Influence of hydrogenation on the magnetic properties of amorphous FeCoZr alloys

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Journal of Magnetism and Magnetic Materials 112 (1992) 334-336 North-Holland

Influence of hydrogenation cn the magnetic properties of amorphous Fe-Co-Zr alloys M. Z~troch, P. Petrovi6, I. Brovko, T. Svec and M. K o n 6 Department of Experimental Physics, P.J. Sajdrik Unicersity, CS-041 54 Ko~ice, Czech and Slocak Federal Rep.

The changes in the hyperfine parameters of rapidly quenched amorphous Fe,~0_.~Co.,Zrlo (x = 0, 5, 10) ribbons induced by hydrogenation have been studied by 57Fe M6ssbauer spectroscopy. Difference~ in spectra of uncbarged and hydrogenated samples were observed at various temperatures. The increase of the magnetic transition temperature of these alloys due to hydrogenation was observed and the influence of the substitution of Fe-atoms by Co-atoms is discussed.

1. Introduction As reported in refs. [1-6] for the amorphous FexZr~00_ x system the onset of magnetic ordering occurs above a critical Fe concentration x c > 45. The Curie temperature is increasing with x up to x = 85 and decreases for x > 85. This results from the competition of two factors. The first one is a volume effect related to the increase of the F e - F e interatomic distances which makes the d-band narrower and higher, promoting ferromagnetism, the second factor is a simple dilution effect. M6ssbauer spectra of the F e - Z r amorphous alloys given in refs. [1,2,5] show significant changes in the hyperfine paramezers due to hydrogenation. Hydrogen atoms, according to ref. [4], prefers sites near the Zr-atoms. The changes in Curie temperature due to hydrogenation were explained in ref. [3] by the formation of Z r - H bounds which weaken the F e - Z r interaction and causes the change ~n the F e - F e interaction. According to ref. [61 the changes of Curie temperature are due mainly to volume cxp~n,;ion effect.

CorresFondence to: Dr. P. Petrovi?:, Department of Experimental Physics, P.J. SafSrik University, n~m. Februfir. Vit'azstva 9, CS-041 54 Ko~ice, Czechoslovakia.

2. Experimental procedure Amorphous Fe90_xCoxZri0 ( x = 0 , 5, 10) rapidly quenched ribbons (thickness about 20 ixm, width 4 mm) were prepared by the melt-spinning technique. Nominal composition of alloys was checked by EDX-analysis. The samples were electrolytically hydrogenated in a H 2 S O 4 solution containing a few ppm of CS 2 at constant electrode potential relative to the standard hydrogen electrode. The contents of hydrogen in the charged sample was determined by measuring the pressure of the gas released from the sample heated to about 750 K. Accuracy of the hydrogen concentration determinated in this way is better then 8%. The M6ssbauer measurements ~ e r e performed using a conventional microcomputer controlled spectrometer in constant acceleration mode with transmission geometry, and perpendicular ~/-ray orientation to the ribbon plane. The 57Co ~,-ray source in the Rh-matrix (nominal activity of 1.85 GBq) was used. The velocity scale was calibrated relative to natural a-iron at room temper~.ture. The simple liquid nitrogen cryostat and furnace, both with a calibrated Cu-thermometer and electronic temperature controller were used for M6ssbauer measurements from

0304-8853/92/$05.00 ~ 1992 - Elsevier Science Publishers B.V. All rights reserved

335

M. Zatroch et aL / Amorphous Fe-Co-Zr alloys and hydrogenation 30

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Fig. 1. Hyperfine magnetic field distributions derived from Mfssbauer spectra of amorphous samples in the as-quenched state and after hydrogenation, recorded at room temperature.

room temperature down to about 135 K and up to about 410 K. The M6ssbauer transmission spectra were collected in 500 channels at every temperature. Our own least-squares fitting program with nongradient quadratic programming method based on the LeCafir-Dubois method was used for evaluation of the hyperfine magnetic field distribution and other hyperfine parameters of the spectra.

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3. Results and discussion

The shape of M6ssbauer spectra of investigated samples changes from an asymmetric broadened double" to a distributed sextet depending on the sample temperature and on the content of Co and H. The first shape corresponds to the quadrupole splitting due to a distribution of electric field gradient at iron nuclei, what is typical for paramagnetic amorphous alloys. The second one corresponds to magnetic splitting due to a distribution of the hyperfine magnetic field in the magnetically ordered amorphous samples: Fig. 1 shows the temperature dependence of an average hyperfine magnetic field obtained by evaluating a set of M6ssbauer spectra collected at

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336

M. Zatroch et al. / Amorphous Fe-Co-Zr alloys and hydrogenation

different temperatures. We see that the hydrogenation promotes ferromagnetism. We assume that the increase of the average hyperfine magnetic field and Curie temperature after hydrogenation of the Fe-rich Fe-Zr amorphous alloys is due mainly to the volume effect. The presence of hydrogen atoms in these alloys results in increased Fe-Fe interatomic distances and leads to a change in the exchange energy. The influence of the hydrogen on the average hyperfine magnctic field decreases with decreasing temperature and with increasing Co-concentration. Amorphous Fc,90Zrl0 alloy behaves as a weak ferromagnet. The substitution of Fe-atoms by Co-atoms leads to an increase of Curie temperature from about 240 K for Feg0Zrm0 to about 345 K for FessCosZr~0 and to about 450 K for FesoCo~0Zrt0. The role of Co-substitution atoms is demonstrated in the hyperfine field distributions (fig. 2). For as-prepared and hydrogenated samples these distributions consist of two parts. The low field hump decreases with lower temper-

atures and with growth of the Co-concentration. The high field component does not change significantly its magnitude and shift to a higher magnetic field with Co-concentration. it is evident that substitution of the Fe-atoms by Co-atoms "shifts" the F e - C o - Z r alloys to the right in the increasing part of the Slater-Pauling curve.

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