Effect of double-cation substitution on the low-energy electrodynamics of M-type hexaferrites/Влияние двухкатионного замещения на низкоэнергетическую электродинамику гексаферритов М-типа тема диссертации и автореферата по ВАК РФ 01.04.07, кандидат наук Ахмед Асмаа Гамаль Мохамед

• Ахмед Асмаа Гамаль Мохамед
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Ахмед Асмаа Гамаль Мохамед

Table of Contents

Acknowledgments

List of Abbreviation and Symbols

Introduction

Chapter 1. Crystal structure, properties, and applications of M-type hexaferrites

Preface

1.1 Crystal structure of hexaferrites

1.1.1 Crystal structure of barium M-type hexaferrites

1.1.2 Building blocks of the barium M-type hexaferrite lattice

1.2 Tunable barium M-type hexaferrite for practical applications

1.2.1 Multiferroics properties of barium M-type hexaferrites

1.2.2 Soft mode phenomena

Chapter 2. Experimental section

2.1 Samples characterization and preparation

2.1.1 Single crystals

2.1.2 Ceramics

2.2 Dielectric spectroscopy techniques

2.2.1 Fourier transform infrared spectroscopy (FTIR)

2.2.2 Terahertz spectroscopy

2.2.2.1 Monochromatic terahertz backward-wave oscillators (BWOs)

2.2.2.2 Terahertz time domain spectrometers (THz-TDS) ''TeraView TPS Spectra-3000'' and ''Menlo Systems Tera K15''

2.2.3 Impedance spectroscopy

2.3 Data analysis of the dielectric spectra

2.3.1 The independent Lorentzian model

2.3.2 Terahertz spectra analysis methods

2.3.3 The dielectric relaxation time x(t)

Chapter 3: Broadband electrodynamic response of single-crystalline Ba1-xPbxFe12O19

Preface

3.1 Infrared spectroscopy

3.1.1 Middle-infrared spectra of single-crystalline Ba1-xPbxFe12O19

3.1.2 Infrared phonon resonances observed in single-crystalline Ba^PbiFe^Ow

3.1.3 Infrared dielectric response of single-crystalline Ba1-xPbxFe12O19

3.2 Terahertz spectroscopy

3.2.1 Terahertz transmissivity spectra of single-crystalline Bai-xPbxFei2Oi9

3.2.2 The dielectric nature of the absorption resonances in lead-substituted barium M-type hexaferrite

3.2.3 Terahertz dielectric permittivity spectra in lead substituted barium M-type hexaferrite

3.2.4 Terahertz ferroelectric-like soft mode

3.2.5 Gigahertz resonance in lead-substituted barium M-type hexaferrite

3.3 Radio-frequency spectroscopy

3.3.1 Dielectric relaxations in radio-frequency spectra of Bai-xPbxFei2Oi9

3.3.2 Temperature dependence of dielectric relaxation spectra of Ba1-xPbxFe12O19 crystals

Summary

Chapter 4: Terahertz electrodynamic response of Ba1-xPbxFenO19 ceramics

Preface

4.1 Transmissivity and reflectivity spectra of Ba1-xPbxFe12O19 ceramics

4.2 Complex dielectric permittivity spectra of Ba1-xPbxFe12O19 ceramics

4.3 Terahertz soft phonon observed in Ba1-xPbxFe12O19 ceramics

4.4 Terahertz soft mode observed in Ba1-xPbxFe12O19 ceramics

4.5 Molecular model of the soft mode

Summary

Chapter 5. The influence of double-cation substitution on low-energy electrodynamics of

barium hexaferrite

Preface

5.1 Infrared spectroscopy of single-crystalline Ba0.20Pb0.80AlyFe12-.yO19

5.2 Terahertz spectroscopy of single-crystalline Ba0.20Pb0.80AlyFe12-yO19

5.2.1 Temperature dynamics of low-energy(terahertz) absorption lines

5.3 Crystal field distortions

Summary

Conclusions

Bibliography

Appendix A

Appendix B

Введение диссертации (часть автореферата) на тему «Effect of double-cation substitution on the low-energy electrodynamics of M-type hexaferrites/Влияние двухкатионного замещения на низкоэнергетическую электродинамику гексаферритов М-типа»

Introduction

Relevance of the topic

Hexaferrite materials have shown a rapid increase of interest in the field of materials science within the last few years [1]. Barium hexaferrite, Ba2+Fe12O19, briefly BaM, is one of the easily accessible and low-cost materials surpassing recently 1/3 of the magnetic materials market, and therefore, is widely used in engineering and instrumentation: in permanent magnets, high-density magnetic recording media, data storage materials, microwave devices materials, phase shifter applications, resonators, electromagnetic wave absorbers, etc. [2]-[7]. The predominant trend in the study of hexaferrites is focusing on their magnetic properties due to mainly two factors. Firstly, they demonstrate brilliant magnetic properties, like high Curie temperature (Tc ~ 750 K), relatively high saturation magnetization (Ms ~ 72 Am2 kg-1), high coercivity (Hc ~ 160-255 k A m-1) [1]. Secondly, these compounds allow for a possibility of tuning of their functional properties by ionic substitution [8], in addition they possess high chemical stability. At the same time, electrodynamic properties of M-type hexaferrites, especially at sub-terahertz and terahertz frequencies, remain practically unstudied and the mechanisms that determine these properties are thus far from being understood.

The motivation in studying the electrodynamics of substituted barium hexaferrites is fuelled by the results of only a few available works on their multiferroic properties [9-12]. In addition, the complex crystal structure of BaM and the high sensitivity of its properties to doping provide vast opportunities for the development of tunable devices functioning in the sub-terahertz and terahertz ranges (phase shifters, signal sources, absorbing coatings, etc.). Also, in some papers [13], [14], assumptions are made about the existence of quantum critical state in hexaferrites BaFe12O19 and SrFe12O19, and this can open a new field in studying complex quantum phenomena in multiferroic materials.

The aim of the present study was to unveil the fundamental physical mechanisms that determine the dielectric properties of M-type hexaferrites. To that end, a set of spectroscopic techniques was utilized that allowed for the first thorough and systematic measurements of dielectric response (spectra of complex dielectric permittivity) of single-and double-cation substituted barium hexaferrites Ba1-xPbxAlyFe12-yO19 in an unprecedentedly broad frequency interval - 1 Hz - 240 THz (with exclusion of 300 MHz - 25 GHz range) - under conditions of doping with various ions, and in the broad temperature interval, from 5 K to 300 K.

The objectives of the dissertation

1- Based on the data obtained, provide recommendations on possible applications of M-type hexaferrites in electronic industry.

2- To obtain detailed quantitative information on the dielectric properties of single-cation substituted barium hexaferrites Ba1-xPbxFe12O19 in their single crystalline form, x(Pb) = 0.10, 0.20, and 0.80, as well as ceramics, x(Pb) = 0.00 - 0.30, with step 0.05, and of double-cation substituted barium hexaferrites Ba0.20Pb0.80Fe12-yAlyO19, y(Al) = 1.2, 3.0, and 3.3, by measuring the broadband spectra of complex dielectric permittivity of the compounds at frequencies 1 Hz - 240 THz (with exclusion of 300 MHz - 25 GHz range) and at temperatures 5 K - 300 K.

3- To unveil the nature of physical phenomena that determine the broadband electrodynamic response of single- and double-cation substituted M-type hexaferrites and show up in the spectra in the form of broad absorption bands and narrow resonance lines.

4- To propose microscopic or phenomenological models that can account for the observed phenomena.

5- Based on the data obtained, provide recommendations on possible applications of M-type hexaferrites in electronic industry.

Scientific Novelty

1- First detailed spectroscopic studies (from radio-frequencies up to infrared with exclusion of 300 MHz - 25 GHz range) are performed of electrodynamic properties of single and double cation substituted barium hexaferrites Ba1-xPbxAlyFe12-yO19, x(Pb) = 0.0-0.8, y(Al) = 0.0-3.3, that allowed for a number of novel findings important in fundamental as well as in applicational aspects.

2- A temperature unstable excitation is discovered in the family of Ba1-xPbxFe12O19 compounds, whose frequency position changes from ~30 cm-1 to ~8 cm-1 when the temperature varies from 300 K to 5 K. A model is proposed that qualitatively explains the origin and the temperature evolution of the excitation.

3- A set of excitations is discovered at terahertz frequencies in the family of double-substituted hexaferrites Ba1-xPbxAlyFe12-yO19, and a model is proposed to interpret their origin as connected with the electronic transitions within the fine-structured ground state of Fe2+ ions located in tetrahedral site-positions.

Scientific Statements for the defence

1. Doping BaFe12O19 hexaferrites with Pb2+ ions leads to an increasing concentration of divalent iron ions Fe2+ in the Ba1-xPbxFe12O19 crystal lattice due to Pb-O-Fe electron exchange.

2. When substituting iron ions in the Ba1-xPbxAlyFe12-.yO19 compounds, aluminium ions tend to occupy octahedral site-positions of hexaferrite crystal lattice.

3. Terahertz electrodynamic response of lead substituted M-type barium hexaferrites BaxPb1-xFe12O19 is governed by a temperature unstable (soft) excitation that is similar to ferroelectric soft modes. Temperature behaviours of frequency and dielectric strength of the observed excitation do not follow the Cochran and Curie-Weiss laws, respectively, known to describe soft modes in regular ferroelectrics.

4. The origin of the excitation is associated with the response of interacting electric dipole moments that appear in the «¿-plane of the Ba1-xPbxFe12O19 crystal structure due to the oscillations of Pb2+ ions in the six-well localizing potential.

5. The frequency position of the soft excitation discovered in Ba1-xPbxFe12O19 follows the power-law temperature dependence ~ (T - Tc)05 with the critical temperature Tc dependent on lead content x(Pb) and taking a minimum value Tc ~ 0 for x(Pb) ~ 0.20 - 0.25 indicating possible emergence of a quantum critical state.

6. Terahertz absorption (frequencies 0.15-2.4 THz) of double-cation substituted M-type hexaferrites Ba1-xPbxAlyFe12-.yO19, x(Pb) = 0.0-0.8, y(Al) = 0.0-3.3, is governed by a set of absorption lines whose intensities grow with increasing of the lead concentration. The origin of the lines can be associated with electronic transitions within the fine-structured ground state of Fe2+ ions located in tetrahedral site positions.

7. Sub-terahertz and terahertz electrodynamic properties of the Ba1-xPbxAlyFe12-.yO19 hexaferrites can be purposefully tuned by changing temperature and/or chemical composition, which makes the family of compounds promising candidates for utilization in terahertz optoelectronics.

The theoretical and practical significance

A microscopic model is proposed to interpret the nature of the discovered ferroelectric-like temperature-unstable terahertz excitation in Ba1-xPbxFe12O19. The frequency position and dielectric strength of the excitation display temperature evolution that does not fall within the framework of the standard for ferroelectrics Cochran soft mode phenomenology. The model considers the dielectric response of coupled electric dipole moments that appear in the ab-plane of the Ba1-xPbxFe12O19 crystal structure due to the oscillations of Pb2+ ions in the six-well localizing potential.

Furthermore, qualitative model of crystal field distortions is proposed to explain the nature of excitations that mainly determine the terahertz absorption processes in the family of double- and single-cation substituted hexaferrites Ba1-xPbxAlyFe12-yO19. The model associates the excitations with the electronic transitions within the fine-structured ground state of Fe2+ ions located in tetrahedral site-positions.

Single- and double-cation substituted barium hexaferrites are considered nowadays as a family of compounds promising for use in the next generation telecommunication systems with operation frequencies in sub-terahertz and terahertz bands. Therefore, the practical value of the results of the thesis is determined by 1) obtained quantitative data on the sub-terahertz/terahertz dielectric characteristics of M-type hexaferrites, on the possibility of their purposeful tuning, and 2) the information on the nature of physical phenomena that are at the origin of these characteristics.

Approbation of the research results

The main results of the dissertation were reported and discussed at seminars of the laboratory of terahertz spectroscopy at the Moscow Institute of Physics and Technology, as well as 9 Russian and 10 international conferences. The list of conferences on the dissertation topic is given at the end of the manuscript.

Publications

The dissertation results are published in 9 papers indexed in Scopus and Web of Science, including 3 articles in highly ranked international peer-reviewed journals [Q1] and 6 theses in materials of international conferences. The list of publications on the dissertation topic is given at the end of the manuscript.

Personal contribution All the results presented in this dissertation were obtained by the author personally or with her direct involvement.

Reliability of experimental results is ensured by full agreement of the data independently obtained on various spectrometers in different Russian and foreign groups, operating in different spectral regions and at different temperatures, their agreement with theoretical models and concepts used to interpret the experimental data. The experimental results were obtained on approved and tested measuring installations and instruments, verified by control measurements, they showed high degree of reproducibility. The research results have been expertly discussed at international and Russian conferences and seminars. The validity of the conclusions is also confirmed by publications in highly ranked peer-review international scientific journals.

Dissertation structure the dissertation contains an introduction, 5 chapters, a conclusion, two

appendices, and a list of 155 references. It is written on 126 pages of typewritten text, includes 66

figures and 20 tables.

List of publications on the dissertation topic

1. A. Ahmed, A.S. Prokhorov, V. Anzin, D. Vinnik, S. Ivanov, A. Stash, YS. Chen, A. Bush, B. Gorshunov, L. Alyabyeva «Terahertz-infrared dielectric properties of lead-aluminium double-cation substituted single-crystalline barium hexaferrite» // Journal of Alloys and Compounds, 898, 162761, 2022, https://doi.org/10.1016/j.jallcom.2021.162761

2. Alyabyeva L.N., Prokhorov A.S., Vinnik D.A., Anzin V.B., Ahmed A.G., Mikheykin A., Bednyakov P., Kadlec C., Kadlec F., de Prado E., Prokleska J., Proschek P., Kamba S., Pronin A.V., Dressel M., Abalmasov V.A., Dremov V.V., Schmid S., Savinov M., Lunkenheimer P., Gorshunov B.P. «Lead-substituted barium hexaferrite for tunable terahertz optoelectronics» // NPG Asia Materials, 13, 63, 2021, 10.1038/s41427-021-00331-x

3. Alyabyeva, L. N., D. A. Vinnik, A. G. Ahmed, V. V. Dremov, and B. P. Gorshunov. «Broadband Electrodynamics of Single-Crystalline Lead-Substituted Barium Hexaferrite » // IEEE, 1-2, 2021, https://doi.org/10.1109/IRMMW-THz50926.2021.9567299

4. Lukianov M.Y., Ahmed A. G, Bush A. A, Prokhorov A. S, Abalmasov V. A, Anzin V. B, Gorshunov B. P, and Alyabyeva L. N. «Terahertz Soft Mode in Ba-Pb M-Type Hexaferrite Ceramics » // IEEE, 1-2, 2021, https://doi.org/10.1109/IRMMW-THz50926.2021.9567545

5. L Alyabyeva, A Ahmed, D Vinnik, M Savinov, P Lunkenheimer, B Gorshunov «Hertz-to-infrared electrodynamics of single-crystalline barium-lead hexaferrite Ba1-xPbxFe12O19» // Journal of Physics: Conference Series. 1984, 012014, 2021, https://doi.org/10.1088/1742-6596/1984/1/012014

6. M Y Lukianov, A G Ahmed, A A Bush, A S Prokhorov, V B Anzin, B P Gorshunov, L N Alyabyeva «Terahertz spectroscopy of lead substituted barium hexaferrites Ba1-xPbxFe12O19» // Journal of Physics: Conference Series. 1984, 012013 2021, https://doi.org/10.1088/1742-6596/1984/1/012013

7. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, Liudmila Alyabyeva «Origin of terahertz excitations in single-crystalline lead substituted M-type barium hexaferrite doped with Al» // Journal of Physics: Conference Series. 1984, 012015 2021, https://doi.org/10.1088/1742-6596/1984/1/012015

8. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, and Liudmila Alyabyeva. «Terahertz-Infrared Electrodynamics of Single-Crystalline Ba0.2Pb0.8Al1.2Fe10.8O19 M-Type Hexaferrite» // Journal of Alloys and Compounds 836:155462.

2020, https://doi.org/10.1016/j.jallcom.2020.155462 9. Asmaa Ahmed, Liudmila Alyabyeva, Victor Torgashev, Anatoly S. Prokhorov, Denis Vinnik, Martin Dressel, and Boris Gorshunov. «Effect of Aluminium Substitution on Low Energy Electrodynamics of Barium-Lead M-Type Hexagonal Ferrites» // Journal of Physics: Conference Series 1389:012044. 2019, https://doi.org/10.1088/1742-6596/1389/1/012044

List of conferences on the dissertation topic

1. Lukianov M.Y., Ahmed A.G., Bush A.A., Prokhorov A.S., Anzin V.B., Gorshunov B.P., Alyabyeva L.N. "Terahertz soft mode in Ba-Pb M-type hexaferrite ceramics"// 46th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), Chengdu, China, August 29 - 3 September 2021, "Online oral". [international].

2. Alyabyeva L.N., Ahmed A.G., Prokhorov A.S., Vinnik D., Savinov M., Lunkenheimer P., Gorshunov B.P. "Broadband electrodynamics of single-crystalline lead-substituted barium hexaferrite"// 46th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), Chengdu, China, August 29 - 3 September 2021, "Online oral". [international].

3. M. Y. Lukianov, A. G. Ahmed, A. A. Bush, A. S. Prokhorov, V. B. Anzin, B. P. Gorshunov and L. N. Alyabyeva, "Terahertz spectroscopy of lead substituted barium hexaferrites Ba1-xPbxFe12O19" SPb Photonic, Optoelectronic, Electronic Materials (SPb-POEM) 25 - 28 May 2021, "Online oral". [international].

4. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, Liudmila Alyabyeva," origin of terahertz excitations in single-crystalline lead substituted M-type barium hexaferrite" SPb Photonic, Optoelectronic, & Electronic Materials (SPb-POEM) 25 -28 May 2021, "Online oral". [international].

5. Alyabyeva L.N., Prokhorov A.S., Vinnik D.A., Anzin V.B., Ahmed A.G., Mikheykin A., Kadlec Ch., Kadlec F., Prado E., Kamba S., Pronin A.V., Dressel M., Abalmasov V.A., Dremov V.V., Schmid S., Savinov M., Lunkenheimer P., Gorshunov B.P., "Hertz-to-infrared electrodynamics of single-crystalline barium-lead hexaferrite Ba1-xPbxFe12O19", SPb Photonic, Optoelectronic, & Electronic Materials (SPb-POEM) 25 - 28 May 2021, "Online oral". [international].

6. Lukianov M.Y., Ahmed A.G., Bush A.A., Prokhorov A.S, Anzin V.B., Abalmasov V.A, Gorshunov B.P., Alyabyeva L.N. "Effect of ionic substitution on terahertz electrodynamics of barium hexaferrites"// Proceedings of III International Conference on Laser&Plasma researches and technologies LaPlas-2021, Russia, Moscow, 2021, March 23-27, "Online oral". [international].

7. Alyabyeva L.N., Ahmed A.G., Gorbachev E.A., Karpov M.A., Lukyanov M.Yu., Vinnik D.A., Bush A.A., Prokhorov A.S., Gudkov V.V., Trusov L.A., Gorshunov B.P. "Synthesis and study of magnetic, electrodynamic and structural properties of single crystals and ceramics of substitutional solid solutions based on M-type hexagonal ferrites" // meeting of the section "Magnetism" of the Scientific Council of the Russian Academy of Sciences on Condensed Matter Physics, December 2020, "Oral". [Russia].

8. A.G. Ahmed, A.S. Prokhorov, V. B. Anzin, V. A. Abalmasov, A.A. Bush, B. P. Gorshunov, L. N. Alyabyeva '' Terahertz soft mode in M-type hexaferrite Ba1-xPbxFe12O19M XXI All-Russian School -Seminar on Problems of Physics of Condensed Matter (SPFKS-21), school-seminar 18 March - March 25, 2021, Yekaterinburg. Russia, "Poster". [Russia].

9. A.G. Ahmed, A.S. Prokhorov, V. Anzin, D. Vinnik, B. P. Gorshunov, L. N. Alyabyeva ''Terahertz electrodynamics of M-type barium hexaferrite single crystals Ba0.2Pb0.8AlxFe12-xO19 with Pb+2 and Al+3 substitutions'' / / XXI All-Russian School - Seminar on Problems of Physics of Condensed Matter (SPFKS-21), school-seminar on March 18 - March 25, 2021, Yekaterinburg. Russia, "Oral". [Russia].

10. M.Yu. Lukianov, A.G. M. Ahmed, A.A. Bush, A.S. Prokhorov, V.B. Anzin, V.A. Abalmasov, B.P. Gorshunov, L.N. Alyabyeva "Terahertz soft mode in substituted barium hexaferrite Ba1-x Pbx Fe12 O19 // 63 All-Russia scientific conference in MIPT, November 23- 29, 2020, Dolgoprudny, Russia, "Online oral". [Russia].

11. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, Liudmila Alyabyeva "Influence of crystal field distortions on the energetic fine structure of divalent iron in single crystalline lead substituted M-type barium hexaferrite" 63 All-Russia scientific conference in MIPT, November 23- 29, 2020, Dolgoprudny, Russia, "Online oral" [Russia].

12. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, Liudmila Alyabyeva "Terahertz-infrared excitations in the Ba0.2Pb0.8Al1.2Fe10.8O19 single crystal"// First virtual Bilateral Conference on Functional Materials (BiC-FM), October 8-9, 2020, "Online oral" [international].

13. Asmaa Ahmed, Anatoly S. Prokhorov, Vladimir Anzin, Denis Vinnik, Alexander Bush, Boris Gorshunov, Liudmila Alyabyeva "Low-energy electrodynamics of single-crystalline M-type barium hexaferrite doped with lead and aluminium"// XVII conference Strongly Correlated Electronic Systems and Quantum Critical Phenomena, Troitsk, Moscow, September 2020. [Russia].

14. A.G. Ahmed, L.N. Alyabyeva, V. I. Torgashev, A.S. Prokhorov, D. Vinnik, M. Dressel, B.P. Gorshunov "Terahertz-infrared dielectric response of single crystalline lead-substituted M-type barium

hexaferrites doped with Al3+"// 62 All-Russia scientific conference in MIPT, November 18-23, 2019, Dolgoprudny, Russia, "Oral" [Russia].

15. L.N. Alyabyeva, A.S. Prokhorov, D. Vinnik, A.G. Ahmed, M. Dressel, B. P. Gorshunov "Soft mode in single crystalline substituted barium hexaferrite"// 62 All-Russia scientific conference in MIPT, November 18-23, 2019, Dolgoprudny, Russia, "Oral" [Russia].

16. L. Alyabyeva, V. Torgashev, D. Vinnik, A.S. Prokhorov, A. Ahmed, B. Miksch, M. Dressel, M. Savinov, B. Gorshunov "Effect of Chemical Pressure on Terahertz Electrodynamics of Barium Hexaferrite - Perspective Multiferroic"// International Congress on Graphene, 2D Materials and Applications, September 30 - October 04, 2019, Sochi, Russia, "Oral" [Russia].

17. A.G. Ahmed, L.N. Alyabyeva, V. I. Torgashev, A.S. Prokhorov, D. Vinnik, M. Dressel, BP. Gorshunov "Effect of Al substitution on low-energy electrodynamics of barium-lead M-type hexagonal ferrites" // The VII Euro-Asian Symposium "Trends in Magnetism" (EASTMAG'2019) September 0813, 2019, Ekaterinburg, Russia, "poster" [international].

18. Liudmila Alyabyeva, Victor Torgashev, Anatoly Prokhorov, Asmaa Ahmed, Denis Vinnik, Martin Dressel, Boris Gorshunov "Unusual terahertz soft mode in multiferroic M-hexaferrite Ba1-xPbxFe12O19" // The Joint European Magnetic Symposia (JEMS'2019), August (26-30) 2019, Uppsala, Sweden, "Oral" [international].

19. L. Alyabyeva, V. Torgashev, D. Vinnik, A. Prokhorov, A. Ahmed, M. Dressel, B. Gorshunov "Terahertz soft mode peculiarity in barium-lead hexaferrite"// Magnetics and Optics Research International Symposium (MORIS'2019), June (23-26) 2019, Prague, Czech Republic, "Oral" [international].

Заключение диссертации по теме «Физика конденсированного состояния», Ахмед Асмаа Гамаль Мохамед

Conclusions

1. Detailed spectroscopic studies are performed of substituted barium hexaferrites that are perspective for developing electronic components of next-generation communication systems. For the first time, the broadband (frequencies 1 Hz - 240 THz with exclusion of 300 MHz - 25 GHz range) temperature-dependent (temperatures 5 K - 300 K) spectra of complex dielectric permittivity of artificial Ba1-xPbxFe12O19, x(Pb) = 0.10, 0.20, 0.80, and Ba0.20Pb0.80Fe12-yAlyO19, y(Al) = 1.2, 3.0, 3.3, single crystals and Ba1-xPbxFe12O19, x(Pb) = 0.00-0.30 with a step 0.05, ceramics are measured using methods of radiofrequency, sub-terahertz, terahertz, and infrared spectroscopies. A rich set of absorption lines is discovered, and their origin is assigned based on the analysis of the temperature and composition dependences of the lines' parameters. Spectroscopic studies are complemented with specific heat and magnetic/electrostatic force microscopy experiments.

2. In the lead-substituted barium hexaferrite, Ba1-xPbxFe12O19, a ferroelectric-like temperature unstable terahertz excitation is discovered. The discovered soft mode reveals unusual temperature behaviour of its frequency and dielectric strength that do not follow the corresponding Cochran and Curie-Weiss laws, respectively, known to describe soft modes in conventional ferroelectrics. The soft mode frequency is shown to follow the power-law temperature dependence v^M(T) ~ (T - Tc)05 with the critical temperature Tc dependent on lead content x(Pb) and taking a minimum value of Tc = 0 ± 5 K for x(Pb) ~ 0.20 - 0.25. This observation indicates the possibility of stabilizing the quantum critical state in Ba1-xPbxFe12O19.

3. To interpret the nature of the observed soft mode, we proposed a microscopic model that qualitatively explains the origin of the observed excitation and its nonstandard temperature evolution. The model connects the soft mode's nature with the response of interacting electric dipoles that appear in the a^-plane of the Ba1-xPbxFe12O19 structure due to the oscillations of Pb2+ ions in the six-well localizing potential.

4. Strongly temperature-dependent relaxational excitations are discovered at radio frequencies in dielectric permittivity spectra of single-crystalline Ba1-xPbxFe12O19, x(Pb) = 0.10, 0.20. The origin of the relaxations is assigned to the dielectric response of electrically polarized magnetic domain's walls. The electrical polarity at the boundaries of magnetic domains is firmly detected in undoped BaFe12O19, and the microscopic mechanism of its emergence is analyzed.

5. The effect of double-cation substitution on the dielectric response of barium hexaferrite is explored. A set of terahertz absorption resonances is discovered in the Ba0.2Pb0.8AlyFe12-.yO19 compounds with y(Al) = 1.2, 3.0 and 3.3, and their temperature and aluminum-doping dependences are analyzed with an account taken of disorder introduced by aluminum. To clarify the nature of the resonances, a model is developed that considers electronic transitions within the fine-structured ground state of four-fold coordinated Fe2+ ions. It is shown that the trigonal distortions of the crystal field lead to lowering of the symmetry of 4f1 and 4e tetrahedral site-positions of Fe2+ ions and, as a result, to further splitting of the ground state spin-orbital sub-levels. The observed absorption lines are associated with the electronic transitions between the corresponding sub-levels. The model is successfully applied to describe the THz spectra of single-cation substituted Ba1-xPbxFe12O19, x(Pb) = 0.10, 0.20, 0.80, hexaferrites.

Список литературы диссертационного исследования кандидат наук Ахмед Асмаа Гамаль Мохамед, 2022 год

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