Field-induced transitions from negative to positive exchange bias in nanoparticles with inverted ferromagnetic-antiferromagnetic core-shell morphology

Abstract

In an antiferromagnetic (core)/ferromagnetic (shell) nanoparticle, the transition behaviors from negative (NEB) to positive exchange bias (PEB) at low temperature after field cooling are studied in detail. The NEB field may exhibit an oscillatory behavior due to the competition between interfacial coupling and cooling field. The critical cooling fields, at which the transitions occur, exhibit a logarithmic decrement with the decrease of interfacial coupling, but indicate a linear decrease with the decrease of antiferromagnetic coupling or with the further dilution in the antiferromagnetic core. With the further increase of cooling field, the PEB field increases linearly and finally levels off. Moreover, the weaker antiferromagnetic coupling may enhance the NEB field value, whereas the suppression of PEB may be observed by diluting the antiferromagnetic core. The magnetization reversal by coherent rotation strongly depends on the variation of the magnetic parameters, because its occurrence just needs to consume a lower additional energy, which is the main reason for these unique phenomena. We have shed new light on the microscopic origin of the peculiar magnetic properties in the nanoparticles with such an inverted magnetic structure.

Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles

“…Nevertheless, Monte Carlo simulations have already probed the effects of different types interface couplings, such as antiferromagnetic (AFM) (i.e., layers antiparallel to each other), from the theoretical point of view. These studies predict a range of novel effects such as positive exchange bias, compensation points or proximity effects [13][14][15][16] . Interestingly, diverse types of couplings have been reported for thin film bilayers and multilayers [17][18][19][20][21][22] and in particular the AFM one, cause numerous significant effects, which have given rise to crucial magnetic devices such as novel types of recording media, advanced read heads in hard disks, magnetic random access memories and biomedical sensors [23][24][25] .…”

Relative-thickness dependence of exchange bias in bilayers and trilayers

“…On the contrary, computer simulations based either on Monte Carlo (MC) methods [18,26,27] or on micromagnetic approach [28,29] have proved useful to gain insight into the microscopic origin of EB. Moreover, in our previous work, the dependence of EB on various magnetic parameters, including size and shape in core/shell [30][31][32], core/ matrix [33,34], and random alloy systems [35], has been investigated. In this paper, we continue studying the thickness-dependent EB in the layered systems and resolve a more complex problem in order to understand how EB is jointly controlled by the dual influences of an FM (or an AFM) layer which increases in thickness and the other which decreases in thickness at the same time.…”

Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals

“…Due to their diverse range of applications-from data storage, energy harvesting and conversion to nano-mechanical devices-nanostructures are of enormous interest to academia and industry [1][2][3][4][5][6][7][8]. At the nanoscale, confinement effects, size variations, strain, defects and shape morphology have been recently shown to play a crucial role in mechanical and magnetic device applications [9][10][11][12][13].…”