Magnetization behavior of RFe3-hydrides (R = Tb, Er and Tm)

Journal of Magnetism and Magnetic Materials 40 (1983) 27-31 North-Holland Publishing C o m p a n y

27

M A G N E T I Z A T I O N B E H A V I O R OF RFe3-HYDRIDES (R = Tb, Er A N D Tm) * S.K. M A L I K **, F. P O U R A R I A N and W.E. W A L L A C E Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 9 May 1983

The R F e 3 c o m p o u n d s (R = heavy rare earth), crystallizing in rhombohedral PuNi 3 type structure, absorb typically three hydrogens per formula unit at room temperature. The hydrogen absorption does not change the structure of these compounds but merely causes an expansion of the lattice. Magnetization studies have been carried out on the hydrides in the temperature range 4.2 to 300 K and in applied fields up to 21 kOe. The parent compounds are known to be ferrimagnetically ordered with reasonably high Curie temperatures. The magnetization of the hydrides at 4.2 K is observed to be smaller than that of the corresponding parent compounds. The compensation temperature also decrease on hydrogen absorption. These results m a y be taken to imply either an increase in m o m e n t on iron a n d / o r a decrease in rare earth moment. In ErFe3-hydride , a sharp increase in magnetization is observed close to room temperature. This is attributed to the change in easy direction of magnetization.

1. Introduction In an earlier communication we reported on the effect of absorbed hydrogen on the magnetic behavior of G d F e 3, D y F e 3 and H o F e 3 [1]. The present work represents the continuation of this work to TbFe 3, ErFe 3 and T m F e 3, completing the series from G d F e 3 to T m F e 3. The RFe 3 compounds crystallize in a rhombohedral PuNi 3 type structure. There are three inequivalent Fe sites and two inequivalent R sites. All these compounds are ordered ferrimagnetically [2,3] with Curie-temperatures ranging from 542 K for T m F e 3 to 648 K for T b F e 3 [3]. The rare earth constituent does not substantially alter either the Curie-temperature or the magnetic moment on Fe. The rare earth, however, influences the easy direction of magnetization of the compounds and the compensation temperatures. The saturation magnetizations of ErFe 3 and TbFe 3 are nearly the same, but their temperature dependencies are quite * This work was supported by a contract with the A r m y Research Office. ** Permanent address: Tata Institute of Fundamental Research, Colaba, Bombay 400 005, India.

different [3]. In fact, ErFe 3 has a compensation at a temperature of 228 K, while compensation in T b F e 3 occurs at 610 K. The compound ErFe 3 also shows a change in easy direction of magnetization at about 50 K [4]. On the other hand, T m F e 3 has the lowest magnetization of any of the RFe 3 systems, and earlier studies [5] could not accurately determine the compensation temperature. Recent measurements by Herbst and Croat [3] yield Tcomp = 112 K for this compound. It was, therefore, of interest to see how these features of the RFe 3 compounds are modified on hydrogen absorption.

2. Experimental The alloy samples were prepared by induction melting stoichiometric amounts of the high purity constituent elements in a water-cooled copper boat under a continuous flow of purified argon gas. The ingots were turned over and melted several times and subsequently annealed for one hour in the induction furnace itself at temperatures just below their respective melting temperatures. Hydrogen absorption measurements were carried out in a system of calibrated volume. After a suitable

0304-8853/83/0000-0000/$03.00 © 1983 North-Holland

S.K. Malik et al. / Magnetization behavior of Fe3-hydrides

28

Table 1 S u m m a r y of the crystal structure and magnetization data for some of the ReFe 3 compounds and their hydrides Compound

a (A)

c (,~)

M a at 21 kOe (/La/f.u.)

TbFe 3 TbFe3H3. 5

5.140 5.370

24.590 26.814

3.19 2.90

ErFe 3 ErFeaH3. 5 TmFe 3 TmFe3H 3

5.089 24.473 5.296 26.246 5.063 24.621 X-ray lines broad

3.42 2.05 1.47 4.08

YFe3 c YFe3H3.5 c

5.137 5.375

5.01 5.7

24.61 26.46

Tc b (K)

Tcomp b

(K) 610 205 (4 kOe) 170 (21 kOe) 228 81 112 13 (4 kOe) 36 (21 kOe) no comp. no comp.

648 300 552 > 300 542 > 300 549 545

a Data Obtained at 4.2 K. b The comPensation and the Curie-temperatures for the parent RFe 3 comp. are taken from the more recent work of ref, [3]. c Data taken from ref. [6].

activation treatment the samples were exposed to highpurity hydrogen at room temperature and 1000 psi hydrogen pressure. The absorption was instantaneous in most cases. The amount of hydrogen uptake was calculated from the change in the pressure in the system. Powder X-ray diffraction patterns o f the parent: compounds and the hydrides were recorded on a Diano-XRD diffractometer using C o K a radiation, which showed that all these compounds are single-phase materials. The lattice parameters ~are given in table 1 . Temperature dependence of the magnetization of the hydride and non-hydride samples was measured using the Faraday method over the temperature interval 4.2 to 300 :K and at two applied fields, namely; 4 kOe and 21 kOe. Isothermal magnetization was also recorded up to an applied field of 21 kOe at temperatures of 4.2, 77 and 300 K. In all Cases the magnetic ordering of the hydride appears to be well above room temperature. Thus, because of H2 released at high temperatures, we were unable to determine the Curie-tempera. tures. The results of magnetic measurments are discussed below and summarized in table 1.

pensation point is observed at a temperature of 228 K [3], implying that the resultant Fe moment is coupled antiparallel to the Er moment, and they cancel each other at this temperature. A change in the easy direction of magnetization in this compound occurs at T = 50 K. Below 50 K the easy direction of magnetization is parallel to the c-axis, while above 50 K it is in the basal plane. This reflects as a small but abrupt jump in the magnetization. The jump is more pronounced in magnetically aligned samples [4]. The change in easy direc-

5

i

i

I

i

I

Er Fe3H3. s

43

~

245K

2

~

L-~e - - ~ w "~° : -- -- :--;:=--" 4'21"

QD

:L

E

O

e~n O t 77

K

3. Results and discussions 3.1. ErFe ~ and hydride

: The compound ErFe 3 is magnetically ordered with a Curie-temperature of about 550 K. Acom-

8

16

24

Mognetic Field H (kOe) Fig. 1. Magnet.~ation vs. applied field for EtFe3-hydnde at 280, 77 and 4.2 K.

S.K. Malik et al. / Magnetization behavior of Fe3-hydrides

tion of magnetization has also been observed by 57Fe M(Sssbauer studies [4]. The nearly constant 57Fe isomer shift indicates that the electronic configuration of the three Fe sites is nearly the same. Magnetization-field data for ErFe 3 hydride at three temperatures are shown in fig. 1. Magnetization of the hydride shows saturation behavior as a function of applied field both at 4.2 K and at room temperature, but not at 77 K (which is close to the compensation temperature in the hydride). The magnetic moment per formula unit in the hydride is smaller than that in the parent ErFe 3. The compensation in the hydride is still present (see fig. 2) and is indicative of the fact that Er and Fe moments continue to couple antiparallel in the hydride. The compensation temperature, however, is lowered to about 80 K by hydrogen absorption in the hydride (and is slightly field-dependent). The reduced /~s=t and Tromp a r e consistent either with a relative decrease in Er moment or a relative increase in Fe moment. An increase in Fe moment has been observed in YFe 3 and GdFe 3 on hydrogen absorption [6]. A jump in the magnetization of the hydride is observed between 200 K and 250 K, the origin of which is not understood. It may reflect a change either in the easy axis of magnetization or in the magnetic structure of Er and Fe sublattices.

5

i

5

i

29

i

i

,•

4

• Tb Fe3 H3. s

oE

=E

4.2K" 300K

2

u

3OO =E

%

I

I

I

8

24

Mognetic Field H (kOe)

3.2. TbFe s and hydride

TbFe 3 is a ferrimagnet with easy axis of magnetization along the b-axis in the basal plane at low temperatures [7]. Unlike the case of ErFe 3 and HoFe 3, there is no change in the easy axis of magnetization up to room temperature. Fig. 3 shows the magnetization as a function of applied field for TbFe 3 and the hydride at 4.2, 77 and 300

5

i

i

i

o TbFe s • Tb F.e3 H3. 5

4

~k ~ 3

3

c

c

E

E

0

0

•

._u

2

._~

c

o

I

Fig. 3. Magnetization vs. applied field for TbFe 3 and TbFe 3hybride at 300, 77 and 4.2 K.

5

:t

I

16

Er Fe3H3. 5

~

i

3

i

4 kOe

i

o Tb F'e3

c

I

o

I

I

O0

I00

200

300

T(K) Fig. 2. Magnetization vs. temperature for ErFe~-hydride in t w o different applied fields.

O0

i

I

I00

i

I

200

i

300

T (K) Fig. 4. Magnetization vs. t e m p e r a t u r e f o r TbFe 3 and TbFe3-hydride in two different applied fields.

30

S.K. Malik et al. / Magnetization behavior of Fes.hydrides

K. The magnetization in each case is not saturated. In the parent TbFe 3 (and most likely in the hydride also) this is due to the effect of rare earth sublattice magnetization. A decrease in the magnetization of the compound is observed on hydrogen absorption. Fig. 4 shows the temperature dependence of the magnetization at two applied fields for TbFe 3 and the hydride. The compensation temperature in TbFe 2 is 610 K, well above room temperature. In the hydride, the compensation occurs between 150 to 200 K, depending upon the applied field. The decrease in magnetization at Zcomp is again consistent with an increase in Fe moment upon hydrogen absorption.

51

r

i

L

i

i

i

TmFe3H3 4.2K

4

~

3

c

E

0

~

i

TmFe3H3

,.2

4

3. ¢-

E 0

2

cO

06

t

I

I

IOO

200

j

30o

T(K)

TmFe 3 is ferrimagnetically ordered with a Curie-temperature of 542 K and a compensation temperature of 112 K [3]. This compound has a low magnetization of 1.47#B per formula unit at 4.2 K. Fig. 5 shows the magnetization as a function of applied field for TmFe3-hydride at 300, 79 and 4.2 K. The magnetization is not saturated in 20 kOe applied field at any of the above temperatures. However, the deviation from saturation is most pronounced at 4.2 K. Fig. 6 shows the variation of magnetization of TmFe3-hydride as a func-

i

i

"T

3.3. T m F e 3 and hydride

F

,

Fig. 6. Magnetization vs. temperature for TmFe3-hydridein two different applied fields.

tion of temperature in two different applied fields. The compensation is observed at very low temperatures, below which the Tm-magnetization takes over and the resulting magnetization rises rapidly in 21 kOe applied field. The decrease in T~omp is similar to that observed in TbFe 3- and ErFe3-hydrides and arises from the same reasons. It is also thought that the T m - F e exchange has been considerably weakened by hydrogen absorption to the extent that the Tm moments show a "fanning" effect. As the temperature is lowered, an appreciable component of the T m moment antiparallel to the Fe moment builds up, resulting in a compensation temperature. The lack of saturation in magnetization and a rapid rise in magnetization at low temperatures may also be the consequence of weak T m - F e exchange and fanning of Tm moments.

I

2 4. C o n c l u d i n g

remarks

c

o

I

O0

~

I 8

I

t 16

I

24

Mognelic Field H (kOe)

Fig. 5. Magnetization vs. applied field for TmFe3-hydrideat 300, 77 and 4.2 K.

A two-sublattice molecular field analysis of the magnetization of parent RFe 3 compounds [3] has revealed that the rare earth magnetization falls off much more rapidly than the Fe magnetization as temperature is increased. Hydrogen absorption in RFe a compounds seems to have two main effects. First, it weakens the R - F e exchange so that the

S.K. Malik et al. / Magnetization behavior of Fe3-hydrides

net rare earth magnetization decreases much more rapidly with increase of temperature. This may be partly responsible for decrease in T~omp. When the R - R e exchange is comparable to the local anisotropy energy of the rare earth, the rare earth moment is strongly coupled to the local crystal field axis randomized by the presence of hydrogen. This may lead to fanning of rare earth moments (excepting Gd) at low temperatures. Such a fanning of the rare earth moments has been inferred from neutron diffraction studies on some RFe2-hydrides [8] a n d f r o m m a g n e t i z a t i o n studies in RFe2_xCox-hydrides [9,10]. By analogy with the situation in RFe2-hydride, it is expected that similar fanning of rare earth moments may occur in RFe3-hydrides. The fanning effect is likely to be larger in TmFe3-hydride than in Tb and Er compounds due to the weaker exchange interaction in the former and may explain the lack of saturation at 4.2 K observed in this compound. The low temperature magnetic structure in such systems is expected to have ferromagnetic Fe moments with rare earth moments showing a fanning and lying opposite to the Fe moments. The second effect of the hydrogen absorption is, in most cases, to in-

31

crease the magnetic moment on Fe, which is thought to arise from the changes in the 3d band structure. This has been observed also in many other R - F e hydrides [1,11].

References [1] S.K. Malik. T. Takeshita and W.E. Wallace, Magnetism Lett. 1 (1976) 33. [2] W.E. Wallace, Rare Earth Intermetallics (AcademicPress, New York, 1973) p. 181. [3] J.F. Herbst and J.J. Croat, J. Appl. Phys. 53 (1982) 4304. [4] A.M. van der Kraan, P.C.M. Gubbens and K.H.J. Buschow, Phys. Stat. Sol. (a) 31 (1975) 495. [5] K.H.J. Buschow,ibid. 7 (1971) 199. [6] K.H.J. Buschow,Sol. State Commun. 19 (1976) 421. [7] W.J. James, K. Hardman, W. Yelon, J. Keem and J. Croat, J. Appl. Phys. 50 (1979) 2006. [8] G.E. Fish, J.J. Rhyne, S.G. Sankar and W.E. Wallace, J. Appl. Phys. 50 (1979) 2003. [9] F. Pourarian, W.E. Wallace and S.K. Malik, J. Magn. Magn. Mat. 25 (1982) 299. [10] F. Pourarian, W.E. Wallace and S.K. Malik, J. Less-Common Met. 83 (1982) 95. [11] E.B. Boltich, F. Pourarian, W.E. Wallace, H.K. Smith and S.K. Malik, SO1.State Commun. 40 (1981) 117.