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Evidence for perpendicular magnetic anisotropy in Fe(110) epitaxial films in the monolayer range on gold
Thi~ Solid Films, 175 (1989) 311-316
311
EVIDENCE FOR PERPENDICULAR MAGNETIC ANISOTROPY IN Fe(110) E P I T A X I A L F I L M S IN T H E M O N O L A Y E R R A N G E O N G O L D G. L U G E R T A N D G. B A Y R E U T H E R
lnstitut fiir Angewandte Physik, Universitdt Regensburg, Universitiitsstrafle 31, 8400 Regensburg (F.R.G.)
We succeeded in growing ultrathin Fe(ll0) films in ultrahigh vacuum sandwiched between Au(lll), which show ferromagnetic behaviour even below monolayer coverage. For the first time, low-temperature magnetization measurements on such A u - F e - A u sandwiches reveal a switching of the easy axis from the film plane to the film normal in iron films thinner than 3 monolayers (ML). w e interpret this as an interface effect. The usually dominating shape anisotropy is overcompensated by a magnetic interface anisotropy at smaller iron coverages. The effective magnetic anisotropy field at 10K is proportional to the itiverse iron thickness down to 2 ML. The related interface anisotropy'field is determined to 40 kOe per single Fe(110)-Au interface.
1. INTRODUCTION
The large interest in magnetic properties of thin 3d-metal films during the last years, both theoretical and experimental, was mainly :focussed on the question of whether the surface and the monolayer show ferromagnetic order and how t h e magnetic moment per atom is changed compared to the bulk moment. Equally interesting seems to be the problem of the direction of magnetization in surfaces and ultrathin films, i.e. magnetic anisotropies. Due to the reduced local symmetry at interfaces one expects surface or interface anisotroi~iesl-4, which in certain cases should dominate a'4 over the volume type anisotropies. However, not only the preliminary results of ab initio band calculations 3'4, also the experimental results on Fe(100) with different methods give a controversial picture52.~°: It is the aim of this paper to study magnetic anisotropies in ultrathin films of iron by magnetometry, where the proportion of ferromagnetic atoms in a broken symmetry configuration is enlarged o r even dominant over the o n e w i t h cub!c symmetry. Besides the fundamental interest, magnetic surface anisotropies (MSA) could be the key for the design of magnetic devices in possible future applications 11. x2. * Paper presentedat the 2nd International Symposiumon Trends and NewApplicationsin Thin leiims. Regensburg, F.R.G., February 27-March 3, 1989. ~ 0040-6090/89 '$3.50
© ElsevierSequoia/Prir~tcdin The Netherlands
312
G. LUGERT, G. BAYREUTHER
2. EXPERIMENTAL DETAILS
2.i. Sample preparation Ultrathin epitaxial iron films were grown on gold films by vapor deposition from beryllium oxide Knudsen cells. Residual gas pressure during film preparation was always less than 5 x 10- to mbar. In order to have a symmetric structure and to protect the iron films from oxidation they were covered by gold. The gold substrate layers (20 60 nm) have been prepared onto extremely smooth quartz plates (r.m.s. roughness _+0.4 nm) after an outgassing procedure of 10 h at 650 K in UHV. Only by this way do we get atomically flat gold films as verified by scanning tunneling microscopy (STM) t 3. The substrate temperatures were 330 K for gold and iron. The evaporation rates were 2 monolayers (ML) rain- 1 and 1 M L min- t in the case of gold and iron, respectively. The protective gold layer was 5-60 nm thick. By transmission electron microscopy [TEM) and electron diffraction we find the lateral size of the gold crystallites to be larger than 400 nm with the (I 11) plane parallel to the surface. The iron films show a (110) orientation parallel to the Au(111) face (d t to (Fe) = 0.202 rim). Similar results were obtained in the case of iron on silver 1,-18. The structural, topological and magnetic properties of the A u - F e - A u sandwiches were very sensitive to the Yacuum conditions, quartz substrate treatment, substrate temperatures and evaporation rates during preparation. F o r example, without the above mentioned outgassing of the quartz substrate prior to the gold preparation we find rough surfaces by STM t3, smaller crystallites by T E M and a largely reduced magnetization for a 5 ML iron film at room temperature compared to iron films on atomically flat gold films t3. Only the latter will be used for the following discussion of magnetic properties. We have used quartz oscillation monitors for in siru rate control and thickness determination. The calibration was done by a direct mass determination and by energy dispersive X-ray fluorescence analysis. By irradiation of the sample through a GaAs secondary target rather than by direct excitation we were able tO reduce the background signal to a value corresponding to a coverage of 0.4 M L Fe(110). This allows us to determine the thickness of our films with an error smaller than 2~o for 1 M L of iron. 2.2. Magnetometry The magnetic measurements were done with a superconducting quantum interference device (SQUID) magnetometer with applied fields up to 50 kOe in the temperature range from 4 to 400 K. Magnetization (M) vs. applied field (H) loops have been measured for several temperatures on samples with an iron thickness of 0.5 ~< tFe ~< 2 3 M L and on a number of "bulk" samples for comparison (tvo > 400 ML). The magnetic field was applied parallel (1[) and perpendicular (_1_)to the film plane. We have also determined the temperature dependence of the saturation magnetization Ms, of the remanent magnetization MR(T) and of the magnetization in constant applied field M(T)t 9. When anisotropy is investigated by M(H) measurements in ultrathin films one should rule out the effect of thermal spin excitations and surface spin waves at elevated temperatures. Additionally, size
PERPENDICULAR MAGNETIC ANISOTROPY IN
Fe(110) EPITAXIAL FILMS
313
effects2°'21, e.g. the thickness dependence of the Curie temperature Tc (t) must be taken into account. This has not been considered sufficiently in previous experimental investigations of magnetic anisotropies in ultrathin ferromagnetic films 5-7'22 where the measurements were done at temperatures not very small compared to T0 In order to eliminate the influence of thermal excitations, we only discuss experiments at 10 K in the following sections. 3. RESULTS AND DISCUSSION The magnetization loops parallel and perpendicular to the film plane depend in a characteristic way on the iron film thickness tF,. In particular we find: (1) ferromagnetic order at 10K down to an iron coverage of 0.5 ML as indicated by strong anisotropy and large remanence in the easy axis (e.a.), comparable to Fig. l(b), (2) a switching of the e.a. from the film plane (11,see Fig. l(a)) to a perpendicular orientation (±, see Fig. l(b)) for tFe < 3 ML, (3) in films with an in plane e.a. (1[, t > 3 ML) a decrease of the saturation field in the hard axis (h.a.) Hsat, n.a. wtth • decreasing tEe, (4) in films with perpendicular e.a. (1, t < 3 ML) an increase of H~t" with decreasing tee. The first result (1) implies a growth of iron on gold in large monolayer patches. This is shown by careful analysis of hysteresis loops in samples with submonolayer iron coverage 23.
'
5ME
Fe/Au
l
~
,L ./H=
. . . . .
j
_.ji~
*
1
~
T=IOK
- 20
(a)
~
Ih.o.~
:
:~= (
kOel
(e.o.) H z,
T=10K
S -20
20
H
2 ML Fe/Au
(b)
2O [kOe)
Fig. 1. Magnetization loops M(H) of epitaxial Au(l11)-Fe(110)-Au sandwiches, magnetization is normalized to saturation M(H)/Ms,with the applied field H parallel (l[) and perpendicular ( / ) to the film plane: (a) 5 ML iron characteristic for all thicker iron films with e.a. in the film plane and h.a. perpendicular, (b) 2 ML iron typical for tv~ < 3 ML with h.a. in the plane and e.a. perpendicular.
In Fig. 1 we show the normalized field dependent magnetization M(H)/M s at 10 K. Qualitatively, the observed dependence of M(HPr) and M(H ±) in Fig. l(a) represents the expected thin film behaviour in an almost ideal form, dominated by the shape anisotropy. But, there is a remarkable difference compared to "bulk" iron films where Hs~at amounts to about 22 kOe ~ 4riMs. In the case of 5 M L Fe-Au as~at = 10-t-2 kOe (Fig. l(a)). Obviously some additional anisotropy relative to the film normal must be present. The results (2) and (3) suggest the presence of a MSA in competition with the shape anisotropy. This is described by an effective anisotropy field HT,ff with respect
314
G. LUGERT, G. BAYREUTHER
to the surface normal and can be written in the form H ~ ff =
4nM~ + H~ + N1---(H~I)+H~ 2))
(1)
where 4re M~ is the shape anisotropy field, Hv a usually small total volume anisotropy field, N the number of atomic layers and H~° represent the surface anisotropy fields at both interfaces of the magnetic layer according to the phenomenological model of NOel 1. H~ ff is identified 2.'25 with the full saturation field in the h.a. H sh.,. a t , and is by convention positive when the e.a. is in the plane (Fig. l(a)) or negative when perpendicular (Fig. l(b)). The data from our Fe(110)-Au sandwiches fit well a linear dependence of H~ff on 1/N, as shown in Fig. 2. Note that relation (1) holds down to a film thickness as small as 2 ML. F r o m the slope in Fig. 2 we get the sum of the anisotropy fields at the two interfaces. Because of symmetry and the small iron film thickness H~) _- --sH~2~ and we therefore get an interface anisotropy field of H F*-A~ = - 40 +_2 kOe for the single F e ( l l 0 ) - A u interface. This corresponds to an anisotropy constant of Ks ve-Au -- - 0.69 + 0.1 erg c m - 2 10ML t,ML
2ML
20 - ~ I0 "k
°l
IML
T = 10K e.a. II
{
e.a
1
°:E~ - 1 0
-20 0.5
I
tIN
Fig. 2. Effective anisotropy field H~~f relative to the film normal for A u - F e - A u sandwiches n u m b e r of iron monolayers 1 / N , d 11 o - 0.202 nm.
vs.
inverse
This is different from earlier results obtained for the Fe(110)-Ag(111) interface, where it could be shown by magnetometry and conversion electron M6ssbauer spectroscopy (CEMS), that the interface anisotropy is small compared to the shape anisotropy, i.e. [H~e-Ag[<<4riMs ~ 22 kOe 14-16. There are only few experiments in the literature related to MSA at iron interfaces. Most of them have been done on Fe(100) interfaces 5-1°. Due to the different orientation they cannot be directly compared to our results. Moreover, they find a variation of Ks with tee measured at room temperature and disagree even in the sign of Ks on equal interfaces ~-8. MSA at Fe(110) interfaces has been investigated by Elmers and G r a d m a n n 22. Their iron films were grown on W(100) and covered by copper, silver or gold. Due to this asymmetry only the sum o f the anisotropy fields related to the two different interfaces can be obtained. The'result is t h a t the difference of Hs between Fe~-Agand Fe Au is zero within the experimental error, i.e. small compared to the positive total interface anisotropy field, .e.g. H~ e-w + H~re-A" ~ + 60__ 10 kOe. In contrast, we observe a different anisotropy at F e - A u a n d F e - A g interfaces w i t h H F*-A"- H~ "-Ag .~ - 40 kOe. T h e r e a s o n for this discrepancy is not clear at the moment. It could be explained with a different microscopic structure of the films due
PERPENDICULAR MAGNETIC ANISOTROPY IN
Fe(110) EPITAXIAL FILMS
315
to the different substrate material. The strain of the iron lattice will not be equal at the Fe-W and the Fe-Au or Fe-Ag interface according to the different lattice mismatch. This is especially true in the case of asymmetric interfaces in the iron films investigated by Elmers and Gradmann 22. Elastic strain in the film is partially released by the formation of misfit dislocations26, depending on details of growth conditions (growth rate, temperature and substrate surface). According to the density and extension of disclocations, lattice deformations will inevitably remain in the vicinity of the interface which then substantially contribute to the magnetic interface anisotropy. 4. CONCLUSION For the first time we have observed an out of plane magnetic anisotropy in ultrathin epitaxial Fe(ll0) films between Au(lll) which varies linearly with the inverse film thickness down to 2 ML. This behaviour shows the presence ofa MSA independent of film thickness 24. It can only be observed if the iron films are grown under very clean conditions on atomically flat gold substrates ~a and if the measurements are performed at low temperature 19'2a. This is also necessary when comparing experimental results to recent band calculations of MSA4 which always refer to the ground state. These theoretical studies look promising, but they could not give decisive results for a realistic film system up to now. In particular, the different anisotropy of Fe(110) on gold, silver and tungsten cannot be explained by available ab initio band calculations. If a more precise determination of anisotropy energies will become possible by more advanced methods realistic interface structures should be used in the calculations including lattice deformations close to the interface. It seems also necessary to carry out more systematic experimental studies in order to distinguish the role of elastic strain and misfit dislocations for magnetic surface anisotropies from the pure breaking of translational symmetry. ACKNOWLEDGMENTS
The authors wish to thank D. Renard and C. Marlirres from the Institut d'Optique, Universit6 Paris Sud, Orsay, France, for their fruitful collaboration on some of the TEM and RHEED experiments and helpful discussions; K. Bauer, F. Schneider, G. Reiss and J. Vancea from the Institut f/ir Angewandte Physik, Universit/it Regensburg, for the STM measurements. REFERENCES 1 L. N~el, Compt. Rend. Acad. Sci., 237 (1953) 1468; J. Phys. Radium, 15 (1954) 225. 2 G.T. Rado, Phys. Rev.,52(1982) l178;withG. T. RadoandLuZhang, Phys. Rev. B, 33(1986) 3 4 5 6
5080. J.G. Gay and R. Richter, Phys. Rev. Lett., 56 (1986) 2728. J.G. Gay and R. Richter, J. Appl. Phys., 61 (1987) 3362. S.D. Bader and E. R. Moog, J. Appl. Phys., 61 (1987) 3729. B. Heindrich, K. B. Urquhart, A. S. Arrot, J. F. Cochran, K. Myrtle and S. T. Purcell, Phys. Rev. Lett., 59 (1987) 1756.
316 7 8 9 10 [1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
G. LUGERT, G. BAYREUTHER
B. Heinrich, K. B. Urquhart. J. R. Dutcher. S. T. Purcell, J. F. Cochran and A. S. Arrott, J. Appl. Phys., 63 (1988) 3863. B.T. Jonker. K. H. Walter, E, Kisker, G. A. Prinz and C. Carbone. Phys. Rev. Lett., 57 (1986) 142. J.J. Krebs. B. T. Jonker and G. A. Prinz. J. Appl. Phys., 63 (1988) 3467. M. Stampanoni, A. Vaterlaus. M. Aeschlimann and F. Meier, Phys. Rev. Lett., 59 (19871 2483. K. R611. J. Vac. Sci. Technol. A. 4 (1986) 14. R.M. White, Science, 229 (1985) 11. G. Lugert. G. Reiss. K. Bauer, F. Schneider_ J. Vancea and H. Hoffmann, to be published. G. Bayreuther and G. Lugert, J. Magn. Mater., 35 (1983] 50. G. Bayreuther. J. Magn. Magn. Mater.. 38 (1983) 273. G. Lugert and G. Bayreuther, Phys. Rev. B. 38 (1988) 11068. S. Norman. T. Anderson, G. Pet6 and S. SogyL Thin Solid Films. 77 (1981) 359. H.C. SnymanandG. H. Olsen. J. Appl. Phys, 44(1973) 889. G. Lugert and G. Bayreuther. to be published. U. Gradmann, Appl. Phys., 3 (1974) 161. U. Gradmann, J. Magn. Magn. Mater.. 6 ~1977) 173. H.J. Elmers and U. Gradmann. J. Appl. Phys., 64 (1988) 5328. G. Lugert and G. Bayreuther, to be published. U. Gradman, J. Magn. Magn. Mater.. 54 57 (1986) 733. R. Koepke and Bergraann, Z. Physik B, 21 (1975) 185. U. Gradmann and M. Przybylski. in R. F. C. Farrow. S. S. P. Parkin. P. J. Dobson, J. H. Neove and A. S. Arrot (eds.), Thin-Film Growth Techniques for Low Dimensional Structures, Plenum, New York. I987. p. 261
311
EVIDENCE FOR PERPENDICULAR MAGNETIC ANISOTROPY IN Fe(110) E P I T A X I A L F I L M S IN T H E M O N O L A Y E R R A N G E O N G O L D G. L U G E R T A N D G. B A Y R E U T H E R
lnstitut fiir Angewandte Physik, Universitdt Regensburg, Universitiitsstrafle 31, 8400 Regensburg (F.R.G.)
We succeeded in growing ultrathin Fe(ll0) films in ultrahigh vacuum sandwiched between Au(lll), which show ferromagnetic behaviour even below monolayer coverage. For the first time, low-temperature magnetization measurements on such A u - F e - A u sandwiches reveal a switching of the easy axis from the film plane to the film normal in iron films thinner than 3 monolayers (ML). w e interpret this as an interface effect. The usually dominating shape anisotropy is overcompensated by a magnetic interface anisotropy at smaller iron coverages. The effective magnetic anisotropy field at 10K is proportional to the itiverse iron thickness down to 2 ML. The related interface anisotropy'field is determined to 40 kOe per single Fe(110)-Au interface.
1. INTRODUCTION
The large interest in magnetic properties of thin 3d-metal films during the last years, both theoretical and experimental, was mainly :focussed on the question of whether the surface and the monolayer show ferromagnetic order and how t h e magnetic moment per atom is changed compared to the bulk moment. Equally interesting seems to be the problem of the direction of magnetization in surfaces and ultrathin films, i.e. magnetic anisotropies. Due to the reduced local symmetry at interfaces one expects surface or interface anisotroi~iesl-4, which in certain cases should dominate a'4 over the volume type anisotropies. However, not only the preliminary results of ab initio band calculations 3'4, also the experimental results on Fe(100) with different methods give a controversial picture52.~°: It is the aim of this paper to study magnetic anisotropies in ultrathin films of iron by magnetometry, where the proportion of ferromagnetic atoms in a broken symmetry configuration is enlarged o r even dominant over the o n e w i t h cub!c symmetry. Besides the fundamental interest, magnetic surface anisotropies (MSA) could be the key for the design of magnetic devices in possible future applications 11. x2. * Paper presentedat the 2nd International Symposiumon Trends and NewApplicationsin Thin leiims. Regensburg, F.R.G., February 27-March 3, 1989. ~ 0040-6090/89 '$3.50
© ElsevierSequoia/Prir~tcdin The Netherlands
312
G. LUGERT, G. BAYREUTHER
2. EXPERIMENTAL DETAILS
2.i. Sample preparation Ultrathin epitaxial iron films were grown on gold films by vapor deposition from beryllium oxide Knudsen cells. Residual gas pressure during film preparation was always less than 5 x 10- to mbar. In order to have a symmetric structure and to protect the iron films from oxidation they were covered by gold. The gold substrate layers (20 60 nm) have been prepared onto extremely smooth quartz plates (r.m.s. roughness _+0.4 nm) after an outgassing procedure of 10 h at 650 K in UHV. Only by this way do we get atomically flat gold films as verified by scanning tunneling microscopy (STM) t 3. The substrate temperatures were 330 K for gold and iron. The evaporation rates were 2 monolayers (ML) rain- 1 and 1 M L min- t in the case of gold and iron, respectively. The protective gold layer was 5-60 nm thick. By transmission electron microscopy [TEM) and electron diffraction we find the lateral size of the gold crystallites to be larger than 400 nm with the (I 11) plane parallel to the surface. The iron films show a (110) orientation parallel to the Au(111) face (d t to (Fe) = 0.202 rim). Similar results were obtained in the case of iron on silver 1,-18. The structural, topological and magnetic properties of the A u - F e - A u sandwiches were very sensitive to the Yacuum conditions, quartz substrate treatment, substrate temperatures and evaporation rates during preparation. F o r example, without the above mentioned outgassing of the quartz substrate prior to the gold preparation we find rough surfaces by STM t3, smaller crystallites by T E M and a largely reduced magnetization for a 5 ML iron film at room temperature compared to iron films on atomically flat gold films t3. Only the latter will be used for the following discussion of magnetic properties. We have used quartz oscillation monitors for in siru rate control and thickness determination. The calibration was done by a direct mass determination and by energy dispersive X-ray fluorescence analysis. By irradiation of the sample through a GaAs secondary target rather than by direct excitation we were able tO reduce the background signal to a value corresponding to a coverage of 0.4 M L Fe(110). This allows us to determine the thickness of our films with an error smaller than 2~o for 1 M L of iron. 2.2. Magnetometry The magnetic measurements were done with a superconducting quantum interference device (SQUID) magnetometer with applied fields up to 50 kOe in the temperature range from 4 to 400 K. Magnetization (M) vs. applied field (H) loops have been measured for several temperatures on samples with an iron thickness of 0.5 ~< tFe ~< 2 3 M L and on a number of "bulk" samples for comparison (tvo > 400 ML). The magnetic field was applied parallel (1[) and perpendicular (_1_)to the film plane. We have also determined the temperature dependence of the saturation magnetization Ms, of the remanent magnetization MR(T) and of the magnetization in constant applied field M(T)t 9. When anisotropy is investigated by M(H) measurements in ultrathin films one should rule out the effect of thermal spin excitations and surface spin waves at elevated temperatures. Additionally, size
PERPENDICULAR MAGNETIC ANISOTROPY IN
Fe(110) EPITAXIAL FILMS
313
effects2°'21, e.g. the thickness dependence of the Curie temperature Tc (t) must be taken into account. This has not been considered sufficiently in previous experimental investigations of magnetic anisotropies in ultrathin ferromagnetic films 5-7'22 where the measurements were done at temperatures not very small compared to T0 In order to eliminate the influence of thermal excitations, we only discuss experiments at 10 K in the following sections. 3. RESULTS AND DISCUSSION The magnetization loops parallel and perpendicular to the film plane depend in a characteristic way on the iron film thickness tF,. In particular we find: (1) ferromagnetic order at 10K down to an iron coverage of 0.5 ML as indicated by strong anisotropy and large remanence in the easy axis (e.a.), comparable to Fig. l(b), (2) a switching of the e.a. from the film plane (11,see Fig. l(a)) to a perpendicular orientation (±, see Fig. l(b)) for tFe < 3 ML, (3) in films with an in plane e.a. (1[, t > 3 ML) a decrease of the saturation field in the hard axis (h.a.) Hsat, n.a. wtth • decreasing tEe, (4) in films with perpendicular e.a. (1, t < 3 ML) an increase of H~t" with decreasing tee. The first result (1) implies a growth of iron on gold in large monolayer patches. This is shown by careful analysis of hysteresis loops in samples with submonolayer iron coverage 23.
'
5ME
Fe/Au
l
~
,L ./H=
. . . . .
j
_.ji~
*
1
~
T=IOK
- 20
(a)
~
Ih.o.~
:
:~= (
kOel
(e.o.) H z,
T=10K
S -20
20
H
2 ML Fe/Au
(b)
2O [kOe)
Fig. 1. Magnetization loops M(H) of epitaxial Au(l11)-Fe(110)-Au sandwiches, magnetization is normalized to saturation M(H)/Ms,with the applied field H parallel (l[) and perpendicular ( / ) to the film plane: (a) 5 ML iron characteristic for all thicker iron films with e.a. in the film plane and h.a. perpendicular, (b) 2 ML iron typical for tv~ < 3 ML with h.a. in the plane and e.a. perpendicular.
In Fig. 1 we show the normalized field dependent magnetization M(H)/M s at 10 K. Qualitatively, the observed dependence of M(HPr) and M(H ±) in Fig. l(a) represents the expected thin film behaviour in an almost ideal form, dominated by the shape anisotropy. But, there is a remarkable difference compared to "bulk" iron films where Hs~at amounts to about 22 kOe ~ 4riMs. In the case of 5 M L Fe-Au as~at = 10-t-2 kOe (Fig. l(a)). Obviously some additional anisotropy relative to the film normal must be present. The results (2) and (3) suggest the presence of a MSA in competition with the shape anisotropy. This is described by an effective anisotropy field HT,ff with respect
314
G. LUGERT, G. BAYREUTHER
to the surface normal and can be written in the form H ~ ff =
4nM~ + H~ + N1---(H~I)+H~ 2))
(1)
where 4re M~ is the shape anisotropy field, Hv a usually small total volume anisotropy field, N the number of atomic layers and H~° represent the surface anisotropy fields at both interfaces of the magnetic layer according to the phenomenological model of NOel 1. H~ ff is identified 2.'25 with the full saturation field in the h.a. H sh.,. a t , and is by convention positive when the e.a. is in the plane (Fig. l(a)) or negative when perpendicular (Fig. l(b)). The data from our Fe(110)-Au sandwiches fit well a linear dependence of H~ff on 1/N, as shown in Fig. 2. Note that relation (1) holds down to a film thickness as small as 2 ML. F r o m the slope in Fig. 2 we get the sum of the anisotropy fields at the two interfaces. Because of symmetry and the small iron film thickness H~) _- --sH~2~ and we therefore get an interface anisotropy field of H F*-A~ = - 40 +_2 kOe for the single F e ( l l 0 ) - A u interface. This corresponds to an anisotropy constant of Ks ve-Au -- - 0.69 + 0.1 erg c m - 2 10ML t,ML
2ML
20 - ~ I0 "k
°l
IML
T = 10K e.a. II
{
e.a
1
°:E~ - 1 0
-20 0.5
I
tIN
Fig. 2. Effective anisotropy field H~~f relative to the film normal for A u - F e - A u sandwiches n u m b e r of iron monolayers 1 / N , d 11 o - 0.202 nm.
vs.
inverse
This is different from earlier results obtained for the Fe(110)-Ag(111) interface, where it could be shown by magnetometry and conversion electron M6ssbauer spectroscopy (CEMS), that the interface anisotropy is small compared to the shape anisotropy, i.e. [H~e-Ag[<<4riMs ~ 22 kOe 14-16. There are only few experiments in the literature related to MSA at iron interfaces. Most of them have been done on Fe(100) interfaces 5-1°. Due to the different orientation they cannot be directly compared to our results. Moreover, they find a variation of Ks with tee measured at room temperature and disagree even in the sign of Ks on equal interfaces ~-8. MSA at Fe(110) interfaces has been investigated by Elmers and G r a d m a n n 22. Their iron films were grown on W(100) and covered by copper, silver or gold. Due to this asymmetry only the sum o f the anisotropy fields related to the two different interfaces can be obtained. The'result is t h a t the difference of Hs between Fe~-Agand Fe Au is zero within the experimental error, i.e. small compared to the positive total interface anisotropy field, .e.g. H~ e-w + H~re-A" ~ + 60__ 10 kOe. In contrast, we observe a different anisotropy at F e - A u a n d F e - A g interfaces w i t h H F*-A"- H~ "-Ag .~ - 40 kOe. T h e r e a s o n for this discrepancy is not clear at the moment. It could be explained with a different microscopic structure of the films due
PERPENDICULAR MAGNETIC ANISOTROPY IN
Fe(110) EPITAXIAL FILMS
315
to the different substrate material. The strain of the iron lattice will not be equal at the Fe-W and the Fe-Au or Fe-Ag interface according to the different lattice mismatch. This is especially true in the case of asymmetric interfaces in the iron films investigated by Elmers and Gradmann 22. Elastic strain in the film is partially released by the formation of misfit dislocations26, depending on details of growth conditions (growth rate, temperature and substrate surface). According to the density and extension of disclocations, lattice deformations will inevitably remain in the vicinity of the interface which then substantially contribute to the magnetic interface anisotropy. 4. CONCLUSION For the first time we have observed an out of plane magnetic anisotropy in ultrathin epitaxial Fe(ll0) films between Au(lll) which varies linearly with the inverse film thickness down to 2 ML. This behaviour shows the presence ofa MSA independent of film thickness 24. It can only be observed if the iron films are grown under very clean conditions on atomically flat gold substrates ~a and if the measurements are performed at low temperature 19'2a. This is also necessary when comparing experimental results to recent band calculations of MSA4 which always refer to the ground state. These theoretical studies look promising, but they could not give decisive results for a realistic film system up to now. In particular, the different anisotropy of Fe(110) on gold, silver and tungsten cannot be explained by available ab initio band calculations. If a more precise determination of anisotropy energies will become possible by more advanced methods realistic interface structures should be used in the calculations including lattice deformations close to the interface. It seems also necessary to carry out more systematic experimental studies in order to distinguish the role of elastic strain and misfit dislocations for magnetic surface anisotropies from the pure breaking of translational symmetry. ACKNOWLEDGMENTS
The authors wish to thank D. Renard and C. Marlirres from the Institut d'Optique, Universit6 Paris Sud, Orsay, France, for their fruitful collaboration on some of the TEM and RHEED experiments and helpful discussions; K. Bauer, F. Schneider, G. Reiss and J. Vancea from the Institut f/ir Angewandte Physik, Universit/it Regensburg, for the STM measurements. REFERENCES 1 L. N~el, Compt. Rend. Acad. Sci., 237 (1953) 1468; J. Phys. Radium, 15 (1954) 225. 2 G.T. Rado, Phys. Rev.,52(1982) l178;withG. T. RadoandLuZhang, Phys. Rev. B, 33(1986) 3 4 5 6
5080. J.G. Gay and R. Richter, Phys. Rev. Lett., 56 (1986) 2728. J.G. Gay and R. Richter, J. Appl. Phys., 61 (1987) 3362. S.D. Bader and E. R. Moog, J. Appl. Phys., 61 (1987) 3729. B. Heindrich, K. B. Urquhart, A. S. Arrot, J. F. Cochran, K. Myrtle and S. T. Purcell, Phys. Rev. Lett., 59 (1987) 1756.
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