Ferromagnetic thickness dependence of exchange bias: breaking of the inverse proportionality law

Low Temperature Physics, 2012

The influence of magnetic anisotropy of ferromagnetic film on the phenomenon of exchange bias is studied. Hysteresis behavior in the 2-spin model of a ferro/antiferromagnet (FM/AFM) bilayer with exchange bias has been investigated in detail. In this model a half-space of AFM with fixed magnetic configuration contacts with a 2-layer FM film. Twelve different types of magnetization curves M(H) (both with and without hysteresis) have been found. Some of the M(H) curves demonstrate unusual features, such as plateaus and inclined segments. The hysteresis loop becomes asymmetric if the surface anisotropy is taken into account. 0 2 2 0 J J J J H .

Journal of Applied Physics, 2004

Physical Review B, 2000

Analytical expressions have been derived for the exchange bias field, coercivity, and effective anisotropy field in ferromagnetic/antiferromagnetic bilayers in the framework of a model assuming the formation of a planar domain wall at the antiferromagnetic side of the interface with the reversal of the ferromagnetic orientation. It is shown that there are five different sets of analytical expressions for the hysteresis loop displacement and coercivity, which depend on the interfacial exchange coupling strength and ferromagnetic anisotropy, and only one expression for the effective anisotropy field. These expressions are compared with the previously reported theoretical results, and the validity of the latter is discussed. It is shown that in the framework of the present model, the hysteresis loop, ac susceptibility, and ferromagnetic resonance measurements of exchange anisotropy should give the same values for the exchange bias field. The difference between the exchange bias field values, estimated experimentally by ac susceptibility and through hysteresis loop measurements for Co/CoO bilayers, is explained as well.

Low Temperature Physics, 2009

Field dependences of the magnetization and exchange bias in ferro/antiferromagnetic systems. II. Continuum model of a ferromagnetic layer

Journal of Physics D: Applied Physics, 2006

The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field H EB and the coercive field H C are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.

Physical review letters, 2015

The exchange bias of antiferromagnetic-ferromagnetic (AFM-FM) bilayers is found to be strongly dependent on the ferromagnetic spin configuration. The widely accepted inverse proportionality of the exchange bias field with the ferromagnetic thickness is broken in FM layers thinner than the FM correlation length. Moreover, an anomalous thermal dependence of both exchange bias field and coercivity is also found. A model based on springlike domain walls parallel to the AFM-FM interface quantitatively accounts for the experimental results and, in particular, for the deviation from the inverse proportionality law. These results reveal the active role the ferromagnetic spin structure plays in AFM-FM hybrids which leads to a new paradigm of the exchange bias phenomenon.

Physical Review B, 2002

The dependence of exchange bias on antiferromagnet thickness has been measured in FeF 2 /Fe and MnF 2 /Fe bilayers. The two fluoride systems have identical crystal structures, similar lattice constants, but anisotropy fields that differ by a factor of 20. Hence, by comparing the antiferromagnetic layer thickness dependence of the exchange bias in the two systems we are able to directly establish the effect of the antiferromagnet anisotropy. We find that the critical antiferromagnet thickness for the onset of exchange biasing is an order of magnitude smaller for the more anisotropic fluoride, confirming the often-used assumption that the anisotropy dictates the critical thickness. By measuring the temperature dependence of the exchange bias and the structural morphology of the layers we are able to prove that the effects we observe are not due to the blocking-temperature thickness dependence or the onset of discontinuity in thin antiferromagnet layers.

Applied Physics Letters, 2009

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet ͑AF/FM͒ thin films exhibiting double hysteresis loops ͑DHLs͒ is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Journal of Applied Physics, 2009

For NiFe/FeMn bilayers, the correlation among the exchange field, the coercivity, the training effect, the hysteretic effect of the angular dependence of the exchange bias, and the rotational hysteresis loss has been studied as a function of the antiferromagnet layer thickness t AFM. With increasing t AFM , all these quantities undergo nonmonotonic variations, except for the monotonic change in the exchange field. The maximal values of the coercivity, its relative change, and the rotational hysteresis loss are almost located at the same t AFM of 3.8 nm. The maximal values of the relative change in the exchange field and of the hysteretic effect of the angular dependence are located at 2.5 and 3.0 nm, respectively. The rotational hysteresis loss and the hysteretic behavior of the angular dependence of the exchange bias have different characteristics. The variations of all physical quantities with t AFM can be ascribed to the irreversible reversal of the antiferromagnet spins, which are governed by the Arrhenius-Néel law, except for that of the rotational hysteresis loss.

Journal of Magnetism and Magnetic Materials, 2005

We study a series of NiFe ð10:0 nmÞ=½Ir 20 Mn 80 ð6:0 nmÞ=Co 80 Fe 20 ð3:0 nmÞ N multilayers with different numbers N of bilayers grown by DC magnetron sputtering. After field-cooling, SQUID and MOKE measurements show a sizable increase of the exchange bias field with N. X-ray specular and diffuse scattering data reveal no significant variation of the lateral correlation length and only a weak dependence of the vertical rms interface roughness on N. Atomic and magnetic force microscopy, however, show a strong reduction of the grain size accompanied by distinct changes of the magnetic domain structure. We conclude that the enhancement of the exchange bias effect is related to the shrinking of the domain size in the antiferromagnet due to the structural evolution in the multilayers. r