Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

Abstract

The origin of the martensitic transition in the magnetic shape memory alloy Ni-Mn-Ga has been widely discussed. While several studies suggest it is electronically driven, the adaptive martensite model reproduced the peculiar nonharmonic lattice modulation. We used femtosecond spectroscopy to probe the temperature and doping dependence of collective modes, and scanning tunneling microscopy revealed the corresponding static modulations. We show that the martensitic phase can be described by a complex charge-density wave tuned by magnetic ordering and strong electron-lattice coupling. DOI: 10.1103/PhysRevLett.115.076402 PACS numbers: 71.45.Lr, 78.47.-p, 81.30.Kf, 81.70.Fy Magnetic shape memory alloys present a new type of multifunctional materials, which display strong coupling between the magnetic and structural degrees of freedom. The ferromagnetic Ni-Mn-Ga alloy serves as a prototype system, displaying a giant 12% magnetic-field-induced strain in its low-temperature (T) martensitic phase (M phase) [1,2]. Since in Ni-Mn-Ga the M-phase transition can be tuned far above the room temperature by changing stoichiometry or doping [3,4], this alloy is of special technological interest [1,2,5].The rich phase diagram of Ni 2þxþy Mn 1−x Ga 1−y is characterized by a complex sequence of phase transitions. In its high-T (austenite) phase, Ni-Mn-Ga has a cubic L2 1 Heusler structure. At the M-phase transition temperature T M , the lattice undergoes a transformation, which can be described by a periodic shuffling of (011) planes along the ½011 direction [6]. Depending on the stoichiometry and the residual stress, the resulting low-T M phase is commonly found to have either tetragonal or orthorhombic symmetry, with a modulation period of ten (10M) or 14 (14M) atomic layers [6,7], respectively. For specific stoichiometries, e.g., Ni 2þx Mn 1−x Ga with x ≲ 0.1, a premartensitic phase (PM phase) is observed above T M [3] with the transition temperature T PM up to 50 K above T M [3,8]. In the PM phase, the lattice displays a harmonic threefold modulation with the wave vector q PM ¼ q max ð 1 3 ; 1 3 ; 0Þ. Inelastic neutron scattering studies of the high-T cubic phase [9] showed a dramatic softening of the TA-2 phonon branch at q PM , indicative of a Kohn anomaly. These observations [9][10][11][12], together with the observed opening of a pseudogap at T PM [8,13], suggest an electronic instability within the Peierls scenario to be driving the PM-phase transition. The observation of a phase mode in the 10M phase [14] also suggested the electronic instability. In fact, the electronic band structure studies [11,15,16] revealed the possible Fermi surface nesting conditions for all of the observed structural modulations. However, for the case of the 14M martensite, it was argued that the modulated phase can be constructed from nonmodulated martensitic unit cells [17,18]. This adaptive martensite scenario [17][18][19] recently received considerable attention.To determine which theory gives a more appropriate physical picture of the pha...

Modulations in martensitic Heusler alloys originate from nanotwin ordering

“…This misfit can be closed by stacking faults 12 that are inserted with a substantially larger period than the modulation period. As this stacking fault period is not solely defined by the lattice constants of martensite but also depends on the austenite, the arrangement is incommensurate when measuring a completely martensitic sample, which is in agreement with several diffraction experiments 5 , 22 , 62 . Compared to a periodic stacking sequence, the presence of stacking faults increases the energy of an incommensurate nanotwin arrangement.…”

“…In modulated structures the atomic positions deviate in a quite periodic way from a simple tetragonal structure. The origin of the modulation is not well-understood, with two competing explanations put forward, so far 5 – 8 .…”

Magnetocaloric materials: From micro- to nanoscale

“…7. 118 Note that recently the origin of the martensitic transition in the MSMA Ni-Mn-Ga was described by Schubert et al, 128 like a complex charge-density wave tuned by magnetic ordering and strong electron-lattice coupling. Moreover Zhang et al 129 showed the advantage of tailoring the thin film composition so that the magnetostructural martensitic transformation and the FM ordering of the austenitic phase occurred simultaneously.…”

Geometry of adaptive martensite in Ni-Mn-based Heusler alloys

“…In this concept, the modulated structures are described as nanotwinned microstructures of the tetragonal non-modulated (NM) phase that form to minimize the elastic energy at the phase boundary. Alternative models base on Fermi surface nesting [16,17] and phonon softening [18] as driving mechanism. A unified description of both concepts, which is based on the ordering of nanotwin boundaries, was proposed recently [19].…”