Spin reorientation in Tm2Fe14B

Spin reorientation in Tm2Fe14B

~ Solid State Com~nunications, Vol.56,No.2, pp.181-]83, Printed ih Great Britain. S P I N REORIENTATIOt~ IN 1985. 0038-1098/85 $3.00 + .00 Pergamo...

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Solid State Com~nunications, Vol.56,No.2, pp.181-]83, Printed ih Great Britain.

S P I N REORIENTATIOt~ IN

1985.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

Tm2Pe14B

R.L. DavAS A u s t r a l i a n I n s t i t u t e o f Nuclear ScAenoe and Engineering Lutes H e i g h t s Researc~h Laboratories, NSW, Australia 2232

R.K. D a y and J.B. Dunlop CSIR0 DivAsion of Applied Physics, Sy~r~y, Australia 2070

( R o c e i v e d : 17 June 1985 b y R A Cowley)

The magnetAo structures of Tm2FeI4B above and below its span reorlentatlon temperature (a~prox. 316 K) have been detentlned from hlgh resolution neutron pov~er diffraction m e a s u ~ m m n t 8 . W/thin ex~erlmen~L1 acC-uracy all magnetio r a n t s lie In the basal plane at 294 X and along the c - a x i s at 340 K. The Tm and 1~ , ~ e n ~ s are antiparallel.

room temperature to c-axle when the temperature wal r a i ~ above 317 K for Er2FeI4B and 310 K for Tm2Fe14B. We report here neutron powder diffrao~ion detezlainatio~ of the ~ j n e t i c stru~cure8 of T~2Fe14B above and b e l o w its spin reorientation temperature.

The outstanding permanent magnetic properties of NdFeB alloys are associated with the pTesen~e of the ternary Intermetalllc phase of ~ I t l o n Nd2Fe14B. The structure of thl8 pha~e wag first d e t e m i n e d using neutron pov~er diffraction methods [ 1 ] an(] then confirmed by X-ray single cTystal work [2,3]. subsequently there have b e e n further crystal an~ magnetic structure determinations of the i8omorphous c~uq~ound8 PT2FeI4B and D~2re14B, again by neutron powder diffraction methods [4]. The magnetic properties of the R2Fe14B co~pounds# where R = a rare earth, are now fairly well understood [5,6,7]. The iron and R atoms may be regarded as forming two 8ublattice8. The e ~ h a n g e interaction between the R and Fe _m~__nts _ favours a parallel arrangement of moments for the light rare earths and an antlparallel arrange-~nt for the heavy rare earths. The easy direction of magnetl~atlon As determined b y the anlsotropAes of the R and Fe eublattices. The 1~ sublattice has an easy c-axle, as do the rare earth sublattloe8 for R-Pr,Nd,Tb,Dy and Ro. For RmSm,Er and T~, however, the R sublattlce has an easy direction perpendlcular to the o-axle and there 18 competition between the R and 1~ 8ublattl~e anlsotropAe8. Although the magnet1~atlon of the latter three (~mpound8 18 in the basal plane at room temperature [5,6,7], At was a n t i c i p a t e d that the Fe anisotropy would eventually dominate as the temperature was raised towards the Curie temperature. Our 57Fe M~esbauer measurements [ 8 ] on Tm2PeI4B did indicate a ma~jnetlc anomaly at T:316 K and neutron powder dif fraction measurements were carried out in order to investigate the character of this anomaly. While this w o r k was in p r o g r e s s , H l r o s a w a a r ~ sagawa [9] reported results of ~mgnetization measurements on single crystals of Er2Pe14B and Tm2FeI4B. T h e y observed that the easy direction of ma(/~eti~ation 0hanged from basal plane at

2. EXPERIMENTAL DETAILS The alloys were prepared from weighed mnount8 of Fe,Tm and B, all of 99.99% purity. The pure elements were first melted together in a titanlum-gettered argon arc furnace, then sample8 were prepared b y single roller melt quenching in a helium atmosphere t o JFroduae a fine w a i n lultiphase alloy. These samples were wraRped in tantalu~ foil, seale~ in quartz tubes unde= an argon atmosphere and annealed at a teIpeEature of 1000°C foe a period of 48 hours. M e t a l l o ~ a p h l c examination of 8ample8 prepared in thl8 w a y revealed only one or two pe~-ent of a second phase. The m m ~ l e used in the neutron pokier dlffrac~ion investigation weighed 20 grams and was sealed under a helitm atmosphere in a thln-walled vanadium can of 12 m R diamstez. This was rotated inslde a 140 mm diameter thln-wallBd steel can and could he heated by a flow of w a r m air. The data were c o l l e ~ e d at 294 K and 340 K usir~ a w a v e l e n ~ h of 0.1893 ne on the high resolution dlffractome~er at Lutes Heights (described in reference i0, but now with eight counters at 6 o intervals). 3. RESULTS AND D I S C U S S I ~

The neutron powder diffraction p a t t e r n s axe shown in Pig. 1. All observed reflectioru~ were ir~]e1~ on the Teflned te~ragonal Ohemlcal unit cell a-0.872 ran, c~1.193 r~ e ~ - e ~ for the [111] reflection of =-Fe Which was shown by two phase reflhe~mnt [II] to constitute 2% of the s a ~ l e and thereafter e~cluded. The data were analyzed by the Rietveld profile refinement ~tho~ [ 12,13 ] on the assn~q~t ion that the cTF~r~al str-c~ure was

181

182

SPIN REORIENTATION IN Tm2Fe]4B

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Figure 1. cal~ulated ( - - ) and observed ( . ) neutron p o w d e r dlffrac~ion pattern8 for TmlFOI4B at (a) 294 K and ( b ) 340 K with markers i n d i c a t i n g calculated 9eak 3x)8£tiona and d i f feren(Ri plots of Y~LIO-~O~I. The =-Iqi(111) peak at 20~55.6 ° has been ex=Inded.

Im~mor~ w i t h Ndl~el4B [~]. This belongs to spa~e group P42/Im, with four formula units (Tml~e14B) per unit ~ell, a total of 68 atoms di~rt]~ed over t~o Inequlvalant thulium s i t e s (4f and 4~), S i X iron S i t e s (16 k I, 16 k 2, 8J1, 8~2 , 4e and ~:), and one boron site ( ~ ) . Pazmm~sz~ refined were the Instru~0ntal ixIEslll~ars of ik3ale factor, 2 - ~ h e ~ a SeZO, p e a k asym~:ry and t h r e e h a l f w i d t h p a r s e C : e r e , and st~u~al L~zsmoters of fourteen atomic ~ o o ~ I n ~ t e s , 4~eo 1 ~ t t i ~ conlVcant8 and, in the final re flnem~nt, eight m a g n e t i c : moment o~l~ononts, The 9~Iferre~ orientation p a r t i e r I t m m ~ l i t t l e d a p a r ~ u r e from r ~ s s and w s s I~Io~. & fIJm~ negative overall temgerature f l ~ o ~ of -1.46 wsl i n o l ~ in the reflnements to ai~roxlmE~e the effect of the ex~erlmlntally ~e~e=lllne~ l~l~la attenuation [14]. At both tml~ez~tuEes the indlVi~Ual atola ilK~opio •A m ~ e r a t u r e factors (B i n T a b l e I ) w e r e f ~ at the valuas given in reference 3 for Nd2Fe14B. The nu¢leaz l~atterlng l e ~ used were 7.05fro f o r Tm, 9 . 5 4 ~ f o r 1~ a n d 5 . 3 5 ~ for B [15] ~ile ~J~ magnetic f o r m f a c ~ r e f o r Tm a n d Pe taken from referenoe8 16 and 17, :~epectlvaly.

Vol.

56, No. 2

W i t h 54 m a g n e t ~ ~ per unit nell l a s s t h a n 8 , o f t h e ~x~41 i ~ i ~ of ~up~e~t~ ocigin we c o n f i n e d our air, ration to a spin configuration in ~ 1 ~ ~ t ~ m o m n t s on crFstallo~aphically equlvalent s i t e s were e q u a l in both magnltude and d l r e o t l o n . Trlal refinm~nts allowed the d t ~ of the mm~nt8 t o v a r y w i t h i n t h e a - ¢ and b m m l p l a n e s . This resulted in f s r r ~ t E structures, w i t h Tm and r e moments a n t t p a r a l l e l , in the basal plane a t 294 K and a l o n g t h e c - a x i s a t 340 K. The tilt of individual rants away f r o m t h e s e di~ion8 was less than, o r comparable w i t h , the s t a t i s t i c a l accuracy o f detsrmlnatlon o f the a n g l e s w h i c h w a s t 30 ° i n t h e w o r s t e a s e . In the f i n a l ~eflmment8 r a n t s were constralned t o be a l o n g the a - a x l s ( ~ ) in the basal plane f o r t h e 294 K d a t a a r ~ a l o n g t h e c - a x i s ( g z ) f o r the 340 K data w i t h no 81gnlffloant reduotlon i n the q u a l i t y o f the f i t . F i n a l 8 t r u c t u r n l values are l i s t e d i n Table I . The atomic coordinate8 a t 294 K a n d 340 K aze in ew~ellerrt agreement with each o~her w i t h the v a l u e s r e p o r t e d f o r DyZ~e148, t h e o n l y o t h e r h e a v y r a r e e a r t h c~=~=ound s t u d i e d [4J. The d i f f e r e n t Fe momen~e o b t a i n e d f o r t h e ( 4 e ) and ( 4 c ) s i t e s a r e i n c o n s i s t e n t w i t h t h e 57~e ~sSbausr m e a s u r e m e n t on T m l F e l 4 B [ 8 ] and on o t h e r R2Fel4B - - t e r l a l s [28,29]. However o u r f i n d i n g t h a t t h e Fe ( ~ 2 ) moment i s l a r ~ e r t h a n the l~(Jl), ~ e ( k 1) a n d F s ( k 2 ) r a n t s Is in agrem~nt with other work [4,20]. Constrained refinement with both thulium mom0nts equal and all SiX iron r a n t s equal did n~ give as good a fit to the data, but the m01mnt values were close to the weiqhte~ averages of the individual values given in Table r and shown here in square brackets for comparison ? i s . ~T,)--Z.V(Z)[-Z.5(~)], at 294K, and ~(T~)--~.?(2)[-1.?(~)], at 34OK.

~e)-l.?(~)[1.9(2)] ~(~e)-l.5(1)[1.5(~)]

The calculated total moment per Tm2Fel4B formula u n i t i s 22.2~B at 294 K and 18~2~B a~ 340 K which may b e ~ p a r e d w i t h a v a l u e o f 22~B from ~tl~ation measurements a l o n g t h e e a s y ( l O 0 ) d l r e o t l o n o f a s i n g l e c r l m ~ a l o f TmlFeI4B at 296 K b y H l r o s a w a and s a g a w a [g]. 4. C011CII/SIOll Our neutron diffraction -malurements SU|M~ort a model of TmlPO14B aJ a bal~l plane f e r r ~ at 294 K and a o-sxls farr~Jilgnst at 3 4 0 K. Bo~mver, we are unable to ex~Indo the poSSlbili~y of tilting of individual ~e or T~ mmalnts at either temperature.

We w i s h t o t h a n k Dr I(.M. E ~ o f NU~C:, l~aw He~Whts ~ Edd:o~ttorie8 for mmful d i m m ~ s i o n 8 and B. Lane o f t h e CSZR0 D i v i s i o n o f PqRplie~ P h y s t O l f o r mlliltml~l with eleGtron ub~-~)seopie mumination of our mm~les.

Vol. 56, No. 2

SPIN REORIENTATION

IN Tm2FeI4B

183

Table I. Final refinement values for the positional paraPet°re, x,y,z, lattice constants a,c and magnetic moments p and P z along a and c respectively, of Tm2FeI4B at 294 K and 340 K. Estimated standard x deviations are given in brackets. R( nuc )~£ l Inuc( ob8 )~Inuc( talc ) I £1nuc( oh8 ) P~ maq )~£ I rmag( ob8 )~Ima~( calo ) I/£1mag( obs ) . R( prof )=£ Iy( ob8 ) ~ y ( talc ) I / £ IY( obs ). Residual=Ew( y( obs )-~y( calc) }2/(N_p4cf ) where y are the point intensities, I the (separated) peak areas, c is the scale factor, N the number of observations, P the number of parameters, Cf the number of constraint factors and w the statistical weight. 294 K x

Atom

Tm(4f) Tin( 4~ ) Fe(16kl) Fe(lSk2) Fe( 831 ) Pe(8~2 ) re(4e) ~(~) B(4g) a c

0.2672(13) 0.1436(12) 0.2249(8) 0.0368(5) 0.0962(7) 0.3153(6) 0.5 0 0.3712(17) O. 87203( 9 ) 1.19414( 12 )

~(n.c) RC~q) R(~o~ )

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1.92

y

x --x 0.5642(6) 0.3597(7) X x 0.5 0.5 -x

340 K z

/~x

0 -2.6(2) 0 -2.4(3) 0.1273(4) 1.9(2) 0.1740(5) 1.7(2) O. 2 0 0 ? ( 5 ) 1 . 7 ( 2 ) 0.2449(6) 1.?(2) 0.1166(10) 0.9(3) 0 3.0(2) 0

B

0.64 0.54 0.55 0.56 0.54 0.59 0.59 0.56 1.40

x

y

0.2675(12) O. 1 4 5 9 ( 1 2 ) 0.2246(7) 0.0358(5) 0.0960(6) 0.3159(6) 0.5 0 0.3716(16 ) O. 8'7206( 9 ) 1.19450( 11 )

x x 0.5650(6 ) o.3692(6) x x 0.6 0.5 --X

z

0 0 O. 1 2 7 2 ( 4 ) o.~61(4) 0.2006(4) 0.2460(5) 0.1155(8) o

Pz

-2.1(3) -1.~3) 1.4(2) 1.1(2) 1.8(2) 1.9(2) 0.~3) 2.9(=)

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n~ort

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1766 (1981).

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