JP2010147439A - Multilayer film, magnetic head, and magnetic storage device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve reproduction precision of information recorded to high recording density by enhancing the sensing capability of a reproducing element. <P>SOLUTION: A multilayer film that the reproducing element includes a nonmagnetic layer 400, and a first ferromagnetic layer 401 and a second ferromagnetic layer 402 between which the nonmagnetic layer 400 is sandwiched, and at least one of those first and second ferromagnetic layers has an atom composition ratio of Co:Fe:B=2:1:1 and also has an L2<SB>1</SB>-ruled crystal structure. Consequently, a high polarization rate is actualized, so that even when element resistance is made small by reducing the film thickness of the nonmagnetic layer to smaller than a certain extent, a high magnetic resistance change rate can be obtained to obtain high sensing capability. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a multilayer film used for a head of a magnetic storage device such as a hard disk drive (HDD), a magnetic head mounted on a hard disk drive, and a magnetic storage device including the magnetic head.

  In recent years, a magnetic reproducing element used in a magnetic head of an HDD (Hard Disk Drive) has been required to have a sensing capability capable of reproducing a fine recording pattern (recording bit) of a storage medium having a high storage density. Examples of magnetic reproducing elements having high sensing capability include giant magnetoresistive elements such as GMR (Giant Magneto Resistive) elements and TMR (Tunnel Magneto Resistance) elements. These TMR elements and GMR elements have a multilayer film structure in which a nonmagnetic layer and a ferromagnetic layer are laminated. By such a multilayer film structure, a minute magnetic field change in which a minute recording pattern (recording bit) is generated. Is converted into an electrical signal.

  In the case of a TMR element, since an insulator is used as the nonmagnetic layer, the resistance is larger than that of the GMR element. Recently, a technique for obtaining a high magnetoresistance ratio (also referred to as an MR ratio) has appeared by using MgO as an insulator of a TMR element and using CoFeB having an amorphous structure as a ferromagnetic layer (for example, non-patented). Reference 1 and Patent Reference 1). Patent Document 1 discloses that the use of amorphous CoFeB can increase the homogeneity of the ferromagnetic layer as compared with the case of using CoFe, and the improvement of the productivity. Is also disclosed.

On the other hand, a Heusler alloy having a high polarizability is also known as one of the materials of a ferromagnetic layer that can provide a high rate of change in magnetoresistance. As the composition of this Heusler alloy, for example, CoFeAl has been proposed (see Patent Document 2). The crystal structure of the Heusler alloy is, for example, a B2 ordered structure or an L2 ₁ ordered structure. It is also known that Si, Ge, As, Sb, Bi, or the like can be added in place of “Co” in CoFeAl (see Patent Document 3).

K. Tsunekawa, et al: App. Phys. Lett. 87, 072503 (2005) JP 2007-142424 A JP 2004-221526 A JP 2007-250977 A

However, as in Non-Patent Document 1, when MgO is used as the insulator of the TMR element and CoFeB having an amorphous structure is used as the ferromagnetic layer, in order to obtain a high magnetoresistance change rate of 100% or more at room temperature, It is necessary to limit the film thickness of the insulator. Specifically, from the experimental results using a TMR element for reproducing a storage medium having a high storage density, the insulating layer needs to have a film thickness of 0.7 nm or more. It becomes difficult to obtain the resistance change rate. Here, when the film thickness of the insulating layer is 0.7 nm, the resistance area product RA (product of the resistance value of the element and the area of the element) is several Ω · μm ² or more. Therefore, in this case, there is a possibility that the magnetoresistive change rate having a characteristic that becomes larger as the RA value is smaller cannot be increased beyond a certain level.

  On the other hand, as the recording pattern (recording bit) on the storage medium becomes finer, the size of the reproducing element (TMR element) that enables reading of information recorded in the recording pattern becomes smaller, and the resistance of the element inevitably increases. To increase. When the resistance of the element increases in this way, impedance matching with a circuit that takes out a resistance change in the element as an electric signal cannot be obtained, and thus the signal characteristics in the high frequency band are deteriorated. Therefore, in the reproducing element, it is an important problem to reduce the resistance value.

  In the GMR element, although the nonmagnetic layer is a conductor and the element resistance is small, the magnetoresistance change rate is smaller than that of the TMR element.

  As described above, in order to ensure the sensing capability for a storage medium with a high storage density, it is necessary that the element resistance is small and the magnetoresistance change rate is large.

  The present invention has been made under such circumstances, and an object of the present invention is to provide a multilayer film that can obtain a high magnetoresistance change rate even when the element resistance is reduced. It is another object of the present invention to provide a magnetic head capable of reading information recorded on an information recording medium at a high recording density with high accuracy. Furthermore, an object of the present invention is to provide a magnetic storage device that can maintain high-precision information reproducing capability over a long period of time.

The multilayer film described in the present specification includes a nonmagnetic layer and ferromagnetic layers provided on both sides of the nonmagnetic layer, and at least one of the ferromagnetic layers has an atomic composition ratio of Co: Fe: B = 2: 1: 1, and having L2 ₁ ordered crystal structure.

In order to solve the above problems, the present inventor has searched for a new material that can improve the magnetoresistance change rate even if the element resistance is reduced, and has analyzed the characteristics of the new material. As a result, when the ferromagnetic layer included in the multilayer film has the above-described composition ratio and the crystal structure is an L2 _1- ordered structure, the s-electron state on the downspin side is near the Fermi level that contributes to the current. It was possible to obtain new knowledge that the density was almost zero and the polarizability (spin) of the s electrons on the upspin side responsible for electrical conduction was almost 100%. In the ferromagnetic layer having this crystal structure, the direction of polarization of d electrons based on the density of states of d electrons is the same as the direction of polarization of s electrons, and the high polarizability of s electrons depends on the polarization state of d electrons. Since it was not killed, it was found that the ferromagnetic layer having the above crystal structure can obtain a high polarizability as a whole of electrons including s electrons and d electrons.

Thus, the ferromagnetic layer has an L2 ₁ ordered crystal structure of CoFeB, and its composition ratio is Co: Fe: B = 2: 1: 1. Through this, a high polarizability can be realized. Thereby, even if the element resistance of the nonmagnetic layer is reduced, a high magnetoresistance change rate can be obtained.

  In addition, since the ferromagnetic layer CoFeB uses B (boron), it is difficult to oxidize compared to CoFeAl or the like, so that deterioration of characteristics due to oxidation can be suppressed. Furthermore, since the ferromagnetic layer CoFeB can be made smaller in lattice size than other materials known for Heusler alloys (for example, CoFeSi and CoFeAl), the nonmagnetic layer and the ferromagnetic layer are joined. It is possible to improve the bondability at the bonding surface. Further, from the above composition ratio, the ferromagnetic layer contains 75% Co and Fe, and the Curie temperature can be set to room temperature or higher, so that there is no problem in practical use.

  The magnetic head described in the present specification is a magnetic head that includes the multilayer film described in the present specification and reproduces information recorded on a magnetic storage medium using a magnetoresistive effect exhibited by the multilayer film.

  According to this, since the multi-layer film having a small element resistance and a high resistance change rate is provided and a high sensing capability can be ensured, even when information is recorded at a high recording density on the magnetic storage medium, the information is stored. It is possible to reproduce with high accuracy. In addition, since a multilayer film that hardly oxidizes and has a Curie temperature of room temperature or higher is used, it is possible to ensure reproduction performance over a long period of time.

  The magnetic storage device described in this specification is a magnetic storage device including the magnetic head described in this specification and a magnetic storage medium that stores information readable by the magnetic head.

  According to this, even when information is recorded on the magnetic storage medium at a high recording density, the information can be reproduced with high accuracy and the magnetic performance is ensured over a long period of time. The reproduction performance of information recorded at a high density can be maintained high over a long period of time.

  The multilayer film described in this specification has an effect that a high magnetoresistance change rate can be obtained even if the element resistance is reduced. In addition, the magnetic head described in this specification has an effect that information recorded on an information recording medium with high recording density can be read with high accuracy. Furthermore, the magnetic storage device described in the present specification has an effect of maintaining a highly accurate information reproducing capability over a long period of time.

  Hereinafter, an embodiment of a multilayer film, a magnetic head, and a magnetic storage device according to the present invention will be described with reference to FIGS.

  FIG. 1 shows an internal configuration of a hard disk drive (HDD) 100 as a magnetic storage device according to the present embodiment. As shown in FIG. 1, an HDD 100 includes a box-shaped housing 11, a magnetic disk 20 as a magnetic storage medium housed in a space (housing space) inside the housing 11, a spindle motor 15, a head stack assembly ( HSA) 16 and a control board (not shown) on which an LSI or the like is mounted. In FIG. 1, the upper lid of the housing 11 is not shown in order to show the inside of the housing 11. The head stack assembly 16 has a magnetic head 30.

  The magnetic disk 20 is driven to rotate at a high speed by the spindle motor 15. A plurality of magnetic disks 20 may be provided along the rotation axis (along the direction orthogonal to the plane of FIG. 1).

  FIG. 2 is a schematic diagram of the magnetic head 30 and the magnetic disk 20. On the glass substrate 20A of the magnetic disk 20, a soft magnetic layer 21, a nonmagnetic intermediate layer 22, a hard magnetic layer 23, a protective layer 24, and a lubricating layer 25 are sequentially laminated.

  The magnetic head 30 includes a recording head 31, an upper magnetic shield 32, a lower magnetic shield 33, and a TMR element 40 as a multilayer film provided between the magnetic shields 32 and 33. The recording head 31 includes a main magnetic pole 311, an auxiliary magnetic pole 312, and an information recording coil 313.

  FIG. 3 is a schematic cross-sectional view of the TMR element 40. The TMR element 40 includes a nonmagnetic layer 400 that is an insulating thin film, and a first ferromagnetic layer 401 and a second ferromagnetic layer that are provided on both sides of the nonmagnetic layer 400 and sandwich the nonmagnetic layer 400 therebetween. 402 and an antiferromagnetic layer 403. On the substrate 40A of the TMR element 40, an antiferromagnetic layer 403, a second ferromagnetic layer 402, a nonmagnetic layer 400, and a first ferromagnetic layer 401 are stacked in this order. Here, as the material of the nonmagnetic layer 400, for example, MgO is adopted.

The first and second ferromagnetic layers 401 and 402 are made of Co ₂ FeB. That is, the atomic composition ratio of Co, Fe, and B is Co: Fe: B = 2: 1: 1. Further, the first and second ferromagnetic layers 401 and 402 have a crystal structure shown in FIG. That is, the first and second ferromagnetic layers 401 and 402 each have an L2 ₁ ordered structure. This L2 ₁ ordered structure is ordered by arranging B (boron) in a substitutional form with respect to the bcc structure (body-centered cubic structure) of CoFe. Further, the lattice size of the L2 _1- ordered crystal is 5.37 mm when estimated from the electronic state calculation by the density functional method using GGA (Generalized gradient approximation) approximation.

  FIG. 5 is a conceptual diagram of characteristics based on the tunnel magnetoresistive effect by the TMR element 40. Here, the magnetization direction of the first ferromagnetic layer 401 changes according to the magnetic field generated from the recording bit of the magnetic disk 20. On the other hand, the spins of the second ferromagnetic layer 402 are aligned in one direction due to exchange interaction with the antiferromagnetic layer 403, so the magnetization direction of the second ferromagnetic layer 402 is fixed. In this case, according to the change in the magnetization direction of the first ferromagnetic layer 401, the magnetization directions of the first and second ferromagnetic layers 401 and 402 are parallel (state (a) shown in FIG. 5) or antiparallel. (State (b) shown in FIG. 5).

  As described above, when the relative directions of the magnetizations of the first and second ferromagnetic layers 401 and 402 change according to the magnetic field generated from the recording bit, the electric resistance of the TMR element 40 changes. The rate at which this electrical resistance changes is called the magnetoresistance change rate. In the TMR element 40, when the magnetization direction of the first ferromagnetic layer 401 is in the state shown in FIG. When a current flows and the state shown in FIG. 5B is reached, only a small current flows. The resistance change indicated by the current is output as an electric signal from the first and second ferromagnetic layers 401 and 402 to a circuit (hard disk controller (HDC) or the like) on the control board. Information recorded on the disk 20 is read. That is, the information stored in the magnetic disk 20 is reproduced using the magnetoresistive effect exhibited by the TMR element 40.

Here, the magnetoresistance ratio, first, the electrical resistance of the TMR element 40 when the direction of magnetization of the second ferromagnetic layer 401 and 402 are parallel to (device resistance) _{R 1,} TMR element when antiparallel 40 of the electrical resistance when the _{R 2,} represented by the following formula (1).
Magnetoresistance change rate = (R ₂ −R ₁ ) / R ₁ (1)

The unit of _{R 1} and _{R 2} are usually represented by 1μm resistance converted per ^{2 (Ω} · ^{μm 2).} Since the TMR element 40 includes the insulating nonmagnetic film 400, the resistance is larger than that of the GMR element having the same element area.

In the present embodiment, the area of the TMR element 40 corresponding to the fine recording bits of the magnetic disk 20 is very small. On the other hand, the film thickness of the nonmagnetic layer 400 can be made thinner than that of the conventional TMR element. For example, the film thickness of the nonmagnetic layer 400 can be less than about 0.7 nm. The element resistance R ₁ of the TMR element 40 is as small as about 2Ω · μm ² or less. For this reason, since impedance matching between the element and the circuit when taking out the resistance change in the element as an electric signal can be taken, the signal characteristic at high frequency can be maintained well.

FIG. 6 shows the distribution of the density of states (DOS) of all electrons in the crystal structures of the first and second ferromagnetic layers 401 and 402. Here, the conduction electrons of the Fermi level E _F vicinity contributing to current, most of which are occupied by majority spin electrons shown by the solid line (where the up-spin electrons). On the other hand, in the Fermi level E _F vicinity, the density of states of the minority spin electrons shown by a broken line (in this case down-spin electrons) is approximately zero. According to the analysis of the first principle electronic state analysis using GGA approximation, polarization of conduction electrons in the Fermi level E _F vicinity, 98% approaching the half-metal. Here, the polarizability P can be expressed by the following equation (2) when the electronic state density at Fermi energy is N _UP (up spin side) and N _DOWN (down spin side).
P = (N _UP −N _DOWN ) / (N _UP + N _DOWN ) (2)

FIG. 7 shows the electron state density distribution of FIG. 6 for each electron. Among these, FIG. 7A shows the d electron state density distribution of Co, FIG. 7B shows the d electron state density distribution of Fe, and FIG. 7C shows the Co ₂ FeB of L2 ₁ ordered structure. The s electron density of states distribution is shown. Here, it is mainly s electrons that contribute to the current in the TMR element. Thus, referring to s electronic state density distribution in FIG. 7 (c), a state density of approximately zero down spin side of the Fermi level E _F (vertical axis negative side), conduction electrons only up-spin end electronic is there. Therefore, the polarizability of s electrons is approximately 100% from the above equation (2).

On the other hand, a diagram showing the electronic state density distribution of CoFeB having an amorphous structure is shown in a paper by P. Paluskar et al. (P. Paluskar et al: Phys. Rev. Let. No. 100, 057205 (2008)). This is referred to in FIGS. 8A to 8C. 8A shows the d electron state density distribution of Co, FIG. 8B shows the d electron state density distribution of Fe, and FIG. 8C shows the s electron state density distribution of CoFeB. Again, referring to FIG. 8 (c) showing the s-electron density of states distribution in the Fermi level E _F vicinity, towards the density of states of up-spin side represented by the vertical axis positive side than the density of states of the down-spin side Is also big. According to the above paper, the polarizability of s electrons in FIG. 8C is 40 to 50%.

As described above, when viewed with s electrons contributing to current in the TMR element, the composition ratio (Co: Fe: B = 2: 1: 1) and the polarization of Co ₂ FeB which is an L2 _1- ordered crystal structure are described. The rate is much higher than the polarizability of CoFeB having an amorphous structure.

  Here, the film thickness of the nonmagnetic layer 400 is very thin. In general, reducing the film thickness in this way leads to loss of the property of transmitting electrons. However, since the nonmagnetic layer 400 made of MgO easily allows only s electrons to pass through and the polarizability of s electrons in the ferromagnetic layers 401 and 402 is remarkably high, the thickness of the nonmagnetic layer 400 is reduced. However, a high magnetoresistance change rate can be obtained. Specifically, the magnetoresistance change rate of the ferromagnetic layers 401 and 402 of this embodiment is 1000% or more.

  In the case of CoFeB having an amorphous structure, the polarization direction of d electrons is reversed from the polarization direction of s electrons shown in FIG. 8C, as shown in FIGS. 8A and 8B. In the ferromagnetic layers 401 and 402 of the form, as shown in FIGS. 7A and 7B, the polarization direction of d electrons is the same as the polarization direction of s electrons shown in FIG. 7C. That is, since the high polarizability of s electrons is not diminished by the polarization state of d electrons, the ferromagnetic layers 401 and 402 of this embodiment have a polarizability as a whole of electrons including s electrons and d electrons. high.

FIG. 9 shows a table comparing the amorphous CoFeB and the L2 ₁ ordered Co ₂ FeB according to the present embodiment. Even if the structure of amorphous CoFeB is crystallized, it is not a clear crystal structure because it is only a polycrystalline part of which is crystallized. Since the polarizability of this amorphous CoFeB is small as described above, a high magnetoresistance change rate cannot be obtained. In contrast, Co 2 _FeB is L2 ₁ ordered crystals, since down-spin state density at the Fermi level E _F as described above is substantially 0, high magnetoresistance ratio.

  Here, the operations and effects as described above are achieved not only by limiting the composition but also by providing a crystal structure related to the composition. This is the significance of the multilayer film described in this specification. As described above, CoFeAl or the like is known as a Heusler alloy, but any material other than Al may be employed. Actually, for example, the website of RIKEN (http://www.riken.jp/lab-www/nanomag/research/heusler_j.html) includes Si, Ga as atoms that can be substituted for Al. , Ge, As, In, Sn, Sb, Tl, Pd, and Bi.

For this reason, when atoms other than those described above are substituted with Al, the same characteristics as the Heusler alloy are not always obtained. For example, when replacing the P and Al, as shown in FIG. 10, does not increase the polarization at the Fermi level E _F (down-spin electrons is very large). The characteristics are not determined only by the combination of Co and Fe, but the formation of the density of states by mixing the s-electron and p-electron orbitals of B with the s-, p-electron, and d-electron orbitals of Co and Fe. This is because how it is done is important.

As described in detail above, according to this embodiment, as the ferromagnetic layer 401 and 402, the atomic composition ratio of Co: Fe: B = 2: 1: 1, and the L2 ₁ ordered crystal structure Since the material having this is used, the high polarizability of the ferromagnetic layers 401 and 402 can realize a high magnetoresistance change rate while reducing the element resistance. Thereby, since the high sensing capability of the magnetic head 30 with respect to the recording bits of the magnetic disk 20 can be ensured, even if information is recorded on the magnetic disk 20 with a high storage density, the information can be reproduced with high accuracy. . Further, since the element resistance is low, impedance matching can be achieved regardless of the high frequency and low frequency, and the SN of the output signal can be maintained well. Thereby, data reproduction using a high-frequency signal can be performed, and the reproduction speed can be increased.

  In the present embodiment, since the ferromagnetic layers 401 and 402 do not include easily oxidizable materials such as Mn, Al, and Si, it is possible to suppress deterioration of characteristics due to oxidation, and to improve the reliability of the magnetic head 30 and the HDD 100. It can be maintained for a long time.

In the present embodiment, since the lattice size of the L2 ₁ ordered crystal of Co ₂ FeB used for the ferromagnetic layers 401 and 402 is small, the junction between the nonmagnetic layer 400 having a small lattice size and the ferromagnetic layers 401 and 402 is used. Can be improved.

Further, Co ₂ FeB used for the ferromagnetic layers 401 and 402 contains 75% of CoFe and has a Curie temperature of room temperature or higher, so that there is no problem in practical use.

  The above-described embodiment is an example of a preferred embodiment of the present invention. However, the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention.

For example, a GMR element can be employed as the multilayer film. FIG. 11 is a schematic cross-sectional view of the GMR element 50. As shown in FIG. 11, an antiferromagnetic layer 403, a second ferromagnetic layer 402, a nonmagnetic layer 500 that is a conductive thin film, and a first ferromagnetic layer 401 are formed on a substrate 50A of the GMR element 50. They are stacked in order. Here, first, second ferromagnetic layer 401 and 402 is a _Co 2 FeB crystal composition ratio and L2 ₁ normalized structures described above. On the other hand, the nonmagnetic layer 500 is made of Cu, for example. The GMR element 50 of the present embodiment is a CPP-GMR (CPP: Current Perpendicular to the Plane) element, and a current flows in the orthogonal direction of each layer of the GMR element 50 in accordance with the magnetic field change of the recording bit of the magnetic disk 20. Flowing.

Here, unlike the nonmagnetic layer 400 of the TMR element 40, the nonmagnetic layer 500 of the GMR element 50 does not easily pass only s electrons. Therefore, the entire electronic density of states, including s-electrons and d electrons is important, it is only electronic Fermi level E _F near the conduction electrons of up spin as shown in FIG. 6, a ferromagnetic layer Since the polarizabilities of 401 and 402 are high, a high magnetoresistance change can be obtained also in the GMR element 50.

The multilayer film can be applied to an MRAM (Magnetic Resistive Random Access Memory) other than the above-described TMR element and GMR element. In the MRAM, MTJ (Magnetic Tunnel Junction) elements as multilayer films are arranged in a matrix. Each MTJ element has a nonmagnetic layer and a ferromagnetic layer sandwiching the nonmagnetic layer. Therefore, by using Co ₂ FeB having the above-described composition ratio and L2 _1- ordered structure as the ferromagnetic layer, it is possible to promote high memory density due to the high polarizability of the ferromagnetic layer even in MRAM. It becomes.

The configuration of the multilayer film is not limited to the above-described example. In the example described above, each of the first and second ferromagnetic layers in the multilayer film is a single layer. However, the present invention is not limited to this, and each layer may be formed of a plurality of films including a nonmagnetic film. An intermediate layer having an arbitrary function may be interposed between the first ferromagnetic layer, the nonmagnetic layer, the second ferromagnetic layer, and the antiferromagnetic layer. Furthermore, only one of the first and second ferromagnetic layers may be formed of Co ₂ FeB having the above-described composition ratio and L2 ₁ ordered structure. Also in this case, since the polarizability can be increased, a high magnetoresistance change rate can be obtained.

It is a figure which shows the structure of the hard-disk drive based on embodiment of this invention. It is a schematic diagram of a magnetic head and a magnetic disk. It is a cross-sectional schematic diagram of a TMR element. The first is a diagram showing the L2 ₁ ordered _Co 2 FeB crystal of the second ferromagnetic layer. It is a conceptual diagram shown about the characteristic based on a magnetoresistive effect. It is a distribution diagram of the density of states of all electrons in the crystal structure of the first and second ferromagnetic layers. The figure which showed the electronic state density distribution of FIG. 6 for every component, respectively. It is a quote from the paper of P. Paluskar et al., Showing the electronic state density distribution of CoFeB having an amorphous structure. The table | surface which compared CoFeB of an amorphous structure with L2 ₁ ordered Co2FeB which concerns on this embodiment is shown. Is a diagram showing characteristics of L2 ₁ ordered _Co 2 FeP crystals. It is a cross-sectional schematic diagram of a GMR element.

Explanation of symbols

20 Magnetic disk (magnetic storage medium)
30 Magnetic head 40 TMR element (multilayer film)
400 Nonmagnetic layer (insulating nonmagnetic layer)
401 First ferromagnetic layer (ferromagnetic layer)
402 Second ferromagnetic layer (ferromagnetic layer)
50 GMR element (multilayer film)
500 Nonmagnetic layer (conductive nonmagnetic layer)

Claims (5)

A non-magnetic layer;
A ferromagnetic layer provided on each of the front and back sides of the nonmagnetic layer, and
Wherein at least one of the ferromagnetic layers, the atomic composition ratio of Co: Fe: B = 2: 1: 1, and the multilayer film characterized by having a L2 ₁ ordered crystal structure.   The multilayer film according to claim 1, wherein the nonmagnetic layer has an insulating property.   The multilayer film according to claim 1, wherein the nonmagnetic layer has conductivity. The multilayer film according to any one of claims 1 to 3,
A magnetic head for reproducing information recorded on a magnetic storage medium by utilizing a magnetoresistive effect exhibited by the multilayer film. A magnetic head according to claim 4,
And a magnetic storage medium for storing information readable by the magnetic head.

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