Temperature dependence of microwave voltage emission associated to spin-transfer induced vortex oscillation in magnetic tunnel junction

Abstract

The temperature dependence of a vortex-based nano-oscillator induced by spin transfer torque (STVO) in magnetic tunnel junctions (MTJ) is considered. We obtain emitted signals with large output power and good signal coherence. Due to the reduced non-linearities compared to the uniform magnetization case, we first observe a linear decrease of linewidth with decreasing temperature. However, this expected behavior no longer applies at lower temperature and a bottom limit of the linewidth is measured.PACS numbers: 75.40.Gb, 85.30.Mn Keywords: magnetic devices, spin-transfer-torque, tunnel magnetoresistance, magnetic vortex dynamics A spin polarized current can exert a large torque on the magnetization of a ferromagnet through a transfer of spin angular momentum 1 . This mechanism offers a new method to manipulate a magnetization, and potentially a stable precession can be reached 2 . By converting such dynamics into a high frequency voltage oscillation through the magnetoresistance effect, the concept of spin transfer-torque nano-oscillator (STNO) has been proposed as a promising device for new technological applications in IC-Technologies. However, despite compelling progresses 3-6 , present challenges remain to both increase the output power and improve the coherence of the emitted signal. Recently, we demonstrated 7,8 that, by considering a magnetic tunnel junction (MTJ) with a vortex ground state (free layer), the so-called spin transfer-torque vortex oscillator (STVO), we obtain large integrated power (P int ≃ 5 nW) for a small linewidth, i.e., full width at half maximum, ∆f < 1 MHz. However, although ∆f is considerably reduced compared to the uniform magnetization case, a major issue is still its origin, and consequently the origin of the phase noise.When the efficiency of the spin transfer-torque induced by I dc exceeds a critical value, the vortex core starts to precess and eventually reaches a stable gyrotropic motion. The frequency of such gyrotropic vortex mode 9 is well-separated from the frequencies of others modes (radial and azimuthal spin waves). This fact, in principle, should allow to avoid the excitation of multiple modes and simplify the shape of the oscillation peak, i.e., reduces ∆f 10 . From non-linear oscillation models 11,12 , ∆f is expected to be proportional to the temperature T and to the nonlinearity of the system, i.e.,Here, ν ∝ N , which is the non-linear frequency shift a) Corresponding author. Electronic address: paolo.bortolotti@thalesgroup.com coefficient N = df /dp, Γ G is the intrinsic damping coefficient and E(p) represents the energy of the autooscillation with power p. In the case of gyrotropic vortex oscillators, N is weak compared to MTJs with uniform magnetization 4 . Hence, we expect a quasi-linear dependence of ∆f with temperature.The samples are circular MTJs of 300 nm diameter, with a 10 nm NiFe thick free layer and a synthetic antiferromagnet (SAF) that acts as in-plane uniformly magnetized spin polarizer. The complete structure (with thickness in nm) is: PtMn(15) / C...

Spin transfer nano-oscillators

“…29,62,91 While micromagnetic simulations can reproduce also quantitatively the experimental linewidth, 53 it is worth to note that the theory developed in Ref. 78 cannot be directly applied and research towards generalizing that model will be very useful for a better understanding of the dynamical properties of the vortex based STNOs.…”

Spintronics technology and device development

“…This phenomenon is known as self-excited oscillation and is also called microwave oscillation because the characteristic oscillation frequency is near the GHz band. 14,15) …”

Designing Large Arrays of Interacting Spin-Torque Nano-Oscillators for Microwave Information Processing

“…Furthermore, it is important to check that the mutual coupling between oscillators, which naturally arises when they are electrically connected or closely packed, does not compromise their individual response [15][16][17]. In this work, our focus is on oscillators with a vortex in the free layer as their properties are well described and understood [18][19][20]. We analytically derive design rules to build large arrays of uncoupled vortex spin-torque nanooscillators with equally spaced frequencies and equal frequency sensitivity that can process microwave inputs on a wide frequency range.…”