Yu Shiratsuchi

Surface Structure of Self-Organized Sapphire (0001) Substrates with Various Inclined Angles

We studied the surface structure of self-organized sapphire (0001) substrates with various inclined angles, using an atomic force microscope (AFM) in UHV. The self-organized surface structure strongly depended on annealing condition, inclined angle and inclined direction. We could successfully produce a highly self-organized step structure by choosing suitable values of these parameters. The surface of the self-organized inclined substrates was characterized by step and terraces. The morphology of step edges depends on the inclined direction of the substrate. In the case of the inclined direction, the step edges were faceted, and the faceted plane was the {20} plane. On the other hand, in the case of the inclined direction, the step edges were straight. The distribution of step heights and terrace widths measured from AFM images are presented. The distribution of step heights had peaks corresponding to the unit cell of the sapphire crystal, while the terrace widths followed a normal distribution.

High Perpendicular Exchange Bias with a Unique Temperature Dependence in Pt/Co/α-Cr2O3(0001) Thin Films

Perpendicular exchange bias was investigated using Pt/Co/α-Cr2O3(0001) thin films prepared by a reactive DC magnetron sputtering method. On the α-Cr2O3(0001) layer, the Pt/Co thin film simultaneously showed perpendicular magnetic anisotropy (PMA) and perpendicular exchange bias (PEB). Although PEB was not observed at room temperature, it suddenly appeared, accompanied by a unique temperature dependence. The suddenly appearing PEB was as large as a unidirectional magnetic anisotropy energy, JK, of about 0.29 erg/cm2. The mechanism of the sudden appearance of JK is discussed on the basis of Meiklejohn and Beans exchange anisotropy model.

Magnetoelectric switching of perpendicular exchange bias in Pt/Co/α-Cr2O3/Pt stacked films

We report the realization of magnetoelectric switching of the perpendicular exchange bias in Pt/Co/α-Cr2O3/Pt stacked films. The perpendicular exchange bias was switched isothermally by the simultaneous application of magnetic and electric fields. The threshold electric field required to switch the perpendicular exchange bias was found to be inversely proportional to the magnetic field, which confirmed the magnetoelectric mechanism of the process. The observed temperature dependence of the threshold electric field suggested that the energy barrier of the antiferromagnetic spin reversal was significantly lower than that assuming the coherent rotation. Pulse voltage measurements indicated that the antiferromagnetic domain propagation dominates the switching process. These results suggest an analogy of the electric-field-induced magnetization with a simple ferromagnet.

Control of the Interfacial Exchange Coupling Energy in Pt/Co/

We investigated the control of the perpendicular exchange bias (PEB) in Pt/Co/α-Cr2O3 (0001) thin films that exhibit a unique temperature dependence of the PEB. Previously, we demonstrated that the onset of the PEB is explained by the competition between the interface coupling energy and the magnetic anisotropy energy in the α-Cr2O3 layer. Based on this finding, in this study, we investigated the influence of a Pt spacer layer at the Co/α-Cr2O3 interface to control the interface exchange coupling. By inserting a Pt spacer layer, the unidirectional magnetic anisotropy energy JK was reduced because of the weakened exchange coupling between Co and α-Cr2O3. The weakened direct exchange coupling caused an increase in the onset temperature of JK, resulting in a reduction in the α-Cr2O3 layer thickness necessary to maintain the PEB up to the Neel temperature. In addition, the change in the interface exchange coupling mechanism between Co and α-Cr2O3 is discussed with respect to the temperature dependence of JK.

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Switching of the perpendicular exchange bias polarity using a magneto-electric (ME) effect of α-Cr2O3 was investigated. From the change in the exchange bias field with the electric field during the ME field cooling, i.e., the simultaneous application of both magnetic and electric fields during the cooling, we determined the threshold electric field to switch the perpendicular exchange bias polarity. It was found that the threshold electric field was inversely proportional to the magnetic field indicating that the EH product was constant. The high EH product was required to switch the exchange bias for the film possessing the high exchange anisotropy energy density, which suggests that the energy gain by the ME effect has to overcome the interfacial exchange coupling energy to reverse the interfacial antiferromagnetic spin.

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We have studied superparamagnetic behavior of ultrathin Fe films grown on an Al2O3(0001) substrate at various growth temperatures. It is demonstrated that 1-nm-thick Fe films are in the superparamagnetic state, and the blocking temperature is strongly dependent on the growth temperature. The blocking temperature has a minimum value of 30 K for a growth temperature of 473 K, while it is ∼70 K at other growth temperatures. In order to clarify the behavior, we consider the Fe growth mechanism and the magnetic interactions between Fe particles. Fe grows as three-dimensional islands at all temperatures studied and forms particles. The volume of the particles is observed via atomic force microscopy to increase with increasing growth temperature. In the case of growth at 323 and 373 K, Fe forms small particles that are close together and that interact with each other. For growth at 673 and 773 K, Fe forms relatively large particles and the magnetic properties are dominated by the individual particles.

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The magnetic coupling at the interface of Co and Cr has been investigated using ultrathin Co film on Cr2O3(0001) film, which is expected to be a promising system to show the magnetoelectronic effect. We mainly examined the temperature dependence of magnetization, the exchange bias at different field directions relative to the Cr spin direction, and the temperature dependence of exchange bias. The temperature dependence of magnetization under 50 Oe indicates the collinear coupling of Co spin and Cr spin at the interface. Furthermore, the exchange bias effect is observed in the coupled direction of Co and Cr, namely, parallel to the Cr spin direction. However, the changes in the magnetic behavior with changing field directions and Co thicknesses imply the existence of biquadratic coupling of Co spin and Cr spin, as well. In agreement with the numerical calculation under the coexistence of collinear and biquadratic couplings, the exchange bias field decreases nonmonotonically with increasing temperature.

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We show experimental evidence of the equilibrium surface magnetization of α-Cr2O3(0001) by studying chromium magnetization at the Co(111)/α-Cr2O3(0001) interface. The soft X-ray magnetic circular dichroism intensity from uncompensated Cr spins was found to be almost independent of the interface roughness and exchange anisotropy energy. Moreover, no exchange bias training effect was found, except in the transition temperature regime, suggesting that the antiferromagnetic spin structure, including the interface spin arrangement, is in equilibrium. Furthermore, the exchange bias polarity was switched by magnetoelectric field cooling, which is also a salient feature of the surface magnetization of α-Cr2O3(0001).

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We investigated perpendicular magnetic anisotropy and exchange magnetic anisotropy in a Pt/Co/α-Cr2O3(0001) thin film grown on an α-Al2O3(0001) substrate. The film exhibits perpendicular magnetic anisotropy below a Co thickness of 1.2 nm at room temperature. Independent of the magnetic easy direction of the Co layer, the perpendicular exchange bias (PEB) appears in a direction perpendicular to the film below 80 K. The maximum unidirectional magnetic anisotropy energy estimated from the exchange bias field is 0.33 erg/cm2, which is higher than the reported PEB strength. The perpendicular exchange bias is accompanied by the in-plane remanent magnetization and an increase in the in-plane coercivity. We speculate that the increases in the in-plane remanent magnetization and the in-plane coercivity are caused by the spin canting of Cr3+ in the α-Cr2O3(0001) layer.

Films by Inserting a Pt Spacer Layer at the Co/

We have investigated the perpendicular magnetic anisotropy (PMA) at the ferromagnetic/antiferromagnetic interface using [Pt/Co]n/α-Cr2O3 superlattices. By changing the number of stacking periods, the contributions of the Co/α-Cr2O3 and Pt/Co interfaces to the PMA are separately estimated. The results indicate that the strong PMA is induced at the Co/α-Cr2O3 interface as well as at the Pt/Co interface. The change in the interface magnetic anisotropy energy density with the ferromagnetic layer composition supports the PMA at the Co/α-Cr2O3 interface. To the best of our knowledge, this is the first demonstration of PMA at the ferromagnetic/antiferromagnetic interface.