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The magnetic structures of planar interfaces
PHYSICA ELSEVIER
Physica B 198 (1994) 169-172
The magnetic structures of planar interfaces M.R. Fitzsimmons a'*, G.S. Smith a, R. Pynn a, M.A. Nastasi a, E.
Burkel b
aLos Alamos National Laboratory, Los Alamos NM, USA bSektion Physik, Lehrstuhl Peisl, Universitiit Miinchen, Miinchen, Germany
Abstract The magnetization density profiles of two planar interfaces are deduced from polarized neutron reflectometry measurements. The first interface is the phase boundary between a thin Fe film and a MgO substrate; the second, a [0 0 1] twist grain boundary between two thin films of Ni that were hot-pressed together. The atomic densities of both boundary regions were determined to be less than those of the neighboring thin films, while the average magnetic moments per atom were increased compared to the bulk films.
1. Introduction A promising new class of advanced materials with unique mechanical, thermal and electromagnetic properties not found in naturally occurring materials are being developed for use in specific industrial applications. Often, the properties of these materials derive from their unusual microstructures. For example, the magnetic hardness of materials with ultra-fine grain sizes (on the order of a few nanometers) can be much greater than conventional materials. This can be a consequence of the increase frequency of magnetic domain wall pinning due to the magnetoanisotropies of grain boundaries, which may comprise half the volume fraction of ultra-fine grained materials. In another example, multilayer structures composed of alternating ferromagnetic and non-ferromagnetic thin films materials can have extremely large magnetoresistances [ 1]. Large magnetoresistances have also
*Corresponding author.
been observed in granular materials composed of nanometer-sized Fe and Co particles in a Cu matrix [1]. All these examples have in common large volume fractions of interfaces, which spatially limit the microstructures of the materials. Since the magnetic properties of spatially limited systems may find use in the magnetic recording industry or as permanent magnets, considerable interest in the role(s) interfaces play in their magnetic properties has been generated. In particular, are their unusual magnetic properties due to the reduced dimensionality imposed upon crystalline grains or thin films by interfaces, or a consequence of the altered magnetic properties of the interfaces themselves? The two experiments reported in this paper are designed to answer the latter question.
2. The magnetization of the Fe-MgO phase boundary A thin film of Fe was grown onto the polished surface of a MgO single crystal, which was heated
0921-4526/94/$07.00 © 1994 ElsevierScience B.V. All rights reserved SSDI 0921-4526(93)E0530-T
170
M.R. Fitzsimmons et al./ Physica B 198 (1994) 169 172
to a temperature of 500 ° C. The F e - M g O sample was later characterized with grazing incidence X-ray diffraction at HASYLAB, Germany. From these measurements the Fe film was determined to be a single crystal with [1 1 0] MgOtl [1 00] Fe. Three satellite reflections about the Fe[-1 1 0] reflection were observed, which suggests the presence of a periodic array of dislocations with a mean spacing of 55/~. Since the satellite reflections were observed only when the penetration depth of the radiation was sufficient to traverse the 2404 thick Fe film, the periodic array is most likely located at the F e - M g O interface, or phase boundary. Like grain boundaries [2,3-], phase boundaries are expected to be regions whose atomic densities are less than the bulk. Peaks of diffuse scattering about the specular X-ray reflectivity of the sample occurring at constant values of perpendicular momentum transfer suggest that the F e - M g O phase boundary and Fe surface are rough, and that the roughnesses of each are correlated [-4]. Since no precautions were taken to preclude its formation, a native oxide is probably present on the Fe surface. The polarized neutron reflectivity of the sample was measured at the Los Alamos Neutron Scattering Facility (LANSCE). The sample reflectivity with the polarization state aligned parallel to the applied magnetic field (1.2 kG), R+(o), and antiparallel, R_(o), are shown in Fig. 1. Calculated reflectivity curves from a model structure, where the Fe film is made of three separate slabs, representing an Fe-oxide, Fe and phase boundary layers, plus the MgO substrate were fitted to the data. The nuclear and magnetic scattering length densities, ft, and tim respectively, of the three slabs, as well as their thicknesses, and the root-mean-square Gaussian roughness of each interface were refined to achieve the scattering length density profile shown in Fig. 2. This profile produced the solid reflectivity curves in Fig. 1. From this analysis, the native oxide surface layer was determined to be non-ferromagnetic and about 33/~ thick. The phase boundary was determined to be about 31 ,~ thick and had a density 82% that of the bulk film. After calculating the number density profile of Fe atoms in the sample from ft,, the average magnetic moment per atom could be determined as a function of depth
10 °
>,
R
i~ 10 -1
"6 N cr 10-2 cO
•
fitted R+ R
,>
a
fitted
10 -a
~
10 -4 N "r_~ 10 -s 0 13_ 10 .8
o
I
I
0.02
0.04
I
I
0.~16z [A04~)8
I
I
I
0.1
0.12
0.14
Fig. 1. Polarized neutron reflectivity of the Fe MgO sample (symbols) with reflectivity curves calculated from the model shown in Fig. 2.
b14 ,r-
_~12 c-
~o c-
~E _J
~
8 6
c-
~4 ¢J
co 2 E O
~o •
z
0
50
100
i .... i..... i.... 150
200
z [A]
250
300
350
Fig. 2. Neutron scattering length density profile for the two polarization states fitted to the data in Fig. 1.
in the sample. A plot of this profile near the phase boundary is shown in Fig. 3. A clear and significant enhancement, ( 8 _ 1)%, of magnetic moment is realized in the region of the phase boundary compared to that of the bulk film. When no enhancement at the phase boundary is allowed, the goodness-of-fit, as measured by X2, is increased by about 40%.
171
M.R. Fitzsimrnons et al. / Physica B 198 (1994) 169-172
,~= 2-1 E ~ 2.05 E 0 ~: 2 •
/ /
~ 1.95 -
~ E
1.9 ,-
0
1.85
.o •~
-
1.8
phase boundary region
1.75 1.7 250
._,_.1 270
290 z [A] 310
330
now in contact with air, was essentially zero, the first reflecting interface from the sample (and the interface over which the largest change of scattering density occurs) is that of the grain boundary interface. The use of Ni films with different scattering lengths facilitated the measurement of the number density profile for the grain boundary. This profile is needed in order to deduce the magnetization of the grain boundary. After its fabrication, the unpolarized (Fig. 4) and polarized neutron reflectivities (not shown) of the
350
Fig. 3. The averagemagneticmoment per atom in the Fe-MgO phase boundary region, showing an enhancement.
100 10-1 >.,
3. The magnetization of a [ 0 0 1 ] Ni twist grain boundary
A [00 1] Ni twist grain boundary was manufactured by hot-pressing together two Ni thin films. One of the films was epitaxially deposited onto a MgO single crystal substrate, while the second film was grown on a freshly cleaved NaCI single crystal. For the film grown on NaC1, a specially prepared Ni charge containing a large fraction of 62Ni was used. The fraction used was determined by the desire to have the neutron scattering length density of the second film be as close to zero as possible, which can be achieved with 62Ni, since this element has a negative scattering length. After the deposition of the films, their neutron reflectivities were measured, from which their scattering length density profiles were deduced. Next the two films, while still attached to their respective substrates, were sintered together. The sintering procedure involved two steps. First, the films were exposed to 1 atm CO at 40 ° C for 15 min. During this exposure, the Ni surfaces were reduced, i.e. the native oxide layer removed. Next, the CO atmosphere was evacuated and the temperature raised to 500 ° C for 2 h. During this period the grain boundary was formed. After cooling, the NaCI substrate was dissolved, yielding a Ni bicrystal on a MgO substrate. Since the scattering length density of the upper portion of the Ni bicrystal, i.e. the film that is
._> 0-2 ~ 10~ c 0
z
104 10.2 10.6
I
0
I
0.04
I
0.08 0.12 Qz [AI]
I
0.16
0.2
Fig. 4. Unpolarized neutron reflectivityof a Ni bicrystal containing a [001] twist grain boundary.
0,
0.08
~'.~c~0.06 a 0.04 .O
"t
z 0.02
0
550
I
i
!
i
600
z65[iDA]
700
750
Fig. 5. Number densityas a functionof distance across the grain boundary interface.
172
M.R. Fitzsimmons et al,/ Physica B 198 (1994) 169-172
4. Conclusion
1.4 ,:=L 1.2 E 0
1
0.8
E E 0
0.6
k~
0.4
gram boundary interface
tt~
0.2 0 55O
I
I
I
I
600
650
700
750
z [A] Fig. 6. The average magnetic moment per atom in the grain boundary region, showing a significant enhancement. Dashed lines indicate uncertainties.
Ni bicrystal were measured. Since the scattering length density profiles of the N i - M g O and Ni air interfaces (which in light of the fact that the scattering length of the Ni film on NaC1 was close to zero, can be assumed to be the same as the original Ni-NaC! interfaces) were deduced from earlier reflection measurements, they were held fixed during the refinement of the bicrystal model. In other words, only the number density profile in the close vicinity of the grain boundary (Fig. 5) was refined to achieve the fitted reflectivity curve shown in Fig. 4. From this analysis the minimum density of the grain boundary was determined to be about 65% of that for the Ni films. The width of the boundary half-way between its minimum density and the Ni film density is about 50/~. Having determined the number density profile of the grain boundary, its magnetization was deduced by refining only the magnetic scattering length profile of the grain boundary to the polarized neutron reflectivity data taken from the bicrystal (not shown). The resulting average magnetic moment per atom in the grain boundary region is shown in Fig. 6. A strong enhancement of the magnetic moment is observed at the grain boundary compared to the value of 0.6/~B for bulk Ni. When averaged over the grain boundary region the magnetic moment per atom in the interface is (0.8 + 0.1)/~B, which is an enhancement of about (33 ___ 17)% compared to the bulk.
Enhancements in the magnetic moments of atoms near the F e - M g O phase boundary (8 _+ 1%) and [00 1] twist grain boundary in a Ni bicrystal (33 + 17%) have been deduced from the polarized neutron reflectivities of these interfaces. The increases are accompanied by corresponding decreases in the number densities of the interracial regions (18 % and 35 % reductions, respectively, for the phase and grain boundaries). The inverse relationship between magnetic moment strength and number density is consistent with the hypothesis that the magnetic moments of other spatially limited systems, particularly clusters, increase as a result of their decreased densities [5]. Based upon the two studies presented here, evidence exists for the conclusion that magnetizations of interfaces in transition metal magnetic systems are increased compared to the bulk. The results of these analyses will be tested further by co-refining neutron and X-ray reflection data taken from both samples. This work is currently underway.
Acknowledgements This study was supported by the U.S. Department of Energy under contract W-7405-ENG-36 with the University of California, and the German Federal Ministry for Research and Technology (BMFT) under the Contract No. 03PE1LMU/2. The Manuel Lujan Jr. Neutron Scattering Center is a national user facility funded by the U.S. Department of Energy Office of Basic Energy Science. The use of the D4 and W1 beam lines at HASYLAB, Germany, and the equipment of Riso National Laboratories, Denmark, are acknowledged. The support of Professor J. Peisl is gratefully acknowledged.
References [1] [2] [3] [4] [5]
P.M. Levy,Science256 (1992)972. M.R.Fitzsimmonsand S.L.Sass,Acta Metall. 36(1988)3103. M.R.Fitzsimmonsand S.L Sass,Acta Metall. 37(1989)1009. R. Pynn, Phys. Rev. B 45 (1992) 602. V.L Moruzzi, Phys. Rev. Lett. 57 (1986) 2211.
Physica B 198 (1994) 169-172
The magnetic structures of planar interfaces M.R. Fitzsimmons a'*, G.S. Smith a, R. Pynn a, M.A. Nastasi a, E.
Burkel b
aLos Alamos National Laboratory, Los Alamos NM, USA bSektion Physik, Lehrstuhl Peisl, Universitiit Miinchen, Miinchen, Germany
Abstract The magnetization density profiles of two planar interfaces are deduced from polarized neutron reflectometry measurements. The first interface is the phase boundary between a thin Fe film and a MgO substrate; the second, a [0 0 1] twist grain boundary between two thin films of Ni that were hot-pressed together. The atomic densities of both boundary regions were determined to be less than those of the neighboring thin films, while the average magnetic moments per atom were increased compared to the bulk films.
1. Introduction A promising new class of advanced materials with unique mechanical, thermal and electromagnetic properties not found in naturally occurring materials are being developed for use in specific industrial applications. Often, the properties of these materials derive from their unusual microstructures. For example, the magnetic hardness of materials with ultra-fine grain sizes (on the order of a few nanometers) can be much greater than conventional materials. This can be a consequence of the increase frequency of magnetic domain wall pinning due to the magnetoanisotropies of grain boundaries, which may comprise half the volume fraction of ultra-fine grained materials. In another example, multilayer structures composed of alternating ferromagnetic and non-ferromagnetic thin films materials can have extremely large magnetoresistances [ 1]. Large magnetoresistances have also
*Corresponding author.
been observed in granular materials composed of nanometer-sized Fe and Co particles in a Cu matrix [1]. All these examples have in common large volume fractions of interfaces, which spatially limit the microstructures of the materials. Since the magnetic properties of spatially limited systems may find use in the magnetic recording industry or as permanent magnets, considerable interest in the role(s) interfaces play in their magnetic properties has been generated. In particular, are their unusual magnetic properties due to the reduced dimensionality imposed upon crystalline grains or thin films by interfaces, or a consequence of the altered magnetic properties of the interfaces themselves? The two experiments reported in this paper are designed to answer the latter question.
2. The magnetization of the Fe-MgO phase boundary A thin film of Fe was grown onto the polished surface of a MgO single crystal, which was heated
0921-4526/94/$07.00 © 1994 ElsevierScience B.V. All rights reserved SSDI 0921-4526(93)E0530-T
170
M.R. Fitzsimmons et al./ Physica B 198 (1994) 169 172
to a temperature of 500 ° C. The F e - M g O sample was later characterized with grazing incidence X-ray diffraction at HASYLAB, Germany. From these measurements the Fe film was determined to be a single crystal with [1 1 0] MgOtl [1 00] Fe. Three satellite reflections about the Fe[-1 1 0] reflection were observed, which suggests the presence of a periodic array of dislocations with a mean spacing of 55/~. Since the satellite reflections were observed only when the penetration depth of the radiation was sufficient to traverse the 2404 thick Fe film, the periodic array is most likely located at the F e - M g O interface, or phase boundary. Like grain boundaries [2,3-], phase boundaries are expected to be regions whose atomic densities are less than the bulk. Peaks of diffuse scattering about the specular X-ray reflectivity of the sample occurring at constant values of perpendicular momentum transfer suggest that the F e - M g O phase boundary and Fe surface are rough, and that the roughnesses of each are correlated [-4]. Since no precautions were taken to preclude its formation, a native oxide is probably present on the Fe surface. The polarized neutron reflectivity of the sample was measured at the Los Alamos Neutron Scattering Facility (LANSCE). The sample reflectivity with the polarization state aligned parallel to the applied magnetic field (1.2 kG), R+(o), and antiparallel, R_(o), are shown in Fig. 1. Calculated reflectivity curves from a model structure, where the Fe film is made of three separate slabs, representing an Fe-oxide, Fe and phase boundary layers, plus the MgO substrate were fitted to the data. The nuclear and magnetic scattering length densities, ft, and tim respectively, of the three slabs, as well as their thicknesses, and the root-mean-square Gaussian roughness of each interface were refined to achieve the scattering length density profile shown in Fig. 2. This profile produced the solid reflectivity curves in Fig. 1. From this analysis, the native oxide surface layer was determined to be non-ferromagnetic and about 33/~ thick. The phase boundary was determined to be about 31 ,~ thick and had a density 82% that of the bulk film. After calculating the number density profile of Fe atoms in the sample from ft,, the average magnetic moment per atom could be determined as a function of depth
10 °
>,
R
i~ 10 -1
"6 N cr 10-2 cO
•
fitted R+ R
,>
a
fitted
10 -a
~
10 -4 N "r_~ 10 -s 0 13_ 10 .8
o
I
I
0.02
0.04
I
I
0.~16z [A04~)8
I
I
I
0.1
0.12
0.14
Fig. 1. Polarized neutron reflectivity of the Fe MgO sample (symbols) with reflectivity curves calculated from the model shown in Fig. 2.
b14 ,r-
_~12 c-
~o c-
~E _J
~
8 6
c-
~4 ¢J
co 2 E O
~o •
z
0
50
100
i .... i..... i.... 150
200
z [A]
250
300
350
Fig. 2. Neutron scattering length density profile for the two polarization states fitted to the data in Fig. 1.
in the sample. A plot of this profile near the phase boundary is shown in Fig. 3. A clear and significant enhancement, ( 8 _ 1)%, of magnetic moment is realized in the region of the phase boundary compared to that of the bulk film. When no enhancement at the phase boundary is allowed, the goodness-of-fit, as measured by X2, is increased by about 40%.
171
M.R. Fitzsimrnons et al. / Physica B 198 (1994) 169-172
,~= 2-1 E ~ 2.05 E 0 ~: 2 •
/ /
~ 1.95 -
~ E
1.9 ,-
0
1.85
.o •~
-
1.8
phase boundary region
1.75 1.7 250
._,_.1 270
290 z [A] 310
330
now in contact with air, was essentially zero, the first reflecting interface from the sample (and the interface over which the largest change of scattering density occurs) is that of the grain boundary interface. The use of Ni films with different scattering lengths facilitated the measurement of the number density profile for the grain boundary. This profile is needed in order to deduce the magnetization of the grain boundary. After its fabrication, the unpolarized (Fig. 4) and polarized neutron reflectivities (not shown) of the
350
Fig. 3. The averagemagneticmoment per atom in the Fe-MgO phase boundary region, showing an enhancement.
100 10-1 >.,
3. The magnetization of a [ 0 0 1 ] Ni twist grain boundary
A [00 1] Ni twist grain boundary was manufactured by hot-pressing together two Ni thin films. One of the films was epitaxially deposited onto a MgO single crystal substrate, while the second film was grown on a freshly cleaved NaCI single crystal. For the film grown on NaC1, a specially prepared Ni charge containing a large fraction of 62Ni was used. The fraction used was determined by the desire to have the neutron scattering length density of the second film be as close to zero as possible, which can be achieved with 62Ni, since this element has a negative scattering length. After the deposition of the films, their neutron reflectivities were measured, from which their scattering length density profiles were deduced. Next the two films, while still attached to their respective substrates, were sintered together. The sintering procedure involved two steps. First, the films were exposed to 1 atm CO at 40 ° C for 15 min. During this exposure, the Ni surfaces were reduced, i.e. the native oxide layer removed. Next, the CO atmosphere was evacuated and the temperature raised to 500 ° C for 2 h. During this period the grain boundary was formed. After cooling, the NaCI substrate was dissolved, yielding a Ni bicrystal on a MgO substrate. Since the scattering length density of the upper portion of the Ni bicrystal, i.e. the film that is
._> 0-2 ~ 10~ c 0
z
104 10.2 10.6
I
0
I
0.04
I
0.08 0.12 Qz [AI]
I
0.16
0.2
Fig. 4. Unpolarized neutron reflectivityof a Ni bicrystal containing a [001] twist grain boundary.
0,
0.08
~'.~c~0.06 a 0.04 .O
"t
z 0.02
0
550
I
i
!
i
600
z65[iDA]
700
750
Fig. 5. Number densityas a functionof distance across the grain boundary interface.
172
M.R. Fitzsimmons et al,/ Physica B 198 (1994) 169-172
4. Conclusion
1.4 ,:=L 1.2 E 0
1
0.8
E E 0
0.6
k~
0.4
gram boundary interface
tt~
0.2 0 55O
I
I
I
I
600
650
700
750
z [A] Fig. 6. The average magnetic moment per atom in the grain boundary region, showing a significant enhancement. Dashed lines indicate uncertainties.
Ni bicrystal were measured. Since the scattering length density profiles of the N i - M g O and Ni air interfaces (which in light of the fact that the scattering length of the Ni film on NaC1 was close to zero, can be assumed to be the same as the original Ni-NaC! interfaces) were deduced from earlier reflection measurements, they were held fixed during the refinement of the bicrystal model. In other words, only the number density profile in the close vicinity of the grain boundary (Fig. 5) was refined to achieve the fitted reflectivity curve shown in Fig. 4. From this analysis the minimum density of the grain boundary was determined to be about 65% of that for the Ni films. The width of the boundary half-way between its minimum density and the Ni film density is about 50/~. Having determined the number density profile of the grain boundary, its magnetization was deduced by refining only the magnetic scattering length profile of the grain boundary to the polarized neutron reflectivity data taken from the bicrystal (not shown). The resulting average magnetic moment per atom in the grain boundary region is shown in Fig. 6. A strong enhancement of the magnetic moment is observed at the grain boundary compared to the value of 0.6/~B for bulk Ni. When averaged over the grain boundary region the magnetic moment per atom in the interface is (0.8 + 0.1)/~B, which is an enhancement of about (33 ___ 17)% compared to the bulk.
Enhancements in the magnetic moments of atoms near the F e - M g O phase boundary (8 _+ 1%) and [00 1] twist grain boundary in a Ni bicrystal (33 + 17%) have been deduced from the polarized neutron reflectivities of these interfaces. The increases are accompanied by corresponding decreases in the number densities of the interracial regions (18 % and 35 % reductions, respectively, for the phase and grain boundaries). The inverse relationship between magnetic moment strength and number density is consistent with the hypothesis that the magnetic moments of other spatially limited systems, particularly clusters, increase as a result of their decreased densities [5]. Based upon the two studies presented here, evidence exists for the conclusion that magnetizations of interfaces in transition metal magnetic systems are increased compared to the bulk. The results of these analyses will be tested further by co-refining neutron and X-ray reflection data taken from both samples. This work is currently underway.
Acknowledgements This study was supported by the U.S. Department of Energy under contract W-7405-ENG-36 with the University of California, and the German Federal Ministry for Research and Technology (BMFT) under the Contract No. 03PE1LMU/2. The Manuel Lujan Jr. Neutron Scattering Center is a national user facility funded by the U.S. Department of Energy Office of Basic Energy Science. The use of the D4 and W1 beam lines at HASYLAB, Germany, and the equipment of Riso National Laboratories, Denmark, are acknowledged. The support of Professor J. Peisl is gratefully acknowledged.
References [1] [2] [3] [4] [5]
P.M. Levy,Science256 (1992)972. M.R.Fitzsimmonsand S.L.Sass,Acta Metall. 36(1988)3103. M.R.Fitzsimmonsand S.L Sass,Acta Metall. 37(1989)1009. R. Pynn, Phys. Rev. B 45 (1992) 602. V.L Moruzzi, Phys. Rev. Lett. 57 (1986) 2211.