In the quest for post-CMOS (complementary metal-oxide-semiconductor) technologies, driven by the need for improved efficiency and performance, topologically protected ferromagnetic 'whirls' such as ...skyrmions
and their anti-particles have shown great promise as solitonic information carriers in racetrack memory-in-logic or neuromorphic devices
. However, the presence of dipolar fields in ferromagnets, which restricts the formation of ultrasmall topological textures
, and the deleterious skyrmion Hall effect, when skyrmions are driven by spin torques
, have thus far inhibited their practical implementation. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling, have recently become the subject of intense focus
, but they have yet to be experimentally demonstrated in natural antiferromagnetic systems. Here we realize a family of topological antiferromagnetic spin textures in α-Fe
O
-an Earth-abundant oxide insulator-capped with a platinum overlayer. By exploiting a first-order analogue of the Kibble-Zurek mechanism
, we stabilize exotic merons and antimerons (half-skyrmions)
and their pairs (bimerons)
, which can be erased by magnetic fields and regenerated by temperature cycling. These structures have characteristic sizes of the order of 100 nanometres and can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways through which further scaling may be achieved. Driven by current-based spin torques from the heavy-metal overlayer, some of these antiferromagnetic textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature
.
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GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
Strain engineering is widely used to manipulate the electronic and magnetic properties of complex materials. For example, the piezomagnetic effect provides an attractive route to control magnetism ...with strain. In this effect, the staggered spin structure of an antiferromagnet is decompensated by breaking the crystal field symmetry, which induces a ferrimagnetic polarization. Piezomagnetism is especially appealing because, unlike magnetostriction, it couples strain and magnetization at linear order, and allows for bi-directional control suitable for memory and spintronics applications. However, its use in functional devices has so far been hindered by the slow speed and large uniaxial strains required. Here we show that the essential features of piezomagnetism can be reproduced with optical phonons alone, which can be driven by light to large amplitudes without changing the volume and hence beyond the elastic limits of the material. We exploit nonlinear, three-phonon mixing to induce the desired crystal field distortions in the antiferromagnet CoF2. Through this effect, we generate a ferrimagnetic moment of 0.2 μB per unit cell, nearly three orders of magnitude larger than achieved with mechanical strain.This paper shows how lattice distortions induced by a laser pulse can create a ferrimagnetic moment in an antiferromagnet. This mechanism gives a magnetic response that is orders of magnitude larger than using mechanical strain.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A helix of dipoles
In magnetic materials, magnetic dipoles typically line up parallel or antiparallel to each other. However, more complex orderings, such as helical, can also occur. Khalyavin
et al.
...found that in the material BiCu
0.1
Mn
6.9
O
12
, a helical order can be formed out of electric rather than magnetic dipoles. The material also harbors an associated structural helical order, which symmetry analysis suggests might be switchable with an applied electric field.
Science
, this issue p.
680
Light hole doping of the quadruple perovskite BiMn
7
O
12
results in an unusual ordering.
Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets and antiferromagnets, to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) collinear alignments of electric dipoles. Here, we report an observation of incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn
7
O
12
. In analogy with magnetism, the electric dipole helicoidal texture is stabilized by competing instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate density wave.
A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at ...temperatures below approximately 130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric (P1), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering (Cc).
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Our recent discovery of three new phases of ice has increased the total number of known distinct polymorphs of ice to fifteen. In this Perspective article, we give a brief account of previous work in ...the field, and discuss some of the particularly interesting open questions that have emerged from recent studies. These include (i) the effectiveness of acid and base dopants to enable hydrogen-ordering processes in the ices, (ii) the comparison of the calorimetric data of some of the crystalline phases of ice and low-density amorphous ice, (iii) the disagreement between the experimental ice XV structure and computational predictions, (iv) the incompleteness of some of the hydrogen order/disorder pairs and (v) the new frontiers at the high and negative pressure ends of the phase diagram.
A theory describing how ferroic properties can emerge transiently in the ultrafast regime by breaking symmetry with light through three-phonon coupling is presented. Particular emphasis is placed on ...the special case when two exactly degenerate midinfrared or THz phonons are resonantly pumped, since this situation can give rise to an exactly rectified ferroic response with damping envelopes of ∼1 ps or less. Light-induced ferroelectricity and ferromagnetism are discussed in this context and a number of candidate materials that could display these phenomena are proposed. The same analysis is also applied to the interpretation of previous femtomagnetism experiments, performed in different frequency ranges (visible and near infrared), but sharing similar symmetry characteristics.
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Abstract
Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are ...robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel proof-of-principle ionic approach to control the spin reorientation (Morin) transition reversibly in the common antiferromagnetic insulator α-Fe
2
O
3
(haematite) – now an emerging spintronic material that hosts topological antiferromagnetic spin-textures and long magnon-diffusion lengths. We use a low-temperature catalytic-spillover process involving the post-growth incorporation or removal of hydrogen from α-Fe
2
O
3
thin films. Hydrogenation drives pronounced changes in its magnetic anisotropy, Néel vector orientation and canted magnetism via electron injection and local distortions. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate reversible control of the room-temperature spin-state by doping/expelling hydrogen in Rh-substituted α-Fe
2
O
3
.
We explore the magneto-optical Kerr effect (MOKE) for different spin configurations of the (CrI3)2 bilayer and (CrBr3/CrI3) mixed bilayer using symmetry arguments and first-principles electronic ...structure calculations. Starting from CrX3 (X = I, Br) monolayers, we considered collinear ferromagnetic (FM) and layered antiferromagnetic (AFM) states for (CrI3)2 and (CrBr3/CrI3) bilayers. The AFM (CrI3)2 bilayer does not show MOKE, consistent with the presence of a symmetry operator combining inversion (I) and time reversal (T) symmetries. The FM state preserves I symmetry but breaks the T symmetry, thus allowing a nonzero Kerr angle, which is reversible by switching the FM spins. The (CrBr3/CrI3) bilayer breaks both the I and T symmetries and thus exhibits MOKE both in the FM state and, remarkably, in the AFM state. In both FM and AFM configurations, the Kerr angle switches by reversing the spins in both layers. Our study demonstrates that the MOKE spectra can help to characterize different magnetic configurations in these emerging two-dimensional (2D) magnetic materials due to a different stacking of the monolayers, even in the AFM case. Note that the present symmetry analyses and MOKE properties apply to more general 2D magnetic van der Waals heterostructures. Furthermore, we propose the (CrBr3/CrI3) bilayer as a promising candidate for AFM spintronics since the two time-reversed AFM states are associated with opposite Kerr rotation, i.e., they could be used as memory elements.
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Theory of Electromagnons in CuO Cao, Kun; Giustino, Feliciano; Radaelli, Paolo G
Physical review letters,
2015-May-15, Volume:
114, Issue:
19
Journal Article
Peer reviewed
We develop a theory of electromagnons in CuO by combining a symmetry analysis based on irreducible corepresentations, ab initio calculations, and simulations of spin dynamics using a model ...Hamiltonian and the Landau-Lifshitz-Gilbert equation. We show that the electromagnon measured in Jones et al., Nat. Commun. 5, 3787 (2014) with the electric field along the 101 direction originates from a magnetoelectric coupling mediated by Dzyaloshinskii-Moriya interactions and consists of a rigid rotation of the Cu spins around the axis defined by the electric field. Furthermore we predict the existence of a second electromagnon originating from exchange striction and coupled to electric fields along the 010 direction in the AF2 phase.
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