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Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni2MnGa over the temperature range 100 K < T < 250 K. All measurements detect clear signatures of the premartensitic transition (TPM ∼ 247 K) and the martensitic transition (TM ∼ 196 K). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at TPM located 0.3 eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3 eV in the UPS spectra for T > TPM is due to the Ni-d minority-spin electrons. Below TM this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni-d and Mn-d electronic contributions to the majority-spin density of states and is accompanied by significant reconstruction of the Fermi surface. [5] as a system undergoing a martensitic transition (MT) in its ferromagnetic phase (T C ∼ 380 K) with little magnetic hysteresis. In the last decade research on these alloys has focused on the structural and magnetic characterization and on their shape-memory applications [6]. First-principles calculations [7,8] and measurements on shape-memory alloys [9,10,11,12] indicate the driving role of the electronic structure and its relation to the lattice dynamics.The lattice dynamics of Ni 2 MnGa has been investigated from ultrasonic measurements [13] and neutron diffraction experiments [14,15,16]. It was found that the transverse TA 2 phonon branch exhibits pronounced softening at 1/3 of the zone boundary on decreasing the temperature, and this softening was described as a Bain distortion in the context of the Wechler, Lieberman, and Read theory of martensite formation [17]. In similar structural shape-memory alloys InTl [9], AuZn [10] and NiAl [11], this softening is associated with nesting features of the Fermi surface [8]. Below a certain temperature, there is a freezing of the displacements associated with this soft phonon so that a micro-modulated phase forms, which is described as a periodic distortion of the parent cubic phase [14]. In Ni 2 MnGa, the premartensitic phase develops with little or no thermal hysteresis and is driven by a magnetoelastic coupling [18]. On further cooling, Ni 2 MnGa transforms to an approximately fivelayered quasi-tetragonal martensitic structure. The lowtemperature phase is incommensurate [14] with a period (0.43,0.43,0) and exhibits well-defined phasons best characterized as charge-density wave (CDW) excitations [16].In this paper we study the role of conduction electrons in the two-step MT in Ni 2 MnGa using photoemission spectroscopy and thermodynamic measurements. LEED and X-ray diffraction Laue measurements show the quality of our sample is appropriate for high-resolution photoemission spectroscopy. Ultraviolet photoemission (UPS) measurements show the opening of a pseudogap at 0.3 eV below the Fermi energy at the MT and provide further evidence that the Fer...

The heat capacity and thermal expansion of a high quality single crystal of MnSi were measured at ambient pressure at zero and high magnetic fields. The calculated magnetic entropy change in the temperature range 0-30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the thermal expansion coefficient in the range 30-150 K, which correlates with an enhancement of the linear electronic term in the heat capacity. A surprising similarity among the variations of the heat capacity, thermal expansion coefficient and temperature derivative of the resistivity is observed through the phase transition in MnSi. Specific forms of the heat capacity, thermal expansion coefficient and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as the combination of sharp first-order features and broad peaks or shallow valleys of as yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state slightly above the transition temperature in MnSi.

Specific-heat and magnetic-susceptibility measurements are reported for the polycrystalline spinel compounds GeNi 2 O 4 and GeCo 2 O 4 in magnetic fields up to 14 T and 0.5 K Յ T Յ 400 K. Both compounds have first-order antiferromagnetic transitions. There are two sharp closely spaced magnetic-ordering anomalies for GeNi 2 O 4 at Néel temperatures T N1 ͑0͒ = 12.080 K and T N2 ͑0͒ = 11.433 K in zero magnetic field. There is also a broad anomaly in the specific heat centered at ϳ5 K, which is present for all fields. Spin waves with an average gap of 10.9 K are associated with this anomaly, which is confirmed by neutron-scattering measurements. An unusual feature of the antiferromagnetism for GeNi 2 O 4 is the simultaneous presence of both gapped and ungapped spin waves in the Néel state, inferred from the specific-heat data. GeCo 2 O 4 has a single anomaly at T N ͑0͒ = 20.617 K in zero magnetic field. Spin waves with an average gap of 38.7 K are derived from fitting the low-temperature specific heat and are also observed by neutron scattering. For both compounds ϳ50% of the derived magnetic entropy is below the ordering temperatures, and the total magnetic entropies are only ϳ60% of that predicted for the Ni 2+ and Co 2+ single-ion ground-state configurations. The missing entropy is not linked to magnetic disorder in the ground state or hidden ordering below 0.5 K. It is postulated that the missing entropy is accounted for by the presence of substantial magnetic correlations well above the Néel temperatures. Fitting the GeNi 2 O 4 susceptibilities to the Curie-Weiss law yields parameters that are consistent with those found for Ni 2+ ions in a crystal-electric-field environment including octahedral and trigonal components. The application of the Curie-Weiss law to the GeCo 2 O 4 susceptibilities is not valid because of low-lying crystal-electric-field states.

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