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Titled and graded anisotropy FePt and FePtCu thin films for the application of hard disk drive and spin torque oscillators
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. (Johan Åkerman)
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

 The FePt and FePtCu thin films with graded anisotropy and titled anisotropy are utilized to solve both the magnetic recording ‘‘trilemma’’ of the hard disk drives (HDDs) and the large field operation problem of spin torque oscillators (STOs). We have successfully realized the FePtCu thin films with graded anisotropy. During deposition a compositional gradient is achieved by continuously varying the Cu content from the top to bottom. After annealing at proper temperatures, the top Cu-poor regions remain at soft A1 phase, while the bottom Cu-rich regions transform into hard L1 0phase. Hence the gradient anisotropy is established through the film thickness. The critical role of the annealing temperatures (TA) on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both the reduced coercivity and moderate thermal stability. In conjunction of the fabrication, the subsequent analysis of the graded material is not trivial. The reversal mechanism of graded anisotropy have been investigated by alternation gradient magnetometer (AGM) and magneto optical Kerr effect (MOKE) measurements with first order reversal curves (FORC) technique. The AGM-FORC analysis, which clearly shows the soft and hard phases, is not able to resolve how these phases are distributed through the film thickness. MOKE-FORC measurement which preferentially probes the surface of the film, reveal that the soft components are indeed located toward the top surface. The TA plays a critical role in the induced anisotropy gradient. We provide a detailed study of the how the anisotropy gradient in a compositional graded FePtCu film gradually develops as a function of the TA. By utilizing the in-situ annealing and magnetic characterization capability of a physical property measurement system, the evolution of the induced anisotropy gradient is elucidated. These results are important and useful for the application of HDDs. In order to achieve the zero-field operation for STOs, we have successfully fabricated pseudo spinvalves based on L1 0(111) textured FePt or FePtCu. We demonstrate magnetoresistance(MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10(111)-oriented FePt fixed layers with a 36 ° out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers.We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers. Futuremore, through optimization of the Cu spacer thickness, we demonstrate MR up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. These results are an important milestone for future applications of tilted spin polarizers in STOs.

Place, publisher, year, edition, pages
Kista: KTH Royal Institute of Technology , 2011. , v, 65 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:08
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-34013ISBN: 978-91-7415-996-7OAI: oai:DiVA.org:kth-34013DiVA: diva2:418547
Presentation
2011-05-31, C1, Isafjordsgatan 22, Kista, 10:00 (English)
Opponent
Supervisors
Note
QC 20110525Available from: 2011-05-25 Created: 2011-05-23 Last updated: 2011-05-25Bibliographically approved
List of papers
1. Continuously graded anisotropy in single (Fe53Pt47)(100-x)Cu-x films
Open this publication in new window or tab >>Continuously graded anisotropy in single (Fe53Pt47)(100-x)Cu-x films
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 18, 182504- p.Article in journal (Refereed) Published
Abstract [en]

We report on continuously graded anisotropy. During deposition, a compositional gradient is achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)(70)Cu-30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after annealing using the composition dependence of the low-anisotropy (A1) to high-anisotropy (L1(0)) ordering temperature. The critical role of the annealing temperature on the resultant anisotropy gradient is investigated. Magnetic measurements support the creation of an anisotropy gradient in properly annealed films which exhibit both a reduced coercivity and moderate thermal stability. These results demonstrate that an anisotropy gradient can be realized, and tailored, in single continuous films without the need for multilayers.

Keyword
MEDIA, CU
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27099 (URN)10.1063/1.3505521 (DOI)000283934100045 ()2-s2.0-78649276605 (ScopusID)
Note
QC 20101209Available from: 2010-12-09 Created: 2010-12-06 Last updated: 2011-05-25Bibliographically approved
2. First-order reversal curve analysis of graded anisotropy FePtCu films
Open this publication in new window or tab >>First-order reversal curve analysis of graded anisotropy FePtCu films
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2010 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 20, 202501- p.Article in journal (Refereed) Published
Abstract [en]

The reversal mechanisms of graded anisotropy FePtCu films have been investigated by alternating gradient magnetometer (AGM) and magneto-optical Kerr effect (MOKE) measurements with first-order reversal curve (FORC) techniques. The AGM-FORC analysis, which clearly shows the presence of soft and hard components, is unable to resolve how these phases are distributed throughout the film thickness. MOKE-FORC measurements, which preferentially probe the surface of the film, reveal that the soft components are indeed located toward the top surface. Combining AGM-FORC with the inherent surface sensitivity of MOKE-FORC analysis allows for a comprehensive analysis of heterogeneous systems such as graded materials.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27960 (URN)10.1063/1.3515907 (DOI)000284545200034 ()2-s2.0-78649234796 (ScopusID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20110104Available from: 2011-01-04 Created: 2011-01-03 Last updated: 2011-05-25Bibliographically approved
3. An In Situ anneal study of graded anisotropy FePtCu films
Open this publication in new window or tab >>An In Situ anneal study of graded anisotropy FePtCu films
2011 (English)In: IEEE Magnetic Letters, Vol. 2Article in journal (Refereed) Published
Abstract [en]

We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475 ◦C reduce the gradient until the film is effectively uniform for TA ≥ 525 ◦C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525 ◦C where the gradient vanishes, and a final increase as the uniform L10 film orders completely.

Keyword
hard magnetic materials, FePtCu films, graded anisotropy, in situ magnetic characterization, magnetic recording media
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-34094 (URN)10.1109/LMAG.2011.2114332 (DOI)000208910300012 ()2-s2.0-79958715767 (ScopusID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20110525

Available from: 2011-05-25 Created: 2011-05-25 Last updated: 2014-09-23Bibliographically approved
4. Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy
Open this publication in new window or tab >>Pseudo spin valves based on L10 (111)-oriented FePt fixed layers with tilted anisotropy
Show others...
2009 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 94, no 16, 163108- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate magnetoresistance (MR) in excess of 4% in FePt/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111)-oriented FePt fixed layers with a 36 degrees out-of-plane tilted magnetization. The high MR is achieved by increasing the spin polarization at the Cu interfaces, using thin CoFe, and optimizing the FePt growth and Cu interface quality using Ta and Ta/Pt underlayers. We observe well-separated switching of the FePt/CoFe fixed layer and the CoFe/NiFe free layer, suggesting that CoFe is rigidly exchange coupled to FePt and NiFe in the respective layers.

Keyword
cobalt alloys, copper, exchange interactions (electron), giant, magnetoresistance, iron alloys, magnetic anisotropy, magnetic, multilayers, magnetic switching, magnetisation, nickel alloys, platinum, alloys, spin polarised transport, spin valves, high areal density, films
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-18394 (URN)10.1063/1.3123003 (DOI)000265823300063 ()2-s2.0-65449120703 (ScopusID)
Note
QC 20100525 QC 20120321Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2012-03-21Bibliographically approved
5. Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers
Open this publication in new window or tab >>Improved magnetoresistance through spacer thickness optimization in tilted pseudo spin valves based on L10 (111)-oriented FePtCu fixed layers
2009 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 106, no 5Article in journal (Refereed) Published
Abstract [en]

Through optimization of the Cu spacer thickness, we demonstrate magnetoresistance (MR) up to 5% in FePtCu/CoFe/Cu/CoFe/NiFe pseudo spin valves based on L10 (111) FePtCu fixed layers with a tilted magnetization. We find an optimum spacer thickness of about 2.4 nm which correlates with a clear onset of strong interlayer exchange coupling below 2.4 nm and spin-independent current shunting in the spacer above 2.4 nm. We argue that yet higher MR should be possible through further reduction in the interlayer exchange coupling.

Keyword
copper alloys, exchange interactions (electron), giant, magnetoresistance, iron alloys, magnetisation, nickel alloys, optimisation, platinum alloys, spin valves, 1 tb/in(2), media, multilayers
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-18767 (URN)10.1063/1.3211964 (DOI)000269850300078 ()2-s2.0-70349330340 (ScopusID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-05-25Bibliographically approved

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