VOLUME 79, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 29 DECEMBER 1997

Ferrimagnetic t-MnAl Co Superlattices on GaAs

G. Lauhoff,1 C. Bruynseraede,2 J. De Boeck,2 W. Van Roy,2,* J. A. C. Bland,1 and G. Borghs2
1Cavendish Laboratory, Madingley Road, Cambridge CB3 OHE, United Kingdom
2IMEC, Kapeldreef 75, B-3001, Leuven, Belgium
(Received 19 July 1996; revised manuscript received 27 May 1997)
We report on the ferrimagnetic superlattice ordering in epitaxial t-MnAl Co superlattices on GaAs.
For ultrathin Co layers (3.57 Å), both the ferromagnetic t MnAl and Co layers display a large
perpendicular magnetic anisotropy together with an antiferromagnetic interface exchange coupling. As
a result, an abrupt transition from the ferromagnetically aligned state at saturation to a ferrimagnetic
superlattice ordering occurs at unusually high fields (37 T) followed by a reversal of the total sample
moment at low fields (0.31.6 T). [S0031-9007(97)04893-X]

PACS numbers: 73.61.At, 75.70.Cn, 78.20.Ls

Magnetic superlattices (SL) are of substantial funda- The samples in this study are grown using a Riber 2300
mental and applied interest since their properties can be molecular beam epitaxy system. Epitaxial growth of
significantly different from those of the component mate- t-MnAl Co heterostructures includes the deposition of
rials. A very wide range of magnetic SL has now been an AlAs buffer layer on GaAs(100) undoped substrates
fabricated including SL of ferromagnetic (FM) with non- using standard conditions. This is followed by low
magnetic (NM) layers (e.g., Ni Mo [1], Fe Si [2], Co Ru temperature deposition of an amorphous template con-
[3]), FM with antiferromagnetic (AF) layers (e.g., Fe Cr sisting of 2.5 monolayers Mn60Al40, a subsequent crys-
[4], Co Mn [5]), AF with AF layers (e.g., Fe3O2 NiO tallization at about 250 ±C, and finally the alternate
[6]), or FM with FM layers. In this last category, di- deposition of t-MnAl and Co layers at a substrate tem-
rect exchange interactions at the interfaces occur leading perature of 220250 ±C. All samples were terminated
to FM or AF coupling. FM interface coupling has been by a t-MnAl cap layer. In situ reflection high-energy
studied in a wide range of multilayers, e.g., Ni Fe [7] electron diffraction monitoring together with transmission
and Ni Co [8], but AF interface coupling is much rarer electron microscopy investigations and x-ray diffraction
and has been reported in structures which include 4d se- (XRD) analysis allow us to calibrate the metal deposition
ries FM layers such as Fe Gd and Co Gd multilayers rates (0.6 6 0.05 nm min for Co and 1.7 6 0.1 nm min
with small magnetic anisotropies and in-plane magnetiza- for t-MnAl) and confirm the epitaxial relationship of the
tion [9]. In this Letter, we describe the unusual exchange metal layers to the III-V substrate. Furthermore, they in-
coupling and magnetic anisotropies in molecular-beam dicate a fourfold in-plane symmetry during the complete
epitaxy (MBE) grown t-MnAl Co SL [10] observed by growth of the multilayers and reveal a close lattice match-
high field magneto-optic Kerr effect (MOKE) and extra- ing between the t-MnAl and Co epilayers. From these
ordinary Hall effect (EHE) measurements. Earlier studies considerations, we expect the bcc Co phase to be the pre-
show that epitaxial t-MnAl thin films are hard ferro- ferred growth mode for the ultrathin Co layers, which
magnets with perpendicular magnetic anisotropy (PMA) has been confirmed by XRD and nuclear magnetic reso-
due to their epitaxial relationship with the underlying nance measurements [10,14]. The XRD data also reveal
GaAs(001) substrate. This relationship forces the c-axis an excellent interface quality and confirm the SL nature
of the tetragonal t-MnAl structure, which is the easy axis of the t-MnAl Co heterostructures. More details about
of magnetization, perpendicular to the film plane [11,12]. the growth and structural characteristics of these SL can
Here, we demonstrate for the first time that an AF cou- be found elsewhere [10].
pling occurs at the t-MnAl Co interface, as previously The polar MOKE and EHE measurements were per-
predicted theoretically [13], which leads to a ferrimagnetic formed at room temperature with a superconducting
superlattice ordering [9] with perpendicular magnetization magnet [15]. Both polar MOKE and EHE measurements
of the FM t-MnAl and Co layer moments up to surpris- provide information on the magnetization component per-
ingly high fields. Moreover, we show that an unusually pendicular to the sample. Figure 1 shows high field EHE
abrupt magnetization reversal occurs at high fields due to data of three samples. The sample structures are given in
the presence of a strong AF coupling in combination with Table I. For t-MnAl Co SL with thicker Co layers
a PMA in both constituent t-MnAl and Co layers. This is (8.5 Å, sample 1, curve a) we find that the Co layers have
in contrast to a more linear transition from the ferrimag- an in-plane and the t-MnAl layers an out-of-plane rema-
netic aligned state to the FM aligned state at saturation nent magnetization [10]. The observed hysteresis loop is
typically observed for Co Gd or Fe Gd SL with small a superposition of a square hysteresis loop at low fields
anisotropies and in-plane magnetization [9]. attributed to the reversal of the t-MnAl moments, and a

VOLUME 79, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 29 DECEMBER 1997

in curve b indicates that the t-MnAl and Co layers are AF
coupled to each other, resulting in Co and t-MnAl spin
orientations perpendicular to the sample surface. This fer-
rimagnetic superlattice ordering is maintained up to high
fields and can only be broken for an applied field larger
than 6.3 T.
Other possible explanations for the observed high field
features can be ruled out. First, a biquadratic coupling
between the t-MnAl and the Co layers could lead to
similar high field behavior. But in this case, an in-plane
remanent magnetization should be observed. AGFM
measurements show that this is not the case and therefore
FIG. 1. EHE measurement with the field applied perpendicu- exclude this possibility. Second, structural disorder in
lar to the film (curve a) of sample 1, (curve b) of sample 2, and the t-MnAl unit cell affects the magnetic properties of
(curve c) of sample 3. The loops are normalized to the satura- this alloy in the sense that the ideal stacking of Mn
tion value of the EHE resistivity and shifted for comparison. and Al on alternating planes of the tetragonal lattice is
no longer achieved, leading to an AF coupling between
hard axis hysteresis loop attributed to the coherent rota- Mn atoms on Mn sites and Mn atoms on Al sites
tion of the Co magnetization from an in-plane orientation [11,12]. These antiferromagnetically aligned Mn atoms
at remanence to an out-of-plane orientation at saturation. in the crystal lattice could then also experience a spin
In the case of the t-MnAl Co SL with ultrathin Co lay- reversal at high fields. This mechanism, however, should
ers (4 Å, sample 2, curve b), a square loop at low fields also be present in single t-MnAl thin films, grown in
is observed. Above the low switching field (50 mT) al- otherwise identical conditions, but was never detected, as
most no change in the EHE signal is observed, indicating shown in Fig. 1(c). Furthermore, theoretical predictions
that no change in the amplitude of the perpendicular mag- by Van Leuken and de Groot [13] indicate that a coupling
netization component occurs. But at an applied field of between t-MnAl and Co layers would be AF for a Mn
about 6.3 T an abrupt switching is observed, and at 7.5 T terminated interface between the t-MnAl and Co layers,
the sample is almost saturated. As a reference, EHE data supporting our conclusion of AF coupling.
for a single crystal 200 Å thick t-MnAl film (sample 3) Polar MOKE measurements on various SL corrobo-
are given in curve c showing an almost square hysteresis rate the AF coupling model and will allow us to evalu-
loop with a coercive field of 0.3 T. The low field fea- ate which of the two sublayers is switching at high fields.
tures of both curve b and curve c show a square hystere- From MOKE measurements on sample 1, given in Fig. 2
sis loop (with different coercive fields, however) which (curve a), we conclude that the t-MnAl and Co layers
might indicate that the low field magnetization reversal for have the same sign of Kerr rotation. With this informa-
sample 2 (curve b) is determined by t-MnAl. Both the tion, a clear AF coupling between the two constituent lay-
absence of a hard axis hysteresis loop and the absence ers of sample 4 can be identified from the data of Fig. 2
of an in-plane remanent magnetization for sample 2, as
determined from in-plane alternating gradient field mag-
netometry (AGFM) measurements at low fields ,1.4 T ,
suggest that the Co magnetization is strongly coupled to
the t-MnAl magnetization which is perpendicular to the
film surface. The observed high field switching as shown

TABLE I. Sample names and their structure descriptions.

Shown in
Name Sample structure figure
Sample 1 63 t-MnAl 36 Å Co 8.5 Å Fig. 1(a);
Fig. 2(a)
Sample 2 63 t-MnAl 18.5 Å Co 4 Å Fig. 1(b);
Fig. 4 FIG. 2. Polar MOKE measurement with the field applied
perpendicular to the film (curve a) of sample 1 and (curve b) of
Sample 3 200 Å t-Mn60Al40 Fig. 1(c) sample 4. The loops are normalized to the saturation intensity
Sample 4 63 t-MnAl 20 Å Co 5 Å Fig. 2(b) and shifted for comparison. The direction of the Co and
Sample 5 63 t-MnAl 20 Å Co 37 Å Figs. 3(a) t-MnAl magnetizations at remanence and at saturation, as
3(e) discussed further in the paper, are added for clarity. A linear
background has been subtracted.

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VOLUME 79, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 29 DECEMBER 1997

(curve b). At the low field reversal (at 1 T), the MOKE AF coupling strength and the possible presence of a
intensity drops, when ramping up the field. This can only PMA in the Co layers using an energy minimization
occur when the magnetic moments of the t-MnAl and approach [17]. We use the bulk magnetization for
Co layers switch simultaneously, remaining strongly AF t-MnAl 490 kA m [12] and fcc Co 1420 kA m and
coupled, and the MOKE contribution from the subset of a magnetocrystalline anisotropy constant for t-MnAl of
layers forced antiparallel to the applied field is largest. At KMnAl 2 MJ m3 [18]. Both the exchange coupling
higher fields the AF coupling between Co and t-MnAl is strength J1 and PMA for Co are fitted numerically
broken and the Kerr intensity rises as the magnetization to the observed magnetization loops. For sample 2 in
directions become aligned. Fig. 4(b) an AF coupling strength of J1 21.9 mJ m2
In order to identify which subset of layers is switching per interface has been extracted. The field range in which
at high fields, the influence of the thickness of the Co lay- the magnetization reversal of the Co layers at high field
ers on the AF coupling in these t-MnAl Co SL was stud- occurs would become progressively larger as the PMA
ied. A special wedged 63 t-MnAl 20 Å Co 3 7 Å for Co is decreased, as can be seen in Fig. 4 (compare
SL (sample 5) was grown for this purpose [16]. Three curve b with curve c). A large effective PMA for Co
typical polar MOKE loops of the wedge are shown in of Keff Co 1.2 MJ m3 has therefore been extracted.
Figs. 3(a)3(c). Very sharp switching is observed both While more accurate values for the magnetizations of Co
at high and low fields. Curve d shows the Co thickness and t-MnAl are required for a precise determination of
dependent field values, at which the AF coupling between the coupling strength and Co PMA, we can nonetheless
the Co and t-MnAl is broken when ramping up the field, conclude from our simulations that the ratio of J1 MCo
whereas curve e gives the field values at which the mag- does not change significantly over a wide range of values
netizations for Co and t-MnAl switch back to the AF ori- for MMnAl and MCo and that a PMA for both t-MnAl
entation. These data clearly show a lower saturation field and Co must always be present.
for thicker Co layers, indicating that it is the Co layers The same energy minimization approach also enabled
which are switching to the FM orientation at high fields. us to model the Co thickness dependent saturation field
We have modeled numerically the magnetization (Fig. 3 lower figure) assuming a slightly reduced magne-
process of the t-MnAl Co SL in order to estimate the tization for the Co layers of 1100 kA m, yielding an ef-
fective PMA for Co of Keff 0.5 MJ m3 and a constant
exchange coupling strength J1 21.15 mJ m2, which
confirms the interfacial origin of the AF coupling between
t-MnAl and Co. Moreover, it was only possible to fit
these Co thickness dependent M-H loops with a constant
value for J1 by assuming a smaller total magnetic moment
for the Co layers than for the t-MnAl layers. This leads
us to the conclusion that it is indeed the Co layers which
are abruptly switching at high fields. The increase in
the low field t-MnAl coercivity with increasing Co layer

FIG. 3. (upper figure) Polar MOKE measurements with the
field applied perpendicular to the surface of the wedged
SL (sample 5). The nominal Co layer thicknesses are 7 Å
(curve a), 5 Å (curve b), and 4 Å (curve c). The loops are
normalized to the saturation value and shifted for comparison.
A linear background has been subtracted. (lower figure) FIG. 4. (curve a) EHE measurement of sample 2 (normalized
Curve d shows the field values, at which a sharp magnetization to saturation value). (curve b) A modeled M-H loop for
reversal is observed at high field when ramping up the field, J1 21.9 mJ m2 and Keff Co 1.2 MJ m3. Note that for
and curve e shows the corresponding field values when ramping a smaller Keff Co the Co moment switches less abruptly at
down. The Co thickness dependent saturation field can be high fields as shown in (curve c) using J 21.9 mJ m2 and
modeled as indicated in the figure (continuous line) using Keff Co 22pM2Co 21.3 MJ m3, which includes only the
J1 21.15 mJ m2 and Keff Co 0.5 MJ m3. shape anisotropy of Co.

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thicknesses (0.3 to 1.6 T) as shown in Figs. 3(a)3(c) can [1] M. Grimsditch et al., Phys. Rev. Lett. 51, 489 (1983).
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increased number of pinning sites for the t-MnAl magne- 64, 2304 (1990).
tization reversal process. For films with Co thicknesses [4] P. Grünberg et al., Phys. Rev. Lett. 57, 2442 (1986).
below 3.5 Å, we find a loss of AF coupling which we at- [5] Y. Henry and K. Ounadjela, Phys. Rev. Lett. 76, 1944
tribute to the breaking of the SL structure and an evolution (1996); K. Uchiyama et al., J. Magn. Magn. Mater. 156,
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23 (1996).
In summary, we have shown that t-MnAl Co SL [6] C. A. Ramos et al., Phys. Rev. Lett. 65, 2913 (1990).
epitaxially grown on GaAs(001) show a strong AF [7] W. L. O'Brian and B. P. Tonner, Phys. Rev. B 52, 15 332
interface exchange coupling combined with a large PMA (1995).
for both the t-MnAl and ultrathin Co layers (3.57 Å). [8] G. H. O. Dalderop, P. J. Kelly, and F. J. A. den Broeder,
This results in an unusually abrupt spin reversal at high Phys. Rev. Lett. 68, 682 (1992).
fields, observed by EHE and polar MOKE measurements. [9] Y. Kamiguchi, Y. Hayakawa, and H. Fujimori, Appl.
From the polar MOKE measurements of a SL with Phys. Lett. 55, 1918 (1989); R. E. Camley and D. R.
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observed magnetization reversal at high field can be [10] J. De Boeck et al., J. Cryst. Growth 150, 1139 (1995); C.
attributed to the Co spin reversal from the antiparallel to Bruynseraede et al., Mater. Res. Soc. Symp. Proc. 384, 85
the parallel orientation with respect to the magnetization (1995).
[11] T. Sands et al., Appl. Phys. Lett. 57, 2609 (1990); W. Van
of the t-MnAl layers. Our work illustrates how the Roy et al., J. Appl. Phys. 78, 398 (1995).
presence of additional magnetic anisotropies together with [12] A. J. J. Koch et al., J. Appl. Phys. 31, 75s (1960).
AF interlayer coupling can lead to new spin reversal [13] H. van Leuken and R. A. de Groot (private communi-
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The authors thank Robert Hicken for valuable discus- (1990).
sions. The authors acknowledge the financial support of [14] G. Strijkers and H. Swagten (private communication).
the EPSRC (U.K.), the Newton Trust (U.K.), the EU under [15] A. J. R. Ives et al., J. Appl. Phys. 75, 6458 (1994).
Grant No. ESPRIT 20.027, the IWT (Flanders, Belgium), [16] C. Bruynseraede et al. (to be published).
and the National Fund of Scientific Research (Belgium). [17] B. Dieny, J. P. Gavigan, and J. P. Rebouillat, J. Phys.
Condens. Matter 2, 159 (1990); B. Dieny and J. P.
Gavigan, J. Phys. Condens. Matter 2, 187 (1990).
[18] The c-lattice constant obtained from XRD measurements
*Present address: Joint Research Center for Atom is 3.38 Å. For this constant, KMnAl 2 MJ m3 is ex-
Technology-Angstrom Technology Partnership, 1-1-4 pected after A. Sakuma, J. Phys. Soc. Jpn. 63, 1422
Higashi, Tsukuba, Ibaraki 305, Japan. (1994).

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