Temperature-dependent relaxation of dipole-exchange magnons in yttrium iron garnet films

Abstract

Low energy consumption enabled by charge-free information transport, which is free from ohmic heating, and the ability to process phase-encoded data by nanometer-sized interference devices at GHz and THz frequencies are just a few benefits of spin-wave-based technologies. Moreover, when approaching cryogenic temperatures, quantum phenomena in spin-wave systems pave the path towards quantum information processing. In view of these applications, the lifetime of magnonsspin-wave quanta-is of high relevance for the fields of magnonics, magnon spintronics and quantum computing. Here, the relaxation behavior of parametrically excited magnons having wavenumbers from zero up to 6 · 10 5 rad cm −1 was experimentally investigated in the temperature range from 20 K to 340 K in single crystal yttrium iron garnet (YIG) films epitaxially grown on gallium gadolinium garnet (GGG) substrates as well as in a bulk YIG crystal-the magnonic materials featuring the lowest magnetic damping known so far. As opposed to the bulk YIG crystal in YIG films we have found a significant increase in the magnon relaxation rate below 150 K-up to 10.5 times the reference value at 340 K-in the entire range of probed wavenumbers. This increase is associated with rare-earth impurities contaminating the YIG samples with a slight contribution caused by coupling of spin waves to the spin system of the paramagnetic GGG substrate at the lowest temperatures.The fields of spintronics and magnonics promote the realization of faster data processing technologies with lower energy dissipation by complementing or even replacing electron charge-based technologies with spin degree of freedom based devices [1][2][3]. Simultaneously, novel fascinating magnetic phenomena-such as, e.g., room-temperature Bose-Einstein magnon condensates [4-6], magnon vortices [7] and supercurrents [8][9][10][11]open a whole new range of research areas [3,12] both for basic and applied spin physics. For these purposes many novel materials have been designed and investigated [13][14][15] whereupon one of the most outstanding ones so far is the insulating ferrimagnet yttrium iron garnet (Y 3 Fe 5 O 12 , YIG).Since its discovery in 1956, YIG has served as a prime example material for its microwave, optical, acoustic, and magneto-optical properties [16] in a wide range of experiments and applications. Nowadays, single crystal YIG films epitaxially grown on gadolinium gallium garnet (Gd 3 Ga 5 O 12 , GGG) substrates [17-19] dominate in theoretical and experimental studies [5-8, 20-24]. Their pertinence ranges from building of devices like microwave YIG oscillators, filters, delay lines, phase shifters, etc. [25] up to the latest high-profile research as in magnonics [26, 27], spintronics [1, 28] and quantum computing [29,30]. Consequently it has become clear that a deep understanding of the magnetic damping properties, determining the magnon lifetimes, is of crucial importance throughout these fields. Given its high Curie temperature at 560 K, YIG is applicable at ambient temperatures, where ...

Microwave magnon damping in YIG films at millikelvin temperatures

“…However, in the case of YIG films grown on GGG, the story is more complex. GGG is a geometrically frustrated magnetic system [22] and it has long been known that at temperatures below 70 K, it exhibits paramagnetic behaviour that has been reported to increase damping in films grown on its surface [23][24][25]. The behaviour of GGG at mK temperatures is yet to be thoroughly characterised [26][27][28], but recent results at mK temperatures have suggested that magnon damping in YIG films grown on GGG is higher than expected if the properties of the YIG system alone are considered arXiv:1903.02527v3 [cond-mat.mes-hall] 25 Oct 2019 Dashed lines are linear fits, the details of these fits are summarised in table I.…”

Magnon Bose–Einstein Condensate and Supercurrents Over a Wide Temperature Range

“…The similar temperature behavior of the magnon BEC and parametrically injected magnons brings hope that an extremely long BEC lifetime can be achieved in ultra pure bulk YIG samples, where, as it has been found in Refs. [21,39], the relaxation rate of parametric magnons monotonically decreases from room to cryogenic temperatures.…”

“…Here, the dimensionless data from Ref. [21] were scaled to coincide with the BEC relaxation rate at one temperature point T = 340 K. As is clearly seen from Fig. 4, the relaxation rates of both groups of magnons have the same qualitative and quantitative temperature dependence.…”

“…4). This fact can be understood as the manifestation of a slightly weaker coupling of the condensed short-wavelength dipoleexchange magnons (BEC wavenumbers q BEC ≈ ±4 · 10 4 rad cm −1 ) [3] to the paramagnetic spin structure of the GGG substrate in comparison with such a coupling of the rather long-wavelength dipolar magnons (q → 0) parametrically excited and characterized in work [21]. This coupling is expected to be smaller in a thicker film than in a thinner one, and, thus, the BEC relaxation rates in the thinner film come closer to the relaxation of long-wavelength parametric magnons in the thicker one.…”

“…Here γ = 2π · 2.8 MHz/Oe is the modulus of the gyromagnetic ratio. Points -damping of magnon BEC, measured in this work; linesdamping of parametrically injected magnons in YIG films of two different thicknesses (from Ref [21]…”