Metamagnetic texture in a polar antiferromagnet

Abstract

The notion of a simple ordered state implies homogeneity. If the order is established by a broken symmetry, elementary Landau theory of phase transitions shows that only one symmetry mode describes this state. Precisely at points of phase coexistence domain states formed of large regions of different phases can be stabilized by long range interactions. In uniaxial antiferromagnets the so-called metamagnetism is an example of such a behavior, when an antiferromagnetic and field-induced spin-polarized paramagnetic/ferromagnetic state co-exist at a jump-like transition in the magnetic phase diagram. Here, combining experiment with theoretical analysis, we show that a different type of mixed state between antiferromagnetism and ferromagnetism can be created in certain acentric materials. In the small-angle neutron scattering experiments we observe a field-driven spin-state in the layered antiferromagnet Ca3Ru2O7, which is modulated on a scale between 8 and 20 nm and has both antiferromagnetic and ferromagnetic parts. We call this state a metamagnetic texture and explain its appearance by the chiral twisting effects of the asymmetric Dzyaloshinskii-Moriya (DM) exchange.The observation can be understood as an extraordinary coexistence, in one thermodynamic state, of spinorders belonging to different symmetries. Experimentally, the complex nature of this metamagnetic state is demonstrated by measurements of anomalies in electronic transport which reflect the spin-polarization in the metamagnetic texture, determination of the magnetic orbital moments, which supports the existence of strong spin-orbit effects, a pre-requisite for the mechanism of twisted magnetic states in this material. Our findings provide an example of a rich and largely unexplored class of textured states. Such textures mediate between different ordering modes near phase co-existence, and engender extremely rich phase diagrams.

Spontaneous cycloidal order mediating a spin-reorientation transition in a polar metal

“…4, revealing a large expansion of the cycloidal phase under small fields. A striking feature of the phase boundary is its similarity with that of the so-called "metamagnetic texture" reported by Sokolov et al [16], who measured incommensurate peaks at (± , 0, 0) in fields above 2 T with small angle neutron scattering (SANS). The connection to the magnetic satellites that we observe can be seen by doubling our wave vector (δ, 0, 1) and then projecting back into the first Brillouin zone through subtraction of the lattice vector (0,0,2), giving (2δ, 0, 0) ≈ ( , 0, 0) [30].…”

“…We show how the frustration that renders the cycloid sensitive to temperature also allows it to be delicately tuned by magnetic field, under which higher harmonic modulations of the fundamental order appear to be generated. This necessitates a reinterpretation of an incommensurate structure previously observed under field [16], unifying it with the magnetic response to doping [17][18][19]. Our results thus demonstrate a transition at the juncture of polar metals, spin-orbit-coupled systems, and magnetic frustration.…”

Prospecting chiral multisite interactions in prototypical magnetic systems

“…Phenomenological considerations can also be used to identify allowed forms for the interactions consistent with the symmetry of a target material. This led to the discovery of chiral four-spin three-site interactions in MnGe [20] (for which a derivation based on multiple scattering theory was also provided), and motivated their existence in an Fe chain on Re(0001) [52], while the magnetism of Ca 3 Ru 2 O 7 was rationalized by invoking higher order Lifshitz invariants in connection to a Ginzburg-Landau theory [53]. Lastly, the energy can also be expanded in a Taylor series in small deviations from a reference magnetic structure.…”

Electronically driven spin-reorientation transition of the correlated polar metal Ca 3 Ru 2 O 7

“…The resistivity exhibits a sudden jump on cooling through TS, but, although its absolute value remains relatively high (ρ ab (5 K) ≈ 50 µΩcm), the in-plane resistivity recovers a metallic temperature dependence to low temperature (16,17). A rich noncollinear magnetic texture has been observed under the application of magnetic fields (20), pointing to an important role of spin-orbit coupling combined with the noncentrosymmetric crystal structure, while strong electronic correlations are expected to also play an important role in shaping the electronic and magnetic properties of this system (the single-layered sister compound Ca2RuO4 is a Mott insulator) (21).…”