Exchange anisotropy and the antiferromagnetic surface order parameter

Journal of Physics D: Applied Physics, 2011

Results from ferromagnetic resonance experiments carried out on epitaxially grown Fe/KNiF3/FeF2 trilayers are presented. Exchange coupling between the KNiF3, a weak anisotropy antiferromagnet, and the Fe leads to shifts in the resonance field of the ferromagnet. The field shifts can be described by a temperature-dependent exchange anisotropy . depends on the orientation direction of the applied field relative to the magnetic anisotropy axis, and a non-monotonic dependence on KNiF3 thickness. Three thickness regimes appear that correspond to different values of exchange bias in each region. A qualitative understanding of the basis for these three thickness regimes due to spin canting at the interfaces is presented. Our results illustrate a method to tune the value of exchange anisotropy using a combination of different antiferromagnets.

Solid State Communications, 2000

Positive exchange bias (PEB) is a remarkable phenomenon, which was recently observed experimentally. Normal (negative) exchange bias (NEB) was discovered more than 40 years ago. Its signature is the shift of the hysteresis loop along the applied field axis by H E Ͻ 0; in systems where a ferromagnet (FM) is in close contact with an antiferromagnet (AFM). This occurs after the system is cooled below the Néel temperature in an external field H cf of a few kOe. As H cf is substantially increased H E adopts positive values. Here we explain this rather unexpected behavior on the basis of an incomplete domain wall model that develops in the FM, for Fe/FeF 2 and Fe/MnF 2 systems. A consistent and unified picture of both NEB and PEB, and satisfactory quantitative agreement with experimental results are obtained on the basis of our theory.

Physical Review Letters, 1996

Physical Review Letters, 2003

The exchange bias and magnetic anisotropies in a Co layer on a single-crystalline FeF 2 film have been determined between 30 and 300 K. By postulating that the coupling between the ferromagnet and the antiferromagnet persists above the Néel temperature (T N ) we develop a model that quantitatively describes the exchange bias and the anisotropies over the whole temperature range, both above and below T N . Using only the measured low temperature exchange bias and a distribution of blocking temperatures we explain (i) the temperature dependence of the bias, (ii) the magnitude of the anisotropies, (iii) the opposite sign of the first and second order anisotropies, (iv) the observed 1=T and 1=T 3 temperature dependencies of the first and second order uniaxial anisotropies above T N , and (v) the decrease of the anisotropies below T N .

Physical Review B, 2002

We have undertaken a systematic study of the influence of in-plane crystalline quality of the antiferromagnet on exchange bias. Polarized neutron reflectometry and magnetometry were used to determine the anisotropies of polycrystalline ferromagnetic ͑F͒ Fe thin films exchange coupled to antiferromagnetic ͑AF͒ untwinned single crystal ͑110͒ FeF 2 , twinned single crystal ͑110͒ FeF 2 thin films and ͑110͒ textured polycrystalline FeF 2 thin films. A correlation between the anisotropies of the AF and F thin films with exchange bias was identified. Specifically, when exchange coupling across the F-AF interface introduces an additional anisotropy axis in the F thin film-one perpendicular to the cooling field, the magnetization reversal mechanism is affected ͑as observed with neutron scattering͒ and exchange bias is significantly enhanced.

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

Physical Review B, 2008

We measured directly the depth-dependent Fe spin rotation upon magnetization reversal in exchangecoupled Fe/ MnF 2 bilayers using nuclear resonant scattering of synchrotron radiation from an 57 Fe-probe layer buried at different depths within the Fe film. Our results show that the exchange-biased ferromagnetic layer develops a noncollinear spin structure along the film normal direction, reminiscent of a partial domain wall parallel to the Fe/ MnF 2 interface. This is contrary to most theoretical models of exchange bias which assume a collinear spin structure in the 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.

Journal of Magnetism and Magnetic Materials, 2006

We have investigated the angular dependence of exchange bias (H ex ) and coercivity (H c ) in polycrystalline CoFe (100 nm)/MnIr(x) bilayers with x ¼ 0, 2, 4, 10 and 20 nm. The angular dependence of H ex and H c show the abrupt kinks at critical angle (y c ) in the samples of H c 4H ex . The profiles of angular dependent H ex is changed at critical antiferromagnetic layer thickness ðt c AF Þ and shows inverse proportional behavior with field angle from 901Ày c to 901+y c . These complex angular dependence of H ex and H c with antiferromagnetic layer thickness can be well explained by using the Stoner-Wohlfarth (S-W) model. r

Physica B: Condensed Matter, 2013

Exchange-biasing of ferromagnetic (F) Fe layers by adjacent antiferromagnetic (AF) Mn layers has been investigated in (Fe/Mn) 10 multilayered films. This study has been focused on the relationship between the evolution of the exchange-bias field and the evolution of the film microstructure as a function of the deposition temperature. The increase of the deposition temperature results in the formation of an Fe-Mn alloy at the interfaces and columnar features whose size increases with the deposition temperature. In parallel, the exchange-bias field decreases significantly, due to interface roughness. (C. Bordel). Physica B 416 (2013) 45-50