hyperfine measurement of magnetic compensation in ferrimagnetic (gdce)fe2 and (gdy)fe2 compounds

Hyperfine Interactions 14(1983)233-241 233

HYPERFINE MEASUREMENT OF MAGNETIC COMPENSATION IN FERRIMAGNETIC

(GdCe)Fe 2 AND (GdY)Fe 2 COMPOUNDS

J.W. ROSS and M.A. CHOUDHRY

Schuster Laboratory, The University, Manchester M13 9PL, U.K.

Received 7 July 1983

Magnetic compensation in the ferrimagnetic series of compounds (GdxYl-x)Fe 2 and (GdxCel-x)Fe 2 has been observed by M6ssbauer measurements of the hyperfine field at Gd and Fe nuclei in external fields (Bap p) up to 6 T. The hyperfine field shows a sharp change of 2Bapp at the compensation composition x c due to the alignment of the magnetization of the compound with the external field. Strong compositional dependence of this change allows an accurate determination of xc in the two systems.

I . In t roduct ion

Ferrimagnetism in the cubic Laves phase compound GdFe 2 is determined by the strong exchange interaction between the Fe moments and the intermediate strength Fe-Gd interaction [I]. Consequently, GdFe2 consists of two antiparallel magnetic sublattices whose net magnetization is a monotonic function of temperature with a unique magnetic ordering temperature (790 K [2]). When Gd (S = {) is replaced by magnetically inert trivalent Y in the pseudobinary GdxYz-xFe2 system, the sublattice magnetizations remain antiparallel and the Curie temperature remains high (Tc=540 K for YFe 2 [3]). Substitution of tetravalent Ce for Gd has more severe effects, reducing the magnetic moment of Fe from 1 .6~ B in GdFe2 [2] to 1 .15UB in CeFe 2 and reducing T c to 230 K [4]. The results of Miskinis et al.[5] indicate the rare earth and Fe sublattice magnetizations remain antiparallel across the GdxCel-xFe 2 series in spite of the differences between the terminal compounds. UGd>2UFe in both series of compounds and each system therefore pos- sesses a unique magnetic compensation composition x c at which the net magnetization is zero. Bulk magnetization measurements [5,6] on such systems do not clearly detect the absence of magnetization and Xc can only be found by considerable extrapolation.

It has long been established that the hyperfine field (hff) at both Gd (+ 43.4 T) and Fe (- 22.6 T) nuclei in GdFe 2 is largely determined by the Fe magnetic moment [7], i.e. by the Fe sublattice magnetization rather than the net magnetization. These values of the hff are slightly modified as the Fe moment changes with x in the pseudobinary systems but significant splitting still occurs for all x at low temperatures [8]. We have made use of this intrinsic accuracy to determine x c in both pseudobinary systems by measuring the M~ssbauer effect at iSSGd and STFe nuclei in applied magnetic fields up to

6 T. The principle of the method may be demonstrated by considering

the variation of the measured hff at iSSGd nuclei as the magnetization of the rare earth sublattice (~Gd) is reduced. At low Y or Ce concen- -w . trations, the magnitude of MGd ZS greater than that of the Fe sub-

�9 J.C. Baltzer Scientific Publishinq Company

234 J.W. ROSS, M.A. Choudhry, Magnetic compensation in RFe~ compounds

lattice and the net magnetization of the M6ssbauer absorber will be in the same direction as MGd. Hence. in a saturating applied magnetic field ~app, ~d will be parallel to ~app and the total measured hff will be the sum of the hff and the external field. (We assume that the hff is independent of applied field, i.e. Knight shifts are neglected.) At high dilution, the magnetization of the Fe sublattice dominates so that the sublattice magnetizations are reversed with respect to the external field and the total measured hff at iSSGd nuclei is now the difference between hff and Bap p. Consequently, a discontinuous change of 2Bap p in the measured hff occurs at the compensation composition Xc. Similar considerations for the hyperfine measurements at SYFe nuclei reveal identical behaviour, since the sign of the hff at Fe is negative, i.e. in the opposite direction to that of the Fe electronic magnetic moment.

2. Experimental

The easy direction of magnetization in GdFe~ is intermediate between the principal crystallographic axes and complicated SYFe M~ssbauer spectra are observed [9]. In order to obtain only a single 6 line M~ssbauer pattern, we replaced 3% of the Gd atoms by Ho to main- tain a [100] easy axis of magnetization across the series, f

The GdxHo0.0~Y0.97-xFe 2 and GdxHOe.0~Ce0.~_xFe: series of pseudobinary compounds were prepared from the metals (metallic purity 4N, Ce metallic purity 3N) by arc melting in a high purity argon atmos- phere, followed by annealing at 850 to 900 ~'C for 150 hours. Typical evaporation losses of 2 to 5 mg in 2000 mg occurred during prepara- tion and allowance for this was made in the initial weights of the metals. X-ray diffractometer measurements on powdered samples showed the compounds possessed the C]5-type structure with the maximum im- purity phase (GdFes-type structure) < 4%. These small amounts of im- purity phase and possible uncompensated evaporation losses on prepa- ration introduce an uncertainty of +0.002 in our values of x.

Appropriate amounts of finely ground compound were thoroughly mixed with epoxy in thin plastic holders to produce absorbers with a M6ssbauer thickness of unity for the l~SGd 87 keV y-ray. SYFe absorbers, having the same M6ssbauer thickness, consisted of finely ground compound mixed with silicone grease in high purity graphite holders. Both the ~SSGd and SYFe experiments were carried out using constant acceleration spectrometers calibrated against pure metallic iron. The comparatively high energy of the 87 keV y-ray used in the ISSGd M6ss- bauer effect meant that the absorption measurements had to be per- formed with both source and absorber at 4.2 K. Uniform magnetic fields up to 6 T were applied to the absorber using a superconducting solenoid whose axis was along the direction of propagation of the y-rays, i.e. normal to the plane of the absorber. Compensating turns at one end of the solenoid ensured the neutron irradiated tS4smPd~ source was always in a field-free region. The ]4.4 keV SYFe M6ssbauer absorption spectra were made with the absorber at ]00 K or room temperature, while the SYCo:Rh source was always at room temperature. At both temperatures, the absorber was subjected to a uniform field up to 0.9 T by an electromagnet. The relative orientation of the electromagnet and the field-free source was such that the absorber was magnetized wholly within its plane and the y-rays were transmitted along

t For the GdxHO0.03Cea.97_xFe 2 series, 32 Ho was insufficiunt to ensure ,~ []C}0] direction of magnetization; the Fe spectra in applied fields ~or com[-.ounds with x > 0.78 could only be fitted by assuming an intemnediate direction of maqnctiza- tion.

J.W. Ross, M.A. Choudhry, Magnetic compensation in RFe 2 compounds 235

the direction of the normal to the plane. The orientation of the absorber magnetization with respect to the direction of propagation of the y-ray ensured longitudinal polarization (Am = 0 transitions absent) of the tSSGd 87 keV M1 y-rays and transverse polarizations of the SYFe 14 keV M1 y-rays.

When fitting the IS5Gd spectra, the intensity of each of the 12 equal width lorentzian lines was assumed to be proportional to the square of the appropriate Clebsch-Gordan coefficient, but the ratio of the intensities of the Am = 0 group of lines to the Am = • 1 groups (R) was taken as a variable parameter. R was minimized along with the hyperfine splitting, quadrupole splitting, isomer shift, line width and the relative absorption of the most intense line in a least squares computer programme. For compounds with x outside the range x c • 0.03, R was found to be small, indicating a high degree of magnetic saturation in an external field of 6 T. The almost complete re- duction from 12 to 8 lines is clearly illustrated by the spectra of Gd0.88Ho0.0~Y0.09Fe2 shown in fig.1. In most zero external field Gd

8"12

8"0E

~o8.04

, - t" O

945

941

937!

9.3. ~

, ~ I I I I I I %, I I I " "

BappyO �9 R=1.2 _ Bapp-0 "~ ~ ' ~ R --- 12 _

J J

Bapp.=6T R = O 0 6

I I I I I I I -2 0 2

Velocity (mm/sec)

C

J J

Fig.l. The ISSGd M6ssbauer spectra of Gdo.ssHOo.03Yo.09Fe2 at 4.2 K (o- experimental points, solid line- computer fit).

236 J.W. ROSS, M.A. choudhry, Ma~inetic compensation in RFe 2 compounds

spectra R was found to be 1.1 to 1.2, indicating a small degree of transverse polarization of the 87 keV y-rays. We attribute this to preferential alignment of permanently magnetized compound particles within the plane of the absorber before the liquid epoxy solidifies, the demagnetization field being less in the plane of the absorber than normal to it. This result indicates that some of the powder particles must be of the order of the size of a magnetic domain, since care was taken not to subject the compound to a magnetic field before the absorber was prepared.

In the S7Fe spectra, all 6 MSssbauer lines are clearly resolved. Consequently, the intensities of the 3 pairs of lines, each corresponding to a particular s were used as unconstrained parameters in the fitting programme.

3 . R e s u l t s and d i s c u s s i o n

The almost discontinuous change of 12 T in the observed hff in an external field of 6 T clearly identifies x c in the GdxHO0.0~Y0.97_xFe: system (fig.2) . A similar sharp increase in the iSSGd hff occurs in the Ce diluted series. The hff value of (43.4 • 0.5)T for Gd0.97Ho0.03Fe 2 is

A

45 11

(lJ

~- 35

7--

25 ~

YFe 2

I I I

x x

x

x

0o% "

x •

•

x

I I I 02 04+ 0-6

Fig.2. Hyperfine field at

i i

x

x x

I 08 G d F e 2

i 5 5Cj in the

Gd.<Ho~ 3~Y:~ We series at 4 . 2 K. �9 ~ . . 9 7 - x - 2

(o - in :zero applied field, X - in 6 T longitudinal field)�9

in excellent agreement with the value of (43.38 • 0.06)T given by Tomala et al.[10] for C<IFe::. (Both measurements were made in the absence of an external magnetic field.) The hff at S7Fe depends on x in a similar manner. Fig.3 shows a precipitous change of 1.8 T in the hff of S7Fe in the Gdxlbc.o~Ce~.~7-• 2 compounds at x< when measured in an applied field of 0.9 T. Hff measurements were also made at several intermediate field strengths for compositions close to Xc in both pseudobinary series and examples are shown in figs.4 and 5. Note that a variation of only a few percent in x results in a change in slope (G(x)) of the hff versus Bap P straight line from -I to + I.

At the compensation composition, the net magnetization of the sample is zero and the application of an external magnetic field of

ROSS, M.A. c'houdhr~l, MagneSic compensation in RFo 2 compounds 237

-o 22

Q/ r-"

2C- k~

--1-

18-

CeFe2

I I I I x q

x

x �9

x x �9

x �9

o � 9 I

�9 ~ xxxx

x

x

0"2 l I I

0.4 0.6 0-8 Gd Fez X

Fig.3. Hyperfine field at S7Fe in ~he

GdxHo0.03Ceo.qT_xFc ? seEies at 100 K

(e - in zero applied field, X - in 0.9 T

transverse field).

3O

. _

I I

I ~ 2 C t CI_

--r-

10

I I I I I X

0 . 4 6 _

0~4 0 4 2

~176 0&0

0 3 8 0 3 6

I I I I I I

I 3 5 Bapp(T)

Fig.4. The variation of hyperfine field

at ISSGd with applied field in the series

GdxHo0.03Ce0.97_xFe 2 at 4.2 K for composi-

tions around x c .

,3 o c3u J.W. ROSS, M,A. Choudhrl], Magnetic compensation in RFc' 2 compounds

~. 21

"10

u_

20

T

19

I I I I I I I I I X

014 8

0 45

a _~_----~ --a 04&

-

0 . 4 3

O.Z.2 -

"--11 0 3 9

1 I I I I I I I I 02 04 06 08 10

Bap p (T)

F/g/5. Tile variation of hyperfine field at F( v:iti~ appli<d fleld ill tile series G - i x H O : . , :r Y <' . " ? - - ] < F ' . ' .l t Y J ~ ]'[ f C ! l v ~ c l m p ~ .h i -

[ ] k;ItS , J . t OUl],J ) [ c "

6 T has little influence on the direction of the sublattice magnetization if the strong antiparallel exchange interaction between Fe and rare earth moments still exists at Xc. Consequently, in polycrystalline absorbers the component of ~apo along the local direction of the hff will be an average over all angles with equal weighting

; C

i . e . :<c i s t h a t c o m p o s i t i o n f o r wh~_ch t h e e x t e r n a l m a g n e t i c f i e l d h a s no effect on the measured hff. Table I shows the interpolated values of x c deduced from G versus x plots. Tise slight difference in x c measured by the iS<Gd (4 .2 K) and STFe (100 and 293 K) resonances for both

Table !

The compensation compositions of the two of the pseudobinary systems determined at various temperatures

GdxHOc.03Yc.97-xFe2

GdxHoj.0sCe0.97_xFe 2

x c (t 0.003)

4.2 K i00 K[ 293 K

0.375 0.393 0.438

0.411 0.422

pseudobinary series is attributed to the different temperature de- pendences of the Fe (S = I) and Gd (S = Z) sublattice magnetizations.

2

.7.w. Ross, M.A. Choudhrg, Magnetic compensation in RFo~ compounds 239

Although the random orientation of ~app with respect to the local hff direction does not produce a first moment contribution, this is not the case for the second moment, and we expect the M6'ssbauer line width to increase by a factor corresponding to

AB ~[(BappCOS{9)2]'~ BaP~

at x c . In the (GdCe)Fe 2 system the substitution of trivalent Gd by

tetravalent Ce results in a distribution of electric field gradients at the iSSGd and STFe nuclei, which causes broadening of the Mdssbauer lines and obscures an}' second moment contribution. Replacement of Gd atoms by similarly sized trivalent Y atoms does not produce such an effect. The expected increases in line width of ~ 25% and ~ 75< were observed on application of 6 T and 0.9 T for the iSSGd and STFe resonances, respectively, in the compounds whose composition was closest to x c .

As mentioned in the experimental section, the degree of magnetic alignment of the absorbing nuclei strongly influences the M6"ssbauer spectra. Both M~ssbauer resonances used in this work are MI transitions and the ratio R of the intensity of the Am : 0 to the s = -+ I transitions is 2 sin 0 (I +cos28) -I. (8 is the angle between the dipole axis of the absorbing nucleus and the direction of propagation of the y-ray.) For a fully magnetized absorber in a longitudinal maqnetic field @ = 0, whereas for a random orientation of the domains (B a =0) s--r~-Sn 8=~ and R is unit}'. For compositions outside the range Xc• the average value of R for lSSGd in a longitudinal field of 6 T is zero for the Ho:(CiiCe)Fe system and 0.09-+ 0.04 for the Ho:(GdY)Fe: system (figs.6 and 7). It is difficult to confirm from magnetization measurements that the (GdY)Fe 2 compounds are magnetically less saturated than the (GdCe)Fe 2 compounds, since the data of both Miskinis et al.[5] and Burzo and Ursu [6] were taken at lower magnetic fields.

The almost @-function dependence of R with composition close to xc clearly shows the lack of alignment of the domains in the magnetic field as the net magnetization of the absorber approaches zero. In the Ho:(GdCe)Fe~ series of compounds, the maximum value of R is ~2. This implies that the easy [100] directions must be in the plane of the absorber (8 =�89 Confirmation of this is found in the zero external field spectra of compounds with x close to x c where the value of R rises to ~ ] .4. One explanation might be that close to xc the domain size is greater than the average particle size (--5~m) so that, although the net atomic magnetization is small, each particle is magnetized more strongly than a similar particle in less compensated compounds where the domain size is assumed to be smaller. The magnetized single domain particles orient themselves within the liquid epoxy of the absorber holder to minimize the demagnetizing field, i.e. with their magnetization in the plane of the absorber. The alignment is more pronounced in the Ho: (CdCe)Fe2 system than the HO:(GdY)Fe 2 system since the lower Curie temperature of the former results in a higher magnetization at room temperature for a compound which is fully compensated at 4.2 K.

4. Sulnlllary and conclusions

We have demonstrated that the magnetic compensation composition of a ferrimagnetic system can be accurately determined by measuring the hff in the presence of an external magnetic field. For low aniso- tropy Laves phase compounds based on GdFe 2, the measured hff varies

~40 J.W. ROSS, N.A. Choudhry, Magnetic compensation if] RFe 2 compounds

06

04

I I I

F- "" ~[

I

0 " "I I I I

YFe 2 (}2 O,L, 0-6 08 6dFe 2 •

Fig.6. The polarization ratio (R) of the 87 key ]'-ray of IF'5Gd in a longitudinal external field of 6 T for the

GdxHOc.03Y0.97_xFe~ system at 4.2 K.

2-4

16- R _

04i0

CeFe 2

I I I I

t -

}

I I I I

0.2 0-4 0.6 08 GdFe2 •

Fig.7. The polarization ratio (R) of the 87 keV y-ray of 155Gd in a longitudinal external field of 6 T for the

GdxHO0.0~Cee.97_xFe 2 system at 4.2 K.

varies rapidly with composition, allowing x c to be determined to with-

in i 0.003. The present work used the M~ssbauer effect to determine the hff,

but PAC or NMR could be used. PAC would revert to angular correlation at x c for a polycrystalline source. Grover et al. [11] carried out 27AI NMR measurements in a field of 1.3 T on Sml-xGdxA1 ~ and showed that a compensation composition existed between x = 0 and 0.1, AI- Assadi et ai.[12] have observed a sharp change in the spin-echo NMR frequency of 16SHo corresponding to twice the applied magnetic field (IT) in the Ho:(OdY)Fe 2 system around x e . Despite its lack of precision, the M6ssbauer technique has several advantages over NMR:

(a) nuclear y-ray resonance originates largely from within domains and does not rely on domain wall enhancement for its observa-

tion;

J.W. Ross, M.A. Choudhry, Maqnetic compensa[ion in RFe 2 compounds ~

(b) it does not suffer from the complication of overlapping resonance signals as observed for 89y and 1~s'15~Gd NMR in (GdY)Fe~ [13];

(c) the magnetic saturation can be monitored by the degree of y-ray polarization, confirming the hff measurements.

In some ferrimagnets, the exchange interactions are such that when the sub]attice magnetizations have different temperature depend- ences, magnetic compensation may occur at a well-defined temperature

less than the magnetic ordering temperature. The lack of variation of hff with applied field could be used to find this temperature in the

same way that x c was determined.

References

[ I ] E. Burzo, Invited lecture at S}~mposium on Metal Physics, Zloty-Potok, Poland (quoted by H. Kirchmayr and C.A. Po]dy in Handbook on Phys. and Chem. of Rare Earths, ed. K.A. Gschneider and L. EyrJn~, vol. 2 (Nortil-Holland, Amsterdam,

1979) p.55) . [2] M.J. Besnus, A. Herr and G. Fischer', J. Phys. F9(1979)7.15. [3] M.,]. Besnus, P. Bauer and J.M. G~nin, J. Phys. F8(197S)191. [4] J. D@portes, D. Givord and K.R.A. Ziebeck, J. AppI. Phys. %2(1981)2074. [5] E.T. Miskinis, K.S.V.L. Narsimhan, W.E. Wallace and R.S. Craig, J. Solid

State Chem. I](1975)]11. [6] E. Burzo and I. Ursu, J. AppI. Phys. 5U(1979)1471. [7] R.E. Gegenwarth, J.l. Budnick, S. Skalski and J.H. Wernick, Phys. Rev. Lett.

18(1967)9. [8] O. Prakash, M.A. Choudhry, J.W. Ross and M.A.H. McCa!is]and, J. Mag. Mag.

:.later. 36(1983)271. [!i] G.J. Bowden, D.S<.P. Hunbury, A.P. Guimarfies and R.E. Snyder, J. Phys. CI

(I<]68) 1376. [10] K. Tomala, G. Oz]zek, J. FJnk and H. Schmidt, So]. Stat{! Commun. 24(1977)~57.

[1]] A.K. Grover, S.h. {.lo. lik, R. Vi]ayaraghavan and k. Shlm~zu, J. ApR]. Phys. 50

(197~ . [ 12] K.F. Ai-Assadi, ].S. Mack<.n::ie and M.A.H. McCaus]and, to be published. [ ]3] V.A. Vasil 'kovski] , N.M. Kovtun, A.K. Kuprianov, S.A. N1ckitin and V.F.

Ostrovskii , JETP {Soy. Phys. ) 38 (1974) 342.

hyperfine measurement of magnetic compensation in ferrimagnetic (gdce)fe2 and (gdy)fe2 compounds

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