MAGNETIC CHARACTERISTICS OF THE " LOW MOMENT COMPOUNDS " ErCo0.3Al1.7 and TbCo0.255Al1.475

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MAGNETIC CHARACTERISTICS OF THE ” LOW MOMENT COMPOUNDS ” ErCo0.3Al1.7 and

TbCo0.255Al1.475

H. Oesterreicher

To cite this version:

H. Oesterreicher. MAGNETIC CHARACTERISTICS OF THE ” LOW MOMENT COMPOUNDS ” ErCo0.3Al1.7 and TbCo0.255Al1.475. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1141-C1- 1143. �10.1051/jphyscol:19711408�. �jpa-00214449�

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JOURNAL DE PHYSIQUE Colloqzte C I, supplkmenf a u no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1

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1141

MAGNETIC CHARACTERISTICS OF THE << LOW MOMENT COMPOUNDS

^B

ErCo,.,Al, ., and T ~ C O , ~ , , ~ A I , .,,,

H. OESTERREICHER, Oregon Graduate Center

9340, S. W. Barnes Rd., Portland, Oregon, U. S. A.

Rburnk. - ErCo~.~All.-r et TbCoo.szsAl1.475 cristallisent dans le syst6me cubique faces centrees type MgCuz.

Les composts prennent un ordre fcrromagnetique dans des champs de 20 kOe avec des moments & saturation bien infe- rieurs au moment de la terre rare trois fois ionisee. De prkedentes etudes de diffraction neutronique sur le compose de I'Er ont montrk que le Co ne porte pas de moment et que I'ion tripositif terre rare prend un ordre ferromagnetique avec un moment moitie. On observait de plus un large fond continu de diffusion a 4,2 ^OK.Nous avons maintenant observe qu'a ⁵⁰⁰Oe 1es composCs presentent une temptrature de NCel. ^A1,6 OK, un accroissement spontane et rtversible d'environ un tiers de I'aimantation est observe pour l'alliage de Tb A 12 kOe environ.

Abstract. - ErCoo. 3A11. 7 and TbCo0.525A11.4~~ both crystallize with the cubic face centered MgCu2 type. The compounds order ferromagnetically in fields of 20 kOe with saturation moments well below the tripositive rare earth moment. Previous neutron diffract~on studies on the Er compound have shown Co not to exhibit a moment and approximately half of the tripositive rare earth moment ordered ferromagnetically in addition to a large remaining diffuse background scattering at 4.2 ^OK.We now fmd at 500 Oe the compounds to exhibit Nee1 temperatures. At 1.6 ^O^Ka spontaneous and reversible increase of about one-third of the magnetization is observed for the Tb alloy at about 12 kOe.

Introduction. - We have previously investigated [I, 21 pseudobinary systems of the form RT,-RAl, (R ⁼rare earth metal, T = transition metal) to establish the influence of the changing valence electron concentration (v. e. c.) on phase stability analogous to classical investigations [3]. ErCo,-ErAl, in particular changes from cubic C15 (MgCu,) t o hexagonal C 14 (MgZn,) and back again t o the C15 structure with increasing A1 content or numbers of free electrons. The present study is concerned with magnetic properties of specimens in the homogeneous region of C15, RAl,(Co). Changes in the v. e. c. should also influence the type of magnetic ordering in cases where an indirect exchange mechanism of the Rudermann- Kittel type is responsible for magnetic exchange. Mattis has pointed out [4] that in simple cubic phases ferromagnetism transforms t o several modes of antiferromagnetism as the v. e. c. is increased.

A neutron diffraction study on ErCo,-ErAl, [5]

indicates C o t o be nonmagnetic beyond 30 mole o/, ErAl,. The neutron diffraction study, furthermore, shows fluctuations in the ordered magnetic moment with composition. An interesting observation concerns the disorder background scattering. Whenever the ordered Er sublattice moment is low, the disorder background scattering is high. A simple model that comes to mind is a ferromagnetic lattice of precessing moments along a cone with the projection of the full Er moment on the cone axis appearing as the ordered contribution. In order to elucidate the nature of an admixture of order and disorder, we have expand- ed our investigation to higher fields, lower tempera- turcs and a study of the ordering in small fields. Fur- thermore, we have included T b alloys anticipating higher temperatures for possibly interesting effects.

Experimental. - Alloys were prepared in much the same fashion as previously described [ I , 2, 51 by induc- tion melting and subsequent annealing at 1 000 OK for 100 to 200 hours. Debye-Scherrer diagrams showed the

presence of pure C15 type. The magnetic measurements employed a vibrating Foner magnetometer.

Results. - ERCO,.,AL,

.,,.

- This compound was investigated with 20 and 40 kOe and rising temperatures and Curie temperatures of 24 and 25 OK were obtained. A t 500 Oe a broad peak in o (magnetization) vs temperature is observed around 10 OK (Fig. 1). All samples were cooled in the absence of a field. If the specimen is cooled in a field of 500 Oe, a curve resembl- ing ferromagnetism is obtained. At higher temperatures the substance obeys Curie-Weiss law with a para-

FIG. 1. - Magnetic moment ( p ) vs temperature for ErCoo. 1.7.

A was obtained after cooling in the ^field.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711408

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C 1

-

¹¹⁴² H . OESTERREICHER magnetic Curie temperature (0) of 12.5 OK and an

effective magnetic moment of 9.5 pB according to the defining relation p,, is 2,839 J X ( ~ - 8). The satura- tion moment at 4.2 OK, extrapolated from fields up to 40 kOe, is 3.50 pB. Previous neutron diffraction data [5]

a t 4.2 OK have shown a ferromagnetically ordered Er sublattice with 5.3 pB and a sizeable diffuse background scattering in comparison to ErAl,. An analysis of the magnitude of this background scattering taking ErA1, as a standard yields pd = 7.2 pB which approximately satisfies the relation p i

+

p i = 9', with po and pd ordered and disordered contributions of magnetic moment. This could be a hint for the geometry of disorder. We should mention that we found stepwise increases in magnetization with field at different compositions in Er specimens previously investigated with neutrons.

Saturation moments of 7.2 pB and 7.6 pB were reported for powders of ErAl, [6, 71. A low saturation moment of ErAl, single crystals has been recently described in terms of crystal field effects [8, 91. There the magnetic moment is shown to be along (1 11) and is still rising in fields of 30 kOe. Neutron diffraction [5, 10, 111 however, indicates values close to the tri- positive ion one. The low saturation moment in the powdered binary can therefore be explained partly by strong anisotropy effects, the low moment found by neutron diffraction for the pseudobinary, however, should indicate a different situation.

T B C O , . ~ ~ ~ A L , . , , ~ .

-

Temperature dependence of magnetization of T ~ C O ~ ~ ~ , ~ A ~ , . , , ~ and reciprocal sus- ceptibility vs temperature are shown in figure 2. Indica-

a 2

s

b

too

50

FIG. 2. - Magnetization ⁽⁰⁾and reciprocal molar suscepti- bility (1/~,) vs. temperature for TbCoo. 525Al f.475. A curve was obtained by cooling in a field of 4 300 Oe. Applied fields were

respectively 19 400, 15 400, 4 300 and 900 Oe.

tions for antiferromagnetism are apparent at all applied fields. Neel temperatures are respectively : 26.4 OK (900 Oe), 13.2 OK (4 300 Oe) 15 OK (15 400 Oe), 8.3 OK (19 380 Oe). If the substance is cooled in a field (4 300 Oe) -the curve resembles ferromagnetic behavior with T , ⁼57 OK. The magnetization follows Curie-Weiss law for T > 50 OK up to room tempera- ture with 0 = 39 OK and an effective moment of 9.6 pB.

Field dependencies of magnetization (Fig. 3) indicate

FIG. 3. - Field dependencies of magnetic moment (p) per Tb at various temperatures for 1) TbCoo.5~5A11.4~5 and 2)

TbCo. 3A11.7.

the material to be metamagnetic (S-type curve). At 2 OK and 1.6 OK the magnetic moment exhibits an almost discontinuous rise in the order of about 2 pB at 12 kOe. Beyond that the magnetization is still rising to saturation values of 5.41 pB (1.6 OK), 5.43 pB (2 OK) and 5.63 pB (4.2 OK). Stepwise increases in magnetiza- tion have also been observed at different compositions ; the case of TbCo0,,Al,., is included in figure 3. This latter preparation has been investigated with neutrons and a situation similar to the Er materials obtaines : Pure ferromagnetism with a substantially quenched ordered moment is observed at 4.2OK although the specimen exhibits a Neel temperature of 35 OK in a field of 500 Oe.

Saturation moments of TbAI, powders extrapolate to 6.89 pB at 1.6 OK and to 6.56 pB at 4.2 OK, similar to previous investigations [6,7]. At 500 Oe the magnetiza- tion is slightly increasing from 1.6 to about 68 OK and then decreasing. At 40 kOe, however, the substance resembles a pure ferromagnet with a Curie temperature of 109 OK. Curie-Weiss behavior is observed at higher temperatures with 0 = 105 OK and p,,, = 9.44 pB.

Discussion. - There are several approaches to explain the two most prominent features of this study : the low ordered moments at cryogenic temperatures (with the inherent peculiarities of spontaneous increases with field) and the peaks in the temperature dependence of magnetization. Since the finding of appreciable Stark splitting in Pr metal [12], there has been a growing interest in crystal field quenching of the orbital magnetic contributions in rare earth jnter- metallics. Bleany [13] e. g. has tried to reason low moments in RNi, [14, 151 compounds in terms of

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MAGNETIC CHARACTERISTICS OF THE << LOW MOMENT COMPOUNDS >> C1 -1143

Stark splitting comparable in magnitude to the exchange interaction. Besides by crystal field effects, low moments would naturally be accounted for by some antiferromagnetic contributions or by the failure to achieve saturation at insufficiently low fields [16].

None of these models seems to be entirely satisfac- tory for the present compounds. The materials are neither clearly antiferromagnetic nor is lack of saturation involved as is evidenced by neutron diffraction [5].

Moreover, crystal fields, if active, should only account for small effects since neutron diffraction [5, 10, 1 I]

and Mossbauer work [7] on ErAl, e. g. show values close to the free tripositive ion value. Our neutron diffraction results, furthermore, show that the <( lost )>

Er sublattice moment is readily ⁽⁽found ⁾⁾again in the disorder background scattering.

The compounds are thought rater to be in a peculiar state between order and disorder. Disordered antiferromagnetic components may be present partially in the pure binaries (see the temperature behavior of magnetization in TbAI,) and are considerably enhanc- ed as the valence electron concentration is decreased by Co replacing Al. Such a change in the type of magnetic order to an antiferromagnetic contribution accords with the principles of indirect exhance, only it is a disordered contribution in this instance rather than an ordered one. The state of the disordered component is speculative beyond this point. It could be either static or dynamic. If dynamic, a possible precession along a preferred axis may happen only with specified angles and could explain the sudden rises in magnetization with field.

References

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[2] OESTERREICHER (H.) and WALLACE (W. E.), LESS (J.), [lo] WILL (G.), Z. Naturforsch, 1968, 23 a , 413.

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