Bismuth-substituted yttrium iron garnets (Bi:YIG) have attracted recent interest due to the possibility of having coexisting perpendicular magnetic anisotropy (PMA) and
ultralow Gilbert damping (~10-4) [1–3]. The existence of ultralow damping, however, has so far been accompanied by the presence of substantial inhomogeneous broadening (~10–100 Oe). The relevant parameter for applications is the total ferromagnetic resonance (FMR) linewidth, and so it is of interest to minimize the inhomogeneous broadening as well as the damping. Here we demonstrate the presence of both ultralow damping (≤ 5 × 10-4) and low inhomogeneous broadening (≈ 10 Oe) in a series of epitaxial PLD-grown Bi:YIG/GSGG(111)films. However, achieving these low linewidths requires the magnetization to be approximately parallel to the [100] crystallographic direction (roughly 55 degrees relative to the film normal), as seen in the figure. It is extremely unusual to observe a minimum linewidth for intermediate angles—typically a sharp maximum is observed where the angle of the magnetization is most sensitive to changes in the angle of the applied field [4]. We show that for a film grown on a GGG(100) substrate, the perpendicular FMR linewidth at 20 GHz (≈ 30 Oe) is much smaller than for the films grown on (111) substrates (≥ 60 Oe). Furthermore, the minimum in the linewidth as a function of angle is not nearly as sharp and occurs at approximately 35 degrees relative to the film normal. For applications, it is crucial for the FMR linewidth to be small for perpendicular magnetization. We discuss potential causes for the anomalous angular dependence of the FMR linewidth and how to minimize it for perpendicular magnetization. This work was supported by SMART, a center funded by nCORE, a Semiconductor Research Corporation program sponsored by NIST.
**References** 
[1] Soumah et al., Nat. Comm. 9, 3355 (2018).
[2] Lin et al., JMMM 496, 165886 (2020).
[3] Caretta et al., Science 370, 1438 (2020).
[4] Woltersdorf and Heinrich, Phys. Rev. B 69, 184417 (2004). 

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Full-width-at-half-maximum FMR linewidth as a function of magnetization angle θ relative to the film normal for a 70 nm Bi:YIG/GSGG(111) film. Angles corresponding to [111] and [100] crystallographic directions are indicated

Anomalous angular dependence of ferromagnetic resonance linewidths in perpendicular Bi:YIG films with ultralow damping

Ferrimagnetic rare earth iron garnets (X3Fe5O12, where X denotes a rare earth element) are promising materials for various applications, such as magneto-optical (MO) isolators, spintronic and spin wave devices. The three types of cation sites (dodecahedral, octahedral and tetrahedral) can be occupied by a wide range of ions resulting in a large variation of the physical properties of the garnet. This can be utilized to tailor its MO response or magnetic properties. 

To determine the contribution of each sublattice to the MO response and therefore to characterize sublattice magnetism, an analysis of spectrally dependent MO hysteresis loops can be used [1]. For this purpose, we measured field dependent polar MO Kerr effect (MOKE) spectra in the range from 1.4 eV to 4.5 eV of various ferrimagnetic garnet thin films, such as terbium iron garnet (TbIG) or yttrium iron garnet (YIG), prepared by pulsed laser deposition on GGG substrates. This allowed us to extract MOKE hysteresis loops with very high spectral resolution. The resulting hysteresis loops exhibited spectrally dependent shapes and were modelled as a sum of two hysteresis loops, shown for 300 nm thick epitaxial TbIG in Fig. 1. Moreover, their contributions to the MOKE spectra were separated as shown in Fig. 2. These results suggest that rather than sublattice contributions this analysis led to the identification of components with two different magnetic anisotropies within this particular film. The TbIG has a strain-induced perpendicular magnetic anisotropy when grown on GGG. This anisotropy, however, may not be retained for thicker samples [2] due to strain relaxation leading to an evolution of magnetic anisotropy across the layer. These results can be compared with those on YIG films, where the spectral response of the octahedral and tetrahedral Fe sublattices can be separated. This work demonstrates field dependent MOKE spectra as an effective tool to separate not only sublattice contributions but also an evolution of magnetic anisotropy across the film. 
**References:** [1] M Deb, E Popova, A Fouchet and N Keller, J. Phys. D: Appl. Phys. 45 455001 (2012) 
[2] V. H. Oritz, M. Aldosary, J. Li, et al. APL Materials 6, 121113 (2018) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/8e25eafd35dbf5ee3c5905bdddbb2577.png) 
Fig. 1 Separation of MOKE hysteresis loop of 300 nm thick TbIG film on GGG 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0a79af8640ed78fe22f84f65879730ca.png) 
Fig. 2 Individual loop contributions to the MOKE spectra of 300 nm TbIG on GGG

Field Dependent Magneto Optical Spectra of Ferrimagnetic Garnets to Identify Magnetic Contributions

Iron-based garnet films that have large magneto-optical (MO) effect are a promising material for MO device applications. In particular, terbium iron garnet (TbIG) can crystallize directly on a Si substrate without garnet seed layers [1-4] and, therefore, recent investigations show quite practical optical isolators taking advantage of TbIG [1,2]. However, crystallization for TbIG requires high temperatures and it is desirable to limit the heating to those regions of the chip where devices are formed. In this study, we compare the structure, magnetic and MO properties of TbIG and Bi doped TbIG (Bi-TbIG) films processed by rapid thermal annealing (RTA) to those annealed locally using an embedded resistor with 40-μm-square area. TbIG (Tb2.77Fe5O12) and Bi-TbIG (Bi0.02Tb2.82Fe5O12) films are made by pulsed laser deposition at room temperature, which allows for liftoff patterning using a photoresist mask. The crystal structure for the films is characterized by micro-Raman spectroscopy and X-ray diffraction (XRD) and the magnetic properties by vibrating sample magnetometry. XRD shows single phase garnet after RTA 1073 – 1273 K. Raman spectra for Bi-TbIG film before/after 1173 K annealing and for the Si substrate are shown in Fig. 1. The Raman spectrum for the as deposited film is almost the same as that of the Si substrate while a Raman peak corresponding to the strongest peak of TbIG (Eg) is observed around 261 cm-1 in the annealed film. Raman spectroscopy enables detection of local crystallization in Bi-TbIG on embedded resistors. Figure 2 shows room temperature in-plane M-H loops of the annealed TbIG and Bi-TbIG films. The TbIG film is not saturated at 1 T while the Bi-TbIG is saturated at 0.7 T. Coercivity in TbIG rapidly increases in the vicinity of the compensation temperature (Tcomp) [3] which is ~250 K for bulk TbIG, and slightly lower for thin film TbIG with Tb deficiency [4]. Bi doping lowers (Tcomp) further [4] explaining the easier saturation of the Bi-TbIG film. The high figure of merit of Bi-TbIG and its higher remanence make this material attractive for integrated photonic devices.

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Crystallization of magneto-optical bismuth terbium iron garnet films

GOD: Magnetization Dynamics IV

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JPB: Ferrites, Garnets and Magneto-Dielectric Materials II

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**Session chair:** Shawn Pollard, The University of Memphis


FPD - Magnetooptics II

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Takian Fakhrul

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Anomalous angular dependence of ferromagnetic resonance linewidths in perpendicular Bi:YIG films with ultralow damping

Field Dependent Magneto Optical Spectra of Ferrimagnetic Garnets to Identify Magnetic Contributions

Crystallization of magneto-optical bismuth terbium iron garnet films

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