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Ferromagnetic resonance, magnetic susceptibility, and transformation of domain structure in CoFeB film with growth induced anisotropy
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 8, 083926- p.Article in journal (Refereed) Published
Abstract [en]

Field dependence of magnetic susceptibility in nanocrystalline CoFeB film was studied in a wide frequency range from 500 kHz to 15 GHz. Anomalies of the susceptibility were detected exciting CoFeB film with a solenoidal coil, microwave strip line, in the tunable microwave cavity as well as employing magneto-optical domains imaging. Critical spin fluctuations in the form of "soft" modes were observed in a whole range of orientations of magnetic field perpendicular to the "easy" magnetic axis. A sequence of domain structure transformations was extensively examined in a "hard" direction in in-plane magnetic field reduced below the field of uniaxial anisotropy H-p = 535Oe. At first, uniformly magnetized state in CoFeB film transforms to stripe domains separated by low angle Neel domain walls (DWs) parallel to the "hard"-axis. Then, at critical field H-cr = 232 Oe, Neel DWs gradually convert to the"easy"-axis oriented Bloch DWs loaded with vertical Bloch lines (VBLs). After field reversal at H = H-cr, backward conversion of VBL-loaded Bloch DWs to Neel DWs results in instantaneous energy release and sharp anomaly of magnetic susceptibility. Appearance of critical spin fluctuations accomplishes domains transformation to the uniformly magnetized state at H = 535 Oe. 

Place, publisher, year, edition, pages
2011. Vol. 109, no 8, 083926- p.
Keyword [en]
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-33972DOI: 10.1063/1.3559732ISI: 000290047000118ScopusID: 2-s2.0-79955710753OAI: diva2:419955
QC 20110530Available from: 2011-05-30 Created: 2011-05-23 Last updated: 2011-11-22Bibliographically approved
In thesis
1. Ferromagnetic resonance in films with growth induced anisotropy
Open this publication in new window or tab >>Ferromagnetic resonance in films with growth induced anisotropy
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis discusses two different magnetic materials: epitaxial yttrium iron garnet (YIG) and heteromorphous CoFeB-SiO2 films.

YIG films were grown by pulse laser deposition (PLD) techniques onto gadolinium gallium garnet (GGG) substrates of (111) and (001) crystal orientations. Using stoichiometric and overstoichiometric ablative targets, we developed two types of YIG submicron films. The films grown from overstoichiometric targets have magnetic properties slightly different from standard liquid phase epitaxy (LPE) YIGs. They also demonstrate good substrate matching and approximately 6% nonstoichiometry. In contrary, films grown from stoichiometric targets posses surprisingly high values of uniaxial anisotropy, meanwhile cubic anisotropy is reduced several times. These films also reveal strong lattice distortions and nonstoichiometry around 17%.

Employing Weiss molecular field theory and single-ion anisotropy model we determined the preferential occupancy of the octahedral [a] positions in the YIG cubic lattices by Fe3+ vacancies. The vacancies were found to be preferentially oriented along the growth direction perpendicular to the film surface. We called this effect “deformation blockade”.

Different magnetostatic surface wave (MSSW) filters were also demonstrated. The filters employ high uniaxial anisotropy in YIG submicron films with magnetic losses ΔH ~ 1 Oe.

 Heteromorphous CoFeB-SiO2 films were deposited onto glass substrates employing carrousel magnetron sputtering. This novel technique allows amorphous films fabrication with record high in-plane anisotropy. The induced anisotropy fields here are approximately dozen times greater the values achieved using conventional growth technique when external bias field is applied during deposition process.

Interesting observations were made studying CoFeB-SiO2 magnetization dynamics in the wide frequency range from 500 kHz up to 15 GHz.  Two different anomalies of the magnetic susceptibility were found at the field of in-plane anisotropy Hp and critical field Hcr (0 < Hcr < Hp). We explained the anomalies appearance by sequence of the domain walls transformations so that Néel-Bloch-Néel domain wall transition stands for the instability at H = ±Hcr and transition from the uniformly magnetized state to the domain state with Néel domain wall and vice versa is responsible for the instability at H = ±Hp.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xv, 114 p.
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:16
ferromagnetic resonance, magnetic anisotropy, single-ion anisotropy model, ferrites, magnetic epitaxial films, magnetostatic waves, pulsed laser deposition, magnetic alloys, domains, domain wall transformations
National Category
Condensed Matter Physics Other Electrical Engineering, Electronic Engineering, Information Engineering Other Materials Engineering
urn:nbn:se:kth:diva-48248 (URN)978-91-7501-193-6 (ISBN)
Public defence
2011-12-14, Ka-C2, KTH-Electrum, Isafjordsgatan 26, Kista, 10:15 (English)
Swedish Research Council
QC 20111122Available from: 2011-11-22 Created: 2011-11-16 Last updated: 2011-11-22Bibliographically approved

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