Comment on "Magnetic field effects on neutron diffraction in the antiferromagnetic phase of UPt3"

Moreno and Sauls [Phys. Rev. B 63, 024419 (2000)] have recently tried to reanalyze earlier neutron scattering studies of the antiferromagnetic order in UPt3 with a magnetic field applied in the basal plane. In their calculation of the magnetic Bragg peak intensities, they perform an average over different magnetic structures belonging to distinct symmetry representations. This is incorrect. In addition, they have mistaken the magnetic field direction in one of the experiments, hence invalidating their conclusions concerning the experimental results.Comment: Revised 5 June 2001: Added group theory analysis and modified discussion of S and K domain

Coupling of phonons and electromagnons in GdMnO_3

The infrared and Terahertz properties of GdMnO_3 have been investigated as function of temperature and magnetic field, with special emphasis on the phase boundary between the incommensurate and the canted antiferromagnetic structures. The heterogeneous incommensurate phase reveals strong magnetodielectric effects, characterized by significant magnetoelectric contributions to the static dielectric permittivity and by the existence of electrically excited magnons (electromagnons). In the commensurate canted antiferromagnetic phase the magnetoelectric contributions to the dielectric constant and electromagnons are suppressed. The corresponding spectral weight is transferred to the lowest lattice vibration demonstrating the strong coupling of phonons with electromagnons.Comment: 5 pages, 4 figure

Non collinear magnetism and single ion anisotropy in multiferroic perovskites

The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction, the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density-functional calculations. Using BiFeO$_3$ and LaFeO$_3$ as model systems, we quantify the relationship between the oxygen octahedra rotations, the ferroelectricity and the weak ferromagnetism (wFM). We recover the fact that the wFM is due to the DM interaction induced by the oxygen octahedra rotations. We find a simple relationship between the wFM, the oxygen rotation amplitude and the ratio between the DM vector and the exchange parameter such as the wFM increases with the oxygen octahedra rotation when the SIA does not compete with the DM forces induced on the spins. Unexpectedly, we also find that, in spite of the $d^5$ electronic configuration of Fe$^{3+}$, the SIA is very large in some structures and is surprisingly strongly sensitive to the chemistry of the $A$-site cation of the $A$BO$_3$ perovskite. In the ground $R3c$ state phase we show that the SIA shape induced by the ferroelectricity and the oxygen octahedra rotations are in competition such as it is possible to tune the wFM "on" and "off" through the relative size of the two types of distortion

Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4

In both spinel compounds GeCo$_2$O$_4$ and GeNi$_2$O$_4$ which order antiferromagnetically (at $T_N = 23.5 K$ and $T_{N_1} = 12.13 K$, $T_{N_2} = 11.46 K$) with different Curie Weiss temperatures ($T_{CW}$=80.5 K and -15 K), the usual magnetic frustration criterion $f=|T_{CW}|/T_N>>1$ is not fulfilled. Using neutron powder diffraction and magnetization measurements up to 55 T, both compounds are found with a close magnetic ground state at low temperature and a similar magnetic behavior (but with a different energy scale), even though spin anisotropy and first neighbor exchange interactions are quite different. This magnetic behavior can be understood when considering the main four magnetic exchange interactions. Frustration mechanisms are then enlightened.Comment: submitted to Phys.Rev.B (2006

Coupling of frustrated Ising spins to magnetic cycloid in multiferroic TbMnO3

We report on diffraction measurements on multiferroic TbMnO3 which demonstrate that the Tb- and Mn-magnetic orders are coupled below the ferroelectric transition TFE = 28 K. For T < TFE the magnetic propagation vectors (tau) for Tb and Mn are locked so that tauTb = tauMn, while below TNTb = 7 K we find that tauTb and tauMn lock-in to rational values of 3/7 b* and 2/7 b*, respectively, and obey the relation 3tauTb - tauMn = 1. We explain this novel matching of wave vectors within the frustrated ANNNI model coupled to a periodic external field produced by the Mn-spin order. The tauTb = tauMn behavior is recovered when Tb magnetization is small, while the tauTb = 3/7 regime is stabilized at low temperatures by a peculiar arrangement of domain walls in the ordered state of Ising-like Tb spins.Comment: 5 pages, 3 figure

First Principles Study of Structural, Electronic and Magnetic Interplay in Ferroelectromagnetic Yttrium Manganite

We present results of local spin density approximation pseudopotential calculations for the ferroelectromagnet, yttrium manganite (YMnO3). The origin of the differences between ferroelectric and non-ferroelectric perovskite manganites is determined by comparing the calculated properties of yttrium manganite in its ferroelectric hexagonal and non-ferroelectric orthorhombic phases. In addition, orthorhombic YMnO3 is compared with the prototypical non-ferroelectric manganite, lanthanum manganite. We show that, while the octahedral crystal field splitting of the cubic perovskite structure causes a centro-symmetric Jahn-Teller distortion around the Mn3+ ion, the markedly different splitting in hexagonal perovskites creates an electronic configuration consistent with ferroelectric distortion. We explain the nature of the distortion, and show that a local magnetic moment on the Mn3+ ion is a requirement for it to occur.Comment: Replacement of earlier version with error in pseudopotentia

Crystal structure and high-field magnetism of La2CuO4

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La2CuO4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined by displacements of the apical oxygen atoms. An in-plane magnetic field is observed to drive a continuous reorientation of the copper spins from the orthorhombic b-axis to the c-axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for non-stoichiometric La2CuO4 is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work

Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator

Neutron scattering was used to determine the spin structure and the magnon spectrum of the Mott--Hubbard insulator YTiO$_3$. The magnetic structure is complex, comprising substantial G-type and A-type antiferromagnetic components in addition to the predominant ferromagnetic component. The magnon spectrum, on the other hand, is gapless and nearly isotropic. We show that these findings are inconsistent with the orbitally ordered states thus far proposed for YTiO$_3$ and discuss general implications for a theoretical description of exchange interactions in orbitally degenerate systems.Comment: to appear in Phys. Rev. Let

Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO$_{3}$ by high-temperature synchrotron X-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100K and 1350K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO$_{3}$.Comment: 10 pages 5 figures. New version, Additional data, Journal of Physics: Condensed Matter, in Pres

Spin fluctuations in the stacked-triangular antiferromagnet YMnO3

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO3 was studied above the Neel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above TN, a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line-width that increases with q like Dq^z and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q-dependence at small wave-vectors.Comment: JETP LETTERS, in pres