Manipulation of the magnetization of a perpendicular ferromagnetic free layer by spin-orbit torque (SOT) is an attractive alternative to spin-transfer torque (STT) in oscillators and switches such as magnetic random-access memory (MRAM) where a high current is passed across an ultrathin tunnel barrier. A small symmetry-breaking bias field is usually needed for deterministic SOT switching but it is impractical to generate the field externally for spintronic applications. Here, we demonstrate robust zero-field SOT switching of a perpendicular CoFe free layer where the symmetry is broken by magnetic coupling to a second in-plane exchange-biased CoFe layer via a nonmagnetic Ru or Pt spacer. The preferred magnetic state of the free layer is determined by the current polarity and the sign of the interlayer exchange coupling (IEC). Our strategy offers a potentially scalable solution to realize bias-field-free switching that can lead to a generation of SOT devices, combining a high storage density and endurance with a low power consumption.

Cubic Mn 2 Ga films with the half-Heusler C1 b structure are grown on V (001) epitaxial films. The phase is a soft ferrimagnet, with Curie temperature T C ¼ 225 K and magnetization M s ¼ 280 kA m −1 , equivalent to 1.65 μ B per formula. Adding ruthenium leads to an increase of T C up to 550 K in cubic Mn 2 Ru x Ga films with x ¼ 0.33 and a collapse of the net magnetization. The anomalous Hall effect changes sign at x ¼ 0.5, where the sign of the magnetization changes and the magnetic easy direction flips from in plane to perpendicular to the film. The Mn 2 Ru 0.5 Ga compound with a valence electron count of 21 is identified as a zero-moment ferrimagnet with high spin polarization, which shows evidence of half-metallicity. DOI: 10.1103/PhysRevLett.112.027201 PACS numbers: 75.30.Gw, 75.60.Ej, 75.70.-i Cubic ferrimagnetic Heusler compounds are a rich family of magnetic materials [1], that can exhibit a higher spin polarization at the Fermi level than any binary 3d ferromagnetic alloy. Several Co 2 YZ compounds, for example, are thought to be half-metals with a gap in the spin-polarized density of states for one spin direction [1][2][3], which makes them particularly suitable for spin valves [4,5] and magnetic tunnel junctions [6,7]. The net spin in Bohr magnetons per unit cell should be an integer for a stoichiometric half-metal, and the moments m of X 2 YZ half-metals with the L2 1 structure are found to follow a modified Slater-Pauling curve [8].where N v is the number of valence electrons per formula. The ordered L2 1 structure illustrated in Fig. 1 Fig. 1(b)], but a variant has X and Y atoms ordered on 8c sites (4c=4d), with 4b sites vacant. The modified Slater-Pauling rule is thenThe original example of a half-metal was NiMnSb (m ¼ 4 μ B ) [12,13].When X and Y atoms carry a moment and the X − Y coupling is antiferromagnetic, the compounds are ferrimagnets. Half-metallic full Heuslers with N v ¼ 24 and half Heuslers with N v ¼ 18 are particularly interesting, because the ferrimagnetism should then be perfectly compensated, with net spin m ¼ 0 μ B . At first, such a metal with perfect spin polarization was termed a "half-metallic antiferromagnet," although the two sublattices are strictly inequivalent, structurally and often chemically. If it existed, it could be useful because the material would be fully spin polarized, yet would be free from shape anisotropy and create no stray field [14].Although there have been numerous electronic structure calculations for hypothetical compounds with perfectly compensated ferrimagnetism [1,[14][15][16][17][18][19], all attempts to make them in practice have failed [14]. Nature seems to shun this unusual variety of magnetic order. Half-metallicity is a zero-temperature property, which can be spoiled by imperfect atomic order of the X, Y, and Z atoms in the structure, spin-orbit coupling, or finite temperature, either due to smearing of the Fermi surface or to the different temperature dependence of the magnetization of the two sublattice magnetizations [16]. Candidate ma...