Exchange Bias HE Research Articles

Large exchange bias and enhanced coercivity in strongly-coupled Ni/NiO binary nanoparticles.

In this study, Ni/NiO binary nanoparticles are synthesized utilizing a reflux method combined with a calcination process. The average size of the nanoparticles is 5–20 nm and the Ni content is 3.55%. Both the microstructures of the Ni/NiO interface and the states of different phases have significant impacts on the magnetic properties. By tuning the temperature and the cooling field during the loop measurement, the change rule of several critical parameters such as coercivity HC and exchange bias HE was complicated in nature. Both large HE (482 Oe) and enhanced HC (1335 Oe) were observed at 5 K, mainly due to the strong coupling interaction between Ni and NiO components. For current studies of the Ni/NiO binary nanoparticles, the complex magnetic behaviors are related to (i) the ferromagnetic contribution of Ni nanoparticles, (ii) the intrinsic antiferromagnetism of the volume phase of NiO, and (iii) the spin-glass-like characteristic corresponding to the frozen disordered state at the surface of partial NiO particles. The comprehensive effect of these three magnetic structures promotes the generation of a strongly-coupled Ni/NiO binary system, and improves the magnetic performance.

Giant spontaneous exchange bias triggered by crossover of superspin glass in Sb-doped Ni50Mn38Ga12 Heusler alloys.

A spontaneous exchange bias (SEB) discovered by Wang et al. [Phys. Rev. Lett. 106 (2011) 077203.] after zero-field cooling (ZFC) has attracted recent attention due to its interesting physics. In this letter, we report a giant SEB tuned by Sb-doping in Ni50Mn38Ga12-xSbx Heusler alloys. Such an SEB was switched on below the blocking temperature of approximately 50 K. The maximum exchange bias HE can arrive at 2930 Oe in a Ni50Mn38Ga10Sb2 sample after ZFC to 2 K. Further studies showed that this SEB was attributable to interaction of superspin glass (SSG) and antiferromagnetic matix, which was triggered by the crossover of SSG from canonical spin glass to a cluster spin glass. Our results not only explain the underlying physics of SEB, but also provide a way to tune and control the SEB performance.

Tuning Exchange Bias Effect in Ni<sub>50</sub>Mn<sub>36</sub>Sn<sub>14</sub> Heusler Alloy

The tuning exchange bias HE at martensitic state of Ni50Mn36Sn14 alloy has been investigated by means of hysteresis loop measurement. It was found that the whole loop can be tuned by HFC from a double-shifted to a single-shifted hysteresis loop, leading to an appearance of maximum HEat HFC = 0.5 kOe. This behavior could be ascribed to the competition between two types of AFM clusters and HFC,which exhibits predominantly atlow HFC range, while the competition between FM clusters and HFC, which becomes predominant at high HFC range.

Theoretical Investigation of Exchange Bias in Compensated Cases

The exchange bias (EB) of the ferromagnetic (FM)/antiferromagnetic (AFM) bilayers in a compensated case is studied by use of the many-body Green's function method of quantum statistical theory. The so-called compensated case is that there is no net magnetization on the AFM side of the interface. Our conclusion is that the EB in this case is primarily from the asymmetry of the interfacial exchange coupling strengths between the FM and the two sublattices of the AFM. The effects of the layer thickness, temperature and the interfacial coupling strength on the exchange bias HE are investigated. The dependence of HE on the FM layer thickness and temperature is qualitatively in agreement with experimental results. HE is nearly inversely proportional to FM thickness. When temperature varies, both HE and HC decrease with temperature increasing. The anisotropy of the FM layer only slightly influence HC, but does not influence HE.

Theoretical investigation of exchange bias

The exchange bias of bilayer magnetic films consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers in an uncompensated case is studied by use of the many-body Green's function method of quantum statistical theory. The effects of the layer thickness and temperature and the interfacial coupling strength on the exchange bias HE are investigated. The dependence of the exchange bias HE on the FM layer thickness and temperature is qualitatively in agreement with experimental results. When temperature varies, both the coercivity HC and HE decrease with the temperature increasing. For each FM thickness, there exists a least AFM thickness in which the exchange bias occurs, which is called pinning thickness.

Quantum Statistical Calculation of Exchange Bias

The phenomenon of exchange bias of ferromagnetic (FM) films, which are coupled with an antiferromagnetic (AFM) film, is studied by Heisenberg model by use of the many-body Green's function method of quantum statistical theory for the uncompensated case. Exchange bias HE and coercivity are calculated as functions of the FM film thickness L, temperature, the strength of the exchange interaction across the interface between FM and AFM and the anisotropy of the FM. decreases with increasing L when the FM film is beyond some thickness. The dependence of the exchange bias HE on the FM film thickness and on temperature is also qualitatively in agreement with experiments.

Variation of domain formation in a 15 nm NiFe layer exchange coupled with NiO layers of different thicknesses

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.

Hysteretic ferromagnetic resonance as a probe for coercivity, exchange bias, and loop asymmetry

The in-plane angular variation of hysteretic ferromagnetic resonance (HFMR) at 9.57 GHz, in increasing and decreasing field sweeps, is used to determine both coercivity HC and exchange bias HE in a ferromagnetic/antiferromagnetic Co/MnPt bilayer. The angular dependence of HC and HE is in excellent agreement with micromagnetic models in which the interfacial exchange energy is approximately equal to the domain-wall energy in the antiferromagnet. The HFMR linewidths show a systematic angular variation, indicating that domain formation is important.

The exchange coupling of a NiO/FeNi bilayer withinterdiffused interface

A series of NiO(110 Å)/FeNi(tF Å)/Cu(24 Å)/FeNi(tF Å)spin-valve-structure samples were fabricated using ion beamsputtering. The exchange bias HE of the pinned FeNi layerincreases as the FeNi thickness tF decreases. For tF<70 Å, however, HE decreases with the decrease of tF.The coercivity HCp varies as 1/tF at room temperature.The M-H loops show that the magnetic moment of the pinned FeNilayer is smaller than that of the free FeNi layer, indicating thatan FeNi layer of about 20 Å is interdiffused with the NiO layerleading to a drop of the magnetic moment of the pinned FeNi layer. Theinterdiffusion between the FeNi and NiO layers may account for thedecrease of the exchange bias HE for tF<70 Å.

Oscillatory Angular Dependence of Exchange Bias for Epitaxial NiO-Co (001) Bilayers

ABSTRACTWe have sputter-deposited NiO-Co bilayers on MgO (001) substrates. NiO grows epitaxially on MgO at 900°C and subsequently the room deposited 80 Åthick Co films have a fcc crystal structure in epitaxy with the oxide underlayer. These samples were warmed up to 300° C and then zero-field or field cooled through the NiO Néel temperature (a 300 Oe magnetic field was applied along the [100] or the [110] MgO axis). Magnetic hysteresis loops were obtained by magneto-optical Kerr effect, the magnetic field being oriented in the plane of the substrate for various angles α with respect to the [100] direction. The usually expected behavior for such experiments is a smooth angular α dependence of the exchange bias HE close to a cosine with only one sign change over 180°. The high crystallographic coherence of our NiO/Co bilayers induces a very unusual oscillatory HE (α) dependence with several sign changes according to the NiO axis field application. Despite of the Co magnetization switching mechanism which is not a pure coherent rotation, we propose a Stoner-Wohlfhart model including four fold anisotropy and unidirectional exchange anisotropy giving a realistic description of these typical magnetic properties.