- Email: [email protected]
Magnetic reentrant behavior of pdfe-films due to disorder
Physica B 165&166(1990) 463-464 North-Holland
MAGNETIC REENTRANT BEHAVIOR OF NFe-FILMS
DUE TO DISORDER
Peter BRULL, Paul ZIEMANN Fakult;it Physik, Universitgt Ronstanx, D-7750 Konstanz, FRG Fe -films (x=0.01) were prepared by vapor-quenching onto liquid-He cooled Insitu measurements of the ac-susceptibility performed in ~&ra~es. additionally applied dc-fields allow to check these films for a possible reentrant behavior. Reentrance is found for disordered films and substantiated by field-cooling experiments. Lowering the degree of disorder by stepwise annealing the films up to 330 K leads to a gradual disappearance of the reentrant behavior.
1. INTRODUCTION Bulk Pdl_xFex is a wellknown giant moment svstem exhibitina ferromaanetism (FM) abode a percolatio; threshold of O.lat% Fe (1). On the other hand recent measurements on corresponding films (2) provided some evidence for a reentrant transition from FM to a cluster glass (CG). To substantiate these observations, further experiments have been performed under improved preparation conditions (the magnetic behavior of PdFe is extremely sensitive to oxygen impurities built-in during the evaporation process (3)) and special emphasis has been put on the influence of superposed dc-magnetic fields applied after cooling the samples (ZFC)-or durins cooling IFCI. The role of disorder on-the reentrant behavior is tested by gradually annealing the films. 2. EXPERIMENTAL The =Fe-films (lat% Fe, typical thickness 55 nm) were prepared by flash-evaporation of small pieces from a mother alloy onto liquid-Be cooled sapphire substrates. The pressure d ring evaporation was less than 7*10-8 mbar, a necessary prequisite to obtain reproducible results. The highly disordered films were stepwise annealed up to TA and the ac-susceptibility x was determined insitu between 5.5 K and T, using a field-amplitude of 0.1 mT at n 1049 Hz. In addition small (<3 mT) magnetic dc-fields were applied either after cooling to 5.5 K (ZFC) or at T (FC). More details on the susceptomeeer can be found in (4). 3. RESULTS AND DISCUSSION Fig.1 shows the temperature depen-
6
Tf 8
IO
12 TIKl
Fig. 1: Temperature dependence of the specific susceptibility X for a quench condensed BFe-film (lat% Fe) annealed at TA=240 K. Different dc-fields as indicated were applied (ZFC). dence of the specific susceptibility x for a film annealed up to TA=240 K. Here different dc-fields as indicated in the figure were applied at 5.5 K. The results exhibit two main features: A steep increase of x below 12 K indicating the transition into the FM-state (a Curie-temperature TC=11.4 K is extrapolated from a tangent-construction) and a pronounced shoulder below Tf=7.2 K as indicated by the arrow in fig.1, which is interpreted as a transition into the CG-state. This interpretation is suggested by the observed effect of the dc-fields, which suppress the FM-part of X above Tf much stronger than the CG-part below Tr (5). The bulk reduced TC-value as comp&d'to samples confirms earlier observations that defects are lowering the Curietemperature (6). The above interpreta-
0921-4526/90/$03.50 @ 1990- Elsevier Science Publishers B.V.(North-Holland)
464
P. Briill, P. Ziemann
x
X
xTc
X X
X
X
0
X
0
0
J-r
0
TtKI
Fig. 2: Influence of dc-field-cooling on the temperature dependence of the susceptibility x for a PdFe-film (lat% Fe). The different cooling fields are indicated in the figure, the x-measurements were performed in a fixed dcfield of 2 Oe. tion is corroborated by FC-experiments, where varying dc-fields are applied at T'TC. The corresponding results are shown in fig.2: the dc-fields practically have no influence on the FM-part of x(T), but lead to a pronounced splitting of the x-curves at low temoeratures starting at Tg, independent of the dc-field for Bd&O mT. It is imoortant to note that for a higher anngaling temperature TA=330 K the FC-experiments still show a splitting of the x(T)-curves depending on the dc-fields, but starting not at a well defined temperature T . One rather observes a gradual onset sfarting at temperatures very close to TC for fields B 10.1 mT. This qualitatively differen ec behavior is also found by the ZFC-experiments: For T 5330 K x(T) exhibits no longer a 8A oulder at low temperatures. This is interpreted as disappearance of the reentrant CG-state due to annealing. Fig. 3 summarizes the magnetic behavior of the SFe-films containing lat% Fe. Here TC and the reentrant temperature Tf are plotted versus T . While its some the T -dependence of T exhi*B* strut+ ure reflecting tfe preferential annealing of a specific type of disorder (6), the corresponding T -behavior is structureless with a smal f positive slope within the observed range (at TA=330 K, according to our criteria, no reentrance was found, at TA=160 K the reentrance could not be observed above the lowest experimentally attained temperature of 5.5 K).
f.X
X
1
1
L
LO
120
200
280 T,(Kl
Fig. 3: Dependence of the Curie- and the reentrant temperatures (TC, Tf) on the degree of disorder as indicated by the annealing temperature TA. The meaning of the arrows is described in the text. 4. CONCLUSIONS The experimental results on MFefilms demonstrate that strong disorder as produced by vapor-quenching onto cold substrates can lead to a reentrant transition from FM to CG even in systems, which,as bulk samples, exhibit is only FM. The reported effect restricted to Fe-concentrations below 2at% suggesting that the coupling between moments via the Pd-host electrons is the most disorder-sensitive part. ACKNOWLEDGEWBNTS We thank Ch. Neumann for his experimental help. This work was partly supported by the Deutsche Forschungsgemeinschaft, SFB 306. REFERENCES (1) G.J.Nieuwenhuys, Adv. Phys. 24, 515 (1975) (2) M.Avirovic, P.Ziemann, Jap. J. Appl. Phys. & 785 (1987) (3) H.Claus, N.C.Koon, Sol. State Commun. 60, 481 (1986) (4) G.Ziebold, D.Korn, J. Phys. E: Sci. Instr. 12, 490 (1979) B.H.Verbeeck, thesis, Leiden (1979) :s5;M.Hitzfeld, P.Ziemann, W.Buckel, H.Claus, Phys. Rev. B 29, 5023 (1984)
MAGNETIC REENTRANT BEHAVIOR OF NFe-FILMS
DUE TO DISORDER
Peter BRULL, Paul ZIEMANN Fakult;it Physik, Universitgt Ronstanx, D-7750 Konstanz, FRG Fe -films (x=0.01) were prepared by vapor-quenching onto liquid-He cooled Insitu measurements of the ac-susceptibility performed in ~&ra~es. additionally applied dc-fields allow to check these films for a possible reentrant behavior. Reentrance is found for disordered films and substantiated by field-cooling experiments. Lowering the degree of disorder by stepwise annealing the films up to 330 K leads to a gradual disappearance of the reentrant behavior.
1. INTRODUCTION Bulk Pdl_xFex is a wellknown giant moment svstem exhibitina ferromaanetism (FM) abode a percolatio; threshold of O.lat% Fe (1). On the other hand recent measurements on corresponding films (2) provided some evidence for a reentrant transition from FM to a cluster glass (CG). To substantiate these observations, further experiments have been performed under improved preparation conditions (the magnetic behavior of PdFe is extremely sensitive to oxygen impurities built-in during the evaporation process (3)) and special emphasis has been put on the influence of superposed dc-magnetic fields applied after cooling the samples (ZFC)-or durins cooling IFCI. The role of disorder on-the reentrant behavior is tested by gradually annealing the films. 2. EXPERIMENTAL The =Fe-films (lat% Fe, typical thickness 55 nm) were prepared by flash-evaporation of small pieces from a mother alloy onto liquid-Be cooled sapphire substrates. The pressure d ring evaporation was less than 7*10-8 mbar, a necessary prequisite to obtain reproducible results. The highly disordered films were stepwise annealed up to TA and the ac-susceptibility x was determined insitu between 5.5 K and T, using a field-amplitude of 0.1 mT at n 1049 Hz. In addition small (<3 mT) magnetic dc-fields were applied either after cooling to 5.5 K (ZFC) or at T (FC). More details on the susceptomeeer can be found in (4). 3. RESULTS AND DISCUSSION Fig.1 shows the temperature depen-
6
Tf 8
IO
12 TIKl
Fig. 1: Temperature dependence of the specific susceptibility X for a quench condensed BFe-film (lat% Fe) annealed at TA=240 K. Different dc-fields as indicated were applied (ZFC). dence of the specific susceptibility x for a film annealed up to TA=240 K. Here different dc-fields as indicated in the figure were applied at 5.5 K. The results exhibit two main features: A steep increase of x below 12 K indicating the transition into the FM-state (a Curie-temperature TC=11.4 K is extrapolated from a tangent-construction) and a pronounced shoulder below Tf=7.2 K as indicated by the arrow in fig.1, which is interpreted as a transition into the CG-state. This interpretation is suggested by the observed effect of the dc-fields, which suppress the FM-part of X above Tf much stronger than the CG-part below Tr (5). The bulk reduced TC-value as comp&d'to samples confirms earlier observations that defects are lowering the Curietemperature (6). The above interpreta-
0921-4526/90/$03.50 @ 1990- Elsevier Science Publishers B.V.(North-Holland)
464
P. Briill, P. Ziemann
x
X
xTc
X X
X
X
0
X
0
0
J-r
0
TtKI
Fig. 2: Influence of dc-field-cooling on the temperature dependence of the susceptibility x for a PdFe-film (lat% Fe). The different cooling fields are indicated in the figure, the x-measurements were performed in a fixed dcfield of 2 Oe. tion is corroborated by FC-experiments, where varying dc-fields are applied at T'TC. The corresponding results are shown in fig.2: the dc-fields practically have no influence on the FM-part of x(T), but lead to a pronounced splitting of the x-curves at low temoeratures starting at Tg, independent of the dc-field for Bd&O mT. It is imoortant to note that for a higher anngaling temperature TA=330 K the FC-experiments still show a splitting of the x(T)-curves depending on the dc-fields, but starting not at a well defined temperature T . One rather observes a gradual onset sfarting at temperatures very close to TC for fields B 10.1 mT. This qualitatively differen ec behavior is also found by the ZFC-experiments: For T 5330 K x(T) exhibits no longer a 8A oulder at low temperatures. This is interpreted as disappearance of the reentrant CG-state due to annealing. Fig. 3 summarizes the magnetic behavior of the SFe-films containing lat% Fe. Here TC and the reentrant temperature Tf are plotted versus T . While its some the T -dependence of T exhi*B* strut+ ure reflecting tfe preferential annealing of a specific type of disorder (6), the corresponding T -behavior is structureless with a smal f positive slope within the observed range (at TA=330 K, according to our criteria, no reentrance was found, at TA=160 K the reentrance could not be observed above the lowest experimentally attained temperature of 5.5 K).
f.X
X
1
1
L
LO
120
200
280 T,(Kl
Fig. 3: Dependence of the Curie- and the reentrant temperatures (TC, Tf) on the degree of disorder as indicated by the annealing temperature TA. The meaning of the arrows is described in the text. 4. CONCLUSIONS The experimental results on MFefilms demonstrate that strong disorder as produced by vapor-quenching onto cold substrates can lead to a reentrant transition from FM to CG even in systems, which,as bulk samples, exhibit is only FM. The reported effect restricted to Fe-concentrations below 2at% suggesting that the coupling between moments via the Pd-host electrons is the most disorder-sensitive part. ACKNOWLEDGEWBNTS We thank Ch. Neumann for his experimental help. This work was partly supported by the Deutsche Forschungsgemeinschaft, SFB 306. REFERENCES (1) G.J.Nieuwenhuys, Adv. Phys. 24, 515 (1975) (2) M.Avirovic, P.Ziemann, Jap. J. Appl. Phys. & 785 (1987) (3) H.Claus, N.C.Koon, Sol. State Commun. 60, 481 (1986) (4) G.Ziebold, D.Korn, J. Phys. E: Sci. Instr. 12, 490 (1979) B.H.Verbeeck, thesis, Leiden (1979) :s5;M.Hitzfeld, P.Ziemann, W.Buckel, H.Claus, Phys. Rev. B 29, 5023 (1984)