Exchange Bias Systems Research Articles

Temperature Dependence of the Exchange Bias Characteristics in aConventional Magnetic Tunnel Junction

The temperature dependence of the exchange bias in magnetic tunnel junctions (MTJs) was measured by using a cryogenic tunneling magneto-resistance (TMR) system and a superconducting quantum interference device (SQUID) at temperatures from 2 K to 300 K. The MTJ device have a typical Ta / NiFe / IrMn / CoFe / AlOx / CoFe / Ta structure. The exchange bias eld (Hex) was estimated from the curves of the TMR ratio and the M-H loop measurements with the SQUID were used to con rm the experimental results. The Hex of the typical exchange bias system was gradually increased up to about 10 K for both as-grown and ex-situ-annealed samples. The increase in Hex with decreasing temperature may be related to a reduction in the interface roughness or to an uncompensated AFM interfacial spin density in the anti-ferromagnetic systems.

Large Exchange Bias IrMn/CoFe for Magnetic Tunnel Junctions

We report on the enhancement of the exchange field, Hex , in IrMn/CoFe systems as a result of the application of field annealing procedures. The exchange field is enhanced up to a 37% on application of a 2 Tesla field for 90 minutes at 225degC. This setting process improves the magnitude of HEX without significant changes in the coercivity, HC, or the median blocking temperature. The sample with largest exchange bias field measured, 3.7 kOe and HC = 0.2 kOe at 20degC, seems to have the largest ratio of HEX to HC ever reported for an IrMn/CoFe exchange bias system. This makes these structures good candidates as pinned layers in magnetic tunnel junctions. The samples show good thermal stability with median blocking temperatures up to 200degC. The results also indicate that the improvement in the quality of the interfaces via the degree of alignment of the interfacial spins improves the coupling across the interface and hence the magnitude of the exchange bias field.

Texture Effects in IrMn/CoFe Exchange Bias Systems

We report an enhancement of the average blocking temperature <T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> > in IrMn/CoFe exchange bias systems due to an increase in the anisotropy constant of the IrMn. This is related to a (111) texture of IrMn parallel to the interface. Si<100>/Seed (5 nm)/IrMn (10 nm)/CoFe (3 nm)/Ta (10 nm) were prepared by dc sputtering. Cu, Ru, and NiCr seed layers were used. X-ray diffraction studies involving both thetas/2thetas and grazing incidence scans revealed a strong (111) texture of the IrMn parallel to the interface for the samples with a NiCr seed layer. Cu and Ru seed layers did not cause a marked texture. Thermal activation measurements found an enhancement of the average blocking temperature from 367 K for the sample with Cu seed layer to 477 K for the sample with a NiCr seed. The anisotropy constant was found to increase from (2.8plusmn0.2) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a Cu seed layer to (3.3 plusmn 0.4) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a NiCr seed layer. We find that the increase in the anisotropy constant leads to an enhanced blocking temperature.

Interfacial spin order in exchange biased systems

The phenomenon of exchange bias in which a ferromagnet is grown in intimate contact with an antiferromagnet leads to a shifted hysteresis loop. All analytical theories developed to explain this effect predict values of the shift of the loop that are too large, often by an order of magnitude. In many numerical models an interface exchange coupling constant C∗ is introduced to account for these discrepancies. In this work we describe a series of experiments, which reveals the temperature and field dependence of this interfacial spin order that account for the weakened coupling between the layers. We also show that this order has a marked temperature dependence at temperatures down to ∼50 K. At temperatures below 50 K we observe a sudden marked increase in the observed exchange bias of up to a factor of 2, which we attribute to a spontaneous spin ordering of the interfacial spins analogous to that which occurs in spin-glass systems.

Control of the setting process in CoFe/IrMn exchange bias systems

The time and temperature dependences of the exchange field achieved by control of the setting conditions in CoFe/IrMn exchange couples have been investigated. This has been achieved by varying the temperature at which the antiferromagnet was set (TSET) and by measuring the time dependence of the exchange field (Hex) at each temperature. The exchange field was observed to vary logarithmically with setting time, indicating that a wide distribution of energy barriers is present in the antiferromagnetic (AF) layer. The results are consistent with an independent AF grain model of exchange bias where the energy barrier distribution to reversal is governed by the grain volume distribution and the temperature dependence of the magnetocrystalline anisotropy. Calculations of the time dependence of the setting process are in good agreement with the grain volume model.

CoO1−δ layers in a reactively sputtered exchange-bias system

In exchange-bias (EB) systems involving CoO as the antiferromagnetic (AF) component, preparation of overoxidized (i.e. CoO1+δ) AF layers has previously been shown to have an important effect on EB properties, which arises due to point defects associated with the non-stoichiometric material. Here, we present a study on the effect of post-deposition annealing to progressively higher temperatures, Tann, on magnetic and structural parameters of a [Co/CoO/Ag]30 multilayer. Our results indicate a different AF layer stoichiometry: CoO1−δ (i.e. underoxidized), with δ=0.1 for the as-deposited sample. The annealing behaviour of the different parameters is explained via a mechanism of thermally induced diffusion of Co ions from the oxide to the Co layers, which is initiated at Tann≈300 °C (=T*) and leads to the approach to stoichiometry (i.e. Co1O1) in the AF layers. We suggest why our sample preparation conditions could have led to this unusual (i.e. CoO1−δ) stoichiometry and comment on possible future studies.

Tuning Exchange Anisotropy of Exchange-Biased System

Exchange anisotropy in FM/AFM bilayers has given a lot of static magnetization properties such as enhanced coercivity and magnetization loop shifts. These phenomena are primarily from the effective anisotropies introduced into a ferromagnet by exchange coupling with a strongly anisotropic antiferromagnet. These effective anisotropies can also be used to explain the dynamic consequences of exchange-biased bilayers. In this article, the dynamic consequences such as exchange-induced susceptibility, exchange-induced permeability, and the corresponding domain wall characteristics in the exchange-biased structures of ferromagnet/antiferromagnet1/antiferromagnet2 are studied. The results show that the second antiferromagnetic layer can largely affect the dynamic consequences of exchange-biased bilayers. Especially in the case of critical temperature, the effects become more obvious. Practically, the exchange anisotropy of biased bilayer system can be tuned by exchange coupling with the second antiferromagnetic layer.

Polarized Neutron Reflectometry for the Analysis of Nanomagnetic Systems

In recent years, polarized neutron reflectometry (PNR) has played an essential role for the exploration of magneto- and spintronic structures. Well known systems extensively studied include exchange coupled magnetic superlattices, exchange bias systems between ferromagnetic and antiferromagnetic films, exchange spring valves between soft and hard magnetic films, and more recently magnetic semiconductors and ferromagnetic Heusler alloy films and superlattices. In addition to studies of laterally extended layered systems, neutron scattering has now been applied to the exploration of periodic magnetic arrays, such as stripes and islands on the submicrometer scale. Although the competition with magneto-optics and X-ray resonant magnetic scattering (XRMS) has increased in recent years, there are some advantages PNR offers that are hard to challenge. One of those is the quantitative analysis of the data via fits to theoretical models based on the distorted wave Born approximation (DWBA), which accounts for both specular and off-specular scattering. The second one is spin flip (SF) scattering, which has no counterpart in XRMS. SF scattering probes magnetization fluctuations transverse to the mean magnetization direction and gives access to magnetic roughness and magnetic domain states. Specular and off-specular PNR work is exemplified by most recent work on spintronic materials and on patterned and functionalized magnetic layers.

Unconventional Exchange Bias and Multiferroics

Exchange bias is conventionally induced between a ferromagnet and an antiferromagnet where the Curie temperature (TC) considerably exceeds the Neel temperature (TN). Under these circumstances, cooling the bilayer in sufficient field to saturate the ferromagnet is sufficient to induce a finite bias below the blocking temperature (TB) which is close to T N. However, there are many systems of technological interest in which T C is not significantly greater than T N or in which it is not possible to field-cool the bilayer through T N; a highly relevant example is that of systems incorporating the multiferroic antiferromagnet BiFeO3which has a T N above the temperature at which interfacial reactions appear to occur with conventional ferromagnets. This paper reports work on several systems which show unconventional bias, including detailed measurements of FeMn/CuNi bilayers in which it is demonstrated that bias can be induced despite TC = TN. The reason for this is that, at least for thin antiferromagnet layers, there are always likely to be regions of the interface in which the interfacial coupling energy exceeds the anisotropy energy of the antiferromagnet. This means that field-cooling the bilayer enables partial micromagnetic reordering of the antiferromagnet and provided that reversal of the ferromagnet during measurement at sufficiently low temperatures (i.e., below TB) cannot remove this then a small but permanent bias is induced. We have previously shown that this can be described in terms of an activation energy model which predicts the correct form of the dependence of bias on antiferromagnet thickness and temperature. This result is vital in understanding the problems associated with attempts to apply the high temperature antiferromagnet multiferroic BiFeO3 in exchange bias systems. Here, a change in the electrical polarization of the antiferromagnet is expected to lead to a corresponding change in the micromagnetic order and hence a change in the exchange interaction. However, this can only lead to the goal of an electrically-controllable exchange bias if the bias can be established in the first place, and the energy changes induced by the electrical reversal exceed the exchange energy at the interface. This paper will examine the limitations which will need to be overcome for successful application of this effect.

MAGNETOELECTRIC Cr2O3 FOR SPINTRONIC APPLICATIONS

ABSTRACT Magnetoelectric (ME) materials being at the same time antiferromagnetic (AF) are proposed to be used in spintronic applications, e.g. for Magnetic Random Access Memory (MRAM). The main idea considers exchange coupling to a ferromagnet at the interface of the ME material and corresponding exchange bias (EB). The proposed electric switching of the EB field has been demonstrated on systems of single Cr2O3 crystalline substrates covered by [Co/Pt] n heterolayers [1]. Thin Cr2O3 thin films have been prepared by Molecular Beam Epitaxy (MBE) in order to be ready for downscaling. Corresponding EB systems show perpendicular EB at temperatures below T B ≈ 265 K. As an origin of the observed difference from the single crystal behavior, the formation of Cr2O3 nanograins at the interface is proposed and confirmed by measurements before and after thermal annealing of the sample [1]. P. Borisov, A. Hochstrat, X. Chen, W. Kleemann, and Ch. Binek, Phys. Rev. Lett. 94 117203 (2005).

Evidence of intrinsic exchange bias and its origin in spin-glass-like disorderedL0.5Sr0.5MnO3manganites (L=Y,Y0.5Sm0.5, andY0.5La0.5)

Exchange-bias (EB) phenomena have been observed in the ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($L=\mathrm{Y}$, ${\mathrm{Y}}_{0.5}{\mathrm{Sm}}_{0.5}$, and ${\mathrm{Y}}_{0.5}{\mathrm{La}}_{0.5}$)-type manganites showing cluster-glass-like and spin-glass-like behavior. The field cooled magnetic hysteresis loops exhibit shifts both in the field and magnetization axes. The values of exchange field $({H}_{E})$, coercivity $({H}_{C})$, remanence asymmetry $({M}_{E})$, and magnetic coercivity $({M}_{C})$ of ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ are found to depend strongly on temperature, measuring field, as well as strength of cooling field ${H}_{\mathrm{cool}}$. ${H}_{E}$ increases sharply with the magnitude of ${H}_{\mathrm{cool}}$ $(\ensuremath{\leqslant}3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, but for larger ${H}_{\mathrm{cool}}$ $(g3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, it decreases due to the growth of the ferromagnetic cluster size. This observed behavior has been explained in terms of interfacial exchange coupling between coexisting ferromagnetic cluster glass and the disordered spin-glass-like phases, where the spin configurations are strongly affected by the cooling field strength. Below the spin and/or cluster glass freezing temperature $({T}_{f})$, both ${H}_{E}$ and ${M}_{E}$ decrease exponentially with temperature. The value of ${H}_{C}$ increases almost exponentially with increasing magnetic-field step size $(\ensuremath{\Delta}H)$. In addition, the observed training effect of the EB behavior has been explained well in terms of the existing relaxation model developed for other classical EB systems. In view of the use of manganites in spintronics, the present observation of EB-like shift even in the mixed valent polycrystalline manganites such as ${L}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ is of paramount importance.

Exchange bias and spin valve systems with Fe–Mn antiferromagnetic pinning layers, obtained by the thermo-ionic vacuum arc method

Exchange bias and spin valve systems with Fe–Mn antiferromagnetic layers of different Fe concentrations were obtained by the thermo-ionic vacuum arc method. The adherence of the multilayer system to the Si substrate depends on the Fe–Mn composition. The overall roughness depends on the growing order of the antiferromagnetic/ferromagnetic layers. Very low blocking temperatures for exchange bias were observed for the considered compositions of the Fe–Mn layer. The coercive forces of both the pinned and the free layers of spin valve structures can be considerably modified along a large set of samples simultaneously prepared.

Angular dependence of magnetization reversal in exchange-biased Co∕Pt multilayer with perpendicular magnetic anisotropy

The magnetization reversal in exchange-biased (Co∕Pt)5∕Co∕FeMn multilayer with perpendicular magnetic anisotropy has been studied depending on the angle between an applied field and the easy axis (the normal of the film plane). The results show different characters from that in most in-plane exchange bias systems. In a large angular range, the magnetization rotates first toward the adjacent direction of easy axis for both descending and ascending branches because perpendicular magnetic anisotropy is much larger than unidirectional exchange anisotropy. With increasing the angle from 0° to 90°, the magnitude of the exchange bias field decreases, but the coercivity increases due to domain nucleation and propagation included in the magnetization reversal process. The angular dependence of magnetization reversal shows no hysteresis between clockwise and counterclockwise rotations.

Microscopic Nature of Ferro- and Antiferromagnetic Interface Coupling of Uncompensated Magnetic Moments in Exchange Bias Systems

Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.

Interfacial spin effects on Hex in metallic polycrystalline exchange biased systems

In this work, we show how the magnitude of the annealing field affects the antiferromagnetic (AF) spin structure in an exchange biased trilayer with two ferromagnetic layers with different thicknesses pinned by a single AF layer. The value of Hex is moderated by the interfacial spin structure, while the mean blocking temperature remains constant. We find a 24% increase in Hex when the setting field (Hset) used in the field cooling procedure is increased from 0.25to20kOe, while all other setting conditions are kept constant. We show that the reversal of the order in the AF gives the same mean blocking temperature, 442±2K, while the maximum value of Hex increases with Hset from 220to286Oe. The superposition of the variation of Hex with the degree of order of the AF reveals the dependence of the blocking temperature TB on the bulk of the AF, while the magnitude of the exchange field is shown to be dependent not only on the order in the AF but also on the degree of alignment of the interfacial spins determined by Hset. A further confirmation that this is an interfacial effect comes from an observation that similar behavior occurs at both AF interfaces in our trilayer system.

Ultrafast laser excitation of spin waves and the permanent modification of the exchange bias interaction in IrMn∕Co

Single mode coherent magnetization oscillations are observed in exchange biased IrMn∕Co thin films by the pump-probe time-resolved magneto-optical Kerr effect, using moderate intensity pump pulses. Unlike past experiments on exchange biased systems, the oscillations produced can be excited when the Co magnetization is saturated along the pinning axis. The oscillation frequencies correspond to ferromagnetic resonance measurements and can be described using a FMR equation. It is also demonstrated that ultrafast laser pulses can induce pinning along the direction of the applied magnetic field.

Spin waves in exchange-biased NiFe∕IrMn circular nanorings

We present the experimental investigation of the static and dynamical properties of NiFe(20nm)∕IrMn(5nm) rings. Magneto-optic Kerr effect measurements show that the IrMn introduces a sizable exchange-bias field HEB≈80Oe, which shifts the hysteresis loop along the excange-bias direction. Measured loops were satisfactorily reproduced using a micromagnetic calculation which includes the presence of HEB. Measurement of high frequency normal modes, by Brillouin light scattering, shows that the presence of the IrMn layer causes an overall up-shift of the spin wave frequency, in qualitative agreement with pervious studies of other exchange-biased systems. Moreover, the frequency asymmetry of the spin wave frequency upon reversing the external field is quantitatively consistent with the presence of the exchange-bias field measured by the Kerr effect.

Effect of pulsed laser irradiation on the structural and the magnetic properties of NiMn∕Co exchange bias system

We report about the influence of pulsed laser irradiation on the structural and magnetic properties of NiMn∕Co thin films. Rocking curve measurements showed a significant improvement of the (111) texture of NiMn after laser irradiation which was accompanied by grain growth. We have studied the ordering transition in as-prepared and irradiated (laser fluence of 0.15J∕cm2) samples during subsequent annealing. The onset of the fcc to fct phase transformation occurs at 325°C irrespective of laser irradiation. Exchange bias fields for the laser irradiated samples are higher than those of the as-prepared samples. The observed increase in the exchange bias field for laser irradiated samples has been attributed to the increased grain size and the improved (111) texture of the NiMn layer after laser irradiation.

Thermal activation of bulk and interfacial order in exchange biased systems

In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately ∼50% of the exchange bias, while the remaining 50% is due to the order in the bulk. For both samples, the median blocking temperature was found to be greater for the interfacial spins by ∼25K. The energy barriers to reversal for both bulk and interfacial spins have been calculated from the magnetic data and we find that the distributions are of similar form but with that for the interfacial spins occurring at higher values.

The effect of field-cooling strength and interfacial coupling on exchange bias in a granular system of ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

A modified Monte Carlo Metropolis method is performed to simulate the effect of field-cooling strength and interfacial coupling on exchange bias and coercivity of a system with ferromagnetic cores embedded in an antiferromagnetic matrix, based on three-dimensional classical Heisenberg model. The results show that the exchange bias changes from negative value to positive value with increasing cooling field as the interfacial coupling is antiferromagnetic, whereas coercivity is almost unchanged due to the small size of ferromagnetic cores. After applying a weak cooling field, the exchange bias of system with antiferromagnetic interfacial coupling may be positive just when the value of interfacial coupling is not large and exhibit a positive maximum. However, the exchange bias is constantly positive and increasing with increasing values of interfacial coupling after applying a strong cooling field. The exchange bias of system with ferromagnetic interfacial coupling is independent of cooling field and increases for larger interfacial coupling, but the sign is negative due to the constantly positive net magnetization of antiferromagnetic surface. It is obvious that the positive exchange bias depends on the field-cooling strength to a large extent. However, the interfacial coupling may change the configuration of antiferromagnetic matrix to influence the exchange bias.