Magnetic field control of the spin Seebeck effect

The origin of the suppression of the longitudinal spin Seebeck effect by applied magnetic fields is studied. We perform numerical simulations of the stochastic Landau-Lifshitz-Gilbert equation of motion for an atomistic spin model and calculate the magnon accumulation in linear temperature gradients for different strengths of applied magnetic fields and different length scales of the temperature gradient. We observe a decrease of the magnon accumulation with increasing magnetic field and we reveal that the origin of this effect is a field dependent change of the frequency distribution of the propagating magnons. With increasing field the magnonic spin currents are reduced due to a suppression of parts of the frequency spectrum. By comparison with measurements of the magnetic field dependent longitudinal spin Seebeck effect in YIG thin films with various thicknesses, we find that our model describes the experimental data very well, demonstrating the importance of this effect for experimental systems

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

Electric field modification of magnetotransport in Ni thin films on (011) PMN-PT piezosubstrates

This study reports the magnetotransport and magnetic properties of 20 nm-thick polycrystalline Ni films deposited by magnetron sputtering on unpoled piezoelectric (011) [PbMg1/3Nb2/3O3](0.68)-[PbTiO3](0.32) (PMN-PT) substrates. The longitudinal magnetoresistance (MR) of the Ni films on (011) PMN-PT, measured at room temperature in the magnetic field range of -0.3T < mu H-0 < 0.3 T, is found to depend on the crystallographic direction and polarization state of piezosubstrate. Upon poling the PMN-PT substrate, which results in a transfer of strain to the Ni film, the MR value decreases by factor of 20 for the current along [100] of PMN-PT and slightly increases for the [01 (1) over bar] current direction. Simultaneously, a strong increase (decrease) in the field value, where the MR saturates, is observed for the [01 (1) over bar] ([100]) current direction. The anisotropic magnetoresistance is also strongly affected by the remanent strain induced by the electric field pulses applied to the PMN-PT in the non-linear regime revealing a large (132 mT) magnetic anisotropy field. Applying a critical electric field of 2.4 kV/cm, the anisotropy field value changes back to the original value, opening a path to voltage-tuned magnetic field sensor or storage devices. This strain mediated voltage control of the MR and its dependence on the crystallographic direction is correlated with the results of magnetization reversal measurements. (C) 2015 AIP Publishing LLC

Length Scale of the Spin Seebeck Effect

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50  μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.United States. Dept. of Energy. Office of Science (Solid-State Solar-Thermal Energy Conversion Center Grant DE-SC0001299)National Science Foundation (U.S.) (Award ECCS1231392

PENGARUH LAMA PERENDAMAN DALAM LARUTAN DENGAN BERBAGAI RASIO EKSTRAK JAHE EMPRIT (Zingiber officinale var. Amarum) DAN SUKROSA TERHADAP KARAKTERISTIK FISIK, KIMIA, DAN ORGANOLEPTIK KERIPIK KELAPA HIJAU (Cocos nucifera L. var. Viridis)

Kelapa merupakan komoditas perkebunan terbesar di Indonesia. Luas area lahan dari perkebunan kelapa merupakan yang terbesar dibandingkan komoditas perkebunan lainnya. Hal ini tidak lepas dari kondisi Indonesia yang merupakan negara tropis sehingga sangat cocok menjadi tempat tumbuh tanaman kelapa. Pada penelitian ini, daging kelapa diolah menjadi keripik dikarenakan keripik merupakan jenis makanan ringan yang sangat digemari masyarakat tidak hanya Indonesia, tetapi juga dunia. Keripik kelapa yang diolah pada penelitian ini juga ditambahkan ekstrak jahe emprit untuk menambah nilai fungsional dari keripik kelapa.Tujuan dari penelitian ini adalah melihat pengaruh lama perendaman keripik kelapa hijau didalam larutan ekstrak jahe emprit dan sukrosa terhadap karakteristik fisik, kimia, dan organoleptiknya. Rancangan percobaan dalam penelitian ini menggunakan Rancangan Acak Lengkap (RAL) dengan satu faktor yaitu lama perendaman dari keripik kelapa (1 jam, 2 jam, dan 3 jam). Data hasil penelitian dianalisis menggunakan metode One Way ANOVA dan jika terdapat beda nyata dilanjutkan dengan analisis DMRT pada taraf signifikansi α = 0,05.Hasil penelitian menunjukkan bahwa keripik kelapa dengan formulasi 25% ekstrak jahe emprit dan 75% sukrosa dengan lama perendaman 2 jam merupakan formulasi terbaik secara organoleptik. Berdasarkan hasil analisis karakteristik kimia, formulasi 25% ekstrak jahe emprit dan 75% sukrosa dengan lama perendaman 2 jam memiliki kadar air 0,9761 %, kadar abu sebesar 0,316 %, lemak 48,4242 %, protein 6,6946 %, karbohidrat 43,474%, dan aktivitas antioksidan sebesar 32,814 %.. Hasil analisis karakteristik fisik menunjukkan nilai hardness sebesar 4,6 N dan nilai apparent modulus sebesar 1,776 N/

Origin of the spin Seebeck effect

Der Spin Seebeck Effekt repräsentiert einen neuartigen Spin kalorischen Effekt mit vorteilhaften und aussichtsreichen Eigenschaften für Anwendungen in den Gebieten der Spintronik und Thermoelektrik.rnIn dieser Arbeit präsentieren wir eine umfangreiche Untersuchung des Spin Seebeck Effekts in isolierenden, magnetischen Granaten und geben Antworten zum kontrovers diskutierten Ursprung des Effekts. Um dieses Ziel zu erreichen, haben wir die Abhängigkeit des Spin Seebeck Effekts von der Dicke des Ferromagneten, der Temperatur, der Stärke des magnetisches Feldes, der Grenzflächen und des Detektormaterials, sowie Kombinationen dieser Parameter gemessen. Im Zuge dessen haben wir das Wachstum der untersuchten magnetischen Granate optimiert und eine umfassende Analyse der strukturellen und magnetischen Parameter durchgeführt, um Einflüsse der Probenqualität auszuschließen. Des Weiteren zeigte eine Untersuchung des magnetoresistiven Effekts, welcher als mögliche Ursache des Effekts galt, in Kombination mit einer Studie des Messaufbaus, dass parasitäre Einflüsse auf das Messergebnis ausgeschlossen werden können. Unsere Ergebnisse zeigen, dass der Spin Seebeck Effekt mit zunehmender Dicke des Ferromagneten eine Sättigung des Signals aufweist. Diese hängt zudem von der Temperatur ab, da mit abnehmender Temperatur die Sättigung erst bei dickeren Filmen auftritt. Außerdem fanden unsere Messungen ein Maximum des Spin Seebeck Effekts für Temperaturen unterhalb der Raumtemperatur, welcher sowohl von der Dicke des Materials als auch der Magnetfeldstärke und dem Detektormaterial beeinflusst wird. In Messungen bei hohen magnetischen Feldstärken beobachteten wir eine Unterdrückung des Messsignals, dessen Ursache mithilfe von Simulationen auf den magnonischen Ursprung des Spin Seebeck Effekts zurückgeführt werden kann. Dies unterstreicht, dass der Effekt auf vom Ferromagneten emittierten Magnonen basiert. Im letzten Abschnitt dieser Arbeit präsentieren wir Messungen in einem bislang nicht untersuchten ferrimagnetischen Material, welche zwei Vorzeichenwechsel des Spin Seebeck Effekts als Funktion der Temperatur aufzeigen. Dieses bisher unbekannte Signalverhalten betont, dass der Effekt aus einem komplexen Zusammenspiel der magnonischen Moden resultiert und zusätzlich vom Detektormaterial abhängt.rnSomit tragen unsere Ergebnisse und Beobachtungen im hohen Maße zur Beantwortung der Frage nach dem Ursprungs des Spin Seebeck Effekts bei und zeigen neuartige bisher nicht beobachtete Effekte, welche ein neues Kapitel für das Gebiet der Spin Kaloritronik eröffnen.The spin Seebeck effect presents a novel spin caloric effect which possesses promising properties for spintronic and thermoelectric applications. Currently the effect leaves open questions about its origin and dependences.rnIn this thesis, we present a comprehensive study of the spin Seebeck effect in insulating magnetic garnets providing answers to its controversially discussed origin. To fulfill this task we probed the dependence of the spin Seebeck effect on the thickness of the ferromagnetic material, the temperature, the magnetic strength field, the interface and the detection material as well as combinations of the afore mentioned parameters. In order to be able to exclude influences of the insufficient sample material quality, we optimized the growth of investigated magnetic garnets and performed a comprehensive analysis of the crystalline and magnetic material parameters. Furthermore the magnetoresistance of the detection layer, previously claimed to be the origin of the effect, has been determined and a systematic study of the self-designed and constructed setups is presented to rule out parasitic signal contributions. Thus our experimental results are of high quality and show a characteristic saturation of the spin Seebeck effect with increasing thickness of the used ferromagnetic material. The observed saturation additionally depends on the temperature as the length scale increases with decreasing temperature. Further temperature dependent studies reveal the occurrences of a thickness, temperature, and magnetic field dependent peaking of the spin Seebeck effect signals, which also can be shifted to different temperatures by the detector material. For measurements performed at high magnetic fields we find a suppression of the spin Seebeck signals. Our observed signal features are in agreement with the results of numerical simulations of a magnon based spin Seebeck effect, highlighting that the spin Seebeck effect is based on magnonic spin currents emitted by the bulk of the ferromagnetic material. In the last part of this thesis we present measurements in a so far not investigated ferrimagnetic material, revealing two sign changes of the spin Seebeck effect as a function of temperature. These novel signal features highlight that the spin Seebeck effect originates from a complex interplay of the magnonic modes of the material, strongly depending on the detection material.rnThereby our results and findings are able to a provide valuable answers to the origin of the spin Seebeck effect and reveal novel, so far unknown effects, opening up a new interesting chapter for the field of spin caloritronics