arXiv subjects

O. Zaharko

Publications and source records attributed to O. Zaharko.

At least 55 records · Page 3Linked to original sources

arXiv2013

Multiferroicity in the geometrically frustrated FeTe2O5Cl

The layered FeTe2O5Cl compound was studied by specific-heat, muon spin relaxation, nuclear magnetic resonance, dielectric, as well as neutron and synchrotron x-ray diffraction measurements, and the results were compared to isostructural FeTe2O5Br. We find that the low-temperature ordered state, similarly as in FeTe2O5Br, is multiferroic - the elliptical amplitude-modulated magnetic cycloid and the electric polarization simultaneously develop below 11 K. However, compared to FeTe2O5Br, the magnetic elliptical envelop rotates by 75(4) deg and the orientation of the electric polarization is much more sensitive to the applied electric field. We propose that the observed differences between the two isostructural compounds arise from geometric frustration, which enhances the effects of otherwise subtle Fe3+ (S = 5/2) magnetic anisotropies. Finally, x-ray diffraction results imply that, on the microscopic scale, the magnetoelectric coupling is driven by shifts of the O1 atoms, as a response to the polarization of the Te4+ lone-pair electrons involved in the Fe-O-Te-O-Fe exchange bridges.

cond-mat.str-el↗

arXiv2013

Magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 as revealed by single-crystal neutron diffraction

The magnetic structure of superconducting Eu(Fe0.82Co0.18)2As2 is unambiguously determined by single-crystal neutron diffraction. A long-range ferromagnetic order of the Eu2+ moments along the c-direction is revealed below the magnetic phase transition temperature Tc = 17 K. In addition, the antiferromagnetism of the Fe2+ moments still survives and the tetragonal-to-orthorhombic structural phase transition is also observed, although the transition temperatures of the Fe-spin density wave (SDW) order and the structural phase transition are significantly suppressed to Tn = 70 K and Ts = 90 K, respectively, compared to the parent compound EuFe2As2.We present the microscopic evidences for the coexistence of the Eu-ferromagnetism (FM) and the Fe-SDW in the superconducting crystal. The superconductivity (SC) competes with the Fe-SDW in Eu(Fe0.82Co0.18)2As2.Moreover, the comparison between Eu(Fe1-xCox)2As2 and Ba(Fe1-xCox)2As2 indicates a considerable influence of the rare-earth element Eu on the magnetism of the Fe sublattice.

cond-mat.supr-con↗

arXiv2013

Temperature-driven Phase Transformation in Y_3Co: Neutron Scattering and First-principles Studies

Contrary to previous studies that identified the ground state crystal structure of the entire R_3Co series (R is a rare earth) as orthorhombic Pnma, we show that Y_3Co undergoes a structural phase transition at T_t=160K. Single crystal neutron diffraction data reveal that at T_t the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the Pnma structure of Y_3Co. In particular, a transversal acoustic phonon mode along the (00z) direction has imaginary frequencies at z<1/4. Employing inelastic neutron scattering measurements we find a strong damping of the (00z) phonon mode below a critical temperature T_t. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y_3Co as a system with the charge density wave instability.

cond-mat.mtrl-sci↗

arXiv2013

Evolution of magnetic and crystal structures in the multiferroic FeTe2O5Br

Neutron diffraction and nuclear quadrupole resonance (NQR) measurements were employed to investigate magnetic order in the non-ferroelectric phase preceding the low-temperature multiferroic state in FeTe2O5Br. Refnement of the neutron diffraction data and simulations of 79,81Br NQR spectra reveal that the incommensurate magnetic ordering in the non-ferroelectric state comprises amplitude-modulated magnetic moments, similarly as in the multiferroic state. The two ordered states differ in the orientation of the magnetic moments and phase shifts between modulation waves. Surprisingly, all symmetry restrictions for the electric polarization are absent in both states. The different ferroelectric responses of the two states are thus argued to arise from the differences in the phase shifts between certain modulation waves, which cancel out in the non-ferrolectric state.

cond-mat.str-el↗

arXiv2013

Magnetic order and low-energy excitations in the quasi-one-dimensional antiferromagnet CuSe$_2$O$_5$ with staggered fields

Ground state and low-energy excitations of the quasi-one-dimensional antiferromagnet CuSe$_2$O$_5$ were experimentally studied using bulk magnetization, neutron diffraction, muon spin relaxation and antiferromagnetic resonance measurements. Finite interchain interactions promote long-range antiferromagnetic order below $T_N=17$ K. The derived spin canted structure is characterized by the magnetic propagation vector $\mathbf{k}=(1, 0, 0)$ and the reduced magnetic moment $\bm{m}=[0.13(7), 0.50(1), 0.00(8)]μ_B$. The values of the magnetic anisotropies determined from the field and angular dependencies of the antiferromagnetic resonance comply well with a previous electron paramagnetic resonance study and correctly account for the observed magnetic ground state and spin-flop transition.

cond-mat.str-el↗

arXiv2012

Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order

An incommensurate elliptical helical magnetic structure in the frustrated coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to 53(3) mK (T/T_N ~ 1/200), according to neutron scattering and muon spin relaxation. In this state, finite spin fluctuations at T -> 0 are evidenced by muon depolarization, which is in agreement with specific-heat data indicating the presence of both gapless and gapped excitations. We thus show that the amplitude-modulated magnetic order intrinsically accommodates contradictory persistent spin dynamics and long-range order and can serve as a model structure to investigate their coexistence.

cond-mat.str-el↗

arXiv2012

Magnetic ground state and 2D behavior in pseudo-Kagome layered system Cu3Bi(SeO3)2O2Br

Anisotropic magnetic properties of a layered kagome-like system Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic susceptibility measurements as well as powder and single-crystal neutron diffraction. At T_N = 27.4 K the system develops an alternating antiferromagnetic order of (ab) layers, which individually exhibit canted ferrimagnetic moment arrangement, resulting from the competing ferro- and antiferro-magnetic intralayer exchange interactions. A magnetic field B_C ~ 0.8 T applied along the c axis (perpendicular to the layers) triggers a metamagnetic transition, when every second layer flips, i.e., resulting in a ferrimagnetic structure. Significantly higher fields are required to rotate the ferromagnetic component towards the b axis (~7 T) or towards the a axis (~15 T). The estimates of the exchange coupling constants and features indicative of an XY character of this quasi-2D system are presented.

cond-mat.str-el↗

arXiv2012

Muon spin rotation study of magnetism and superconductivity in Ba(Fe1-xCox)2As2 single crystals

Using muon spin rotation (muSR) we investigated the magnetic and superconducting properties of a series of Ba(Fe1-xCox)2As2 single crystals with 0 =< x =< 0.15. Our study details how the antiferromagnetic order is suppressed upon Co substitution and how it coexists with superconductivity. In the non-superconducting samples at 0 < x < 0.04 the antiferromagnetic order parameter is only moderately suppressed. With the onset of superconductivity this suppression becomes faster and it is most rapid between x = 0.045 and 0.05. As was previously demonstrated by muSR at x = 0.055 [P. Marsik et al., Phys. Rev. Lett. 105, 57001 (2010)], the strongly weakened antiferromagnetic order is still a bulk phenomenon that competes with superconductivity. The comparison with neutron diffraction data suggests that the antiferromagnetic order remains commensurate whereas the amplitude exhibits a spatial variation that is likely caused by the randomly distributed Co atoms. A different kind of magnetic order that was also previously identified [C. Bernhard et al., New J. Phys. 11, 055050 (2009)] occurs at 0.055 < x < 0.075 where Tc approaches the maximum value. The magnetic order develops here only in parts of the sample volume and it seems to cooperate with superconductivity since its onset temperature coincides with Tc. Even in the strongly overdoped regime at x = 0.11, where the static magnetic order has disappeared, we find that the low energy spin fluctuations are anomalously enhanced below Tc. These findings point toward a drastic change in the relationship between the magnetic and superconducting orders from a competitive one in the strongly underdoped regime to a constructive one in near optimally and overdoped samples.

cond-mat.supr-con↗

arXiv2012

Coupling of magnetic and ferroelectric hysteresis by a multi-component magnetic structure in Mn2GeO4

The olivine compound Mn2GeO4 is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity (FE), and a canted commensurate order leads to weak ferromagnetism (FM). Symmetry suggests that the direct coupling between the FM and FE is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multi-component magnetic structures found in magnetically-frustrated materials like Mn2GeO4 provide a new route towards functional materials that exhibit coupled FM and FE.

cond-mat.mtrl-sci↗

arXiv2012

Multiferroic FeTe$_2$O$_5$Br: Alternating spin chains with frustrated interchain interactions

A combination of density functional theory calculations, many-body model considerations, magnetization and electron spin resonance measurements shows that the multiferroic FeTe$_2$O$_5$Br should be described as a system of alternating antiferromagnetic $S=5/2$ chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer models. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization.

cond-mat.str-el↗

arXiv2011

Incommensurate Magnetic Order in TbTe$_3$

We report a neutron diffraction study of the magnetic phase transitions in the charge-density-wave (CDW) TbTe$_3$ compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe$_3$ at $T_{mag1}$ = 5.78 K as a result of continuous phase transitions. We observe that near the temperature $T_{mag1}$ the magnetic Bragg peaks appear around the position (0,0,0.24) (or its rational multiples), that is fairly close to the propagation vector $(0,0,0.29)$ associated with the CDW phase transition in TbTe$_3$. This suggests that correlations leading to the long-range magnetic order in TbTe$_3$ are linked to the modulations that occur in the CDW state.

cond-mat.str-el↗

arXiv2011

Magnetic structure of CuCrO2: a single crystal neutron diffraction study

This paper presents results of a recent study of multiferroic \CCO\ by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T\nsub{N1}=24.2$ K and $T\nsub{N2}=23.6$ K. The low temperature magnetic structure below $T\nsub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T\nsub{N1}$ and $T\nsub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.

cond-mat.mtrl-sci↗

arXiv2011

120° Helical Magnetic Order in the Distorted Triangular Antiferromagnet alpha-CaCr2O4

α-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+ ions which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below T_N=42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wavevector k=(0,~1/3,0) and angles close to 120° between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the ac plane perpendicular to k. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest neighbor interactions are equal, in spite of the fact that α-CaCr2O4 is distorted with two inequivalent Cr3+ ions and four different nearest neighbor interactions. By simulating the magnetic order as a function of these four interactions it is found that the special pattern of interactions in α-CaCr2O4 stabilizes 120° helical order for a large range of exchange interactions.

cond-mat.str-el↗

arXiv2010

Magnetic phase diagram of the multiferroic FeTe$_2$O$_5$Br

The low-temperature magnetic phase diagram of the multiferroic system FeTe$_2$O$_5$Br down to 300 mK and up to 9 T is presented. Short-range magnetic correlations within the crystal layers start to develop already at $\sim$50 K, i.e., far above $T_{N1} \sim$ 11.0 K, where the system undergoes a magnetic phase transition into the high-temperature incommensurate (HT-ICM) phase. Only 0.5 K lower, at $T_{N2}$, the system undergoes a second phase transition into the low-temperature incommensurate amplitude-modulated (LT-ICM) phase accompanied by a spontaneous electric polarization. When the magnetic field is applied, the transition temperatures shift depending on the field orientation. In the case of $B||b$ and $B >$ 4.5 T, the HT-ICM phase disappears along with the electric polarization in the LT-ICM phase. The field dependence of the magnetic transition temperatures is explained in the context of the magnetic susceptibility behavior. Similarities and differences between the novel amplitude-modulated and well-established helicoidal magnetoelectrics are discussed.

cond-mat.str-el↗

arXiv2010

Evolution of magnetic states in frustrated diamond lattice antiferromagnetic Co(Al1-xCox)2O4 spinels

Using neutron powder diffraction and Monte-Carlo simulations we show that a spin-liquid regime emerges at $all compositions in the diamond-lattice antiferromagnets Co(Al1-xCox)2O4. This spin-liquid regime induced by frustration due to the second-neighbour exchange coupling J2, is gradually superseded by antiferromagnetic collinear long-range order (k=0) at low temperatures. Upon substitution of Al3+ by Co3+ in the octahedral B-site the temperature range occupied by the spin-liquid regime narrows and TN increases. To explain the experimental observations we considered magnetic anisotropy D or third-neighbour exchange coupling J3 as degeneracy-breaking perturbations. We conclude that Co(Al1-xCox)2O4 is below the theoretical critical point J2/J1=1/8, and that magnetic anisotropy assists in selecting a collinear long-range ordered ground state, which becomes more stable with increasing x due to a higher efficiency of O-Co3+-O as an interaction path compared to O-Al3+-O.

cond-mat.str-el↗

arXiv2009

Ni3TeO6 - a collinear antiferromagnet with ferromagnetic honeycomb planes

We report a comprehensive study of magnetic properties of Ni3TeO6. The system crystallizes in a noncentrosymmetric rhombohedral lattice, space group R3. There are three differently coordinated Ni atoms in the unit cell. Two of them form an almost planar honeycomb lattice, while the third one is placed between the layers. Magnetization and specific heat measurements revealed a single magnetic ordering at TN = 52 K. Below TN the susceptibility with the magnetic field parallel to the c-axis drops towards zero while the perpendicular susceptibility remains constant, a characteristic of antiferromagnetic materials. Neutron diffraction confirmed that the system is antiferromagnet below TN with ferromagnetic ab-planes stacked antiferromagnetically along the c-axis. All Ni moments are in the S = 1 spin state and point along the c-axis.

cond-mat.mtrl-sci↗

arXiv2009

Diffuse scattering from the lead-based relaxor ferroelectric PbMg_1/3Ta_2/3O_3

The relaxor ferroelectric PbMg_1/3Ta_2/3O_3 was studied by single-crystal neutron and synchrotron x-ray diffraction and its detailed atomic structure has been modeled in terms of static Pb-displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg_1/3Nb_2/3O_3 or PbZn_1/3Nb_2/3O_3 the diffuse scattering in the [H,0,0]/[0,K,0] scattering plane has a butterfly-shape around the (h,0,0) Bragg reflections and is transverse to the scattering vector for (h,h,0) peaks. In the [H,H,0]/[0,0,L] plane the diffuse scattering is elongated along the <1,1,2> directions and is transverse to the scattering vector for (h,h,h) reflections. We find that a model consisting of correlated Pb-displacements along the <1,1,1>-directions reproduces the main features of the diffuse scattering in PbMg_1/3Ta_2/3O_3 adequately when the correlation lengths between the Pb-ion displacement vectors are longest along the <1,1,1> and <1,-1,0> and shortest along <1,1,-2> directions.

cond-mat.mtrl-sci↗

arXiv2009

Spin amplitude modulation driven magnetoelectic coupling in the new multiferroic FeTe$_2$O$_5$Br

Magnetic and ferroelectric properties of layered geometrically frustrated cluster compound FeTe$_2$O$_5$Br were investigated with single-crystal neutron diffraction and dielectric measurements. Incommensurate amplitude modulated magnetic order with the wave vector $\bf{q}$=($\half$, 0.463, 0) develops below $T_N=10.6(2) {\rm K}$. Simultaneously, a ferroelectric order with the spontaneous polarization perpendicular to ${\bf q}$ and to Fe$^{3+}$ magnetic moments emerges. The observed ferroelectricity and extraordinary linear scaling of the ferroelectric and magnetic order parameter are provoked by the striction of the intercluster Fe-O-Te-O-Fe bridges leading to the shift of Te$^{4+}$ ions and polarization of their lone-pair electrons.

cond-mat.mtrl-sci↗