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Magnetic investigations of the system: Zr(Fe1−xAlx)2
MAGNETIC
INVESTIGATIONS
G. HILSCHER
OF THE SYSTEM:
Zr(Fe, _,AQ2
and R. GRbSSINGER
Institut fir Experimentalphysik,
Technische Uniwrsit;it,
A-1040
Vienna, Austria
The concentration dependence of the Curie temperature, the magnetisation and the crystal structure was determined for the whole range of compositions. Both, T, and the magnetisation were found to decrease with x almost linearly but with three varying slopes according to the crystal structure. In the hexagonal structure range magnetocrystalline anisotropy was detected which decreases with X, while the influence of magnetic clusters becomes more important as x approaches the critical range for the onset of magnetism at x = 0.55.
much work has been done on Recently, R(FeAI), (R = rare earth) and Y(FeAl), which is summarised by Kirchmayr and Poldy [I]. This paper presents results of the not fully investigated Zr(FeAl), system [2] concerning the concentration dependence of the magnetisation, the Curie temperature T, and a discussion of the onset of magnetism. This pseudobinary system was chosen to study the influence of the tetravalent Zr upon the onset of magnetism in this series and for a future comparison with trivalent Y. The samples were prepared stoichiometrically by high frequency melting in a cold crucible under argon atmosphere. The magnetic measurements were performed in a low frequency extraction magnetometer and in a pulsed field magnetometer. Curie temperatures were determined by initial susceptibility measurements and from extrapolation of MZ versus H/M plots (Arrott plots). In fig. 1 the results of the magnetic measurements are summarised together with the crystallographic structure changes: Zr(Fe, _,A& crystallises in a cubic MgCu, up to x = 0.25; within the concentration range 0.25 < x < 1.0 the hexagonal Laves phase appears to be stable except that for 0.85 < x < 0.95 again the cubic Cl5 structure occurs. A discontinuity of both the magnetic moment and T, is observed at x = 0.25 due to the structural change from cubic to hexagonal. Contrary to this, Muraoka et al. [2] claimed a rather gradual variation of the magnetic properties at x = 0.25 from an investigation of only four samples. Their magnetisation and T,, however, are in reasonably good agreement with our experiments. Furthermore fig. 1 shows that T, and the spontaneous magnetic moment Me, determined from extrapolation of the high field slope of M* = H/M plots to H = 0, decreases almost linearly but with three different slopes. Whereas dT,/dx is almost
Journal of Magnetism
and Magnetic Materials
unchanged above and below x = 0.25 (i.e. dT,/dx = 11.4 K/at% Al), the concentration dependence of dM,Jdx is reduced from 0.064 &at% Al to 0.046 &at% Al for 0 < x < 0.25 and 0.25 < x < 0.42, respectively. The additional change of dT,/dx and dM,/dx at x > 0.42 may be correlated with the disappearance of magnetocrystalline anisotropy at x = 0.43. In this hexagonal concentration range magnetocrystalline anisotropy fields H, were measured [3] using the singular point detection technique [4]. These fields are of the order of 10 down to 2 kG and become unmeasurable small at x = 0.42 where HA = 2 kG, and where the critical concentration range for the breakdown of the magnetic order is approached (xr = 0.55). The onset of magnetic order appears at x = 0.55 and is determined from M* versus H/M plots (Arrott plots) at 4.2 K, which are shown for various x values in fig. 2. The curvature of the Arrott plots shown in fig. 2 is due to heterogeneities which give rise to the observed downturns. Spatially nonuniform magnetisations have been discussed either including additional terms in the Landau free energy expansion of the magnetisation [5], or considering the presence of magnetic clusters described by Brillouin function in a background homogeneous magnetisation [6]. We analysed the magnetic isotherms in terms of the law of approach to saturation, where the a/H term gives a measure of the spatial nonuniform magnetisation and from the term b/H* the anisotropy field can be determined. From this analysis the inhomogeneity parameter a is shown to increase as x changes from x = 0.4 to x = 0.56. For x values anisotropy below x = 0.42, the magnetocrystalline becomes dominant, as discussed above, and is responsible for pronounced downturns in the Arrott plots. Recently [3], the magnetic isotherms were successfully and quantitatively
15-18 (1980) 1189-I 190 0North
Holland
1189
G. Hi&her,
R. G&singer/
Magnetic inoestigatiom
of Zr(Fe, _,Al,),
f M21emu/g12 . .
190,
.
.
150,
x=OG
. .
130;
.
. *
ZrCFe 1.X%$ T:L2K
l
.
Fig. 1. Concentration dependence of T,, the magnetic moment at 72 kG (G), the spontaneous magnetic moment (0) and the structural phase changes. Values indicated as (+) are from ref 121. 30'
described by the assumption that the itinerant magnetism described within the Stoner Edwards Wohlfarth (SEW) model is strongly influenced by the magnetocrystalline anisotropy. The analysis of the magnetic isotherms in terms of the Landau free energy expansion of the magnetisation (F = F, + iAM + iBM4 - HM) with A and B the usual Landau expansion coefficients which can be correlated within the SEW model with the density of states (N(E,) and its derivatives [7]) give decreasing values of l/B as x tends to xr. Except for x = 0.46, where the anisotropy vanishes, a kink in the l/B versus x dependence is observed. l/B, the slope of the Arrott plots, is a first approximation proportional to N3(EF). Therefore the homogeneous background-magnetisation, which is first modified by the magnetic anisotropy and for higher x-values by the increasing influence of magnetic clusters, is lowered by decreasing IV(&) values as x approaches xr. Finally, we conclude from the partially linear concentration dependence of MO and T, that the onset of magnetism cannot be described easily within the frame work of simple models either of the itinerant magnetism or the Jaccarino-Walker type. While the magnetic isotherms for 0.26 < x <
101 ,A
Fig.
2. Arrott
plots
of Zr(Fe,_,Al,) concentrations.
at 4.2
K for
variou
0.40 are described nicely within itinerant mag netism combined with the dominant influence o the magnetocrystalline anisotropy, the additiona change of dM/dx and the curvature of the Arrot plots may be understood by the increasing effect o magnetic clusters as the critical concentration range is approached.
References [1] H. R. Kirchmayr and C. Poldy, J. Magn. Magn. Mat. (1978) 1. [2] Y. Muraoka, M. Shiga and Y. Nakamura, Phys. Stat. Sol. (a 42 (1977) 369. [3] R. G&singer, G. Hilscher, C. Schmitzer and E. F Wohlfarth, IEEE, MAG-15(1979) 1302. [4] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [S] H. Yamada and E. P. Wohlfarth, Phys. Lett. 51 A (1975) 65 (61 F. Acker and R. Huguenin, J. Magn. Magn. Mat. 12 (1979 58. (71 E. P. Wohlfarth, Physica 91B (1977) 305.
INVESTIGATIONS
G. HILSCHER
OF THE SYSTEM:
Zr(Fe, _,AQ2
and R. GRbSSINGER
Institut fir Experimentalphysik,
Technische Uniwrsit;it,
A-1040
Vienna, Austria
The concentration dependence of the Curie temperature, the magnetisation and the crystal structure was determined for the whole range of compositions. Both, T, and the magnetisation were found to decrease with x almost linearly but with three varying slopes according to the crystal structure. In the hexagonal structure range magnetocrystalline anisotropy was detected which decreases with X, while the influence of magnetic clusters becomes more important as x approaches the critical range for the onset of magnetism at x = 0.55.
much work has been done on Recently, R(FeAI), (R = rare earth) and Y(FeAl), which is summarised by Kirchmayr and Poldy [I]. This paper presents results of the not fully investigated Zr(FeAl), system [2] concerning the concentration dependence of the magnetisation, the Curie temperature T, and a discussion of the onset of magnetism. This pseudobinary system was chosen to study the influence of the tetravalent Zr upon the onset of magnetism in this series and for a future comparison with trivalent Y. The samples were prepared stoichiometrically by high frequency melting in a cold crucible under argon atmosphere. The magnetic measurements were performed in a low frequency extraction magnetometer and in a pulsed field magnetometer. Curie temperatures were determined by initial susceptibility measurements and from extrapolation of MZ versus H/M plots (Arrott plots). In fig. 1 the results of the magnetic measurements are summarised together with the crystallographic structure changes: Zr(Fe, _,A& crystallises in a cubic MgCu, up to x = 0.25; within the concentration range 0.25 < x < 1.0 the hexagonal Laves phase appears to be stable except that for 0.85 < x < 0.95 again the cubic Cl5 structure occurs. A discontinuity of both the magnetic moment and T, is observed at x = 0.25 due to the structural change from cubic to hexagonal. Contrary to this, Muraoka et al. [2] claimed a rather gradual variation of the magnetic properties at x = 0.25 from an investigation of only four samples. Their magnetisation and T,, however, are in reasonably good agreement with our experiments. Furthermore fig. 1 shows that T, and the spontaneous magnetic moment Me, determined from extrapolation of the high field slope of M* = H/M plots to H = 0, decreases almost linearly but with three different slopes. Whereas dT,/dx is almost
Journal of Magnetism
and Magnetic Materials
unchanged above and below x = 0.25 (i.e. dT,/dx = 11.4 K/at% Al), the concentration dependence of dM,Jdx is reduced from 0.064 &at% Al to 0.046 &at% Al for 0 < x < 0.25 and 0.25 < x < 0.42, respectively. The additional change of dT,/dx and dM,/dx at x > 0.42 may be correlated with the disappearance of magnetocrystalline anisotropy at x = 0.43. In this hexagonal concentration range magnetocrystalline anisotropy fields H, were measured [3] using the singular point detection technique [4]. These fields are of the order of 10 down to 2 kG and become unmeasurable small at x = 0.42 where HA = 2 kG, and where the critical concentration range for the breakdown of the magnetic order is approached (xr = 0.55). The onset of magnetic order appears at x = 0.55 and is determined from M* versus H/M plots (Arrott plots) at 4.2 K, which are shown for various x values in fig. 2. The curvature of the Arrott plots shown in fig. 2 is due to heterogeneities which give rise to the observed downturns. Spatially nonuniform magnetisations have been discussed either including additional terms in the Landau free energy expansion of the magnetisation [5], or considering the presence of magnetic clusters described by Brillouin function in a background homogeneous magnetisation [6]. We analysed the magnetic isotherms in terms of the law of approach to saturation, where the a/H term gives a measure of the spatial nonuniform magnetisation and from the term b/H* the anisotropy field can be determined. From this analysis the inhomogeneity parameter a is shown to increase as x changes from x = 0.4 to x = 0.56. For x values anisotropy below x = 0.42, the magnetocrystalline becomes dominant, as discussed above, and is responsible for pronounced downturns in the Arrott plots. Recently [3], the magnetic isotherms were successfully and quantitatively
15-18 (1980) 1189-I 190 0North
Holland
1189
G. Hi&her,
R. G&singer/
Magnetic inoestigatiom
of Zr(Fe, _,Al,),
f M21emu/g12 . .
190,
.
.
150,
x=OG
. .
130;
.
. *
ZrCFe 1.X%$ T:L2K
l
.
Fig. 1. Concentration dependence of T,, the magnetic moment at 72 kG (G), the spontaneous magnetic moment (0) and the structural phase changes. Values indicated as (+) are from ref 121. 30'
described by the assumption that the itinerant magnetism described within the Stoner Edwards Wohlfarth (SEW) model is strongly influenced by the magnetocrystalline anisotropy. The analysis of the magnetic isotherms in terms of the Landau free energy expansion of the magnetisation (F = F, + iAM + iBM4 - HM) with A and B the usual Landau expansion coefficients which can be correlated within the SEW model with the density of states (N(E,) and its derivatives [7]) give decreasing values of l/B as x tends to xr. Except for x = 0.46, where the anisotropy vanishes, a kink in the l/B versus x dependence is observed. l/B, the slope of the Arrott plots, is a first approximation proportional to N3(EF). Therefore the homogeneous background-magnetisation, which is first modified by the magnetic anisotropy and for higher x-values by the increasing influence of magnetic clusters, is lowered by decreasing IV(&) values as x approaches xr. Finally, we conclude from the partially linear concentration dependence of MO and T, that the onset of magnetism cannot be described easily within the frame work of simple models either of the itinerant magnetism or the Jaccarino-Walker type. While the magnetic isotherms for 0.26 < x <
101 ,A
Fig.
2. Arrott
plots
of Zr(Fe,_,Al,) concentrations.
at 4.2
K for
variou
0.40 are described nicely within itinerant mag netism combined with the dominant influence o the magnetocrystalline anisotropy, the additiona change of dM/dx and the curvature of the Arrot plots may be understood by the increasing effect o magnetic clusters as the critical concentration range is approached.
References [1] H. R. Kirchmayr and C. Poldy, J. Magn. Magn. Mat. (1978) 1. [2] Y. Muraoka, M. Shiga and Y. Nakamura, Phys. Stat. Sol. (a 42 (1977) 369. [3] R. G&singer, G. Hilscher, C. Schmitzer and E. F Wohlfarth, IEEE, MAG-15(1979) 1302. [4] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [S] H. Yamada and E. P. Wohlfarth, Phys. Lett. 51 A (1975) 65 (61 F. Acker and R. Huguenin, J. Magn. Magn. Mat. 12 (1979 58. (71 E. P. Wohlfarth, Physica 91B (1977) 305.