An electron hopping on non-coplanar spin sites with spin chirality obtains a complex phase factor (Berry phase) in its quantum mechanical amplitude that acts as an internal magnetic field, and is predicted to manifest itself in the Hall effect when it is not cancelled. The present combined work of transport measurement, neutron scattering, and theoretical calculation provides evidence that the gigantic anomalous Hall effect observed in Nd2Mo2O7, a pyrochlore ferromagnet with geometrically frustrated lattice structure, is mostly due to the spin chirality and the associated Berry phase originating from the Mo spin tilting.
Temperature and magnetic field dependences of the anomalous Hall effect have been investigated for single crystals of Mo-based ferromagnets with pyrochlore structure. The Hall resistivity of Nd 2 Mo 2 O 7 compound shows unconventional temperature dependence whereas Gd 2 Mo 2 O 7 exhibits rather normal behaviour. The Berry phase model can explain the difference well; it is attributed to the difference in nature of the anisotropy of the rare-earth moment and the resultant Mo spin state. The Hall resistivity of Nd 2 Mo 2 O 7 changes its sign with increasing field applied along the [111] direction, while it monotonically approaches zero with the field applied along the [100] or [110] direction. This behaviour is also in accord with the prediction of the Berry phase theory.
The high temperature paramagnetic state in an antiferromagnetic (AFM) insulator Pr1−xCaxMnO3 is characterized by the ferromagnetic (FM) spin fluctuations with an anomalously small energy scale. The FM fluctuations show a precipitous decrease of the intensity at the charge ordering temperature TCO, but persist below TCO, and vanish at the AFM transition temperature TN. These results demonstrate the importance of the spin ordering for the complete switching of the FM fluctuation in doped manganites. 71.27.+a, 71.30.+h, 75.25.+z For hole doped perovskite manganites R 1−x A x MnO 3 (R is a trivalent rare earth ion, and A is a divalent alkaline earth ion.), the ferromagnetic (FM) metallic state has been qualitatively explained with the double exchange (DE) model, in which the doped holes gain the kinetic energy by aligning the spins on the Mn sites ferromagnetically.1 This DE picture implies that, regardless of its FM or antiferromagnetic (AFM) ordering in the ground state, the FM spin correlation must be the characteristic feature of spin fluctuations in the paramagnetic state of doped manganites. When the charge ordering is formed, however, it will suppress the hopping of the holes, and suppresses the FM fluctuations which are assisted by the DE interactions. In addition, the orbital ordering may turn on the AFM super exchange interactions between localized spins on the Mn sites.Being consistent with such a DE picture, it was recently reported that the FM spin fluctuations exist in the paramagnetic phase of the insulating AFM (Bi,Ca)MnO 3 , and they change over to the AFM spin fluctuations below the onset of the charge ordering. 2Very recently, it is discovered that the spin correlation in the A-type AFM manganite Nd 0.45 Sr 0.55 MnO 3 is ferromagnetic in the paramagnetic state, and it exhibits a metallic AFM state with the orbital ordering at low temperatures.3 These recent results manifest the importance of the FM spin fluctuations in the paramagnetic state of the doped manganites. In addition, the switching of spin fluctuations from FM to AFM correlations at the charge and/or spin ordering temperature typifies the interplay between the orbital, charge, and spin orderings in doped manganites.In order to confirm the existence of the FM spin fluctuations in the paramagnetic state of doped manganites with an AFM ground state and to elucidate the characteristic feature of the FM spin fluctuations in the paramagnetic state, we have studied an insulating AFM system Pr 1−x Ca x MnO 3 . We chose this material because its transport, optical, and magnetic properties are all wellcharacterized. 4,5,6,7 Furthermore, this system is convenient to examine the mechanism of the switching of spin fluctuations because its well-separated charge and spin ordering temperatures, 5,6 for example, the x = 0.35 sample shows the CE-type charge ordering (CO) at T CO ∼ 230 K, while the CE-type AFM spin ordering at T N ∼ 165 K, as shown later. Consequently, this system allows us to experimentally distinguish the influence of the spin orderi...
In order to find out more about the suppression of ferromagnetic (FM) interactions in Sr1-xLaxRuO3, electronic structures and magnetic properties have been investigated upon changing x from 0.0 to 0.5, using an XRD method with Rietveld analysis, a SQUID magnetometer and a DV-Xα computational method. In comparison with magnetic properties in Sr1-xCaxRuO3, FM interactions in Sr1-xLaxRuO3 are found to be suppressed very rapidly against x. Neither structural distortion nor cation-size disorder can account for such rapid suppression. Instead, this may be attributed to the effect of La-O hybridization created by La substitution for Sr. This hybridization effect weakens the FM order around Ru ions and, as a result, the long-range FM states are suppressed even if x is small. The DV-Xα cluster method was employed to estimate the energy difference between the up and down spin density of states in SrRuO3 and Sr0.5La0.5RuO3. This calculation predicts that Sr1-xLaxRuO3 contains La-O hybridization which suppresses FM interaction even at small x.
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