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Structural and magnetic properties of mechanically alloyed (FexCu1 − x)93Zr7 (x = 0.5, 0.7) solid solutions
jR ,~ ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 214-215
Journalof
magnetism and magnetic materials
Structural and magnetic properties of mechanically alloyed (FexCux-x)93Zr7 (x = 0.5, 0.7) solid solutions M. Multigner a, A. Hernando a, P. Crespo a'*, C. Stiller b, J. Eckert b, L.
Schultz b
~lnstituto de Magnetismo Aplicado. RENFE-UCM, P.O. Box /55, Las Rozas, 2,~¢230Madrid, Spain ~'IFM, Helmoltzstr. 20, 01069, Dresden. Germam'
Abstract Nanocrystalline FCC-(Feo.sCuo.5)93Zr7 and BCC-(Feo.TCuo.3)93Zr; (at%) solid solution, have been obtained by mechanical alloying. Both alloys are ferromagnetic at room temperature. By heating, the alloys decompose into FCC and BCC phases which, with further annealing, evolve into BCC-Fe, F C C - C u and the intermetallic Fe3Zr. This process leads to an increase of the coercive field originated by a decoupling of the ferromagnetic grains and to the appearance of the high anisotropy Fe3Zr phase. ,.~ 1999 Elsevier Science B.V. All rights reserved. Keywords." Mechanical alloying; FeCu-based alloys; Metastable systems
The high-energy ball milling technique, HEBM, allows to synthesize new phases far from the equilibrium. For example, it is possible to overcome the immiscibility between Fe-Cu, obtaining solid solutions of FeCu in all the compositional range [1]. In this work we have produced and characterized FeCu(Zr) nanocrystalline solid solutions. (FexCul -x)93Zr7 (x = 0.5, 0.7) alloys were obtained by HEBM of Fe and Cu powders and files of Zr. Structural studies were performed by X-ray diffraction. Magnetic properties were studied by means of a S Q U I D and vibrating sample magnetometer. After 120 h of milling, the XRD patterns correspond to a single nanocrystalline phase with an average grain size of 6 nm. The crystalline structure depends on the Fe/Cu ratio in a similar way as reported for FexCu, x [1]. For .r = 0.5, the alloy presents a F C C structure with a lattice parameter of 0.3633 nm, whereas for x = 0.7 it is found that the alloy has a BCC structure and its lattice parameter is 0.2902 nm. Both lattices are expanded with respect to FCC-Cu or BCC-Fe.
*Corresponding author. [email protected]
Fax:
91-6301625: e-mail:
From the shape of the hysteresis loops, it can be concluded that the alloys behave as a single magnetic phase. The temperature dependence of the magnetization measured under a field of 1 T, is shown in Fig. 1. The magnetization decreases with temperature, as occurs in a ferromagnetic material. Further increase in temperature produces an increase of the magnetization, which is related to the decomposition process of the alloys. For the FCC-(Feo.sCuo.s)93Zr7 alloy, the Curie point was estimated to be around 310 K, a value significantly lower than that of the Zr-free alloys [2,3], around 500 K. The BCC-(Feo.vCuo.3)93Zr~, alloy decomposes before reaching its Curie point, hence the estimation was not possible. The experimental data measured under a field of 5 T can be fitted to a Bloch law indicating that the thermal demagnetization process is mainly due to spin wave excitations at low temperatures. The magnetic moment per Ee atom, laFe is 1.8 laB and 2.16 laB, for x = 0.5 and 0.7, respectively. In both cases, laFe is smaller than that of Zr-free samples [2,3], indicating that the Zr addition changes the electronic structure of the FeCu alloys. The calorimetric trace for both alloys shows that the decomposition process takes place in two steps. The first one is characterized by a heat release over a broad temperature range, 400-850 K, while the second one is characterized by a well-defined peak at around 920 K.
0304-8853/99/$ - see front matter C' 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 6 2 - 8
215
M. Multigner et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 214-215 140120== 100-
o= so~
~
300
--0 240 - - • %0 _~ 180
uo~)~Zr~
\\
"~ 801 ~
40-
~
20-
x=0.5 x=0.7
'\
12o 60
0 0
200
400
600
800
200
1000
Fig. 1. Thermomagnetization curves measured with an applied field of la0H = 1 T.
fcc-(Feo.sCuos)=3Zr7
~
o b=
~•
-'-"~
\
•
/
/
t823 K
973 K
1273 K
4'0
6'0
400
600
800
1000 1200 1400
annealing temperature (K)
T e m p e r a t u r e (K)
80
20 (degree)
Fig. 2. Evolution of XRD patterns and grain sizes (inset) with heating temperature for the FCC-(Fe0.sCuo.5)9aZr7alloy. Symbols indicate : (O) FCC, (O) BCC and (*) Fe3Zr phases.
The XRD patterns indicate that the first process corresponds to the phase separation of the metastable FeCuZr solid solutions into FCC and BCC phases. From the evolution of the lattice parameter it can be concluded that they do not correspond to pure Cu or Fe, hence they should be FCC-Curich and BCC-Fe,ioh phases, respectively. The increase of the volume fraction of the Fe,ich phase as well as its Fe content at the expense of the initial alloy explain the observed increase of the magnetization shown in Fig. 1. As an example, the XRD patterns for the
Fig. 3. Evolution, with the thermal treatment, of the coercive field at 300 K.
x = 0.5 composition is shown in Fig. 2, together with the evolution of the average grain sizes. After heating above the 850 K the presence of the intermetallic Fe3Zr is resolved in the pattern, whereas the FCC and BCC phases have evolved toward pure BCC-Fe and FCC-Cu. The microstructural evolution produces important changes in the values of the coercive field, He, see Fig. 3. First, the coercive field increases due a progressive exchange isolation of the ferromagnetic Fe-rich nanocrystals due to the FCC-Cu-rich phase as well as to the compositional changes of the BCC phase. After heating above 850 K the drastic increase of Hc is due to the appearance of the high anisotropy Fe3Zr phase. The subsequent softening of the material should be related to the growth of the ferromagnetic grains above the single domain limit. It has been shown that HEBM allows to produce new alloys based on immiscible elements such as Fe and Cu. In this case the decomposition of alloys produces a gradual change of the microstructural features allowing to cover a broad range of magnetic behaviors.
References [1] K. Uenishi, K.F. Kobayashi, S. Nasu, H. Hatano, K.N. Ishihara, P.H. Shingu, Z. Metallkd. 83 (1992) 132. [2] P. Crespo, A. Hernando, R. Yavari, O. Drbohlav, A. Garcia Escorial, J.M. Barandiar/m, I. Or~e, Phys. Rev. B 48 (10) (1993) 7234. f3] P. Crespo, I. Navarro, A. Hernando, A. Garcia Escorial, P. Rodriguez, J.M. Barandiar~n, O. Drbohlav, R. Yavari, J. Magn. Magn. Mater. 159 (1995) 409.
Journal of Magnetism and Magnetic Materials 196-197 (1999) 214-215
Journalof
magnetism and magnetic materials
Structural and magnetic properties of mechanically alloyed (FexCux-x)93Zr7 (x = 0.5, 0.7) solid solutions M. Multigner a, A. Hernando a, P. Crespo a'*, C. Stiller b, J. Eckert b, L.
Schultz b
~lnstituto de Magnetismo Aplicado. RENFE-UCM, P.O. Box /55, Las Rozas, 2,~¢230Madrid, Spain ~'IFM, Helmoltzstr. 20, 01069, Dresden. Germam'
Abstract Nanocrystalline FCC-(Feo.sCuo.5)93Zr7 and BCC-(Feo.TCuo.3)93Zr; (at%) solid solution, have been obtained by mechanical alloying. Both alloys are ferromagnetic at room temperature. By heating, the alloys decompose into FCC and BCC phases which, with further annealing, evolve into BCC-Fe, F C C - C u and the intermetallic Fe3Zr. This process leads to an increase of the coercive field originated by a decoupling of the ferromagnetic grains and to the appearance of the high anisotropy Fe3Zr phase. ,.~ 1999 Elsevier Science B.V. All rights reserved. Keywords." Mechanical alloying; FeCu-based alloys; Metastable systems
The high-energy ball milling technique, HEBM, allows to synthesize new phases far from the equilibrium. For example, it is possible to overcome the immiscibility between Fe-Cu, obtaining solid solutions of FeCu in all the compositional range [1]. In this work we have produced and characterized FeCu(Zr) nanocrystalline solid solutions. (FexCul -x)93Zr7 (x = 0.5, 0.7) alloys were obtained by HEBM of Fe and Cu powders and files of Zr. Structural studies were performed by X-ray diffraction. Magnetic properties were studied by means of a S Q U I D and vibrating sample magnetometer. After 120 h of milling, the XRD patterns correspond to a single nanocrystalline phase with an average grain size of 6 nm. The crystalline structure depends on the Fe/Cu ratio in a similar way as reported for FexCu, x [1]. For .r = 0.5, the alloy presents a F C C structure with a lattice parameter of 0.3633 nm, whereas for x = 0.7 it is found that the alloy has a BCC structure and its lattice parameter is 0.2902 nm. Both lattices are expanded with respect to FCC-Cu or BCC-Fe.
*Corresponding author. [email protected]
Fax:
91-6301625: e-mail:
From the shape of the hysteresis loops, it can be concluded that the alloys behave as a single magnetic phase. The temperature dependence of the magnetization measured under a field of 1 T, is shown in Fig. 1. The magnetization decreases with temperature, as occurs in a ferromagnetic material. Further increase in temperature produces an increase of the magnetization, which is related to the decomposition process of the alloys. For the FCC-(Feo.sCuo.s)93Zr7 alloy, the Curie point was estimated to be around 310 K, a value significantly lower than that of the Zr-free alloys [2,3], around 500 K. The BCC-(Feo.vCuo.3)93Zr~, alloy decomposes before reaching its Curie point, hence the estimation was not possible. The experimental data measured under a field of 5 T can be fitted to a Bloch law indicating that the thermal demagnetization process is mainly due to spin wave excitations at low temperatures. The magnetic moment per Ee atom, laFe is 1.8 laB and 2.16 laB, for x = 0.5 and 0.7, respectively. In both cases, laFe is smaller than that of Zr-free samples [2,3], indicating that the Zr addition changes the electronic structure of the FeCu alloys. The calorimetric trace for both alloys shows that the decomposition process takes place in two steps. The first one is characterized by a heat release over a broad temperature range, 400-850 K, while the second one is characterized by a well-defined peak at around 920 K.
0304-8853/99/$ - see front matter C' 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 6 2 - 8
215
M. Multigner et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 214-215 140120== 100-
o= so~
~
300
--0 240 - - • %0 _~ 180
uo~)~Zr~
\\
"~ 801 ~
40-
~
20-
x=0.5 x=0.7
'\
12o 60
0 0
200
400
600
800
200
1000
Fig. 1. Thermomagnetization curves measured with an applied field of la0H = 1 T.
fcc-(Feo.sCuos)=3Zr7
~
o b=
~•
-'-"~
\
•
/
/
t823 K
973 K
1273 K
4'0
6'0
400
600
800
1000 1200 1400
annealing temperature (K)
T e m p e r a t u r e (K)
80
20 (degree)
Fig. 2. Evolution of XRD patterns and grain sizes (inset) with heating temperature for the FCC-(Fe0.sCuo.5)9aZr7alloy. Symbols indicate : (O) FCC, (O) BCC and (*) Fe3Zr phases.
The XRD patterns indicate that the first process corresponds to the phase separation of the metastable FeCuZr solid solutions into FCC and BCC phases. From the evolution of the lattice parameter it can be concluded that they do not correspond to pure Cu or Fe, hence they should be FCC-Curich and BCC-Fe,ioh phases, respectively. The increase of the volume fraction of the Fe,ich phase as well as its Fe content at the expense of the initial alloy explain the observed increase of the magnetization shown in Fig. 1. As an example, the XRD patterns for the
Fig. 3. Evolution, with the thermal treatment, of the coercive field at 300 K.
x = 0.5 composition is shown in Fig. 2, together with the evolution of the average grain sizes. After heating above the 850 K the presence of the intermetallic Fe3Zr is resolved in the pattern, whereas the FCC and BCC phases have evolved toward pure BCC-Fe and FCC-Cu. The microstructural evolution produces important changes in the values of the coercive field, He, see Fig. 3. First, the coercive field increases due a progressive exchange isolation of the ferromagnetic Fe-rich nanocrystals due to the FCC-Cu-rich phase as well as to the compositional changes of the BCC phase. After heating above 850 K the drastic increase of Hc is due to the appearance of the high anisotropy Fe3Zr phase. The subsequent softening of the material should be related to the growth of the ferromagnetic grains above the single domain limit. It has been shown that HEBM allows to produce new alloys based on immiscible elements such as Fe and Cu. In this case the decomposition of alloys produces a gradual change of the microstructural features allowing to cover a broad range of magnetic behaviors.
References [1] K. Uenishi, K.F. Kobayashi, S. Nasu, H. Hatano, K.N. Ishihara, P.H. Shingu, Z. Metallkd. 83 (1992) 132. [2] P. Crespo, A. Hernando, R. Yavari, O. Drbohlav, A. Garcia Escorial, J.M. Barandiar/m, I. Or~e, Phys. Rev. B 48 (10) (1993) 7234. f3] P. Crespo, I. Navarro, A. Hernando, A. Garcia Escorial, P. Rodriguez, J.M. Barandiar~n, O. Drbohlav, R. Yavari, J. Magn. Magn. Mater. 159 (1995) 409.