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V. E. ANTONOV al.: The Magnetic Properties of Ni-RIn-H Solid Solutions et
phys. stat. sol. (a) 59, 673 (1980) Subject classification: 18.2; 21.1

673

Institute of Solid State Physics, Academy of Sciences of the USSR, Chernogolovka')

The Magnetic Properties of Ni-Mn-H Solid Solutions
BY

V. E. ANTONOV, T. BELASH, V. G. THIESSEN I. and
The behaviour of the Curie points at PH? 15 kbar and of the magnetization at atmospheric pressure and 4.2 5 T 5 100 K is investigated for hydrogen solid solutions on the base of disordered f.c.c. Ni-Mn alloys containing 10, 20, and 30 at?; Mn. The samples for the magnetization measurements are hydrogenated at PH,5 70 kbar. It is shown that the observed effects may be explained within the framework of the band theory of magnetism considering hydrogen as a donor of a fractional qiiantit'y of electrons.

5- IJacTBOpOB BOnOpOJIa Ha 6a3e HeyIIOpl~OqeHHhIX u K CIIJIaBOB Ni-bfn, COEepWaUlfX r 10, 20 xi 30 atyo Mn, MccneHoBaHo noseneme ToqeK HIOPHnpn PH?& 15 kbar ~l HaMaTHH9eHHOCTIl IIPn aTMOC@epHOM AaBJIeHHlT II 4,2 2 T & 100 K. OGpa3ub1 AJIH 113nprl J1e~)eHMHHaMaTHMqeHHOCTEl HaC,bIUaJIM BOAopo~o~ PH, 5 70 kbar. nOIGi3aH0, 9TO HaGnIonaemIe 3@@e~~h1 G H T ~ 06%lCHeHbI pawax ~OHHOB MoryT B Tepoiiri ~ a r ~ e ~ m ~ a ,
eCJI1l

CqHTaTb

BOHOPOJI HOHOPOM

JIpO6HOrO qIiCJIa 3JIeKTpOHOB.

1. Introduction
The effect of hydrogen on the magnetic properties of the Xi-Fe [l], Fe65(Xil-.$hz)35 [?I, and Ni-Co [3] y-alloys (with f.c.c. crystal lattice of the metal) turned out, to be similar to that on changing the composition of t>he starting samples towards the enrichi s electrons. Since the depending with the elements having a greater number of 3d ence of the properties of most of these alloys (at least of Ni-Co and Ni-Fe non-invar alloys) upon composition is determined mainly by the electron concentration [4],the results of [l to 31 allowed a qualitatjive description of the magnet'ic properties of the mentioned Me-H solid solutions with hydrogen considered as a donor of a fractional quantity of elect'rons [2, 31. As has been noted in [3], the calculations of the band structures of y-hydrides of soine transition metals [5 to 81 may serve as a basis for such a description. These calculations indicate t>hatakhough the rigid band niodel cannot be used to describe the Me-H solid solutions, t'he changes of the states of d-symmetry of the host ixet'als under hq-drogenat,ionare considerably weaker t,han bhose of sp-states. At the at'omic ratio H-to-metal n 2 1, no new (hydrogen) elect,ronic band is formed below the Fermi energy, an increase of hydrogen content in t,he metal resulting in an increase of the degree of occupation of d-st)at>es, while it is the d-band structure and the degree of its occupation that mainly det>erminethe magnetic properties of transition metals. The available experimental evidence makes it possible now to consider t,he more involved case of hydrogen solid solutions in Ni-Cr and Xi-Mn alloys, where the con]position dependence of the magnetic properties is st>ronglyaffected by the changes of the band structure [4].Hydrogenation of the Ni-Cr y-alloys containing 5 7 at yo Cr has been shown t,o lower the Curie points T, and t.he spontaneous magnetization Go at T = 0 I( [S]. The Ni-Mn-H solutions have been studied in [lo, 111 at temperatures close to room teniperature. The present work is devoted to the investigation of the. behaviour of T, and cr, of Ni-Mn-H solid solutions based on the disordered f.c.c Ni-Mn alloys cont.aining 10, 20, and 30 atyo Mn. On the grounds of t.he data of [I to . __ I) Chernogolovka 142 433, Moscow district, USSR.

+


674

V. E. ANTONOV, T. BELASH, V. G. THIESSEN I. and

3, 91 and the present work, a description of t,he dependence of magnetic propert,ies of Me-H y-solutions based on transition metals and their alloys upon hydrogen concentration has been proposed within the franiework of the band t'heory of ferroniagnetisni.

2. Exporimcntal
The ingots were melted in an induction furnace under an argon atmosphere from electrolytical nickel and manganese. After a 6 h homogenization at, 1400 K and water quenching, these ingots were rolledinto thick sheets = 0.5 mni. Then the sheets were subjected to stress-relief annealing at 1400 B for 3 niin, quenched in water to obtain the disordered &ate at) nornial conditions [13], and cheniically etched to the thickness of =: 0.3 nini. The saniples were prepared from t.his foil. Alloying of nickel with manganese lowers t,he kinetic stability of Me-H solid solutions in regard to the desintegration into nietal and molecular hydrogen [lo]. In our case, the solutions on the base of the alloy wit>h30 at% Mn began to lose hydrogen with not'iceable rat,e at temperatures beyond = 170 to 220 K (at room temperature the samples completely decompose in about 3 min). Hydrogenation of t,he samples \vas conducbed by several hours exposure under hydrogen pressures up to 70 kbar and T = 530 K with subsequent quenching down to 150 K [3]. The inagnet,ization was measured with an accuracy of 8% at normal pressure in a pulsed magnetic field H 5 45 kOe by the induct>ion method [13] in the temperature range 4.2 to 100 K, the pulse duration being = 0.01 s. The Curie points of the alloys under high pressure were determined by t.he different.ia1 transfornier niet,hod 114, 31 b>- controlling the position of the anomalies of the t,emperat>ure dependence curve of the initial magnetic permeability p,,( T) with an accuracy of +3 K in an inert niediuni (silicon) and +5 K in hydrogen. At pressures up to 30 kbar the experimental errors did not exceed f l and 5 3 I( in temperature and 10.2 and 50.5 kbar in pressure, in an inert. medium and in hydrogen, respectively. Then t.he measurements were performed in hydrogen, the thermocouple (chroniel-alumel) and the nianganin wire gauge were protected from direct exposure to hydrogen. The hydrogen content) in the Me-H samples was measured with 5% error by collecting the released hydrogen to a scaled glass vessel (silicon being expelled from t'he vessel) at, at'niospheric pressure and room temperature.

3. Results
3.1 Phase !/'-PH,diugrum of the iVi-3fn-H system

'

In nickel under high hydrogen pressure the isomorphic transition y1 & yz [15] occurs whose line in the T-PH2 diagram terminates in a critical point with the coordinates 620 T,, 700 K, 16 (PH,),,19 kbar [16, 171. Alloying of nickel with manganese results in lowering T,, to room temperature at = 18 at yo of Mn [18], solubility of hydrogen at T 2 298 K thus becoming a continuous function of pressure in alloys with a greater manganese content. Fig. 1 shows the electric resistance isotherms for the alloy with 10 at./, Mn at a step-wise increase and decrease of hydrogen pressure. At every point presented in Fig. 1 the sample was exposed for a time A7 up to the terniination of the resistance drift R(7) caused by the diffusional nature of the formation of Ni-Mn-H solid solutions, and the final value of R was plotted in the figure. The characteristic values of A7 were of the saine order of magnitude as in the Ni-Fe-H system [I(;]. It is seen frotii Fig. 1 that at 423 K there are well localized resistance jumps of the R(P,I*)isotheriiis corresponding to the y1 --r yz and ya 4 y1 phase transitions. The transformation possesses a noticeable hysteresis (= 1 kbar) that points to its first-order nature at

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The Magnetic Properties of Ni-Mn-H Solid Solutions

675

2
I-.

fi

I

Fig. 1

Fig. 2

Fig. 1. Electrical resistance isotherms of the Ni,,Mn,, alloy in hydrogen atmosphere. 0 - increasing pressure, o - decreasing pressure. R, is the resistance of the sample at normal conditions
0 y?

Fig. 2. T-PH?phase diagram of the Ni,,Mn,,-H system. 0 pressures of the y1 regions of the supercritical anomalies of electrical resistance, at a step-wise increase and decrease of pressure, respectively
~

..

yp transition, Curie points

this temperature. At 563 K the hysteresis disappears, and the resistance anoinaly becomes more sloping. Further increase of temperature results in a decrease of the amplitude of this anomly. This kind of hehaviour is typical for supercritical resistance isothernis [l6]. A rough estimate yields T,, = (600* 30) I(.The positions of the anomalies of the isotherms R(P,,)are plotted in the T-PH2 phase diagrani Fig. 2. The obtained curves of y1 y2 and y2 y1 transformations for the Ni,,Mn,-H system do not differ strongly from those for the Ni-H system [17]. Note that. the Tcr(x;lln) dependence for the studied system is considerably non-linear : the rate of T,,decreasing grows with the increase of the atomic fraction x m in Ni-Mn alloys. For instance, a 10 at yoMn addition to nickel results in a decrease of the critical temperature by T,,(O) - Tc,(O.l)x T$ - 600 & 100 K, while a further 8 atyo Mn dependence for the addition results in Tc,(O.l)- T,,(0.18) x 300 K. The Tcr(xFe) Ni-Fe-H system has a siniilar character [16, 191.

-

4

3.2 The C'uviepoints and sponfaneous magnetization of"-Mn-H

solid solutions

The o ~ ( xdependence for the disordered f.c.c. Ni-Mn alloys is non-monotonic. For ~~~) pure nickel o0(0)x 0.606 pB/atom; as manganese is subst,ituted for nickel a,, grows, reaches the maximum value 0,(0.1) x 0.8 pu,/atomfor ~ ~ ji= 0.1, subsequently beginIl ning to decrease [20]. The Curie points steadily decrease from 630 K for nickel to helium temperatures at xnIll x 0.26. The measurements carried out in the present work at P 5 15 kbar have shown that in an inert medium the Curie points of the Ni-Mn alloys with 10 and 20 atyo Mn approximately linearly decrease with pressure (the dTc/dP)i, values


676
Table 1
XJin

V. E. ANMNOV, T. BELASH, V. G. THIFXBEN I. and

Tc (10
506 353
-

OobBlatom) 0.80 0.62 0

(dTc/dP)i, (I< kbar-') -0.65 & 0.2 -0.30 & 0.1

6
0.27 0.16

i -~
-

0.1 0.2 0.3

-

0.81 0.48

are shown in Table 1. At a high hydrogen pressure, due to the diffusional nature of the forination of Ni-Mn-H solid solution, time dependences Tc(z)appeared at a fixed PIT, after a change of pressure. As well as in the case of resistance measuring, the Samples were exposed at each selected P H up to the termination of the time drift T,(T) ~ and the final T, value was plotted in the diagram. The obtained Tc(PI12) dependences for the alloys with 10 and 20 atyo Mn are shown in Fig. 2 and 3, respectively. The a,,(n) dependences for the studied y-solutions Ni-Mn-H are shown in Fig. 4. As in [2, 31, the a, and T, values of Me-H specimens metastable at atmospheric pressure were obtained using the equations of the theory of very weak itinerant ferrornagnetisni

pi].

0.81

300

t
0 2 4

6 8 P ikbar)4

i

b
0
02

04

06 08

10

n+

Fig. 3

Fig. 4

Fig. 3. Curie point dependence on hydrogen pressure for the Ni80Mn2, alloy. Tc of the starting samples; A, A data obtained at increasing and lowering pressure, respectively, after initial satura= tion of the sample with hydrogen at T = 420 K and PIC, 0.8 kbar; 0, o the same for the sample initially saturated with hydrogen at T = 420 K and PH,= 4.8 kbar Fig. 4. Spontaneous magnetization u,, at T = 0 K in dependence on hydrogen content n in the Ni-&In alloys with a 10, 20, o 30 at"/b Mn; 0 u,, values used to calculate 6 and c; r: calculated values of a,(%), see the text. The magnetization is given in pB/atom of the Ni-Mn alloy


The Magnetic Properties of Ni-Mn-H

Solid Solutions

677

3.2.1 Nig,Mnl,-H solutions

As is seen from Fig. 2, Tc of the yl phase of this system steadily decreases with hydrogen pressure until the intersection of the Curie point line with the yl yz transition curve at PH, =: 9 kbar, AT, = T,(PH2 9 kbar) - T,(P = 1 bar) being = -100 K. = In an inert medium a pressure of 9 kbar results in lowering the Curie point of the NiS,Mnlo alloy by = I(dT,/dP)li, 9 = (6 f 2) K (see Table 1).Consequently, the considerable decrease of T, observed in hydrogen atmosphere is caused by an increase of the hydrogen solubilit,yin the yl-phase of t'he NisoMn,,-H system with increasing pressure. Measurement under high hydrogen pressure indicated that the y,-phase of this system is paramagnetic at T 2 300 K and PH, 5 20 kbar. The hydrogen mobility in Ni-H solutions was shown to be so large that even at T < 250 K the separation into the yl and yz phases occurs in accordance with the equilibrium T-C diagram, the compositions of the phases coexisting at P = 1 atm being 12mas < 0.02 and n p = 0.7 0.05 [22]. A similar separation should be observed in YI solutions based on the Nis,Mnloalloy due t.o their high T,, value. Since the teniperat.ure of our Ni-Mn-H specimens at P = 1 atm never exceeded 150 K (see Section 2), the observed values of n y and n:'" are the equilibrium ones for T 150 K, if of course, the kinetics of the separation allows to approach equilibrium at these temperatures (note that t.he nyraxand n.zn values at. norrnal pressure may differ st.rongly from those at' high hydrogen pressure: so, in the Ni-H system, as iiient'ioned above, nK" = x 0.7 at' P = 1 atm and T < 250 I( whereas n,zn = 1 under nearly equilibrium hydrogen pressure at) room temperature [17, 231). As will be shown later, in the Xi,,Mn,-H system the hydrogen mobilit,y seems t.o be large enough to ensure an eqiiilihriuni phase coniposition of the samples at' P = 1 atm and T & 150 K. An X-ray study (P = 1 atm, T = 77 K) has shown the Ni,,Mn,,-H specimens with )/ 2 0.75 to be single-phase 1181, i.e. n p 0.75. According to the similar studp at P = 1 atni and T = 83 K, the specimens with n 5 0.5 consist of two phases: pure metal and hydride. Since hydrogenation leads to an increase of the unit- cell volunie T7, of the Ni-Mn f.c.c. sublattice with the slope /3 = (a/&) Vo x 10 A3 [18], and the accuracy of the X-ray nieasiirenient, was 8JT, 5 0.3 A3, the observed absence of volunie expansion due to the presence of hydrogen in the yl phase of t.he t,wo-phase specimens with n. 5 0.5 gives n y 5 8Tr0/,9 S 0.03. The oo(n) dependence at) atmospheric pressure for the alloy with 10 atyo Mn is shown in Fig. i a . At n 5 0.66 (or in t'heregion of hydrogen contents where the sainple should consist of a mixture of the yl and y, phases) o, approximat,ely linearly decreases as hydrogen concentrat,ion increases. The Nig,Mn,o-H solutions with n 2 0.8 do not possess a spontaneous magnetization in the range of temperatures studied.
+

s

Fig. 5. Magnetization isotherms a(H) for some of the Ni90Mnlo-H solid solutions. (1) n = 0, T = 4.2 K; (2, 3) n = 1.11, T = 4.2 and 73 K; (4, 5) n = 0.66, T = 4.2 and 100 K; (6) (U::;'(H))~, dependence, see the text


67s

V. E. ANTONOV, BELASH, V. G. THIESSEN I. T. and

The o(H) dependences for some of the Nig,Mnl0-H samples are shown in Fig. 5. It is seen (curve 1) that the inagnetizat.ion of t,he Nig,Mnl, alloy at H 5 45 kOe does not depend upon the magnetic field within the experiniental error. Within this error the t,emperature dependence of the magnetization of the Ni,,Mn,, sample is absent, too. The magnetization of the Ni,,Mn,-H solid solution with hydrogen content n y should behave similarly due to the smallness of t'he latter value. In particular, since n y 0 at T 0 K, 395 K = T,(PH? 9 kbar) T, T,(P = 1 kbar) = 506 K in such a sample = (see Fig. 2). Hence, samples with n 2 0.8 for which a, = 0 (Fig. 4a) consist of the y,-phase only. At 0.8 (= .n (= 1.11 the paramagnetic sussceptibilityof the Ni,,MB,,-H solutions weakly depends on hydrogen cont'ent. The al.ll(H)dependences of t'he sample with n = 1.11 at T 1 4 . 2 and 73 H are shown inFig.5 (curves 2 and 3). It) is seen that as temperature increases, t>heparamagnetic susceptibility of this sample decreases, and in the chosen scale of axses the al.ll(H)dependence approaches a linear one (in t,his section the subscripts and superscripts of a show the t,eniperature and hydrogen concent,ration, respect,ively). For t.he sample 3vit.h ?z = 0.66, q,=+= 0 (Pig. 4a), the niagnetizat,ion at 4.3 K is st.rongly dependent on the magnetlie field (Fig. 5, curve 4). When temperature increases up to ==: 70 Ti, this "paraprocess" noticeably decreases, and t>hefurther increase of T (to 130 K) does scarcely change neither tjhe spontaneous magnetization nor the a(H) dependence. Taking into account' that under conditions of the experinient 0.5 ny,ax and that a,, = 0 at n 2 0.8, one should expect the phase of the n&, coniposit'ion to have T, 298 K. So, the sample wit>hn = 0.66 is twophase. Independent of temperature at T 70 K spontaneous magnetization of this sample is caused by the presence of the y,-phase (of ii~laxcomposition), the magnetisame teniperat>uresbeing a consequence zat,ion dependence on the magnetic field at t>he of the large paramagnetic suscept)ibilit>y the y,-phase (of nRin coniposition). In of order t'o determine whether the y,-phase beconie ferromagnet,ic below 70 B let us estimate the niagnetizabion ay2(H) the part of the specimen with n = 0.66 which is of in the y,-&ate. At T = 100 K (Fig. 5, curve 5), when the y,-phase is known t,o be paramagnet,ic, the spontaneous magnetization of the mixture of tlhe y1 and y2 phases is aO.GG(a:$$)y, = 0.08 pB/atoni (extrapolation of the a:$(H) dependence to H = 0 100 z is shown hy a dotted line). Since the magnetization of the phase of iiy composition at T 100 I( is nearlyindependent of H and T (see above), ('T~."(H)),~a0.6G = (HI - a:$ (note, that the sample wit>h = 0.66 contains only a:$:/a; n x 0.0SjO.S = 1/10 of t'heyl, phase). The (aj:!6(H))y2 dependence is shown in Fig. 5 with a dash-dotted line (curve 6). Extrapolat'ion of t'his dependence to H = 0 using equations [31] yields (a4.2)y, 0. Thus, at atmospheric pressure the y,-phase of the Pu'i,,Mn,,-H = soliit'ion (as well as bhe y2-phase of the Ni-H solut'ion [32]) is paramagnetic at T 2 4.2 K up to the boundary composition nEn. The spont,aneoiis magnet>izationof t>hecquilihriu ni t,wo-phase mixture is a linear function of its coniposition. As it is seen from Fig. l a , t'hese demands are well fulfilled for the Ni,,Mn,,-H syst,em. So, the hydrogen mobility in this system is large enough for the coniposition of the (yl y2)-phase mixt,ure to approach near-equilibrium state even at. T 150 B (see above). An approximation of the experiment,al dependence ao(n)for t,he Xi,,Mn,,-H solutions with n 5 0.66 by a straight line (dashed line in Fig. 4a) yields t'he value of = 0.7 = for a hydrogen cont>ent which a, = 0 at) (due to smallness of ny, ignored the change of a, in t>heregion of honiogenity of we the y,-phase which does not, necessarily follow the dependence shown by a dashed line). Thus, alloying of nickel with manganese up to 10 at yo Mn changes weakly both T,,(Section 3.1) and n?".

-

<

<

-

nEn >

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+

ne


The Magnetic Properties of Ni-Mn-H Solid Solutions

679

3.2.2 Th,e Ni,,Nn,,-H

solutions

As mentioned above (Section 3.1), at q I n 0.1 the critical temperature in the XiMn-H system rapidly decreases and at z",, = 0.18 reaches room temperature. No separation int,o t,he y1 and yz phases is observed for the samples with xhrn = 0.193 at. atniospheric pressure even at 77 K [18]. Thus, in the range of temperatures and pressures used in the present work, hydrogen must) form a continuous series of solid solutions wit'h Ni,,Mn,, and Ni,,Mn,, alloys. The behaviour of the Curie points of the alloy with 20 at yo Mn under high hydrogen pressure is shown in Fig. 3. In contrast to t>healloy with 10 atyo Mn (Fig. a), hydrogenation at t'his alloy may result in irreversible changes of T,. The starting Ni,,Mn,, alloy has T, = (353 3) K. Rapid heating at PH,= 0.8 khar up to 120 K with exposure at this temperature for 30 niin t'o saturate the sample with hydrogen up to t'he equilibrium value resulted in an increase of its Curie point up t>o=r 370 K (t>riangles in Fig, 3). A similar procedure at PH,= 4.8 kbar resulted in an increase of T, up to m 420 K (circles in Fig. 3), the anomaly of the po(T) dependence corresponding t,o T,, becoming great,ly stretched in temperature. On furt.her step-wise increase of pressure with steps of = 1 kbar, the Curie points steadily decreased in b0t.h cases. On lowering the pressure, the Curie points fell approximately to the sanie curves as for pressure alloy. Such a phenomena increase and did not ret>urn the T, values of t,he st>arting bo had not been observed before on studying hydrogen solutions in Ni [17], Ni-Fe [16, 241, Ni-Co 125, 31, and Ni-Mn alloys with 10 atyo Mn (see Fig. 2). The Curie points of the Ni-Mn alloys may undergo a drastic change at plastic deformation of the samples. For instance, rolling of t>hesample witah20 atyo Mn at room temperature from a thickness of 0.2 to 0.08 mm resulted in a decrease of T, from 353 to 317 K. The detected increase of T, on t'he initial rapid sat>urationof this alloy with hydrogen is apparently associated with just the crystalline structure defects the formed during this process (introduction of n = 0.7 of hydrogen int>o Xi-Mn alloys produces an = 18% increase of their volume [18]). An increase of the Curie points of our alloys niight have been caused by t,heir ordering [12]. However, this possibilit,p should be excluded, since aft,er the exposure of t.he specimens which were used t.0 and at,mospheric pressure for ohtain t,he data shown in Fig. 3, t,o room t,emperat>ure several weeks their Curie points returned to t>heinit>ialvalue of T, = 353 R, the latter would have been impossible in the case of ordering (no additional hydrogen was given off from the specimens). It should be noted that the possibility of irreversible changes of magnetic propert'ies of transit'ion metal alloys on hydrogenation shown in this work makes one to t>reatwith care the results of t>heexperiments wit,h Me-H samples obt'ained using elect'rochemical techniques, i.e. under the conditions which are known to he non-equilibrium and hardly cont>rollableones. So, if we discard t>heeffect>sassociated with the irreversible changes of T,, the increase of hydrogen concentration, as is seen from Fig. 3, results in lowering of the solutions to room temperature already at PH, Curie points of the Ni,,Mn,-H 10 kbar. The sample saturated with hydrogen t.0 n = 0.91 at. pfr, = 70 kbar and T = 530 K had T, = 100 K at atmospheric pressure. Thus, t,he Curie p0int.s of the Si,,)Mn,,-H solution steadily decrease with hydrogenation. The irreversible changes of T, observed for the alloy with 20 at yo Mn are not accompanied, however, by such changes of o, exceeding the measurement' error. It is seen from Fig. 4a that on hydrogenation of t'his alloy o, increases, reaches its maximum value of = 0.82 pB/atjoni n = 0.37, and begins t,o decrease. A similar change at of a, for bhe Ni,,Mn,, alloy also occurs on substitut>ionof nickel by manganese: a, increases to = 0.8 pu,/at'oni at xyn = 0.1, subsequently decreasing to op = 0.606 ,uB/atom 1201.

>


680

V. E. ANTONOV, BELASH, V. G. THIESSEN I. T. and

<

Thus, similar to the previous observations for hydrogen solutions in the Ni-Fe [I], Fes5(Nil-,Mn),, [2], and Ni-Co [3] alloys, an increase of hydrogen concentration in the Ni,,Mn,, alloy results in t>hesame change of a, as the increase of the content of 4s electrons. Following [l to 31, we int>roduce an element with a greater number of 3d t'he coefficients 5 = -Axt.,,jAn and = ANe/An connecting t>hechanges of conipo4s elechons per atom of the sition and, respect,ively, Ne concentrations of t>he3d starting alloy, with the changes of n resulting in the same changes of a, (in t>hecase of the Ni-Mn alloys i= 35). Then for t'he position of the maximum of the a,(n) dependence for t>he Ni,,Mn,,-H solution (0 Fig. 4a) we have 5 x 0.1/0.37 = 0.27; in = 0.81. Assuming that E does not depend on n., we obtain n = 0.2/,t x 0.74 for the cornposition of the solution for which a, = a:i. The obtained point is ninrked with a cross in Fig. 4a and agrees with the experinlental dependence ao(n)for the alloy studied. It may be relevant) to note here that) the nionotonous decrease of the spontaneous alloy at teniperatures close t.0 inagnetizat ion on hydrogenat,ion of tlhe NiR0.7Mn19,3 room temperature [lo, 111 does not contradict the o,(n)dependence for the Ni,,Mn,, alloy found in the present) work (Fig. 4a). In fact, according to our data an increase of hydrogen content) in the Ni,,Mn,,-H solution should lead to a decrease of the Curie point and, consequently, to a reduction of the spont>aneousmagnetization at room teInperat.ure due to an increase of the relative temperat.ure TIT, of the nieasureinent. -4s is seen from Fig. 2 and 3, T, of t,his solution should drop below 300 B at PH,& 10 kbar in good agreement with the data of [ll].

+

+

3.2.3 ATi,,ilfn,,-H solutions The Xi,,Mn,, alloy is paramagnetic at T 2 4.2 K. As is seen from Fig. 4b, the introduction of hydrogen leads to its ferromagnetic ordering. At 'n = 0.9, a, of the Ni,,Mn,,solution reaches values close to the niaxinlum ones in t>heKi-Mn alloys, the Curie point' growing to = 250 K. Similar to the case of the alloy with 20 atyo Mn, hydrogenation of the saniples wit>h 30 atyo Mn results in irreversible changes of their magnetic properties, the effect being st,rongly dependent. both on the condit,ions of the Ni,,Mn,,-H solution preparation, and on the conditions of their decomposition into nietal and iiiolecular hydrogen. For instance, after a complete release of hydrogen, a,( n) disappearing the samples sometimes possessed a spontaneous niagnetization a, aft,er exposing them for several days to normal condit,ions. The occurrence of irreversible changes of a, on hydrogenation allows to consider the data shown in Fig. 4b as only seniiquantitative est>imates. Assuming the mean value of a, of the four samples grouped close to n = 0.85 in Fig. 4b, to be t,he a, for the Ni,,Mn,,-H solution of this concentration (0 Fig. 4h) in and t,aking into account, that. it is t.he Ni-Mn alloy wit,h 16.5 at.% Mn which has such a value of spont'aneous magnetization [20], we obtain 5 = (0.3 - 0.165)/0.85 z 0.16. The a,(n)values ca1culat)edusing this value of 5 on the basis of the ao(mm) dependence for the Ni-Mn alloys [30] are marked with crosses in Fig. 4b, and a cont,inuous curve is drawn through them. As is seen from the figure, t>heexperimental a, values do not contradict. the calculated o&) dependence. Thus, in t,his alloy the effect of hydrogen on a, also turns out to be similar to that on t>hesubstitution of manganese by nickel. Moreover, in the case of the Ni,,Mn,, alloy, as well as in the case of the Xi-Fe [l], Pes5(Nil-,Mn,),, [2],and Ni-Co [3] alloys, such an analogy (if only by the order of magnitude) also takes place for the Tc(n,)dependence. In particular, on hydrogenation of the Ni70Mn,, alloy up to n. = 0.85 (the a, value for this composition of the solution is marked by 0 in Fig. 4b), its Curie point st>eadilyincreases up to T, = 250 K, and the Ni83,5Mn16,5 alloy (which has the same value of a,) shows a T, as high as = 400 K [12].

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The Magnetic Properties of Ni-Mn-H Solid Solutions

681

4. Discussion

The experimental data show that the a,(%) dependences for all the investigated Nibased Me-H solid y-solutions except for the Ni-Cr-H [9] are similar to the corresponding a,(z~i) dependences for the starting alloys. Such a similarity among the Tc(n) and TC(z~i) dependences is violated already for the Ni-Cr alloys as well as for the Ni-Mn alloys with 10 and 20 atyo Mn (see Section 3.2). It will be shown below that the band theory allows to give at least a qualitative account for the behaviour of the ao(n) and T,(n) dependences for all the investigated Me-H systems considering hydrogen as a donor of a fractional quantity of electrons.
4.1 Strong itinerant ferromagnets

The assumption [2,3] that the effect of hydrogen on the magnetic properties of yalloys of transition metals may be described by considering the increasing degree of occupation of the d-states of the starting metal as themaineffect, was basedon the fact that in the case of Ni-Fe [l], Fe6,(Nil-,Mn),, [2], and Ni-Co [3] alloys this effect is similar to that on enriching the starting alloys with the element having a greater number of 3d 4s electrons. This, in its turn, suggests that the magnetic properties of the studied alloys (without hydrogen) in sufficiently wide ranges of coniposition are mainly determined by their electron concentration, Ne. The latter assumption is to be true in the case of the Ni-Co alloys and Ni-Fe alloys containing & 60 atyo Fe. In fact, near T = 0 K nickel can be considered as a strong itinerant ferromagnet [26], one half of its d-band (df with spins up) being completely filled and the other (dJ,with spins down) only partially. On alloying nickel with cobalt or iron, a, approximately linearly increases with concentration, in agreement with the Pauling-Slater curve, and has a slope aa,/aNe = -1 pB/electron [27]. This is precisely the behaviour which should be expected if alloying does not result in a change of the band structure of the metal, onlydecreasing the degree of occupation of its d$-subband (i.e. the rigid band model for strong itinerant ferromagnets is valid). Note that in the rigid band approximation hydrogenation of these alloys would lead to ao;/an = - -1 p.,/atom. The experimental values of au,/an deviate noticeably from auh/an and amount to -0.6 to -0.4 p.,/atom for the Ni-Fe alloys containing 10 to 60 atyo Fe [I], -0.76 and -0.71 pB/atom for the Ni-Co alloys with 30 and 60 atyo Co [3]. Thus, hydrogen essentially distorts the metal band structure. According to the band structure calculations [5 to 81, an increase of the number of protons in interstitial sites of transition metals lowers the energy of the states of sp-symmetry resulting in an increase of the number of states below the Fermi energy and, therefore, only a part of electrons (0.4 to 0.1 electron/proton for Pd and Ni [5]),supplied by hydrogen atoms into the conduction band, populates the states above the Fermi energy, the rest occupying the mentioned additional states below it. For strong itinerant ferromagnets this yields 0 > aa,/an > -1 pB/atom, 1 > 5 > 0 in agreement with the slopes of the experiment,al dependences cro(n)and T,(n) [l, 31.

+

4.2 Weak ifinerant ferromagnets

The uo(Ne) dependence for the Ni-Fe alloys with zFe2 0.62 [l] Fe65(Nil-xMnz)35 [2], and Ni-Mn alloys with xM,, 2 0.1 deviates from the Pauling-Slater curve towards lower u, values [27], the effect being associated probably with the appearance of holes in the dt-subband (e.g. according to [%I, holes in both d-subbands of the Ni-Fe alloys appear at xFe 2 0.63). In this case the role of tlhe electron concentration in changing the magnetic properties appears to be more probleniatic than in the case of alloys in which holes exist. only in one d4-subband. The situation is rather reverse,


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V. E. ANTONOV, T.BELASH, V. G. THIESSEN I. and

the similarity of t'he behaviour of a, (and, in niost cases, of T,)on hydrogenation and on increase of t'he nickel content, detected for the in Ni-Fe [1], Fe65(Nil-zMn&5 [a], and Ni-Mn alloys indicates that in wide ranges of coniposition (AN" & [rim,, 0.5) the degree of occupation of the d-band, rather than its deformation, mainly determines t>he niagnet'ic properties of the start>ingalloys (without hydrogen). In fact, if the deviations of the oo(Ne)dependence from the Pauling-Slater one (wit'h t'he slope i3a/,aNe =r -1 ,un/electjron)were due to different changes of the band structure in the above alloys, t'hen in each case t>he effect, of hydrogen would be different in principle (in the Ni-Fe alloys it should be opposite to the effect of Fe, in Ni-Mn and Fe65(Nil-zMnz)35 opposite to t>hatof Mn), which is unlikely. Note, however, that the coefficient,s 5 may be quite phenomenological and do not give any est.imate of the fraction of electrons supplied by hydrogen atoms above the existing Fermi level, since these coefficients describe the total effect of hydrogenat ion upon the magnetic properties including, in particular, t,hat due tlo an increase of the alloy volume (approaching = 18% at n = 0.7 [15, 18, 1 to 31). For inst.ance, the T,(T') dependence seems to play a dominant role in T, varing under hydrogenation of the Ni-Fe alloys containing more than 10 at)%Fe [l, 241. The situat'ion with account of the a,( V) dependence is more involved. In fact), the maximum values a. achieved on hydrogenat>ionapproxiniately equal those for the st>arting alloys without hydrogen =r 0.8 pB/atjom for Ni,,Mn,, alloy for Ni-Mn-H and Ni-Fe-H systems (that' is, and hydrogenated Ni,,Mn,, alloy, see Section 3.2.2., =r 1.8 pu,/atonifor Ni,,Fe,, alloy and hydrogenated alloys containing 66.1 and 67.5 atyo Fe [I]). This is a rather unexpected result), particularly in the case of the Ni-Fe alloys due to the st'rong dependence of their a, values upon pressure (and, therefore, upon volume). For instance, under the assumption that the ao(V) dependence is the same on expansion as on compression, the volume expansion on hydrogenat'ion leads to aa:/ai~I,=o =3.4 ,uu,/atom for the Ni33,$eGfi,l alloy [a]having the a, value close to t'he niaxiniuni one, whereas the experiment) yields for such a sample aa,/an =r 0 up to n = 0.5 [I]. Thus, t'he o0(n) dependence for the Ni-Fe alloys behaves so as if at normal presure these alloys have the maximum values of a, possible at a given electron concentration. Unfortunately, any acceptable t'heory accounting for the behaviour of the magnetic properties of invars is absent at) present, and any further discussion in this field is still premature. Another question to be considered in this section concerns the ao(rz,) dependence for the Ni,,Mn,, alloy. Since the yz phase of the Ni,,Mn,,-H solution is paramagnetic at n 2 0.7 (Section 3.2.1), and 01 = 0.8 pB/atom at n = 0 (Fig. .la), (aao/8n)averagr & -a:/n = -1.1 pu,/atonifor t.his alloy. Such a large value of I aa,/anl, however, does not contradict' t'he assumption that hydrogen atoms supply a fractional quantity (less than unity) of elect.rons above the existing Fermi level, since the Ni,,Mn, alloy itself is not a strong itinerant ferroniagnet (in particular, it.s a, value lies below the Pauling-Slaber curve [37]). In a weak itinerant ferromagnet large values of 1 bo,/ii~[ are possible due to theappearance of holes in thedT-subband in addit,ion to the filling of the dj,-subband.

avx

4.3 Ni-Cv-like alloys

Alloying of nickel with chroniiuni and 3d-elements left of it in the Periodic Table sharply decreases a,. Since the decrease of Ne occurring in this case should result in the opposite effect if the rigid band model is valid, the changes of the band structure are to determine the behaviour of a, of these alloys to a considerable degree. According to Friedel [4] a perturbation potential introduced into nickel by an impurity with nuclear charge 2; 5 Zcr is repulsive enough to substract a bound state from the d? -subband and to shift it above the Fermi energy. Such a virtual state will


The Magnetic Properties of Ni-Mn-H Solid Solutions

683

empty itself into the conduct>ionband, mainly in the obher half of the d-band (with spins down) because of its high density of st,at>es, which results in a decrease of 5, of the alloy. Consequently, the electrons supplied by the dissolving hydrogen will niainly fill bhe d$-subband, too, lowering the magnet'ization and decreasing the Curie point>s.This is the behaviour of 5,(n) and T,(n,) which was observed for t'he Ni-Cr-H solid solutions containing 7 atyo Cr in t'he very good work [9]. As to the Ni-Mn alloys, summarizing t.he above facts, one can conclude t,heiii to exhibit an intern1ediat.e kind of behaviour, their Tc(zICn)and CTJZAI~) dependence being determined by the increase of electron concent,ration, as will as by the changes of the band structure. N0t.e t,hat the role of elect,ron concent,ration seems t,o increase as xJI,, increases. For instance, both T, and 5, grow on hydrogenation of the Ni,,JIn,, alloy in agreement, with their dependence for the starting Ni-Mn alloys (Sect'ion3.9.3). So, hydrogenation should result in a decrease of a, and T, of the alloys of nickel and 3d-niet)als with 2 sZcr. The sanie effect should be observed, e.g. on hydrogenation of f.c.c. cobalt alloys with niet'als with 2 5 ZA~,,, t'he oo(Ne) as dependences for these alloys also have a posit'ive slope. Due to t>helarge density of st'ates at the Fermi level of nickel and cobalt, the electrons supplied by hydrogen will also fill the dJ,-subbandin f.c.c. alloys of these metals with non-transition elements, again decreasing 5, and T,.

5. Conclusion
The behaviour of a, and T, of hydrogen solid so1ut)ionsin f.c.c. alloys of 3d-metals based on nickel shows t'hat their d-bands considerably weaker deforms on hydrogenation than bhe sp-bands, the degree of occupation of the d-bands growing as n increases. The a,( n) dependences may be qua1itat)ively described by considering hydrogen as a donor of a fractional quant'ity of electrons. Such an approach makes it possible to predict the behaviour of magnetic properties of a number of Me-H solut,ions. For instance, as n grows a decrease of 5, and T, should occur for st.rong it,inerant ferromagnets (Ni-Zn, ('o-Fe, see Sections 4.1, 4.2; for t,he Ni-Cu alloys the disappearance of ferroniagnetisni on hydrogenation was shown in [30, 31]),for alloys with a5,/aNe 0 at. a low impurity content (Ni-V, Ni-Ti, Co-Mn, Co-Cr, etc., see Section d.3), and for alloys with non-transition elements (Ni-Al, Ni-Be, Ni-Sb, etc. see Sect,ion 4.3). An increase of 5" on hydrogenation should, probably, be observed only for the Ni-Fe and Ni-Mn alloys (as well as for t'he alloys based on them, in particular, 0. Fe65(Nil-zMn,),,) in the region of conipositions where 85,/8Ne

>

>

>

Acknoudedgements

The authors wish to thank E. G. Ponyatovskii for stating the problem of investigation, constant support of the work and useful discussions, B. B.Ponomarev who supplied the equipment for magnetic measurements, K. A. Peresada, A. I. Anielin, and A. N. Grachev for their assistance in preparing and conducting the experiments, as well as V. G. Glebovskii in whose group the nickel-manganese alloys were melted.

References
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