The nature of magnetic correlation at low temperature in two‐dimensional artificial magnetic honeycomb lattice is a strongly debated issue. While theoretical researches suggest that the system will develop a novel zero entropy spin solid state as T → 0 K, a confirmation to this effect in artificial honeycomb lattice of connected elements is lacking. This study reports on the investigation of magnetic correlation in newly designed artificial permalloy honeycomb lattice of ultrasmall elements, with a typical length of ≈12 nm, using neutron scattering measurements and temperature‐dependent micromagnetic simulations. Numerical modeling of the polarized neutron reflectometry data elucidates the temperature‐dependent evolution of spin correlation in this system. As temperature reduces to ≈7 K, the system tends to develop novel spin solid state, manifested by the alternating distribution of magnetic vortex loops of opposite chiralities. Experimental results are complemented by temperature‐dependent micromagnetic simulations that confirm the dominance of spin solid state over local magnetic charge ordered state in the artificial honeycomb lattice with connected elements. These results enable a direct investigation of novel spin solid correlation in the connected honeycomb geometry of 2D artificial structure.

Artificial magnetic honeycomb lattices are expected to exhibit a broad and tunable range of novel magnetic phenomena that would be difficult to achieve in natural materials, such as long-range spin ice, entropy-driven magnetic charge-ordered state and spin-order due to the spin chirality. Eventually, the spin correlation is expected to develop into a unique spin solid state density ground state, manifested by the distribution of the pairs of vortex states of opposite chirality. Here we report the creation of a new artificial permalloy honeycomb lattice of ultra-small connecting bonds, with a typical size of 12 nm. Detail magnetic and neutron scattering measurements on the newly fabricated honeycomb lattice demonstrate the evolution of magnetic correlation as a function of temperature. At low enough temperature, neutron scattering measurements and micromagnetic simulation suggest the development of loop state of vortex configuration in this system. arXiv:1802.06631v1 [cond-mat.mes-hall]

The conductivity of a neodymium‐based artificial honeycomb lattice undergoes dramatic changes upon application of magnetic fields and currents. These changes are attributed to a redistribution of magnetic charges that are formed at the vertices of the honeycomb due to the nonvanishing net flux of magnetization from adjacent magnetic elements. It is suggested that the application of a large magnetic field or a current causes a transition from a disordered state, in which magnetic charges are distributed at random, to an ordered state, in which they are regularly arranged on the sites of two interpenetrating triangular Wigner crystals. The field and current tuning of electrical properties are highly desirable functionalities for spintronics applications. Consequently, a new spintronics research platform can be envisaged using artificial magnetic honeycomb lattices.

Artificial magnetic honeycomb lattice provides a two-dimensional archetypal system to explore novel phenomena of geometrically frustrated magnets. According to theoretical reports, an artificial magnetic honeycomb lattice is expected to exhibit several phase transitions to unique magnetic states as a function of reducing temperature. Experimental investigations of permalloy artificial honeycomb lattice of connected ultra-small elements, 12 nm, reveal a more complicated behavior. First, upon cooling the sample to intermediate temperature, 175 K, the system manifests a non-unique state where the long range order co-exists with short-range magnetic charge order and weak spin ice state. Second, at much lower temperature, 6 K, the long-range spin solid state exhibits a re-entrant behavior. Both observations are in direct contrast to the present understanding of this system. New theoretical approaches are needed to develop a comprehensive formulation of this two dimensional magnet.

A magnetic system, which can exhibit unidirectional current biasing at a modest current (resulting in reasonably small output power) without the application of magnetic field, is still elusive. Additionally, any such device must also demonstrate the operational ability at room temperature for practical applications. In a surprise, we have found the diode-type rectification in nanostructured 2D honeycomb lattice, made of ultrasmall permalloy (Ni 0.81 Fe 0.19 ) magnet, which satisfies most of the aforementioned criteria. Electronic measurements on newly fabricated permalloy honeycomb lattice of ultrasmall connecting elements (≈12 nm) have revealed unidirectional electrical properties without the application of magnetic field and with the output power of the order of ≈30 nW. In fact, magnetic field application changes the asymmetric behavior into a symmetric phenomenon. In zero field, electrical differential conductivity increases by more than two orders of magnitude for a modest unidirectional current application of ≈10 µA, compared to the negligible value near zero bias. Electronic measurement for the oppositely directed current yields a very small or negligible differential conductivity. The observation of diode-type function in permalloy honeycomb lattice of ultrasmall element possibly hints of new and unexplored properties of magnetic charges (monopoles and multipoles) that are argued to exist in this geometrically frustrated system. [14,15] The design of 2D artificial honeycomb lattice was originally envisaged to complement the study of spin ice phenomenon and associated magnetic monopoles in 3D geometrically frustrated magnets. [14,16] Since then, it has evolved in a new research arena to explore theoretically predicted novel magnetic properties involving monopoles and multipoles. [17,18] For a moderate aspect ratio, defined by l (length)/t (thickness), of the connecting element of the honeycomb, the magnetic moment lies along the length of the element due to magnetostatic interaction and shape anisotropy. [15,19] Consequently, two types of local spin configurations emerge: all moments either point to or away from a vertex of the honeycomb, also called "all-in or all-out" configuration or, two moments pointing in and one point away from the vertex (or vice versa), also called "two-in and one-out" (or vice versa) configuration. [15] The latter spin arrangement is also termed as the spin ice configuration, which results in a net magnetic charge of ±1 unit to a given vertex. Instead of using electron beam lithography, commonly used by researchers to create 2D magnetic honeycomb lattice,The magnetic analog of a semiconductor diode, demonstrating unidirectional electrical transport, is a highly desirable functionality for spintronics application, as it can play a dual role as magnetic memory device and logic element. However, creating such a functional material or device with operation ability at room temperature in the absence of any external tuning parameter, for instance a magnetic field, is a challenge till da...