Ni 2 MnGa(001) Surface Observed by STM

In Figure 6.3(a) a topographic STM image of the Ni2MnGa surface in the aus-tenitic state is presented. The surface resembles large flat terraces, which are separated by sharp steps. It is apparent that these steps are preferentially run-ning along the [100] and [010] direction, which reveals the cubic symmetry of the austenitic L21 unit cell. In Figure 6.3b a height profile marked in (a) is displayed graphically. The line profile reveals mainly steps with an average step height of3.0±0.2Å. This value corresponds well to a half of the austenitic L21 unit cell witha_A = 5.825Å (see Fig. 2.1). Also steps with a step height cor-responding to the lattice parameter of a full L21 unit cell occur (see Fig. 6.3b).

6.2. Austenitic Phase

2 nm

20 nm 0 nm

L2₁ [100] a_A

aA/2 b)

Mn-Ga layer

Mn-Ga layer Ni layer a)

Figure 6.3.: (a)STM topography (165×165nm²) of the Ni2MnGa(001) surface in the austenitic state (U_T = 1.0V,I_T = 0.16nA). Large terraces and sharp steps running along the L21[100] and [010] direction are visible.(b)Height profile corresponding to the line scan in (a).

Steps between two adjacent atomic layers, with a step height corresponding to a_A/4, were observed only very rarely. This finding leads to the conclusion that steps between terraces occur between two adjacent equivalent atomic layers, i.e. between two neighboring Ni layers or two neighboring Mn-Ga layers (this case is schematically shown in Fig. 6.3b).

More detailed atomically resolved STM images in Figure 6.4 can give more insight into the the atomic arrangement and the termination of the observed ter-races. In the STM images 6.4a and b an arrangement of bright features (atoms) in a quadratic lattice, which is rotated by45^◦ with respect to the image border, can be observed. The image border is aligned along the [100]-direction of the Ni2MnGa L21structure, which in turn means that the imaged surface atoms are aligned along the [110]- and the[¯110]-direction. The nearest-neighbor distance between two atoms is4.1±0.4Ź. This value agrees well with the Mn-Mn or Ga-Ga interatomic distance of4.12Å (see Fig. 6.4d) [12]. Also the orientation of the detected surface unit cell corresponds to the lattice consisting either of Mn or Ga atoms (compare Fig. 6.4a and d, unit cell). For a Ni-terminated surface one would also expect a quadratic arrangement of the lattice, but with

1 The relatively large standard deviation is due to drift during the measurement.

Ni Mn Ga

L2[010]1

2.91 Å

a=A5.83 Å aA=5.83 Å

4.12 Å

1 c)

1 a)

b)

1 nm L2 [100]₁

d)

e)

Figure 6.4.: (a)and(b)Atomically resolved topographic images showing a Mn-Ga terminated (001)-oriented surface of Ni2MnGa in the austenitic state. The images were obtained on different terraces at different tunneling voltages (a:

U_T = 50mV,I_T = 35nA, b:U_T = 15mV,I_T = 12nA).(c)Height profile corre-sponding to the linescan in (a).(d)Orientation of different (001) atomic planes of the L21 Ni2MnGa unit cell including the interatomic distances [12] are de-picted.(e)Calculated element-specific electronic density of states of cubic L21

Ni2MnGa obtained by Entelet al.[74]. Mainly Mn and Ni contribute to the total DOS nearE_F.

atoms aligned along the [100]- and [010]-direction with an interatomic distance of2.91Å [12] (compare Fig. 6.4a and d, unit cell). From these results it can be therefore concluded that a Mn-Ga-terminated (001)-oriented surface is ob-served with only one atomic species (Mn or Ga) visible in STM measurements.

The observation of a Mn-Ga-terminated Ni-Mn-Ga surface is also supported by a photoemission study by Dhakaet al.[100].

The specific atomic contrast observed in STM measurements of the Ni2MnGa surface can be attributed to two effects:

6.2. Austenitic Phase

I Electronic effect of element specific DOS at the Fermi level.

II Topographic effect of buckling of one sort of atoms.

As it can be seen from first principles calculations of the element-specific DOS [74, 96], the contribution of Ga states to the total DOS is almost negligible com-pared to the Ni and Mn states (see Fig. 6.4(e)). This could explain the observa-tion, that for a Mn-Ga-terminated surface one sort of atoms (Ga) is rendered invisible to the STM tip for the tunneling bias used for imaging. This consid-eration is highlighted in the height profile presented in Fig. 6.4c. However, no considerable changes in the STM contrast were observed for a wide range of tunneling parameters. Consequently, a topographic effect has to be taken into account. A buckling phenomenon of the Mn-Ga-terminated surface has been discussed in the previous section in connection with LEED measurements. A possible relaxation of the atomic positions could result in an outward protru-sion of one atomic species. This possible buckling is significant for the STM image, since it makes the lower lying sort of atoms invisible to the STM tip [155]. This effect could explain the obtained STM results.

Finally also the issue of large dark features, which can be frequently ob-served in atomically resolved images (see Figs. 6.4a and b), shall be addressed.

These features have lateral dimensions of 1 to several nanometers and appear to be distributed randomly on the surface. Height profiles show, that a topo-graphical depth of≈ 100to≈150pm can be assigned to these features. This corresponds approximately to a distance between two adjacent (001)-oriented atomic planes of the L21unit cell. Also no considerable change of this height information was observed for a wide range of tunneling voltage parameters.

Therefore it is concluded that these dark features indicate defects of the Mn-Ga layer, e.g. missing Mn and Mn-Ga atoms. This conclusion is also supported by the observation, that the surface defect density decreases with increasing annealing temperature of the sample.

6.2.3. Summary

The study of the (001)-oriented surface of the stoichiometric Ni2MnGa per-formed in the austenitic state at room temperature can be summarized as it

follows: A reconstruction-free Mn-Ga terminated surface was observed, and a pronounced geometric outward protrusion of Mn or Ga atoms leads to STM images with only one atomic species visible.