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MAGNETIC PROPERTIES OF AMORPHOUS
MANGANESE ALUMINOSILICATES : SPIN-GLASS
LIKE BEHAVIOUR AT LOW TEMPERATURE
J. Renard, J. Pommier, J. Ferre, K. Knorr
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-936
MAGNETIC PROPERTIES OF AMORPHOUS MANGANESE ALUMINOSILICATES : SPIN-GLASS LIKE BEHAVIOUR AT LOW TEMPERATURE
« 3E9€ K3C 35 XH J.P. Renard , J. Pommier , J. Ferre and K. Knorr
* Institut d'Eleatronique Fondamentale, Laboratoive AssoaiS au CNRS, Batiment 220, Universite Paris-Sud, 91405 ORSAI CEDEX (Frame)
zs Laboratoire de Physique des Solides, Laboratoire Assoeie au CNRS, BStiment 510 Universite Paris-Sud, 91405 ORSAI CEDEX (France)
sss Institut fiir Rristallographie, Tubingen, W.Germany
Résumé.- On a mesuré la susceptibilité magnétique à 70 Hz en champ nul et en champ appliqué, l'aimantation en fonction du champ et l'aimantation rémanente d'aluminosilicates amorphes Mn0.Al2O3.SiO2 de concentration atomique en Mh 18,9 % et 15,1 %, aux températures supérieu-res à 1,2 K. Malgré une forte interaction antiferromagnétique moyenne entre ions M n2 +, les propriétés magnétiques sont très similaires à celles des alliages dits "verres de spin". Abstract.- AC susceptibility in zero and in applied magnetic field, magnetization versus applied field and remanent magnetization of aluminosilicate glasses MhO.Al203Si02 with atomic Mn concentrations 18.9 % and 15.1 %, have been studied down to 1.2 K. In spite of the large mean value fo antiferromagnetic interactions between Mn + ions, the magnetic pro-perties are very similar to those of spin—glass alloys.
Recent measurements of Verhelst et Al./l/ have shown that cobalt and manganese aluminosili-cate exhibit a relatively sharp peak in the low field AC susceptibility, very similar to that obser-ved for the spin-glass alloys as Au-Fe /2/.In order to obtain a more detailed comparison, we have ex-tended this work on amorphous manganese aluminosi-licates and performed detailed measurements which are now classical for spin-glass alloys : effect of applied fields on the AC susceptibility; high field magnetization and remanent magnetization measure-ments .
Two samples 1 and 2 of MnO.Al203 .Si02 glasses of good optical quality with respective atomic Mn concentrations 18.9 % and 15.1 % have been prepared following the method previously des-cribed /3/. Most of the experiments were performed on the sample 1.
AC SUSCEPTIBILITY : AC susceptibilities were measu-red using a -mutual inductance bridge operating at 70 Hz. The amplitude of the AC field was kept below 5 Oe. Measurements were done at room temperature, 77 K and in the range 1.2 - 4.2 K.
From the high temperature measurements, we found respective Curie-Weiss temperatures 9 =-100 K and - 130 K for samples 1 and 2. Measured suscepti-bilities versus temperature in the range 1.2 - 4.2K are shown in figure 1. Peaks are observed at respec-tive temperature TM = 3.23 K and 2.25 K for samples
1 and 2. An additional small static field of a few hundred oersteds produces a net rounding of the peaks as for the Au-Fe alloys 111. The de-crease of the maximum susceptibility value is about 5 % for sample 1 and 2 % for sample 2 at 340 Oe.
Fig. I : The AC susceptibility of samples 1 (183 Mn at %) and 2 (15.1 %) versus temperature and the effect of weak additional field.
I n a d d i t i o n , AC s u s c e p t i b i l i t y has been measured v e r s u s s t a t i c f i e l d H up t o 36 kOe a t three
f i x e d temperatures : 1.4 K, 4.2 K and t h e peak tem- p e r a t u r e TM. The s u s c e p t i b i l i t y s t r o n g l y d e c r e a s e s when i n c r e a s i n g f i e l d . Above 10 kOe, t h e x(H) cur- ves a r e i d e n t i c a l f o r t h e t h r e e s e l e c t e d temperatu- res.Below 10 kOe, x(H) depends on temperature and e x h i b i t s t h e s h a r p e s t d e c r e a s e f o r T = TM.
MAGNETIZATION VERSUS FIELD : The v a r i a t i o n of t h e magnetization M w i t h t h e a p p l i e d f i e l d H has been deduced from Faraday r o t a t i o n measurements f o r sample I ( f i g u r e 2 ) . I n t h e p r e s e n t c a s e , t h e r o t a - t i o n angle 9 can be expressed a s 0 = aM
+
bH. The a and b c o e f f i c i e n t s a r e temperature independent and have been determined from low f i e l d m a g n e t i z a t i o n curves a t 295 K and 4 K.Fig. 2 : Magnetization of t h e sample 1 v e r s u s a p p l i e d f i e l d around TM.
The most s t r i k i n g f e a t u r e s a r e t h e low f i e l d beha- v i o u r of t h e magnetization f o r T << T ( f i g u r e i n i w M s e r t ) and t h e h i g h f i e l d m a g n e t i z a t i o n much s m a l l e r t h a n i t s c a l c u l a t e d s a t u r a t i o n v a l u e M = 203 emu/g. H y s t e r e s i s curves very s i m i l a r t o t h o s e of spin-glass a l l o y s such a s Y Fe2 / 4 / were o b t a i n e d f o r zero o r i n - f i e l d c o o l i n g through TM.
REMANENT MAGNETIZATION : Remanent'magnetization below T was measured by a f l u x m e t r i c method and by Faraday r o t a t i o n . The time v a r i a t i o n of t h e thermo- remanent m a g n e t i z a t i o n / 5 / (TRM) of sample 1 a t
1.35 K i s shown i n f i g u r e 3. Between t = 3 s and 104 s t h e TRM i s a l i n e a r f u n c t i o n of l o g t . The s l o p e of t h e l o g t l i n e c l e a r l y depends on a p p l i e d f i e l d f o r small f i e l d v a l u e s .
Fig. 3 : mermoremanent m a g n e t i z a t i o n of t h e sam- p l e 1 a t 1.35 K f o r a s e r i e s of a p p l i e d f i e l d v a l u e s , a s a f u n c t i o n of time.
S i m i l a r f e a t u r e s were observed f o r t h e i s o t h e r m a l remanent m a g n e t i z a t i o n (IRM). F i g u r e 4 shows TRM and IRM measured a t t = 100 s and 1.35 K, v e r s u s magnetic f i e l d . The I R M curve i s below t h e TRM one which reaches i t s s a t u r a t i o n v a l u e f o r r e l a t i v e l y
small f i e l d s of about 1 kOe. This s a t u r a t e d
TRM
v a l u e v a r i e s s t r o n g l y w i t h temperature and cannot be d e t e c t e d f o r T > 2.4 K which corresponds t o about 0.75 TM. I n t h e temperature range 1.2-
2.4K, i t can b e f i t t e d by t h e law M r(T)
= 1 . 7 exp (-2.5 T) emu/g. s oj-emu/g'
(a) TRM 0.05-
0 T 0 0.5 1 (kOe) 1 2 300 F i g . 4 : a ) F i e l d dependence of t h e thermoremanent and t h e i s o t h e r m a l remanent magnetization of t h e sample 1 measured a t t = 100 s and T = 1.35 K .b ) S a t u r a t e d remanent magnetization a t t = 100 s v e r s u s temperature Mr (T) and t h e empi- r i c a l law 1.7 exp(- 2.5 T) (sol18 c u r v e ) .
DISCUSSION : Our 9 and TM v a l u e s a r e about two ti- mes s m a l l e r than those V e r h e l s t e t a l . This may be due t o t h e r a t h e r d i f f e r e n t sample p r e p a r a t i o n . Since superexchange i s very s e n s i t i v e t o t h e a n g l e of Mn-0-Mn bonds, s l i g h t s t r u c t u r a l changes may s t r o n g l y a f f e c t t h e magnetic p r o p e r t i e s . The small v a l u e of h i g h f i e l d magnetization compared t o
Mo
i s c o n s i s t e n t w i t h s t r o n g a n t i f e r r o m a g n e t i c mean i n t e r a c t i o n s i n d i c a t e d by
i
91
= I00 K, l a r g e compa- r e d t o TM. I n s p i t e of t h e s e i n t e r a c t i o n s comple-temperature and field behaviour of AC susceptibili-
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