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A. I. Smirnov

Publications and source records attributed to A. I. Smirnov.

At least 37 records · Page 2Linked to original sources
arXiv2010

Modes of magnetic resonance of S=1 dimer chain compound NTENP

The spin dynamics of a quasi one dimensional $S=1$ bond alternating spin-gap antiferromagnet Ni(C$_9$H$_{24}$N$_4$)NO$_2$(ClO$_4$) (abbreviated as NTENP) is studied by means of electron spin resonance (ESR) technique. Five modes of ESR transitions are observed and identified: transitions between singlet ground state and excited triplet states, three modes of transitions between spin sublevels of collective triplet states and antiferromagnetic resonance absorption in the field-induced antiferromagnetically ordered phase. Singlet-triplet and intra-triplet modes demonstrate a doublet structure which is due to two maxima in the density of magnon states in the low-frequency range. A joint analysis of the observed spectra and other experimental results allows to test the applicability of the fermionic and bosonic models. We conclude that the fermionic approach is more appropriate for the particular case of NTENP.

cond-mat.str-el↗
arXiv2010

New high magnetic field phase of the frustrated $S=1/2$ chain compound LiCuVO$_4$

Magnetization of the frustrated $S=1/2$ chain compound LiCuVO$_4$, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).

cond-mat.str-el↗
arXiv2008

Co-existing chiral and collinear phases in a distorted triangular antiferromagnet

The entire magnetic phase diagram of the quasi two dimensional (2D) magnet on a distorted triangular lattice KFe(MoO4)2 is outlined by means of magnetization, specific heat, and neutron diffraction measurements. It is found that the spin network breaks down into two almost independent magnetic subsystems. One subsystem is a collinear antiferromagnet that shows a simple spin-flop behavior in applied fields. The other is a helimagnet that instead goes through a series of exotic commensurate-incommensurate phase transformations. In the various phases one observes either true 3D order or quasi-2D order. The experimental findings are compared to theoretical predictions found in literature

cond-mat.str-el↗
arXiv2008

Spin Dynamics of $Ni Cl_2-4SC(NH_2)_2$ in the Field-Induced Ordered Phase

$Ni Cl_2$-$4SC(NH_2)_2$ (known as DTN) is a spin-1 material with a strong single-ion anisotropy that is regarded as a new candidate for Bose-Einstein condensation (BEC) of spin degrees of freedom. We present a systematic study of the low-energy excitation spectrum of DTN in the field-induced magnetically ordered phase by means of high-field electron spin resonance measurements at temperatures down to 0.45 K. We argue that two gapped modes observed in the experiment can be consistently interpreted within a four-sublattice antiferromagnet model with a finite interaction between two tetragonal subsystems and unbroken axial symmetry. The latter is crucial for the interpretation of the field-induced ordering in DTN in terms of BEC.

cond-mat.str-el↗
arXiv2007

Spin-liquid dynamics of pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7

The spin-liquid phase of two highly frustrated pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7 is probed using electron spin resonance in the temperature range 1.3 - 30 K. The deviation of the absorption line from the paramagnetic position ν=γH observed in both compounds below the Curie-Weiss temperature Θ_CW ~ 10 K, suggests an opening up of a gap in the excitation spectra. On cooling to 1.3 K (which is above the ordering transition T_N ~ 1.0 K) the resonance spectrum is transformed into a wide band of excitations with the gap amounting to Δ~ 26 GHz (1.2 K) in Gd2Ti2O7 and 18 GHz (0.8 K) in Gd2Sn2O7. The gaps increase linearly with the external magnetic field. For Gd2Ti2O7 this branch co-exists with an additional nearly paramagnetic line absent in Gd2Sn2O7. These low lying excitations with gaps, which are preformed in the spin-liquid state, may be interpreted as collective spin modes split by the single-ion anisotropy.

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arXiv2007

Triplet spin resonance of the Haldane compound with interchain coupling

Spin resonance absorption of the triplet excitations is studied experimentally in the Haldane magnet PbNi2V2O8. The spectrum has features of spin S=1 resonance in a crystal field, with all three components, corresponding to transitions between spin sublevels, being observable. The resonance field is temperature dependent, indicating the renormalization of excitation spectrum in interaction between the triplets. Magnetic resonance frequencies and critical fields of the magnetization curve are consistent with a boson version of the macroscopic field theory [Affleck 1992, Farutin & Marchenko 2007], implying the field induced ordering at the critical field, while contradict the previously used approach of noninteracting spin chains.

cond-mat.str-el↗
arXiv2006

Magnetic phase diagram, critical behaviour and 2D-3D crossover in a triangular lattice antiferromagnet RbFe(MoO4)2

We have studied the magnetic and thermodynamic properties as well as the NMR spectra of the Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature dependence of the order parameter, the critical indices and the overall magnetic H-T phase diagram are all in a good agreement with the theoretical predictions for a 2D XY model. The temperature dependence of the specific heat at low temperature demonstrates a crossover from a T^2 law characteristic of a two-dimensional antiferromagnet to a three-dimensional T^3 law.

cond-mat.str-el↗
arXiv2006

Magnetic resonance in a pyrochlore antiferromagnet Gd2Ti2O7

Electron spin resonance study of frustrated pyrochlore Gd2Ti2O7 is performed in a wide frequency band for a temperature range 0.4-30 K, which covers paramagnetic and magnetically ordered phases. The paramagnetic resonance reveals the spectroscopic g-factor about 2.0 and a temperature dependent linewidth. In ordered phases magnetic resonance spectra are distinctive for a nonplanar cubic (or tetrahedral) antiferromagnet with an isotropic susceptibility. In the high-field saturated phase, weakly-dispersive soft modes are observed and their field evolution is traced.

cond-mat.str-el↗
arXiv2005

ESR study of the single-ion anisotropy in the pyrochlore antiferromagnet Gd2Sn2O7

Single-ion anisotropy is of importance for the magnetic ordering of the frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy parameters for the Gd2Sn2O7 were measured using the electron spin resonance (ESR) technique. The anisotropy was found to be of the easy plane type, with the main constant D=140mK. This value is 35% smaller than the value of the corresponding anisotropy constant in the related compound Gd2Ti2O7.

cond-mat.str-el↗
arXiv2005

Magnetic resonance study of the spin-reorientation transitions in the quasi-one-dimensional antiferromagnet BaCu2Si2O7

A quasi-one dimensional antiferromagnet with a strong reduction of the ordered spin component, BaCu2Si2O7, is studied by the magnetic resonance technique in a wide field and frequency range. Besides of conventional spin-flop transition at the magnetic field parallel to the easy axis of spin ordering, magnetic resonance spectra indicate additional spin-reorientation transitions in all three principal orientations of magnetic field. At these additional transitions the spins rotate in the plane perpendicular to the magnetic field keeping the mutual arrangement of ordered spin components. The observed magnetic resonance spectra and spin-reorientation phase transitions are quantitatively described by a model including the anisotropy of transverse susceptibility with respect to the order parameter orientation. The anisotropy of the transverse susceptibility and the strong reduction of the anisotropy energy due to the quantum spin fluctuations are proposed to be the reason of the spin reorientations which are observed.

cond-mat.str-el↗
arXiv2005

Single-ion anisotropy in the gadolinium pyrochlores studied by an electron paramagnetic resonance

The electron paramagnetic resonance is used to measure the single-ion anisotropy of Gd3+ ions in the pyrochlore structure of (Y{1-x}Gdx)2Ti2O7. A rather strong easy-plane type anisotropy is found. The anisotropy constant D is comparable to the exchange integral J in the prototype Gd2Ti2O7, D~0.75J and exceeds the dipolar energy scale. Physical implications of an easy-plane anisotropy for a pyrochlore antiferromagnet are considered. We calculate the magnetization curves at T=0 and discuss phase transitions in magnetic field.

cond-mat.str-el↗
arXiv2004

Magnetocaloric effect in pyrochlore antiferromagnet Gd2Ti2O7

An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional paramagnetic salts, this compound can exhibit a temperature decrease by a factor of ten in the temperature range below the Curie-Weiss constant. The most efficient cooling is observed in the field interval between 120 and 60 kOe corresponding to a crossover between saturated and spin-liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy survives in the strongly correlated state. According to the theoretical model, this entropy is associated with a macroscopic number of local modes remaining gapless till the saturation field. Monte Carlo simulations on a classical spin model demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a well-known material for low temperature magnetic refrigeration.

cond-mat.str-el↗
arXiv2003

Spin-resonance modes of the spin-gap magnet TlCuCl_3

Three kinds of magnetic resonance signals were detected in crystals of the spin-gap magnet TlCuCl_3. First, we have observed the microwave absorption due to the excitation of the transitions between the singlet ground state and the excited triplet states. This mode has the linear frequency-field dependence corresponding to the previously known value of the zero-field spin-gap of 156 GHz and to the closing of spin-gap at the magnetic field H_c of about 50 kOe. Second, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field. Finally, we have observed antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field H_c. This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field.

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arXiv2002

Quasi two-dimensional antiferromagnet on a triangular lattice RbFe(MoO4)2

RbFe(MoO4)2 is a rare example of a nearly two-dimensional Heisenberg antiferromagnet on a triangular lattice. Magnetic resonance spectra and magnetization curves reveal that the system has a layered spin structure with six magnetic sublattices. The sublattices within a layer are arranged in a triangular manner with the magnetization vectors 120 degree apart. The H-T phase diagram, containing at least five different magnetic phases is constructed. In zero field, RbFe(MoO4)2 undergoes a phase transition at T_N=3.8 K into a non-collinear triangular spin structure with all the spins confined in the basal plane. The application of an in-plane magnetic field induces a collinear spin state between the fields H_c1=47 kOe and H_c2=71 kOe and produces a magnetization plateau at one-third of the saturation moment. Both the ESR and the magnetization measurements also clearly indicate an additional first-order phase transition in a field of 35 kOe. The exact nature of this phase transition is uncertain.

cond-mat.str-el↗
arXiv2001

Paramagnetic and antiferromagnetic resonances in the diamagnetically diluted Haldane magnet PbNi2V2O8

The impurity-induced antiferromagnetic ordering of the doped Haldane magnet Pb(Ni{1-x}Mg{x})2V2O8 (0 < x <0.06) was studied by electron spin resonance (ESR) on ceramic samples in the frequency range 9-110 GHz. Below the Néel temperature a transformation of the ESR spectrum was found, indicating an antiferromagnetic resonance mode of spin precession. The excitation gap of the spin-wave spectrum increases with increasing Mg-concentration $x$ in the same manner as the Néel temperature, reaching its maximum value of 80 GHz at x > 0.04. At small concentrations x < 0.02 the signals of antiferromagnetic resonance were found to coexist with the signal of the paramagnetic resonance indicating a microscopic separation of the magnetic phases.

cond-mat.str-el↗
arXiv2001

Magnetic Resonance of Spin Clusters and Triplet Excitations in a Spin-Peierls Magnet with Impurities

The magnetic resonance spectrum of spin clusters formed in spin-Peierls magnets in the vicinity of impurity ions was investigated. The observed temperature dependences of the effective g-factor and the linewidth of the electron spin resonance (ESR) in crystals of Cu(1-x)Ni(x)GeO(3) are described in the model of the exchange narrowing of the two-component spectrum with one component ascribed to spin clusters and exhibiting an anomalous value of the $g$-factor and the other related to triplet excitations. An estimation of the size of the suppressed dimerization region around the impurity ion is obtained (this region includes about 30 copper ions). The dependence of the effective g-factor and the ESR linewidth on the impurity concentration at low temperatures indicates the interaction of clusters.

cond-mat.str-el↗
arXiv2001

Separation of the magnetic phases at the Néel point in the diluted spin-Peierls magnet CuGeO3

The impurity induced antiferromagnetic ordering of the doped spin-Peierls magnet Cu(1-x)Mg(x)GeO(3) was studied by ESR technique. Crystals with the Mg concentration x<4% demonstrate a coexistence of paramagnetic and antiferromagnetic ESR modes. This coexistence indicates the separation of a macroscopically uniform sample in the paramagnetic and antiferromagnetic phases. In the presence of the long-range spin-Peierls order (in a sample with x=1.71%) the volume of the antiferromagnetic phase immediately below the Néel point T_N is much smaller than the volume of the paramagnetic phase. In the presence of the short-range spin-Peierls order (in samples with x=2.88%, x= 3.2%) there are comparable volumes of paramagnetic and antiferromagnetic phases at T=T_N. The fraction of the antiferromagnetic phase increases with lowering temperature. In the absence of the spin-Peierls dimerization (at x=4.57%)the whole sample exhibits the transition into the antiferromagnetic state and there is no phase separation. The phase separation is explained by the consideration of clusters of staggered magnetization located near impurity atoms. In this model the areas occupied by coherently correlated spins expand with decreasing temperature and the percolation of the ordered area through a macroscopic distance occurs.

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