Giant magnetoresistance in NiFe-Ag granular alloys

Some FeNi-Ag granular films of composition Fe ld%.w‘h8z.2z (sample A) and Fe7.62Ni16.4Ag75.98 (B) were prepared by using rf magnetron sputtering, gnd once deposited were rapidly annealed at 600, 650, and 750 “C. All samples displayed giant magnetoresistance. The zero-field-cooled and field-cooled processes evidence the segregation of ferromagnetic particles with a broad size distribution. The temperature and magnetic field dependence of the resistance is analyzed. The magnetoresistance follows a H” law at high fields and it decays from its maximum value with a T”’ behavior, with m approaching 1 at high fields.


I. INTRODUCTION
The discovery of giant magnetoresistance (GMR) effects in a variety of antiferromagnetically coupled transition-metal multilayers has opened a new amazing research field not only from the fundamental point of view but also from the technological one.Recently, this extraordinary effect has also been found in granular alloys' consisting of a distribution of nonaligned nanocrystalline ferromagnetic particles embedded in a nonmagnetic metallic matrix.In both kinds of systems, the resistivity strongly drops as the magnetic field orients the magnetic moments.Concerning theoretical explanations, both the existence of a spin-dependent potential scattering either at the interfaces or in the bulk of ferromagnetic layers (or particles) and the role of the unfilled d bands of the transition metal constituent (through an asymmetric density of states for majority-and minority-spin d bands)3P have been taken into account in order to correlate GMR with the microscopic parameters.The magnitude of GMR has been found to be a sensitive function of both the size of the ferromagnetic particles and the concentration of the ferromagnetic material in the alloy.The former effect is postulated to be due to the existence of an optimum particle size, determined by the conduction electron mean free path or spin diffusion length.Larger particles result in a reduction of GMR as a result of the decrease in particle surface-tovolume ratio.3The latter effect is believed to be due to the onset of percolation, which acts to couple the particles ferromagnetically.l"We present in this article the temperature and magnetic field dependence of the resistance of NiFe-Ag granular alloys presenting GMR.' II.EXPERIMENT Ag-Ni-Fe lY.ms of thickness 200-300 nm were rf sputtered onto glass microscope slides using a Nordic0 2000 sputtering system.The base pressure was less than 2X10m7 Torr, the sputtering pressure was 8 mTorr of argon and the sputtering power was 300 W. The target used consisted of a 4 in.Ag (99.999%) disc onto which were placed Ni,,Fe, and Fe 0.25 cm2 squares arranged in a mosaic pattern.In order to promote post-deposition phase segregation and magnetic particle growth, strips of about 7 mm wide were rapidly thermally annealed in a custom built vacuum system.Three an-nealing temperatures were investigated: 600, 650, and 750 "C, and these were reached in 20 s, 2 min, and 3 min, respectively.Resistance and magnetoresistance (MR) of all samples were measured by an ac four-point probe technique in the temperature range 20-300 K and in magnetic fields up to 12 kOe.The relative geometry among the flm plane, the electrical current, and the magnetic field was set by three ways: (a) the electrical current and the magnetic field are parallel to the film plane (parallel geometry); (b) the in-plane magnetic field is perpendicular to the electrical current (transversal geometry); and (c) the magnetic field is perpendicular to both the electrical current and the film plane (perpendicular geometry).The zero-field-cooled (ZFC) and fieldcooled (FC) processes at low fields and the magnetization curves at 5 K up to 55 kOe were carried out by applying the magnetic field along the film plane using a superconducting quantum interference device magnetometer.

Ill. RESULTS AND DISCUSSION
The structure of some of the thin films (d=50 nm) were investigated by transmission electron microscopy (TEM) in a modified JEOL 2000 electron microscope.Films were deposited onto Si substrates into which a SiN covered window had been etched and were found to have a strong (111) texture.A number of films were also investigated by using a Philips x-ray diffraction (XRD) system.This confirmed the strong (111) texturing but in neither the TEM nor the XRD was any clear evidence of phase segregation of the Ni or Fe from the Ag matrix.Magnetic and transport properties were measured on films which had composition Fel,.43Ni,,,Ags2,22 (sample A) and Fe7.62Ni16.4Ag75.98(sample B).As both samples were rapidly annealed at 600, 650, and 750 "C, we will refer to them as A(as cast), A(600), A(650), A(750), B(as cast), B(600), B(650), and B(750), respectively.
Figure 1 shows the ZFC-FC processes for sample A(650) measured at 100 Oe.The ZFC displays a broad maximum at TM-22 K, suggesting the existence of a broad size distribution of ferromagnetic particles.As magnetic irreversibility persists up to high temperature, we expect very large particles to be present in the sample.We plot in Fig. 2   ated with an intrinsic in-plane magnetocrystalline anisotropy) because there is no difference between the MR in the parallel and transversal geometry.All measurements have been recorded with increasing and decreasing field, and we observe a slight irreversibility at low temperatures below the coercive field.
We have analyzed the temperature dependence of the MR as Mattson et aL6 by defining the MR as

RM(T,H)=R(T,H=O)-R(T,H),
where R(T,H) is the resistance measured at a temperature T and in an applied field H.The total resistance at T and H is assumed to be given by R(T,H)=R,+R,,(T)-tR,(T,H), where R. is the resistance due to defects, R,,(T) is the temperature dependent resistance due to phonons and magnons.We show in Fig. 5 the temperature dependence of R&T,H) at different fields for sample A(650).R,(T,H) displays a monotonic increase as temperature goes down, which is in agreement with the progressive blocking of the ferromagnetic particles (Fig. I).We plot in the inset of Fig. 5 the temperature dependence of R,&",H)V3 at various fields for the same sample.We notice that R,(T,H)U3 is perfectly linear with T in the whole temperature range 20-290 K when the magnetic field is the maximum available in our experimental setup (H,,=12 kOe), and that the linear law R,(T,H)" versus T is followed in a smaller temperature range as we reduce the magnetic field.The same l/3 exponent and temperature dependence of R,(T,H) is found for sample B(650).If we extrapolate the data at T=O, we obtain the R,(T=O,H) and we may define hR,= R,(T=O,H) -R&T,H).
The log-log plot of ARM versus T is displayed in Fig. 6  law gives us an idea of the underlying scattering mechanism.It is evidenced that the temperature range in which the power law is accomplished increases with magnetic field (as expected, since the MR saturates at large fields).The exponent m slightly increases with H and seems to tend to about 1, which is smaller than the T3" and T2 laws found by Mattson et aL6 in FelCr multilayers.These behaviors are attributed to the thermal excitation of magnons.The temperature dependence of the MR of granular materials is complicated by there being a distribution of particle sizes and therefore blocking processes.We might tentatively attribute the temperature dependence of &, at high fields to the thermal excitation of magnons with a smaller exponent in the T" law due to the reduction of the magnetic system dimensionality.
Concerning the field dependence of the MR, we have observed that RM(T,H)/R(T,O) follows a Hn behavior at high fields (above about 6000 Oe), as was found by Nigam et aL7 in Aus7Fer3 cluster glass.Solids lines in Figs. 3 and 4 indicate the best fit of the data to the H" law.The e-exponent monotonically increases with temperature, ranging from 0.18 at 21.6 K to 0.86 at 290 K for samples A(650), and from 0.13 at 21.5 K to 0.76 at 282.1 K for sample B(650).The error in n is about 0.02.This monotonic temperature behavior evidences the-progressive blocking of the ferromagnetic particles, without being a freezing state corresponding to a spin glass behavior.Also, it is always smaller than the n =2 value expected for a pure paramagnetic state,7 signaling that magnetic correlations persist even at room temperature and/or larger particles are still blocked at this temperature, since the size distribution of ferromagnetic particles seems to be very broad (see Fig. 1).
FIG. 1. ZFC and FC processes measured at 100 Oe for sample A(650).
FIG.6.Log-log plot of the temperature dependence of AR,-R,(T=O,H)-R,(T,H), at (a) 2 kOe and at (a) 12.1 kOe for the sample A(650).Solid lines correspond to the best fit of the data to a T'" law.