Hyperfine Interactions 94(1994)1861-1865 1861
Temperature dependence of the iron hyperfine field distribution in amorphous Fe-rich F e - Z r alloys
D. Kapt~is, T. Kem6ny, J. Balogh, L.F. Kiss, L. Grfimisy and I. Vincze Solid State Physics Institute, Central Research Institute for Physics,
P.O. Box 49, H-1525 Budapest, Hungary
The temperature dependence of the iron hyperfine field distribution is reported in melt-quenched amorphous Fe-Zr alloys. The most remarkable feature is the compositinal change in the shape of the average hyperfine field versus temperature curves. The unusual increase in the average hyperfine field below about 85 K is a characteristic feature of the RSG systems; however, no anomaly is observed in the width of the hyperfine field distribution as a function of temperature. The results cannot be properly explained in the framework of the existing spin glass models.
1. Introduction
T h e compositional collapse o f ferromagnetism in alloys is often assumed to be the result o f a significant amount o f antiferromagnetic e x c h a n g e interactions.
Theoretically, these e x c h a n g e frustrated systems show magnetic anomalies similar to the observed behaviour. Little is known, however, o f the existence o f the supposed antiferromagnetic e x c h a n g e interactions. The distribution o f exchange interactions d e t e r m i n e the temperature d e p e n d e n c e o f the individual magnetic moments, thus presenting a possibility for the investigation o f the magnetic interaction distribution.
The aim o f the present paper is to study the temperature d e p e n d e n c e o f iron magnetic m o m e n t (i.e. hyperfine field) distribution in amorphous Fel00_xZrx alloys where the collapse of ferromagnetism is observed in a rather narrow 6 at.% concentration range.
2. E x p e r i m e n t a l
A m o r p h o u s ribbons o f Fej00_xZrx (7 < x < 12) were melt spun in v a c u u m f r o m ingots prepared from the pure metals by induction melting in a cold crucible u n d e r argon. T h e absence o f crystallinity was c o n f i r m e d by X-ray diffraction and r o o m temperature M6ssbauer spectroscopy.
Magnetization was measured by a Foner-type magnetometer in an electromagnet up to 2 T. The magnetic measurements show a single magnetic transition both at
9 J.C. Baltzer AG, Science Publishers
1862 D. Kapt6s et aL / Iron hyperfine field distribution in F e - Z r
12 at.% Zr and 7 at.% Zr content (paramagnetic (PM) to ferromagnetic (FM) and PM to spin glass (SG), respectively) and re-entrant (RSG) behaviour in between.
M6ssbauer spectra were obtained by a conventional constant-acceleration spectrometer with a 1.8 GBq 57CoCr source at room temperature. For the evaluation of the spectra the binomial distribution method [1] was used, where the shape of the binomial distributions is adjusted to the spectra and linear correlation is assumed between the hyperfine field, the isomer shift and the quadrupole splitting. The intensity of the 2 - 5 lines was determined by iteration.
3. Results
Typical M6ssbauer spectra and hyperfine field distributions p(B) were published in refs. [2,3]. The hyperfine field distributions can be separated [2,4,5] into high- field (HF) and low-field (LF) contributions. Near to the paramagnetic transition, the shape of the spectra is characteristic of magnetic spin relaxation. In this temperature region, the spectra are extremely sensitive to a small external field (Bex t = 13 mT), proving the existence of magnetic cluster relaxation [3].
The reduced average hyperfine fields a ( T ) = (B(T))/(B(O)) are shown in fig. 1. Our data for x = 9 are in good agreement with those of Ghafari et al. [6], but for x = 7, 8 and 9, disagree with the results of Ryan et al. [7,8]. In the case of x = 10, the overall agreement is satisfactory with ref. [9]. The most remarkable feature of the o'(T) curves is the change from the usual convex shape (x = 12; FM state) to the concave form (x = 9, 8; RSG states) and then the change back to the convex shape for x = 7 (SG state). A similar change in the o-(T) shapes as a function of composition was reported, for example, for the F M - R S G transition [10] in crystalline FeNiMn and for the S G - R S G transition [1 1,12] in amorphous RuFeB and FeScZr alloys.
The unusual increase in the reduced average hyperfine field or(T) at low temperatures is a characteristic feature of the RSG systems. It is well reflected in the o'(T) v e r s u s T 312 or T 2 curves. The latter is shown in fig. l(b), since this plot is linear in larger temperature intervals, including F e 9 3 Z r 7 where the T 3/2 plot is curved.
On the other hand, the standard widths O'a of the hyperfine field distributions shown in fig. 2 decrease monotonously with increasing temperature and do not reflect the anomalous temperature dependences of a(T) for Fe92Zrs and Fe91Zr9.
4. Discussion
The anomalous increase in or(T) is observed at around 85 K for both x = 8 and 9. This temperature is significantly higher than the temperatures (61 and 31 K, respectively) where the ferromagnetic breakdown is observed in the low field magnetization measurements.
D. Kaptt~s et al. / Iron hyperfine field distribution in F e - Z r 1863
1.0
0.8
0.6
0.4
0.2
0 0 50 100 150 200 250
(o) T (K)
1.0
0.8 0.6 0.4
0.2
•
L $ +++,o6oo 2o6oo 30600 40600 so6oo 6o6o0 (b) r" (K ~)
Fig. 1. The reduced average iron hyperfine field (r(T)= (B(T))/(B(O)> of amorphous Fel0o_xZr x as a function of temperature (a), and T 2 (b) (O: x= 7; A: x---8; O: x = 9;
I-1: x = 10; +: x = 12). Full and empty symbols denote different samples with the same nominal compositions. The inset shows the T 2 dependence of the reduced high field components at 8 and 9 at.% Zr contents, respectively. The continuous lines are guides to the eye.
The unusual b e h a v i o u r at a b o u t the s a m e t e m p e r a t u r e for both c o m p o s i t i o n s c a n n o t be e x p l a i n e d by a s u p p o s e d m a g n e t i c p h a s e separation [13, 14]. T h e a b s e n c e o f any significant a m o u n t o f p a r a m a g n e t i c contribution at higher t e m p e r a t u r e s and the result that the high field c o m p o n e n t BHF shows the s a m e t y p e o f a n o m a l y (see the inset o f fig. I (b) and ref. [15]) rules out this possibility. Since these o b s e r v a t i o n s indicate that all iron a t o m s contribute to the t e m p e r a t u r e a n o m a l y , it c a n n o t be a s s o c i a t e d with a given type o f (e.g. the iron-rich) e n v i r o n m e n t s . Since all iron m a g n e t i c m o m e n t s contribute to the a n o m a l o u s or(T) behaviour, it is very attactive
1864 D. Kapttts et al. /Iron hyperfine field distribution in Fe-Zr
8 i ~ esla)
4 Zr7 2
O . . . . . . . . . . . . . . . . . . . . . .
4 2 0
4 **
2 0
4
2 Zqo
O . . . .
ol
. . . -r . ( K ) / .0 50 100 150 200 250
Fig. 2. Temperature dependence of the standard deviation of the hyperfine field distribution tra of amorphous Fel00_xZr x.
to attribute it [5, 16] to the broad distribution of the exchange integrals, i.e. to the presence of weakly coupled magnetic moments having distorted magnetization curves.
However, it is difficult to reconcile this kind of explanation with the observed composition dependence. For x = 7, an even larger anomaly would be expected due to the increased amount of antiferromagnetic interactions resulting in the direct P M - S G transition.
In the spin canting model [17], the (B(T)) value extrapolated to 0 K from the high (T > Tf) temperature range is smaller than the value extrapolated from the low (T < Tf) temperature region (Tf is the spin freezing temperature). It is attributed to the freezing of the transversal components of the magnetic moments at low temperatures which would average out at higher temperatures. A similar increase is expected for
D. Kapt6s et al. / Iron hyperfine field distribution in F e - Z r 1865
the w i d t h o f the d i s t r i b u t i o n s o'B, w h i c h is n o t o b s e r v e d . This d i s c r e p a n c y q u e s t i o n s that a n t i f e r r o m a g n e t i c i n t e r a c t i o n s are r e s p o n s i b l e f o r the o b s e r v e d a n o m a l o u s t e m p e r a t u r e d e p e n d e n c e s .
Acknowledgement
T h i s i n v e s t i g a t i o n w a s s u p p o r t e d b y the H u n g a r i a n N a t i o n a l R e s e a r c h F u n d O T K A - 2 9 3 3 a n d T 4 4 6 4 .
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