RMnO3 Compounds Research Articles

Magnetic interaction induced splitting of bands in hexagonal RMnO3 (R = Lu, Y, and Sc) compounds

In this work, calculations using non-collinear spin density functional theory were performed to study the electronic band structures for different antiferromagnetic orders in hexagonal RMnO3 (R = Lu, Y, and Sc) compounds. These are an important class of multifunctional materials with interesting properties for various applications. By comparing the band structure of different magnetic configurations, it is observed that some of them exhibit band splitting that can be attributed to ferromagnetic interplane coupling. The energy splitting is significantly larger (331 meV) for the ScMnO3 compound, which probably has a higher interplane magnetic interaction. In addition, we report that the spin–orbit coupling also induces a splitting of bands in some regions of the Brillouin zone for those magnetic configurations with a weak magnetic moment along the hexagonal c-axis. These findings offer an intriguing insight into the topological characteristics of hexagonal manganites’ band structure, which was previously unexplored in existing literature. This discovery opens avenues for future investigations in this field.

Magnetic properties of SmMn1-xCoxO3 and Sm1-yRyMn0.6Co0.4O3

Polycrystalline SmMn1-xCoxO3 and Sm1-yRyMn0.6Co0.4O3, which have a single orthorhombic perovskite-type structure without any impurities, were prepared by complex polymerization to investigate the effects of A- and B-site element substitutions on the crystal structure and magnetic properties. The unit-cell volume of SmMn1-xCoxO3 decreased with increasing x and that of Sm1-yRyMn0.6Co0.4O3 increased with increasing average radius of A-site ions ⟨rR⟩, and the chemical pressure due to element substitution was observed in these two systems. At 0.0 ≤ x ≤ 0.4 in SmMn1-xCoxO3, a marked increase in the field-cooled magnetization MFC and an increase in the canted-antiferromagnetic transition temperature Tc were observed with increasing x, but they decreased for 0.5 ≤ x ≤ 1.0. However, no specific spin reversal phenomenon was observed, unlike in other RMnO3 compounds. The magnetic susceptibility of both compounds obeyed the Curie–Weiss law with constant susceptibility above 200 K, which was not common in other Sm compounds, and the effective magnetic moment Peff could be easily evaluated. By analyzing Peff of SmMn1-xCoxO3, we determined the electronic states of the ions constituting the sample as Mn3+(high spin, HS), Mn4+, and Co2+(HS) at 0 < x ≤ 0.5 and Mn4+, Co3+(low spin), and Co2+(HS) at 0.5 < x ≤ 0.9. The MFC of Sm1-yRyMn0.6Co0.4O3 decreased with increasing Y and Lu concentrations and increased with La concentration. The Tc and Weiss temperature of Sm1-yRyMn0.6Co0.4O3, both of which increased linearly with increasing ⟨rR⟩, could be attributed to the effects of chemical pressure. Although M − H measurements showed that Sm3+ in Sm1-yRyMn0.6Co0.4O3 was paramagnetic, the Peff analysis results suggested that the excited state of Sm3+ was affected.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d 5 − d 3 system can be significantly strengthened by reducing the virtual exchange gap (G ex) between occupied and empty e g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO3 (R = Sc, Y, and Lu) compounds with the T C that is as high as ∼1510 K (YMnO3). The oxygen breathing motions open a band gap and create an unusual Mn2+/Mn4+ charge ordering of the Mn-d electrons, which play an important role in altering the G ex. Interestingly, the rhombohedral RMnO3 compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO3. These results not only deepen the understandings of magnetic couplings in d 5 − d 3 system, but also provide a way to design room-temperature FM–FE multiferroics.

Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (−3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

The evolution of spin-glass behavior in hexagonal Lu1-xCaxMnO3 single crystals

In this research, we have successfully synthesized Lu1-xCaxMnO3 (x = 0.1, 0.3, and 0.5) single crystal samples with space group P63cm, and studied their magnetic properties with multiple methods. On the basis of AC susceptibility and magnetic relaxation rate, we can reveal the evolution of spin-glass (SG) behavior, which occurs in Ca doping with x = 0.1 and x = 0.3 and disappears at x = 0.5. Such SG behavior is induced by the competition between super-exchange and double-exchange interactions among Mn ions and the magnetic transition occurs at about 100 K. This study would be helpful to study the magnetism in RMnO3 compounds and the SG behavior in multi-valence doping.

First-principles study on structural, electronic, and ferroelectric properties of high-temperature RMn2O5 (R = Sm, Gd, Dy)

As a class of type-II multiferroics, RMn2O5 compounds show intriguing magnetoelectric coupling behaviors. However, the crystallographic characteristics of the high-temperature paramagnetic phase remain controversial. In present work, by employing PBEsol and SCAN exchange-correlation functionals, we have performed first-principles calculations to study the structural, electronic, and ferroelectric properties of the high-temperature RMn2O5 (R = Sm, Gd, Dy) compounds. According to our calculations, it is revealed that the high-temperature phase of RMn2O5 should be viewed as a super-paraelectric state due to the degeneracy of total energy for the centrosymmetric Pbam and polar Pm/P2 structures. Due to its unique structural and electronic properties, under certain external stimuli, the high-temperature RMn2O5 can develops the so-called electronic ferroelectricity through charge transition/transfer between Mn4+ and Mn3+ sites. In addition, the predicted ferroelectric polarization in the monoclinic Pm/P2 structures is no more than 1 μC/cm2. Our results may offer an alternative understanding for the complicated magnetoelectric behaviors occurred in the RMn2O5 family.

Characteristics of Coherent Optical Phonons in a Hexagonal YMnO3 Thin Film

This paper reviews our recent study on a coherent optical phonon in a hexagonal YMnO3 thin film together with related optical studies in hexagonal RMnO3 (R = Y, Lu, Ho) compounds. Coherent phonons have been observed in RMnO3 compounds by pump-probe spectroscopy with subpicosecond laser pulses, whereas the observation of coherent optical phonons was reported only in LuMnO3. Recently, we succeeded in the observation of the coherent optical phonon in a YMnO3 thin film. The generation process of the coherent optical phonon is assigned to a displacive mechanism, which is identical to that in LuMnO3. The coherent optical phonon is observed in the temperature range from 10 K to room temperature, while the oscillation intensity strongly decreases as the temperature increases to the Néel temperature of ~70 K from a lower temperature range. It is interesting that the temperature dependence is largely different from that in LuMnO3. We describe that the result can be qualitatively explained by the property of an isostructural transition around the Néel temperature in RMnO3 compounds. In addition, we briefly discuss ultrafast incoherent responses of excited electronic states from the viewpoint of the excitation photon energy of laser pulses.

Analysis of the Anisotropic Magnetocaloric Effect in RMn2O5 Single Crystals

Thanks to the strong magnetic anisotropy shown by the multiferroic RMn2O5 (R = magnetic rare earth) compounds, a large adiabatic temperature change can be induced (around 10 K) by rotating them in constant magnetic fields instead of the standard magnetization-demagnetization method. Particularly, the TbMn2O5 single crystal reveals a giant rotating magnetocaloric effect (RMCE) under relatively low constant magnetic fields reachable by permanent magnets. On the other hand, the nature of R3+ ions strongly affects their RMCEs. For example, the maximum rotating adiabatic temperature change exhibited by TbMn2O5 is more than five times larger than that presented by HoMn2O5 in a constant magnetic field of 2 T. In this paper, we mainly focus on the physics behind the RMCE shown by RMn2O5 multiferroics. We particularly demonstrate that the rare earth size could play a crucial role in determining the magnetic order, and accordingly, the rotating magnetocaloric properties of RMn2O5 compounds through the modulation of exchange interactions via lattice distortions. This is a scenario that seems to be supported by Raman scattering measurements.

Magnetic dynamics of phase separation domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn2O5 and Gd0.8Ce0.2Mn2O5 are compared with the properties of the previously investigated RMn2O5 (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn2O5 compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn2O5 is built.

Revealing the Formation Mechanism and Effect of Pressure on the Magnetic Order of Multiferroic BiMn2O5 Through Neutron Powder Diffraction

The crystal and magnetic structures of the strong magnetoelectric BiMn2O5 have been studied as a function of pressure up to 5.7 GPa in the temperature range from 10 K to 60 K by means of neutron powder diffraction. The results reveal that the Pbam orthorhombic crystal structure remains unchanged in the investigated thermodynamic range. At ambient pressure, a long-range commensurate antiferromagnetic (AFM) phase with propagation vector q = (1/2, 0, 1/2) formed below TN = 41(2) K, accompanied by anomalies in the temperature dependence of structural parameters including the lattice parameters, interatomic distances, and bond angles. This AFM phase remained stable␣in the studied pressure range, and the relevant pressure coefficient of the Neel temperature was determined to be 3.0(4) K/GPa. No incommensurate AFM phase was detected. The magnetic properties of BiMn2O5 and their difference from most other RMn2O5 compounds were analyzed in terms of competing exchange interactions.

Impact of temperature-dependent local and global spin order in RMnO3 compounds for spin–phonon coupling and electromagnon activity

The orthorhombic rare-earth manganite compounds RMnO3 show a global magnetic order for , and several representatives are multiferroic with a cycloidal spin ground state order for . We deduce from the temperature dependence of spin–phonon coupling in Raman spectroscopy for a series of RMnO3 compounds that their spin order locally persists up to about twice TN. Along the same line, our observation of the persistence of the electromagnon in GdMnO3 up to is attributed to a local cycloidal spin order for , in contrast to the hitherto assumed incommensurate sinusoidal phase in the intermediate temperature range. The development of the magnetization pattern can be described in terms of an order–disorder transition at Tcycl within a pseudospin model of localized spin cycloids with opposite chirality.

Pressure‐Induced Multiferroics via Pseudo Jahn–Teller Effects and Novel Couplings

Multiferroic materials are currently attracting a lot of interest because of the cross-coupling between their electrical and magnetic properties, which has the potential to lead to novel devices. There is a relatively small number of multiferroics and, hence, various strategies have been proposed to design materials possessing both ordered electric and magnetic dipoles. However, one strategy that has been scarcely investigated so far is the application of hydrostatic pressure because such external factor has the tendency to suppress, rather than enhance or create, electrical polarization. Here, first-principles calculations are used to demonstrate that hydrostatic pressure can, in fact, stabilize a novel polar and magnetic – and thus multiferroic – phase; the occurrence of such a phase seems very general, as it is found in many presently investigated RMnO3 compounds, where R is a rare-earth ion. The predicted structure features original pseudo Jahn–Teller effects that induce a spontaneous electrical polarization via a previously unknown structural coupling of a rather convenient form. Such a discovery opens a new route to create multiferroics and tunable electronic devices.

Thermoelectric and phononic properties of (Gd, Tb) MnO3 compounds: DFT calculations

We have investigated the structural, electronic, magnetic, phononic and thermoelectric properties of rare earth multiferroic manganite RMnO3 (R = Gd and Tb) compounds by the density functional theory within the spin polarized GGA, LDA and mBJ + U + SOC. The full-potential linearized augmented plane wave and pseudopotential methods are used. The combined mBJ + U + SOC study indicates that the obtained results for the band gap are in close agreement with the experimental results. The Bader analysis show that the ionic nature of the MnO bonds cause the large charge transfer from Mn to O atoms. Obtained band gaps are different for two spin channels therefore we have calculated the spin polarization dependence of transport properties. The directional thermoelectric study shows that these compounds are anisotropic. We have also presented the phononical properties such as Raman spectra and Born effective charges.

New RMnO3+δ (R=Y, Ho; δ≈0.35) phases with modulated structure

A series of hexagonal RMnO3 (R=Y, Ho–Lu) compounds are first annealed in various highly oxidizing conditions to show that for the largest R constituents, Y and Ho, it is possible to load the phases with large amounts of excess oxygen. Within the oxygen-annealed samples we identify new heavily-oxygenated phases, that is, YMnO3.35 and HoMnO3.34. The former is then thoroughly characterized by thermogravimetric analysis, X-ray diffraction and electron diffraction. These studies evidence a hexagonal sub-cell (aS≈3.578Å (aH/3), cS≈11.18Å, space group P63mc) and satellites, associated with two modulation vectors q→1⁎≈0.41a→S⁎+0.41b→S⁎+1/3c→S⁎andq→2⁎=(1/2). The results are interpreted by the formation of a modulated composite structure, resulting from the evolution of the hexagonal parent YMnO3+δ structure toward a reduced pyrochlore-type structure Y2Mn2O7−x, in long- and short-range ordering.

Anisotropy of optical properties of hexagonal RMnO3 Manganites (R = Ho, Er, Tm, Yb)

The evolution of real and imaginary parts of the complex permittivity of hexagonal single crystals of RMnO3 manganites (R = Ho, Er, Tm, Yb) differing in the radius of rare-earth ion r R has been studied using spectroscopic ellipsometry in the range of 0.5–5.0 eV at room temperature. Spectra of dielectric functions show a strong polarization dependence. The optical-absorption edge for polarization E ⊥ c is determined by the intense narrow peak at 1.6 eV, whereas for polarization E ‖ c, the peak is shifted toward high energies by 0.15–0.20 eV and its intensity is suppressed greatly. It has been shown that, when r R decreases, the energy position of the intense peak at 1.6 eV in the spectrum of the imaginary part of the dielectric function for polarization E ⊥ c shifts toward low energies by no more than 0.1 eV, which reflects changes in the local surroundings of the Mn3+ ion. For the both polarizations, a broad absorption band with the center at 2.4 eV has been revealed; the band was detected earlier in the antiferromagnetic phase in nonlinear spectra upon the optical generation of the second harmonic. Spectra of permittivity have been analyzed within available concepts on the electronic structure of hexagonal RMnO3 compounds and have been compared with corresponding spectra of previously studied orthorhombic RMnO3 compounds.

Neutron diffraction studies of nanoparticle RMnO3 compounds (R=Pr, Nd)

The neutron diffraction measurements of the nano-size RMnO3 (R=Pr, Nd) manganites have been performed. The obtained results indicate that these compounds crystallize in the orthorhombic crystal structure described by the space group Pnma. Low temperature data indicate both the absence of a long-range magnetic order down to 1.5K in the PrMnO3 sample annealed at 800°C and the CxFy magnetic structure with a very low value of the Mn magnetic moment as compared with bulk sample in the sample annealed at 900°C. In the NdMnO3 samples annealed at 850°C and 900°C the magnetic order is described by the CxFy mode in the Mn sublattice and the Fy mode in the Nd sublattice. The values of the Mn and Nd magnetic moments are lower that those in a bulk sample. Based on the model proposed in López-Quintela et al. [1] and Balcells et al. [2] the thickness of nonmagnetic shell was determined to be 7.1(4)nm in PrMnO3 and 4.6(5)nm in both the NdMnO3 compounds.

Magnetic structure and physical properties of the multiferroic compound PrMn2O5

RMn2O5 (R=lanthanide, Bi, Y) multiferroic compounds are intensively studied for their potential application in the spintronic field. In these systems, the key issue is to understand the origin of the strong coupling between the ferroelectric and magnetic orders and to investigate the influence of the nature of the R ions in this coupling. While the phase diagram of RMn2O5 compounds with small R size is well established, this of large R size compounds is missing due to the lack of samples originating with difficulties of synthesis. We present in this paper the first investigation of the thermodynamic, structural and magnetic properties of high quality polycrystalline PrMn2O5 samples. Our work shows that PrMn2O5 presents two magnetic transitions corresponding to commensurate magnetic orderings. We also evidence a weak lattice effect coupled to the magnetic order. Our results point out that the physical properties of PrMn2O5 differ from those of the parent compounds with magnetic R ions.

Theoretical investigation of NMR spectra in rare-earth manganites

The work is aimed to theoretical investigation of orbital and magnetic structure of manganites. The orbitally-dependent exchange interaction model was used in order to describe the superexchange interaction in all range of RMnO3 (R = La, Pr, Nd, Tb, Dy, Ho) orfhorhombic compounds. The model of nearest-neighbour and next-nearest neighbour exchange was used to describe the magnetic structures of manganites with small rare-earth ion sublattice (R = Dy, Tb, Ho). We investigate different types −A, E, non-collinear - of magnetic structures. The models of NMR spectra calculation of Mn3+ ion in RMnO3 compounds are proposed. The theoretical study of the magnetic structure could distinguish The NMR-spectra of antiferromagnetic (A or E) or strongly non-collinear ordering type.

Spin-phonon coupling in multiferroic manganites RMnO3: comparison of pure (R = Eu, Gd, Tb) and substituted (R = Eu1- x Y x ) compounds

For rare-earth manganite RMnO3 compounds spin-phonon coupling manifests itself as a phonon softening in the temperature range of the magnetically ordered phases. Within this class of materials, a continuous tuning of the lattice and thus also of the magnetic properties of multiferroic manganites is achieved by Y doping in substituted Eu1- x Y x MnO3. We compare the impact on spin-phonon coupling within this partial-substitution approach in a series of Eu1- x Y x MnO3 samples 0 ≤ x ≤ 0.5) with the effect of a complete exchange of the rare earth ions R3+ in a series of pure RMnO3 compounds (R = Eu, Gd, Tb). For this purpose we employ polarized Raman scattering in the 10–300 K temperature range. The low-temperature results show phonon softening in all investigated compounds. For decreasing R3+ radius, i.e. an increasing orthorhombic distortion and magnetic frustration, we observe in both systems a weakening of the spin-phonon coupling. For known sublattice magnetization within the MnO2-plane, quantitative results for the spin-phonon coupling constant are derived for both cases within a molecular field approximation. Our results show, that the spin-phonon coupling strength in the magnetically ordered phases of the various investigated manganites does not correlate with the magnetization pattern. Instead, the pure RMnO3 compounds and the substituted Eu1- x Y x MnO3 fit excellently within a common scheme, in which the weakening of the spin-phonon coupling reflects the degree of tilting of the MnO6 octahedra due to the orthorhombic distortion of the crystal lattice.

Magnetic excitations in multiferroicLuMnO3studied by inelastic neutron scattering

We present data on the magnetic and magneto-elastic coupling in the hexagonal multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization and thermal expansion measurements. We measured the magnon dispersion along the main symmetry directions and used this data to determine the principal exchange parameters from a spin-wave model. An analysis of the magnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare the results for LuMnO3 with data on other hexagonal RMnO3 compounds.