Session V39: Magnetism in Rare Earth Materials

Controlling magnetism and magneto-structural phase transformation in rare earth containing materials

In rare earth element based materials the alloyed p-block elements provide chemical formation and stability, and control magnetism exhibited by rare earths. We report here two examples of how non-magnetic elements substitution pinpointed enhanced and controlled magnetism and phase transformation. First example is Eu₂In, in which the Eu 5d states strongly hybridize with the In 5p states exhibiting large exchange splitting in ferromagnetic configuration. This unexpectedly high exchange splitting is the root cause of the sharp paramagnetic to ferromagnetic transition [1]. The other example is scandium substituted Gd₅Ge₄. Total energy calculations show that Sc favors the Gd atoms located inside the slabs and predict a stable ferromagnetic ground state with the 20 atomic % Sc substituted Gd₅Ge₄. The predicted phase is Pu₅Rh₄-type which is intermediate between the Sm₅Ge₄-type and Gd₅Si₄-type. The delocalized 3d electronic states of Sc strongly hybridize with delocalized 5d electronic states of Gd, thus promoting indirect interslab 4f-4f exchange interactions [2].

[1] Nature Comm. 9, 2925 (2018)
[2] Chem. Mater. 29, 3962 (2017)   

Spin dynamics in magnetic reorientation transitions

The Nd-Fe-B magnet is known as a permanent magnet with strong coercive field and commercially used for motors and generators, etc. It shows a magnetic reorientation transition at around 150 K (magnetic Tc is 600~700 K) [1]. The origin of the reorientation transition is higher order magnetic anisotropy constants, which causes a tilted spin state at low temperatures. We study the temperature dependence of ferromagnetic resonance frequency of an atomistic spin model of the magnet. We estimate the power spectrum by using the stochastic Landau-Lifshitz-Gilbert equation [2]. We find non-monotonic temperature dependence of the peak position of the frequency. We analyze the cause of this phenomenon and show that this non-monotonicity is universal for this type of magnetic reorientation transitions.

[1] M. Nishino et al., Phys. Rev. B 95, 094429 (2017).
[2] M. Nishino and S. Miyashita, Phys. Rev. B 91, 134411 (2015).   

Magnetic Neutron Scattering Study of Rare-Earth Titanates

The Mott-insulating rare-earth (R) titanates (RTiO₃) are known to host ferromagnetic-antiferromagnetic transitions in solid-solution systems such as Y_1-xLa_xTiO₃, as well as metal-insulator transitions in charge carrier-doped systems such as Y_1-xCa_xTiO₃. Neutron scattering studies have been largely restricted to the undoped YTiO₃ and LaTiO₃. Here, we report on our neutron scattering work on the magnetic order and excitations in the doped systems Y_1-xLa_xTiO₃ and Y_1-xCa_xTiO₃, using triple-axis spectrometry and small-angle neutron scattering (SANS). We find a strong reduction in the transition temperature and ordered magnetic moment, but accompanied by very little change in the spin-wave dispersion in the Y_1-xLa_xTiO₃ system as we approach the critical doping x~0.3. SANS results suggest a rotation of the easy axis with substitution, development of magnetic inhomogeneities on ~10 nm scales, and distinct changes in critical scattering as the magnetic crossover is approached. These results suggest significant changes in the magnetic ground state as one approaches the crossover. Initial studies on the Y_1-xCa_xTiO₃ system will also be reported.   

Phase diagram of emergent orders of rare-earth nickelates

Phase diagram of Rare-earth nickelates, thin films and heterostructures, is a paradigmatic problem of strongly correlated material having strong coupling in between spin, orbital, lattice degrees of freedom. [1]
Recently, there have been suggestions on possible new emergent orders includes antiferromagnetism. [2]
In this talk, based on low energy hamiltonian of eg
manifold,[3] we discussed possible emergent orders of Rare-earth nickelates includes superconductivity, charge disproportionation, and magnetism. These emergent orders would be accessible by tuning material parameters such as interaction strength and effective crystal field.

[1] S. Catalani et al., Rep. Prog. Phys. 81 <a href="tel:<a href="tel:(2018) 046501">(2018) 046501</a>">(2018) 046501</a>
[2] H. Gui et al., Nature Communications 9 (2018)
[3] A. Subedi et al., Phys. Rev. B 91, 075128 (2015)   

Energetics of the coupled electronic-structural transition in the rare-earth nickelates

Rare-earth nickelates exhibit a metal-insulator transition (MIT) accompanied by a structural distortion that breaks the symmetry between formerly equivalent Ni sites. The quantitative theoretical description of this coupled electronic-structural instability is extremely challenging. Here, we address this issue by simultaneously taking into account both structural and electronic degrees of freedom using a charge self-consistent combination of density functional theory and dynamical mean-field theory, together with screened interaction parameters obtained from the constrained random phase approximation. Our total energy calculations show that the coupling to an electronic instability towards a charge disproportionated insulating state is crucial to stabilize the structural distortion, leading to a clear first order character of the coupled transition. Decreasing octahedral rotations across the series suppress this electronic instability weakening the correlation effects responsible for the MIT. Our approach allows to obtain accurate values for the structural distortion and thus facilitates a comprehensive understanding, of the complex interplay between structural properties and electronic correlation effects across the nickelate series.   

Interplay Between Magnetic Order and Lattice Distortions in Rare-Earth Nickelates

We study the magnetic order appearing in the rare-earth nickelates at low temperatures, the exact nature of which is still debated vigorously in the literature. While there is an established link between the metal-insulator transition and lattice distortions in the nickelates, to our knowledge there has been little work on the impact of lattice distortions on the magnetic order, e.g. whether they favor some orders over others. To address this, we consider a two-band Hubbard model for the nickelates, coupled to the lattice distortions (which are treated semiclassically), and use the Hartree-Fock approximation to find the resulting phase diagram. Aside from quantitatively demonstrating how lattice distortions drive bond disproportionation, we find that a variety of 4-site magnetic orders are self-consistent within the model, including both collinear and non-collinear orders. However, in our model a magnetic order can only couple to the lattice distortions if there is an asymmetry in average charge occupation of neighbouring sites. As a result, we find that coupling to the lattice distortions broadly favors the collinear ↑-o-↓-o order in large sectors of the parameter space.   

Cooperative exchange coupling of rare-earth spins with a vacuum magnon field in ErFeO3

Dicke cooperativity is a many-body effect in quantum optics describing the process of N oscillators in a photonic cavity developing macroscopic coherence amongst themselves by interacting with a single light field, while exhibiting a N^1/2 enhancement of the light-matter coupling rate. Here, we have discovered that cooperatively enhanced coupling similar to this canonical light-matter interaction effect occurs in a magnetic solid in the form of matter-matter interaction. We studied an ErFeO₃ single crystal with terahertz magnetospectroscopy at low temperatures and high magnetic fields, and discovered that the exchange coupling of the paramagnetic Er³⁺ ions with a Fe³⁺ magnon field within the crystal quantitatively follows the scaling behavior expected by the Dicke cooperativity effect. This scaling behavior also enables quantitative determination of the Er³⁺-Fe³⁺ exchange coupling coefficients. Our finding provides a novel route for understanding, controlling, or predicting new phases of condensed matter using concepts and tools available in quantum optics.
  

Structural and magnetic properties of Sc-doped rare earth ferrites

We have studied crystal structure and magnetic properties of Sc-doped rare earth ferrites R1-xScxFeO3 (R=Ho, Er, Yb). For R=Ho, pure structural phases, orthorhombic, garnet, and bixbyte, were observed for x=0, 0.5, and 1 respectively. Pure garnet phase was also found for x=0.5 and R=Er. In contrast, when x=0.5 and R=Yb, mixed phases dominated with hexagonal and garnet were observed. Magnetic properties of Ho0.5Sc0.5FeO3 show incomplete magnetic compensation, with a magnetization minimum at a temperature lower than the compensation temperature of the undoped Ho-Fe-O garnet. These results indicate that Sc atoms occupy both Fe and rare earth sites due to the intermediate atomic radius.   

Optimizing the intrinsic magnetic properties of REFe12-xMxphases

Collecting energy from renewable resources, using e.g. wind power plants has become important and goes hand in hand with a growing demand for new permanent magnets (PM) with magnetic performance comparable to Nd-Fe-B magnets. A key task is to identify new PM with less rare earth (RE). Systems with ThMn₁₂ structure are of special interest. They have the lowest RE concentration of all known phases. However, Fe-based 1:12 systems with light RE need a phase stabilizer which goes on the Fe sites.
Two types have been studied, (Nd_1-xY_x)Fe_12-yTi_y and SmFe_12-yV_y, using state of the art density functional theory methods (VASP; FP LMTO RSPt). The phase stability and the magnetic properties were calculated depending on the Ti and V concentration. Aiming to reduce the RE amount the performance depending on the Nd/Y ratio was monitored.
We find that the Nd-Y (Sm) system is stable down to y = 0.5 (y=1) which leads to an increase of the magnetization by 17% compared to the commonly used concentrations of Ti (V). In viev of the MAE a replacement of Nd by Y turned out to be preferable over a reduction of Ti. MAE values of 1.3 MJ/m³ ((NdY)Fe₁₁Ti) and 1.7 MJ/m³ (SmFe₁₁V) are predicted. The latter could be verified in recent experiments.   

Nanoscale characterization of magnetic microstructure in Gd3Fe5O12

A key barrier to the understanding of magnetic phenomena and design of functional magnetic materials is the limited scope of nanoscale characterization of magnetic microstructure. We utilize scanning x-ray diffraction microscopy to image the magnetic domain morphology in rare-earth iron garnets as a function of temperature using an x-ray wave plate and zone plate focusing optics to generate a 100 nm beam of circularly polarized synchrotron light. Rare earth iron garnets such as Gd₃Fe₅O₁₂ are magnetic systems that can exhibit spin Seebeck effect (SSE) voltage generation with an efficiency that could depend on the morphology of the magnetic domain structures. We have obtained linear and circular x-ray magnetic dichroism maps of the micron- to submicron-scale distributions of magnetic domains recorded in a prototype SSE device composed of epitaxial layers of Gd₃Fe₅O₁₂ capped with 10 nm of Pt for temperatures ranging from 4 K to 250 K. These results bring insight into the relationship between the temperature-dependent magnetic domain morphology and the efficiency of SSE devices based on rare-earth iron garnets.
  

Large magneto-crystalline anisotropy in strain-tuned epitaxial BiYIG thick films

Bi-doped Yttrium Iron Garnet (Bi:YIG) with large Magneto-optical (MO) activity and low optical losses still needs to get probed for its magnetization dynamics. The fact that the ablation laser fluence strongly affects the lattice constant of the films, was used to grow two sets of BiYIG/GGG(111) samples with different strain states. The set-A films with lower thicknesses (10.2, 37.0, and, 92.5 nm), grown using ∼ 1 J/cm² laser fluence, show large magnetocrystalline anisotropy (MCA) due to larger rhombohedral distortion induced strain. The set-B films with larger thicknesses (120 and 150 nm) grown using higher laser fluence of ∼ 2 J/cm² show large strain irrespective of film thicknesses and large magnitude of MCA, comparable to the value of a film as-thin-as ∼ 37 nm from Set-A. Interestingly, the substitution of strong spin orbit coupling Bi³⁺ ions enhances the magnetoelastic coupling (MEC) but weakly affects the precessional damping (∼ 9.7×10^-4). Coexistence of two mutually exclusive parameters i.e., low precessional damping and strong MEC, combined with large MO activity and large MCA in BiYIG films may provide a possible material platform for light dependent magnonics and other spintronics applications.   

Magnetic and Magnetocaloric Properties of ( Nd1-xCex)YFe15 Alloys

Magnetocaloric materials exhibiting a second-order phase transition (SOPT) near room temperature have attracted much attention because these materials are free from magnetic and thermal hysteresis, and may show large cooling efficiency and moderate magnetic entropy change. We have investigated (Nd_1-xCe_x)YFe₁₅ (x = 0 - 1) alloys prepared by arc melting and vacuum annealing. The x-ray diffraction patterns analyzed using the Rietveld method shows that the samples crystallized in the rhombohedral Th₂Zn₁₇-type structure with space group R-3m. The thermomagnetic curves M(T) measured at μ₀H = 1T show smooth SOPT with Curie temperatures near 300 K. The values of peak magnetic entropy change (ΔS_M,max) and relative cooling power (RCP) measured at 3 T increase with increasing Nd concentration reaching 4.20 Jkg^-1K^-1 and 285 Jkg^-1 for (Nd_0.7Ce_0.3)YFe₁₅. The measured ΔS_M and RCP values are relatively high as compared to those of other Gd-free compounds showing magnetocaloric effect due to SOFT near room temperature. This suggests that the (Nd_1-xCe_x)YFe₁₅ alloys have potential for room temperature magnetic refrigeration.