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V. E. Aru~oh-ov al. : The Magnetic Properties of Ni-Co-H Solid Solutions et
phys. stat. sol. (a) 57, 75 (1980) Subject classification: 18.2; 21.1
lnstitute of Solid State Physics, Academy of Sciences of the USSR, Chrnogolovka
1)

75

The Magnetic Properties of Ni-Co-H Solid Solutions
BY V. EL ANTOJOV,I. T. BELASH, K. PONONAREV, POKYATOVSKJI, B. E. G. and V. G. THIESSEN
The behavionr of the Curie points at PH? 67 kbar and of the magnetization at atmospheric 5 pressure and 80 5 T 5 220 K is investigated for hydrogen solutions on the base of the f.c.c. Ni-Co alloys with 30 and 60 at";o Co. The samples for the magnetization measurement are hydrogenated at PI{, 5 60 kbar.

Y PaCTBOpOB BOnOpOjla Ha 6a3e r m ClIJIaBOB Ni-CO, CO~cpWaUMX 30 U 60 ato/b CO, MCCJleAOBaHO IIOBe~eHHC TOgeK &OpM IIpM PH, 5 67 kbar M HaMarHWleHHOCTM IIpH aTMOC$lepHOMjlaBJIeHMH M 80 5 T 5 220 K. 06pa3111,1 &Tlfi H3MePeHMH IIaMaTHI.IYeHHOCTH Hacmuam BOAO~OAOMIIPH PH, 5 60 kbar.

1. Introduction
The investigation of thc Ni-Fe-H solid y-solutions (with f.c.c. sublattice of the metal) has shown [l I that a change of spontaneous magnetization a, at T = 0 K and of the Curie points T, on hydrogenation is qualitatively analogous to that on substitution of Fe by Xi. In y-alloys of the invar system Fees(Nil-.Mn,),, the effect of hydrogenon the magnetic properties turns out qualitatively analogous to that of Mn substitution by Ni. In either case, the effects on hydrogenation are analogous to those on substitution of atoms of the host metal by atoms with greater number of 3d 4s electrons. The band structure calculations for the y-hydridcs of transition metals (for instance [3,4]) show that at atomic ratio H-to-metal n 5 1 no new hydrogen band is formed below the Fermi energy, but the presence of the protons in interstitial sites of the metal crystal lattice distorts strongly the band structure of the host metal. Now, the energy of states of sp-symmetry considerably decreases whereas the d-states (that predominantly determine the magnetic properties of transition metals) change insignificantly. The facts mentioned allow to expect that magnetic properties of hydrogen solid y-solutions in many transition metals and their alloys can be qualitatively described considering hydrogen as a donor of a fractional quantity of electrons [2]. Indeed, to verify this suggestion one has to study a number of several Me-H systems since, for example, it cannot be excluded that the observed dependences a,(%) and Tc(n) the Xi-Fe-H solution result from thespecificpropertiesof just this of system. The present work is devoted to the investigation of hydrogen y-solutions in the Ni-Co alloys containing 30 and 60 atyo Co.

+

2. Experimental
The ingots were melted from electrolytical Ni and Co in an induction furnace in argon atmosphere. After a 10 h homogenization in vacuum at 1400 K and water-quenching these ingots were rolled into foils 0.2 mm thick. Then these foils were subjected to stress-relief annealing at 1400 K for 15 min in vacuum and again water-quenched. The samples of 2: 20 mg for the magnetic measurements were assemblies of 6 X x 1 mm2 bands cut from such foils. The magnetization of the samples was measured
I)

142432 Chernoqolovlia, Moscov district, USSR.


76

V. E. ANTQNOV al. et

by the induction method [5] in a pulsed magnetic field up to 45 kOe in the temperature range 80 to 220 K, the pulse duration being z 0.01 s (at normal pressure the Xi-Co-H solutions are kinctieall yunstable in regard to the desintegration into metal and molecular hydrogen at temperatures above x 240 K). Hydrogenation of the Xi-Co speeimens was conducted by several hours exposure to hydrogen pressures up to 60 kbar and T = 520 K with following quenching down to 240 K [l, 21. The Curie points of the alloys at high hydrogen pressure were determined by the differential transformer technique [6]. The transformer core was a ring with an external diameter about 4 mm made from the foil of the investigated alloy. The temperature was measured by a chromel-alumel thermocouple insulated against the exposure to hydrogcn. The pressure was measured with an accuracy of &3 kbar, the temperature *15 I<. No corrections due to the pressure effect on the thermocouple readings were made. The temperature was kept constant within +2 K during investigation of the electric resistance isotherms. The hydrogen concentration in the Xi-Co-H samples was measured in the process of their desintegration at normal conditions with 3 yo error by collecting the released hydrogen into a scaled glass vessel, silicon being expelled from the vessel.

3. Results and Discussion
3.1 The T-PH,phase diagram of the NhoCo6o-H system

Under high hydrogcn pressure nickel experiences an isomorphous y1 yz transformation followed by a jump-like increase of hydrogen solubility and the parameter of the f.e.c. nickel sublattice [7]. In the T-YH~ diagram the line of this transformation terminates in a critical point with the coordinates 620 < T,, < 700 I<, 16 (P& 19 kbar [8, 91. An investigation of the T-PH~ phase diagrams of thc Ni-Co-H solutions with 5 30 atyo Co has shown the doping of nickel with cobalt to increase the critical pressure [lo, 111. Fig. 1 represents the electric resistance isotherms of the alloy with 60 at yo Co at step-wise increase and decrease of hydrogen pressure. In every point the sample was exposed up to the end of the resistance time drift ll(t) occurring after a pressure change in hydrogen atmosphere due to the diffusional nature of the formation of Ni-Co-H solid solutions. The finite values of R are plotted in the figure. One can see the prominent jumps of R(PH,)dependences near PH,= = 42 kbar at 523 K, the time At needed for approaching the equilibrium values of R sharply increasing in the pressure intervals where the resistance jumps take place. Such a behaviour of electric resistance is characteristic of a phase t)ransfor-

<

<

<

Fig. 1. Electrical resistance isotherms of the Ni-Co alloy with 60 atyo Co in hydrogen atmosphere; at increasing pressure, o at decreasing pressure. The digits show the times At in minutes for the resistance drift to terminate at fixed T and PH,after the presslire has been changed. The values A7 at pressure increases are left of the CUPVCS R(PH,),at decrease right. R,, the resistance of the samples at normal is conditions. (a) T = 623, (b) 598, (c) 573, (d) 548, (e) 523 I<


The Magnetic Properties of Ni- Co-H Solid Solutions

77

I
g

700

Fig. 2. T-Plr, phase diagram of the Ni,,,Co,,,-H system. pressures of the y, y2transition, 0 of y2 - yl. A Curie points
+

mation of the first kind. An X-ray study (P= 1 atm, T = 83 K) of the specimen Ni,oCo,o-H hydrogenized at 550 K and PH2 = 67 kbar up to n = 0.7 f 0.02 has shown that metal atoms form an f.c.c. sublattice with 500 unit cell volume being increased by AVO= (6.9 & 0.5)A3 as compared to that in a hydrogen-free alloy. This "20 30 10 50 60 70 agrees with the dependence AVo(n) for hydrogen ypH2 lkbarl solutions in Ni-Fe [l] and Ni-Fe-Mn [2] alloys. So, the first-order phase transformation observed in the Ki40C060 alloy at 523 K and PH, = 42 kbar is likely to be the isomorphous yl y, transformation. yz transformation has no hysteresis already at As it is seen from Fig. 1, this yl 523 K. Above 573 K the behaviour of the resistance isotherms becomes more smooth, and the pressure intervals where At increases up to the values peculiar for yl yz and ya --f y1 transformations rapidly increase. It is the behaviour of electric resistance that should be expected in Me-H systems at t,emperatures near T,,owing to the decrease of the rate of hydrogen diffusion in the vicinity of the critical point [12]. The positions of the anomalies of R(PH,) isotherms for the alloy with 60 atyo Co are plotted in the T-PH2 phase diagram Fig. 2.
550

:

m

-

-

3.2 The spoiitaneoits niagnetixution ao(ii)

equilibrium T-C diagram. With decreasing temperature the two-phase region in the T-C diagram expands; in particular, the value of the minimum hydrogen solubility in the y,-phase increases. In order to study at T (= 220 K the magnetization of onephase Ni-Co-H samples only the alloys were hydrogenized at temperature520 K T,, and pressures different from those of the y1 y, transformation by not less than 3 to 5 kbar. The one-phase nature was verified by X-ray measurement in the present work for the Ni40C060-I specimen with n = 0.7 and in [lo] for the Ni,,Co,-H specimen with n = 0.65. The values of a, represented in Fig. 3 for hydrogen solutions in the Ni-Co alloys with 30 and 60 atyo Co and of the Curie points for the Xi,Co,-H solutions were

It was shown [13] that the hydrogen mobility in Xi-H solutions is so large that even at T < 250 I< the separation into y1 and yz phases occurs in accordance with the

<

6 =? ID' a,
-a

1

I?-

0.5

0

\!
'\
200
700
\

05

I

I

\

'\

10

15'

Fig. 3. Spontaneous magnetization a,, at T = 0 K (0 for 30 atyo Co, c j for goatyo Co) and the Curie points (A for 30 atyo Co) in dependence on hydrogen concentration 7t for the Ni-Co-H system. (The magnetization is given in p13/atom of the Xi-Co alloy)


78

V. E. ANTQNOV al. et

obtained using the equations of the theory of very weak itinerant ferromagnetism [14]. Note, that within the error of = 5% an extrapolation of the dependences a(T) to T = 0 K by the low T3I2of the spin wave theory yields the same values a, for the investigated samples. As it is seen from Fig. 3, the magnitude of a, for both the alloys decreases monotonously with increasing n. The a,(n) dependence for the alloy with 30 atyo Co is nonlinear with concavity towards the n-axis. Analogous deviation from linearity was observed for Ni-Fe non-invar y-alloys at n 2 0.8. The concentration n = 0.8 is peculiar for the Me-H solutions with f.c.c. crystal sublattice of the transition metal since in all the investigated solutions the character of the dependences AV,(n) changes near this n [2, 151. The nature of this effect is still obscure. For instance, an assumption exists that at n 0.8 hydrogen in these metals begins to populate not only octahedral, but also tetrahedral interstitial sites [9]. It is not excluded, too, that even at n 1 hydrogen continues to populate octapores alone, their number increasing with respect to the number of the lattice points 0.8 due to the increase of the number of vacancies in the f.c.c. metal lattice at n (i.e. due to the formation of a f.c.c. subtractional solution by the metal atoms) [16]. Restricting ourselves to the data for n 0.8 only, with a linear approximation of the dependences a,(n) by the least squares technique we obtain au,/an = -0.76 and -0.71 pB/atom for the Ni,,Co,-H and Ni,,Co,,-H solutions, respectively. Nickcl is described well by the model of itinerant ferromagnetism, and near T = 0 K it can be considered as a strong itinerant ferromagnet [17]. The dependence cr,(x~~) for Ni-Co alloys is close to linear, agrees with the Pauling-Slater curve, and has the slope ao,/ax,-, ==. 1.05pB/atom= -1.05p,,/clectron [18], i.e. it is described well by the rigid band model for strong ferromagnets (xco is the atomic fraction of Co in the Ni-Co alloys). The ao(xco) dependence for these alloys seems to be mainly determined by the changes in electron concentration, the band structure varying weakly. According to [3,4], hydrogenation of the f.c.c. transition metals increases the degree of occupathe tion of the d-band (by 0.4 to 1.0 electron/proton for Pd and Xi [3]). Hence, one can expect 0 > aa,/8n > -l,uH/atoni for the Ni-Co-H solid solutions. The sign and the order of magnitude of the experimental C?o,/Bn values agree with these calculations.

>

>

<

<

3.3 The Curie points TC

Fig. 2 shows the T,(PE,) dependence for the Ni,,Co,, alloy. The Curie temperature is seen to decrease on hydrogenation. The curve T,(P=,) in the T-PnI diagram intersects the prolongation of the line of yl % y2phase transformation in the supercritical region (T, u 1200 K at P = 1 atm for the Ni,,Co,, alloy [lS, 191). Hence, this dependence is a continuous function of hydrogen pressure (and hydrogen concentration in the sample). As it is seen from Fig. 3, the Curie temperature of the Ni,Co,, alloy (at P = 1 atm) also decreases with increasing hydrogen concentration. The error in Tc determination using Wohlfarth's equations [14] is +lo K when T 300 K and grows up to 550 K at T = 500 K. The equations [14] are valid for very weak itinerant ferromagnets. In the case of the Ni,,Co,o-H system they yield somewhat overestimated values for the Curie points : the measurements under high hydrogen pressure have shown that in the y2-region of the T-PHz diagram of this system T, 350 K. Following El], we introduce the coefficients 5 relating the values of n and Axco for every Ni-Co alloy:

<


The Magnetic Roperties of Ni-Co-H Solid Solutions

79

This equation establishes the correspondence between alloys with the same a,, one containing xc0 cobalt and n hydrogen, the other one containing xc0 - Axco cobalt and no hydrogen. Under the assumption that the corresponding alloys have also close values of T, and (dT,/dP)in the semiquantitative description was given in [l] forthe ATFp dependences in the Ni-Fe-H system, where the index "in" designates an inert medium, ATFp is the difference between the values of T, in hydrogen and in an inert medium at the same pressure. Under such an assumption in the case of Ni-Co alloys we have [l]

AT?' = AT2

+

AT:

=

{TC(x~o 5n) - TC(zco)) -

+

since T,(P) dependences for the Xi-Co alloys are nearly linear [19]. For the alloy with xc0 =- 0.6 at n = 0.7 equation (1) gives xc0 - 5n = 0.6 - (0.71/1.05)0.7 = 0.1. Using the values of Tc and (dT,/dP)i, from [18 and 191, at P = 67 kbar we obtain: -450 K . (750 - 1200) (0.66 - 0.74) 67 x -450 - 5 AT?'

+

An analogous calculation shows that at n = xC,,/~ 0.3(1.05/0.76)= 0.4 the Curie x temperature of the Ni,,Co3,-II solution must decrease down to TZi = 630 K [18, 19, 91 (compare with the data of Fig. 3). So, in the cases of both Ni,oCo,o and Ni,,Co,,, alloys, formula (I) yields the correct sign and order of magnitude of AT,. The observed corrclation between the behaviour of a, and T, of the Xi-Co-H solid y-solut,ions (and Xi-Fe-H solutions, too, [l]) is of additional interest because these values are determined by the different physical parameters. The tendencies of the magnetization changes in Ni-Cr-H [20] and Ni-Mn-II [21] solutions obtained by electrochemical techniques show that such a correlation may be absent in these systems. It is to be noted that the magnetic properties of Mi-Cr and Ni-Mn alloys are anomalous [IS], and further studies are needed for a better understanding of the properties of hydrogen solutions on their base.
4. Conclusion

ATE and AT: are of the same order of magnitude, in the case of the Ni-Co-H system [ATCI IT61 due to the small values (dT,/dP),,. So, at n = 0.7 the Ni,,Co,,-H solution has to have T, x 750 K. This composition is attained at PH,= 67 kbar and T = T, (PHz 67 kbar) x 560 K (see Section 3.1). =

Note, that in contrast to the hydrogen solutions in the Ni-Fe invars [l, 21, where

<

Despite of a strong deforniation of the band structures of transition metals on hydrogenation, the bchaviour of the spontaneous magnetization at T = 0 I< and of the Curie points for hydrogen solid y-solutions on the base of a number of alloys (Ni-Fe, Fe,,(Nil -%Mn2),,, Ni-Co) can be qualitatively described considering hydrogen as a donor of a fractional quantity of electrons.
Acknowledgement

The authors are thankful to A. I. Amelin for his kind assistance in preparing the experiment. References
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V. E. AKTOKOV al. : The Magnetic Properties of Xi-Co-H Solid Solutions et

[3] A. c. SwrTExDIcK, ner. Bunsenges. phys. Chem. 76, 535 (1972). [4] N. I. KuLmov, V. N. BORZUNOV, A. I). ZVONKOV, and phys. stat. sol. (b) 86, 83 (1978). [5J L. JACOBS LAWRENCE. sci. Instrum. 28, 713 (1958). and P. Rev. [6] G. T. DCBOVKA E. G. POOLYATOVSKII, and Fiz. Metallov i Metallovedenie 33, 540 (1972). [71 B. BARANOWSKI R. WISNIEWSKI, and Bull. Acad. Polon. Sci., SBr. Sci. chim. 14, 273 (1966). [SJ E. G. PONYATOVSKII,ANTONOV, I. T. BELASH, V. E. and Dokl. Akad. Nauk SSSR 230, 649 (1976). [9] V. E. ANTONOV. T. BELASH. E. G. PONYATOVSRII, Acad. Nauk SSSR 233, 1114 I. and Dokl. (1977). [lo] V. E. AKTOOLOV,T. BELASH, I. and E. G. PONYATOVSKII, Metallov i Metallovedenie 46, Fiz. 882 (1978). [111 S. FIIJPEK, B. BARANOWSKI, 11. YONEDA, and Roczniki Chemii 61, 2243 (1977). [I21 Y. RIBAUPIEHRE F. D. MANCHESTER, and J. Phys. C 7, 2126 (1974). [13] V. G. THIESSEOL. E. ANTONOV, T. BELASH. K. PONOMAREV, E. G. PONYATOVSV. I. B. and HII, Dokl. Akad. Nauk SSSR 242,1390 (1978). [14] D. M. EDWARDS E. P. WOHLFARTH, and Proc. Roy. SOC. A303, 127 (1968). [15] B. B~ANOWSKI, S. MAJCHRZAK, T. R. FLANAQAN,Phys. F 1, 259 (1971). and J. [16] S. SIT.SHILSHTEIN, private communication. [17J B. K. PONOMAREV G. THIESSEN, ekspcr. teor. Fiz. 75, 332 (1977). and V. Zh. [18] S. V. VONSOVSKII, Magnetizm, Izd. Nauka, Moskva 1971 (p. 617). [19] J. M. LEGER, LORIERS-SUSSE, B. VODAR, C. and Phys. Rev. B 6, 4250 (1972). [20] G. J. ZIMMERMANN H. J. BAUER, Phys. 229, 154 (1969). and Z. [21] H. J. SCHENK H. J. BAUER, and Proc. Internat. Meeting Hydrogen in Metals, Vol. 1, Miinster (RRD) 1979 (p. 350). (Ileeeived July 31, 1979)