Exchange Bias Systems Research Articles

Magnetic iron oxide nanoparticles with tunable size and free surface obtained via a “green” approach based on laser irradiation in water

Crystalline structure, size and surface coating are the relevant parameters influencing the catalytic and magnetic properties of iron oxide nanoparticles (FeOxNPs). The development of a “green” method for the synthesis of FeOxNPs with good crystallinity, controlled size and uncontaminated surface all at the same time is challenging. Here we show that laser ablation in water can be combined with laser irradiation at 355 nm to obtain FeOxNPs with average size tunable in the 10–30 nm range and a percentage of ferrimagnetic phase up to the 94% of crystalline phases, while maintaining the particles surface free from stabilizers or other chemical by-products and without using harmful or toxic reagents. The final FeOxNPs show good magnetization values and have the typical magnetic behaviour of exchange bias systems, due to their highly polycrystalline structure and small (ca. 3 nm) crystalline domains. The one-step coating with phosphonate ligands conferred long time stability in physiological medium to FeOxNPs.

Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements

Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the major detrimental factors to the performance of a magnetic tunnel junction (MTJ), is difficult to avoid during the fabrication process of the MTJ’s tunnel barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs, overoxidation of the tunnel barrier results in the formation of a CoO antiferromagnetic (AF) interface layer which couples with the bottom FM electrode to form a typical AF/FM exchange bias (EB) system. In this work, surface oxidation of the CoFe and CoFeB bottom electrodes was detected via magnetometry measurements of EB characterizations including the EB field, training effect, uncompensated spin density, and enhanced coercivity. Variations in these parameters were found to be related to the surface oxidation of the bottom electrode, among them the change in coercivity is most sensitive. Annealed samples show evidence for an oxygen migration back to the MgO tunnel barrier by annealing.

Measurement of the attempt frequency in antiferromagnets

The characteristic relaxation frequency of a granular antiferromagnet f0 in the Néel–Arrhenius expression has been calculated from a high resolution measurement of the time dependence of the median blocking temperature of a CoFe/IrMn exchange biased system. Calculations based on a granular model give f0=(2.1±0.4)×1012 s−1, three orders of magnitude greater than values commonly found in the literature.

Angular dependences of the switching field and coercivity for magnetic multilayer films with strain caused by lattice mismatch

The angular dependences of the switching field and coercivity for magnetic multilayer films with strain caused by lattice mismatch are studied in detail with the help of the Stoner–Wohlfarth model. The expressions or equations for determining the switching field and the coercivity are obtained analytically, taking the strain effect into consideration. With these analytical expressions, the roles of the strain and the unidirectional anisotropy (exchange coupling) are displayed clearly. Compared to the standard Stoner–Wohlfarth model (without strain), the centers of the curves of the switching field and coercivity are found to shift from π/2 to π/2+θ∗ due to the in-plane strain caused by the lattice mismatch between the film and substrate. For the ferromagnetic/antiferromagnetic exchange bias systems, the angularly dependent amplitude of the switching field or coercivity exhibits an obvious asymmetry. The staggering or separation of the peaks of the two branches of the coercivity results in a jump phenomenon in the curves for the exchange bias field and the coercivity near θ∗.

Using a zero-magnetization ferromagnet as the pinning layer in exchange-bias systems

Exchange bias properties have been investigated in a $({\text{Sm}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Al}}_{2}/{\text{SmAl}}_{2})$ bilayer (for $x=0.028$) with perpendicular magnetization, an original exchange-coupled system based on the zero magnetization ferromagnet $({\text{Sm}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Al}}_{2})$ as the pinning layer. This unusual magnet exhibits a magnetic compensation temperature where it presents both a zero magnetization and a long-range spin ferromagnetic order. In the $({\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}/{\text{SmAl}}_{2})$ bilayer, a large positive exchange bias has been observed for the ${\text{SmAl}}_{2}$ magnetization reversal, attesting for the exchange coupling between both compounds, even at the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ magnetic compensation temperature; the magnetization of the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ part of the bilayer appears to be pinned but the pinned component is obviously smaller than the one observed in a single uncovered ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$. X-ray Magnetic Circular Dichro\{\i}sm experiments have been undertaken to investigate the magnetization reversal in both layers independently and more particularly to probe the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ behavior in its zero-magnetization state. They reveal that approximately 20% of the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ layer is driven to reverse by exchange coupling to ${\text{SmAl}}_{2}$. The presence of lateral domains in the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ layer likely accounts for the relatively large proportion of rotatable moments and for the observed temperature dependence of exchange-bias field.

Polarized neutron reflectivity study of a thermally treated MnIr/CoFe exchange biassystem

It has recently been found that the exchange bias of a MnIr/CoFe system can be increasedsignificantly by adding a thermal treatment to the bilayer. To reveal the origin of thehigher exchange bias, we performed polarized neutron reflectivity measurements at theJRR-3 neutron source. The magnetization vector near the MnIr/CoFe interface forthermally treated samples differed from that for samples without the treatment. Wepropose a model in which the pinned spin area at the interface is extended due to theincreased roughness and atomic interdiffusion that result from the thermal treatment.

Exchange Bias and Domain Evolution at 10 nm Scales

For a fixed 2 μm×2 μm area of a Co/Pt-CoO perpendicular exchange bias system we image the ferromagnetic (FM) domains for various applied fields with 10-nm resolution by magnetic force microscopy (MFM). Using quantitative MFM we measure the local areal density of pinned uncompensated spins (pinUCS) in the antiferromagnetic (AFM) CoO layer and correlate the FM domain structure with the UCS density. Larger applied fields drive the receding domains to areas of proportionally higher pinUCS aligned antiparallel to FM moments. The data confirm that the evolution of the FM domains is determined by the pinUCS in the AFM layer, and also present examples of frustration in the system.

Recovery of the training effect in exchange bias systems within a coherent rotation model

Brems et al., [Phys. Rev. Lett. 95, 157202 (2005)] reported that the training effect in Co/CoO bilayers can be recovered by performing a hysteresis loop with an in-plane field perpendicular to the initial cooling field without raising the temperature above the N\'eel temperature. We performed numerical simulations of this effect within the coherent rotation model with multiple antiferromagnetic (AFM) easy axes. Both the ferromagnetic (FM) and AFM layers are divided into multi domains, and each FM domain couples with one AFM domain. In combination with the anisotropic magnetoresistance calculation, we find a strong dependence of the training recovery effect on the magnitude of the perpendicular field, in good agreement with the experimental findings.

A physical model of exchange bias in [Pd/Co]5/FeMn thin films with perpendicular anisotropy

A physical model of perpendicular exchange bias (PEB) has been established based on the total energy equation per unit area of an exchange bias system by assuming coherent rotation of the magnetization. The anisotropy energy of antiferromagnetic (AFM) layer, KAFM×tAFM, as well as ferromagnetic (FM) multilayers, KFM,eff×tFM, and the interfacial exchange coupling energy, Jex were considered as primary physical parameters in building up the physical model of PEB phenomenon. It was proposed that the PEB is a result of the energy competition between KAFM×tAFM, KFM,eff×tFM, and Jex; where KAFM×tAFM≥Jex, is a critical condition to observe exchange bias in the system. In particular, it was revealed that Jex is directly relevant to the net magnetization of FM and AFM spin structure, Jex∝cos αAFM×cos βFM, in the perpendicular direction rather than the magnetization angle difference observed in an in-plane system. The physical role of perpendicular anisotropy energy, KFM,eff×tFM was also found to be significant to enhance the PEB. These physical characteristics are completely different from those are observed from an exchange bias system with in-plane anisotropy. The physical validity of the proposed PEB model was confirmed using different structures of exchange biased [Pd/Co]5/FeMn thin films with perpendicular anisotropy. The experimentally analyzed results demonstrated that the physical model of PEB proposed in this work is agreed well with the experimentally observed PEB phenomenon. Furthermore, the proposed model was found to be effective to design and to predict a new PEB system for the advanced spintronics applications.

Extracting uniaxial anisotropy of ferromagnetic layer in exchange-biased system

Effective anisotropy of the ferromagnetic pinned layer of ferro(FM)-antiferromagnetic (AF)-coupled NiFe(FM)/FeMn(AF) exchange-biased system was investigated in a broad frequency range (100 MHz–5 GHz) using a complex permeability spectrum. The exchange bias and effective uniaxial anisotropy fields of the thin film have been computed theoretically using the Landau–Lifs chitz–Gilbert (LLG) equation. From the measurements, uniaxial anisotropy of the pinned FM layer has been extracted to understand the nature of the exchange bias in the system. It is found that the uniaxial anisotropy field of NiFe layer when exchange biased with the AF layer increases from 5 to 15 Oe at different external magnetic fields.

Manipulation of uncompensated moments in trained exchange bias system

In an exchange coupled trained antiferromagnet (AF)–ferromagnet (FM) system, we could directly manipulate the uncompensated moments in the AF—resulting in inducing a magnetism within the AF layer. This is made possible by applying an in-plane magnetic field perpendicular to the cooling field HFC. Depth-sensitive polarized neutron scattering experiments in specular and off-specular mode have revealed that this interfacial manipulation could neither restore the untrained state nor could it reinduce the initial asymmetry in magnetization reversal. Our results demonstrate an initiative toward user defined exchange-coupled interfaces for the future.

Ultra-high ferromagnetic resonance frequency in exchange-biased system

Static and dynamic magnetic properties of exchange-biased FeCo/MnIr multilayers were systematically investigated. A huge increased ferromagnetic resonance frequency up to 10 GHz was observed in the case of thin MnIr layers being possibly interpreted as the contribution of the rotatable anisotropy. The rotatable anisotropy as well as the resonance frequency and frequency linewidth can be tuned by changing the thickness of MnIr layer. The correlation between the variation of rotatable anisotropy and enhancement of coercivity and magnetic damping is discussed.

Enhanced exchange anisotropy in IrMn/CoFeB systems and its correlation with uncompensated interfacial spins

Bottom pinned exchange bias systems of IrMn/CoFe and IrMn/CoFeB on CoFe seed layers were studied. Enhanced exchange anisotropy has been observed for IrMn/CoFeB samples annealed at 350 °C. The ferromagnetic and antiferromagnetic layers of both samples are polycrystalline and textured {110} for the CoFe and CoFeB, and {111} for IrMn. Results demonstrated that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins as evidenced by the enhanced Mn x-ray magnetic circular dichroism (XMCD) signal strength. A quantitative correlation between the Mn XMCD signal and the exchange anisotropy constant Jk was observed.

Texture and magnetic properties of exchange bias systems

We report on the magnetic and structural properties of IrMn/CoFe exchange bias systems deposited onto a dual seed layer of NiCr(X)/Ru(5 nm), with X=2, 6, and 20 nm. Samples with the structure NiCr (Xnm)/IrMn (7 nm)/CoFe (3 nm)/Ta (10 nm) with X=2, 6, and 20 nm were prepared by dc sputtering for magnetic characterization. A second set of samples with structure NiCr (Xnm)/IrMn (10 nm) with X=2, 6, and 20 nm were deposited onto TEM grids for structural characterization by TEM. A method of manipulating of the TEM grid to allow a qualitative analysis of the in-plane texture of the samples is described and used to analyze the microstructure of these samples. The microstructure and particularly the texture are correlated with the anisotropy constant (KAF) of the antiferromagnet (AF) layer, with an optimum NiCr seed layer of 6 nm to give a maximum value of KAF of 1.2×107 ergs/cc.

Interfacial and bulk order in polycrystalline exchange bias systems

A study of the effect of the setting conditions on the two coercive fields HC1 and HC2 in an exchange bias system is presented. The study was performed on a polycrystalline thin film sample of composition Si/Cu(5 nm)/IrMn(10 nm)/CoFe(2 nm)/Ta(5 nm) with a median blocking temperature ⟨TB⟩=240 K. An experiment analogous to a thermoremanence experiment, where order is quenched in by field cooling, is compared to a setting field experiment at constant temperature. These processes can be used to probe the order of the interfacial spin clusters and the bulk of the antiferromagnet. HC1 and HC2 follow different setting temperature dependences, indicating that different mechanisms are dominant in the two branches of the loop.

Antiferromagnetic thickness dependence of the CrTe–MnTe exchange-bias system

We report the antiferromagnetic (AFM) thickness dependence study of zinc-blende CrTe/ZnTe/MnTe layered structure grown on GaAs (100) by low temperature molecular beam epitaxy. The coercivity (HC) and exchange bias field (HE) show a nonmonotonic dependence on the AFM thickness. Both (HC) and HE exhibit a maximum value at the AFM thickness of ∼15 nm. With decreasing AFM thickness, we found that the blocking temperature drops from 70 (Néel temperature of bulk MnTe) to 22 K, while the Curie temperature is found to increase from 100 to 190 K. Additionally, a vertical magnetization shift is observed in the M-H loop, which can be attributed to the presence of frozen ferromagnetic spins at the interface.

Wheatstone bridge sensor composed of linear MgO magnetic tunnel junctions

A full Wheatstone bridge sensor composed of linear MgO based magnetic tunnel junctions (MTJ) was designed and achieved. The magnetization direction of reference layers in the required bridge arms was successfully switched by using local current heating method, also demonstrating a viable method of manipulation of pinning direction for exchange bias system on a chip level. The final bridge output shows approximately full signal of individual MTJ but almost null output in the absence of any applied sensing field and small offset of voltage and field.

Measurement of the antiferromagnet activity in exchange bias systems

In this work, we describe the results of a recent set of experimental measurements on samples of CoFe/IrMn ferromagnet/antiferromagnet (AF) bilayer exchange bias materials. Careful magnetic measurement protocols and detailed particle size measurements have enabled us to confirm the link between the exchange field (Hex) and the distribution of grain volumes in the IrMn AF layer. This can be achieved by fitting the blocking temperature curve to obtain an effective value for the Néel temperature (TN). The value of (TN) is then used to determine the stable and set fraction of the grain volume distribution. Through a comparison of samples with different seed layer structures, we demonstrate control of the exchange bias where almost all the AF grains contribute to Hex. These samples have an AF with a mean grain diameter of 8.7 nm and an anisotropy constant of 7.8×106 ergs/cc. Our results indicate a reduction in the value of the Néel temperature (TN) of up to 100 K.

Thermal stability of exchange bias nanostructures

The thermal stability of an array of square exchange bias nanoelements has been investigated as a function of both the element size and the microstructure of the antiferromagnetic layer. The thermal stability of the nanostructures is measured in terms of their median blocking temperature ⟨TB⟩. It is shown that, generally, ⟨TB⟩ is lower for nanostructures than for the equivalent thin film system. However, the opposite behavior can be observed depending on the measurement conditions, specifically the temperature of measurement and the setting temperature. These results emphasize the importance/limitations of the setting process in exchange bias systems.

Coupling of Fe and uncompensated Mn moments in exchange-biased Fe/MnPd

A bilayer exchange-bias system composed of Fe/MnPd is investigated using x-ray magnetic circular dichroism (XMCD) and soft x-ray resonant magnetic reflectometry (XRMR). The absorption and XMCD data at the $\text{Fe}\text{ }L$ and $\text{Mn}\text{ }L$ edges are used to derive the optical and magneto-optical properties of the individual layers. Then the structural and magnetic depth profiles of the sample are obtained from XRMR. From reflectivity measurements at the $\text{Fe}\text{ }L$ and $\text{Mn}\text{ }L$ edges, a precise magnetic depth profiling of the interface region between the ferromagnet (F) and antiferromagnet (AF) was carried out. It reveals rotatable and pinned uncompensated Mn moments in the AF. By comparing the signs and magnitudes of the absorption, the relative coupling directions in the system are determined. It is found that rotatable Mn and the ferromagnetic Fe couple antiparallel. The pinned Mn moments are oriented antiferromagnetic to the neighboring rotatable Mn and ferromagnetic with respect to the Fe during the field cooling process.