Strength Of Cooling Field Research Articles

Exchange bias behavior in cobalt ferrite-cobalt oxide CoFe2O4/CoO nanocomposites for data storage applications

In this study cobalt ferrite-cobalt oxide CFO/CoO nanoparticles were synthesized using hydrothermal chemical method for studying the exchange bias (EB) phenomena, a key parameter for data storage applications. The structural analysis was carried out by Scanning electron microscopy (SEM), x-ray diffraction (XRD) and Energy dispersive spectroscopy (EDS), which confirm formation of platelets like CFO/CoO nanoparticles with appropriate chemical composition of different elements with an average crystallite size of 15 nm. Magnetic study revealed the ferromagnetic behaviour of nanoparticles from (5 K–360 K) and blocking temperature has found to be above 360 K, which decreases nonmonotonically with increasing the applied field HFC. The field cool (FC) magnetic hystersis (MH) loop studies have revealed that the samples exhibit very large exchange bias effect. The values of exchange bias (HEB) is found to be maximum at low temperature and decreases with increasing temperature before vanishing at the blocking temperature. The aim of the study is to determine role of cooling field strength for the exchange bias in magnetically contrasted CFO/CoO nanoparticles suitable for data storage applications.

Oscillatory exchange bias and training effects in nanocrystalline Pr0.5Ca0.5MnO3

We report on exchange bias effects in 10 nm particles of Pr0.5Ca0.5MnO3 which appear as a result of competing interactions between the ferromagnetic (FM)/anti-ferromagnetic (AFM) phases. The fascinating new observation is the demonstration of the temperature dependence of oscillatory exchange bias (OEB) and is tunable as a function of cooling field strength below the SG phase, may be attributable to the presence of charge/spin density wave (CDW/SDW) in the AFM core of PCMO10. The pronounced training effect is noticed at 5 K from the variation of the EB field as a function of number of field cycles (n) upon the field cooling (FC) process. For n > 1, power-law behavior describes the experimental data well; however, the breakdown of spin configuration model is noticed at n ≥ 1.

Dependence of exchange coupling direction on cooling-field strength

We studied the dependence of exchange coupling on cooling-field strength in an exchange-biased spin valve with a synthetic antiferromagnetic layer by experiment and theory. Our theory calculates magnetic anisotropy energies in each magnetic layer composing the spin valve during the field-cooling process, finds the minimum state of total energy, and explains how the magnetizations in the layers interact with one another during field-cooling under various cooling-field strengths. Calculations based on the theory well match results of the experimental measurements. Our observation shows that one has to carefully choose the cooling-field strength optimal for designing exchange-biased spin devices having a synthetic antiferromagnetic layer; otherwise the exchange coupling direction can significantly deviate from the cooling-field direction, which impairs performance.

Angular dependence of the exchange bias direction and giant magnetoresistance on different cooling-field strengths

We studied the effect of different cooling-field strengths on the exchange bias by measuring the angular-dependent sheet resistance and the giant magnetoresistance of exchange-biased spin valves using a PtMn antiferromagnetic and a synthetic antiferromagnetic layer. When we annealed the spin valve at a cooling-field of 100 Oe, the exchange bias was antiparallel to the cooling-field. As we increased the cooling-field to 4000 Oe, the exchange bias direction gradually rotated and it ended up parallel to the cooling-field direction. The giant magnetoresistance also changed with the cooling-field strength. In the cooling-field range between 100 and 4000 Oe, the magnetoresistance ratios measured along the cooling-field direction were significantly reduced. However, the magnetoresistance ratios measured along the exchange bias direction increased, although still remaining smaller than those of the spin valve annealed at 100 or 4000 Oe. On the other hand, the exchange bias strength did not change significantly with the cooling-field strength.

Effect of cooling field strength and ferromagnetic shell shape on exchange bias in nanoparticles with inverted ferromagnetic–antiferromagnetic core‐shell morphology

AbstractThe dependence of exchange bias (EB) effects on cooling field strength and particle shape in nanoparticles with antiferromagnetic (AFM) interfacial coupling and inverted AFM core with a fixed radius and ferromagnetic (FM) shell with various thicknesses are investigated by using a modified Monte Carlo Metropolis method. It is found that with the increase of cooling field, field‐cooled exchange bias field (HE) fluctuates in the range of negative values initially, and then has an abrupt jump from the negative value to the positive value, finally levels off. However, HE decreases as the FM shell shape varies from No. 1 to No. 13 regardless of the strength of cooling field. Coercivity is affected by cooling fields and shapes indicating distinct behaviors. Because the AFM core is almost unaffected by shape and frozen completely during measuring hysteresis loops, the effect of ferromagnets on EB, negligible in most of other systems, is ambiguously manifested in such an unconventionally structural system. Moreover, the phenomena are interpreted well by presenting the snapshots of microscopic spin energy distributions, which make us observe directly and vividly the movement of domains and the competition of energies. This work will shed new light into the microscopic origin of peculiar magnetic properties of nanoparticles with special structures.

Evidence of intrinsic exchange bias and its origin in spin-glass-like disorderedL0.5Sr0.5MnO3manganites (L=Y,Y0.5Sm0.5, andY0.5La0.5)

Exchange-bias (EB) phenomena have been observed in the ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($L=\mathrm{Y}$, ${\mathrm{Y}}_{0.5}{\mathrm{Sm}}_{0.5}$, and ${\mathrm{Y}}_{0.5}{\mathrm{La}}_{0.5}$)-type manganites showing cluster-glass-like and spin-glass-like behavior. The field cooled magnetic hysteresis loops exhibit shifts both in the field and magnetization axes. The values of exchange field $({H}_{E})$, coercivity $({H}_{C})$, remanence asymmetry $({M}_{E})$, and magnetic coercivity $({M}_{C})$ of ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ are found to depend strongly on temperature, measuring field, as well as strength of cooling field ${H}_{\mathrm{cool}}$. ${H}_{E}$ increases sharply with the magnitude of ${H}_{\mathrm{cool}}$ $(\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, but for larger ${H}_{\mathrm{cool}}$ $(g3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, it decreases due to the growth of the ferromagnetic cluster size. This observed behavior has been explained in terms of interfacial exchange coupling between coexisting ferromagnetic cluster glass and the disordered spin-glass-like phases, where the spin configurations are strongly affected by the cooling field strength. Below the spin and/or cluster glass freezing temperature $({T}_{f})$, both ${H}_{E}$ and ${M}_{E}$ decrease exponentially with temperature. The value of ${H}_{C}$ increases almost exponentially with increasing magnetic-field step size $(\ensuremath{\Delta}H)$. In addition, the observed training effect of the EB behavior has been explained well in terms of the existing relaxation model developed for other classical EB systems. In view of the use of manganites in spintronics, the present observation of EB-like shift even in the mixed valent polycrystalline manganites such as ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ is of paramount importance.

Coexistence of exchange-bias fields and vertical magnetization shifts in ZnCoO∕NiO system

Exchange fields accompanying vertical magnetization shifts were observed in the epitaxial ZnCo0.07O (dilute magnetic semiconductor)/NiO system after field cooling. Transitions of exchange fields and magnetization shifts were observed at 50K, above which the magnetization shift disappeared and the exchange field was significantly reduced. Both the exchange field and the magnetization shift increased with increasing cooling-field strength at temperatures below 50K, which might be attributed to the existence of “frozen” spins in ZnCoO. The observed linear dependence of the exchange field on the magnetization shift may directly elucidate the role of pinned spins on the exchange fields.

Loop bifurcation and magnetization rotation in exchange-biasedNi∕FeF2

Exchange biased Ni/ FeF2 films have been investigated using vector coil vibrating sample magnetometry as a function of the cooling field strength H_FC. In films with epitaxial FeF2, a loop bifurcation develops with increasing H_FC as it divides into two sub-loops shifted oppositely from zero field by the same amount. The positively biased sub-loop grows in size with H_FC until only a single positively shifted loop is found. Throughout this process, the negative/positive (sub)loop shift has maintained the same discrete value. This is in sharp contrast to films with twinned FeF2 where the exchange field gradually changes with increasing H_FC. The transverse magnetization shows clear correlations with the longitudinal sub-loops. Interestingly, over 85% of the Ni reverses its magnetization by rotation, either in one step or through two successive rotations. These results are due to the single crystal nature of the antiferromagnetic FeF2, which breaks down into two opposite regions of large domains.

Effects of cooling field strength on exchange anisotropy at permalloy/CoO interfaces

We have studied exchange anisotropy at ferromagnet/antiferromagnet (FM-AFM) interfaces as a function of cooling field in Ni80Fe20 (permalloy) films deposited on bulk single-crystal CoO substrates. Hysteresis loops measured after cooling through the antiferromagnetic ordering temperature in different magnetic fields show that the exchange bias has little dependence on cooling field. Large cooling fields produced hysteresis loops with larger remanent magnetization and larger coercivity. These trends can be explained using a model that assumes that the FM anisotropy axis directions can be influenced by the magnetic history of the AFM. Large cooling fields also led to larger CoO susceptibility, which implies rotation of the CoO spin axes. The simultaneous rotation of the FM anisotropy axes and the AFM spin axes suggests perpendicular coupling at the FM-AFM interface.