Abstract
Due to the complicated magnetic and crystallographic structures of BiFeO3, its magnetoelectric (ME) couplings and microscopic model Hamiltonian remain poorly understood. By employing a first-principles approach, we uncover all possible ME couplings associated with the spin-current (SC) and exchange-striction (ES) polarizations, and construct an appropriate Hamiltonian for the long-range spin-cycloid in BiFeO3. First-principles calculations are used to understand the microscopic origins of the ME couplings. We find that inversion symmetries broken by ferroelectric and antiferroelectric distortions induce the SC and the ES polarizations, which cooperatively produce the dynamic ME effects in BiFeO3. A model motivated by first principles reproduces the absorption difference of counter-propagating light beams called non-reciprocal directional dichroism. The current paper focuses on the spin-driven (SD) polarizations produced by a dynamic electric field, i.e. the dynamic ME couplings. Due to the inertial properties of Fe, the dynamic SD polarizations differ significantly from the static SD polarizations. Our systematic approach can be generally applied to any multiferroic material, laying the foundation for revealing hidden ME couplings on the atomic scale and for exploiting optical ME effects in the next generation of technological devices such as optical diodes.
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The exceptional characteristics exhibited by BiFeO3 include its high ferroelectric ( 1100 K [1]) and magnetic ( 640 K [2]) transition temperatures, both well above room temperature, and its large ferroelectric (FE) polarization (∼90 μC cm−2 [3]) below . Below the magnetic ordering temperature , antiferromagnetic order develops with a long-wavelength ( [2]) cycloid. Surprisingly, the same characteristics that make BiFeO3 so extraordinary have also hampered our understanding of the magnetoelectric (ME) effects driven by spin ordering below . Despite strenuous effort [2, 4–9] and the strong ME effects recently revealed by neutron-scattering [10] and Raman-spectroscopy [11] measurements, little is known about the microscopic origins of the spin-driven (SD) polarizations and ME couplings in bulk rhombohedral BiFeO3 (space group R3c).
Due to the lack of spatial inversion and time reversal symmetries in multiferroics, the coupling between spins and local electric dipoles creates strong ME effects [12]. Mostly studied in the static limit, ME effects are resonantly enhanced at the so-called ME spin-wave excitations or electromagnons characterized by the coupled dynamics of spins and local electric dipoles [12]. The different absorption of counter-propagating light beams called non-reciprocal directional dichroism (NDD) has proven to be a powerful tool to investigate intrinsic ME couplings in several multiferroics [13–17]. Dynamical studies are especially suited to leaky ferroelectrics where static magneto-capacitance measurements are not feasible and to type-I multiferroics like BiFeO3 where static magneto-capacitance measurements are often hindered by the large preexisting FE polarization.
BiFeO3 has two distinctive structural distortions that remove inversion centers and couple to the electric component of light. One is the FE distortion [111] that breaks global inversion symmetry (IS). The other is the antiferroelectric (AF) octahedral rotation [111] that breaks the local IS between nearest neighbor spins. Using a first-principles approach tied to a microscopic Hamiltonian, we demonstrate that all ME couplings are microscopically driven by distinct combinations of these two inherent structural distortions.
The first-principles approach described in this paper has already laid the foundation for two previous studies of BiFeO3. This approach was used [18, 19] to predict the dynamic NDD observed in BiFeO3 even at room temperature. As discussed in section 3, four spin-current (SC) polarizations associated with FE and AF distortions cooperatively induce the strong NDD in BiFeO3. This approach was also used [20] to predict that the static SD polarization −3 μC cm−2 in BiFeO3 points opposite to the preexisting FE polarization. A record high among all known multiferroics, this SD polarization is produced by the ES contribution discussed in section 4.
1. Microscopic spin-cycloid model for BiFeO3
The FE polarization emerging below can take eight different orientations along the four cubic diagonals . For a given , the three possible orientations for the cycloidal modulation wavevectors are , , and with corresponding . In magnetic domain m, the cycloidal ordering wavevectors are
where is the wavevector for simple G-type antiferromagnet and is the pseudo-cubic lattice constant. Hence, the ordering wavevectors are , , and . In terms of , the cycloidal period is .
FE and AF distortions create the DM interactions and . Including all magnetic anisotropies produced by these distortions, the spin Hamiltonian can be written
where and represent nearest and next-nearest neighbor spins, respectively. This is the most general Hamiltonian that includes the allowed distortions in R3c BiFeO3 but neglecting exchange anisotropy terms of the form
which are usually small for transition metal ions with half-filled d-shell such as Fe3+. Moreover, due to the long wavelength of the cycloid, exchange anisotropy can be effectively absorbed into the single-ion anisotropy (SIA) , which favors spin alignment along . All terms in this Hamitlonian are also essential to explain the spin modes of BiFeO3 observed using THz spectroscopy [21, 22].
Since the FE distortion is uniform, the sum is translation invariant. Due to the translation-odd [111] AF octahedral rotation, the sum contains the coefficient , which alternates from one hexagonal layer to the next. Simplified forms for the DM terms and are given in appendix
By ignoring the higher cycloidal harmonics but including the tilt [23] τ produced by , the classicsl spin state can be approximated [24] as
so that the spins on each hexagonal layer depend only on the integer . The weak FM moment of the canted antiferromagnetic phase above Hc is related to the tilt by [21] . For [5, 25] , or . By comparison, the local spin-density approximation (LSDA)+U (U = 5 eV) calculations described in the next section yield . Because higher harmonics are neglected, averages taken with the tilted cycloid in zero magnetic field introduce a very small error of order 10−5. Quantum fluctuations about the classical spin state are expected to be small for Fe3+ ions.
2. First-principles method
First-principles calculations were performed using density functional theory (DFT) from the VASP code within LSDA+U. The Hubbard U = 5 eV and the exchange parameters were optimized for Fe3+ in BiFeO3 [26, 27]. We employed projector augmented wave potentials [28, 29]. To integrate over the Brillouin zone, we constructed a supercell made of 2 × 2 × 2 perovskite units (40 atoms, 8 f.u.) and a 3 × 3 × 3 Monkhorst–Pack k-points mesh. The DM interactions and were evaluated with 4 × 2 × 2 units (80 atoms, 16 f.u.) and a 1 × 3 × 3 Monkhorst–Pack mesh. The wave functions were expanded with plane waves up to an energy cutoff of 500 eV. To calculate exchange interactions (Jn), we applied four different magnetic configurations (G-AFM, C-AFM, A-AFM and FM). We estimated and by replacing all except four of the Fe3+ cations with Al3+ [26] in the 80 atom unit cell. As shown in table 1, the LSDA+U results are in excellent agreement with recent neutron-scattering measurements [22].
Table 1.
Calculated magnetic interaction parameters (meV) compared to spin model results fitted to neutron-scattering measurements [22]. splits into two components parallel (A = 0.042) and perpendicular (B = 0.075) to spin bond direction. The components A and B are explained in appendix
meV | J1 | K | ||
---|---|---|---|---|
LSDA+U | −6.1 | 0.089 | 0.042, 0.075 | 3.5 × 10−3 |
Neutron | −5.3 | 0.103 | 0.064 | 4.1 × 10−3 |
After obtaining the exchange, DM, and SIA interactions, we calculate their derivatives with respect to an applied electric field parallel to a cartesian direction. A dielectric constant is used to estimate the SD polarizations when the electric field is perpendicular to the rhombohedral axis [30]. To simulate atomic displacements driven by the applied field Eα, we evaluate the lowest-frequency polar eigenvector from the dynamical matrix by forcibly moving the atoms incrementally from the ground state R3c structure. The resulting energy difference between the two structures is divided by the induced electric polarization . The major difference in the polar eigenvectors obtained from the dynamic and the force-constant matrices arises from the Fe–O–Fe bond angle. The eigenvectors of the dynamic matrix reduce the bond-angle while the eigenvectors of the force-constant matrix raises that angle (see appendix
3. SC polarizations
The cross products modulate the Fe–O–Fe bond angle and produce the SC polarizations [31]. These SC polarizations are simply obtained from electric-field derivatives of Dzyaloshinskii–Moriya interaction Hamiltonian. In BiFeO3, FE and AF distortions generate SC polarizations and associated with the electric-field derivatives of the DM interactions and . These are calculated using the procedure described in [20].
The first SC polarization is induced by the response of the FE distortion to an external electric field:
where is a sum over nearest neighbors with and , , or cubic axis. The electric-field derivatives of the DM interactions are given in appendix
Table 2.
SD polarizations from ES, SC and SIA. Shown are the calculated (LSDA+U) electric-field derivatives of J1, , and K. The upper left and right scripts denote the directions of the spin bond and electric field, respectively. , , and by R3c symmetry as in appendix
SC polarization from | SC polarization from | ES polarization from J1 | |||||
---|---|---|---|---|---|---|---|
+ 2 | |||||||
LSDA+U | 9 | 17 | 14 | 17 | −19 | −250 | −350 |
NDD | 36 | 29 | 29 | 28 | −7.2 | — | — |
In the lab reference frame , regrouping terms for domain 2 with using equations(7)–(9) yields
where
with , , and given in table 2. Fits to the NDD [19] described in section 6 imply that g = h.
The second SC polarization alternates in sign due to the alternating AF rotations along [111]:
The SC polarization components are evaluated in table 2. While the derivative of between spins and with parallel to the electric field is nearly anti-parallel to , the derivative () of between spins with perpendicular to the electric field is perpendicular to , as shown in figure 1.
Appendix
where
with and given in table 2.
4. ES polarizations
The absence of an inversion center between neighboring spin sites induces the ES bond polarizations. Since the scalar product is altered by external perturbations such as temperature, electric field, or magnetic field, FE and AF distortions each generates its own ES polarization.
For symmetric exchange couplings, ES is dominated by the response of the nearest-neighbor interaction J1:
The two ES polarizations and are closely related to one another. The electric-field derivatives are given in the cubic coordinate system by
where () and for spin bonds perpendicular and parallel to the electric field, respectively.
Because the AF octahedral rotation is perpendicular to , the ES polarization associated with AF rotations is also perpendicular to with
Unlike W1u, W2u alternates in sign due to the opposite AF rotations on adjacent hexagonal layers.
The first ES polarization parallel to with coefficient modulates the FE polarization that already breaks IS above . The second ES polarization perpendicular to has coefficient . The AF rotations affect the bonds between nearest-neighbor spins in the plane normal to because each oxygen moves along the directions , , and , perpendicular to . Thus, the second ES polarization is associated with atomic displacements perpendicular to and parallel to the AF rotation.
Figure 2 demonstrates the strong anisotropy in the response of magnetic exchange to an electric field. While arises from the change in Fe–O–Fe bond angle due to a polar distortion, arises from bond contraction. As shown, is much more sensitive to an electric field than . Since the ME anisotropy produces an ES polarization, the AF rotation angle is changed by the spin ordering. In particular, the negative sign of nC cm−2 indicates that the rotation angle increases with the dot product because oxygen atoms moving in the AF plane have a negative effective charge .
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Standard image High-resolution imageThe anisotropic ES polarization components and cooperatively induce the ES polarization along under the IS broken by the FE polarization. In contrast to our previous study [20] on the response to a static electric field ( nC cm−2), we obtain a negative nC cm−2 in a dynamic electric field. Appendix
We recently predicted [20] that the static SD polarization of BiFeO3 is about −3 μC cm−2 along a cubic diagonal opposite to the FE polarization emerging below . The electronic plus atomic contribution to the SD polarization is −1.3 μC cm−2 and the lattice-deformation contribution is −1.7 μC cm−2, which were slightly underestimated (−1.0 and −1.3, respectively) in previous literature [33, 34]. The total SD polarization (−3 μC cm−2) is higher than observed in any other known multiferroic material [20].
5. Origin of NDD
The most stringent test yet for the microscopic model proposed above is its ability to predict the NDD which is the asymmetry in the absorption of light when the direction of light propagation is reversed. The absorption of THz light is given by , where [35, 36]
is the complex refractive index for a linearly polarized beam, , and are the dielectric, magnetic, and magnetoelectric susceptibility tensors describing the dynamical response of the spin system [13, 15, 17, 35] and is the dielectric constant related to the charge response. Subscripts i and j are fixed by the electric and magnetic polarization directions, respectively. The second term, which depends on the light propagation direction and produces NDD, is separated from the mean absorption by writing .
Summing over the spin-wave modes n at the cycloidal ordering wavevector , is given by
where is the magnetization, is the volume per Fe site, is the net SD polarization given in units of, nC cm−2, and .
For each field orientation, the integrated weight of every spectroscopic peak at is compared with the measured values, thereby eliminating estimates of the individual peak widths. Experimental results for the NDD with field along are plotted in figure 3(a) for . Fits to the NDD are based on the plotted 2, 4, 6, 8, 10, and 12 T data sets. For each data set, we evaluate the integrated weights for the eight modes [22] , , , , and between roughly 12 and 35 cm−1.
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Standard image High-resolution imageComparing figures 3(a) and (b), the NDD for is dominated by the two sets of SC polarizations and . Table 2 indicates that the fitting results are not significantly changed by including the ES polarizations. Figure 3(c) attempts to fit the experimental data using the ES polarizations alone. Clearly, the ES polarizations by themselves cannot produce the observed NDD.
Figures 3(a) and (b) indicate that the various components of the SC polarizations in BiFeO3 are captured by first-principles calculations and that the NDD is not strongly affected by the ES terms This selectivity originates from the spin dynamics of this nearly collinear antiferromagnet. Due to the very small SIA on the S = 5/2 Fe3+ spins, each magnon mode can be described as the pure precession of Fe3+ spins: the oscillating component of the spin on site i is perpendicular to its equilibrium direction . Since neighboring spins in the long-wavelength spin cycloid of BiFeO3 are close to collinear, a dynamic polarization is effectively induced by SC terms such as . However, the dynamic polarization generated by ES terms is almost zero. The spin stretching modes observed in strongly anisotropic magnets [35, 38] do not appear in BiFeO3.
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Standard image High-resolution imageRecent work [37] explains the observed static polarization perpendicular to by the term proportional to h − f. Although the fitting and LSDA+U values for nC cm−2 in table 2 are an order of magnitude smaller than required by that work, keep in mind that the SC parameters given in table 2 were evaluated or fitted for a dynamic electric field.
Although DFT calculations underestimate the ME coefficients compared to the NDD fitting results in table 2, they nicely demonstrate which of the symmetry-allowed ME couplings are relevant and which are negligibly small. Combining the two methodologies allows a more unambiguous determination of these coupling parameters. There are several possible explanations for the difference between the results obtained from DFT calculations and the NDD fitting. First, a larger dielectric constant could produce better agreement between DFT and NDD since the SD polarizations are proportional to through
Second, higher-frequency polar modes not considered here also can affect NDD. Third, a smaller Hubbard U will increase the SD polarizations and improve the agreement with the experimental fits. Fourth, magnon modes were observed between and 40 cm−1 while we calculated the SC coupling constants in the dynamical limit. The crossover frequency between static and dynamical behavior lies between 0 and the polar phonon at cm−1. If lies in the middle of the measured frequencies, then the SC fitting parameters may differ from the dynamical couplings obtained from LSDA +U.
6. Discussion
In order to study the ME couplings in complex multiferroic systems, first-principles calculations must be anchored to the right microscopic Hamiltonian. With two sets of SC polarizations derived from the two distinct structural distortions, BiFeO3 is a good example of how our atomistic approach works for complex materials. This paper calculated only the ionic displacement contribution to the ME coupling which is typically larger than the purely electronic contribution [34, 39]. The lattice deformation contribution to the SD polarization was discussed in our previous work [20].
The higher-frequency polar modes contribute to the electric-field induced displacement. Their contributions are proportional to the mode strength , where Z is the mode effective charge and ω its frequency. From our dynamical matrix calculations, the mode strengths of the higher frequency modes are less than 30% smaller than the strength of the lowest mode. Therefore, the lowest mode makes dominates the electric-field induced polar displacement.
The advantages (large FE polarization, high , and high ) of BiFeO3 are also major obstacles to understanding the ME couplings that produce the SD polarizations below . Leakage currents and the preexisting large FE polarization at high temperatures have hampered magneto-capacitance measurements and hidden the SD polarizations. Although recent neutron-scattering measurements [10] imply a large ES polarization, most other ME polarizations are unknown. We show that NDD measurements combined with first-principles calculations based on a microscopic model reveal the hidden SC polarizations. In particular, this approach allows us to disentangle the delicate SC polarizations and the hidden ES polarizations associated with AF rotations. We envision that intrinsic methods such as NDD will reveal hidden ME couplings in many materials and rekindle the investigation of type-I multiferroics.
Acknowledgments
We acknowledge discussions with H Kim, E Bousquet, Nobuo Furukawa, S Miyahara, J Musfeldt, U Nagel, S Okamoto, S Bordács and T Rõõm. Research sponsored by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. IK was supported by the Hungarian Research Fund OTKA K 108918. JHL was in part supported by the year of 2015 Research Fund (1.150132.01) of the UNIST (Ulsan National Institute of Science and Technology). We also thank Hee Taek Yi and Sang-Wook Cheong for preparation of the BiFeO3 sample.
Appendix A.: Simplified form for the Dzyaloshinskii–Moriya interactions
A.1. FE-induced Dzyaloshinskii–Moriya interaction
Since the FE vectors are given by (0, , ), (, , 0), and (, , 0) between nearest spins along , , and , respectively, the FE-induced DM interaction can be transformed as:
where meV is now larger by than in previous work [21].
A.2. AF-induced Dzyaloshinskii–Moriya interaction
The AF interactions along , , and can be written
For domain 2 with ,
where the primed sum over is restricted to either ni odd or even hexagonal layers. Based on the tilted cycloid of equations (7)–(9) the term dominates because is of order .
Previously, the second DM term was written
which also uses equations (7)–(9). Therefore, meV, which is in excellent agreement with previous work [21].
Appendix B.: Eigenvectors of dynamic and force-constant matrices
As noted in section 4, is negative from the eigenmode of the dynamic matrix while it is positive from the eigenmode of the force-constant matrix [20]. This difference originates from the opposite changes to the Fe–O–Fe bond angle: the bond angle increases in the static limit while it decreases in the dynamic limit.
Appendix C.: Spin-current polarization components
Defining ,
where , and .
Defining ,
where , and .
Appendix D.: SC polarization from antiferrodistortive Dzyaloshinskii–Moriya coupling
For domain 2 with
which use
The SD polarization associated with is
Similarly
So in the local frame
plus a correction of order .
The polarization matrix used to evaluate the NDD is given by
where nC cm−2 and nC cm−2 are obtained from first principles as given in table 2. ( nC cm−2, nC cm−2, nC cm−2, nC cm−2, and nC cm−2).
Footnotes
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