- Email: [email protected]
Magnetic properties and 57Fe Mössbauer effect of uranium pseudoternaries with the ThMn12-type structure
~ ELSEVIER
Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
Jeurnalof
roll natlsm
magnetic materials
Magnetic properties and 57Fe MiSssbauer effect of uranium pseudoternaries with the ThMn12-type structure W. Suski a,b,*, F.G. Vagizov a,~, K. Wochowski a, H. Drulis a, T. Mydlarz b a Polish Academy of Sciences, W. Trzebiatowski Institute of Low Temperature and Structure Research, P.O. Box 937, PL-50-950 Wroctaw 2, Poland b International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw, Poland
Received 27 June 1995; revised 12 September 1995
Abstract The magnetic properties and the 57Fe MiSssbauer effect (ME) of UFe9.9Cr0.1Si 2 and UFeloSil.9Sn0. l have been investigated in a broad temperature range and under magnetic fields up to 14 T in the case of the magnetization measurements. Both compounds exhibit ferromagnetic properties below about 650 K. The saturation magnetizations amount to about 15.9/zB/f.u. (magnetization) and about 15.7/xB/f.u. (ME) for the first compound, and 1o about 18.2/Xa/f.u. (magnetization) and about 15.9/xB/f.u. (ME) for the second. These values suggest that the uranium atom contributes to the magnetism of these materials.
I. Introduction The f-electron Fe compounds with the ThMn~2type tetragonal structure have recently received considerable attention because of their high Curie points and magnetization. They exist as ternaries with various additional elements that function mostly to stabilize the lattice, although their influence on the magnetic properties is also significant (see e.g. Ref. [1]). In the ThMnl2-type structure ( I 4 / m m m space group) there are four crystallographic positions: the 2(a) sites are occupied by the f-electron atoms, while the Fe atoms and those of the stabilizing element are distributed over the 8(f), 8(j) and 8(i) positions.
* Corresponding author. Fax: +48-4871-44-10-29; email: [email protected]. I Permanent address: Russian Academy of Sciences, Physicotechnical Institute, Kazan, Russia.
The uranium compounds belong to the same class of compounds, although one cannot expect any applications for the 5f-electron materials. Research on these intermetallics providing results of fundamental character helps to understand the properties of the rare earth alloys which are promising hard magnetic materials. The compounds described in this paper are derivatives of UFel0Si 2, which exhibits a high Curie point and magnetization but very low remanence [2]. For the rare earth compounds, alloying with a fourth element frequently improves the magnetic data; for example, the addition of Co increases the Curie points of numerous rare earth-based materials. For the uranium compounds, the addition of Y or Tb instead of U, Ni or A1 instead of Fe and Re and Mo instead of Si rather reduces their respective parameters. Alloying with Co instead of Fe increases the Curie point, and with Ni slightly increases the remanence, but the influence on the other data is apparently negative (for a review, see Ref. [1]).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD10304-8853(95)00591-9
208
W. Suski et al./ Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
We therefore decided to admix in turn Cr instead of Fe, and Sn instead of Si, and to check their influence on the Curie point, remanence, etc. Originally, we planned to obtain these alloys in a broad concentration range but unlike the majority of uranium alloys investigated earlier, the U(Fe,Cr)10Si 2 and UFel0(Si,Sn) 2 systems are single phase (according to the X-ray examination, XRD) in a very narrow range of existence, probably as the UFel0(Si,Mo) 2 system [3]. It should be noted that the U(Fe,Mn)loSi 2 system does not form at all. In this paper we report on the magnetic properties and the 57Fe MiSssbauer effect of UFe9.9Cr0.1Si2 and UFe=0Sil.9Sn0. ~ intermetallics. Preliminary results concerning crystallographic and some magnetic properties have been already ~eported in poster form only [4]. These investigations have shown UFeg.gCr0 iSi 2 and UFel0SiL9Sn0. I to be ferromagnetic below 652 and 657 K, with saturation magnetizations amounting to 15.9 and 18.2/zB/f.u., respectively.
2. Experimental The intermetallics were prepared by melting the elements in stoichiometric quantities in an arc furnace under a protective argon atmosphere. This procedure was followed by prolonged annealing at 800900°C for two weeks. The obtained samples were single phase with the following lattice parameters: a = 0.82941 and 0.83607 nm, and c = 0.46856 and 0.47436 nm for the Cr- and Sn-substituted compounds, respectively. The magnetic measurements were carried out using standard equipment in our laboratories in the temperature range 4.2-1000 K and under magnetic fields of 0.5 T at high temperature, up to 14 T at low temperature, and in zero magnetic field close to the Curie points. The SVFe Mtissbauer measurements were performed using 57Co in a Cr matrix source between 17 K and Tc. The velocity scale was calibrated using an o~-Fe absorber at room temperature (RT). The experimental spectra were least-squares fitted, as in our previous work [5], by the three sextets resulting from three nonequivalent positions of the Fe atoms [8(0, 8(i) and 8(j)]. The distribution of the hyperfine fields
resulting from the varying amounts of Si, Sn and Cr in the close vicinity of the MGssbauer atom was taken into account by fitting the experimental spectra through different line widths of the external and internal lines. It was assumed that Hhf(8i) > Hhf(8j) > Hhf(8f); this relation has been determined in other Fe-rich intermetallics with the ThMn~2-structure [6] and has been confirmed by band structure calculations [7,8].
3. Results and discussion
3.1. Magnetic properties The magnetic properties of UFe9.9Cr0.1Si 2 and UFel.9Sil.9Sn0.1 are presented in Figs. 1-3. Fig. 1 shows the magnetization, M, in the temperature range above 300 K (right-hand scale) and the 57Fe MGssbauer hyperfine field, Hhf, in the temperature range 17-700 K (left-hand scale). The Curie points determined from both sets of experiments are close to each other and to the Curie point of pure UFel0Si 2, amounting to 650 K. It can therefore be concluded that if magnetic order also exists in the U sublattice it cannot persist above 650 K. On the other hand, the smooth decrease in the magnetization with increasing temperature presented for UFe9.9Cr0.1Si 2 in Fig. 2 seems to exclude a magnetic transition in the U sublattice below 650 K. However, both M(T) plots above Tc present tails due to the presence of cx-Fe in the samples according to the MGssbauer results (see 300
57Fe/,toe HoagUFe1~i~Sn0.1 z~ . UF~rO~i 2
250 -
200
"'"
12 10 ..=
o
B 100
"~
50 o
6
~ 200
Z+00
, ~
6 00
Ternperoture(K)
4 2 ~, - " - - ~ ' - ~ - ~
B00
1000
Fig. 1. Magnetization, M (right-hand scale, solid symbols) and the 57Fe hyperfine field, Hh~ (left-hand scale, open symbols) versus temperature for UFeg.9Cro.lSi 2 (triangles) and UFeloSil.9Sno. l
(circles).
209
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
100
16.0 15.0
96
% ~lk.0
9/*
o IE
,
v
V
,
,
,
,
,
'
'
'
'
"UFec9Cr0~lSi2 i
U Feg.9Cro.lSi2
12.0
~ 97 eTemperature (K}
o
2;o
95 99
Fig. 2. Magnetization, M, versus temperature below room temperature for UFeg.9Cr0jSi2.
~
,
UFel0Si z ' '
~
97 95
below), which are not seen in the X-ray patterns. For simplicity, we show only the results of M(T) at T > 7 0 0 K in Fig. 1 and M(T) in Fig. 2 for UFe9.9Cr0jSi2; the results for UFel0Sil.9Sn0.1 are very similar. Fig. 3 presents the magnetization, M, versus magnetic field at 4.2 K for both pseudoternaries and for the parent UFe~0Si 2. The results for the latter material, obtained by Andreev et al. [9], were determined on an aligned powder sample, and the M(H) curves were obtained for magnetic fields applied parallel and perpendicular to the axis of alignment. The results for the pseudoternaries were determined from bulk samples. One can see that above about 5 T, the magnetizations for both magnetic field directions are very close; this field is assumed to be an anisotropy
~-~16~
•~ 12.0
~, 8.0 .~/ ,fl 6 z,.O ~'
0
2:0
T=4.2 K " * UF%Siz o U Fel0Sn~lSh.~ "UFe~gC ro.1Si2
4.=0
G~.O 810 1(~.0 12.0 14.0"
Magnetic fietd (T)
Fig. 3. Magnetization, M, versus magnetic field at T = 4.2 K for UFel0Si 2 (crosses, easy direction; diamonds, hard direction in a field-preoriented powder sample according to Ref. [9]), for UFeg.9Cro.lSi2 (triangles) and UFeloSil.9Sno. 1 (circles).
,UFeloSi195no i "1 ' velocity (m m/s)
Fig. 4. 57Fe MiSssbauer spectra at 17 K for UFe99Cro iSi 2 (upper panel),
UFeloSi;
(middle
panel)
and
UFeloSil9Snot
(lower
panel). The solid lines at the top of upper and lower panels correspond to the spectrum of free a-Fe.
field. Therefore, magnetization values obtained above this field can be considered as saturation fields and can be compared with the respective magnetizations for the substituted alloys obtained at the highest available fields. Without discussing the numerical values for the time being, it can be observed that the substitution of Cr instead of Fe reduces slightly the magnetization (magnetic moment 15.9 versus 16.4/ZB/f.u.). This result, together with the 57Fe ME (15.7/xB/f.u.) to be discussed below, strongly suggests that the Cr atom substitutes for the Fe atom in the 8(0 position (see Fig. 4). As is well known, the substitution of the Fe atom on the 8(i) site by a non-magnetic or a weakly magnetic atom reduces the magnetic moment of the Fe sublattice. In turn, the magnetization (magnetic moment 18.2/XB/f.u.) for UFel0Sil.9Sn0j is markedly higher than that for the parent compound. According to the ME experiment (see Fig. 4) the admixed Sn atom enters the 8(j) position. There are two possible explanations for this behavior. The first concerns the supposedly antiferromagnetic coupling of the U sublattice (to the Fe sublattice) [9]. By some unknown mechanism the antiferromagnetic U sublattice could be compen-
210
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
sated. This hypothesis is even more tempting in light of the ME experiment, which gives values of the magnetic moment, determined from the hyperfine field values (15.7 and 15.9/XB/f.u.), that are similar for both pseudoternaries and very close to that of UFe]0Si 2. This means that changes in the magnetization are due mostly to other than the Fe sublattice. The influence of free oL-Fe cannot be the reason for these changes because its concentrations in the two samples are very similar.However, one should remember that the magnetization computed from the ME is a sum of the magnetization or hyperfine fields of the individual sites. Then, if some Fe sites are coupled antiferromagnetically the simple ME experiment alone cannot detect this fact. Thus, one can assume that the Fe atoms located on the 8(j) sites couple antiferromagnetically to their surroundings. The geometrical conditions of this site are favourable for such coupling. The replacement of the Fe by Sn atoms on this site reduces the antiferromagnetic contribution and increases the total magnetization. However, the two hypotheses need to be confirmed by other methods, such as neutron diffraction.
98
96
U F89.gCro.lSi z 9/. 100
' -
' - •
. ~
.
.
.
, "~"
c 98 o
m
~
96
g ~ 94 ,oo
96 UFe10Si1'gSno I 94 L
,
,
,
,
,
,
-8
-6
-4
-2
0
2
4
velocity
"
,
6
8
(rn m/s)
Fig. 5.57Fe Mtissbauer spectra at 585 K for UFe9.9Cro.lSi2 (upper panel), UF%oSi 2 (middle panel) and UFe]oSil.9Sno. 1 (lower panel).
3.2. 57Fe Mrssbauer effect study In spite of the fact that an X-ray diffraction (XRD) study showed both pseudoternaries to be single phase, the present ME spectra revealed weak hyperfine patterns, the parameters of which are close to that of oL-Fe (322 kOe for Cr-substituted and 318 kOe for Sn-substituted samples). In the MiSssbauer spectra in Fig. 4 the hyperfine patterns of the impu-
Table 1 Magnetic moments of Fe atoms and the hyperfine parameters of the
rity phase are indicated by solid lines at the top of each panel. The area of the M~Sssbauer spectrum of this phase corresponds to an et-Fe content of approximately 6-7%. The results of the measurements of the 57Fe ME are shown in Fig. 1 discussed above, and in Figs. 4 and 5. In the last two figures the ME spectra of both pseudoternaries and of UFel0Si 2 obtained at 17 K (Fig. 4) and at 585 K (Fig. 5), are
57Fe
MiSssbauer spectra at 17 K
Compound
Site
Occupation (%)
Hhf (kOe)
IS (mm/s)
M ( ~B/Fe atom)
UFe9.gCr0. ]Si 2
8(i) 8~j) 8(f)
39.5(2) 30.5(2) 28.7(3)
240 222 215
0.25 0.01 0.17
1.66 1.53 1.48
UFet0 Si2
8(i) 8(j) 8(f)
40.0(2) 31.0(2) 29.0(2)
243 226 217
0.26 0.01 0.16
1.68 1.56 1.50
UFe l0 Si 19 Sn 0.J
8(i) 8(j) 8(f)
40.0(2) 30.7(2) 29.3(2)
243 226 219
0.25 0.02 0.17
1.68 1.56 1.51
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
shown for comparison. The solid lines joining the experimental points represent the results of the least-squares fitting. 3.2.1. UFe9.9Cro3Si 2
The 27Fe ME spectra of UFe9.9Cr0.1Si 2 are shown in the top panels of Figs. 4 and 5. Comparing these spectra with that for UFel0Si 2, one can conclude that in spite of the considerable similarities, there are some differences in the amplitudes of the subspectra, which can be seen most clearly for the part with high velocity. This is related mostly to the diminution in the number of the Fe atoms in the 8(i) positions. The locations of the lines corresponding to these atoms are indicated by arrows in the Fig. 4. One can see that the amplitude of this line decreases in relation to UFel0Si 2. This observation suggests that the Cr atoms exhibit a preference to occupy the 8(i) site, which is the most important site from the point of view of magnetic properties. This assumption is confirmed by the above-mentioned diminution of the saturation magnetic moment of the Cr-substituted alloy against pure UFel0Si 2 (16.4/z~/f.u. [9]). Table 1 presents the occupation, magnetic moment values and parameters of the hyperfine structure of Fe atoms in various crystallographic positions. The values of the isomer shift and the quadrupole splitting of all the investigated compounds are very close to those detected for UFel0Si 2, which proves that the local electronic environment of the Fe atom does not change substantially when alloying with small quantities of Cr or Sn. However, it should be noted that even such a low admixture of Cr reduces the magnetic moment. A change in the number of Fe atoms on the 8(i) sites reduces almost equally the values of the magnetic moment in all three crystallographic positions. This is related to the uniform distribution of atoms located in the 8(i) positions around each site of the ThMn ~2-type crystal lattice. In fact, the atoms on the 8(f), 8(j) and 8(i) sites have 4, 4 and 5 nearest-neighbor atoms of the 8(i) type. 3.2.2. U F e l o S i l . 9 S n o . 1
The MiSssbauer spectra are shown in the bottom panels of Figs. 4 and 5. One can see that these spectra are also slightly different from those for
211
UFe~0Si 2. An admixture of Sn instead of Si apparently results in the rearrangement of the last component. As shown previously [3,10], in UFel0Si 2 the Si atoms are distributed non-uniformly in the 8(j) and 8(f) positions; there are three times as many Si atoms in the former position than in the latter. Therefore, the amplitude of the subspectrum corresponding to the Fe atoms occupying the 8(j) sites is slightly larger than that of the lines for the 8(f) sites. This is clearly seen in Fig. 4, where the locations of the outermost lines corresponding to the atoms on the 8(j) sites are indicated by arrows. It can be observed that in the Sn-substituted alloy, the intensity of these lines decreases relative to the lines corresponding to the 8(f) sites. This suggests that in UFel0Sil.9Sn0 l, the substitution of some of the Si atoms by Sn atoms makes the distribution of the Si atoms more uniform in the 8(j) and 8(f) positions, and at the same time the number of Fe atoms in the 8(j) position decreases. This, in turn should reduce the intensity of this line in the spectrum, which is indeed observed. Moreover, it can be assumed that the Sn atoms populate preferentially the 8(f) sites. The location in the 8(f) position of Sn atoms, which exhibit considerably larger atomic radii (Rsn = 0.162 nm) than Fe atoms (RFe = 0.126 rim), can result in the expansion of the crystal lattice along the c-axis, because this parameter depends mostly on the separation between the atoms located in the 8(f) positions, i.e. c / 2 = es(f)_8(f). An X-ray examination has shown that in this compound an expansion of the c-parameter amounting to 0.0023 nm is indeed detected, while a change in the a-parameter is very small (Aa = - 0.0009 nm). The observed small diminution of the a-lattice parameter apparently results from the condition of the preservation of the lattice volume. From Table 1 it follows that the magnetic moment determined from the hyperfine field using the conversion factor 14.5 T / / x B [11] for the Sn-substituted alloy (15.9/zB/f.u.) does not change relative to UFe~0Si 2, in contrast with the value obtained from the magnetic measurements. This means that in spite of the fact that the numbers of atoms in the 8(f) and 8(j) positions change, the number of Fe atoms on the 8(i) sites does not change in relation to UFe~oSi 2, and the occupation is 100%. These observations do not support any of the above hypotheses about the differences between the values of the magnetic moment
212
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
determined in the magnetometric and ME experiments.
4. Conclusions 1. The alloys exhibit very limited ranges of existence and the materials investigated here are border compositions. 2. An admixture of Cr and Sn to UFemSi 2 does not increase the remanence and does not change the Curie points considerably. 3. For the Cr alloy the Fe atom is substituted in the 8(i) position and reduces the saturation magnetization. 4. The Sn atom substituted for Si entering the 8(j) site, does not change the magnetization of the Fe sublattice and increases the total magnetization measured in the magnetometric experiment. Neutron diffraction studies are now necessary to explain the substantial difference between the saturation magnetization values determined in the 57Fe ME and magnetic measurements.
References [1] W. Suski, in: Handbook on the Physics and Chemistry of Rare Earths, vol. 22, eds. K.A. Gschneidner, Jr. and L. Eyring (North-Holland, Amsterdam, 1996). [2] W. Suski, A. Baran and T. Mydlarz, Phys. Lett. A 136 (1989) 89. [3] W. Suski, F.G. Vagizov, H. Drulis, J. Janczak and K. Wochowski, J. Magn. Magn. Mater. 117 (1992) 203. [4] W. Suski, K. Wochowski and T. Mydlarz, Presented at EMMA '95 (contribution no. 507). [5] F.G. Vagizov, H. Drulis, W. Suski and A.V. Andreev, J. Alloys Compounds 191 (1993)213. [6] H.S. Li and J.M.D. Coey, in: Handbook of Magnetic Materials, vol. 6, part 1, ed. K.H.J. Buschow (Elsevier, Amsterdam, 1991), p. 1. [7] R. Coehoorn, Phys. Rev. B 41 (1990) 11790. [8] S.S. Jaswal, Y.G. Ren and D.J. Sellmyer, J. Appl. Phys. 67 (1990) 4564. [9] A.V. Andreev, W. Suski and N.V. Baranov, J. Alloys Compounds 187 (1992) 293. [10] T. Berlureau, B. Chevalier, P. Gravereau, L. Fournes and J. Etourneau, J. Magn. Magn. Mater. 102 (1991) 166. [11] Th. Sinnemann, K. Erdmann, M. Rosenberg and K.H.J. Buschow, Hyperfine Interactions 50 (1989) 675.
Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
Jeurnalof
roll natlsm
magnetic materials
Magnetic properties and 57Fe MiSssbauer effect of uranium pseudoternaries with the ThMn12-type structure W. Suski a,b,*, F.G. Vagizov a,~, K. Wochowski a, H. Drulis a, T. Mydlarz b a Polish Academy of Sciences, W. Trzebiatowski Institute of Low Temperature and Structure Research, P.O. Box 937, PL-50-950 Wroctaw 2, Poland b International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw, Poland
Received 27 June 1995; revised 12 September 1995
Abstract The magnetic properties and the 57Fe MiSssbauer effect (ME) of UFe9.9Cr0.1Si 2 and UFeloSil.9Sn0. l have been investigated in a broad temperature range and under magnetic fields up to 14 T in the case of the magnetization measurements. Both compounds exhibit ferromagnetic properties below about 650 K. The saturation magnetizations amount to about 15.9/zB/f.u. (magnetization) and about 15.7/xB/f.u. (ME) for the first compound, and 1o about 18.2/Xa/f.u. (magnetization) and about 15.9/xB/f.u. (ME) for the second. These values suggest that the uranium atom contributes to the magnetism of these materials.
I. Introduction The f-electron Fe compounds with the ThMn~2type tetragonal structure have recently received considerable attention because of their high Curie points and magnetization. They exist as ternaries with various additional elements that function mostly to stabilize the lattice, although their influence on the magnetic properties is also significant (see e.g. Ref. [1]). In the ThMnl2-type structure ( I 4 / m m m space group) there are four crystallographic positions: the 2(a) sites are occupied by the f-electron atoms, while the Fe atoms and those of the stabilizing element are distributed over the 8(f), 8(j) and 8(i) positions.
* Corresponding author. Fax: +48-4871-44-10-29; email: [email protected]. I Permanent address: Russian Academy of Sciences, Physicotechnical Institute, Kazan, Russia.
The uranium compounds belong to the same class of compounds, although one cannot expect any applications for the 5f-electron materials. Research on these intermetallics providing results of fundamental character helps to understand the properties of the rare earth alloys which are promising hard magnetic materials. The compounds described in this paper are derivatives of UFel0Si 2, which exhibits a high Curie point and magnetization but very low remanence [2]. For the rare earth compounds, alloying with a fourth element frequently improves the magnetic data; for example, the addition of Co increases the Curie points of numerous rare earth-based materials. For the uranium compounds, the addition of Y or Tb instead of U, Ni or A1 instead of Fe and Re and Mo instead of Si rather reduces their respective parameters. Alloying with Co instead of Fe increases the Curie point, and with Ni slightly increases the remanence, but the influence on the other data is apparently negative (for a review, see Ref. [1]).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD10304-8853(95)00591-9
208
W. Suski et al./ Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
We therefore decided to admix in turn Cr instead of Fe, and Sn instead of Si, and to check their influence on the Curie point, remanence, etc. Originally, we planned to obtain these alloys in a broad concentration range but unlike the majority of uranium alloys investigated earlier, the U(Fe,Cr)10Si 2 and UFel0(Si,Sn) 2 systems are single phase (according to the X-ray examination, XRD) in a very narrow range of existence, probably as the UFel0(Si,Mo) 2 system [3]. It should be noted that the U(Fe,Mn)loSi 2 system does not form at all. In this paper we report on the magnetic properties and the 57Fe MiSssbauer effect of UFe9.9Cr0.1Si2 and UFe=0Sil.9Sn0. ~ intermetallics. Preliminary results concerning crystallographic and some magnetic properties have been already ~eported in poster form only [4]. These investigations have shown UFeg.gCr0 iSi 2 and UFel0SiL9Sn0. I to be ferromagnetic below 652 and 657 K, with saturation magnetizations amounting to 15.9 and 18.2/zB/f.u., respectively.
2. Experimental The intermetallics were prepared by melting the elements in stoichiometric quantities in an arc furnace under a protective argon atmosphere. This procedure was followed by prolonged annealing at 800900°C for two weeks. The obtained samples were single phase with the following lattice parameters: a = 0.82941 and 0.83607 nm, and c = 0.46856 and 0.47436 nm for the Cr- and Sn-substituted compounds, respectively. The magnetic measurements were carried out using standard equipment in our laboratories in the temperature range 4.2-1000 K and under magnetic fields of 0.5 T at high temperature, up to 14 T at low temperature, and in zero magnetic field close to the Curie points. The SVFe Mtissbauer measurements were performed using 57Co in a Cr matrix source between 17 K and Tc. The velocity scale was calibrated using an o~-Fe absorber at room temperature (RT). The experimental spectra were least-squares fitted, as in our previous work [5], by the three sextets resulting from three nonequivalent positions of the Fe atoms [8(0, 8(i) and 8(j)]. The distribution of the hyperfine fields
resulting from the varying amounts of Si, Sn and Cr in the close vicinity of the MGssbauer atom was taken into account by fitting the experimental spectra through different line widths of the external and internal lines. It was assumed that Hhf(8i) > Hhf(8j) > Hhf(8f); this relation has been determined in other Fe-rich intermetallics with the ThMn~2-structure [6] and has been confirmed by band structure calculations [7,8].
3. Results and discussion
3.1. Magnetic properties The magnetic properties of UFe9.9Cr0.1Si 2 and UFel.9Sil.9Sn0.1 are presented in Figs. 1-3. Fig. 1 shows the magnetization, M, in the temperature range above 300 K (right-hand scale) and the 57Fe MGssbauer hyperfine field, Hhf, in the temperature range 17-700 K (left-hand scale). The Curie points determined from both sets of experiments are close to each other and to the Curie point of pure UFel0Si 2, amounting to 650 K. It can therefore be concluded that if magnetic order also exists in the U sublattice it cannot persist above 650 K. On the other hand, the smooth decrease in the magnetization with increasing temperature presented for UFe9.9Cr0.1Si 2 in Fig. 2 seems to exclude a magnetic transition in the U sublattice below 650 K. However, both M(T) plots above Tc present tails due to the presence of cx-Fe in the samples according to the MGssbauer results (see 300
57Fe/,toe HoagUFe1~i~Sn0.1 z~ . UF~rO~i 2
250 -
200
"'"
12 10 ..=
o
B 100
"~
50 o
6
~ 200
Z+00
, ~
6 00
Ternperoture(K)
4 2 ~, - " - - ~ ' - ~ - ~
B00
1000
Fig. 1. Magnetization, M (right-hand scale, solid symbols) and the 57Fe hyperfine field, Hh~ (left-hand scale, open symbols) versus temperature for UFeg.9Cro.lSi 2 (triangles) and UFeloSil.9Sno. l
(circles).
209
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
100
16.0 15.0
96
% ~lk.0
9/*
o IE
,
v
V
,
,
,
,
,
'
'
'
'
"UFec9Cr0~lSi2 i
U Feg.9Cro.lSi2
12.0
~ 97 eTemperature (K}
o
2;o
95 99
Fig. 2. Magnetization, M, versus temperature below room temperature for UFeg.9Cr0jSi2.
~
,
UFel0Si z ' '
~
97 95
below), which are not seen in the X-ray patterns. For simplicity, we show only the results of M(T) at T > 7 0 0 K in Fig. 1 and M(T) in Fig. 2 for UFe9.9Cr0jSi2; the results for UFel0Sil.9Sn0.1 are very similar. Fig. 3 presents the magnetization, M, versus magnetic field at 4.2 K for both pseudoternaries and for the parent UFe~0Si 2. The results for the latter material, obtained by Andreev et al. [9], were determined on an aligned powder sample, and the M(H) curves were obtained for magnetic fields applied parallel and perpendicular to the axis of alignment. The results for the pseudoternaries were determined from bulk samples. One can see that above about 5 T, the magnetizations for both magnetic field directions are very close; this field is assumed to be an anisotropy
~-~16~
•~ 12.0
~, 8.0 .~/ ,fl 6 z,.O ~'
0
2:0
T=4.2 K " * UF%Siz o U Fel0Sn~lSh.~ "UFe~gC ro.1Si2
4.=0
G~.O 810 1(~.0 12.0 14.0"
Magnetic fietd (T)
Fig. 3. Magnetization, M, versus magnetic field at T = 4.2 K for UFel0Si 2 (crosses, easy direction; diamonds, hard direction in a field-preoriented powder sample according to Ref. [9]), for UFeg.9Cro.lSi2 (triangles) and UFeloSil.9Sno. 1 (circles).
,UFeloSi195no i "1 ' velocity (m m/s)
Fig. 4. 57Fe MiSssbauer spectra at 17 K for UFe99Cro iSi 2 (upper panel),
UFeloSi;
(middle
panel)
and
UFeloSil9Snot
(lower
panel). The solid lines at the top of upper and lower panels correspond to the spectrum of free a-Fe.
field. Therefore, magnetization values obtained above this field can be considered as saturation fields and can be compared with the respective magnetizations for the substituted alloys obtained at the highest available fields. Without discussing the numerical values for the time being, it can be observed that the substitution of Cr instead of Fe reduces slightly the magnetization (magnetic moment 15.9 versus 16.4/ZB/f.u.). This result, together with the 57Fe ME (15.7/xB/f.u.) to be discussed below, strongly suggests that the Cr atom substitutes for the Fe atom in the 8(0 position (see Fig. 4). As is well known, the substitution of the Fe atom on the 8(i) site by a non-magnetic or a weakly magnetic atom reduces the magnetic moment of the Fe sublattice. In turn, the magnetization (magnetic moment 18.2/XB/f.u.) for UFel0Sil.9Sn0j is markedly higher than that for the parent compound. According to the ME experiment (see Fig. 4) the admixed Sn atom enters the 8(j) position. There are two possible explanations for this behavior. The first concerns the supposedly antiferromagnetic coupling of the U sublattice (to the Fe sublattice) [9]. By some unknown mechanism the antiferromagnetic U sublattice could be compen-
210
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
sated. This hypothesis is even more tempting in light of the ME experiment, which gives values of the magnetic moment, determined from the hyperfine field values (15.7 and 15.9/XB/f.u.), that are similar for both pseudoternaries and very close to that of UFe]0Si 2. This means that changes in the magnetization are due mostly to other than the Fe sublattice. The influence of free oL-Fe cannot be the reason for these changes because its concentrations in the two samples are very similar.However, one should remember that the magnetization computed from the ME is a sum of the magnetization or hyperfine fields of the individual sites. Then, if some Fe sites are coupled antiferromagnetically the simple ME experiment alone cannot detect this fact. Thus, one can assume that the Fe atoms located on the 8(j) sites couple antiferromagnetically to their surroundings. The geometrical conditions of this site are favourable for such coupling. The replacement of the Fe by Sn atoms on this site reduces the antiferromagnetic contribution and increases the total magnetization. However, the two hypotheses need to be confirmed by other methods, such as neutron diffraction.
98
96
U F89.gCro.lSi z 9/. 100
' -
' - •
. ~
.
.
.
, "~"
c 98 o
m
~
96
g ~ 94 ,oo
96 UFe10Si1'gSno I 94 L
,
,
,
,
,
,
-8
-6
-4
-2
0
2
4
velocity
"
,
6
8
(rn m/s)
Fig. 5.57Fe Mtissbauer spectra at 585 K for UFe9.9Cro.lSi2 (upper panel), UF%oSi 2 (middle panel) and UFe]oSil.9Sno. 1 (lower panel).
3.2. 57Fe Mrssbauer effect study In spite of the fact that an X-ray diffraction (XRD) study showed both pseudoternaries to be single phase, the present ME spectra revealed weak hyperfine patterns, the parameters of which are close to that of oL-Fe (322 kOe for Cr-substituted and 318 kOe for Sn-substituted samples). In the MiSssbauer spectra in Fig. 4 the hyperfine patterns of the impu-
Table 1 Magnetic moments of Fe atoms and the hyperfine parameters of the
rity phase are indicated by solid lines at the top of each panel. The area of the M~Sssbauer spectrum of this phase corresponds to an et-Fe content of approximately 6-7%. The results of the measurements of the 57Fe ME are shown in Fig. 1 discussed above, and in Figs. 4 and 5. In the last two figures the ME spectra of both pseudoternaries and of UFel0Si 2 obtained at 17 K (Fig. 4) and at 585 K (Fig. 5), are
57Fe
MiSssbauer spectra at 17 K
Compound
Site
Occupation (%)
Hhf (kOe)
IS (mm/s)
M ( ~B/Fe atom)
UFe9.gCr0. ]Si 2
8(i) 8~j) 8(f)
39.5(2) 30.5(2) 28.7(3)
240 222 215
0.25 0.01 0.17
1.66 1.53 1.48
UFet0 Si2
8(i) 8(j) 8(f)
40.0(2) 31.0(2) 29.0(2)
243 226 217
0.26 0.01 0.16
1.68 1.56 1.50
UFe l0 Si 19 Sn 0.J
8(i) 8(j) 8(f)
40.0(2) 30.7(2) 29.3(2)
243 226 219
0.25 0.02 0.17
1.68 1.56 1.51
W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
shown for comparison. The solid lines joining the experimental points represent the results of the least-squares fitting. 3.2.1. UFe9.9Cro3Si 2
The 27Fe ME spectra of UFe9.9Cr0.1Si 2 are shown in the top panels of Figs. 4 and 5. Comparing these spectra with that for UFel0Si 2, one can conclude that in spite of the considerable similarities, there are some differences in the amplitudes of the subspectra, which can be seen most clearly for the part with high velocity. This is related mostly to the diminution in the number of the Fe atoms in the 8(i) positions. The locations of the lines corresponding to these atoms are indicated by arrows in the Fig. 4. One can see that the amplitude of this line decreases in relation to UFel0Si 2. This observation suggests that the Cr atoms exhibit a preference to occupy the 8(i) site, which is the most important site from the point of view of magnetic properties. This assumption is confirmed by the above-mentioned diminution of the saturation magnetic moment of the Cr-substituted alloy against pure UFel0Si 2 (16.4/z~/f.u. [9]). Table 1 presents the occupation, magnetic moment values and parameters of the hyperfine structure of Fe atoms in various crystallographic positions. The values of the isomer shift and the quadrupole splitting of all the investigated compounds are very close to those detected for UFel0Si 2, which proves that the local electronic environment of the Fe atom does not change substantially when alloying with small quantities of Cr or Sn. However, it should be noted that even such a low admixture of Cr reduces the magnetic moment. A change in the number of Fe atoms on the 8(i) sites reduces almost equally the values of the magnetic moment in all three crystallographic positions. This is related to the uniform distribution of atoms located in the 8(i) positions around each site of the ThMn ~2-type crystal lattice. In fact, the atoms on the 8(f), 8(j) and 8(i) sites have 4, 4 and 5 nearest-neighbor atoms of the 8(i) type. 3.2.2. U F e l o S i l . 9 S n o . 1
The MiSssbauer spectra are shown in the bottom panels of Figs. 4 and 5. One can see that these spectra are also slightly different from those for
211
UFe~0Si 2. An admixture of Sn instead of Si apparently results in the rearrangement of the last component. As shown previously [3,10], in UFel0Si 2 the Si atoms are distributed non-uniformly in the 8(j) and 8(f) positions; there are three times as many Si atoms in the former position than in the latter. Therefore, the amplitude of the subspectrum corresponding to the Fe atoms occupying the 8(j) sites is slightly larger than that of the lines for the 8(f) sites. This is clearly seen in Fig. 4, where the locations of the outermost lines corresponding to the atoms on the 8(j) sites are indicated by arrows. It can be observed that in the Sn-substituted alloy, the intensity of these lines decreases relative to the lines corresponding to the 8(f) sites. This suggests that in UFel0Sil.9Sn0 l, the substitution of some of the Si atoms by Sn atoms makes the distribution of the Si atoms more uniform in the 8(j) and 8(f) positions, and at the same time the number of Fe atoms in the 8(j) position decreases. This, in turn should reduce the intensity of this line in the spectrum, which is indeed observed. Moreover, it can be assumed that the Sn atoms populate preferentially the 8(f) sites. The location in the 8(f) position of Sn atoms, which exhibit considerably larger atomic radii (Rsn = 0.162 nm) than Fe atoms (RFe = 0.126 rim), can result in the expansion of the crystal lattice along the c-axis, because this parameter depends mostly on the separation between the atoms located in the 8(f) positions, i.e. c / 2 = es(f)_8(f). An X-ray examination has shown that in this compound an expansion of the c-parameter amounting to 0.0023 nm is indeed detected, while a change in the a-parameter is very small (Aa = - 0.0009 nm). The observed small diminution of the a-lattice parameter apparently results from the condition of the preservation of the lattice volume. From Table 1 it follows that the magnetic moment determined from the hyperfine field using the conversion factor 14.5 T / / x B [11] for the Sn-substituted alloy (15.9/zB/f.u.) does not change relative to UFe~0Si 2, in contrast with the value obtained from the magnetic measurements. This means that in spite of the fact that the numbers of atoms in the 8(f) and 8(j) positions change, the number of Fe atoms on the 8(i) sites does not change in relation to UFe~oSi 2, and the occupation is 100%. These observations do not support any of the above hypotheses about the differences between the values of the magnetic moment
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W. Suski et al. / Journal of Magnetism and Magnetic Materials 154 (1996) 207-212
determined in the magnetometric and ME experiments.
4. Conclusions 1. The alloys exhibit very limited ranges of existence and the materials investigated here are border compositions. 2. An admixture of Cr and Sn to UFemSi 2 does not increase the remanence and does not change the Curie points considerably. 3. For the Cr alloy the Fe atom is substituted in the 8(i) position and reduces the saturation magnetization. 4. The Sn atom substituted for Si entering the 8(j) site, does not change the magnetization of the Fe sublattice and increases the total magnetization measured in the magnetometric experiment. Neutron diffraction studies are now necessary to explain the substantial difference between the saturation magnetization values determined in the 57Fe ME and magnetic measurements.
References [1] W. Suski, in: Handbook on the Physics and Chemistry of Rare Earths, vol. 22, eds. K.A. Gschneidner, Jr. and L. Eyring (North-Holland, Amsterdam, 1996). [2] W. Suski, A. Baran and T. Mydlarz, Phys. Lett. A 136 (1989) 89. [3] W. Suski, F.G. Vagizov, H. Drulis, J. Janczak and K. Wochowski, J. Magn. Magn. Mater. 117 (1992) 203. [4] W. Suski, K. Wochowski and T. Mydlarz, Presented at EMMA '95 (contribution no. 507). [5] F.G. Vagizov, H. Drulis, W. Suski and A.V. Andreev, J. Alloys Compounds 191 (1993)213. [6] H.S. Li and J.M.D. Coey, in: Handbook of Magnetic Materials, vol. 6, part 1, ed. K.H.J. Buschow (Elsevier, Amsterdam, 1991), p. 1. [7] R. Coehoorn, Phys. Rev. B 41 (1990) 11790. [8] S.S. Jaswal, Y.G. Ren and D.J. Sellmyer, J. Appl. Phys. 67 (1990) 4564. [9] A.V. Andreev, W. Suski and N.V. Baranov, J. Alloys Compounds 187 (1992) 293. [10] T. Berlureau, B. Chevalier, P. Gravereau, L. Fournes and J. Etourneau, J. Magn. Magn. Mater. 102 (1991) 166. [11] Th. Sinnemann, K. Erdmann, M. Rosenberg and K.H.J. Buschow, Hyperfine Interactions 50 (1989) 675.