Unidirectional Anisotropy Constant Research Articles

Fabrication and characterization of IrMn/PtMn/PtCr/PtMn antiferromagnets in terms of the role of each layer

Herein, we investigated the exchange couplings in ferromagnet/antiferromagnet film systems of structures CoFe/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50, and CoFe/Ir20Mn80/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50. These investigations revealed that the backed(b)-PtMn(2.5–3.5 nm), the interface(i)-PtMn(2 nm), and the IrMn(0.8 nm)/i-PtMn(1.2 nm) significantly increased the unidirectional anisotropy constant (Jk) up to 0.67 erg/cm2. The insertion of an ultra-thin IrMn(0.8–1.2 nm) layer at the CoFe interface improved the CoFe hysteresis, i.e. the ratio of exchange bias field (Hex) to coercivity (Hc) was increased. The effects associated with thin b-PtMn, i-PtMn, and IrMn/i-PtMn layers were presumably due to a mismatch between lattice constant and a/c ratio of the PtCr. In other words, the larger lattice constants and lattice deformation during L10 transformation of PtMn may promote a dynamic-stress-induced PtCr ordering-assisting effect, though the PtCr thickness was reduced to 13 nm. The real cause of the effect of ultra-thin IrMn layers has not been properly explained to date. The blocking temperature (Tb) for the CoFe/i-PtMn(2 nm)/PtCr(25 nm)/b-PtMn(3 nm) film was 500 °C. Although the additional insertion of IrMn(0.8 nm) reduced Tb to 410 °C, the Tb and the normalized Hex did not depend on the PtCr thickness, which varied from 13 to 25 nm. Remarkably, an insertion of IrMn(0.8 nm) had no influence on the normalized Hex at temperatures below 300 °C.

Composition dependence of exchange anisotropy in PtxMn1−x/Co70Fe30 films

We systematically investigated the exchange anisotropy for ferromagnetic Co70Fe30 and antiferromagnetic Pt-Mn bilayer films. We focused on the relevance between the exchange bias and the composition of the PtxMn1−x (14 < x < 22 and 45 < x < 56 at%) films, and we successfully optimized the composition. The crystal structure of the PtxMn1−x films was FCC for 14 < x < 22 at% and FCT for 45 < x < 56 at% after annealing at 370 °C for 6 h. The unidirectional anisotropy constant (Jk) for fcc-Pt15Mn85 (20 nm) and fct-Pt48Mn52 (20 nm) prepared under optimum conditions in composition were 0.16 and 0.20 erg cm−2, respectively. Both Pt15Mn85 and Pt48Mn52 films showed a larger unidirectional anisotropy constant (Jk) than in other reports for the bilayer system that uses Pt-Mn antiferromagnetic layer. They also showed a flatter surface than that of other antiferromagnetic/ferromagnetic materials, which is suitable to avoid unexpected interlayer coupling in spin-valve structure. The obtained Pt-Mn films with a large exchange anisotropy and slight roughness are useful as an antiferromagnetic layer in spintronic applications.

Compositional Dependence of Exchange Anisotropy in Pt$_x$Mn$_{100-}$$_x$/Co$_y$Fe$_{100-}$$_y$ Films

Bilayer films consisting of a 15 nm thick Pt x Mn 100-x (x = 48 or 15) layer and a 2 nm thick Co y Fe 100-y (0 ≤ y ≤ 90) layer were fabricated by ultrahigh-vacuum magnetron sputtering. The Pt 48 Mn 52 and Pt 15 Mn 85 films consisted of face-centered tetragonal (FCT) and face-centered cubic (FCC) structures, respectively. The effect of composition of CoFe films on the unidirectional anisotropy constant (Jk) in the Pt x Mn 100-x /Co y Fe 100- y bilayer films and the optimum conditions for producing large Jk were determined. The FCT and FCC PtMn films annealed at 370 °C for 6 h both exhibited maximum J k when the film composition was Co 70 Fe 30 . Blocking temperatures of the FCT and FCC PtMn films were 410 °C and 160 °C, respectively. All the bilayer films possess flat surfaces. These results indicate that the obtained Pt-Mn films-with low roughness, high exchange bias, and high blocking temperature-will be practical candidates as the antiferromagnetic layer in spintronic devices.

Magnetic and structural properties of MnRh thin films

A systematic study of magnetic and structural properties of MnRh thin films fabricated onto MgO substrates and amorphous SiO2 has been conducted. All the MnRh thin films thus fabricated are found to be of the CsCl type structure, and exhibit the ferromagnetism at room temperature. The coercivity of about 1.1 kOe was observed at 5 K for films grown onto SiO2 substrates, while coercivity measured at 300 K in all the films were less than 200 Oe. The temperature dependence of magnetization shows thermal hysteresis for all the samples ranging from 150 K to 250 K that varies with the substrates used. The maximum of exchange bias field of 270 Oe and unidirectional magnetic anisotropy constant of 0.35 erg/cm2 at 5K was observed for films grown onto SiO2 substrates better than that observed for the films grown onto MgO substrates. This enhanced exchange bias and unidirectional magnetic anisotropy constant in film grown onto SiO2 is attributed to the strong lattice distortion in such a case.

Magnetic and structural properties of MnRh thin Films

A systematic study of magnetic and structural properties of MnRh thin films fabricated by sputter-deposition onto silica glass has been conducted. The MnRh thin films are found to be of the CsCl-type structure, and ferromagnetic at room temperature. The MnRh thin film undergoes the magnetic phase transition between antiferromagnetic and ferromagnetic states at a temperature around 175K and 310K during the cooling and heating process, respectively. The temperature dependence of the magnetization shows a thermal hysteresis of about 120K. An exchange bias field of about 450Oe at 5K was observed with the coercivity of 900Oe and unidirectional anisotropy constant of about 0.45erg/cm2. The magnetic field dependence of M–T shows that the transition temperature of about 230K remains unchanged with increasing field during the temperature variation process. A detailed STEM-EDS analysis indicates a non-uniform compositional distribution of Mn and Rh with an average composition of Mn58Rh42at%. A high resolution STEM-HAADF analysis reveals the compositional variations within the CsCl-type MnRh grains. It is proposed that the origin of exchange bias effect is resulted from the exchange coupling between the ferromagnetic region with Mn-rich and the antiferromagnetic region with nearly the equiatomic composition.

Effect of Surface Oxidation on the Magnetization Reversal of Cobalt Planar Wires

The influence of the antiferromagnetic layer of cobalt oxide on the magnetization reversal of a submicron cobalt planar wire was studied using the magneto-transport measurements. For pure Cobalt (Co) planar wires of width less than 1.2 μm, length of 30 μm, and thickness of 30 nm, the shape anisotropy dominates the magnetic behavior revealing all characteristics of a single domain structure. With oxidation, there is a thin layer of CoO on top of the Co layer and the exchange coupling between the CoO (antiferromagnet) and Co (ferromagnetic) layers may suppress the shape anisotropy induced single domain structure and the typical switching behavior of magnetization reversal. The magnetic configuration and magnetization reversal are determined by the competition of unidirectional anisotropy and exchange coupling constant.

Perpendicular Exchange Anisotropy in Mn-Ir/Fe-Co/[Pt/Co]$_{4}$ Multilayers

The perpendicular unidirectional anisotropy constant, JKperp, of Mn-Ir/Fe-Co/[Pt/Co] 4 multilayers was investigated by systematically changing the Fe-Co layer composition, and was compared with the in-plane unidirectional anisotropy constant, JKin, of Mn-Ir/Fe-Co bilayers. Observed trends were similar for the variation of JKperp and JKin against the compositional change. This result indicates that the perpendicular exchange anisotropy and in-plane exchange anisotropy are induced by similar mechanisms. On the other hand, an apparent discrepancy was pointed out between the absolute values of JKperp and JKin in particular for the compositional region where Fe-Co forms body-centered-cubic (bcc) structure. This significant discrepancy in the bcc-Fe-Co region was explained by the magnetic anisotropy of the Fe-Co layer, and it is expected that JKperp could be increased as large as JKin when strong perpendicular magnetic anisotropy is induced in the Fe-Co layer.

Correlation between Exchange Anisotropy and Crystal Structure of Ferromagnetic Layer in γ-Mn-Ir/Fe-Co-Ni Bilayers

The unidirectional anisotropy constant, JK, of γ-Mn-Ir / FM (FM = Fe-Co-Ni) bilayers was investigated by systematically changing the composition of the Fe-Co-Ni layers. The FM layers were grown in the crystal structures corresponding to their composition on the (111)-oriented face centered cubic (fcc) Mn-Ir layer. JK was significantly changed around the phase boundary of the FM layer between the body-centered cubic (bcc) phase and fcc-phase. The experimental results revealed a general feature where bcc-structured FM layers were more likely to induce stronger exchange anisotropy than fcc-structured FM layers. The JK values and the lattice mismatch between the Mn-Ir layer and the FM layer were increased with increasing Co concentration in the bcc-FM region. The dependence of JK on FM composition have been explained qualitatively with lattice distortion at the interface between the Mn-Ir and FM layers.

Systematic Study for Magnetization Dependence of Exchange Anisotropy Strength in Mn-Ir/FM (FM${=}$Ni-Co, Co-Fe, Fe-Ni) Bilayer System

The unidirectional anisotropy constant, JK, of the Mn76Ir24/FM (FM =Ni-Co, Co-Fe, Fe-Ni) bilayers was investigated, systematically changing the FM layer composition. On the (111)-oriented face centered cubic (fcc) Mn-Ir layer, the FM layers were grown in the respective crystal structure, corresponding to their composition. The phase boundaries between fcc and body centered cubic (bcc) structures existed around Co80Fe20 and Fe70Ni30, roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, Ms, of the FM layer showed a broad peak around Fe60Co40 and a shallow dip at Fe70Ni30 , corresponding well with the Slater-Pauling curve of bulk alloys. On the other hand, the JK reached a maximum around Co70Fe30 and showed different variation from that of the Ms against the FM layer composition. Therefore, the correlation plot between JK and Ms did not provide any clear relationship. From the experimental results, we found a general feature that bcc structured FM layers are more likely to induce strong exchange anisotropy than fcc structured FM layers.

Temperature Dependence of Magnetic Properties in Epitaxial FePt/FeRh Bilayer

A systematic investigation of structural and magnetic properties of FePt/FeRh bilayers on dependence of temperature has been carried out. The transition temperature of single FeRh layer was higher than that of FePt/FeRh bilayer. A sharp decrease in coercivity is observed at transition temperature for FePt/FeRh bilayer. The thickness FePt and FeRh dependence of exchange bias field and unidirectional anisotropy constant are discussed. Of interest is that the exchange bias effect appears in FePt/FeRh bilayer and unidirectional anisotropy constant is very large up to 1 erg/cm 2, suggesting that this system can be applied to spin-valve sensors and heat assisted magnetic recording medium.

Exchange bias of ferromagnetic/antiferromagnetic in FePt/FeRh bilayers

A systematic investigation of structural and magnetic properties of FePt/FeRh bilayers has been carried out. The transition temperature of single FeRh layer is higher than that of FePt/FeRh bilayer. A sharp decrease in coercivity is observed at transition temperature for FePt/FeRh bilayer. The thickness FePt dependence of exchange bias field and unidirectional anisotropy constant are discussed. Of interest is the exchange bias effect that appears in FePt/FeRh bilayer and the unidirectional anisotropy constant, which is very large (up to 0.8 erg/cm2), suggesting that this system can be applied for spin-valve sensors and heat assisted magnetic recording medium. It may also provide some useful information for better understanding of the mechanism of exchange bias.

In-Plane and Out-of-Plane Exchange Biases in Epitaxial FePt-FeMn Multilayers With Different Crystalline Orientations

A systematic study of in-plane and out-of-plane exchange biases in FePt/FeMn multilayers epitaxially deposited onto MgO(100), MgO(110), MgO(111) substrates by ion beam sputter-deposition system was performed. It is found that the surface and volume anisotropy constants are largest when samples are growth in (100) direction. While the FePt thickness dependence of the blocking temperature show a similar behavior for samples grown with different crystalline orientations, the dependences of exchange bias field and the unidirectional anisotropy constant on FePt thickness behave differently for different orientations. The ratio of in-plane and out-of-plane exchange biases is not only strongly dependent on crystalline orientation but also on thickness of FePt layers suggesting that the spin structure at the interface in multilayer thin films may not be the same as that of the antiferromagnetic bulk.

Enhanced Exchange Anisotropy by Ultra-Thin Co$_{x}$ Fe$_{100 - x}(x&gt; 80)$ Layer Insertion at the Interface of ${\rm L1}_{2}$-Ordered Mn$_{3}$Ir/Co$_{65}$Fe$_{35}$ Bilayers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have investigated the exchange anisotropy in <formula formulatype="inline"> <tex Notation="TeX">${\rm L1}_{2}$</tex></formula>-ordered Mn<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula>Ir/Co<formula formulatype="inline"> <tex Notation="TeX">$_{65}$</tex></formula>Fe<formula formulatype="inline"> <tex Notation="TeX">$_{35}$</tex></formula> bilayers with inserting ultra-thin Co<formula formulatype="inline"><tex Notation="TeX">$_{x}$</tex></formula>Fe<formula formulatype="inline"><tex Notation="TeX">$_{100 - x}(x &gt; 65)$</tex></formula> layer at the interface between <formula formulatype="inline"><tex Notation="TeX">${\rm L1}_{2}$</tex></formula>-Mn<formula formulatype="inline"><tex Notation="TeX">$_{3}$</tex> </formula> Ir and Co<formula formulatype="inline"><tex Notation="TeX">$_{65}$</tex> </formula>Fe<formula formulatype="inline"><tex Notation="TeX">$_{35}$</tex> </formula> layer. The unidirectional anisotropy constant, <formula formulatype="inline"> <tex Notation="TeX">$J_{K}$</tex></formula>, was enhanced from 0.93 erg/cm<formula formulatype="inline"><tex Notation="TeX">$^{2}$</tex></formula> to 1.2erg/cm<formula formulatype="inline"><tex Notation="TeX">$^{2}$</tex></formula> at the range from 85 to 98 at.% of Co composition in Co<formula formulatype="inline"><tex Notation="TeX">$_{x}$</tex></formula>Fe<formula formulatype="inline"><tex Notation="TeX">$_{100 - x}$</tex></formula> insertion layer. The crystal structure of the ultra-thin Co<formula formulatype="inline"><tex Notation="TeX">$_{90}$</tex> </formula>Fe<formula formulatype="inline"><tex Notation="TeX">$_{10}$</tex> </formula> insertion layer was transformed to bcc structure following the bcc structured Co<formula formulatype="inline"><tex Notation="TeX">$_{65}$</tex> </formula>Fe<formula formulatype="inline"><tex Notation="TeX">$_{35}$</tex> </formula> layer during thermal annealing. These results suggest that the enhancement of exchange anisotropy with ultra-thin Co<formula formulatype="inline"> <tex Notation="TeX">$_{x}$</tex></formula>Fe<formula formulatype="inline"> <tex Notation="TeX">$_{100 - x}(x&gt; 80)$</tex></formula> insertion layer is attributed to the Co-rich bcc phase formation in the insertion layer. </para>

Ｃｏ基ホイスラー合金を用いたＧＭＲ積層膜の作製

GMR multilayer films using Co2MnGe(CMG) and Mn-Ir/CMG exchange-coupled films were fabricated on thermally oxidized Si substrate, and their crystal orientation, magnetic properties, and interfacial roughness were investigated. The main results are stated as follows. (1) (100) oriented MnIr/CMG exchange-coupled films were realized when Ni-Ta underlayer/Cr buffer layer were deposited on thermally oxidized Si substrate. After thermal annealing for 2h at 250°C in a magnetic field of 5 kOe, the unidirectional anisotropy constant (JK) of bilayers was 0.13 erg/cm2. (2) Dependence of saturation field (Hs) on Cr thickness for GMR films with (100) oriented CMG/Cr/CMG trilayer was investigated. Hs was minimum when Cr thickness was 1 nm and 5 nm. (3) Surface roughness (Ra) of Ni-Ta 5 nm on Al-Cr bottom electrode layer(100 nm) was 0.108 nm. (4) Flat and (100) oriented CMG/Cr films was formed on Al-Cr bottom electrode layer (100 nm)/Ni-Ta underlayer (15 nm)/Cr buffer layer (60 nm) when Cr content of the Al-Cr layer was 18.7-24.6 at%.

Compositional Dependence of Exchange Anisotropy in .GAMMA.-Mn-Ir-Rh-Ru / Co70Fe30 Bilayers

Exchange anisotropy of quaternary γ-Mn-Ir-Rh-Ru / Co70Fe30 bilayer has been investigated. Unidirectional anisotropy constant, JK, exhibited gradual change against the chemical composition of the quaternary system. Contour map of JK shows a broad peak at the compositional range from Mn75Ir25 to Mn79Ir15Rh6. The JK decreases mainly along the direction to which the Ru content increases. The thinnest critical thickness of the antiferromagnetic layer is obtained for Mn-Ir alloy among the respective terminal binary alloy systems. We concluded that Mn-Ir is the most promising antiferromagnetic material for exchange-biased bilayers in ultra-high density hard disk drives among the present γ-Mn-Ir-Rh-Ru quaternary system.

Enhancement of exchange bias by ultra‐thin Mn layer insertion at the interface of Mn‐Ir/Co‐Fe bilayers

AbstractEffects of ultra‐thin Mn, Ru, Pd, Ta, Gd, and Tb layer insertion at the interface on the exchange bias of Mn75Ir25/Co70Fe30 bilayers were investigated. Unidirectional anisotropy constant, JK, of the bilayer was significantly enhanced from 0.46 erg/cm2 to 0.84 erg/cm2 with the insertion of 0.5‐nm‐thick Mn layer, while it was reduced with increasing the inserted layer thickness for the other materials. The value of 0.84 erg/cm2 was not obtained in the variation of JK with respect to the chemical composition of Mn‐Ir layer in a range of Ir content of 0 ∼ 40 at.%. The cause of the enhancement of JK with ultra‐thin Mn insertion might be due to the modification of antiferromagnetic spin structure at the interface. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Giant parallel and perpendicular exchange biases in MnPd/Co bilayers

AbstractA systematic study of exchange bias in MnPd/Co bilayers has been carried out in both parallel and perpendicular directions, where the dependences of blocking temperature, exchange bias and unidirectional anisotropy constant on the thicknesses of MnPd and Co layers were investigated. One of the particular interests is that the blocking temperature of parallel exchange bias is higher than that of perpendicular exchange bias which can be interpreted as the difference of the ordering in parallel and perpendicular direction. The other is that a huge unidirectional anisotropy constant, JK = 5.5 erg/cm2 was observed, which is in reasonable agreement with the theoretical prediction based on the model by Meiklejohn and Bean. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Giant exchange bias in MnPd/Co bilayers

A systematic study of exchange bias in MnPd/Co bilayers has been carried out, where the dependences of exchange bias, unidirectional anisotropy constant and coercivity on the thicknesses of MnPd and Co layers were investigated. A huge unidirectional anisotropy constant, J K = 2.5 erg / cm 2 was observed, which is in reasonable agreement with the theoretical prediction based on the model by Meiklejohn and Bean. The angular dependences of exchange bias field and coercivity have also been examined showing that both exchange bias and coercivity follow 1 / cos α rule.

Soft x-ray magnetic circular dichroism study of Mn–Ir∕Co–Fe bilayers with giant exchange anisotropy

Soft x-ray magnetic circular dichroism (XMCD) and element-specific magnetic hysteresis (ESMH) measurements were performed in transmission mode on Mn73Ir27 10nm∕Co70Fe30 2nm bilayers with different chemical orderings of the Mn–Ir layer.The unidirectional anisotropy constant was 0.55erg∕cm2 for the disordered Mn–Ir layer and 1.18erg∕cm2 for the ordered Mn–Ir layer. The XMCD signal of Mn was observed, which means the induced ferromagnetic component of Mn spins through the exchange coupling at the interface. No vertical offset of the Mn ESMH loops was observed for either the disordered or the ordered bilayers, which means that insignificant uncompensated Mn spin was pinned at the interface to induce exchange bias on the Co–Fe layer.

Exchange Anisotropy of L1$_2$-Mn$_3$X (X$=$Ir, Rh, Ru)/Co-Fe Bilayers

The L1 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_2$</tex> -phase formation and resultant exchange anisotropy were examined in Mn-Rh/Co-Fe and Mn-Ru/Co-Fe bilayers, varying the substrate temperature, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_ sub$</tex> , during the deposition of Mn-Rh or Mn-Ru layer. The superlattice diffractions from the Mn <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_3$</tex> Rh were detected. The unidirectional anisotropy constant, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J_ K$</tex> , was enhanced more than double with increasing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_ sub$</tex> from room temperature to 130 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$sim$</tex> 170 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C, similarly to Mn-Ir/Co-Fe bilayers. The maximum <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J_ K$</tex> value obtained was 0.77 erg/cm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^2$</tex> for the Mn-Rh/Co-Fe bilayer and 0.56 erg/cm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^2$</tex> for the Mn-Ru/Co-Fe bilayer. The blocking temperature, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_ B$</tex> , was increased about 80 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C to 100 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ$</tex> C, in accordance with enhancing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J_ K$</tex> . Variations of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J_ K$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_ B$</tex> as a function of antiferromagnetic layer thickness, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d_ AF$</tex> , were also studied. The critical thickness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d_ AF$</tex> was less than 5 nm for both the bilayers.