Exchange Bias Effect Research Articles

Structural, magnetic and magnetocaloric effect studies of Nd0.6Sr0.4AxMn1-xO3 (A=Co, Ni, Zn) perovskite manganites

The effect of B-site substitution by other transition elements on the structural, magnetic, and magnetocaloric effect properties of Nd0.6Sr0.4AxMn1-xO3 (x = 0.1 and x = 0.2, A=Co, Ni, and Zn) nanopowders has been reported in this study. Auto-combustion sol-gel method was used to synthesize all the nanopowder samples. Room temperature X-ray diffraction shows that all the nanopowders have orthorhombic structure of Pnma space group crystallography, confirming the purity of the single phase. Also, the average nanosized scale of these powder samples is ~ 43 nm, revealing a spherical shape with a packed and homogenous structure. The Mn4+/Mn3+ ratio in these nanopowder compounds is highly dependent on the dopants concentration (x), which shows a significant diminishes as the dopants introduce into the parent compound (Nd0.6Sr0.4MnO3). The decrease in the Mn4+/Mn3+ ratio enhances in the antiferromagnetic (AFM) super exchange interaction (SE) due to the lack of Mn3+— O—Mn4+ ferromagnetic (FM) double exchange (DE). The magnetization measurements indicate that all samples exhibit a ferromagnetic (FM) to paramagnetic (PM) transition with increasing temperature. The Curie temperature (TC) is also affected by the dopants concentrations where considerable decrease has been noticed for all compounds compared to the parent compound. The M-H curves for all compounds reveal PM behavior at 300 K, whereas FM behavior characterized the magnetic hysteresis at low temperature (2 K) which also shows the presence of exchange bias effect. The magnetic entropy change |−∆SMMax| has shown bigger effect for Co dopant (5.92 J/kg.K) compared to Ni and Zn dopants with values of 5.19 J/kg.K, and 4.28 J/kg.K respectively at an applied magnetic field of 9 T. Despite substituting at Mn-site by other transition elements lower the Tc, |−∆SMMax| and RCP, the obtained results are promising for magnetic cooling materials and further investigation.

Conical order, magnetic compensation, and sign reversible exchange bias in spinel structured AB2O4 compounds: A Monte Carlo study

$A{B}_{2}{\mathrm{O}}_{4}$ are a special class of spinel structured compounds exhibiting unique multiferroic properties. Recent experimental observation confirms switchable polarization in $A{\text{Cr}}_{2}{\text{O}}_{4}$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe})$, arising out of noncollinear magnetic spin order. In this paper we unfold the microscopic origin behind such magnetic spin order, hysteresis, polarization, and the so-called magnetic compensation effect in $A{\text{Cr}}_{2}{\text{O}}_{4}$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ using Monte Carlo simulation. We have carefully constructed a set of exchange interaction parameters, which help to explain various experimental findings such as magnetization vs temperature $(T)$, conical stability, magnetic ordering, and polarization in a representative compound ${\text{CoCr}}_{2}{\text{O}}_{4}$. When substituted with Fe, ${\text{CoCr}}_{2}{\text{O}}_{4}$ shows few exotic phenomena such as sign reversible exchange bias effect and magnetic compensation. ${\text{CoMn}}_{2}{\text{O}}_{4}$ and ${\text{CoFe}}_{2}{\text{O}}_{4}$ are two other compounds in the spinel family where we found no conical spin order and polarization, and thus facilitate a distinct contrast compared to others.

Spin–orbit torque controllable complete spin logic in a single magnetic heterojunction

The realization of complete spin logic within a single nonvolatile memory cell is a promising approach toward next-generation low-power stateful logic circuits. In this work, we demonstrate that all 16 Boolean logic functions can be realized within a single four-state nonvolatile IrMn/Co/Ru/CoPt magnetic heterojunction, where controllable field-free spin–orbit torque switching of the perpendicularly magnetized CoPt alloy is obtained, relying on the interlayer exchange coupling and exchange bias effect. By assigning different values to four variables of the four-state memory, that is, the initial control current pulse, the initial control magnetic field, and the input electrical potential of two terminals, in sequentially three steps, the complete Boolean logic functions are realized, while the anomalous Hall voltage of the devices is considered as logic output. The coexistence of nonvolatile four-state memory and complete spin logic functions holds promising application for future computing systems beyond von Neumann architecture.

Structural characterization and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires

AbstractHerein, structural and magnetic properties of single‐crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires synthesized via a hydrothermal process are reported. The nanowires are crystallized in an orthorhombic structure (Pnma space group). Their lattice parameters follow the relationship a ≈ c ≈ b/√2 These nanowires exhibited a clean and smooth surface with diameters of 60‐120 nm and an average length of approximately 2.0 μm. High‐resolution transmission electron microscopy images confirmed the single‐crystalline nature of the nanowires growing along the [100] direction. The nanowires demonstrated magnetic hysteresis loops at low temperatures and a weak exchange bias (EB) effect. Paramagnetic (PM)–ferromagnetic (FM) phase transition occurred at a Curie temperature (TC) of 224 K, and strong irreversibility between zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization (MZFC and MFC, respectively) curves was observed at 273 K. The MZFC curve exhibited a significantly broad peak with a maximum at a freezing temperature (Tf) of 134 K. Relative difference between MFC and MZFC in the nanowires [(MFC − MZFC)/MFC] rapidly increased below Tf and reached approximately 50% below 35 K. The effective magnetic moment deduced from the Curie constant is larger than the theoretical value, indicating short‐range FM interactions in the nanowires. A positive PM TC (θp) implies dominant FM interactions in the nanowires, and θp > TC observed herein indicates the existence of short‐range ordered states above TC.

Electric-Field Reversible Switching of the Exchange Spring and Exchange Bias Effect in SrCoO3-x/La0.7Sr0.3MnO3 Heterostructures.

The technique of electrical field to manipulate physicochemical properties of oxide heterostructures has ample potential in electronic and ionitronic devices. SrCoO3-x is a famous "sponge" material displaying topotactic structural phase transition from perovskite (0 ≤ x ≤ 0.25) to brownmillerite (x = 0.5) accompanied by the magnetic phase transition from ferromagnetism to antiferromagnetism, which can be controlled reversibly by electric field via the ionic liquid gating method. Here, the exchange spring effect can be observed at the perovskite SrCoO3-x (P-SCO)/La0.7Sr0.3MnO3 (LSMO) bilayer, while the exchange bias effect is received at the brownmillerite SrCoO2.5 (B-SCO)/LSMO bilayer. The reversible and nonvolatile switching of the exchange spring and exchange bias effect can be achieved in these SCO3-x/LSMO bilayers by utilizing ionic liquid gating to control the annihilation or generation of oxygen vacancies. In addition, the variations in the stacking orders of these SCO3-x/LSMO bilayers are investigated because the previous SCO3-x layer always acts as the cover layer. It is worth noting that LSMO/SCO3-x bilayer magnetization is strongly suppressed when the SCO3-x layer is used as the bottom layer. Combined with the X-ray line dichroism measurements, it is suggested that the bottom SCO3-x layer would induce the spin arrangements in the LSMO layer to have the tendency toward the out-of-plane orientation. This is the reason for the sharp decrease in magnetization of LSMO/SCO3-x bilayers. Our investigations accomplish a reversible control of the exchange coupling transition in all-oxide bilayers and provide the foundation for further electric-field control of magnetic properties.

Free tuning of exchange bias via resettable alignment of antiferromagnetic Néel axes using mechanical vibrations

Absence of net magnetization in antiferromagnet (AFM) renders magnetic anisotropy of AFM firmly robust against external magnetic field. Therefore, uniaxial realignment of spin structure in AFM requires the field-cooling procedure, which heats AFM to above Néel temperature overcoming the existing anisotropy followed by cooling in a magnetic field to reinstate its anisotropy. Ferromagnet (FM) coupled to the uniaxially aligned AFM spins exhibits unidirectional anisotropy, which is conventionally reflected in the exchange bias effect along the cooling-field direction. Here, we report that alternating mechanical vibrations with relatively low frequencies in the range of few kHz can substitute the inconvenient heating step in the conventionally well-known field-cooling process for the spin alignment of AFM. Mechanical vibration and external magnetic field have been applied simultaneously to AFM IrMn3/FM Co or CoFeB bilayers deposited on Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 (PMN-PZT) piezoelectric single-crystalline substrate in an ambient temperature. Exchange bias of the bilayers can arbitrarily be set along a desired direction as determined by the applied magnetic field. The simple technique without a heating step enables local control of exchange bias effect at selected regions only within complex microdevice structures. Furthermore, the capability of repeated post-readjustment of exchange bias effect in various directions and magnitudes would promote wider integrations of exchange-biased system in novel magnetic devices.

Core Size and Interface Impact on the Exchange Bias of Cobalt/Cobalt Oxide Nanostructures

Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.

Role of short-range magnetic interaction between core and surface spins in exchange bias and memory effect in nanocrystalline doped manganite

• Synthesis of bulk and nanocrystalline La 0.4 (Ca 0.5 Sr 0.5 ) 0.6 MnO 3 compounds. • Enhancement of ferromagnetic clusters in core part with size reduction is noticed. • Magnetic inhomogeneity induced glassy magnetic phase is observed in nanomaterial. • Magnetic disorder results in strong memory and exchange bias effect in nanocrystal. • Theoretical model for core-shell structure corroborates the short-range correlation. A comparative study of low temperature magnetic properties for bulk and nanocrystalline La 0.4 (Ca 0.5 Sr 0.5 ) 0.6 MnO 3 compounds has been presented here. Considerable enhancement of exchange bias as well as magnetic memory effect in nanocrystalline material has been observed compared to bulk. Substantial increase of ‘A’ -site size-disorder parameter ( σ 2 ) with ‘Sr’ doping concentration in La 0.4 Ca 0.6 MnO 3 compound is found to assist in destabilizing the charge ordering state along with a grow of ferromagnetic clusters in these systems. In nanomaterial, the short-range magnetic interaction between the increased ferromagnetic cluster and the antiferromagnetic matrix in the core part of the core-shell structure assists to develop strong magnetic disorder (glassy phase) in the system. Additionally, the magnetic interaction of uncompensated shell spins and its interaction with adjacent core ferromagnetic clusters also attribute to grow glassy phase in the system, resulting in strong memory effect and exchange bias effect. The experimental findings have been described in terms of different theoretical models developed for core-shell structure.

Unusual positive exchange bias after Zero-Field cooling at room temperature in the Co-CoO film composed of amorphous Co + CoO and crystallized Co/CoO

The exchange bias (EB) effect is the physical cornerstone of magnetic storage and spin valves. The traditional EB effect is always realized under cooling field at low temperatures, which greatly restricts the practical application of the EB effect. In this work, an unusual positive EB effect after the zero-field cooling can be observed in Co-CoO film at room-temperature. The reason for this phenomenon is that the film is composed of crystallized Co/CoO and amorphous Co + CoO, and thus there is an exchange interaction between ferromagnetic, antiferromagnetic and uncompensated antiferromagnetic.

Strong Interfacial Perpendicular Magnetic Anisotropy in Exchange-Biased NiO/Co/Au and NiO/Co/NiO Layered Systems.

The ability to induce and control the perpendicular magnetic anisotropy (PMA) of ferromagnetic layers has been widely investigated, especially those that offer additional functionalities (e.g., skyrmion stabilization, voltage-based magnetization switching, rapid propagation of domain walls). Out-of-plane magnetized ferromagnetic layers in direct contact with an oxide belong to this class. Nowadays, investigation of this type of system includes antiferromagnetic oxides (AFOs) because of their potential for new approaches to applied spintronics that exploit the exchange bias (EB) coupling between the ferromagnetic and the AFO layer. Here, we investigate PMA and EB effect in NiO/Co/Au and NiO/Co/NiO layered systems. We show that the coercive and EB fields increase significantly when the Co layer is coupled with two NiO layers, instead of one. Surrounding the Co layer only with NiO layers induces a strong PMA resulting in an out-of-plane magnetized system can be obtained without a heavy metal/ferromagnetic interface. The PMA arises from a significant surface contribution (0.74 mJ/m2) that can be enhanced up to 0.99 mJ/m2 by annealing at moderate temperatures (~450 K). Using field cooling processes for both systems, we demonstrate a wide-ranging control of the exchange bias field without perturbing other magnetic properties of importance.

High-state spin in Ru4+: Play the role in the interface coupling of the La0.7Sr0.3MnO3/SrRuO3 heterostructure

We have investigated the exchange bias effect in La0.7Sr0.3MnO3/SrRuO3 bilayers that epitaxial grown in different directions and various substrates. A linear-tunable exchange bias effect, which related to the spin states of the SrRuO3 layers have been discovered. The spin state of Ru4+ play the centre role in the interaction, and proposes mechanisms to understand the Mn-O-Ru antiferromagnetic coupling. This study emphasizes the effect of the spin state as well as the exchange strength on the interfacial coupling, which may open a constructive approach in improving the performance of the related spintronic devices.

Size dependence of magnetic and dielectric properties in YCr0.5Fe0.5O3 ceramics

The YCr0.5Fe0.5O3 ceramics with different sintering temperatures (TS) have been synthesized by a sol-gel method, and the structural, magnetic and dielectric properties are studied. With the decrease of TS, continuous decrease of grain size is evidenced via the micromorphology analysis. Due to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interaction, the total magnetization becomes zero at the compensation temperature (TComp) in the presence of a lower magnetic field, and the TComp exhibits size dependent behavior. With the decrease of grain size, the TComp becomes lower due to the increase of uncompensated spins. Meanwhile, the TComp is adjustable by the magnetic field, and a larger external field drives the TComp to lower temperature. After cooling the samples in magnetic field, the magnetic anisotropy is established, which results in a positive exchange bias (EB) effect below the TComp. The decrease of TS would increase the ferromagnetic component, which leads to the decrease of EB field (HEB). The dielectric measurements suggest that the samples of TS = 900 and 800 °C possess large permittivity accompanied with dielectric relaxation near room temperature, and the dielectric relaxation which arises from the single-ionized oxygen vacancies moves to higher temperature with the decrease of TS.

Comparative studies on the structural, magnetic, and optical properties of perovskite Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) manganite nanoparticles synthesized by sol–gel method

Comparative studies of the structural, magnetic, and optical properties of the sol–gel synthesized Ln0.67Ca0.33MnO3 (Ln = La, Pr, Nd, and Sm) nanoparticles were carried out focusing on the effect of the A-site average cation size ⟨rA⟩. Rietveld refinements of x-ray diffraction data demonstrate all nanoparticles crystallize in an orthorhombic crystal structure (Pnma space group). Their unit cell volumes and the Mn–O–Mn bond angle decreased with reducing ⟨rA⟩, whereas the Mn–O bond length increased. The morphologies of nanoparticles evolved from spherical to irregular shapes, and their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching mode of the Mn–O bond near 600 cm−1, and the softening of this phonon mode as reducing ⟨rA⟩ is ascribed to the elongation of the Mn–O bond length. X-ray photoelectron spectroscopy reveals the mixed Mn3+ and Mn4+ cations with a content ratio of Mn3+/Mn4+ = 2:1, divalent Ca cations, and trivalent rare earth Ln cations in all nanoparticles and oxygen element existing as lattice oxygen and chemically absorbed oxygen. The La0.67Ca0.33MnO3 nanoparticles exhibited ferromagnetic behavior, whereas Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles displayed antiferromagnetic behavior and strong exchange bias effect. Temperature dependence of dc magnetizations suggests the spin-glass behavior established in the La0.67Ca0.33MnO3 nanoparticles, while magnetic cluster-glass behavior formed in the Ln0.67Ca0.33MnO3 (Ln = Pr, Nd, and Sm) nanoparticles, in which the charge-ordered and antiferromagnetic phases were completely suppressed. Electronic bandgaps of the nanoparticles were about 1.55 eV–1.66 eV, which was ascribed to the electronic charge transfer between two eg bands of the Mn cation with up-spins and down-spins separated by Hund’s coupling energy.

Antiferromagnet-mediated spin–orbit torque induced magnetization switching in perpendicularly magnetized L1-MnGa

Current-induced magnetization switching plays an essential role in spintronic devices exhibiting nonvolatility, high-speed processing, and low-power consumption. Here, we report on the spin–orbit torque-induced magnetization switching in perpendicularly magnetized L10-MnGa/FeMn/Pt trilayers grown by molecular-beam epitaxy. An antiferromagnetic FeMn layer is inserted between the spin current generating Pt layer and spin absorbing MnGa layer. Due to the exchange bias effect, the trilayers show field-free spin–orbit torque switching. Overall, the spin transmission efficiency decreases monotonically as the FeMn thickness increases. It is found that the spin current can be transmitted through an 8 nm-thick FeMn layer as evidenced by partial switching of the L10-MnGa. The damping-like spin–orbit torque efficiency shows a peak value at tFeMn = 1.5 nm due to the enhanced interfacial spin transparency and crystalline quality of the FeMn. These results help demonstrate the efficacy of emerging spintronic devices containing antiferromagnetic elements.

Magnetostriction and extraordinary exchange spring and exchange bias effects in Ni48Mn39Sn13 Heusler alloy

Experimental investigation of the Ni48Mn39Sn13 Heusler alloy at low temperature range revealed remarkable magnetovolume effects in vicinity of the martensite low temperature state of the alloy. Forced volume magnetostriction, Joulian-like magnetostriction, thermal expansion, and exchange spring and exchange bias effects have been studied including the high pressure effect on magnetization of the alloy. The observed value of forced volume magnetostriction of martensite phase of the alloy, dω/dH = (6.85 ± 0.05)*10−6 T−1 can be well compared with early presented pronounced decrease of magnetization under pressure. The Joulian magnetostriction at low temperature (with ΔL||< 0 and ΔL┴> 0) confirms a competition between strengths of magneto-crystalline anisotropy and elastic energy in martensite of the alloy during a field induced changes of its twin structure. The exchange spring and exchange bias effects in martensite phase of the alloy, Hsp = 510 Oe and Heb = 200 Oe, are not accompanied by any volume change. Field and temperature dependence of magnetostriction is discussed in detail with respect to magnetic behavior of the alloy.

The Exchange Bias Effect of CoFe2O4@NiO Core–Shell Nanofibers Based on Annealing Temperature

The Exchange Bias Effect of CoFe2O4@NiO Core–Shell Nanofibers Based on Annealing Temperature

Magnetic properties of Ni5Sn(O2BO3)2 ludwigite

The magnetic properties of site-disordered ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite have carefully been studied using local probes and bulk experimental methods in temperatures down to 1.6 K and magnetic fields up to 9 T. Our results have clearly shown the role of the effect of order/disorder site occupancy in these complex compounds with spin moments $S=1$. Different from most ludwigites, site-disordered ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigite has shown three magnetic transitions at 80, 50, and 5 K. Local probes ($^{119}\mathrm{Sn}$ M\"ossbauer and muon spin spectroscopy) have indicated a partial long-range magnetic order with onsets at 80 and 50 K with predominant ferromagnetic interactions at 80 K and antiferromagnetic ones at 50 K. At lower temperatures, a partial spin-glass-like freezing of the moments has been developed concomitantly with fraction of the moments ordered at higher temperatures, a spin-glass-like and long-range magnetic order coexist and a spontaneous exchange bias effect has been observed. Applied magnetic fields have gradually reduced magnetic frustration, leading to a magnetically ordered state of the complete structure at 51 K under 9 T. The absence of double-exchange interactions in order/disorder ${\mathrm{Ni}}_{5}\mathrm{Sn}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ ludwigites shows how the magnetic interactions that compete as direct exchange and double exchange are modified by the site occupancy of nonmagnetic Sn ions. Features as a third magnetic transition, the appearance of spontaneous exchange bias, and the enhancement of the coercive field at low temperatures were not found in the site-ordered isomer compound.

Suppression of multiple magnetic ordering induced by Nb and Ru substitution in SrCoO3-δ systems

Abstract In this paper, we report detailed study on magnetic behavior of perovskite SrCoO3-δ compound with partial substitution of non-magnetic Nb and magnetic Ru at Co site. All the samples show single-phase nature with tetragonal structure of P4/mmm space group. The semiconducting behavior is evident from temperature dependent resistivity (ρ) measurement. The resistivity exhibits Variable Range Hopping and Small Polaron Hopping mechanisms in different temperature regimes. The magnetoresistance increases with substitution and reaches up to ~35% at 10 K with 70 kOe magnetic field. The samples show weakly ferromagnetic-type transition below room temperature. The saturation magnetization and exchange bias (EB) increase with Nb concentration but the effect is less prominent for Ru substituted samples. Different magnetic properties are explained on the basis of valence states of Nb5+ and mixed valence of Ru4+/5+ replacing Co3+. The existence of ferromagnetic correlated region in antiferromagnetic matrix and irreversible glass state with competing exchange interactions impart the EB effect in these samples. The EB value HEB~7.45 kOe at 10 K is quite significant which might be useful for further technological implementation in electronics/spintronics devices.

Dynamic magnetic response of exchange-biased [formula omitted]FeAl/IrMn nanostructures

We report an experimental investigation of the quasi-static and dynamic magnetic properties of exchange-biased Co2FeAl/IrMn nanostructures. Specifically, we explore the exchange bias effect and the broadband ferromagnetic resonance spectrum of Co2FeAl/IrMn multilayers and verify their dependency with the thickness of the antiferromagnet. We observe an evolution of the dynamic magnetic response when samples are compared, and show it is experimentally tunable by varying the IrMn thickness and the angle between the external and bias fields. Our results thus provide further insights on the effect of antiferromagnetic thickness on the dynamical properties of Co2FeAl films. Additionally, they place exchange-biased Co2FeAl/IrMn nanostructures as interesting systems for applications based on the magnetization dynamics at high frequencies.

Exchange Bias in Weakly Interlayer-Coupled van der Waals Magnet Fe3GeTe2.

van der Waals (vdW) magnetic materials provide an ideal platform to study low-dimensional magnetism. However, observations of magnetic characteristics of these layered materials truly distinguishing them from conventional magnetic thin film systems have been mostly lacking. In an effort to investigate magnetic properties unique to vdW magnetic materials, we examine the exchange bias effect, a magnetic phenomenon emerging at the ferromagnetic-antiferromagnetic interface. Exchange bias is observed in the naturally oxidized vdW ferromagnet Fe3GeTe2, owing to an antiferromagnetic ordering in the surface oxide layer. Interestingly, the magnitude and thickness dependence of the effect is unlike those expected in typical thin-film systems. We propose a possible mechanism for this behavior, based on the weak interlayer magnetic coupling inherent to vdW magnets, demonstrating the distinct properties of these materials. Furthermore, the robust and sizable exchange bias for vdW magnets persisting up to relatively high temperatures presents a significant advance for realizing practical two-dimensional spintronics.