Showing papers by "Matthias Opel published in 2008"

Voltage controlled inversion of magnetic anisotropy in a ferromagnetic thin film at room temperature

Abstract: The control of magnetic properties by means of an electric field is an important aspect in magnetism and magnetoelectronics. We here utilize magnetoelastic coupling in ferromagnetic/piezoelectric hybrids to realize a voltage control of magnetization orientation at room temperature. The samples consist of polycrystalline nickel thin films evaporated onto piezoelectric actuators. The magnetic properties of these multifunctional hybrids are investigated at room temperature as a function of the voltage controlled stress exerted by the actuator on the Ni film. Ferromagnetic resonance spectroscopy shows that the magnetic easy axis in the Ni film plane is rotated by 90 degree upon changing the polarity of the voltage Vp applied to the actuator. In other words, the in-plane uniaxial magnetic anisotropy of the Ni film can be inverted via the application of an appropriate voltage Vp. Using SQUID magnetometry, the evolution of the magnetization vector is recorded as a function of Vp and of the external magnetic field. Changing Vp allows to reversibly adjust the magnetization orientation in the Ni film plane within a range of approximately 70 degree. All magnetometry data can be quantitatively understood in terms of the magnetic free energy determined from the ferromagnetic resonance experiments. These results demonstrate that magnetoelastic coupling in hybrid structures indeed is a viable option to control magnetization orientation in technologically relevant ferromagnetic thin films at room temperature.

In situ manipulation of magnetic anisotropy in magnetite thin films

Abstract: We show that the ferromagnetic anisotropy of a thin crystalline Fe3O4 film can be manipulated in situ via the application of tunable stress. The stress is exerted by a piezoelectric actuator, onto which the Fe3O4 film is cemented. The strain in the sample is quantified as a function of the voltage applied to the actuator using high-resolution x-ray diffraction, and the corresponding evolution of the magnetic anisotropy is determined by ferromagnetic resonance spectroscopy. By this means, we are able to directly correlate structural and magnetic properties. The experimental results demonstrate that a piezoelectric actuator allows to substantially modify the magnetic anisotropy of a crystalline ferromagnetic thin film, enabling a voltage control of magnetization orientation. The possibility to orient the main elongation axis of the actuator along any given direction in the film plane opens a pathway for the investigation of the magnetoelastic properties of ferromagnetic thin films under tunable stress, shear, or combinations of both stress and shear.

Ga 1−x Mn x As/piezoelectric actuator hybrids : A model system for magnetoelastic magnetization manipulation

Abstract: We have investigated the magnetic properties of a piezoelectric actuator/ferromagnetic semiconductor hybrid structure Using a GaMnAs epilayer as the ferromagnetic semiconductor and applying the piezo stress along its [110] direction, we quantify the magnetic anisotropy as a function of the voltage ${V}_{p}$ applied to the piezoelectric actuator using anisotropic magnetoresistance techniques As the magnetic anisotropy in GaMnAs substantially changes as a function of temperature $T$, the ratio of the magnetoelastic and the magnetocrystalline anistropies can be tuned from approximately 1/4 to 4 Thus, GaMnAs/piezoelectric actuator hybrids are an ideal model system for the investigation of different piezoelastic magnetization control regimes At $T=5\text{ }\text{K}$ the magnetoelastic term is a minor contribution to the magnetic anisotropy Nevertheless, we show that the switching fields of $\ensuremath{\rho}({\ensuremath{\mu}}_{0}H)$ loops are shifted as a function of ${V}_{p}$ at this temperature At 50 K---where the magnetoelastic term dominates the magnetic anisotropy---we are able to tune the magnetization orientation by about $70\ifmmode^\circ\else\textdegree\fi{}$ solely by means of the electrical voltage ${V}_{p}$ applied Furthermore, we derive the magnetostrictive constant ${\ensuremath{\lambda}}_{111}$ as a function of temperature and find values consistent with earlier results We argue that the piezo voltage control of magnetization orientation is directly transferable to other ferromagnetic/piezoelectric hybrid structures, paving the way to innovative multifunctional device concepts As an example, we demonstrate piezo voltage-induced irreversible magnetization switching at $T=40\text{ }\text{K}$, which constitutes the basic principle of a nonvolatile memory element

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

Abstract: The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.

Nanosized superparamagnetic precipitates in cobalt-doped ZnO

Abstract: The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc... These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution x-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.

Anomalous Hall effect in magnetite: Universal scaling relation between Hall and longitudinal conductivity in low-conductivity ferromagnets

Abstract: The anomalous Hall effect (AHE) has been studied systematically in the low-conductivity ferromagnetic oxide ${\text{Fe}}_{3\ensuremath{-}x}{\text{Zn}}_{x}{\text{O}}_{4}$ with $x=0$, 0.1, and 0.5. We used (001), (110), and (111) oriented epitaxial ${\text{Fe}}_{3\ensuremath{-}x}{\text{Zn}}_{x}{\text{O}}_{4}$ films grown on MgO and sapphire substrates in different oxygen partial pressure to analyze the dependence of the AHE on crystallographic orientation, Zn content, strain state, and oxygen deficiency. Despite substantial differences in the magnetic properties and magnitudes of the anomalous Hall conductivity ${\ensuremath{\sigma}}_{xy}^{\text{AHE}}$ and the longitudinal conductivity ${\ensuremath{\sigma}}_{xx}$ over several orders of magnitude, a universal scaling relation ${\ensuremath{\sigma}}_{xy}^{\text{AHE}}\ensuremath{\propto}{\ensuremath{\sigma}}_{xx}^{\ensuremath{\alpha}}$ with $\ensuremath{\alpha}=1.69\ifmmode\pm\else\textpm\fi{}0.08$ was found for all investigated samples. Our results are in agreement with recent theoretical and experimental findings for ferromagnetic metals in the dirty limit, where transport is by metallic conduction. We find the same scaling relation for magnetite, where hopping transport prevails. The fact that this relation is independent of crystallographic orientation, Zn content, strain state, and oxygen deficiency suggests that it is universal and particularly does not depend on the nature of the transport mechanism.

Piezo-voltage control of magnetization orientation in a ferromagnetic semiconductor

Abstract: The possibility to control magnetic properties via electrical fields is investigated in a piezoelectric actuator/ferromagnetic semiconductor thin film hybrid structure. Using anisotropic magnetoresistance techniques, the magnetic anisotropy and the magnetization orientation within the plane of the ferromagnetic film are measured quantitatively. The experiments reveal that the application of an electrical field to the piezoelectric actuator allows to continuously and reversibly rotate the magnetization orientation in the ferromagnet by about 70°. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

All oxide ferromagnet/semiconductor epitaxial heterostructures

Abstract: Oxide based ferromagnet/semiconductor heterostructures offer substantial advantages for spin electronics. We have grown (111) oriented Fe3O4 thin films and Fe3O4/ZnO heterostructures on ZnO(0001) and Al2O3(0001) substrates by pulsed laser deposition. High quality crystalline films with mosaic spread as small as 0.03°, sharp interfaces, and rms surface roughness of 0.3 nm were achieved. Magnetization measurements show clear ferromagnetic behavior of the magnetite layers with a saturation magnetization of 3.2μB/f.u. at 300 K. Our results demonstrate that the Fe3O4/ZnO system is an intriguing and promising candidate for the realization of multifunctional heterostructures.

All Oxide Ferromagnet/Semiconductor Epitaxial Heterostructures

Abstract: Oxide based ferromagnet/semiconductor heterostructures offer substantial advantages for spin electronics. We have grown (111) oriented Fe3O4 thin films and Fe3O4/ZnO heterostructures on ZnO(0001) and Al2O3(0001) substrates by pulsed laser deposition. High quality crystalline films with mosaic spread as small as 0.03 degree, sharp interfaces, and rms surface roughness of 0.3 nm were achieved. Magnetization measurements show clear ferromagnetic behavior of the magnetite layers with a saturation magnetization of 3.2 muB/f.u. at 300 K. Our results demonstrate that the Fe3O4/ZnO system is an intriguing and promising candidate for the realization of multi-functional heterostructures.

Suppression of hole-mediated ferromagnetism in Ga1−xMnxP by hydrogen

Abstract: We report the passivation of the Mn acceptors in Ga1−xMnxP upon exposure to a hydrogen plasma. The as-grown films are nonmetallic and ferromagnetic with a Curie temperature of TC=55 K. After hydrogenation the sample resistivity increases by approximately three orders of magnitude at room temperature and six orders of magnitude at 25 K. Furthermore, the hydrogenated samples are paramagnetic, which is evidenced by a magnetization curve at 5 K that is best described by a Brillouin function with g=2 and J=5/2 expected for Mn atoms in the 3d5 configuration. Upon annealing, partial depassivation and a recovery of ferromagnetism are observed. These observations unambiguously demonstrate that the ferromagnetism in Ga1−xMnxP is carrier-mediated similar to Ga1−xMnxAs.