UPS Studies of Ni-Mn-Ga

In the previous chapter it was demonstrated that structural properties of stoi-chiometric and off-stoistoi-chiometric Ni-Mn-Ga samples differ significantly with respect to the character of the martensitic superstructure. It is therefore of par-ticular interest to compare also the electronic properties of these both classes of Ni-Mn-Ga samples. This was performed using temperature-dependent ul-traviolet photoelectron spectroscopy.

The (001)-oriented surface of the epitaxial film sample EF1 with bulk

compo-8.3. UPS Studies of Ni-Mn-Ga sitionNi_48.8Mn_25.6Ga_25.6was prepared by repeated cycles of Ne⁺sputtering and annealing (see also section 4.3 for further details of surface preparation). LEED measurements performed at room temperature (Fig. 8.7b) have demonstrated a well ordered austenitic surface. At low temperatures (i.e.130K) STM mea-surements revealed a twinned martensitic surface (see Fig. 8.7c). The surface structure clearly demonstrates characteristics of a 7M nanotwinned martensi-tic structure (twinning angle:≈3^◦, twinning periodicity:λ_7M ≈2nm). UPS data were recorded in normal emission geometry with±1^◦angle of acceptance that provides partial Brillouin zone integrated spectra. Using a pass energy of 5eV the analyzer resolution was around50meV. Both samples were measured using same settings.

Figure 8.7a illustrates the normal-emission UPS spectra of the EF1 sample for temperatures from299K to 161K. The measurements show one sudden redistribution of the intensity for a temperature around235K, which is iden-tified as the martensitic transformation temperature T_M and coincides with the transition temperature measured by SQUID (see Table 4.1). Below and aboveT_M the UPS spectra remain unchanged and indicate, which rules out the existence of an intermediate premartensitic phase. The shape of the235 K-spectrum shall be described by the fact that a martensitic transition in epitaxial films is broad an hence this spectrum is a combination of signals from auste-nitic and martensitic parts of the sample surface. STM measurements of the martensitic phase transition, which did not reveal any intermediate phases, support this hypothesis. On the right hand side of the figure only the spectra for the highest and the lowest temperature are presented. Both spectra exhibit the characteristic peak located at1.3eV BE, which originates primarily from Ni 3d-Mn3dhybridized states [26, 27]. As the temperature is lowered belowT_Mits position slightly shifts to a higher BE but the intensity increases. The austenitic spectrum additionally exhibits a prominent peak located at0.3eV BE and an intensity dip0.6eV BE. Also a weak shoulder can be recognized around0.8eV BE. BelowT_M the intensity of the0.3eV peak drops and UPS intensity around the0.7eV region increases significantly. The difference spectra presented in Figure 8.8b vividly show that a spectral weight transfer occurs from the dip at

0.3eV BE to the region between0.6eV and0.9eV BE. However, also a transfer of states aboveE_Fcan not be excluded.

10 nm

gradient

80 nm

topography

austenite T=293K

austenite martensite

martensite T=130K a)

b) c)

E = 89.5 eV E = 22.7 eV

Intensity (arb. units) Intensity (arb. units)

Figure 8.7.: (a) The ultraviolet photoemission spectra (UPS) of an off-stoichiometric Ni-Mn-Ga film sample (EF1) recorded as a function of temper-ature while cooling (He I,hν = 21.2eV).(b)LEED patterns recorded at room temperature in the austenitic state of the sample.(c) STM topography and a topography gradient of the Ni-Mn-Ga surface obtained at130K revealing a 7M structure with a nanotwinned superstructure in the martensitic state of the sam-ple (U_T= 1V,I_T= 0.15nA).

UPS measurements were also performed for the stoichiometric Ni2MnGa sample, which exhibits a 5M martensitic structure, both in austenitic and mar-tensitic state. The results, which are in good agreement with those reported in Ref. [26], are illustrated in direct comparison to the UPS spectra of the 7M sample in Fig. 8.8a. The spectra were normalized with respect to the inten-sity of the main peak. The spectra of the austenitic state are similar in shape, although a difference is visible at0.6eV BE. In the Ni2MnGa the intensity of this peak was shown to be related to a flat majority-spin band (see previous section) and it appears as if this band is missing in the austenitic 7M sample.

8.3. UPS Studies of Ni-Mn-Ga However other UPS measurements of off-stoichiometric 7M samples which were performed integrating over larger portions of the BZ did show a evidence for a peak in intensity in this region, but still the intensity was lower than in the case of a stoichiometric sample. Spectra of the martensitic state both show a decrease of the0.3eV peak, which was shown to be related to the minority-spin band crossingE_F. The difference is again found around0.65eV BE. While the 5M structure shows here a distinct peak, which could be also observed in ARPES measurements (see Fig.8.6), the 7M structure is featureless for the energy region of energies1eV below the Fermi energy.

a)

b)

austenite 5M 7M

5M 7M

5M-L2₁ L1 -L20 1

martensite 5M 7M

c)

Intensity (arb. units)Intensity (arb. units) Intensity (arb. units)Intensity (arb. units)

Figure 8.8.: (a)Comparison of UPS spectra of an off-stoichiometric Ni-Mn-Ga film sample (EF1) with a 7M structure and a stoichiometric Ni2MnGa single crystal with a 5M structure in austenitic and martensitic state. The intensity of the main peak at1.3eV has been normalized to unity (He I,hν = 21.2eV).

(b) Difference spectra obtained for each sample by subtracting the spectrum of the austenitic state from the spectrum of the martensitic state.(c) Theoreti-cal difference spectra Theoreti-calculated from first-principles total DOS spectra of the L2₁ (austenitic), 5M and L1₀ (martensitic) structure. Data were adapted from Ref. [96].

In the context of an adaptive martensitic phase, which has been discussed for off-stoichiometric samples in chapter 7, also the electronic properties should be addressed. The DOS of the adaptive martensitic phase should be - apart from fine details⁶ - nearly identical with the DOS of the NM L10 martensitic phase, since the L10unit cell represents the building block of the nanotwinned adap-tive phase [79]. The absence of features in the UPS spectrum at0.6eV binding energy of the martensitic phase of the 7M structure can be interpreted as a fingerprint of a L10 structure. This difference is expected from first-principles calculations of the DOS for the martensitic 5M and L10structure (see Fig. 8.6b).

While the DOS of the L10 structure is featureless for BE betweenE_Fand0.8eV, the 5M DOS exhibits a peak in the minority-spin channel at0.6eV ( ). And this difference can be also observed in the experimental data of the martensitic state for the stoichiometric sample with 5M structure and the off-stoichiometric sam-ple with the nanotwinned 7M structure, where the electronic feature at≈0.6eV is absent for the nanotwinned structure (see fig. 8.8a). However, the measure-ments should be questioned critically. The difference spectra for both sample types displayed in Fig. 8.8b show a similar spectral weight transfer as sample transforms from the austenitic to the martensitic state. Also the theoretical difference spectra in Fig. 8.8c, which are in good agreement with experimental data (Fig. 8.8b), are very similar for both types of the martensitic structure. It is therefore reasonable to conclude, that more precise measurements of electronic properties such as spin-resolved photoemission experiments are required to identify the electronic structure of the nanotwinned martensitic phase.

8.4. Summary

In summary, temperature dependent photoemission measurements of sev-eral Ni-Mn-Ga samples were presented. Angle-resolved measurements pro-vided band structure and 2D cuts of the Fermi surface for the stoichiometric Ni2MnGa. A nesting feature, which is believed to trigger the (pre)martensitic transition, was identified in the austenitic phase. BelowT_Ma dramatic

deple-6 The differences are due to (101) nanotwin boundaries, which serve as defects.

8.4. Summary tion of states located up to0.3eV belowE_F,associated with the appearance of an energy gap at the Fermi level for the band of the nested FS sheet and a shift of its minimum, was observed. Along with the observation of charge-density waves by means of STM these observations provide for this alloy a complete picture of a martensitic transition driven by FS nesting and strong electron-phonon coupling. The analysis was extended also to off-stoichiometric Ni-Mn-Ga sam-ples showing 7M martensitic structure. Angle-integrated photoemission mea-surements revealed similar modifications of the electronic structure over the transition from austenite to martensite for both types of the resulting martensi-tic structure, i.e. 5M and 7M. This observation suggests similar driving forces of the martensitic transition in stoichiometric and off-stoichiometric Ni-Mn-Ga alloys.