Bloch line dynamics within magnetic domain walls

Touko Herranen

Bloch line dynamics within magnetic domain walls

Julkaisun otsikon käännös: Magneettisen seinämän Bloch-viivojen dynamiikka

Touko Herranen

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Ferromagneetissa on magneettisia alueita, joissa atomien magneettiset momentit ovat samansuuntaiset ja joita erottaa magneettinen seinämä. Seinämän sisäiset vapausasteet voidaan virittää asettamalla se ulkoiseen magneettikenttään, jonka voimakkuus on suurempi kuin niin kutsutun Walkerin kentän. Tämän seurauksena seinämä kokee Walkerin romahduksen, mikä ilmenee keskimääräisen nopeuden romahduksena ja seinämän magnetisaatio alkaa kiertyä syklisesti. Jos mekanismi käynnistetään magneettisessa seinämässä, jonka jokin ulottuvuus on suurempi kuin materiaalille tyypillinen Bloch-viivan pituus, magneettisen seinämän sisään muodostuu seinämiä joita kutsutaan Bloch-viivoiksi. Magneettisten seinämien dynamiikkaa on tutkittu laajasti sekä käyttäen erinäisiä laskennallisia menetelmiä että kokeellisesti. Tässä väitöskirjassa käytämme mikromagneettista ohjelmistoa tutkiaksemme Bloch-viivojen dynamiikka sekä niiden vaikutusta seinämien dynamiikkaan kohtisuoran magneettisen anisotropian näytteissä. Ensimmäisessä julkaisussa tutkimme eri levyisten CoPtCr-ohutkalvojen seinämän dynamiikkaa. Havaitsemme Bloch-viivojen nukleaatiota seinämissä sekä epäjärjestyneissä että puhtaissa näytteissä, kun ulkoinen kenttä on asetettu Walkerin kenttää voimakkaammaksi. Luomme järjestelyn, joka rajoittaa seinämän kahden loven väliin, ja käytämme sitä tutkiaksemme Bloch-viivojen dynamiikkaa pinnan suuntaisen kentän vaikutuksen alaisena. Lopuksi demonstroimme sähkövirralla ajetun Bloch-viivamuistin potentiaalista suorituskykyä. Toisessa julkaisussa tutkimme reunaehtojen ja näytteen paksuuden vaikutusta seinämän dynamiikkaan magneettisessa granaatissa, ja huomaamme että nämä ominaisuudet määrittävät seinämän sisäistä dynamiikkaa. Näytteen paksuus rajoittaa seinämän suurinta vakaan dynamiikan nopeutta ennen romahdusta. Huomaamme myös maksiminopeuden kytkeytyvän Bloch-viivojen leveyteen. Kolmannessa julkaisussa lähestymme Barkhausenin kohinaa ja vyörystatistiikkaa ohuissa Pt/Co/Pt kerrosrakenteissa mikromagnetismin keinoin. Seinämää kuljetetaan käyttäen kvasistaattista vakionopeutta. Uudenlainen lähestymistapa mahdollistaa ensimmäistä kertaa magnetisaation havaitsemisen seinämän osasta, josta vyöry on saanut alkunsa. Seinämän magnetisaation sisäinen dynamiikka osoittaa, että Bloch-viivojen aktiivisuus on voimakkaampaa vyöryjen aikana kuin itse seinämän etenemiseen liittyvä aktiivisuus. Aktiivisuussignaaleista määritetyt vyöryjen koko- ja kestojakaumat noudattavat potenssilakia, ja vastaavat piirteet ovat havaittavissa myös seinämän nopeudesta määritetyissä jakaumissa. Analyysi osoittaa myös että mikromagneettisista simulaatioista saadut tulokset ovat lähellä pelkistetymmällä mallilla saatuja tuloksia. Pelkistetty malli kuvaa lyhyen kantaman vuorovaikutuksia elastisessa langassa epäpuhtaassa väliaineessa.

Julkaisun otsikon käännös	Magneettisen seinämän Bloch-viivojen dynamiikka
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	Alava, Mikko, Vastuuprofessori Laurson, Lasse, Ohjaaja
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-8133-5
Sähköinen ISBN	978-952-60-8134-2
Tila	Julkaistu - 2018
OKM-julkaisutyyppi	G5 Artikkeliväitöskirja

Tutkimusalat

mikromagnetismi
magneettiset seinämät
Bloch-viivat
Barkhausenin kohina

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-8134-2Lisenssi: Määrittelemätön

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@phdthesis{b9f65e52666c40d6ba4ef9b4e79f0c0f,

title = "Bloch line dynamics within magnetic domain walls",

abstract = "Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.",

keywords = "micromagnetics, domain wall dynamics, Bloch lines, Barkhausen noise, mikromagnetismi, magneettiset sein{\"a}m{\"a}t, Bloch-viivat, Barkhausenin kohina, micromagnetics, domain wall dynamics, Bloch lines, Barkhausen noise",

author = "Touko Herranen",

year = "2018",

language = "English",

isbn = "978-952-60-8133-5",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "153",

address = "Finland",

school = "Aalto University",

}

TY - BOOK

T1 - Bloch line dynamics within magnetic domain walls

AU - Herranen, Touko

PY - 2018

Y1 - 2018

N2 - Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.

AB - Magnetic domains are uniformly magnetized regions within a ferromagnet separated by magnetic domain walls. The internal degrees of freedom of a domain wall can be excited by applying a magnetic field greater than so called Walker field. As a result the domain wall velocity experiences the Walker breakdown, an abrupt drop of the average velocity, and the magnetization of the domain wall starts a cyclic rotation. If this mechanism is triggered in a domain wall with a dimension greater than a material characteristic Bloch line width, the excitations become non-uniform, which results in nucleation of domain walls within the domain wall called Bloch lines. The dynamics of domain walls in disordered media have been studied extensively using various computational methods as well as experimentally. In this doctoral dissertation we use a micromagnetics software to simulate the Bloch line dynamics and the effects of Bloch lines on domain wall dynamics in samples with perpendicular magnetic anisotropy. In Publication I we study the domain wall dynamics in CoPtCr thin films with different widths. We observe nucleation of Bloch lines within domain walls in disordered and perfect samples when driven with a magnetic field higher than the Walker field. We construct a geometry to confine the domain wall between two notches, and use it to study Bloch line dynamics under an in-plane field. Finally we demonstrate the performance potential of an electrical current operated Bloch line memory. In Publication II we study the effects of boundary conditions and thickness effects on the domain wall dynamics in a magnetic garnet, and we find they determine the internal dynamics allowed for the magnetization of the domain wall. The sample thickness limits the maximum achievable stable velocity before the breakdown. The velocity limit is also found to be related to the spatial width of Bloch lines. In Publication III we use a micromagnetics approach to study the Barkhausen effect and avalanche statistics in a thin Pt/Co/Pt multilayer. The domain wall is driven using a quasistatic constant velocity. The novel approach enables us to determine magnetization of the domain wall segment where avalanches are triggered. Internal magnetization dynamics show that during avalanches the activity of in-plane magnetization, i.e. the Bloch line motion, is higher than the activity related to the domain wall motion. Avalanche size and duration distributions obtained from the activity signals follow power law scaling, and the corresponding features extracted from the domain wall velocity show no significant difference. The analysis also shows that the results obtained using micromagnetic simulations are close to the values expected from a simpler model describing a short-range elastic string in a random medium.

KW - micromagnetics

KW - domain wall dynamics

KW - Bloch lines

KW - Barkhausen noise

KW - mikromagnetismi

KW - magneettiset seinämät

KW - Bloch-viivat

KW - Barkhausenin kohina

KW - micromagnetics

KW - domain wall dynamics

KW - Bloch lines

KW - Barkhausen noise

M3 - Doctoral Thesis

SN - 978-952-60-8133-5

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Bloch line dynamics within magnetic domain walls

Abstrakti

Tutkimusalat

Pääsy asiakirjaan

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