P. Tong

Magnetic transition broadening and local lattice distortion in the negative thermal expansion antiperovskite Cu1−xSnxNMn3

The local distortion indicated by the split of the Cu/Sn-Mn bonds for the negative thermal expansion (NTE) materials Cu1−xSnxNMn3 (x = 0.1 and 0.5) was observed using neutron pair distribution function. The distribution of Cu/Sn-Mn bonds upon Sn doping is suggested to be attributable to the fluctuation in the hybridization of Mn d with Sn p orbitals. Accordingly, the antiferromagnetic (AFM) coupling mediated by the p-d hybridization fluctuates in strength. Consequently, the AFM transition closely coupled with the volume change is broadened, leading to the NTE.

Structural, magnetic, electrical transport properties, and reversible room-temperature magnetocaloric effect in antipervoskite compound AlCMn3

We report the structural, magnetic, electrical transport properties, and magnetocaloric effect (MCE) of antipervoskite compound AlCMn3. It exhibits a second-order ferromagnetic–paramagnetic phase transition around (TC) 287 K. The electronic resistivity (ρ) shows a good metallic behavior except for a slope change around TC. At lower temperatures (below 130 K), ρ∝T2 indicates that the electron-electron scatterings domain. At evaluated temperatures (130–270 K), ρ is linear dependence on temperature, implying that the phonon scatterings boost up greatly. Furthermore, a broad distribution of the magnetic entropy change (−ΔSM) peak is found to about 100 K with the magnetic field change ΔH=45 kOe. The relative cooling power are ∼137 J/kg and ∼328 J/kg (or ∼68 K2 and ∼162 K2) with ΔH=20 kOe and 45 kOe, respectively. All these values are comparable with the typical MCE associated with a second-order transition. It suggests that AlCMn3 may be considered as a candidate material for near room-temperature magnetic ref...

Tunable negative thermal expansion related with the gradual evolution of antiferromagnetic ordering in antiperovskite manganese nitrides Ag1−xNMn3+x (0 ≤ x ≤ 0.6)

The thermal expansion and magnetic properties of antiperovskite manganese nitrides Ag1-xNMn3+x were reported. The substitution of Mn for Ag effectively broadens the temperature range of negative thermal expansion and drives it to cryogenic temperatures. As x increases, the paramagnetic (PM) to antiferromagnetic (AFM) phase transition temperature (TN) decreases. At x~0.2, the PM-AFM transition overlaps with the AFM to glass-like state transition. Above x=0.2, two new distinct magnetic transitions were observed: one occurs above room temperature from PM to ferromagnetic (FM), and the other one evolves at a lower temperature (T*) below which both AFM and FM orderings are involved. Further electron spin resonance measurement suggests that the broadened volume change near T* is closely related with the evolution of {\Gamma}5g AFM ordering.

Tunable room-temperature zero temperature coefficient of resistivity in antiperovskite compounds Ga1−xCFe3 and Ga1−yAlyCFe3

The effects of the Ga content and the substitution of Al for Ga on the temperature coefficient of resistivity (TCR) of antiperovskite GaCFe3 have been investigated systematically. Our results indicate the value of TCR and its temperature range can be tuned by altering chemical compositions. With decreasing the Ga content in Ga1−xCFe3 or increasing Al dopant in Ga1−yAlyCFe3, the sign of TCR changes from positive to negative and room-temperature zero TCR material can be achieved. Typically, the optimized TCR values are about −5.72 ppm/K(265–315 K) and −14.68 ppm/K(280–320 K) for Ga0.95CFe3 and Ga0.85Al0.15CFe3, respectively. The possible mechanisms for the observed low TCR are discussed.

Magnetic and electrical/thermal transport properties of Mn-doped Mn+1AXn phase compounds Cr2-xMnxGaC (0 《 x 《 1)

In this paper, we report the effects of partial substitution of Mn for Cr on the structural, magnetic, and electrical/thermal transport properties of Mn+1AXn phase compounds Cr2−xMnxGaC (0 ≤ x ≤ 1). As a result, the unit cell volume and the thermal conductivity decrease while the resistivity increases with increasing x. Interestingly, the magnetism of Cr2−xMnxGaC changes from the nonmagnetic Cr2GaC (x = 0) to the ferrimagnetic CrMnGaC (x = 1). In order to shed light on the discrepancy observed between Hall coefficient and Seebeck coefficient of Cr2GaC, the electrical conductivity, Hall coefficient, and magnetoresistance are analyzed within a two-band model. Furthermore, an upturn is observed in low-temperature specific heat of Cr2−xMnxGaC, which may be related with the magnetic Mn dopant.

The magnetic, electrical transport and thermal transport properties of Fe-based antipervoskite compounds ZnCxFe3

The effects of carbon concentration on the crystal structure, magnetic, and electrical/thermal transport properties of ZnCxFe3 (1.0 ≤ x ≤ 1.5) have been investigated systematically. Both the Curie temperature and the saturated magnetization decrease firstly and then reach saturation with increasing x. The investigations of heat capacity and resistivity indicate that ZnC1.2Fe3 displays a strongly correlated Fermi liquid behavior considering its Kadowaki-Woods ratio (∼0.64 a0). Around the ferromagnetic-paramagnetic phase transition (∼358 K), a reversible room-temperature magnetocaloric effect is observed. The relative cooling power (RCP) is ∼164 J/kg (∼385 J/kg) with the magnetic field change ΔH = 20 kOe (45 kOe). Considering the considerable large RCP, inexpensive and innoxious raw materials, ZnC1.2Fe3 is suggested to be a promising candidate for room-temperature magnetic refrigeration. Furthermore, the studies of thermal transport properties indicate that ZnC1.2Fe3 can also be a potential thermoelectric m...

Composition dependent-magnetocaloric effect and low room-temperature coefficient of resistivity study of iron-based antiperovskite compounds Sn1−xGaxCFe3 (0 ≤ x ≤ 1.0)

We present the magnetic/structural phase diagram of Sn1-xGaxCFe3 (0 ≤ x ≤ 1). With increasing x, Curie temperature (TC) and saturated magnetization increase while lattice constant decreases. The results indicate that GaCFe3 may be a promising high-temperature soft magnetic material. Around TC, chemical composition-dependent magnetocaloric effect is studied. The relative cooling power increases with increasing x, reaching a maximum of ∼3.22 J/cm3 (∼61% of Gd) around 345 K. Considering their remarkable advantages, Sn1-xGaxCFe3 are suggested to be promising magnetic refrigerant materials. The low-T resistivity displays a metallic behavior for x ≤ 0.80 and a semiconductor-like behavior for GaCFe3. Furthermore, room-temperature coefficient of resistivity is comparative (∼46.2 ppm/K for GaCFe3, 250-310 K).

Colossal negative thermal expansion with an extended temperature interval covering room temperature in fine-powdered Mn0.98CoGe

MnM′X (M′ = Co, Ni; X = Ge, Si, etc.) alloys usually present a large volumetric change during the Martensitic (MA) transformation. This offers a great opportunity for exploring new negative thermal expansion (NTE) materials if the temperature interval of NTE can be extended. Here, we report colossal NTE in fine-powdered Mn0.98CoGe prepared by repeated thermal cycling (TC) through the MA transition or ball milling. Both treatments can expand the MA transformation, and thus broaden the NTE temperature window (ΔT). For the powders that have gone through TC for ten times, ΔT reaches 90 K (309 K–399 K), and the linear expansion coefficient (αL) is about −141 ppm/K, which rank among the largest values of colossal NTE materials. The difference between two kinds of treatments and the possible mechanisms of the extended MA transformation window are discussed based on the introduced strain.

Giant negative thermal expansion covering room temperature in nanocrystalline GaNxMn3

Nanocrystalline antiperovskite GaNxMn3 powders were prepared by mechanically milling. The micrograin GaNxMn3 exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50 K as the average grain size (⟨D⟩) is reduced to ∼30 nm. The corresponding coefficient of linear thermal expansion (α) reaches ∼ −70 ppm/K, which are comparable to those of giant NTE materials. Further reducing ⟨D⟩ to ∼10 nm, ΔT exceeds 100 K and α remains as large as −30 ppm/K (−21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling ⟨D⟩, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions.

Spin-glass behavior and zero-field-cooled exchange bias in a Cr-based antiperovskite compound PdNCr3

We report the synthesis, structure, and magnetic and electrical/thermal transport properties of a Cr-based antiperovskite compound PdNCr3, which crystallizes in MgCNi3-type cubic structure (space group Pmm, No. 221). Interestingly, the spin-glass (SG) behavior, which is confirmed by the corresponding characteristic parameters (the freezing temperature T0 = 61.4(2) K, the dynamical exponent zν = 7.103(3), and the flipping time τ0 = 2.714(2) × 10−11 s), is observed in PdNCr3. Furthermore, the value of the Sommerfeld–Wilson ratio (RW ∼ 1.024(3)) for PdNCr3 is much smaller than those of cluster glass systems (RW > 100) and Kondo cluster glass systems (RW = 20–30), indicating that PdNCr3 is a canonical SG system. Density functional theory calculation shows that the origin of SG in PdNCr3 is attributed to the disordering located N vacancies, which is further confirmed by the measurement of sample PdN0.75Cr3 with more N deficiency. On the other hand, infrequently, the zero-field-cooled exchange bias (ZFC-EB) with an exchange bias field (HE) of about 350 Oe is observed after zero-field cooling from an unmagnetized state in PdNCr3. The values of HE are found to depend strongly on temperature and measuring magnetic field. For PdNCr3, the ferromagnetic unidirectional anisotropy, which is the origin of our ZFC-EB effect, is formed around the ferromagnetic–SG interface isothermally during the initial magnetization process below the blocking temperature. In addition, the training effect of ZFC-EB in PdNCr3 is observed after the zero-field cooling process and has been explained well in terms of the spin configurational relaxation model.