arXiv subjects

E. Pomjakushina

Publications and source records attributed to E. Pomjakushina.

148 records · Page 9Linked to original sources

arXiv2005

Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states

A gradual spin-state transition occurs in LaCoO3 around T~80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering (INS), and found that with increasing temperature an excitation at ~0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal field interaction and spin-orbit coupling we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (IS, S=1) vs. high-spin (HS, S=2) states. Since the g-factor obtained from the field dependence of the INS is g~3, the second interpretation looks more plausible.

cond-mat.str-el↗

arXiv2005

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.

cond-mat.str-el↗

arXiv2004

Spin structure and magnetic phase transitions in TbBaCo2O5.5

Spin ordering in TbBaCo2O5.5 and its temperature transformation is studied. Polymorphism due to the oxygen ordering with the average content close to 5.5 is investigated. One of ceramic samples (I), in addition to the main phase ap x 2ap x 2ap, Pmmm, (Z=2), contained about 25% of the phase ap x ap x 2ap, Pmmm, (Z=1), with statistical distribution of oxygen over the apical sites, where ap is parameter of perovskite cell. The other sample (II) contained only a main phase with well defined octahedral and pyramidal sublattices. Treatment of neutron diffraction patterns of the sample I itself gives a sophisticated spin structure. Knowing the structure of sample II, one can chose only proper magnetic lines, which give exactly the same results as for sample II. Above the Neel temperature TN ~ 290K, there is a structural transition to the phase 2ap x 2ap x 2ap, Pmma. At TN, the spins order with the wave vector k19 = 0 (Phase 1). At T1 ~ 255K, a magnetic transition takes place to the phase 2 with k22 = b3/2. At T2 ~ 170K, the crystal structure changes to 2ap x 2ap x 4ap, Pcca (Z=4). The wave vector of the spin structure becomes again k19 = 0 (Phase 3). The basis functions of irreducible representations of the group Gk have been found. Using results of this analysis, the magnetic structure in all phases is determined. The spins are always parallel to the x axis, and the difference is in the values and mutual orientation of the moments in the ordered non-equivalent pyramidal or octahedral positions. Spontaneous moment at T = 260 K is due to ferrimagnetic ordering of the moments in pyramidal sites. (Dzyaloshinskii-Moriya canting is forbidden by symmetry.) The moment values together with the ligand displacements are used to analyze the picture of spin-state/orbital ordering in each phase.

cond-mat.mtrl-sci↗

arXiv2004

Evidence for charged critical fluctuations in underdoped Y-123

We report and analyze in-plane penetration depth measurements in YBa2Cu3O7-x taken close to the critical temperature Tc. In underdoped YBa2Cu3O6:59 we find consistent evidence for charged critical behavior. Noting that the effective dimensionless charge scales as 1/sqrt(Tc), this new critical behavior should be generically observable in suitably underdoped cuprates.

cond-mat.supr-con↗