Ippei Suzuki

Stability of ferromagnetic state of epitaxially grown ordered FeRh thin films

We report on the magnetic properties of B2-type ordered FeRh epitaxial thin films deposited on MgO(001) substarates as a function of film thickness. All the films show a clear magnetic phase transition from the antiferromagnetic state to the ferromagnetic state with increasing temperature while the transition temperature of a 10-nm-thick film decreases down to 300K. The 10-nm-thick film also shows a large magnetization even in the antiferromagnetic state compared with other thicker films. These magnetization data indicate that the ferromagnetic state is becoming more stable than the antiferromagnetic state with decreasing film thickness. Such thickness dependent magnetic properties are qualitatively compatible with a theoretical prediction for FeRh(001) thin layers.

Elastically controlled magnetic phase transition in Ga-FeRh/BaTiO3(001) heterostructure

We demonstrate elastically induced ferromagnetic to antiferromagnetic phase transition of Ga-substituted FeRh thin films on BaTiO3(001). It is found that two abrupt changes of magnetization occur at the successive phase transitions from the tetragonal to orthorhombic and the orthorhombic to rhombohedral phases of BaTiO3. Magnetization and magnetoresistance together clearly reveal that a ferromagnetic to antiferromagnetic phase transition is induced due to the compressive lattice strain accompanied by the orthorhombic to rhombohedral structural phase transition, while the tetragonal to orthorhombic phase transition causes a change in the symmetry of the magnetic anisotropy in the ferromagnetic phase of FeRh.

Strain-induced reversible and irreversible magnetization switching in Fe/BaTiO3 heterostructures

Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.

Effect of spin polarized current on magnetic phase transition of ordered FeRh wires

We report on spin polarized current injection from Co wires into a B2-ordered FeRh wire, which shows the magnetic phase transition from the antiferromagnetic (AF) state to the ferromagnetic state at 370 K. Current–voltage characteristics of a Co/FeRh wires junction sample show a marked feature in the AF state below 380 K, while the feature disappears above 380 K, indicating that the noticeable feature is associated with the antiferromanetic–ferromagnetic phase transition induced by the spin polarized current injection into the FeRh wire. We attribute the spin polarized current induced phase transition to a possible modulation of the s-d exchange interaction arising from the spin polarization of itinerant electrons and Rh atoms in the FeRh wire.

Clear correspondence between magnetoresistance and magnetization of epitaxially grown ordered FeRh thin films

Magnetoresistance and magnetization of the CsCl-type ordered FeRh epitaxial thin films grown on MgO(001) substrates are investigated as a function of temperature and film thickness. All the films show a clear first-order magnetic phase transition from the antiferromagnetic state to the ferromagnetic state at around 380 K. A large negative variation in the field-dependent magnetoresistance of the FeRh thin films, which is accompanied by the field-induced magnetic phase transition, is found to be well scaled with the magnetization squared M2. The results indicate that the magnetoresistance primarily arises from spin-dependent scattering through the s–d exchange interactions between conduction electrons and the localized magnetic moments.

Controllable exchange bias in Fe/metamagnetic FeRh bilayers

We report the studies of tuning the exchange bias at ferromagnetic Fe/metamagnetic FeRh bilayer interfaces. Fe/FeRh(111) bilayers show exchange bias in the antiferromagnetic state of FeRh while no exchange bias occurs at Fe/FeRh(001) interface. The contrasting results are attributed to the spin configurations of FeRh at the interface, i.e., the uncompensated ferromagnetic spin configuration of FeRh appears exclusively for (111) orientation. The exchange bias disappears as the bilayers are warmed above the antiferromagnetic-ferromagnetic transition temperature. The direction of the exchange bias for Fe/FeRh(111) is also found to be perpendicular to the cooling-field direction, in contrast to the commonly observed direction of exchange bias for ferromagnetic/antiferromagnetic interfaces. In view of these results, the exchange bias in Fe/FeRh bilayers with the (111) crystallographic orientation should be useful for the design of rapid writing technology for magnetic information devices.

Current induced antiferro–ferromagnetic transition in FeRh nanowires

We investigated the antiferromagnetic–ferromagnetic phase transition of FeRh nanowires induced by an electric current. The critical current Icr for the phase transition decreases with increasing the sample temperature. It is found that the , where Rcr is the resistance at Icr, linearly decreases as the sample temperature increases, suggesting that the temperature raised by the Joule heating is responsible for the transition. We also studied for various wire widths to characterize the heat dissipation of the nanowires. Our results provide a way to explore the Joule heating in high density nanosized electrical devices where significant electric current flow is involved.

Barkhausen-like antiferromagnetic to ferromagnetic phase transition driven by spin polarized current

We provide clear evidence for the effect of a spin polarized current on the antiferromagnetic to ferromagnetic phase transition of an FeRh wire at Co/FeRh wire junctions, where the antiferromagnetic ground state of FeRh is suppressed by injecting a spin polarized current. We find a discrete change in the current-voltage characteristics with increasing current density, which we attribute to the Barkhausen-like motion of antiferromagnetic/ferromagnetic interfaces within the FeRh wire. The effect can be understood via spin transfer, which exerts a torque to the antiferromagnetic moments of FeRh, together with non-equilibrium magnetic effective field at the interface. The conclusion is reinforced by the fact that spin unpolarized current injection from a nonmagnetic Cu electrode has no effects on the antiferromagnetic state of FeRh.

Electric field driven variation in magnetoresistance of Co/Cu/Fe/BaTiO3 heterostructure

We demonstrate control of the magnetoresistance of Co/Cu/Fe giant magnetoresistance structures on BaTiO3(001) by electric field. The magnetoresistance clearly increases with electric field strength in the three different ferroelectric phases of BaTiO3, while the electric field dependence is more significant in the orthorhombic and rhombohedral phases with hysteretic behaviors, associated with the different ferroelectric domain reversal processes of BaTiO3. The results clearly show that electrically induced lattice distortion at the Fe/BaTiO3 interface can provide an alternative means to manipulating the electronic structure of the Fe layer by electric field.

Transmission of spin waves in ordered FeRh epitaxial thin films

We report on B2-ordering dependence of magnetostatic surface spin waves in ferromagnetic FeRh at room temperature. Spin waves transmit over a distance longer than 21 {\mu}m in highly ordered FeRh alloys even with relatively large spin-orbit interaction. The long-range transmission likely arises from the induced Rh moments of the ordered FeRh due to ferromagnetic exchange interaction between Fe and Rh. The results indicate a potential of using FeRh in spintronic and magnonic applications by integrating with other fascinating magnetic characteristics of FeRh such as electric field induced magnetic phase transition.