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STUDIES OF MAGNETIC ANISOTROPY OF MIXED Eu-Sm RARE EARTH IRON GARNETS
U. Atzmony, K. Hardy, J. Walker, E. Loh
To cite this version:
U. Atzmony, K. Hardy, J. Walker, E. Loh. STUDIES OF MAGNETIC ANISOTROPY OF MIXED Eu-Sm RARE EARTH IRON GARNETS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-920- C1-921. �10.1051/jphyscol:19711327�. �jpa-00214360�
JOURNAL DE PHYSIQUE Colloque C 1, supplktnent au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 920
STUDIES OF MAGNETIC ANISOTROPY OF MIXED Eu-Sm RARE EARTH IRON GARNETS
U. ATZMONY (*), K. HARDY, and J. C . WALKER The Johns Hopkins University, Baltimore, Maryland, U. S. A.
and E. LOH
Towson State College, Baltimore, Maryland, U. S. A.
Rhume. - Nous avons utilise I'effet Mossbauer de la transition a 21,6 keV de l5lEu pour etudier l'anisotropie magnetique de plusieurs grenats de terres rares de formule : RZSm~/2-xEu~l~IG, 0 < x < 4 A unetempkrature de 4,2 OK.
La direction de facile aimantation du cristal peut Etre determinke en utilisant le spectre Mijssbauer de I'europium.
La direction de facile aimantation correspond au plan [Ill] pour la plupart des grenats, et au plan [I101 dans le cas du grenat de samarium.
En modifkant les proportions de R et du samarium dans le grenat R x S m ~ ~ 2 - z E ~ ~ ~ ~ , nous avons reussi a changer la direc- tion de facile aimantation. Cette methode nous permet de tirer des conclusions sur la valeur relative de l'anisotropie de plusieurs grenats de terres rares, a une temperature de 4,2 OK. Les resultats obtenus pour l'erbium sont sp6cialement intt- ressants. Nous avons Ctabli que la direction de facile aimantation dans le grenat d'erbium A 4,2 OKcorrespond au plan [loo]. Ce resultat semblerait trancher un debat prolong6 sur la question.
Abstract. - Mossbauer effect studies of the 21.6 keV transition of l5lEu have been carried out on many mixed compounds of rare earth iron garnets of the formula R z S m ~ l ~ - z E ~ ~ ~ ~ I G , 0 < x B 4 at 4.2 OK. The direction of easy magne- tization in the crystal is easily detected, using Mossbauer spectra of the Eu transition. The direction of easy magnetization is [Ill] in most of the RIG and [I101 in the SmIG. By changing the relative amounts of the R and the Sm in the
RzSn1112-zEui~zIG ,
we succeeded in changing the direction of easy magnetization. This method enables us to draw conclusions as to the relative strength of the anisotropy of various RIG of the heavy rare earth elements at 4.2 OK. Of special interest are the results with Er. Here we showed that the direction of easy magnetization in ErIG at 4.2 OK is the [loo], a result about which there is some debate in the literature.
I. Introduction. - The magnetic anisotropy of the rare-earth iron garnets is not well understood.
One of the consequences of this anisotropy is the exis- tence of a preferred direction of easy magnetization.
For the cubic symmetry of the garnets, the bulk aniso- tropy energy can be expressed as
where a,, a,, and a, are cosines of the direction of the magnetization. It can be seen that the direction of easy magnetization lies parallel to one of the principal cubic directions : [I 1 I], [I 101 and [I001 depending on the values of K, and K 2 .
The principal source of the anisotropy is the rare earth-iron interactions [I, 21. Therefore, when one has a mixed garnet system with several rare earth atoms, one can assume, to first order, that the bulk anisotropy constants K , and K 2 arc weighted averages of the corresponding constants for the pure garnets.
In most of the pure rare earth garnets, the easy magnetization direction is [I1 11 [3, 41. One exception is samarium iron garnet where the direction of easy magnetization is [110]. There is some uncertainty about the direction of easy magnetization in erbium iron garnet. It has been reported to be the [I 111 direction from previous Mossbauer effect experiments [4] while optical measurements favor the [I001 direction [5].
The present work strongly supports the [loo] choice.
11. Analysis. - We have used Mossbauer effect measurements of the 21.6 keV transition in Eut5' t o detect the direction of easy magnetization in mixed rare-earth iron garnet systems. The effective magnetic hyperfine field acting on the europium nucleus in
magnetically ordered compounds is mainly due t o the exchange interaction which mixes the J = 1 excited state into the J = 0 ground state. Due to the anisotropy of the exchange interaction, the six inequi- valent rare-earth sites may experience different hyper- fine interactions determined by the direction of the spin of the iron atoms. This direction lies along the direction of easy magnetization or is slightly canted with respect t o it. For the [Ill], [110] and [I001 directions, the europium ions will be grouped in magnetically inequivalent sites according to the ratios 3 : 3, 4 : 1 : 1, and 4 : 2 respectively (If there is canting the number of inequivalent sites may increase to a maximum of six.)
Atzmony et al. [6] carried out measurements with (Sm-Eu) IG and found a spectrum completely diffe- rent from that of pure EuIG. The result could be explai- ned satisfactorily only by assuming that the direction of easy magnetization had changed from [ I l l ] to [110] with the addition of as little as 5 % samarium to the EuIG. It follows that the anisotropy of the Eu-Fe interaction is small compared to that of the Sm-Fe interaction. There was, however, some slight depen- dence of the spectrum on the amount of samarium which was explained as the result of some canting of the iron spins with respect to the [I 101 direction. The assumption of some canting improved the agreement between the theoretical and experimental spectrum.
For the present work Mossbauer studies were carried out using the 21.6 keV transition of "'Eu and polycrystalline absorbers of the form
with
(*) New Address : Nuclear Res. Center, Beer-Sheva, Israel. All measurements were made with the absorbers at
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711327
STUDIES OF MAGNETIC ANISOTROPY OF MIXED Eu-Sm RARE EARTH IRON GARNETS C 1 - 921
4.2 OK. In this expression R is one of the heavy rare earths : Gd, Tb, Dy, Er, Tm, and Yb. Examination 1 0 0 0 of the resulting Mossbauer spectra yields information about the relative strengths of the anisotropy of the R-Fe interactions compared to the Sm-Fe interaction.
Typical spectra are shown in the figures. The points 9 9 0 -
are experimental and the curves are theoretical fits. ?I 6L1V
The experimental spectra obtained in these studies E~,,,,s~,.E~,,,I o 42.K
can be divided into three groups from which some aualitative results can easily be deduced. The first 9 8 0 case involves gadolinium as the R in the mixed garnet formula. For all amounts of gadolinium down to only 5 % samarium the spectra indicated a [I101 b o o o easy direction of magnetization characteristic of (Sm-Eu)IG. There were slight changes depending on x. A second group with R = Tb, Dy, Tm, and Yb, ,,,
showed spectra of the [ l l l ] type for x larger than g
113 and spectra of the [I 101 type for x smaller than
5
116. In general there was a continuous change of the
9 8 0
Ef,,,Sm,,,E*,,,IO 42.K
spectra for values of x between these two limits. - g The third group of spectra are those with B R = erbium, shown in figure I. For x > $ the spectra are completely different from either the [I 1 I] or [I I01 types. This suggests that the easy magnetization in these cases is the [loo] direction. Indeed, the Er%Eu,/,IG spectrum can be fit reasonably well assum- ing two inequivalent sites with relative populations 4 : 2 which is expected for the [I001 direction of easy
magnetization. As in the case of the second group, "O Er,,,sm,,, Eu,,,IG 42.K
the spectra for x < $ were of the [I101 type and change continuously to the [I001 type with increasing x.
A quantitative analysis of the data produces some difficulties. In order to determine the anisotropy '"
tensor Atzmony et al. [6] fit the (Sm-Eu) IG spectra - 8 -6 -4 -2 o 2 4 6 8
to three sites with the 4 : 1 : 1 population ratio cha- Veloc~ty C ~ I S
racteristic of the [I 101 direction with no canting. With FIG. - (Top) Spectrum is characteristic of [1001 direction
small canting in the [loo] plane leading to four sites of easy magnetization. (Middle) A transition spectrum between
with 2 : 2 : 1 : 1 population, a better fit was achieved. [loo] and [I101 direction of easy magnetization. (Bottom)
The anisotropy tensor which resulted was in total Spectrum more closely resembles S ~ ~ E U I / Z I G suggesting a disagreement with recent NMR measurements ver- 11 101 direction of easy magnetization.
formed with a single crystal of EuIG at 4.2 OKa[7].
In the present work we have been able to get good fits to the Sm,/,Eu%IG data by changing to a 1 : 1 : 2 : 2 fit, i. e., the more populated sites experience the smaller fields. This analysis yields results which are consistent with the NMR results and with the results for Erl/,Eu%
I G which can be fit to two sites with intensities 4 : 2.
The disadvantage of this analysis of the (Sm-Eu)IG data is that it implies large canting (about 150 with respect to the [I101 direction. If one analyzes the Er%Eu%IG in the same way, the canting angle is 150 with respect to the [loo] direction.
Attempts to interpret spectra of the second group in this manner are not very meaningful as one can no longer assume that the canted iron spins be in the [loo] plane as in the previous cases. Also, several
different assumptions about the anisotropy tensor yield essentially equivalent fits.
111. Conclusions. - One can get some useful information about the magnetic anisotropy of the rare-earth iron garnets from polycrystalline samples.
Such measurements strongly indicate that the direction of easy magnetization in ErIG is [I001 in agreement with optical data, but in disagreement with previous Mossbauer effect measurements. One can reconcile Mossbauer effect measurements with NMR results if canting of the iron spins is assumed. The main disadvantage of this analysis is that the fit is not unique in all cases. To clarify this ambiguity, Moss- bauer effect measurements on single crystals are needed.
References
[I] NOWIK (I.), OFER (S.), Phys. Rev., 1967, 153, 409. [5] HARRISON (F.), THOMPSON (J.), and LUNG (G.), J . [2] ATZMONY (U.), BAUMINGER (E.), EINHORN (B.), HESS Appl. Phys., 1965, 36, 1014.
(J.), MUSTACHI (A.), and OFER (S.), J. Appl. [6] ATZMONY (U.), BAUMINGER (E.), MUSTACHI (A.), Phys., 1968, 39, 1250. OFER (S.) and TASSA (M.), Phys. Rev., 1969, [3] PEARSON (R.), J. Appl. Phys., 1961, 33, 1236. 179, 514.
[4] GELLER (S.), REMEIKA (J.), SHERWOOD (R.), WILLIAMS [A STREEVER (R.) and CAPLAN (P.), Phys. Rev. Lett., (H.), and ESPINOSA (G.), Phys. Rev., 1965, 137 1970, 24, 978.
A, 1034.
