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Spin-diode effect and thermally controlled switching in magnetic spin-valves
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit.

The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 85 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:11
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-91300ISBN: 978-91-7501-287-2 (print)OAI: oai:DiVA.org:kth-91300DiVA: diva2:509309
Public defence
2012-03-30, FB52, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Supervisors
Note
QC 20120313Available from: 2012-03-13 Created: 2012-03-12 Last updated: 2012-03-13Bibliographically approved
List of papers
1. Tunneling spectroscopy of magnetic double barrier junctions
Open this publication in new window or tab >>Tunneling spectroscopy of magnetic double barrier junctions
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2007 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 43, no 6, 2818-2820 p.Article in journal (Refereed) Published
Abstract [en]

Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers.

Keyword
current rectification, diode effect, magnetic tunnel junctions (MTJs), tunneling spectroscopy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-39451 (URN)10.1109/TMAG.2007.893313 (DOI)000246706200242 ()2-s2.0-34249058694 (Scopus ID)
Note
Conference: 10th Joint Magnetism and Magnetic Materials Conference/International Magnetics Conference. Balitmore, MD. JAN 07-11, 2007. QC 20110911Available from: 2011-09-12 Created: 2011-09-09 Last updated: 2017-12-08Bibliographically approved
2. Spin diode based on Fe/MgO double tunnel junction
Open this publication in new window or tab >>Spin diode based on Fe/MgO double tunnel junction
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2008 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 8, no 3, 805-809 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a spin diode consisting of a semiconductor-free nanoscale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magnetoresistance is a record high > 1000%, essentially making the structure an on/off spin switch. This, combined with the strong diode effect, similar to 100, demonstrates a new device principle, promising for memory and reprogrammable logic applications.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-33382 (URN)10.1021/nl072676z (DOI)000253947400007 ()2-s2.0-61449211124 (Scopus ID)
Note

QC 20110505

Available from: 2011-05-05 Created: 2011-05-05 Last updated: 2017-12-11Bibliographically approved
3. Exchange coupling and magnetoresistance in CoFe/NiCu/CoFe spin valves near the Curie point of the spacer
Open this publication in new window or tab >>Exchange coupling and magnetoresistance in CoFe/NiCu/CoFe spin valves near the Curie point of the spacer
2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 9, 09D711- p.Article in journal (Refereed) Published
Abstract [en]

Thermal control of exchange coupling between two strongly ferromagnetic layers through a weakly ferromagnetic Ni-Cu spacer and the associated magnetoresistance is investigated. The spacer, having a Curie point slightly above room temperature, can be cycled between its paramagnetic and ferromagnetic states by varying the temperature externally or using joule heating. It is shown that the giant magnetoresistance vanishes due to a strong reduction in the mean free path in the spacer at above similar to 30% Ni concentration-before the onset of ferromagnetism. Finally, a device is proposed which combines thermally controlled exchange coupling and large magnetoresistance by separating the switching and the readout elements.

Keyword
NI-CU ALLOYS, ELECTRICAL-RESISTIVITY, MAGNETIC MULTILAYERS, MOMENTS, MEMORY, NICKEL, LAYERS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-29840 (URN)10.1063/1.3340509 (DOI)000277834300288 ()2-s2.0-77951697796 (Scopus ID)
Note
QC 20110222 11th Joint MMM-Intermag Conference, Washington, DC, JAN 18-22, 2010Available from: 2011-02-23 Created: 2011-02-17 Last updated: 2017-12-11Bibliographically approved
4. Thermoelectrically Controlled Spin-Switch
Open this publication in new window or tab >>Thermoelectrically Controlled Spin-Switch
2010 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, 2140-2143 p.Article in journal (Refereed) Published
Abstract [en]

The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

Keyword
Joule heating, magnetic thin-film switches, Ni-Cu alloys
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-27589 (URN)10.1109/TMAG.2010.2041053 (DOI)000278037800224 ()2-s2.0-77952835990 (Scopus ID)
Note
QC 20101214Available from: 2010-12-14 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved
5. Thermoelectrical manipulation of nanomagnets
Open this publication in new window or tab >>Thermoelectrical manipulation of nanomagnets
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2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 12, 123706- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermoelectronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermoelectric manipulation of nanomagnets). Supporting experimental evidence of such thermally-controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel to antiparallel switching causes magnetoresistance oscillations whose frequency can be controlled by proper biasing from essentially dc to GHz. We discuss in detail an experimental realization of a device that can operate as a thermomagnetoresistive switch or oscillator.

Keyword
MAGNETIC MULTILAYER, ELECTRIC-CURRENT, SPIN-WAVES, DRIVEN, EXCITATION, MEMORY
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-29461 (URN)10.1063/1.3437054 (DOI)000279993900068 ()2-s2.0-77954182379 (Scopus ID)
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme, FP7-ICT-2007-C 225955
Note
QC 20110208Available from: 2011-02-08 Created: 2011-02-02 Last updated: 2017-12-11Bibliographically approved
6. Thermal-magnetic-electric oscillator based on spin-valve effect
Open this publication in new window or tab >>Thermal-magnetic-electric oscillator based on spin-valve effect
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2012 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 111, 044315- p.Article in journal (Refereed) Published
Abstract [en]

A thermal-magnetic-electric valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-91298 (URN)10.1063/1.3686735 (DOI)000300948600093 ()2-s2.0-84863238065 (Scopus ID)
Note
QC 20120313Available from: 2012-03-12 Created: 2012-03-12 Last updated: 2017-12-07Bibliographically approved
7. Exchange-induced phase separation in Ni-Cu films
Open this publication in new window or tab >>Exchange-induced phase separation in Ni-Cu films
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2012 (English)In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 324, no 13, 2131-2135 p.Article in journal (Refereed) Published
Abstract [en]

Magneto-structural properties of films of diluted ferromagnetic alloys NixCu1-x in the concentration range 0.7 < x < 1.0 are studied experimentally. Films deposited by magnetron sputtering show partial phase separation, as evidenced by structural analysis and ferromagnetic resonance measurements. The phase diagram of the NixCu1-x bulk system is obtained using numerical theoretical analysis of the electronic structure, taking into account the interatomic exchange interactions. The results confirm the experimentally found partial phase separation, explain it as magnetic in origin, and indicate an additional metastable region connected with the ferromagnetic transition in the system.

Keyword
Dilute ferromagnet, Thin film, Phase diagram, Ferromagnetic resonance, Curie temperature
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-91299 (URN)10.1016/j.jmmm.2012.02.027 (DOI)000301798900024 ()2-s2.0-84858446128 (Scopus ID)
Funder
EU, FP7, Seventh Framework Programme
Note
QC 20120423.  Updated from manuscript to article in journal.Available from: 2012-03-12 Created: 2012-03-12 Last updated: 2017-12-07Bibliographically approved

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