ELSEVIER

Journal of Magnetism and Magnetic Materials 140-144 (1995) 1757-1758 ~ Journal of

magnetism and magnetic materials

Effect of coherent anisotropy on the H-T phase diagram of random anisotropy magnets of amorphous DyGdFe

T. Saito *, H. Miyano, K. Shinagawa, T. Tsushima Dept. of Physics, Faculty of Science, Toho Univ., Funabashi City, Chiba, 274 Japan

Abstract For two amorphous (a-) DyGdFe films, one with a weak coherent anisotropy (CA) and the other with a strong CA, we

measured magnetization and ac susceptibility in small dc fields (H), and determined the H - T phase diagrams. The results suggest that a ferrimagnetic-like phase is induced by the strong CA, and the system shows a double transition from paramagnetic to a ferrimagnetic-like to a randomly-frozen state for H = 0. Without strong CA, such a double transition disappears.

Random Anisotropy Magnets (RAMs) possess both randomness and frustration, and a RAM with a weak ratio of the local random anisotropy, D, to the exchange con- stant, J, resembles the Heisenberg spin glass (SG) [1]. However, unlike the SG, it is considered [2] that the weak RAM has characteristic phases such as correlated spin glass (CSG) and ferro(ferri)magnet with wandering axes (FWA) phases; in the CSG, the total magnetization is zero although a region with a ferromagnetic correlation is pre- sent; an external dc field (H) or 'coherent' anisotropy (CA) may convert the CSG phase into the FWA phase. Though residual CA is inevitably present in amorphous samples prepared by sputtering, evaporation, or by splat cooling etc., the effect has been scarcely investigated experimentally [3,4]. In this paper, we studied the effect of uniaxial CA ('perpendicular anisotropy' normal to the amorphous film) on the critical behavior in weak RAMs by measuring magnetization and ac susceptibility in small dc fields.

Amorphous DyloGdaFes6 films were prepared by the rf sputtering technique onto glass substrates. X-rays were used to check the amorphous structure. By measurements of magnetic hysteresis loops, we selected two samples: one with strong CA above 50 kOe of anisotropy field and the other with weak CA of less than 4 kOe of anisotropy field. Judging from ICPS analysis, the difference in composition between the two samples is within 0.4 at%. The tempera- ture dependence of both field-cooled (FC) and zero-field- cooled (ZFC) magnetizations was measured by a vibrating-sample magnetometer. Longitudinal (X l) and

* Corresponding author. Fax: +81-474-751855.

transverse (X t) ac susceptibilities were measured with dc fields H parallel and perpendicular to the excitation field h 0 ( 2 0 e , 270 Hz), respectively. Both H and h 0 were applied parallel to the film plane to minimize the demagne- tizing field.

Figs. l(a) and (b) show X l and X t as a function of H for a-DyloGdaFe86 with weak CA. For H = 0, a large peak is observed. When H is increased, the magnitude of X l decreases and the peak separates into two peaks (we define the peak temperatures as T c and Ti), while in X t only one peak exists, and the peak temperature ( - T f ) slightly shifts toward lower T with increasing H. The behavior in both ac susceptibilities under H is similar to that of a-Dy16Fe84 [1] with a weak CA.

Figs. l(c) and (d) show X t and X t for the sample with strong CA. Even for H = 0 two anomalies, a peak and a shoulder, are present. Comparing Fig. l(c) with Fig. l(a), we considered that the peak in H = 0 is induced by the strong CA, and that the peak temperature corresponds to T c and the shoulder temperature in X l corresponds to T i. For X t (Fig. l(d)), when H ~ 0, the shoulder temperature is thought to be Tf by taking into consideration the results of magnetization measurement described below.

Figs. 2(a) and (b) show the characteristic temperatures in the H - T plane for both samples. In Fig. 2, the open circles denote the results of magnetization measurements. The temperature where the irreversibility in magnetization occurs is close to the critical line H(Te), while the temper- ature where ZFC magnetization has an inflection point is close to the H(T i) line. The tendency of the H(T i) and H(Tf) lines of the weak CA sample in Fig. 2(a) is similar to that of Heisenberg SG as discussed in our previous work for a-Dy16Fe84 [1]. On the other hand, for the strong CA sample (Fig. 2(b)), the H(T c) line separates from the

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1758 T. Saito et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1757-1758

H(Tf) line, suggesting that a ferrimagnetic-like phase is induced by the strong CA in this region. In fact, the irreversibility in magnetization occurs not on the separated H(T c) line but near the lower H(Tf) line. Our previous Hall resistivity measurement on a-Dy25Fe75 [3] with strong perpendicular anisotropy also supports the idea that the induced higher phase is ferrimagnetic.

Theoretically, it has been proposed [2,5] that a magni- tude of coherent field strongly affects the critical behavior of random magnetic systems, and that the long-range order broken by random fields is restored by coherent fields. The results in this paper suggest that uniaxial CA such as 'perpendicular anisotropy' stabilizes the long-range order and induces a FWA-like phase above a randomly frozen CSG or speromagnetic phase, so that the system with CA exhibits a double transition. (We note that a double transi-

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tion may also occur in several RAMs with small D / J [6] by another unknown mechanism.)

References

[1] T. Saito, Y. Matsumaru, K. Shinagawa and T. Tsushima, J. Magn. Magn. Mater. 130 (1994) 346, and references therein.

[2] E.M. Chudnovsky, W.M. Saslow and R.A. Serota, Phys. Rev. B 33 (1986) 251.

[3] T. Saito, J. Maedomari, K. Shinagawa and T. Tsushima, J. Phys. (Paris) Suppl. C 8 (1988) 1235.

[4] T. Saito, N. Ohmura, J. Maedomari, K. Shinagawa and T. Tsushima, Solid State Commun. 72 (1989) 147.

[5] Y.Y. Goidschmidt and A. Aharony, Phys. Rev. B 32 (1985) 264.

[6] M.J. O'Shea and K.M. Lee, J. Magn. Magn. Mater. 99 (1991) 103.