Joel Cramer

Length Scale of the Spin Seebeck Effect

The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained by any effects originating from the interface, such as magnetic proximity effects in the spin detector (Pt). Comparison to theoretical calculations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite effective magnon propagation length so that the origin of the effect can be traced to genuine bulk magnonic spin currents ruling out parasitic interface effects.

Influence of Thickness and Interface on the Low-Temperature Enhancement of the Spin Seebeck Effect in YIG Films

The temperature dependent longitudinal spin Seebeck effect (LSSE) in heavy metal (HM)/Y3Fe5O12 (YIG) hybrid structures is investigated as a function of YIG film thickness, magnetic field strength, and different HM detection material. The LSSE signal shows a large enhancement with reducing the temperature, leading to a pronounced peak at low temperatures. We find the LSSE peak temperature strongly depends on the film thickness as well as on the magnetic field. Our result can be well explained in the framework of magnon-driven LSSE by taking into account the temperature dependent effective propagation length of thermally excited magnons in bulk. We further demonstrate that the LSSE peak is significantly shifted by changing the interface coupling to an adjacent detection layer, revealing a more complex behavior beyond the currently discussed bulk effect. By direct microscopic imaging of the interface, we correlate the observed temperature dependence with the interface structure between the YIG and the adjacent metal layer. Our results highlight the role of interface effects on the temperature dependent LSSE in HM/YIG system, suggesting that the temperature dependent spin current transparency strikingly relies on the interface conditions.

Multiphoton dressing of an anharmonic superconducting many-level quantum circuit

We report on the investigation of a superconducting anharmonic multilevel circuit that is coupled to a harmonic readout resonator. We observe multiphoton transitions via virtual energy levels of our system up to the fifth excited state. The back-action of these higher-order excitations on our readout device is analyzed quantitatively and demonstrated to be in accordance with theoretical expectation. By applying a strong microwave drive we achieve multiphoton dressing within our anharmonic circuit which is dynamically coupled by a weak probe tone. The emerging higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit are observed. Experimental results are in good agreement with master-equation simulations.

Thermal generation of spin current in epitaxial CoFe2O4 thin films

The longitudinal spin Seebeck effect (LSSE) has been investigated in high-quality epitaxial CoFe2O4 (CFO) thin films. The thermally excited spin currents in the CFO films are electrically detected in adjacent Pt layers due to the inverse spin Hall effect. The LSSE signal exhibits a linear increase with increasing temperature gradient, yielding a LSSE coefficient of ∼100u2009nV/K at room temperature. The temperature dependence of the LSSE is investigated from room temperature down to 30u2009K, showing a significant reduction at low temperatures, revealing that the total amount of thermally generated magnons decreases. Furthermore, we demonstrate that the spin Seebeck effect is an effective tool to study the magnetic anisotropy induced by epitaxial strain, especially in ultrathin films with low magnetic moments.

Full angular dependence of the spin Hall and ordinary magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films

Electrical readout of the orientation of an antiferromagnet by detecting the Neel vector is necessary to exploit this class of materials for applications. Here, the authors study bilayers of thin films of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt. The observed three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) is analyzed, considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic

Magnon mode selective spin transport in compensated ferrimagnets

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Magnon detection using a ferroic collinear multilayer spin valve

and

Temperature dependence of the non-local spin Seebeck effect in YIG/Pt nanostructures

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Tunable long-distance spin transport in a crystalline antiferromagnetic iron oxide

domains in the NiO(001). The authors find that a careful subtraction of the ordinary magnetoresistance contribution in Pt is crucial to determine the SMR amplitude and thus the Neel vector average orientation.

Complex Terahertz and Direct Current Inverse Spin Hall Effect in YIG/Cu1-xIrx Bilayers Across a Wide Concentration Range

We investigate the generation of magnonic thermal spin currents and their mode selective spin transport across interfaces in insulating, compensated ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal exhibits a nonmonotonic temperature dependence with two sign changes of the detected voltage signals. Using different ferrimagnetic garnets, we demonstrate the universality of the observed complex temperature dependence of the spin Seebeck effect. To understand its origin, we systematically vary the interface between the ferrimagnetic garnet and the metallic layer, and by using different metal layers we establish that interface effects play a dominating role. They do not only modify the magnitude of the spin Seebeck effect signal but in particular also alter its temperature dependence. By varying the temperature, we can select the dominating magnon mode and we analyze our results to reveal the mode selective interface transmission probabilities for different magnon modes and interfaces. The comparison of selected systems reveals semiquantitative details of the interfacial coupling depending on the materials involved, supported by the obtained field dependence of the signal.