Research articlesEvolution of spatial spin-modulated structure with La doping in Bi1-yLayFeO3 multiferroics
Introduction
Promising prospects of Ι type multiferroic BiFeO3 (BFO) application in spintronics, data storage and various multifunctional devices generate keen interest in BFO-based compounds over the past 30 years [1], [2], [3], [4]. Pure undoped BiFeO3 possesses rhombohedral distorted perovskite structure (sp. gr. R3c), high Curie temperature of 1100 K and Neel temperature of 670 K [5], [6]. In paper [7] the authors observed a complicated cycloid type spatial spin-modulated structure (SSMS) with relatively big period λ = 620 ± 20 Å incommensurate with the crystal lattice period by means of time-of-flight neutron diffractometry. Each magnetic moment of the trivalent iron ion Fe3+, surrounded by neighbouring Fe3+ ions with spins antiparallel to the spin of the central atom, rotates along the direction of the modulated wave in a plane perpendicular to the hexagonal basal plane. The presence of SSMS reduces to zero the total magnetization and linear magnetoelectric effect [8]. One can conclude that for practical applications of these ferrites SSMS must be suppressed. Various methods for suppressing SSMS are described in the literature, for example, replacing bismuth or iron with other elements [8], [9], [10], synthesizing samples in the form of nanocrystallites or thin films [2], [11], [12], [13], and also using an external magnetic field [14], [15].
The replacement of bismuth atoms by rare-earth atoms affects the physical properties (including magnetoelectric). This effect is due to the difference between ionic radii, valency and other parameters of Bi3+ ions and substitute ions [16]. It was shown in [17] that doping of BFO with lanthanum leads to a structural transition from the rhombohedral to the orthorhombic phase due to chemical compression. Evidence of a decrease in the concentration of charge defects, dielectric losses, and leakage current upon doping with La is given in [18], [19]. It was also found that certain concentrations of various elements replacing bismuth lead to the destruction of SSMS in compounds based on BFO [11], [20], [21].
In [22], a mathematical description of SSMS in BFO is given through the function of the anharmonic cycloid:where θ is the angle of rotation of the antiferromagnetism vector relative to the c axis; x is the coordinate along the direction of propagation of the cycloid; sn(x, m) is the Jacobi elliptic function with the anharmonicity parameter m; K(m) is a complete elliptic integral of the first kind. The SSMS period λ is determined by the exchange stiffness A, the effective constant of the uniaxial magnetic anisotropy Ku, and the anharmonicity parameter m [22], [23].
As follows from Eqs. (1), (2), the SSMS anharmonicity leads to a non-uniform distribution of the magnetic moments of iron over the angle θ, leading to their higher concentration near the c axis or in the perpendicular direction, depending on the sign of the Ku magnetic anisotropy constant. Moreover, magnetic anisotropy also leads to a weak anisotropy of local hyperfine fields at the nuclei 57Fe Hhf (θ) [22], [24]:where Hiso is the isotropic contribution to the hyperfine magnetic field Hn, determined mainly by the Fermi contact interaction with s-electrons localized on the nucleus and polarized by the atomic spin. Han is the anisotropic contribution due to the magnetic dipole–dipole interaction with localized magnetic moments of atoms and the anisotropy of the hyperfine magnetic interaction of the nucleus with the electrons of the ion core of its own atom. From Eq. (3) one can obtain a relation between the values of the hyperfine fields for the orientation of the magnetic moment of the iron atom parallel to (H‖) and perpendicular (H⊥) to the crystal symmetry axis the with isotropic and anisotropic contributions by simple relations [24]:
Experimental methods that can detect SSMS and observe its evolution caused, for example, by replacement of bismuth or iron by other ions or by distortions of the crystal structure are of significant importance for study perovskites based on BiFeO3 ferrite. Such methods are neutron diffraction, nuclear magnetic resonance (NMR), and nuclear gamma resonance (Mössbauer effect).
The existence of SSMS in BiFeO3 was revealed by the Mössbauer effect method on 57Fe nuclei [24], [25]. In [25], when processing the Mössbauer spectra of BiFeO3 taking into account the lattice εlat and magnetic εmag contributions to the quadrupole shift, the anharmonicity parameter was estimated as m = 0.5 at room temperature and m = 0.6 at 90 K, i.e. SSMS is highly anharmonic. In [24], an estimate of the additional contribution from the magnetic component εmag to the quadrupole shift showed that this contribution is small and can be neglected when processing the Mössbauer spectra, which leads to a decrease in the estimate of the anharmonicity parameter to m = 0.26 at 4.2 K.
One of the most obvious and direct methods for observing SSMS is NMR spectroscopy. The rotation of magnetic moments modifies the line shape in such a way that the spectrum becomes frequency distributed within 2 MHz and acquires a characteristic shape with two peaks at the edges of the spectrum of different (in the general case) intensities and a gap between them. In [26], NMR spectra were obtained for samples of the Bi1-yLayFeO3 series with y = (0, 0.1, 0.2, 0.9 and 1.0). In the first two samples with the lowest La content, the magnetic order of the SSMS type of the cycloid type was preserved.
Up to now, only a few papers report on the effect of replacement of bismuth by rare-earth atoms on SSMS studied by neutron diffraction, NMR, or Mössbauer spectroscopy.
This work aims to systematic study of effect of replacement of trivalent bismuth atoms by trivalent lanthanum atoms on SSMS, local magnetic and electrical states of Fe atoms in the rhombohedral phase R3c of the Bi1-yLayFeO3 system (y = 0, 0.015, 0.03, 0.05 and 0.10) by means of NMR at 4.2 K and the Mössbauer effect at room temperature on 57Fe nuclei. In the present paper, we combined NMR spectroscopy at low temperature and Moessbauer spectroscopy to study the region of drastic changes in the magnetic structure of the Bi1 – yLayFeO3: the destruction of SSMS and anisotropy type change at room temperature.
Section snippets
Materials and methods
Polycrystalline samples of Bi1-yLayFeO3 multiferroics (y = 0, 0.015, 0.03, 0.05 and 0.10) with a relative content of the stable 57Fe isotope of 10% mol percent (for all samples except y = 0.10) were prepared using solid-state ceramic technology. Sample with y = 0.10 was characterized by a higher 57Fe isotope content of ≈ 95%. A mixture of oxide powders of ferrite components in appropriate proportions was pressed into tablets, which were annealed for 25 h at a temperature of 700 – 830° C in the
Crystal structure
As mentioned above, the samples contained impurity phases Fe2O3 and Bi2Fe4O9 at the amount of a few percents (<5%). The presence of these phases in the studied samples was also confirmed by Mössbauer spectroscopy. X-ray diffraction measurements of the studied samples showed that the replacement of bismuth atoms by lanthanum atoms leads to a slight decrease in the parameters of the rhombohedral (sp.gr. R3c) lattice: a = b decreases from 5.581 Å (for y = 0) to 5.578 Å (for y = 0.1); c decreases
Conclusions
X-ray diffraction studies of Bi1-yLayFeO3 multiferroic samples (y = 0, 0.015, 0.03, 0.05 and 0.10) defined the crystal structure of the samples as rhombohedral with the space group R3c. The lattice parameters decrease with increasing lanthanum content. This effect is caused by a smaller value of the effective ionic radius R of trivalent La3+ lanthanum ions relative to the radius R of bismuth Bi3+ for N = 12-oxygen environment.
The magnetic structure of Bi1-yLayFeO3 multiferroics (y = 0, 0.015,
CRediT authorship contribution statement
V.S. Pokatilov: Conceptualization, Formal analysis. A.O. Makarova: Data curation. A.A. Gippius: Project administration, Supervision. A.V. Tkachev: Software, Data curation. S.V. Zhurenko: Investigation, Software. A.N. Bagdinova: Visualization. N.E. Gervits: Writing - original draft, Writing - review & editing, Methodology.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
V.S. Pokatilov and A.O. Makarova appreciate the support of the RFBR grant No. 20-02-00795. N.E. Gervits, A.A. Gippius, A.V. Tkachev and S.V. Zhurenko are grateful for the support of the RFBR grant No. 17-52-80036; A.A. Gippius and A.N. Bagdinova thank the Russian Foundation for Basic Research (RFBR) for the financial support of the project № 19-29-10007. Also, the team of authors is grateful for the fruitful discussion with S.N. Polulyakh.
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