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We report here the electrical transport and magnetic properties of the newly synthesized quaternary Heusler compounds CoFeTiSn and CoFeVGa. Our study indicates a striking change in the electronic transport properties of CoFeTiSn as the system undergoes the paramagnetic to ferromagnetic transition. While the sample shows an activated semiconducting behaviour in the paramagnetic phase, it turns abruptly to a metallic phase with the onset of ferromagnetic transition. We have compared the system with other Heusler compounds showing similar anomaly in transport, and it appears that CoFeTiSn has much similarities with the Fe 2 VAl compound having pseudogap in the paramagnetic phase. On the other hand, CoFeVGa shows a semiconducting behaviour from high temperature to 90 K. On cooling below 90 K, a broad hump like feature is observed. Both the compositions show negative Seebeck coefficient varying linearly with temperature. The value of the Seebeck coefficient of CoFeTiSn is comparable to that of many Heusler alloys identified as potential thermoelectric materials.

Heusler compounds are a large group of intermetallic compositions with versatile material properties. In recent times, they are found to be important for their practical applications in the fields of spintronics and shape memory effect. Interestingly, their physical properties can be easily tuned by varying the valence electron concentration through proper doping and substitution. Empirical laws concerning the valence electron concentration, such as Slater-Pauling or Hume-Rothery rules are found to be useful in predicting their electronic, magnetic and structural properties quite accurately. Electrical transport measurements are simple laboratory-based techniques to gather a handful of information on the electronic properties of metals and semiconductors. The present review aimed to provide a comprehensive view of the transport in 3dand 4dtransition metal-based bulk Heusler compositions. The main emphasis is given on resistivity, magnetoresistance, Hall effect, thermopower and spin-dependent transport in spintronics devices. The review primarily focuses on magnetic Heusler compounds and alloys, albeit it also addresses several non-magnetic materials showing superconductivity or large thermopower.

The complex metallic alloy ${\mathrm{Mn}}_{3}\mathrm{In}$ has recently attracted attention for being a probable candidate for the fully compensated half-metallic ferrimagnet. The compound is associated with a rather unusual negative coefficient of resistivity approximately above the ferrimagnetic, ${T}_{C}=80\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our investigation indicates that that the ground state of the alloy is spin-glass like, which possibly coexists with the ferrimagnetic state. The carrier density obtained from the Hall-effect measurement shows a jump below ${T}_{C}$; afterward, it gradually increases with lowering temperature and shows a saturating tendency at the lowest temperature. The electronic structure calculations indicate that, at the Fermi energy, dominant contributions come from minority-spin channels with larger bandwidth. In contrast, very weak but nonzero states arise from the majority-spin channel, and half metallicity can be ruled out. The calculations also found that the antiferromagnetic alignment of Mn spins with nonzero resultant magnetic moment confirms the ferrimagnetic nature consistent with the experimental results. The high-temperature phase with negative temperature coefficient of resistivity can be related to the localization of charge carriers through electron-phonon interactions.

The quaternary Heusler compound NiFeTiSn can be considered to be derived from the exotic pseudogap-compound Fe2TiSn by the replacement of one Fe atom by Ni. In contrast to Fe2TiSn, which shows a disorder-induced ferromagnetic phase, the ground state of NiFeTiSn is antiferromagnetic with the signature of spin canting. Interestingly, NiFeTiSn shows a Griffiths-like phase characterized by isolated ferromagnetic clusters before attaining the antiferromagnetic state. The Griffiths-like phase is possibly associated with the antisite disorder between Fe and Ti sites as evident from our powder X-ray diffraction study. The compound also shows rather an unusual temperature dependence of resistivity, which can be accounted for by the prevailing structural disorder in the system. NiFeTiSn turned out to be a rare example where a Griffiths-like phase is observed in a semiconducting 3d transition metal-based intermetallic compound with an antiferromagnetic ground state., Comment: 12 pages, 9 figures

Mn2RuGa is known to be a ferrimagnetic compound below 460 K, while Ru2MnGa is an antiferromagnet with Neel temperature as low as 15 K. In the present work, we have studied the solid solutions of these two compositions, namely Mn 2 - x Ru 1 + x Ga (x = 0.2, 0.5), intending to design a fully compensated ferrimagnet. We successfully synthesized the compositions which crystallize with an inverse Heusler (XA) structure. The magnetic transition temperatures of both the alloys are found to be just above room temperature, and the magnetization data indicate a ferrimagnetic ordering. The compositions show spin glass like state at low temperature, which possibly emerges from the disorder and random magnetic correlations due to doping. Most interestingly, the x = 0.5 alloy shows the vanishingly small moment at low temperature, which can be explained on the basis of antiparallel alignment of moments at two inequivalent sites of Mn. The composition also shows a significant amount of exchange bias at low temperature. The observation of exchange bias for a system with a low value of net magnetization is much preferred for magneto-electric applications. Interestingly, both the samples show a transition from a high-temperature semiconducting phase to a low-temperature metallic phase. The electrical transport in the high-temperature phase is found to be dominated by small polaron hopping.

The electronic structure of PrAg in the ferromagnetic phase has been studied as a function of pressure. The magnetic instability is analysed by calculating the total energy in the local density approximation. From the spin-polarized calculation, it is found that the magnetic moment decreases under pressure. The constant f occupation number as a function of pressure indicates that the moment reduction is not due to the f electrons' delocalization.

Mn11Ge8 undergoes long range ferromagnetic ordering at 274 K followed by non-collinear antiferromagnetic structure below 150 K. Our investigations indicate the existence of large positive magnetoresistance which changes sign depending upon the applied field and temperature. In the temperature variation of resistivity and heat capacity, the sample shows very large coefficients of quadratic and linear terms, respectively. This indicates the existence of giant spin fluctuation despite the fact that the sample is in a magnetically ordered state. The Mn-moments possibly have both localized and itinerant character and the anomaly appears to be associated with the local moment spin fluctuations mediated via itinerant electrons. The large positive magnetoresistance is believed to be originated from the possible development of short range antiferromagnetic clusters on application of magnetic field.