Zero-field Cooled Exchange Bias Research Articles

Emergence of exchange bias effect in double perovskite La2Cu0.9Fe0.1IrO6 originated from Fe doping

The effect of partial electron doping (10% Fe) in the antiferromagnetic (AFM) double perovskite La2CuIrO6 (LCO) has been investigated at the macroscopic level using X-ray diffraction (XRD), and DC Magnetic measurements to study the evolution of complex magnetic order. The experimental observations indicate a significant modification in the nature of microscopic magnetic interaction induced by charge imbalance resulting in strong ferromagnetic (FM) ordering in the parent antiferromagnetic LCO. The nature of magnetic interaction results in a cluster-glass state (CG) with novel zero-field-cooled exchange bias (ZEB) and conventional exchange bias (CEB) effect at low temperature. Obtained ZEB field value is 0.735 kOe at 3 K, while an unusually large CEB field of 9.24 kOe for a bias cooling field (HB) of 50 kOe has been observed at the same temperature. The doped LCO also shows the training effect in field cooled M(H) measurements, following the frozen spin relaxation behavior, indicating the cluster-glass (CG) nestle in the interface/pinning boundary. The emerged complex exchange interaction among different magnetic ordering, i.e., FM, AFM, and CG, significantly influences the CEB as well as ZEB.

Absence of zero-field-cooled exchange bias effect in single crystalline La2−xAx CoMnO6 (A=Ca,Sr) compounds

Magnetic properties of A2BB'O6 (A = rare or alkaline earth ions; B,B' = transition metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero field cooled exchange bias (ZEB) effect observed for the hole doped La2-xAxCoMnO6 polycrystalline samples. In this work we synthesize La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca and Sr doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples.

Maximizing Zero-Field-Cooled Exchange Bias in Crystallized Co@CoO Nanocluster Assembled Thin Film by Varying Film Thickness

In this paper, crystallized core–shell Co@CoO nanocluster assembled thin films with different thicknesses were prepared by magnetron sputtering. A large zero-field-cooled exchange bias (ZEB) around...

Effect of Ge substitution on the magnetocaloric effect and exchange bias in Mn–Ni–Sn-Ge alloys: The role of Mn–Mn distance

Abstract The effect of Ge substitution on the phase transitions, magnetocaloric effect, and exchange bias is studied in Mn50Ni40Sn10-xGex alloys by structural and magnetic analysis. Due to the lattice contraction induced by Ge substitution, the martensitic transformation temperature increases gradually, and the reentrant spin glass ground state is suppressed at x = 5 where zero-field-cooled exchange bias (ZEB) emerges. The maximum values of magnetic entropy change and ZEB are 19.2 J/kg·K for the field change of 5 T and 1.49 kOe at 10 K, which are achieved in Mn50Ni40Sn8Ge2 and Mn50Ni40Sn4Ge6, respectively. Importantly, the second-nearest Mn–Mn distance is found to be a key parameter affecting the magnetization change across the martensitic transformation and the ground state of these alloys. This work sheds new light on understanding the magnetism in Ni–Mn-based ferromagnetic shape memory alloys.

Zero field cooled exchange bias effect in nano-crystalline Mg-ferrite thin film

The Zero Field Cooled (ZFC) Exchange Bias (EB) effect in a single phase nanocrystalline Mg-ferrite thin film, deposited on an amorphous quartz substrate using pulsed laser ablation technique, is reported. The film showed a high ZFC EB shift (HE ∼ 190 Oe) at 5 K. The ZFC EB shift decreased with increasing temperature and disappeared at higher temperatures (T > 70 K). This Mg-ferrite thin film also showed the Conventional Exchange Bias (CEB) effect, but unlike many CEB systems, the film showed a decrease in coercivity (HC) under the Field Cooled measurements. The film also showed the training effect in ZFC measurements, which followed the frozen spin relaxation behavior. The observed exchange bias could be attributed to the pinning effect of the surface spins of frozen glassy states at the interface of large ferrimagnetic grains.

Zero-field-cooled exchange bias effect in phase-segregated La2−xAxCoMnO6−δ (A=Ba,Ca,Sr;x=0,0.5)

In the zero-field-cooled exchange bias (ZEB) effect the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of external magnetic field. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ stands out as presenting the largest ZEB reported so far, while for La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ the exchange bias field ($H_{EB}$) is one order of magnitude smaller. Here we show that La$_{1.5}$Ba$_{0.5}$CoMnO$_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with intermediate $H_{EB}$ value in respect to Ca- and Sr-doped samples. In order to figure out the microscopic mechanisms responsible for this phenomena, these compounds were investigated by means of synchrotron X-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, AC and DC magnetization, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The parent compound La$_{2}$CoMnO$_{6}$ was also studied for comparison, as a reference of a non-ZEB material. Our results show that the Ba-, Ca- and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states Co$^{2+}$/Co$^{3+}$ and Mn$^{4+}$/Mn$^{3+}$ are already present at the La$_{2}$CoMnO$_{6}$ parent compound, and that Ba$^{2+}$/Ca$^{2+}$/Sr$^{2+}$ partial substitution at La$^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples (0.05$\leq \delta \leq$0.1). Our XMCD results show a great decrease of Co moment for the doped compounds, and indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.

Unusual exchange bias in Sr2FeIrO6/La0.67Sr0.33MnO3 multilayer

Here, we study interface induced magnetic properties in a 3d-5d based multilayer made of La0.67Sr0.33MnO3 and double perovskite Sr2FeIrO6, respectively. Bulk La0.67Sr0.33MnO3 is metallic and shows ferromagnetic (FM) ordering above room temperature. In contrast, bulk Sr2FeIrO6, is an antiferromagnet (AFM) with a Néel temperature around 45 K () and exhibits an insulating behavior. Two set of multilayers have been grown on SrTiO3 (1 0 0) crystal with varying thickness of FM layer. A multilayer with equal thickness of La0.67Sr0.33MnO3 and Sr2FeIrO6 (∼10 nm) shows exchange bias (EB) effect both in conventionally field cooled (FC) as well as in zero field cooled (ZFC) magnetic hysteresis measurements which is rather unusual. The ZFC EB effect is weakened both with increasing maximum field during initial magnetization process at low temperature and with increasing temperature. Interestingly, a multilayer with reduced thickness of La0.67Sr0.33MnO3 (∼5 nm) does not exhibit ZFC EB phenomenon, however, the FC EB effect is strengthened showing much higher value. We believe that an AFM type exchange coupling at the interface and its evolution during initial application of magnetic field causes this unusual EB in present multilayers.

Influence of spin glass-like magnetic relaxation on the zero-field-cooled exchange bias effect

The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon recently reported for some reentrant spin glass-like compounds. In this work, the time-evolution of magnetization is thoroughly investigated for two ZEB materials in order to figure out the role played by the spin glass-like phase on such effect. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ and La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ were chosen as representative samples of ZEB systems, since the former compound presents the largest ZEB reported so far, while the second has a much smaller effect, despite being structurally/chemically similar. Comprehensive magnetic measurements were carried on both samples, and the results are discussed in terms of the amount and time-evolution of the spin glass-like phase under the influence of a varying field. We also propose a phenomenological model, based on the pinning of spin glass-like moments and on the dynamics of their magnetic relaxation, to explain the asymmetry observed in the hysteresis loops. The good agreement between the simulated and experimental results confirms our hypothesis that the spin glass-like phase is key to the ZEB effect.

Zero-field cooled exchange bias and magnetization reversal in La1.5Sr0.5Co0.4Fe0.6MnO6

The polycrystalline sample La1.5Sr0.5Co0.4Fe0.6MnO6 (LSCFMO) was prepared by sol-gel method and its magnetic properties were studied. The interesting magnetization reversal phenomenon and the zero-field cooled exchange bias (ZEB) effect were simultaneously observed in LSCFMO. ZEB effect can exist in a wider temperature range (0–200 K) compared with La1.5Sr0.5CoMnO6 (0–10 K), which is very important in the potential applications. A schematic diagram based on the coupling between the Fe3+ spins, Mn3+ spins and Co2+ or Co3+ spins is used to understand the ZEB and the reversal behaviors. Due to the doping of 60% Fe ion, the magnetic microstructure of LSCMFO is more complex than that of LSCMO, resulting in the meta-stable spin structure and more interesting magnetic phenomenon.

Negative magnetization and zero-field cooled exchange bias effect in Eu0.9Pr0.1CrO3 ceramics

Interesting magnetic behaviors, including negative magnetization, zero field fooled and field cooled exchange bias effects, have been observed in Eu0.9Pr0.1CrO3. The negative magnetization at low temperature results from the antiparallel coupling between the Pr3+ moment and the canted moment of Cr3+ sublattice. Left shift of zero field cooled M-H loops, and right shift of field cooled M-H loops have been observed, due to the growth and competition of two types of magnetic structures with GxFz (EuCrO3) and GzFx (PrCrO3) orderings under magnetic field. When the cooling field is high enough, the Pr3+ moment is frozen antiparallel with the applied field during the cooling process, giving rise to a positive exchange bias (right shift of M-H). However, when the cooling field is near zero, magnetic clusters with GxFz (EuCrO3) or GzFx (PrCrO3) orderings are formed and distribute randomly during the cooling process, and the net Pr3+ moment get aligned along the applied field during the initial magnetization process to lower the Zeeman energy, and then negative exchange bias (left shift of M-H) appears.

Particle Size-Dependent Zero-Field Exchange Bias in LaFeO 3 Nanoparticles

The present communication deals with particle size-dependent zero-field cooled exchange effect in LaFeO3 (LFO). LFO nanoparticles were prepared by PVA-based sol-gel method. The sample was calcined at different temperatures (700, 750, and 800 ∘C) to tune the average particle size. The detailed crystal structural and their geometrical parameters were examined by means of Rietveld refinement using x-ray diffraction data. The different techniques such as Scherrer’s method, Williamson Hall method, FESEM, and TEM were applied to accurately determine the average particle size. Zero-field cooled exchange bias and weak ferromagnetism in antiferromagnetic nanoparticles was observed for all samples using hysteresis loop measurement. The training effect was studied to confirm that the loop shift originates due to exchange bias. Exchange bias as well as coercivity are strongly related to the particle size at low and room temperature. The results clearly demonstrate that the process of calcination tailor the overall properties of material which makes it a promising candidate for device applications.

Probing the ground state and zero-field cooled exchange bias by magnetoresistance measurement in Mn50Ni41Sn9 ribbon

Recently, a new type of exchange bias (EB) after zero-field cooling has attracted considerable interest mainly in bulk magnetic competing systems. Here, we use a detailed magnetotransport investigation to probe the ground state and zero-field cooled EB (ZEB) in Mn50Ni41Sn9 ribbon. Both ZEB and field cooled EB were detected in magnetoresistance results consistent with magnetic measurement. A pure spin-glass ground state is proposed based on parabolic shape of low-field magnetoresistance combined with AC magnetization, memory effect. The appearance of ZEB is attributed to the field-induced nucleation and growth of ferromagnetic domains in the spin glass matrix forming unidirectional anisotropy at the interface.

Positive to negative zero-field cooled exchange bias in La0.5Sr0.5Mn0.8Co0.2O3 ceramics.

Exchange bias effect obtained after zero-field cooling from unmagnetized state usually exhibits a shift of hysteresis loop negative to the direction of the initial magnetic field, known as negative zero-field cooled exchange bias. Here, positive zero-field cooled exchange bias is reported in La0.5Sr0.5Mn0.8Co0.2O3 ceramics. In addition, a transition from positive to negative exchange bias has been observed with increasing initial magnetization field and measurement temperature. Based on a simple spin bidomain model with variable interface, two type of interfacial spin configuration formed during the initial magnetization process are proposed to interpret the observed phenomenon.

Doping induced zero-field cooled exchange bias effect in hexagonal Y0.95Eu0.05MnO3 single crystal

The crystal structure and magnetic properties of hexagonal Y0.95Eu0.05MnO3 single crystal were systematically investigated. The refinement structure analysis of Y0.95Eu0.05MnO3 using single crystal XRD shows that Eu doping results in an expansion of the MnO5 bipyramids along the c axis, and the contraction of the Mn forming trimerization in the ab plane. Magnetization measurements show that Eu dopant leads to the magnetic anisotropy change. Meanwhile, a spin glass behavior and a large zero-field cooled (ZFC) exchange bias (EB) effect of 735Oe are observed due to the Eu doping. Based on the results, we proposed that (1): the spin glass phase of Y0.95Eu0.05MnO3 mainly results from the intrinsic magnetic frustration and the disorder coming from Eu dopant; (2) the coupling between ferromagnetic spin glass phase and the frustrated antiferromagnetic phase leads to the exchange bias effect.

Negative magnetization and zero-field cooled exchange bias effect in Co0.8Cu0.2Cr2O4 ceramics

The negative magnetization and zero-field cooled exchange bias (ZFC EB) effect are observed in Co0.8Cu0.2Cr2O4 polycrystalline ceramics. 20% Cu substitution for Co in CoCr2O4 leads to the evident magnetization reversal at the compensation temperature (Tcomp ∼ 50 K) with applied magnetic field of 500 Oe. Besides, Tcomp decreases monotonously with increasing applied field, and the negative magnetization finally disappears when the field increases to 9000 Oe. Different temperature dependence of sublattice magnetization at different crystallographic sites is proved to induce the magnetization reversal. In addition, ZFC EB effect can be tuned by measuring temperature and presents the maximum of exchange bias field (HEB) with ∼2300 Oe at 50 K. This unconventional EB effect can be attributed to the coupling interaction between the two sublattices.

Large zero-field cooled exchange bias in Ba1+δMn8O16 nanoribbons

A large zero-field cooled exchange bias was obtained in single-crystal Ba1+δMn8O16 nanoribbons. Magnetization measurements indicate the coexistence of ferromagnetism and spin glass-like behavior in antiferromagnetic Ba1+δMn8O16 nanoribbons, which gives rise to a path-dependent exchange bias below the spin freezing temperature. Depending on the sign of the starting field [either positive (p) or negative (n)] as well as the maximum measurement field for the loop trace, the exchange bias (HE) varies significantly with |−HEp|>|+HEn|. HE has a monotonic decrease when increasing the maximum measurement field, |Hmmax|, to 3–6T. All features are attributed to the superexchange coupling between the ferromagnetic phase and the canted antiferromagnetic matrix via spin glass-like interface.

Zero-field cooled exchange bias effect in LaxSm1−xCrO3 (x=0–0.9) ceramics

The zero-field cooled exchange bias (ZEB) and conventional field cooled exchange bias (CEB) effects are investigated in single phase bulk SmCrO3 ceramics below the spin reorientation transition temperature. The ZEB field is found to be ~5.065kOe at 20K, revealing a large value compared with other ZEB systems. Nonmagnetic La3+ as a dopant is introduced into the system to confirm the influence of Sm3+ on the ZEB effect. A schematic diagram based on the coupling between the internal magnetic sublattices of Sm3+ and Cr3+ is used to understand the ZEB effect.

Giant zero field cooled spontaneous exchange bias effect in phase separated La1.5Sr0.5CoMnO6

We report a giant zero field cooled exchange bias (ZEB) effect (∼0.65 T) in La1.5Sr0.5CoMnO6 sample. Magnetic study has revealed a reentrant spin glass ∼90 K, phase separation to spin glass and ferromagnetic phases below 50 K and canted antiferromagnetic transition ∼10 K. A small conventional exchange bias (CEB) is established with the advent of spontaneous phase separation down to 10 K. Giant ZEB and enhanced CEB effects are found only below 10 K and are attributed to the large unidirectional anisotropy at the interface of isothermally field induced ferromagnetic phase and canted antiferromagnetic background.

Zero-field cooled exchange bias in hexagonal YMnO3 nanoparticles

Effect of varying particle size and maximum applied field during initial magnetization (HM) on zero-field cooled (ZFC) exchange bias (EB) in hexagonal YMnO3 nanoparticles prepared by conventional solid state method has been studied. EB phenomenon is observed after ZFC from an unmagnetized state without any remanent magnetization. EB field (Heb) and Vertical shift (ME) were found to vary nonmonotonically with both particle size and HM. Large Heb of 1024 Oe is obtained for the particle size of 55 nm. These features are attributed to the exchange interaction between the compensated antiferromagnetic spins and uncompensated surface spins of the nanoparticles.

Large zero-field cooled exchange-bias in bulk Mn2PtGa.

We report a large exchange-bias effect after zero-field cooling the new tetragonal Heusler compound Mn(2)PtGa from the paramagnetic state. The first-principles calculation and the magnetic measurements reveal that Mn(2)PtGa orders ferrimagnetically with some ferromagnetic inclusions. We show that ferrimagnetic ordering is essential to isothermally induce the exchange anisotropy needed for the zero-field cooled exchange bias during the virgin magnetization process. The complex magnetic behavior at low temperatures is characterized by the coexistence of a field-induced irreversible magnetic behavior and a spin-glass-like phase. The field-induced irreversibility originates from an unusual first-order ferrimagnetic to antiferromagnetic transition, whereas the spin-glass-like state forms due to the existence of antisite disorder intrinsic to the material.