Spin lattice coupling in multiferroic hexagonal YMnO3

Aiming to shed light on the possible existence of hybrid phonon-magnon excitations in multiferroic manganites, neutron scattering measurements have been undertaken at LLB and ILL on the particular case of hexagonal YMnO₃. Our experiments focused on a transverse acoustic phonon mode polarized along the ferroelectric axis. The neutron data show that below the magnetic transition, this particular phonon mode splits in two different branches. The upper branch is found to coincide with a spin wave mode. This manifestation of a strong spin-lattice coupling is discussed in terms of a possible hybridization between the two types of elementary excitations, the phonon and magnons.      

Spin phonon coupling in hexagonal multiferroic YMnO3

Inelastic neutron scattering measurements have been performed on YMnO3, aiming to study the interplay between spin and lattice degrees of freedom in this hexagonal multiferroic material. Our study provides evidence for a strong coupling between magnons and phonons, evidenced by the opening of a gap below T(N) in the dispersion of the transverse acoustic phonon mode polarized along the ferroelectric axis. The resulting upper phonon branch is found to lock on one of the spin-wave modes. These findings are discussed in terms of a possible hybridization between the two types of elementary excitations.     

Structure and interaction of antiferromagnetic domain walls in hexagonal YMnO3

The structure of antiferromagnetic (AFM) domain walls and their interaction with lattice strain are derived taking the multiple-order-parameter compound YMnO3 as a model example. Contrary to the conviction that AFM domain walls are energetically unfavorable, their interaction with lattice strain lowers the total energy of the system and leads to a piezomagnetic clamping of the electric and magnetic order parameters in good agreement with the available experimental data.     

Raman scattering studies of the magnetic ordering in hexagonal HoMnO3 thin films

We present the results of the temperature dependence of the Raman spectra of hexagonal HoMnO₃ thin films in the 13–300 K temperature range. The films were grown on Pt(111)//Al₂O₃ (0001) substrates using the laser ablation method. In the HoMnO₃ thin films, we initially observedseveral broad Raman peaks at ∼510, 760, 955, 1120, and 1410 cm⁻¹. These broad Raman peaks display an anomalous behavior near the magnetic transition temperature, and the intensity difference of the Raman spectra at different temperatures shows several pairs of negative and positive peaks as the temperature is lowered below the Néel temperature. Our analyses indicate that all the broad peaks are correlated with magnetic ordering, and we have assigned the origin of all the broad peaks. Purely on the basis of the Raman analysis, we have deduced the Néel temperature and the spin exchange integrals of HoMnO₃ thin films. We also investigated the effects of the growth condition on the strongest broad peak at ∼760 cm⁻¹, which is related with pure magnetic ordering. This result indicates that the oxygen defect in the HoMnO₃ sample has negligible effect on magnetic ordering. Copyright © 2009 John Wiley & Sons, Ltd.     

Anomalously broad Raman scattering spectrum due to two-magnon excitation in hexagonal YMnO3

A strong and broad Raman scattering (RS) spectrum is observed from two-magnon processes in YMnO3. The spectrum is analyzed by taking account (i) the magnon-exciton interaction and (ii) the magnon-phonon coupling in the intermediate state. Anomalous broadening of the RS is attributed to the large superexchange interaction between manganite ions and subsequent modification of magnons under the presence of the exciton and phonons in the intermediate state.     

Nonlinear optical spectroscopy of electronic transitions in hexagonal manganites

The hexagonal rare-earth manganites RMnO₃ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) are a group of materials with an unusual combination of magnetic, electric and optical properties. The electronic structure of these materials was studied by second harmonic (SH) spectroscopy in the range from 1.2 to 3.0 eV. Faraday rotation and absorption spectra were measured in the range from 1.0 to 1.6 eV. Broad bands at ∼1.7 eV and ∼2.7 eV are assigned to electronic transitions between Mn³⁺(3d⁴) levels. The SH spectra are discussed on the basis of a recently developed microscopic theory. Received: 26 April 2001 / Published online: 18 July 2001     

Electronic structure of hexagonal rare-earth manganites RMnO<Subscript>3</Subscript>

The optical spectra of single crystals of hexagonal rare-earth manganites RMnO₃ (R=Sc, Y, Er) are studied in the range from 0.7 to 5.4 eV. It is found that the spectra substantially differ from the spectra of orthorhombic manganites in both the positions of spectral features and their polarization anisotropy. It is shown that the optical absorption edge is determined by an abnormally strong (k?1) and narrow electric dipole transition with the center at approximately 1.6 eV with light polarization in the basal plane of the crystal. This transition can be treated with confidence as charge transfer from oxygen to manganese. The experimental results are in many instances substantially different from the first-principles calculations of the electronic structure of YMnO₃ published recently and, hence, may serve as a reliable basis for the further improvement of computational methods.     

Nature of Ho magnetism in multiferroic HoMnO3

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho3+ in multiferroic HoMnO3. In zero field, Ho3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1 x 10(7) V/m, the magnetic structure of Ho3+ remains unchanged.     

Magnetoelastic effects in multiferroic YMnO3

We have investigated magnetoelastic effects in multiferroic YMnO(3) below the antiferromagnetic phase transition, T(N) ≈ 70 K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO(3) decreases normally above T(N), but decreases anomalously below T(N), whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T(N). The unit cell volume also undergoes an anomalous contraction below T(N). By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO(3) couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition.     

Determination of the magnetic symmetry of hexagonal manganites by second harmonic generation

Optical second harmonic spectroscopy is introduced as a powerful supplement for the determination of complex magnetic structures. Experimental efforts are simplified and new degrees of freedom are opened. Thereby, some principal or technical restrictions of neutron or magnetic x-ray diffraction experiments are overcome. High spatial resolution leads to additional information about magnetically ordered matter. As an example, the noncollinear magnetic structure of the hexagonal manganites RMnO3 ( R = Sc, Y, Ho, Er, Tm, Yb, Lu) is analyzed. The results show that some earlier conclusions on their magnetic symmetry and properties should be revised.     

Magnetic order and spin dynamics in ferroelectric HoMnO3

Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.     

Effect of piezomagnetism on coupling of electric and magnetic domain walls in hexagonal manganites

The profiles of antiferromagnetic domain walls in hexagonal manganites RMnO₃ are obtained numerically depending on anisotropy and internal strain due to the lattice distortion at the ferroelectric domain walls. It is found that the piezomagnetism can lower the free energy of the system thus it favors the coupling between electric and magnetic domain walls. Due to the piezomagnetic effect, the clamped antiferromagnetic domain walls with spin orientation angle ψ changing from 0 to π have different profiles comparing with those of ψ changing from 0 to -π, and the former is energetically more favorable than the latter when the internal strain is tensile at the FEL domain walls while it is the contrary for compressive strain. Moreover, the strongest coupling between the FEL domain walls and the favorable AFM domain walls can be achieved at an optimized internal strain.     

EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La_0.67Ca_0.33MnO_3?δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.     

Comparison of electronic structures of orthorhombic and hexagonal manganites studied by X-ray absorption spectroscopy

Electronic structures of orthorhombic perovskite-type manganites (OM) and hexagonal manganites (HM) are compared by using X-ray absorption near edge structure spectroscopy. This study reveals that the environments around Mn and O ions in both the systems are different. These are reflected primarily in the lifting of degeneracy of the Mn 3d orbitals. Interestingly, the unoccupied Mn 3dx²−y² orbital in HM is strongly hybridized with O 2p states and appears to be responsible for both long range orderings: ferroelectric ordering at high temperatures and ferromagnetic ordering at low temperatures.     

Magnetocaloric effect and magnetic properties in YMnO3 perovskite

The magnetic properties and magnetocaloric effect in YMnO₃ have been investigated using Monte Carlo simulations. The thermal magnetization, specific heat and magnetic entropy have been obtained for different values of exchange interactions and for a several external magnetic field values. The variation of adiabatic temperature change with the temperatures has been obtained for several values of external magnetic field. It has been found that the sample exhibited a paramagnetic to ferromagnetic phase transition at 30 K. The transition temperature of YMnO₃ has been deduced for different values of size (1/L) and different values of exchange interactions. The relative cooling power with several values of external magnetic field has been established.     

Epitaxial growth and in-plane dielectric properties of orthorhombic HoMnO3 films

Orthorhombic HoMnO3(HMO) thin films were grown epitaxially on LaAlO3(001) substrates by using pulsed laser deposition technique. The films showed perfect orthorhombic crystallization and were well-aligned with the substrates. The in-plane dielectric constant and loss of HMO films were measured as functions of temperature(80–300 K) and frequency(120 Hz–100 kHz) by using coplanar interdigital electrodes. Two thermally activated dielectric relaxations were found, and the respective peaks shifted to higher temperatures as the measuring frequency increased. The in-plane dielectric properties of epitaxial orthorhombic HMO films were considered as universal dielectric response behavior, and the dipolar effects and the hopping conductivity induced by the charge carriers were used to explain the results.     

Muon method for structural defects investigation in ferromagnetic metals

Equations describing depolarization of positive muons implanted in a pure crystalline ferromagnetic metal with structural defects are derived. In particular cases, solutions of these equations are obtained. In terms of the proposed model some experimental consequences are discussed.     

Perturbed angular correlations investigations on YMnO3 multiferroic manganite

The Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO₃ multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure.