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

Abstract

The pyrite-type antiferromagnet NiS 2 exhibits a non-coplanar antiferromagnetic (NAF) spin ordering with four spin sublattices in the temperature region between T N1 = 38 K and T N2 = 30 K, and forms a weak ferromagnetic phase below T N2 . We have carried out detailed magnetization measurements using high-quality single crystals of NiS 2 to reveal magnetic properties associated with the NAF spin structure. Our results obtained in the field cooling sequence under various magnetic fields μ 0 H FC 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 μ 0 H FC dependence of the magnetic hysteresis found in the NAF ordered phase strongly suggests that the weak ferromagnetic moment comes from the domain wall.Non-coplanar spin arrangements in frustrated magnets have attracted great interest because the associated vector and=or scalar spin chirality may induce exotic phenomena such as multiferroicity, 1) topological Hall transport, 2-4) and chiral spin liquid states. 3,5) While most non-coplanar magnets are characterized by their ferromagnetic (FM) component, a purely antiferromagnetic (AF) all-in=all-out (AIAO) structure (all moments on a tetrahedron point either inwards or outwards to its center) has recently been discovered in several 5d transition metal pyrochlore oxides such as Eu 2 Ir 2 O 7 , 6-8) Nd 2 Ir 2 O 7 , 9) and Cd 2 Os 2 O 7 . 10) While this magnetic structure has zero net magnetic moment owing to the cancellation of the moments on the four vertexes of a tetrahedron, it has two types of AF variant, which are connected by the time-reversal operation, and permits a multi-domain state as in the case of ferromagnetism. The domain wall (DW) breaks such a constraint and thus may alter the electronic structure locally. 11) In particular, the AIAO state in the Ir system is interesting because various topologically nontrivial states such as Weyl semimetals and topological Mott insulators have been predicted to arise on the basis of the interplay of Coulomb interactions, spin-orbit coupling, and the topology of band structure. 12-15) In addition, it has been theoretically proposed 16) that the magnetic domain hosts a gapless interface state with Fermi arcs and induces net uniform magnetization as the origin of the spin-glass-like magnetic signature observed in various pyrochlore iridates. 6,17) Experimentally, the anomalous magnetoconductivity and the unconventional weak ferromagnetic (WF) moment realized in the AIAO ordered phase have been observed in Nd 2 Ir 2 O 7 18) and Eu 2 Ir 2 O 7 . 6) As the domain physics is inherent to the AIAO structure, similarly interesting physics associated with the DW should generally emerge in other materials that host the AIAO structure. However, to date, there have been almost no reports on studies of domain phenomena related to the AIAO structure.The AF insulator NiS 2 has the pyrite structure (Pa " 3; T 6 h ) where Ni and S pairs form the NaCl structure. The magne...

Frustrated magnetism in the Heisenberg pyrochlore antiferromagnets $A$Yb$_2$$X$$_4$ ($A$ = Cd, Mg, $X$ = S, Se)

“…3(a)). Experimentally, a similar hysteretic behavior was reported in non-coplanar antiferromagnets which have the pyrochlore and fcc lattices [59][60][61][62][63] .…”

Magnetic structure ofNiS2−xSex

“…Previous studies argued for the presence of a weak ferromagnetic component along with the M2 phase 26 . It is puzzling to see how a simple binary compound can demonstrate such a complex magnetic state.…”

Gating-Induced Mott Transition in NiS$_2$

“…At T N ∼ 38 K, NiS 2 undergoes a phase transition to a noncollinear antiferromagnetic insulating phase with S = 1 moments formed by the parallel alignment of the spins of the two electrons on the Ni e g orbitals [44,71,72]. While it remains an insulator in the paramagnetic phase, a Mott-insulator to metal transition can be induced by doping [39], pressure [73,74], or isovalent substitution of S with Se [38,40,52].…”