There is currently much interest in the development of 'spintronic' devices, in which harnessing the spins of electrons (rather than just their charges) is anticipated to provide new functionalities that go beyond those possible with conventional electronic devices. One widely studied example of an effect that has its roots in the electron's spin degree of freedom is the torque exerted by a spin-polarized electric current on the spin moment of a nanometre-scale magnet. This torque causes the magnetic moment to rotate at potentially useful frequencies. Here we report a very different phenomenon that is also based on the interplay between spin dynamics and spin-dependent transport, and which arises from unusual diode behaviour. We show that the application of a small radio-frequency alternating current to a nanometre-scale magnetic tunnel junction can generate a measurable direct-current (d.c.) voltage across the device when the frequency is resonant with the spin oscillations that arise from the spin-torque effect: at resonance (which can be tuned by an external magnetic field), the structure exhibits different resistance states depending on the direction of the current. This behaviour is markedly different from that of a conventional semiconductor diode, and could form the basis of a nanometre-scale radio-frequency detector in telecommunication circuits.

Magnetoresistance (MR) ratio up to 230% at room temperature (294% at 20 K) has been observed in spin-valve-type magnetic tunnel junctions (MTJs) using MgO tunnel barrier layer fabricated on thermally oxidized Si substrates. We found that such a high MR ratio can be obtained when the MgO barrier layer was sandwiched with amorphous CoFeB ferromagnetic electrodes. Microstructure analysis revealed that the MgO layer with (001) fiber texture was realized when the MgO layer was grown on amorphous CoFeB rather than on polycrystalline CoFe. Since there have been no theoretical studies on the MTJs with a crystalline tunnel barrier and amorphous electrodes, the detailed mechanism of the huge tunneling MR effect observed in this study is not clear at the present stage. Nevertheless, the present work is of paramount importance in realizing high-density magnetoresistive random access memory and read head for ultra high-density hard-disk drives into practical use.

When an electric current passes from one ferromagnetic layer via a non-magnetic layer into another ferromagnetic layer, the spin polarization and subsequent rotation of this current can induce a transfer of angular momentum that exerts a torque on the second ferromagnetic layer 1-4 . This provides a potentially useful method to reverse 3,5-7 and oscillate 8 the magnetic momenta in nanoscale magnetic structures. Owing to the large current densities required to observe spin-torqueinduced magnetization switching and microwave emission (∼10 7 A cm −2 ), accurately measuring the strength, or even the direction, of the associated spin torque has proved difficult. Yet, such measurements are crucial to refining our understanding of the mechanisms responsible and the theories that describe them 9,10 . To address this, we present quantitative experimental measurements of the spin torque in MgO-based magnetic tunnel junctions 11-14 for a wide range of bias currents covering the switching currents. The results verify the occurrence of two different spin-torque regimes with different bias dependences that agree well with theoretical predictions 10 .Magnetic tunnel junctions (MTJs) consisting of a MgO insulating layer sandwiched between two ferromagnetic layers (S 1 and S 2 in Fig. 1a) were used to provide very large magnetoresistance 11,14 . Such MTJs are now useful as data storage cells in magnetic random-access memories (M-RAMs) and as magnetic-field sensors in magnetic hard disk drives [11][12][13] . The MTJs with a layer structure of Ir-Mn/Co-Fe/Ru/Co 60 Fe 20 B 20 /MgO/Co 60 Fe 20 B 20 were prepared on a MgO substrate using an ultrahigh-vacuum sputtering system (C-7100; Canon ANELVA). The 3-nm-thick bottom Co-Fe-B layer (S 1 ) acts as a spin polarizer. The top Co-Fe-B layer (S 2 ), a 2-nm-thick free layer, is excited by the spin torque. The MgO tunnel barrier is about 1 nm thick. The MTJs are rectangular with dimensions of approximately 70 nm × 250 nm (see the Methods section for preparation details).Resistance-magnetic-field (R-H ) curves measured at a small bias voltage (0.1-0.3 mV) and different in-plane field directions, that is, θ H = 0 and 45 • , are shown in Fig. 1b. θ H is the angle between the applied field direction and the easy axis of the magnetic cell along the long axis of the rectangular cell (see Fig. 1a). The magnetoresistance ratio is defined as MR = (R AP − R P )/R P , where R P and R AP respectively represent resistance in the parallel and antiparallel magnetization alignments of S 1 and S 2 . A positive bias current denotes electron flow from S 2 to S 1 . The magnetoresistance ratio and R P at a small bias voltage are, respectively, 154% and about 120 (R P × (Junction area) = 2 µm 2 ). Figure 1c shows the bias voltage, V b , dependence of the tunnelling resistance, as measured in four different fields (A-D), which are indicated by arrows in Fig. 1b. For antiparallel alignment (curves A and B), the resistance decreases with increasing V b because new tunnelling channels open at higher bias voltages 1...

Spin-momentum transfer between a spin-polarized current and a ferromagnetic layer can induce steady-state magnetization precession, and has recently beenproposed as a working principle for ubiquitous radio-frequency devices for radar and telecommunication applications. However, to-date, the development of industrially attractive prototypes has been hampered by the inability to identify systems which can provide enough power. Here, we demonstrate that microwave signals with device-compatible output power levels can be generated from a single magnetic tunnel junction with a lateral size of 100 nm, seven orders of magnitude smaller than conventional radio-frequency oscillators. We find that in MgO magnetic tunnel junctions the perpendicular torque induced by the spin-polarized current on the local magnetization can reach 25% of the in-plane spin-torque term, while exhibiting a different bias-dependence. Both findings contrast with the results obtained on all-metallic structures -previously investigated -, reflecting the fundamentally different transport mechanisms in the two types of structures.Mizuguchi for discussions.

Shifting electrically a magnetic domain wall (DW) by the spin transfer mechanism [1-4] is one of the future ways foreseen for the switching of spintronic memories or registers [5,6]. The classical geometries where the current is injected in the plane of the magnetic layers suffer from a poor efficiency of the intrinsic torques [12,13] acting on the DWs. A way to circumvent this problem is to use vertical current injection [7,8,11]. In that case, theoretical calculations [9] attribute the microscopic origin of DW displacements to the out-of-plane (field-like) spin transfer torque [17,18]. Here we report experiments in which we controllably displace a DW in the planar electrode of a magnetic tunnel junction by vertical current injection. Our measurements confirm the major role of the out-of-plane spin torque for DW motion, and allow to quantify this term precisely. The involved current densities are about 100 times smaller than the one commonly observed with in-plane currents [10].Step by step resistance switching of the magnetic tunnel junction opens a new way for the realization of spintronic memristive devices [14][15][16].We devise an optimized sample geometry for efficient current DW motion using a magnetic tunnel junction with an MgO barrier sandwiched between two ferromagnetic layers, one free, the other fixed. Such junctions are already the building block of magnetic random-access memories (M-RAMs), which makes our device suitable for memory applications. The large tunnel magnetoresistance [19,20] allows us to detect clearly DW motions when they propagate in the free layer of the stack [21]. The additional advantage of magnetic tunnel junctions is that the out-of-plane field-like torque T OOP can reach large amplitudes, up to 30% of the classical in-plane torque T IP [22,23], in contrast to metallic spin-valve structures, in which the out-of-plane torque is only a few % of the in-plane torque [24,25]. This is of fundamental importance since theoretical calculations predict that, when the free and reference layers are based on materials with the same magnetization orientation (either inplane or perpendicular), the driving torque for steady domain wall motion by vertical current injection is the OOP field-like torque [9]. Indeed, T OOP is equivalent to the torque of a magnetic field in the direction of the reference layer, that has the proper symmetry to push the DW along the free layer. On the contrary, the inplane torque T IP can only induce a small shift of the DW of a few nm. In magnetic tunnel junctions with the same composition for the top free and bottom reference layers, the OOP field-like torque exhibits a quadratic dependence with bias [22,23], which could not allow us to reverse the DW motion by current inversion. Therefore we use asymmetric layer composition to obtain an asymmetric OOP field-like torque [26,27].The magnetic stack is sketched in Fig.