Exchange Bias Energy Research Articles

Reduced thermal dependence of the sensitivity of a planar Hall sensor

The ability to stabilize the sensitivity of a magnetoresistance sensor in unstable thermal environments is a key parameter in many high precision measurements. Here, we propose a method to stabilize the sensitivity of a highly sensitive and low noise magnetic sensor based on a planar Hall Effect crossed junction. The stability is achieved by controlling the interplay between Zeeman energy, exchange bias energy, and anisotropy energy as a function of the temperature of the sensor stack comprising a trilayer structure NiFe/Cu/IrMn (10/0.12/10 nm). The high thermal stability of the sensor sensitivity of 4.5 ± 0.15 × 10−3 V/A/T/K is achieved when the external magnetic field is set around ±2 ± 0.04 mT and the applied current is fixed at 20 mA in the temperature range of 110 K–360 K. This method improves the magnetic sensor detection by about an order of magnitude, enabling its deployment in various research fields, particularly to study magnetic properties of small quantities of magnetic materials toward the detection of single magnetic objects, which was impossible before.

Direct observation of frozen moments in the NiFe/FeMn exchange bias system

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin–orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energy.

Size dependent structural and magnetic properties of FeO–Fe3O4 nanoparticles

The magnetic properties of monodisperse FeO-Fe3O4 nanoparticles with different mean sizes and volume fractions of FeO synthesized via decomposition of iron oleate were correlated to their crystallographic and phase compositional features by exploiting high resolution transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy and field and zero field cooled magnetization measurements. A model describing the phase transformation from a pure Fe3O4 phase to a mixture of Fe3O4, FeO and interfacial FeO-Fe3O4 phases as the particle size increases was established. The reduced magnetic moment in FeO-Fe3O4 nanoparticles was attributed to the presence of differently oriented Fe3O4 crystalline domains in the outer layers and paramagnetic FeO phase. The exchange bias energy, dominating magnetization reversal mechanism and superparamagnetic blocking temperature in FeO-Fe3O4 nanoparticles depend strongly on the relative volume fractions of FeO and the interfacial phase.

강자성 공명법을 이용한 CoFe/MnIr 박막의 교환 결합 에너지 분석

본 연구에서는 CoFe/MnIr 이중층 구조의 계면에 존재하는 비상보성 반강자성(AF) 스핀들에 의한 교환 결합 에너지 특성을 강자성 공명(FMR) 측정법을 이용하여 분석하였다. MnIr의 두께가 <TEX>$t_{AF}$</TEX>= 0, 3 및 10 nm 재료를 열처리한 한 후 이들 재료의 FMR 신호를 측정하였으며, <TEX>$t_{AF}$</TEX>= 0 nm 재료를 기준으로 하여 <TEX>$t_{AF}$</TEX>= 3 및 10 nm 재료의 교환 바이어스 자기장(<TEX>$H_{ex}$</TEX>)과 회전 이방성 자기장(<TEX>$H_{ra}$</TEX>)을 도출하였다. <TEX>$300^{\circ}C$</TEX>에서 열처리한 재료의 경우, <TEX>$H_{ex}$</TEX>과 <TEX>$H_{ra}$</TEX>의 합은 MnIr의 임계 두께에 무관하게 동일한 값을 보이는데, 이는 비상보성 AF 스핀들이 모두 한 방향으로 정열 되었음을 의미한다. 이들 결과로부터 비상보성 AF 스핀들 중에서 일부분은 CoFe/MnIr의 계면에 고정되어 <TEX>$H_{ex}$</TEX>로 발현되어 나타나고, 나머지 부분은 자기장의 방향에 따라서 회전하므로 <TEX>$H_{ra}$</TEX>로 발현되고 있음을 알 수 있었다. 따라서 교환 결합 에너지는 교환 바이어스 에너지 및 회전 이방성 에너지의 합으로 표현될 수 있음을 보였다. We measure the ferromagnetic resonance signals in order to analyze the exchange coupling energy due to the uncompensated antiferromagnetic spins in exchange coupled CoFe/MnIr bilayers. The exchange bias fields (<TEX>$H_{ex}$</TEX>) and rotatable anisotropy fields (<TEX>$H_{ra}$</TEX>) are obtained from the ferromagnetic resonance fields measured with in-plane angle in thermal annealed samples with <TEX>$t_{AF}$</TEX>= 0, 3, and 10 nm. The sum of the <TEX>$H_{ex}$</TEX> and <TEX>$H_{ra}$</TEX> do not depend on the MnIr thickness, which means that all the uncompensated AF spins are aligned to one direction in <TEX>$300^{\circ}C$</TEX> annealed samples. Therefore, the uncompensated AF spins are divided into two different parts. One parts are fixed at the interface between CoFe/MnIr bilayers and induces the <TEX>$H_{ex}$</TEX>, other parts are rotatable with magnetic field and induces the <TEX>$H_{ra}$</TEX>. Finally, the exchange coupling energy can be expressed by the sum of the exchange bias energy and rotatable anisotropy energy.

Exchange bias anisotropy in Co nanowires electrodeposited into polycarbonate membranes

We report on exchange bias effects and time evolution in Co nanowires electrodeposited into the pores of track etched polycarbonate membranes. The slow variation of the exchange bias anisotropy, as extracted from field cooled hysteresis loops, is due to the delayed in time oxidation of the nanowires embedded in the polymer template. A model has been proposed to account for the drop of magnetization due to the presence of an oxide layer at the periphery of the nanowires, within the nanowires/polymer interface. The measured exchange bias energy is about four times smaller than the energy in planar bilayers because of the cylindrical geometry of the wires.

Investigation of the Perpendicular Exchange Bias in the Cu/Ni/FeMn Structure

The perpendicular exchange bias (Hex) in epitaxial Cu/Ni/FeMn trilayers with various Ni and FeMn thicknesses has been investigated. All the samples were prepared by dc-magnetron sputtering. We analyzed the behaviors of the exchange bias eld and the coercivity as functions of the Ni and the FeMn thicknesses. The maximum perpendicular exchange bias eld (305 Oe) was obtained for Ni(4 nm)/FeMn(5 nm) lms. The exchange bias energy, ex, was calculated in a good argument that exchange bias eld depends on Ni thickness. The crystalline orientation is also reported.

Exchange bias energy in Co/Pt/IrMn multilayers with perpendicular and in-plane anisotropy

The magnetization reversal process in perpendicularly biased [ Pt / Co ] 3 / d Pt Pt/IrMn and in-plane biased Co / d Pt Pt/IrMn multilayers with 0 nm ⩽ d Pt ⩽ 1.2 nm was investigated using Kerr magnetometry and Kerr microscopy. For the system with in-plane magnetic anisotropy, the exchange bias field decreases monotonically with Pt insertion layer thickness, while its coercivity remains constant. The samples with perpendicular magnetic anisotropy, on the other hand, exhibit maximum exchange bias and minimum coercivity for d Pt = 0.1 nm . In both cases, the existence of large exchange bias fields correlates with a high domain density during magnetization reversal. The interface exchange coupling energy is larger for the in-plane biased films than for the perpendicularly biased multilayers.

Structural characterization of Co100−xFex nano-oxide layer

For the structural characterization of a Co100−xFex nano-oxide layer (NOL), the exchange bias properties of the Co100−xFex-natural oxidized NOL in the specular spin-valve (SPSV) system were investigated. The exchange bias energy (Jex) increased monotonically with the increasing Fe content for the Co100−xFex-NOL. The enhancement of both the magnetoresistance ratio and the exchange bias field (Hex) was realized by increasing the Fe content in the Co100−xFex-NOL. It should be mentioned that Hex more than 800Oe is obtained by the insertion of Co30Fe70-NOL, even in NOL-SPSV, which is a remarkably higher pinning field than that ever reported on IrMn-SV. This high exchange bias field is considered to be realized by the formation of an Fe-rich fcc phase at the interface of IrMn.

Enhancement of room temperature-exchange biasing in specular spin valves

The exchange bias properties of the Co/sub 1-x/Fe/sub x/ natural-oxidized nano-oxide layer (NOL) in the specular spin valve system are investigated. The exchange bias energy (J/sub ex/) increased with the Fe content of the CoFe NOL. This result suggests that the enhancement of the pinning field/energy at room temperature was induced by the inserted NOL into the pinned layer. The enhancement of both the magnetoresistance ratio and the exchange bias field was realized by increasing the Fe content in the Co/sub 1-x/Fe/sub x/ NOL.

CoFe/IrMn exchange biased top, bottom, and dual spin valves

The present work is a comparative study of CoFe/IrMn exchange biased top, bottom, and dual spin valves. For as-deposited top spin valves the exchange bias energy Jk was determined to be 0.14 erg/cm2 for IrMn 50 Å. The exchange field (Hex) decreases rapidly upon heating, resulting in a blocking temperature (Tb) of 250 °C. For bottom IrMn based spin valves the exchange bias energy is much higher (Jk=0.28 erg/cm2) while ΔR/R (6.9%) is reduced with respect to top spin valves (&amp;gt;8.0%). This is interpreted as showing that Hex is dominantly dependent on fcc(111) crystalline texture while ΔR/R is sensitive to both crystalline texture and interface roughness. Dual spin valves based on the above results showed two exchange loops due to the difference in the exchange bias energy between the top and bottom pinning layers.

Exchange-bias systems with compensated interfaces

When a ferromagnetic metal (F) is in contact with an antiferromagnet (AF), often a shift of the hysteresis loop away from its normal, symmetric position around H=0, to HE≠0 does occur. This phenomenon is known as exchange bias (EB). We put forward an analytic model, for compensated AF interfaces, based on the AF interface freezing into a metastable canted spin configuration. The EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall, in the F slab. Our theory yields the right values of HE and its F thickness dependence HE∝tF−1. It also predicts the F layer by layer magnetization profile.

Exchange coupling and GMR properties in ion beam sputtered hematite spin-valves

We have succeeded in fabricating hematite (/spl alpha/-Fe/sub 2/O/sub 3/) antiferromagnetic (AF) thin films by the ion beam sputtering method, which give unidirectional anisotropy to the adjacent ferromagnetic layer. Its exchange bias energy (J/sub ex/) is 0.03 erg/cm/sup 2/, and blocking temperature (T/sub b/) is 250/spl deg/C. Spin-valve films with the hematite AF layer were also fabricated. They exhibit GMR property with unidirectional anisotropy of the pinned layer. Spin- valve films of which magnetic layers consist of only NiFe exhibit relatively low MR ratio and poor thermal stability. However, the spin-valve using Co as the pinned layer and the free layer at the Cu interface exhibits MR ratio of more than 6% and excellent thermal stability.