SPIN ROTATION IN GdCo2

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SPIN ROTATION IN GdCo2

U. Atzmony, G. Dublon

To cite this version:

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SPIN ROTATION IN

GdCo,

U. ATZMONY (*) and G. DUBLON

Nuclear Research Centre-Negev, P. 0. Box 9001, Beer-Sheva, Israel

R6sum6. - Une Btude de l'effet Mossbauer de 57Fe dans GdCoz est prksentke. Les mesures revelent un spin de l'axe cubique [loo] en dessous de 200 K, a I'axe [I101 au-dessus de 290 K.

Dans l'intervalle entre ces deux tempbatures, la magnktisation prend des directions non majeures.

IR

spin dans GdCo2 peut Etre expliquk par le traitement phknom6nologique de l'bnergie aniso- tropique magnbto-cristalline.

Abstract.

-

A Mossbauer study of 57Fe in GdCon is reported. The measurements reveal a spin rotation from the [loo] cubic axis below 200 K, to the [I101 axis above 290 K. Over the spin rotation temperature interval, the easy magnetization assumes non-major directions. The spin rotation in GdCoz can be acounted for in terms of a phenomenological treatment of the magnetocrystalline anisotropy energy.

1. Introduction.

-

The RM, compounds, where R is a rare earth element and M is a transition metal

(Al, Fe, Co or Ni), while possessing the same MgCu,

cubic Laves structure with very similar lattice parameters, exhibit a variety of complex properties of magnetic origin [I]. Most of these properties may be ascribed mainly to the anisotropic interaction of the 4f localized shells with the crystal fields [2-91. This approach obvioulsy excludes the Gd-containing compounds which have the S-state ion. The RM, compounds further display an interesting magnetic behavior from the point of view of magnetism in transition metals 110-131. This is best observed by substituting the R element by non-magnetic Y or Lu. The Mossbauer effect of 57Fe has been established as a highly effective tool in the study of magnetic anisotropy effects in cubic Laves RFe, compounds [2-61. Data on the directions of easy magnetization and spin rotation processes, were in turn used to gain insight into crystalline electric field (CEF) effects and to determine crystal field parameters [8]. This technique is currently applied to the study of RCo, compounds doped with 57Fe [9]. The present Mossbauer investigation of 57Fe-doped GdCo, is meant for the study of sources of magnetocrystalline anisotropy in the RCo, compounds, other than the 4f-CEF interaction. The presence of these additional contributions to the magnetic anisotropy free energy, has been indicated by slight discrepancies between the experimental and single ion calculated spin orientation results of non-S-state RM2 compounds p-101.

GdCo, is a ferrimagnet with a Curie point of (*) Also at Materials Engineering Department, Ben-Gorion,

University of the Negev, Beer-Sheva, Israel.

T,

= 395 [ll], or, as elsewhere reported, 409 K [13]. The ferrimagnetism of GdCo, is due to the negative coupling of the R and M spins, which is commonly found [6] in the RM, compounds [ l l , 121. s 4 n g in the S-state, the Gd3+ moment in GdCo, is only negligibly affected by the CEF and should thus be close to its free-ion value of 7 pB. From the saturation moment of GdCo,, one finds [ll-141 1.0 ,uB per cobalt in GdCo,. It should be noted that the cobalt moments in RCo, are believed to be essentially paramagnetic [lo-131, and are polarized by the strong localized magnetic moment of the rare earth.

2. Experimental.

-

The compounds

G ~ ( C O , - , ~ ~ F ~ , ) , with 0.01 < E

<

0.05

,

were prepared by arc melting and subsequent anneal- ing of the arc-cast samples containing stoichiometric amounts of 3 N pure Gd, 4 N Co and 80

%

isotopi- cally enriched 57Fe. Microprobe and x-ray analyses established the presence of less than 1

%

of foreign phases and compositional homogeneity within 0.5

%.

The lattice parameters of the 57Fe-containing alloys were in good agreement with the previously reported values for iron-free GdCo,, and varied by less than 0.05

%

with the iron content in the 1-5

%

Fe range. The Mossbauer measurements were made using a constant acceleration type spectrometer with a 15 mCi "Co-Pt source. The 14.4 keV y-ray transition of 57Fe was detected after passing through a 300- mesh, 22 mg/cm2 GdCo, (57Fe) absorber. The temperature was stabilized to within 0.5 K in helium or nitrogen flow cryostats, over the 4.2-300 K temperature range. The Mossbauer parameters of G d c ~ , ( ' ~ F e ) were only negligibly dependent upon the iron content

40

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C6-626 U. ATZMONY AND G. DUBLON

in the 1-5

%

Fe range. The presence of low 57Fe concentrations, as has been verified by similar MOSS- bauer measurements of other R C O , ( ~ ~ F ~ ) compounds [9], increases only slightly the Curie points as compared w ~ t h the iron free compounds [ll-131. This is possibly due to to a weak fluctuating spin polarization caused by the 57Fe moments, which becomes noticeable only as the R-Co and R-R exchange is sufficiently diminished. The presence of 57Fe in RCo, is thus registered in the

Mossbauer spectra only very close to T,, and is reflect- ed by small wings that are found [9] around an essentially quadrupole-split Mossbauer doublet.

3. Procedure of analysis.

-

The interpretation of the Mossbauer spectra follows the pattern employed in previous studies of spin rotation in ternary [3] R:-,R;F~, and binary [4, 51 RFe, and [9] RCo, Laves compounds. The direction of the magnetization, n, relative to the cubic cell axes of these compounds, determines the number of inequivalent iron sites and their population ratio. Since the iron atoms evidently occupy the Co sites [9] in GdCo,, and exclud- ing deviations from cubic symmetry [14], the same approach is applicable here. A detailed description of

the least-squares computer fitting procedure for one-, two-, or three-site 57Fe Mossbauer spectra, was given elsewhere [4]. It takes into acount the angle between the direction of the magnetic exchange field and the CEF gradient. The computer program calculates the positions and intensities of the absorption lines by diagonalizing the Hamiltonian for each inequivalent site. It is assumed that : (i) the effective magnetic field is parallel to n ; (ii) all inequivalent iron sites posses the same isomer shift and lattice quadrupole constant $ eq, Q , but differ in the value of the magnetic hyperfine constant go p,, He,, ; (iii) the total quadrupole interaction is the sum of a lattice field contribution, with an axis ,of symmetry parallel to the local axis of symmetry, and a magnetically induced contribution with an axis parallel to the effective field, He, ; (iv) the absorption lines have a ~orenzian shape and equal width at half maximum, To.

4. Results.

-

A cursory study of the Mossbauer

spectra of 5 7 ~ e in GdCo, (Fig. I), reveals the presence of three temperature dependent magnetic states. Below 200 K, simple six-line patterns were recorded which are characteristic of a single iron (i. e. cobalt) site, and interpretable in terms of a [loo] easy axis of magnetization. A typical spectrum of this type, at 60 K, is

shown in figure 1. Spectra of a more complex structure were obtained above 200 K. These are superpositions of three six-line patterns with relative intensity 2 : 1 : 1 up to 290 K. At higher temperatures, spectra consist- ing of two six-line patterns with relative intensity 2 : 2 are obtained. Figure 1 presents the Mossbauer results of G d c ~ , ( ' ~ F e ) at 60 to 300 K, and least-squares computer simulations (solid lines) which were made

- L - 2 0 2 4 V E L O C I T Y ( r n r n l s )

FIG. 1.

-

Mossbauer spectra of 57Fe in GdCoz between 60 and 300 K. The least-squares computer fits (full lines) are [I001 and [llO]-type simulations at 60 and 300 K, respectively. The intermediate spectra were fitted assuming a [uuw] direction of

easy magnetization.

following the above considerations. These results thus imply the presence of a spin rotation in GdCo,, from the [loo] cubic axis below 200 K, to the [110] axis above 290 K. In the intermediate 200-290 K range, the easy magnetization, n, assumes non-major directions. The corresponding spectra could be adequately fitted (Fig. 1) by assuming that n is contained in the (110) plane. The possibility that n is not confined to a cubic symmetry plane while rotating from one major cubic axis to another, could not be verified using the present Mossbauer data. Such a direction of n produces four inequivalent Co sites (1 : 1 : 1 : 1).

A

four-site computer fit, employing more adjustable parameters, should evidently be at least as adequate as the fits given in figure 1. Corrections that were made for a broadening of the absorption lines due to both random iron occu- pation of the Co sites and 57Fe saturation effects, proved insignificant and of negligible dependence on the iron content. The temperature dependence of the 57Fe effective hypedine fields, He,,, at the various Co

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FIG. 2. -Effective hyperfine fields of 57Fe in GdCo2 as a function of temperature. Up to 200 K a single iron site is found. Three inequivalent iron sites with relative intensity 2 : 1 : 1, and two sites (2 : 2) are found at 200-290 K and at 300 K, respectively. The dashed line is the intensity-weighed average

of the two and three hypefine fields above 200 K.

following the fitting procedures described above. The validity of these procedures of analysis is reflected by the smooth and normal behavior of the He,, vs T curve, which is very similar to the temperature dependence of the magnetization of [ l l ] GdCo,. It should be noted (Fig. 2) that the intensity-weighed average of the two and three hyperfine effective fields above 200 K, coincides with an extrapolation to high T of the single

He, vs T curve below 200 K. The isomer shift and quadrupole constants of 57Fe in GdCo, at 4.2 and 300 K, are listed in table I.

Isomer shift and quadrupole constants (in mm/s)

of 57Fe in GdCo, at 4.2 and 300 K

4.2 K 300 K

-

Isomer shift - 0.13

+

0.01 - 0.27 & 0.01 Lattice quadrupole cons-

tant 0.10 & 0.01 0.19

*

0.01 Induced quadrupole cons-

tant 0.058

+

0.005 0.078 & 0.004

5. Discussion.

-

The occurrence of various easy axes of magnetization in non-S-state FRe, [2-5, 151, RCo, [9] and RAl, [16], and in particular spin rota- tions that are found in some mixed rare earth ternary R:-,R:F~, compounds [3], in binary CeFe, [4], HoFe, [5], SmFe, [15] and in NdCo, and HoCo, [9], were successfully reproduced by single ion crystal field calculations. These calculations, that were recently expanded [8] to yield also non-major axes of easy magnetization, involve the diagonalization of the single (rare earth) ion Hamiltonian for various direc-

exchange interactions are isotropic and that the magnetocrystalline anisotropy of the rare earth containing alloy is entirely due to the 4f-CEF interaction. The observed spin rotation in GdCo, cannot be described in the same fashion. Further, the presence of a [I101 axis of easy magnetization in GdCo, at high temperatures, is unusual among the binary and ternary RM, compounds. SmFe, is the only binary compound in which n is parallel to the [I101 axis below about [15] 140 K, and unlike the other RFe, alloys, exhibits a sizeable crystal field induced admixture of the Sm3+ J- states [6]. Some of the ternary R: -,R:F~, compounds posses [I101 axes of n only at low temperatures [3, 81, and in NdCo, n is close, but not parallel, to the [I101 axis below about [9] 40 K.

With the CEF interactions excluded, two further major possible sources of magnetic anisotropy energy in RM, compounds should be considered. One is the magnetic dipole-dipole interaction between the localized moments, and the other is the exchange, direct or indirect, between spins. Due to the cubic point symmetry of the R sites, anisotropic parts of both the R-R dipole-dipole and the lowest term of the exchange between rare earths, vanish in the RM, compounds. The importance of anisotropies attributed to higher term R-R exchange interactions has been indicated [16] in HoAl, and perhaps also in GdA1,. It should, howe- ver, be noted that these contributions are expected to more significant in the presence of non-S-state R3 +

ions. Interactions which involve a transition metal element cannot be rejected offhand from symmetry considerations, as possible sources of magnetocrystalline anisotropy. A noticeable contribution to the

anisotropy energy, due to Fe-Fe interactions, has indeed been found in the RFe, system [3, 41. In this respect, the RCo, compounds apparently present some difficulties. The dominant exchange is between the R ~ + and cobalt ions, and is believed to result, essentially, in the polarization of the cobalt spins [lo]. The Co-Co interaction in RCo, is basically paramagnetic [lo, 111, as indicated by the strongly enhanced Pauli paramagnetism of YCo, and LuCo,. Though the possibility that the Gd-Co, Co-Co interactions and high-term Gd-Gd exchange, contribute significantly to the magnetic anisotropy of GdCo, is highly specula- tive, we are at the moment unable to present a more plausible explanation of the complex magnetocrystalline anisotropy properties of this compound. A comparison with other Gd-containing RM, compounds is also difficult. Though n in GdFe, is also along [loo] at low temperatures [4], it does not change its direction up to 300 K. In GdAl, n is parallel to the [ I l l ] axis.

-

.

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C6-628 U. ATZMONY AND G. DUBLON direction cosines (a,, a,, a,) of the direction of easy

magnetization with respect to the cubic axes :

The inclusion of the 8th power term in the above expression has been shown 181 to yield also non-major axes of easy magnetization in a cubic crystal. The restrictions, imposed on the temperature dependent bulk anisotropy constants, Ki, so as to obtain minima for E(n, T) for major as well as non-major directions of

n, are &en elsewhere 181. Figure 3 shows, in the

FIG. 3. -Boundaries of regions with different easy axes of K; = K,/K,, K; = _{K J K ~ plane, regions with different magnetization ofa}_cubic_crystal,_{in the K; =}_{K 1 / K 3 ,}_Ki₌_KZIK3 possible axes of easy magnetization in a cubic crystal. plane. For a detailed discussion of the figure see ref. 181.

Using the above spin orientation data, figure 3 enables a rough estimate of the temperature dependence of

K; and K; of GdCo,, i. e. at low temperatures K; and triangle LDO of figure 3. At 200 K < T < 290 K

K; are located at the upper part of the K;K; area. these values continously cross the transition region Above 200 K they are located at the right end in the (MFDF area of figure 3).

References

[I] TAYLOR, K. N. R., Adv. Phys. 20 (1971) 551.

[2] BOWDEN, G. J., BUNBURY, D. St P., GUIMARAES, A. P.

and SYNDER, R. T., J. Phys. C 2 (1968) 1367.

[3] ATZMONY, U., DARIEL, M. P., BAUMINGER, E. R., LEBEN-

BAUM, D., NOWIK, I. and OFER, S., Phys. Rev. B 7

(1973) 4220.

[4] ATZMONY, U. and DARIEL, M. P., Phys. Rev. B 10 (1974) 2060.

[5] ATZMONY, U. and DARIEL, M. P., AZP Con$ Proc. 24 (1974) 662.

[6] DUBLON, G., DARIEL, M. P. and ATZMONY, U., Phys.

Lett. 51A (1975) 262.

[7] DUBLON, G., KLIMKER, H., ATZMONY, U., DARIEL, M. P., ROSEN, M., GRAYEWSKI, A. and FEKETE, D., Phys.

Lett. 53A (1975) 23.

[8] ATZMONY, U. and DARIEL, M. P., Phys. Rev. B 13 (1976) 4006.

[9] ATZMONY, U., DARIEL, M. P. and DUBLON, G., Phys. Rev.

B (in press).

[lo] BLOCH, D., EDWARDS, D. M., SHIMIZU, M. and Vorrto~, J.,

J. Phys. F 5 (1975) 1217.

[ l l ] BURZO, E., Zntl. J. Magnetism 3 (1973) 161.

[12] FARRELL, J. and WALLACE, W. E., Znorg. Chem. 5 (1966) 106.

[13] HARRIS, I. R., MANSEY, R. C. and RAYNOR, G. V., J. Less Common Metals 9 (1965) 270.

[14] MOON, R. M., KOEHLER, W. C. and FARRELL, J., J. Appl.

Phys. 36 (1965) 978.

[15] ATZMONY, U. and DARIEL, M. P., Proc. 10th Rare Earth Conf., Carefree, Arizona, April 1973, p. 605. [16] KAPLAN, N., DORMANN, E., BUSCHOW, K. H. J. and