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The x-T magnetic phase diagram of the LaMn2 − xFexGe2 (0≤x≤1) system by neutron diffraction study
ELSEVIER
Journal
of Alloys and Compounds 248 ( 1997) 1 I2- 120
The x-T magnetic phase diagram of the LaMn,-,Fe,Ge, system by neutron diffraction study M.N. Norlidah”, G. Venturini”‘*,
(0 5 x 5 1)
B. Malaman”, E. Ressoucheb
1. Introduction The magnetic properties of the ThCr,Si,-type structure RMn,Ge, compounds (R = rare earths) have been extensively studied during the past few decades (for a review
seeRef. [I]). Recently,neutrondiffraction and Miissbauer studies of the light rare earth compounds have pointed out several unknown phenomena [2-71. According to these investigations, the magnetic bchaviour of the Mn sublattice may be described as follows. Firstly, it has been observed that the in-plane Mn-Mn magnetic coupling is apparently correlated to the in-plane Mn-Mn spacing. Antiferromagnetic (001) Mn plaes occur in compounds characterized by large Mn-Mn inter-
l Comspondlng aulhor 09x-8388/97/$17.00 8 1997 Elsevier Science S.A. AU righIs reserved PII SO925-8388(96)02662-O
atomic distances (dMnmMn2 2.84 A) whereas shorter distances yield ferromagnetic Mn planes. In this latter case, the ferromagnetic (001) Mn planes couple antiferromag-
netically with adjacentplanesgiving rise to the so-called AFil magneticstructure(Fig. I). The interlayercoupling of antiferromagnetic(001) Mn planes either lead to an I-centrcd commensurate (c) structure (AFI) or to an incommensurate (i) structure (AFfs) characterized by a propagating vector k = (0.0.93. Both strucllcturesare presentcd in Fig. 1. It was further observed that at Tc
becomecanted,giving way to non-collinearMn moments and henceto a ferromagneticcomponentwithin the (001) planes.Such Mn planeswere describedas mixed planes 151. The third remark concerns rhe in-plane ferromagnetic component which may bc ferro- or antiferromagnetically
M.N. Norlidnh er al. I lounud
of Alloys and Compounds 248 (1997) Ii.?-I.20 planes become canted and the in-plane fe component couples ferromrgnaically with the ferromagnetic componems of adjacent (001) Mn planes, i.e. an Fmi-type magnetic stmctmc.
Fig. I Magnetic ordering of the. compounds 12-61.
in TbCr,Si,-type RMIl,X,
coupled with the ferromagnetic components of adjacent planes. These interplane interactions are apparently also dependent on the Mn-Mn in-plane distance. For rare earth compounds, it was shoyn that an Mn-Mn in-plane distance greater than 2.87 A yields a ferromagneik interplatt$ coupling whilst an Mn-Mn distance shorter than 2.87 A yields an antiferromagnetic coupling [4]. The difference in the interplane coupling for the ferromagnetic and antiferromagnetic components lead to four other magnetic structures characterized by mixed planes (m) denoted as the Fmi, Fmc, AFmi. AFmc structures (Fig. I). According to this review, the magnetic behaviour of the LaMn,Ge, compound [2,7] may be described as follows: I. below TN a420 K, the manganese “ublattice orders antiferromagnetically and the magnetic stntctttre is constituted of antiferromagnetic (001) Mn planes. The interplane coupling gives rise to an incommensurate arrangement leading to the AFfs-type magnetic structure: the Mn moments within the (001) 2. below Tc -3lOK.
Recently, magnetizatkn measwements on the LaMn,_,Fe,Ge, solii solution (05~~5 I) and a neutron diffraction study of th LaMn,~,Fe,,,Ge, compound have recently been repotted [it]. It was shown that the substitution of iron drastically ittguences the magnetk pqertks of the Mn sublattice. Magnetixation measurements show that an increase in iron content results in a fast decmase of Tc (corresponding to the occurrence of a ferromagnetic component, i.e. mixed planes). Furthemmm, neutron diffraction pointed out that the lncommettsurate inkrpkne arrangement no longer persisted for an iron concentntion of x = 0.5. At 2 K, LaMn,,,Fe,,Ge, exhibits att AFatctype magnetic structure (Fig. 1) with an easy plane ferromagnetic component, whereas an easy plane Af/ arrangement cccurs above Tc [S]. A more recent study on the L.aMn,$u,Ge, solii solution (05x5 I) [9] indicates that+ in this system, Tc decreases much more slowly, when increasing x, than in the LaMn,_,Fe,Ge, system [8]. Moteover. the neutron diffraction study of the L&In,-,Cu,Ge, (x = 0.25, 0.5, 1.0) compounds points out that the helical magnetic structure (Fmi), due to the htcommensurate interpkne arrangement, oersists beyond x = 0.5. An X-ray crystallogmphic study of RT,Ge, single crystals (R = La. Nd. Y; T=Mn, Fe, Cu) proved the Ge-Ge interatomic distances to vary greatly as a function oftheTmetal[lO,ll].Itissuggested~thediffmncein magnetic behaviottr of the iron- and copper-substiMed compounds may arise from changes in bond strength. In order to obtain a more detailed comparison between the iron- and copper-substituted compounds, the magnatk phase diagram of the L&n,-,Fe$e, system was determined by neutron diffraction experiments.
2. ExperhnentaI
procedure
Samples were prepared in an induction furnace from stoichiomettic mixtures of high purity ekmettts. The resuhing ingots were anneakd for 4 days at 1273 K. The purity of samples was dctctmincd with the use of a powder X-my diffraction technique (Guinier Cu Ku). Neutron experiments were carried out at the Silas reactor of the Centm d’Btudes Nu~l&ire~ de Grenoble (CENG) and at the Institut Laue Lattgevin (B-E, Grertoble). The diffmctkn p&tents were recorded with the Onedimensionat curved muhhktectors DNS (A = 2.497 A) and DlB (A = 2.531 A). In order to correct textute effects, following the March procedure, during the refinetnent we
M.N. Norlidah ct al. I Jowml of Alloys and Compounds Z4R (1997) l/Z- I.20
114
used a fitted coefficient r,,, which reflected the importance of the preferential orientation. Using the scattering lengths b, = 8.185 fm, b,, = 3.73 fm, b, = 9.45 fm and b,, = 8.24 fm, and the form factor of Mn from Ref. [12], the scaling factor, the zo. atomic position, rc0- and the magnetic moments were refined by the MIXED crystallographic executive for diffraction (MXD) using a least squares fitting procedure 1131. In the ThCr,Si,-type structure (14/mmm), the La and Ge atoms occupy the 2(a) (0.0.0) and 4re) (O.O,z, = 0.38) sites respectively. The T metal (Mn or Fe), however, occupies the special position 4(d) (0.1/2,1/4) with an additional C translation mode. Hence, magnetic contributions to observed intensities will merely affect nuclear lines which respect the following limiting reflection condition . (i) (hkl) with h + k = 2n for ferromagnetic ordering of the Mn sublattice and (ii) (hkl) with h + k = 2n + 1 for antiferromagnetic ordering of the Mn moment within the (001) planes. Finally, in all cases, the occupancy factor of the 4(d) transition metal site,&,, was also relined. fair agreement with the corresponding formula.
3. Neutron
diffraction
I. Neutron diffraction
compound at
yielding values in LaMn,-,Fe,Ger
results.
LaMnFeGez and LaMn, ,,Fe,?,Ge? were studied in their whole ordered range, whilst LaMn, 625Feu ),sGe2 was studied between 2 and 250 K. Neutron patterns of LaMn ,.deo 125Ge, were rapidly recorded in the temperature range 200-450 K to determine the commensurate to incommensurate transition temperature T,, and the Niel point TN. 3.
Fig. 2. Neutron diffraction palterns of LaMn, ,.Fe,,,,Ge, K.
450. 300 and 2
study of LaMn, ,5Fe0 &ez
Three neutron diffraction patterns, recorded at 450, 300 and 2 K, are presented in Fig. 2. From 450 to 300 K, one observes a considerable increase of the (101) line intensity indicating the appearance of antiferromagnetic (001) Mn planes. At lower temperature, the presence of the 101 satellites are proof of a transition from a commensurate to an incommensurate state. The higher intensity of the (I 12) line at 2 K is probably related to the ferromagnetic ordering of the inplane ferromagnetic component. The thertrai variation of the intensities of some characteristic lines below room temperature is presented in Fig. 3. As can be seen, intensities of the (lOl)satellite and (112) peak start to increase simultaneously T = Tc = 225 K. The thermal variation of the miensity above room temperature is presented illustrating a N&l temperature of TN = 420 K. The observed and calculated intensities and able parameters at various temperatures are
T WI Fig. 3. LaMn, ,,Fe,,#e2: temperature dependence of the (101). (101) and (I 12) line intensities between 300 and 2 K.
x=0.125
at around (101) peak in Fig. 4 the adjustgathered in
Fig. 4. Temperature dependence of the (101) line inwnsity in the Lahfn,_,Fe,Cie, series forx=O.l25, 0.250. 0.5 and I.
of Alloys and Cmnpounds 248 119971112-120
MN. Nor/i&h et al. I Jownd
Table 1. At 289K the (001) Mn planes are antiferromagnetic with Mn moments lying in the (001) plane, i.e. an easy plane AFLtype magnetic structure (Fig. 1). Below 225 K the Mn moments become canted, yielding a ferromagnetic component aligned along the E axis. Concurrently, the magnetic structure becomes incommensurate with an antiferromagnetic in-plant component rotating in the (001) plane, i.e. an Fmi-type structure (Fig. 1). Refinement of moment values yields 2.32(9) k per 4d site for the antiferromagnetic component and L.l7(39) llg per 4d site for the ferromagnetic component. The total moment value is 2.59( I I) cc, per 4d site, or otherwise 2.96(11) LC, per Mn atom. Thermal variation of the 4: wavevector component is plotted in Fig. 5. 3.2. Neutron diffraction bemeen 2 and 250 K
study
2
02-
xr0.125
--_
2 ” P
o,,-
x10.25 ----Y‘i :
o,o,“......-..‘...‘.‘.t 0
200
100
300
400
VI0 Fig. 5. Tempenture dependenceof the 9: componem of Ihe wavevec~or in the Lahk.,Fe,Gt, scrks forr=O.O. 0.125.0.250.
of LaMn, ,,,Fe, J,JTez
Neutron diffraction patterns. recorded at 220, 191 and 2 K, are presented in Fig. 6. It is obvious that a commensurate magnetic structure persists down to 2 K. Thermal variation of the (101) and (112) line intensities is presented in Fig. 7. The (112) line intensity begins to increase at Tc = 200 K and continues to do so from this temperature downwards. Below Tsr = 175 K, one simulTable I L.&h, ,,Fe,, 2.Cie,: calculated and observed intenWe\ and adJustable pmmten ilf 439.289 and 2K hkl
002 101. 101 101’ 110 103 103 004 103' II2 200 II4 105202 I05
(1 (A) = (A)
h,.(+a)AF per 4(d) site s~m.WdF
per4(d) site IhI. wd per 4(d) site R (%)
439 K
289 K
2K
F;
F:
F;
F;
Ff
F;
59.9
59X(7)
52.6
53.4(S)
I7
22(2)
90
WI)
559
562(S)
548
548(3)
57 41 18 43 559 41
54.2(6) 42.4(9) 24(l) 43(l) 560(3) 3M2)
476
483(9)
500
500(3)
26 250 I2
35(4) 233(9) -
23 238 IO
25(2) 238(3) l8(2)
468 49 50 269
471(3) 4lC3) 51(2) 246(5)
195
785(16)
754
744(7)
4.182(2) io.%( I ) 1.02(l) 0.3803(91
0.9wI) 2.4
4.184(2) 10.940(5) 1.043(4) 0.3804(4) 0.907(5) 2.55 2.2314)
-
2.55 2.23(4) I.2
48
54(5)
800
822(7)
4.172(l) 10.908(6) 0.165(l) 1.02(l) 0.381(I) 0.88( I) 2.65 2.32(9) 1.34 l.l7(39) 2.96 2.59(11) 3.1
Fig. 6. Neutron diffraction pancms of L&n, 220. I91 and 2K.
.,,Fe. .,,Cc, compound81
(112) O!....,..............,....l...~ 0
50
100
180 T WI
200
250
300
Fig. 7. LaMn,,,,Fe,,,,Ge,: fevnp8hr~ depnden~e of the (101) and (I 12) line invnsities bctwan 260 and 2 K.
116
M.N. Norlidah e, al. I Jounral o/A/low
taneously observes a decrease in the (101) line intensity and a continuous regular increase of the (112) line intensity. The decrease in intensity of the (101) line may be interpreted as being due to a rotation of the antiferromagnetic component from the [OOl] to the [ 1001 direction. Inversely, the increase of the (I 12) intensity may be the consequence of a rotation of the ferromagnetic component from the [ 1001 to the [OOI] direction. Refinements were performed taking into account these observations. The observed and calculated intensities along with adjustable parameters at various characteristic temperatures are gathered in Table 2. A”.m ““.S T c = 200 K, the structure is constituted of purely antiferromagnetic Mn planes with Mn moments aligned along the c axis (easy axis AFI). It is noteworthy that, according to Fig. 4. the Ntel temperature may he interpolated to TN -400 K. At T = 191 K, the magnetic structure is constituted of mixed Mn planes. Here, the AF component remains aligned along the c axis whilst the ferromagnetic component lies in the (001) plane (Fmc). At 2 K, simultaneous rotations of the ferro- md antifetromagnetic components were considered during the refinements. In order to limit the number of refined parameters, the r,,, factor and the zoc posttton . were fixed at the values obtained for the 191 K neutron pattern. The deviation angle from the (001) plane of the ferromagnetic component was refined to @= 55( 11)” (and 35” for the AF component) with moment values of 2.12( 11) k and Table 2 L&In , .:,Fe,, &&: calculated and observed intensities and adjustable parameters at 220. 191 and 2 K hkl
002 IO1 I IO 103 004 II2 200 II4 202 I05 a (Ii, c (A) ‘c,.. G,. f MC I’M, (i+r) AF per4Cd) site /JMM.(H)F per 4(d) site @ @kg)’ PM” WB) per 4(d) site R (S) * see text.
220 K
I91 K
2K
F:
F:
F:
F:
F,?
F:;
37 II9 489
39(i) IIW) 48X5)
40 119 486
‘WI) 1 l9t2) 4910)
40 102 502
39(l) 103(2) 526(5)
4% 62 338 40
497(6) 64!4)
504 61
35(6)
46
498(6) 69(4) 349(4) 3W5)
526 86 345 53
683
663ClO)
661
647(9)
694
35X8) 336
4.158(3) 10.862(S) 1.04(l) 0.37-H(8) 0.801(9) 2.47 2.01(6) 2.47 2.01(6) 2.7
4.157(3, IO.860(7) 1.04(l) n.3711.2(7) 0.801 2.51 2.04(4) 1.03 0X4(18) 2.71 2.20(9) 2.6
and Cmpmds
248 (1997) I12- 120
T>XtOK
200K>T>17SK
T < 17SK
Fig. 8. Magnetic stmctures of LaMn, c.lrFe,, -,,Ge, as a function of the
l&(24) & per 4d site for the antiferromagnetic and ferromagnetic components respectively. The total moment is thus 2.56(13) /.tr, per 4d site leading to 3.15( 13) k per Mn atom. The magnetic structures at 220, 191 and 2 K are illustrated in Fig. 8.
3.3. Neutron
diffraction
study
of LaMnFeGe,
The neutron diffraction patterns recorded at 2 and 300 K are presented in Fig. 9. They are mainly characterized by an increase of the (101) line intensity. This indicates the presence of antiferromagnetic (001) Mn planes, giving rise to a commensurate magnetic arrangement (AFI). The observed and calculated intensities together with the corresponding refined parameters are gathered in Table 3. The moments are aligned along the c axis. The moment value is of 1.38(6) cc, per 4d site leading to a total moment value of 2.76( 12) h per Mn atom. The magnetic structure at 2 K is similar to that of LaMn,,,rsFee,,,,Ges at 220 K (Fig. 8).
5@%6) 80(3) 37W)
44W 6-W’)
4.149(l) 10.&12(4) 1.04 0.3785 0.801 2.61 2.12(11) I .71 1.44(24) 55(ll) 3.15 2.56(13) 3.8
Fig. 9 Neutron diffraction patterns of LaMnFeGe, compound at 2K.
3oamd
MN. Nor/i&h et al. I Journal of Alloys and Compounds 248 (1997) 112-120 Table 3 LaMnFeGe,: calculated and observed intersities and adjustable panmetersarZ%mdZK hkl
2%K
002 101 110 103 004 112 200 114 202 105 a (A, c A ‘.... :ar f IILL I%. (I%) per 4(d) sue CLH~(I+,)F per 4(d) site PM” WB) per 4(d) site R (S)
2K
Ff
F.2,
F:
fi
3.3 24 374
3.8(4)
388(3)
3.4 102 312
IO2( I, 3W3)
787 428 979 394
79of4) 45ti3) 93W) 414(6)
786 464 1013 405
78%4) 488(4) 97X6) 4120)
819
7W7)
859
837(7)
27(l)
4.143(l) 10.816(Z) 1.02(l) 0.376i I ) 0.4% I ) -
2.W)
4.138(l) 10.756(3) 1.04(l) 0.378( I ) 0.48( I , 2.76 I.38161 -
-
2.76 1.38(6) 3.1
4.4
Thermal variation of the (101) line intensity, presented in Fig. 4, yields a Neel temperature of T,., = L30 K. 3.4. Neutron diffraction study between 200 and 450 K
of LaMn, ,,.qFe, ,z3Ge,
The magnetic behaviour of this compound is strictly similar to that of LaMn ,,,,Fe, Jiez (see Section 3. I ) Tbe thermal evolution of the (lOI) line intensity above the room temperature is shown in Fig. 4 with TN i= 435 K (Para+ AFI). Thermal variation in intensity of the (I l2),
240
260
200 T
117
(101) and (lOI)- peak8 is pmsemed in Fig. IO yielding T,, = Tc J 280 K (AFl+ Fmi). The tbemtai evolution of the ‘I; wavevector component, below Tk. is pmsenkd in Fig. 5.
4. Discussion 4.1. Magnetic phase dkagram syste.m (0 _C x _C I)
of the
The neutron diffraction study of the La&,-,Fc,Ge, compounds for x = 0.0, 0.125, 0.25, 0.375, 0.5, 1.0 and magnetization measurements on compounds of the LaMn,-,Fe,Ge, (OSXC I) solii soh&n [8] en&k us to construct the partial x-T magnetk phase diagram of the LaMn,_,Fe,Gez system, as ilhtstmkd in Fig. 11. Four different magnetic strucmms tlUybCObSCWCdill this diagram. Between 320 and 420 K, the incommensurate AFfs-type magnetic stmcture (characterixed by antiferromagnetic (001) Mn planes) is observed in LaMn,Ge,. We predict that this structure persist8 within the limited range of compounds characterized by a small ircn content (.r< 0.1). For x z 0.125, the Mfs magnetic arrangement is mplaced by the AFI-type magnetic structme. also characterized by antiferromagnetic Mn (001) planes. This type of magnetic order remains beyond x = 1.0. TN does not vary linearly with iron content x, and seems to exhibit a maximum for x -0.1. In spite of the decmase in the up to x = 0.251 Accordhtg to molecular geld approximation, this trend suggests that the bon substitution enhance8 the values of the mean-held interlattice Coupling constants. At low temperature and for weak iron content (x < 0.35 at 2 K), the Fmi magnetic structure is stabilized. The ~ahtes of TC and T,, for LaMn,,,,sFeO,,,,Ge, and
300
W
Fig. 10. LaMn, .,sFe,,I,Ck,: cemperamrc dependence of the (101). (lOI)-’ and (112) lim intensities between 280 and 25OK.
mqnetic Fig. Il. Partial r-T Lath ~,_,Fe,Cae,(x = 1).
phase dia@‘sn
of
the
system
118 LaMn, ,sWO,,Ge, would encourage us to establish the commensurate to htcommensurate transition and the canting transition as concurrent transitions. at ieast for an iron content between x = 0. I25 and x = 0.25. This observation is probably related to the sharp decrease of 4: in LaMnzGez at Tc (Fig. 5). Obviously. it appears that the transition from mixed planes to purely antiferromagnetic planes destabilizes the incommensurate magnetic arrangement. At Tc, changes in the Fermi surface may occur and could be held responsib!e for this effect. A higher iron content (~~0.3) seems to stabilize the Fmc magnetic structure. However, with a marked decrease in Tc, this structure does not remain stable beyond x = 0.8. Another characteristic in the magnetic properties of the LaMtt-,Fe,Gez solid solution is the rotation of the ferroand antiferromagnetic Mn moment components as a function of the iron content. For small x values (less than 0.3). the ferromagnetic component is aligned along the c axis whilst the antiferromagnetic component lies in the (001) plane. For x = 0.375 and above Tsr = 175 K, these directions are inverted with the AF component aligned along the c axis and the F component in the (001) plane. Below TSF, a spin flip-flop transition occurs. The antiferromag-
u.0
0,s
1.0
2.0
I.5
X Rg. 12. Pan~al x-T magnew LaMn,~,Cu,Ge, (XC I) ISI.
phase diagram of
the system
is in progress. A change in the Mn moment component direction would reverse the sign of the quadrupolar splitting of the “Fe nucleus.
pounds via Mn compounds, it clearly appears that in-plane T-T and Ge-Ge distances vary quite linearly, while T-Ge and interplane T-T distances exhibit a maximum for the Mn compounds. The variation in T-Ge contacts is obviously related to the elemental radius of the transition metal involved. The peculiar variation in Ge-Ge contacts could be related to the variation in the P-P bonds observed in isotypic phosphides. As suggested by the theoretical work of Hoffmann and Zheng [ 141, moving from the left-hand side of the transition metal series to the right-hand side should enhance the P-P bond. With manganese compounds as an exception, we have shown this trend to be quite accurate in the case of isotypic germanides [ IO.1 I]. The variation of in-plane and interplane T-T distances may be the result of constraints imposed by the T-Ge and Ge-Ge contacts. In the effort to establish an interdependence between crystallographic properties and magnetic behaviwr, we
4.2. Effects of the substitution in various solid solutions and correlations with the change in the interatomic distances
firstly remark that the interplane T-T and the Ge-Ge distances are longer in LaPeaGe, than in LaCu2Ge2, and secondly that the fetmmagnetic component is more destabilized in iron-substituted compounds than in the Cu-
netic component rotates to the (001) plane whilst the ferromagnetic component rotates to the c axis. Finally, LaMnFeGe, exhibits an easy axis AFI magnetic structure in its whole ordered range. It is noteworthy that the LaMn, sFe,,,,GeZ compound disagrees with this thermal evolution of the easy direction, since the antiferromagnetic component was found lying in the (001) plane at high temperature [8] even with an iron content greater than that of LaMn, ,z,Fe,375Ge2. In order %?a r*..l.r 1.l rlm.x.. *I.:. -in.
4.2.1. Iron and copper substitutiofi in LuMn,Ge, The x-T magnetic phase diagram of
the
LaMtt-,Cu,Ge, system deduced from a previous study [9] is presented in Fig. 12. It is worth noting the considerable reduction of the AFI range with respect to the corresponding range in the LaMn,-,Fe,Ge2 system. This is attributed to the more rapid decrease of TN for low x values and to the slower decrease of T, which in turn indicates that the ferromagnetic component is less destabilized in copper compounds than in iron compounds. This behaviour may be related to variations in interatomic distances of the various RT,Ge, compounds. Single crystal X-ray analysis on a number of compounds allowed us to scrutinize these variations (Table 4 and Fig. 13) [IO,1 11. Moving from Cu compounds to Fe com-
Table 4 Interatomic distances in RT,Ck, and R(T,T’),Ge, La. Nd: T,-f’ = Mn. Fe. Cu) Il0.l I]
compounds (R = Y.
Distance (A)
YCqie, YMll,GC, YFe,Gc, NdCU,Ge, NdMn,Ge, NdFe,Ck, f-@h% IAhlncuctc, LaMn,cIe, LaMllFcGe, t--P*
T-Ge
ck-Ge
T-T in-plane
T-T interplane
2.425 2.470 2.393 2.444 2.511 2.408 2.461 2.m 2.540 2.476 2.423
2.431 2.511 2.524 2.478 2.551 2.618 2.536 2.592 2.612 2.679 2.706
2.845 2.820 2.801 2.915 2.899 2.854 2.979 2.974 2.964 2.926 2.903
5.137 5.425 5.210 5.103 5.451 5.245 5.080 5.298 5.482 5.4@0 5.281
MN.
CU ’
5,s
Norlidah
CU,Mll ’
Yn ’
5-5
Mn.F1
T-T
61
.
3900
A
A
890 2.80
112-120
Fe .
interplane
A A
T-T
2,70
A
in-plane
n . p
n 0
0
q
T-Ge
2,40
2,30 CU n
.
o
Fig. 14. Partial r-T magnetic NdMn,.,Fe,Ge, (XS 1.7) [I?.[.
n
Go-G0
5w WJ
Cu,Yn
0 ’
Yn
n
Mn,Fe
n
Fe
Fig. 13. Variation of the T-T. T-G+ and Ge-GL irtentomic distances in L.aT,Ge, and LSl”Gq compounds as a function of the TIT’ ( = Mn. Fe. Cu) transition metal [IO.1 I].
substituted compounds. We thus suggest that the ferromagnetic component originates from additional ferromagnetic interplane interactions which tend to align the Mn moments. These interactions could probably be of an indirect type via a polarization of conduction electrons or of ‘supersupemxchange-type’ via the Mn-Ge-Ge-Mn path. The complicated magnetic behaviour of ThCr$iZ-type rare earth manganese germanides could be the result of a highly frustrated situation. Here, the in-plane interactions give rise to an antiferromagnetic in-plane order, whilst interplane interactions tend to align Mn moments in the (001) plane. Any change in interplane interactions within the ironand copper-substituted compounds may also explain variations in the extent of the incommensurate ranges in both series. The stabilization of such helimagnetic structures involves long range interplane interactions and a change in these could drastically act on the value of the propagating vector. Iron
248 (19%‘)
.
52
4.2.2.
and Compounds
A A
5.3
0
’
of Alloys
A
5,4
45
et al. I Journal
substitution
in LaMn,Ge,
plane ferromagnetic component occurs for x = 0.4, giving rise to the AFmc magnetic structme. It is notcwotthy that the magnetic behavi&r observed at low tempera&e is related to the neodymium magnetic contribut&. Modifications inthe chemi&l bonds are likely to be the source of these differences. Variations in interatomic distances for various RTzGe, compounds (R = Y, Nd La; T = Fe, Mn) and for LaMnFeGe, are p&cd in Fig. 15. It is obvious that iron substituti& lea& to a cons~Lkrable decrease in T-T and T-Ge distances and to an increase of Ge-Ge distances. The reduction of the T-Ge and T-T contacts is due to the smaller size of the iron atoms and 10 the weakening of Ge-Ge bonds, which enables a rearrangement of the T-Ge polyhedron surrounding the rare earth atom.
and NdMn,Gr,
The magnetic phase diagram of the NdMn,-,Fe,CeZ system [IS], presented in Fig. 14. shows an even larger variation of the Mn magnetic behaviour compared with that observed in the lanthanum compounds. For this series of compounds, and in the case of iron-rich compounds, the antiferromagnetic in-plane component vanishes at x 5 0.8, giving rise to the AFil magnetic structmz. It is also observed that the FwAF interplane transition of the in-
rW+)
(4
Fig. IS. R ionic radius r acpndmn MI the T-T, T-f& and Ge-Ge interatomic distances in L.aT,Ge, and LalT’Ge, canpovnds (T,T’ = Mn. Fe, Cu) [10.111.
The characteristic features of the NdMn,-,Fe,Ge2 magnetic phase diagram probably arise from the contraction of Mn-Ge and ivIn-?Zn distances due to iron substitution. In lanthanum compounds, this contraction is not sufficient to obtain the critical Mn-Mn and/or Mn-Ge distances below which the in-plane Mn-Mn coupling becomes ferromagnetic and the interplane coupling of the ferromagnetic component becomes antiferromagnetic.
5. Conclusions Correlations between magnetic properties and interatomic distances in compounds of several RMnz-,T,Ge, solid solutions (R = Nd, La: T = Fe, Cu) indicate a possible interplay between the chemical bond and the Mn magnetic behaviour. Iron and copper substitution seem to perturb the LaMnzGez system in a not altogether an identical manner. It is suggested that the occurrence of mixed planes in RMn,Ge, (R = rare earth) could be the result of additional interplane interactions. The strengthening of Ge-Ge bonds by copper substitution may in addition reinforce these additional interplane interactions in LaMn,_,Cu,Ge2 solid solution. On the contrary, the weakening of Ge-Ge bonds by iron substitution in NdMn2Ge2 leads/enables a rearrangement of the T-Ge polyhedron around the rare earth element and thus yields shorter T-T and T-Ge contacts giving rise to ferromagnetic (001) Mn planes. These effects are probably the result of subtle modifications in the chemical bonds within these compounds. Finally, the study of these solid solu-
tions has led us to suggest interplay between Mn-Ge,
the existence of a complicated Mn-Mn and Ge-Ge contacts.
References [I] A. Szytula and J. Leciejewicz. Magnetic properties of ternary intermetallic compounds of the RT,X, type. in K.A. Gschneider. Jr. and L. Eyring (eds.). Handtmoh on the Physics and Chemisrq of Rare Eut/w. Vol. 1.2. Elsevier. Amwrdam. 1989. Chapter 83, p. 133. 121 G. Venturini. R. Welter, E. Ressauche and B. Malaman. J. Allow Cmpd.. 210 (1994) 213. I31 R. Welter. G. Venturini, E. Ressouche and B. M&man. J. ARow Cmnpd.. 218 (1995) 204. 141 G. Venturini. R. Welter. E. Rewuche and B. Malaman, 1. Allou cmpd.. 223 (1995) 101. I51 G. Venturini. R. Welter. E. Ressouche and B. M&man. J. Magn. Mqn. Muter.. I50 ( 1995) 197. 161 G. Venturini. E. Ressouche and B. Malaman. J. Alloys Compd.. in press. 171 1. Nowik. Y. Levi, 1. Felner and E.R. Bauminger. J. Magtr. Md.qn. M&T.. 147 (1995) 373. 181 G. Ventmini, R. Welter. E. Rebrouche and 8. Mnlamnn. J. A/lob’s Cmpd.. 224 ( 1995) 262. I91 M.N. Norlidah. G. Venturini. E. Ressouche and B. Malaman. J. Alloys Compd.. in pres. ] IO] G. Venlunni. B. M&man and B. Roque.r. 1. Solid Sme Chum.. 79 (1989) 136. [I II G. Venlunni and B. Molarnan. 1. A//on Cmpd.. 2.75 (1996) 201. [I21 C.G. Shull and Y. Yam&. J. P/Iw. SW. Jpn.. 22 (I%21 1210. 1131 l? Wolfers 1. A&. Crr:rru//o~r.. 2-T (1990) 554. [I41 R. Hoffmann and C. Zheng. 1. P/y. Chon.. 89 (1985) 4175. I I51 G. Venturini. E. Rebboucheand B. Malaman. J. Alloys Cmpd.. 2.37 (1996) 61.
Journal
of Alloys and Compounds 248 ( 1997) 1 I2- 120
The x-T magnetic phase diagram of the LaMn,-,Fe,Ge, system by neutron diffraction study M.N. Norlidah”, G. Venturini”‘*,
(0 5 x 5 1)
B. Malaman”, E. Ressoucheb
1. Introduction The magnetic properties of the ThCr,Si,-type structure RMn,Ge, compounds (R = rare earths) have been extensively studied during the past few decades (for a review
seeRef. [I]). Recently,neutrondiffraction and Miissbauer studies of the light rare earth compounds have pointed out several unknown phenomena [2-71. According to these investigations, the magnetic bchaviour of the Mn sublattice may be described as follows. Firstly, it has been observed that the in-plane Mn-Mn magnetic coupling is apparently correlated to the in-plane Mn-Mn spacing. Antiferromagnetic (001) Mn plaes occur in compounds characterized by large Mn-Mn inter-
l Comspondlng aulhor 09x-8388/97/$17.00 8 1997 Elsevier Science S.A. AU righIs reserved PII SO925-8388(96)02662-O
atomic distances (dMnmMn2 2.84 A) whereas shorter distances yield ferromagnetic Mn planes. In this latter case, the ferromagnetic (001) Mn planes couple antiferromag-
netically with adjacentplanesgiving rise to the so-called AFil magneticstructure(Fig. I). The interlayercoupling of antiferromagnetic(001) Mn planes either lead to an I-centrcd commensurate (c) structure (AFI) or to an incommensurate (i) structure (AFfs) characterized by a propagating vector k = (0.0.93. Both strucllcturesare presentcd in Fig. 1. It was further observed that at Tc
becomecanted,giving way to non-collinearMn moments and henceto a ferromagneticcomponentwithin the (001) planes.Such Mn planeswere describedas mixed planes 151. The third remark concerns rhe in-plane ferromagnetic component which may bc ferro- or antiferromagnetically
M.N. Norlidnh er al. I lounud
of Alloys and Compounds 248 (1997) Ii.?-I.20 planes become canted and the in-plane fe component couples ferromrgnaically with the ferromagnetic componems of adjacent (001) Mn planes, i.e. an Fmi-type magnetic stmctmc.
Fig. I Magnetic ordering of the. compounds 12-61.
in TbCr,Si,-type RMIl,X,
coupled with the ferromagnetic components of adjacent planes. These interplane interactions are apparently also dependent on the Mn-Mn in-plane distance. For rare earth compounds, it was shoyn that an Mn-Mn in-plane distance greater than 2.87 A yields a ferromagneik interplatt$ coupling whilst an Mn-Mn distance shorter than 2.87 A yields an antiferromagnetic coupling [4]. The difference in the interplane coupling for the ferromagnetic and antiferromagnetic components lead to four other magnetic structures characterized by mixed planes (m) denoted as the Fmi, Fmc, AFmi. AFmc structures (Fig. I). According to this review, the magnetic behaviour of the LaMn,Ge, compound [2,7] may be described as follows: I. below TN a420 K, the manganese “ublattice orders antiferromagnetically and the magnetic stntctttre is constituted of antiferromagnetic (001) Mn planes. The interplane coupling gives rise to an incommensurate arrangement leading to the AFfs-type magnetic structure: the Mn moments within the (001) 2. below Tc -3lOK.
Recently, magnetizatkn measwements on the LaMn,_,Fe,Ge, solii solution (05~~5 I) and a neutron diffraction study of th LaMn,~,Fe,,,Ge, compound have recently been repotted [it]. It was shown that the substitution of iron drastically ittguences the magnetk pqertks of the Mn sublattice. Magnetixation measurements show that an increase in iron content results in a fast decmase of Tc (corresponding to the occurrence of a ferromagnetic component, i.e. mixed planes). Furthemmm, neutron diffraction pointed out that the lncommettsurate inkrpkne arrangement no longer persisted for an iron concentntion of x = 0.5. At 2 K, LaMn,,,Fe,,Ge, exhibits att AFatctype magnetic structure (Fig. 1) with an easy plane ferromagnetic component, whereas an easy plane Af/ arrangement cccurs above Tc [S]. A more recent study on the L.aMn,$u,Ge, solii solution (05x5 I) [9] indicates that+ in this system, Tc decreases much more slowly, when increasing x, than in the LaMn,_,Fe,Ge, system [8]. Moteover. the neutron diffraction study of the L&In,-,Cu,Ge, (x = 0.25, 0.5, 1.0) compounds points out that the helical magnetic structure (Fmi), due to the htcommensurate interpkne arrangement, oersists beyond x = 0.5. An X-ray crystallogmphic study of RT,Ge, single crystals (R = La. Nd. Y; T=Mn, Fe, Cu) proved the Ge-Ge interatomic distances to vary greatly as a function oftheTmetal[lO,ll].Itissuggested~thediffmncein magnetic behaviottr of the iron- and copper-substiMed compounds may arise from changes in bond strength. In order to obtain a more detailed comparison between the iron- and copper-substituted compounds, the magnatk phase diagram of the L&n,-,Fe$e, system was determined by neutron diffraction experiments.
2. ExperhnentaI
procedure
Samples were prepared in an induction furnace from stoichiomettic mixtures of high purity ekmettts. The resuhing ingots were anneakd for 4 days at 1273 K. The purity of samples was dctctmincd with the use of a powder X-my diffraction technique (Guinier Cu Ku). Neutron experiments were carried out at the Silas reactor of the Centm d’Btudes Nu~l&ire~ de Grenoble (CENG) and at the Institut Laue Lattgevin (B-E, Grertoble). The diffmctkn p&tents were recorded with the Onedimensionat curved muhhktectors DNS (A = 2.497 A) and DlB (A = 2.531 A). In order to correct textute effects, following the March procedure, during the refinetnent we
M.N. Norlidah ct al. I Jowml of Alloys and Compounds Z4R (1997) l/Z- I.20
114
used a fitted coefficient r,,, which reflected the importance of the preferential orientation. Using the scattering lengths b, = 8.185 fm, b,, = 3.73 fm, b, = 9.45 fm and b,, = 8.24 fm, and the form factor of Mn from Ref. [12], the scaling factor, the zo. atomic position, rc0- and the magnetic moments were refined by the MIXED crystallographic executive for diffraction (MXD) using a least squares fitting procedure 1131. In the ThCr,Si,-type structure (14/mmm), the La and Ge atoms occupy the 2(a) (0.0.0) and 4re) (O.O,z, = 0.38) sites respectively. The T metal (Mn or Fe), however, occupies the special position 4(d) (0.1/2,1/4) with an additional C translation mode. Hence, magnetic contributions to observed intensities will merely affect nuclear lines which respect the following limiting reflection condition . (i) (hkl) with h + k = 2n for ferromagnetic ordering of the Mn sublattice and (ii) (hkl) with h + k = 2n + 1 for antiferromagnetic ordering of the Mn moment within the (001) planes. Finally, in all cases, the occupancy factor of the 4(d) transition metal site,&,, was also relined. fair agreement with the corresponding formula.
3. Neutron
diffraction
I. Neutron diffraction
compound at
yielding values in LaMn,-,Fe,Ger
results.
LaMnFeGez and LaMn, ,,Fe,?,Ge? were studied in their whole ordered range, whilst LaMn, 625Feu ),sGe2 was studied between 2 and 250 K. Neutron patterns of LaMn ,.deo 125Ge, were rapidly recorded in the temperature range 200-450 K to determine the commensurate to incommensurate transition temperature T,, and the Niel point TN. 3.
Fig. 2. Neutron diffraction palterns of LaMn, ,.Fe,,,,Ge, K.
450. 300 and 2
study of LaMn, ,5Fe0 &ez
Three neutron diffraction patterns, recorded at 450, 300 and 2 K, are presented in Fig. 2. From 450 to 300 K, one observes a considerable increase of the (101) line intensity indicating the appearance of antiferromagnetic (001) Mn planes. At lower temperature, the presence of the 101 satellites are proof of a transition from a commensurate to an incommensurate state. The higher intensity of the (I 12) line at 2 K is probably related to the ferromagnetic ordering of the inplane ferromagnetic component. The thertrai variation of the intensities of some characteristic lines below room temperature is presented in Fig. 3. As can be seen, intensities of the (lOl)satellite and (112) peak start to increase simultaneously T = Tc = 225 K. The thermal variation of the miensity above room temperature is presented illustrating a N&l temperature of TN = 420 K. The observed and calculated intensities and able parameters at various temperatures are
T WI Fig. 3. LaMn, ,,Fe,,#e2: temperature dependence of the (101). (101) and (I 12) line intensities between 300 and 2 K.
x=0.125
at around (101) peak in Fig. 4 the adjustgathered in
Fig. 4. Temperature dependence of the (101) line inwnsity in the Lahfn,_,Fe,Cie, series forx=O.l25, 0.250. 0.5 and I.
of Alloys and Cmnpounds 248 119971112-120
MN. Nor/i&h et al. I Jownd
Table 1. At 289K the (001) Mn planes are antiferromagnetic with Mn moments lying in the (001) plane, i.e. an easy plane AFLtype magnetic structure (Fig. 1). Below 225 K the Mn moments become canted, yielding a ferromagnetic component aligned along the E axis. Concurrently, the magnetic structure becomes incommensurate with an antiferromagnetic in-plant component rotating in the (001) plane, i.e. an Fmi-type structure (Fig. 1). Refinement of moment values yields 2.32(9) k per 4d site for the antiferromagnetic component and L.l7(39) llg per 4d site for the ferromagnetic component. The total moment value is 2.59( I I) cc, per 4d site, or otherwise 2.96(11) LC, per Mn atom. Thermal variation of the 4: wavevector component is plotted in Fig. 5. 3.2. Neutron diffraction bemeen 2 and 250 K
study
2
02-
xr0.125
--_
2 ” P
o,,-
x10.25 ----Y‘i :
o,o,“......-..‘...‘.‘.t 0
200
100
300
400
VI0 Fig. 5. Tempenture dependenceof the 9: componem of Ihe wavevec~or in the Lahk.,Fe,Gt, scrks forr=O.O. 0.125.0.250.
of LaMn, ,,,Fe, J,JTez
Neutron diffraction patterns. recorded at 220, 191 and 2 K, are presented in Fig. 6. It is obvious that a commensurate magnetic structure persists down to 2 K. Thermal variation of the (101) and (112) line intensities is presented in Fig. 7. The (112) line intensity begins to increase at Tc = 200 K and continues to do so from this temperature downwards. Below Tsr = 175 K, one simulTable I L.&h, ,,Fe,, 2.Cie,: calculated and observed intenWe\ and adJustable pmmten ilf 439.289 and 2K hkl
002 101. 101 101’ 110 103 103 004 103' II2 200 II4 105202 I05
(1 (A) = (A)
h,.(+a)AF per 4(d) site s~m.WdF
per4(d) site IhI. wd per 4(d) site R (%)
439 K
289 K
2K
F;
F:
F;
F;
Ff
F;
59.9
59X(7)
52.6
53.4(S)
I7
22(2)
90
WI)
559
562(S)
548
548(3)
57 41 18 43 559 41
54.2(6) 42.4(9) 24(l) 43(l) 560(3) 3M2)
476
483(9)
500
500(3)
26 250 I2
35(4) 233(9) -
23 238 IO
25(2) 238(3) l8(2)
468 49 50 269
471(3) 4lC3) 51(2) 246(5)
195
785(16)
754
744(7)
4.182(2) io.%( I ) 1.02(l) 0.3803(91
0.9wI) 2.4
4.184(2) 10.940(5) 1.043(4) 0.3804(4) 0.907(5) 2.55 2.2314)
-
2.55 2.23(4) I.2
48
54(5)
800
822(7)
4.172(l) 10.908(6) 0.165(l) 1.02(l) 0.381(I) 0.88( I) 2.65 2.32(9) 1.34 l.l7(39) 2.96 2.59(11) 3.1
Fig. 6. Neutron diffraction pancms of L&n, 220. I91 and 2K.
.,,Fe. .,,Cc, compound81
(112) O!....,..............,....l...~ 0
50
100
180 T WI
200
250
300
Fig. 7. LaMn,,,,Fe,,,,Ge,: fevnp8hr~ depnden~e of the (101) and (I 12) line invnsities bctwan 260 and 2 K.
116
M.N. Norlidah e, al. I Jounral o/A/low
taneously observes a decrease in the (101) line intensity and a continuous regular increase of the (112) line intensity. The decrease in intensity of the (101) line may be interpreted as being due to a rotation of the antiferromagnetic component from the [OOl] to the [ 1001 direction. Inversely, the increase of the (I 12) intensity may be the consequence of a rotation of the ferromagnetic component from the [ 1001 to the [OOI] direction. Refinements were performed taking into account these observations. The observed and calculated intensities along with adjustable parameters at various characteristic temperatures are gathered in Table 2. A”.m ““.S T c = 200 K, the structure is constituted of purely antiferromagnetic Mn planes with Mn moments aligned along the c axis (easy axis AFI). It is noteworthy that, according to Fig. 4. the Ntel temperature may he interpolated to TN -400 K. At T = 191 K, the magnetic structure is constituted of mixed Mn planes. Here, the AF component remains aligned along the c axis whilst the ferromagnetic component lies in the (001) plane (Fmc). At 2 K, simultaneous rotations of the ferro- md antifetromagnetic components were considered during the refinements. In order to limit the number of refined parameters, the r,,, factor and the zoc posttton . were fixed at the values obtained for the 191 K neutron pattern. The deviation angle from the (001) plane of the ferromagnetic component was refined to @= 55( 11)” (and 35” for the AF component) with moment values of 2.12( 11) k and Table 2 L&In , .:,Fe,, &&: calculated and observed intensities and adjustable parameters at 220. 191 and 2 K hkl
002 IO1 I IO 103 004 II2 200 II4 202 I05 a (Ii, c (A) ‘c,.. G,. f MC I’M, (i+r) AF per4Cd) site /JMM.(H)F per 4(d) site @ @kg)’ PM” WB) per 4(d) site R (S) * see text.
220 K
I91 K
2K
F:
F:
F:
F:
F,?
F:;
37 II9 489
39(i) IIW) 48X5)
40 119 486
‘WI) 1 l9t2) 4910)
40 102 502
39(l) 103(2) 526(5)
4% 62 338 40
497(6) 64!4)
504 61
35(6)
46
498(6) 69(4) 349(4) 3W5)
526 86 345 53
683
663ClO)
661
647(9)
694
35X8) 336
4.158(3) 10.862(S) 1.04(l) 0.37-H(8) 0.801(9) 2.47 2.01(6) 2.47 2.01(6) 2.7
4.157(3, IO.860(7) 1.04(l) n.3711.2(7) 0.801 2.51 2.04(4) 1.03 0X4(18) 2.71 2.20(9) 2.6
and Cmpmds
248 (1997) I12- 120
T>XtOK
200K>T>17SK
T < 17SK
Fig. 8. Magnetic stmctures of LaMn, c.lrFe,, -,,Ge, as a function of the
l&(24) & per 4d site for the antiferromagnetic and ferromagnetic components respectively. The total moment is thus 2.56(13) /.tr, per 4d site leading to 3.15( 13) k per Mn atom. The magnetic structures at 220, 191 and 2 K are illustrated in Fig. 8.
3.3. Neutron
diffraction
study
of LaMnFeGe,
The neutron diffraction patterns recorded at 2 and 300 K are presented in Fig. 9. They are mainly characterized by an increase of the (101) line intensity. This indicates the presence of antiferromagnetic (001) Mn planes, giving rise to a commensurate magnetic arrangement (AFI). The observed and calculated intensities together with the corresponding refined parameters are gathered in Table 3. The moments are aligned along the c axis. The moment value is of 1.38(6) cc, per 4d site leading to a total moment value of 2.76( 12) h per Mn atom. The magnetic structure at 2 K is similar to that of LaMn,,,rsFee,,,,Ges at 220 K (Fig. 8).
5@%6) 80(3) 37W)
44W 6-W’)
4.149(l) 10.&12(4) 1.04 0.3785 0.801 2.61 2.12(11) I .71 1.44(24) 55(ll) 3.15 2.56(13) 3.8
Fig. 9 Neutron diffraction patterns of LaMnFeGe, compound at 2K.
3oamd
MN. Nor/i&h et al. I Journal of Alloys and Compounds 248 (1997) 112-120 Table 3 LaMnFeGe,: calculated and observed intersities and adjustable panmetersarZ%mdZK hkl
2%K
002 101 110 103 004 112 200 114 202 105 a (A, c A ‘.... :ar f IILL I%. (I%) per 4(d) sue CLH~(I+,)F per 4(d) site PM” WB) per 4(d) site R (S)
2K
Ff
F.2,
F:
fi
3.3 24 374
3.8(4)
388(3)
3.4 102 312
IO2( I, 3W3)
787 428 979 394
79of4) 45ti3) 93W) 414(6)
786 464 1013 405
78%4) 488(4) 97X6) 4120)
819
7W7)
859
837(7)
27(l)
4.143(l) 10.816(Z) 1.02(l) 0.376i I ) 0.4% I ) -
2.W)
4.138(l) 10.756(3) 1.04(l) 0.378( I ) 0.48( I , 2.76 I.38161 -
-
2.76 1.38(6) 3.1
4.4
Thermal variation of the (101) line intensity, presented in Fig. 4, yields a Neel temperature of T,., = L30 K. 3.4. Neutron diffraction study between 200 and 450 K
of LaMn, ,,.qFe, ,z3Ge,
The magnetic behaviour of this compound is strictly similar to that of LaMn ,,,,Fe, Jiez (see Section 3. I ) Tbe thermal evolution of the (lOI) line intensity above the room temperature is shown in Fig. 4 with TN i= 435 K (Para+ AFI). Thermal variation in intensity of the (I l2),
240
260
200 T
117
(101) and (lOI)- peak8 is pmsemed in Fig. IO yielding T,, = Tc J 280 K (AFl+ Fmi). The tbemtai evolution of the ‘I; wavevector component, below Tk. is pmsenkd in Fig. 5.
4. Discussion 4.1. Magnetic phase dkagram syste.m (0 _C x _C I)
of the
The neutron diffraction study of the La&,-,Fc,Ge, compounds for x = 0.0, 0.125, 0.25, 0.375, 0.5, 1.0 and magnetization measurements on compounds of the LaMn,-,Fe,Ge, (OSXC I) solii soh&n [8] en&k us to construct the partial x-T magnetk phase diagram of the LaMn,_,Fe,Gez system, as ilhtstmkd in Fig. 11. Four different magnetic strucmms tlUybCObSCWCdill this diagram. Between 320 and 420 K, the incommensurate AFfs-type magnetic stmcture (characterixed by antiferromagnetic (001) Mn planes) is observed in LaMn,Ge,. We predict that this structure persist8 within the limited range of compounds characterized by a small ircn content (.r< 0.1). For x z 0.125, the Mfs magnetic arrangement is mplaced by the AFI-type magnetic structme. also characterized by antiferromagnetic Mn (001) planes. This type of magnetic order remains beyond x = 1.0. TN does not vary linearly with iron content x, and seems to exhibit a maximum for x -0.1. In spite of the decmase in the up to x = 0.251 Accordhtg to molecular geld approximation, this trend suggests that the bon substitution enhance8 the values of the mean-held interlattice Coupling constants. At low temperature and for weak iron content (x < 0.35 at 2 K), the Fmi magnetic structure is stabilized. The ~ahtes of TC and T,, for LaMn,,,,sFeO,,,,Ge, and
300
W
Fig. 10. LaMn, .,sFe,,I,Ck,: cemperamrc dependence of the (101). (lOI)-’ and (112) lim intensities between 280 and 25OK.
mqnetic Fig. Il. Partial r-T Lath ~,_,Fe,Cae,(x = 1).
phase dia@‘sn
of
the
system
118 LaMn, ,sWO,,Ge, would encourage us to establish the commensurate to htcommensurate transition and the canting transition as concurrent transitions. at ieast for an iron content between x = 0. I25 and x = 0.25. This observation is probably related to the sharp decrease of 4: in LaMnzGez at Tc (Fig. 5). Obviously. it appears that the transition from mixed planes to purely antiferromagnetic planes destabilizes the incommensurate magnetic arrangement. At Tc, changes in the Fermi surface may occur and could be held responsib!e for this effect. A higher iron content (~~0.3) seems to stabilize the Fmc magnetic structure. However, with a marked decrease in Tc, this structure does not remain stable beyond x = 0.8. Another characteristic in the magnetic properties of the LaMtt-,Fe,Gez solid solution is the rotation of the ferroand antiferromagnetic Mn moment components as a function of the iron content. For small x values (less than 0.3). the ferromagnetic component is aligned along the c axis whilst the antiferromagnetic component lies in the (001) plane. For x = 0.375 and above Tsr = 175 K, these directions are inverted with the AF component aligned along the c axis and the F component in the (001) plane. Below TSF, a spin flip-flop transition occurs. The antiferromag-
u.0
0,s
1.0
2.0
I.5
X Rg. 12. Pan~al x-T magnew LaMn,~,Cu,Ge, (XC I) ISI.
phase diagram of
the system
is in progress. A change in the Mn moment component direction would reverse the sign of the quadrupolar splitting of the “Fe nucleus.
pounds via Mn compounds, it clearly appears that in-plane T-T and Ge-Ge distances vary quite linearly, while T-Ge and interplane T-T distances exhibit a maximum for the Mn compounds. The variation in T-Ge contacts is obviously related to the elemental radius of the transition metal involved. The peculiar variation in Ge-Ge contacts could be related to the variation in the P-P bonds observed in isotypic phosphides. As suggested by the theoretical work of Hoffmann and Zheng [ 141, moving from the left-hand side of the transition metal series to the right-hand side should enhance the P-P bond. With manganese compounds as an exception, we have shown this trend to be quite accurate in the case of isotypic germanides [ IO.1 I]. The variation of in-plane and interplane T-T distances may be the result of constraints imposed by the T-Ge and Ge-Ge contacts. In the effort to establish an interdependence between crystallographic properties and magnetic behaviwr, we
4.2. Effects of the substitution in various solid solutions and correlations with the change in the interatomic distances
firstly remark that the interplane T-T and the Ge-Ge distances are longer in LaPeaGe, than in LaCu2Ge2, and secondly that the fetmmagnetic component is more destabilized in iron-substituted compounds than in the Cu-
netic component rotates to the (001) plane whilst the ferromagnetic component rotates to the c axis. Finally, LaMnFeGe, exhibits an easy axis AFI magnetic structure in its whole ordered range. It is noteworthy that the LaMn, sFe,,,,GeZ compound disagrees with this thermal evolution of the easy direction, since the antiferromagnetic component was found lying in the (001) plane at high temperature [8] even with an iron content greater than that of LaMn, ,z,Fe,375Ge2. In order %?a r*..l.r 1.l rlm.x.. *I.:. -in.
4.2.1. Iron and copper substitutiofi in LuMn,Ge, The x-T magnetic phase diagram of
the
LaMtt-,Cu,Ge, system deduced from a previous study [9] is presented in Fig. 12. It is worth noting the considerable reduction of the AFI range with respect to the corresponding range in the LaMn,-,Fe,Ge2 system. This is attributed to the more rapid decrease of TN for low x values and to the slower decrease of T, which in turn indicates that the ferromagnetic component is less destabilized in copper compounds than in iron compounds. This behaviour may be related to variations in interatomic distances of the various RT,Ge, compounds. Single crystal X-ray analysis on a number of compounds allowed us to scrutinize these variations (Table 4 and Fig. 13) [IO,1 11. Moving from Cu compounds to Fe com-
Table 4 Interatomic distances in RT,Ck, and R(T,T’),Ge, La. Nd: T,-f’ = Mn. Fe. Cu) Il0.l I]
compounds (R = Y.
Distance (A)
YCqie, YMll,GC, YFe,Gc, NdCU,Ge, NdMn,Ge, NdFe,Ck, f-@h% IAhlncuctc, LaMn,cIe, LaMllFcGe, t--P*
T-Ge
ck-Ge
T-T in-plane
T-T interplane
2.425 2.470 2.393 2.444 2.511 2.408 2.461 2.m 2.540 2.476 2.423
2.431 2.511 2.524 2.478 2.551 2.618 2.536 2.592 2.612 2.679 2.706
2.845 2.820 2.801 2.915 2.899 2.854 2.979 2.974 2.964 2.926 2.903
5.137 5.425 5.210 5.103 5.451 5.245 5.080 5.298 5.482 5.4@0 5.281
MN.
CU ’
5,s
Norlidah
CU,Mll ’
Yn ’
5-5
Mn.F1
T-T
61
.
3900
A
A
890 2.80
112-120
Fe .
interplane
A A
T-T
2,70
A
in-plane
n . p
n 0
0
q
T-Ge
2,40
2,30 CU n
.
o
Fig. 14. Partial r-T magnetic NdMn,.,Fe,Ge, (XS 1.7) [I?.[.
n
Go-G0
5w WJ
Cu,Yn
0 ’
Yn
n
Mn,Fe
n
Fe
Fig. 13. Variation of the T-T. T-G+ and Ge-GL irtentomic distances in L.aT,Ge, and LSl”Gq compounds as a function of the TIT’ ( = Mn. Fe. Cu) transition metal [IO.1 I].
substituted compounds. We thus suggest that the ferromagnetic component originates from additional ferromagnetic interplane interactions which tend to align the Mn moments. These interactions could probably be of an indirect type via a polarization of conduction electrons or of ‘supersupemxchange-type’ via the Mn-Ge-Ge-Mn path. The complicated magnetic behaviour of ThCr$iZ-type rare earth manganese germanides could be the result of a highly frustrated situation. Here, the in-plane interactions give rise to an antiferromagnetic in-plane order, whilst interplane interactions tend to align Mn moments in the (001) plane. Any change in interplane interactions within the ironand copper-substituted compounds may also explain variations in the extent of the incommensurate ranges in both series. The stabilization of such helimagnetic structures involves long range interplane interactions and a change in these could drastically act on the value of the propagating vector. Iron
248 (19%‘)
.
52
4.2.2.
and Compounds
A A
5.3
0
’
of Alloys
A
5,4
45
et al. I Journal
substitution
in LaMn,Ge,
plane ferromagnetic component occurs for x = 0.4, giving rise to the AFmc magnetic structme. It is notcwotthy that the magnetic behavi&r observed at low tempera&e is related to the neodymium magnetic contribut&. Modifications inthe chemi&l bonds are likely to be the source of these differences. Variations in interatomic distances for various RTzGe, compounds (R = Y, Nd La; T = Fe, Mn) and for LaMnFeGe, are p&cd in Fig. 15. It is obvious that iron substituti& lea& to a cons~Lkrable decrease in T-T and T-Ge distances and to an increase of Ge-Ge distances. The reduction of the T-Ge and T-T contacts is due to the smaller size of the iron atoms and 10 the weakening of Ge-Ge bonds, which enables a rearrangement of the T-Ge polyhedron surrounding the rare earth atom.
and NdMn,Gr,
The magnetic phase diagram of the NdMn,-,Fe,CeZ system [IS], presented in Fig. 14. shows an even larger variation of the Mn magnetic behaviour compared with that observed in the lanthanum compounds. For this series of compounds, and in the case of iron-rich compounds, the antiferromagnetic in-plane component vanishes at x 5 0.8, giving rise to the AFil magnetic structmz. It is also observed that the FwAF interplane transition of the in-
rW+)
(4
Fig. IS. R ionic radius r acpndmn MI the T-T, T-f& and Ge-Ge interatomic distances in L.aT,Ge, and LalT’Ge, canpovnds (T,T’ = Mn. Fe, Cu) [10.111.
The characteristic features of the NdMn,-,Fe,Ge2 magnetic phase diagram probably arise from the contraction of Mn-Ge and ivIn-?Zn distances due to iron substitution. In lanthanum compounds, this contraction is not sufficient to obtain the critical Mn-Mn and/or Mn-Ge distances below which the in-plane Mn-Mn coupling becomes ferromagnetic and the interplane coupling of the ferromagnetic component becomes antiferromagnetic.
5. Conclusions Correlations between magnetic properties and interatomic distances in compounds of several RMnz-,T,Ge, solid solutions (R = Nd, La: T = Fe, Cu) indicate a possible interplay between the chemical bond and the Mn magnetic behaviour. Iron and copper substitution seem to perturb the LaMnzGez system in a not altogether an identical manner. It is suggested that the occurrence of mixed planes in RMn,Ge, (R = rare earth) could be the result of additional interplane interactions. The strengthening of Ge-Ge bonds by copper substitution may in addition reinforce these additional interplane interactions in LaMn,_,Cu,Ge2 solid solution. On the contrary, the weakening of Ge-Ge bonds by iron substitution in NdMn2Ge2 leads/enables a rearrangement of the T-Ge polyhedron around the rare earth element and thus yields shorter T-T and T-Ge contacts giving rise to ferromagnetic (001) Mn planes. These effects are probably the result of subtle modifications in the chemical bonds within these compounds. Finally, the study of these solid solu-
tions has led us to suggest interplay between Mn-Ge,
the existence of a complicated Mn-Mn and Ge-Ge contacts.
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