Anharmonic Spin Research Articles

Magnetic ground state of NdB4 : Interplay between anisotropic exchange interactions and hidden order on a Shastry-Sutherland lattice

The neodymium tetraboride NdB4, crystalizing into a tetragonal crystal structure, has the magnetic ions located on a Shastry-Sutherland lattice. The compound undergoes several magnetic phase transitions and exhibits a complex multi-k ground state that has so far been elusive. Here we report the solution and quantitative refinement of the magnetic ground state of NdB4 based on neutron diffraction data. The magnetic structure consists of two separate components; one of them is confined within the ab plane and makes an orthogonal all-in-all-out spin configuration maintaining translation symmetry of the crystal. Another is pointing along the c axis, and this component forms an anharmonic spin density wave consisting of three-up-two-down sequences with fivefold periodicity. The in-plane and out of plane spin structures represent distinct magnetic instabilities which normally would not coexist. Their simultaneous presence in the same phase is an interesting phenomenon associated with anisotropic exchange interactions and coupling to a symmetry breaking nonmagnetic order parameter undetectable by the available diffraction data. Published by the American Physical Society 2024

Electric, Dielectric, and Phonon Properties of Cu2OCl2

Investigating the multiferroic properties of , their temperature and magnetic‐field dependencies around the magnetic and ferroelectric phase transition temperatures and are analyzed using a microscopic model and Green's function technique. Near the Neel temperature , a kink in the specific heat and also in the phonon energy and damping due to strong anharmonic spin–phonon interaction is observed. The kink in disappears with increasing magnetic field h. The polarization shows also a small anomaly at and decreases with increasing magnetic field h. This behavior strongly suggests that exhibits multiferroic characteristics. A kink at can be seen in the real part of the dielectric constant , too. It should be noted that demonstrates a significant increase with increasing field strength h below , which is a pronounced magneto‐dielectric effect. These findings qualitatively agree with the experimental data.

Competition between Electron–Phonon and Spin–Phonon Interaction on the Band Gap and Phonon Spectrum in Magnetic Semiconductors

Using the microscopic s-f model and Green’s function theory, we study the temperature dependence of the band gap energy Eg and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of Eg and the phonon spectrum, there is a kink at the phase transition temperature TC or TN due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I>0,R>0, Eg decreases with increasing SPI and EPI, whereas for I<0,R>0, there is a competition; Eg increases with raising the EPI and decreases for enhanced SPI. For R<0, in both cases, the SPI and EPI reduce Eg. The magnetic field dependence of Eg for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO (I>0, R<0), whereas ω = 151 cm−1 in EuSe (I>0, R>0) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data.

Field theory for zero temperature soft anharmonic spin glasses in a field

We introduce a finite dimensional anharmonic soft spin glass in a field and show how it allows the construction a field theory at zero temperature and the corresponding loop expansion. The mean field level of the model coincides with a recently introduced fully connected model, the KHGPS model, and it has a spin glass transition in a field at zero temperature driven by the appearance of pseudogapped non-linear excitations. We analyze the zero temperature limit of the theory and the behavior of the bare masses and couplings on approaching the mean field zero temperature critical point. Focusing on the so called replicon sector of the field theory, we show that the bare mass corresponding to fluctuations in this sector is strictly positive at the transition in a certain region of control parameter space. At the same time the two relevant cubic coupling constants g 1 and g 2 show a non-analytic behavior in their bare values: approaching the critical point at zero temperature, g 1 → ∞ while g 2 ∝ T with a prefactor diverging at the transition. Along the same lines we also develop the field theory to study the density of states of the model in finite dimension. We show that in the mean field limit the density of states converges to the one of the KHGPS model. However the construction allows a treatment of finite dimensional effects in perturbation theory.

Robust cycloid crossover driven by anisotropy in the skyrmion host GaV4S8

We report on the anomalous magnetization dynamics of the cycloidally-modulated spin textures under the influence of uniaxial anisotropy in multiferroic $\mathrm{GaV_4S_8}$. The temperature and field dependence of the linear ac susceptibility [$\chi_{1\omega}^{\prime}(T,H)$], ac magnetic loss [$\chi_{1\omega}^{\prime\prime}(T,H)$], and nonlinear ac magnetic response [$M_{3\omega}(T,H)$] are examined across the magnetic phase diagram in the frequency range $f = 10-10000$ Hz. According to recent theory, skyrmion vortices under axial crystal symmetry are confined along specific orientations, resulting in enhanced robustness against oblique magnetic fields and altered spin dynamics. We characterize the magnetic response of each spin texture and find that the dynamic rigidity of the N\'eel skyrmion lattice appears enhanced compared to Bloch-type skyrmions in cubic systems, even in the multidomain state. Anomalous $M_{3\omega}$ and strong dissipation emerge over the same phase regime where strong variations in the cycloid pitch were observed on lowering temperature in recent small-angle neutron scattering experiments [White et al., Phys. Rev. B 97, 020401(R) (2018)]. Here, we show that strong anisotropy also drives an extended crossover of the zero-field cycloid texture in $\mathrm{GaV_4S_8}$. The frequency dependence of these dynamic signatures is consistent with that of a robust anharmonic spin texture exhibiting a correlated domain arrangement. The results underpin the essential role of magnetic anisotropy in enhancing the rigidity of topological spin textures for diverse applications.

Changes in the Magnetic Structure of Multiferroic BiFe0.80Cr0.20O3 with Temperature

The results of a Mossbauer study of the magnetic structure of multiferroic BiFe0.80Cr0.20O3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe0.80Cr0.20O3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Neel temperature, TN = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe0.80Cr0.20O3 multiferroic.

Температурные изменения магнитной структуры мультиферроика BiFe-=SUB=-0.80-=/SUB=-Cr-=SUB=-0.20-=/SUB=-O-=SUB=-3-=/SUB=-

AbstractThe results of a Mössbauer study of the magnetic structure of multiferroic BiFe_0.80Cr_0.20O_3 in the temperature range of 5–550 K are presented. It is found that a collinear antiferromagnetic structure of the G type is present in BiFe_0.80Cr_0.20O_3 at temperatures below 260 K. Above 260 K, an anharmonic spin wave with a magnetic anisotropy of the easy-axis type with a high value of the anharmonicity parameter m arises. With a further increase in the temperature, the m parameter decreases and tends to zero at T ~ 420 K, at which a harmonic spin wave comes into existence. Above a temperature of about 420 K, the m parameter increases again and the spin wave becomes anharmonic with an easy-plane magnetic anisotropy. At the Néel temperature, T _N = 505 ± 10 K, the sample undergoes a transition from the magnetically ordered to the paramagnetic state. The change in the type of magnetic anisotropy at T ~ 420 K is explained by competing contributions of different signs to the effective magnetic anisotropy constant and their different temperature dependence for the BiFe_0.80Cr_0.20O_3 multiferroic.

Size and doping dependence of the phonon properties of SnO2 nanoparticles

Using a microscopic model, taking into account the spin–phonon interactions and the Green’s function technique, we have studied the phonon properties of pure and ion-doped SnO2 nanoparticles (NPs). The phonon energy of SnO2 NPs decreases whereas the damping increases with decreasing particle size. Near the Curie temperature in the NPs, there appears an anomaly in the phonon energy [Formula: see text] and damping [Formula: see text]. The phonon properties are very sensitive to the anharmonic spin–phonon interaction R. They can increase or decrease for [Formula: see text] or [Formula: see text] with increasing of temperature, respectively. In dependence of the radius of the doping ions, the phonon energies [Formula: see text] could be reduced (Co, Fe, Sm, Nd) or enhanced (Cu). The phonon damping is always enhanced in the doped NPs. In summary, due to the size and temperature effects, we obtain changes in the phonon energy and damping in pure SnO2 NPs, whereas in ion-doped ones, the doping effects strengthen these properties.

Temperature Mössbauer study of the spatial spin-modulated structure in the multiferroic BiFeO3

57Fe Mössbauer detailed study of the spatial spin-modulated structure of the multiferroic BiFeO3 was carried out in a wide temperature range including the temperature of magnetic phase transition. The Mossbauer spectra have been analysed by fitting in terms of the anharmonic spin cycloid mode. It is established that at temperatures below ~330 K a magnetic anisotropy of the "easy axis" type is realized and above is the magnetic anisotropy of the "easy plane" type. An explanation for the change in the type of magnetic anisotropy is proposed, based on taking into account the different temperature dependences of the two contributions to the effective uniaxial magnetic anisotropy constant: a crystal anisotropy of net antiferromagnet and a weak ferromagnetism.

Spontaneous decays of magneto-elastic excitations in non-collinear antiferromagnet (Y,Lu)MnO3

Magnons and phonons are fundamental quasiparticles in a solid and can be coupled together to form a hybrid quasi-particle. However, detailed experimental studies on the underlying Hamiltonian of this particle are rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored, although it is essential for a proper understanding of their diverse thermodynamic behaviour and intrinsic zero-temperature decay. Here we show that in non-collinear antiferromagnets, a strong magnon–phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon–phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it.

ChemInform Abstract: Generalized Theory of Spin Fluctuations in Itinerant Electron Magnets: Crucial Role of Spin Anharmonicity

AbstractReview: 24 refs.

Generalized theory of spin fluctuations in itinerant electron magnets: Crucial role of spin anharmonicity

The paper critically overviews the recent developments of the theory of spatially dispersive spin fluctuations (SF) in itinerant electron magnetism with particular emphasis on spin-fluctuation coupling or spin anharmonicity. It is argued that the conventional self-consistent renormalized (SCR) theory of spin fluctuations is usually used aside of the range of its applicability actually defined by the constraint of weak spin anharmonicity based on the random phase approximation (RPA) arguments. An essential step in understanding SF in itinerant magnets beyond RPA-like arguments was made recently within the soft-mode theory of SF accounting for strong spin anharmonicity caused by zero-point SF. In the present paper we generalize it to apply for a wider range of temperatures and regimes of SF and show it to lead to qualitatively new results caused by zero-point effects.

Room temperature ferromagnetism and phonon properties of pure and doped TiO2 nanoparticles

We have considered the origin of RTFM in TiO2 nanoparticles (NPs). Further we have studied the properties of the Eg1 phonon mode. The phonon frequency of anatase TiO2 NPs increases whereas in the case of rutile TiO2 NPs it decreases as the particle size decreases. The phonon damping is always enhanced in the nanosized materials. The hardening of the Eg1 mode and the softening of the Eg3 mode in anatase TiO2 NPs could be explained with the different anharmonic spin–phonon interaction constants of these modes. The doping effects with different transition metal ions on the Eg1 phonon mode are also discussed.

A possible spin-fluctuation mechanism in the emergence of superconductivity in a region of magnetic instability, using the example of PuCoGa5

The spin-fluctuation superconductivity mechanism of strongly correlated d-f electrons is considered within the Hubbard model. It is shown that the emergence of superconductivity is possible under conditions of strong spin anharmonicity. Evaluations of the superconductivity temperature in the model developed for the electron structure of PuCoGa5 agree with the experimental data.

Anomalous influence of an external magnetic field on spin fluctuations in magnetic semiconductors with strong p-d hybridization and the colossal magnetoresistance effect

The colossal magnetoresistance effect in magnetic semiconductors based on lanthanum manganites has been investigated in terms of the model allowing for the effects of p-d hybridization and electronelectron Coulomb correlations. The influence of an external magnetic field on spin fluctuations has been considered under the conditions where the chemical potential is in a narrow heavy-fermion band formed in the hybridization gap. It has been shown that, in the vicinity of the Curie point TC, the strong spin anharmonicity leads to an anomalously strong suppression of spin fluctuations by the external magnetic field, a phenomenon contributing significantly to the formation of colossal negative magnetoresistance.

Extended Dynamic Spin-Fluctuation Theory with Application to Iron

The problem of discontinuous phase transition in the dynamic spin-fluctuation theory is resolved by taking into account large anharmonic spin fluctuations and nonlocality of the mean Green function. The extended theory is applied to the calculation of magnetic properties of iron.

Theory of phonon properties in doped and undoped CuO nanoparticles

We have studied the phonon properties of CuO nanoparticles and have shown the importance of the anharmonic spin–phonon interaction. The Raman peaks of CuO nanoparticles shift to lower frequency and become broader as the particle size decreases in comparison with those of bulk CuO crystals owing to size effects. By doping with different ions, in dependence of their radius compared to the host ionic radius the phonon energies ω could be reduced or enhanced. The phonon damping is always enhanced through the ion doping effects.

ION DOPING EFFECTS ON THE DIELECTRIC AND PHONON PROPERTIES OF (Ba, Sr)TiO3 THIN FILMS

The exact mechanism by which doping ions change the properties in (BST) thin films is not fully understood. We have shown based ( Ba , Sr ) TiO 3 on the transverse Ising model including anharmonic spin–phonon interactions and using a Green's function technique that the dielectric constant and the phonon properties can be changed due to different exchange interactions between the doping ions and the host ions in BST thin films. They are highly dependent upon the magnitude and the kind of the stress (tensile or compressive) due to the different radii of the doping ions. The phonon energies can be larger or smaller for different doping ions, whereas their damping is always enhanced.

Impact of the spin–phonon interaction on the phonon properties of multiferroic hexagonalRMnO3 thin films

The phonon properties of multiferroic hexagonalRMnO3 thin films are studied based on a microscopic model including anharmonic spin–phononinteractions. We obtain an anomaly near the magnetic phase transition temperatureTN which can be attributed to the magnetoelectric nature ofRMnO3 and a strong spin–phonon interaction. The size and surface effects on the phononproperties are discussed. It is demonstrated that the phonon energies can be larger orsmaller for different substrates or doping ions, whereas their damping is alwaysenhanced. The influence of an external magnetic field on the phonon properties isobserved.

Phenomenological model of longitudinal spin fluctuations in itinerant antiferromagnets

We present the phenomenological analysis of the spectrum of longitudinal spin fluctuations in isotropic itinerant electron antiferromagnets with account of spin anharmonicity giving rise to coupling of transverse and longitudinal normal modes. The spectrum consists of a quasielastic part forming a central peak or a dip, depending on temperature and the Landau relaxation rate. Effects of spin fluctuation coupling also give rise to an inelastic part of the spectrum which has a form of resonances or antiresonances near the magnon frequencies related to non-propagating longitudinal excitations.