http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

We report the discovery of a large negative thermal expansion (NTE) up to α=−25×10−6K−1 (α: coefficient of linear thermal expansion) in Ge-doped anti-perovskite manganese nitrides Mn3AN (A=Cu,Zn,Ga). This gigantic NTE is several to ten times higher than that of commercially available NTE materials. The discontinuous lattice expansion seen in pure Mn3AN is broadened by Ge substitution over a wide temperature window, at widest ΔT∼100K, around room temperature. Such a large, isotropic and nonhysteretic NTE is desirable for practical applications.

Giant negative thermal expansion in Ge-doped anti-perovskite manganese nitrides

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a “perfect” NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Caloric effects in magnetic materials are promising for many applications. A significant barocaloric effect is observed in Mn3GaN and shown to be promoted by frustration arising from its antiferromagnetism.

Giant barocaloric effect enhanced by the frustration of the antiferromagnetic phase in Mn3GaN.

The perovskite manganites with generic formula RE1?xAExMnO3 (RE = rare earth, AE = Ca, Sr, Ba and Pb) have drawn considerable attention, especially following the discovery of colossal magnetoresistance (CMR). The most fundamental property of these materials is strong correlation between structure, transport and magnetic properties. They exhibit extraordinary large magnetoresistance named CMR in the vicinity of the insulator?metal/paramagnetic?ferromagnetic transition at relatively large applied magnetic fields. However, for applied aspects, occurrence of significant CMR at low applied magnetic fields would be required. This review consists of two sections: in the first section we have extensively reviewed the salient features, e.g.?structure, phase diagram, double-exchange mechanism, Jahn?Teller effect, different types of ordering and phase separation of CMR manganites. The second is devoted to an overview of experimental results on CMR and related magnetotransport characteristics at low magnetic fields for various doped manganites having natural grain boundaries such as polycrystalline, nanocrystalline bulk and films, manganite-based composites and intrinsically layered manganites, and artificial grain boundaries such as bicrystal, step-edge and laser-patterned junctions. Some other potential magnetoresistive materials, e.g.?pyrochlores, chalcogenides, ruthenates, diluted magnetic semiconductors, magnetic tunnel junctions, nanocontacts etc, are also briefly dealt with. The review concludes with an overview of grain-boundary-induced low field magnetotransport behavior and prospects for possible applications.

https://iopscience.iop.org/article/10.1088/0953-8984/20/27/273201/pdf

Low field magnetotransport in manganites

ZnNMn 3 has an unusual cubic-to-cubic structural phase transition near 160 K, which coincides with the onset of the antiferromagnetic (AFM) ordering. The phase transformation involves the formation of microcracks induced by the magnetic moment-dependent lattice expansion, which is also accompanied by an upturn in the resistivity curve p(T) near the Neel temperature. With an increasing number of warming and cooling cycles, the p( T) curve shifts to progressively higher resistivity values due to the formation of microcracks. The origin of this peculiar behavior is discussed in terms of a reduced mean-free path associated with electron scattering on cracks at constant carrier concentration. In addition, we show that the AFM alignment caused by the interatomic distance-dependent exchange inversion leads to lattice expansion.

Cracks induced by magnetic ordering in the antiperovskite ZnNMn 3

The magnetoresistances (MR's) and magnetic properties of layered manganese oxides, ${\mathrm{Sr}}_{1.6}{R}_{1.4}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ ($R=\mathrm{L}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d}$; $R$-327), have been investigated. The La-, Pr-, and Nd-substituted compounds show metal insulator transitions below 150 K, whereas isostructural Gd-327 compound is semiconducting and paramagnetic. For La-327, the transition temperature corresponds well to the onset of ferromagnetic ordering and the MR ratio is fairly large compared with perovskite-type $A{\mathrm{MnO}}_{3}$ oxides. The Pr and Nd analogs show negative MR's, but both compounds display other anomalous transitions to semiconductors below 50 K. Neither material shows a transition to a ferromagnetic state below 350 K. The observation of colossal magnetoresistance in the absence of bulk ferromagnetism suggests that the double-exchange interaction alone is not sufficient to explain the resistivity of $R$-327 near ${T}_{c}.$

Effect of lanthanide ions on the magnetotransport properties in layered Sr 1.6 R 1.4 Mn 2 O 7 ( R = L a , P r , N d , G d )

Lattice effects on the magnetic and transport properties in La1.4Sr1.6−xAxMn2O7 (A=Ca, Ba)

Neutron diffraction studies on a ferromagnetic layered manganite La 2-2x Sr 1+2x Mn 2 O 7 (x=0.3) prepared by a ceramic method

The metal–nonmetal (MN) transition data of mixed valent manganese oxide perovskites A1−xAx′MnO3 (A: trivalent lanthanide metals, A′: divalent metals, 0.2 0.906 and σ≲σ=232t−202 (pm). Large t and small σ are favorable for high TMN. TMN data of A0.7Ba0.15Sr0.15MnO3 (A=La, Pr, Nd, Sm), A0.7Ba0.15Ca0.15MnO3 (A=La, Pr, Nd, Sm), (La0.7Ca0.3)x(Gd0.7Ba0.3)1−xMnO3 (x=0.1, 0.3, 0.5, 0.7, 0.9), and (Nd0.7Ca0.3)x (Gd0.7Ba0.3)1−xMnO3 (x=0.25, 0.3, 0.5, 0.55, 0.7, 0.75) newly synthesized are reported.

Eun-Ok Chi

Papers

Cracks induced by magnetic ordering in the antiperovskite ZnNMn 3

Effect of lanthanide ions on the magnetotransport properties in layered Sr 1.6 R 1.4 Mn 2 O 7 ( R = L a , P r , N d , G d )

Lattice effects on the magnetic and transport properties in La1.4Sr1.6−xAxMn2O7 (A=Ca, Ba)

Neutron diffraction studies on a ferromagnetic layered manganite La 2-2x Sr 1+2x Mn 2 O 7 (x=0.3) prepared by a ceramic method

Overall and local distortion effects on the metal–nonmetal transitions of mixed valent perovskite type manganese oxides