Giant Magnetoresistance Effect in the Metal-Insulator Transition of Pyrochlore Oxide Nd2Ir2O7

We investigated the magnetoresistance (MR) effect of the pyrochlore oxide Nd2Ir2O7, which shows a metal-insulator transition at T_MI =33 K. A small positive MR effect was observed in the metallic state above T_MI, while a large negative MR effect was observed in the insulating state below T_MI . MR effects exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed the crossover from the insulating state to a state with a small or partial band gap in a field up to 56 T. Furthermore, from the MR effect in Eu2Ir2O7 (T_MI = 120 K) and Gd$_2$Ir$_2$O$_7$ (T_MI = 127 K), we revealed that the large negative MR effect of the pyrochlore iridate Ln2Ir2O7 depends on the magnetism of the lanthanide Ln^{3+} ion. The d-f interaction plays a significant role in the large negative MR effect in the insulating state.Comment: 10 pages, 4 figure

Metal–Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln2Ir2O7 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal-insulator transitions (MITs) at different temperatures. The transition temperature TMI increases with a reduction in the ionic radius of Ln. The ionic radius　boundary for MITs in Ln2Ir2O7 lies between Ln = Pr and Nd. MITs in Ln2Ir2O7 have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (»10−3 μB/f.u.) caused by Ir 5d electrons is observed below TMI.The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/K mole, respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J /K mole] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln2Ir2O7 is discussed

Metal–Insulator Transitions in Pyrochlore Oxides Ln2Ir2O7

We report the physical properties of Ln2Ir2O7 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Ho), which exhibit metal-insulator transitions (MITs) at different temperatures. The transition temperature TMI increases with a reduction in the ionic radius of Ln. The ionic radius　boundary for MITs in Ln2Ir2O7 lies between Ln = Pr and Nd. MITs in Ln2Ir2O7 have some common features. They are second-order transitions. Under the field cool condition, a weak ferromagnetic component (»10−3 μB/f.u.) caused by Ir 5d electrons is observed below TMI. The entropy associated with MITs for Ln = Nd, Sm, and Eu is estimated to be 0.47, 2.0, and 1.4 J/K mole, respectively. The change in entropy is much smaller than 2R ln 2 [11.5 J /K mole] expected in a magnetic transition due to localized moments of S = 1/2. The feature of continuous MITs in Ln2Ir2O7 is discussed

Observation of a Liquid-Gas-Type Transition in the Pyrochlore Spin Ice Compound Dy2Ti2O7 in a Magnetic Field

Low temperature magnetization measurements on the pyrochlore spin ice compound Dy2Ti2O7 reveal that the ice-rule breaking spin flip, appearing at H∼0.9  T applied parallel to the [111] direction, turns into a novel first-order transition for T<0.36   K which is most probably of a liquid-gas type. T-linear variation of the critical field observed down to 0.03 K suggests the unusual situation that the entropy release across the transition remains finite [∼0.5   (J/K)⋅mol−Dy] as T→0, in accordance with a breaking of the macroscopic degeneracy in the intermediate “kagomé ice” state

Effect of pressure on single-chain magnets with repeating units of the MnIII-NiII-MnIII trimer

The single-chain magnet (SCM) system [Mn2(saltmen)2Ni(pao)2(L)2](A)2 (L: intrachain attaching ligand of NiII ion; A-1: interchain counteranion) is a ferromagnetic one-dimensional network system with repeating units of the MnIII-NiII-MnIII trimer which itself behaves as a single-molecule magnet with an S=3 spin ground state and negative uniaxial single-ion anisotropy (D) parallel to the bridging direction. The slow relaxation of the magnetic moment in this SCM system originates in an energy barrier for spin reversal (ΔE), which is closely related to the ferromagnetic interaction between the trimers (Jtrimer) as well as to the D of the trimer. We have investigated the effects of pressure on three compounds representative of the above SCM family through ac susceptibility measurements under hydrostatic pressures up to P=13.5 kbar and crystal structural analysis experiments up to P=20.0 kbar, and have observed a pronounced enlargement of ΔE when J was artificially increased. The application of hydrostatic pressure brought about the systematic enhancement of EΔ (a maximum increase of 10% within the pressure region of the experiments). The pressure dependence of EΔ varied according to the kind of attaching ligand L involved and the intrachain structure, and we have experimentally found that isotropic lattice shrinkage is desirable if a continuous increase of ΔE in this system is aimed at

Low-Temperature Spin Diffusion in a Highly Ideal S= Heisenberg Antiferromagnetic Chain Studied by Muon Spin Relaxation

The organic radical-ion salt DEOCC-TCNQF4 contains linear chains of stacked molecules with significant Heisenberg antiferromagnet interactions along the chain and extremely weak interactions between the chains. Zero-field µSR has confirmed the absence of long-range magnetic order down to 20 mK and field-dependent µSR is found to be consistent with diffusive motion of the spin excitations. The anisotropic spin dynamics and the upper boundary for magnetic ordering temperature both indicate interchain magnetic coupling |J|<7 mK. As the intrachain coupling J is 110 K, |J/J| is significantly less than 10-4. This system therefore provides one of the most ideal examples of the one-dimensional S=1/2 Heisenberg antiferromagnet yet discovered

Pressure-Induced Ferromagnetic to Nonmagnetic Transition and the Enhancement of Ferromagnetic Interaction in the Thiazyl-Based Organic Ferromagnet γ-BBDTA·GaCl4

A thiazyl-based ferromagnet, the γ-phase of BBDTA (i.e., benzo[1,2- d :4,5- d \u27]bis[1,3,2]dithiazole)·GaCl 4 , has a high ferromagnetic ordering temperature of 7.0 K in organic radical ferromagnets. In this system, pressurization generated more compact molecular packing, resulting in that the ferromagnetic state at P = 16.2 kbar is stabilized over a temperature range of more than twice of the initial range. However, the saturation magnetic moment was reduced with increasing pressure, decreasing to about 12% of the initial value even at the low pressure level of P = 1.0 kbar. This suggests that the ferromagnetic molecular packing of the monoclinic γ-phase is easily transformed into that of the diamagnetic phase. Powder X-ray diffraction experiments revealed that the diamagnetic non-monoclinic (α- or β-) phase became stable instead of the monoclinic γ-phase across the pressure of 2.5–5.8 kbar. The increase in the temperature of onset of ferromagnetic state occurs in the surviving ferromagnetic domain surrounded by the diamagnetic domains

High-pressure dc magnetic measurements on a bisdiselenazolyl radical ferromagnet using a vibrating-coil SQUID magnetometer

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations

Effects of Hydrostatic Pressure and Uniaxial Strain on Spin-Peierls Transition in an Organic Radical Magnet, BBDTA·InCl4

We investigated the effects of hydrostatic pressure and uniaxial strain on the spin-Peierls (SP) transition of an organic radical magnet, benzo[1,2-d:4,5-d\u27]bis[1,3,2]dithiazole(BBDTA)·InCl 4 . It has a one-dimensional coordination polymer structure along its c -axis and its SP transition occurs at 108 K. The SP transition temperature T SP decreased to 99 K at a hydrostatic pressure of 10 kbar, while it increased to 132 K at a uniaxial strain along the c -axis of 8 kbar. The pressure dependences of T SP under these two conditions were discussed by evaluating two parameters, namely, the intrachain interaction 2 J / k B and the effective spin–lattice coupling parameter η, that are related to T SP by the equation T SP =1.6η J / k B . Under ambient pressure, the a - and c -axes of this material shortened monotonically with decreasing temperature, while the b -axis elongated below T SP . In this study, we found the correlation between η and the change in the lattice constant b . 2 J / k B increased with increasing hydrostatic pressure and uniaxial strain, suggesting that the contraction along the c -axis does not depend on the manner of pressurization. From the evaluation of η, the observed variation in T SP is explained by the difference between the changes in b under the two pressurization conditions

Hydrostatic Compression Effects on Fifth-Group Element Superconductors V, Nb, and Ta Subjected to High-Pressure Torsion

In fifth-group element superconductors V, Nb, and Ta, the increase in superconducting transition temperature (Tc) was attempted by using both high-pressure torsion (HPT) and additional hydrostatic pressure (HP) compression. The former brings about the grain refinement and strain accumulation in the unit-cell level. The additional compression for severely strained superconductors triggers strengthening intergrain-contact and/or structural deformation in the unit-cell level. The manner of the appearance of the above two effects depends on the kind of elements: First, in V, there is no prominent effect of HPT, comparing to the hydrostatic compression effects on its non-strained material. Next, in Ta, the effect of strengthening intergrain-contact appears at small hydrostatic compression, resulting in temporal increase in Tc. Finally, Nb exhibits prominent increase in Tc by both effects and, in particular, the structural deformation in the unit-cell level promotes the increase in Tc. Thus, the accumulation of residual strain in the level of starting material can be a promising work to manipulate Tc under HP compression