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Magnetic circular dichroism of Gd-Co and Gd-Ni alloys in the Gd 3d, 4d and the (Co, Ni) 2p, 3p core excitation regions: Antiferromagnetic coupling between Gd and (Co, Ni) moments
Journal of Electron Spectroscopyand Related Phenomena78 (19%) 217-220
Magnetic circular dichroism of Gd-Co and Gd-Ni alloys in the Gd 3d, 4d and the (Co, Ni) 2p, 3p core excitation regions: Antiferromagnetic coupling between Gd and (Co, Ni) moments Tadashi Hatano,a Semg-Yerl Park,b* Takaaki Hanyu’ and Tsuneaki Miyahara” ‘Photon Factory, National Laboratory for High Energy Physics, Oho l-l, Tsukuba, Ibaraki, 305, Japan bDepartment of Synchrotron Radiation Science, The Graduate University for Advanced Studies, Oho l-l, Tsukuba, Ibaraki, 305, Japan ‘Department of Physics, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa l-l, Hachioji, Tokyo, 192-03, Japan
Magnetic circular dichroism(MCD) spectra of Gd-Co and Gd-Ni alloys were measured in core excitation regions using evaporated film and arc-melted bulk samples. Antiferromagnetic coupling between spins of rare-earth and transition metals was demonstrated in Gd-Co but it was not the case in bulk Gd-Ni, which gives rise to a possible spin-glass-like state at the liquid nitrogen temperature with the help of hidden ferromagnetic interaction.
1.
information of magnetism in alloy systems. Hence
INTRODUCTION
we can separately measure the local magnetizations It is well known that the spin magnetic moments antiferromagnetically
In the present study the MCD spectra of Gd-Co
intermetallic compounds of 3d-transition metals with
and Gd-Ni evaporated films were measured in the Gd
rare-earth metals, which has been concluded from the
4d-4f and (Co, Nil 3p-_3d core excitation regions. The
dependence of absolute value of magnetization on 4f
MCD spectra of arc-melted bulks were measured also
electron number of the rare-earth metal [I] or on the
in the inner core excitation, Gd 3d-4f ‘and(Co, Ni)
temperature [2]. Since synchrotron radiation was
2p-3d regions in addition to those regions.
co11pled
in
of respective elements by means of this technique.
some
are
developed as a circularly polarized VUV ‘and X-ray source,
the
measurement
of magnetic
circular
2.
EXPERIMENTAL
PROCEDURES
dichroism(MCD) due to core level excitation has been a powerful method to investigate magnetic materials.
One
of
the
advantages
measurement is the provision
of
The experiment was performed using circularly
MCD
polmized synchrotron radiation at two beamlines
of site selective
equipped with a helical undulator. BL-28A of the
*Present address; Basic Science Research Institute, Pohang University of Science and Technology, San 31. Hyoja Dong. Pohang, 790-784, Republic of Korea 0368-2048/%/$15.00 0 19% Elsevier Science B.V. All rights reserved PII SO368 - 2048 (%) 027 16-8
218
Photon Factory [3] and BL-NElB of the TRISTAN Accumulation Ring [4] cover the Gd N,., (Co, Ni) M?? edges and the Gd M,,, (Co, Ni) Lz 3 edges, respectively. The apparatus with a permanent magnet flipper is illustrated elsewhere [5]. We have made an additional construction
of a double evaporation
source in order to prepare binary alloy samples. Each film sample of about 2OOA thickness was prepared by simultaneous evaporation on a Si(lO0) substrate cooled to the liquid nitrogen temperature. Then it was immediately transported to the sample holder also cooled to the liquid nitrogen temperature and the measurement was performed in-situ. The other type of sample was a bulk made with Ar-‘arc furnace. The bulk samples were filed in the vacuum chamber just before the measurement. We employed the total photoelectron method because absorption
yield(TY)
measurements
were
impossible for either types of sample. Here we define the TY-MCD as the difference between the photocurrents under the two opposite directions of external magnetic fields with the same circular polarization of incident light. In order to make a quantitative
analysis
approximation
of
the TY-MCD
of so-called absorption
as
MCD,
an it
should be noted that TY is not always proportional
ke---+ 780
800 ’ 1160 I I 1180 I I 1200 I I 1220 I
PHOTON ENERGY (eV)
to absorption and depends on the attenuation length
FIG. 1. MCD spectra of Gd-Co alloy system. (a) Film
of excited electrons [6] in the alloy.
samples at the Gd Nhs and Co MLj cdgcs. (h) Bulk samples at the Gd Na5 and Co M,, edges. (c) Bulk
3. RESULTS
AND DISCUSSIONS
samples at the Gd M,, and Co L,, edges.
Figure l(a) shows the examples of TY-MCD for
magnetic moment of Gd atomic site is antiparallel to
Gd-Co films. One can observe MCD’s due to the Gd
that of Co atomic site, which gives a direct evidence
4d-4f and Co 3p-3d transitions around 15OeV and
to ferrimagnetism of this alloy system. The results
just above 6OeV. respectively. The structures around
of Gd-Co bulk samples were consistent with those
50eV and 15eV were caused by the higher order
of film samples as shown in Figs. l(b) and (c).
diffraction of the grating monochromator. We found
Figure 2(a) shows the TY-MCD spectra of Gd-Ni
that the MCD of GdCo, is reversed in the Gd 4d-4f
film samples, which have similar characteristics to
region and that the MCD of GdCo, is reversed in the
those of Gd-Co. On the other hand the MCD spectra
Co 3p-3d region. These results indicate that averaged
of Gd-Ni bulk
samples
give
the
orientation
219
r
I
rlu
I
I
I
I
I
I
(a) Cd-Ni film in VUV region p, =
V
Ni
V
Jcid
ld-4f
(1)
x
II u3
d.E
w?Ni,7
where l,(E) is the TY-MCD spectrum of Gd,Co,., or Gd,Ni,.,. This formula was derived from the least I I
I I1
I
#\I Ih
I I
I I
I I
I1 I
I
-
(b) Cd-Ni bulk in VUV region
square fitting of I,(E) to p&,(E)
change in spectral shape. Then p, was normalized so that the maximum moment,7.55p,,
x10 w
GdNi,
4
where &, is the Bohr magneton.
also been carried out. Figures 3(a) and 4(a) show the moments thus estimated for the Gd atomic site in
Cd I I I Iti 50 60 70 80 ”
of pu, equals the pure Gd
The similar calculation in the Gd 3d-4f region has
G&Nil7
x10
neglecting the
I I I I I 130 140 150 160 170 180
Gd-Co ‘andGd-Ni respectively. The moments of Co and Ni sites were also nnalyzed in the similar manner and are shown in Figs. 3(b) and 4(b), where the moments of pure Co and Ni are assumed to be 80 20 40 t’..“..~..“*..‘...j
II
11
830
850
1
h.8
11
11
I1
60
80
100
60
80
100
II
870 ” 1160 1180 1200 1220 PHOTON ENERGY (eV)
FIG. 2. MCD spectra of Gd-Ni alloy system. (a) Film samples at the Gd Nd5 and Ni M,, edges. (b) Bulk samples at the Gd N.,5and Ni ML1edges. Ten times magnification has been made in the Ni region. (c) Bulk samples at the Gl M,, and Ni LL,edges.
independent of the concentration as shown in Figs. 2(b) ‘and(c), about which some discussions will be made later. We analyzed the concentration dependence of the averaged magnetic moments of Gd site as follows. The magnitude of MCD per Gd atom in the Gd 4&4f region was evaluated by
0
20
40
ATOMIC % Cd FIG. 3. Gd concentration
dependence
of magnetic
moments (a) ofG3 site and (b) of Co site in Gd-Co.
220
8 0
20
40
60
80
100
t’,“““““..“‘.‘)
state in the film GdN& may be due to a delicate balance between the two opposite interactions. There are some reasons why the absolute values of Gd, Co and Ni magnetic moments decrease as the Gd concentration increases, First the applied magnetic field of 1.05T is not sufficient to saturate pure Gd. Second the Curie
temperatures
are remarkably
lowered by adding (Co, Ni) to Gd and even close to -8 0.8,.
the liquid nitrogen temperature.
. . , . . . , . . . , . . . , . . . ,
(b) 0.4 :
4. CONCLUSION
l
.
MCD spectra of Gd-(Co, Ni) alloys were measured
0.2 : n 1
. -0.2 : -0.4.
l
.
in core excitation regions for evaporated film and .
arc-melted bulk samples. It was found that the sign
’
. . * . ’ . * * ’ . * . ’ * . * ’ * * . * 0 20 40 60 80 100
ATOMIC % Gd FIG. 4. Gd concentration dependence of magnetic moments (a) of Gd site and (h) ofNi site in Gd-Ni.
In Fig. 3, one can see the switching of the sign between 20 and 25 at. % Gd with the absolute value continuous.
In other words, the compensation
temperature
was close
the liquid
reversed when the concentration of (Co, Ni) or Gd increases except in the bulk Gd-Ni samples, which indicates the antiferromagnetic coupling between Gd and (Co, Ni) moments. As for Gd-Ni bulk sample, ferromagnetic interaction is prevalent. These findings implies a possible spin-glass-like sate at particular
1.71~~ and 0.604~,, respectively.
to
of the MCD due to Gd or (Co, Ni) moment is
nitrogen
concentrations caused by coexistence of ferro- ‘and antiferro-magnetic interactions.
REFERENCES
temperature in Gd-Co of this cd concentration, We found both Gd and Ni atomic magnetizations
1. E. A. Nesbitt, H. J. Williams, J. H. Wernick and
were positive in Gd-Ni bulk samples as shown in
R. C. Sherwood: J. Appl. Phys. 3 3. 1674 ( 1962).
Fig. 4, which suggests that ferromagnetic interaction
2. P. Chaudhari, J. J. Cuomo and R. J. Gambio;
appears
and
overcomes
antiferromagnetic
one.
Another remarkable feature in Fig. 4 is apparent instability of the film samples at x=1/6 (GdNi,). The
Appl. Phys. Lett. 2 2, 337 (1973). 3. T. Koide, T. Shidara, T. Miyahara and M. Yuri; Rev. Sci. Instrum. 66, 1923 (1995).
origin of small magnetic moments in some of them
4. Y. Kagoshima, T. Miyahara, S. Yamamoto, H.
can be considered as spin-glass-like alignment of
Kitamura, S. Muto, S. Y. Park and J. D. Wang;
magnetic moments, because the result of the Gd-Ni
Rev. Sci. Intsrum. 6 6, 1696 (1995).
bulk sample suggests the ferromagnetic interaction which is hidden in most film samples but might reappem when the antiferromagnetic interaction is much reduced. Therefore the above spin-glass-like
5. S. Muto, Y. Kagoshima and T. Miyahara: Rev. Sci. Instrum. 6 3, 1470 (1992). 6. C. R. Crowell, W. G. Spitzer, L. E. Howarth and E. E. LaBate; Phys. Rev. 127, 2006 (1962).
Magnetic circular dichroism of Gd-Co and Gd-Ni alloys in the Gd 3d, 4d and the (Co, Ni) 2p, 3p core excitation regions: Antiferromagnetic coupling between Gd and (Co, Ni) moments Tadashi Hatano,a Semg-Yerl Park,b* Takaaki Hanyu’ and Tsuneaki Miyahara” ‘Photon Factory, National Laboratory for High Energy Physics, Oho l-l, Tsukuba, Ibaraki, 305, Japan bDepartment of Synchrotron Radiation Science, The Graduate University for Advanced Studies, Oho l-l, Tsukuba, Ibaraki, 305, Japan ‘Department of Physics, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa l-l, Hachioji, Tokyo, 192-03, Japan
Magnetic circular dichroism(MCD) spectra of Gd-Co and Gd-Ni alloys were measured in core excitation regions using evaporated film and arc-melted bulk samples. Antiferromagnetic coupling between spins of rare-earth and transition metals was demonstrated in Gd-Co but it was not the case in bulk Gd-Ni, which gives rise to a possible spin-glass-like state at the liquid nitrogen temperature with the help of hidden ferromagnetic interaction.
1.
information of magnetism in alloy systems. Hence
INTRODUCTION
we can separately measure the local magnetizations It is well known that the spin magnetic moments antiferromagnetically
In the present study the MCD spectra of Gd-Co
intermetallic compounds of 3d-transition metals with
and Gd-Ni evaporated films were measured in the Gd
rare-earth metals, which has been concluded from the
4d-4f and (Co, Nil 3p-_3d core excitation regions. The
dependence of absolute value of magnetization on 4f
MCD spectra of arc-melted bulks were measured also
electron number of the rare-earth metal [I] or on the
in the inner core excitation, Gd 3d-4f ‘and(Co, Ni)
temperature [2]. Since synchrotron radiation was
2p-3d regions in addition to those regions.
co11pled
in
of respective elements by means of this technique.
some
are
developed as a circularly polarized VUV ‘and X-ray source,
the
measurement
of magnetic
circular
2.
EXPERIMENTAL
PROCEDURES
dichroism(MCD) due to core level excitation has been a powerful method to investigate magnetic materials.
One
of
the
advantages
measurement is the provision
of
The experiment was performed using circularly
MCD
polmized synchrotron radiation at two beamlines
of site selective
equipped with a helical undulator. BL-28A of the
*Present address; Basic Science Research Institute, Pohang University of Science and Technology, San 31. Hyoja Dong. Pohang, 790-784, Republic of Korea 0368-2048/%/$15.00 0 19% Elsevier Science B.V. All rights reserved PII SO368 - 2048 (%) 027 16-8
218
Photon Factory [3] and BL-NElB of the TRISTAN Accumulation Ring [4] cover the Gd N,., (Co, Ni) M?? edges and the Gd M,,, (Co, Ni) Lz 3 edges, respectively. The apparatus with a permanent magnet flipper is illustrated elsewhere [5]. We have made an additional construction
of a double evaporation
source in order to prepare binary alloy samples. Each film sample of about 2OOA thickness was prepared by simultaneous evaporation on a Si(lO0) substrate cooled to the liquid nitrogen temperature. Then it was immediately transported to the sample holder also cooled to the liquid nitrogen temperature and the measurement was performed in-situ. The other type of sample was a bulk made with Ar-‘arc furnace. The bulk samples were filed in the vacuum chamber just before the measurement. We employed the total photoelectron method because absorption
yield(TY)
measurements
were
impossible for either types of sample. Here we define the TY-MCD as the difference between the photocurrents under the two opposite directions of external magnetic fields with the same circular polarization of incident light. In order to make a quantitative
analysis
approximation
of
the TY-MCD
of so-called absorption
as
MCD,
an it
should be noted that TY is not always proportional
ke---+ 780
800 ’ 1160 I I 1180 I I 1200 I I 1220 I
PHOTON ENERGY (eV)
to absorption and depends on the attenuation length
FIG. 1. MCD spectra of Gd-Co alloy system. (a) Film
of excited electrons [6] in the alloy.
samples at the Gd Nhs and Co MLj cdgcs. (h) Bulk samples at the Gd Na5 and Co M,, edges. (c) Bulk
3. RESULTS
AND DISCUSSIONS
samples at the Gd M,, and Co L,, edges.
Figure l(a) shows the examples of TY-MCD for
magnetic moment of Gd atomic site is antiparallel to
Gd-Co films. One can observe MCD’s due to the Gd
that of Co atomic site, which gives a direct evidence
4d-4f and Co 3p-3d transitions around 15OeV and
to ferrimagnetism of this alloy system. The results
just above 6OeV. respectively. The structures around
of Gd-Co bulk samples were consistent with those
50eV and 15eV were caused by the higher order
of film samples as shown in Figs. l(b) and (c).
diffraction of the grating monochromator. We found
Figure 2(a) shows the TY-MCD spectra of Gd-Ni
that the MCD of GdCo, is reversed in the Gd 4d-4f
film samples, which have similar characteristics to
region and that the MCD of GdCo, is reversed in the
those of Gd-Co. On the other hand the MCD spectra
Co 3p-3d region. These results indicate that averaged
of Gd-Ni bulk
samples
give
the
orientation
219
r
I
rlu
I
I
I
I
I
I
(a) Cd-Ni film in VUV region p, =
V
Ni
V
Jcid
ld-4f
(1)
x
II u3
d.E
w?Ni,7
where l,(E) is the TY-MCD spectrum of Gd,Co,., or Gd,Ni,.,. This formula was derived from the least I I
I I1
I
#\I Ih
I I
I I
I I
I1 I
I
-
(b) Cd-Ni bulk in VUV region
square fitting of I,(E) to p&,(E)
change in spectral shape. Then p, was normalized so that the maximum moment,7.55p,,
x10 w
GdNi,
4
where &, is the Bohr magneton.
also been carried out. Figures 3(a) and 4(a) show the moments thus estimated for the Gd atomic site in
Cd I I I Iti 50 60 70 80 ”
of pu, equals the pure Gd
The similar calculation in the Gd 3d-4f region has
G&Nil7
x10
neglecting the
I I I I I 130 140 150 160 170 180
Gd-Co ‘andGd-Ni respectively. The moments of Co and Ni sites were also nnalyzed in the similar manner and are shown in Figs. 3(b) and 4(b), where the moments of pure Co and Ni are assumed to be 80 20 40 t’..“..~..“*..‘...j
II
11
830
850
1
h.8
11
11
I1
60
80
100
60
80
100
II
870 ” 1160 1180 1200 1220 PHOTON ENERGY (eV)
FIG. 2. MCD spectra of Gd-Ni alloy system. (a) Film samples at the Gd Nd5 and Ni M,, edges. (b) Bulk samples at the Gd N.,5and Ni ML1edges. Ten times magnification has been made in the Ni region. (c) Bulk samples at the Gl M,, and Ni LL,edges.
independent of the concentration as shown in Figs. 2(b) ‘and(c), about which some discussions will be made later. We analyzed the concentration dependence of the averaged magnetic moments of Gd site as follows. The magnitude of MCD per Gd atom in the Gd 4&4f region was evaluated by
0
20
40
ATOMIC % Cd FIG. 3. Gd concentration
dependence
of magnetic
moments (a) ofG3 site and (b) of Co site in Gd-Co.
220
8 0
20
40
60
80
100
t’,“““““..“‘.‘)
state in the film GdN& may be due to a delicate balance between the two opposite interactions. There are some reasons why the absolute values of Gd, Co and Ni magnetic moments decrease as the Gd concentration increases, First the applied magnetic field of 1.05T is not sufficient to saturate pure Gd. Second the Curie
temperatures
are remarkably
lowered by adding (Co, Ni) to Gd and even close to -8 0.8,.
the liquid nitrogen temperature.
. . , . . . , . . . , . . . , . . . ,
(b) 0.4 :
4. CONCLUSION
l
.
MCD spectra of Gd-(Co, Ni) alloys were measured
0.2 : n 1
. -0.2 : -0.4.
l
.
in core excitation regions for evaporated film and .
arc-melted bulk samples. It was found that the sign
’
. . * . ’ . * * ’ . * . ’ * . * ’ * * . * 0 20 40 60 80 100
ATOMIC % Gd FIG. 4. Gd concentration dependence of magnetic moments (a) of Gd site and (h) ofNi site in Gd-Ni.
In Fig. 3, one can see the switching of the sign between 20 and 25 at. % Gd with the absolute value continuous.
In other words, the compensation
temperature
was close
the liquid
reversed when the concentration of (Co, Ni) or Gd increases except in the bulk Gd-Ni samples, which indicates the antiferromagnetic coupling between Gd and (Co, Ni) moments. As for Gd-Ni bulk sample, ferromagnetic interaction is prevalent. These findings implies a possible spin-glass-like sate at particular
1.71~~ and 0.604~,, respectively.
to
of the MCD due to Gd or (Co, Ni) moment is
nitrogen
concentrations caused by coexistence of ferro- ‘and antiferro-magnetic interactions.
REFERENCES
temperature in Gd-Co of this cd concentration, We found both Gd and Ni atomic magnetizations
1. E. A. Nesbitt, H. J. Williams, J. H. Wernick and
were positive in Gd-Ni bulk samples as shown in
R. C. Sherwood: J. Appl. Phys. 3 3. 1674 ( 1962).
Fig. 4, which suggests that ferromagnetic interaction
2. P. Chaudhari, J. J. Cuomo and R. J. Gambio;
appears
and
overcomes
antiferromagnetic
one.
Another remarkable feature in Fig. 4 is apparent instability of the film samples at x=1/6 (GdNi,). The
Appl. Phys. Lett. 2 2, 337 (1973). 3. T. Koide, T. Shidara, T. Miyahara and M. Yuri; Rev. Sci. Instrum. 66, 1923 (1995).
origin of small magnetic moments in some of them
4. Y. Kagoshima, T. Miyahara, S. Yamamoto, H.
can be considered as spin-glass-like alignment of
Kitamura, S. Muto, S. Y. Park and J. D. Wang;
magnetic moments, because the result of the Gd-Ni
Rev. Sci. Intsrum. 6 6, 1696 (1995).
bulk sample suggests the ferromagnetic interaction which is hidden in most film samples but might reappem when the antiferromagnetic interaction is much reduced. Therefore the above spin-glass-like
5. S. Muto, Y. Kagoshima and T. Miyahara: Rev. Sci. Instrum. 6 3, 1470 (1992). 6. C. R. Crowell, W. G. Spitzer, L. E. Howarth and E. E. LaBate; Phys. Rev. 127, 2006 (1962).