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                                             PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒

             Thickness dependence of Hall transport in Ni1.15Mn0.85Sb thin films on silicon
                                      W. R. Branford,* S. K. Clowes, and Y. V. Bugoslavsky
                    Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom

                                           S. Gardelis, J. Androulakis, and J. Giapintzakis
     Foundation for Research and Technology—Hellas, Institute of Electronic Structure and Laser, P.O. Box 1527, Vasilika Vouton,
                                               711 10 Heraklion, Crete, Greece

                                                 C. E. A Grigorescu and S. A. Manea
                        National Institute for Research & Development for Optoelectronics, Bucharest, Romania

                                                               R. S. Freitas
                    ´                                                                      ´
      Instituto de Fısica, Universidade Federal Fluminense, Campus da Praia Vermelha, Niteroi, 24210-340 Rio de Janeiro, Brazil

                                                                S. B. Roy
                     Low Temperature Physics Laboratory, Centre for Advanced Technology, Indore 452013, India

                                                               L. F. Cohen
                    Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom
                                         ͑Received 12 December 2003; published 18 May 2004͒
                 Highly spin polarized Heusler alloys, NiMnSb and Co2 MnSi, attract a great deal of interest as potential spin
              injectors for spintronic applications. Spintronic devices require control of interfacial properties at the ferro-
              magnet:semiconductor contact. To address this issue we report a systematic study of the ordinary and anoma-
              lous Hall effect, in Ni1.15Mn0.85Sb films on silicon, as a function of film thickness. In contrast to the bulk
              stoichiometric material, the Hall carriers in these films become increasingly electron-like as the film thickness
              decreases, and as the temperature increases from 50 K toward room temperature. High field Hall measurements
              confirm that this is representative of the majority transport carriers. This suggests that current injected from a
              NiMnSb:semiconductor interface may not necessarily carry the bulk spin polarization. The films also show a
              low temperature upturn in the resistivity, which is linked to a discontinuity in the anomalous Hall coefficient.
              Overall these trends indicate that the application of Heusler alloys as spin injectors will require strictly
              controlled interfacial engineering, which is likely to be demanding in these ternary alloys.

              DOI: 10.1103/PhysRevB.69.201305                      PACS number͑s͒: 73.50.Jt, 85.75.Ϫd, 73.61.At, 75.50.Ϫy


    It has long been known that there is a component of the                  A primary motivation for performing this study was to
Hall resistivity of ferromagnets proportional to the magneti-            determine whether bulk-like transport could be achieved in
zation, ␳ xy ϭR O BϩR S ␮ 0 M , where R O and R S are known,             thin films and hence evaluate NiMnSb as a potential spin
respectively, as the ordinary and anomalous Hall coefficients             injector. Here, we report a systematic study of the longitudi-
and ␮ 0 M is the magnetization. The recent development1,2 of             nal ( ␳ xx ) and Hall ( ␳ xy ) electrical resistivities of
                                                                         Ni1.15Mn0.85Sb films on silicon as a function of film thick-
theories, based on the Berry3 ͑or Pancharatnam͒ phase, that
                                                                         ness. We demonstrate that the film transport properties close
quantitatively describe the behavior of R S in a number of               to the interface vary quite drastically compared to bulk—like
different systems has resulted in a resurgence of interest in            behavior and this has important implications for using this
the anomalous Hall effect ͑AHE͒. The observation of a non-               material in spintronic applications. Thin films of
zero anomalous Hall velocity requires a finite spin polariza-             Ni1.15Mn0.85Sb, with thicknesses of 5, 45, 80, 110 and 400
tion of the transport current and spin-orbit coupling. This              nm, were grown on Si͑100͒ by pulsed laser deposition10 at
holds out the intriguing possibility that the transport spin             200 °C from a stoichiometric target. All films were shown by
polarization can be extracted from anomalous Hall measure-               energy dispersive x-ray analysis to be slightly off-
ments in well characterized systems.                                     stoichiometric, formulated Ni1ϩx Mn1Ϫy Sb; xϭ0.15Ϯ0.05,
    The half Heusler4 alloy NiMnSb is ferromagnetic with a               yϭ0.15Ϯ0.05. The x-ray diffraction patterns were consistent
Curie temperature (T C ) of 728 K,5 and band structure calcu-            with ͑220͒ oriented polycrystalline NiMnSb Heusler phase
lations predict that it is half-metallic6 with the spin-polarized        with lattice parameter 5.99 ÅϮ0.02 Å, compared to
carriers holes derived from the Sb 6s 2 band. Further calcu-             5.9320 ÅϮ0.0028 Å for the target. No second phase was
lations have shown the transport spin polarization ( P t ) of            observed. The rocking curve of the ͑220͒ reflection indicated
NiMnSb to be highly sensitive to atomic disorder7 and sur-               that the out-of-plane alignment was imprecise, with a spread
face effects.8,9 Ideally spin injection for spintronic applica-          of orientations of 12 around ͓110͔.
tions will require the carriers close to the injection interface             Magnetotransport data were collected in a square geom-
to carry the bulk spin polarization.                                     etry by the van der Pauw method. The geometry led to a

0163-1829/2004/69͑20͒/201305͑4͒/$22.50                         69 201305-1                           ©2004 The American Physical Society
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W. R. BRANFORD et al.                                                                     PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒




                                                                          FIG. 2. ͑a͒ Longitudinal resistivity vs temperature for the series
    FIG. 1. Hall resistivity ͑open squares͒ vs field for 80 nm film at   of films. ͑b͒ Ordinary Hall coefficient R O vs temperature, solid lines
50, 60, 70, 80, 90, 100, 110, 130, 150, 200, 250, and 290 K. Solid     are a guide to the eye.
lines show fit to ␳ xy ϭR O BϩR S M at each temperature. Inset: Hall
resistivity vs field for 5 nm film at selected temperatures.             The crossover from positive to negative R O corresponds to a
                                                                       crossover from hole dominated to electron dominated Hall
strong mixing of the Hall and MR components, which were                transport, and hence that the Hall data must be considered
separated by their opposite symmetries with respect to inver-          within a two-carrier model. Band structure calculations6 pre-
sion of the magnetic field. The temperature and field depen-             dict that the spin polarized carriers are Sb holes, so the ob-
dence of the magnetoresistance were reported previously.11             servation of electron dominated transport at room tempera-
The field dependence of the magnetization of the films was               ture in thin films suggests that NiMnSb may not be an
measured at the same temperatures and in the same geometry             efficient spin injector.
͑field perpendicular to the film surface͒ as the Hall measure-              In a two-carrier system, R O is only constant in the low
ments, in an Oxford Instruments vibrating sample magneto-              field limit ͑when ␮ e,h 2 B 2 Ӷ1), in this limit R O is given by
meter. In this geometry the magnetic anisotropy of the films            Eq. ͑1͒, where n and p are the electron and hole carrier
is dominated by the shape anisotropy. A reliable magnetiza-            concentrations and ␮ e and ␮ h are the respective mobilities.
tion could not be obtained for the 5 nm film.                           In the high field limit ( ␮ e,h 2 B 2 ӷ1) the dependence on the
    The Hall resistivity was measured for all the films at se-          mobility ratio z disappears and R O ϭ1/(pϪn)e; hence it be-
lected temperatures between 50 and 290 K, the data for the             comes a direct measure of the majority carriers. Therefore, if
80 nm film, which is typical of all the films, is shown in Fig.          the low-field Hall resistivity has been dominated by a high
1. An iterative procedure was used to fit the measured Hall             mobility minority carrier, then there must be a strong curva-
resistivity to the expression for ␳ xy ϭR O BϩR S ␮ 0 M , using        ture of ␳ xy at intermediate fields with an eventual change of
independently measured magnetization, which was measured               sign,
at the same temperature, and in the same geometry ͑with
field perpendicular to film surface͒. We previously used this                                         pϪnz 2               ␮e
                                                                                        R Oϭ                    ,   zϭ      .           ͑1͒
method to report12 the Hall transport of the thickest film.                                     ͉ e ͉ ͑ pϩnz ͒ 2          ␮h
With this field orientation the demagnetization factor (N) is
unity, hence the flux density, Bϭ ␮ 0 ͓ Hϩ4 ␲ (1ϪN)M ͔                      Hence, the low-field Hall mobility is not necessarily rep-
ϭ ␮ 0 H, where H is the applied magnetic field in A/m. The              resentative of the majority transport carriers. For example,15
fitting procedure was limited to the range of data 1.5 T                in CrO2 there are a small number of high mobility holes and
у ␮ 0 Hу0 T, because a slight curvature was observed in ␳ xy           around 500 times more low mobility electrons, the low-field
at larger fields, indicating that the low-field limit model ͑dis-        Hall is hole-like, and the high-field Hall is electron-like, in
cussed in the following͒ becomes inappropriate above 1.5 T.            agreement with the thermopower. To investigate whether the
The solid lines in Fig. 1 are fits to the data for the 80 nm film        low-field Hall is representative of the majority transport car-
at a selection of temperatures. The temperature dependence             riers, the high field Hall resistivity of the 5 nm sample was
of the low field limit ordinary Hall coefficient R O obtained            measured, this is plotted in the inset to Fig. 1. There is a
from this fitting procedure, for all the films, is shown in Fig.         slight curvature toward less negative slope with increasing
2. The temperature dependence of ␳ xx is also shown in Fig.            field, but unlike CrO2 ͑Ref. 15͒ neither a sign change nor the
2, for comparison. Note an increasingly strong low tempera-            high field limit is reached by 8 T. This strongly suggests that
ture upturn in ␳ xx is observed with decreasing thickness.             the low-field Hall is representative of the majority transport
    In the stoichiometric bulk material R O remains positive at        carriers. The curvature can be fit to the two band model16,15
all temperatures below T C . 13,14 It is immediately apparent          but the refined parameters are strongly correlated and unique
from Fig. 2͑b͒ that the transport in all these films is different       fit could not be obtained. This is consistent with the
to that material, as R O is increasingly negative as the tem-          observation15 that a reliable fit can only be obtained by re-
perature increases from 50 K, and as the thickness decreases.          lating the band parameters to the measured low-field limit,

                                                                 201305-2
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THICKNESS DEPENDENCE OF HALL TRANSPORT IN . . .                                           PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒

high field limit and crossover point values. There is no fea-
ture in the temperature dependence of R O associated with the
resistivity upturn; this suggests that the resistivity upturn is
not due to a freezing out of carriers, but to a decrease in
carrier mobility.
    Detailed knowledge of the transport carriers as a function
of thickness is important for understanding spin injection
processes at ferromagnet:semiconductor interfaces. Four re-
lations are required to determine the four band parameters,
the Hall and the zero field resistivity provide two. Two-
carrier transport analysis is routine in high mobility semicon-
ductors, where the other two relations are obtained from the
Shubnikov–de Haas oscillations and the MR. In these films
that information is not accessible because the two-carrier MR
is masked by the anomalously large positive MR17 and in
metals Shubnikov–de Haas oscillations are only observed in
extremely high fields. Therefore, only a qualitative analysis
of the band parameters can be made. The sign reversal of the
low field R O with increasing temperature, even in the thick-              FIG. 3. ͑a͒ a and ͑b͒ b coefficients obtained from the fits to
est film, shows that at low temperature pϾnz 2 and at high             R S / ␳ xx ϭaϩb ␳ xx for all films. Because the magnetization of the
temperature pϽnz 2 . z is unlikely to change dramatically             5nm film could not be measured directly, the magnetization loop of
with temperature and n/p is almost certainly increasing with          the 45 nm film was scaled by volume to obtain R S of the 5 nm film.
temperature. The small amount of curvature in the high-field           Bulk values taken from Otto et al. ͑Ref. 13͒. Inset to ͑b͒ R S / ␳ xx vs
Hall indicates that, unlike CrO2 , 15 z is close to unity. The        ␳ xx for the 45 nm film, dashed line is a guide to the eye. Solid lines
band structure of stoichiometric NiMnSb contains both holes           show fits to R S / ␳ xx ϭaϩb ␳ xx in regions above and below upturn.
and electrons,6 with holes dominating the Hall resistivity,13,14
although the thermopower14 indicates a crossover to                   low the resistivity upturn two different straight lines are ob-
electron-dominant transport. For the films studied here, a             tained. The a and b coefficients obtained from linear fitting
likely hypothesis is that the holes result from the bulk band         of R S / ␳ xx vs ␳ xx above and below the upturn for all the films
structure and their concentration is only weakly temperature          are shown in Figs. 3͑a͒ and 3͑b͒, respectively. Above the
dependent, whereas the electron concentration seems to be             resistivity upturn, the coefficients are, within error, the same
derived partly from the band structure and partly from a ther-        as the stoichiometric bulk values13 of aϭϪ6.5ϫ10Ϫ4 TϪ1
mally activated process, such as thermal excitation of donor          and bϭ21 500 TϪ1 ⍀ Ϫ1 mϪ1 , which were previously inter-
states. The off-stoichiometry in these Ni1.15Mn0.85Sb films            preted as side-jumps dominating over skew scattering. How-
will result in a large number of atomic site defects, which are       ever, below the resistivity upturn, the magnitudes of both the
predicted7 to affect the band structure, and the difference           slope and the intercept increase dramatically as the thickness
between the stoichiometric bulk and the 400-nm-thick film is           is increased from 5 to 110 nm, driven by the temperature
likely to be a result of the stoichiometry. The increasingly          dependence of ␳ xx . The implicit assumption in the tradi-
electron dominated transport as a function of thickness is not        tional R S / ␳ xx vs ␳ xx analysis13 is that R S is only indirectly a
attributed to off-stoichiometry in our films as this did not           function of temperature via its dependence on the resistivity
change systematically with thickness. The trend can only be           ͑scattering͒. It appears that both in the stiochiometric bulk
explained by the increasing significance of electronic surface         material, and these films, that assumption and the validity of
or interface states, arising from either the reduced symmetry         that model breaks down around 100 K. No change was ob-
at the interfaces or strain induced defects. Note that unlike         served by room temperature in the sign of R S in any of our
the silver chalcogenides,18 there is no evidence of a cross-          films. In a number of simple ferromagnets ͑such as Fe, Co,
over of majority carrier at the MR maximum ͑resistivity up-           Gd͒ the skew-scattering and side jump terms are of opposite
turn͒.                                                                sign, but it is not known if that is the case in our material.
    Now let us turn to R S . The anomalous Hall effect has                It is important to note that the anomalous Hall effect
historically been ascribed19 to a scattering anisotropy, al-          arises from an asymmetric deflection of the carriers, resulting
though there can also be an intrinsic20 ͑scattering indepen-          in an anomalous Hall conductivity, ( ␴ A ); the anomalous Hall
dent͒ term, which is discussed in the following. In the scat-         resistivity ( ␳ A ) is derived from the conductivity by ␴ A
tering model, it was proposed13 that R S was derived from             ϭ ␳ A /( ␳ xx ϩ ␳ 2 )Ϸ ␳ A / ␳ xx because ␳ xx ӷ ␳ xy . Since a qua-
                                                                                 2
                                                                                        xy
                                                                                                     2

contributions from side-jump scattering and skew scattering,          dratic behavior of R S in ␳ xx only requires a temperature in-
and that these terms were proportional to ␳ xx and ␳ xx , re-
                                                   2
                                                                      dependent ␴ A , it is not clear that any inferences can be made
spectively. In bulk NiMnSb, this model accounts for the ex-           about the scattering.
perimental data at high temperatures, but there is a disconti-            Recently, theories describing the anomalous Hall effect
nuity in R S / ␳ xx vs ␳ xx at around 100 K.13,14 The inset to Fig.   as an intrinsic Berry3 phase effect, have given a good quan-
3͑b͒ shows a typical R S / ␳ xx vs ␳ xx plot, from the 45 nm film      titative agreement with experiment1,2 that was never
␳ xx which is non-monotonic. At temperatures above and be-            achieved with the scattering model. Two types of Berry

                                                                201305-3
RAPID COMMUNICATIONS

W. R. BRANFORD et al.                                                                     PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒

phase induced AHE have, thus far, been reported, one is                stiochiometric material. The thickness dependence is likely
related to spin chirality in magnetically frustrated21 systems         to be due to the increasing significance of interface or free
and the other is associated with thermally induced topologi-           surface electronic states, and indicates that controlled inter-
cal defects that show an exponential temperature                       facial engineering will be required for the use of NiMnSb as
dependence1 around T C . In our films there is no feature in            a spin injector. The anomalous Hall conductivity cannot be
the magnetization at the resistivity upturn temperature, and           interpreted within the traditional scattering model at low
this temperature is far from the Curie temperature, so the             temperatures, because of an additional contribution that
anomalous behavior of R S below the resistivity upturn is              comes into play, which is attributed to a change in the spin-
dissimilar to previously reported Berry systems. Although a            dependent scattering. Unlike the silver chalcogenides, there
Berry phase component cannot be ruled out, the change in               appears to be no correlation between the large positive MR
␴ A at the resistivity upturn is probably due to a change in           found in these films17 and the sign reversal in the ordinary
spin dependent scattering.
                                                                       Hall coefficient.
   In summary, the room temperature electrical transport in
non-stoichiometric Heusler thin films becomes increasingly                 We acknowledge the E.U. programme G5RD-CT-2001
electron dominated with decreasing thickness, in marked                and the EPSRC GR/S14061 and EPSRC GR/R98945 for
contrast to the spin-polarized holes predicted for the bulk            funding.


*Electronic address: w.branford@imperial.ac.uk                             Roy, and L. F. Cohen, Appl. Phys. Lett. ͑in press͒.
1                                                                      12
   H. Yanagihara and M. B. Salamon, Phys. Rev. Lett. 89, 187201           W. R. Branford, S. B. Roy, S. K. Clowes, Y. Miyoshi, Y. V. Bugo-
    ͑2002͒.                                                                slavsky, S. Gardelis, J. Giapintzakis, and L. F. Cohen, J. Magn.
 2
   T. Jungwirth, J. Sinova, K. Y. Wang, K. W. Edmonds, R. P. Cam-          Magn. Mater. ͑in press͒.
                                                                       13
    pion, B. L. Gallagher, C. T. Foxon, Q. Niu, and A. H. Mac-            M. J. Otto, R. A. M. Vanwoerden, P. J. Vandervalk, J. Wijngaard,
    Donald, Appl. Phys. Lett. 83, 320 ͑2003͒.                              C. F. Vanbruggen, and C. Haas, J. Phys.: Condens. Matter 1,
 3
   M. V. Berry, J. Mod. Opt. 34, 1401 ͑1987͒.                              2351 ͑1989͒.
 4
   F. Heusler, Verh. Dtsch. Phys. Ges. 5, 219 ͑1903͒.                  14
                                                                          C. Hordequin, D. Ristoiu, L. Ranno, and J. Pierre, Eur. Phys. J. B
 5
   M. J. Otto, H. Feil, R. A. M. Vanwoerden, J. Wijngaard, P. J.           16, 287 ͑2000͒.
                                                                       15
    Vandervalk, C. F. Vanbruggen, and C. Haas, J. Magn. Magn.             S. M. Watts, S. Wirth, S. von Molnar, A. Barry, and J. M. D.
    Mater. 70, 33 ͑1987͒.                                                  Coey, Phys. Rev. B 61, 9621 ͑2000͒.
 6                                                                     16
   R. A. de Groot, F. M. Mueller, P. G. van Engen, and K. H. J.           R. G. Chambers, Proc. Phys. Soc., London, Sect. A 65, 903
    Buschow, Phys. Rev. Lett. 50, 2024 ͑1983͒.                             ͑1952͒.
 7                                                                     17
   D. Orgassa, H. Fujiwara, T. C. Schulthess, and W. H. Butler,           W. R. Branford, S. K. Clowes, M. H. Syed, Y. V. Bugoslavsky, S.
    Phys. Rev. B 60, 13237 ͑1999͒.                                         Gardelis, J. Androulakis, J. Giapintzakis, A. V. Berenov, S. B.
 8
   D. Ristoiu, J. P. Nozieres, C. N. Borca, B. Borca, and P. A. Dow-       Roy, and L. F. Cohen ͑unpublished͒.
    ben, Appl. Phys. Lett. 76, 2349 ͑2000͒.                            18
                                                                          M. Lee, T. F. Rosenbaum, M. L. Saboungi, and H. S. Schnyders,
 9
   G. A. de Wijs and R. A. de Groot, Phys. Rev. B 64, 020402               Phys. Rev. Lett. 88, 066602 ͑2002͒.
    ͑2001͒.                                                            19
                                                                          L. Berger and G. Bergmann, in The Hall Effect and its Applica-
10
   J. Giapintzakis, C. Grigorescu, A. Klini, A. Manousaki, V. Zorba,       tions, edited by C. L. Chien and C. R. Westgate ͑Plenum, New
    J. Androulakis, Z. Viskadourakis, and C. Fotakis, Appl. Phys.          York, 1979͒.
    Lett. 80, 2716 ͑2002͒.                                             20
                                                                          J. M. Luttinger, Phys. Rev. 112, 739 ͑1958͒.
11                                                                     21
   W. R. Branford, S. K. Clowes, M. H. Syed, Y. V. Bugoslavsky, S.        Y. Taguchi, Y. Oohara, H. Yoshizawa, N. Nagaosa, and Y. Tokura,
    Gardelis, J. Androulakis, J. Giapintzakis, A. V. Berenov, S. B.        Science 291, 2573 ͑2001͒.




                                                                 201305-4

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  • 1. RAPID COMMUNICATIONS PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒ Thickness dependence of Hall transport in Ni1.15Mn0.85Sb thin films on silicon W. R. Branford,* S. K. Clowes, and Y. V. Bugoslavsky Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom S. Gardelis, J. Androulakis, and J. Giapintzakis Foundation for Research and Technology—Hellas, Institute of Electronic Structure and Laser, P.O. Box 1527, Vasilika Vouton, 711 10 Heraklion, Crete, Greece C. E. A Grigorescu and S. A. Manea National Institute for Research & Development for Optoelectronics, Bucharest, Romania R. S. Freitas ´ ´ Instituto de Fısica, Universidade Federal Fluminense, Campus da Praia Vermelha, Niteroi, 24210-340 Rio de Janeiro, Brazil S. B. Roy Low Temperature Physics Laboratory, Centre for Advanced Technology, Indore 452013, India L. F. Cohen Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2BZ, United Kingdom ͑Received 12 December 2003; published 18 May 2004͒ Highly spin polarized Heusler alloys, NiMnSb and Co2 MnSi, attract a great deal of interest as potential spin injectors for spintronic applications. Spintronic devices require control of interfacial properties at the ferro- magnet:semiconductor contact. To address this issue we report a systematic study of the ordinary and anoma- lous Hall effect, in Ni1.15Mn0.85Sb films on silicon, as a function of film thickness. In contrast to the bulk stoichiometric material, the Hall carriers in these films become increasingly electron-like as the film thickness decreases, and as the temperature increases from 50 K toward room temperature. High field Hall measurements confirm that this is representative of the majority transport carriers. This suggests that current injected from a NiMnSb:semiconductor interface may not necessarily carry the bulk spin polarization. The films also show a low temperature upturn in the resistivity, which is linked to a discontinuity in the anomalous Hall coefficient. Overall these trends indicate that the application of Heusler alloys as spin injectors will require strictly controlled interfacial engineering, which is likely to be demanding in these ternary alloys. DOI: 10.1103/PhysRevB.69.201305 PACS number͑s͒: 73.50.Jt, 85.75.Ϫd, 73.61.At, 75.50.Ϫy It has long been known that there is a component of the A primary motivation for performing this study was to Hall resistivity of ferromagnets proportional to the magneti- determine whether bulk-like transport could be achieved in zation, ␳ xy ϭR O BϩR S ␮ 0 M , where R O and R S are known, thin films and hence evaluate NiMnSb as a potential spin respectively, as the ordinary and anomalous Hall coefficients injector. Here, we report a systematic study of the longitudi- and ␮ 0 M is the magnetization. The recent development1,2 of nal ( ␳ xx ) and Hall ( ␳ xy ) electrical resistivities of Ni1.15Mn0.85Sb films on silicon as a function of film thick- theories, based on the Berry3 ͑or Pancharatnam͒ phase, that ness. We demonstrate that the film transport properties close quantitatively describe the behavior of R S in a number of to the interface vary quite drastically compared to bulk—like different systems has resulted in a resurgence of interest in behavior and this has important implications for using this the anomalous Hall effect ͑AHE͒. The observation of a non- material in spintronic applications. Thin films of zero anomalous Hall velocity requires a finite spin polariza- Ni1.15Mn0.85Sb, with thicknesses of 5, 45, 80, 110 and 400 tion of the transport current and spin-orbit coupling. This nm, were grown on Si͑100͒ by pulsed laser deposition10 at holds out the intriguing possibility that the transport spin 200 °C from a stoichiometric target. All films were shown by polarization can be extracted from anomalous Hall measure- energy dispersive x-ray analysis to be slightly off- ments in well characterized systems. stoichiometric, formulated Ni1ϩx Mn1Ϫy Sb; xϭ0.15Ϯ0.05, The half Heusler4 alloy NiMnSb is ferromagnetic with a yϭ0.15Ϯ0.05. The x-ray diffraction patterns were consistent Curie temperature (T C ) of 728 K,5 and band structure calcu- with ͑220͒ oriented polycrystalline NiMnSb Heusler phase lations predict that it is half-metallic6 with the spin-polarized with lattice parameter 5.99 ÅϮ0.02 Å, compared to carriers holes derived from the Sb 6s 2 band. Further calcu- 5.9320 ÅϮ0.0028 Å for the target. No second phase was lations have shown the transport spin polarization ( P t ) of observed. The rocking curve of the ͑220͒ reflection indicated NiMnSb to be highly sensitive to atomic disorder7 and sur- that the out-of-plane alignment was imprecise, with a spread face effects.8,9 Ideally spin injection for spintronic applica- of orientations of 12 around ͓110͔. tions will require the carriers close to the injection interface Magnetotransport data were collected in a square geom- to carry the bulk spin polarization. etry by the van der Pauw method. The geometry led to a 0163-1829/2004/69͑20͒/201305͑4͒/$22.50 69 201305-1 ©2004 The American Physical Society
  • 2. RAPID COMMUNICATIONS W. R. BRANFORD et al. PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒ FIG. 2. ͑a͒ Longitudinal resistivity vs temperature for the series FIG. 1. Hall resistivity ͑open squares͒ vs field for 80 nm film at of films. ͑b͒ Ordinary Hall coefficient R O vs temperature, solid lines 50, 60, 70, 80, 90, 100, 110, 130, 150, 200, 250, and 290 K. Solid are a guide to the eye. lines show fit to ␳ xy ϭR O BϩR S M at each temperature. Inset: Hall resistivity vs field for 5 nm film at selected temperatures. The crossover from positive to negative R O corresponds to a crossover from hole dominated to electron dominated Hall strong mixing of the Hall and MR components, which were transport, and hence that the Hall data must be considered separated by their opposite symmetries with respect to inver- within a two-carrier model. Band structure calculations6 pre- sion of the magnetic field. The temperature and field depen- dict that the spin polarized carriers are Sb holes, so the ob- dence of the magnetoresistance were reported previously.11 servation of electron dominated transport at room tempera- The field dependence of the magnetization of the films was ture in thin films suggests that NiMnSb may not be an measured at the same temperatures and in the same geometry efficient spin injector. ͑field perpendicular to the film surface͒ as the Hall measure- In a two-carrier system, R O is only constant in the low ments, in an Oxford Instruments vibrating sample magneto- field limit ͑when ␮ e,h 2 B 2 Ӷ1), in this limit R O is given by meter. In this geometry the magnetic anisotropy of the films Eq. ͑1͒, where n and p are the electron and hole carrier is dominated by the shape anisotropy. A reliable magnetiza- concentrations and ␮ e and ␮ h are the respective mobilities. tion could not be obtained for the 5 nm film. In the high field limit ( ␮ e,h 2 B 2 ӷ1) the dependence on the The Hall resistivity was measured for all the films at se- mobility ratio z disappears and R O ϭ1/(pϪn)e; hence it be- lected temperatures between 50 and 290 K, the data for the comes a direct measure of the majority carriers. Therefore, if 80 nm film, which is typical of all the films, is shown in Fig. the low-field Hall resistivity has been dominated by a high 1. An iterative procedure was used to fit the measured Hall mobility minority carrier, then there must be a strong curva- resistivity to the expression for ␳ xy ϭR O BϩR S ␮ 0 M , using ture of ␳ xy at intermediate fields with an eventual change of independently measured magnetization, which was measured sign, at the same temperature, and in the same geometry ͑with field perpendicular to film surface͒. We previously used this pϪnz 2 ␮e R Oϭ , zϭ . ͑1͒ method to report12 the Hall transport of the thickest film. ͉ e ͉ ͑ pϩnz ͒ 2 ␮h With this field orientation the demagnetization factor (N) is unity, hence the flux density, Bϭ ␮ 0 ͓ Hϩ4 ␲ (1ϪN)M ͔ Hence, the low-field Hall mobility is not necessarily rep- ϭ ␮ 0 H, where H is the applied magnetic field in A/m. The resentative of the majority transport carriers. For example,15 fitting procedure was limited to the range of data 1.5 T in CrO2 there are a small number of high mobility holes and у ␮ 0 Hу0 T, because a slight curvature was observed in ␳ xy around 500 times more low mobility electrons, the low-field at larger fields, indicating that the low-field limit model ͑dis- Hall is hole-like, and the high-field Hall is electron-like, in cussed in the following͒ becomes inappropriate above 1.5 T. agreement with the thermopower. To investigate whether the The solid lines in Fig. 1 are fits to the data for the 80 nm film low-field Hall is representative of the majority transport car- at a selection of temperatures. The temperature dependence riers, the high field Hall resistivity of the 5 nm sample was of the low field limit ordinary Hall coefficient R O obtained measured, this is plotted in the inset to Fig. 1. There is a from this fitting procedure, for all the films, is shown in Fig. slight curvature toward less negative slope with increasing 2. The temperature dependence of ␳ xx is also shown in Fig. field, but unlike CrO2 ͑Ref. 15͒ neither a sign change nor the 2, for comparison. Note an increasingly strong low tempera- high field limit is reached by 8 T. This strongly suggests that ture upturn in ␳ xx is observed with decreasing thickness. the low-field Hall is representative of the majority transport In the stoichiometric bulk material R O remains positive at carriers. The curvature can be fit to the two band model16,15 all temperatures below T C . 13,14 It is immediately apparent but the refined parameters are strongly correlated and unique from Fig. 2͑b͒ that the transport in all these films is different fit could not be obtained. This is consistent with the to that material, as R O is increasingly negative as the tem- observation15 that a reliable fit can only be obtained by re- perature increases from 50 K, and as the thickness decreases. lating the band parameters to the measured low-field limit, 201305-2
  • 3. RAPID COMMUNICATIONS THICKNESS DEPENDENCE OF HALL TRANSPORT IN . . . PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒ high field limit and crossover point values. There is no fea- ture in the temperature dependence of R O associated with the resistivity upturn; this suggests that the resistivity upturn is not due to a freezing out of carriers, but to a decrease in carrier mobility. Detailed knowledge of the transport carriers as a function of thickness is important for understanding spin injection processes at ferromagnet:semiconductor interfaces. Four re- lations are required to determine the four band parameters, the Hall and the zero field resistivity provide two. Two- carrier transport analysis is routine in high mobility semicon- ductors, where the other two relations are obtained from the Shubnikov–de Haas oscillations and the MR. In these films that information is not accessible because the two-carrier MR is masked by the anomalously large positive MR17 and in metals Shubnikov–de Haas oscillations are only observed in extremely high fields. Therefore, only a qualitative analysis of the band parameters can be made. The sign reversal of the low field R O with increasing temperature, even in the thick- FIG. 3. ͑a͒ a and ͑b͒ b coefficients obtained from the fits to est film, shows that at low temperature pϾnz 2 and at high R S / ␳ xx ϭaϩb ␳ xx for all films. Because the magnetization of the temperature pϽnz 2 . z is unlikely to change dramatically 5nm film could not be measured directly, the magnetization loop of with temperature and n/p is almost certainly increasing with the 45 nm film was scaled by volume to obtain R S of the 5 nm film. temperature. The small amount of curvature in the high-field Bulk values taken from Otto et al. ͑Ref. 13͒. Inset to ͑b͒ R S / ␳ xx vs Hall indicates that, unlike CrO2 , 15 z is close to unity. The ␳ xx for the 45 nm film, dashed line is a guide to the eye. Solid lines band structure of stoichiometric NiMnSb contains both holes show fits to R S / ␳ xx ϭaϩb ␳ xx in regions above and below upturn. and electrons,6 with holes dominating the Hall resistivity,13,14 although the thermopower14 indicates a crossover to low the resistivity upturn two different straight lines are ob- electron-dominant transport. For the films studied here, a tained. The a and b coefficients obtained from linear fitting likely hypothesis is that the holes result from the bulk band of R S / ␳ xx vs ␳ xx above and below the upturn for all the films structure and their concentration is only weakly temperature are shown in Figs. 3͑a͒ and 3͑b͒, respectively. Above the dependent, whereas the electron concentration seems to be resistivity upturn, the coefficients are, within error, the same derived partly from the band structure and partly from a ther- as the stoichiometric bulk values13 of aϭϪ6.5ϫ10Ϫ4 TϪ1 mally activated process, such as thermal excitation of donor and bϭ21 500 TϪ1 ⍀ Ϫ1 mϪ1 , which were previously inter- states. The off-stoichiometry in these Ni1.15Mn0.85Sb films preted as side-jumps dominating over skew scattering. How- will result in a large number of atomic site defects, which are ever, below the resistivity upturn, the magnitudes of both the predicted7 to affect the band structure, and the difference slope and the intercept increase dramatically as the thickness between the stoichiometric bulk and the 400-nm-thick film is is increased from 5 to 110 nm, driven by the temperature likely to be a result of the stoichiometry. The increasingly dependence of ␳ xx . The implicit assumption in the tradi- electron dominated transport as a function of thickness is not tional R S / ␳ xx vs ␳ xx analysis13 is that R S is only indirectly a attributed to off-stoichiometry in our films as this did not function of temperature via its dependence on the resistivity change systematically with thickness. The trend can only be ͑scattering͒. It appears that both in the stiochiometric bulk explained by the increasing significance of electronic surface material, and these films, that assumption and the validity of or interface states, arising from either the reduced symmetry that model breaks down around 100 K. No change was ob- at the interfaces or strain induced defects. Note that unlike served by room temperature in the sign of R S in any of our the silver chalcogenides,18 there is no evidence of a cross- films. In a number of simple ferromagnets ͑such as Fe, Co, over of majority carrier at the MR maximum ͑resistivity up- Gd͒ the skew-scattering and side jump terms are of opposite turn͒. sign, but it is not known if that is the case in our material. Now let us turn to R S . The anomalous Hall effect has It is important to note that the anomalous Hall effect historically been ascribed19 to a scattering anisotropy, al- arises from an asymmetric deflection of the carriers, resulting though there can also be an intrinsic20 ͑scattering indepen- in an anomalous Hall conductivity, ( ␴ A ); the anomalous Hall dent͒ term, which is discussed in the following. In the scat- resistivity ( ␳ A ) is derived from the conductivity by ␴ A tering model, it was proposed13 that R S was derived from ϭ ␳ A /( ␳ xx ϩ ␳ 2 )Ϸ ␳ A / ␳ xx because ␳ xx ӷ ␳ xy . Since a qua- 2 xy 2 contributions from side-jump scattering and skew scattering, dratic behavior of R S in ␳ xx only requires a temperature in- and that these terms were proportional to ␳ xx and ␳ xx , re- 2 dependent ␴ A , it is not clear that any inferences can be made spectively. In bulk NiMnSb, this model accounts for the ex- about the scattering. perimental data at high temperatures, but there is a disconti- Recently, theories describing the anomalous Hall effect nuity in R S / ␳ xx vs ␳ xx at around 100 K.13,14 The inset to Fig. as an intrinsic Berry3 phase effect, have given a good quan- 3͑b͒ shows a typical R S / ␳ xx vs ␳ xx plot, from the 45 nm film titative agreement with experiment1,2 that was never ␳ xx which is non-monotonic. At temperatures above and be- achieved with the scattering model. Two types of Berry 201305-3
  • 4. RAPID COMMUNICATIONS W. R. BRANFORD et al. PHYSICAL REVIEW B 69, 201305͑R͒ ͑2004͒ phase induced AHE have, thus far, been reported, one is stiochiometric material. The thickness dependence is likely related to spin chirality in magnetically frustrated21 systems to be due to the increasing significance of interface or free and the other is associated with thermally induced topologi- surface electronic states, and indicates that controlled inter- cal defects that show an exponential temperature facial engineering will be required for the use of NiMnSb as dependence1 around T C . In our films there is no feature in a spin injector. The anomalous Hall conductivity cannot be the magnetization at the resistivity upturn temperature, and interpreted within the traditional scattering model at low this temperature is far from the Curie temperature, so the temperatures, because of an additional contribution that anomalous behavior of R S below the resistivity upturn is comes into play, which is attributed to a change in the spin- dissimilar to previously reported Berry systems. Although a dependent scattering. Unlike the silver chalcogenides, there Berry phase component cannot be ruled out, the change in appears to be no correlation between the large positive MR ␴ A at the resistivity upturn is probably due to a change in found in these films17 and the sign reversal in the ordinary spin dependent scattering. Hall coefficient. In summary, the room temperature electrical transport in non-stoichiometric Heusler thin films becomes increasingly We acknowledge the E.U. programme G5RD-CT-2001 electron dominated with decreasing thickness, in marked and the EPSRC GR/S14061 and EPSRC GR/R98945 for contrast to the spin-polarized holes predicted for the bulk funding. *Electronic address: w.branford@imperial.ac.uk Roy, and L. F. Cohen, Appl. Phys. Lett. ͑in press͒. 1 12 H. Yanagihara and M. B. Salamon, Phys. Rev. Lett. 89, 187201 W. R. Branford, S. B. Roy, S. K. Clowes, Y. Miyoshi, Y. V. Bugo- ͑2002͒. slavsky, S. Gardelis, J. Giapintzakis, and L. F. Cohen, J. Magn. 2 T. Jungwirth, J. Sinova, K. Y. Wang, K. W. Edmonds, R. P. Cam- Magn. Mater. ͑in press͒. 13 pion, B. L. Gallagher, C. T. Foxon, Q. Niu, and A. H. Mac- M. J. Otto, R. A. M. Vanwoerden, P. J. Vandervalk, J. Wijngaard, Donald, Appl. Phys. Lett. 83, 320 ͑2003͒. C. F. Vanbruggen, and C. Haas, J. Phys.: Condens. Matter 1, 3 M. V. Berry, J. Mod. Opt. 34, 1401 ͑1987͒. 2351 ͑1989͒. 4 F. Heusler, Verh. Dtsch. Phys. Ges. 5, 219 ͑1903͒. 14 C. Hordequin, D. Ristoiu, L. Ranno, and J. Pierre, Eur. Phys. J. B 5 M. J. Otto, H. Feil, R. A. M. Vanwoerden, J. Wijngaard, P. J. 16, 287 ͑2000͒. 15 Vandervalk, C. F. Vanbruggen, and C. Haas, J. Magn. Magn. S. M. Watts, S. Wirth, S. von Molnar, A. Barry, and J. M. D. Mater. 70, 33 ͑1987͒. Coey, Phys. Rev. B 61, 9621 ͑2000͒. 6 16 R. A. de Groot, F. M. Mueller, P. G. van Engen, and K. H. J. R. G. Chambers, Proc. Phys. Soc., London, Sect. A 65, 903 Buschow, Phys. Rev. Lett. 50, 2024 ͑1983͒. ͑1952͒. 7 17 D. Orgassa, H. Fujiwara, T. C. Schulthess, and W. H. Butler, W. R. Branford, S. K. Clowes, M. H. Syed, Y. V. Bugoslavsky, S. Phys. Rev. B 60, 13237 ͑1999͒. Gardelis, J. Androulakis, J. Giapintzakis, A. V. Berenov, S. B. 8 D. Ristoiu, J. P. Nozieres, C. N. Borca, B. Borca, and P. A. Dow- Roy, and L. F. Cohen ͑unpublished͒. ben, Appl. Phys. Lett. 76, 2349 ͑2000͒. 18 M. Lee, T. F. Rosenbaum, M. L. Saboungi, and H. S. Schnyders, 9 G. A. de Wijs and R. A. de Groot, Phys. Rev. B 64, 020402 Phys. Rev. Lett. 88, 066602 ͑2002͒. ͑2001͒. 19 L. Berger and G. Bergmann, in The Hall Effect and its Applica- 10 J. Giapintzakis, C. Grigorescu, A. Klini, A. Manousaki, V. Zorba, tions, edited by C. L. Chien and C. R. Westgate ͑Plenum, New J. Androulakis, Z. Viskadourakis, and C. Fotakis, Appl. Phys. York, 1979͒. Lett. 80, 2716 ͑2002͒. 20 J. M. Luttinger, Phys. Rev. 112, 739 ͑1958͒. 11 21 W. R. Branford, S. K. Clowes, M. H. Syed, Y. V. Bugoslavsky, S. Y. Taguchi, Y. Oohara, H. Yoshizawa, N. Nagaosa, and Y. Tokura, Gardelis, J. Androulakis, J. Giapintzakis, A. V. Berenov, S. B. Science 291, 2573 ͑2001͒. 201305-4