Ivan Schuller

University of California, San Diego, Physics, Faculty Member

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Papers by Ivan Schuller

The role of interface crystalline and magnetic structure in exchange anisotropy (abstract)

Journal of Applied Physics, 1996

Large exchange bias effects were measured in antiferromagnetic FeF2-ferromagnetic Fe bilayers. Th... more Large exchange bias effects were measured in antiferromagnetic FeF2-ferromagnetic Fe bilayers. The interface spin structure and roughness was controlled by using different substrates and growth temperatures. The samples were characterized ex situ using low and high angle x-ray diffraction, in-plane x-ray diffraction, and atomic force microscopy. We discovered that the exchange bias is largest in the (110) FeF2 spin-compensated surface films, in contradiction with simple models that predict a low exchange anisotropy for spin-compensated surfaces. For these samples, the exchange anisotropy decreases as the interface roughness increases.

Magnetization Reversal Processes in Long Chains of Co Dots

APS March Meeting Abstracts, Mar 1, 1997

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Influence of antiferromagnet anisotropy on critical thickness in TMF2/Fe

APS March Meeting Abstracts, Mar 1, 2002

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Determination of Magnetic Ordering in Antiferromagnetic FeF 2 - Ferromagnetic Fe Interfaces Using Exchange Anisotropy

APS, Mar 1, 1996

Dynamic Effects O F Magnetic Multilayer Interlayer Coupling

Coupling between magnetic layers in multilayer samples gives rise to dynamic effects which are ma... more Coupling between magnetic layers in multilayer samples gives rise to dynamic effects which are manifest as anomalous modes in ferromagnetic resonance (FMR) spectra. According to the model presented, antiferromagnetic (ferromagnetic) coupling would produce such modes on the high (low) field side of the uniform FMR mode. Anomalous modes, observed in Fe/Cu and Fe/Cr multilayer samples, are analyzed to obtain coupling constants as a function of nonmagnetic layer thickness. The modes are shown to arise from 180 out-of-phase interlayer magnetization precession. The coupling is observed to be antiferromagnetic in all samples and to be an order of magnitude greater in the Cr system than in the Cu.

Ferromagnetic resonance in a Ni-Mo superlattice

Journal of Applied Physics, Apr 15, 1985

Ferromagnetic resonance (FMR) measurements, at room temperature and at 4.2 K, have been made on a... more Ferromagnetic resonance (FMR) measurements, at room temperature and at 4.2 K, have been made on a layered Ni (249 Å)-Mo(83 Å) superlattice. We have examined the resonance position as a function of the angle between the film normal and the applied field. The measured g value agrees with that of bulk Ni, but the magnetization is lower than that obtained for bulk Ni and also for this sample using both light scattering and direct measurement techniques. This low magnetization contrasts with FMR measurements on compositionally modulated Ni-Cu samples, where the magnetization was reported to be greater than that of bulk Ni. We show that a reduced value of the magnetization is consistent with perpendicular uniaxial anisotropy. When the applied field is less than 20° from the surface normal, additional lines appear that move to higher fields than the main resonance. These lines are consistent with the existence of nonuniform regions of distinct magnetization. An observed resonance, which is suggestive of a spin-wave mode, is discussed.

Role of the antiferromagnetic bulk spins in exchange bias

Journal of Magnetism and Magnetic Materials, Oct 1, 2016

This "Critical Focused Issue" presents a brief review of experiments and models which describe th... more This "Critical Focused Issue" presents a brief review of experiments and models which describe the origin of exchange bias in epitaxial or textured ferromagnetic/antiferromagnetic bilayers. Evidence is presented which clearly indicates that inner, uncompensated, pinned moments in the bulk of the antiferromagnet (AFM) play a very important role in setting the magnitude of the exchange bias. A critical evaluation of the extensive literature in the field indicates that it is useful to think of this bulk, pinned uncompensated moments as a new type of a ferromagnet which has a low total moment, an ordering temperature given by the AFM Néel temperature, with parallel aligned moments randomly distributed on the regular AFM lattice.

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Temperature Dependence of Magnetization and Anisotropy in Fe/Cu Multilayers

APS, Mar 1, 1996

ABSTRACT Magnetic properties of Fe/Cu multilayers have been investigated as a function of tempera... more ABSTRACT Magnetic properties of Fe/Cu multilayers have been investigated as a function of temperature using dc magnetization and ferromagnetic resonance techniques. The samples were prepared via dc sputtering resulting in equal thickness elemental layers ranging in modulation wavelength from 1 to 30 nm. The interface anisotropy remains nearly constant at 0.32 erg/cm^2 through the 4 K to 300 K temperature range, which is in stark contrast to large thermal effects observed in Ni based systems. (Michael J. Pechan, J. Appl. Phys. 64, 5754 (1988)) Both the interface (1.5 monolayers) and volume Fe magnetizations follow Bloch T^3/2 behavior, but whereas the volume Fe coefficient matches the bulk value of 3.5x10-6K-3/2, the interface coefficient is 17 times greater. This indicates an interface Fe-Fe exchange which is much weaker than would be expected at a free Fe surface. Possible explanations for this decrease will be discussed.

Deposition of epitaxial α-Fe<sub>2</sub>O<sub>3</sub> layers for exchange bias studies by reactive dc magnetron sputtering

Philosophical Magazine B-physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties, Dec 1, 2001

We describe the deposition and characterization of epitaxial thin ®lms of the iron oxide haematit... more We describe the deposition and characterization of epitaxial thin ®lms of the iron oxide haematite (a-Fe 2 O 3 ), a promising candidate for fundamental studies of exchange anisotropy as well as for high thermal stability applications. The ®lms were deposited by reactive dc magnetron sputtering. Structural characterization was by large-angle X-ray di raction and grazing-incidence re¯ectivity (with the scattering vector normal to the sample plane), in-plane grazing-incidence di raction (scattering vector in the sample plane) and re¯ection high-energy electron di raction. Optimized sputtering conditions result in good epitaxy both in the growth direction and in the plane, as well as very smooth surfaces.

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Thermal effects in magnetization, anisotropy, and interface width in Fe/Cu multilayers (abstract)

Journal of Applied Physics, Apr 15, 1996

Magnetic properties of Fe/Cu multilayers have been investigated as a function of temperature usin... more Magnetic properties of Fe/Cu multilayers have been investigated as a function of temperature using dc magnetization and ferromagnetic resonance techniques. The samples were prepared via dc sputtering resulting in equal thickness elemental layers ranging in modulation wavelength from 1 to 30 nm. The interface anisotropy remains nearly constant at 0.32 erg/cm2 through the 4–300 K temperature range, which is in stark contrast to large thermal effects observed in Ni-based systems. Relative increases in magnetization with decreasing temperature range from 3% in the 30 nm sample to 27% in the 1.0 nm sample. This indicates an interface region in which the magnetization is more strongly temperature dependent than the interior of an Fe layer. By plotting magnetization as a function of the inverse layer thickness, we obtain the temperature dependence of the width of this interfacial region.

Magnetic response of FeF2 films on (100) MgO (abstract)

Journal of Applied Physics, Apr 15, 1997

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Determination of magnetic anisotropy in Fe/Cu multilayers: Equivalence of dynamic and static measurements

Physical review, Aug 1, 1995

The magnetic anisotropy of Fe/Cu multilayers has been investigated using dc magnetization, fer- r... more

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Spin waves in exchange-biased Fe/FeF2

Physical review, Feb 1, 1999

The unidirectional anisotropy in exchange biased Fe/FeF 2 bilayers is measured by Brillouin light... more The unidirectional anisotropy in exchange biased Fe/FeF 2 bilayers is measured by Brillouin light scattering ͑BLS͒ from spin waves in the Fe layer. This technique, where the ferromagnetic layer is always saturated, allows for a straightforward, accurate determination of the exchange bias field. The value obtained from BLS is 25% larger than the one obtained from conventional magnetometry. Moreover, a large broadening of the spin-wave mode due to the presence of the antiferromagnet is observed. From the field dependence of this broadening we conclude that the mode broadening is unidirectional. ͓S0163-1829͑99͒00506-8͔

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Physical review, Aug 28, 2003

An analytical model of exchange anisotropy in epitaxial ferromagnetic/antiferromagnetic bilayers ... more An analytical model of exchange anisotropy in epitaxial ferromagnetic/antiferromagnetic bilayers was developed. The model demonstrates that the high symmetry exchange anisotropy terms in ferromagnetic/ antiferromagnetic bilayers originate from a partial domain wall in the antiferromagnetic layer. Application of the model to the experimental data analysis enables one to separately determine the fraction of uncompensated interfacial spins in the antiferromagnetic layer and the interfacial exchange coupling energy between spins in the ferromagnet and in the antiferromagnet. The model provides a quantitative description of complex exchange anisotropy recently observed in Fe/MnF 2 bilayers.

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In-Plane and Depth Dependent Magnetization in FeCr Multilayer

APS, Mar 1, 2004

We have used specular and off-specular neutron surface scattering with polarization analysis to s... more We have used specular and off-specular neutron surface scattering with polarization analysis to study the in-plane magnetic structure and depth profile of magnetization of a series of [Fe(30 nm)/Cr(1.2 nm)] multilayer prepared with varying degrees of roughness. The measurements were taken as a function of magnetic field around the hysteresis loop. The results show strong diffuse scattering on a ridge in reciprocal space through the normal wavevector component corresponding to AF order between the Fe layers, but no off-specular AF multilayer Bragg peak, indicating AF coupled domains with no net in-plane magnetization at low field values. Work of I. R and I. S is supported by DOE.

Static and Dynamic Characterizaton of Interfacial Anisotropy in Magnetic Superlattices

We have observed large discrepancies between the DC magnetization and ferromagnetic resonance (FM... more We have observed large discrepancies between the DC magnetization and ferromagnetic resonance (FMR) magnetic anisotropy in Mo/Ni superlattices. We show that higher order anisotropy is present in both measurements and develop an analysis by which the first order and second order energies can be treated independently. The low temperature results show a systematic divergence of the first order FMR and DC anisotropy as the Ni layer thickness decreases. It is shown that the source of the divergence is an enhanced surface anisotropy measured by FMR. Possible sources of this enhancement are discussed.

EA-04. Measurements of the Ferromagnetic/Antiferromagnetic Exchange Coupling Energy

Two different measurements relying on small reversible rotations of the magnetization have been u... more Two different measurements relying on small reversible rotations of the magnetization have been used to determine the magnitude of the direct F/AF exchange coupling energy (FAECE) in bilayers. One technique is to use the anisotropic magnetoresistance (AMR) to monitor rotations of the magnetization away from the uniaxial exchange axis easy direction,' where the rotation is accomplished by applying external magnetic fields on the order of 0.1 or less of the effective F/AF exchange field. The FAECE can also be determined by measuring the response of the ac susceptibility as a function of the angle between the ac field and the unidirectional exchange field. * For this measurement, the applied magnetic field may be 0.001 or less of the effective F/AF exchange field. Both of these differ from the traditional technique which is to relate a shift in the hysteresis loop to the FAECE. The AMR and ac susceptibility techniques utilize small fields and involve reversible rotations of the mag...

XPS Analysis of Thin Insulating Barriers in Magnetic Tunnel Junctions

Magnetic Storage Systems Beyond 2000, 2001

Large efforts have been lately devoted to the study of the structural, magnetic and transport pro... more Large efforts have been lately devoted to the study of the structural, magnetic and transport properties of magnetic tunnel junctions (MTJ) [1] due to their potential applications as low magnetic field sensors and magnetic random access memories. MTJ are based on trilayered systems of the type FM1/I/FM2, where FMI and FM2 are ferromagnetic electrodes and I is a thin insulating barrier (1–3 nm in thickness). Much research have been undertaken on insulating barriers based on an Al layer which is oxidized either after being deposited (e.g., by natural or plasma oxidation) or while being deposited (e.g., by reactive sputtering). The technological needs for the application of MTJ are strongly dependent on the thickness and surface roughness of the Al layer, as well as on its oxidation, which has to be uniform and complete (without pinholes).

Rotation of the Magnetization in Exchange Biased FeF_2/Fe Bilayers

Influence of antiferromagnet anisotropy on critical thickness in TMF2/Fe

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Ivan Schuller

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