Magnetic properties of amorphous Fe4Ni74Si9B13

Nuclear Instruments and Methods 199 (1982) 237-240 North-Holland Publishing C o m p a n y

237

MAGNETIC PROPERTIES OF AMORPHOUS Fe4Ni74SigBi3 T. S H I G E M A T S U 2, W. K E U N E l, V. MANNS l, j. L A U E R 1, H.-D. PFANNES 1 and M. NAKA 3 J Laboratorium fiir Angewandte Physik, Universiti~t Duisburg, 4100 Duisburg 1, Fed. Rep. Germany 2 Institute for Chemical Research, Kyoto University, UjL Kyoto-fu 611, Japan 3 The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal Japan

57Fe MOssbauer effect measurements have been performed on the amorphous spin-glass alloy Fe4Ni74Si9Bi3. Magnetic hyperfine-split spectra are found below an average magnetic transition temperature ~ ~ 11 K, the transition being not sharp but smeared over several degrees. In the spin-glass phase the hyperfine field Hin t near saturation has an average value of (230± 5) kOe and shows a distribution P(H) which is zero for H~< 140 kOe. The most probable hyperfine field is found to follow the relation Hin t ( T ) : ( 2 6 0 k O e ) . ( 1 - T / T f ) B over a large temperature range, with f l - 0 . 4 0 . An external magnetic field strongly affects the spin orientation in the spin-glass phase. For T > Tf and in the presence of an external field our measurements indicate that superparamagnetic clusters exist at least up to 77 K, with an average cluster moment of about 70/~B.

1. Introduction

2. Experimental

Nickel-based amorphous alloys of the type NixP~.Bz (x = 76-80, y + z = 20-24) show no bulk magnetic order since the Ni concentration is below that needed for ferromagnetism. In these alloys the substitution of Ni by Fe leads to a sequence of different magnetic states which proceed (with increasing Fe concentration) from spin-glass behaviour to mictomagnetism and homogeneous ferromagnetic ordering [1]. Thus low-field susceptibility and specific heat investigations have shown that amorphous (FexNil_x)v9Pi3Bs, FexNis0_ x P j 4 B 6 , and (FexNi a_x)75P16B6A13 alloys at low Fe concentrations are spin glasses at low temperature [1-4]. It was found that the magnetic moment in these alloys is not carried by individual Fe atoms alone but in addition by magnetic polarization clouds of surrounding Ni atoms [1]. The object of the present study was to obtain information (via the hyperfine field) on the magnetic properties of an amorphous (dilute) spin-glass system by using the M6ssbauer effect as a local, microscopic method. Our interest was focused on the hyperfine-field distribution function P ( H ) and the temperature dependence of Hint in the spinglass phase, and we looked for possible magnetic cluster behaviour above the spin-glass freezing temperature Tf.

Our amorphous Fe4Ni74Si9BI3 samples were prepared by the roller-quenching (melt-spinning) technique in the form of ribbons - 2 0 ~m thick and 1-2 mm wide. The iron was 90% enriched in 57Fe in order to increase the resonance absorption. The amorphous nature of the samples was confirmed by X-ray diffraction. For the measurements the sample was mounted in the He-gas filled sample chamber of a variabletemperature bath cryostat. The sample temperature was measured with an accuracy of -+0.05 K by using a calibrated carbon-glass resistor and was electronically feedback-controlled by resistance heating. Conventional MOssbauer spectroscopy in transmission geometry was used with sources of 57Co in rhodium. The measured spectra were fitted by a computer program similar to that of Window [5] in order to determine the distribution of hyperfine fields P ( H ) and the most probable hyperfine field (peak in the distribution curve). 3. Results and discussion

3.1. Spectra at T <~~ Typical MOssbauer spectra at various temperatures (in zero external field) are shown in fig. 1. A

0167-5087/82/0000-0000/$02.75 © 1982 North-Holland

lII. HYPERFINE FIELDS/MAGNETIC PROPS.

12 Shigematsu et al. / Magnetic properties of arnorphous F e 4 N i 7 4 S i g B I 3

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magnetic transition temperature of Tr ~ 11 K can be estimated from these spectra. A b o v e Tf quadrupole-split spectra are observed. At and below Tr line broadening occurs which develops into a well resolved magnetic hyperfine splitting with decreasing T as a result of the freezing of Fe

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Fe4Ni74SigB13

T. S h i g e m a t s u et al. / M a g n e t i c p r o p e r t i e s o f a m o r p h o u s

tra indicating a random spin orientation. The shape of P(H) is similar to the Gaussian distribution of molecular fields adopted in theoretical models of spin-glasses [7,8]. The most probable hyperfinefield value obtained at 2.7 K (and 1.5 K) is (230 -+ 5) kOe which corresponds to the saturation hyperfine field H ~ t. Thus Hsat in our amorphous alloy is remarkably smaller than that of dilute Fe in crystalline (pure) Ni which is 285 kOe [9]. As can be seen in fig. 2 the P ( H ) - c u r v e is zero for hyperfine fields ~< 140 kOe. We may conclude that the molecular field also has a minimum value below which the molecular field distribution is zero. Such a behaviour of the molecular field distribution was suggested on theoretical grounds [7,10,11]. Since the sharpness of the spin-glass freezing temperature Tf is of importance we have applied the thermal-scan technique to determine Tr. For this purpose the c o u n t - r a t e , N(t~p), at constant velocity Vp as well as the count-rate off resonance, N(v~), were measured as a function of temperature near T~ (% is the peak velocity at the quadrupole line near zero velocity which is seen above T~, fig. 1 and fig. 7). The normalized countrate, N(~p)/N(v~), is plotted versus T in fig. 3. As can be seen the spin-glass transition does not occur abruptly at a definite'T~ by lowering the temperature but is a gradual process which starts at ~ 13 K and extends down to ~ I0 K. We can find an average freezing temperature (at the inflec-

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tion point) of T~ -- 11 K. Our result is similar to that of Wagner [12] for dilute AuFe. It was found that the most probable hyperfinefield values (as obtained from the P(H)-curves) when plotted versus temperature closely follow the relation Him(T ) = H ~ t ( 1 - T/Tf) ~ within a large temperature range (fig. 4). The fitted curve was obtained by taking fl = 0.40 -+ 0.05 and Ha, = 260 kOe. It is interesting that the hyperfine field in ferromagnetic amorphous Ni-based alloys (with higher Fe content) similarly has been described by a critical exponent fl of 0.4 [13]. This agreement could mean that short-range ferromagnetic interaction is also important at dilute Fe concentrations in amorphous Ni-based alloys. In fig. 5 a comparison is made between the temperature dependence of our measured Hi2,,-values (squared most probable hyperfine field) and recent theoretical (Monte Carlo) results for the Edwards-Anderson order parameter q(T) calculated by Morgenstern and Binder [8]. We find reasonable agreement between theoretical and experimental points if we use an exchange interaction parameter J = (8.4 -+ 0.5) k B. Application of an external magnetic field strongly influences the spin orientation in the spin-glass phase. Fig. 6 shows spectra measured at 4.6 K in the presence of various fields (parallel to the y-direction). The preferential alignment of the spins parallel to the external field direction is I11. HYPERFINE F I E L D S / M A G N E T I C PROPS.

T. Shigematsu et aL / Magnetic properties of amorphous Fe4Ni74SioB13

240

indicated by the decreasing relative intensity of the intermediate lines number 2 and 5. The spins align only gradually, and no complete polarization is achieved at H~,, = 7 T. Our observation is remarkable since crystalline AuFe spin-glass alloys do not show polarization effects at all [14,15]. This means that either the local anisotropy field in our sample is much less than in AuFe, or (more likely) the local moment in Fe4Niv4SigBL3 which interacts with the applied field is much larger than in AuFe.

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Typical spectra above Tf without and in the presence of an external field are shown in fig. 7 for T = 16.3 K. The fairly temperature-independent asymmetric quadrupole-split spectrum has broadened lines with an average splitting A EQ of 0.45 m m / s . Spectra obtained in external fields indicate that the effective field at the nucleus (as measured from the splitting of the outer lines) is not equal to the applied field. This observation means that fluctuating (superparamagnetic) cluster moments exist above Tf which are aligned by the external field. The broadening of the outer lines in the external field spectra might be caused by a distribution of moments (and hyperfine fields) combined with quadrupole interactions, or possibly by magnetic relaxation effects. These magnetic clusters are found to exist up to at least 77 K. Assuming (as usual) that the hyperfine field Hi. t is antiparallel to the external field we have determined Hi~~ from the most probable effective field values. Our normalized data points for Hi,t/H~ t plotted versus H I T ( H = external field, fig. 8) can be well described by a Langevin function yielding H~, =

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2 3 /. H /kOe T K Fig. 8. Measured normalized hyperfine field H i n t / H s a t as a function of H / T ( H = external magnetic field) for T > Tf. The drawn curve is the Langevin function B~(gH/kBT ) with H~. t =240 kOe and ~ = 7 0 ~B-

240 kOe and a very large (average) cluster moment of ~ = 70 /~B surprizingly independent of temperature within the range 16.3 K ~ T ~ < 7 7 K. Our value for g should be considered as an upper limit since interaction between cluster moments has been neglected in the analysis. Valuable discussions with Dr R.A. Brand are appreciated. This work was supported by the Minister ftir Wissenschaft und Forschung, Dtisseldorf, Fed. Rep. Germany. References [1] J. Durand, in: Amorphous magnetism II, eds., R.A. Levy and R. Hasegawa (Plenum, New York, 1977) p. 305. [2] D.G. Onn, T.H. Antoniuk, T.A. Donnelly, W.D. Johnson, T. Egami, J.T. Prater and J. Durand, J. Appl. Phys. 49 (1978) 1730. [3] J.T. Prater and T. Egami, J. Appl. Phys. 50 (1979) 1706. [4] S.M. Bhagat, J.A. Geohegan and H.S. Chen, Sol. St. Commun. 36 (1980) 1. [5] B. Window, J. Phys. E:4 (1971) 401. [6] S. De Benedetti, J.A. Rayne, A. Zangwill and R.A. Levy, AIP Conf, Proc. 29 (1975) 241. [7] L.J. Schowalter and M.W. Klein, J. Phys. C:12 (1979) L935. [8] I. Morgenstern and K. Binder, Phys. Rev. B22 (1980) 288. [9] C.E. Johnson, M.S. Ridout and T.E. Cranshaw, Proc. Phys. Soc. London 81 (1963) 1079. [10] R.G. Palmer and C.M. Pond, J. Phys. F:9 (1979) 1451. [11] H.J. Sommers, J. Magn. Magn. Mat. 13 (1979) 139. [12] H.G. Wagner, Dissertation (1980) Universit~it des Saarlandes, Saarbrticken, unpublished. [13] S.N. Kaul, Phys. Rev. B22 (1980) 278. [14] R.J. Borg, Phys. Rev. B1 (1970) 349. [15] G. Chandra and J. Ray, J. Phys. 39 (1978) C6-914.