Journal of Magnetism and Magnetic Materials 143-144 (1995) 297-298
Temperature and external magnetic field dependence of the spin freezing in amorphous Fe,, Zr,
I. Vincze alb* * , D. Kaptis a, T. Kem6ny a, L.F. Kiss a, J. Balogh a
a Research Instilute for Solid State Physics, P.O.B. 49, H-1525 Budupes~, Hungary b Solid State Physics Department, Eiitviis University, Bt!dogest, Htqwy
Abstract
The rotation of the Fe magnetic moments toward the collinear state was studied by “Fe Miissbauer spectroscopy as a fttnrtion of temperature in applied magnetic fields up to 7 T in amorphous Fe,& exhibiting a single paramagnetic to spin glass transition. The spin freeeziiig L ‘cmpera!ure decreases linearly as a function of the external magnetic field.
The spin glass transition is manifested in a cusp of the susceptibility which is smeared out in an external magnetic field, B,,,. At larger fields the spin freezing temperanne rr is not well defined, The transformation can be observed by
“Fe MSssbauer spectroscopy, which characterizes the di- rection of the Fe magnetic moments via the line intensities of the spectra. The relative intensity of the second and fifth lines, 12,s (corresponding to the Am = 0 nuclear transition) is given by 12,s = 4 sir&/(1 + cos*O), where 0 is the angle between the magnetic moment and the magnetic field (applied parallel to the y-beam direction). A random Fe spin orientation corresponds to &, = 2, while I,,5 = 0 indicates that all the magnetic moments are collinear to B ext *
Miissbauer investigation of the Tr vs. BeX, dependence in moderate magnetic fields (above 2 T) was performed only in a few systems showing re-entrant spin glass (RSG) behaviour, i.e. the transition from the high temperature collinear ferromagnetic structure to the low temperature canted state was studied [l-3]. A linear relationship be- tween Tr and B,,, was found in Cr,sFe, [I] and no measurable influence of the external magnetic fields up to 3 T on the RSG state was reported for Fe6Ni7&Br3 [2]
and for Au o~aszFe0,t6R 131. Different T&B,,) dependence is expected theoretically for the direct paramagnetic (PM) to spin glass (SG) transition (the critical exponent is 2/3 for the Ising model and 2 for the isotropic model of m-compo- nent spins) and for the RSG transition (linear B,,, depen- dence) [I].
In the present work the spin freezing in amorphous
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Fe,,Zr, was studied by Miissbauer spectroscopy as a function of temperature and external magnetic field. The amorphous ribbons were prepared by melt-spinning in vacuum. Besides X-ray diffraction and Miissbauer spec- troscopy, magnetization measurements were performed by a vibrating sample magnetometer. The magnetization mea- sured in I mT as a function of temperature exhibits the well-known cusp-type behaviour characteristic of a single PM-SG transition [4]. The Miissbauer measurements were performed using a conventional constant acceleration spcc- trometer with a 50 mCi “CoRh source at room tempera- ture. The external magnetic field was applied parallel to the y-beam using a 7 T Janis superconducting magnet.
Typical Mtissbauer spectra are shown in Fig. 1. The broad lines reflect a distribution of the hyperfine fields p(B,,) the shape of which was fitted by a double binomial distribution [S]. Linear correlation was assumed between the hyperfine field, the isomer shift, and the quadrupoie splitting. The linewidth of the individual six line patterns, together with the intensity of the 2-5 lines was determined by least-squares fitting of the spectra. The disappearance of the 2-5 lines is obvious in Fig. la in larger fields (2 4 T) and in Fig. lb at higher temperatures; the iz5 values of these spectra are shown in Fig. 2. Similarly to our earlier Miissbauer investigation at 4.2 K [6], the value of B,, necessary to reach collinearity was determined from 12,&$.,J at each temperature. The Z’r@,) dependence is presented in Fig. 3. It is remarkable that the T&B,,, = 0) value extrapolated linearly from the low temperature data (T I 40 K) agrees well with the value measured directly.
The slope is about -22 K/T. At high temperature (TT 100 K) magnetic relaxation plays a dominant role [7]
resulting in a large increase of the magnetization in exter- nal magnetic field. Consequently the value of the demag- netization field in the direction perpendicular to the sample 0304.8853/95/$09.50 8 1995 Elsevier
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298 I. Vincze et al./Journal of Magnetism and Magnetic Materials 140-144 @95) 297-298
.
I1, I I I I I ! I I I I I I
-6 -4 -2 0 2 4 6
velocity (mm/s)
Ib) 4
-6 -4 -2 0 2 4 6
velocity (mm/s)
Fig. 1. Typical MSssbauer spectra of amorphous Fe,Zr, at 40 K in different extema! fields applied parallel to the y-beam direction (a) and at different temperatures in 3 T (b). The calculated positions of the second and fifth lines are marked.
plane will be larger than that expected on the base of the zero field magnetization curve. The value of B,,, can be corrected for the demagnetization effect using the propor- tionality between the Fe hyperfine field to the absolute value of the Fe magnetic moment with a conversion factor of about 14.5 T/pe [8]. Even after this correction Tr
Fig. 2.
0
20 40 60 60 100Relative intensities in Fig. lb.
of the 2-5 lines Iz,5 of the spectra
0 2 4
Lt U,6
Fig. 3. Spin freezing temperature Tr as a function of the applied magnetic field B,,.. (0) and after correcting for the demagnetiza- tion field (0). The lines are guides to the eye.
follows a linear relationship with the effective magnetic field as shown in Fig. 3, but the slope has a somewhat more negative vaiue i&out -25 K/T).
Acknowledgement: This investigation was supported by the Hungarian National Research Fund OTKA-2933 and T4464.
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