MAGNETIC ANISOTROPY AND RESONANCE LINE WIDTH IN RARE EARTH DOPED NiFe2O4 CRYSTALS

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MAGNETIC ANISOTROPY AND RESONANCE LINE WIDTH IN RARE EARTH DOPED NiFe2O4

CRYSTALS

R. Krishnan

To cite this version:

R. Krishnan. MAGNETIC ANISOTROPY AND RESONANCE LINE WIDTH IN RARE EARTH DOPED NiFe2O4 CRYSTALS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-148-C1-149.

�10.1051/jphyscol:1971148�. �jpa-00214475�

JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 148

MAGNETIC ANISOTROPY AND RESONANCE LINE WIDTH IN RARE EARTH DOPED NiFe,04 CRYSTALS

R. KRISHNAN

Laboratoire de MagnCtisme, C. N. R. S., 92, Bellevue, France

Resum&. - Les constantes d'anisotropie magnktocristalline XI, Kz et la largeur de raie A H des monocristaux NiFe I, 966Er0,o ^{1 4 0 4} et NiFe 1, 66Yb0,0 ^{1 4 0 4} ont Bte determinees par la resonance ferrimagnktique entre 4,20 et 323 OK.

La contribution des ions Er3+ et Yb3+ 5 K1 est positive B toutes les temperatures et a 0 OK correspond a 4- 0,18 x 10-16 et + 0,34 x 10-16 ergs/ion respectivement. Ces valeurs sont plus petites que celles observees dans le cas de YIG. On constate que Kz est negligeable dans ces cristaux. Ces cristaux montrent une variation de AH en fonction de la tempera- ture semblable & celle que Yon trouve dans le grenat dope avec ces terres rares. Alors que la contribution des ions Er3+

au AH est plus petite par rapport B celle trouvke dans le YIG, celle des ions Yb3+ est plus grande. Ainsi, chaque % des ions Yb3i- dans le site octaedrique contribue au pic de AH d'environ 200 et 160 Oe suivant les directions [loo] et [Ill]

respectivement.

Abstract. - Magnetocrystalline anisotropy constants K I , Kz and the line width A H of NiFel.966Er0.01404 and NiFe1.966Yb0.~1404 single crystals have been measured by ferrimagnetic resonance techniques-in the range 4.2 to 323 ^{O K .} The contribution from Er3+ and Yb3+ ions to K1 is posltive at all temperatures and single ion contribution at 0 OK is found to be + 0.18 x 1016 and + 0.34 x 10-16 ergs respectively. These values are smaller compared to those obtained for these ions when doped in YIG. However K 2 in these crystals are very small down to 4.2 OK. The temperature depen- dence of AH in these crystals resembles strongly that of YIG doped withthese rare earth ions. While Er3+ ion contribution to AH is smaller than that in YIG, that of Yb3+ ion is found to be higher. Each. % of Yb3+ ion in theoctahedralsite contributes of the order of 200 and 160 Oe along [loo] and [ I l l ] directions at the line width peak.

I. Introduction. - The rare earth ions of the 4-f group have been extensively studied in the garnet structure, commencing with their discovery by Ber- taut and Forrat [ l ] and their magnetic moment inves- tigation by Pauthenet [2]. The magnetization in these materials has been well interpreted by NCel's theory [3]. Further the study of magnetic properties such as anisotropy, line width, g-factor and some giant anisotropies associated with certain rare earth ions have received great attention since over a decade [4].

In these garnets these rare earth ions occupy the dode- cahedral (24c) sites. We report here for the first time the magnetic behaviour of some of the rare earth ions in the spinel lattice. The present work concerns the rare earth ions Er3

(4f11) and Yb3

(4f13) substi- tuted for Fe3+ ions in NiFe,04 single crystals. The preparation of these crystals (by flux method) and their characterization have been described else where [5].

11. Experimental procedure and results. - Ferrima- gnetic resonance was observed in 1) NiFe, .,,6Ero.01404 and 2) NiFel,,6,Yb,~,140, crystals in the X-band in the temperature range 4.2 to 323 ^OK. From the field for resonance along the symmetry axes K1 and K2 were calculated. Line widths were obtained from either cavity perturbation techniques or by tracing the resonance curve using a shorted wave guide.

Fig. 1 shows the temperature dependence of K,/M for NiFe204 crystal with and without the rare earth oxides. While the effect Er3+ ion on K1/M is evident only at lower temperatures that of Yb3+ is so at all temperatures. In both cases I K , / M decreases as the temperature is lowered showing a positive contri- bution from the rare earth ions. The angular depen- dence of H,,, showed a normal behaviour in the case of Er3+ ion at all temperatures, whereas in the case of Yb3+ ion, a small anomaly was observed below 20 ^OK. A small H,,, minimum occurring near 400 (measured from [OOl]) instead of at [I 111. We have however ignored this anomaly in computing K , and K,. Fig. 2 and 3 show the temperature dependence of

20j/ ^Ni Fe1.986 E r O , O 1 ~ O L ' Ni Fe1.98fj b O . O 1 ~ O L

f ^C

0 100 200 300 T°K

RG. I. - Tempbrature 'dependence of K I / M for NiFezO4, NiFel.966Er 0.01404 and NiFe1.966Ybo.01404.

Fro. 2. - Tempbrature dependence of line width for NiFe1.~6~

Ero.01404 crystal.

-

0 100 200 300 T°K

FIG. 3. - Tempkrature dependence of line width for NiFel.966

Yb0.01404.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971148

MAGNETIC ANISOTROPY AND RESONANCE LINE WIDTH C 1 - 149 the Iine width for Er3+ and Yb3

doped NiFe20, crys-

tals. It might be mentioned that in undoped crystals the Iine width is temperature independent and is of the order of 10 Oe, thus indicating that the peaks observed are only due to rare earth ion.

111. -Discussion. 1) Er3 ' ^ION : Er3 ' ion contribu- tion to K, is positive and begins to appear only below about 150 OK. However below 10 OK the contribution increases sharply. It is interesting to compare our results with those obtained in Er3' ion doped YIG [6]. The behaviour of Er3+ ion is qualitatively much the same in both host lattices. Pearson [63 has measured K1 in Er3+ ion doped YIG by torque methods and it is seen that Kl from Er3+ ions has positive sign and is measurable only below 100 OK.

The one ion contribution of Er3 to K1 at 0 OK calcu- lated in YIG is + ^4.7 x 10-l6 ergs as compared to + ^0.18 x 10-l6 ergs in NiFe204 crystals. It is seen that Er3+ contribution in YIG is about 25 times higher thanthat in NiFe,O,. However K, in our case is small enough to be determined with any accuracy, as against in YIG : Er, where it was negative and quite high. Temperature dependence of line width beha- viour is also qualitatively similar to that observed by Clarke et al. [7] in Er3

doped YIG. Line width along the symmetry axes shows a maximum in the temperature range 40 to 50OK, the peak being the highest along [ I l l ] just as in the case of YIG. A rough estimation shows that Er3+ ion contribution to the line width peak in NiFe,O, is smaller than that in YIG. All these properties of Er3+ ion is to be attri- buted to its high spin orbit coupling of the order of 2 350 cm-l [8].

2) Yb3+ ION : Yb3

ion also profoundly affects the magnetic properties again due to its very high spin orbit coupling 2 940 cm-l [8]. K, arising from Yb3' ion is positive and is measurable even a t 200 OK.

However unlike for Er3+ ion it increases rapidly even below 1000K. Comparing this behaviour of Yb3+ in NiFe,04 with that in YIG [6], it is seen that in the latter the sign is negative. The one ion contri- bution to K , at 0 OK in NiFe,O, is + 0.34 x 10-16 ergs as against - 5.2 x 10-l6 ergs in YIG obtained from ref. [6], which is only an order of magnitude higher. Using the theoretical calculation of Kl f(T) of Henderson and White (Fig. 5 in ref. 6) one can compare the behaviour of Yb3+ ion in YIG and NiFe20,. Fig. 4 shows that K, is prominent even around 100 OK and that it increases less rapidly in the range 20-100OK as compared to that in garnet.

This could be due to the fact that in spinel Yb3* ion is in the ferric lattice itself thus subjected to the exchange field even a t high temperatures whereas in garnet it is not so. Assuming these ions to be present in octahedral sites, the sign of K, could be expected to be positive from symmetry considerations [9].

K2 was found to be negligible for Yb3+ ions also.

FIG. 4. - One ion contribution to K1 in ergs of Yb3+ ion in garnet and spinel host lattices.

Fig. 3 shows the temperature dependence of line width along [I001 and [ l l l ] axes. Comparing this with results of Teale and a1 [lo] obtained also in the X band, it is seen that the behaviour is qualitatively the same though in our case line width peaks are at lower tem- peratures. The peak along [ I l l ] is at a lower tempera- ture unlike in YIG : Yb. Taking the percentage of octahedral sites occupied by Yb in our case line width contribution at the peak can be expressed as 210 Oe and 160 Oe along [loo] and [I1 11 respectively for each % of yb3' ion. However a similar approach yields only 20 and 40 Oe for each % of Yb3+ ion in YIG [lo]. Besides no anomaly was found in the angu- lar dependence of line width in [I101 plane at 4.2 OK unlike Dillon who found a peak at 32.50 from [001J axis 1111. The above arguments indicate that the beha- viour of Yb3+ is some what different when in octa- hedral sites.

In conclusion, the rare earth ions Er3+ and Yb3' have been substituted for Fe3+ ions in NiFe204 and they are shown to contribute a positive Kl that from Yb3+ being higher. Further the resonance line width behaviour in these crystals strongly resembles that in rare earth doped garnets.

Aknowledgements. - It is a pleasure to acknow- ledge the kind interest of Dr Charles Guillaud in this work and interesting discussions with Dr Roger Vautier.

References BERTAUT (E. F.) and FORRAT (F.), C. R. Acad. Sci.,

Paris, 1956, 242, 382.

PAUTHENET (R.), Ann. Physique Paris, 1958, 3, 424.

NEEL (L.), Ann. Physique, 1948, 3, 137.

See for example. Hand book of Microwave Ferrite Materials. Edited by W. H. von Aulock, Academic Press, N. Y., 1965.

KRISHNAN (R,), Phys. Stat. Sol. (a) 1970, 3, k.

PEARSON (R. F.), J. AppE Phys., 1962, 33 supplement to 3, 1236.

171 CLARKE (B. H.), PEARSON (R. F.), TEALE (R. W.) and TWEEDALE (K.), J. Appl. Phys., 1963, 34, 1269.

---. .

[8] BLEANEY (B.), Porc. Phys. Soc. (London), 1955, A68, 937.

[91 STURGE (M. D.), GYORGY (E. M.), LECRAW (R. C . ) and REMEIKA (J. P.), Phys. Rev., 1969, 180, 413.

[lo] TEALE (R. W.), PEARSON (R. F.) and HIGHT (M. J.), J. Appl. Phys., 1961, 32, 150 S.

[ll] DILLON (J. F.) Jr, J. Appl. Phys., 1961, 32, 159s.