Magnetic Phase Transitions in Epitaxial Fe/Cr Superlattices
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T (K) Figure 3: Resistivity of an [Fe (14A)/Cr(70A)] 1 3 superlattice. (a) p vs. T measured at H=500 Oe. The dashed line is a linear extrapolation of the data above 280 K. (b) The difference between the measured p and the linear extrapolation plin normalized to Plin. (c) Derivative of p smoothed for clarity. The minimum in dp/dT locates TN.
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T (K) Figure 4: Temperature dependent magnetization results for the superlattice shown in Fig. 3. (a) Squareness ratio Mr/Ms; (b) coercivity, (c) saturation field defi'ed at 90% of Ms, and (d) the magnetoresistance. The vertical dashed line locates TN of the Cr interlayers.
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TN from a value of 0.53Ms for T>TN to =0.95Ms for TTN, reaches a maximum at -230K, and then decreases sharply and approaches zero at TN. The MR in Fig. 4(d) also shows an anomaly at TN, and its decrease is consistent with a loss of interlayer coupling below TN. The Mr value for T>TN is consistent with a 90' alignment of the magnetization of adjacent Fe layers, indicative of biquadratic interlayer magnetic coupling, while the Mr value for TTN [2 1]. Utilizing both the transport and magnetic properties to identify TN, Fig. 5 shows TN
tCr for a series of (001)-oriented Fe(14A)/Cr(tCr) superlattices. For tCr42A TN increases rapidly and reaches a value of 265K for a 165-A Cr thickness. For a 3000-A thick Cr film grown in similar fashion to the superlattices, a TN value of 295K was obtained. A number of factors can alter the TN value of Cr, including impurities, strains and defects.11, 19 Studies of Cr(001) films on LiF(001) substrates reported thickness dependent TN values attributed to epitaxial strain. 19 The behavior of TN vs. tCr reported herein can be understood as arising from a combination of finite-size effects within the Cr spacer and spin-frustration effects at the Fe-Cr interface, as is discussed in what follows. Vs.
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Figure 5: TN for a series of [Fe(14A)/Cr(tCr)]N superlattices vs. Cr thickness. The dashed and solid lines are fits to Eq. (2) and (3), respectively. The inset shows a possible spin configuration of Cr on a stepped Fe surface in which the region of spin frustration at the Fe-Cr interface is shown schematically by the shaded ellipse to the right of the atomic step.
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In thin films, magnetic properties are altered due to the surface contribution to the free energy [221. Since this contribution is generally positive, the magnetic order is weakened at the surface and the ordering temperature is reduced. Scaling theory predicts that TN should have the form: TN(-o) - TN(tCr) = b tcr TN(tCr)
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where X,-/v is the shift exponent, b is a constant and v is the correlation-length exponent for the bulk system. The theoretically expected , values are 1/0.7048=1.419 [23] and 1/0.6294 = 1.5884 [24] for the 3D Heisenberg and Isi
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