Spin-phonon coupling in Ba₂Zn₂Fe₁₂O₂₂ Y-type hexaferrite

Abstract

The spin-phonon coupling is frequently observed in magnetic materials and can be investigated using Raman spectroscopy. Surprisingly, there are no studies using Raman spectroscopy to confirm the existence of spin-phonon coupling in the Y-type Hexaferrite Ba₂Zn₂Fe₁₂O₂₂ (Zn₂Y), despite its ferrimagnetic nature. In this context, this paper aims to fill this gap. Raman spectra of Zn₂Y were measured from 296 K to 438 K, and a hardening of the mode at 660 cm^-¹ was observed near the Curie temperature. This behavior was associated with spin-phonon coupling.

Keywords:
Spin-phonon coupling; Hexaferrite; Raman spectroscopy

INTRODUCTION

The hexaferrite, discovered in the 1950s¹, can be described as a stacking of a sequence of three basic blocks: block S (Me²⁺Fe₄O₈; spinel block, where Me denotes a divalent metal ion such as Fe²⁺, Mg²⁺, Zn²⁺, Co²⁺, etc.); block R [(Ba, Sr)Fe₂O₁₁]²^-; and block T [(Ba, Sr)₃Fe₈O₁₄]. Hexagonal ferrites have been classified according to their structure and composition into several classes, including M-type ((Ba, Sr)Fe₁₂O₁₉), W-type ((Ba, Sr)Me₂Fe₁₆O₂₇), Y-type ((Ba, Sr)₂Me₂Fe₁₂O₂₂), X-type ((Ba, Sr)₂Me₂Fe₂₈O₄₆), and Z-type ((Ba, Sr)₂Me₂Fe₂₄O₄₁)². Among the Y-type hexaferrites, attention has been focused on Zn-based Y-type hexaferrite (Ba₂Zn₂Fe₁₂O₂₂ or Zn₂Y) due to its low insertion loss, wide bandwidth, low biasing field, and high anisotropy field (HA) for microwave device applications³.

Contrary to Ba₂Mg₂Fe₁₂O₂₂, in which Mg is distributed in the octahedral and tetrahedral sites⁴ in the Ba₂Zn₂Fe₁₂O₂₂, zinc is only in two different tetrahedral sites⁵. Zn₂Y hexagonal planar ferrite in that it has an easy plane of magnetization perpendicular to the c axis⁵. This compound is trigonal (space group $R\overline{3}m$); expressed in the hexagonal axes, the cell parameters are: a=b=5.875(1) Å and c=43.571(6) Å. Zn₂Y has three unit cells per unit formula (Z=3). Its magnetic structure is characterized by alternate stacks of L blocks and S blocks along c. Within each block, the magnetic moments of Fe³⁺(S=5/2) are ferrimagnetically coupled and collinearly aligned in the ab plane. This yields a large magnetic moment on the L block and a small magnetic moment on the S block^{1), (2}.

Spin-phonon coupling plays a crucial role in hexaferrites, as it is associated with the magnetodielectric effect in Z-type hexaferrites⁶. However, spin-phonon coupling is frequently investigated and observed in magnetic compounds^{7), (8), (9), (10), (11}. Despite its well-documented ferrimagnetic behavior (T_C=403 K)^{1), (12}, spin-phonon coupling studies in Zn₂Y, as well as its temperature-dependent phonon spectra, have not been reported in the literature. One of the most employed techniques for investigating spin-phonon coupling is Raman spectroscopy. There have been few studies on the Raman spectroscopy of the Y-type hexaferrite Zn₂Y, except for some investigations involving the isostructural compounds: Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂¹³, Ba₂Mg₂Fe₁₂O₂₂¹⁴ and BaSrCoZnFe₁₁AlO₂₂¹⁵. This paper aims to investigate spin-phonon coupling in the Zn₂Y hexaferrite and discuss its temperature-dependent vibrational properties.

EXPERIMENTAL

Ba₂Zn₂Fe₁₂O₂₂ (Zn₂Y) was prepared using the solid-state reaction method according to a stoichiometric mixture of BaCO₃, ZnO, and Fe₂O₃. The starting materials were mixed according to their molecular weight proportions and ground in an agate mortar, then sieved to obtain a homogeneous powder. The mixed powders were calcined at 1200 °C in the atmosphere for 8 hours. The resulting powders were investigated by X-ray diffraction (XRD) at room temperature using a Bruker D8 Advance powder diffractometer with Cu-Kα radiation (40 kV, 40 mA) in a 15° to 80° range with a step size of 0.02°. The X-ray diffraction pattern of the powders was compared with data from the ICSD (Inorganic Crystal Structure Database, FIZ Karlsruhe, and NIST) from the international diffraction database (ICSD#14239), confirming the formation of the Zn₂Y phase.

For the Raman spectroscopy measurements, the polycrystalline powder was uniaxially compressed into 7 mm pellets at 4 metric tons. The pellets were placed in alumina crucibles with sacrificial powder of the same composition and sintered in air at 1200°C for 16 hours to obtain dense ceramics. The Raman spectroscopy measurements were carried out using a Horiba iHR550 Raman spectrometer coupled with an Olympus BX41 microscope, equipped with a long working distance objective (20x, 20.5 mm). The 662 nm line from a diode laser operating at 17 mW was used to excite the Raman signal, which was collected by an air-cooled Synapse CCD. High temperature measurements were performed by furnace TS 1200 LIKAM model in the range 296 K to 438 K.

RESULTS AND DISCUSSIONS

Figure 1 shows the XRPD pattern of the Zn₂Y sample. The experimental data was refined using the software GSAS. The X-ray diffraction pattern of the powders was compared with data from the ICSD (Inorganic Crystal Structure Database, FIZ Karlsruhe, and NIST) from the international diffraction database (ICSD#14239), confirming the formation of the Zn₂ Y phase. Additionally, the diffraction pattern indicated a small amount of ZnFe₂O₄ (3.3%) and BaFe₂O₄ (3.0%) as residual phase. These secondary phases are commonly observed in the synthesis of Zn₂Y hexaferrite¹.

Figure 1
Observed, calculated, and diference XRPD profiles for Zn₂Y at room temperature. The low-intensity peaks identified with the cross and the asterisk represent the subphases ZnFe₂O₄ and BaFe₂O₄, respectively. These peaks are shown in an enlarged view in the inset.

Figure 2a shows the Raman spectrum at room temperature corresponding to the Zn₂Y sample. Group theory predicts 32 Raman-active phonons in Zn₂ Y, with their distribution in the irreducible representations of the factor group being 14 A_1g+18 E_g¹⁶. We observe sixteen modes at room temperature at the following wavenumbers: 723, 697, 660, 494, 461, 425, 385, 360, 330, 289, 280, 254, 221, 188, 174, 660and 130 cm^-1. Figures 2b and 2c show the evolution of the phonon spectra with increasing temperature. As we can see, except for the expected broadening of the bands due to anharmonic effects, there is no significant change in the spectra with increasing temperature, allowing us to rule out a structural phase transition in Zn₂Y within the investigated temperature range.

Figure 2
(a) Zn₂Y spectrum collected at room temperature. Temperature-dependent evolution of the Zn₂Y phonons: (b) low wavenumber (c) high wavenumber.

The symmetries of the main bands can be elucidated by comparing them with the Raman spectra of the isostructural compound Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂¹³, which allows us to conclude that in Zn₂Y, the bands at 697, 657, 494, 461, 360, and 188 cm^-1 are A_1g, while the bands at 289, 174, and 130 cm^-1 are E_g. Regarding the classification of the bands, this task is quite complicated for Zn₂Y due to its complex crystal structure. It has tetrahedral and octahedral sites, in addition to the MeO₆ groups forming chains that share faces and edges, making it very difficult to assign the corresponding vibration modes to specific sites. Despite this, we can qualitatively classify some of these modes by considering the crystal structure of Zn₂Y, which is formed by octahedral FeO₆ and tetrahedral FeO₄ groups. In this context, classification can be done by comparing the Raman spectrum of Zn₂Y with that of magnetoplumbite (BaFe₁₂O₁₉)¹⁷ and spinel-type ferrites^{18), (19}, which are materials that have octahedral and tetrahedral groups in their structures. It is known from the literature that ferrites (AFe₂O₄, where A is a divalent cation) are characterized by A_1g bands in the 660-720 cm^-1 region, regardless of the nature of the divalent ion¹⁸, which are related to the stretching of Fe³⁺O₄ tetrahedra.

Additionally, studies show that the A_1g modes in the 460-640 cm^-1 region¹⁷ in ferrites with a normal spinel structure are dominated by Fe³⁺O₆ octahedra. Thus, we suggest that the bands at 697 and 660 cm^-1 present in Zn₂Y are associated with the vibrations of the MeO₄ tetrahedra in the S and T blocks (with Me=p_ZnZn²⁺+(1-p_Zn) Fe³⁺, where p_Zn is the population of zinc ions). While the modes at 494, 461, 360, 289, and 188 cm^-¹ are due to the vibration of the MeO₆ octahedron and the vibration of the O-Me-O bonds, respectively, the low-frequency bands are often associated with heavy cations. Therefore, we assign the mode at 130 cm^-¹ to Ba-O vibrations. Finally, the band at 174 cm^-¹ can be attributed to the vibrations of the entire S block, as it is in this region that the modes for the S block are found in spinel ferrites¹⁷. The classification of these modes is summarized in Table 1.

Table 1
Assignment of the main Raman-active modes observed in Zn₂Y.

Wavenumber (cm-1)	Symmetry	Assignment
697	A1g	Stretching MeO4
660	A1g	Stretching MeO4
494	A1g	Stretching MeO6
461	A1g	Stretching MeO6
360	A1g	Vibration MeO6
289	Eg	Vibration MeO6
188	A1g	O-Me-O bridge
174	Eg	Whole spinel block
130	Eg	Vibration Ba-O

Figure 3 shows the dependence of the position of the most intense mode at 660 cm^-¹ as a function of temperature, obtained through a fitting process. It is observed that, up to the Curie temperature (403 K), the phonon wavenumbers decrease. As observed, near this temperature, an anomaly occurs in the position of this active Raman mode. This happens because, at room temperature, Zn₂Y is ferrimagnetic, which leads to a contribution from the interactions between the Fe³⁺-O-Fe³⁺magnetic ions in the phonon dependence on temperature, in addition to anharmonicity²⁰. The deviation at the Curie temperature indicates that there is spin-phonon coupling in Zn₂Y. It is known from the literature that this coupling can be interpreted as a modulation of the exchange integral by phonons, which leads to a renormalization of the phonon. The spin-phonon contribution (∆ω_s-ph) to the phonon frequency variation due to magnetic ordering can be written as^{20), (21), (22}:

(1)

where ρ is the spin-phonon coupling constant and is the scalar spin correlation function. In the case of a ferrimagnetic material, ρ and are negative. As the temperature increases, the spin-phonon coupling is suppressed, since thermal agitation hinders magnetic ordering, and thus anharmonicity becomes more important at higher temperatures. With the spin-phonon coupling no longer existing, a change in the slope of the curve of the mode position at 660 cm^-¹ is expected at the Curie temperature. The contribution of the subphases ZnFe₂O₄ and BaFe₂O₄ to the observed anomaly can be readily dismissed, as the Néel temperatures of these ferrites are 10 K²³ and 227 K²⁴, respectively.

Figure 3
Temperature dependence of the Raman - active mode at 660 cm^-1. The dots represent the data points with error bars and the dashed line indicates the temperature at which the abnormality occurs. The continuous curve is a guide to the eyes

A similar result is found for BiFeO₃²⁵, where there is softening and hardening for the A₁ and E modes, respectively, above the Néel temperature (643 K). F. M. Silva Júnior and C. W. A. Paschoal²⁶ observed spin-phonon coupling at the Curie temperature of BaM (723 K). M. A.P. Buzinaro, et al.²⁷ also observed the spin-phonon coupling at the Curie temperature (T_c=720 K) of the M-type SrFe₁₂O₁₉ hexaferrite. Y.P. Santos et al.²⁸ observed strong spin-phonon coupling in the Z-type hexaferrite Ba_1.6Sr_1.4Co₂Fe₂₄O₄₁ using Raman spectroscopy, related to three magnetic transitions: conical-planar, uniaxial planar, and ferromagnetic-paramagnetic. Similarly to this work, these authors observed the spin-phonon coupling in the most intense Raman band associated with the stretching vibration of the FeO₄ tetrahedra. Finally, Raju et al.²⁹ also observed spin-phonon coupling in the SrFe₈Co₂Ti₂O₁₉ hexaferrite at 330 K, which they attributed to a magnetic transition from a collinear phase to a conical phase¹. All spin-phonon couplings in these hexaferrites were observed in modes with A_1g symmetry. These results support our hypothesis of spin-phonon coupling in Zn₂Y in the high-temperature regime.

CONCLUSION

The Y-type hexaferrite Ba₂Zn₂Fe₁₂O₂₂ was obtained successfully by solid state reaction. The transition from the ferrimagnetic to the paramagnetic phase, which occurs at 403 K, is reflected in the temperature dependence of the phonon observed near 660 cm^-1. Its frequency increases abruptly above Tc due to spin-phonon coupling.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of the funding agencies CAPES, CNPq, and FAPEMA.

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Edited by

• AE:
  Daniel Z. de Florio.

Publication Dates

• Publication in this collection
  09 June 2025
• Date of issue
  2025

History

• Received
  31 Jan 2025
• Reviewed
  24 Mar 2025
• Accepted
  14 Apr 2025