Compensated Ferrimagnetic Tetragonal Heusler Thin Films for Antiferromagnetic Spintronics

Abstract

Spintronics is a large field of research that involves the generation, manipulation and detection of spin currents in magnetic heterostructures and the use of these currents to excite and to set the state of magnetic nano-elements. [1,2] The field of spintronics has focused on ferromagnetic thin film structures in which charge currents can be spin-polarized via interfacial and volume spin dependent scattering. However, ferromagnets produce magnetostatic dipole fields, which increase in size as devices are scaled to smaller dimensions. These must be minimized or eleminated to enable operation of spintronic field sensing and magnetic memories. [2] One way to do this is to take advantage of the phenomenon of long-range oscillatory interlayer coupling [3] to create synthetic antiferromagnetic heterostructures [1] or the use of ferrimagnetic materials such as rare-earthtransition metal alloys at their compensation point. [4] The latter ferrimagnetic materials are of special inerest because they can completely eliminate dipole fields volumetrically, even on the atomic scale. However, materials are needed that can operate over a wide temperature window and not solely at or in the vicinity of a compensation temperature. Here we show that Heusler compounds can be designed for this purpose.Heusler compounds, YZ X 2 (where X , Y are transition metals and Z is a main-group 2 element), are well known for their potential applications in spintronics, especially in spin-torque based devices. [5] These materials crystallize in both cubic and tetragonal crystal structures with multiple magnetic sub-lattices, and hence are good candidates for engineering a wide range of complex magnetic structures. In particular all the known tetragonal Heuslers are ferrimagnetic with at least two magnetic sub-lattices whose magnetizations are aligned anti-parallel to one another, or, as has been shown recently, can be non-collinear to one another. The Mn based tetragonal Heuslers are of particular interest because their magnetic ordering temperatures can be well above room temperature, but the magnetizations of their two sub-lattices are typically distinct, leading to a net uncompensated magnetization. One of the most interesting of these materials is Mn3Ga which has a Curie temperature of ~750 K. [6] By tuning the magnetization of the two sublattices in Mn3Ga by changes in composition, we propose that a fully compensated ferrimagnetic (CFI) Heusler is possible. To help identify the needed compositional variations we have carried out density functional calculations of the electronic structures of Mn3-xYxGa   1 0   x (at zero temperature) for the elements = Y Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au, which are non-magnetic or nearly non-magnetic when substituted into Mn3Ga. We show that in all these cases it is theoretically possible to obtain a CFI , which we have experimentally validated for the case of Y=Pt.The present calculation is based on the experimental lattice parameters of the bulk Mn 3 Ga lattice. [6] We find that a compensated mag...

Antiferromagnetic spintronics

“…The growth of novel materials with high spin polarization and low damping will also be an important step (Hu, 2012;Sahoo et al, 2016). A recent striking and stimulating example of how two fields of condensed matter physics may envision a common future is the prediction that Dirac quasiparticles [with the example of Dirac quasiparticles in the CuMnAs semimetal (Tang et al, 2016)] can be controlled by spin-orbit torque reorientation of the Néel vector in an antiferromagnet (Šmejkal et al, 2017).…”

Heusler 4.0: Tunable Materials

“…We have proposed that a fully compensated ferrimagnetic state with a zero net magnetic moment can be achieved by systematically tuning the sub-lattice magnetic moments in Mn3Ga (66). From theoretical calculations we have shown that the compensated magnetic state can be achieved for a wide range of Heusler materials by substituting a late transition metal element in Mn3−xYxGa including Ni, Cu, Rh, Pd, Ag, Ir, Pt and Au (67). A specific example of magnetic compensation when Y = Pt is shown in Figure 6 Figure 6 (a), with increasing Pt concentration, the total magnetic moment decreases and becomes zero at x = 0.6 before further increasing for higher Pt concentrations.…”

Magnetic antiskyrmions above room temperature in tetragonal Heusler materials

“…Indeed, Heusler compounds, X 2 YZ (where X, Y are transition metals and Z is a main-group element), are well known for their potential applications in spintronics, especially in tunneling based devices 26 . Mn 2 YZ based magnetic Heuslers provide a perfect platform for the design of anisotropic and acentric room-temperature magnets with flexible magnetic configurations [27][28][29] .…”