Exchange Bias Training Effect Research Articles

The martensitic structures, exchange bias effect and training effect in Ni-Mn-Ga ribbons

The exchange bias effect is the physical basics of spin valves and magnetic recording devices. A stable exchange bias effect would be suitable for practical application. In this work, Ni50Mn50-xGax (11≤ x ≤16) ribbons were obtained by melt spinning. The grain size and microstructure have been tuned through melt spinning, which changes the exchange interaction in ribbons and thus gets a giant and stable exchange bias effect. As the increase of x, the structure of Ni50Mn50-xGax (11≤ x ≤16) ribbons undergoes a transition from non-modulated martensite to non-modulated martensite &10 layers modulated martensite to 10 layers modulated martensite. The average grain size of all ribbons is less than 10 μm, and the martensitic plate widths are smaller than 200 nm. With the increasing x or the cooling fields, the magnetization of the ribbons has been enhanced. As a result, the competition between ferromagnetic and antiferromagnetic interactions has been changed and thus influencing spin glass behavior. Furthermore, a low degree of attenuation for the exchange bias effect after continuous cycles has been achieved which arises from the stable spin configuration with a larger contribution of frozen spin which has a slow relaxation rate. For example, the exchange bias field of 8.78 kOe in Ni50Mn38Ga12 ribbons can be achieved under the 40 kOe field cooling at 10 K, and the exchange bias field after infinite cycles can be kept at 8.58 kOe, the degree of attenuation is only 3.4 %. This work enriches the research on the exchange bias effect of Heusler alloys and provides the experimental basis and theoretical support for designing a stable exchange bias effect.

Formula omitted]-antisite disorder driven exchange bias effect and Griffiths-like phase in 25% Sr-doped [formula omitted

The unusual magnetic behaviour and multiple magnetic phases in double perovskite systems mainly originate from B-antisite disorder and various exchange couplings. Here we report the detailed exchange bias study in the hole doped Sm1.5Sr0.5CoMnO6 double perovskite system. The existence of structural antisite-disorder (Co-O-Co-O-Mn or Mn-O-Mn-O-Co) in the system is revealed. The ordering of Co2+ and Mn4+ gives rise to two ferromagnetic transitions, Tc1∼133 K and Tc2∼114 K. A Griffiths-like phase at ∼153 K is identified by the dc magnetization data above the ferromagnetic ordering temperature. The isothermal magnetization measurements further probe the exhibition of the conventional exchange bias effect and training effect phenomena in our sample. Maximum field shift ∼3.43 kOe in exchange bias effect is observed owing to the exchange couplings at the interfaces of FM/AFM at low temperature induced by B-antisite-disorder. In the 64–300 K temperature range, this disordered double perovskite exhibits insulating characteristics with a variable range hopping type of conduction mechanism. We also notice a negative magnetoresistance of −2.74% at 64 K and 60 kOe magnetic field.

Exchange bias effect and inhomogeneous magnetism in 6H Ba3CoFeRuO9: Role of structural site disorder

Structural, magnetic, electrical and thermal investigations on ternary barium ruthenate Ba3CoFeRuO9 (BCFRO) reveal the role of the two different octahedral B-site in creating locally varying competing antiferromagnetic and ferromagnetic interactions. BCFRO forms in 6H hexagonal structure in the P63/mmc space group. Neutron diffraction and specific heat capacity measurements affirmed the absence of any long-range ordering and structural transition in the compound. DC thermomagnetic studies evidences spin glass magnetism with Tg ∼ 50 K. A large exchange bias (EB) field of 3.48 kOe is obtained at 3 K under a cooling field of 50 kOe. Careful analysis indicates, EB in our system stems from inhomogeneous magnetism owing to spatially varying chemical composition. Competing ferromagnetic and antiferromagnetic phases giving rise to spin glass behaviour is the manifestation of site disorder induced by mixed valences of magnetic ions. The exchange bias training effect in our system follows spin relaxation model of rotatable and frozen spins which are exchange coupled at the interface. Strong electron localisation occurs by virtue of the specific 6H structure with corner shared octahedral 2a site and face shared octahedral 4f site. The electrical and thermal transport in BCFRO follows Efros-Shklovskii variable range hopping (ES VRH) model below 150 K. The high resistivity of BCFRO at low temperatures is also reflected in the low thermal conductivity and huge Seebeck coefficient.

Investigation of Exchange Bias in Irmn/Nife Bilayer Thin Films Using Magnetoresistance Technique

A bottom pinned Si/SiO2/Ta(20)/IrMn(15)/NiFe(10)/Ta(5) film has been fabricated using an ion beam sputtering system at room temperature. Post-deposition magnetic annealing was performed on sputtered multilayer thin film in a vacuum of 2´10-6 mbar at 250˚C for one hour in an in-plane magnetic field of 1.8 kG. The X-ray diffraction and X-ray reflectivity measurements have been performed for the structural and thickness analysis, respectively. The roughness of individual layers was determined using XRR measurements. The longitudinal MOKE measurements have been performed to measure the exchange bias, coercivity, and magnetic anisotropy at room temperature. The magnetoresistance measurement technique is employed to demonstrate the thermal evaluation of the exchange bias effect, training effect, and magnetization reversal asymmetry at room temperature and low temperature. The magnetoresistance measurement revealed a large exchange bias, pronounced magnetization reversal asymmetry, and enhancement in the training effects at 20 K. The experimental result exhibits the occurrence of temperature-dependent change from uniaxial to biaxial exchange anisotropy.

Exchange bias training effect in IrMn-layer/ferromagnetic-ribbon heterostructures probed with magnetoimpedance

The exchange-bias training effect in IrMn-layer/ferromagnetic-ribbon heterostructure is studied by performing magnetoimpedance (MI) measurements. Asymmetric, hysteretic and single peak behavior of the MI response and a shift in the MI peak to zero fields accompanied by 52% increase in MI has been detected as the signature of the exchange bias (EB) and training effect (TE), respectively. Also, during the consecutive filed sweep of MI response, both EB field and the degree of asymmetry of MI decrease that is another reason for existence of TE in our sample. The analysis of the magneto-optical Kerr effect (MOKE) and MI behavior establish that Hoffmann s model (Phys. Rev. Lett. 93, 097203 (2004)) is a good description of our experimental data, due to the existence of a strong single cycle TE in our IrMn-layer/ferromagnetic-ribbon system.

Structural and magnetic properties of molecular beam epitaxy grown chromium selenide thin films

Chromium selenide thin films were grown epitaxially on Al${_2}$O${_3}$(0001) and Si(111)-(7${\times}$7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 {\AA}. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a N\'eel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

Exchange bias effect in hierarchical NiO/NiFe2O4 sub-microcubes fabricated via a self-sacrifice template process

In this study, we report a facile in situ synthesis of hierarchical NiO/NiFe2O4 sub-microcubes by annealing Ni3[Fe(CN)6]2 sub-microcubes fabricated via a one-step solution-precipitation method. The temperature-dependent magnetization curves show that there exists a collective freezing of the moments of interface spin clusters at TB≈296K, which further indicates the appearance a coupling interaction between the ferrimagnetic NiFe2O4 and antiferromagnetic NiO nanoparticles. Furthermore, hysteresis loops measurements reveal the existence of an exchange bias effect in the NiO/NiFe2O4 composite system. Interestingly, no training effect of exchange bias is observed, which indicates that the magnetic pinning interaction between NiFe2O4 and NiO is stable and robust. Exchange bias field shows an exponential dependence on temperature and vanishes at about 150 K, which reveals the strong coupling interaction does not improve effectively the thermal stability of exchange bias effect in the current system.

Exchange bias in Fe/Ir20Mn80 bilayers: Role of spin-glass like interface and ‘bulk’ antiferromagnet spins

We have performed magnetic measurements such as temperature (T), cooling field (μ0HFC) dependence and training effect of exchange bias (EB) to investigate the magnetic nature of the interface in the Fe/Ir20Mn80 system. Bilayers of different thicknesses of Ir20Mn80 (antiferromagnetic) have been prepared by dc magnetron sputtering at room temperature. The variation of exchange bias field (μ0HEB) with Ir20Mn80 thickness indicates that the ‘bulk’ spins of the antiferromagnet contribute to EB. Exponential decay of μ0HEB and coercive field (μ0HC) with T reveals the possible presence of a spin glass (SG) like interface. The decrease of μ0HEB with increasing μ0HFC provides further evidence of SG like behaviour at the interface. The results of training effect data indicate the presence of frozen and rotatable spins at the magnetically frustrated interface of these Fe/Ir20Mn80 bilayers.

Exchange Bias Training Effect in GdBaCo2O5.5 Cobaltite

The nature of the training effect in exchange bias compounds, which consists in a decrease in the exchange bias field HEB under magnetic field cycling, is clarified. It is shown that the training effect in GdBaCo2O5 + δ cobaltite with δ ≈ 0.5 is caused by a decrease in magnetization. Depending on time, magnetization M(t) decreases sharply after the first cycle and slowly in subsequent N cycles. A direct relationship (proportionality) is established between the decrease in the magnetization M(t) of ferromagnetic particles and the change of the field HEB(t). It is assumed that the cooling of a sample in a magnetic field gives rise to small (single-domain) and large ferromagnetic clusters in an antiferromagnetic environment. The sharp decrease in M(t) and HEB(t) and the deviation of the function HEB(N) from the well-known power-law relation ΔHEB ∝ N–1/2 with N > 1 is explained by the transformation of large ferromagnetic clusters into a multidomain state under magnetic field cycling. The results are consistent with the phenomenological model of Meiklejohn and Bean, which predicts a linear dependence of the exchange bias on the ferromagnetic magnetization of a ferromagnet/antiferromagnet structure.

The exchange bias behavior of BiFeO3 nanoparticles with natural core-shell structure

The surface and interface effects of small antiferromagnetic nanostructures are important on the modulation of their magnetic properties. In this report, temperature and particle size dependent magnetic exchange bias effect was investigated in BiFeO3 (BFO) nanoparticles that possess natural core-shell structure. Nonmonotonic variation of exchange bias field, interesting surface spin-glass state and improved exchange bias training effect are only obtained in 18 nm BFO particles. Based on comparative experiments on particles with different sizes, we found that the surface spins and the interaction among them show great effect on the interfacial exchange coupling of the core-shell structure, and thus are responsible for the peculiar exchange bias behavior in small BFO nanoparticles. Our work provides the effect of surface spin state on the magnetic characteristics of nanomaterials and will favor their applications on spintronic devices.

Exchange bias training effect in phase separated polycrystalline Sm0.1Ca0.7Sr0.2MnO3

Magnetic properties of antiferromagnetic (AFM) electron doped manganite Sm0.1Ca0.7Sr0.2MnO3 have been investigated, focusing mainly on the exchange bias (EB) effect and associated training effect. The studied compound exhibits the ground state with heterogeneous spin configuration, consisting of the C-type antiferromagnetic phase with the Néel temperature TN-C ≈ 120 K, the G-AFM phase with the Néel temperature TN-G ≈ 60 K, and ferromagnetic-like phase with a very weak spontaneous magnetic moment. Measurements of hysteresis loops have shown that the exchange bias field monotonously decreases with increasing temperature and vanishes above 40 K, while the coercivity disappears only above 70 K. The temperature variation of the exchange bias field has been successfully described by an exponential decay form. The stability of EB has been evaluated in the studies of the training effect, which has been discussed in the frame of the spin relaxation model, elucidating the important role of the AFM domain rearrangement at the interface. The complex phase separation and possible contributions from different interfaces between coexisting magnetic phases to the EB effect have also been discussed.

A Simple Model to Describe Different Types of Exchange Bias Training Effect

The exchange bias (EB) training effect, referring to the exchange field (H E) and/or the coercivity (H C) decreasing with the magnetic cycle (n), is often accompanied with EB. Usually, the EB training effect has different types, showing that H C1 (coercive field at the descending branch) and H C2 (coercive field at the ascending branch) change with n differently. In order to understand the origin producing the training type, a phenomenological model is therefore proposed. According to this model, how H C1 (or H C2) changes with n is determined by the change of pinning magnetization at the descending (or ascending) branch during the training process. For verifying the validity of our model, various experimental training results with respect to different exchange-biased systems are selected for fitting and all the fitting results are nearly perfect.

The Loop-Stage-Dependent Exchange Bias Training Effect in FeNi/FeMn Bilayers

The loop-stage-dependent exchange bias training effect has been systematically studied in the polycrystalline FeNi/FeMn bilayers. According to the ferromagnet (FM) magnetization states, an entire magnetic hysteresis loop can be divided into four different stages: 1) positive saturation stage; 2) descending branch; 3) negative saturation stage; and 4) ascending branch. The loop-stage-dependent training effect was studied by probing the instant variation of the exchange field and the coercivity after adding a short waiting time $\Delta t$ on a specific loop-stage. Our experimental results unambiguously show that this training effect strongly depends on the stage that the waiting time is assigned on and the quantity of $\Delta t$ as well. Based on the macroscopic two-level relaxation model, these phenomena can be well explained by considering different switching probabilities of the interfacial uncompensated antiferromagnetic spins during different FM magnetization reversal processes.

Temperature dependence of exchange bias and training effect in Co/CoO film with induced uniaxial anisotropy

The exchange bias effect and training effect of the Co/CoO film with induced uniaxial anisotropy were investigated as functions of temperature. It was found that both effects exhibited drastic differences along the easy and the hard axes. Along the easy axis, the magnetization reversal was dominated by domain wall motion throughout the whole temperature range. However, along the hard axis, the magnetization reversal was dominated by domain wall motion and domain rotation at temperatures below and above 150 K, respectively. The crossover of the two reversal modes characterized with significant asymmetry in the hysteresis loop was observed along the hard axis at 150 K due to the interplay between the exchange and uniaxial anisotropies. Significant difference of training effect in the two directions was observed and ascribed to the differences of the duration and intensity of the interaction between ferromagnetic and antiferromagnetic spins in the two magnetization reversal modes.

Giant exchange bias behavior and training effect in spin-glass-like NiCr2O4/NiO ceramics

NiCr2O4/NiO ceramic has been synthesized successfully through co-precipitation method to study the magnetic properties. Field cooling (FC) magnetic hysteresis loops measured at low temperature show significant shift in both coercive field and remnant magnetization. It has been plotted that the exchange bias (EB) field can be as large as 11.86 kOe at 10 K followed with an enhanced coercive field. The variation of EB field and shift of remnant magnetization after FC processes shows a regular tendency with increasing temperature and cooling field. Known as training effect, the EB behavior also presents a strong dependence on loop cycle. The cycle dependence of large EB field, vertical shift of magnetization, and coercive field at low temperature can be ascribed to the pinned spins in spin-glass-like (SGL) phase of the system interface. The SGL phase has been proved by Almeida–Thouless line and high field relaxation effect. The disorder structure is responsible for the frustration interface and the formation of SGL phase in the interface.

Temperature dependent exchange bias training effect in single-crystalline BiFeO3/Co bilayers

Single-crystalline BiFeO3 (BFO)/Co bilayers were prepared by combined pulsed laser deposition and magnetron sputtering on (001) SrTiO3 substrates. Exchange bias (EB) and accompanying training effect have been studied as a function of temperature (T) between 5 K and 300 K. A non-monotonic exchange field variation with sharp increase below 100 K has been observed. In the meanwhile, strong training effect was recorded when T < 100 K and it weakens monotonically with increasing T up to 300 K. These temperature dependent EB and training effect may be caused by the uncompensated spins in both the interfacial spin-glass (SG) phase at low temperature and the antiferromagnetic BFO layer at higher temperature. The low temperature EB training results can be well fitted by a modified Binek's model considering asymmetric changes of the pinning SG spins at the descending and the ascending branches.

Metastable magnetic state and exchange bias training effect in Mn-rich YMnO3 thin films

The magnetic nature of stoichiometric and Mn rich hexagonal YMnO3 films grown at different oxygen partial pressures is investigated. The stoichiometric YMnO3 thin film showed antiferromagnetic ordering below 72 K; while the film having excess Mn content revealed metastable magnetic behaviour at a temperature of less than 42 K. The metastable magnetic behaviour in this sample is attributed to the competing anti-ferromagnetic and ferromagnetic phase fractions (caused by Mn2+ ions that originated due to excess Mn content). Also, in the metastable regime the exchange bias effect is observed, which is further supported by the training effect. Both the samples reveal room temperature ferroelectric behaviour.

Origin of the exchange bias training effects in magnetically coupled soft/hard synthetic bilayers at low temperature

Hysteresis loops of the nanoscale magnetic layer Co90Fe10 and Ni81Fe19 and bilayer Co90Fe10/Ni81Fe19 and Ni81Fe19/Co90Fe10 films were measured as a function of external dc magnetic field and the thickness dependence of these films were plotted as a function of temperature. Time evolution of the minor/middle/major hysteresis loops of 5/5nm-thick Ni81Fe19/Co90Fe10 monolayer have been observed at 10K. The spin valve, exchange bias training and Barkhausen effects for magnetic layer and bilayer films have been analysed at various temperatures, thicknesses and different orientations according to the substrate. The exchange-bias training effects have been observed only in positive magnetization region. Origin of the exchange-bias training effects and asymmetric hysteresis loops are related to the relaxation mechanism of a pinning layer in magnetically coupled soft/hard bilayers.

Equilibrium surface magnetization of α-Cr2O3 studied through interfacial chromium magnetization in Co/α-Cr2O3 layered structures

We show experimental evidence of the equilibrium surface magnetization of α-Cr2O3(0001) by studying chromium magnetization at the Co(111)/α-Cr2O3(0001) interface. The soft X-ray magnetic circular dichroism intensity from uncompensated Cr spins was found to be almost independent of the interface roughness and exchange anisotropy energy. Moreover, no exchange bias training effect was found, except in the transition temperature regime, suggesting that the antiferromagnetic spin structure, including the interface spin arrangement, is in equilibrium. Furthermore, the exchange bias polarity was switched by magnetoelectric field cooling, which is also a salient feature of the surface magnetization of α-Cr2O3(0001).

Asymmetric exchange bias training effect in spin glass (FeAu)/FeNi bilayers

A significant exchange bias (EB) training effect has been observed in sputter deposited FeAu/FeNi bilayers, wherein the exchange field (HE) exhibits a special sign-changeable temperature dependence. Very interestingly, despite the absence of multiple easy axes in the FeAu spin glass (SG) layer, HE drops abruptly between the first and second magnetic cycles, which is followed by a more gradual continuous change in the subsequent cycles. This training behavior cannot be described by the empirical n−1/2 law because of the asymmetric magnetization reversal processes. We propose modifying Binek's model to include the asymmetric changes of the pinning SG spins at the descending and ascending branches. This new model successfully describes the EB training effect in FeAu/FeNi bilayers.