Roshnee Sahoo

Magnetic antiskyrmions above room temperature in tetragonal Heusler materials

Magnetic skyrmions are topologically stable, vortex-like objects surrounded by chiral boundaries that separate a region of reversed magnetization from the surrounding magnetized material. They are closely related to nanoscopic chiral magnetic domain walls, which could be used as memory and logic elements for conventional and neuromorphic computing applications that go beyond Moore’s law. Of particular interest is ‘racetrack memory’, which is composed of vertical magnetic nanowires, each accommodating of the order of 100 domain walls, and that shows promise as a solid state, non-volatile memory with exceptional capacity and performance. Its performance is derived from the very high speeds (up to one kilometre per second) at which chiral domain walls can be moved with nanosecond current pulses in synthetic antiferromagnet racetracks. Because skyrmions are essentially composed of a pair of chiral domain walls closed in on themselves, but are, in principle, more stable to perturbations than the component domain walls themselves, they are attractive for use in spintronic applications, notably racetrack memory. Stabilization of skyrmions has generally been achieved in systems with broken inversion symmetry, in which the asymmetric Dzyaloshinskii–Moriya interaction modifies the uniform magnetic state to a swirling state. Depending on the crystal symmetry, two distinct types of skyrmions have been observed experimentally, namely, Bloch and Néel skyrmions. Here we present the experimental manifestation of another type of skyrmion—the magnetic antiskyrmion—in acentric tetragonal Heusler compounds with D2d crystal symmetry. Antiskyrmions are characterized by boundary walls that have alternating Bloch and Néel type as one traces around the boundary. A spiral magnetic ground-state, which propagates in the tetragonal basal plane, is transformed into an antiskyrmion lattice state under magnetic fields applied along the tetragonal axis over a wide range of temperatures. Direct imaging by Lorentz transmission electron microscopy shows field-stabilized antiskyrmion lattices and isolated antiskyrmions from 100 kelvin to well beyond room temperature, and zero-field metastable antiskyrmions at low temperatures. These results enlarge the family of magnetic skyrmions and pave the way to the engineering of complex bespoke designed skyrmionic structures.

Compensated Ferrimagnetic Tetragonal Heusler Thin Films for Antiferromagnetic Spintronics

Fully compensated ferrimagnets with tetragonal crystal structure have the potential for large spin-polarization and strong out-of-plane magnetic anisotropy; hence, they are ideal candidates for high-density-memory applications. Tetragonal Heusler thin films with compensated magnetic state are realized by substitution of Pt in Mn3-x Ptx Ga. Furthermore, the bilayer formed from compensated/uncompensated Mn-Pt-Ga layers is utilized to accomplish exchange bias up to room temperature.

Structural, electronic, and magnetic properties of perpendicularly magnetised Mn2RhSn thin films

Epitaxial thin films of Mn2RhSn were grown on a MgO(0 0 1) substrate by magnetron co-sputtering of the constituents. An optimised range of temperature for heat treatment was used to stabilise the tetragonal structure and to prevent the capping Rh layer from diffusing into the Heusler layer. Electronic and magnetic properties were analysed by hard x-ray photoelectron spectroscopy as well as field- and temperature-dependent Hall and resistivity measurements. The measured valence spectra are in good agreement with the calculated density of states. The measured saturation magnetisation corresponds to a magnetic moment of 1 μB in the primitive cell. The magnetisation measurements revealed an out-of-plane anisotropy energy of 89 kJ m−3 and a maximum energy product of 3.3 kJ m−3. The magnetoresistance ratio is 2% for fields of 9 T. The lattice parameter of the compound has a very small mismatch with MgO, which makes it promising for coherent electron tunnelling phenomena.

Magnetic phase coexistence and metastability caused by the first-order magnetic phase transition in the Heusler compound Mn2PtGa

The Heusler compound Mn2PtGa exhibits a first-order ferrimagnetic (FI) to antiferromagnetic (AFM) phase transition, in contrary to the conventional martensitic structural transition displayed by their close relatives, the Heusler shape-memory alloys. With the help of isofield and isothermal magnetization experiments as well as magnetic relaxation measurements, we exemplify the presence of a magnetic-phase coexistence and a metastable magnetic behavior below the FI-AFM phase transition. Field cooling across the FI-AFM transition leads to a nucleation of a supercooled FI phase below the transition temperature, where the fraction of the supercooled phase depends upon the strength of the cooling field.

In-plane and out of plane magnetic properties in Ni46Co4Mn38Sb12 ribbons

Magnetic, magnetocaloric, and exchange bias properties have been systematically investigated in Ni46Co4Mn38Sb12 ribbon by applying magnetic field along in-plane (IP) and perpendicular out of plane (OP) to the ribbon plane. From the thermo-magnetization curves, the sharpness of the martensitic transition is observed to be nearly the same for both IP and OP ribbons. The thermomagnetic irreversibility region is found to be larger in the OP ribbon at 500 Oe, indicating that the magnetic anisotropy is larger in this case. The OP ribbon shows the Hopkinson maximum at 500 Oe, both for the field cooled cooled and zero field cooled modes. The magnetization curve for IP ribbon shows a faster approach to saturation, compared to the OP ribbon. Isothermal magnetic entropy change at 50 kOe has been found to be nearly same for both the ribbons. At 5 K, the coercivity and exchange bias values are larger for the OP ribbon. Crystallographic texturing of the ribbons and its effect in the easy magnetization direction are fou...

Titanium nitride as a seed layer for Heusler compounds

Titanium nitride (TiN) shows low resistivity at room temperature (27 μΩ cm), high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by x-ray diffraction and 4-terminal transport measurements. Element specific x-ray absorption spectroscopy revealed pure TiN inside the thin films. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode in magnetic tunnel junctions, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device and anomalous Hall effect for Mn2.45Ga. Magneto optical Kerr effect measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and square...

Heteroepitaxial growth of tetragonal Mn2.7−xFexGa1.3 (0 ≤ x ≤ 1.2) Heusler films with perpendicular magnetic anisotropy

This work reports on the structural and magnetic properties of Mn2.7−xFexGa1.3 Heusler films with different Fe content x (0 ≤ x ≤ 1.2). The films were deposited heteroepitaxially on MgO single crystal substrates, by magnetron sputtering. Mn2.7−xFexGa1.3 films with the thickness of 35 nm were crystallized in a tetragonal D022 structure with (001) preferred orientation. Tunable magnetic properties were achieved by changing the Fe content x. Mn2.7−xFexGa1.3 thin films exhibit high uniaxial anisotropy Ku ≥ 1.4 MJ/m3, coercivity from 0.95 to 0.31 T, and saturation magnetization from 290 to 570 kA/m. The film with Mn1.6Fe1.1Ga1.3 composition shows high Ku of 1.47 MJ/m3 and energy product (BH)max of 37 kJ/m3 at room temperature. These findings demonstrate that Mn2.7−xFexGa1.3 films have promising properties for mid-range permanent magnet and spintronic applications.

Enhanced magnetization and anisotropy in Mn-Ga thin films grown on LSAT

Epitaxial thin films of MnxGa1−x (x = 0.70, 0.74) grown on single crystal (LaAlO3)0.3(Sr2TaAlO6)0.7 [LSAT] substrates exhibit an enhanced magnetic moment and magnetic anisotropy in comparison to films of the same composition grown epitaxially on SrTiO3 [STO] single crystal substrates. Atomic and magnetic force microscopy revealed films exhibiting uniform grains and magnetic domain structures, with only minor differences between the films grown on different substrates. High resolution transmission electron microscopy on the x = 0.74 sample grown on LSAT showed a well-ordered, faceted film structure with the tetragonal c-axis oriented out of the film plane. Further, misfit dislocations, accommodating the lattice mismatch, were evidenced at the film/substrate interface. The out of plane c lattice parameter is larger for all x in the films grown on LSAT, due to the smaller substrate lattice parameter compared to STO. The increase in c generates a larger distortion of the tetragonal lattice which promotes the ...

Effect of annealing on the structural, microstructural and magnetic properties in Ni46Co4Mn38Sb12 ribbons

The effects of annealing on the structural, microstructural and magnetic properties of Ni46Co4Mn38Sb12 alloy ribbons have been investigated. It was observed that the room temperature structure changes from partially ordered B2 to fully ordered L21 phase upon annealing. At low temperatures both the as spun and the annealed ribbons exhibit 5M modulated martensitic structure. Significant changes in the martensitic transition and the magnetic properties in the annealed ribbon arise due to structural changes brought after annealing the ribbon.

Effect on magnetic and magnetocaloric properties with Ge substitution in Ni-Mn-Sb Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric and transport properties of Ni45Co5Mn38Sb12− xGex (x=0-3) has been investigated. The decrease in the exchange interaction brought by Ge substitution can be seen from the reduction in the magnetization of austenite phase and the increase in the martensitic transition temperature. Large magnetocaloric effect has been observed at room temperature, making it a potential material system for various applications.