Exchange Bias Behavior Research Articles

Crystal structure and magnetism of BiFeO3 nanoparticles regulated by rare-earth Tb substitution

Tb-doped BiFeO3 nanoparticles were prepared using sol–gel method. The effect of Tb substitution on crystal structure and magnetism of BiFeO3 nanoparticles were investigated. It is shown that the crystal structure and magnetism of BiFeO3 nanoparticles are regulated by rare-earth Tb substitution. Particularly, the sizes of the particles are reduced to smaller than 100 nm after doping with Tb. The magnetization of Tb-doped BiFeO3 nanoparticles has been enhanced in magnitude, which is mainly attributed to the suppression of spin cycloid structure belonging to R3c phase fraction in the process of rhombohedral-to-orthorhombic structural phase transformations. At the meantime, the magnetic hysteresis loops show exchange bias towards negative axis. The exchange bias behaviors originate from the coupling interaction between antiferromagnetic core and ferromagnetic surface. The present work provides a route regulating the magnetization of BiFeO3 particles as well as further promoting its applications in multiferroic materials.

Charge ordering and exchange bias behaviors in Co3O4 porous nanoplatelets and nanorings

We present the synthesis of α-Co3O4 porous nanoplatelets and hexagonal nanorings using microwave-assisted hydrothermal and conventional chemical reaction methods. The x-ray diffraction (XRD) and refinement analyses indicate the α-Co3O4 crystal structure, and the x-ray photoelectron spectrum (XPS) indicates the high purity of the samples. The M–T (including 1/χ–T) curves indicate an antiferromagnetic transition at about 35K in both kind of samples but the interesting finding was made that a charge-ordered (CO) state appears at 250K for the nanoplatelets sample whereas it is inattentive for the nanorings. The antiferromagnetic transition temperature TN is lower than that of the bulk α-Co3O4 single crystal due to the nanosized structures. We observed quite significant exchange bias for nanorings. The exchange bias behavior of the α-Co3O4 hexagonal nanorings is consistent with an antiferromagnetic (AFM) Co3O4 core and spin-glass like shell.

Structural and magnetic properties of NiCr1.9Mn0.1O4

Polycrystalline NiCr1.9Mn0.1O4 within nanoscale has been synthesized by sol–gel method. X-ray diffraction results prove pure formation of the sample without any other impurities. Characterizations of scanning electron microscopy and X-ray fluorescence reveal the good crystallinity and desired element ratio of the phase. Magnetic hysteresis loop measured at 10 K after cooling the sample in applied field of 30 kOe shows asymmetry both in the field and magnetization axes with a bias field about 3316.5 Oe and a magnetization shift about 0.381 emu/g. This exchange bias (EB) behavior has also been studied as the function of measuring temperature and applied cooling field. The values of EB field and magnetization shift are found to depend strongly on temperature as well as the cooling field. The accompanying training effect is also observed in the sample. Irreversibility in temperature dependence of magnetization at different applied fields indicates the presence of spin-glass-like (SGL) phase. High field relaxation measurement confirms the existence of SGL state. The results suggest that different formation of ferromagnetic and antiferromagnetic sublattice orders with spin frustration due to the doping of Mn cations in the sample leads to these interesting magnetic properties.

The Abnormal Optical Property and Room‐Temperature Exchange Bias Behavior in Na‐ and Ru‐Codoped BiFeO3 Nanoparticles

Bi0.98Na0.02Fe1−xRuxO3 (x = 0, 0.01, 0.02) samples were prepared via a sol–gel method. The formation of the desired materials was confirmed using X‐ray diffraction. The tetravalent Ru doping could effectively reduce the number of oxygen vacancy, and subsequently reduce the leakage current. Interestingly, the optical band gap was narrowed gradually in our samples, which seems contradictory with the variation in oxygen vacancy. To better understand this abnormal optical property, the effect of bandwidth was analyzed based on the change in Fe–O bond length and Fe–O–Fe bond angle, and the corresponding phenomenological qualitative model was proposed. The magnetization was increased with Ru substitution. In addition, an exchange bias (EB) phenomenon without field cooled process was observed at room temperature for all the samples, which was explained by introducing a core–shell structure model. Moreover, the EB behavior becomes more pronounced at 5 K for 1% Ru‐doped sample.

Exchange bias and spin glass behavior in biphasic NiFe2O4/NiO thin films

Magnetic exchange bias and coercivity of nanogranular NiFe2O4/NiO thin films, prepared using flow-stabilized microplasmas and post-deposition annealing, have been investigated as a function of ferrimagnet/antiferromagnet phase fraction, grain size, and temperature. Exchange bias (EB) and vertical shifts in hysteresis loops observed in the as-deposited and low-T annealed (≤600°C) films were attributed to exchange coupling between nanocrystalline NiFe2O4 (~8–10nm) and a structurally-disordered spin glass (SG)-like phase. At higher annealing temperature (850°C), the observed EB was found to arise from exchange coupling between NiFe2O4 and NiO, rather than a SG phase, most likely due to reduction of structurally-disordered interfaces and a substantial increase in NiFe2O4 grain size (~26nm).

Study of Exchange Bias in Mn-Doped YFeO3 Compound

Single-phase samples of orthorhombic YFe1−xMnxO3 (x = 0 to 0.3) were prepared using solid-state route. Temperature variation of magnetization measurements show antiferromagnetic transition with Neel temperature (TN) in the range of 646 K for x = 0 to 456 K for x = 0.3. Spin canting-induced weak ferromagnetism is observed for T < TN. These materials exhibit an exchange bias behavior as per magnetic hysteresis loop measurement under field-cooled condition. The magnitude of maximum exchange bias field at 413 K is around 1.2 kOe. The observed exchange bias is explained by considering the exchange coupling across the interface of two magnetic phases having a dominant collinear antiferromagnetism and a weak ferromagnetism, respectively.

Influence of antisite disorders on the magnetic properties of double perovskite Nd2NiMnO6

Abstract The structural and magnetic properties of polycrystalline Nd2NiMnO6 (NNMO) synthesized by solid state reaction method have been investigated. Rietveld refinement of Powder X-Ray diffraction data suggests that NNMO compound crystallizes in monoclinic phase with space group P21/n. Magnetic measurements reveal an additional magnetic transition T C ׳ ~85 K due to to Ni3+–O–Mn3+ antiferromagnetic superexchange interaction along with ferromagnetic transition ~196 K in M–T curve, leading to antiferromagnetic interactions in this compound. High sintering temperature (upto 1100 °C) leads to the formation of antisite disorders and antiferromagnetic antiphase boundaries in the present sample. The magnetic hysteresis measurements show ferromagnetic behavior with low saturation magnetization value (~2 μB/f.u.) than the theoretical spin-only value (5 μB/f.u.) of Ni/Mn ions. Hence, antisite disorders diminish the long range ferromagnetic ordering and also affect the magnetic properties of this compound. Exchange bias behavior has been observed in the hysteresis loops when the sample was field cooled from 300 K to 5 K. Exchange coupling between the ferromagnetic moments and antiferromagnetic antiphase boundaries is responsible for the observation of exchange bias in the present system.

Exchange bias effect in CuCr2O4/Cr2O3 nanogranular systems

We have successfully synthesized the CuCr2O4/Cr2O3 nanogranular system through a high-temperature phase separation route from Cu doped Cr2O3 nanoparticles, which are confirmed by X-ray diffraction measurements. Magnetic susceptibility shows two magnetic transitions at 130 K and 310 K corresponding to the ferrimagnetic and antiferromagnetic phases of CuCr2O4 and Cr2O3, respectively. Significant exchange bias behavior could be observed in the field cooling magnetic hysteresis loop at low temperatures. Both exchange bias field (HEB) and coercive field (HC) decrease with increasing temperature and disappear near 200 K. This exchange bias behavior is discussed in terms of the existence of exchange coupling between the ferrimagnetic CuCr2O4 and antiferromagnetic Cr2O3. The uncompensated spins in antiferromagnetic phase frozen along field cooling direction provide the pinning force on the ferrimagnetic spins leading to the exchange bias effect.

Exchange bias driven by the structural/magnetic transition in Mn-doped SrRuO3

Magnetic properties of bulk polycrystalline Mn-doped SrRu1−xMnxO3 perovskite, at doping range 0.2≤x≤0.3, were studied by both magnetization and ac-susceptibility measurements. It was found that the exchange bias (EB) effect emerges with increasing Mn doping in SrRu1−xMnxO3 at x≈0.25, following the ferromagnetic (FM) to antiferromagnetic (AFM) phase transition. This transition is accompanied with the change in structure symmetry and then the EB field, HEB, increases significantly in doping range 0.25<x≤0.3. The EB effect was verified by both cooling magnetic field, Hcool, dependence of the HEB and training effect. A markedly nonmonotonic HEB vs. Hcool dependence with maximum at around 40kOe was found, resembling the behavior of phase-separated EB systems. Moreover, a clear analogy with behavior of classic EB system of Pr1/3Ca2/3MnO3 was noticed, strongly suggesting that the EB effect in SrRu1−xMnxO3 originates from exchange interactions at the interface of nanoscale FM clusters (size of ~1.6nm at x=0.3) coexisting together with dominant AFM phase at the boundary of the first-order FM/AFM transition. The training effect observed is well understandable within the spin-configuration relaxation model and indicates important contribution to the EB behavior from the AFM domains rearrangement at the interface.

Exchange bias and magnetization reversal in Ni(Cr1−xFex)2O4 (x=0–0.20)

Exchange bias and magnetization reversal in single phase samples of Ni(Cr1−xFex)2O4 (x=0–0.20) were studied through magnetic measurements. Substitution of Fe for Cr changes the crystal structure at room temperature from tetragonal (space group: I41/amd) to cubic (space group: Fd3¯m) form. Temperature variation of magnetization measurements show that these samples undergo ferrimagnetic transitions and the transition temperature (TC) increases from 73K for x=0.0 to 314K for x=0.20. An interesting magnetization reversal phenomenon was observed for x=0.06 sample with a magnetic compensation temperature of 49K. M–H loop measurements at different temperature show the signature of presence of strong antiferromagnetic interactions especially at low temperature (T<50K). Tunable exchange bias behavior with a maximum exchange bias field of 5670Oe is observed for x=0.06 sample and it is explained by considering the exchange anisotropy between the ferrimagnetic and the antiferromagnetic components. The exchange bias field and the vertical shift in magnetization decrease exponentially with increase in temperature.

Layer Effects on the Magnetic Behaviors of Aurivillius Compounds Bin+1Fen−3Ti3O3n+1 (n = 6, 7, 8, 9)

Bin+1Fen−3Ti3O3n+1 (BFT‐n, n = 6, 7, 8, 9) ceramics were prepared using Pechini's method and their magnetic behaviors were investigated. XRD and HRTEM results suggested that BFT‐n have the pure Aurivillius structure. Magnetic properties of BFT‐n depend largely on their layer numbers. BFT‐6 exhibits a paramagnetic (PM)‐like characteristics with a small portion of weak antiferromagnetic (AFM) property; while spin glass (SG)‐like characteristics was observed for the BFT‐n (n ≥ 7) samples. An amazing exchange bias phenomenon was observed in BFT‐n (n ≥ 7) ceramics, of which shifts in both vertical and horizontal direction were found in the magnetic hysteresis loops as the functions of testing temperature and the layer number (n). For BFT‐8, the maximum HE is 3911 Oe when measured at 5 K. The AFM/SG interface should be the origin for the observed exchange bias effect, which was aggravated with the increased layer numbers for the enhanced coupling probability among Fe3+ ions. The fantastic exchange bias behavior of BFT‐n (n ≥ 7) makes them a good single‐phase candidate for future spintronic devices.

Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible "On" and "Off" Exchange Bias Behavior.

The syntheses, structures, and magnetic properties of four new complex salts, (PPN){[Mn(III)(salphen)(MeOH)]2[M(III)(CN)6]}·7MeOH (Mn2M·7MeOH) (M = Fe, Ru, Os and Co; PPN(+) = bis(triphenylphosphoranylidene)ammonium cation; H2salphen = N,N'-bis(salicylidene)-1,2-diaminobenzene), and a mixed metal Co/Os analogue (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH were undertaken. It was found that all compounds exhibit switchable single-molecule magnet (SMM) and exchange-bias behavior depending on the interstitial methanol content. The pristine (PPN){[Mn(salphen)(MeOH)]2[Os(CN)6]}·7MeOH (Mn2Os·7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (Ueff/kB), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of Ueff/kB to 42.0 K and an opening of the hysteresis loop observable at 1.8 K. Mn2Os·7MeOH shows also exchange-bias behavior with magnetic hysteresis loops exhibiting a shift in the quantum tunneling to 0.25 T from zero-field. The Fe(III) and Ru(III) analogues were prepared as reference compounds for assessing the effect of the 5d versus 4d and 3d metal ions on the SMM properties. These compounds are also SMMs and exhibit similar effects but with lower energy barriers. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their slow relaxation of the magnetization properties. The (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)(CN)6]}·7MeOH (Mn2Co·7MeOH) analogue with a diamagnetic Co(III) central atom and the mixed Co/Os (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH (Mn2Co/Os·7MeOH) "magnetically diluted" system with a 9:1 Co/Os metal ratio were prepared in order to further probe the nature of the energy barrier increase upon desolvation of Mn2Os. In addition, inelastic neutron scattering and frequency-domain Fourier-transform THz electron paramagnetic resonance spectra obtained on Mn2Os·7MeOH and Mn2Os in combination with the magnetic data revealed the presence of anisotropic exchange interactions between Mn(III) and Os(III) ions.

Monte Carlo Investigation of the Magnetic and Thermal Properties of Four Sublattice Ferromagnetic– Antiferromagnetic Double-Layer Superlattices

Using Monte Carlo simulation, we have studied the magnetic and thermal properties of a four sublattice ferromagnetic (FM) and anti-FM (AFM) double-layer superlattices system in a longitudinal magnetic field. The effects of the intralayer AFM exchange coupling $J_{{2}}$ , the single-ion anisotropy ${D}$ and the longitudinal magnetic field ${h}$ on the magnetization ${M}$ , the susceptibility ${x}$ , the internal energy ${U}$ , and the specific heat ${c}$ of the system are discussed in detail. Particular attention has been paid to the variations of the hysteresis loops of the system with the AFM exchange coupling $J_{ {2}}$ and the anisotropy ${D}$ . Some interesting results, such as the triple hysteresis and the exchange bias behavior, have been found under certain conditions.

Exchange bias and training effect in NiCr2O4/Cr2O3 composite

Abstract

Investigation of spin canting phenomena in perpendicularly exchange biased Pt/Co/Pt/Cr2O3 thin films

We investigated perpendicular exchange bias and spin canting phenomena in Pt/Co/Pt/Cr2O3 thin films grown on Si (111) substrates. Unusual decrease of perpendicular exchange bias has been observed below 160K in ultra-thin Co (5Å) and Cr2O3 (0001) exchange coupling system. We also examined the possible deviation of antiferromagnetic spin axis from the c-axis of Cr2O3 (0001) layer by using a thicker Co (50Å) film with a magnetic easy axis lies on the sample plane. The temperature-dependent exchange bias measurements of both samples exhibited that the canting of uncompensated Cr spins at the Cr2O3 surface is responsible for the unusual behavior of perpendicular exchange bias in Co (5Å)/Cr2O3 (0001) ultra-thin film. Furthermore, in-plane angle (φ)-dependent exchange bias experiments with thicker Co (50Å)/Cr2O3 (0001) sample indicated that neither the deviation of antiferromagnetic spin axis from the c-axis of Cr2O3 nor spin–flop transition mechanisms is present in antiferromagnetic Cr2O3 layer. We also found that uncompensated canted Cr spins at the Cr2O3 surface are magnetically disordered (paramagnetic-like) and reduce the exchange coupling between the ferromagnetic Co and antiferromagnetic Cr2O3 (0001) layers. The magnetically disordered uncompensated canted Cr spins negatively affect the perpendicular exchange bias starting from 220K down to 160K, and further decrease of temperature dramatically reduces perpendicular exchange bias in Pt/Co/Pt/Cr2O3 thin films.

Scrutinizing the role of size reduction on the exchange bias and dynamic magnetic behavior in NiO nanoparticles

NiO nanoparticles (NPs) with a nominal size range of 2–10 nm, synthesized via high-temperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak ( K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ( K) is a signature of a SG-like freezing of the surface spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the magnetic moments freeze into a SG-like state below K, with no evidence of EB effect.

Oscillatory exchange bias effect in La0.67Sr0.33MnO3/G-SrMnO3/La0.67Sr0.33MnO3 sandwiches

We investigate the exchange bias effect in La0.67Sr0.33MnO3/G-SrMnO3/ La0.67Sr0.33MnO3 (LSMO/SMO/LSMO) sandwiches, where the spacer is a G-type antiferromagnet. Because of the cycloid bulk spins in the SMO modulated by the top LSMO layer, the sandwich structure exhibits an oscillatory exchange bias behavior as a function of the SMO thickness. This unprecedented phenomenon vanishes in LSMO/SMO bilayers, where the exchange bias saturates as the SMO thickness increases, giving rise to the antiferromagnetic anisotropy constant of SMO, 1.0 –2.0 × 105 erg cm−3. The results provide a broad opportunity to tailor antiferromagnetic moments by interfacial design for antiferromagnet spintronics.

Effect of Fe substitution at the Ni and Mn sites on the magnetic properties of Ni50Mn35In15 Heusler alloys

The structural and magnetic properties of Ni48Fe2Mn35In15 and Ni50Mn34FeIn15 Heusler alloys have been investigated. At room temperature, Ni48Fe2Mn35In15 has L21 cubic structure, whereas Ni50Mn34FeIn15 shows a two-phase structure due to the martensitic transition. In the case of Ni48Fe2Mn35In15, there is only one magnetic transition at 316 K with no martensitic transition. However, in Ni50Mn34FeIn15, we observe the martensitic transition at about 280 K. The Curie temperatures for austenite and martensite phases are 314 and 200 K, respectively. The maximum magnetic entropy changes are found to be 5.5 and 4.5 J kg−1 K−1 for Ni48Fe2Mn35In15 and Ni50Mn34FeIn15, respectively, for 50 kOe. Ni50Mn34FeIn15 exhibits exchange bias behavior, with a bias field of 130 Oe at 5 K. Both the alloys satisfy the empirical relation between the martensitic transition and the valence electron concentration (e/a) ratio.

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.

Effect of L12 ordering in antiferromagnetic Ir-Mn epitaxial layer on exchange bias of FePd films

Two series of samples of single-layer IrMn and IrMn/FePd bilayer films, deposited on a single-crystal MgO substrate at different IrMn deposition temperatures (Ts = 300–700 °C), were investigated using magnetron sputtering. L12 ordering was revealed for the 30 nm-thick IrMn epitaxial (001) films with Ts ≥ 400 °C, determined by synchrotron radiation x-ray diffractometry (XRD). XRD results also provide evidence of the epitaxial growth of the IrMn films on MgO substrate. Increasing Ts from 400 to 700 °C monotonically increases the ordering parameter of L12 phases from 0.17 to 0.81. An in-plane exchange bias field (Heb) of 22 Oe is obtained in a 10 nm-thick FePd film that is deposited on the disordered IrMn films. As the L12 ordering of the IrMn layers increases, the Heb gradually decreases to 0 Oe, meaning that the exchange bias behavior vanishes. The increased surface roughness, revealed by atomic force microscopy, of the epitaxial IrMn layers with increasing Ts cannot be the main cause of the decrease in Heb due to the compensated surface spins regardless of the disordered and ordered (001) IrMn layers. The change of antiferromagnetic structure from the A1 to the L12 phase was correlated with the evolution of Heb.