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.

Ferrimagnetic Mn 3 Ga exhibits a unique combination of low saturation magnetization (M s = 0.11 MA m −1 ) and high perpendicular anisotropy with a uniaxial anisotropy constant of K u = 0.89 MJ m −3 . Epitaxial c-axis films exhibit spin polarization as high as 58%, measured using point-contact Andreev reflection. These epitaxial films will be able to support thermally stable sub-10-nm bits for spin-transfer torque memories. 14 The magnetization of these films lies in plane, but tetragonally distorted compounds could offer high perpendicular anisotropy, necessary for thermally stable sub-10-nm tunnel junctions and spin valves. Here we present epitaxially grown tetragonal Mn 3 Ga films, which exhibit a spin polarization of up to 58%, together with uniaxial anisotropy (K 1 = 0.89 MJ m −3 ) and low magnetization (M s = 0.11 MA m −1 ), a combination of properties that may be ideal for tiny perpendicular spin-torque switchable elements that will allow for scalable magnetic memory and logic.The general composition of the L2 1 cubic Heusler alloys is X 2 YZ. In the perfectly ordered state, illustrated in Fig. 1(a), the Y and Z atoms occupy two interpenetrating face-centeredcubic lattices, where each is octahedrally coordinated by the other, and the X atoms form a simple cubic lattice, where they are tetrahedrally coordinated by both Y and Z atoms, at the corners of a cube. The X-Y, X-X, and Y-Y bond lengths are √ 3a 0 /4, a 0 /2, and a 0 / √ 2, respectively, where a 0 is the cubic lattice parameter. The material we discuss here, Mn 3 Ga, forms two stable crystal structures. The high-temperature hexagonal D0 19 phase is a triangular antiferromagnet, easily obtained by arc melting. 15,16 The tetragonal D0 22 phase is a ferrimagnet, usually obtained by annealing the hexagonal material at 350-400• C for 1-2 weeks. 15,17,18 The spin polarization at the Fermi level has been calculated to be 88% for the tetragonal phase, which has been suggested as a potential material for spintransfer-torque (STT) applications. 19 To this end, thin films with appropriate magnetic properties were needed.The D0 22 structure is a highly distorted tetragonal variant of the L2 1 Heusler unit cell, which has been stretched by ∼27% along the c axis. The unit cell is outlined in Fig. 1(a) by the red (solid) line; the lattice parameters of the unit cell (space group I4/mmm), which contains two Mn 3 Ga formula units, are a = 394 pm and c = 710 pm. As a result of the tetragonal structure, both 4d tetrahedral X sites and 2b octahedral Y sites are subject to strong uniaxial ligand fields, which lead to uniaxial anisotropy at both Mn positions. Imperfect atomic order results in some Mn population of the 2a Z sites, which are similarly distorted.Generally, the Mn moment and Mn-Mn exchange in metallic alloys are very sensitive to the interatomic distances. Widely spaced Mn atoms with a bond length 290 pm tend to have a large moment, of up to 4 μ B , and couple ferromagnetically. 20Nearest-neighbor Mn atoms have much smaller moments, and couple antiferromagnet...

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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...