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Ni3d-Gd4f correlation effects on the magnetic behaviour of GdNi
ELSEVIER
Physica B 223&224 (1996) 382 384
Ni3d-Gd4f correlation effects on the magnetic behaviour of GdNi P i . Paulose, Sujata Patil, R. Mallik, E.V. Sampathkumaran*, V. Nagarajan Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay-5, India
Abstract The results of magnetization and heat-capacity measurements on the alloys, Gdl-xYxNi (x = 0.0, 0.25, 0.5, 0.75 and 0.9) are reported. The data suggest that there is a Gd induced magnetic moment on Ni, which may in turn enhance G d - G d exchange interaction strength in GdNi. The induced moment (on Ni) apparently exhibits itinerant ferromagnetism in the magnetically ordered state of GdNi.
It has been generally believed that Ni ions in the alloys RNi (R = rare earths) do not possess any magnetic moment [1, 2]. Although it may be true for R = La and Y, it is possible that, for those alloys with moment-bearing R ions, there may be some induced moment on Ni. Generally speaking, such an induced moment (though presumably small in the case of light rare earths) can interfere with the magnetic behaviour associated with R ions, thereby making the understanding of the physics of strongly correlated f-electron systems very complex. In order to address this question in the RNi series, we have undertaken investigations on GdNi by dilution of the Gd sub-lattice by Y. It may be recalled that GdNi forms in the CrB-type orthorhombic structure, whereas YNi crystallizes in FeB-type orthorhombic structure [.2]. Thus Y substitution offers an ideal opportunity to compare the induced magnetic behaviour for two different crystal structures. The samples Gdl-xYxNi (x = 0.0, 0.25, 0.5, 0.75, 0.9 and 1.0), were made by arc melting and were characterized by X-ray diffraction. We find that there is a transformation from CrB to FeB type structure at x = 0.5. DC
* Corresponding author.
magnetic susceptibility (Z) (2-300 K in a field of 0.2 T) and isothermal magnetization (M) behaviour (upto 55 kOe at selected temperatures) were obtained with a commercial Superconducting Quantum Interference Device (SQUID). Heat-capacity (C) measurements (2-300 K) were also performed by a semi-adiabatic heatpulse method. The results of Z and M measurements for all the alloys are shown in Figs. 1 (the plot has been restricted to the temperature range 0-150 K for the sake of clarity) and 2, respectively. As known earlier [3], for GdNi, Z diverges at about 70 K and tends to saturate at lower temperatures; in addition the plot of M versus H saturates at about 1 T. These features confirm that GdNi orders ferromagnetically at about (To = ) 70 K. The plot of inverse Z versus T is linear above 80 K. The value of the effective moment (Peel) is much larger (8.47/@ than that expected for the tripositive Gd ion, as reported in the literature [,1-5]. We propose that this excess moment arises from Ni 3d band polarized by Gd moment. The value of #eff on Ni (#Ni) has been calculated from the formula, ~eff = (~2Ni"~ ]/2d)1/2, assuming the free ion effective moment for Gd 3+ (/~Gd= 7.94/~B) and it turns out to be 2.92#B, which is incidentally the spin-only moment on Ni 2+. The induced Ni moment is ferromagnetically
0921-4526/96/$15.00 ~ 1996 Elsevier Science B.V. All rights reserved PII S092 1 - 4 5 2 6 ( 9 6 ) 0 0 1 29-9
383
P.L. Paulose et al. /Physica B 223&224 (1996) 382-384 20
40
20
40
60
80
1 O0
120
140
60
80
I00
120
140
1 O0
50
'-8 o
"~
10
0
0
Temperature
(K)
Fig. 1. Inverse susceptibility as a function of temperature (0-150 K) for the alloys, Gdl -xYxNi. The continuous lines represent the least square fit of the data to Curie-Weiss behaviour in the paramagnetic state.
I
1
I
I
I
.+.a
dt'_x Y
x = 0 (sK)
6 0
0.25
(st)
05
(6K)
I:z:l v
zi.
4
coupled to that of Gd at low temperatures, as the saturation moment (/~B) at 5 K is larger (by about 0.7pB) than that expected for Gd (see Fig. 2). The excess saturation moment is clearly several times smaller than the value of /~Ni derived in the paramagnetic state. The behaviour observed is similar to that in Ni metal. This observation is consistent with the predictions of theories on itinerant magnetism [6]. Thus, GdNi presents a situation where there is a coexistence of localized magnetism (due to Gd) and induced itinerant ferromagnetism (from Ni 3d electrons). Apparently, the moment induced on Ni in turn enhances the G d - G d exchange interaction strength in GdNi (see below). With respect to the Y substituted alloys, the values Of 0p closely follow those of Tc (see Table 1) as a function of composition and this suggests insignificant antiferromagnetic component in any of these alloys. The values of Z obtained employing a magnetic field of 100 Oe for both the field-cooled and zero-field-cooled specimens are found to be the same down to 2 K for all the compositions. This finding suggests the absence of spin-glass ordering even in dilute limit (x = 0.9). We emphasize on the following observations: (i) 0p is a non-monotonic function of x, with a sharp rise at x = 0.5; the value for x = 0.5 indicates that GdNi should order ferromagnetically at least around 140 K if it crystallizes in the FeB structures; this suggests that the strength of exchange interaction is relatively stronger in the FeB structure; (ii) the values of 0p, normalized to Gd concentration, gets reduced with the dilution of Gd sublattice (in the FeB structure), in contrast to the behaviour expected on the basis of Rudermann-Kittel-Kasuya-Yosida interaction and (iii) excess /~s decreases with increasing x (excess /~s = 0.7, 0.54, 0.43, 0.1 and 0.04pB for x = 0.0, 0.25, 0.5, 0.75 and 0.9, respectively). The findings (ii) and (iii) viewed together imply that Ni 3d contributes significantly to the exchange interaction strength at the Gd rich end in the FeB structure. It appears that the 3d induced moment has a similar influence on the Gd sublattice magnetic ordering even in the CrB structure, as evidenced by the depression of Tc by La substitution for Gd
o .,-q
0 . 7 5 (2K)
Table 1 The values of the saturation moment (#s) per formula unit, paramagnetic Curie temperature (0p) and Curie temperature (To) for the alloys, Gd,-xYxNi
0.9
x
#2 (#B)
0p (K)
Tc (K)
0.0 0.25 0.5 0.75 0.9
7.7 5.79 3.93 1.85 0.74
71 59 69 29 10
70 55 65 27 6
[q 2 q~
ea~
:
:
:
Z~ f"---"--~
'
I
I
: I
1 2 3 Applied magnetic
I
= (2K) I
4 5 f i e l d (T)
Fig. 2. Isothermal magnetization behaviour of the alloys, Gdl _xYxNi.
384
P.L. Paulose et al. / Physica B 223&224 (1996) 382 384 I
0.4
I
shown in Fig. 3. Distinct anomalies are seen at Tc. However, the upturn in Cm with decreasing temperature begins well above T c and the entropy at Tc is at least 20% less than the full value. Thus these results indicate the existence of short range magnetic correlations over a wide range of temperature above T o giving rise to broadening of the plots of C versus T around T o It is to be noted that, for x = 0.9, Cm/T increases from 20 K reaching a value of about 800 m J / G d mol K 2 at 6 K (at which long range magnetic order sets in) mimicking the behaviour in heavy fermions. Thus, these results suggest that the magnetic precursor effects can give rise to heavyfermion-like anomalies in the C data. To conclude, it is worthwhile to carry out neutron diffraction studies in the alloys, Gdt xYxNi, to probe the complexities of the ferromagnetic state arising from both the localized and the itinerant magnetism.
r
It
i
oo
I e
_~
20
L
• x
x=
0.0
09
O0
10
20
T (K)
.
¢ v v
Gdl_xYxNi
0.25 v
I - -a v
~D
v
v
10
vVVv ~v
v
•
v
One of the authors (SP) would like to thank Council of Scientific and Industrial Research (India) for a financial support.
v
i
v
References
20
40
60
80
1O0
T (K) Fig. 3. The 4f contribution (C) heat capacity as a function of temperature for the alloys, Gd~ -xYxNi. The plot of C/T versus T for x = 0.9 is also shown in the inset.
[5], though in this case Tc decreases monotonically with decreasing Gd concentration. We now discuss some features in the heat-capacity data. The 4f contribution (Cm) to C obtained by subtracting the C values of YNi for the lattice contribution [7] is
[1] C.A. Poldy, K.N.R. Taylor, Phys. Stat. Sol. 18 (1973) 123. [2] H.R. Kirchmayr and C.A. Poldy, in: Handbook on the Physics and Chemistry of Rare Earths, eds. K.A. Gschneidher Jr. and L. Eyring (North-Holland, Amsterdam, 1979) 55. [3] J.A. Blanco, J.C. Gomez Sal, J.R. Fernandez, D. Gignoux, D. Schmitt and J.R. Carvajal, J. Phys: Condens. Matter 4 (1992) 8233. [4] R.E. Walline and W.E. Wallace, J. Chem. Phys. 41 (1964) 1587. [5] E. Gratz, G. Hilscher, H. Sassik and V. Sechovsky, J. Magn. Magn. Mater. 54 57 (1986) 459. [6] P. Rhodes and E.P. Wohlfarth, Proc. Roy. Soc. 273 (1963) 247. [7] J.A. Blanco, J.C. Gomez Sal, J.R. Fernandez, M. Castro, R. Burriel, D. Gigoux and D. Schmitt, Solid State Commun. 89 (1994) 389.
Physica B 223&224 (1996) 382 384
Ni3d-Gd4f correlation effects on the magnetic behaviour of GdNi P i . Paulose, Sujata Patil, R. Mallik, E.V. Sampathkumaran*, V. Nagarajan Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay-5, India
Abstract The results of magnetization and heat-capacity measurements on the alloys, Gdl-xYxNi (x = 0.0, 0.25, 0.5, 0.75 and 0.9) are reported. The data suggest that there is a Gd induced magnetic moment on Ni, which may in turn enhance G d - G d exchange interaction strength in GdNi. The induced moment (on Ni) apparently exhibits itinerant ferromagnetism in the magnetically ordered state of GdNi.
It has been generally believed that Ni ions in the alloys RNi (R = rare earths) do not possess any magnetic moment [1, 2]. Although it may be true for R = La and Y, it is possible that, for those alloys with moment-bearing R ions, there may be some induced moment on Ni. Generally speaking, such an induced moment (though presumably small in the case of light rare earths) can interfere with the magnetic behaviour associated with R ions, thereby making the understanding of the physics of strongly correlated f-electron systems very complex. In order to address this question in the RNi series, we have undertaken investigations on GdNi by dilution of the Gd sub-lattice by Y. It may be recalled that GdNi forms in the CrB-type orthorhombic structure, whereas YNi crystallizes in FeB-type orthorhombic structure [.2]. Thus Y substitution offers an ideal opportunity to compare the induced magnetic behaviour for two different crystal structures. The samples Gdl-xYxNi (x = 0.0, 0.25, 0.5, 0.75, 0.9 and 1.0), were made by arc melting and were characterized by X-ray diffraction. We find that there is a transformation from CrB to FeB type structure at x = 0.5. DC
* Corresponding author.
magnetic susceptibility (Z) (2-300 K in a field of 0.2 T) and isothermal magnetization (M) behaviour (upto 55 kOe at selected temperatures) were obtained with a commercial Superconducting Quantum Interference Device (SQUID). Heat-capacity (C) measurements (2-300 K) were also performed by a semi-adiabatic heatpulse method. The results of Z and M measurements for all the alloys are shown in Figs. 1 (the plot has been restricted to the temperature range 0-150 K for the sake of clarity) and 2, respectively. As known earlier [3], for GdNi, Z diverges at about 70 K and tends to saturate at lower temperatures; in addition the plot of M versus H saturates at about 1 T. These features confirm that GdNi orders ferromagnetically at about (To = ) 70 K. The plot of inverse Z versus T is linear above 80 K. The value of the effective moment (Peel) is much larger (8.47/@ than that expected for the tripositive Gd ion, as reported in the literature [,1-5]. We propose that this excess moment arises from Ni 3d band polarized by Gd moment. The value of #eff on Ni (#Ni) has been calculated from the formula, ~eff = (~2Ni"~ ]/2d)1/2, assuming the free ion effective moment for Gd 3+ (/~Gd= 7.94/~B) and it turns out to be 2.92#B, which is incidentally the spin-only moment on Ni 2+. The induced Ni moment is ferromagnetically
0921-4526/96/$15.00 ~ 1996 Elsevier Science B.V. All rights reserved PII S092 1 - 4 5 2 6 ( 9 6 ) 0 0 1 29-9
383
P.L. Paulose et al. /Physica B 223&224 (1996) 382-384 20
40
20
40
60
80
1 O0
120
140
60
80
I00
120
140
1 O0
50
'-8 o
"~
10
0
0
Temperature
(K)
Fig. 1. Inverse susceptibility as a function of temperature (0-150 K) for the alloys, Gdl -xYxNi. The continuous lines represent the least square fit of the data to Curie-Weiss behaviour in the paramagnetic state.
I
1
I
I
I
.+.a
dt'_x Y
x = 0 (sK)
6 0
0.25
(st)
05
(6K)
I:z:l v
zi.
4
coupled to that of Gd at low temperatures, as the saturation moment (/~B) at 5 K is larger (by about 0.7pB) than that expected for Gd (see Fig. 2). The excess saturation moment is clearly several times smaller than the value of /~Ni derived in the paramagnetic state. The behaviour observed is similar to that in Ni metal. This observation is consistent with the predictions of theories on itinerant magnetism [6]. Thus, GdNi presents a situation where there is a coexistence of localized magnetism (due to Gd) and induced itinerant ferromagnetism (from Ni 3d electrons). Apparently, the moment induced on Ni in turn enhances the G d - G d exchange interaction strength in GdNi (see below). With respect to the Y substituted alloys, the values Of 0p closely follow those of Tc (see Table 1) as a function of composition and this suggests insignificant antiferromagnetic component in any of these alloys. The values of Z obtained employing a magnetic field of 100 Oe for both the field-cooled and zero-field-cooled specimens are found to be the same down to 2 K for all the compositions. This finding suggests the absence of spin-glass ordering even in dilute limit (x = 0.9). We emphasize on the following observations: (i) 0p is a non-monotonic function of x, with a sharp rise at x = 0.5; the value for x = 0.5 indicates that GdNi should order ferromagnetically at least around 140 K if it crystallizes in the FeB structures; this suggests that the strength of exchange interaction is relatively stronger in the FeB structure; (ii) the values of 0p, normalized to Gd concentration, gets reduced with the dilution of Gd sublattice (in the FeB structure), in contrast to the behaviour expected on the basis of Rudermann-Kittel-Kasuya-Yosida interaction and (iii) excess /~s decreases with increasing x (excess /~s = 0.7, 0.54, 0.43, 0.1 and 0.04pB for x = 0.0, 0.25, 0.5, 0.75 and 0.9, respectively). The findings (ii) and (iii) viewed together imply that Ni 3d contributes significantly to the exchange interaction strength at the Gd rich end in the FeB structure. It appears that the 3d induced moment has a similar influence on the Gd sublattice magnetic ordering even in the CrB structure, as evidenced by the depression of Tc by La substitution for Gd
o .,-q
0 . 7 5 (2K)
Table 1 The values of the saturation moment (#s) per formula unit, paramagnetic Curie temperature (0p) and Curie temperature (To) for the alloys, Gd,-xYxNi
0.9
x
#2 (#B)
0p (K)
Tc (K)
0.0 0.25 0.5 0.75 0.9
7.7 5.79 3.93 1.85 0.74
71 59 69 29 10
70 55 65 27 6
[q 2 q~
ea~
:
:
:
Z~ f"---"--~
'
I
I
: I
1 2 3 Applied magnetic
I
= (2K) I
4 5 f i e l d (T)
Fig. 2. Isothermal magnetization behaviour of the alloys, Gdl _xYxNi.
384
P.L. Paulose et al. / Physica B 223&224 (1996) 382 384 I
0.4
I
shown in Fig. 3. Distinct anomalies are seen at Tc. However, the upturn in Cm with decreasing temperature begins well above T c and the entropy at Tc is at least 20% less than the full value. Thus these results indicate the existence of short range magnetic correlations over a wide range of temperature above T o giving rise to broadening of the plots of C versus T around T o It is to be noted that, for x = 0.9, Cm/T increases from 20 K reaching a value of about 800 m J / G d mol K 2 at 6 K (at which long range magnetic order sets in) mimicking the behaviour in heavy fermions. Thus, these results suggest that the magnetic precursor effects can give rise to heavyfermion-like anomalies in the C data. To conclude, it is worthwhile to carry out neutron diffraction studies in the alloys, Gdt xYxNi, to probe the complexities of the ferromagnetic state arising from both the localized and the itinerant magnetism.
r
It
i
oo
I e
_~
20
L
• x
x=
0.0
09
O0
10
20
T (K)
.
¢ v v
Gdl_xYxNi
0.25 v
I - -a v
~D
v
v
10
vVVv ~v
v
•
v
One of the authors (SP) would like to thank Council of Scientific and Industrial Research (India) for a financial support.
v
i
v
References
20
40
60
80
1O0
T (K) Fig. 3. The 4f contribution (C) heat capacity as a function of temperature for the alloys, Gd~ -xYxNi. The plot of C/T versus T for x = 0.9 is also shown in the inset.
[5], though in this case Tc decreases monotonically with decreasing Gd concentration. We now discuss some features in the heat-capacity data. The 4f contribution (Cm) to C obtained by subtracting the C values of YNi for the lattice contribution [7] is
[1] C.A. Poldy, K.N.R. Taylor, Phys. Stat. Sol. 18 (1973) 123. [2] H.R. Kirchmayr and C.A. Poldy, in: Handbook on the Physics and Chemistry of Rare Earths, eds. K.A. Gschneidher Jr. and L. Eyring (North-Holland, Amsterdam, 1979) 55. [3] J.A. Blanco, J.C. Gomez Sal, J.R. Fernandez, D. Gignoux, D. Schmitt and J.R. Carvajal, J. Phys: Condens. Matter 4 (1992) 8233. [4] R.E. Walline and W.E. Wallace, J. Chem. Phys. 41 (1964) 1587. [5] E. Gratz, G. Hilscher, H. Sassik and V. Sechovsky, J. Magn. Magn. Mater. 54 57 (1986) 459. [6] P. Rhodes and E.P. Wohlfarth, Proc. Roy. Soc. 273 (1963) 247. [7] J.A. Blanco, J.C. Gomez Sal, J.R. Fernandez, M. Castro, R. Burriel, D. Gigoux and D. Schmitt, Solid State Commun. 89 (1994) 389.