Bolometers are rectification devices that convert electromagnetic waves into direct current voltage through a temperature change. A superconducting bolometer has a responsivity of approximately 106–107 V/W under cryogenic temperatures at infrared wavelengths; however, no devices have realized such a high responsivity in the sub-GHz frequency region. We describe a spin bolometer with a responsivity of (4.40 ± 0.04) × 106 V/W in the sub-GHz region at room temperature using heat generated in magnetic tunnel junctions through auto-oscillation. We attribute the unexpectedly high responsivity to a heat-induced spin-torque. This spin-torque modulates and synchronizes the magnetization precession due to the spin-torque auto-oscillation and produces a large voltage output. In our device, heat-induced spin-torque was obtained because of a large heat-controlled magnetic anisotropy change: −2.7 µJ/Wm, which is significant for enhancing dynamic range and responsivity. This study can potentially lead to the development of highly sensitive microwave detectors in the sub-GHz region.
We studied nonlinear magnetic anisotropy changes to the DC bias voltage of magnetic tunnel junctions (MTJs) with capping layers of different thermal resistances. We found that increasing the thickness of MgO capping layers (in the range 0.3–0.5 nm) in MTJs enhances the Joule heating-induced magnetic anisotropy change, which indicates an enhancement of the interfacial thermal resistance at the FeB|MgO capping layer interface. This enhanced interfacial thermal resistance may be attributed to roughness at the FeB|MgO interface. Moreover, we observed a larger power-driven magnetic anisotropy change of 3.21 µJ W−1m−1 in the MTJ with a composite MgO (0.3 nm)|W (2 nm)|MgO (0.4 nm) capping layer. This research supports methods of efficient spin manipulation of spintronic devices such as microwave devices and magnetic memories.
In this study, bi-stable toggle magnetization switching in magnetic tunnel junctions induced by electrically injected sub-nanosecond unipolar heat pulses was demonstrated. The switching probability of magnetization between bi-stable states was estimated by applying 0.5 ns voltage pulses and measuring the perpendicular component of the magnetization direction. The maximum switching probability was approximately 70 %, suggesting that bi-stable toggle magnetization switching was induced by the torque created by the Joule-heat-induced magnetic anisotropy change. Joule-heat-driven magnetization switching has the potential to become a fundamental technology for fast spin control.
We investigated the heat controlled magnetic anisotropy (HCMA) in magnetic tunnel junctions with various junction sizes. We evaluated the HCMA from perpendicular magnetic anisotropy under a direct current voltage measured by the spin-torque diode technique. The maximum HCMA magnitude of 5.4 μJ (Wm)−1 was observed, and the HCMA increased with increasing diameter. Our results can be explained by a simple heat dissipation model and suggest that the in-plane heat current affects HCMA.
Quasi-maser operations using magnetic tunnel junctions (MTJs) that amplified the transmission of radio frequency (RF) signals through themselves are demonstrated. We measured the transmittance of heat-driven MTJs using a vector network analyzer and observed that transmittance S21 >1 at sub-GHz frequencies. Furthermore, we installed two of these MTJs in a feedback-loop circuit and investigated the RF signal in the circuit. The results showed that the auto-oscillation mode of the RF signal appeared without phase synchronization of each MTJ because of the amplification effect. Our results will help develop a mechanism for producing coherent microwave signals using multiple MTJs.
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