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STRUCTURAL AND MAGNETIC STUDIES OF THE PSEUDO-BINARY COMPOUNDS (Gd1-xNdx) Co2

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HAL Id: jpa-00214056

https://hal.archives-ouvertes.fr/jpa-00214056

Submitted on 1 Jan 1971

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STRUCTURAL AND MAGNETIC STUDIES OF THE PSEUDO-BINARY COMPOUNDS (Gd1-xNdx) Co2

G. Primavesi, K. Taylor, I. Harris

To cite this version:

G. Primavesi, K. Taylor, I. Harris. STRUCTURAL AND MAGNETIC STUDIES OF THE PSEUDO-

BINARY COMPOUNDS (Gd1-xNdx) Co2. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-661-

C1-662. �10.1051/jphyscol:19711229�. �jpa-00214056�

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JOURNAL DE PHYSIQUE

Colloque C I, supple'ment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1 - 661

STRUCTURAL AND MAGNETIC STUDIES

OF THE PSEUDO-BINARY COMPOUNDS (Gd,-Pd,) Co,

by G. J. PRIMAVESI (*), K. N. R. TAYLOR (*) and I. R. HARRIS (**)

Rksumk.

- Nous avons effectue des mesures d'aimantation et de parametre du reseau pour les compods Gd1-~Nd~Co2. On interprete le comportement magnetique en termes de moment bloqu6 du nbdyme dont l'amplitude varie avec la temp6rature et la composition. On peut mettre en relations les variations de parametre du rkseau et les don- nQs magnetiques.

Abstract. - Measurements have been made of the magnetization and lattice parameters of the compounds Gd I-,NdzCo2. The magnetic behaviour has been interpreted in terms of a quenched neodynium moment whosemagnitude varies with temperature and composition. The changes in lattice parameter may be correlated with the magnetic data.

Introduction. - In the course of a series of inves- magnetization curves. Saturation moment data of tigations into the magnetic properties of compounds figure 1 for less than 50 % neodymium, suggests a and alloys formed with the heavy and light rare earth slightly higher value for the equilibrium concentration metals, and of the effects of magnetic order on the at

x =

0.57. A specimen of this composition was lattice parameters of Laves phase compounds, we prepared and gave the magnetization temperature data have examined the pseudobinary series Nd,Gd, -,Co,. given in figure 2. On increasing the temperature the

Experimental.

-

The techniques used for speci- men preparation, the determination of lattice para- meters and magnetization measurements have been described previously [I]. Figure 1 shows the variation

I

I

FIG. 1. -

Composition dependance of the room temperature lattice parameters and saturation magnetization at

4.2 OK

for

Gdl-zNdzC02.

with neodymium concentration of the room tempera- ture lattice parameters and the saturation moments a t 4.2 OK. The observed Curie temperatures decrease linearly between those of the terminal compounds (GdCo,

=

416OK, NdCo,

=

113 OK) to better than 3 %. For neodymium concentrations between x

=

0.57 and 0.67 the magnetization-temperature curves exhi- bit compensation points suggesting an equilibrium concentration of x = 0.55. Reversal of the magnetiza- tion was only observed in the variation of remanence with temperature ; for applied fields greater than zero the compensation points appeared as minima in the

(*)

University of Durham.

(**)

University of Birmingham.

Ro.

2.

- Temperature dependance of the remanent magne- tization and magnetization at

4

kOe applied field for composition

x = 0.57.

observed remanence reversed sign at T = 90 OK and 205OK, as shown in the figure. In an applied field there is a slight minimum in the magnetization at approximately 80 OK but the rapid rise in magnetiza- tion for T > 1000K obscures any feature which may occur near 200 OK.

The lattice parameter variation across the series shows a change in slope at about

x =

0.33 and a sudden rise at

x FZ

0.57.

Discussion. - Previous workers [2, 31 have shown that the observed molecular moments of the AB, compounds can be understood in terms of an exchange interaction which causes the spins of the rare earth and transition metal ions to be anti-parallel. With the heavy rare earths this leads to a ferrimagnetic structure and with the light rare earths to ferro- magnetism. Use of this model leads to values of the ionic cobalt moment close to 1.0 pB for GdCo, and 0.8 pB for NdCo,. In addition it is frequently neces- sary to invoke a crystal field quenched rare earth

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711229

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C

1

-

662 G . J. PRIMAVESI, K. N. R. TAYLOR A N D I, R. HARRIS

moment to give a favourable comparison with the

experimental magnetization data. In this way pNd in NdCo, is found to be 2.6 p,. The molecular moment data shown in figure 1 may be examined in terms of these values. For x < 0.7 the observed results agree well with a total moment for the compound calculated on the basis of ,uGd

=

7.0 p,, pNd

=

2.1 p,,

PC,,

varying linearly from 1.0 to 0.8 p,, and an antiparallel alignment of the gadolinium moments with respect to the neodymium and cobalt moments. These values, however, predict a NdCo, moment of approximately 3.7 p, which is appreciably less than that observed.

The neodymium rich specimens

( x

> 0.75) are better fitted by pNd

=

2.6 p,, and it would appear that the rapid rise in the total moment in the vicinity of x

=

0.72 originates from this change in the neodymium moment.

The reduced moments of the neodymium ions almost certainly arise because of the effects of crystal field splitting of the J

=

912 level of this ion ; this has been discussed in the past by Bleaney [4]. In these compounds the exchange field is comparable to the crystal field, and no quantitative calculations of the combined effects of both these fields on the J

=

912 manifold have been made.

By computing the energy level diagram for various crystal and exchange field strengths we have been able to account for the behaviour described above. Using the Hamiltonian.

X

=

X,, + X,,

agreement with experiment is only possible provided the easy direction of magnetization is parallel to the 11 1 direction. This Hamiltonian has been eveluated for exchange fields up to 2 MOe. The ground state level a t Hex

=

500 kOe has a moment of about 2.3 p,, the first excited state lying some tens of degrees above this and having a moment in excess of 3 p,. By allowing a continuous variation of the exchange and crystal field parameters the degree of admixing of the lower levels changes and in NdCo, can give an increase in the neodymium groundstate moment of 0.5 p,. Although the absolute magnitudes of these moments are higher than the experimental values it is encouraging that moment increases of the

type observed can be predicted. Excitation into the excited states is also thought to be responsible for the anomalous results of figure 2. The

x =

0.57 specimen is predicted to have zero magnetization at 0 OK from the measurements made at the gadolinium rich end of the series. This is found to be the case, and with increasing temperature the increase in remanence arises from the detailed differences between the magne- tizations of the different sublattices with temperature.

This would not normally become zero again until the Curie temperature is reached. However, due to ther- mal excitation into the excited states of the J = 912 energy level system the neodymium moment is increased and upsets the magnetization balance in the system.

Assuming that at low temperatures the net magnetiza- tion is parallel to that of the gadolinium sublattice, then the increase in pNd causes the molecular moment to decrease to zero and subsequently to increase with the net magnetization parallel to the neodymium and cobalt sublattices. We estimate that the change in pNd is of order 0.5 p, to account for the total change in remanence between the turning points. Having attained the higher neodymium moment the individual sublattice magnetizations no longer give a compensa- tion point at 0 OK, and it is possible that the second reversal at 205 OK may be the compensation point corresponding to the new moment values.

The magnetization of this specimen increase rapi- dly for T > 100 OK, reaching a maximum of 0.34 p, at 200 OK, which is of the correct order for a neody- mium moment change of 0.5 p,.

The change in slope of variation of the lattice para- meter with composition at

x =

0.33 coincides with the composition at which the Curie temperature becomes comparable to room temperature. Conse- quently the gadolinium rich specimens are magnetically ordered at the measuring temperature, while those for x > 0.33 are not. Changes of this type have been observed previously in other AB, compounds [l]

and may be attributed to magnetostriction effects.

The discontinuous change in lattice parameter near to the equilibrium composition may be associated with the changes in the 4 f state caused by both composition and temperature.

References

[I] HARRIS (I. R.)

et

al., J. Less Comm. Metals, 1969, [3] LEMAIRE (R.), Cobalt, 1966,

33,

201.

19, 437. [4] BLEANEY (B.), Rare Earth Research ed. K. S. vorres [2] FARREL

(J.)

and WALLACE (W. E.), Znorg. Chem. Gordon and Breach. 1964,

p.

499.

1966. 5, 195.

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