NiO Spin Research Articles

Transfer of magnetic anisotropy in epitaxial Co/NiO/Fe trilayers

The magnetic properties of Co(10 Å)/NiO(40 Å)/Fe trilayer epitaxially grown on W(110) substrate were investigated with use of x-ray magnetic linear dichroism (XMLD) and x-ray magnetic circular dichroism (XMCD). We showed that magnetic anisotropy of Fe film that can be controlled by a thickness-driven spin reorientation transition is transferred via interfacial exchange coupling not only to NiO layer but further to ferromagnetic Co overlayer as well. Similarly, a temperature driven spin reorientation of Fe sublayer induces a reorientation of NiO spin orientation and simultaneous switching of the Co magnetization direction. Finally, by element specific XMCD and XMLD magnetic hysteresis loop measurements we proved that external magnetic field driven reorientation of Fe and Co magnetizations as well as NiO Néel vector are strictly correlated and magnetic anisotropy fields of Fe and Co sublayers are identical despite the different crystal structures.

Control of spin orientation in antiferromagnetic NiO by epitaxial strain and spin–flop coupling

A ferromagnet/antiferromagnet (FM/AFM) Fe/NiO bilayer was grown using molecular beam epitaxy on MgO(001) and Cr buffered MgO(001) substrates. X-ray linear dichroism measurements showed a dominating out-of-plane component for the NiO spins in Fe/NiO/MgO and an in-plane spin direction for NiO layers grown on the Cr buffer. Furthermore, systematic studies on the magnetic properties of Fe/NiO grown on the wedge-shaped Cr buffer revealed a continuous strain-induced spin reorientation transition from out-of-plane to in-plane NiO spin directions when the Cr thickness increased from 0 nm to 3.5 nm. The analysis of the in-plane magnetic structure of NiO in Fe/NiO/Cr showed a pronounced uniaxial anisotropy in thin AFM layers. The AFM spins are perpendicular to the Fe spins due to spin–flop interaction. These results demonstrate the feasibility of using strain and coupling with FMs to manipulate spin structures in NiO.

The effect of spin reorientation transition of antiferromagnetic NiO on the Py magnetic anisotropy in Py/NiO/CoO/MgO(0 0 1)

Using X-ray Magnetic Linear Dichroism (XMLD) measurement, we investigate the antiferromagnetic (AFM) NiO spin reorientation transition (SRT) in epitaxial NiO/CoO/MgO(0 0 1) system at room temperature and constructed a complete NiO spin phase diagram. Then utilizing Magneto-Optic Kerr Effect (MOKE) combined with a rotation magnetic field (ROTMOKE), we investigated the effect of NiO SRT on the magnetic anisotropy of a ferromagnetic (FM) Py overlayer in Py/NiO/CoO/MgO(0 0 1) system. We find that the Py coercivity and anisotropy only slightly enhanced for Py on top of out-of-plane NiO spins but greatly enhanced for Py on top of in-plane NiO spins. Both the uniaxial and fourfold anisotropies of the Py showed a clear dependence on the NiO spin orientation.

Exchange bias effects of NiFe/NiO bilayers through ion-beam bombardment on the NiO surface

The influence of ion-beam bombardment of the NiO surface on the exchange bias behavior of NiFe/NiO bilayers was systemically investigated with different bombardment energies and durations. The results show that by varying the bombardment energies, different crystallographic orientations are created which modifies the NiO spin structures. This results in the changes in the coupling type in NiO when it is in contact with the NiFe layer. The NiFe/NiO bilayers exhibited either enhanced or decreased exchange bias filed, depending on the uncompensated moments or misaligned NiO spin created by ion-beam bombardment. The variations in coercivities of NiFe/NiO bilayers imply that the NiO anisotropy could be tuned by ion-beam bombardment.

Continuous spin reorientation transition in epitaxial antiferromagnetic NiO thin films

Fe/NiO/MgO/Ag(001) films were grown epitaxially, and the Fe and NiO spin orientations were determined using x-ray magnetic dichroism. We find that the NiO spins are aligned perpendicularly to the in-plane Fe spins. Analyzing both the in-plane and out-of-plane spin components of the NiO layer, we demonstrate unambiguously that the antiferromagnetic NiO spins undergo a continuous spin reorientation transition from the in-plane to out-of-plane directions with increasing of the MgO thickness.

In-plane spin reorientation transition in Fe/NiO bilayers on Ag(1,1,10)

Magnetic properties of Fe/NiO bilayers grown on a (1,1,10) vicinal surface of Ag(001) were studied by magneto-optical Kerr effect (MOKE) and x-ray magnetic linear dichroism (XMLD). The orientation of the antiferromagnetically (AFM) aligned spins of NiO films shows an in-plane spin-reorientation transition (SRT) from parallel to perpendicular to the steps with increasing NiO thickness. Two in-plane SRTs of Fe moments are found in dependence of the NiO thickness. The first SRT of the Fe magnetization from parallel to perpendicular to the steps is observed at NiO thickness where antiferromagnetic order is found, the second at the NiO thickness of the SRT in the NiO film. Also, the in-plane SRT of Fe spins driven by a temperature increase is observed. The perpendicular coupling between the Fe and NiO spins is proven. The induced uniaxial anisotropy energy is estimated to be 0.12 erg/cm${}^{2}$.

Anisotropic x-ray magnetic linear dichroism and spectromicroscopy of interfacial Co/NiO(001)

Photoemission electron microscopy (PEEM) with linearly polarized x rays is used to determine the orientation of antiferromagnetic domains by monitoring the relative peak intensities at the $3d$ transition metal ${L}_{2}$ absorption edge. In such an analysis it is necessary to take into account the orientations of the x-ray polarization $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and magnetization $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ with respect to the crystalline axes. We address this problem by presenting a general expression of the angular dependence for both x-ray absorption (XA) spectroscopy and x-ray magnetic linear dichroism (XMLD) for arbitrary directions of $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ in the (001) cubic plane. In cubic symmetry the angular-dependent XMLD is a linear combination of two spectra with different photon energy dependencies. As a result the intensity maxima of the XA and XMLD are no longer found for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ parallel to $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ and their directions will vary with photon energy. When $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ or $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ is along a high-symmetry axis there is only one spectrum. The angular-dependent XMLD can be separated into an isotropic part, which is symmetric around $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$, and an anisotropic part, which depends on the orientation of the crystal axes. The anisotropic part has maximal intensity when $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ have equal but opposite angles with respect to the [100] direction. The Ni${}^{2+}$ ${L}_{2}$ edge has the peculiarity that the isotropic part vanishes, which has the interesting consequence that the maximum in the XMLD intensity is obtained not only for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [100] but also for ($\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [110], $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [1$\mathrm{1\ifmmode \bar{}\else \={}\fi{}}$0]). We apply the angular-dependent theory to determine the spin orientation near the Co/NiO(100) interface. The PEEM images show that the ferromagnetic Co spins and antiferromagnetic NiO spins are aligned perpendicular to each other. By rotating the sample with respect to the linear x-ray polarization we furthermore find that the perpendicular coupling with the ferromagnetic Co layer at the interface causes a canting of the antiferromagnetic Ni moments. This shows that taking into account the angular dependence of the XMLD in the detailed analysis of PEEM images can lead to an accurate retrieval of the spin axes of the antiferromagnetic domains.

Ferromagnetic resonance study of Permalloy/Cu/Co/NiO spin valve system

Ferromagnetic resonance, in 7 to 18 GHz frequency range, is used to investigate a series of rf sputtered Permalloy (Py)/Cu/Co trilayers and Py/Cu/Co/NiO spin valve system. The experimental data, frequency versus applied magnetic field, agree quit well with the theoretical model. The magnetic coupling between Py and Co is found to be ferromagnetic with effective magnetic coupling parameter values increasing from 0.05 to 0.1 erg/cm2 as the interlayer Cu thickness decreases from 10 to 2 nm. While the magnetic coupling leads to a decrease in the resonance fields, HR, of the modes, the exchange anisotropy at the Co/NiO interface shifts the mode upwards. However the shift is more important for the optical mode than for the acoustic one. Hysteresis curves, qualitatively, confirm the ferromagnetic coupling. The antiferromagnetic NiO layer leads to a slight increase in the coercive field.

Effect of NiO spin orientation on the magnetic anisotropy of the Fe film in epitaxially grown Fe/NiO/Ag(001) and Fe/NiO/MgO(001)

Single-crystalline Fe/NiO bilayers were epitaxially grown on Ag(001) and on MgO(001), and investigated by low-energy electron-diffraction, magneto-optic Kerr effect, and x-ray magnetic linear dichroism (XMLD). We find that while the Fe film has an in-plane magnetization in both Fe/NiO/Ag(001) and Fe/NiO/MgO(001) systems, the NiO spin orientation changes from in-plane direction in Fe/NiO/Ag(001) to out-of-plane direction in Fe/NiO/MgO(001). These two different NiO spin orientations generate remarkable different effects that the NiO induced magnetic anisotropy in the Fe film is much greater in Fe/NiO/Ag(001) than in Fe/NiO/MgO(001). XMLD measurement shows that the much greater magnetic anisotropy in Fe/NiO/Ag(001) is due to a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling between the in-plane NiO spins and the in-plane Fe spins.

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

A series of polycrystalline Ag-doped Ni 1− x Ag x O/Ni bilayers with x up to 0.2 were prepared by magnetron sputtering. X-ray diffraction, atomic force microscopy and transmission electron microscopy analyses reveal that Ag doping significantly reduces the mean NiO grain size and leads to the appearance of Ag nanoparticles on the surface of the Ag-doped NiO films. As x increases, the exchange bias field and coercivity at room temperature decrease as a consequence of the reduced thermal stability of smaller NiO grains and the screening effect resulting from the interfacial Ag nanoparticles. At lower temperatures, a slight enhancement of the exchange bias field is observed in the Ag-doped sample, indicating that the Ag doping increases the uncompensated NiO spin density. In addition, our studies find that the training effect of the Ag-doped sample can be well described by a spin configurational relaxation model, regardless of the presence of Ag nanopartiles at the interface.

Element-specific study of the anomalous magnetic interlayer coupling across NiO spacer layer in Co/NiO/Fe/Ag(001) using XMCD and XMLD

Co/NiO/Fe trilayers are grown on Ag(001) substrate using molecular-beam epitaxy and investigated by element-specific magnetic domain images using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism techniques. By comparing the Co, Fe, and NiO magnetic domain images, we identify that the anomalous Co-Fe interlayer coupling from a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling to a collinear coupling with increasing the NiO film thickness is due to a transition from a collinear to $90\ifmmode^\circ\else\textdegree\fi{}$ coupling at the NiO/Fe interface while retaining a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling at the Co/NiO interface. Uncompensated Ni spins are found at the Co/NiO interface but are absent at the NiO/Fe interface. No evidence of spiral NiO spin structure is found in this Co/NiO/Fe sandwich.

Antiferromagnetic coupling and perpendicular anisotropy in TbFeCo∕NiO multilayers

For Tbx(Fe90Co10)100−x(40nm)∕NiO(3.3nm) multilayers with x=17 and 19, antiferromagnetic coupling is revealed by both in-plane and out-of-plane hysteresis loops at low temperatures. Magnetic perpendicular anisotropy and out-of-plane coercivity of multilayers are enhanced, in comparison to those of TbFeCo single layer films. The strength of the antiferromagnetic coupling, the perpendicular anisotropy, and out-of-plane coercivity are all reduced at high temperatures. These results can be ascribed to the weakening of antiferromagnetic ordering of NiO spins.

Spin-Polarized Scanning Electron Microscopy

Spin-polarized scanning electron microscopy is one of the magnetic domain observation methods. It has excellent capabilities, including high spatial resolution, high surface sensitivity, and detection of magnetization direction. Taking advantage of these capabilities, we studied the magnetism of various materials. In this paper, I select three topics from our research works, and show the results. The first is a ferromagnetism appearing at the bulk-paramagnetic material surface. The sample is FeO formed by oxidation of Fe(110). We found that the FeO surface is reconstructed (111), and ferromagnetism is localized at this surface with magnetization antiparallel to that of the underlying Fe(110). The second is an exchange coupling between ferro-and antiferromagnetic materials. Using Fe/NiO(001) we found 90o coupling between ferromagnetic Fe spin and antiferromagnetic NiO spin, which is proposed by the theory. The third is the magnetic structure of a layered manganite of La1.4Sr1.6Mn2O7. We found that it was spatially separated into two phases: a major phase of layered antiferromagnetism and a minor phase of ferromagnetism. The spin direction of the antiferromagnetic phase area is laid down form perpendicular to parallel to the layer plane with the increase of temperature.

Effect of varying ferromagnetic anisotropy on exchange-bias in [Pt/Co]3/NiO(11 Å) multilayers

An investigation has been performed on the effect of varying ferromagnetic anisotropy on the exchange biasing field, its temperature dependence, and blocking temperature in [Pt(5Å)/Co(tCo)]3/NiO(11Å) multilayers. The ferromagnetic easy axis switches from out-of-plane to in-plane on increasing the Co layer thickness tCo. After field cooling along out-of-plane and in-plane directions, respectively, a number of interesting features have been observed. The in-plane exchange field is much lower than the out-of-plane one, and goes to zero at a much lower blocking temperature. Both the in-plane and out-of-plane blocking temperatures drop with increasing Co layer thickness. The temperature dependence of the out-of-plane exchange field displays two distinct regions, which are separated by a temperature TCr close to the in-plane blocking temperature TB=35K at tCo=20Å. The exchange field decreases linearly in both regions but with a much steeper slope below TCr. These features are considered to be related to the variation of interfacial NiO spin structures with the ferromagnetic anisotropy easy axis and the temperature behavior of the NiO anisotropies.

Effect of thermal stability on magnetoresistance in NiO spin valve

The interface structure and magnetoresistance (MR) for the single spin valve (SV) of Co/Cu/Co structure with a NiO layer at the top or under the bottom were investigated. Glancing incident x-ray reflectivity and x-ray diffuse scatter studies show that the interface roughness of NiO on Co is much larger than that of Co on NiO. The large roughness may be one of the main factors of suppressing spin reflectivity. The interface between NiO and Co becomes more flat after annealing. Different temperature dependence of MR was observed for the top and the bottom SV. We attribute the above phenomena to the competition between the roughness and the exchange effects of the NiO/Co interface.

Track width definition of giant magnetoresistive sensors by ion irradiation

We describe the concept of using irradiation with light energetic ions in conjunction with a mask to define the edges of giant magnetoresistive sensors without milling. The irradiated material undergoes substantial interfacial mixing, and consequent loss of magnetoresistance (MR), accompanied by small changes in resistivity, and no significant change in magnetic moment. The ion species and energy may be chosen to minimize lateral scattering as they pass through the sensor film. Modeling of a perfect mask edge above a sensor film of thickness /spl sim/240 /spl Aring/ irradiated with a well-collimated ion beam indicates that sensor edge definition of /spl plusmn/20 /spl Aring/ is achievable-a vast improvement over sensor edges generated using ion milling, which may run to hundreds of Angstroms in width. Experimental data from irradiation of typical NiO, IrMn, NiMn and PtMn-based spin valves using 700 keV N/sup +/ ions indicate that doses between 10/sup 15/ and 10/sup 17/ ions/cm/sup 2/ are sufficient to diminish the MR of the films to below 5% of the as-grown value. Changes in MR caused by the irradiation are stable against annealing at temperatures typical for setting these sensors.

Enhancement of giant magnetoresistance properties of NiO spin valves with an oxidized free layer naturally in the air

The effect of specular electron scattering on the natural oxidation of a free layer in NiO spin valves has been investigated. The magnetoresistance ratio and the exchange biasing field (Hex) of NiO(600 Å)/Ni81Fe19(50 Å)/Co(7 Å)/Cu(20 Å)/Ni81Fe19(70 Å) spin valves were increased from 4.9% to 7.3%, and 110 to 170 Oe after the natural oxidation in the atmosphere for 80 days, respectively. The sheet resistance ρ decreased from 28 to 17 μΩ m, but Δρ did almost not change after the oxidation. The spin valves enhanced by the specular electron scattering in the naturally oxidized Co/NiFe/NiFeOx free layer were confirmed from the depth profiles using Auger electron spectroscopy.

Enhanced blocking temperature in NiO spin valves: Role of cubic spinel ferrite layer between pinned layer and NiO

We compare the blocking temperature for simple permalloy (80% Ni)/Au/permalloy/NiO spin valves with and without an interfacial, oxidized Fe layer (Fe layer 12 Å thick) inserted at the permalloy/NiO interface. We find a significantly increased blocking temperature and pinning field for spin valves with the interfacial Fe-oxide layer, as a result of moderate (230 °C/1 h) annealing of the Au/permalloy/Fe-oxide/NiO part of the spin valve. High resolution transmission electron microscopy and electron energy loss microanalysis reveal that the Fe-oxide layer is converted to a cubic spinel ferrite (Ni0.8Fe2.2O4) layer by solid state reaction with the NiO.

Antiparallel pinned NiO spin valve sensor for GMR head application (invited)

NiO antiferromagnetic material possesses certain advantages for spin valve applications and has attracted considerable attention. Some of the key advantages are its insulating properties, very high corrosion resistance, less sensitivity to composition, and its low reset temperature. This material, however, has a low blocking temperature which prevents its application to simple spin valve designs. The use of this material in spin valve structures required significant improvements in thermal stability, blocking temperature, and the spin valve design. In the present study, the blocking temperature and the blocking temperature distribution of the NiO films have been improved by depositing the films reactively using ion beam sputtering. A number of improvements in the processing method and deposition system had to be made to allow full NiO spin valve deposition for mass production. Another critical part was the use of antiparallel pinned design in place of the simple design to improve the thermal stability of the NiO spin valves as read elements at disk drive temperatures. The selection of the ferromagnetic pinned layers and the Ru spacer thickness in AP-pinned spin valves has significant impact on the behavior of the devices. These spin valves are all bottom type, NiO/PL1/Ru/PL2/Cu/Co/NiFe/Ta, where the metallic portion of the spin valve is deposited on top of the NiO AF layer. The PL1 and PL2 are ferromagnetic layers comprising NiFe and Co layers. Read elements have been made using these spin valves that delivered areal densities of 12 Gbit/in. These topics and other improvements which resulted in successful use of NiO spin valves as GMR heads in hard disk drives will be discussed.

Anisotropic roughness effects of magnetoresistive hysteresis in scratched NiO spin valves

Magnetoresistance (MR), exchange biasing, and coercivity of NiO/NiFe/Cu/NiFe spin valves, deposited over etched glass, scratched glass, and Si 3N 4 buffer layers were measured in order to investigate the effect of roughness slope and anisotropic roughness at the NiO/NiFe interface. It was found that the MR ratio and the biasing field were not influenced by etching and scratching the substrate. This was due to a constant value of the average slope of roughness, despite an increased rms roughness. In the case of samples with Si 3N 4 buffer layer, the MR ratio and exchange biasing field increased due to an increasing slope or roughness. The coercive field of the unbiased NiFe layer was reduced by perpendicular scratching. This was probably due to refinement of the magnetic domains.