Exchange bias in the IrMn/Co structures with alternative sequences of antiferromagnetic and ferromagnetic layers

The magnetic properties of Mo/IrMn/Co/Mo/SiO₂/Si structures with alternative sequences of the antiferromagnetic and ferromagnetic layers have been studied by measuring the angular dependence of the high-frequency radiation absorption in the ferromagnetic resonance region. The layers have been prepared by pulsed laser deposition in the absence of a magnetic field. It has been found that thermal annealing and cooling make it possible to create the exchange bias in the structure with the upper antiferromagnetic layer at a temperature much below the Néel temperature. At the same time, the identical heat treatment does not induce the exchange bias in the structure with the upper ferromagnetic layer. The possible mechanisms of the phenomena observed are discussed.     

Exchange bias in the IrMn/Co structures with alternative sequences of antiferromagnetic and ferromagnetic layers

JETP Letters - The magnetic properties of Mo/IrMn/Co/Mo/SiO2/Si structures with alternative sequences of the antiferromagnetic and ferromagnetic layers have been studied by measuring the angular...     

Investigation of ferromagnetic coupling between Co layers in a Co/Pt multilayer with perpendicular anisotropy

The coercivity of a Co/Pt multilayer with out-of-plane anisotropy can be lowered greatly if it is grown onto an ultrathin NiO underlayer . By making use of this characteristic, a series of samples glass/NiO(10 Å)/[Co(4 Å)/Pt(5 Å)]₃/Pt(x Å)/[Co(4 Å)/Pt(5 Å)]₃ with different Pt spacer thickness have been prepared to determine the ferromagnetic (FM) coupling between Co layers across the Pt layer. The measurements of major and minor hysteresis loops have shown that the FM coupling between the top and bottom Co/Pt multilayers decreases monotonically with the Pt layer thickness and disappears above the Pt layer thickness of 40 Å. This thickness of 40 Å is much larger than that in the literature. In addition to the FM coupling between the top and bottom Co/Pt multilayers across the Pt spacer, there exists a weak biquadratic coupling, which induces the broad transition of the bottom Co/Pt multilayer.     

Spin-flop coupling and exchange anisotropy in ferromagnetic/antiferromagnetic bilayers

By investigating the antiferromagnetic spin configuration, the exchange anisotropy and the interfacial spin-flop coupling in ferromagnetic/antiferromagnetic (FM/AF) bilayers have been discussed in detail. The results show that there are four possible cases for the AF spins, namely the reversible recovering case, irreversible half-rotating case, irreversible reversing and irreversible half-reversing cases. Moreover, the realization of the cases strongly depends on interface quadratic coupling, interface spin-flop (biquadratic) coupling and AF thickness. The magnetic phase diagram in terms of the AF thickness t_AF, the interfacial bilinear coupling J₁ and the spin-flop coupling J₂ has been constructed. The corresponding critical parameters in which the exchange bias will occur or approach saturation have been also presented. Specially, the small spin-flop exchange coupling may result in an exchange bias without the interfacial bilinear exchange coupling. However, in general, the spin-flop exchange coupling can weaken or eliminate the exchange bias, but always enhances the coercivity greatly.     

Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

A study of the induced anisotropy in a ferromagnetic grain from an exchange coupled antiferromagnetic grain with uniaxial anisotropy

Approximate analytic solutions for the energy of an antiferromagnetic (AF) grain, which experiences an external exchange torque from a ferromagnetic grain have been obtained for a wide range of AF thickness. The accuracy of the analytic expression is within 2.0% of the exact solution, which do not have a closed analytical form. The model predicts that there are two critical AF grain thickness for each particular exchange energy strength. Below the first critical thickness, the induced anisotropy energy is well approximated by an uniaxial anisotropy term. Above the second critical thickness the induced anisotropy is unidirectional. In the intermediate range the induced anisotropy can not be expressed simply as uniaxial or unidirectional. The exchange bias and coercivity in NiFe/IrMn films have been studied as a function of the IrMn thickness, and the results are consistent with the proposed theory.     

Magnetism in structures with ferromagnetic and superconducting layers

The influence of superconductivity on ferromagnetism in the layered Ta/V/Fe_1–x V _x /V/Fe_1–x V _x /Nb/Si structures consisting of ferromagnetic and superconducting layers is studied using polarized neutron reflection and scattering. It is experimentally shown that magnetic structures with linear sizes from 5 nm to 30 μm are formed in these layered structures at low temperatures. The magnetization of the magnetic structures is suppressed by superconductivity at temperatures below the superconducting transition temperatures in the V and Nb layers. The magnetic states of the structures are shown to undergo relaxation over a wide magnetic-field range, which is caused by changes in the states of clusters, domains, and Abrikosov vortices.     

Magnetic reordering in the vicinity of a ferromagnetic/antiferromagnetic interface

The magnetic arrangement in the vicinity of the interface between a ferromagnet and an antiferromagnet is investigated, in particular its dependence on the exchange couplings and the temperature. Applying a Heisenberg model, both sc(001) and fcc(001) lattices are considered and solved by a mean field approximation. Depending on the parameter values a variety of different magnetic configurations emerge. Usually the subsystem with the larger ordering temperature induces a magnetic order into the other one (magnetic proximity effect). With increasing temperature a reorientation of the magnetic sublattices is obtained. For coupled sc(001) systems both FM and AFM films are disturbed from their collinear magnetic order, hence exhibit a similar behavior. This symmetry is absent for fcc(001) films which, under certain circumstances, may exhibit two different critical temperatures. Analytical results are derived for simple bilayer systems.     

Magnetic anisotropy of Co/Cu/Co films with indirect exchange coupling

The effect of isothermal annealing on the magnetic anisotropy, bilinear and biquadratic exchange coupling energies, and domain structure of Co/Cu/Co trilayer fiilms with d_Co=6 nm and d_Cu=1.0 and 2.1 nm prepared by magnetron sputtering has been studied. It is shown that, under isothermal annealing, the biquadratic coupling energy decreases by more than an order of magnitude in films with d_Cu=1.0 nm and increases in films with d_Cu=2.1 nm. The fourth-order magnetic anisotropy is shown to be related to the existence of biquadratic exchange energy.     

铁磁/反铁磁双层薄膜中应力各向异性研究

采用铁磁共振方法,研究了铁磁/反铁磁双层薄膜中交换各向异性和应力各向异性对其物理性质的影响.结果表明,单向各向异性来源于界面交换作用,应力各向异性对材料的磁化难易程度有较大影响.当外磁场方向与应力场方向平行时,应力场的存在将促进该方向的磁化.反之,应力场将会阻碍该方向的磁化.     

Electrical measurement of antiferromagnetic moments in exchange-coupled IrMn/NiFe stacks

We employ antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift, which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals an electrically measurable memory effect in an antiferromagnet.     

Magnetic anisotropy of ferromagnetic metals in low-symmetry systems

We have constructed an analytic formula to treat the perpendicular magnetic anisotropy energy in ferromagnetic metals with low symmetry, such as $C_{4\mathrm{V}}$ and $C_{3\mathrm{V}}$. We find that the anisotropy energy is proportional to a part of the expectation values of the orbital angular momentum and magnetic dipole operator. Although the result is similar to the model proposed by Laan (1998) [2], we have derived a concrete expression for the spin-flip virtual excitation process term, which can be dominant in atoms with small magnetic moments and/or small exchange splitting. Pt monatomic layer with proximity-induced spin polarization grown on Fe is an example of this. Other multilayer systems such as Co/Pd and Co/Ni, and bilayer systems such as Fe(CoB)/MgO can be discussed similarly. Moreover, the relation between perpendicular magnetic anisotropy energy and measurable physical parameters is discussed based on X-ray magnetic circular dichroism spectroscopy.     

Magnetic moments of ferromagnetic and antiferromagnetic bcc and fcc iron

Magnetic moments and total energies of ferromagnetic and antiferromagnetic bcc and fcc iron are obtained as a function of atomic volume by means of self-consistent band-structure calculations using the local spin-density-functional approximation.     

Magnetic phase transition in Heisenberg antiferromagnetic films with easy-axis single-ion anisotropy

The staggered susceptibility of spin-1 and spin-3/2 Heisenberg antiferromagnet with easy-axis single-ion anisotropy on the cubic lattice films consisting of n=2, 3, 4, 5 and 6 interacting square lattice layers is studied by high-temperature series expansions. Sixth order series in J/k_BT have been obtained for free-surface boundary conditions. The dependence of the Néel temperature on film thickness n and easy-axis anisotropy D has been investigated. The shifts of the Néel temperature from the bulk value can be described by a power law n^−λ with a shift exponent λ, where λ is the inverse of the bulk correlation length exponent. The effect of easy-axis single-ion anisotropy on shift exponent of antiferromagnetic films has been studied. A comparison is made with related works. The results obtained are qualitatively consistent with the predictions of finite-size scaling theory.     

Magnetic splitting of valence states in ferromagnetic and antiferromagnetic lanthanide metals

The magnetic splitting of Delta(2) valence states in the heavy lanthanide metals Gd, Tb, Dy, and Ho was studied in epitaxial films by angle-resolved photoemission, revealing an essentially Stoner-like temperature dependence in all cases. It scales linearly with the 4f spin moment, even in the case of the helical antiferromagnet Ho. Such a behavior can be explained by a substantial localization of the corresponding wave function in the c direction. The helical magnetic structure was confirmed for the thin Ho films by in situ resonant magnetic x-ray diffraction.     

Proximity effect in multilayer structures with alternating ferromagnetic and normal layers

The character of the penetration of superconducting correlations into multilayer FF…F, FNFN…FN, and NFNF…NF structures being in contact with a superconductor with the singlet pairing potential has been studied theoretically. Analytical expressions for the effective superconductivity penetration depth in such structures have been obtained in the limit of small layer thicknesses. Numerical calculations taking into account self-consistently the suppression of the superconductivity in the superconductor owing to the proximity effect have been performed at arbitrary thicknesses. A simple analytical dependence approximating the spatial variation of the Green’s function in a multilayer has been proposed. It has been shown that superconductivity is induced by the generation of two channels existing in parallel, one of which is characterized by the smooth (as in SN sandwiches) decay of the superconductivity, while damped oscillations (as in SF structures) take place in the second one.     

A study of the effective magnetic anisotropy and the corresponding spin configurations in two dimensional ferromagnetic/antiferromagnetic system

The spin configurations of two dimensional ferromagnetic/antiferromagnetic system were investigated using model calculations and Monte-Carlo simulation methods. The lowest energy state was obtained under various coupling conditions to investigate the role of interfacial interaction on anisotropy. We found that the total ferromagnetic layer anisotropy is contributed not only from its own crystalline anisotropy but also from the antiferromagnetic layer spin flop effect. The overall ferromagnetic layer effective anisotropy is calculated as a function of the exchange energy of antiferromagnetic layer and the interfacial interaction energy. If the effective anisotropy from the spin flop effect is comparable with the crystalline anisotropy, the asymmetric spin configuration is generated. In this configuration, the magnetization direction of the ferromagnetic layer is neither perpendicular nor parallel to the antiferromagnetic spin direction. Temperature effect on the perpendicular-to-collinear coupling transition was also investigated using Monte-Carlo simulation, and the relationship between the effective anisotropy and the temperature was obtained.     

Tunable magnetic anisotropy of antiferromagnetic superlattice and resultant exchange bias of ferromagnetic layer on it

Exchange biasing of ferromagnetic layer deposited on the antiferromagnetic superlattice was investigated in (Co₇₀Fe₃₀/Ru)_29.5/Ru/Co₉₀Fe₁₀ multilayers. Uniaxial magnetic anisotropy (K_AF) was induced and tuned in the antiferromagentic superlattice by uniaxial substrate bending method through the inverse effect of magnetostriction. The exchange bias increased and tended to be saturated with increasing the K_AF, while it was not observed at K_AF=0.