Phase diagrams as a function of anisotropy D and magnetic field H are obtained for discommensurations and surface states for an antiferromagnet in which H is parallel to the easy axis, by modeling it using the ground states of a one-dimensional chain of classical XY spins. A surface spin-flop phase exists for all D, but the interval in H over which it is stable becomes extremely small as D goes to zero. First-order transitions, separating different surface states and ending in critical points, exist inside the surface spin-flop region. They accumulate at a field H (depending on D) significantly less than the value HSF for a bulk spin-flop transition. For H < H < HSF there is no surface spin-flop phase in the strict sense; instead, the surface restructures by, in effect, producing a discommensuration infinitely far away in the bulk. The results are used to explain in detail the phase transitions occurring in systems consisting of a finite, even number of layers. http://publish.aps.org/abstract/prb/v59/p6239
Magnetic multilayers which incorporate ultrathin ferromagnetic films are
physical realizations of classical, one dimensional spin systems, with spins
coupled via exchange mediated by spacer layers or interactions at interfaces,
and subject to anisotropy. Here by `spin', we refer to the total spin angular
momentum of an ultrathin film in the structure. Such systems can undergo a
rich range of phase transitions, in reponse to an external magnetic field, or
change in temperature. Since interfilm exchange is weak, modest magnetic
fields can induce spin reorientation phase transitions. We thus have a new
and diverse class of magnetic materials, with phase diagrams subject to
design, since both thickness, composition, or growth conditions. The paper
reviews selected examples, including recent studies of the dynamic response
(AC susceptibility) of an antiferromagnetically coupled Fe/Cr system which
undergoes the surface spin-flop transition.
Keywords: Magnetic multilayers; Spin reorientation transition; Phase
transitions
Corresponding author. Tel.: +1-949-8245148; fax: +1-949-8242174
We report experimental and theoretical studies of the magnetization curves of Fe/Cr(211) superlattices with antiferromagnetic interlayer coupling and uniaxial in-plane anisotropy. There are substantial differences between structures with an even and an odd number of Fe layers, when the magnetic field is applied along the easy axis. For even layered superlattices, the inequivalence of the terminal Fe layers gives rise to a surface spin-flop transition that evolves into a bulk spin-flop arrangement with increasing magnetic field, as originally envisioned by Keffer and Chow [Phys. Rev. Lett. 31, 1061 (1973)].
We use the numerical self-consistent mean-field method to examine the ground-state configurations of finite-size multilayers constructed from ferromagnetic films which are antiferromagetically coupled. At intermediate fields, a surface spin-flop state, as suggested by Keffer and Chow, has been found for even numbered multilayers, but not for odd numbered ones. These are confirmed by experimental observations on systems such as Fe/Cr. A simple macroscopic approach has been devised in calculating the spin-wave excitations for the various field-induced ground states. Special emphasis is placed on the influence of interfilm dipolar couplings on the spin-wave spectrum. We note, for example, that there are substantial differences for the two cases k||D << 1 and k||D >~ 1, with D the thickness of the finite superlattice, and k|| the wave vector of the spin waves parallel to its surface.
A epitaxial orientation of Fe/Cr superlattices-Fe/Cr(211) on MgO(110)-is grown by magnetron sputtering. Its structural and magnetic characterizations are presented and compared to those for Fe/Cr(100) superlattices grown simultaneously onto MgO(100) substrates. The epitaxial orientation of the Fe/Cr(211) superlattices is Fe/Cr[01-bar1]||MgO[001] and Fe/Cr[1-bar11]||MgO[11-bar0], while that for Fe/Cr(100) is Fe/Cr[001]|| MgO[011]. A uniaxial, in-plane surface anisotropy for the Fe/Cr(211) superlattices along the Fe[01-bar1] of 0.06 erg/cm2 is obtained from analysis of the magnetization hysteresis loops. Four oscillations in the antiferromagnetic interlayer coupling and giant magnetoresistance (GMR) are observed with a period of 18 Å for both orientations. The strength, oscillation period, and phase of the magnetic coupling are identical for the two orientations. The GMR values increase by a factor of ~4 to 5 on cooling from room temperature to 4.2 K. At 4.2 K the maximum GMR value of the [Fe(14 Å)/Cr(8 Å)]50 superlattice is 70% for the (211) orientation and 150% for the (100) orientation.