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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Temperature-induced sign change of the exchange bias in Fe0.82Zn0.18F2/Co bilayers

Hongtao Shi, D. Lederman, N. R. Dilley, R. C. Black, J. Diedrichs, K. Jensen, and M. B. Simmonds

J. Appl. Phys. 93, 8600 (2003); http://dx.doi.org/10.1063/1.1557858 (3 pages) | Cited 16 times

Online Publication Date: 9 May 2003

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A single crystal, (110)-oriented dilute antiferromagnet (AF) Fe0.82Zn0.18F2 film was grown via molecular beam epitaxy on a (110)-MgF2 substrate by codepositing FeF2 and ZnF2, followed by 1.0 nm pure FeF2 and 18 nm Co layers. The exchange bias (HE) and coercivity (HC) of the Co film strongly depend on the cooling field (HCF) and temperature. For 0⩽HCF<2 kOe, HE<0 in the whole temperature range before reaching zero at the blocking temperature (TB). For HCF⩾15 kOe, HE>0 for T<TB. In both cases, HC peaks when T∼TB. For 2 kOe⩽ HCF<15 kOe, HE<0 at low temperatures, and then suddenly becomes positive at a characteristic switching temperature TS(<TB). TS decreases as HCF is increased and HC peaks at both T∼TS and T∼TB. These results indicate that for intermediate HCF unstable domains are created in the AF during the cooling procedure. The domains suddenly disappear as enough thermal energy is available to switch them when perturbed by the rotation of the ferromagnetic magnetization. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.50.Ee Antiferromagnetics
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Stationary antiferromagnetic domains during magnetization reversal in an exchange-biased FeMn/Fe76Mn6C18 bilayer

R. D. Shull, A. J. Shapiro, V. S. Gornakov, V. I. Nikitenko, and Hong-Wu Zhao

J. Appl. Phys. 93, 8603 (2003); http://dx.doi.org/10.1063/1.1557814 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 Å)/FeMn (100 Å) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during the prior field application, so that at H=0 the domain structure was the same as that in the original ground state. If H was off-axis, during the original field application domain growth occurred in that same direction; upon field reduction, the preferred domain growth direction was at an angle with reversed sign. These results prove the AF domain walls do not move during the motion of FM domain walls, and that an exchange spring is created parallel to the AF/FM interface as the ferromagnetic layer reverses. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
75.60.Jk Magnetization reversal mechanisms
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.50.Bb Fe and its alloys
75.30.Et Exchange and superexchange interactions
78.20.Ls Magneto-optical effects
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Metastable antiferromagnetic domain configurations in exchange biased bilayers

Luc Thomas and Béatrice Negulescu

J. Appl. Phys. 93, 8606 (2003); http://dx.doi.org/10.1063/1.1557792 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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We report on exchange bias relaxation in NiO/FeNi bilayers. FeNi films have been sputtered on top of NiO films, previously grown on quartz substrates using pulsed laser deposition. As-prepared bilayers show enhanced coercivity but no bias. Their hysteresis loops are essentially isotropic within the sample plane. Exchange bias relaxation is observed after the samples have been magnetized once, and let in their remanent state: exchange bias progressively builds up in zero external magnetic field, over a few days timescale. This behavior may be related to the reorientation of antiferromagnetic domains triggered by the exchange coupling with the ferromagnetic domains throughout the interface. Relaxed samples are exchange biased along the magnetization direction, which becomes an easy axis. Along the perpendicular direction, a two-step magnetization reversal is observed. This two-step process is erased when the samples are field-annealed at elevated temperatures, and it is replaced by the rotation process usually observed along the hard axis. These results suggest that two different antiferromagnetic domain structures can be stabilized at room temperature, both giving rise to exchange bias. This may be related to the competition between the field-induced exchange bias direction, and the antiferromagnetic anisotropy easy axis within the NiO layer. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Jk Magnetization reversal mechanisms
75.60.Nt Magnetic annealing and temperature-hysteresis effects
75.30.Gw Magnetic anisotropy

Magnetic relaxation in exchange-coupled Co/CoO bilayers measured with ac-anisotropic magnetoresistance

D. Venus, F. Hunte, I. N. Krivorotov, T. Gredig, and E. Dan Dahlberg

J. Appl. Phys. 93, 8609 (2003); http://dx.doi.org/10.1063/1.1557793 (3 pages) | Cited 4 times

Online Publication Date: 9 May 2003

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The complex ac-susceptibility of field-cooled Co/CoO bilayers has been measured by perturbing the magnetization with a small (5 Oe) ac field, and detecting the response through anisotropic magnetoresistance using a double modulation technique. Samples of Ta(3 nm)/Co(8 nm)/CoO(3 or 10 nm)/Cu(3 nm) grown by sputter deposition on an amorphous silicon nitride substrate have been studied using a frequency of 100 Hz. The relaxation of the exchange anisotropy in response to the perturbation allows the activation energies and relaxation times of the CoO grains to be investigated. Both the real and imaginary response show prominent thickness-dependent peaks at the blocking temperature, as well as a peak near 80 K which is insensitive to CoO film thickness. The measurements are well-described by a generic linear relaxation model of the anisotropy field, using a bimodal distribution of activation energies for the CoO grains. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.47.Pq Other materials
75.30.Cr Saturation moments and magnetic susceptibilities
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Effect of interfacial coupling on the magnetic ordering in ferro-antiferromagnetic bilayers

Shan-Ho Tsai, D. P. Landau, and Thomas C. Schulthess

J. Appl. Phys. 93, 8612 (2003); http://dx.doi.org/10.1063/1.1557794 (3 pages) | Cited 15 times

Online Publication Date: 9 May 2003

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Monte Carlo simulations have been used to study magnetic ordering in coupled anisotropic ferro/antiferromagnetic (FM/AFM) films of classical Heisenberg spins. We consider films with flat interfaces that are fully uncompensated as well as rough interfaces that are compensated on average. For both types of interfaces above the “Néel temperature” we observed order in the AFM with the AFM spins aligning collinearly with the FM moments. In the case of rough interfaces there is a transition from collinear to perpendicular alignment of the FM and AFM spins at a lower temperature. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Gw Magnetic anisotropy
75.10.Jm Quantized spin models, including quantum spin frustration
75.40.Mg Numerical simulation studies
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
02.70.Uu Applications of Monte Carlo methods

Numerical simulation of unidirectional anisotropy in ferro(F)/antiferromagnetic(AF) exchange coupled layers with a compensated AF-interface

Yutaka Sakurai and Hideo Fujiwara

J. Appl. Phys. 93, 8615 (2003); http://dx.doi.org/10.1063/1.1557813 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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We have performed a Zero-K Monte Carlo simulation for F/AF coupled systems of simple cubic structure. No combination of parameters, JF-AF/JF-F and JAF-AF/JF-F (JF-F, JAF-AF and JF-AF are exchange constant between F-F, AF-AF, and F-AF spins, respectively) ranging between 0.1 and 1 has been found, assuming Ku/JF-F=0.01 (Ku: uniaxial anisotropy coefficient), which gives unidirectional anisotropy in the sense that the energy exhibits 360° symmetry dependence on the F-magnetization direction. However, by assuming the existence of a plane in the AF-layer across which the AF-spins couple loosely with each other, it is demonstrated that a typical 360° symmetry anisotropy appears without assuming any AF-spin constraint in the plane. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.40.Mg Numerical simulation studies
02.70.Uu Applications of Monte Carlo methods

Micromagnetic calculations of bias field and coercivity of compensated ferromagnetic antiferromagnetic bilayers

Dieter Suess, Markus Kirschner, Thomas Schrefl, Werner Scholz, Rok Dittrich, Hermann Forster, and Josef Fidler

J. Appl. Phys. 93, 8618 (2003); http://dx.doi.org/10.1063/1.1557859 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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Exchange bias in polycrystalline IrMn/NiFe was found at perfectly compensated interfaces. The energy associated with unidirectional anisotropy is stored in lateral domain walls in the antiferromagnet. In addition to exchange bias, this mechanism leads to a training effect. The bias field shows a maximum of μ0Hb=4 mT at an antiferromagnetic layer thickness of 22 nm. The coercivities are on the order of μ0Hc=10 mT. The coercive field increases with decreasing intergrain exchange interactions within the ferromagnet. © 2003 American Institute of Physics.
Show PACS
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
75.60.Ch Domain walls and domain structure
75.70.Kw Domain structure (including magnetic bubbles and vortices)
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