Strong Spin-lattice Coupling Research Articles

Magnetic and dielectric properties of quasi-two-dimensional manganese oxide Sr7Mn4O15

Using dc magnetic susceptibility, electron spin resonance (ESR) and dielectric spectroscopy techniques, we investigate the magnetic and dielectric properties of Sr7Mn4O15, which has a layered structure with pairs of face-sharing octahedra Mn2O9. Magnetic susceptibility reveals the Néel temperature TN∼70.5K, above which Sr7Mn4O15 manifests intense short range order. ESR spectra indicate the existence of a transition at about 370K, confirming the suggested spin pairing in the Mn2O9 units. In the dielectric spectra, a sudden drop of permittivity is observed at TN, revealing the existence of strong spin–lattice coupling in Sr7Mn4O15.

Spin-lattice coupling in the frustrated antiferromagnet ZnCr2Se4probed by ultrasound

Ultrasound and magnetization studies of the frustrated spinel ZnCr${}_{2}$Se${}_{4}$ are performed as a function of temperature and magnetic field up to 14 T. In zero field, the sound velocity and attenuation reveal significant anomalies at the antiferromagnetic transition at ${T}_{N}\ensuremath{\approx}$ 21 K indicating strong spin-lattice coupling. External magnetic fields shift these anomalies to lower temperatures concomitantly with the reduction of the N\'eel temperature. At 2 K, the sound velocity as a function of magnetic field manifests three pronounced anomalies: a deep minimum at 5.4 T related to an inflection point of the magnetization followed by two plateaus with distinct stiffness at fields above 7 and 10 T. The first plateau is ascribed to a transformation from a tetragonal to a cubic phase, while the second one corresponds to a state with fully polarized magnetization. The evolution of magnetic and structural states is discussed within a $H\ensuremath{-}T$ phase diagram and compared with related frustrated magnetic spinels with strong spin-lattice coupling.

Weak ferromagnetism and internal magnetoelectric effect in LiFeP2O7

The magnetic, thermodynamic, and pyroelectric properties of LiFeP$_2$O$_7$ single crystals are investigated with emphasis on the magnetoelectric interaction of the electrical polarization with the magnetic order parameter. The magnetic order below T$_N\simeq$ 27 K is found to be a canted antiferromagnet with a weak ferromagnetic component along the $b-$axis. A sharp peak of the pyroelectric current at T$_N$ proves the strong internal magnetoelectric interaction resulting in a sizable polarization decrease at the onset of magnetic order. The magnetoelectric effect in external magnetic fields combines a linear and a quadratic field dependence below T$_N$. Thermal expansion data show a large uniaxial magnetoelastic response and prove the existence of strong spin lattice coupling. LiFeP$_2$O$_7$ is a polar compound with a strong interaction of the magnetic order parameter with the electric polarization and the lattice.

Coupled structural and magnetic antiphase domain walls on BaFe2As2

High-resolution scanning tunneling microscopy measurements on the (001) surface of BaFe${}_{2}$As${}_{2}$, a parent compound of Fe-based high-${T}_{c}$ superconductors, reveal a complex surface with coexisting structural phases. Large areas display a squarelike $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})R{45}^{\ensuremath{\circ}}$ structure with surface-pinned antiphase domains. The domain walls exhibit ${C}_{2}$ symmetry, in contrast with the bulk ${C}_{2v}$ geometric structure. We argue that the strong spin-lattice coupling at the surface results in the coexistence of structure and spin antiphase domain boundaries with ${C}_{2}$ symmetry.

Partially irreversible metamagnetic behavior induced by magnetic field in La0.27Nd0.4Ca0.33MnO3

Measurements of the isothermal magnetization on La0.27Nd0.4Ca0.33MnO3 have been performed in a field up to 7T at various temperatures. A distinct field-induced metamagnetic transition is observed within a temperature region of 2–180K, resulting from the coexistence of charge ordered (CO), antiferromagnetic (AFM) and ferromagnetic (FM) phases. A temperature and magnetic field dependent irreversible CO–FM transition appears in the lower temperature region. Accompanying the transition an obvious lattice strain was observed, indicating that the transition is related to the change of lattice strain due to strong spin–lattice coupling in La0.27Nd0.4Ca0.33MnO3. These properties make the doped manganites a good candidate for a magnetostriction sensor material.

Continuous and discontinuous quantum phase transitions in a model two-dimensional magnet.

The Shasty-Sutherland model, which consists of a set of spin 1/2 dimers on a 2D square lattice, is simple and soluble but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu(2)(BO(3))(2), in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution X-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure-dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.

Magnetoelectric properties of quantum paraelectric EuTiO3 materials on the strain effect

Because of the strong coupling between the magnetic and dielectric properties, the study of quantum paraelectric EuTiO3 has attracted more and more attention in both theoretical and experimental research recently. In this paper, the first principles based on the density functional theory within the generalized gradient approxiamtion is used to investigate the magnetic and electronic structure of quantum paraelectric EuTiO3, and to analyze the effects of the strain on the magnetic and strutural phase transition, in turn to discuss the possible magnetoelectric coupling mechanism of this material. The calculations show that EuTiO3 with the strain-free is in a paraelectric cubic and G-type antiferromagnetic state at low temperature, while appling either compressive or tensile strain along the c-axis to it, the balance of hybridization between Ti 3d and O 2p orbit will be breaken and EuTiO3 will transite from paraelectric and G-antiferromagnetic to ferroelectric-ferromagnetic structure as the strain is increased to a certain value. All those indicate the strong spin-lattice coupling effect in EuTiO3.

Ferroelectricity generated by spin-orbit and spin-lattice couplings in multiferroic DyMnO3

While the ferroelectricity in type-II multiferroic rare-earth manganites is believed to be generated by the inverse Dzyaloshinskii-Moriya (DM) interaction (spin-orbit coupling) associated with the Mn spiral spin order, recent results revealed the strong spin-lattice coupling arising from the Dy-Mn spin interaction in DyMnO3, which may also be an ingredient contributing to the ferroelectricity. In this work, we summarize our recent experiments on this issue by performing a series of rare-earth site nonmagnetic Y and magnetic Ho substitutions at Dy site for DyMnO3. It is demonstrated that the Dy-Mn spin interaction contributes to the ferroelectric polarization through the symmetric exchange striction mechanism (spin-lattice coupling). A coexistence of the spin-orbit coupling and spin-lattice coupling in one compound is confirmed. At the same time, the independent Dy antiferromagnetic spin order at low temperature can be effectively suppressed by the substitutions, beneficial to the polarization enhancement.

Thermodynamic properties and resistivity of the ferromagnetic semiconductor EuC2

EuC2 is a ferromagnet with a Curie temperature of TC ≃ 15 K. It is semiconducting with the particularity that the resistivity drops by about five orders of magnitude on cooling through TC, which is therefore called a metal–insulator transition. In this paper, we study the magnetization, specific heat, thermal expansion and the resistivity around this ferromagnetic transition on high-quality EuC2 samples. At TC we observe well-defined anomalies in the specific heat cp(T) and thermal expansion α(T) data. The magnetic contributions of cp(T) and α(T) can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field, the magnetic contributions of the specific heat and thermal expansion fulfil a Grüneisen scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of TC via Ehrenfest's relation, we expect a considerable increase of TC under applied pressure due to a strong spin–lattice coupling. Furthermore, the influence of weak off-stoichiometries δ in EuC2±δ was studied. It is found that δ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behaviour of EuC2 strongly resembles that of EuO.

Concurrent transition of ferroelectric and magnetic ordering near room temperature

Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic Néel temperature of the multiferroic compound BiFeO(3) is suppressed to around room temperature by heteroepitaxial misfit strain. Remarkably, the ferroelectric state undergoes a first-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our findings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.

Antiferromagnetic superexchange via3dstates of titanium in EuTiO3as seen from hybrid Hartree-Fock density functional calculations

A superexchange mechanism between Eu${}^{2+}$ 4$f$ spins via the 3$d$ states of nonmagnetic Ti${}^{4+}$ ions is proposed through first-principles calculations based on a hybrid Hartree-Fock density functional approach to explain $G$-type antiferromagnetism in EuTiO${}_{3}$. This mechanism is supported by systematic calculations for related Eu${}^{2+}$-based perovskite oxides. In EuTiO${}_{3}$, the competition between the antiferromagnetic superexchange and an indirect ferromagnetic exchange via the Eu 5$d$ states leads to a delicate balance between antiferromagnetic and ferromagnetic phases. The superexchange mechanism involving the Ti 3$d$ states hints at the microscopic origin of the strong spin-lattice coupling in EuTiO${}_{3}$.

Strong magnetorefractive and quadratic magneto-optical effects in(Pr0.4La0.6)0.7Ca0.3MnO3

We have characterized the spectroscopic magneto-optical response of a ${({\text{Pr}}_{0.4}{\text{La}}_{0.6})}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ thin film in the visible. We find, in addition to the conventional linear magneto-optical response, two distinct nonlinear contributions corresponding to the quadratic magneto-optical and the magnetorefractive effect, respectively. These nonlinear effects appear with dissimilar intensity depending on the temperature and the light frequency. Strikingly, these nonconventional responses can be one or two orders of magnitude larger than the linear magneto-optical effect. This phenomenon is related to the complex electronic behavior of this manganite and probably can be expected also in other correlated electronic systems with strong spin-lattice coupling.

THz and infrared studies of multiferroic hexagonal Y1− x Eu x MnO3 (x = 0–0.2) ceramics

We report an investigation of hexagonal Y1− x Eu x MnO3 ceramics with x = 0, 0.1, and 0.2 using infrared (IR) and terahertz (THz) spectroscopies in the temperature range between 5 and 900 K. The temperature dependence of the THz permittivity reveals a kink near the antiferromagnetic (AFM) phase transition temperature T N ≈ 70 K, giving evidence of a strong spin–lattice coupling. Below T N, two absorption peaks were revealed in the THz spectra close to 43 and 73 cm−1. While the first peak corresponds to a sharp AFM resonance exhibiting softening on heating towards T N, the second one may be attributed to an impurity mode or a multiphonon absorption peak. High-temperature THz spectra measured up to 900 K reveal only small gradual increase of the permittivity in agreement with a weak phonon softening observed in the IR reflectance spectra upon heating. This corresponds to an improper ferroelectric character of the phase transition proposed from first principle calculations by Fennie and Rabe (Phys. Rev. B 72: 100103(R), 2005).

Strong spin-lattice coupling in multiferroic hexagonal manganite YMnO3 probed by ultrafast optical spectroscopy

We report the observation of spin-lattice coupling in multiferroic YMnO3 by femtosecond near-infrared pump and probe spectroscopy. A coherent 31 GHz acoustic phonon was detected above the magnetic ordering temperature, and a higher frequency coherent mode was observed in the antiferromagnetic phase. This temperature-dependent measurement demonstrates that the acoustic phonon excitation is coupled to spin ordering.

Local spin-coupled distortions in multiferroic hexagonalHoMnO3

Local structural measurements have been performed on hexagonal HoMnO3 in order to ascertain the specific changes in bond distances which accompany magnetic ordering transitions. The transition from paramagnetic to the antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn (nearest and next nearest) and Ho-Ho correlations. These changes in bond distances reveal strong spin-lattice coupling. The similarity in magnitude of the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The structural changes are interpreted in terms of a model of competing spin order and local structural distortions. Density functional calculations are used to estimate the energies associated with ionic displacements. The calculations also reveal asymmetric polarization of the charge density of Ho, O3 and O4 sites along the z-axis in the ferroelectric phase. This polarization facilitates coupling between Ho atoms on neighboring planes normal to the z-axis.

Spin-Lattice Coupling and Superconductivity in Fe Pnictides

We consider strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular on the dependence of the magnetic moment on the atomic volume of Fe defined by the positions of the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner criterion for the onset of magnetism. We suggest that the coupling between electrons and phonons through the spin channel may be sufficiently strong to be an important part of the superconductivity mechanism in Fe pnictides.

Lattice Distortion Coupled with Magnetic Ordering in a Triangular Lattice Antiferromagnet CuCrO2

We investigated magnetostrictive properties of a triangular lattice antiferromagnet CuCrO 2 showing an incommensurate proper-screw spin order by means of x-ray diffraction and strain gauge measurements. A deformation of the triangular lattice plane leading to a lowered crystallographic symmetry was found upon the magnetic ordering, which demonstrates a strong spin–lattice coupling in this system. The resultant multiple structural domains can be successfully arranged by magnetic field cooling procedures. We discuss the relationship between the lattice distortion and incommensurability in CuCrO 2 .

Field-dependent dielectric and magnetic properties in multiferroicCdCr2S4

Based on the results of field-dependent dielectric and magnetic measurements, we observe several interesting behaviors and phase transitions in this geometrically frustrated spinel system ${\text{CdCr}}_{2}{\text{S}}_{4}$. (1) A glassy dipolar state occurs near ${T}_{C}\ensuremath{\sim}85\text{ }\text{K}$, which is induced by the onset of ferromagnetic ordering. (2) A ferroelectric ordering occurs near ${T}_{p}\ensuremath{\sim}56\text{ }\text{K}$, which is enhanced by externally applying electric field. (3) Both the magnitude and step-up temperature of dielectric constant $({\ensuremath{\epsilon}}^{\ensuremath{'}})$ near ${T}_{C}$ are suppressed by an electric field yet are increased by the magnetic field. (4) Both electric and magnetic fields colossally enhance the magnitude of dielectric constant $({\ensuremath{\epsilon}}^{\ensuremath{'}})$ near ${T}_{p}$. (5) A clear dip of magnetization under various electric fields is observed near ${T}_{p}$ indicating the different spin-dipole interactions near ${T}_{C}$ and ${T}_{p}$. Attempts are made to analyze these field-dependent properties by considering the strong spin-lattice coupling and exchange striction effect in this novel multiferroic material.

Lattice strain and heat capacity anomalies at the spin reorientation transitions of ErFeO3 orthoferrite

We have investigated the thermal expansivities of an ErFeO3 single crystal along the three crystallographic orientations using a high-resolution capacitance dilatometer and found clear signatures of lattice distortions in the spin rotation region as evidence for strong spin-lattice coupling. The observed lattice strain is consistent with the smooth rotation of the magnetization and it reveals the importance of the magnetoelastic interaction. Heat capacity measurements show a well-defined plateau-like enhancement in the spin re-orientation regime as predicted by the Landau theory of second order phase transitions. The data are used to estimate microscopic anisotropy parameters. Heat capacity is also measured near the low-temperature erbium magnetic ordering transition.

Electronic structure of dimerized spinel ZnV 2O 4

Electronic structure calculations were performed for ZnV 2O 4, a material close to a metal–insulator transition. Structural optimization leads to the formation of V–V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained.