The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.

Ion irradiation is an excellent tool to modify magnetic properties on the submicrometer scale, without modification of the sample topography. We utilize this effect to magnetically pattern exchange bias double layers using resist masks patterned by electron-beam lithography. Ion irradiation through the masks leads to a lateral modification of the magnetization reversal behavior and allows one to study the magnetization reversal as a function of the exchange bias field strength on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and magnetic force microscopy, respectively.

Second harmonic generation magneto-optic Kerr effect (SHMOKE) experiments, sensitive to buried interfaces, were performed on a polycrystalline NiFe/FeMn bilayer in which areas with different exchange bias fields were prepared using 5 KeV He ion irradiation. Both reversible and irreversible uncompensated spins are found in the antiferromagnetic layer close to the interface with the ferromagnetic layer. The SHMOKE hysteresis loop shows the same exchange bias field as obtained from standard magnetometry. We demonstrate that the exchange bias effect is controlled by pinned uncompensated spins in the antiferromagnetic layer.PACS numbers: 33.55. Fi, 75.30.Gw The magnetic exchange interaction between an antiferromagnetic (AF) and an adjacent ferromagnetic (F) layer may lead to the exchange bias effect discovered in 1956 [1,2]. Among other various intriguing features, this effect leads to a shift of the F hysteresis loop along the field axis by the so-called exchange bias field H eb . For recent reviews see Refs. [3,4,5]. Proposed models to account for the exchange bias involve (i) domain walls or partial domain walls in the AF layer which are either parallel [5,6] or perpendicular [7] to the interface, and/or (ii) uncompensated AF layer magnetic moments at the interface [7,8,9] and/or in the bulk [9,10]. In most exchange bias models, the interfacial uncompensated spins are linked to roughness, structural defects, or disoriented grains. Although uncompensated spins have been already evidenced [11], their behavior during the F layer magnetic reversal has not been reported so far and, experimentally, the relationship between uncompensated spins and exchange bias is still unclear. In the special case where artificial random defects can be introduced in the AF layer (such as in a diluted antiferromagnet), the so-called "domain state model" [9,10] showed that the exchange bias effect stems from the volume AF spin arrangement triggered by non magnetic defects. In this model, AF interfacial reversible and irreversible uncompensated spins (creating M F/AF rev and M AF irr respectively) are distinguished. Some of the interfacial AF uncompensated spins reverse under the action of an external magnetic field and the additional effective interface exchange field originating from the magnetized F layer, whereas the rest of the AF uncompensated spins remain frozen in the same range of applied fields. The reversible uncompensated spins hysteresis loop is found to be shifted along the field axis by H eb and along the magnetization axis by an amount directly proportional to M AF irr , which scales with H eb [10]. Using superconducting quantum interference device magnetometry, this vertical shift of the hysteresis loop of F/AF bilayers has already been measured and related to the exchange bias field sign [12], although its origin was not determined.Here we study the second-harmonic magneto-optic Kerr effect (SHMOKE) in an exchange-bias system. A second-harmonic signal in centrosymmetric materials is selectively generated at their inter...

He þ ion irradiation is an excellent tool to modify the magnitude and direction of the exchange bias field on the sub-micrometer scale without affecting the sample topography. This effect has been utilized to magnetically pattern NiFe/FeMn exchange bias double layers using resist masks patterned by electron beam lithography. Ion irradiation through the masks leads to a local modification of the magnetization reversal behavior and allows to study the magnetization reversal as a function of the exchange bias field strength and the pattern dimensions on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and Lorentz microscopy.

The effect of He ion irradiation on the magnetic properties of NiFe exchange coupled to different antiferromagnetic alloys (FeMn, CrMn, and PtMn) with the same layer thickness is investigated. All systems exhibit an enhanced coercivity prior to irradiation. An exchange bias field is only observed for FeMn and PtMn. Upon ion irradiation the FeMn-based system shows with increasing ion dose an enhancement followed by a decrease and finally a full suppression of the exchange bias field. For systems exchange coupled to PtMn only a decrease and suppression of the bias field is found. This can be attributed to the ion induced chemical disordering of the antiferromagnetic phase in the latter case. In the case of CrMn the antiferromagnetic layer thickness is too small to induce an exchange bias field, but an enhanced coercivity is observed which is caused by the exchange coupling between the antiferromagnetic and ferromagnetic layers. For all systems, this enhanced coercivity of the exchange coupled bilayer system is modified by ion irradiation.