Bistability and relaxor ferrimagnetism in off-stoichiometric NiCrO3

© 2017 Elsevier B.V. NiCrO 3 has been proposed as a likely candidate for antiferromagnetic half metallic behaviour. A sample prepared at high pressure adopts the corundum structure with Ni/Cr cation disorder, and is found to have off-stoichiometric composition Ni 0.80 Cr 1.20 O 3 . This material shows complex local magnetic ordering phenomena at temperatures below 120 K but without any long range spin order observed by neutron diffraction. The transition to local ferrimagnetism occurs at 50–100 K, with two distinct regimes at T C1  = 95 and T C2  = 53 K evidencing electronic phase separation driven by variations in local composition. At low temperature the system undergoes a further transition at T C3  = 22 K, assigned to potential freezing of a cluster glass-like state, that results in a substantial increase in magnetic anisotropy. Ni 0.80 Cr 1.20 O 3 is a bistable relaxor ferrimagnet where magnetic properties are linked to the lattice strain manifold that is determined by the peculiarities in the local chemical composition

Incommensurate spin order in the metallic perovskite MnVO3

Incommensurate Mn spin order has been discovered in the perovskite MnVO3 containing localized 3d5 Mn2+ and itinerant 3d1 V4+ states. This phase has a distorted Pnma crystal structure (a = 5.2741(6) Å, b = 7.4100(11) Å, and c = 5.1184(8) Å at 300 K) and is metallic at temperatures of 2-300 K and at pressures of up to 67 kbar. Neutron scattering reveals a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (kx 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions. © 2011 American Physical Society

Metastable and localized Ising magnetism in α−CoV2O6 magnetization plateaus

$\alpha$-CoV$_{2}$O$_{6}$ consists of $j_{\mathrm{eff}}={1 \over 2}$ Ising spins located on an anisotropic triangular motif with magnetization plateaus in an applied field. We combine neutron diffraction with low temperature magnetization to investigate the magnetic periodicity in the vicinity of these plateaus. We find these steps to be characterized by metastable and spatially short-range ($\xi\sim$ 10 $\r{A}$) magnetic correlations with antiphase boundaries defining a local periodicity of $\langle \hat{T}^{2} \rangle =\ \uparrow \downarrow$to$\langle \hat{T}^{3} \rangle =\ \uparrow \uparrow \downarrow$, and$\langle \hat{T}^{4} \rangle=\ \uparrow \uparrow \downarrow \downarrow$or$\uparrow \uparrow \uparrow \downarrow$ spin arrangements. This shows the presence of spatially short range and metastable/hysteretic, commensurate magnetism in Ising magnetization steps.Comment: 9 pages, 6 figures, to be published in Phys. Rev.

Spin-orbital correlations from complex orbital order in MgV2_{2}O4_{4}

MgV$_{2}$O$_{4}$ is a spinel based on magnetic V$^{3+}$ ions which host both spin ($S=1$) and orbital ($l_{eff}=1$) moments. Owing to the underlying pyrochlore coordination of the magnetic sites, the spins in MgV$_{2}$O$_{4}$ only antiferromagnetically order once the frustrating interactions imposed by the $Fd\overline{3}m$ lattice are broken through an orbitally-driven structural distortion at T$_{S}$ $\simeq$ 60 K. Consequently, a N\'eel transition occurs at T$_{N}$ $\simeq$ 40 K. Low temperature spatial ordering of the electronic orbitals is fundamental to both the structural and magnetic properties, however considerable discussion on whether it can be described by complex or real orbital ordering is ambiguous. We apply neutron spectroscopy to resolve the nature of the orbital ground state and characterize hysteretic spin-orbital correlations using x-ray and neutron diffraction. Neutron spectroscopy finds multiple excitation bands and we parameterize these in terms of a multi-level (or excitonic) theory based on the orbitally degenerate ground state. Meaningful for the orbital ground state, we report an "optical-like" mode at high energies that we attribute to a crystal-field-like excitation from the spin-orbital $j_{eff}$=2 ground state manifold to an excited $j_{eff}$=1 energy level. We parameterize the magnetic excitations in terms of a Hamiltonian with spin-orbit coupling and local crystalline electric field distortions resulting from deviations from perfect octahedra surrounding the V$^{3+}$ ions. We suggest that this provides compelling evidence for complex orbital order in MgV$_{2}$O$_{4}$. We then apply the consequences of this model to understand hysteretic effects in the magnetic diffuse scattering where we propose that MgV$_{2}$O$_{4}$ displays a high temperature orbital memory of the low temperature spin order.Comment: 21 pages and 13 figure

Incommensurate spin order in the metallic perovskite MnVO₃

Incommensurate Mn spin order has been discovered in the perovskite MnVO3 containing localized 3d(5) Mn2+ and itinerant 3d(1) V4+ states. This phase has a distorted Pnma crystal structure (a = 5.2741(6) angstrom, b = 7.4100(11) angstrom, and c = 5.1184(8) angstrom at 300 K) and is metallic at temperatures of 2-300 K and at pressures of up to 67 kbar. Neutron scattering reveals a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (k(x) 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions

Incipient Ferromagnetism in Tb2Ge2O7:Application of Chemical Pressure to the Enigmatic Spin-Liquid Compound Tb2Ti2O7

The origin of the spin liquid state in Tb$_2$Ti$_2$O$_7$ has challenged experimentalists and theorists alike for nearly 20 years. To improve our understanding of the exotic magnetism in Tb$_2$Ti$_2$O$_7$, we have synthesized a chemical pressure analog, Tb$_2$Ge$_2$O$_7$. Germanium substitution results in a lattice contraction and enhanced exchange interactions. We have characterized the magnetic ground state of Tb$_2$Ge$_2$O$_7$ with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to Tb$_2$Ti$_2$O$_7$, there is no long-range order in Tb$_2$Ge$_2$O$_7$ down to 20 mK. The Weiss temperature of $-$19.2(1) K, which is more negative than that of Tb$_2$Ti$_2$O$_7$, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of Tb$_2$Ge$_2$O$_7$ reveals that at 3.5 K liquid-like correlations dominate in this system. However, below 1 K, the liquid-like correlations give way to intense short-range ferromagnetic correlations with a length scale related to the Tb-Tb nearest neighbor distance. Despite stronger antiferromagnetic exchange, the ground state of Tb$_2$Ge$_2$O$_7$ has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in Tb$_2$Ti$_2$O$_7$.Comment: 6 pages, 4 figures, accepted for publication in PR