Magnetic properties of evaporated Fe-(Pr, Nd, Sm) amorphous alloys

Journal of Magnetism and Magnetic North-Holland. Amsterdam

MAGNETIC

Materials

PROPERTIES

T. MIYAZAKI,

K. HAYASHI,

71 (1987) 83-89

83

OF EVAPORATED

Fe-@,

Nd, Sm) AMORPHOUS

ALLOYS

T. OTAKI

Department of Applied Physics, Tohoku University, Sendai 980, Japan

M. TAKAHASHI

and T. SHIMAMORI

Department of Electronic Engineering, Tohoku University, Sendai 980, Japan Received

5 August

1987

Magnetization and perpendicular magnetic anisotropy have been investigated in the temperature range between 77 and 400 of these K for evaporated Fe lca_xRx (R = Pr, Nd, Sm; 10 < x < 80) films. The magnetic moment and Curie temperature samples were compared with those of amorphous ribbon samples. It seems that there exist no difference between film and ribbon samples in magnetic moment. The iron moment in the amorphous Fe-R alloys decreases with increasing R content. However, the Curie temperature of films is much lower than that of ribbons, especially in x > 30 samples. The dependence of perpendicular magnetic anisotropy on temperature is nearly the same in these amorphous films. The magnetic anisotropy at 77 K exhibits a maximum for Fe-Pr and Fe-Nd films and a minimum for Fe-Sm films at x = 30-40.

1. Introduction During the last 15 years many studies reported on the magnetic properties of the amorphous alloys of heavy rare earth-3d transition metal alloys because these alloys showed a strong potential for magneto-optic storage and bubble devices [l-5]. The proposal of interesting magnetic structures in amorphous rare earth-3d transition metal alloys, asperomagnetic, speromagnetic and sperimagnetic structures [6,7] and the usefullness of the rapid quenching technique to prepare amorphous samples accelerated the study of magnetism for light rare earth-3d transition metal alloys [S-11]. In addition, following a report by Talyor et al. [12], the light rare earth-3d transition metal films attracted much attention because of their perpendicular magnetic anisotropy and significant polar Kerr or Faraday rotations [13-161. In these studies described above the amorphous samples are fabricated by rapid quenching, sputtering or evaporation methods. Even though the X-ray diffraction patterns of these samples exhibit a broad halo and they can be identified as an amorphous structure, it is not clear whether 030~8853/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

their magnetic properties such as spontaneous magnetization and Curie temperature, which are the material constants, depend on the fabrication method or not. Therefore, in order to clarify this question it requires a systematic study of magnetic properties in a wide composition range and various alloy systems. The present paper describes the magnetization, Curie temperature and perpendicular magnetic anisotropy for evaporated FeiW_,R, (R = Pr, Nd, Sm; 10 Q x Q SO) alloys and discusses these magnetic properties in comparison with those of ribbon samples prepared by rapid quenching.

2. Experiment The electrolytic iron 99.5% in purity and light rare earths (99.9%) were arc-melted in an argon atmosphere. The ingot of l-l.5 g was melted in a vacuum of 2 x lop6 Torr and amorphous films were prepared by means of one electron beam evaporation for Fe-(Pr, Nd) and a dual one for Fe-Sm alloys. Films were 3000-6000 A thick and deposited on glass or Kapton substrates at amB.V.

T. Miyazaki et al. / Evaporated Fe - (Pr, Nd, Sm) amorphous alloys

84

3. Experimental

bient temperatures. The evaporation rate was 20-30 A/s. The structure of the film samples was investigated by X-ray diffraction measurements using Cu-Ka and MO-Ka radiation. The composition was analyzed by the SEM-EDX method. The analysis of oxygen was carried out using Augerelectron spectroscopy for the Fe,,Ndx3 and Fe,,Sm,, films which were kept in a vacuum of 1 X 10-l Torr for about 2 months after deposition. The oxygen exists on both the film surface and the substrate side. The total thickness of the oxygen was equivalent to about 10%15% of the sample thickness. The result implies that the oxygen does not much influence the magnetic properties described here after. The magnetization was measured for the samples deposited on the Kapton substrate over the temperature range 77-450 K using a vibrating sample magnetometer in magnetic fields up to 15 kOe. The magnetic anisotropy was examined in the temperature range from 77 K to their Curie temperatures using a torque magnetometer in magnetic fields up to 20 kOe.

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results and discussion

3. I. X-ray structural analysis Amorphous films were obtained in the composition range 25 < x < 60 for R = Pr, 15 < x < 70 for Nd and 20 < x < 60 for Sm. At lower concentrations of R, the films were mixtures of amorphous + a-Fe. 3.2. The dependence ature

of magnetization

on temper-

Figs. la, b and c show the 15 kOe magnetization extrapolated to 0 K for Fe-Pr, Fe-Nd and Fe-Sm amorphous alloy systems, respectively. In the figure, the magnetization for intermetallic compounds [18] is also shown for comparison. As can be seen in the figure, the data of evaporated film samples in Fe-Pr and Fe-Nd systems scatter but there may be no essential difference of the magnetization between film and ribbon samples. The broken line in fig. lc shows the magnetization calculated by assuming simple dilution of Sm to the constant iron moment of 2.2~~. Since the experimental results decrease much steeper than calculated, the moment of iron can be considered

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Fig. 1. (a) 15 kOe magnetization at 0 K as a function of Pr for Fe-Pr amorphous alloys. l : film (amorphous), @: film (Am+ a-Fe), 0: ribbon [9], A: ribbon [17], 0: Fe,,Pr, compound [18]; (b) 15 kOe magnetization at 0 K as a function of Nd for Fe-Nd amorphous alloys. 0: film (amorphous), c): film (Am+ a-Fe), 0: ribbon [8], A: ribbon [19], 0: Fe,,Nd, compound [18]; (c) 15 kOe magnetization at 0 K as a function of Sm for Fe-Sm amorphous alloys. 0: film (amorphous), @: film (Am+ a-Fe), 0: ribbon [20], 0: Fe,Sm and Fe,Sm compounds [18], - - - - - - simple dilution.

T. Miyazaki et al. / Evaporated Fe - (Pr, Nd, Sm) amorphous alloys

to decrease with increasing Sm content. The extrapolation of u vs. x to x = 1.0 predicts that the Sm moment does not contribute to the total magnetization. The magnetic moment of Sm orients at random within a cone (speromagnetic) owing to the negative effective field. On the other hand, Pr and Nd may be regarded to contribute to the magnetization from figs. la and b. The magnetic moments of Pr and Nd are estimated to be 0.35 and 0.26pB, respectively. The average magnetic moment jWifor Feioo_x R, alloys can be expressed as

(1) where iiFe and j& are the magnetic moment of the iron and the rare earth element, respectively. In general j& and j& depend on the content x. For simplicity, we assume that the moment of R is independent of x. By using &,, = 0.35jiB, ,iiNd= 0.26& and ,i&, = 0, we evaluated the dependence of pFe on concentration (fig. 2). In this calculation we used the solid curves as the magnetization data in figs. la, b and c, since the scattering of magnetization data for the samples is considered to be due to the scattering of composition. For the Fe,,Sm,, and Fe&m, ribbon samples, j& obtained from the MSssbauer data [21] is also shown. It is clear from fig. 2 that E_iFedecreases with R content in these alloy systems. The value of J&e exhibits a finite value above x = 70, being different from that of Fe-Y systems [22], dotted line in fig. 2. Further, the agreement of jLiFe between magnetization and Miissbauer data supports the above assumption.

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Fig. 2. Magnetic moment per iron atom as a function content in Fe,,_,R, alloys. 0: Fe,,Sm,, and F%$m, x : Fe,,_,Y, alloys [22].

of R [21],

85

3.3. The dependence of Curie temperature on concentration Figs. 3a, b and c show the Curie temperature as a function of R content for both film and ribbon samples. Also shown in the figure are the data for the compounds [23]. It is noted that T, of the films exhibits a broad maximum around x = 20-30 and deviates from that of the ribbon samples with increasing x. The trend of T, vs. x is very similar to that of Fe-heavy rare earth element amorphous films [5]. From these data it is expected that the R-R negative interaction in films is much stronger than that in ribbon samples. 3.4. The dependence of perpendicular magnetic anisotropy on temperature and concentration Figs. 4a, b and c show the intrinsic perpendicular magnetic anisotropy constant K, as a function of temperature for Fe-Pr, Fe-Nd and Fe-Sm alloy systems, respectively as a function of temperature. In Fe-Pr and Fe-Nd alloys K, is positive, while it is negative (the net magnetization is in the film plane) for Fe-Sm alloys. Besides Fe,,Nd,, and FessSm,, alloys, K, decreases or increases monotonically with increasing temperature and is nearly zero at the Curie temperature. The behavior of K, below 350 K and its steep decrease above 375 K in the Fe,,Sm,, alloy may reflect the existence of a-Fe. The reason of the broad maximum of K, around 130 K for the Fe,,Nd,, sample is unclear at present. In order to see whether the perpendicular magnetic anositropy in these three alloy systems is of the same origin or not, these K, vs. T curves are normalized by the value of K, (0) extrapolated to 0 K, and q. Fig. 5 shows the normalized value of K I (T), K I (T)/K I (0), as a function of normalized temperature, T/T,. As seen in the figure, although the data points scatter, they lie on one curve, suggesting that the origin of perpendicular magnetic anisotropy for these Fe-R (R = Pr, Nd, Sm) alloy systems is the same. Fig. 6 shows the intrinsic perpendicular magnetic anisotropy constant at 77 K as a function of R content. The value of K, for R = Pr and Nd is nearly the same and exhibits a broad maximum

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around x = 30-40. On the other hand, K, for R = Sm exhibits a minimum at x = 30. Concerning the origin of perpendicular magnetic anisotropy, a few models has been proposed for the (Fe, Co)-heavy rare earth systems [24,25]. The typical models proposed are the magnetostrictive effect and pair ordering model. Recently, a single ion model has been proposed to explain the perpendicular magnetic anisotropy for sputtered Gd-R-Co (R = rare earth) amorphous films [26,27]. To support the model is the fact that the dependence of the perpendicular magnetic anisotropy on R is similar to that calculated for rare earth metals based on the single ion model. As a possible mechanism for introduction of the anisotropic atomic arrangement, Suzuki et al. [27] propose the anelasticity model. The key point of the prospect to introduce the anisotropic atomic arrangements is that due to a temperature rise of the films during deposition, an anelastic deformation can easily occur assisted by thermal excitations. In the present case, it is not expected that the temperature of the films increases as high as in the sputtering case. Futhermore, recently Sellmyer et al. [28] investigate magnetic properties of Nd/Fe multilayer films which possess excess Fe-Fe and Nd-Nd pairs in the plane of the film and Nd-Fe pairs perpendicular to the plane of the film but the net magnetization lies in the plane of the film and no perpendicular magnetic anisotropy is observed. This result means that the significant anisotropic distribution of atom pairs does not introduce perpendicular magnetic anisotropy. Thus the origin of the perpendicular magnetic anisotropy in Fe-rare earth amorphous alloy films cannot be decided completely at present. Further measurements, including temperature dependence of the perpendicular magnetic anisotropy in Fe-Dy, Fe-Er and other Fe-rare earth systems together with that of Co-rare earth systems, are in progress.

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Fig. 3. Curie temperature as a function of R content for Fe-Pr (a), Fe-Nd (b) and Fe-Sm (c) alloy systems. 0: film (amorphous), @: film (Am+ a-Fe), 0: ribbon (Fe-Pr 191, Fe-Nd 181, Fe-Sm 1201, A: ribbon [19], 0: compounds [23].

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Fig. 4. The dependence of perpendicular magnetic anisotropy constant on temperature for Fe-Pr systems.

(a), Fe-Nd

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T. Miynzaki et al. / Evaporated Fe - (Pr, Nd, Sm) amorphous alloys

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4. Summary

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Magnetization and perpendicular magnetic anisotropy were investigated for evaporated Fe,,, _ ,~ R, (R = Pr, Nd, Sm, 10 Q x G 80) amorphous films and their magnetic properties were compared with those of amorphous ribbon samples. The main results obtained are as follows: (i) There exist no difference of magnetic moment between the film and ribbon samples. The iron moment of the amorphous Fe-R alloys decreases with increasing R content. (ii) The Curie temperature of the films is much lower than that of ribbons, especially in x > 30 samples. (iii) The net magnetization lies perpendicular to the plane of the film in Fe-Pr and Fe-Nd films and in the plane of the film in Fe-Sm films. The dependence of the perpendicular magnetic anisotropy on temperature in these amorphous films is nearly the same, being irrespective to the kinds of rare earth element. The result implies that the perpendicular magnetic anisotropy in evaporated FeIoO_,R, (R = Pr, Nd, Sm) amorphous films has the same origin. (iv) The perpendicular magnetic anisotropy at 77 K exhibits a maximum for Fe-Pr and Fe-Nd films and a minimum for Fe-Sm films at x = 10-40.

References 5 0 \ ?

VI P. Chaudhary,

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J, --cr

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-2010

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Fig. 6. Perpendicular magnetic anisotropy constant at 77 K as a function of R content for Fe-(Pr, Nd, Sm) alloy systems.

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