- Email: [email protected]
The effect of absorbed hydrogen on the magnetic properties of DyFe11Ti
ELSEVIER
Journal of Magnetismand Magnetic[¢laIedals150(!995) 393-398
The effect of absorbed hydrogen on the magnetic properties of DyFe tlTi A. Apostolov, R. Bezdushnyi, R. Damianova *, N. Stanev, I. Naumova F~r~!~. ay Physics. Department of Solid State PhyMcs. Sofia Universi~, Sofia 1126, Ilul~aria
Received26 N o v ~
i993; in fin;! ,,~..~.~Aream: 14 March 199S
Abstract
The effect of absorbed hydrogen on the structural and magnetic properties of DyFeHTi has been investigated. The compound DyFel,'l'i was hydrogenated m the composition DyFeuTiH ~, where x ranges from 0 to i.5. '1~ ~.,orot'on hydrogen pressure-composition isotherms of the hydrides indicale solid solution behavior. Hydrogen absorption does not change the tetragonal structure of the host compound but is ~mompanied by an expansion in unit cell volume. Maga~ie characteristics of the host material and its hydrides (measured in situ in a hydrogen ammsphere) were inve~ig;ged over tree temperature range 77-700 K and at applied fields up to 12 kOe. Ulmn hydrogen~aionthe magnetic ordering temper'~ure as well as the spin-reorientation teraperatums Ts~are observed to increase. Moreover, the incorporation of hydrogen ~ to an expansion of the temperature range in which the ma~letic anisotropy of the rare-earth subl~ttice clominates over t ~ of lhe iron sul31attice.
1. Introduction The search for novel magnetic materials has led to the discovery of RFetz_~Tx ( R = r a r e earth, T = stabilizing element) ternary compounds. Among them the RFettTi system has attracted considerable attention. It was believed ',hat the R - F e - T i type of ternary compounds might serve as alternatives to existing permanent magnet systems [1,2]. The R - F e - T i series turns out to be very interesting from a fundamental point of view, because it provides a very nice opportunity to study the crystalline-electric-fieldeffect as well as the spin reofienration transitions induced by temperature and applied magnetic field in R-intermetallic compounds [3].
" Correspondingautlwr.
The expetimcmal results show that among the R F o . T i series only SmFetiTi appears to be an altraclive candidate for permanent magnet fabrication [4,5]. These materials have sufficiently high values of T~ for practical application, ranging from 480 to 610 K [6,7]. In connection with the above, a matter of interest is the investigation of the influence of hydrogen on the magnetic behavior of RFenTI type comlmunds. As it is known, many rare-ea.,~ intemmmllk: compounds undergo considerabIe ch~-,;;ge$ i~ their magnetic properties upon hydrogenation [8]. Though the intensive study of the R(Fe,T)t:: system began in 1987, there are only a few papers concerning h y d ~ sen abs~tvdon in these compounds [9-12]. The investigation of the first RFeltTi type hydrides (R = Y, Sin) showed that these compounds readily absorb hydrogen a ~ form hydrides at room
030~8853/95/$09.50 © 1995 E l . ~ r Sc~n~ B.V. All rightsr¢~¢~cd
SSDI 0304-8853(95)00286-3
A ~,z,
et aL/.loarnat v f M a g ~
B e s ~ aH the changes that occur in fl~e eE'c~ s~mc~ure of ~ confluent elements and ~n ~e ~ag~:: ~ of these c o m p o m ~ upon h ~ r o g e a ~ o a , ~he raos~ ~mv~ting ~¢sull that has ~ ~s ~ hydro~n entering the SmTiFean z ~ Y'I'~e,a ! ~ z e s s~gn~fw,antly ~ncteases the over~ y field. Da~ ~ m the hyd~des of R T ~ e . im~n-~,lalt~c ca~pounds contaimng heavy r~'e-em'~ ~ in [9913 [g2]. The hydrides were formed by exposing me ho~ metats at room temperarare ~o ~,~gh-pur/~yF.ydrogen at about 40 a~n. For the ~,a",zclu~ and m ~ f i c ~ n ~ the samples ~-¢¢e hy~m~ena~c~ a.~ d'~en poisoned using SO z to /~rever~ hy~"ogen escape. In ~ s paper we t~esent the results of an investiof fire magnetic properties of DyFenTiH,
a~l Magnetic Materials 150 f 19951 393 -398 •
,
.
.
.
,
.
,
T
•
,
•
,
,
.
~-~e~m.
chaxac~enzes c0.u- inve~gadons is that the measureme~ts, in contrast to the those wesented in the review paixr [12], were carried out in situ in a hydroge~ am~o~phere and the hydrogen concentration was k ~ ; within the required acgtwimy. To date there ~ no quantitative results on the lnf~uencc of hydrogen c~ncentration upon the magne~c behavior of compounds exh/b~fing spin-reorienL.~k-m t,-ar~s~fions, The presem investigation appears be f~-s~ s~u~y of ff~Js r~w subject area and reports • ~e ra~gn~c c.har~tefimics of DyFe,mTiH ~ hydrides as a fu~cm~a of file hydrogen concentration. ~ e present paper is also a continuation of oar previous investigations and is iniended to extend our prei]mie~--y dz~z [13].
2= E
~
de~s
The DyF%tTi polycrys~l]ne samples were prepared by am meliing of dm constitgent materials (Dy. Fe and TD of 99.9% parity under a purified Ar ~ t ~ _ ~ r e . In order to achieve beuer homogeneity, the buaons were remelted four times. Bolh #~tse and s~ag~..1 ana~ys~swere p e r f ~ at room temperat~e ~ , + ~ n [ y v ~ powders with the use of a ~ - a - d X-ra2," diffractomc~r and Co K,, ¢adialion. X-ray ar~lys[s showed that DyFe,~Ti I%lsa telragonai T~A~n:, =ype smic~ure. A sraal] amoum of Fe ireFul l y phase was ~.c-rvexi in ~ a~loy. The quantity of
DyFe~l 0
t .
I~
0.1
O.Ol o . o ' 0'.2 ' 0.4 Hydroqen
• - , 0~ Tffi473 K • ..-, T=294 K
0.6 0.8 1.o 1.z t., concentration, ot.H/f.u.
1.6
~g.
!. Hydmgvnabsorptionpt~um-conc~ntraZionimth~nns for the DyFe n=Ti compo'a~.
this impurity phase was estimated by X-ray and magnetic phase analysis to be about 3%. A volumetric type experimental set-up was used to study the hydrogen absorption-desorption woperties at hydrogen pressure up to 13 arm in the temperatare range from 300 to 700 K. The magnetization measurements were carded out in the temperature range 77-700 K in permanent magnetic fields up to 12 kOe using an apparatus for magnedc measurements under gas pressure up to 12 arm, consffucted on the principle of a vibratlng-sample magnetometer [14].
3. Results and discussion The hydrogen absorption pressure-concentration is~herms for DyFetnTiH , are shown in Fig. I. In the pressure and temperature ranges investigated no plateau regions are observed to be formed in tbe DyFellTi-hydrogen system. This indicates that only the solid solution is possible. These solid solutions are stable at room temperature at hydrogen concentrations not less than x -- 0.92 and for this reason we use the common name 'hydrides' in the present work. The X-ray diffraction patterns showed that both DyFeuTi and its hydrides have a ThMn12 type structure, i.e. hydrogenation does not change the structural type of the compound. The lattice parameters for a reinesentative hydride and its host material are p~seated in Table I. It is seen that the introduction
A. Apostolov e~aL /.io~wnal of Magnetism and Magnetic Male~qals 150 (1995) 393-J98 Tablc I Lattice pararr~tcr data for 1IraDyFc,=Ti and CeF¢,Ti [Ill cony pounds and their hy~h'ides
Compound DyFe.Ti DyFe,~TiH,.92
a(A) 8.505 8.517 8.532 8.564
CeF¢=,Ti CeFe, =Till a..
4O0
•
,
.
,
.
,
.
,
,
,
.
,
r
c (/~) u (A:~) c/a 4.7c~0 346.485 0.563 4.796
347.898
4.778
347.814 0.560 351.968 0.560
4.799
0.563
**De, lit
of hydrogen to the lattice leads to an increase in the lattice parameters and in turn the unit cell volume without a change in tbe ratio c / a . A similar behavior was previously observed by us for the CeFe,TiH, series t i l l and the results are included here foe comparison (Table 1). Representative curves of magnetization 0" versus temperature behavior of DyFeHTiH . in the temperature range from 78 to 700 K are shown in Fig. 2. In the whole temperature range studied the hydrogen concentration was kept within an accuracy of 15% by maintaining an appropriate pressure. The curves for hydrides with hydrogen concentration x < 0 . 6 hydrogen atoms per formula unit e;.hibit well defined wide maxima restricted by two temperatures T~t and T~,2, which characterize the spin-reorientation transitions~ The transition temperatures were identified with the inflection points of the curve of o-(T) (see the insert in Fig. 2). The values of T~= and Tsr2 depend strongly on the hydrogen concen~'a-
80
,,,
.
,
.
,
-
,
[ DyFe~,TiH, / H = 2 kOe
_
i= J
.....
.
,
.
,
'.
,
Hydrogen concentration, aLH/f.u. Fig. 3. Shift of the spin-moriemmion tem~r~ttw.s with ~ hydfoge~eo~.c~mtien.
lion. With increasing hydrogen concentration. T~ and T~,z shift towards higher temperatures (Fig. 3). The magnetic oedcring tempera~res of th~ DyFe,Ti hydrides are shown in Fig. 4. As is evidenced in the figure., T~ is strongly dependent on the hydrogen composition. On hydrogen absorption d'm value of Tc of the host compound is observed to increase from T¢ = 530 K for x = 0 to Tc = 575 K for x = 1.4. This increase is rela~l to an expansion of the lattice parameters and may be am'ibuted to a redaction of the negative exchange interactions between Fe ions. The hysteresis loops measured at 77 and 300 K in magnetic fields up to 12 kOe for a ~ n ~ v e hydride and the host material are displayed in Fig. 5. Our experimental results for the DyFeuTiH~
.''-
,4
c
°0
Tt/ 100
x = 0,6
tg
200 ~ 400 5o0 Temperoture, K
~
?
Fig. 2. Tem~mmre dependence of Ihe magueliza~a of some DyFex,TiH~ hydrides. In the insert, d~e derivative ~¢/~T x = 0.45. obl~inedon the basis of the experimental data t~n8 the least-squares
o'(T) curve.
method.
It
has
extmma
in
the
infl~Joft
p o h ~
of
Ii1¢:
Hydrogen concentra'dan, aLH/f.u. F~8. 4. TI~ ~ p ¢ ~ c ~ c of" ~
~
the DyFe=,~ h y d e i ~ on the ~ n
~nng
~mp¢~
¢aecee~tioL
~_
A. ~'~w*#~w et J-/Yeurrdzl af Mognetism ond Magnetic Materials 150 (1995~ .193-398
o
- -
at low temperatures and that of Fe being dominant al higher ones [16]. The overall effective anisotropy cm~stant for both subtattiees is defined as:
Oy~e.T~
K,~rt = Km + glFe,
~
--~
.
12
T =
2g,;
~c
b) '
~ " ~
1~
F~g ~ H~-~ere~ t~,,~ ~eas~,-t-d,x 77 and 300 K for a reprcsen~'d~'e hydride a,-~ ~hcho~ n~cttM.
comp~3unds raise a question about file ~p~.ndenceof ~ . : r . and T~ on the hydrogen concentcation as ~veii ~ ~he ~emperature behavior of the total magnetization upon hyd~-ogenalion. da~a for the iX~s~mmeri~l oblained previously [6.gA3.]5At] show that in the irttermetallic comp ~ a d DyFeIITi d~e Dy and Fe sub[auices cou#e ~alipara]lel. and ~he magnetic moments of both subkmices at T > T~z are orien,.ed along the q,O01.~di~,~cdc,n. T'rm Fe sublalt[ce exhibhs uniaxiai anisoirop:r at atl temper&ures, white that of the Dy sublattice depends on the tetragonal symmetry of tlm latter. The anis~tropy energy of the Dy sublatfiee (W~R) is g~,~cn by: W,R = K m s i n : 0 + Kza s [ n ~ + K'_,a sinaO cos 4~, ~'here K~R, KzR and K~a are the anisotropy cons ~ s of the rare-earth sublatfice, and ~ and $ refer ~o r2~eoriemation of the magnedzatiun direction. The ~empcralure depet'Mcr~ce of the rare-earth WaR and tr~sifi~n metal W~F~ sublattiee anisotropies are gener~iy quite differem, tha~ of the Dy being dominant
where K m and Ktr ¢ have opposite signs. The change of K~zf with temperature depends on K m and Kt~~. It may be supposed that 7",:_ is the temperature at which /¢.~fr changes its sign and the Dy snbtattice magnetization deviates from the c-axis. As a resvll, a spin-reorientatiGn transition axis-to-cone (referred to decreasing temperature) lakes place. For the DyFe,tTi alloy the value of this transition temperature corresponding to the inflection point of lhe curve of o'(T) is nearly 200 K. With decreasing temperature the contribution of K m and K', R to the total anisotropy energy becomes more significant. It leads to a change in the magnetic structure of the investigated compound and at T = 80 K the easy magnetization direction becomes perpendicular to the c-axis, i.e. a spin-reorientation transition coneto-ptane takes place [16]. As mentioned above hydrogen absorption does not change the symmetry of the investigated hydrides. That is why the anisotropy energy of the hydride and the host compound is given by one and the same expression. The hydrogen entering the lattice leads to the following mutually connected effects: firstly, a reduction of the negative exchange interactions between Fe ions, which manifests itself as raising the value of T~; secondly, a change in the interrelation between the rare-earth and Fe sublattice anisotrop.;es, which results in shifting T.,~l and T~,z towards higher temperatures. The investigation of CeFe t tTill., [ 17] showed that the change in ~he properties of the hydrides of this ',ype containing Ti is due to the modification in the localization of the 4f electrons and in 3d(Fe)-4ffR) hybridization. In Ref. [18] the neutron diffraction experiments performed on the ErFe m.5M°I.~D, [ i 8], with ThMn r, type structure showed that the site occupied by the interstitial element (D) is an oetabedral hole which has rare-earth atoms and 8j iron atoms as nearest neighboars. It may be supposed that in our ease the hydrogen atoms entering the host compound DyFettTi (belonging to the same type of structure)
A.
Aposrolot, et at./ Journal of Magnetism and Magnetic Materials 150¢!995) 393-398
occupy, in all probability, the same positions. After hydrogenation, the uniaxial 3d transition metal anismropy is larger and controls g'~ increase of anisotropy field at high tempet~atures, Hydrogenalion leads to an increase of the planar contribution to the anisotropy from the R site. These facts can explain the considerable change of T.~ in the hydrides investigated, its shift to higher temperatures. The influence of hydrogen mainly on the Dy sublauice anisotropy energy can be expected became the energy needed for the formation of the Fe-H pair exceeds by several hundred de~ees the energy of the lbrmation of the Dy-H pair. According to our experimeutal data, at hydrogen concentration x > 0.8 the temperatures T,,~ and T,~z practically coincide with T~, i.e. spin-reorientation transitions disappear. It is believed that in this case the magnetic moments of the Dy ions at T_< T¢ lie in the (00i) plane. This means that the incorporation of hydrogen leads to an expansion of the temperature range in which the magnetic anisolropy of the rareearth sublattice dominates over that of the iron sublattice. The considerable difference between our results and those of Wallace and co-workers [t2] (i.e. Ihe increase in the temperature of the spin-reorientation transitions and comparatively small increase in Tc on hydrogenation) lies, in all probability, in the method of keeping up the hydrogen concentration in the samples investigated, used in Ref. [12]. Poisoning, using SO~ to prevent hydrogen escape, does not guarantee the maintenance of hydrogen coacena'alion in the investigated temperature range. The influence of small fluctuations of Fe and Ti content oa the magnetic anisotropy of DyFe,TiH~ cannot he excluded either. Hydrogen's change of the overall bulk aniso~opy due to hydrogen is found by comparison of the o-(H) curves measured at different temperatures (Fig. 5). In the low-temperature range the magnitude of the overall bulk anisolropy is higher than that observed at room temperature. The increase of the magnetic anisotropy at low temperatures manifests itself also as an increase in the value of the coercive force. The increase in total magnetization upon hydrogenation as well as in ehe average Fe moment was also observed in the hydrides investigated. The mag-
397
netic behavior of the hydrogenated DyFel~T~ cc~apound can be ascribed to the following effects: firs~y, to varimions of the intcmtomic distances a~L secondly, to strengthening of the 3d-3d and weakening of the 4f-3d exchange interactions. It shouht also be noted that the increase of the average Fe mom¢~ is typical of rare-earth - 3d transition-metal ing'mm~lic compounds and is undoubtedly an e l ~ / c effect. Presumably there is a transfer of etec~ons f~,ra H to 3d subbands resulting in an increas~ of ~he F¢ sublattice magnetic moment and hence in azt increase in the overall magnetic moment.
4. Co~'lusion We have synthesized and studied the DyFe.Ti intennetallic compound and its hydrides. For the first time the quantitative dependence of the magnetic ordering temperature and the spin-reo~ien~ion temperature on the hydrogen concentration ~ave been investigated. R has been found that both T~ and T~ increase monotonically with increasing hydrogen concentration. The experimental results ob~ncd arc useful for the better understanding of the changes in the magnetic anisotropy of both the iron and raceearth sublattices as well as in the exchange imeractions upon hydrogen absorption.
Acknowledgment This work was supported by the National Foundation for Fundamental Research, grant No TN 105.
References Ill K. Ob~shi, T. Yakoym~aa, R. Sugi and Y. T~m-a. IEEE Trans. Magn. 23 (1987) 310t. 12l K.H.L Bcschow, L AppL Phys. 63 (1958) 3130. [3] X.C. Kou. T_S_ ~ . ~ , R. Gt'w~jnger, H.R. K i r ¢ ~ y r , X. Li F.R. de Boer. ~Phys.Rev. B 47 (1993) 323l. [4] J. Ding and M. Ros~:rdJcrg,J, Magn. Magn. M ~ r . 83 ( [ 9 ~ ) 257. [5] L. Sch-o|tz. K. Schnhzk¢ and J. Wccker, J. Magn. Me,g~. Mler. 83 (1990) 254. [6] Bo-Ping Hu. Hong-S,'~aoLi. J.P. Gavigaa ~ d LM.D. Coey. J. Phys. Coadea~ M~tcx ! (t'~89) 755.
39g
A_ .4r~-~,to,_,e', al. / lo~maf of Magnetism ~
[9~ L'~'. Y~,r~. W.F__W a ~ _ J. Less-Comva~ M ~ 149 (1989) 371. [~,Cq S. ~ . S. M ~ D. F r , ~ t m ~ R l'Heri6~r. Z -~3x ~ N~ 163 (t9~9) i6l.
Magr,etic Materials 150 f 1995~ 393-398
[14] [15] [16]
[t7] [~2t LY_ ;Z~a~. S.G S , ~ ,
W.E Wa~h-~e anc~ S.g. MaliL 6~ [18]
SP33 ~ C , ~ ~¢gk~ Univctsi~. P/C,shurgh, PA). ~3] g .%~_zcqov.R. B e . ~ k ' ~ , V. Kr'~¢~ev.N. Sma~v m~d M.
Smycehev. A~u~rc de i'Uniuersite de Sofia St. Kliment O ~ k i . vol. 86 (1994) 129. R. Bezdushnyi a ~ N. Sto~nev,to bc published, Bo-piag Ha. Hong-Shuo Li. l.P. Gavigan, and J.MD. Coey, Phys. Rev. B 41 (1~1)) 2221. E.B. Bo~ich. BM. Ma. L.Y. Zhang, F. Poum,-ian~S.K. MaliL S.G. Sanlu~ and W.E. Wallace, J. Magn. Magn~ Ma~er. 78 (1989) 365. J. L'-'l~boy.A. M~r~lli and L Bozukov, Jpn. J. Appl. Phys, 32 (1993) ,Suppl. 32-2, 758. E. Towey. M 8~m,-mn, D. Fmcham S. Miraglia, I.L. Sou~yroux. D. Gignoux, E. Palaeios. IEEE Tmns, Magn. {1994), tu be published.
Journal of Magnetismand Magnetic[¢laIedals150(!995) 393-398
The effect of absorbed hydrogen on the magnetic properties of DyFe tlTi A. Apostolov, R. Bezdushnyi, R. Damianova *, N. Stanev, I. Naumova F~r~!~. ay Physics. Department of Solid State PhyMcs. Sofia Universi~, Sofia 1126, Ilul~aria
Received26 N o v ~
i993; in fin;! ,,~..~.~Aream: 14 March 199S
Abstract
The effect of absorbed hydrogen on the structural and magnetic properties of DyFeHTi has been investigated. The compound DyFel,'l'i was hydrogenated m the composition DyFeuTiH ~, where x ranges from 0 to i.5. '1~ ~.,orot'on hydrogen pressure-composition isotherms of the hydrides indicale solid solution behavior. Hydrogen absorption does not change the tetragonal structure of the host compound but is ~mompanied by an expansion in unit cell volume. Maga~ie characteristics of the host material and its hydrides (measured in situ in a hydrogen ammsphere) were inve~ig;ged over tree temperature range 77-700 K and at applied fields up to 12 kOe. Ulmn hydrogen~aionthe magnetic ordering temper'~ure as well as the spin-reorientation teraperatums Ts~are observed to increase. Moreover, the incorporation of hydrogen ~ to an expansion of the temperature range in which the ma~letic anisotropy of the rare-earth subl~ttice clominates over t ~ of lhe iron sul31attice.
1. Introduction The search for novel magnetic materials has led to the discovery of RFetz_~Tx ( R = r a r e earth, T = stabilizing element) ternary compounds. Among them the RFettTi system has attracted considerable attention. It was believed ',hat the R - F e - T i type of ternary compounds might serve as alternatives to existing permanent magnet systems [1,2]. The R - F e - T i series turns out to be very interesting from a fundamental point of view, because it provides a very nice opportunity to study the crystalline-electric-fieldeffect as well as the spin reofienration transitions induced by temperature and applied magnetic field in R-intermetallic compounds [3].
" Correspondingautlwr.
The expetimcmal results show that among the R F o . T i series only SmFetiTi appears to be an altraclive candidate for permanent magnet fabrication [4,5]. These materials have sufficiently high values of T~ for practical application, ranging from 480 to 610 K [6,7]. In connection with the above, a matter of interest is the investigation of the influence of hydrogen on the magnetic behavior of RFenTI type comlmunds. As it is known, many rare-ea.,~ intemmmllk: compounds undergo considerabIe ch~-,;;ge$ i~ their magnetic properties upon hydrogenation [8]. Though the intensive study of the R(Fe,T)t:: system began in 1987, there are only a few papers concerning h y d ~ sen abs~tvdon in these compounds [9-12]. The investigation of the first RFeltTi type hydrides (R = Y, Sin) showed that these compounds readily absorb hydrogen a ~ form hydrides at room
030~8853/95/$09.50 © 1995 E l . ~ r Sc~n~ B.V. All rightsr¢~¢~cd
SSDI 0304-8853(95)00286-3
A ~,z,
et aL/.loarnat v f M a g ~
B e s ~ aH the changes that occur in fl~e eE'c~ s~mc~ure of ~ confluent elements and ~n ~e ~ag~:: ~ of these c o m p o m ~ upon h ~ r o g e a ~ o a , ~he raos~ ~mv~ting ~¢sull that has ~ ~s ~ hydro~n entering the SmTiFean z ~ Y'I'~e,a ! ~ z e s s~gn~fw,antly ~ncteases the over~ y field. Da~ ~ m the hyd~des of R T ~ e . im~n-~,lalt~c ca~pounds contaimng heavy r~'e-em'~ ~ in [9913 [g2]. The hydrides were formed by exposing me ho~ metats at room temperarare ~o ~,~gh-pur/~yF.ydrogen at about 40 a~n. For the ~,a",zclu~ and m ~ f i c ~ n ~ the samples ~-¢¢e hy~m~ena~c~ a.~ d'~en poisoned using SO z to /~rever~ hy~"ogen escape. In ~ s paper we t~esent the results of an investiof fire magnetic properties of DyFenTiH,
a~l Magnetic Materials 150 f 19951 393 -398 •
,
.
.
.
,
.
,
T
•
,
•
,
,
.
~-~e~m.
chaxac~enzes c0.u- inve~gadons is that the measureme~ts, in contrast to the those wesented in the review paixr [12], were carried out in situ in a hydroge~ am~o~phere and the hydrogen concentration was k ~ ; within the required acgtwimy. To date there ~ no quantitative results on the lnf~uencc of hydrogen c~ncentration upon the magne~c behavior of compounds exh/b~fing spin-reorienL.~k-m t,-ar~s~fions, The presem investigation appears be f~-s~ s~u~y of ff~Js r~w subject area and reports • ~e ra~gn~c c.har~tefimics of DyFe,mTiH ~ hydrides as a fu~cm~a of file hydrogen concentration. ~ e present paper is also a continuation of oar previous investigations and is iniended to extend our prei]mie~--y dz~z [13].
2= E
~
de~s
The DyF%tTi polycrys~l]ne samples were prepared by am meliing of dm constitgent materials (Dy. Fe and TD of 99.9% parity under a purified Ar ~ t ~ _ ~ r e . In order to achieve beuer homogeneity, the buaons were remelted four times. Bolh #~tse and s~ag~..1 ana~ys~swere p e r f ~ at room temperat~e ~ , + ~ n [ y v ~ powders with the use of a ~ - a - d X-ra2," diffractomc~r and Co K,, ¢adialion. X-ray ar~lys[s showed that DyFe,~Ti I%lsa telragonai T~A~n:, =ype smic~ure. A sraal] amoum of Fe ireFul l y phase was ~.c-rvexi in ~ a~loy. The quantity of
DyFe~l 0
t .
I~
0.1
O.Ol o . o ' 0'.2 ' 0.4 Hydroqen
• - , 0~ Tffi473 K • ..-, T=294 K
0.6 0.8 1.o 1.z t., concentration, ot.H/f.u.
1.6
~g.
!. Hydmgvnabsorptionpt~um-conc~ntraZionimth~nns for the DyFe n=Ti compo'a~.
this impurity phase was estimated by X-ray and magnetic phase analysis to be about 3%. A volumetric type experimental set-up was used to study the hydrogen absorption-desorption woperties at hydrogen pressure up to 13 arm in the temperatare range from 300 to 700 K. The magnetization measurements were carded out in the temperature range 77-700 K in permanent magnetic fields up to 12 kOe using an apparatus for magnedc measurements under gas pressure up to 12 arm, consffucted on the principle of a vibratlng-sample magnetometer [14].
3. Results and discussion The hydrogen absorption pressure-concentration is~herms for DyFetnTiH , are shown in Fig. I. In the pressure and temperature ranges investigated no plateau regions are observed to be formed in tbe DyFellTi-hydrogen system. This indicates that only the solid solution is possible. These solid solutions are stable at room temperature at hydrogen concentrations not less than x -- 0.92 and for this reason we use the common name 'hydrides' in the present work. The X-ray diffraction patterns showed that both DyFeuTi and its hydrides have a ThMn12 type structure, i.e. hydrogenation does not change the structural type of the compound. The lattice parameters for a reinesentative hydride and its host material are p~seated in Table I. It is seen that the introduction
A. Apostolov e~aL /.io~wnal of Magnetism and Magnetic Male~qals 150 (1995) 393-J98 Tablc I Lattice pararr~tcr data for 1IraDyFc,=Ti and CeF¢,Ti [Ill cony pounds and their hy~h'ides
Compound DyFe.Ti DyFe,~TiH,.92
a(A) 8.505 8.517 8.532 8.564
CeF¢=,Ti CeFe, =Till a..
4O0
•
,
.
,
.
,
.
,
,
,
.
,
r
c (/~) u (A:~) c/a 4.7c~0 346.485 0.563 4.796
347.898
4.778
347.814 0.560 351.968 0.560
4.799
0.563
**De, lit
of hydrogen to the lattice leads to an increase in the lattice parameters and in turn the unit cell volume without a change in tbe ratio c / a . A similar behavior was previously observed by us for the CeFe,TiH, series t i l l and the results are included here foe comparison (Table 1). Representative curves of magnetization 0" versus temperature behavior of DyFeHTiH . in the temperature range from 78 to 700 K are shown in Fig. 2. In the whole temperature range studied the hydrogen concentration was kept within an accuracy of 15% by maintaining an appropriate pressure. The curves for hydrides with hydrogen concentration x < 0 . 6 hydrogen atoms per formula unit e;.hibit well defined wide maxima restricted by two temperatures T~t and T~,2, which characterize the spin-reorientation transitions~ The transition temperatures were identified with the inflection points of the curve of o-(T) (see the insert in Fig. 2). The values of T~= and Tsr2 depend strongly on the hydrogen concen~'a-
80
,,,
.
,
.
,
-
,
[ DyFe~,TiH, / H = 2 kOe
_
i= J
.....
.
,
.
,
'.
,
Hydrogen concentration, aLH/f.u. Fig. 3. Shift of the spin-moriemmion tem~r~ttw.s with ~ hydfoge~eo~.c~mtien.
lion. With increasing hydrogen concentration. T~ and T~,z shift towards higher temperatures (Fig. 3). The magnetic oedcring tempera~res of th~ DyFe,Ti hydrides are shown in Fig. 4. As is evidenced in the figure., T~ is strongly dependent on the hydrogen composition. On hydrogen absorption d'm value of Tc of the host compound is observed to increase from T¢ = 530 K for x = 0 to Tc = 575 K for x = 1.4. This increase is rela~l to an expansion of the lattice parameters and may be am'ibuted to a redaction of the negative exchange interactions between Fe ions. The hysteresis loops measured at 77 and 300 K in magnetic fields up to 12 kOe for a ~ n ~ v e hydride and the host material are displayed in Fig. 5. Our experimental results for the DyFeuTiH~
.''-
,4
c
°0
Tt/ 100
x = 0,6
tg
200 ~ 400 5o0 Temperoture, K
~
?
Fig. 2. Tem~mmre dependence of Ihe magueliza~a of some DyFex,TiH~ hydrides. In the insert, d~e derivative ~¢/~T x = 0.45. obl~inedon the basis of the experimental data t~n8 the least-squares
o'(T) curve.
method.
It
has
extmma
in
the
infl~Joft
p o h ~
of
Ii1¢:
Hydrogen concentra'dan, aLH/f.u. F~8. 4. TI~ ~ p ¢ ~ c ~ c of" ~
~
the DyFe=,~ h y d e i ~ on the ~ n
~nng
~mp¢~
¢aecee~tioL
~_
A. ~'~w*#~w et J-/Yeurrdzl af Mognetism ond Magnetic Materials 150 (1995~ .193-398
o
- -
at low temperatures and that of Fe being dominant al higher ones [16]. The overall effective anisotropy cm~stant for both subtattiees is defined as:
Oy~e.T~
K,~rt = Km + glFe,
~
--~
.
12
T =
2g,;
~c
b) '
~ " ~
1~
F~g ~ H~-~ere~ t~,,~ ~eas~,-t-d,x 77 and 300 K for a reprcsen~'d~'e hydride a,-~ ~hcho~ n~cttM.
comp~3unds raise a question about file ~p~.ndenceof ~ . : r . and T~ on the hydrogen concentcation as ~veii ~ ~he ~emperature behavior of the total magnetization upon hyd~-ogenalion. da~a for the iX~s~mmeri~l oblained previously [6.gA3.]5At] show that in the irttermetallic comp ~ a d DyFeIITi d~e Dy and Fe sub[auices cou#e ~alipara]lel. and ~he magnetic moments of both subkmices at T > T~z are orien,.ed along the q,O01.~di~,~cdc,n. T'rm Fe sublalt[ce exhibhs uniaxiai anisoirop:r at atl temper&ures, white that of the Dy sublattice depends on the tetragonal symmetry of tlm latter. The anis~tropy energy of the Dy sublatfiee (W~R) is g~,~cn by: W,R = K m s i n : 0 + Kza s [ n ~ + K'_,a sinaO cos 4~, ~'here K~R, KzR and K~a are the anisotropy cons ~ s of the rare-earth sublatfice, and ~ and $ refer ~o r2~eoriemation of the magnedzatiun direction. The ~empcralure depet'Mcr~ce of the rare-earth WaR and tr~sifi~n metal W~F~ sublattiee anisotropies are gener~iy quite differem, tha~ of the Dy being dominant
where K m and Ktr ¢ have opposite signs. The change of K~zf with temperature depends on K m and Kt~~. It may be supposed that 7",:_ is the temperature at which /¢.~fr changes its sign and the Dy snbtattice magnetization deviates from the c-axis. As a resvll, a spin-reorientatiGn transition axis-to-cone (referred to decreasing temperature) lakes place. For the DyFe,tTi alloy the value of this transition temperature corresponding to the inflection point of lhe curve of o'(T) is nearly 200 K. With decreasing temperature the contribution of K m and K', R to the total anisotropy energy becomes more significant. It leads to a change in the magnetic structure of the investigated compound and at T = 80 K the easy magnetization direction becomes perpendicular to the c-axis, i.e. a spin-reorientation transition coneto-ptane takes place [16]. As mentioned above hydrogen absorption does not change the symmetry of the investigated hydrides. That is why the anisotropy energy of the hydride and the host compound is given by one and the same expression. The hydrogen entering the lattice leads to the following mutually connected effects: firstly, a reduction of the negative exchange interactions between Fe ions, which manifests itself as raising the value of T~; secondly, a change in the interrelation between the rare-earth and Fe sublattice anisotrop.;es, which results in shifting T.,~l and T~,z towards higher temperatures. The investigation of CeFe t tTill., [ 17] showed that the change in ~he properties of the hydrides of this ',ype containing Ti is due to the modification in the localization of the 4f electrons and in 3d(Fe)-4ffR) hybridization. In Ref. [18] the neutron diffraction experiments performed on the ErFe m.5M°I.~D, [ i 8], with ThMn r, type structure showed that the site occupied by the interstitial element (D) is an oetabedral hole which has rare-earth atoms and 8j iron atoms as nearest neighboars. It may be supposed that in our ease the hydrogen atoms entering the host compound DyFettTi (belonging to the same type of structure)
A.
Aposrolot, et at./ Journal of Magnetism and Magnetic Materials 150¢!995) 393-398
occupy, in all probability, the same positions. After hydrogenation, the uniaxial 3d transition metal anismropy is larger and controls g'~ increase of anisotropy field at high tempet~atures, Hydrogenalion leads to an increase of the planar contribution to the anisotropy from the R site. These facts can explain the considerable change of T.~ in the hydrides investigated, its shift to higher temperatures. The influence of hydrogen mainly on the Dy sublauice anisotropy energy can be expected became the energy needed for the formation of the Fe-H pair exceeds by several hundred de~ees the energy of the lbrmation of the Dy-H pair. According to our experimeutal data, at hydrogen concentration x > 0.8 the temperatures T,,~ and T,~z practically coincide with T~, i.e. spin-reorientation transitions disappear. It is believed that in this case the magnetic moments of the Dy ions at T_< T¢ lie in the (00i) plane. This means that the incorporation of hydrogen leads to an expansion of the temperature range in which the magnetic anisolropy of the rareearth sublattice dominates over that of the iron sublattice. The considerable difference between our results and those of Wallace and co-workers [t2] (i.e. Ihe increase in the temperature of the spin-reorientation transitions and comparatively small increase in Tc on hydrogenation) lies, in all probability, in the method of keeping up the hydrogen concentration in the samples investigated, used in Ref. [12]. Poisoning, using SO~ to prevent hydrogen escape, does not guarantee the maintenance of hydrogen coacena'alion in the investigated temperature range. The influence of small fluctuations of Fe and Ti content oa the magnetic anisotropy of DyFe,TiH~ cannot he excluded either. Hydrogen's change of the overall bulk aniso~opy due to hydrogen is found by comparison of the o-(H) curves measured at different temperatures (Fig. 5). In the low-temperature range the magnitude of the overall bulk anisolropy is higher than that observed at room temperature. The increase of the magnetic anisotropy at low temperatures manifests itself also as an increase in the value of the coercive force. The increase in total magnetization upon hydrogenation as well as in ehe average Fe moment was also observed in the hydrides investigated. The mag-
397
netic behavior of the hydrogenated DyFel~T~ cc~apound can be ascribed to the following effects: firs~y, to varimions of the intcmtomic distances a~L secondly, to strengthening of the 3d-3d and weakening of the 4f-3d exchange interactions. It shouht also be noted that the increase of the average Fe mom¢~ is typical of rare-earth - 3d transition-metal ing'mm~lic compounds and is undoubtedly an e l ~ / c effect. Presumably there is a transfer of etec~ons f~,ra H to 3d subbands resulting in an increas~ of ~he F¢ sublattice magnetic moment and hence in azt increase in the overall magnetic moment.
4. Co~'lusion We have synthesized and studied the DyFe.Ti intennetallic compound and its hydrides. For the first time the quantitative dependence of the magnetic ordering temperature and the spin-reo~ien~ion temperature on the hydrogen concentration ~ave been investigated. R has been found that both T~ and T~ increase monotonically with increasing hydrogen concentration. The experimental results ob~ncd arc useful for the better understanding of the changes in the magnetic anisotropy of both the iron and raceearth sublattices as well as in the exchange imeractions upon hydrogen absorption.
Acknowledgment This work was supported by the National Foundation for Fundamental Research, grant No TN 105.
References Ill K. Ob~shi, T. Yakoym~aa, R. Sugi and Y. T~m-a. IEEE Trans. Magn. 23 (1987) 310t. 12l K.H.L Bcschow, L AppL Phys. 63 (1958) 3130. [3] X.C. Kou. T_S_ ~ . ~ , R. Gt'w~jnger, H.R. K i r ¢ ~ y r , X. Li F.R. de Boer. ~Phys.Rev. B 47 (1993) 323l. [4] J. Ding and M. Ros~:rdJcrg,J, Magn. Magn. M ~ r . 83 ( [ 9 ~ ) 257. [5] L. Sch-o|tz. K. Schnhzk¢ and J. Wccker, J. Magn. Me,g~. Mler. 83 (1990) 254. [6] Bo-Ping Hu. Hong-S,'~aoLi. J.P. Gavigaa ~ d LM.D. Coey. J. Phys. Coadea~ M~tcx ! (t'~89) 755.
39g
A_ .4r~-~,to,_,e', al. / lo~maf of Magnetism ~
[9~ L'~'. Y~,r~. W.F__W a ~ _ J. Less-Comva~ M ~ 149 (1989) 371. [~,Cq S. ~ . S. M ~ D. F r , ~ t m ~ R l'Heri6~r. Z -~3x ~ N~ 163 (t9~9) i6l.
Magr,etic Materials 150 f 1995~ 393-398
[14] [15] [16]
[t7] [~2t LY_ ;Z~a~. S.G S , ~ ,
W.E Wa~h-~e anc~ S.g. MaliL 6~ [18]
SP33 ~ C , ~ ~¢gk~ Univctsi~. P/C,shurgh, PA). ~3] g .%~_zcqov.R. B e . ~ k ' ~ , V. Kr'~¢~ev.N. Sma~v m~d M.
Smycehev. A~u~rc de i'Uniuersite de Sofia St. Kliment O ~ k i . vol. 86 (1994) 129. R. Bezdushnyi a ~ N. Sto~nev,to bc published, Bo-piag Ha. Hong-Shuo Li. l.P. Gavigan, and J.MD. Coey, Phys. Rev. B 41 (1~1)) 2221. E.B. Bo~ich. BM. Ma. L.Y. Zhang, F. Poum,-ian~S.K. MaliL S.G. Sanlu~ and W.E. Wallace, J. Magn. Magn~ Ma~er. 78 (1989) 365. J. L'-'l~boy.A. M~r~lli and L Bozukov, Jpn. J. Appl. Phys, 32 (1993) ,Suppl. 32-2, 758. E. Towey. M 8~m,-mn, D. Fmcham S. Miraglia, I.L. Sou~yroux. D. Gignoux, E. Palaeios. IEEE Tmns, Magn. {1994), tu be published.