Surface spin–flop transition in a uniaxial antiferromagnetic Fe/Cr superlattice induced by a magnetic field of arbitrary direction

Abstract

We studied the transition between the antiferromagnetic and the surface spin-flop phases of a uniaxial antiferromagnetic [Fe(14Å)/Cr(11Å] x20 superlattice. For external fields applied parallel to the in-plane easy axis, the layer-by-layer configuration, calculated in the framework of a meanfield one-dimensional model, was benchmarked against published polarized neutron reflectivity data. For an in-plane field H applied at an angle ψ = 0 with the easy axis, magnetometry shows that the magnetization M vanishes at H = 0, then increases slowly with increasing H. At a critical value of H, a finite jump in M (H) is observed for ψ < 5 o , while a smooth increase of M vs H is found for ψ > 5 o . A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for ψ ≥ 5 o . The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low ψ values, while the transition becomes continuous for ψ greater than a critical angle, ψ max ≈ 4.75 o . This is in fair agreement with the experimentally observed results.

Robust A -Type Order and Spin-Flop Transition on the Surface of the Antiferromagnetic Topological Insulator MnBi2Te4Saß1, Kim2, Vanderbilt3 et al. 2020Phys. Rev. Lett.877610View full textAdd to dashboardCite

“…The robust A-type order is further corroborated by the observation of two SSF transitions on domains with opposite terminations revealed by the magnetic field dependence of the domain contrast. Although they have been theoretically studied for decades [25,28,29], SSF transitions have only been observed in synthetic antiferromagnets, not in natural ones [26,27,30,31]. Our results not only shed new light on the realization of topological states in antiferromagnets, but also open up exciting explorations of surface metamagnetic transitions in functional antiferromagnets.…”

“…Thus, it is crucial to reveal the nature of surface magnetism of MnBi2Te4 in order to resolve the dichotomy between the observations of QAH transport and gapless topological surface states [18][19][20][21][22][23]. The magnetic imaging of A-type domain structures in MnBi2Te4 also enable explorations of the long-sought surface spin-flop (SSF) transition in a natural antiferromagnet [25][26][27][28][29][30][31]. In this letter, we report the observation of alternating termination-dependent magnetic signals on the surface of MnBi2Te4 single crystals using cryogenic MFM, which provides direct evidence of the persistence of uniaxial A-type antiferromagnetic order all the way to the surface.…”

“…The observation of robust A-type order on the MnBi2Te4 surface also provides a rare opportunity to explore the interesting SFF transition (or inhomogeneous spin-flop), which was first proposed by Mills decades ago using an effective one-dimensional spin-chain model with AFM nearest-neighbor exchange coupling [25,29]. However, later studies suggested an intriguing scenario of inhomogeneous spin-flop state due to finite size effect [28,30,38]. The SSF transition was observed in synthetic AFMs, which are superlattices of antiferromagnetically coupled ferromagnetic layers [26,27], but not in natural AFMs [28,31].…”

Uniaxially anisotropic antiferromagnets in a field on a square lattice

“…On the experimental side, especially layered cuprates exhibit interesting properties due to an interplay of spin and charge [1,2,3,4,5,6,7,8]. Intriguing phase diagrams have been obtained for other quasi two-dimensional antiferromagnets as well, showing, typically, multicritical behaviour [9,10,11,12].…”

Biconical structures in two-dimensional anisotropic Heisenberg antiferromagnets

“…In case of the anisotropic XY model, the degeneracy of the ground state, at a critical field, in antiferromagnetic, spin-flop, and bidirectional structures seems to result, as in the case of the XXZ model, in a narrow disordered phase between the antiferromagnetic and spin-flop phases, dominated by bidirectional fluctuations. Recently, two-dimensional uniaxially anisotropic Heisenberg antiferromagnets in a magnetic field along the easy axis have been studied theoretically rather intensively 1,2,3,4,5,6,7,8,9 , motivated by experiments on intriguing magnetic properties of layered cuprates 1,10,11,12,13 and by experimental findings on complex phase diagrams for other quasi twodimensional antiferromagnets 14,15,16,17,18 exhibiting, typically, multicritical behavior.A generic model describing such systems is the XXZ Heisenberg antiferromagnet on a square lattice, with the Hamiltonianwhere we consider the classical variant, with the spin at site i,, being a vector of length one. S i is coupled to its four neighboring spins S j at sites j.…”