Exchange bias in NiFe∕FeMn∕NiFe trilayers

Abstract

Ni Fe ∕ Fe Mn ∕ Ni Fe trilayer structure forms an integral part of many conventional and tunneling magnetoresistance spin valve structures with FeMn antiferromagnetic layer. A systematic investigation of the exchange bias variations of the seed and top pinned NiFe layers in the NiFe∕FeMn∕NiFe trilayer structure is reported as a function of thickness of all the three constituting layers, in multilayers prepared by rf magnetron sputtering. X-ray diffraction patterns show the (111) texture for the NiFe and FeMn layers, necessary for the development of antiferromagnetic γ-fcc phase. In thickness variation studies of all the three magnetic layers, seed NiFe layer shows greater bias (150Oe) than the top pinned NiFe layer (80Oe only). The exchange bias shows the expected 1∕t behavior for increasing NiFe layer thickness after initial maxima at low thickness. In the FeMn antiferromagnet layer thickness variation on the other hand, the large bias values attained around 5nm thickness is nearly retained up to a thickness of 25nm and the bias for the top NiFe layer is again substantially lower. The greater bias observed for the seed NiFe layer in all the three thickness variation studies is attributed to its growth over a saturated (111) oriented NiFe seed layer, which induces formation of interfacial FeMn layers with a net parallel spin ordering, in presence of the constant applied field. On the other hand, at the top FeMn∕NiFe surface, the rigid FeMn surface with compensated bulk spin ordering formed already, is not easily biased and reoriented along the top NiFe layer, to develop as much parallel net spins in the antiferromagnetic material, and hence lower bias.

Exchange biased FeNi/FeMn bilayers with coercivity and switching field enhanced by FeMn surface oxidation

“…There is an opinion, that it does not affect the properties of the base (bottom) ferromagnetic layer, 21 but there are data, supporting its active influence. [22][23][24] It can not be ruled out, that under the influence of oxidation not only planar, but also more complex chemical and magnetic structures are formed. Taking into account the enhanced oxidation rate of Mn near the grain boundaries, 6 and also the columnar structure formation in the films prepared by the magnetron sputtering, 25,26 we can not exclude the formation of bridges between the ferromagnetic FeNi and f-FeMn layers through the FeMn layer.…”

Interdiffusion effect on exchange coupling in annealing NiFe/FeMn and FeMn/NiFe systems

“…FeMn must be deposited on a fcc seed layer such as Cu, NiFe in order to crystallize the antiferromagnetic, γ-fcc phase [4,14]. Therefore, Type I structures are a better AFM/FM system and Type II structures are a worse AFM/FM system [5].…”

“…The phenomenon of exchange bias coupling between antiferromagnetic (AFM) and ferromagnetic (FM) thin films, first reported by Meiklejohn and Bean [1,2], is now of major importance for both magnetic recording read heads and for data storage applications such as magnatic random access memory (MRAM) [3][4][5]. The exchange coupling between AFM/FM films, which has been shown to be primarily an interfacial phenomenon, is dependent on the microstructural characteristics which include AFM orientation [6], crystal texture [7], interfacial roughness [5,7], grain size [8], impurities, etc.…”

Role of the Antiferromagnetic Bulk Spin Structure on Exchange Bias

“…On the contrary, the domain state model [9] considers the entire volume of the AF in order to account for EB, suggesting that the internal magnetic structure of the AF is crucial for the emergence of EB. Several techniques have been applied to study AF domains [10][11][12], and 90 or 180 domain walls were claimed for different systems [13][14][15][16][17][18][19][20]. Although recent experiments [21,22] suggest that the bulk AF sets the EB, whether EB is purely controlled by the interface or AF bulk is still controversial.…”