent. 
The effect of interstitial incorporation of carbon into compounds of the 
formula R.sub.2 Fe.sub.17 on the crystal lattice parameters a(nm) c(nm), 
Curie temperature Tc(K), anistropy and magnetic moment M(.mu.B/f.u) is 
shown in Table II below. h represents compounds of the hexagonal crystal 
structure and r compounds of the rhombohedral crystal structure. The 
composition of the carbides is R.sub.2 Fe.sub.17 C.sub.y' where y' is 
between 2.1 and 2.8. 
TABLE II 
______________________________________ 
Aniso- 
R Structure 
a(nm) c(nm) T.sub.c (K) 
tropy M(.mu..sub.B /f.u.) 
______________________________________ 
Y h 0.866 0.840 668 plane 35.8 
Ce r 0.873 1.256 589 plane 33.8 
Pr r 0.880 1.259 653 plane 34.5 
Nd r 0.879 1.260 659 plane 35.1 
Sm r 0.875 1.257 668 axis 34.5 
Gd r 0.870 1.261 711 plane 20.1 
Tb r 0.867 1.264 680 plane 21.3 
Dy h 0.865 0.842 674 plane 17.1 
Ho h 0.861 0.843 672 plane 17.5 
Er h 0.860 0.841 663 T.sub.sr = 
17.9 
124 k 
Tm h 0.860 0.843 656 T.sub.sr = 
21.2 
226 K 
Lu h 0.857 0.842 657 plane 36.4 
______________________________________ 
The effect on magnetic properties and crystal lattice parameters of 
interstitial incorporation of carbon into compounds of the formula 
RFe.sub.11 Ti is shown in Table III below. In the table the value of y is 
between 0.6 and 0.9. In preferred compounds the value of y is 0.8. 
TABLE III 
__________________________________________________________________________ 
.DELTA.V/V 
.DELTA.T.sub.c /T.sub.c 
M.sub.s (.mu..sub.B /f.u.) 
Anisotropy 
a(nm) 
c(nm) 
V(nm).sup.2 
(%) T.sub.c (K) 
(%) 42 K 
273 K 
at 300 K 
__________________________________________________________________________ 
Y(Fe.sub.11 Ti) 
0.851 
0.479 
0.347 
-- 524 -- 19.0 
16.6 
axis 
Y(Fe.sub.11 Ti)C.sub.y 
0.857 
0.481 
0.353 
1.7 678 29.4 19.4 
14.8 
axis 
Nd(Fe.sub.11 Ti) 
0.857 
0.478 
0.351 
-- 547 -- ? 16.8 
axis 
Nd(Fe.sub.11 Ti)C.sub.y 
0.862 
0.482 
0.358 
2.0 670 22.5 18.9 
19.2 
axis 
Sm(Fe.sub.11 Ti) 
0.855 
0.479 
0.350 
-- 584 -- ? 17.1 
axis 
Sm(Fe.sub.11 Ti)C.sub.y 
0.858 
0.480 
0.353 
0.9 698 19.5 16.9 
? plane 
Gd(Fe.sub.11 Ti) 
0.854 
0.480 
0.350 
-- 607 -- ? 12.5 
axis 
Gd(Fe.sub.11 Ti)C.sub.y 
0.856 
0.480 
0.352 
0.6 734 20.9 14.4 
11.9 
axis 
Tb(Fe.sub.11 Ti) 
0.851 
0.479 
0.347 
-- 552 -- 9.7 
10.6 
axis 
Tb(Fe.sub.11 Ti)C.sub.y 
0.857 
0.481 
0.353 
1.7 714 29.3 10.9 
11.3 
axis 
Dy(Fe.sub.11 Ti) 
0.849 
0.478 
0.344 
-- 534 -- 9.7 
11.6 
axis 
Dy(Fe.sub.11 Ti)C.sub.y 
0.857 
0.479 
0.352 
2.3 697 30.5 8.7 
9.5 
axis 
Ho(Fe.sub.11 Ti) 
0.849 
0.479 
0.345 
-- 520 -- ? ? axis 
Ho(Fe.sub.11 Ti)C.sub.y 
0.855 
0.479 
0.350 
1.4 691 32.9 8.0 
8.9 
axis 
Er(Fe.sub.11 Ti) 
0.848 
0.479 
0.344 
-- 505 -- 9.2 
12.4 
axis 
Er(Fe.sub.11 Ti)C.sub.y 
0.856 
0.479 
0.351 
2.0 685 35.6 10.7 
12.6 
axis 
Tm(Fe.sub.11 Ti) 
0.847 
0.478 
0.343 
-- 496 -- ? ? axis 
Tm(Fe.sub.11 Ti)C.sub.y 
0.855 
0.478 
0.349 
1.7 686 38.3 15.5 
17.9 
axis 
Lu(Fe.sub.11 Ti) 
0.846 
0.478 
0.342 
-- 488 -- 17.4 
15.5 
axis 
Lu(Fe.sub.11 Ti)C.sub.y 
0.855 
0.478 
0.349 
2.0 682 39.7 16.8 
16.0 
axis 
__________________________________________________________________________ 
The effect on magnetic properties of interstitial incorporation of boron 
into Sm.sub.2 Fe.sub.17 and of carbon into Nd(Fe.sub.11 Ti) are shown in 
Table IV below. 
TABLE IV 
______________________________________ 
T.sub.c (.degree.C.) 
.mu..sub.o M.sub.s (T) 
B.sub.a (T) 
______________________________________ 
Sm.sub.2 Fe.sub.17 
116 1.17 easy plane 
Sm.sub.2 Fe.sub.17 C.sub.2.2 
395 1.46 14 T 
Sm.sub.2 Fe.sub.17 B.sub.1.6 
325 1.40 &gt;5 T 
Nd(Fe.sub.11 Ti) 
274 1.28 1 T 
Nd(Fe.sub.11 Ti)C.sub.0.7 
397 1.32 7 T 
______________________________________ 
The interstitial incorporation of an element of Group IVA of the Periodic 
Table, for which the example is carbon, into selected intermetallic 
compounds of the formula R.sub.2 Fe.sub.17 or RFe.sub.11 Ti and the 
altered properties achieved thereby are further demonstrated in the 
figures in which: 
FIG. 1(a) shows the rhombohedral crystal structure of Sm.sub.2 Fe.sub.17 
C.sub.y ' where the 9e site is occupied by carbon and FIG. 1(b) shows the 
tetragonal crystal structure of Nd(Fe.sub.11 Ti)C.sub.y showing the 2b 
site occupied by carbon; 
FIG. 2 shows X-ray diffraction patterns of Sm.sub.2 Fe.sub.17 powder (a) 
before (b) after treatment in methane for 2 hours at 550.degree. C. and 
(c) after treatment and orientation in a magnetic field of 0.3T. In FIG. 
2(b) a lattice expansion of about 6% is apparent after interstitial 
corporation of carbon and in FIG. 2(c) easy c-axis anisotropy is shown 
after orientation; 
FIG. 3 shows the difference in unit cell volume between compounds having 
the formula R.sub.2 Fe.sub.17 C.sub.y ' where 1.5&lt;y'&lt;3.0 and those having 
the formula R.sub.2 Fe.sub.17 where R is Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, 
Er, Tm or Lu. A substantial increase in unit cell volume is observed for 
those compounds having the formula R.sub.2 Fe.sub.17 C.sub.y '; 
FIG. 4 shows the Curie temperature of compounds of the formula R.sub.2 
Fe.sub.17 C.sub.y ' where 1.5&lt;y'&lt;3.0 and R.sub.2 Fe.sub.17 where R is Ce, 
Pr, Nd, Sm, Cd, Tb, Dy, Ho, Er, Tm or Lu. A substantial increase in Curie 
temperature is observed for those compounds having the formula R.sub.2 
Fe.sub.17 C.sub.y '; 
FIG. 5 shows magnetization curves at room temperature of powder of Sm.sub.2 
Fe.sub.17 C.sub.y ' where 1.5&lt;y'&lt;3.0 magnetically aligned in an applied 
field of 1T and fixed in epoxy resin. The anisotropy field deduced from 
the data shown in FIG. 5 is 16T; 
FIG. 6 shows X-ray diffraction patterns of Sm.sub.2 Fe.sub.17 before 
treatment (solid line) and after treatment (broken line) at 475.degree. C. 
for 2 hours in benzene vapour showing a lattice expansion of 5.5%; 
FIG. 7 shows the difference in cell unit volume between compounds having 
the formula R(Fe.sub.11 Ti) and compounds having the formula R(Fe.sub.11 
Ti)C.sub.y where y is 0.6&lt;y&lt;0.9 and where R is Ce, Dr, Nd, Sm, Gd, Tb, Dy, 
Ho, Er, Tm or Lu. A substantial increase in unit cell volume is observed 
where carbon has been interstitially incorporated by heating in butane; 
FIG. 8 shows the Curie temperatures of compounds of the formula R(F.sub.11 
Ti) and R(Fe.sub.11 Ti)C.sub.y where 0.6&lt;y&lt;0.9 prepared by the process of 
the invention and where R is Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm or Lu. 
Again a substantial increase in Curie temperature is observed where carbon 
has been interstitially incorporated; 
FIG. 9 shows the X-ray diffraction pattern of an arc-melted and unannealed 
Sm.sub.2 Fe.sub.17 ingot containing 5% weight Nb, showing a substantial 
absence of free iron. The solid line is the trace of the Sm.sub.2 
Fe.sub.17 ingot with additive and the broken lines indicate where the 
.alpha.-Fe peak would appear in an ingot without additive. 
FIG. 10 shows the variation of anisotropy field as a function of neodymium 
content for the series of compounds Y.sub.1-z Nd.sub.z)(Fe.sub.11 
Ti)C.sub.0.8. 
It will be readily apparant from the data presented herein that the process 
of the invention has substantial advantages over hitherto known processes 
for bringing about interstitial incorporation of another element into 
intermetallic magnetic compounds of the rare-earth/iron type and that the 
materials produced thereby have improved magnetic properties. Specifically 
the increase in Curie temperature, the uniaxial anisotropy and increase in 
spontaneous magnetization make the compounds of the invention very well 
suited for the manufacture of permanent magnets. The high Curie 
temperatures of these materials means that magnets made from them can be 
used in apparatus or processes requiring high temperature conditions and 
the magnetization of the magnet will not be lost. 
Magnets may be formed from the materials of the invention by orienting the 
interstitially modified intermetallic compound in powder form in a 
magnetic field with a polymer resin to make a polymer-bonded magnet. More 
specifically the powder of the interstitially-modified intermetallic 
compound may optionally be milled to a finer powder, with particle size of 
10 .mu.m or less and then mixed with a polymeric material (e.g. a 
thermosetting resin or an epoxy resin) and optionally oriented in a 
magnetic field sufficient to align the easy axes of the grains of powder. 
The resin is then set and the composite is subject to a large magnetizing 
field sufficient to saturate the remanent magnetization. 
In a modification of this process, the composite may be formed into a 
desired shape by compression or injection moulding, prior to applying the 
magnetizing field. 
An alternative is to make the composite with a metal matrix rather than a 
polymer matrix. In this case, a low-melting point metal, such as Zn, Sn or 
Al, or a low-melting alloy, such as a solder may be used. The metal is 
mixed with the milled intermetallic powder, which may be oriented in a 
magnetic field prior to heat treatment at a temperature sufficient to melt 
the metal and form a metal-metal composite. The preferred metal is zinc, 
which reacts with any free .alpha.Fe to form a nonmagnetic Fe-Zn alloy, 
thereby enhancing the coercivity of the magnet. 
A further way in which magnets can be formed from the materials is to forge 
with a soft metal under a stress which tends to mechanically orient the 
crystallites of the material. In particular a shear stress is applied to 
the intermetallic powder, which is optionally mixed with a soft metal such 
as Al. The shear stress aligns the c-axes of the intermetallic 
crystallites and thereby increases the remanent magnetization of the 
magnet.