HAL Id: jpa-00214490
https://hal.archives-ouvertes.fr/jpa-00214490
Submitted on 1 Jan 1971
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
CANTED SPIN PHASE IN GADOLINIUM-IRON-GARNET
J. Bernasconi, D. Kuse
To cite this version:
J. Bernasconi, D. Kuse. CANTED SPIN PHASE IN GADOLINIUM-IRON-GARNET. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-205-C1-206. �10.1051/jphyscol:1971163�. �jpa-00214490�
JOURNAL DE PHYSIQUE Colloque C I, supple'ment a u no 2-3, Tome 32, Fe'urier-Mars 1971, page C 1 - 205
CANTED SPIN PHASE
IN GADOLINIUM-IRON-GARNET (*) J. BERNASCONI and D. KUSE
Brown Boveri Research Center, 5401, Baden (Switzerland)
Rhsumi.. - Nous ktudions le comportement des ferrimagnktiques A trois sous-reseaux dans un champ exterieur.
Dans le plan champ-tempkrature nous determinons les limites de stabilitk des phases colineaires vis-8-vis dela formation d'angles entre les differents moments des sous-rkseaux. L'application de notre analyse bas& sur la theorie du champ molkulaire au GdIG montre que la formation d'un angle entre les deux sous-rkseaux de fer fortement couples ne peut pas ttre negligke. En mesurant la rotation de Faraday dans des champs magnktiques s'elevant jusqu'a 10 kOe nous avons pu observer la phase de spins non colinkaires dans GdlG au voisinage du point de compensation.
Abstract. - The behavior of three-sublattice ferrimagnets in an external field is investigated. In the field-temperature plane we determine the stability limits of the collinear phases with respect to angle formation between the various sublat- tice moments. The application of our molecular field analysis to GdlG shows that the angle formation between the two strongly coupled iron sublattices cannot be neglected. By measuring the Faraday rotation in fields up to.10 kOe we were able to observe the angled spin phase in GdIG in the vicinity of the compensation point.
A magnetic field applied t o a ferrimagnet tends to align the sublattice moments parallel to itself, whilst the exchange interactions try to maintain an anti- parallel configuration. Under certain conditions of field and temperature this competition can result in spin configurations in which the sublattice moments form angles with each other and with the field [l-41. It should be noted that this angled structure occurs even in the absence of magnetic anisotropy.
G d I G is a three-sublattice ferrimagnet with a compensation point. The exchange coupling of the rare-earth sublattice with the iron sublattices is weak compared with the dominant coupling between the two iron sublattices. Therefore one expects that, in small fields, angles are formed predominantly between the rare-earth moment and the net iron moment, and that the antiparallel alignment of the two iron moments is broken u p only in much higher fields. Thus, when molecular field theory has been applied to describe the behavior of garnets in an external field [1-31, an angle formation between the two iron moments has been neglected. This simplification leads t o an instabi- lity criterion for the collinear phases which resembles the corresponding two-sublattice relationship [I]. We have found, however, that even in small fields the simplified treatment can lead t o wrong results.
We have, within molecular field theory, derived the correct instability criterion for three-sublattice systems.
Together with thc molecular field equations this cri- terion determines the boundaries of the angled spin phase in the field-temperature plane. For the applica- tion t o G d I G we used Anderson's [5] molecular field coefficients which were obtained from a fit of magneti- zation measurements. The resulting phase diagram for fields smaller than 2 000 kOe is shown in figure 1. Due t o the very strong exchange interactions the angled phase exists only in extremely high fields, except a t temperatures near the compensation point. In the right hand collinear region the two sublattice moments
(*) This is a short version for the conference proceeding.
A detailed report will be published elsewhere.
v . 1
0 100 200 /300
TEMPERATURE IN O K TcOmp
FIG. 1. - Low field part of the phase diagram for GdIG.
Arrows 1, 2, 3 denote the directions of the a, d, c sublattice moments respectively. At P I the c-sublattice is completely demagnetized. The broken lines give the slopes of the phase
boundaries near the compensation point Tcomp 1 and 3 which (below the Curie point) are antiparallel t o the field Ho first decrease in magnitude as H, increases. Then the rare-earth moment vanishes and subsequently increases parallel t o Ho. A t much higher fields the same happens to the moment of the octa- hedrally coordinated iron.
I t may be remarked a t this point that for G d I G the simplified procedure, which treats the two iron moments a s strictly antiparallel, gives wrong results
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971163
C 1 - 206 J. BERNASCONI AND D. KUSE even in very small fields. Our calculations show that
the slopes of the phase boundaries (near to the com- pensation point) would have the wrong sign. Thus the angle formation between the two iron moments cannot be neglected although the angles are presumably very small in low fields.
According t o the phase diagram of figure 1 the sublattice reorientation in fixed field takes place in a finite temperature interval the width of which is nearly proportional to the field for fields up to 100 kOe. We have determined this width as a function of field by measuring the temperature dependence of the optical Faraday rotation in fields of up to 9.7 kOe.
The Faraday effect at optical wavelengths is related to the individual sublattice moments rather than to the total magnetization, and the sublattice reorientation is clearly demonstrated by a corresponding reversal of the sense of the rotation. Three typical recorder traces are shown in figure 2. The rotation angle changes sign in a temperature interval which is about 2 O wide at 9.7 kOe and l o a t 4.6 kOe. The experimental values of
+ 0.1 deg/kOe for the slopes of the phase boundaries at the compensation point compare favorably with
the calculated ones of f 0.094 deg/kOe. It should be recalled in this context that the molecular field coefficients were not fitted to our results but were taken from magnetization measurements [5].
FIG. 2. -Temperature dependence of the Faraday rotation
in GdIG. Sample thickness d
-
0.49 mrn, wavelength 1-
1.15pm.References
[I] CLARK (A. E.) and CALLEN (E.), J. Appl. Phys., 1968, [4] CLARK (A. E.) and ALUEN (R. S.), J. Appi. Phys.,
39, 5972. 1970, 41, 1195.
A number of references to earlier work is compiled
herein. [5] ANDERSON (E. E.), Proceedings of the International
[2] KHARCHENKO W. F.), EREMENKO (V. V.) and B E L Y ~ Conference on Magnetism, Nottingham, 1964 (L. I.), Soviet Physics JETP, 1969, 28, 219. (The Institute of Physics and the Physical Society, [3] BUCHANAN (R. A.) and CLARK (A. E.), Solid State London, 1965), 660.
Commun., 1969, 7, 1087.
