Topologically stable nontrivial spin structures, such as skyrmions and antiskyrmions, display a large topological Hall effect owing to their quantized topological charge. Here, we present the finding of a large topological Hall effect beyond room temperature in the tetragonal phase of a Mn-Ni-Ga based ferrimagnetic Heusler shape memory alloy system. The origin of the field induced topological phase, which is also evidenced by the appearance of dips in the ac-susceptibility measurements, is attributed to the presence of magnetic antiskyrmions driven by D 2d symmetry of the inverse Heusler tetragonal phase. Detailed micromagnetic simulations asserts that the antiskyrmionic phase is stabilized as a result of interplay among inhomogeneous Dzyaloshinskii-Moriya interaction, the Heisenberg exchange, and the magnetic anisotropy energy. The robustness of the present result is demonstrated by stabilizing the antiskyrmion hosting tetragonal phase up to a temperature as high as 550 K by marginally varying the chemical composition, thereby driving us a step closer to the realization of ferrimagnetic antiskyrmion based racetrack memory.
We report on the magnetic, thermodynamic, dielectric, and pyroelectric measurements on the hitherto unreported Fe4Ta2O9. This system is seen to exhibit a series of magnetic transitions, many of which are coupled to the emergence of ferroelectric order, making Fe4Ta2O9 the only genuine multiferroic in its material class. We suggest that the observed properties arise as a consequence of an effective reduction in the dimensionality of the magnetic lattice, with the magnetically active Fe 2+ ions preferentially occupying a quasi 2D buckled honeycomb structure. The low temperature H-T phase diagram of Fe4Ta2O9 reveals a rich variety of coupled magnetic and ferroelectric phases, in similarity with that observed in the distorted Kagome systems.
Structural and magnetic properties of a quasi-one-dimensional spin-1/2 compound NaVOPO4 are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and 31 P nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of NaVOPO4 may be compatible with the gapless uniform spin chain model, detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio α 0.98 and the ensuing zero-field spin gap ∆0/kB 2.4 K directly probed by field-dependent magnetization measurements. No longrange order is observed down to 50 mK in zero field. However, applied fields above the critical field Hc1 1.6 T give rise to a magnetic ordering transition with the phase boundary TN ∝ (H − Hc1)
We report on the coexistence of magnetic order and disorder in the atomically disordered double perovskites Ca2FeRuO6 and CaSrFeRuO6. Powder x-ray and neutron diffraction were used to investigate the crystal structure and magnetic ordering of these oxides. Both compounds are described by the orthorhombic space group Pbnm down to 3 K, where the B-site is found to be statistically occupied by Fe 3+ and Ru 5+ ions. The compound Ca2FeRuO6 shows a G-type antiferromagnetic ordering at TN ≈ 220 K, where the moments are aligned parallel to the c axis.The exchange of Ca by Sr suppresses long-range ordering in this system with the consequence that CaSrFeRuO6 shows a diffuse scattering pattern, indicating only the presence of a short-range order of the magnetic moments. Mössbauer measurements additionally reveal the coexistence of long-range ordered and paramagnetic phase in Ca2FeRuO6, and spin-glass behavior in CaSrFeRuO6. The random occupancy of iron and ruthenium atoms at the B-site gives rise to locally varying competing magnetic exchange interactions which favors the emergence of reentrant magnetism with a spin-glass-like transition at Tf ≈ 87 K for Ca2FeRuO6 and a spin-glass transition at ~65 K for CaSrFeRuO6, as evidenced by frequency dependent ac susceptibility measurements. Our results are an interesting example for crossing the borderline between antiferromagnetism and spin-glass behavior in a 3d-4d hybrid perovskite system by rather modifying structural details described by the tolerance factor of the perovskite structure than by changing the concentration of magnetic ions.
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