Fine tuning of the Magnetic Anisotropy results in Temperature Independent Ferromagnetic Resonance Frequency for Bi-YIG thin films grown by Pulsed Laser Deposition

Abstract

Yttrium Iron Garnet (YIG) is the benchmark material for ferromagnetic resonance; it is up to today the only magnetic material that is being integrated in on-shell electronic devices for its radiofrequency properties that combine high resonance quality factor (104) and frequency tunability. It is the goal of YIG magnonics to utilize this potential for a large variety of applications ranging from beyond complementary metal-oxide semiconductor (CMOS) computation to radiofrequency front-end and back-end analogue signal processing. YIG devices have however a severe drawback that is that they should include thermal stabilization. This is a necessity since YIG has a relatively low Curie temperature (∼550K) which results in a high sensitivity of its magnetization to temperature (∼4 Gauss/Kelvin). Here, we present a novel approach to solve this long standing issue by engineering the magnetic anisotropy in Bi substituted YIG. We evidence for those films that a vanishing effective magnetization yields high thermal stability over very wide temperature range: from 260K to 400K, where the frequency thermal drift is 50 times smaller than that of YIG. These types of properties can be leveraged for new applications where the temperature dependence of YIG is detrimental, while keeping the extremely low damping and insulating character of YIG[1]. For instance, this was key for the excitation of coherent spin-waves using SOT[2]. We anticipate that fabricating a material whose FMR frequency is temperature independent open new opportunities for the field of spintronics.In his seminal article[3] of 1948 Charles Kittel established the Ferromagnetic resonance (FMR) conditions of magnetic samples with arbitrary shapes stressing the importance of correcting the effective magnetic field that sets the Larmor frequency by taking into account the demagnetization field and the effect of uniaxial anisotropy. In today notations the FMR frequency can hence be written for a thin film in the in-plane magnetization configuration as: f=γ*(Ha*(Ha+(MS-Hu)))1/2. Where γ is the gyromagnetic factor, Ha is the applied magnetic field, MS is the saturation magnetization and Hu is the out-of-plane magnetization and the effective magnetization is defined as Meff = MS - Hu. It arises from this equation that the only temperature dependent terms are MS(T) and Hu(T). For most ferromagnetic materials Hu depends on MS with power law dependence defined by Callen and Callen theory[4]. Nonetheless, by engineering the magnetic anisotropy in Bi-YIG films, we were able to balance the saturation magnetization with the uniaxial anisotropy between 260K and 400K. These measurements were done on thin Bi1Y2Fe5O12 films of 21nm grown on (111)-oriented sGGG substrates with Pulsed Laser Deposition. We varied the substrate growth temperature within a range of 100°C and probed the films effective magnetization using FMR: we evidenced a magnetization state transition from in-plane to out-of-plane as the substrate growth temperature decreases from 500°C to 400°C. To ensure the nominal Bi content as well as the magneto-elastic anisotropy were maintained over all the growth temperature range, X-Ray Diffraction measurements were done and evidenced a perfect structural match between thin Bi1Y2IG films. Magneto-Optical Kerr Effect (MOKE) measurements were performed complementary to visualize not only the magnetic domains but also the magnetic anisotropy transition. With room temperature FMR measurements, we obtained the optimal substrate growth temperature range to be 420°C to 430°C: temperatures for which the effective magnetization Meff ≈ 0G. Interestingly, for thin films closest to the compensation, FMR measurements performed at lower temperatures evidenced that effective magnetization stay constant in a range of 140K: suggesting Hu(T)=Ms(T) contradictory with Callen and Callen theory[4]. Finally we measured the frequency thermal drift was about 1mT between 260K and 400K for films close to the compensation, FMR frequency is temperature independent. **