Tomoya Higo

Large anomalous Hall effect in a non-collinear antiferromagnet at room temperature

In ferromagnetic conductors, an electric current may induce a transverse voltage drop in zero applied magnetic field: this anomalous Hall effect is observed to be proportional to magnetization, and thus is not usually seen in antiferromagnets in zero field. Recent developments in theory and experiment have provided a framework for understanding the anomalous Hall effect using Berry-phase concepts, and this perspective has led to predictions that, under certain conditions, a large anomalous Hall effect may appear in spin liquids and antiferromagnets without net spin magnetization. Although such a spontaneous Hall effect has now been observed in a spin liquid state, a zero-field anomalous Hall effect has hitherto not been reported for antiferromagnets. Here we report empirical evidence for a large anomalous Hall effect in an antiferromagnet that has vanishingly small magnetization. In particular, we find that Mn3Sn, an antiferromagnet that has a non-collinear 120-degree spin order, exhibits a large anomalous Hall conductivity of around 20 per ohm per centimetre at room temperature and more than 100 per ohm per centimetre at low temperatures, reaching the same order of magnitude as in ferromagnetic metals. Notably, the chiral antiferromagnetic state has a very weak and soft ferromagnetic moment of about 0.002 Bohr magnetons per Mn atom (refs 10, 12), allowing us to switch the sign of the Hall effect with a small magnetic field of around a few hundred oersted. This soft response of the large anomalous Hall effect could be useful for various applications including spintronics—for example, to develop a memory device that produces almost no perturbing stray fields.

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

The magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials as a powerful probe for electronic and magnetic properties1–3 and for magneto-optical technologies4. The MOKE can be additionally useful for the investigation of the antiferromagnetic (AF) state, although thus far limited to insulators5–9. Here, we report the first observation of the MOKE in an AF metal. In particular, we find that the non-collinear AF metal Mn3Sn (ref. 10) exhibits a large zero-field Kerr rotation angle of 20 mdeg at room temperature, comparable to ferromagnetic metals. Our first-principles calculations clarify that ferroic ordering of magnetic octupoles11 produces a large MOKE even in its fully compensated AF state. This large MOKE further allows imaging of the magnetic octupole domains and their reversal. The observation of a large MOKE in an AF metal will open new avenues for the study of domain dynamics as well as spintronics using antiferromagnets12–16.The magneto-optical Kerr effect is demonstrated in an antiferromagnetic metal. Large rotation angles, magnetic octupole domain imaging was enabled.

Evidence for an exotic magnetic transition in the triangular spin system FeGa2S4

We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T^* \simeq 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below T_f \simeq 16 K. These results are compared to those found for the isomorph NiGa2S4. It is argued that the fate of the transition, which has been interpreted in terms of the Z_2 topological transition in this latter system, is probably different in FeGa2S4.

Large spontaneous Hall effects in chiral topological magnets

Abstract As novel topological phases in correlated electron systems, we have found two examples of non-ferromagnetic states that exhibit a large anomalous Hall effect. One is the chiral spin liquid compound , which exhibits a spontaneous Hall effect in a spin liquid state due to spin ice correlation. The other is the chiral antiferromagnets and that exhibit a large anomalous Hall effect at room temperature. The latter shows a sign change of the anomalous Hall effect by a small change in the magnetic field by a few 100 G, which should be useful for various applications. We will discuss that the magnetic Weyl metal states are the origin for such a large anomalous Hall effect observed in both the spin liquid and antiferromagnet that possess almost no magnetisation.

Frustrated magnetism in a Mott insulator based on a transition metal chalcogenide

A three dimensional geometrically frustrated Mott insulator of tetrahedral cluster compound GaTa4Se8 has been studied by X-ray diffraction, susceptibility and specific heat measurements. Temperature dependence of the magnetic susceptibility shows an anomaly peak at 53 K, which is supposed to be the structural transition from cubic to tetragonal structure on the analogy of GaNb4S8. Moreover, small g factor compared with GaNb4S8 compound revealed strong spin orbit coupling (SOC) which is inherent in this 5d compound.

Magnetic and Transport Properties of Frustrated γ-MnPd alloys

We have synthesized γ-Mn1–xPdx alloys with x = 0.22 and 0.24 and carried out the susceptibility and resistivity measurements. The γ-Mn1–xPdx shows the Neel transition to the non-coplanar antiferromagnetic 3Q phase at ~ 290 K for x = 0.22 and at ~ 250 K for x = 0.24. Below the Neel temperature, two characteristic temperatures, which support the existence of the magnetic and structural phase transitions, were observed. These results were used to construct a phase diagram of the γ-Mn1–xPdx alloys with high x concentrations where non-collinear and/or non-coplanar spin structures are expected.

Magnetization Anomaly due to the Non-Coplanar Spin Structure in NiS2

The pyrite-type antiferromagnet NiS2 exhibits a non-coplanar antiferromagnetic (NAF) spin ordering with four spin sublattices in the temperature region between TN1 = 38 K and TN2 = 30 K, and forms a weak ferromagnetic phase below TN2. We have carried out detailed magnetization measurements using high-quality single crystals of NiS2 to reveal magnetic properties associated with the NAF spin structure. Our results obtained in the field cooling sequence under various magnetic fields μ0HFC reveal that the magnetic domains due to the NAF structure may be controlled through the field cooling procedure into the low-temperature weak ferromagnetic phase. Unusual μ0HFC dependence of the magnetic hysteresis found in the NAF ordered phase strongly suggests that the weak ferromagnetic moment comes from the domain wall.