JOURNAL OF APPLIED PHYSICS
VOLUME 89, NUMBER 11
1 JUNE 2001
In-plane spin reorientation transitions in epitaxial Fe„110…ÕGaAs„110… films R. Ho¨llinger, M. Zo¨lfl, R. Moosbu¨hler, and G. Bayreuthera) Institut fu¨r Experimentelle und Angewandte Physik, Universita¨t Regensburg, 93040 Regensburg, DE, Germany
Epitaxial Fe films in a thickness range from 4 to 64 monolayers 共ML兲 were grown by molecular beam epitaxy on GaAs共110兲 at room temperature. The growth was characterized by reflection high energy electron diffraction. The magnetic in-plane anisotropy was investigated by alternating gradient magnetometry in a temperature range from 150 to 295 K. For a 64 ML thick Fe共110兲 film the 关001兴 axis is the easy axis, the 关⫺110兴 the intermediate axis, and the hard axis is between 关⫺110兴 and 关⫺111兴. For Fe films with a thickness below 24.2⫾1.2 ML the 关⫺110兴 becomes an easy axis at room temperature. A 24 ML Fe film shows a reorientation of the easy axis with decreasing temperature: Above the critical temperature of 共251⫾3兲 K 关⫺110兴 is the easy axis, for lower temperatures it becomes an intermediate axis. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1354585兴
the islands at 4 ML. Coalescence at 4 ML has been observed before for Fe共001兲/GaAs共001兲4 and FeCo共001兲/GaAs共001兲5 grown at room temperature. This means that the growth mode is 共roughly兲 the same in all these cases at room temperature. In order to investigate the in-plane magnetic anisotropy a stepped wedge sample was prepared with a shadow mask with Fe thicknesses ranging from 4 to 64 ML Fe共110兲 in 4 ML steps 共⫽16 fields兲. Finally the Fe film was covered with 30 ML Au as a protective layer for the ex situ measurements. The 12⫻12 mm2 sample was cleaved into 16 parts corresponding to 16 Fe thickness values in order to measure the magnetic properties with an alternating gradient magnetometer 共AGM兲 in a temperature range from 150 to 295 K. The magnetization curves for a 64 ML 共128 Å兲 Fe共110兲 film on GaAs共110兲 is shown in Fig. 2 in different directions.
In epitaxial ferromagnetic films the knowledge of the in-plane magnetic anisotropy is crucial for the interpretation of the magnetization reversal. In epitaxial Fe共110兲 films grown on single crystal 共110兲 oriented substrates, previous investigations1–3 found a reorientation transition of the magnetic easy axis of the magnetization from the 关001兴 to 关⫺110兴 axis with decreasing film thickness. The present work investigates the reorientation transition of the easy axis for epitaxially grown Fe共110兲 films on GaAs共110兲 and explains this reorientation transition based on the thickness and temperature dependence of the anisotropy constants. The Fe films were grown by molecular beam epitaxy on a GaAs共110兲 wafer at a pressure below 4⫻10⫺10 Torr during evaporation. Annealing in UHV at 900 K for 1 h, Ar ion etching 共500 eV兲 at 900 K for 30 min, and final annealing at 900 K for 1 h resulted in a carbon- and oxygen-free GaAs surface according to Auger electron spectroscopy 共AES兲 as reported earlier.4 The reflection high energy electron diffraction 共RHEED兲 pattern of the GaAs共110兲 surface after cooling down to 300 K is shown in Fig. 1. The patterns in the 关⫺100兴 and 关001兴 direction do not show any surface reconstruction as expected. The different distance of the diffraction spots along the two directions are consistent with the symmetry of the 共110兲 surface as reported earlier.1 The position of streaky spots on a Laue circle in the 关⫺100兴 direction indicates a very smooth surface. The Fe films were grown at room temperature with a deposition rate of 0.5 ML/min controlled by a quartz monitor which was calibrated ex situ by x-ray fluorescence spectroscopy 共XFS兲. From the evolution of the RHEED patterns during the Fe deposition two steps of the growth process can be distinguished: First the intensity of the sharp RHEED pattern of the GaAs surface disappears with increasing thickness at 1 ML coverage; this is ascribed to the formation of disordered three-dimensional 共3D兲 Fe nuclei. RHEED patterns characteristic of the bcc structure appear during the coalescence of
FIG. 1. Reflection high energy electron diffraction 共RHEED兲 patterns of GaAs共001兲 in the 关001兴 and 关⫺110兴 directions.
a兲
Electronic mail:
[email protected]
0021-8979/2001/89(11)/7136/3/$18.00
7136
© 2001 American Institute of Physics
Downloaded 19 Oct 2001 to 132.199.101.24. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
Hollinger et al.
J. Appl. Phys., Vol. 89, No. 11, 1 June 2001
FIG. 2. Magnetization curves of 64 ML Fe共110兲 on GaAs共110兲 in the easy axis 关001兴, the intermediate axis 关⫺110兴, and in 关⫺111兴 direction at 295 K.
As expected for bulk cubic Fe, the easy axis is along the 关001兴, and the intermediate axis along 关⫺110兴. The hard axis, however, is rotated from its orientation in bulk Fe, 关⫺111兴, towards 关⫺110兴 due to the presence of a uniaxial term. In contrast, the hysteresis loop of a 4 ML Fe共110兲 shows the hard axis in 关001兴 and the easy axis in 关⫺110兴 共see Fig. 3兲. This means that a reorientation transition takes place at a critical thickness t crit somewhere between 64 ML and 4 ML. The high remanence of the 4 ML film indicates the absence of noticeable ‘‘共magnetically兲 dead layers’’ and hence very little intermixing at the Fe/GaAs interface during the growth at room temperature.4 The magnetic energy density of an 共110兲 oriented Fe film according to Gradmann et al.2 can be described by eff 2 4 2 ⑀ ⫽ 14 K eff 1 关 sin 共 2 兲 ⫹sin 共 兲兴 ⫹K u sin 共 兲
⫺HM S cos共 ⫺ ␣ 兲
共1兲
including the Zeeman term for an applied field, where ␣ is the angle between external field and 关001兴 and is the angle between magnetization and 关001兴. For ␣⫽90°, i.e., the field along the 关⫺110兴 direction, minimizing the energy given by Eq. 共1兲 leads to an analytical expression for the inverse magnetization loop, H(m), valid in the field range of coherent rotation of the magnetization, i.e., with the field in the 关⫺110兴 direction for thick films (t⬎t crit), and along the 关001兴 direction 共␣⫽0°兲 for thin films (t⬍t crit), respectively:
FIG. 3. Magnetization curves of 4 ML Fe共110兲 on GaAs共110兲 in the hard axis 关001兴 and the easy axis 关⫺110兴 directions at 295 K.
7137
eff FIG. 4. Thickness dependence of the anisotropy constants K eff 1 and K u at room temperature.
H储关 ⫺110兴 : H储关 001兴 :
H共 m 兲⫽ H共 m 兲⫽
eff 2K eff 1 ⫹2K U
Ms
eff K eff 1 ⫺2K U
Ms
m⫺
m⫺
3K eff 1 Ms
3K eff 1 Ms
m 3,
m 3,
共2兲 共3兲
where ⫺1⬍m⬍1, and m(⫽M /M s ) denotes the magnetization component along the axis of the applied field normalized to the saturation magnetization M s . The magnetization curves can be fitted in the intermediate direction, i.e., 关⫺110兴 above the critical thickness of the reorientation transition and the 关001兴 direction below. In Fig. eff 4 the values of K eff 1 and K u are plotted versus the inverse film thickness. Both anisotropy constants are fitted by assuming a superposition of a volume term (K vol) and an interface contribution (K int) with t as film thickness: E eff⫽K vol⫹
K int , t
共4兲
where K int includes contributions from both interfaces. The 5 3 value for K vol 1 ⫽(3.6⫾0.7)⫻10 erg/cm is close to the cubic anisotropy constant of bulk bcc Fe of 4.2⫻105 erg/cm3. The ⫺2 erg/cm2 causes a interface term of K int 1 ⫽(⫺3.9⫾0.9)⫻10 eff change of sign of K 1 at a critical thickness of 6 ML indicating a change of the easy axis of the cubic anisotropy to 关⫺111兴. However, due to the presence of a second in-plane anisotropy, the uniaxial contribution, which is much stronger than the cubic anisotropy in this thickness range, the resulting anisotropy has a uniaxial character with the easy axis along 关⫺110兴. A critical thickness of 6 ML for the reorientation transition of K eff 1 was also observed in the systems Au/Fe/ZnSe共001兲 by Reiger et al.6 and Au/Fe/GaAs共001兲 by Brockmann et al.7 and even at the system Au/FeCo/ GaAs共001兲 by Dumm et al.5 This critical thickness for the reorientation of K eff 1 therefore seems to be a fundamental value. For the uniaxial contribution we obtain K vol u ⫽(2.7 ⫺2 ⫽(⫺14.1⫾2.0) 10 erg/cm3. ⫾0.8)⫻105 erg/cm3 and K int u For the 4 ML Fe共110兲 共1/N⫽0.25兲 we observe a breakdown of the uniaxial anisotropy K eff u due to the incomplete coalescence of the film and the reduced Curie temperature. The critical thickness t crit for the reorientation transition of the easy axis of the resulting anisotropy of both contributions, cubic and uniaxial, according to Refs. 2 and 8 is determined by the equation:
Downloaded 19 Oct 2001 to 132.199.101.24. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
7138
Hollinger et al.
J. Appl. Phys., Vol. 89, No. 11, 1 June 2001
eff FIG. 5. Temperature dependence of the anisotropy constants K eff 1 and K u of a 24 ML Fe共110兲 film on GaAs共110兲.
eff K eff 1 ⫽⫺4K u ,
共5兲
where the energies along the 关001兴 and 关⫺110兴 direction become equal. Applying Eq. 共5兲 to the data of Fig. 4 yields a value t crit⫽(20.6⫾8.7) ML. On the other hand, the critical thickness for the reorientation transition of the easy axis can also be obtained by extrapolation of the saturation field in the intermediate axis with decreasing film thickness. By this procedure we obtain a more precise value of t crit⫽(24.2⫾1.2) ML. The present value of the critical thickness for the reorientation transition of the easy axis differs strongly from previous results where it was found at about 50 ML Fe共110兲.1,2,8 This difference may be connected with the growth temperature which was held at 423 K8 or 448 K1 in contrast to 300 K in the present study. eff Next the temperature dependence of K eff 1 and K u was determined by fitting the experimental data with Eqs. 共2兲 or 共3兲, respectively. The values are plotted in Fig. 5 for a 24 ML
film, i.e., for a thickness close to the critical thickness for the reorientation transition at room temperature. A reorientation can occur as a function of temperature because of the temperature dependence of both anisotropy constants K eff 1 and 9 as suggested by Fruchart et al. for Fe共110兲 films. In the K eff u eff eff present case, both K 1 and K u increase with decreasing temperature. The critical temperature for the reorientation transition can be determined by using Eq. 共5兲. Here we obtain a critical temperature T crit⫽(250⫾30)K for a 24 ML Fe共110兲 film. By using the saturation field method we get T crit ⫽(251⫾3)K. It is interesting to note that with decreasing temperature the magnetic anisotropy in the films seem to become more bulk-like. This behavior which is also observed in ultrathin Fe films on GaAs共001兲 remains to be studied in more detail. Supported by the German Ministry of Technology 共BMBF兲 is gratefully acknowledged 共Grant No. 13N7334/7兲.
G. A. Prinz, G. T. Rado, and J. J. Krebs, J. Appl. Phys. 53, 2087 共1982兲. U. Gradmann, J. Korecki, and G. Waller, Appl. Phys. A: Solids Surf. 39, 101 共1986兲. 3 H. Fritzsche, H. J. Elmers, and U. Gradmann, J. Magn. Magn. Mater. 135, 343 共1994兲. 4 M. Zo¨lfl, M. Brockmann, M. Ko¨hler, S. Kreuzer, T. Schweinbo¨ck, S. Miethaner, F. Bensch, and G. Bayreuther, J. Magn. Magn. Mater. 175, 16 共1997兲. 5 M. Dumm, M. Zo¨lfl, R. Moosbu¨hler, M. Brockmann, T. Schmidt, and G. Bayreuther, J. Appl. Phys. 87, 5457 共2000兲. 6 E. Reiger, E. Reinwald, G. Garreau, M. Ernst, M. Zo¨lfl, F. Bensch, S. Bauer, H. Preis, and G. Bayreuther, J. Appl. Phys. 87, 5923 共2000兲. 7 M. Brockmann, M. Zo¨lfl, S. Miethaner, and G. Bayreuther, J. Magn. Magn. Mater. 198Õ199, 384 共1999兲. 8 M. Gester, C. Daboo, R. J. Hicken, S. J. Gray, A. Ercole, and J. A. C. Bland, J. Appl. Phys. 80, 347 共1996兲. 9 O. Fruchart, J.-P. Nozieres, and D. Givord, J. Magn. Magn. Mater. 165, 508 共1997兲. 1 2
Downloaded 19 Oct 2001 to 132.199.101.24. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/japo/japcr.jsp
