CA2040686A1 - Magnetic materials - Google Patents

Abstract

ABSTRACT:
IMPROVED MAGNETIC MATERIALS
A new magnetic material of the general formula:
RxFeyxlazb is derived from an intermetallic compound of rhombohedral, hexagonal or tetragonal crystal structure wherein R is one or more rare earth elements, X' is an element of groups IIIA, IIIB, IVA
or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 0.5 to 2, y is a value from 9 to 19, a is a value from 0 to 3, b is a value from 0.3 to 3 and wherein when the magnetic material of said general formula is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure Fe is unsubstituted or partially substituted by another element and when the magnetic material of said general formula is derived from an intermetallic compound of tetragonal crystal structure Fe is partially substituted by any element of group IIIA or IVA of the periodic table or a transition metal from another group with the further proviso that in the case of materials derived from said rhombohedral or hexagonal crystal structures the element X' is not boron when the component Z is antimony or bismuth.
In these new materials the element Z is an interstitial addition to the existing structures and is introduced by a gas-solid reaction. The materials exhibit increased Curie temperatures, magnetic strength and easy uniaxial anisotropy and are therefore suitable for fabricating into permanent magnets.

Description

IMPROVED MAGNETIC MATERIALS

The invention relates to new magnetic materials having improved magnetic properties, to processes for their production and to the use of the new 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 (Ms) and the easy uniaxial anisotropy conventionally represented by an anisotropy field Ba. 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 :, .. ~.

- 2 - 20406~6 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, Nd2Fel4B 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 320C 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 300C in practice they are not suitable for operating at temperatures greater than 150C.
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 1987 Higano et al (IEEE Transactions on Magnetics, vol Mag-23, No. 5 Sept 1987) reported an attempt to carry out a nitriding reaction by exposure of powders of Sm2Fel7 alloy to gaseous nitrogen at temperatures of 500 and 1100C. The experiment was intended to produce a compoùnd of the formula -Sm2Fel7-N which it was hoped would have improved magnetic properties. However Higano et al found no evidence that such a material was produced by this process but instead found that the nitriding process simply decomposed the rare-earth iron alloy starting material to produce iron and nitrides of the rare earth elements.
The present inventors have now produced a new magnetic material of improved properties which includes at least a rare earth element, iron and a group VA element with optionally one or more other elements. The successful production of these materials is unexpected having regard to the teaching of Higano et al.
In accordance with one aspect of the invention there is provided a magnetic material of the general formula:
RxFeyx~azb which is dervied from an intermetallic compound of rhombohedral, hexagonal or tetragonal crystal structure wherein R is one or more rare earth elements, X' is an element of groups IIIA, IIIB, IVA
or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 0.5 to 2, y is a value from 9 to 19, a is a value from 0 to 3, b is a value from 0.3 to 3 and wherein when the magnetic material of said general formula is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure Fe is unsubstituted or partially substituted by another element and when the magnetic material of said general formula is derived from an intermetallic ~ 4 ~ ~040686 compound of tetragonal crystal structure Fe is partially substituted by any element of group IIIA or IVA of the periodic table or a transition metal from another group with the further proviso that in the case of materials derived from said rhombohedral or hexagonal crystal structures the element X' is not boron when the component Z is antimony or bismuth.
It is to be understood that herein the term rare earth element includes the elements yttrium and thorium, and that the groups IIIA, IIIB, IVA, IVB and V of the periodic table are those defined by the CAS
version of that table. By hexagonal, rhombohedral and tetragonal crystal structure is meant intermetallic compounds having a crystal structure 15 analogous to Th2Nil7, Th2Znl7 and ThMnl2 respectively.
In the case where the material is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure the element R may be samarium alone or a combination of samarium with one or more other rare earth elements selected from lanthanum, cerium, neodymium, praseodymium, erbium, thulium, yttrium, and mischmetal. R may also be yttrium, cerium, neodymium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a mixture of two or more thereof. In the case where the material is derived from an intermetallic compound of tetragonal crystal structure, R may be any rare earth element but preferred elements for R are cerium, praseodymium, neodymium, terbium, dysprosium, holmium or a mixture of two or more thereof. Particularly preferred are neodymium or praseodymium alone or in combination with other elements.
In the case of the hexagonal or rhombohedral materials as aforementioned the iron may be substituted by up to 50%, most preferably up to 33%
with another element or elements. The element is preferably a magnetic transition metal, most preferably cobalt.
In the case of the tetragonal materials as aforementioned the iron is substituted with any element of group IIIA or IVA of the periodic table or with a transition metal not already included in those groups. Preferred substituents are silicon or lo aluminium or any of the transition metals titanium, vandium, molydenum or chromium.
Where an element X' is included in the materials it is preferably carbon, boron, silicon or zirconium and the value of "a" may be as low as 0.1 with a maximum of 3. Preferably the value of a+b is 3.
The component Z may be nitrogen, phosphorus arsenic, antimony or bismuth or mixtures thereof and of these the particularly preferred element is nitrogen. For example, three materials in accordance with this aspect of the invention which demonstrate the requisite improved magnetic properties are Sm2Fel7N2.3, Sm2Fe17Cl.lNl.l and NdFellTiNo.8-The magnetic materials of the invention display considerably improved magnetic properties over materials hitherto known. Firstly they have Curie temperatures in excess of 400C. Secondly, they have improved easy uniaxial anisotropy as demonstrated by X-ray diffraction patterns of the material after a magnetic field has been applied.
Thirdly, the magnetic moment is increased and finally the magnetic moment is subject to little variation with time or temperature around ambient temperature.
These increased intrinsic magnetic properties are all very favourable for permanent-magnet applications.

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~ 6 - I 2040686 , ~n accordan~e Wlth a ue~ond -~e~t io~ tho lnv-nt!ion there i~ prov~d-d a proae-- ror ~odl~yin~
~ t~- ma~n-tia propertlo- Or an lntermet~ c compound aompri'oing at lea~t ono or moro ~r- art ~ ent-an~ th~ el-mcnt lron ln whloh t~he lron 1- pptlonally aub~titut-d with anothor ol~ment whlch proc-~aomprl~o- hoatlng aAid lnt-rm-t,allic compo~nd wlth o ga- oontaining ~t l-a-t ono group VA l-m-~t Z ln the ub-tahtlal abeonc- Or oxy~-n to incorporalt- th- ~aid nt l-aot one el-m-nt Z lnt-r-tltl-lly lnto'lth-ary-tD~ lattlao of~the lntormQtallia compolnd by o gao-eo~ld r-aetlon Whoro tho lntermeeall~c compound 1~ Or tetragonal cry-tal tructur- th- lroh may be ~ub-tituted by an al-m-nt o~ ~roup IIIA or'group IVA
lS Or th-jperiodio toblo or by a tran~ition m~tal not olxe~dy lnclud-d ln thoae group~ j ~ Th- ~ampl- mat~rlal i- prefera~ly p~doed in a ~eal-d'aontaln-r rrom which th- oxygen canlbe pumped and th~ roac'~v- ga~ ~dded n~ heated rrom¦th-- 20 out~id~ It~ tomporatur- 1~ ral~ed to a m~ximum not oxoeodlng about 600C Optlonally th-interm~tAlllo 6tartinq mat-rlal i- ground f~om,- ror xam~lt, an insot to a partlcle,~lze Or rrolm 0 5 to " 50 mla~on- dlam-t-r bofor4 heatln~ ln u~tabl-2S gaO Tho pr-rerrod ran~ 0 5 to 20 mlo~on~
S~ocif~c additiv-- uch a- niob~um or vana,~ u~ mav be e~_ inaot ~aallitatlna the~dev-lopm-nt of ooeroivity ln 't,h~_F~ultlna modifled motallic materlal 30 Alt-rna,tiv-ly, th- tartlng mater~al may bo prepared lnto t~ln rlaxo~ or rlbbon~ by m-lt opinnlng or lnto powder by m chanical alloylng or ~pray ca-ting The heatln~ may proc--d for a p~rlod not eYao-ding 8 ' houro,lbut th- x~at tlma will dop-nd upon th~ qas '' 35 and th~ ~olid ~oomotry o~ tho ~tartlnq matQrlal, The pr~cl~ h-atin~ tlmo rOr any ~tartin~ mat-rial i~
thororo,r- roadlly c41culabl~
suitablo ~a~eo to b- u~od in the procOs~
lnclud-!tho~o whiCh produc- radlc~l~ oontainin~ -alnqlo atom~ Or a group VA olom-nt on contact W~th o~az6z6~a~0 ~a~a~so7lLo : wda~ 6~ ' OZOL laTdoaalal xolax ~S lN3S

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--hot surfaces such as metal or quartz or by exposure to high frequency radiation, for example gaseous hydrides of the group VA elements.
The preferred magnetic materials of the invention, in which the group VA component Z
interstitially inserted into the crystal lattice is nitrogen, may be made from the appropriate intermetallic starting material using gaseous nitrogen, ammonia or hydrazine. When an intermetallic compound of the formula R2Fel7, or RFel1Ti for example, is heated with nitrogen and the gas pressure monitored, a decrease in pressure occurs which begins at about 350C and continues until the temperature reaches 650C. The initial decrease in pressure is attributed to the reaction of nitrogen with the exemplified intermetallic compound and its incorporation into the R2Fel7 or R(FeTi)12 crystal lattice. That a new compound incorporating R, Fe and N has been formed is borne out by the fact that after the heating process the sample has increased weight and there is an increase in the crystal lattice parameters i.e. unit cell volume, as shown by X-ray diffraction.
The process of producing the new preferred materials may also be carried out using ammonia instead of nitrogen. In this case there is a rise in pressure starting at approximately 350C. The rise in pressure is explained by the fact that at 350C
the ammonia decomposes to nitrogen and hydrogen. The nitrogen is taken up by the intermetallic sample as evidenced by a weight gain and increased crystal lattice parameters. It appears that once the temperature exceeds about 650C the newly for~ed material decomposes to alpha-Fe and nitrides of the rare earth element or elements.
In accordance with a third aspect of the .
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20406~36 invention the new magnetic material produced as described herein is used for fabricating permanent magnets.
A preferred process by which this may be achieved comprises milling the magnetic material with a metal such as aluminium, copper or zinc or a solder or an organic powder or resin, magnetically aligning the material by applying a magnetic field and then heating to a temperature not sufficient to decompose the material. Preferably the magnetic material is milled with zinc. This process of forming a magnet serves to increase the coercivity essential for forming magnets.
The following figures and tables give data relating to the magnetic properties of certain preferred intermetallic compounds of the invention and by way of example demonstrate the improvement in magnetic properties over known magnetic materials.

Table 1 The data in this table demonstrate the effect of incorporating nitrogen interstitially into the crystal lattice of compounds of the formula R2Fel7 with respect to crystal lattice parameters, Curie temperature (Tc) and spontaneous magnetization per unit mass (6s). These nitrogen-containing compounds were prepared by the process of heating in nitrogen gas in accordance with the invention.
The lattice parameters are determined by X-ray diffraction. R is represented by 12 different rare earth elements.
The spontaneous magnetization per unit mass (6s) is converted to spontaneous magnetization per unit volume (Ms) by multiplying the value 6s by the density of the magnetic material.

Structure a c Tc 6s 5 Compound type (nm) (nm) (C) (JT-lKg-1) ce2Fel7 Th2Znl7 0.847 1.232 -32 0 Ce2Fel7N2.8 0.873 1.265 440 160 Pr2Fel7 " 0.857 1.242 17 82 Pr2Fel7N2~5 0.877 1.264 455 167 Nd2Fel7 0.856 1.244 57 77 Nd2Fel7N2.3 0.876 1.263 459 178 Sm2Fel7 0.854 1.243 116 100 Sm2Fel7N2.3 0.873 1.264 476 159*
Gd2Fel7 " 0.851 1.243 204 46 Gd2Fel7N2.4 0.869 1.266 485 115 Tb2Fel7 0.845 1.241 131 51 Tb2Fel7N2.3 0.866 1.266 460 96 Dy2Fel7 Th2Nil7 0.845 0.830 94 50 Dy2Fel7N2.8 0.864 0.845 452 115 Ho2Fel7 0.844 0.828 54 49 Ho2Fel7N3.o 0.862 0.845 436 115 Er2Fel7 0.842 0.827 23 32 Er2Fel7N2.7 0.861 0.846 424 134 Tm2Fe17 0.840 0.828 -13 0 Tm2Fel7N2.7 0.858 0.847 417 137 LU2Fel7 0.839 0.826 -18 0 LU2Fel7N2.7 0.857 0.848 405 147 Y2Fel7 " 0.848 0.826 52 92 Y2Fel7N2.6 .. 0.865 0.844 421 164 *Extrapolated value - lo- ; 2040686 ~ ~he dAtl~ pr~ont-d ln ~ablo 1 domo~trat~ thAt th- lntor~tlti~l nitrldo ph~e RaFel7Nb~ ~hor-b l~ ~bout 2~6, XiYt~ ~aro6- tho ontlre raro--arth o~rle~ ~rom C- to ~.u. Tho unlt cell volu~e Or the 5 ~rystal lattlae lnc~eaoQe by S to 9% on ~rming thc nltriqe and tho Curi- t-mp-ratur- Ta an~ ~pontanoous magnotlzatlon 6~ ~r- ~r-atly lnor-a~-d ~ata 1'urther lndla~te that eub~tltutlon6 ~xiot Ib-two-n nitride~ o~ dirr-ren~r- o~r~he eo tb~,t~ e~t~e uch ~?,m~gn,tizAtio~n or nni-otroPY rlold imay b~_ oDtlm~d ~or n~ a~ a~ lon~ ~gyl~ roa~rd to tht oo~t o~ the par~ioulhr rare arth ~omponent ~Z
~ !h- dat~ ln th- table damon~trAte ~h~ ot on ory-t~l lattiao ~r~m-t-r~, Ourl- t~ rature ~To) and tn- BpOntAn-ou~ ma~notio ~oment p-r u~it volumo (M~) or lnoorporating nitro~en lnto th~ ~y~tal Attl~e o~ compound- Or the ~ormula Y2Fo$7Cl,0 nnd 8~2Fel7Cl l Agaln tho no~l oompoun4-~ 20 w-r- ~r-par-d by hoAtlng ln n nitrogon-oontainln~ ga~
in ac~ordanoo with the lnventlon 8tructur-A 'C TC
Compou~d type ~nm~ ~nm) ~C) oM~ J

25 Y2F~17~1~0 Th2Nil7 0 8S5 0 833 239 1 2S
- Y2F-17Cl ONl 4 ~ 0 867 0 851 42a 1 ~6 s~2Fel7cl 1 ~ Th2Znl7 0 8~8 1 244 207 1 11 8~2F-17~ 1 1 0 873 1 270 471 1 53 , at ~8C
~ v~u-- are ~enoltlve to condltlon- o~ h- t tr-at~!nt after m41tlng th- alloy- I -Tho data aqaln demon~trat~ t~ lmprovement in - 35 ~agn-tlc prop-rtles, To, magnotlzatlon and unlt coll volum-, by lnterstitial incorporation of nltroqon into tho cry~tal lattloe of oompounda of the ~enoral for~u~ RXF--yX a-#'0~8~e~6~8~0~0 ~B~SO~lLD ' ~d6E ~ ' ~6-~ -8~' O~OL laT~oaalal x~aX ~6 lN3S

2C~40686 T ble 3 The data presented in this table demonstrate the improved easy uniaxial anisotropy with, as an example, compounds where R is samarium. The value for easy uniaxial anisotropy represented by the anisotropy field Ba~ in Tesla was obtained by aligning the rhombohedral c-axis in the direction of an applied magnetic field. From magnetization curves on oriented powders with the field applied parallel and perpendicular to the alignment direction the values for Ba shown in this table were obtained.

Compound sa( T ) sm2Fel7 < 1. 0 Sm2Fel7N2.3 >12.0 Sm2Fel7cl. 1 Sm2Fe17Cl.lNl.O >8.0 Table 4 The data in this table presented give deduced values for iron-iron and iron-rare earth exchange interactions based on the variation in Curie temperature for the different heavy rare earths.
Compound nR-Fe(~O) nFe-Fe(~o) R2Fe17 225 181 R2Fel7NY 208 515 It is deduced that the iron-iron interactions are enhanced by a factor of 2.5 in the new nitride compounds while iron-rare earth interactions are only slightly decreased.

Table 5 The data presented in the table demonstrate the effect of incorporating nitrogen interstitially into the crystal lattice of compounds of the formula RFellTi with respect to crystal lattice parameters, (a and c), Curie temperature (Tc), average hypefine field Bhf, in Tesla, and anisotropy. The starting materials were prepared by heating in a nitrogen-containing gas in accordance with the process of the invention. The particular process conditions in each case are given in the table.

15 compound a(nm)c(nm) TC(C) 13hf(T) anisotropy processing Nd(FellTi) 0.856 0.478 270 21.5 c-axis Nd(Fel lTi)No 7 0.879 0.487 475 28.0 c-axis 40' at 450C in N2 Sm(FellTi) 0.855 0.479 311 25.5 c-axis Sm(Fel lTi)N0.8 0.864 0.484 496 29.1 c-plane 30' at 480C in N2 Sm(Fel lTi)No 9 0.865 0.486 490 heat to 550C @ 10/min in NH3 Dy(l~el lTi) 0.850 0.478 257 24.4 c-axis Dy(Fel lTi)No 6 0.867 0.480 473 28.2 c-axis 60' at 450C in N2 Tb(Fel lTi) 0.852 0.479 281 24.2 c-axis Tb(Fel lTi)No 5 0.864 0.482 477 28.5 c-axis 40' at 450C in N2 Y(FellTi) 0.851 0.479 251 23.5 c-axis Y(FCllTi)N 0.8 0.862 0.481 460 28.8 c-axis 60'at480CinN2 20406~36 The interstitial incorporation of an element of group VA of the periodic table, for which the example is nitrogen, into selected intermetallic compounds of the formula R2Fel7 or R2Fe17X a or R(FeM)12 or R2(FeM)17 where M is a substituent element as hereinbefore defined and the improved magnetic properties achieved thereby is further demonstrated by data presented in the figures in which:-Figure 1 is a thermopiezic curve for absorptionof nitrogen gas by Y2Fel7 showing the drop in pressure of gas in the chamber as nitrogen is taken up by the sample. The pressure values on cooling demonstrate that the nitrogen remains absorbed by the Y2Fel7 sample;
Figure 2 shows the isothermal reaction of nitrogen with Y2Fel7 powder, having an average grain size of approximately 2 microns diameter at 400C, 450C and 500C, the value y being the number of moles of nitrogen atoms incorporated into a mole of the sample. The data indicate that the optimum temperature range for the operations of the process of the invention is between about 450C and 600C;
Figure 3 is a thermopiezic curve for absorption of ammonia gas by Y2Fe17 at an atmosphere of approximately 1 bar. The curves of heating demonstrate an increase in pressure due to uptake of nitrogen from the ammonia. There is an increase in weight after heating the sample to 550C which is attributed to nitrogen absorption.
Figure 4 shows 57Fe Mossbauer spectra at room temperature of Y2Fel7 before (a) and after (b3 heating to 500C in l bar ammonia. The changes in Curie temperature and magnetic moment are reflected - 14 - !

ln tht 57F- Mo~bauer ~p-ctra in whlah th~ av-ra~o hypor~lne rl-ld at a~oc, ~Bh~ inorea~- 2 'rom 10 ~081a rOr Y~Fol? to 30 Tesl~ ror Y2F~ltN~ 6l Fl~ur- 5 how~ the X-ray dif~raotlc ~n pattern-o~ YaFol7 powd-r h~at-d in ~ th-rmopio~lol analy~-r ln nltrog-n t 10C/mlnuto up tolthe temp-~aturo- Or 500C, S50C, 600C, 700~
and 8$0C Powdor~ Or the rormula R~F-17¦whoro R i- ~noth-r rar- ~nrth l-m~nt behave ~l~llarly ' The ~lgure ~how~ the ap~earana- o~la phase with xpan4-d lattloo param~t-rs which co-exi~s wlth the un xp~nd-d pha-- aft~r tr~atm~nt up to 5s~ IC
Y2F-17N2,6 pha~- rorm~ olearly at 600C ar ~d on h-~ting up to 700C or ~ov~ the alloy deoompo--~ to ~N and ~F-~ ' Flgur- 6 ~how~ X-ray dirrr~ction p~ttern~ Or Y2F-17, ~owder art-r h-atlng in nitro~en g~o l~othtrmallY ae S00C rOr two houro~ Tho ¦-xtond0d heat ~r-atm-nt produoee the YaFol7N2~6 oompound at a ~ow-r tom~-rature than ,~h~wn in the ~-viou~
~i~uro but rurth-r h-at tr-atment to ~50~ rosulto ln d-~ompo41tion to YN and ~Fe , Flguro 7 1- a thermopi~zlo ourv- ~c ~r Y2F q ~Cl o h-~ted rrom roo~ tomp-ratur- ir I an atmo~ph-r- or approxlmat-ly 1 ~ar ammonia~ A~in an increj~- ln pr---ura at ~bout ~70C i- ob~erved;
Flgure 8 how~ thQ d-p-ndence Or t~- Curio tomp-~atur- ta~ ~o~C) and th~junit oell Yolumo of 30 t~ ttlo- ~b) v(A3) on th~ maximum heat~nq tem~-~atur- Tm or Y2F-17C For th- ~am~lo troattd at 450C and 500C the~ oo--xl-t two R2Folt-typ- pha~-- ono wlth the larger unit c~ll volume and hlgher Curlo temperqtur~ and the oth~
with the ~maller unlt o-ll volume an~ low-r Curi-t-mperatur- The mor- th~ ory~tal lattloe i~
expanded th- hi~hor the Curio t~mperatur- ~hero ls # 0~8~EZEI~O~D ~B~SO~L0 ' Wd6~ 6-V -5~' O~OL la~oaalal xolax ~S 1~3S

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also a substantial increase in spontaneous magnetic moment (~oMS) to 1.46 Tesla (see Table 2);
Figure 9 shows Mossbauer spectra at room temperature of Y2Fe17C1.0 before (a) and after (b) heating in 1 bar ammonia at 550C. The average hyperfine field at 18C <Bhf> increases from 25.3 Tesla to 30.8 Tesla after the ammonia treatment;
Figure 10 is a thermopiezic curve for Sm2Fe17Cl.l heated from room temperature in an atmosphere of approximately 1 bar ammonia. Again an increase in pressure is shown at about 350C.
Analysis of the sample after heating to 600C
reveals that the material retains the rhombohedral (Th2Znl7-type) structure with increased lattice parameters. From the increase in mass the nitrogen content is estimated to be 1.1 nitrogen atoms per Sm2Fe17C1.1 formula unit;
Figure 11 is an X-ray diffraction pattern of Sm2Fel7C1.1N1.1 powder before (a) and after (b) orientation in an applied field of 1.2 Tesla for one hour. The figure demonstrates the strong uniaxial anisotropy possessed in particular where R
is samarium;
Figure 12 shows magnetization curves at 18C
of oriented samples of Sm2Fel7C1.1 before (a) and after (b) treatment in 1 bar ammonia up to 600C. Curves are shown for the field applied parallel ( 11 ) and perpendicular (l) to the axis of orientation. From these magnetization curves the values for ~oMs shown in Table 2 and Ba shown in Table 3 are obtained;
Figure 13 shows the X-ray diffraction patterns of a) Sm2Fel7 powder with an average particle size of l~m and b) the same powder heated at 500C in nitrogen gas for two hours to form Sm2Fe17N2.4;

20~0686 - 16 - ~

Figur~- 14~ and b ~how ~h- radial di~trl~utlon ~unct$on~ d-duc-d ~ro~ oxtondo~ X-ray Abs~rptlon ~lno ~tructur- dat~ on tho ~ om~ n- Flgur- 131 Th~
s poak ~pp-arlnq at 2 5 ~ ~how~ th- prec-nc~ o~
approxi~at-ly thr-- nltro~-n atom- At a d~tanc~
2 5 A ~rom ~ amarlum ~tom ln tho nitrlde~
~ Figuroo lSa and b uhow th- c~y~t~ ructur~ O~
thc r~o~boh~dral ~nd h~xagon~l ~s~7 ~truc~uro, 10 lnd~o~ting ~h~ ~lt-- oecupled by nltrog~n; Fl~uro lS~ ~ th~ rhom~oh-dr~l cryotal ~tructureland Fiqure 15~ lt th- hoxAgonal cry-tal ~ructur- ~ rgo oirclq~ roprecont raro eartha, ~mall ~had~d clrcl--r-prooont lron nd mall blAok oirale~ repre~ent lS nitro~on ~ te~ 9e or 6h.
I Flgur- 16 1~ a hl-to~raD o~ tho pa~tlcl- ~iz~
A dl-tr~butlon o~ a typical Sm~F~17 2owd-r ~--d ror nltro~-n ab-orpt~on~ , Flguro 17 how~ th~ ~ariatlon o~ ~ o dirfu~lon 20 coorriclant ~or nitrog~n ln th~ 6m2Fol~ pc wd~r a8 - a ~un~tlon o~ lnv-r~- t-mpcrature ; Flgur- 1~ how- m~gn-tiiation curvl- ~t 18C
~or a~ orlented eampla ~ 5m2F~17N2 3 a~tdr tr-at~-nt wi~h am~onla ~al~ ourvQ~ are l~hown ~or 25 th- ~$-1d ppli-d parall~ ) and p-rp-~diaul~r ~1) to tht axl- o~ ori-ntatlon From th--- t~o valu~s of tho anl~otropy ~leld ~a ar~ obtalne~ ~ u~own in Tabl- 3 Th- valu- o~ Da for ~mzF-l7N2 3 I~-giv-n la~ ~12 0 ~-ala but ln ~ct th- CUrVR~ ~hown 30 in th- ~lgur- indloat- it may b- a~ hlgh a~ zo T~Dla;
rigur- l9~a) 1- a thqr~opi-zio ourv~ for a powd-r m~d- ~ro~ a oa~t in~ot o~ 8m2Fel7 hLat~d in nitrog-n Flgur- l9tb) 1~ a th~r~opi~ic aurve ror a ~owdor ~adq from ~n ingot and AnneAl~d for loo 3s hours Iat 9Soc and h-nt-d ~n nltrogen ~ht dlfforrnc-s ln the two ~ct~ or cUrV08 CleAily n~7e~L~LnLn ~lfiLcn~Ll~n ! W~n~ LR-~ -c~ n~nl,~aT~o~aIal xo~ax AR

, .

- 17 - ' 2040686 !
demonstr~te that th- trL~tm~nt te~paratur~ requlr~d to form th~ R2Fel7Nb ~ha~a vario~i d~p~ndi~g on tho m~t~llur~ial c~mpo~ltion c~ th- ~ngo~ uLi~d to mako the powd-r~
~lqur- 20 ahowa X-rAy dl~raction pattorn- o~
th- compound~ Nd2F-17N2,3, 8m2F~17~2 3 and Er2~l7~a 7 Aft~r an appll~d rl~ld o~¦l z Tosla` In tha ca~-. of ~m2F-17N2 . 3 th- G-~Yi~i 1~ aliwn~ rall~l to the appliod ~leld ~ndlcatlng atrong unlaxiAl a~l~otropy Howev~r in the ~asa wh-re ~ d or Er there 1~ a tendonoy ~pr th~
c-axi- to be all~nod p~rpendlaular to theldirectlon of th~ appliQd mn~not~c field Figure 21 ~how- tha arystal ~truct~ro of the t-tragon~l ls12 aompound showl~ lta- occuplsd by nltrog-n The oodin~ Or tho oircl~ a~ do6cribed for Fl~ur-- 15n ~n~ lSbl Flgur- 22 showo a th-rmopl~zlc tr~p~ ~or a~60rptlon o~ nitrogon ga~ by 8m~F~llTi) i Tho m~ter~al wa~ ted ~t a rato of 10C/~inutQ ~t approxlmately 1 bas nltrogon ~ ur-l d-mon~trateo that th- optlmum tomporatur-lran9~ ~or operation of th- procedo 1~ imilar to th~t o~ the R2F-lj compound-t i Flgure 23 shows room t-mpurature 51 Mossbaunr sp-ct~a o~ sm~FallT~ or- (a) and sfter (b) h-atlpg in A nltrogo~ ~ontalning ga- in a~cordance wlth th~ inventlon Tho avora~ hyp-fin-,fiold incr~ fro~ 25 5 Te~la ln ta) to 29 1 Te~la in ~, r~ oting th- ch~ngo~ in Curlo tQmparatur~ and lron mAgnetlo momant Flgure 24 ~hows X-r~y dl~rnction patt~rn- o~
powd-~- o~ Sm(F-~ nd 9m~Fe11Tl)NO,~
t~j o~i-nt~d ln a ma~n-tio flald o~ 1 2 T~ he 3S ~trong uniax~al anl~otropy o~ Sm~F-11Ti) la tran-~orm-d to a~y-plan~ anl~otropy ln tho intorYtitial nitrlda SmF~ iNo~B demon~tratlng a a ~:o~oz~z~s~olo l9~6~50~LO ' Wd~ 6-~ -S~' OZOL ~a~oaalal xolax ~ lN~S

~ la 1 20A0686 ~ang~ ln i~n o~ th~ ~ocond-ordor cry~ta~ rl-ld ao-~floiont A20 from n~gatlv- to po-ltiv- ~eno~
th- strong uniaxiAl ~nl~otropy ob~ex~-d r~ )r int-r~t~tlally~modifiod 1 12 ~truatur- OOt Ipount- o~
s r~ro-oarth- wlth ~ negatlv- 8tov~n- oOorr ol-nt ~Nd,lEr,~m), n-odymium ln p~rtiaulaxt Flguro 25 1- An lllustratlon o~ ~nf-r~tltlal nitro~n ~tom- around the raro ~rth ln t~-rhomb~hedrAl or hoxagonAl 2 17 otruatur- ~a~ an~ ln tb- t~tragonal 1 1~ ~tructur~ ~b) ~h- ~ootrlc ~l-ld gradi-nt oxp-rieno-d by th- raro-ea~th, ~uant~ri-~ in the parAmeter A20, i~ malnl~ produced by u~round1ng inter~titial atom~ ln th- ~Atarlal~ o~
th- i~vention _ is noqativu for the of th~ 2 17 com~ound- and ~o~itive ror co~ l~u~atiQn of t ~ 1 12 compoundst ' Figur- 26 lllu~trate~ ~m- of tho t~reot- o~
- cobAlt sub-titutlon Sor lron in matoriAlo!o~ tho inv~ntion h~vlng th- rhombohod~al or h-xa~onal ZO cry-t~l trUatur~ I
- ~ Flgur- 26~A) lnaicAtoe ~h- nltrog~r ~ oontont ~cbi-y-d by tr-~ting rln-ly-gr~und pOwaor~ I o~ Sho R2tF~ ococ)Nb tYF- ror~uls whor- o io tl I-numb-~ Or oob~lt Atomo ln nltrogen g~- At t~mp-~a~uro- rangin~ from 400-600CC
Flgur- 26~b) illustrat-s ~ bro4d m~ximum ln magn-~izAtion wlth ~ tran~ition m~tal ~ubttituta whore R lo Y ~nd o-O,~, , , Flguro 26~a) ~howo that tho tr~n~i~lon m~tal ub-tttu-nto mak- ~ po~itlv~ contr$butlonIto tho anl-o~ropy wh-n o 1- >O 1~ 1 ~ Flgur- 2~ 1~ an illu~tr~tlon o~ th~ developm-nt o~ hy~t-ro~la in A ~owd-r Or Sm2Fel7N2,3 I
comprlulng ~irst and econd gusdr~nt dom~gnotlzlng 35 ourv-- Or ~mpl~u Allgn~d And magn-ti~-d $n A ~ul~od ~lold o~ 8 To-la The data roprQeonted a~ ~s rOllOwo2- 1 A) Powd-r o~ Sm2Fel7N2 3 di-poroed ln #'0~73~E~EL3L0~0 ~3LBL90~L0 ' Wd~ B-~ -5L' 0~0L lal~ooalal xolax ~6 lN~

:
.

!
epoxy r--in b) Powder c~ ~m2Fol~N2,3 m~lled ~th Zn pow~er (25 wt %) c) Powdes o~ S~Fel7Na,3 mlll-d w~th Zn , powd-r ~15 wt %) and h-~t tre~ta~ ~t 400C
~or two houro Flgur- 27 ~ndioate- th- magnetlo p~opor~ o~
~o~a~nt ~not- produoed ~rom th- m~gn-~lo mAter~al- o~ th~ lnv~ntlon and ~thod- by !whioh th-lo ooerc~v~ty and hy~t~r~ may be develop-~ For x~npl;- ln 21(c) tbe mst-slal i~ mllled w~th 15 wt Zn and heat-d to 400~ to produa- a ma~n-~ h~ving a oo-ro~vlty Or 0 5 ~ An~ a maximum ~ne~j~y product Or 36XJn~3 ~h datA hown ln Figuro ~7 e~tsbl~be~
conolu-lv-ly th8t 8m2Fel7~ 3 nnd the rel~ted compound- Or th- lnventlon aan ~- r~-ctiv-ly yroo-a~ed to m~X- magn-tD
Further, thln fll~o ot mat~rlAl~ o~ thQ
lnvention maY be oxPloited for magneti~ or m~gn-to-optic_r-cording ,~ "' ~:, ~' 35 , OLJ~'D~OZe~eL~OLO ~o~a~so~LLo ~ w~ LO-7 -5~' O~OL ~laTdooalal xolax:~ lN3S

:: , :: , ,,

Claims (31)

1. A magnetic material of the general formula:

RxFeyX'aZb which is dervied from an intermetallic compound of rhombohedral, hexagonal or tetragonal crystal structure wherein R is one or more rare earth elements, X' is an element of groups IIIA, IIIB, IVA
or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 0.5 to 2, y is a value from 9 to 19, a is a value from 0 to 3, b is a value from 0.3 to 3 and wherein when the magnetic material of said general formula is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure Fe is unsubstituted or partially substituted by another element and when the magnetic material of said general formula is derived from an intermetallic compound of tetragonal crystal structure Fe is partially substituted by any element of group IIIA or IVA of the periodic table or a transition metal from another group with the further proviso that in the case of materials derived from said rhombohedral or hexagonal crystal structures the element X' is not boron when the component Z is antimony or bismuth.

2. A magnetic material as claimed in claim 1 wherein when the material of said general formula is derived from an intermetallic compound of tetragonal crystal structure a=0.

3. A magnetic material as claimed in claim 1 or 2 wherein R is samarium or neodymium.

4. A magnetic material as claimed in claim 1 or claim 3 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure R is samarium.

5. A magnetic material as claimed in claim 1 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure R is samarium in combination with one or more rare earth elements selected from yttrium, lanthanum, cerium, neodymium, erbium, thulium and mischmetal.

6. A magnetic material as claimed in claim 1 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure R is yttrium, lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, dysprosium, erbium, thulium, ytterbium or lutetium or a mixture of two or more thereof.

7. A magnetic material as claimed in any of claims 1,2 or 3 wherein when the material is derived from an intermetallic compound of tetragonal crystal structure R is yttrium, thorium, cerium, praseodymium, neodymium, terbium, dysprosium or holmium or a mixture of two or more thereof.

8. A magnetic material as claimed in any preceding claim wherein the element Fe is up to 33%
substituted with a transition metal.

9. A magnetic material as claimed in claim 8 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure the transition metal is cobalt.

10. A magnetic material as claimed in any one of claims 1,2,3 or 7 wherein when the material of said general formula is derived from an intermetallic compound of tetragonal crystal structure the element Fe is partially substituted by titanium, vanadium, molydenum or chromium.

11. A magnetic material as claimed in any one of claims 1,2,3 or 7 wherein the material of said general formula is derived from an intermetallic compound of tetragonal crystal structure the iron is partially substituted by aluminium or silicon.

12. A magnetic material as claimed in any of claims 1,3,4,5 or 6 wherein when the material is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure X' is carbon, boron, silicon or zirconium and a is a value from 0.1 to 3.

13. A magnetic material as claimed in claim 12 wherein a+b ? 3.

14. A magnetic material as claimed in any preceding claim wherein Z is nitrogen.

15. A magnetic material as claimed in any one of claims 1 to 13 wherein component Z is a combination of nitrogen with one or more other group VA elements.

16. A magnetic material as claimed in any one of claims 1 to 13 wherein component Z is one or more of P, As, Sb and Bi.

17. A magnetic material as claimed in claim 14 which has the formula Sm2Fe17N2.3 or Sm2Fe17C1.1N1.1 or NdFe11TiN0.8.

18. A process for modifying the magnetic properties of an intermetallic compound comprising at least one or more rare earth elements and the element iron in which the iron is optionally substituted with another element which process comprises heating said intermetallic compound with a gas containing at least one group VA element Z in the substantial absence of oxygen to incorporate the said at least one element Z interstitially into the crystal lattice of the intermetallic compound by a gas-solid reaction.

19. A process as claimed in claim 18 wherein when the intermetallic compound is of tetragonal crystal structure the iron is substituted by any element of group IIIA or group IVA of the periodic table or by a transition metal from another group.

20. A process as claimed in claim 18 or claim 19 wherein the gas is one which produces radicals containing single atoms of the group VA element Z on contact with hot surfaces such as metal or quartz or by exposure to high frequency radiation.

21. A process as claimed in claim 20 wherein the gas is a gaseous hydride of the group VA element Z.

22. A process as claimed in any one of claims 18 to 21 which produces a magnetic material as defined in any one of claims 1 to 17.

23. A process as claimed in claim 20 wherein the group VA element Z is nitrogen and the gas is nitrogen, ammonia or hydrazine.

24. A process as claimed in any one of claims 18 to 23 wherein said intermetallic compound is heated to a temperature not exceeding 650°C.

25. A process as claimed in any one of claims 18 to 24 wherein said intermetallic compound is ground to a particle size of 1 to 50 microns diameter.

26. A process as claimed in claim 25 wherein the said ground compound is heated for up to 8 hours.

27. Use of the magnetic material as claimed in any one of claims 1 to 17 for fabricating a permanent magnet.

28. The use as claimed in claim 27, wherein a magnet is formed by a process comprising the steps of:-a) milling said magnetic material with a metal such as aluminium, copper or zinc or an organic powder or resin b) generating magnetic alignment in the said material by applying a magnetic field and c) heating the milled product to a temperature sufficiently low to prevent decomposition of the magnetic material.

29. The use as claimed in claim 28 wherein in the magnet fabricating process the magnetic material is milled with from 5 to 20 wt % zinc.

30. A permanent magnet comprising a magnetic material as claimed in any one or claims 1 to 17.

31. A permanent magnet comprising a magnetic material which is produced by the process of any one of claims 18 to 26.