Anomalous temperature and interfacial‐coupling dependence of exchange bias in antiferromagnetic (core)/ferromagnetic (shell) nanoparticles

Abstract

In a single composite ferromagnetic/antiferromagnetic (AFM) nanoparticle with an unconventional core/shell morphology, the effects of temperature and interfacial coupling on the exchange bias (EB) are studied by performing a modified Monte Carlo method. With the decrease of temperature, three distinct behaviors of the EB are obtained as a result of the state transitions of the AFM spins from completely free via partially free and partially frozen, and, finally, to completely frozen. The behaviors of the coercivity are nonmonotonic due to the energy competitions and the thermal fluctuations. Moreover, the cooling field affects strongly the low-temperature behaviors of the EB. The EB field obtained after field cooling is negative and almost constant, independent of the interfacial coupling. However, after zero-field cooling, the EB field may exhibit a sign-reversal behavior with increasing interfacial coupling at low temperature. On the basis of the theory of the surplus magnetization, it is revealed that the drastic temperature dependence and the energy competition mechanism at the interface in the cooling process are responsible for this unique property in such novel nanostructures.

Influence of time dependent longitudinal magnetic fields on the cooling process, exchange bias and magnetization reversal mechanism in FM core/AFM shell nanoparticles: a Monte Carlo study

“…In figure 6, we display the effect of the thickness of the AFM shell on the simulated equilibrium hysteresis loops of the particle corresponding to different regions. It is clearly shown in this figure that in order to observe a prominent shift in the hysteresis loops, the shell thickness should be well above a certain critical size which agrees well with the previously published works [2,8,11,18,19,23,24,26]. The magnetization of the interface region follows the reversal of the core magnetization exhibiting the same coercivity.…”

“…There are also exchange bias systems that show a temperature dependent transition from negative to positive exchange bias values [25]. In the pioneering works by Meiklejohn and Bean [2], a certain oxide thickness is claimed to be necessary to produce the shift in the hysteresis loop which has also been supported by subsequent studies for thin films and nanoparticles [8,11,18,19,23,24,26]. Besides, the type of the magnetic interactions at the FM/AFM interface is complicated, and either FM or AFM couplings have been considered in theoretical invest igations [10,14,24].…”

“…For detailed reviews about EB in FM/AFM bilayers, nanoparticles, as well as alloys and compounds, see [3][4][5][6][7]. Among the theoretical works, Monte Carlo (MC) simulation method is verified as a versatile approach utilized for the investigation of microscopic mechanism peculiar to EB systems, and it provides a testing ground for performing comp uter experiments [8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Thanks to these efforts, basic picture behind this phenomenon is now well established: First of all, either in the computer or in the laboratory experiments, the sample should be cooled down to the temperatures below the Néel temperature of the AFM surface.…”

Antiferromagnetic layer thickness dependence of the coercivity of the ferromagnetic/antiferromagnetic two‐phase system

“…In recent years, the magnetic two‐phase system has attracted the attention of many authors . Many kinds of materials belong to the magnetic two‐phase system, such as ferromagnetic/antiferromagnetic (FM/AFM) bilayers , FM/AFM core–shell nanoparticles , and exchange‐spring composite systems . A magnetic two‐phase system has many unique properties, such as the wasp‐waisted hysteresis loop, etc., and has achieved important applications in spintronics devices.…”