A. Aqeel

Surface sensitivity of the spin Seebeck effect

We have investigated the influence of the interface quality on the spin Seebeck effect (SSE) of the bilayer system yttrium iron garnet (YIG)–platinum (Pt). The magnitude and shape of the SSE is strongly influenced by mechanical treatment of the YIG single crystal surface. We observe that the saturation magnetic field ( HsatSSE) for the SSE signal increases from 55.3 mT to 72.8 mT with mechanical treatment. The change in the magnitude of HsatSSE can be attributed to the presence of a perpendicular magnetic anisotropy due to the treatment induced surface strain or shape anisotropy in the Pt/YIG system. Our results show that the SSE is a powerful tool to investigate magnetic anisotropy at the interface.

Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO

We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on antiferromagnetic NiO(111) single crystals. An SMR with a sign opposite to conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25 T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnetic field within the easy plane (111) of the antiferromagnet. This correlation of magnetic moment alignment and the external magnetic field direction is realized just by the easy-plane nature of the material without the need of any exchange coupling to an additional ferromagnet. The SMR signal strength decreases with increasing temperature, primarily due to the decrease in Neel order by including fluctuations. An increasing magnetic field increases the SMR signal strength as there are fewer domains, and the magnetic moments are more strongly manipulated at high magnetic fields. The SMR is saturated at an applied magnetic field of 6 T, resulting in a spin-mixing conductance of similar to 10(18) Omega(-1) m(-2), which is comparable to that of Pt on insulating ferrimagnets such as yttrium iron garnet. An argon plasma treatment doubles the spin-mixing conductance. Published by AIP Publishing.

Spin-Hall magnetoresistance and spin Seebeck effect in spin-spiral and paramagnetic phases of multiferroic CoCr2O4 films

We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts=28K) and the spin lock-in (Tlock?in=14K) transitions. The SMR and SSE responses in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc=94K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the noncollinear magnetic state. At T>Tc, magnetic-field-induced SMR and SSE signals are observed, which can be explained by a high interface susceptibility. Our results show that the spin transport at the Pt|CCO interface is sensitive to the magnetic phases but cannot be explained solely by the bulk magnetization.

Self-Assembly of Ferromagnetic Organic–Inorganic Perovskite-Like Films

Perovskite-based organic-inorganic hybrids hold great potential as active layers in electronics or optoelectronics or as components of biosensors. However, many of these applications require thin films grown with good control over structure and thickness--a major challenge that needs to be addressed. The work presented here is an effort towards this goal and concerns the layer-by-layer deposition at ambient conditions of ferromagnetic organic-inorganic hybrids consisting of alternating CuCl4-octahedra and organic layers. The Langmuir-Blodgett technique used to assemble these structures provides intrinsic control over the molecular organization and film thickness down to the molecular level. Magnetic characterization reveals that the coercive field for these thin films is larger than that for solution-grown layered bulk crystals. The strategy presented here suggests a promising cost effective route to facilitate the excellently controlled growth of sophisticated materials on a wide variety of substrates that have properties relevant for the high density storage media and spintronic devices.

Electrical detection of spiral spin structures in Pt vertical bar Cu2OSeO3 heterostructures

We study the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in a noncollinear insulating magnet-Pt heterostructure. We show that SMR can be used as an all-electric probe of complex spin states exhibited by the chiral magnet, Cu2OSeO3, under an applied magnetic field. The slope of the magnetic field dependence of the SMR signal changes sign at the transition between the helical and conical spiral states and shows another discontinuity when the conical spiral turns into a collinear ferromagnetic state. We demonstrate that the amplitude of the SMR signal is controlled by the cone angle theta, and that it changes sign at theta similar to 55 degrees. The angular dependence of the SMR in the multidomain helical spiral state is markedly different from the simple sinusoidal dependence observed in the monodomain conical spiral and ferromagnetic states. This complex behavior is explained within the framework of the SMR theory initially developed for collinear magnets. The SSE displays unconventional behavior where not only the magnitude but also the phase of the angular dependence of the SSE varies with the applied magnetic field.

Helimagnon Resonances in an Intrinsic Chiral Magnonic Crystal

We experimentally study magnetic resonances in the helical and conical magnetic phases of the chiral magnetic insulator Cu_{2}OSeO_{3} at the temperature T=5  K. Using a broadband microwave spectroscopy technique based on vector network analysis, we identify three distinct sets of helimagnon resonances in the frequency range 2  GHz≤f≤20  GHz with low magnetic damping α≤0.003. The extracted resonance frequencies are in accordance with calculations of the helimagnon band structure found in an intrinsic chiral magnonic crystal. The periodic modulation of the equilibrium spin direction that leads to the formation of the magnonic crystal is a direct consequence of the chiral magnetic ordering caused by the Dzyaloshinskii-Moriya interaction. The mode coupling in the magnonic crystal allows excitation of helimagnons with wave vectors that are multiples of the spiral wave vector.

Optically probed symmetry breaking in the chiral magnet Cu2OSeO3

We report on the linear optical properties of the chiral magnet Cu2OSeO3, specifically associated with the absence of inversion symmetry, the chiral crystallographic structure, and magnetic order. Through spectroscopic ellipsometry, we observe local crystal-field excitations below the charge-transfer gap. These crystal-field excitations are optically allowed due to the lack of inversion symmetry at the Cu sites. Optical polarization rotation measurements were used to study the structural chirality and magnetic order. The temperature dependence of the natural optical rotation, originating in the chiral crystal structure, provides evidence for a finite magnetoelectric effect in the helimagnetic phase. We find a large magneto-optical susceptibility on the order of V(540 nm) similar to 10(4) rad/Tm in the helimagnetic phase and a maximum Faraday rotation of similar to 170 deg/mm in the ferrimagnetic phase. The large value of V can be explained by considering spin cluster formation and the relative ease of domain reorientation in this metamagnetic material. The magneto-optical activity allows us to map the magnetic phase diagram, including the skyrmion lattice phase. In addition to this, we probe and discuss the nature of the various magnetic phase transitions in Cu2OSeO3.

New magnetic phase of the chiral skyrmion material Cu2OSeO3

A new magnetic phase is reported in the chiral magnet, Cu2OSeO3, which is predicted to affect its physical properties. The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements, we observe a new multidomain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify the physical origin of the new state and discuss its multiferroic properties.

Probing current-induced magnetic fields in Auj vertical bar YIG heterostructures with low-energy muon spin spectroscopy

We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet - YIG) by using low energy muon spectroscopy (LE-muSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement to the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modelling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explains our observations. Our results, therefore, gauge the limits on the spatial resolution and the sensitivity of LE-muSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-Hall effect.

Probing current-induced magnetic fields in Au|YIG heterostructures with low-energy muon spin spectroscopy

We investigated the depth dependence of current-induced magnetic fields in a bilayer of a normal metal (Au) and a ferrimagnetic insulator (Yttrium Iron Garnet - YIG) by using low energy muon spectroscopy (LE-muSR). This allows us to explore how these fields vary from the Au surface down to the buried Au|YIG interface, which is relevant to study physics like the spin-Hall effect. We observed a maximum shift of 0.4 G in the internal field of muons at the surface of Au film which is in close agreement to the value expected for Oersted fields. As muons are implanted closer to the Au|YIG interface the shift is strongly suppressed, which we attribute to the dipolar fields present at the Au|YIG interface. Combining our measurements with modelling, we show that dipolar fields caused by the finite roughness of the Au|YIG interface consistently explains our observations. Our results, therefore, gauge the limits on the spatial resolution and the sensitivity of LE-muSR to the roughness of the buried magnetic interfaces, a prerequisite for future studies addressing current induced fields caused by the spin-Hall effect.