MICTOMAGNETISM IN Zr(Fe-Co)2

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MICTOMAGNETISM IN Zr(Fe-Co)2

Y. Muraoka, M. Shiga, Y. Nakamura

To cite this version:

Y. Muraoka, M. Shiga, Y. Nakamura. MICTOMAGNETISM IN Zr(Fe-Co)2. Journal de Physique

Colloques, 1979, 40 (C2), pp.C2-213-C2-215. �10.1051/jphyscol:1979276�. �jpa-00218674�

JOURNAL DE PHYSIQUE

Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-213

MICTOMAGNETISM IN Zr(Fe-Co)

2

Y. Muraoka, M. Shiga and Y. Nakamura

Department of Metal Science and Technology, Kyoto University, Kyoto 60S, Japan

Résumé.- Des mesures magnétiques et par effet Mossbauer ont été faites sur le système pseudobinaire Zr(Fe xC ox)?. On trouve qu'il y a apparition d'un comportement mictomagnétique pour x = 0,5 ~ 0,7, avec diminution progressive de l'ordre ferromagnétique.

Abstract.- Magnetic and Mossbauer measurements have been made for the pseudobinary Zr(Fej_xCox)2

system. Both measurements clearly show that mictomagnetism appears for x = 0.5 ~ 0.7, as ferromagne- tism vanishes.

The Zr(Fe C o x)2 system crystallizes in the cubic Laves phase structure (C15) over the whole com- position range. ZrFe is a ferromagnet with a Curie

temperature of 630 K and spontaneous magnetization of 1.6 Ur,/Fe atom, whereas ZrCo2 is a Pauli parama- gnet. The magnetization of the pseudobinary system decreases linearly with increasing Co concentration, indicating that the Fe and Co moments are 1.6 and 0.6 y_, respectively /!/. The magnetization decreases sharply as x exceeds 0.3 and becomes very small around x=0.6, although the system is still ferroma- gnetic up to x=0.8 111• Contradictory results have been reported about the magnetism near the critical concentration. From magnetic measurements Kanematsu III proposed an antiferromagnetic ordering in the re- gion (x=0.55~0.6) where the collapse of ferromagne-

tism occurs. Whereas, Hilscher and Kirchmayr /3/ re- ported very weak itinerant ferromagnetism near the critical concentration of x=0.75. In order to make clear the magnetism around the critical concentration, we have measured the magnetization and the Mossbauer effect of 57Fe.

The magnetic measurements were done up to 60 kOe at 4.2 K, indicating that the system is ferro- magnetic for x ^ 0 . 4 . On the other hand, the M2 vs.

H/M plot clearly shows that no spontaneous magneti- zation exists for x=0.5. However, a relatively large magnetization is induced by an applied magnetic field

for x=0.5 and a hysteresis is found in the magnetiza- tion vs. magnetic field curve. The thermomagnetic curve of this compound shows a broad maximum at about 50 K. This maximum becomes more pronounced as the field is reduced. These magnetic behaviors sug- gest that mictomagnetism appears in this compound and its magnetic moment freezes below the tempera- ture of maximum magnetization. Similar magnetic behaviors were also observed for the compounds with

x=0.6 and 0.7, whose temperatures of maximum magne- tization are about 30 and 15 K, respectively. No maximum magnetization was observed for x>0.8. The magnetic phase diagram estimated from these magnetic measurements is shown in figure 1.

Fig. 1 : Magnetic phase diagram of the Zr(Fej_xCox)2

system.

The Mossbauer measurements were made in order to know whether the magnetic moments really freeze below the temperature of maximum magnetization for mictomagnetic alloys. Besides the conventional mul-

tichannel Mossbauer spectra at several temperatures, the temperature variation of the counting rate was measured at the constant velocity corresponding to the maximum absorption of a paramagnetic spectrum.

The observed Mossbauer spectrum of ZrFe2 was similar to that reported by Wertheim et al. /4/,who interpreted the spectrum at 4.2 K as arising from two Fe sites due to the dipolar field and the com- bined effect of the crystal field direction and the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979276

c2-214 JOURNAL DE PHYSIQUE

< I l I > e a s y d i r e c t i o n of magnetization. On t h e o t h e r hand, t h e s p e c t r a f o r &.2 c o n s i s t of a s e t of broad s i x l i n e s a t t r i b u t a b l e t o a s i n g l e Fe s i t e and i t can be b e t t e r f i t t e d by assuming a <loo> easy d i - r e c t i o n of magnetization. The d i s t r i b u t i o n of i n t e r n a l f i e l d s -shown a s t h e hystogram- was estimated from t h e observed s p e c t r a f o r x=0.2'0.7 by u s i n g a c o n s t a n t quadrupole s p l i t t i n g determined from t h e paramagnetic s p e c t r a .

The Mzssbauer s p e c t r a of t h e compounds w i t h x=0.5, 0.6 and 0 . 7 , which have no spontaneous magne- t i z a t i o n , a r e not simply paramagnetic a s shown i n f i g u r e 2 ( a ) .

As seen i n f i g u r e 2 ( b ) , the d i s t r i b u t i o n of t h e in- t e r n a l f i e l d s a t 4.2 K a r e c o n s i d e r a b l y wide and tk average h y p e r f i n e f i e l d s a r e r e l a t i v e l y l a r g e , i . e . 94, 73 and- 43 kOe f o r x=0.5, 0 . 6 and 0 . 7 , respec- t i v e l y . This i m p l i e s t h a t t h e magnetic moment i s f r o - zen a t 4.2 K w i t h o u t r e v e a l i n g bulk m a g n e t i z a t i o n . As shown i n f i g u r e 3 , t h e counting r a t e i n a cons- t a n t v e l o c i t y mode d e c r e a s e s w i t h i n c r e a s i n g tempe- r a t u r e due t o a r e d u c t i o n of i n t e r n a l f i e l d and t h e i n f l e c t i o n p o i n t i n t h e a b s o r p t i o n v s . temperature curve i n d i c a t e s t h e temperature where t h e i n t e r n a l f i e l d d i s a p p e a r s . These temperatures almost coincide w i t h those of maximum magnetization i n magnetic meas- urements.

2

1 j

Temperature (K)

Fig. 3 : Counting r a t e a t t h e v e l o c i t y corresponding t o the maximum a b s o r p t i o n of paramagnetic spectrum a s a f u n c t i o n of temperature. Arrows i n d i c a t e t h e temperature below which t h e magnetic moment i s f r o - zen.

For x=0.8 a paramagnetic spectrum i s observed down t o 4.2 K , which a l s o a g r e e s w i t h magnetic measure- ments. These Mzssbauer r e s u l t s make t h e magnetic phase diagram shown i n f i g u r e I more r e l i a b l e .

The temperature dependence of t h e Mzssbauer s p e c t r a f o r t h e f e r r o m a g n e t i c compounds ( f i . 4 ) in- d i c a t e s an i n t e r e s t i n g f e a t u r e a s shown, f o r example, i n f i g u r e 4 f o r x=0.3.

F i g . 4 : 5 7 ~ e M k s b a u e r s p e c t r a of Zr(Feo. ,Coo. ³⁾² ( a ) and t h e i r h y p e r f i n e f i e l d d i s t r i b u t i o n s ( b ) .

The distribution of the internal field becomes wide with increasing temperature and the paramagnetic component appears at 298 K, well below the Curie temperature (420 K). As was observed previously / 2 / , the magnetization of the ferromagnetic compounds de- creases sharply with increasing temperature, which has been interpreted as originating from a wide d i s

tribution of the Curie Temperature. The observed

~gssbauer spectrum at high temperatures, especially its paramagnetic component, agrees well with the magnetic measurement.

References

/ I / Kanematsu, K . , J. Phys. Soc. Japan

31

/ 2 / Piegger, E . and Craig, R.S., J. Chem. Phys.

2

(1963) 137.

/ 3 / Hilscher, G. and Kirchmayr, K.E., Phys. Status Solidi (a)

9

/ 4 / Wertheim, G.K., Jaccarino, V. and Wernick, H., Phys. Rev.

135